HANDBOOK  OF  FABRIC FILTER  TECHNOLOGY.   VOLUME
I.   FABRIC  FILTER SYSTEMS  STUDY

Charles  E.  Billings,  et al

GCA Corporation
Bedford,  Massachusetts

December 1970
                                              Distributed .., 'to foster, serve
                                                 and promote the nation's
                                                    economic development
                                                       and technological
                                                          advancement.'
        NATIONAL TECHNICAL INFORMATION SERVICE

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                     VOLUMC I
            TABRIC FILTEK SYSTFM5 STUDY
             "'.    •     -  by  "•    .'  .v
              "Charles E, B:!!lh9s, Ph.D.  '
               ,   John Wilder, Sc.D. ;•/  '.
                                                        fx#' :•:..'•"  -'^m
                                                        •  ' • i, i*!-,-,'.  'r ..,';
             Contract NO.GPAJ22-69-38
    D!V!S!C-.,'  •
h'ATK )"':.*•!  "f
                    ...,:
                    ADMINISTRATION  ,
                    .vN'AKI'
                                                                          v
i..".' . -.P«1 :.

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 STANDARD TITLE PAGE
 FOR TECHNICAL REPORTS
I« R6pOft No.
 APTD-0690
3TRtelpltnt'i Catalog No.
 47Title and Subtitle
  FABRIC FILTER  SYSTEMS  STUDY - Volume I
  HANDBOOK OF  FABRIC FILTER TECHNOLOGY
                                           5. Report Date
                                            December  1970
                                                              6. PertonMfig Org anlxatlon Coda
 /. Autnorts)
  Charles E. Billings, Ph.D.. John  Wilder.  ScD.
                                           8. Performing Organization Rcpt. No.
 9. Performing Organization Nine and Address
  GCA  Corporation
  GCA  Technology  Division
  Bedford,  Massachusetts
 71. Sponsoring Agency Name and Addma        "
  Division of Process Control Engineering
  National Air Pollution  Control Administration
  U. 3.  Department  of Health, Education and  Welfare,
  Consumer Protection and  Environmental Health Service
  Washington, D.  C.   20201	
 15. Supplementary Notes  ~~   ——————————————
                                           10. Project/Task/Work Unit No.
                                           TT.
                                            CPA 22-69-38
                                           13. Type of Report & Period Covered
                                       PHS
                                            . Sponsoring Agency
          fncyTSTde
 16. Abstracts
 A report  is presented on  fabric filter technology in which the  following
 areas  are discussed extensively:   1.  Current  developments in  fiber fabric
 treatments and applications;   2. The  pressure  drop, efficiency  and per-
 formance  characteristics  observed  in  operating fabric  filters and their
 relationship to  the underlying physical and  chemical phenomena  of the
 collection process;  3. The description of models of currently  available
 fabric  collectors;   4. Fabric and  filter properties of  importance in
 application for  industrial  gas filtration;   5. Engineering factors in-
 volved  in the design of the overall system;   6.  Data on pressure drop
 performance and  its variation;  7. Practical  guidelines for estimating the
 cost of various  fabric filteration approaches;  8. Operation  and mainte-
 nance  of  fabric  filters. •••'•• /•,                ».-.«.
 17. Key Words and Document Analysis, (a). Descriptors
 Air pollution control equipment
 Filters
 Filter materials
 fabrics
 Design
 Mechanical  efficiency
 Maintenance
 17b. Identlflers/OpefrEnded Terms
 Air pollution control
 Fabric  filters
 17c. COSATI Field/Group
                  13/B
 18. Distribution Statement
   Unlimited
                                 19.Security Class(Thls Report)
                                     UNCLASSIFIED
                                                    /((.Security Class. (This Page)
                                                        UNCLASSIFIED
        21. No. of Pages
            649
                                                   22. Price

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         DISCLAIMER

This report was furnished to the Air. Pollution
Control Office by
   GCA Corporation
   GCA Technology Division   .
   Bedford, Massac.huse.tt8    .

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GCA-TR-70-17-G
                              HANDBOOK OF
                       FABRIC FILTER TECHNOLOGY

                               VOLUME I
                      FABRIC FILTER SYSTEMS STUDY

                                 by
                       Charles E.  Billings, Ph.D.
                            John Wilder, ScD.
                            GCA CORPORATION
                        GCA TECHNOLOGY DIVISION
                        Bedford,   Massachusetts
                       Contract  No.  CPA-22-69-38


                             December  1970
                                    ^      I
                            Prepared  for
               DIVISION OF PROCESS CONTROL ENGINEERING
            NATIONAL AIR POLLUTION CONTROL ADMINISTRATION
           U.S. DEPARTMENT OF HEALTH, EDUCATION AND WELFARE
                        Public Health Service

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                                FOREWORD

      This  document  is  submitted to the  Department  of Health,  Education
 and Welfare,  National  Air Pollution Control  Administration,  in  partial
 fulfillment of  the  requirements under Contract  CPA 22-69-38.  The
 principal  technical objectives  under the  contract  were:  (1)  to  evaluate
 the status of engineering technology currently  available  to  the researcher,
 manufacturer  and  user  of  fabric filter  systems;  (2)  investigate the
 current practices in the  application of fabric  filtration;  (3)  investigate
 major air  pollution control  areas  which could be amenable to  control
 by fabric  filtration;  (4)  make  a critical  review and engineering evaluation
 of the major  types  of  fabric filter devices  currently available in order
 to assess  the strength and weakness of  each  type of  device;  (5)  prepare
 a comprehensive report containing  the information  collected  in  the task
 areas cited above;  and (6) develop five-year research and development
 programs specifying the research and development efforts  required to
 fill  the stated technical  gaps.  The results of the  contract  efforts
 are presented in  the following  four volumes:
      Volume I - Handbook  of  Fabric  Filter Technology
      Volume II -  Appendices  to  Handbook of Fabric  Filter  Technology
      Volume III - Bibliography  Fabric Filter Systems  Study
      Volume IV -  Final Report Fabric Filter  Systems  Study
      The following  professional  staff members of the  GCA  Technology Division
 contributed to the  study and preparation of  this report:  Dr.  Charles E.
 Billings, Mr.  Richard  Dennis, Dr. Leonard M. Seale,  and Dr. John Wilder.
 The results of the  contract  efforts, partially presented  in this document,
 covered the period  from January  1969 to January 1971.
     Mr.  Dale Harmon of the  Process Control Engineering Division, National
Air Pollution Control  Administration, served as the Contract  Project
Officer.

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                          TABLE OF  CONTENTS




CHAPTER 1   INTRODUCTION                                          1-1




1..1  GENERAL DESCRIPTION                                         1-3




1.2  HISTORICAL ASPECTS OF  FABRIC FILTRATION                     1-6




I.!]  Ol'KKATINC PRINCIPLES                                        I-'J7




1.4  FABRIC FILTER APPLICATIONS AREAS                            1-43




1.5  SUMMARY                                                     1-55




1.6  REFERENCES                                                  1-56




CHAPTER 2   FABRIC FILTRATION TECHNOLOGY                          2-1




2.1  INTRODUCTION                                                2-3




2.2  DESCRIPTIVE AEROSOL TECHNOLOGY                             2-5




2.3  FABRIC FILTRATION PROCESSES                                 2-78




2.4  FLOW THROUGH POROUS MEDIA                                   2-112




2.5  SYSTEM PRESSURE AND FLOW                                    2-199




2.6  REFERENCES                                                  2-208




CHAPTER 3  TYPES OF FABRIC  FILTERS	                      3-1




3.1  STANDARD AVAILABLE FABRIC FILTER EQUIPMENT                  3-3




3.2  FILTER CONFIGURATIONS                                       3-20




3.3  CLEANING MECHANISMS                                         3-32




3.4  CONSTRUCTION AND MATERIALS                                  3-53




3.5  EXTENSIONS OF FABRIC FILTRATION EQUIPMENT                   3-58




3.6  REFERENCES                                                  3-62





CHAPTER 4  FABRIC SELECTION                                      4-1




4.1  INTRODUCTION                                                4-3




4.2  MATERIALS                                                   4-3





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4.5  FABRIC PRODUCTION                                             4"35




4.6  FABRIC PHYSICAL CHARACTERISTICS                               4-46




4.7  AVAILABLE FABRICS                                             4-59




4.8  REFERENCES                                                    4-60




CHAPTER 5  ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS             5-1




5.1  DESCRIPTION OF PROCESS EFFLUENT TO BE FILTERED                5-4




5.2  DUST COLLECTOR DESIGN                                         5-8




5.3  FAN AND DUCTING DESIGN                                        5-16




5.4  PERIPHERAL EQUIPMENT, INSTRUMENTS AND CONTROLS                5-22




5.5  FINAL SYSTEM DESIGN                                           5-26




5.6  PROCUREMENT AND RESPONSIBILITY                                5-28




5.7  REFERENCES                                                    5-30




CHAPTER 6  FABRIC FILTER PERFORMANCE                               6-1




6.1  INTRODUCTION                                                  6'3




6.2  LABORATORY PERFORMANCE OF CLEANED EQUIPMENT                   6-9




6.3  LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT          6-85




6.4  LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE  CLEANED COLLECTORS 6-99




6.5  FIELD PERFORMANCE                                             6-101




6.6  REFERENCES                                                    6-105




CHAPTER 7 ECONOMICS                                                7-1




7.1  INTRODUCTION                                                  7-3




7.2  INITIAL COSTS                                                 7-8




7.3  OPERATING AND MAINTENANCE COSTS                               7-29




7.4  CLOTH AND BAG COSTS                                           7-39




7.5  ACCOUNTING COMPARISONS OF COSTS                               7-44

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7.6  ECONOMY IN FABRIC FILTER OPERATION                             7-48




7.7  REFERENCES                                                     7-48




CHAPTER 8  OPERATION AND MAINTENANCE                                8-1




8.1  OPERATION OF BAGHOUSE SYSTEM                                   8-4




8.2  MAINTENANCE OF BACHOUSE SYSTEM                                 8-9




8.3  ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION PROBLEMS        8-24




8.4  REFERENCES                                                     8-33

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                            LIST OF FIGURES
Figure No.                                                    Page No.

   1.1         Cost of Gas Cleaning Equipment Treating
               Fine Industrial Dust (From Stairmand,
               Ref. 2) (Basis, 8000 hours per year). . .        1-5
   1.2         Early Fabric Filter for Zinc Oxide
               Control	        1-10

   1.3         Early Filters for Dusts	        1-11
   1.4         Bag Houses for Lead and Zinc Smelting . .        1-13

   1.5         Early Automatic Shaker Mechanism and
               Filter .	        1-15

   1.6         Pre-1940 Equipment Similar to Present
               Designs	        1-19

   1.7         Pulse-jet Configuration (ca 1957) ....        1-23

   1.8         Historical Development and Present Status
               of Filtering Velocity 	        1-24

   1.9         Development of Cleanable Filtration . . .        1-26

   1.10        Estimates of Number of Fabric Filter
               Manufacturers (1950-1969)  	        1-31

   1.11        Average Air Pollution Control Equipment
               Industry  Growth Rate As Estimated from
               Reported  Sales of Two Mature Manufacturers
               of Gas Cleaning Products	         1-33

   1.12        Particulate Air Pollution  Control and
               Fabric Filter Sales  	         1-35

   1.13        Typical Fabric Filter Arrangement.  .  .   .         1-39


   2.1         Characteristics of Particles and Particle
               Dispersions	         2-6
   2.2         Typical Concentrations of  Particulate
               Suspensions	         2-8
   2.3         Photomicrographs of Various Atmospheric
               Particles	         2-15

   2.4         Photomicrographs of Typical Aerosol
               Particles	         2-16

   2.5         Scanning  Electron Photomicrographs  of
               Aggregates  of Monodisperse Test  Particles
               Used to Determine Density  and Aerodynamic
               Diameter	         2-18

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Figure No.                                                     Page No.

   2.6         Coagulation of Metal Oxide Fume	         2-20

   2.7         Effect of Average Particle Diameter of
               Atomized Aluminum on Explosibility
               Index	         2-28
   2.8         Effect of Average Particle Diameter of
               Atomized Aluminum on Explosion Parameters         2-29
   2.9         Calculated and Measured Single Particle
               Electron Charges	         2-33

   2.10        Experimental Measurement of Particle
               Electrostatic Charge	         2-40
   2.11        Simple Method for Measurements of Aerosol
               Net Charge	         2-41
   2.12        Brunauer's Five Types of Adsorption
               Isotherms	           2-44
   2.13        Experimental Methods for Measurement of
               Particle Adhesion Forces	           2-50
   2.14        Adhesion of Spherical Fe Particles of
               4 Microme :er Diameter to Fe Substrate
               at Room Temperature in Air as a Function
               of Applied Force	           2-51

   2.15        Distribution of Adhesion Forces,  logarithmic
               probability plot,  for classified  crushed
               quartz particles collected by filtration
               at V = 42 cm/sec on polyamide (nylon)
               fiber, 50 micrometer diameter, ~50% R.H.           2-53
   2.16        Adhesion of Quartz and Pyrex Particles at
               957= Relative Humidity	           2-54
   2.17        Variation of Particle Adhesion With
               Relative Humidity  	           2-56
   2.18        Integral Adhesion  Curves for Spherical
               Glass  Particles of Different Diameters
               Adhering to a steel  surface of the 13th
               class  of units  of  g  (a), and in absolute
               measure  (b)	           2-57
   2.19        Effect of Particle Size  and Relative
               Humidity on Adhesion for Various

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Figure No.                                                    Page No.

  2.20         Adhesive Force as a Function of the
               Charge Determined on Detaching Glass
               Spheres of Various Diameters (by the
               Vibration Method) From a Painted
               Metal Surface	           2-61

  2.21         Effect of Fiber Size on Adhesion of
               Quartz Particles to Pyrex Fibers .  . .           2-63

  2.22         Effect of Surface Roughness on
               Particle Adhesion	           2-68

  2.23         Effects of Surface Roughness on
               Particle Adhesion	           2-70
  2.24         Schematic of Cake Build-Up and Removal
               in Fabric Filters	           2-73
  2.25         Adhesion of Quartz Particles to Wool
               Felt Fabric and All-Glass Filter Paper           2-77

  2.26         Typical Woven Filter Fabrics 	           2-79

  2.27a        Deposits of 1.305-micron Polystyrene
               Latex Spheres on 8.7 micron Diameter
               Glass Fiber Operated at 13.8cm/sec
               at an Approximate Concentration of
               1000 p/cm3.   Aerosol Flow into
               Photograph	            2-82

  2.27b        Sames as 2.27a, but 9.7 Micron
               Diameter Fiber and  29 cm/sec	            2-83
  2.28         Electron Micrograph of Methylene
               Blue Particles Caught on Glass
               Fibers	             2-84
  2.29a        Magnesium Oxide Fume on Glass  Fiber
               Filter  Paper.  .	             2-85
  2.29b        Zinc Oxide Fume on  Glass Fiber
               Filter  Paper	             2-85
  2.30         Photomicrographs of Fiber 30G  Loaded
               in Observation Chamber (Aerosol  Flow
               from Left to Right).	             2-86

  2.31a        Photomicrographs of Pads Containing
               Aerosol  Special  Low Concentration
               Run;  30  ft/sec	             2-87
  2.31b        Photomicrographs of Pads Containing
               Aerosol  Various  Velocities  and
               Loadings	             2-87

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Figure No.                                                     Page No.

  2.31c        Photomicrographs of Pads Containing
               Aerosol Loaded at 1 ft/sec., followed
               by Clean Air at 3, 10, and 30 ft./sec             2-88

  2.31d        Photomicrographs of Pads Containing
               Aerosol	          2-89
  2.32         Five-ounce Cotton Cloth, the Lower
               Half Partially Plugged with Silica
               Dust.  The Unplugged Meshes in the
               Upper Half Average 0.2 to 0.4 mm ....          2-92

  2.33         Mechanisms of Mechanical Filtration. .  .          2-96

  2.34         Particle Slip Correction Factor  ....          2-99

  2.35         Impaction Efficiency for Sphere-
               Sphere System	          2-101

  2.36         Impaction Efficiency for Spherical
               Particles and Various Obstacles in
               Potential Flow, After Langmuir and
               Blodgett	          2-101

  2.37         Grade-efficiency Curve for Fiber
               Filter Before Particles Accumulate.  .   .          2-104

  2.38         Deposition of Particles in Ascending
               and Descending Streams	          2-107

  2.39         Filtration of Aerosols Through Lead
               Shot	          2-107

  2.40         Operating Fabric Filter Efficiency.  .   .          2-109

  2.41         Fractional Efficiency of Collector-
               N.B.S.  Fly Ash Layer on Cotton
               Sateen Using  Methylene Blue and  Uranine
               Test Aerosols	            2-111

  2.42         Comparison of Theories for Flow
               Relative  to Circular Cylinders                     2-118

  2.43         Permeability-porosity Relationships
               for Kozeny-Carman  and  Brinkman Models .            2-119

  2.44         Typical  Filter Cloth Weaves                       2-124
  2.45         Air Flow  Permeability  vs  Fabric Open
               Area (at  0.5"water pressure differential
               across  fabric)	            2-130

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Figure No.                                                     Page No.

  2.46         Bulking Properties of Various Powders. .  .        2-135

  2.47         Bulkiness of Powders	  .  .        2-135
  2.48         Spherical Packing Arrangements	        2-142

  2.49         Tricuspid Interstices in the Osculatory
               Packing of Finite Areas with Circles.  .  .        2-143
  2.50         Screen Discharge Coefficients, Plain
               Rectangular-Mesh Screens	  .  .        2-147
  2.51         Screen and Fabric Data	        2-148

  2.52         Discharge Coefficient - Reynolds Number
               Relationships for 45 Fabrics	        2-149

  2.53         Correlation of Bed Density with a function
               of Pressure Drop and Superficial Gas
               Velocity	        2-152
                              i
  2.54         Nomograph for K2                                  2-161
  2,55         Specific Resistance Determined by Particle
               Size and Deposit Porosity	        2-163

  2.56         Resistance Factors for Dust Layers.
               Theoretical curves Given are Based on a
               Shape Factor of 0.5 and a true Particle
               Specific Gravity of 2.0	         2-165

  2.57         Measured Filter Resistance Coefficient
               vs  Particle Size	          2-171
  2.58         Resistance Coefficient (K*) vs Particle
               Size for Operating Fabric Filters Surveyed
               in  1969	          2-172
  2.59         Resistance Coefficient (Kl ) vs  Particle
               Size for Crushed  Materials	          2-173
                                        i
  2.60         Resistance Coefficient (K-) vs Particle
               Size for Fumes	          2-175
                                        t
  2.61         Resistance Coefficient (K2) vs Particle
               Size for Fly Ash;  slope = -2 positioned by
               eye	          2-176
                                        i
  2.62         Resistance Coefficient (K?  ) vs.  Particle
               Size for Irregular Particles;  slope  =
               -2;  positioned  by eye	          2-177

  2.63         Resistance Coefficient (Kl)  vs Particle
               Size for Soft Collapsible  Materials;  slope
               = -2 positioned by eye	          2-179

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Figure No.                                                     Page No.
  2.64         Effect of Filtering Velocity on Resistance
               Coefficient (K*2)	     2-181
  2.65         Effect of Fiber Glass Construction on
               Filter Pressure	     2-182
  2.66         Effect of Nominal Velocity on Dust Permeability
               for Fiberglass Fabrics	     2-182
  2.67         Effect of Dust Load on Pressure Drop of Various
               Fabrics	     2-183
  2.68         Clogging of Various Types of Filter Material.     2-183
  2.69         Probable Effects of Fabric Structure on
               K!, the Specific Dust-Fabric Filter Resistance
               Coefficient and Performance in Service During
               Filtration                                        2-186
  2.70         Changes in Specific Resistance Due to Fabric
               Surface	     2-192
  2.71         Influence of New Fabric Air-Flow Permeability
               On Resist. Coef.  (Kp	     2-194
  2.72         Effect of Fabric Charge on Pressure Drop .   .     2-195
                                                  t
  2.73         K2 as Obtained from Prediction vs K~  observed
               from Field Data	     2-197
  2.74         Comparison of Specific Resistances, Predicted
               and Observed,  for NAPCA Data	     2-198
  2.75         Conveying Velocities 	     2-201
  2.76         Typical Fan Curves	     2-207

  3.1           Configurations of Fabric Filters 	     3-5
  3.2           Types of Fabric Filter Systems  Depending
               on Cleaning Method	     3-6
  3.3a         High Temperature  Glass Cloth Baghouse.  .  .   .     3-9
  3.3b         Unit Collector Manually Cleaned	     3-11
  3.3c         Typical Shake-Type Baghouse.-	     3-13
  3.3d         Pulsing Flow Baghouse	     3-14
  3.3e         Reverse Flow Envelope Collector	      3-15
  3.3f         Reverse Jet Baghouse	      3-17
  3.3g         Reverse Plenum Pulse Collector	      3-18
  3.3h         Reverse Flow Cylindrical  Collector  	      3-19

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Figure No.                                                    Page No.
  3.3i        Reverse Pulse Collector	      3-21
  3.3j        Ultrafiltration System 	      3-22
  3.4         Shake Cleaning Process and Associated Pressure
              Cycles	       3-36
  3.5         Reverse Flow Cleaning	       3-39
  3.6         Schematic for Reverse Flow Cleaning During
              Continuous Filter Operation .  	       3-41
  3.7         Reverse Pulse Cleaning 	       3-44
  3.8         Examples of Some Styles of Fabric Filter
              Compartment Joints 	       3-55
  3.9         Types of Hopper Discharge Equipment ....       3-57
  3.10        Ducon Sand and Screen Filter Cleaned by
              Back Flow	       3-59
  4.1         Cross Sections of Filtration Fibers .   .   .       4-6
  4.2         Diagram of Fiber Lay	       4-31
  4.3         Diagram of S and Z Twist  in Yarn	       4-32
  4.4         Plied Yarns	       4-35
  4.5         Weaving Styles for Filtration  Fabrics.  .   .       4-36
  4.6         Effect of Graphite on Glass Fabric Tempera-
              ture Endurance	       4-46
  4.7         Stress Strain Curves for  Fibers  	       4-56
  5.1         Some Fabric Filter System Centrifugal Fans        5-20
  6.1         Parameters Controlling and/or  Describing  the
              Performance of Fabric Filter Systems .  .   .       6-4
  6.2         Fabric Filter Internal Component Parameters       6-6
  6.3a        Values of Pressure Drop vs.  Deposit Weight
              (LTV) in Filtering Fine Petroleum Coke
              Dust	        6-12
  6.3b        Values of Pressure Drop vs.  LTV  covering
              Dust Generated in  Abrasive  Blasting  of Steel
              Paint Drums  on High Twist,  unnapped  Orion
              with an  Extremely  Low Fiber  Surface  Area
              Per  Square Fooot and  Fiberstock  Orion .  .  .        6-12

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Figure No.                                                     Page No.

   6.3c        Effect of Particle size and Shape on
               Filter Resistance of Cotton Sateen
               Cloth.  Silica gel dust, 43% less
               than 10 microns; Limestone dust, 32?u
               less than 48 microns	       6-12

   6.4         Pressure Response in Constant Flow
               Rate Gas Filtration	       6-13

   6.5         Schematic Representation of Basic
               Performance Parameters for Fabric
               Filters	       6-15
   6.6a        Improved Mass Probe	       6-21

   6.6b        Filter Gas Velocity Probe	       6-21

   6.7         Corresponding Mass and Filter Velocity
               Profiles	       6-22

   6.9         Variation in Specific Dust-Fabric Filter
               Resistance Coefficient with Height in
               a Filter Tube	       6-23

   6.10a       Development of Dust Mass Profile Through
               a Filtration Period 	       6-24

   6.10b       Development of Dust Drag Profile  .  .  .  .  .       6-24

   6.11        Variation of Average Specific Dust-Fabric
               Resistance Coefficient During a Filtration
               Cycle	       6-26
   6.12        Decrease in Dust Mass  Permeability Through
               a Filtration Period 	       6-27
   6.13a       Effect of Cleaning on  Residual  Dust  Mass
               Profiles	       6-28
   6.13b       Effect of Cleaning on  Residual  Drag  Profiles       6-28

   6.14        Residual Dust Mass Variation with Cleaning
               Duration	         6-29

   6-15        Cost Analysis in Fabric Filter  Cleaning   .         6-31
   6-16        Schematic Drawing of 2-Bag  Test Unit .  .  .         6-32

   6-17        Input Accelerations in Fabric Filter Cleaning      6-33

   6-18        Effect of Cleaning Duration on  Residual
               Filter Drag for  Several Shaking Conditions         6-34

   6-19a       Effect of Shaker Acceleration on Residual
               Filter Drag	         6-35

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Figure No.                                                      Page No.
  6.19b        Minimum Residual Drag as a Function of
               Shaker Acceleration	         6-35
  6.20         Effect of Dust on Residual-Drag-Cleaning
               Duration Relationship. . 	         6-37
  6.21         Effect of Acceleration and Shaking Duration
               on Residual Drag	         6-37
  6.22         Effect of Shaker Amplitude on Cleaning
               Duration Required for Minimum Residual
               Drag	         6-38
  6.23         Effect of Cleaning Duration on Filter
               Capacity for Several Shaking Conditions  .         6-39
  6.24         Effect of Shaker Acceleration on Filter
               Capacity	          6-39
  6.25         Effect of Acceleration and Shaking Duration
               on Residual Deposit 	          6-40
  6.26         Fabric Filter Performance with Intermittent
               Mechanical Shaking  	          6-42
  6.27         Filter Pressure Drop History  	          6-44
  6.28         Fabric Filter Performance with Intermittent
               Mechanical Shaking 	            6-45
  6.29         Filter Pressure Drop History 	            6-46
  6.30         Determination of Duration for Maximum
               Capacity Per Shaking Stroke 	          6-48
  6.31         Local  Filter Velocity after Various
               Cleaning Durations	          6-48
  6.32a        Effect of Fiberglass  Fabric Fill Count Varia-
               tion 	          6-51
  6.32b        Effect of Fiberglass  Fabric Construction.          6-51
                               p
  6.32c        Effect of Dacron  Fabric  Fill Count Variation      6-51
  6.33         Correlation Between Average Specific Dust
               Fabric Filter Resistance  Coefficient and  Fil-
               tration Rate	          6-52
  6.34a        Effect of Nominal Velocity  on Dust  Discharge
               for  Fiberglass  Fabrics  	          6-55
  6.34b        Effect of Nominal Velocity  on Dust  Discharge
               for  DacronR Fabric	           6-55
  6.35         Effect of Fabric on  Filter  Performance  .           6-59

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Figure No.                                                    Page No.
                                                   n
  6.36         Effect of Shaking Duration on Dacron
               Fabric Filter Performance	       6-60

  6.37         Effect of Relative Humidity on Specific
               Dust-Fabric Filter Resistance Coefficient        6-65

  6.38         Effect of Relative Humidity on Outlet
               Dust Concentration	       6-67

  6.39         Relationship of Particle Concentration
               and Filter Time at Various Relative
               Humidities	       6-68

  6.40         Effect of Filtration Rate on Particle
               Size in Deposit at four bag altitudes. . .       6-71

  6.41         Effect of Filtration Rate on Particle
               Size Distribition in Deposit at Center
               of Bag	       6-71

  6.42         Successive Deposit Collapse Observed
               On Pilot-Scale 2 Bag Filter Unit	       6-73

  6.43         Deposit Puncture Observed on Bench-Scale
               Filter	       6-75

  6.44         Efficiency and Pressure Drop;  New Cotton
               Bags with Atmospheric Dust	       6-77

  6.45         Filter Pressure Drop During Filtering
               and Shaking	       6-81
  6.46         Effect of Dust Loadings on Rate of Filter
               Pressure  Drop Increase 	       6-82
  6.47         Three Compartment Baghouse 	       6-84
  6.48         Fly Ash Fallout vs.  Gas Throughput for
               Top and Bottom Feed	       6-85

  6.49         Particle  Size Distribution of  Fly Ash. .  .        6-86
  6.50         Dust Cake Compression .	        6-86

  6.51         Instantaneous Filter  Drag Profile for Six
               Compartment  Baghouse	        6-88

  6.52         Velocity  Pattern in  Six Compartment Baghouse
               as  a Function of Time	        6-88

  6.53         Schematic Pressure Differential  Curve  for
               Multicompartment Baghouse	        6-89

  6.54         Cloth Tube  Filter Cleaned by Mechanical
               Rapping and  Back Flow Air	        6-91

  6.55         Variation of  Filter with  Inlet Dust
               Concentration	        6-98

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Figure No.                                                   Page No.
   7.1         Trade-off in Costs Due to Collector
               Size, for Fixed Gas Flow	          7-4
   7.2         Gas Cleaning Costs for Dusts of
               Various Particle Sizes, for
               300,000 CFM and 8000 hrs/year	          7-5
   7.3         Fabric Filter Annual Cost Distribution          7-7
   7.4         Filter Installation Cost Data	          7-10
   7.5         Initial Fabric Filter Costs - 1969 Basis        7-13
   7.6         Cost Per CFM of 12 Different Dust
               Collector Designs Compared to the
               Total Volume Handled Per Minute. . .  .          7-16
   7.7         Fan and Blower Costs, Including Motor,
               Starter, etc 1969 Basis	          7-18
   7.8         Total Operating and Maintenance Cost  .          7-30
   7.9         Typical Filter Pressure Drops	          7-32
   7.10        Air Power Costs	          7-33
   7.11        Reported Labor Costs, CCA Fabric Filter
               Systems Survey, 1969 (Wages,before
               Overhead)	          7-34
   7.12        Plant Floor Area Required per Filter
               Capacity	          7-37
   7.13        Fabric Usage Reported and Costs. .  .  .          7-41
   7.14        Approximate Temperature Capability/
               Cost Relationship for Filtration Fabric
               Materials	           7-43

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                            LIST OF TABLES
Table No.                                                     Page No.
    1.1        Early History of Fabric Filter Applications
               and Related Technology	       1-8
    1.2        Development of Principal U.S.  Fabric Filter
               Manufacturers	       1-16
    1.3        Industrial Gas Cleaning Equipment—
               Manufacturers' Shipments by End Use, 1967  .       1-46
    1.4        Summary of the Manufacturers'  Report of Air
               Pollution Control Equipment Sales (Particulate)    1-47
    1.5        Summary of IGCl Report of Fabric Filter Sales
               for Air Pollution Control	       1-50
    1.6        Sales of Two Fabric Filter Manufacturers by
               Industrial Category	       1-52
    2.1        Elements in the Analysis of Fabric Filter
               Technology	  .       2-4
    2.2        Major Shapes of Airborne Particles and
               Typical Concentration Ranges 	       2-14
    2.3        Particle Densities for Agglomerates   ....       2-23
    2.4        Typical Density Ratios for Aerosol Particles
               and Precipitated Smokes 	       2-23
    2.5        Explosion Characteristics of Various Dusts  .       2-26
    2.6        Characteristic  Charges on Some  Representative
               Dispersoids	   .       2-32
    2.7        Distribution of Charges on Particles in Equi-
               librium with a Bipolar Ion Atmosphere .  . .        2-36
    2.8        Number  of Unit Charges Acquired  by Particles       2-38
    2.9        Physical Properties cf Typical  Carbon Blacks.       2-45
    2.10        Adhesive Force of Particles Determined  by
               Various Methods for Various Air  Humidities  .       2-65
    2.11        Effects of Surface  Roughness in  Adhesion .   .       2-66
    2.12        Adhesive Force of a Powder Layer	       2-75
    2.13        Observation of the  Structure of  Solid Aerosol
               Particle Deposits on Fibers 	       2-90
    2-14        Terminal Velocities and Diffusion Coefficients
               of  Rigid Spheres  of Unit  Density in  Air  .   .       2-106

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Table No.                                                     Page No.
   2.15       Effect of Deposited Dust on Filtration
              Efficiency	      2-112
   2-16       Theoretical Values of the Kozeny Constant
              from Different Cell Models	      2-117
   2.17       Measurements of Characteristic Geometrical
              Properties of Porous Media 	      2-127
   2.18       Typical Values of Permeability for Various
              Substances	      2-129
   2.19       Representative Values of Porosity for
              Various Substances 	      2-132
   2.20       Apparent Density and Porosity for Some
              Industrial Dusts Collected in Fabric Filters      2-133
   2.21       Particle Size Distribution,  Bulk Density,
              Porosity,  and Flowability of Some Typical
              Powders	      2-134
   2.22       Representative Values of Specific Surface for
              Various Substances 	      2-136
   2.23       Values of  Ap/Vp for Spherical Particles in
              Fabric Filter Dust Cake	      2-137
   2.24a      Physical Properties of Fabric Samples
              Investigated 	      2-139
   2.24b      Fabric Porosity Data	      2-139
   2.25       Pore Sizes of Various Filter Media	      2-141
   2.26       Shape Factor for Typical Granular Porous Packing
              Materials	      2-144
   2.27       Experimental Resistance Coefficients for Fiber
              Filters	      2-151
   2.28       Porosity Function for Granular Porous Media        2-156
   2.29       Calculated Values of the Specific Dust-Fabric
              Filter Resistance Coefficient, the Depth of
              Deposit, and Resulting Pressure  Drop ....      2-164
   2.30       Filter Resistance Coefficients for Certain
              Industrial Dusts   	       2-166
   2.31       Specific Dust-Fabric-Filter  Resistance Co-
              efficients for Operating Collectors Surveyed
              in Fabric  Filter System Study 	       2-168

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Table No.                                                      Page  No.
2.32
2.33
2.34
3.1
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
4.12
4.13
4.14
4.15
4.16
4.17
Summary of Studies on Granular Beds Used
Spherical Particle Sizes Transported by
Conveying Velocities for Dust Collecting . .
Manufacturers and Trade Names of Industrial
Filter Fibers 	
Characteristics, Properties, and Forms of
Acrylic Fiber for Industrial Filtration. . .
Characteristics, Properties, and Forms of
Modacrylic Fiber for Industrial Filtration *
Chemical Composition of Polyamides 	
Characteristics, Properties, and Forms of
Characteristics, Properties, and Forms of
Nomex Nylon Fiber for Industrial Filtration.
Characteristics, Properties, and Forms of
Polypropylene Fiber for Industrial Filtration
Characteristics, Properties, and Forms of
Polyester Fibers for Industrial Filtration .
Characteristics, Properties, and Forms of
Teflon fiber for Industrial Filtration . .
Characteristics, Properties, and Forms of
Vinyon Fiber for Industrial Filtration . . .
Characteristics, Properties, and Forms of
Glass Fibers for Industrial Filtration . . .
Relative Properties of Man-made Fibers . . .
Summary of Physical and' Chemical Properties
Resistance of Fibers to Chemical Reagents. .
Some Types of Mechanical Wear in Fabrics . .
2-191
2-201
2-202
3-34
4-5
4-8
4-11
4-12
4-13
4-14
4-16
4-17
4-19
4-21
4-23
4-24
4-25
4-27
4-30
4-47
4-53

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Table No.                                                     Page No,
   5.1        Effluent and Filtering Requirements 	     5-5
   5.2        Methods of Temperature Conditioning „  .  .  .  .     5-24
   5.3        Approximate Optimizing Exponents of Costs  .  .     5-29
   6.1        Effects on Performance of Fabric Filter
              Internal Component Configuration    	     6-7
   6.2a       Properties of Various Orion Filter Fabrics.  .     6-11
   6.2b       Filtration Characteristics of Napped and
              Unnapped Sides of Orion	     6-11
   6.3        Adhesion of Various Particles to Substrates
              of Various Materials in Water	     6-19
   6.4        Summary of Cleaning Equations for Fabric
              Filter Drag and Dust Deposit During Shaking
              Without Air Flow	     6-43
   6.5        Fabric Filter Media Specifications and Perform-
              ance with Constant Particle Flux	     6-57
   6.6        Fabric Characteristics 	     6-63
   6.7        Effect of Relative Humidity on Specific
              Resistance Coefficient,  Effective Drag,  and
              Terminal Drag	     6-64
   6.8        Effect of Relative Humidity on Outlet  Dust
              Concentration and Efficiency  	      6-66
   6.9        Effectiveness of Filter  Aids for Low Particu-
              late Loadings	      6-78
   6.10       Reduction in Efficiency  of Asbestos Flocked
              Bags During Shaking	      6-79
   6.11       Efficiency of Bag Collector for Various
              Aerosols	      6-80
   6.12       Efficiency and Pressure  Drop of "Dustube" Filter
              at Various Loadings of "Micronized" Talc .  .      6-83
   6.13       Fabric Comparisons with  Light Loadings   . .  .      6-93
   6.14       Fabric Comparisons Using Asbestos  Floats as  a
              Filter Aid   	      6-93
   6.15       Effectiveness of Filter  Aids for Light Loadings
              of Copper Sulfate Microspheres  	      6-95
   6.16       Effect of Decreasing the Number of Raps  in the
              Standard Cleaning Cycle  on Pressure  Drop and
              Efficiency	   	        6-96

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Table No.                                                     Page No.
   6.17       Effect of Variation in Cleaning Cycle Fre-
              quency on Pressure Drop on a Multicompartment
              Collector	     6-97
   6.18       Effect of Changes in Reverse Air Volume on
              Pressure Drop and Penetration	     6-97
   6.19       Effects of Inlet Dust Concentration on Fly
              Ash Penetration	     6-98
   6.20       Individual Compartment Effluent Concentrations
              When Filtering an Inlet Dust Concentration of
              1.1 Grains Per Cubic Foot of Fly Ash	     6-100
   6.21       Comparison of Five Fabrics Filtering Heavy Dust
              Loadings	     6-100
   6.22       Average Inlet and Outlet Dust Concentrations
              for a Variety of Aerosols Tested on the Reverse-
              Jet Collector	     6-102
   6.23       Field Test Results for Cloth Screen(s)  and
              Tube Collectors Cleaned by Intermittent
              Mechanical Shaking 	     6-103
   6.24       Field Test Results for Cloth Bag Collectors
              Cleaned by Reverse-Jet Air	     6-104
   7.1        Marshall and Stevens Index for Updating
              Equipments Costs 	     7-4
   7.2        Approximate Characteristics of Dust and Mist
              Collection Equipment 	     7-6
   7.3        Typical Costs of Fabric Filtration  	     7-9
   7.4        Estimates  of Capital Cost  Breakdown	     7-12
   7.5        Approximate Cost for Bag-Houses  of  the  Indicated
              Construction	0	      7-17
   7.6        Approximate Duct Costs	     7-21
   7.7        Reported Estimates  of  Conveyor Costs   ....     7-24
   7.8        Typical Instrumentation Catalog  Costs -  1969 .     7-26
   7.9        Reported Labor  Distribution Costs  	     7-34
   7.10        Typical Filtration  Fabric  Costs  	     7-40
   7.11        Costs  of Typical Bag	    7-41
   7.12        Fiber,  Temperature  Range and Relative Cost . .    7-42

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Table No.                                                     Page No.

   8.1        Types and Frequency of Problems Reported .  .       8-26

   8.2        Maintenance Problems and Practices Reported
              in the Fabric Filtration Literature ...  .       8-30

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                               CHAPTER 1

                             INTRODUCTION


TABLE OF CONTENTS

1.1  GENERAL DESCRIPTION                                             1-3
1.2  HISTORICAL ASPECTS OF FABRIC FILTRATION                         1-6
     1.2.1  Early Filtration Equipment                               1-6
            1.2.1.1  Early Designs                                   1-?
            1.2.1.2  Early Cleaning Methods                          1-14
            1.2.1.3  Early Commercial Manufacturing                  1-14
     1.2.2  Fabric Developments                                      1-25
     1.2.3  Manufacturers and Market Developments                    1-29
            1.2.3.1  Manufacturers                                   1-30
            1.2.3.2  Fabric Filter Industry                          1-32
1.3  OPERATING PRINCIPLES                                            1-37
1.4  FABRIC FILTER APPLICATIONS AREAS                                1-43
1.5  SUMMARY                                                         1-54
1.6  REFERENCES FOR CHAPTER 1                                        1-56

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                                 CHAPTER 1
                                INTRODUCTION
 1.1  GENERAL DESCRIPTION
      A fabric filter consists of a porous flexible layer of textile material
 through which a dusty gas is passed to separate particles from the gas
 stream.  As particles accumulate, resistance to gas flow increases.  Deposits
 are removed periodically by vigorous cleaning of the cloth to maintain the
 pressure drop across the filter within practical operating limits.   Provision
 of methods for cleaning the fabric in place is a distinguishing characteristic
 of this class of gas filter.
      Fabric filters are commonly used for control of dust concentrations in
                9     ^                               7    *^St
 the range of 10  ng/m  (urban atmospheric dust) to 10  g/n   (pneumatic con-
 veying).   They provide effective removal of particles whose sizes range from
 submicron fumes to >200 micron (|im) powders.  Fabrics are available to permit
 operation at gas temperatures up to about 550°F and to provide chemical re-
 sistance  against specific constituents of the gas or particulate.  Fabric
 filters can provide a substrate for support of granular reactants or adsor-
 bents ,  to recover gaseous components.
      The  quantity of ventilation air or process gas, and the dust concentration,
 in conjunction with specific flow-resistance properties of the particulate de-
 posit,  determine the amount of cloth area required for any selected value of
 operating pressure differential.   An operating pressure drop is  generally
 selected  in the range of 3 to 4 inches of water,  for economic  reasons,  but
 some  designs operate substantially in excess of 10 inches of water.   Super-
 ficial  filtration velocities (total air volume filtered/total  cloth area),
 commonly  called the air-to-cloth  ratio,  are generally in the range  of 1 to
          0                      ~JU*t*
 If. cfm/ft ,  i.e.,  1 to 15 ft/min.   However,  values in excess of  50  ft/min
 can be  achieved at moderate flow  resistance with  certain cleaning devices
 on coarse dusts.
*  One gram per cubic meter = 0.435 grains per cubic foot.
** Average for shaker type, reverse air and pulse jet are approximately
   2.5/1, 2/1 and 6/1, respectively.

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      Collectors are readily available in sizes from a few square feet of
 cloth up to several hundred thousand square feet.  Total gas flows handled
 by individual units range from < 100 cfm to >10  cfm.  Units up to a few
 hundred square feet of cloth are fabricated and assembled by semi-automatic
 production line techniques, produced in relatively large quantities, and
 shipped assembled.  Larger units, which are carefully designed to meet the
 riV;uiivmonts of spocit'ic applications,  arc frequently fabricated for assembly
 at the installation si to.
      Costs vary in proportion to .size and with respect to kind and arrange-
 ment  of fabric and cleaning apparatus;  initial collector costs are typically
 in the range of 0.35 to 1.25 dollars per cfm.   Actual installed costs, in-
 cluding necessary auxiliary equipment,  may be  2 to 3 times the base cost of
 the filter,  a figure of 2.25 times being suggested as a reasonable average
 estimating approximation.   Total annual operating costs relative to weight
 removal efficiency for fabric filters and other gas cleaning equipment are
                                            2
 shown in Figure 1.1 adapted from Stairmand.    Fabric filters are seen to
 yield lower  penetrations (higher efficiency) at costs lower  than those pre-
 dicted from  the guideline shown.   This  favors  fabric filters in those situa-
 tions where  their use is not precluded  by other factors, and where very high
                                                      2
 collection efficiency is required for fine particles.   Generally,  high ef-
 ficiency at  reasonable cost is  an inherent characteristic of the separation
 process utilized  in fabric  filters.   If they are  properly designed,  installed,
 operated  and  most important,  properly maintained,  fabric filters  will col-
 lect  more  than 99.9% of the incoming dust on most  applications.
      In many industrial applications, the discharge from the fabric  filter
 can be returned to the interior  of the  plant,  as  it will be  respirably ac-
 ceptable  if  the conveying gas is  respirable.   This  effects a saving  on the
 heating or cooling of make-up air.   The  collector discharge  dust  concentra-
                                                       2
 tion  will  frequently be found to  be  less  than  100 |_ig/m  which is  as  low  or
 lower than ambient atmospheric dust  concentrations  found in  many major U.S.
 cities.   In  general,  increased outlet concentrations  are associated with
 higher inlet  concentrations, more  cleaning energy,  and higher filtering
velocities.  Performance and cost relationships are explored and considered
in more detail in subsequent chapters.

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                                           Voft4
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                                           K>viro«-J
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 1.2  HISTORICAL ASPECTS  OF  FABRIC  FILTRATION
     1.2.1   Early Filtration  Equipment
             The use  of textile  fabrics  as  filters  for  the  separation  of  air-
 borne  dust  and fume  includes  much  of  the recorded  history  of man, probably
 encompassing the  textile and  metallurgical technologies  developed in  the
 Egyptian  Old Kingdom (ca. 5000  B.C.)-   Earliest  recorded applications of fab-
 rics as filters appear in the use  of  a  cloth drawn over  the mouth and nose to
 prevent inhalation of dusts by  millers  and bakers, miners  and stone cutters,
 and in lead refining and non-ferrous  metallurgy.   Surviving records from Bib-
 lical  times mention  the  use of  woven  sacks (bags)  placed over the head and
 tied round  the neck  for  protection from dusts in mining  and refining of  lead
 oxide.  This  primitive practice persists essentially unchanged to the present
                                3
 under  certain dusty  conditions.
                   4
            Davies   has  reviewed the  early history of  the  use of fabrics and
 fibrous materials  for respiratory  protective devices,  (see summary in Appen-
 dix 1.1), together with  some  early contributions to the  present day under-
 standing  of aerosol  filtration.  The major stimuli to  the  development of an
 understanding of  the filtration process, especially of the initial step of
 particle  deposition  in a cleanable fabric  filter, were associated with the
 protection  of workers from dust.   This  includes medical respirators for
 reduction of  airborne infectious agents; fire-fighters smoke protection;  and
 respiratory protection from chemical, biological and radiological aerosols
 in industrial and military environments.   Development of industrial fabric
 filters has proceeded independently for the processing of large volumes of
 gas for product recovery, protection of machinery or other equipment,  as  well
as for worker health or nuisance control.   The recovery of valuable products
 from dusts and fumes in non-ferrous smelting and refining operations was  pro-
bably one of  the earliest applications of  industrial fabric filters.   The
 escaping  fume represented an economic loss.  In addition, in the case of  lead
and arsenical ores,  the fume was the source of injury to the surrounding
populace and livestock and damage  to the surrounding property,  resulting  in
damage suits and further economic penalties.

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             Table  1.1  summarizes  some  of  the  developments  of  fabric  filters
 for  industrial  dust  and  fumes.
             1.2.1.1  Early Designs.- Figure 1.2  illustrates a single  bag
 design  patented in 1852  by Jones  (sec  Appendix  1.2)  for  the recovery  of zinc
 oxide fume,  probably used in  New  Jersey zinc  works of  that time.  A bag at
 least 8 feet in diameter by 70  feet long  was  suggested for sixty  small retorts.
 The  filtering velocity is estimated to have been about 0.4 ft/min.  Fabrics sug-
 gested  were  closely  woven cloths  of cotton, wool,  flax,  or other  fibrous  tex-
 tiles.   Cleaning was accomplished by striking the  bag  on the  outside  during
 operation or by stopping the  blower, allowing the  bag  to collapse and then
 striking it-  The  use  of reverse  air flow cleaning was suggested  by turning
 the  bag inside  out and applying the pressure  to  the  opposite  side.  The branch
 tubes from the  main  bag  were  emptied during operation  by tying the top and
 opening the  bottom.  Jones cites  the prior use of  a  series of canvas  bags
 open at one  end for  natural draft,  in  the collection of  lamp-black (carbon
 black)  from  incomplete combustion of oils.  (Lamp  black  was produced  for  ink
 and  paint pigment  in ancient  Chinese civilizations,  more than 5,000 years
 ago.)   The Jones patent  contains  discussions  of  the  basic design requirements
 of all  fabric filters:   a closely woven fabric, means  for retaining or sup-
 porting the  fabric,  consideration of the  amount of fabric area necessary, a
 blower  or exhauster, means for cleaning the fabric of accumulated dust and
 fume, and provision  for  discharge of the  collected product.   Subsequent fabric
 filter  patents  relate  to new  and  useful improvements or modifications involv-
 ing  these same  basic requirements.
             Other  early  uses  of fabric filters include the configuration
                                              12
 shown in Figure  1.3a employing natural draft,    and  "long filter sleeves
 hung in rows, tied together at the bottom, while the dirty gases were ducted
 into the top	  At  intervals the sleeves were shaken manually and  emp-
 tied."   Figure  1.3b illustrates  an early filter related to the fabric fil-
 ter but using instead  granules and screens.  More will be said about  this
 type of  filtration later in this  report.
             Initial mining and smelting of zinc in the Missouri, Kansas and
Oklahoma areas  in the  late 19th century resulted in  further developments of

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                               Table  1.1

       EARLY  HISTORY  OF  FABRIC  FILTER  APPLICATIONS AND  RELATED  TECHNOLOGY
Date
 1852
 1867
 1870
1876
1881
1885
1888
1892
                   Event

Collection  of  ZnO  fume  in  8  ft. x  40  ft.
bag  of woven cotton,  wool, or  flax,  forced
 (or  drawn)  through by fan-blower,  approxi-
mate filtering velocity  0.5  ft/min.

Screens  in  the side of a house  substituted
for  the  bags previously  employed  as  filters
for  ZnO, Wetherill and Hall, U.S.  Patent
72,032.

Fabric collection  suggested  for lead  fume,
Percy, "Metallurgy",  p.  449.   Initial
mining of Zinc in  the Joplin, Missouri
area.

Lone  Elms Works  (A.S. &  R.), Joplin, Mo.
installed two-section bag house for  recov-
ery  of fume from Scotch  Hearth  lead  smelt-
ing  furnace; concrete tube sheet  containing
(18  in.  diam.)   thimbles, bags  fastened at
top  on downstream  side of fan, cleaned
by airing or allowing the bags to  tremble
in the current of  air created by opening
the  thimble floor  doors  and  the stack dampers
after shutting off gas flow  into the chamber;
later hand  shaken.

Beth  patent, German, mechanical vertical
motion to top  of bags, cement mill dust,
downstream of  fan, wooden casing,  2 bag
compartmented  unit  (est. 1 ft. diam. x
6 ft high).

Cyclone apparently first patented  by
Jackson.

Solvay, Eng. Patent 18,573, Sand filter
(probably to treat  lime  kiln dust  for
purification of C0£ to ammoniated  brine
for sodium bicarbonate and soda ash).

Large metallurgical bag house developed
by lies and associates,  (U.S. Patents
  Reference
 Ebaugh  (5)
Ebaugh  (5)
Ebaugh  (5)
Labbe &
Donoso (6)
Strauss (7)
Ihlefeldt (8)
Lapple (9)


Gibbs (10)




Ebaugh (5)

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                            Table  l.l  (Continued)
1893

1894
1907-8
1909
1948
                  Event

475,774; 480,834; 484,016;-017)  for  lead
and  zinc furnaces (not  for  copper  furnaces
or roasters) consisting  of  lower dust
chambers 14  feet high cotnparttnented, and
with  provision  for  closure  of  inlet, and
upper bag chamber 30 to  45  feet high,  one
room, walls  and partitions  ot  brick, 18
compartments showing in  lower  chamber, bag-
houso 140 ft. long  x 55  ft. wide;  tube
shoot,  iron  or steel plate  with.18 inN
diameter nipples for wool  (earlier cotton)
bags, outlet initially direct  through
ventilators  and later through  200  ft.
stack;  capacity 150 to 250,000 cfm,  3000
to 4500 bags 18 in. x 30-33 ft (130  ft2/bag,
filtering velocity  0.3  ft/min), note sug-
gested  fiber blends in warp and  fill (U.S.
Patent 485,797) and lower section  of bag to
be of wool reinforcement nearest thimble.
Temperatures of 70  to 270°F used,  but  160°
most satisfactory,  cleaned  by  hand,  then by
external hand-operated lever beaters.

Reverse air  flow in use  for cleaning.

Rings around centers of  bags attached to
wire led outside oE building,  hand-shaken
by end of rope, Rourke,  (U.S.  Patent
530,553).

Early tests of Rhodes and Sprague  on
neutralization of copper furnace and
roaster gases passed to  bag house  by addi-
tion of zinc oxide  to protect  bags.

Bags attached to short levers  extended from
a central shaft, whose outer end projects
beyond the bag house wall,  there provided
with a larger lever to shake the bags by hand.

Development of venturi scrubber.
                                                                  Reference
 Ihlefeldt  (8)

 Ebaugh
Ebaugh (5)
Ebaugh (5)
Ekman and
Johns tone (11)

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                  rty ,'
           _fif/ jj ta
fit/ /
                                                    V
       v
                             U. -.'.
      ?V
    _/v V/,><*.
Figure   1.2  -Early Fabric Filter  for  Zinc Oxide Control

              (See also Appendix 1.2.)

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      ., METAL DECK
   -df--\	*
Ctoth
Air
               V
                                V
                (a) A filter bag,
                    an early type of dust
                    removing apparatus.
                (b) Section through an
                    old time coke air
                    filter.
               Figure  1.3 -Early Filters for Dusts,
                            (From Lewis, Ref. 12.)

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 fabric  filters.   Labbe  and  Donoso   cite  a two-section bag house for recovery
 of fume from Scotch Hearth  lead  smelting furnaces  constructed in 1876 at
 the Lone  Elms Works of  Joplin, Missouri.   Lead fume  was  removed from the
 bags initially  by shutting  the damper  between the  bag house  and the fan dis-
 charge, and  then airing the bags,  causing them to  tremble.   Lead fume dropped
 into the  lower  chamber  (smoke cellar)  and waa sintered in place.
             During the  period of rapid economic development,  that  followed  the
 completion of the trans-continental railway  in about 1869, large metallurg-
 ical production  facilities  were  built  in the  western and inter-mountain
 states  (e.g., Montana,  Utah).  Later during  the period 1830  to  1890 large
 metallurgical bag houses  containing several  thousand bags  were  constructed
 for  use in zinc  and lead  smelting  ,such  as shown in  Figure 1.4.  Thirty foot
 bags  of cotton or wool  were hung from  supports  near  the  top  of  the  building
 and  the lower open ends were tied  to nipples  or thimbles  projecting from the
 floor of  the  bag  compartment.  The  lower  dust chamber  was  partitioned into
 as many compartments as there were  flues  leading to  the  bag house,  enabling
 a  part  of the system to be  shut  down as  required for  cleaning.   Cleaning
 probably  involved some  reverse flow of gas through the fabric.   The dimen-
 sions of  the housing shown  in Figure 1.4  are  approximately 140  ft. long by
 60  ft. high, with  a depth  of the  order  of  90 ft.(est).  Filters of this type
were  suitable for  flue  gas  volumes  of  about 50,000 to  150,000 cfm depending
                                       I        '
upon  the  application (e.g.  Scotch Hearth  or lead blast fume).
            Copper  furnace  and roaster gases  containing high concentrations
of sulfur dioxide  and sulfur trioxide could not  be filtered because of acid
deterioration of  the fabric.  However, the feasibility of fabric treatment
by neutralization  of the  smelter smoke with zinc oxide or lime dusts was
demonstrated  in the 1907-1908 period and  suggested for effluents from copper
blast furnaces,  reverberatories,  and converters  in 1909.  Claims for damage
to vegetation and  cattle  from smelter smoke became an  important factor, dur-
ing this period and required the simultaneous removal of fine metallic
oxide fume particles (lead,  arsenic) and  the  reduction of sulfur dioxide
   .   .    13
emissions.

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0

                                          nn
n
                                                   OKI'
                            (a) lies'  Bag House  (from Ref. 5)
                                   baghanger
                          (b) Typical Bag House Filtering System
                               (from Ref. 15)
                         Figure  1.4 - Bag Houses for Lead and  Zinc  Smelting.

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             The vigorous  application of  Cottrell's  electrostatic  precipita-
 tion process,  beginning about  1910  and continuing through  the  1920's,  re-
                                                                           14
 suited  in the  virtual  cessation of  development  of fabric filter equipment.
 The simultaneous  construction  of high stacks  and the  development  of  sulfur
 dioxide flue gas  recovery plants (chamber,  contact, sorption)  during the
 period  1910 to 1920 reduced  ground  level concentrations of noxious  fumes
 and sulphurous gases and,  in turn,  tended to  inhibit  further development on
 fabric  filtration systems.   Fabric  filters  continued  in use for lead and zinc
 furnace applications,  however,  but  it wasn't  until  the early 1950's  that sig-
 nificant new development  efforts resumed.   These efforts were  directed prim-
 arily toward new  methods  of  fabric  cleaning and new synthetic  fabrics capable
 of  withstanding both higher  temperatures and  corrosive conditions.
             1.2.1.2 Early Cleaning Methods.- In the  early equipment, the
 removal  of  accumulated dust  and fume from the interior surfaces of bags was
 accomplished by shaking or beating  by hand  or with  a  simple lever-bar.  For
 lead  and arsenical  fumes, however,  this  was a particularly unhealthy and un-
 satisfactory method.   Hand or  foot  operated shaker  bars are still used in
                                     2
 small unit  dust collectors (<  300 ft  ).  Automatic  shakers, as shown in
            78
 Figure  1.5,  '  were developed  during  the period 1880  to 1910.  By 1909, bags
 were  suspended from short  levers attached to a central shafting which could
 be  pivoted  back and forth with  an external lever.   This was soon automated
 by  tying all shaker shafts in one chamber to a central shaft outside the
 housing  and with  a  motor and eccentric arrangement imparting either a vertical
 or horizontal  reciprocating motion  to the bags.   Subsequent modern compart-
mented fabric  filters  contain electro-mechanical devices for automatically
 stopping  the flow,  shaking, admitting reverse air when required,  dwelling for
 a period  to allow settling, and finally  restoring flow.
            1.2.1.3  Early Commercial Manufacturing.-  Large bag houses  for
 lead and  zinc  dusts and fumes were commonly designed and built  by the works
 engineers of the individual smelter companies.  '    In addition,  continuing
engineering development work by the individual companies was  directed toward
                                                                2
defining the relationships between pressure drop, volume (tip  ~  Q  ),  dust  con-
centration and collection efficiency  (Seitz, A.S.  & R.,  1929).6  Much attention

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o

c
                               Figure 1.5  -Early Automatic Shaker Mechanism and Filter
                                            (From Ref.  7.)
                  was also given to fabric life, acid resistance, and strength.   Fabric sub-
                  stitutes were considered for higher temperature operation as alternatives
                  to cooling by water sprays or added air.  The construction of automatic
                  fabric filters by works engineers is still a common practice, particularly
                  in the non-ferrous metallurgical industry.
                              The commercial production of fabric filters for control of many
                  types of dusts and fumes developed within several industrial firms who ini-
                  tiated manufacture of these devices for their own specific applications.
                  Estimates of the founding date and principal product lines of some of the
                  present U.S. suppliers of fabric filters are shown in Table 1.2.  Sly,
                  Pangborn, and Wheelabrator were originators of foundry casting cleaning
                  equipment such as tumbling mills (Sly) and compressed air sand blast and
                  airless abrasive shot blast chambers (Pangborn, Wheelabrator).  Large quan-
                  tities of metallic and sand dusts generated during casting cleaning required
O

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                                 Table  1.2

             DEVELOPMENT OF PRINCIPAL U.S. FABRIC FILTER MANUFACTURERS
1877
1881
1905
1907
1907
1908
            Firm

            W.W. Sly Co.,
            Cleveland, Ohio
 Buffalo  Forge
 Co.,   Buffalo,
 New York
Day Co.,
Minneapolis,
Minn.
Pangborn Corp.
Hagerstown,
Mel.
Western Pre-
c ipitation
Corp., Los
Angeles, Cal.
Research-
Cottrell,
Bound Brook,
N.J.

Wheelabrator
Corp., Mish-
awaka, Ind.
                            Remarks                     Re L'erence
                                                             i'f
                   Dcvelopment  of  tumbling mills         C.L.  , 22
                   for  small  casting  cleaning
                   (1880-7),  original  patents  on
                   fabric  collectors,  envelope  screen
                   design  (1920),  travelling reverse
                   flow air  cleaning  device  since  1951.
                                      C.L., 22
                              Blowers, fans, machine tools,
                              inertials and scrubbers; acquired
                              Aeroturn reverse-jet from Koppers
                              1965, publishers of fan engineer-
                              ing handbook since 1925.
Sheet metal shop, probably cyclones,  C.L.
for sawdust and shavings from
Mississippi River lumbering
operations, Hersey reverse-jet
collector since 1948.

Sand and shot blast cleaning          C.L.
equipment, continuous multi-
bag and envelope screen de-
signs, 1920, reverse flow clean-
ing since 1952,  pulse-jet design
1959,  acquired by Carborundum
Co. , Buffalo, N.Y.  1965.

Electrical precipitators,  Hersey      14
reverse-jet collectors built
since 1952, glass fabric bag with
collapse cleaning since 1957,
acquired by Joy Mfg.  Co.,  1959.

Electrical precipitators,  glass       14
bag filters since 1965e, Flex-
kleen, (1959) acquired with pulse-
jet type, 1968.

Shot blast casting  cleaning cab-      C.L.
inets; airless abrasive shot blast
systems,  1920; dust  collecting
equipment built since 1924.
      Company Literature

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Date
1910
1917
1923
 Firm

 Northern  Blower
 Co. , Cleveland,
 Ohio
Dracco Corp.,
Cleveland,
Ohio
Pulverizing
Machinery Co.,
Summit, N.J.
1925
American Air
Filter Co.,
Inc., Louis-
ville, Ky.
Table 1.2 (Continued)

        Remarks                     Reference

 Fans, blowers, cement mill pro-       C.L.
 cess equipment, merged with
 Buell Engineering Co., Inc.,
 1960.

 Fans, blowers, pneumatic con-         C.L.
 voying systems, cement, metal-
 lurgical process equipment,
 merged with Fuller Co., 1957;
 first glass fabric bag collec-
 tors, 1957, with sonic cleaning
 1960, plenum pulse 1968.

 Pulverizing mills, powder clas-       C.L.
 sifiers, pneumatic conveying
 systems, reverse-jet collectors
 built since 1945,  original pulse-
 jet  design  introduced 1957, ac-
 quired Airetron Co., scrubber,
 inertials,  electricals 1958.
 plenum pulse-jet designs 1969,
 acquired Menardi & Co.  (1954) in
 1969.

 Filtration  devices for space  heat-     C.L.
 ing  and air-cbnditioning,  fixed
 fabric,  paper  types,  inertials,
 scrubbers for  most dusts;  for
 fibrous  dusts  (1947), reverse-
 jet  filter  built 1953, glass
 fabric  collector since  1959,
 pulse-jet introduced 1969.

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ventilation and dust control for protection of machinery as well as for

recovery of abrasive.  As a result, envelope shaped fabric geometries sup-

ported on a screen or frame were developed by Sly and Pangborn in the 1920*s.

Wheelabrator began manufacture of cylindrical bag filters about 1916 for use

with blast cleaning equipment.  Northern Blower (Buell) and Dracco (Fuller)

began manufacture of fabric filters for cement mill dust recovery during the

period 1910 to 1920.

            The fabric configurations, fabric supports, and automatic clean-

ing nechanisms, developed during the 1920's and 30*s, have in general with-

stood the test of time and are still commercially available today.   Some of

these early production designs developed prior to about 1940 are shown in

Figure 1.6.10'15'19'20  The British units (Figure 1.6a, 1.6b) illustrate the

use of conical bags with a larger inlet at the bottom tapering toward the

top hanger.

            The Hersey reverse jet filter using dense woolen felt bags was

developed during the 1940*s:

            "It grew out of an effort 1:o provide a means of collecting
            silicious ore crushing plant dust in the Coeur d'Alene  mining
            district of Idaho (ca.  1939) which would produce a constantly
            permissible silica dust count, not possible with shaken woven
            cloth bags operating on a time switch.   The first attempt  was
            with various means of pressure controlled shaking of  large
            wool felt bags.   Although the wool felt was dense enough  to
            prevent leakage after shaking, it finally blinded.  This was
            followed by variations  of reverse jet  applications, some of
            which broke down the filter medium,  and some of which bailed
            to achieve a level porosity balance.

            Finally in 1941 a reverse jet filter consisting of one  36"
            diameter by 16 ft.  bag,  lengthened later to 32 feet,  was
            started in a large grain elevator on an experimental  basis.
            After three years  of service it was  considered proven that
            wool felt would stand the service and  that  constant air flow
            and pressure drop  could be maintained with  the right  combina-
            tion of reverse jet action and filter medium base.  By  1950,
            several hundred reverse jet filters  were applied  to various
            problems  in widely diversified industries  including chemical,
            metallurgical,  pigments,  abrasives,  textiles,  grain,  flour,
            cereals,  sugar, ceramics,  cosmetics, confections  and  drugs.

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   1.6a -Mechanically Cleaned  Bag  Filter, ca 1924,
          British Design (from Ref,  10).
                         ^Of^trMrrli
       1.6b -Waring Dust Collector (suspended conical
             bag unit), with cyclone separator, ca 1940,
             British Design (from Ref. 19).
Figure  1.6 -Pre-1940 Equipment  Similar to Present Designs,

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 Clean air chamber^
 Dustchamber
 Cloth"
 filter bags
 Dust hopper

1.6c -Sly Dust Filter. (W.W. Sly Manufacturing Co.)
      (from Ref. 15).


     1.6d -Dracco "Perfecto" Filter.  (Courtesy of
          Fuller Company).
0
                    1-20

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 o
-o
 o


                            1.6e  -Cloth Screen Dust Collector. (Pangborn
                                   Corporation).   (From Ref. 20.)


                            1.6f  -Bag Filter (Dracco Corporation).
                                   (From Ref. 20.)

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             Although the patent literature has disclosed numerous con-
             ceptions of reverse flow filters  of both the blow thru jet
             and the vacuum sweeper type,  none of them reached commer-
             cial success or significant  practical use so far as is
             known.   They lacked the combinations of features 	
             which provided high-capacity  one-step separation without
             leakage."21
             Typical fabric dust and fume  collectors of principal U.S.  man-
                                                                       19
 ufacturcrs  during the period 1945  to 1950 were summarized by Silverman
                               9
 (Appendix  1.3a) and by Lapple.   The reverse-jet filter was  manufactured  by
 several  U.S.  firms  (as indicated in Appendices 1.3b and 1.3c),  during the
 1950*s.  Appendix 3.1 presents  a 1969 survey  of U.S.  manufacturers and
 Chapter  3  includes  a description of the major products available in that  year.
             The pulse-jet  filter shown in Figure 1.7  was developed in the mid
                                                                       24
 50's  and introduced in 1957 by  T.V.  Reinauer  of Pulverizing  Machinery.
 This  device,  which  employs a short pulse  of compressed air to cause fabric
 motion (with  reverse  flow) to remove dust from the  outside of felt filter
 bags, requires  no moving parts  inside the collector.   At this time (1970)
 most major U.S.  manufacturers of fabric filters have  introduced some form of
 air-jet  or pulse cleaning  configuration in their fabric  filter  product  lines.
            For many  years fabric  filter  developments  have been directed  at
 increasing the  filtering velocity  while maintaining a  reasonable  pressure
 drop of  a few inches  of  water.   A  continuing  approach  to accomplishing  this
 end has  been  on-stream cleaning.   The  early manufacturers of  envelope geome-
 tries) introduced continuous  back-flow  air  cleaning  carriage configurations
 in the early  1950's.  (Operating  gas  velocities  have increased from about
 0.2 ft/min, on  early  infrequently  hand shaken units, to nearly  3 ft/min on
modern automatic  compartmented fabric  filters.   The relatively  recent reverse-
 jet and  reverse-pulse units, which are continuously cleanable on-stream, per-
mit operation at velocities up to  10 to 20 ft/min, depending on conditions.
 Figure 1.8 shows  these developments  in filtering velocity increases and sug-
gests near-future increases of one or even two  orders of magnitude.  Develop-
ment studies were reported in 1962 on cleanable industrial filter configura-
tions with gas velocities up to 300 ft/min for high temperature metallurgical
furnace  fume  (1000 F) utilizing low energy shock wave cleaning of fibrous

-------
  Air —
Tlntir
Bo» . - .
•upper I
                                   Sol«noi4 vol»« .
                                              Cluon
                                              go*
                    Solids
  Figure  1.7   _
Pulse-jet Configuration  (ca  1957).

(Mikro-Pulsairex  design;  from

Kef.  23).

-------
u
1

ff  mil.
Ul
11.
o
I
a
UJ
5
                      1	1	1	1	1	1	r
                                          (nit tm\ K-pi>tt '.n
                                          ft-rl'irroafi' * «t
                                          HI H(I v*f... Jty
                                          Kf|Hirt<-
                                               I I It. l K«|>»H. J

                                               itinH- (Mnrr.
-------
mats.     Studies  continue  on cleannblu refractory granular filter media  '
                                                                      27
and devices  for utilization of  granular media with pulse-jet cleaning   to
meet the  requirements  of demanding physical and chemical environments.
Those unusual  filter configurations are not subject to the same limitations
in filtration  velocity as  are fabric filters.   This point will be discussed
further in Chapter  2.

     1.2.2   Fabric  Developtnents
             As noted above,  fabric filter  designs  have been modified  to take
advantage of the  rapid growth over the past 25  years  of fabric  technology.
This technology has included  completely new developments  in fibers, yarn,
and textile  processes;  most  notably the synthetic  fibers  and glass  fabrics
enabling  filtration both at higher temperatures  and under more  corrosive
conditions.  Figure 1.9 shows the  development of  filtration temperature
capability,  for example.   Because  of  the outlook  for  still  more chemically
and thermally  stable fibers  is  improving,  the simultaneous  evolution  of
fabric technology and  collector and  cleaning designs  seems  likely to  continue.

            Woven cotton and woolen  flannel fabrics used in  the early non-
ferrous metallurgical  bag houses  were  suitable for temperatures below
200 F.  Attempts to use woven asbestos  fabrics for higher temperature appli-
cations were unsuccessful because  the materials were not mechanically strong
                                                                19
enough to resist the vigorous shaking required to dislodge dust.    Fine
               28                         18
steel wool mats   or stitched blanket mats" , used for metallurgical furnace
                                                                         19
fumes in the 1920's,were not sufficiently cleanable by rapping or shaking  ,
and  since they also oxidized rapidly at temperatures  in the vicinity  of
300°F  they have not found  wide  application.
            Man-made polymer textile  fibers having  improved temperature and
chemical resistance have been developed and marketed commercially since the
                                 R*
latter part of the 1940's.   Orion   acrylic multi-filament fabrics were tested
prior to 1950 in fabric filters, '    '   for acidic atmospheres up to 300 F.
Fabric weaves were developed for optimum permeability and dust retention
properties,  and techniques  were  perfected for heat setting the fabric  prior
     *
      Registered trademark

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§52000
"1000
8: 900
3 800
<700
£-600

2 500

  400
  300
  200
                                                              Ta, Co
                  Limited Experi-
                  ence , or
                  Experimental
                  Quantities
                  Available
100
  1910
1920      1930       1940     1950
              APPROXIMATE  DATE
I960
                                                                 1970
MO
               Figure 1.9.   Development  of Cleanable  Filtration.

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 to sewing it into bags in order to eliminate shrinkage under operating condi-

 tions.    Orion  fabrics were being used successfully at 275 F by Wheelabra-
                       31 32 33
 tor and others by 1951  '  '   for numerous applications.  These included

 carbon black, foundry cupolas, electric furnaces, rock dryers, dry grinding

 mills, spray driers, sinter machines, and brass refining furnaces, with

 auxiliary cooling as reqviired.

             By 1954, several other man-made fiber textiles had been evalu-
 ated for their usefulness in filtration at higher temperatures,   including
                     R                                                R
 nylon, vinyon, Dynel  (~60% vinyl chloride, acrylonytride) and Dacron

 (polyester):

                  "In no case were these fabrics found to be capable
                  of withstanding continuous operation in the tempera-
                  ture ranges for which Orlon^ and fiberglass fabrics
                  are suitable.  However, some of these fabrics possess
                  resistance to certain chemicals which has justified
                  their use in special cases.  For example, nylon
                  possesses superior resistance to alkaline materials
                  and is often used in gas filtration at a maximum
                  temperature of 225°F.  Vinyon possesses satisfac-
                  tory resistance to certain chlorides and, to a
                  lesser extent, fluorides; however,  its maximum
                  operating temperature is 200 F, and this limit has
                  sharply reduced its general applicability."30

            Glass  fibers  in mat form have  been widely  used  in the  filtration

 of low concentrations of  atmospheric dusts  in  building ventilation  air  clean-
                             34
 ing,  from the 1930's onward."    Early attempts  were  made  to produce  durable

 filtration fabrics  from glass  fiber,  but experience  soon  showed that  these
 filter  media  were  entirely unsuccessful  from the standpoint of  resistance to

 mechanical damage  during handling  and shaking.   Despite this  drawback,  the
 use of  fiberglass had an obvious advantage  in  that it  could withstand operat-
 ing temperatures in  the neighborhood of  400 to 450°F.    USPHS-sponsored
 research  efforts were directed to  high temperature bag filter developments

 through use of colloidal graphite  lubricants for fiberglass  filter fabrics to
                                                     35
 improve flexure life (A.D. Little,  Inc., 1957-1959).    In  1950 Dracco  in-

 stalled the first full scale commercial  fiberglass installation at Hudson Bay

Mining and Refining.  This large,  shaker and reverse air  cleaned, installation
marks a significant  milestone  in the  employment  of graphite and siliconized
glass filter fabrics.



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            Subsequent PHS evaluation (1963) of thu effects of added col-
loidal graphite treatment to commercial Hiliconc-treatucl glasti fnbri.cu  In-
dicated HubMtantlal improvements in flexure life below 550 F.    An a con-
sequence, the majority of fiberglass textiles used for gas filtration today
(1970) arc generally treated with sillcones and graphite for high tempera-
ture operation.  Fabric lives comparable to those of other fabrics are  now
achieved.  Newer proprietary glasu fabric treatments are now undergoing
evaluation and pilot scale testing for operation at temperatures up to
approximately 1000°F.

            DuPont's Nomex , a high temperature (450 F) nylon was tested in
filter bags on a small scale during 1962 and 1963, and was made commercially
                  37
available in 1964.    It has been used in a number of filtration applications
including carbon black, calcining effluents, and non-ferrous metal fumes in
                                      P
both woven and felted designs.  Teflon  (fluorocarbon) filter fabric is
available, with chemical properties and heat resistance of considerable merit,
but its applications have been limited by cost.  Other high temperature media,
                                  P
including ceramics (e.g. Piberfrax  by the Carborundum Co.) and fine mono-
                                   p
filament metal fiber (e.g. Brunsmet  by Brunswick Corp.) have been woven and
tested on bench and pilot scale for high temperature applications.

            Other equally important advances have occurred in textile  tech-
nology during the period 1945 through 1970.   It has  been necessary  to  develop
spinning, weaving and finishing techniques for each  of the man-made  fibers
consistent with the required fabric permeability and dust retention  proper-
ties.  Methods for heat setting, finishing,  and sewing have been developed
to provide long life under a wide range of environmental exposures.   Felts
suitable for use in pulse-jet and reverse-jet fabric filters  have been de-
veloped (Smith, MIT, ca. 1957) by mechanical needle  punching of staple fila-
                                     38
ments through woven fabric substrate.     Processes for the texturizing or
bulking of filament yarns have been developed to provide numerous tiny fila-
ments within the woven interstices necessary for good  dust retention.   Chap-
ter 4 discusses in greater detail the  available fibers,  yarns,  and  fabrics
and their production and properties with respect to  filtration performance.

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      ' • ^  '  Maniil MC'turt'i'3  and Market.  l)eve 1 opine nt s
            Tin- hlHtory of the  |>renenl  U.S.  I'uhrli-  filter industry In the
.•mm o( Llie v.rowlh!i "'  Individual  I I ruiH  that  have deve I oped sped I it- devices,
ronf Inurnt i<>n«, and  products Lo  meel  l:he  needs  ol  temporal nmrUots.  As
Indicated al>ove , mn )or MtepH  In  ;ippl lcal. Innu  growth  havi1  been associated
with  Improved cleaning methods  and  devices  for  higher  air-to-cloth ratio
(compartmented automatic and reverse  air  on-stream devices)  and with recent
developments in fiber and  fabric  technology.  Apart  from  relatively small
amounts of independent research  efforts,  the  growth  of the fabric  filter in-
dustry has been achieved through  applications-oriented developments by indivi-
dual users and fabric filter manufacturers.
            Although not a widespread practice  today,  early  filters (prior
to about  1910) were  largely constructed by  individual  users  for applications
such as metallurgical fume recovery.  The demonstration (1916)  that fine
respirable (< 10 |.im) mineral dusts were dangerous  (by  many investigators,
including Higgins, Lanza,  Laney,  and Rice of  the Public Health  Service and
the Bureau of Mines) led to increased requirements for silicious dust con-
trol in foundry, granite,  pottery and similar industries.  Toxic metallic
compounds  (e.g. Pb,  As) had long been recognized as  hazardous,  but the impor-
tance  of  particle  size in  occupational  health had  not  heretofore been estab-
lished.   The use of  toxic  particulate smokes  as military  tactical  agents
during the first World War (1916-1918)  also provided substantial impetus to
research  and development in basic aerosol science, respiratory  deposition
and,fate, and in methods for generation,  sampling, analysis,  and most impor-
tantly collection of fine  particulates.   These  efforts have  been further
augmented by the advent of production of nuclear materials after 1945-1950.
           Simultaneous developments in foundry sand blast and  casting
cleaning operations also resulted in filter development by firms such as
Pangborn, Wheelabrator and Sly.   Other developments  of  fabric filters have
been associated with powdered materials technology,  such  as  pneumatic trans-
port and fine grindings, as in cement production by  Fuller Company and
Norblo (Buell).  Since about 1945-1950, control of particulate  air pollu-
tants has been largely responsible for the steady growth  in  the fabric filter

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market.   This  growth  has  been made possible by advances in fabric resis-
tance  to  temperature  and  to  other physical and chemical environmental
factors.

             1.2.3.1  Manufacturers.- A census or enumeration of U.S.  fabric
 filter manufacturers  has  been estimated periodically since 1950 for the
Atomic Energy  Commission, as shown in Figure 1.10.   Manufacturers for the
years  1950,  1954,  and 1961 are listed in Appendix 1.3,  along with charac-
teristics of their equipment.  Manufacturers and equipment for the year 1969
are  given in detail in an Appendix to Chapter 3.  Together these summaries
 indicate  the growth of the industry through the continual  addition of new
products, and  also through increasing numbers of manufacturers.    For the
purpose of these  tabulations, manufacturers have been included who have
product capability tor a  fixed installation greater  than about 10 square
feet of fabric (i.e.,  greater than portable commercial  industrial vacuum
cleaners  which would  contribute an estimated additional 30 manufacturers).
Included  is  the range from bin-vent and unit dust collectors (one to  a few
bags,  < 100  cfm)  through  major installations of the  order  of 10  sq.  ft.
As Figure 1.10 shows,  the fabric filter industry has  in the vicinity  of 50
producers at present.**
             It  should  be  emphasized  that the  individual producers represent
a wide spectrum of  capability for  product  and application,  design engineer-
ing, service,  industrial  application, and market  specialization and penetra-
tion.  They  also vary  in  ability to  respond  to new or novel application
requirements and specifications.   For example, only a limited number of  the
four dozen manufacturers  are  large enough or qualified;to respond to a major
high temperature air pollution control application such a kiln or furnace
                           /
effluent,  and able to  provide the engineering design, fabrication, construc-
tion, installation and operational shal.e down required for turn-key respon-
sibility.   On the other hand, these  same firms might not be in a position
*  See also Appendices 1.4 and 1.5.
** It is important to note, however, that the IGCI members in 1969
   accounted for 83 percent of the total fabric filter system sales.

-------
u>
  100

9°-
1 80-

8 70

0 60
         g  50
         oc
         UJ

         b
         o
         a:
         CD

         if
         a:
         UJ
         m
             30-
             20h
             10
                           Source


                    1.   L. Silver-man, 1950, AIHAQ, U., 1 (Principal Mnfrs.  only)  Ref. 19

                    2.   C.E. Billings, et  al., 1954, USAEC-HACL Report No.  NYO-1590, Ref. 44.

                    3.   C.E. Billings, et  al., I960, USAEC-HACL Report No.  NYO-1590R, Ref. 45.

                    4.   C.E. Billings & J.E. Wilder, 1969,  NAPCA-GCA Fabric Filter

                        Systems Study

                    5.   DOC/BDSA Reports on APCE Sales, F.F. Mnfrs., Ref. 40.
                                       V
                                                        'o
1950     1955
                                                I960
                                                   1965
1970
1975
I960
                            Fia.uro 1.10.  L*.t imatt's of  Number of Fabric  Filter


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 to  respond or compete profitably in small-  to medium-sized units,  where
 other producers  are especially qualified.   Although producers have tradi-
 tionally tended  to concentrate in specific  related markets,  these  distinc-
 tions are gradually disappearing with the increase of filter applications
 and the tendencies toward acquisition and merger  characteristic  of the
 1960's.   Furthermore, major  suppliers of fabric sewn into  ready-to-use
 filter elements  have traditionally been independent of the fabric  filter
 manufacturers, but this  characteristic of the industry is  also disappearing.
            Appendix 1.4 estimates the fabric filter equipment sales and also
 the total  air pollution  control  equipment sales of each identified manufac-
 turer.   These estimates  illustrate two major  factors associated  with the
 manufacture of fabric filters.   First, no single  producer  has  a  majority of
 the market; rather the total  sales are divided among some  45  to  50 large and
 small  firms.   Second,  nearly  all  manufacturers have other  products, and fab-
 ric filters are  estimated to  provide  a relatively small fraction of their
 total  annual  sales.   These two factors have important  consequences  relative
 to  research funding.   No single producer is large enough to be able to  fund
 more than  a modest program in fabric  filter research,  typically  1  to 2 per-
 cent of  FF sales.   When  funded, these  programs tend to  be  concerned with
 specific markets,  applications problems, or product  development.  Most open
 literature technical  publications  presented by industry representatives
 emphasize  engineering  applications, e.g.,  "How XYZ  Co.  solves  its Fume Pro-
 blem," rather than the fundamental aspects of filtration.  The orientation
 of  these companies  to applications rather than to research has undoubtedly
 been a significant  factor  in  the growth pattern of  the  fabric  filter industry.
            The  larger fabric filter producers, for the most part,  manufac-
 ture a full range  of air pollution control equipment including inertial
collectors, scrubbers, and electrostatic precipitators.  Thus, their in-
 terests have not been confined only to the development of fabric filter
equipment.
            1.2.3.2  Fabric Filter Industry.-  An estimate of the average
 growth rate of the fabric filter industry during the past 20 years  is  shown
 in Figure 1.11.  Reported total annual sales for two major producers who

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I
W
Co
                          B50
1955
I960
1965
1970
1975
1980
                     Figure 1.11.  Average Air Pollution Control Equiprnent  Industry Growth

                                  Rate As Estimated  from Reported Sales  of Two M«ur- Manu-

                                  fnd^fil8^ ^ Cleanin§ Products   (Relative f, 1951  (A)

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 supplied  sales  data were  normalized  to  the  initial  year  Indicated.   Both
 producers  are prominent in  the  fabric filter  industry having well  developed
 product lines,  world-wide marketing  organizations,  access  to necessary
 operational and expansion capital, responsible  service,  and experienced
 engineering staffs.   It appears that the  average  sales of  well  established
 firms, in  the air pollution control  equipment business,  have tended  to
 double in  about 10 years  (7.2 percent annual  growth rate).  In  neither in-
 stance does there seem to be any major  perturbation associated  with  accel-
 erated air pollution  control sales at least through 1969.  Both seem to be
 increasing at approximately the same rate.  This  growth  rate is considered
 as typical for  modern U.S.  industry; for  example, the average sales  of the
 ten  largest industrial companies in  the U.S. were $3.3 billion  in  1954 and
 $6.5 billion in 1964.
            Figure 1.12 estimates the growth of the particulate air  pollu-
 tion control equipment market,  and also that part of it  associated with
 fabric filters.  Curve A  represents  an  estimate of  the growth rate of the
mature air pollution  control equipment  industry derived  from Figure  1.11.
Curve B represents total  particulate control equipment sales estimated
 from various reporting sources  as indicated.  Sales prior  to 1963  seemed
 to follow  the growth  rate estimation given in Curve A.  However, from 1963
 through 1967 (last available data),  sales have tended to increase more
 rapidly as a consequence  of legislative and social pressures for greater
 air pollution controls.   From this data base Curve B has been extrapolated
 at the same rate to 1970  as shown, indicating estimated 1969 sales of $140
millions.   From the standpoint  of future estimates or projections it seems
unlikely that this rate of  increase can continue, so Curve B has been con-
servatively estimated to return, after  1970, to the mature industry growth
rate defined in Curve A.

            Sales of  fabric filters  for air pollution control  purposes are
shown associated with Curve C as reported by Business and Defense Services
Administration,  (1963, 1967) and the Industrial Gas Cleaning Institute in

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   1000
    900

    800

    7

    600
    500  _
   200  _
en
ui
 too
_l  90
<  80
i  70
«*
    f>0
O
UJ  r)0
_  40  -
CO
UJ
    20  _
    10
            1958 estimate (x 1/2.25)
Dept.  of Commerce  BDSA estimates of total  shipments
of industrial  gas  cleaning equipment  flange-to-flange, particulate
only
Industrial  Gas Cleaning Institute,  estimates  (x4/3) as re-
ported in 1967 CA  Act Hearings, flange-to-flange, partic-
ulate  only
               IGCI members stated sales, or estimated from above
               testimony, particulate  only
               Fabric Filter sales,  reported or estimated
               from above sources
               Estimated fabric filter  sales, 1969
               IGCI reported sales,  1969, 1968
                  1950
             1955
1960
I960
1970
1976
I960
                 Figure  1.12.   Particulate Air Pollution Control
                                  and  Fabric Filter  Sales.

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 conjunction  with  the  Department  of Commerce  and  NAPCA  (1966,  1967).    Filter
 sales  have reportedly increased  from $14 million (1963)  to  approximately $20
 million (1967).   The  slope  of  the growth estimate Curve  C is  not greater than
 about  10% above what  would  be  predicted from the Curve A mature industry
 growth rate  estimate.   In view of the relatively rapid growth in sales shown
 in Curve  B for all  particulate control equipment, this seems  to indicate a
 decreasing relative share of the control equipment market for fabric  filters.
 This is rather surprising considering the expansion of application markets
 through fabric and  filter device technological developments.   Curve C  indi-
 cates  an  estimated  pollution control  fabric  filter sales of $25 million In
 1969.   Curve C/ has been included to  demonstrate the growth rate of pollution
 control fabric filter sales, had they grown  from 1962 at a  rate parallel to
 Curve  B or total  control equipment sales (est. 1969 FF sales  $45 million).
 The asterisk at $55.5 million  for estimated  1969 aggregate  sales of all fab-
 ric filters  results from a  consideration of  individual fabric  filter manufac-
 turers  in Appendix  1.4.
            This  estimate of $55.5 million for the annual fabric filter sales
 is of  the order of  twice the Figure  1.12 estimate provided  by  extrapolation
 of Department of  Commerce BDSA and IGCI sales.   The reasons for the difference
 in the  two independent  estimates are  not evident, except for  a possible
 qualification to  the  data provided in the earlier surveys.  There, process
 equipment applications  have been excluded by  definition  as  not related to
 air pollution control.  For the purposes of estimating total numbers of
 fabric  filters in use,  the  lower figures have been used,  recognizing that
 the estimates so  provided may  be low  by a factor of order 2.
            Further statistics  of the  fabric filter industry and its devel-
opment  are presented  in Appendix 1.5.  An analysis of these data yields the
 following estimates:
*These surveys were directed toward fabric filter air pollution control
applications and simultaneous product recovery.  The distinction is fre-
quently unclear, however, and the surveys may include some non-air
pollution control sales.

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            Number of Fabric
              Filters:
            Average filter size
              (ft2):
            Total filter area
              (ft2):
            Fabric sold  (ft2):
            Gas  volume  filtered
              (cfm):
            Filter sales:
            Fabric sales:
  Fabric
 Filters
 In use,
 ca. L910
   750

   400

0.3xl06
0.15x10
Sold  in
  19b9
  7500
 3000
22x10
             100x10
 In  Use,
 as  of
  1969
100,000
243x10
           750x10
             $22x10
             $33x10*
                   6
 It  is emphasized  that  these  estimates are  tentative, and are not as precise
 as  could be generated  by a national census or register of fabric filter
 devices.  Sizes and costs of fabric filter equipment will be the subject
 of  later sections of this handbook.
 1.3  OPERATING PRINCIPLES
     A fabric filter is made up of a woven or felted textile material in the
 shape of a cylindrical bag or flat supported envelope.  The textile material
 is  contained in a metal housing having inlet and outlet gas connections, fi
 dust storage hopper, and means for cleaning the fabric periodically.
     Woven fabrics consist of parallel rows of yarns in a square array.
 Open spaces between adjacent  yarns are occupied by projecting fibers.
 Felted fabrics consist of close, randomly intertwined fibers compacted to
 provide fabric strength.  In  operation, dusty gas passes through the filter
 normal to the fabric and dust particles, at the start of filtration, deposit
 on individual fibers and yarn surfaces.  Additional particles then deposit
 and accumulate on already deposited particles forming filamentous aggregate
 structures which project into the gas stream.  As deposition and accumula-
 tion continue, openings between yarns and individual fibers become occupied
by aggregates and reduced in  size.  Eventually a more or less continuous

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deposit  forms, analogous to the  filter cake common  in  liquid  filtration.
Particle collection  then occurs  by mechanisms associated with porouH  gran-
ular media.  Thus, the  fabric  filtration  process may be considered  to con-
sist of  at least  three  distinct  phones:   (1) initial or early deposition,
when depositing particles  land on individual fibers, fibrils  or  filament.s
of the yarn; (2)  intermediate  deposition  when particles accumulate  on pre-
viously  deposited particles, long filamentous particle aggregates form, and
bridging of interweave  and interstitial spaces occurs  leading to the  forma-
tion of  a more or less  continuous deposit; and (3)  the continued deposition
of particles on a matrix similar to a granular layer,  leading to the  forma-
tion and consolidation  of  a filter cake.
     Accumulation of dust  causes an increase in gas flow resistance,  as a
consequence of the particle drag forces and interparticle pore (capillary)
resistance.  The  properties of the particle bed (porosity, permeability)
also change during the  formation of the deposit.  Aggregates  deform,  bend,
collapse, or reorient under the  action of the gas flow and the increasing
compressional pressure  in  the  bed as its  depth increases.  Fundamental
mechanisms of deposit formation  and cake mechanisms are considered  in
greater  detail in Chapter  2.
     The filter bag  (sleeve, tube, or envelope) is  supported by  external
and sometimes internal  structures to permit the dusty gas to flow through
the housing from  an  inlet  section to a clean gas outlet section.   This is
shown in Figure 1.13 in one of several typical arrangements.   In the parti-
cular arrangement the bag  is fastened at the bottom to a tube sheet or cell
plate and held up vertically by  a top hook on an overhead rack.   The bag
                                                       I
may be pulled to  the desired tightness by means of a tension adjustment.
Gas flows into the inside of the bag from the bottom and passes  up the in-
terior and through the bag to the outlet ducting.   The gas may then go to
the suction inlet of a blower and hence out of the system,  either returning
to the ventilated space or to the outdoor atmosphere.   Alternatively,  the
blower can be located on the inlet side of the fabric  filter  so  that it
draws air or gas from the ventilated system or process  and discharges  it  to
the bag collector under pressure.  In this latter  case, as  before,  the gas

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c






                           Figure 1.13  -Typical Fabric Filter Arrangement
                                        (Courtesy of Wheelabrator Corp.)

O

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 passes  through  the  fabric  leaving the dust on the inner surface,  and the
 gas  is  then discharged.
      There  are  several methods  of removing the dust  from the  inside of the
 filter  bags.  In  the  design shown the dust may be removed by  stopping the
 flow and  allowing the dust to fall off by its own weight.   More  typically
 the  dust  is removed by vigorously oscillating the suspension  rack back and
 forth (••-  5  cpa) through  on amplitude  of a few Inchon.   This shaking mot lor,
 imparted  to the bags  in  the absence of flow,  causes  sufficient forces to
 be transmitted  to the deposit to  separate most of it from the fabric sub-
 strate.   The dust then drops down the interior of the  bag,  through the mouth
 of the  bag  at the tube sheet, and into the hopper.   Collected dust is re-
 moved from  the hopper periodically or continuously for disposal  or return
 back into the production process.
      Cohesion and adhesion forces,  holding individual  particles  together or
 to the  underlying fibers or yarns,  exist  over a range  of values.   The appli-
 cation  of a cleaning  force by shaking initially removes a  large  amount of
 the  deposit on  the  first few shake cycles but then progressively  less and
 less  on succeeding  cycles.   A substantial amount  of  residual  deposited
 particulate material  remains within the  fabric  interstitial spaces.   The
 remaining deposit produces a residual (cleaned) pressure drop across  the
 fabric.   It also  provides  a fairly high  initial particle co?.lection  efficiency
 upon resumption of  flow  of the  dusty  gas  on the next filtration cycle  as
 the  individual residual  particles  and particle  aggregates act as objects
 for  collection of the incoming  dust.   Upon resumption  of filtration  the
 dust  deposit  more rapidly  acquires  a  continuous character because  of  the
 residual  material.  These  deposition  processes  proceed' at different  rates
 within  the  total  bag  structure  at  different areas of the fabric in response
 to the  amount of  flow passing locally, which  depends upon the amount of
 residual dust.  Thus a local area which has a more porous or open residual
 structure will tend to allow more flow and, initially at least,  material will
deposit faster.   These initial differences tend to even out so that filtra-
tion velocities  and dust  deposition rates tend to become uniform over the
entire fabric surface.

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     As the  gas  flows  outward  through  the  bag the  velocity inside the  bag
decreases.   Since  the  particle-gas motion  is  vertical,  there are flow  and
particle  stratification  effects which  enter into the  analysis of the macro-
scopic processes of  deposition, accumulation,  and  gas flow through the
media.  In the configuration depicted  in Figure 1.13, the  upward flow  within
                         \
the bag and  the  turn of  the gas in leaving the hopper and  entering an  indi-
vidual bag tend  to separate larger particles  out of the flow by  gravity  or
inertia.  Larger particles carried into the tube are  deposited in the  lower
end of the tube  as the gas velocity, progressively decreases as  it uses.
This stratification  of particle sizes  has  important consequences on deposit
characteristics  and  on fabric  performance  and  life over the  entire bag
length, but  especially at the  bottom near  the  entry.

     The  operation cycle of the fabric filter  thus contains  two  phases.   The
first is  a filtration  phase during which material  is  depositing  and accumu-
lating on the fabric while pressure drop across the deposit  is increasing,
and os a  consequence total flow is decreasing.  The second is  a  cleaning
phase with no filtration flow  during dust  removal.  By  compartmenting  sec-
tions of  bags and isolating these one  at a time from  the gas  flow  for  clean-
ing, the  total flow  from the process ventilated can be  maintained  reasonably
constant.   This cleaning procedure,can be  automatic and the  operation  con-
tinuous.  The residual dust deposited  and retained within the  fabric inter-
stices gradually reaches an equilibrium value after numerous  filtration and
cleaning  cycles, after which the residual  pressure drop remains more or less
                                                      4     7
constant  throughout  the useful life of the fabric  (~10  - 10  cleaning
cycles).
     Particles entering a new  fabric initially contact  individual fibers,
fibrils,  and yarn filaments and arc-, separated from the  gas by several
filtration mechanisms  (see Chapter 2).   Deposited particles serve as addi-
tional obstacles for further capture of other particles.  After cleaning,
the residual dust provides a substantial number of obstacles for  further
particle collection.   The collection efficiency of a  fabric filter is  de-
fined as
            „,.,-.  .        ,    outlet dust concentration       „             .,  ,.
            Efficiency =1	. :•" '  j—:	~—~.— = 1 - Penetration  (1.1)
                              inlet dust concentration

-------
 It  may be determined for  an operating filter  from simultaneous measurements
 of  inlet  and outlet  dust  concentrations  by appropriate stack sampling tech-
 niques.
      The  basic  collection efficiency  of  new filter fabric  is generally in
 the range of 50 to 75% for submicron  atmospheric  dust  (0.5 um count  median
 diameter,  and an  inlet  concentration  of  75  ug/m )  at 3  to  8  ft/min.  As dust
 accumulates, efficiency rises.  By the time the deposit amounts  to 2 to 3
 grams  of  dust per  square meter  (~ 3 to 5 grains per square foot) the collec-
 tion efficiency usually exceeds 90%.  This happens  in about  one minute at
                                      3         3
 an  inlet  dust concentration of  2.3 g/m   (1 gr/ft  ), a common  industrial dust
 leading.   Overall  efficiency then continues to increase with dust accutnula-
                                                                    2
 tion,  and  will generally exceed 997. when the deposit reaches  150 g/m
           2
 (0.03  Ib/ft  ), i.e., after an hour or less at common industrial dust con-
 centrations.  After a period of cyclic filtration  and cleaning ranging from
 a few hours  to a few days the residual deposit will stabilize and thereafter
 efficiency will remain  greater  than 99%.  During usual operating conditions,
 fabric filter overall weight collection efficiency will exceed 99.9%.  The
 collection efficiency for the submicron fraction of the inlet dust is usually
 at  least greater than 90%.
     The high overall efficiency is related to the nature of the accumulated
 deposit.   Overcleaning  in order to reduce flow resistance will frequently
 cause a reduction  in overall efficiency and a slight visible puff of dust
may be observed upon resumption of filtration.  The dust collection efficiency
 for the submicron  fraction may then be less than 80% until cake formation.
Collection efficiency and pressure drop performance relationships between
dust, fabric, cleaning, and design parameters  are  discussed in subsequent
sections.   Overcleaning can also shorten the life  of the fabric and thereby
contribute substantially to the cost of maintaining the filter system.   Fab-
ric life averages  about one year, although it can vary from a few weeks up
to 20 years.   The actual fabric life and indeed the entire filter performance
depend on the design of the filter, the application conditions and the care
given the filter system.  These are all the subjects of later sections of
this handbook.

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 1.4   FABRIC  FILTER APPLICATIONS  AREAS
      It  is the  purpose  of  this section to  cite  several  general  application
 areas  of  fabric  filters  and  of  alternative  particulate  control  equipment.
                                                                      It
                                                                      ,6
As indicated earlier there are estimated to be at least 10  fabric filter
 particulate  collectors  in use  (1969)  treating  on  the  order  of  750 x  10
 cfm.   These  fabric  filters range  over more  than a hundred different  appli-
 cations,  and in size  over six  orders  of magnitude.  The  spectrum of  sizes
 in  use is also  estimated, insofar as  possible, within the limits of  data
 now available.   Later in  Chapters 6 and 9,  more specific applications will
 be  described.
              39
      Friedrich   has  estimated "that  approximately  80 percent  of all manu-
 facturing plants produce  dust  loadings and  particles  small  enough to warrant
 or  require the  use  of..." fabric  filters.   Since  there are  some  311,000 man-
 ufacturing plants in  the  U.S.  (1969 Statistical Abstracts)  this  provides an
 approximation of the  potential usage  of fabric filters.  This  estimate, of
 course, represents  a  reasonable approximation of  the  total  market for parti-
 culate control  devices  including  fabric filters,  electrostatic precipitators,
 scrubbers  and mechanical  collectors.  Since each  system has unique qualifi-
 cations for  specific  applications, the performance  and economic  aspects of
 e-.ach  type of device must  be evaluated in the context  of the applicable par-
 ticulate  control requirements.  Addressing our attention specifically to
 fabric filter systems,  however, we emphasize that this approach  to particu-
 late  control is  highly  versatile.  With design modifications they may be
 used  in the  treatment of  process  gases, for the recovery of powdered pro-
 ducts, and for  the  recovery of  nuisance or toxic dusts and  fumes  for the
 protection of the environment  as  in air pollution control.   As indicated
 earlier these applications include mining and minerals processing  (both
metallic and non-metallic), chemical and allied manufacturing, food products,
 rubber and plastics, metal refining, primary metallurgy,  and machinery.
 Fabric collectors handle dusts from crushing,  grinding, pulverizing,  convey-
 ing, milling, drying; fumes from cement kilns,  iron melting cupolas,  rever-
beratory and electric arc furnaces; and carbonaceous smoke  from incomplete
combustion of chemical process streams as in carbon black manufacture and
fuel combustion, to mention just a few.

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     The principal advantages of fabric filter systems in such installa-

tions are:

     .  Particle collection efficiency is very high and can be main-
       tained at high levels.

     .  Efficiency and pressure drop are relatively unaffected by large
       changes in inlet dust loadings for continuously cleaned filters.

     .  Filter outlet air may be recirculated within the plant in many
       oases.
     .  The  collected material is recovered dry for subsequent processing
       or disposal.
     .  There are no problems of liquid waste disposal, water  pollution,
       or liquid freezing.
     .  Corrosion and rusting of components is usually not a problem.
     .  There is no hazard of high voltage, simplifying maintenance  and
       repair and permitting collection of flammable  dusts.

     .  Use  of selected  fibrous or granular filter  aids permit the high
       efficiency collection of submicron smokes and  gaseous  contaminants.

     .  Filter collectors are available in a large  number  of configurations
       resulting in  a range of dimensions and inlet and outlet flange
       locations to  suit installation requirements.

     Some limitations in the use of fabric filters include:

     .  Fabric life may  be shortened in the presence of acid or alkaline
       particle or gas  constituents,  and at elevated  temperatures.

     .  Temperatures  much in excess  of 500 F require special refractory
       mineral or metallic  fabrics  that are still  in  the  development
       stage.
     .  Hygroscopic materials,  condensation of moisture, or  tarry,
       adhesive components  may cause  crusty caking or  plugging of the
       fabric,  or require special additives.

     o Certain  dusts  may  require  fabric  treatments to reduce seeping of
      the dust  or in other  cases to  assist  in the removal of the col-
       lected dust.

     . Concentrations of  some dusts in  the  collector (~ 50 g/m ) may
      represent  a fire or explosion hazard  if spark or flame is admitted
      by accident.   Fabrics can  burn  if readily oxidizable dust is  being
      collected.

     . Replacement of fabric may  require respiratory protection for  main-
      tenance  personnel.

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     KilttT applications can be categorized to reflect similarities of
industrial processes, unit operations, or filter usage patterns.  Surveys
of fabric filter applications by some categories are available, such as the
Business and Defense Services Administration (BDSA) survey of fabric filter
                                                40
sales for the year 1967 summarized in Table 1.3.
     Table 1.4 presents a summary of sales reported by member companies of
                                                                  41
the Industrial Gas Cleaning Institute for the years 1966 and 1967.    Data
are presented by Standard Industrial Category Number (S.I.C. No.)* for
value and number of collectors sold.   Fabric filters represent the largest
number ot" devices sold in each year,  and were employed in all categories
except domestic, commercial, and industrial heating plants.
     Data obtained from these two surveys may underestimate total fabric
filter usage because:  (1) questionnaires are believed to have been speci-
fically directed only to air pollution control sales;  and (2) the number of
manufacturers of fabric filters surveyed (23 by BDSA,  15 by IGCI) was less
than half the total number of about 50 manufacturers.
     Data from the IGCI particulate air pollution control equipment summary
in Table 1.4 have been combined in ten categories as shown in Table 1.5
to illustrate present usage patterns  and for later estimation of potential
application areas.  In addition, these combined figures may be compared to
similar figures developed by BDSA in  Table 1.3 for some of the categories.
Several conclusions may be drawn from Table 1.5:
     (1)  Major 1967 application areas for fabric filters included all
          categories except Combustion,  Pulp and Paper, and Petroleum
 For more  S.I.C.  detail,  see  "Standard  Industrial Classification Manual,"
 Bureau  of the  Budget, Washington, D.C.

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                                                  Table 1.3

          INDUSTRIAL GAS CLEANING EQUIPMENT--MANUFACTURERS' SHIPMENTS BY END USE,  1967

                                              (Thousands of dollars)
End use
Iron and steel 	
Utilities 	
Chemica Is , 	
2
Rock products 	
Pulp and paper 	
Mining and metallurgical
Re f i ne ry 	 	
3
All other 	
Exports 	

Total shipments
Electrostatic
precipitators
5 783
15 506
1 207
2 760

. cs
687


36,509
Scrubbers,
particulate
7,423

3 709
1 142
989
825
3 901
651

19,229
Mechanical
collectors
2,300
2,476
3,130
1,038
802
389
C1)
8 408


22,381
Fabric
filters
4,536

5,344
3 60?
122
1,855
4 959
1 081

21,730
% Fabric Filters
of all Types
22.5

40.5
42.4

	
27.6


21.8
 1
  Not  published  to  avoid  disclosure.
 >
 ""Rock products" includes  cement  and  asbestos  plants.

  "All other"  includes  shipments to distributors  where  end  use can not be identified.
  (From Ref. 40)



-------
                                                                  TABLE  1.-
                                            SUMMARY OF. THE MANTFACTURERS' REPORT OF AIR POLLUTION
 i
•P-
*
CONTROL EQUIPMENT SALES ( PART I Ct' LATE)

INDUSTRIAL CLASSIFICATION
ALL EQUIPMENT, NEC **
DOMESTIC & COMMERCIAL
HEATING PLANTS
INDUSTRIAL HEATING PLANTS
MINING
fS.I.C. No. : 1C)
MINING AND QUARRYING OF
NONMETALLIC MINERALS,
EXCEPT FUELS
(S.I.C. No. : 14)
FOOD AND KINDRED PRODUCTS
(S.I.C. No. 20)
TEXTILE MILL PRODUCTS
(S.I.C. No. : 22)
APPAREL & RELATED PRODUCTS
(S.I.C. No. : 23)
(From Ret . VO .
NEC = Not Elsewhere C

Year
1966
1967
1966
1967
1966
1967
19-56
1967
1966
1967
1966
1967
1966
1967
1966
1967
lassif led.


ELECTROSTATIC
Number Amount
1
0
0
0
6
5
6
0
0
0
0
0
0
0
0

63,680
0
o
300.533
-36.327
395,000
559, "00
0
0
0
1^
0
0
0
1
u



MECHANICAL
Number Amount
936
1,066
7
16
113
106
33
27
224
178
119
£5
5
2
0
0

953,121
551,012
12 , 649
115,783
907,468
885,153
194,222
185,843
388,733
338,136
407,330
298,277
7,374
1,335
0
0



FABRIC FILTER
Number Amount
1,250
1,380
0
0
0
0
35
24
55
46
394
315
45
20
0
1

601,548
907,779
0
n
O
0
409,676
187,622
334.594
378,234
1,074,862
949,096
84,741
73,618
0
428


WEI SCR:.r±I5,
Sur.ber A.rcur.t

143 --;.::•
112 54 C. 4,4
1 6.30C
0 0
9 33.000
36 2-1.100
58 -90,: 14
50 -06.25'
28 286.64.5
20 124.950
65 199. £17
8? 35C'.2cS
0 0
0 0
0 0
0 0

-------
 I
-p-
co
                                                       TABLE 1.4  (Continued)



                                    SUMMARY OF THE MANUFACTURERS REPORT OF AIR POLLUTION



                                            CONTROL EQUIPMENT SALES  (PARTICULATE) (C-int Ur,r 
-------
                    TARL-  !.- (Continued)



SUMMARY OF THE MANUFACTURERS REPORT CF AIR PCLU'TICN




        CONTROL EQUIPMENT SALES (PARTICULATE) (Continued)
INDUSTRIAL CLASSIFICATION
STONE, CIAY, GLASS 6,
CONCRETE PRODUCTS
(S.I.C. No.: 32)
PRIMARY METAL
(S.I.C. No.
INDUSTRIES
: 33)
FABRICATED METAL PRODUCTS
(S.I.C. No. 34)
MACHINERY, EXCEPT
ELECTRICAL
(S.I.C. No.: 35)
ELECTRICAL MACHINERY
(S.I.C. No.: 36)
TRANSPORTATION
(S.I.C. No.
INSTRUMENTS
(S.I.C. No.
MISCELLANEOUS
INDUSTRIES
(S.I.C. No.
ELECTRIC, GAS
SERVICES
(S.I.C. No.
TOTAL
EQUIPMENT
: 37)
: 38)
MFG.
: 39)
& SANITARY
: 49)

Year
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
1966
1967
ELECTROSTATIC
Number Amount
11.
13
20
16
. 0
0
0
0
0
0
0
0
0
0
0
0
55
63
127
123
2,351,850
2,054,695
3,966.483
2,366,368
0
0
0
0
0
0
0
0
0
0
0
0
15.998,885
22.774,900
26.617,381
31,078.140
MECHANICAL
Nur.ber Ar-.eur.t
99
91
386
356
162
0
521
375
245
203
149
147
121
108
89
77
139
129
4.281
3.950
PARTICULATE CO
380,909
332,825
927,599
704,457
109,286
0
329,444
270,210
146,923
140,671
95,848
105,321
72,184
75.836
58,204
53,927
1.854,969
1,798,669
12.040.582
9,985.349
NTROL TOTAL
FAS3IC FILTER -T
Nu.Tber A.-our.t N-jr're
405
276
819
833
226
264
855
704
376
321
200
187
141
160
107
121
33
20
6.237
5.963
196f> 11
1967 11
2,81-,
3,450,
5,738,
6,587,
207,
251.
574,
581,
337,
258,
272;
342,
HI,
221.
Ill,
91.
369.
221,
952
635
669
496
725
527
5^6
670
055
873
056
152
763
117
070
2%
361
670
18,173,618
H.595,543
io50
69.
71.
IS
261
305
5
5
0
0
3
7
2
2
0
0
0
4
25
115
879
1,014
846,899
397,186


r A-c.r.t
: -- ;- i
:I:-.N.?
- T« •>=;-
. . - . .

3-.--1
0
14,995
i.3,09?
432 , 500
39 50C
0
0
0
70,770
833,534
787.031
13,015.318
10,738.154

-------
                                                                              Table 1.5

                                                   SUlfcfARY OF IGCI REPORT OF FABRIC FILTER SALES FOR AIR POLLLTIOfi CONTROL

                                           1-J66                          1967

I.

2.

3.

4.


5.


6.

7.


8.


9.


10.





Combustion
SIC 010,020,49
Food & Feed
SIC 20
Pulp & Paper
SIC 26,27
Inorganic
Chen.
SIC 28/
Organic
Chem.
SIC 28/30
Petrol Ref.
SIC 29
Non-metal-
lic Mln Ind
SIC 14,32
Iron & Steel,
Fdy SIC 10/
33/
Non-Ferrous
Met SIC 10/
33/
Misc NEC
SIC 00,22,23,
24,25,31,34,
35,36.37,
38,39
No.

33
394

81

156


821


44

461


725


129


3393




% by Category
$ x W3 No." 1 $ %

369 0.5
1,075 6.3

179 1.3

681 2.5


3,576 13.2


184 0.7

3,150 7.4


5219 11.6


929 2.1


2812 54.4





2
5

1

3


19


1

17


28


5



.0
.9

.0

.8


.7


.0

.3


.7


.1


15.5








No.

20
315

62

146


766


27

322


735


122


3447




$x!03

221
949

149

661


3466


259

3828


5755


1019


3288




7. by Category
No. \

0.3
5.3

1.0

2.5


12.8


0.5

5.4


12.3


2.0


57.9




$ %

1.1
4.8

0.8

3.4


17.7


1.3

19.5


29.4


5.2


16.8




I FT $ of all APCE
$ in Category

1
60

6

24


70


8

56


42


35


55




Average FF
Cost, $

11,000
3,000

2,400

4,500


4,500


9.600

11,900


7,800


8,400


960




                           TOTALS        6237   18,174   100.0    100.0    5962   10,595   100.0   100.0

                        $/FF                    $2.910                         $3290

                        •/. FF $ of Total           26*                             271
                          APCE $


                          *
                           APCE $ - Sales of all  types  of  air  pollution equlpnent for particulate control.
I

-------
           Refining, which in 1967 represented in all Leas than about 37,,
           of filter sales.

      (2)  Fabric filters apparently represent more than 507, of particulate
           control device sales in Food and Feed, Chemicals, and Non-
           metallic Minerals, and between 35 and 45% for Metallurgical
           applications.

      (3)  The average cost  of a fabric filter sold in 1967 was approximately
           $3290 (as compared to $253,000 for the average electrostatic
           procipitator,  $2530 for the average mechanical collector,  and
           $10.600 for the average, scrubber).  Using the previously assumed
           average cost ot' $1 per sq.  ft.,  the average filter size was 3290
           sq.  ft.
      (4)  Approxomately  58% of the units sold in 1967 cost less than $1000
           (category 10),  while less than 6% of the units sold cost more
           than $10,000 (categories I  and 7).  The dollar volume represented
           by these groups was approximately the same ("-20% of sales), a
           characteristic  of the size  distribution of fabric filter installa-
           tions .   This pattern is further  illustrated in the discussion which
           follows.

      Two major U.S.  manufacturers have provided information on typical 1967

and  1968 sales,  including product model,  application area and size of col-

lector,  for  each  of  some  1200 units sold during this period.  This sample

is believed  to represent  5  to 107. of  the  total number of fabric filters

produced in  the U.S.  for  this period,  and  approximately the same fraction

of total dollar value.  The  distribution of fabric  filters  by application
ar.d  size is  presented in  Table 1.6.   These  figures  indicate  that  manufac-

turer C  had  major  product penetration (  >10% of  area sold,  column 2)  in
categories 7 and  8,  and that  manufacturer D had  major  penetration (column 4)

in categories  2,  5,  and 7.  Manufacturer C  produced  a  larger number of

collectors with areas  >3200  sq.  ft.,  whereas  manufacturer D  produced  more

collectors with areas  <320  sq.  ft.  Statistical  analyses  of  number-size

data  indicate  the  following median  sizes:


                                       Mnfr.  C       Mnfr. D

           Median Size by Area:        5500  sq.  ft.    1100 sq.  ft.

           Median Size by Number:      1500  sq.  ft.    250 sq.  ft.

The median collector  area produced  for the  two manufacturers  combined  for

both  years was approximately  650  sq.  ft.  The  differences in medians  for

the two manufacturers cannot  be directly translated  to gas volume  treated

-------
I
Ol
                                                       Table  1.6

                                  SALES  OF TWO FABRIC FILTERS MANUFACTURERS BY  INDUSTRIAL CATEGORY

                                                  (1967 -  1968 Combined Sample)
          Category        Manufacturer C
                        % Tot.  Area  % Tot.  No.
  Manufacturer D        Distribution of Fabric Filter Size, Sq.Ft.  % Nos
% Tot. Area  % Tot.No.  <100   100-   320-   1000-   3200-     1QOO-
1.

2.

3.
4.
5.
6.

7 .

8.

9.

10.

Combustion

Food & Feed

Pulp & Paper
Inorganic Chem.
Organic Chem.
Petrol Refining

Non-metallic Minerals

Iron & Steel Foundry

Non-Ferrous Metals

Not Elsewhere Classi-
fied
TOTALS
8.0
	
4.7
	
2.0
3.0
9.7
0
	
39.3
	
10.0
	
4.9
	
18.4
	
100
6.0
	
2.3
	
0.8
3.8
13.3
0
	
45.1
	
4.5
---
1.7
	
22.5
—
100

2.2
«• • •
22.6
0
8.0
28.9
. _ _
0
— - —
28.2
.__
2.2
• — _
2.1

3.8
100

1.3
---
24.3
0
14.0
24.2
___
0
. * _
21.8
• -. —
1.0
. — -
2.9
._-
10.5
100
CUM % Nos,

0
0
0
23
0
0
0
32
0
29
0
0
0
17
0
0
0
47
0
33
14
14
320
0
25
8
34
40
0
0
34
3
29
0
0
7
38
4
33
30
0
14
27
18
32
1000 3200
15
25
30
30
60
0
23
27
50
19
0
0
34
30
8
67
20
41
35
38
31
63
29
50
8
9
0
0
50
7
20
18
0
0
29
9
34
0
30
12
17
2
19
82
10,000
53
0
23
4
0
0
27
0
25
5
0
0
26
6
23
0
10
0
32

13
95
32 . 000
3
0
31
0
0
0
0
0
1
0
0
0
4
0
31
0
1C
0
2
0
5
100

-------
 or sales dollar volume because of differences in applications and design.
 For example,  as will be seen in Chapter 7,  a 650 sq.  ft.  collector could
 have a cost range of 2 to 10 $/sq.  ft.  or more.
      The analyses presented above have  been directed  to:   (a) an estimate
 of the total  number of fabric filters  in use and their cost to enable a
 reasonable estimate of the likely yield from research and development
 investment; and (b) an estimate of the  distribution of the use of fabric
 filters  (categories,  sizes,  and numbers)  to establish the state-of-the-art
 and gap  areas  in applications to be identified with research needs.   For
 maximum  confidence  it  would  be extremely valuable to  have additional  data
 on each  application,  or on a reasonably representative sample.   Since the
 number of fabric filter Installations  is  probably in  excess of 100,000,  a
 representative  sample  would  require at  least several  hundred installations
 within the ten  use  categories to characterize adequately  the range of var-
 iables in each  process and each operation.
      Limited surveys of this kind have  been made including a number of NAPCA
 programs,  recently  completed or presently underway, and an independent
                                                                       42
 survey of  the Non-metallic Minerals  Industry reviewed in  Appendix 1.6.
 NAPCA systems studies  completed as  of early 1970 include:   Category 1,  In-
 cineration (Arthur  D.  Little,  Inc.); 2,  Pulp and Paper (A.A.  Sirrine);  8,
 Iron and  Steel  (Battelle Memorial Institute); 9,  Non-Ferrous  Metallurgy
 (Arthur  G.  McKee).  Others are planned,  programmed  or  in  progress.  Each
 of the completed studies  present  substantial  detail of  the  technical  re-
 quirements  for  particulate pollution control; e.g.  operations, processes,
 gas  temperatures and flows,  particulate  rates, etc.  However,  it  isn't pos-
 sible  from a study  of  these  reports, to obtain technical  details  about more
 than  a few variables associated with specific fabric  filter  installations.
     Technical  aspects  of  the  utilization of  fabric filters have  been des-
cribed for many types  of applications in the  "Air Pollution Engineering
       43
Manual"    based  on  typical installatipns in Los Angeles County.   Source
inventory data which might be  used to estimate the whole  population is
not given, however.  Similar  presentations have  been prepared by  and  for

-------
the  iron  foundry  industry.   It has not been  possible  to deduce  the  extent
or magnitude of controls  for major application categories  from  these manuals.
1.5  SUMMARY
     Fabric Filter dust collectors are one of 4 major types of  particulate
control equipment.  They  are highly efficient, economically attractive  in
many applications and are widely used throughout industry.  Their origin is
traceable back many centuries and some of the initial design approaches de-
veloped in the late 19th  century are still found in new installations.  With-
in the last 20 years, however, innovations have been made which have been
widely accepted and which have resulted in significant improvements in
system performance.
     There are now over 100,000 fabric dust collectors in use, in hundreds
of different dust applications and over a size range of six orders of mag-
nitude.

     Approximately 50 U.S. manufacturers are now supplying in the vicinity
of 7,500 fabric filters a year with sales of$25 to 50 million.  The fabric
for these filters has a similar sales volume.  Apparently, as a direct con-
sequence of this broad supply of manufacturers and the fact that no single
manufacturer has a large percentage of the market,  there is an economically
based reluctance to undertake the research programs necessary to fully ex-
ploit the applications potential of fabric filter systems.
     The analyses presented  in this volume, and the conclusions derived
therefrom, have been hampered by a lack of available and reliable informa-
tion.  As will be shown later, better fabric filtration equipment is needed
for both present and potential applications.  This  need may justify a more
penetrating survey of the present art from any of several standpoints -
performance of fabrics, methods of cleaning the fabric, varieties of dust,
similarities of application, manufacturers' R &D experience,  etc.  In fact
any such survey must cover several of these standpoints to some degree in
order to be effective, as they are so interrelated.
     The principal conclusions which we have reached based on foregoing dis-
cussion and tabulations, and the information presented in the appendices,
are:

-------
1.  The fabric  filter market  Ln  1969 including both hardware and
    fabric sales is estimated to have been about $50 x  10  .  It may
    very well have been twice this as a result of conservative re-
    porting and estimating.

2.  Historically the fabric filter market has approximately doubled
    every ten years and indications are that the recent growth rate
    has been about the same, although the growth rate of all air pol-
    lution control equipment has been approximately twice  this.

3-  There are now approximately 50 manufacturers of fabric filter
    equipment, and the number is increasing at the rate of 2 or 3
    per year.

4.  Fabric sales are a large proportion of the total fabric filtra-
    tion market.

5.  Because the fabric filter industry is composed of a large number
    of producers of equipment, each producer has a small portion of
    the market and apparently cannot invest more than a modest amount
    on research.  (As will be seen in Chapter 4, a similar comment
    applies to the manufacturers and processors of filter  fabrics
    and fibers.)

6.  Research and development efforts tend to be concerned with specific
    markets, applications problems, and equipment improvement,  rather
    than with basic engineering investigations which might result in
    significant advances,in fabric filtration technology.

-------
1.6  REFKRENCKS TOR CHAPTER 1


     1.    E.  L. Wilson,  Statement of Earl L.  Wilson, President. Industrial Caa
          Cleaning  Institute...,  in Air Pollution - 1467 (Air Quality Act),
          Hearings,  U.  S.  Senate, Ninetieth  Congr., First Session, on S. 780,
          Part  4, page  2629 (1967).

     2.    C.  J. Stairmand,  Selection of Gas Cleaning Equipment: A Study of
          Basic Concepts,  in Filtration Society Conference, Choosing the Right
          Equipment  for Dust Control and Air Cleaning,  Preprint of Papers,
          (Sept.  1969).

     3.    H.  G. Guyton,  H.  M.  Decker,  and G.  T.  Anton,  Emergency Respiratory
          Protection Against Radiological and Biological Aerosols, AMA Archiven
          of  Industrial Health,  20:  91 (1959).

     4.    C.  N. Davies,  Fibrous  Filters for Dust and Smoke, The Ninth Inter-
          national  Congress on Industrial Medicine, London, Sept.  13-17.  1948.
          John  Wright and  Sons,  Ltd.,  London  (1949).

     5.    W.  C. Ebaugh,  the Bag  House  and Its Recent Applications, J.  Ind.
          and Eng.  Chem. 1_:  686  (Oct.  1909).

     6.    A.  L. Labbe and  J.  J.  Donoso,  Modern Baghouse Practice for the Re-
          covery of  Metallurgical Fumes,  J. of Metals,  Trans.  AIME,  188;  792
          (May, 1950).

     7.   W.  Strauss, Industrial  Gas  Cleaning, p.  245,  Pergamon Press,  London,
          1966.

     8.    H.  Ihlefeldt,  Technical Status  of Fabric  Dust Collection in Cement
          Plants  (Ger),  Staub. 2,1:  9,  448 (Sept.  1961).

     9.    C.  E. Lapple;  Dust  and  Mist  Collection,  in Chemical  Engineers'
          Handbook,  J.  H.  Perry,  ed. 3rd  ed.,  pp.  1013-1050, McGraw-Hill  Book
          Co.,  New York, 1950.

   10.   W.  E. Gibbs,  Clouds and Smokes,  pp.  129-161,  P. Blakiston's Son and Co.,
          Philadelphia,  1924.

   11.   F.  0. Ekman and H. F. Johnstone, Collection of Aerosols  in a Venturi
          Scrubber,   Ind. and  Engg. Chem.,  43;  6,  1358 (June 1951).

   12.   S.  R. Lewis, Dust Filters and Air Cleaning Apparatus,  in The
         Aerolegist, E. V. Hill,  ed., The Aerologist Publishing Co., Chicago,
          (October,   1933).

   13.   R. E.  Swain, Smoke and  Fume Investigations, Ind.  and Engg.  Chem.,  41:
         2384 (November 1949).

-------
 14.   H.  J.  White, Industrial Electrostatic Precipitation, Addison-Wesley
      Publishing Co., Reading, Mass., 1963.

 15.   E.  Anderson, Separation of Dusts and Mists, in Chemical Engineers'
      Handbook, J. H. Perry, ed. 2nd edition,  p.  1865,  McGraw-Hill Book
      Co.,  New York, 1940.

 16.   S.  K.  Friedlander, K.  Silverman, P.  Drinker,  and  M.  W.  First,  Hand-
      book  in Air Cleaning -- Participate  Removal,  Harvard University and
      U.S.  Atomic Energy Commission (Sept.  1952)  U.S. Government Printing
      Ol'rioo, Washington, D.C.

 17.   0.  Field. Tho Steel-Wool Industry, Mech.  Eng'g.  (Dec.  1927).

 18.   J.  J.  Donoso, American Smelting and  Refining  Co.,  Salt  Lake City, Utah,
      personal communication, I960.

 19.   L.  Silverman,  Filtration through Porous  Materials,  Ind.  Hygiene  Q.
      l_l:ll  (March 1950).

 20.   P.  Drinker and T.  Hatch, Industrial  Dust,  1st ed., McGraw-Hill  Book
      Co., New York, 1936.

 21.   H.  J.  Hersey, Jr.,  personal  communication (22 June 1950).

 22.   100 Years of Metalworking, Iron Age.  Anniversary  issue, (June,  1955).

 23.   Pulverizing Machinery  Division,  Summit, New Jersey,  No Moving  Parts
      Inside  Hot Dust Collector, Chem.  Eng'g. pp. 188-190, 204-5  (August,  1957)

 24.   C.  E.  Billings and L.  Silverman, High Temperature Filter Media
      Performance with Shock Wave Cleaning, Int. J. of Air and Water
      Pollution,  6_:  455-466  (November-December 1962).

 25.   A.  M.  Squires  and  R. Pfeffer, Dept.  of Chem.  Engg. City College,
      City University of New York, Panel Bed Filters for Simultaneous Removal
      of Fly Ash  and Sulphur Dioxide: Part I: Introduction, preprint #69-202,
      66nd Annual Meeting, Air  Pollution Control Assoc., New York, June  1969.

 26.   Adv.  Chem.  Processes Section, AVCO Applied Tech.  Div., Lowell,
      Massachusetts, Evaluation of Granular Bed Devices, USPHS Final Report,
      Contract No. PH-86-67-51, Phase III, June  1969.

27.   H.  Krochta, The Ducon Co., Mineola,  New York, personal communication,
      1970.   See alsq: Granular-bed Dust Filter Withstands High Temperatures,
      Chem.  and Engg. News, p. 57 (December 1969).


28.   Kling  and Wfeidlein, U.  S. Patent No. 1,395,833 (1921).  Also see
      "Kling-Weidlein Filter,"  in Mechanical Engineers' Handbook. L. S.
      Marks, ed.  1st edition, McGraw-Hill Book Co., New York, p.  813,  1930.

-------
29.  C. E. Lapple, G. E. Alves, et al., Fluid and Particle  Mechanics,
     1st Edition, University of Delaware, Newark, Delaware,  (1950).

30.  R. T. Pring. American Wheelabrator and Equipment  Corp.,  Filtration of
     Hot Gases,  in Air Pollution  Control Association  47th Annual  Meeting
     Proceedings, Chatanooga, Tennessee, May 3-6, 1954.


31.  R. T. Pring, Bag Filtration  of  Aerosols, Heating and Venti Hating
     p. 97  (1952).

32.  R. T. Pring, Air Pollution Control  Equipment for Melting Operations
     in the Foundry  Industry, Transactions  of  the .Am.  Foundrymen's Society,
     Des  Plaines, Illinois,  6£  (1952).

33.  R. T. Pring, Am. Wheelabrator and Equipment Corp.,  Bag-type  Cloth
     Dust and  Fume Collectors, U.S.  Tech. Conf. on  Air Pollution  Chapt,  35
     McGraw Hill Book Co., New York  (1951).

34.  A. Marzocci, F. Lachut, and  W.  H. Willis,  Glass  Fibers and their
     Use ad Filter Media, J. Air  Pollution  Control  Assoc.,  12:  1,  p.  38
     (January  1962).

35.  Arthur D. Little,  Inc., High Temperature  Bag Filter Development,
     USDHEW Public Health Service TR A61-34, Taft  Sanitary  Engg.  Center,
     Concinnati, 1961.

36.  P. W.  Spaite, J. E.  Hagan, and  W. F. Todd, A  Protective Finish for
     Glass-Fiber Fabrics,  Chem. Engg.  Prog. ,  5_9: 4, p. 54 (April  1963).

37.   W. C.  Long, "NomexR" Nylon  - a New High Temperature Filter Medium.
      55th National Meeting,  American Institute of  Chem. Engrs., Paper #46B
      Houston, Texas, Feb 7-11,  1965.

38.   A.A. Dambeck,  Guidebook to Man-made Textile Fibers and Textured Yarns
      of the World,  3rd edition,  The United Piece Dye Works, New York,  1969.

3').  H. E. Friedrich, Primer on Fabric Dust Collectors, Air Engineering,
     p. 26 (May  1967).

40.  Business  and Defense Services Administration,  U.S. Department of Com-
     merce,  Industrial  Gas Cleaning Equipment Shipments and  End Use - 1967.

41.  Industrial Gas  Cleaning Institute, Inc., Summary of the Manufacturers'
     Report  of Air Pollution Control Equipment  Sales (Particulate), The
     Institute, Box 448, Rye, New  York, 10580 (1969).

42.  J. G. Kostka, Dust  Collector  Survey, Pit and Quarry Publications,  Inc.
     Chicago,   111. (1968).

-------
43.  U.S. Public Health Service Publication No. 999-AP-40, Cincinnati,
     Ohio (1967).

44.  C. E. Billings, M. W. First, R. Dennis, and L. Silverman, Laboratory
     Performance of Fabric Dust and Futne Collectors, U.S. Atomic Energy
     Commission Report NYO-1590, p. 114, Harvard University Air Cleaning
     Laboratory, August 31, 1954.

*5.  Same as Ref. No. 44, revised January, 1961, App. II.

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                                             CHAPTER 2

                                   FABRIC FILTER TECHNOLOGY
 1-
              2.1  INTRODUCTION                                                     2-3

              2.2  DESCRIPTIVE AEROSOL TECHNOLOGY                                   2-5
                   2.2.1  Particle Size, Concentration and Terminology              2-5
3
                   2.2.2  Significant Characteristics of Particles with
                          Respect to Filtration                                     2-10
                                                                                    2-10
                                                                                    2-12
                                                                                    2-13
                                                                                    2-21
                                                                                    2-24
                                                                                    2-30
                                                                                    2-42
                                                                                    2-44
                                                                                    2-76

              2.3  FABRIC FILTRATION PROCESSES                                      2-78
                                                                                    2-78


                                                                                    2-95

                                                                                    2-98
                                                                                    2-100
                                                                                    2-102
                                                                                    2-104
                                                                                    2-105
                                                                                    2-105
                          2.3.2.7   Other Collecting Mechanisms                       2-108

                   2.3.3   Measurements  of Fabric  Filter  Collection Efficiency        2-108

              2.4   FLOW THROUGH  POROUS  MEDIA                                         2-J.12
                   2.4.1   Introduction                                               2-112
                   2.4.2   Permeability  of Rigid Media                                2-114

                          2.4.2.1   Kozeny-Carman Theory                              2-115
                          2.4.2.2   Brinkman Theory                                   2-117
                          2.4.2.3   Other  Permeability Theories                       2-119

                   2.4.3   Permeability  of Changing Media                             2-120

                   2.4.4   Permeability  in Non-uniform Beds                           2- 122

                   2.4.5   Resistance vs.  Permeability                                2-123

                   2.4.6   Characteristic  Geometric Properties of Porous Media        2-124
                                               2-1









FABRIC
2.3.1
2.3.2





2.2.2.1 Particle Size
2.2.2.2 Shape and Structure
2.2.2.3 Coagulation
2.2.2.4 Density
2.2.2.5 Surface Area
2.2.2.6 Electrostatic Charge
2.2.2.7 Adsorption
2.2.2.8 Adhesion
2.2.2.9 Summary of Particle
FILTRATION PROCESSES
Introduction
Particle Capture in Fibrous,
Filters
2.3.2.1 Inert ial Impact ion
2.3.2.2 Diffusion
2.3.2.3 Direct Interception
2.3.2.4 Sieving or Straining








Properties


Fabric, and Granular





2.3.2.5 Collection by Electrostatic Mechanisms

2.3.2.6 Sedimentation

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                                CHAPTER 2
             2.4.6.1   Porosity                                         2-129
             2.4.6.2   Specific  Surface                                 2-132
             2.4.6.3   Pore  Size Distribution                           2-136
             2.4.6.4   Particle  (grain)  size                            2-140
             2.4.6.5   Pore  Structure                                   2-140
             2.4.6.6   Shape Factors                                   2-143
             2.4.6.7   Granule Surface Roughness                        2-145

     2.4.7  Working Equations                                         2-145
     2.4.8   Flow and  Pressure  Drop in  Fabric Filters                  2-157
     2.4.9  Analysis  of the Specific Dust-Fabric Filter
             Resistance Coefficient (K^)                               2-162
             2.4.9.1   Data  for  K$                                      2-167
             2.4.2.2   Effect of Particle Size                          2-167
            2.4.9.3   Effect of Particle Shape                         2-174
            2.4.9.4   Effect of  Filtering Velocity                     2-178
            2.4.9.5   Fabric Surface Effects                           2-180
            2.4.9.6   Clean Air-Flow Permeability                      2-193
            2.4.9.7   Other Effects                                    2-193

2.5 SYSTEM PRESSURE AND FLOW                                         2-199
    2.5.1  Flow in a  Single Bag                                      2-199
    2.5.2  Flow in a  Single Compartment                              2- 203
    2.5.3  System Flow                                               2- 205

2.6 REFERENCES FOR CHAPTER 2                                         2- 208

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                                CHAPTER 2
                       FABRIC FILTRATION TECHNOLOGY
 2.1   INTRODUCTION
      The  purpose  of  this  chapter  is  to relate  the  pressure  drop,  effi-
 ciency  and  performance characteristics observed  in operating  fabric  fil-
 ters  to the underlying physical and  chemical phenomena  of the  collection
 process.  The  latter may  be  examined at three  analytical levels,  as  out-
 lined in  Table  2.1,  macroscopic,  microscopic and molecular.
      From the macroscopic point of view,  the filtration process can  be
 considered  to consist  of  a collection phase, in which dust  is  removed from
 the gas that penetrates the  filter media,  and  a cleaning phase during
 which the gas flow through the  media may or may not be  interrupted depend-
 ing on  the  cleaning  system.  Analysis of the process is presented below,
 with  respect to a single  bag element,  by considering the macroscopic
 aspects such as the  fabric material  type,  geometry, weave and  treatment;
 and by  the  form and  transmission  characteristics of the application  of
 cleaning  energy including frequency,  amplitude and duration.  The perform-
 ance  of the  complete collector  and further characteristics of  fabric and
 cleaning  on  single bag collectors are  discussed in Chapters 4 and 6.
      The  underlying  features of the  filtration and cleaning processes have
 been  approached at the microscopic level by consideration of fluid-particle
 mechanics, deposition  phenomena,  accumulation structures, particle removal
 efficiency and resulting  pressure drop.  The special single particle-
 collector case, essentially  a two-body  problem for which many theories
 have  been developed, remains under continuing investigation.  Analysis of
 collection and pressure drop in a practical multi-particle system with
 cake  formation, however,  is  less well-developed.
      Processes at the  molecular level  include effects of absorbed or ad-
 sorbed vapors, humidity,  adhesion, cohesion and electrostatic charge on
 particles and the fabric  filter elements.  Analyses of these effects
which are properly in  the domain of colloid and interfacial phenomena,

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                                TABLE 2.1

           ELEMENTS IN THE ANALYSIS OF FABRIC FILTER TECHNOLOGY
Level o£ Analysis
 Typical
Dimension
                    Elements of Analysis
   MACROSCOPIC
> 10
                          0
cm      Fabric design^material, yarn type, weave,
        finishes, bag configuration (length/dia-
        meter), design, suspension, cleaning
        process variables (type, frequency,
        amplitude, duration, repetition rate,
        energy input pattern and amount),  flow
        patterns in bag, deposit relationship
        to pressure drop, macroscopic  deposit
        flow property (drag).
   MICROSCOPIC
    _2
~ 10   cm
        Yarn characteristics,  treatments,  (tex-
        turizing,  bulking,  napping,  needling,
        felting);  deposition of particles  on
        individual yarn elements,  deposition
        on previously deposited particles,
        structure  of cake;  deposition in and on
        cake; compaction of cake,  collapse,
        reorientation;  reentrainment in flow,-
        cleaning,  adhesion  of  individual particles
        and of aggregate structures, size  of
        deposit structures  removed,  mass-energy-
        force relationships in cleaning.
   MOLECULAR
< 10"  cm
        Fiber  surfaces,  particle  surface,
        adhesion,  humidity  effects,  electro-
        static charge, adsorption,  forces
        and  phenomena of attachment  arising
        from molecular considerations.

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 are  summarized in this  chapter,  in forms  suitable for engineering esti-
 mates.   The approach taken in this chapter is  to discuss  the interrela-
 tionships  between these molecular and microscopic processes  and the ob-
 servable (macroscopic)  performance of a single filter element (bag),
 i.e.,  pressure drop  and efficiency,  fabric life and  fabric  filtration
 costs.
 2.2  DESCRIPTIVE  AEROSOL TECHNOLOGY
      2.2.1   Particle Size,  Concentration,  and  Terminology
             An aerosol  is defined as  a stable  system of solid or  liquid
 particles  suspended  in  a gas.   If the gas  is relatively of  turbulence and
 essentially motionless,  stability obtains  only when  particles are  about
 1  um in  diameter  and of unit  density.   In  the  case of highly turbulent and
 fast flowing gas  streams, however,  particles in the  100 um diameter range
 may  remain  in suspension.  Thus,  in  the context of this discussion,  use  of
 the  term "aerosol" refers to  particulate dispersions  which effectively
 undergo  no  change in particle  concentration or size  properties  during
 transit  to  a dust collection  device.   A particle is  considered  to  be  a
 single continuous unit  of solid or liquid  matter, composed of many mole-
                                                              ^
 cules and hence,  much larger  than molecular size (~  0.001 um)  .  A particle
 may  also consist  of  two  or  more unit  structures  (agglomerates)  held  in
 contact  by  interparticle  adhesive  forces,  such that  it behaves  as  a single
 entity while  aerosolized.   Typical sizes of particulate dispersions of
 concern  are  shown in Figure 2.1.  Sizes range  from macromolecular  to  about
 one-tenth millimeter (100 um)  in  diameter with  an approximate range of
             — 91       — f»
 mass from 10    to 10    grams  per  particle.  The maximum size that remains
 suspended for  appreciable periods depends upon  the relative magnitudes of
 Jzhe  fluid forces  acting upon the  particle and  the external forces  tending
 to remove particles  from  the system.
            The presence  of a dust particle represents a local discontin-
 uity which modifies  the continuum properties of  the gas,  e.g., viscosity
 (u), density  (p), and thermal conductivity (K).  Alterations of the gas
 properties may be significant  if the particle concentration is high or
*                   -6
 um = micrometer, 10   m (formerly micron, (a)

-------
Figure 2-1.  Characteristics of Particles and Particle Dispersions
             (Courtesy Stanford Research Institute)

-------
 if the fluid velocity gradient is large.   Figure 2.2 presents  the concen-
                                                                         2
 trations  encountered in several industrial and atmospheric environments.
                                        3
             A concentration of 100 gm/m (typical of that found in pneumatic
 conveying systems anil representing about  10 percent of the supporting nir
 density)  will  decrease the air viscosity  by about 10 percent.   Similar
 behavior  is  observed in hydrosol  systems.   However, the more  common con-
 centrations  encountered in filtration .practice (~ 10 grams/m  )  will have
 very  little  measurable effect  on  the  continuum properties  of  the  fluid as
 far as macroscopic  analysis of motion,  matter  and energy are  concerned,
 except in systems where the rate  of change of  energy is high.  '  '  '
             Aerosol behavior can  be considered as a part of the general
 study of  disperse systems  in fluids.  Analysis of the  physical  character-
 istics and kinetic  properties  of  an aerosol  system will contain both  the
 properties of  the particles (e.g.,  size,  shape,  density)  and  the  continuum
 properties of  the fluid.   For  particle  size  approaching the average  inter-
 molecular distance  in the  gas  (molecular mean  free  path,  A ), motion  of
 the particle with respect  to the  fluid  is  greater than estimated  from its
 continuum behavior.   Analysis  of  the  motion  will  contain a slip correction
 factor for small  particles  ( < 1  pm at  NTP)  or at reduced  gas pressure,
 depending upon  the  particle Knudsen number,  K   =  2X/D  .
            An  important characteristic of particulate  systems, from  the
 physical  and chemical  points of view, is the greatly increased area per
 unit  of mass available  for  interaction with  surfaces as  in adhesion and
with  molecular  constituents  of  the  gas  as  in adsorption  and catalysis.
Two or  three phases  coexisting  may  lead to chemical reaction at high  rates,
 combustion and  explosion, heterogeneous reactions,  catalysis,  condensa-
 tion,  health effects,  etc.   Interfacial characteristics  of  a particulate
 system are also of  interest  in  the  effectiveness  of inter-particle con-
tacts, molecular  accommodation, adhesion and interactions with boundaries.
            A particle  is defined above as a discrete kinetic  unit of
solid  or  liquid matter, and an aerosol as  a system of particles suspended
*X = 65 x 10" m, NTP

-------
                                           GRAMS  PER  CUBIC  METER
NJ
I
oo
o5

10
LOW-PRESSURE PNEUMATIC C(
<— EXPLOSIVE C
1C
)NVEYIN6— *
ONC.OF AIR-BORNE OUSTS 	 >
FLOUR COAL
SAND 8 STONE DRYING E
) i '
^-CLOUDBURST— ^ ! ^-MODERATE

< 	 FOUNDRY - VK
SMAKEOUT CLEA>
<- COTTON- Ml
BREAKING
MJCKING HAULING
WELDING

Fc,0j Z.O
2 ' ' ' io-
	 > < 	 INDUSTRIAL- DIJ
ORKROOM AIR 	 >
IING POURING MOLDING
PICKING CARDING WCA\
SPINNING
C0(
M>
io-
STRICT AIR 	 > !
j < 	 POLLEN

riNG
AIR-CONDITIONING FILTERS
io-s
viscous FILTERS _._ INDUSTRIAL CLOTH FILTERS
L SILICATES
St As Pi CojHtCcO,


ICV»
' 10 « 	 10

FABRIC FILTERS

;' ' K>-«
CELLULOSE 	 AStESTOS MPEMS
IO*
I0f»


I p»"*
10
— TV
ii • • i
RESHOLD LIMITS FOR
RA"*
W
RADIO)
*
CTIVE
ci curuTe

to-"!
Ti
 «*• i-***
                                          GRAMS  PER  CUBIC  MET{R
                    Figure 2.2.  Typical Concentrations of  Particulate Suspensions

-------
in a gas.  Aerosol systems (see Figure 2.1) are usually classified by
common terms relating to particle origin or occurrence, such as:.

                    " solid particles arising from mechanical disinte-
              gration or resuspension of solid materials in comminution
              processes (drilling, crushing, grinding, pulverizing), in
              which the individual particles usually have the same chemi-
              cal composition as the parent material.  Sizes range from
              macroscopic (visible powders) to microscopic, and shape is
              determined by the crystalline nature of the parent sub-
              stance .

              Fogs, mists,  clouds - liquid particles formed by mechanical
              disintegration of a liquid or by condensation of molecular
              constituents  in gas upon suitable nuclei.   Size range is
              generally above one micron.   When they become larger than
              50 to 100 urn  by collision and coalescence,  they tend to fall
              as drizzle or rain, or if the gas is saturated, the particles
              may persist.

              Fumes -  arise from combustion, sublimation or distillation
              processes; particle composition is frequently different from
              the bulk parent material through oxidation or hydrolysis  in
              the gas.   Size is generally  below one micron  and typical
              fumes such as iron oxide or  zinc oxide from metallurgical
              operations will coagulate rapidly to form loose aggregate
              particles. Sizes formed will vary with temperatures,  air
              motion,  or rate of cooling.

              Smoke -  visible material arising from combustion of organic
              materials typically;  particles may consist  of fine solid
              inorganic ash constituents with condensed  carbonaceous
              pyrolysis fragments such as  tarry liquids  of  low vapor  pres-
              sure.  Size is generally below 1 urn.   Chimney smoke from
              combustion of fossil fuels may also contain fly ash or  soot
              flakes of relatively larger  sizes,  above 1  urn,  depending
              on the combustion conditions,  fuel  use pattern, or particle
              deposition and reentrainment in the flue gas  passages.

              Smog - deonotes an aerosol of the type associated with atmo-
              spheric  pollution that consist mainly of a combination of
              smoke and fog.  Photochemical smog implies  a  complex aero-
              sol formed from condensation of gaseous hydrocarbons from
              combustion products of liquid fuels,  through  chemical reac-
              tion processes in urban atmospheres,  potentiated by solar
              radiation, and has typical biological manifestations (eye
              irritation, plant damage) in addition to visibility reduc-
              tion.

              Atmospheric dust or the atmospheric aerosol - is the sum
              of all types  of particulate  matter  suspended  in the atmo-
              sphere (e.g.,  haze),  including many of the  components
              above, with a wide variety of chemical constitutents de-

-------
                pending upon source and history.   Size is typically
                submicron to about 10 ^m for well mixed,  aged material.
                In addition, local anthropogenic  sources  may release
                relatively large particles  that may not remain airborne
                over Large distances.   Injection  of large quantities of
                material into the atmosphere at high altitudes (strato-
                sphere, > 3 x 10^ ft)  may give  rise to a  large-sized com-
                ponent sufficient to appear throughout the global  cir-
                culation.   Natural sources  of atmospheric particles in-
                clude sea spray solid residues; smoke from forest  fires;
                blown dust from prairie erosion;  dust from volcanic erup-
                tion; and pollen, spores and plant exudates.

      2.2.2   Significant Characteristics of Particles with Respect to
             Filtration

             2.2.2.1  Particle  Size.-  The size  of particles  and  their chemi-

 cal  composition,  physical state, and  concentration are major variables  in
 industrial  gas  cleaning systems.  Most natural aerosol particles  are poly-
         *
 disperse with  respect to size,  and frequently with respect  to  shape,
 density,  and chemical composition.   In some special situations, such as

 the  preparation of  sediments,  glass spheres, and powdered metals  or resins,
 one  may  encounter size distributions  which approach the  monodispersed

 state.   Methods for characterization  of the size spectra which were indi-

 cated earlier in  Figure 2.1, are summarized in the  following discussion.

             Techniques and  size  analysis apparatus  for area  and stack
                                                     8  9
 measurements  are  described  in  more  detail  elsewhere.  '    The  most  recent
 instrument  available for  fundamental  particle  size,  surface,  and  composi-
                                                           a
 tion spectral analysis is  the  scanning  electron  microscope.

             For many purposes  in dust  collector  design,  size  analysis is
                        **
 performed with  the  Bahco  micro-particle analyzer which  employs a  cen-

 trifugal winnowing  configuration to separate size fractions  by aerodynamic
 behavior  in a flowing  gas stream.   Other techniques  in current use  in-
 clude centrifugation,  sedimentation,  and electrical  resistance changes
     *
      The terms polydisperse and monodisperse are often used to describe
aerosols containing a range of sizes and a single particle size, respec-
tively.
     **
      H.S. Dietert Co., Detroit, Michigan.  See also ASME Power Test
Codes. PTC-28-1965.


-------
 (Coulter)  in  liquids,  optical microscopy,  and  screen  analyses with  con-
                                Q
 vcntional  and micrpmesh  sieves.   Most  of  the  available  instruments have
 an  effective  lower  limit of  about 1  (am,  as  shown  in the  following, summary
 table:

            Sizing  Technique               Size Capability  Range
     Common Testing Sieves               20 mesh  (= 840  (am)  to  325
                                         mesh  (=  44 (am)
     Micromesh  Sieves                    ~5 (am -  50 (am
     Sedimentation,  Elutriation        > 50 (am
     Cyclones,  respirably size          > 50 (.im
        selective
     Centrifugation -  Whiting MSA      >  5 urn
     Bahco Micro-Particle             ;>  1 ^m
                                                  2
     Aerosol spectrometers,              0.2 - 10 urn
        Goetz, Stober
     Light microscope                  > 0.2 (am
                                                   2
     Electron microscope                0.001 -»  10   (am
     Scanning electron microscope      > 0.05  urn
Size analyses below 1  jam are usually performed with an electron microscope,
which provides  resolution down  to approximately 0.001 urn.
            Particle size refers to  some characteristic  dimension of  the
physical geometry,  as  for example a  diameter measured in a consistent.
manner.  It may, however, reflect the particle  behavior  in a fluid,  as for
example in sedimentation, from which a diameter can be inferred.
            The particle size distributions of many naturally occurring
aerosols tend to be somewhat similar, because  of  the  commonly acting
mechanisms of particle formation and removal.  On the other  hand, the
size distributions  of  particles generated by industrial  processes are
highly variant, depending on the source  and age of the material.  Size
distributions are frequently described by one  or more of the following
statistical parameters,  among others:
            .  Range - the size of the largest  and the smallest particle.
            .  Mean size  - the arithmetic average  sizes of all partiales.

-------
             „  Median size  (by number)  -  that  size  for which there  are an
               equal  number  of smaller  and  larger particles.
             .  Median size  (by weight)  -  that  size  for which the  weight
               of all smaller  particles equals the  weight  of all  larger
               particles.
             .  Standard  deviation or  a  geometric  standard  deviation -  a
               measure of  the  range with  respect  to average  particle size
               (see any  standard  statistics  text).
 It  is  important  that there  be no misunderstanding  as  to which  diameters
 (mean  or median,  by  number, weight or  surface) are used to  characterize  a
 dust since  their numerical  values can  easily  vary  by  a factor  of ten,and
 up  to  one hundred or greater,with highly polydisperse materials.
             2.2.2.2   Shape  and Structure.-  Individual particle  shape
 depends upon the methods of particle formation.  The  shapes of particles
 formed by disintegration operations  (as  in  comminution, attrition,  pul-
 verizing, grinding)  are determined largely  by the  nature  of the  parent
 material.   Usually,  they appear  as irregular  crystalline  granules.  Part-
 icles  formed by  condensation  processes,  from  vapor phase  reactions, e.g.
 metallurgical  fumes  and smokes,  or from  high  temperature  combustion pro-
 cesses, e.g.,  fly ash, are  frequently  regular in shape (spherical,  cubic).
 Subsequent  changes take place  in particle shape and structure as a  function
 of  the history of the particle in its  environment, and may  include  crystal-
 lization, hydration,  collisions  and coagulation, or chemical reaction  (con-
 densation of moisture, oxidation) with gas  phase constitutents.  Spherical
 liquid particles  produced by condensation may coalesce on contact to form
 larger droplets.  Volatilization  of metallurgical  vapors   proceeds simul-
 taneously with oxidation, followed by condensation and solidification of
                       _2
 primary particles (< 10   urn)  and further coagulation to   form larger chain-
 like floccs  of spheres or cubes  (as with iron oxide, magnesium oxide).
Carbon black is produced as soot  from pyrolysis of liquid  or gaseous
hydrocarbons and  involves vapor  phase decomposition and condensation of
                                                         _2
molecular carbon to  form small semi-graphitic  spheres  (10    - 0.5 urn),
typical of condensation aerosols, with  formation followed  by rapid  coagu-
lation.
            Particle  size and  particle  structure  are functions  of the
formation process and of concurrent  and/or  subsequent  events.  There-

-------
 fore,  changes in size and structure must be considered in the design or
 specification of fabric filters,  especially if the  material  to be  col-
 lected is freshly-formed or likely to undergo chemical or physical changes
 while  on the fabric,  making subsequent removal difficult.
             Typical  shapes  and  observed concentrations   of  airborne
 materials likely to  be encountered in urban atmospheres are  shown  in
 Table  2.2.   A more comprehensive  presentation of  morphological features
 and  other characteristics of some 500 specific dusts  is presented  in
 McCrone's Particle Atlas.     Light optical  and electron photomicrographs
 of typical  particulate materials  are  shown  in Figures  2.3, 2.4 and
             The  following  text  discusses  analytical  solutions  for  particle
 capture mechanisms  and  for pressure  drop,  based on models  for  spherical
 particles.   It  is evident  from  the photomicrographs  that particles  likely
 to  be  encountered in  practice are seldom  uniform spherical entities, but
 rather are  irregular  granular or highly chain-like floccs.  These  natural
 forms  may be  compared to those  cited in Table 2.2.   The shape  and  struc-
 ture of a particle will influence its collection, its  interaction with
 the fabric,  and  its behavior as an element  in a granular deposited  layer
 or  cake.  Characterization of the size, shape, and structure for most
 particles of  concern requires costly and  sophisticated analysis and little
 has been done to relate properties of the  particulate  system to behavior
 in  a fabric  filter deposit, or  to effects  on filter  performance.  Most
 dry dusts from manufacturing operations involving product handling, vent-
 ing, and the  related processes  consist of highly aggregated systems of
 single particles.  Since they are often compacted so that their envelope
 shapes are approximately spherical,  their  aerodynamic behavior can be
 predicted adequately from  spherical  models.  Most analytical treatments
 are based on  resistance forces arising from spherical shape.   Irregular
 shapes will experience greater resistance  forces which counter the
 gravitational force and lead to reduced settling velocity.
            2.2.2.3  Coagulation. - Formation of fine solid particles in
high concentrations by vapor phase condensation is generally followed by
rapid coagulation.   Coagulation occurs in all aerosol systems at a rate


-------
                                                      TABLE  2.2
                                                                                              v
                         MAJOR SHAPES  OF AIRBORNE  PARTICLES   AND TYPICAL CONCENTRATION RANGES
          Shape
Appearance
Kinds
  Wt 7, in dust
Range     Average
                                                                                                  Shape factors
                                                                                             a
                                                                    a
                                                                                                               V
i
H-"
*>
spherical



cubical


irregular-cub ical


flakes
smokes, pollen, fly ash
liquid droplets

salt crystals,
MgO indiv. part.

mineral dusts,
cinders

minerals,graphite
epidermis, mica
                                                                            0-20
                                                            10
          *from Whitby and Liu, Ref. 10 with additions

         **  s = surface shape factor

             v = volume shape factor
                                                                           10-90
                                                                            0-10
                                                            40
                                                        n/6
                                            3-8        0.2-0.5


                                            1.5-2      0.02-0.1
acicular,
spiny
fibrous ^"
&
&
condensation
floCCS kp*4
W
i^iMflly zinc oxide,
5^-^ ammonium sulfate
s^tZT\ lint, plant fibers,
""^r""S ] asbestos, talc, fiber 3-35 10 nDJ, £ Df L
_* )) L glass, man-made fiber
***^^>^
— • -"
k^ carbon smokes
./C>t coagulated metal oxide 0-40 15 ' 0.2-2 0.01-0.1
§L*l \ fumes, e.g. iron oxide

-------

   •
.
   c
   -~  -



                                                                            ClMHllMl CMlHtlWI WO* POlP
                       d. Mechanical coniferous wood pulp    e. Straw  „*.





                                 \
                        |. Mineral wool
                             ^'^ •«   .
                           h. Cayenne pepper ,
                                                                                 * .
                                                                           i. Potato starch  t* J-
                      e
                        j. Gypsum
                               .'*"\
                           Jf  *«
    -^

          %«4>
                       .4-*
  -    . -•   * — fr     ^
  m. Pulverized coal  ^  *^.,   | n. Graphite

                                   *>
                                                                           o. Incinerator flyash


                                                                            m *L_    "" • •
                          ..           ^         /—                     __
                        p. Soot blowing coal boiler       r q. Spreader $toker flyash r/f^    r. Oil soot r
   O
Figure  2.3.  Photomicrographs of  Various Atmospheric Particles

              (from McCrone and Saltzenstei, Ref.  12).

-------
                                                A  •*
                                                       *   /
                                               •%•              *.
(a)  Electron micrograph  of crocidollte
    needles
     (b)  Photomicrograph of coal  dust
          (Watson,  1953)
(c)  Electron micrograph of zinc
     oxide smoke
(d)   Electron micrograph of magnesium
     oxide  smoke  (gold-palladium shadowed)
        Figure 2.4.  Photomicrographs  of  Typical Aerosol Particles (From
                     Green and Lane, Ref.  13).

-------
c
           (e)   Electron micrograph of iron oxide
                Smoke  Ceold-Dal InrHiim

(f)   Electron micrograph of gold shadowed
     polystyrene particles formed by
     spraying 1% solution in carbon
     tetrachloride and evaporating the
     droplets.
-
           (g)  Electron micrograph  of methylene
               blue  particles showing crystalliza-
               tion
(h)  Electron micrograph of sodium
     chloride crystals formed by
     spraying from solution in water
     and evaporating the droplets
     (gold palladium shadowed)
                    Figure 2.4.  Photomicrographs of Typical Aerosol Particles  (Continued),

-------
                  (a)  Micrograph  of  triplet  aggregates  of  latex
                      spheres  of  7.9 x  lO'^cm diameter  (Stereoscan
                      scanning electron microscope)
I
I—•
00
(c)   Micrograph of tetrahedral aggregates of four
     latex spheres of 7.9 x 10~5cm diameter (Stereoscan
     scanning electron microscope)
                    (b)  Micrograph  of  octahedral  aggregates of
                        six latex spheres  of  7.9  x  10~->cm diameter
                         (Stereoscan, scanning electron microscope)
     (d)  Micrograph of a giant aggregate of  latex
          spheres of 7.9 x 10~5cm diameter  (Stereoscan,
          scanning electron microscope)
                                         Figure 2.5.  Scanning electron photomicrographs of aggregrates of monodisperse
                                                      test particles used to determine density and aerodynamic diameter.
                                                      (From StSber et al., Ref. 14).




-------
 proportional to the concentration of particles and, as shown above,  has
 a marked effect on particle mass, size,  shape, and structure.   Coagula-
 tion arises as a consequence of relative motion and collisions among
 individual particles.  As will be discussed below, impact forces between
 small particles and surfaces are sufficiently large so that collisions
 usually result in the formation of an adhesive bond.   For particles  less
 than about 1 um, the principal phenomenon promoting contact arises from
 Brownian (thermal) motion of the particles caused by impact with the sus-
 pending gas molecules.
             For simple  systems of monodisperse spherical  particles,  the
                       13
 rate of coagulation is
                                - dn/dt = Kn2                       (2.1)

 where n is the particle number concentration per cc,  K is the  coagulation
 coefficient,  and t is the time.
             For thermal motion of the particles,

                              K = 4 k T C /3 u,                     (2.2)
 where  k is  the  Boltzmann constant,  T  the  absolute  temperature,  uf  the
 fluid  viscosity,  and  C   the  Cunningham-Millikan  slip  correction factor.
 Integration yields  the  expression
                                    4kT   r     t
                                    3u   ' Cc  '  t                    (2<3)
for the concentration particles, n, at any time, t, with the  initial
condition of n = n  particles at the onset of coagulation, t = 0.  The
theoretical value of the coefficient (4kT/3uf) has been found to be in
general agreement with measured values, to within a factor of 2,  (of the
               -8
order of 5 x 10   per cc-min) for particles in the range 0.1 < D  < I um.
          2                                         6           P
For t ~ 10 , the particle concentration must be > 10 /cc for substantial
coagulation to occur.  The experimental verification of thermal coagula-
tion is illustrated in Figure 2.6, taken from the pioneering studies of
Whytlaw-Gray and Patterson.    This figure shows the large, loose,


-------
9 minutes after dispersal
                                         29 minutes after dispersal
49 minutes after dispersal
                                        244 minutes  after  dispersal
Figure 2.6.  Coagulation of Metal Oxide Fume (From Whytlaw-Gray and
             Patterson, (Ref. 15)).

                               2-20

-------
 irregular  masses of chain-like aggregates  formed from metal  oxide smokes
 of cadmium and zinc oxide.   Less  tendency  to form string-like  complexes
 was observed in oxides  of lead, copper,  magnanese and chromium,  some-
 what greater in iron, while  oxides  of  magnesium,  aluminum, and antimony
 appeared similar to clusters shown  in  Figure 2.6.   Later  investigations
 utilizing  electron microscopy have  established  the  metal  oxides  structures
                                                                13
 as large loose irregular  stringy  chains  as  shown in Figure 2.4.
             These observations are  doubly  significant in  the study of
 fabric  filter  technology.  First, the  capture of  these  large aggregate
 particles  by fibrils and  by  the previously  deposited granular  cake should
 be substantially greater  than would be estimated  on the basis  of  the
 individual  particles forming the  complex.   Secondly,  these aerosolized
 aggregate  structures are  very similar  to the kinds  of aggregates  formed
 upon collection of particles  in fabric and  granular filters.   Typical
 material removed by the cleaning  of fabric  filters  also appear as highly
 complex structures.
             More recent analytical  treatments of  coagulation for  aged
 disperse systems are available.   '    These  have  led  to a model of the
 particle size  distribution for the  coagulating  aged  atmospheric aerosol
                                       18
 which has  been experimentally verified.
             The process of coagulation and  the  resulting structure  of
 aerosol aggregates  is influenced  by several  factors  including  particle
 shape, electrical  charge  on  the interacting  particles, adsorbed vapors,
 and  fluid  shear gradients as  a consequence of stirring or mixing  in the
 aerosol, sonic  fields, and also because of sedimentation of larger  part-
 icles or collision  of particles on  walls or  collection on obstacles in
 the  flow.  The  production of  aerosol aggregates as  an aid to the  build-up
 of  particle  size  to make  the  particles easier to remove from suspension
has  been investigated.  The use of  filters as agglomerators has been
 shown to be  practical in  the  filtration of both solids and liquid  aerosols.
             2.2.2.4  Density.- Suspended particulate density is related
 to the method of  formation and subsequent events.  Initially dust  formed
by attrition of  a solid will have the density of the parent material.
However, if  it  undergoes subsequent surface oxidation or hydratibn, for

-------
example,  its density will  change.  Particles  formed by  condensation
processes undergo  substantial coagulation,  as  shown above  for metallurgical
fumes such as ZnO, MgO,  Fe»0»,  or  carbon blacks.  The density of  these
agglomerates will  be less  than  that of  the  particle materials due  to  air
                                   15          19
inclusions, as shown in  Tables  2.3   and 2.4.    Density of  these  aggregates
is about  one-tenth of  the  density  of  the parent material because  of encap-
sulated void volume- (up  to 90%).   Other recent studies  of  the aerodynamic
behavior  or particle aggregates  include those  of Stober et al.,    Johnstone
          20
and Sehmel   (who  confirmed the  earlier finding for MgO given in  Table  2.3)
           21                      22
and Kunkel,   and Megaw  and Wiffen.    Aerodynamic properties of  fine
fibrous particles  are under intensive investigation in  conjunction with
observations of excess numbers of  these particles in the lungs of  urban
dwellers.
            Difficulties have been encountered in measuring  the discrete
particle  densities of organic dyes used as  test aerosols (uranine  and
                                                   23        24
methylene blue) according  to Stein, Esmen and, Corn,   Sehmel,   and
                     25
KcKnight  and Tillery.     Inconsistencies in reported dimensions are at-
tributed  to differences  in manner  of generation, degree of drying,  and
sampling.

             Fly  ash particles  from combustion of  pulverized  coal  typically
contain fused  hollow  spheres  (cenospheres)  having densities  substantially
below that  predicted  from  material properties  alone.  These  differences
are  of  significance in predicting  the atmospheric transport  of released
particulate materials  from anthropogenic sources with respect to  environ-
mental  effects.   They  are  very  important in predicting  and interpretating
fabric  filter  performance.
            From a practical point of view,  particle density can  be
assumed to range  from  1  to 0.1  that of the  true density of the parent
material.  Estimates of  density and particle  size of aerosol aggregates
are  required in  order  to relate  particle mass  to particle collection  in
inertial  systems  and to  predict  the effects of acceleration  and fluid
forces  in the cleaning of  fabric filters.

-------
                               TABLE 2.3
                                                      *
                   PARTICLE DENSITIES FOR AGGLOMERATES
Material
Silver
Mercury
Cadmium Oxide
Magnesium oxide
Mercuric chloride
Arsenic trioxide
Lead monoxide
Antimony trioxide
Aluminum oxide
Stannic oxide
Floe
Density
R/cc
0.94
1.70
0.51
0.35
1.27
0.91
0.62
0.63
0.18
0.25
Normal
Density
g/cc
10.5
L3.6
6.5
3.65
5.4
3.7
9.36
5.57
3.70
6.71
          "
           From Whytlaw-Gray and Patterson,  Ref.  15.



                         TABLE 2.4

 TYPICAL DENSITY RATIOS FOR AEROSOL PARTICLES AND PRECIPITATED SMOKES*
Material
PbO
Sb2o3
A12°3
AS2°3
NaCl
MgO
HgCl2
Au
W
Density Ratio**
0.089-0.049
0.11
0.19
0.049
0.045
0.064-0.145
0.115-0.517
0.0109-0.34
0.07
Reference
Kohlschutter and Tuscher (1921)
Precipitated smokes
Kohlschutter and Tuscher (1921)
Precipitated smokes
Kohlschutter and Tuscher (1921)
Precipitated smokes
Kohlschutter and Tuscher (1921)
Precipitated smokes
Moffat and Mclntosh (1957)
Precipitated smokes
Johnstone (1961) and Whytlaw-
Gray and Patterson (1932)
Aerosol Particles
Whytlaw-Gray and Patterson (1932)
Whytlaw-Gray and Patterson (1932)
Johnstons (1961)
*
 From Beekmans, Ref. 19.
**
  Ratio of observed particle densities to the true density of the material,
  i.e., solids fraction, a.  This ratio for a material composed of equal,
  randomly packed spheres is 0.61, i.e., 39% porosity, c = 1-u.



-------
             A fabric  filter  collector  reduces  a  gaseous  dispersion of
 particles  to a powder  mass.   The  porosity of  the powder  (void  fraction),
 ..  =  1  - a  =  1 -  bulk  density/true  density)  affects  the collected  cake
 pressure drop through  its  effect  on  permeability.   Powder  bulk density
 and  porosity data  for  a  variety of materials  are given in  Tables  2.19,
 2.20,  and  2.21.
             In summary statements  of aerosol  size properties  (dimensions
 alone) are not adequate  for  prediction of particle  aerodynamic behavior
 or for estimation  of  filter  cake  porosity.  Unless  the particles  are  of
 reasonably regular shape,  the  size,  shape,  and structure must  be  con-
 sidered simultaneously to  explain  physical  measurements.
             2.2.2.5  Surface Area.- The large amount of surface area  per
 unit mass  (specific surface)  is a  characteristic  feature of dusts,  powders
 and  aerosol  systems.   For  example, a cubic  centimeter of unit  density
 material distributed as  1  ^m spheres will have a  specific  surface  of
       2
 11.5 m /gram.  Surface areas of some common mineral dusts  are  given in
             27
 Figure 2.6a.     Fine particulate matter, which may have specific  surface
                     -22                   32
 areas ranging from 10   m  /gram (sands)  to  10  m  /gram (carbon black),
 provide extensive  area for chemical and physical reactions with gas phase
 constitutents.  Materials  of practical  interest are usually found  to  have
 greater surface  areas  than those predicted  by geometrical  considerations
 alone  for the  reason that  such considerations do not ordinarily take
 surface roughness  or interstitial  surface into account.
            Organic and inorganic  combustible materials may burn or ex-
 plode when finally subdivided.  Effects  of  particle characteristics (shape,
 size, and concentration) on the intensity of explosions for a wide variety
 of materials  of  technical  interest have  been presented by Hartmann, Nagy
                                     <"     29-35
 and co-workers at  the U.S. Bureau  of Mines.       Hartmann's summary  of
 earlier (1916-1957) Bu. Mines studies of explosive characteristics of
                                                      o /•
 representative dust dispersions is given in Table 2.5.     These studies
 indicate that explosibility is inversely proportional to  particle size,
                             32
 as illustrated in Figure 2.7.    Minimum explosive concentrations in air
                                     3                 33
are in the range of 10 to 100 grams/m   (10 to 100 oz/10  ft  or 4.3 to
           3
43 grams/ft ), c.f. Figure 2.2.  For the aluminum data shown in


-------
: oo.ooo r—p
30.000 I  v
60.000
 40.000
 30,000
                                          S S 3
                                   Average diameter (/),,) micron?
8888
CM  co *r  \o
  Figure  2.6.   Measured specific surface  of five common minerals
                 (From G. G. Brown and Associates, Unit Operations,
                  John Wiley and Sons, New  York, 1950,  by permission)
                 (From Reference 27.)

-------
                             TABLE  2.5
               EXPLOSION CHARACTERISTICS  OF VARIOUS  DUSTs'
Typo of 
IMI II.
40
45
11
1)5
105
200
120
10
20
20
12)
100
75
80
190
45
70
60
60
220
480
40
81
100
100
190
600
20
110
400
140
10
35
21
21
21
11
10
JO
40
10
20
25
11
21
30
45
41
20
40
11
31
30
23
30
70
20
31
100 v
100
10
40
60
60
41
•1

Ma»
rxpltwf'tn
lirrwrnit*,
l»i
90
70
100
90
10
45
41
80
91
k
101
10
60
35
«0
'11
V.
45
351
10
65
60
80
70
80
71
81
41
90
15
90
105
10
110
101
85
90
95
80
mo
100
80
80
WO
90
85
65
80
90
90
75
95
75
80
81
60 1
85
40
651
100
111
100
60
611
10

Ra
prrei
1*1 |
A vie
3,100
2.000
10.1)00
900
1.500
500
800
2.000
3.000
3,400
1.100
2.000
1.400
2.100
100
3.400
3.800
l,t>00
2,900
200
2 4JQ
2,100
800
2,600
400
3 600
1.400
1,500
2,200
4.000
i HOO
1,400
i HOO
i i>oo
2. HOO
1,100
I.5IIII
1 71111
l.'ioii
r,nu
1.700
1.900
2000
1 800
2.200
100
1 500
1,600
2.400
1.200
1.100
1,400
2 300
'800
500
1 700
'200
100
900
1 400
1.200
400
110
110

r*of
urn rlne.
**r e«T
Mm
10,000 +
4.250
10.000 1
2,100
7.000
1.000
1,710
5,210
10.000 1-
10.000 +
2.750
10.000
3.210
6.710
1,250
10.000 1-
10.000 +
3,500
6.210
300
1 710
8,750
8,710
8.100
2.100
10,000 +
10,000 +
10 000 +
4,210
3.600
9.100
10.000 +
10 000 +
4.000
6 710
4'710
8.100
10.000
5,000
4 750
t;??S
6.MO
7 500
y' ooo
(.,000
1.000 ,
1 MO i
7.100
7060
1000
t 000
3.JOO
9 5110 '
2 QUO
1 000
4 100
SOO
1 000

4*000
i!ooo
1 200
250

Uinitii*
OxyKIlD
IMTcuni-
IIKI! 10
|>l'rvi?tlt
nrnitiun
of .lll«t
nluuil *>y
(•Irrtric
optirkl
7
4
ID
1)
1)
3
,1
13
t
6
16
|
1)
0.5
It
10
t
II
t
19
1)
t

||

14
14
17
14
14

13
13
14
15
14

17
14




11
)}

From Harttnann, Ref.  36.

-------
                             TABLE  2.5  (Continued)


Tyi» of duel



Corncob meal 	
Coraetaroh 	
Cotton aeed 	
Deitrln, eora 	
Nrlural raj idue . . . . .
Gnuldult 	
Gulf awd . • ,
Lffoopodltira 	
Nutihelli 	
Oaten, dehyclraiod 	
Fu,oiMrated 	
PMtia 	
Potato itareh 	
Pyrathrum ....
Soybean'.'. .'.'.'..... '. ' .
8uiar 	

Whfatduit 	
Wbettfluur 	

Mlicelianaoue
AdipicaaM.. . . ..
Aluminum itearatu . .
Alpirin 	
Bark Hurt (l>niuhwlir>
Beryllium acetvt*.
Calcium lifiiin auliihnnie uciil.
Cirbon, aotlvHtod
datiB, rennet 	 ....
OlluldM 	
CbaronaKiilnewiNiil)
QoaUo* tuUtlln 	
Coal, medium volatile 	
Qoal,biph volalik (I'uli. WHIII)
Coal.iubhitufmnaiii . .
Cork 	
DuioamliiarMniinii
Drnitro-ortlio-i-n-enl
Djchenyl 	
Qttonit* 	
HemnMtbylenrUlritmim'
Laetalbumin 	
Urnlte 	
Livtr protein 	
Napalm . •
hraforninliloliyUi

rVnlaeryihrikil
Phenol luiuitw
Pbthalir nnliydridr
•hytMitrnl
Pltah. COB! tar (!>8% vol mill-
TrooaineiMMiirillin
Robbi'T. cnule, bun!
SwoberliiUil HiMlium
goap 	
Sodium nlkyliiryUiiMiomii,' . .
Sodium aarbtixymeiliyl m-llii-

Borbi'oaciJ. . . •

Vitamin Hi 	
Wood flour 	

Ignliiun
trinpuf
llunt
CHMIC1.


400
380
470
400
440
430
300
480
410
410
560
420
440
480
440
520

440
471)
180
520
550
400
660
540
620
540
660
520
480
620
6)5
605
610
455
470
550
440
650
560
410
570
440
520
450
410
460
450
540
650
310
710
450
)50
520
430
540
560

)50
470
190
500
4)0
1
•mercy
(or Igni-
tion ut
iliiat
milli '

jouloa
60
30
80
40
40
30
60
40
30

'40
35
25
80
40
50
30
240
50
50
50
70
15
25
40
100
100
60
80


120
60
60
45
20
80
60
25
10
50
60
45
40
20
50
10

15
10
20

50
43
60
80

560
15
15
80
20

Mln as-
(llftllVA
minn*ii-
tiHllon.
'1,000
cu ft


30
40
55
40
40
53
40
25
30
130
50
75
45
100
45
3*,
35
70
70
50
50
35
13
35
30
80
160
'45
35


120
55
45
35
15
25
35
20
15
40
45
45
20
40
43
30
13
13
25
35
25
25
105
45
130
55

150
25
35
105
40

M*v
IVIRX
nxpliialon
iwl


120
MO
40
105
105
45
'05
85
105
60 1
100
MO
45
80
45
100
90
III]
103
45
105
73
93
83
40
10
10
40$
63
100
40(
45
60
85
95
100
40
55t
55
40
100
40
40
80
85
100
85
40
80
70
75
95
50t
80
55
85
75t
85

60
90
SO
SO
no
Uatei of
lircaaum rian,
iwl pnr no

Av«


1,200
2.200
800
I.SflO
1.400
1 000
1 400
2,300
1,900
400
2,100
I.SOO
2.300
600
1,000
1,200
1,600
1 400
1,300
1.200
1,000
1,200
1,200
2,000
2,900
600
600
200
400
1.100
200
300
300
800
1,200
2.000
2.900
1.100
400
1,200
2,400
900
SOD
too
1.000
2,500
900
1,700
1,400
1,300
1.500
1,400
1.000
1,200
250
600
400
1,800

300
3.000
1,700
1,000
1,600

Ma*


5,750
6.750
2.500
7.000
4.000

4/40
7!ooo
4,000
1 250
6:666
8,000
8.000
1.500
2,750
3,250
3,000
J 500
3.300
3.730
2,300
2,750
4,750
10,000 +
9,500
2,000
2,000
JOO
1,000
2.750
230
600
600
2,250
3.000
5,500
10,000 +
2,250
1,500
3,750
10,000 +
2,750
2,750
2,250
1.000
10,000 +
2,250
9,500
4,250
4,250
1,000
6.000
2.000
3,800
300
1,750
1,250
10,000 +

600
10.000 +
4.750
2,250
5,500
Umitln«
oxygen
petoanv
prevent
ftnition
of duat
oloud by
elootrio
aparka

10
15




11





15









17

17
13


IS
16


15

14
11
15
12


14
16
14
15

15






II
17
* When uranium, uranium hvilriile, anil zire.ifiiiiim were tltifiiurMeil into iiir at. rnum tfimiMiritluro, the duat
lloudaia'nlU'd uiitler H,nne rnmUtnuiM.
            » riMiHMt.th  iiit'iiiiurud l>y aa itldvr tcflitng toftlii

-------
               2 20
               o>
              |  10
              x   4
              UJ   H
              o
              ?   2
              5
              1
              Q.
              X
              u
              O
              I
 .4
 .2
  .1
.04
                   10       20   30 40   60 80 100      200
                      AVERAGE PARTICLE DIAMETER, microns
         Figure 2.7.   Effect of Average  Particle Diameter of Atomized
                       Aluminum on fixplosibil ity index  (from Jacobson
                       ot  al. ,  Ref.  32).
 Figure 2.7,  the particle  size,  concentration, and energy  input effects
 are  shown in Figure  2.8.
             Dust  explosions  occasionally occur  in fabric  filters as the
 result of the  accidental  admission of a spark from tramp  iron, welding
 or grinding  operations, or during maintenance or repair.  Disastrous
 effects of explosions can be minimized by providing burst diaphrams or
 pressure  reflief  panels and  large vent ducting to the outside of the
 plant.  The  loss-prevention  staff of most major insurance companies can
 recommend adequate designs, and fabric^, filter manufacturers can supply
 proper components.
            The extensive specific surface area associated with powdered
materials is also used as a reaction surface for the  recovery of  SO
or SO.J from flue gases.   By generating an aerosol of  reactive powdered
material, such as CaCCL, and collecting  it on a  fabric  filter,  the
filter-dust combination functions as  a fixed bed reactor for  partial

-------
   0.4
        400
            400
    .2
 8


I   '

-------
removal of  sulfurous  flue  gases.   Other  relevant  characteristics  of  fine
particulates  associated with  size  and  specific  surface,  and  not necessarily
restricted  to filter  applications,  include  pigment  color and cover prop-
erties, flow  properties, caking characteristics,  drying,  heating, surface
cover, abrasiveness,  potency  of therapeutic  properties,  setting time  of
                                                                  37 38
cements,  bulk powder  density, magnetic properties and solubility.  '

            2.2.2.6   Electrostatic  Charge.-  Electrostatic charge  assoc-
iated with  suspended  particles consists  of an excess ( -  ) or deficiency
( 4- ) of  electrons on the  particle.  Most small particles have naturally
acquired  charges from electron transfer  during contact and separation
or because  of  free ion diffusion.   This  charge may be assumed to  reside
            .  T     e    13,39,40,41
on the particle surface.
                      39
            Mechanisms    that usually produce a charge  on aerosol part-
 icles are:
                Electrolytic Mechanisms - Electron exchange at a high-
                dielectric liquid-solid interface followed by separation
                as from a jet, over a surface, or by impact of liquids
                on solids.

                Contact Potential - Free electron transfer across a po-
                tential barrier because of a differential work function
                of two metals in contact.

                Spray Electrification - Separation of liquids by atomiza-
                tion leads to formation of charged droplets due to ion
                concentration in the drops.

             .  Contact-Separation (Tribo-) Electrification - Separation
                of contacting, dry, non-metallic surfaces (surface work
                function).

                Ion Diffusion in Gases - Air ions may be created by elec-
                trical discharges in air, by natural radioactivity, or by
                flame ionization.  These ions diffuse rapidly in air and
                become attached to particles.
                                       j-.

            Charges of both signs usually appear in equal number after
dispersion of small particles so that the net charge of the aerosol may

be quite small even when individual particles in the cloud are highly
        42 43 44
charged.  '   '     Collision and adhesion of oppositely charged particles

affect  behavior of dust clouds and therefore bear upon the inter-

pretation of size analyses.

-------
            Representative  charge  levels on dusts of  interest  are given
             40
 in Table  2.6.    The number of  charges  acquired by  particles  is  limited
 by the breakdown strength of the surrounding medium.   In the case of
                                    2              10                 2
 dry air,  this  is about  8 esu per cm , or 1.66 x 10    electrons per  cm  .
 It is possible to  create charges on particles which exceed this  value
 under certain  conditions, e.g., electrostatic precipitation, but observed
 levels are usually considerably less.
            Figure 2.9  shows maximum likely particle  charge based on
        2                                           13 40 43 41
 8 esu/cm  and  some experimentally  determined values.   '  '  '    Figure 2.9
 illustrates two important characteristics of electrostatic charge phen-
 omena in  aerosols: 1) charge levels can exist up to the theoretical maxi-
 mum as set by breakdown environment and particle surface area  and 2) charges
 observed  on natural and artificial  aerosols are usually of the order of
 l/10th of the maximum value.  For  estimating purposes, when measurements
 are not available, the data of  Figure 2.9 allow an approximation of the
 likely charge  level.  The accompanying notes indicate the methods used in
 the generation of  the particulate  charge carriers.  Liquid break-up and
 dry powder dispersal are seen to produce high residual charge on particles
 (e.g. see items 5,6).  Presence of  an excess ion cloud tends to reduce
 residual  charge, and conversely, residual small particles remaining air-
 borne, after electrical charging and partial precipitation of the larger
 fraction, are observed to carry larger numbers of electrons (e.g. see
 items 19, 20).
                          46
            Whitby and Liu   calculated the theoretical distribution of
 charge on particles between  0.01 and 1.0 urn in equilibrium with a bipolar
 ion atmosphere based on diffusion charging, as shown  in Table 2.7.   The
 probabilistic nature of the  acquisition of particle charge has been con-
                              41     '       [
 sidered by Boisdron and Brock,   who illustrate the spectrum of particle
 charge for various conditions and the underlying analysis.
            When particles greater than about  1 urn in diameter are  passed
 through a corona discharge,   they acquire charges from electrons and ad-
 sorbed gas ions in proportion to the square of the particle diameter,  D ,
                                           42                           P
and the strength of the charging field,  E .    For conducting particles:
                              ne = (3/4) EQD 2                     (2.4)


-------
                                                          TABLE  2.6

                                   CHARACTERISTIC CHARGES  ON SOME REPRESENTATIVE DISPERSOIDS'
I
U)
NJ
Dispersoid
Raw cement mix
Gypsum dust
(Schumacher
Plant, L.A.)
Copper smelter
dust (Tooele,
Utah)
Fly ash
(Stateline,
Chicago)
Fly ash
(Rochester
Electric)
Gypsum dust
(U.S. Gypsum
Phila, Pa.)
Lead fume
(Tooele,
Utah)
Laboratory
oil fume
Method
of Dispersal
Agitation in air
stream
Grinding, drying
in flash dryer



Grinding and
drying in a
rotary kiln
Dwight -Lloyd
sintering machine
Condensation
from vapor
Charge Distribution Specific Charge
Pos. Neg. Neutral (esu/g)*"*
(%1 Positive Negative
35 35 30 0.7 x 104 0.7 x 104
44 50 6 1.6 1.6
40 50 10 0.2 0.4
31 26 43 1.9 2.1
40 44 16 4.8 4.2
0.2 0.2
25 25 50 0.003 0.003
0 0 100 0 0
               'From White, Ref. 40.

                *              9

-------
    IO7
   I06
-a
o
fc
tr
a.
UJ
s
o
a:

§
UJ
o
UJ
00
*
z
   IO8
   *
IO2
    10'
   10°
             SEE THE FOLLOWING
             PAGE FOR DATA ENTRY
             CODES
            .X
               f
     ic
        2
                              
-------
                           Notes for Figure 2.9
No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

12.
13.
14.
15.
16.
17.
Ref.
13
13
13
13
13
L3
13
43
43
43
43

43
43
45
46
40
40
Source
Green & Lane
Green & Lane
Green & Lane
Green & Lane
Green & Lane
Green & Lane
Green & Lane
Dalla Valle
Dalla Valle
Dalla Valle
Dalla Valle

Dalla Valle
Dalla Valle
Megaw & Wells
Whitby & Liu
White
White
Material
Theor.
Theor.
Ammonium
Chloride
MgO
Silica
H20
H20
Tobacco Smoke
MgO
Clay
Stearic Acic

Ammonium
Chloride
Ammonium
Chloride
Dow PSL
Theor.
Theor.
Theor.
Remarks, Generation Method
2 kV/cm, Field chg.
6 kV/cm, Field chg.
Thermal cond. (Gillespie)
Combustion fume (Gillespie)
25 psig air jet redispersion
(Gillespie)
Fog, dry (Wigand)
Fog, damp, unstable (Wigand)
Combustion
Combustion
Air jet redispersed, elutriation
Laller-Sinclair vap'n-cond'n
generator
Laller-Sinclair vap'n-cond'n
generator
Spray alcohol solution
Sprayed, measured ion
equil distbn.
Equil. charge, bipolar ion
atmosphere
• 3
515 ions/cm , t» 1, diff. limited
4 3
6.7x10 ions/cm , t » 1, diff.
18.
40
White
Oil spray
19.
20.
21.
22.
23.
24.
47
47
47
47
47
48
Schroter
Schroter
Schroter
Schroter
Schroter
Mercer
MgO
MgO
Pie:
PVC
PSL
Thee
                           Plexiglas (PMMA)
limited
Exptl. ESP, 506 kV/cm,t « 1
field limited (p. 145)
Combustion fume, naturally occur.
Comb, fume, after charging
and propnl. larger fraction
Spray dried from benzene
Spray dried from cyclohexanone
Spray dried from benzene
                                             Sprayed distilled  water,  10
                                             ions/cmr
                                                                        14

-------
No.
25.

26.

27.

28.
29.
30.
31.

32.

33.

34.

35.

36.
Ref.
49

49

49

66
66
66
66

66

66

66

66

65
     Notes
  Source
Lundgren

Lundgren

Lundgren

Kunkee
Jutzi
Jutzi
Jutzi

Jutzi

Jutzi

Jutzi

Jutzi
Deryagin
& Zimon
for Figure 2.9
   Material
  Meth.  Blue:
  Uranine
  Meth.  Blue:
  Uranine
  Meth.  Blue:
  Uranine
  Quartz
  Quartz
  Quartz
  Quartz

  Zinc

  Zinc

  Zirconium

  Aluminum

  Glass
(Continued)
  Remarks, Generation Method
  4:1,  Spinning disc  as  gen-
  erated
  Neutralized,  10% >  6 e/p

  1:1 Collision sprayer  and
  impactor
  Quoted in  Zimon, p.  78
  Quoted in  Zimon, p.  78
  Quoted in  Zimon, p.  78
  Quoted in  Zimon, p.  78,
  15 kV corona  discharge,
  7.6 x ID'5 A
  Quoted in  Zimon, p.  78,
  15 kV corona  discharge,
  7.6 x 10-5 A
  Quoted in  Zimon, p. 78,
  15 kV corona  discharge,
  7.6 x 10-5 A
  Quoted in  Zimon, p. 78,
  15 kV corona  discharge,
  7.6 x 10-5 A
  Quoted in  Zimon,  p. 78,
  15 kV corona  discharge,
  7.6 x 10-5 A
  Blow-off surface, quoted  in
  Corn  p. 380.

-------
NJ
I
                                                 TABLE 2.7


                            DISTRIBUTION OF CHARGES ON PARTICLES IN EQUILIBRIUM
                                         WITH A BIPOLAR ION ATMOSPHERE*
D
P
(u)
0.01
0.015
0.02
0.03
0.06
0.1
0.3
1.0

0
0.993
0.955
0.900
0.763
0.550
0.424
0.241
0.133

1
0.007
0.045
0.100
0.236
0.430
0,48
0.41
0.253
Number of Charges on particle
Average
2 3 4 5 6789 10 Charge
0.007
0.045
0.10
0.001 0.238
0.020 0.470
0.09 0.006 0.677
0.232 0.093 0.024 0.005 1.247
0.214 0.162 0.109 0.065 0.035 0.017 0.007 0.003 0.001 2.36
          *

-------
 where n = number of electron charges
       e = charge on one electron (4.8 x 10    statcoulomb)
 Particles of insulating materials  acquire charges  to 50 to  60 percent of
 this value.   For particles less  than about 0.2 urn,  diffusion charging
             40 46
 predominates  '    according to the following relation:
                                                     2
                             D  kT          n  D  c NQ  e t
                        np  = -V  to(l+_B_-)           (2.5)

 where n  = charges  on an initially neutral particle after time,  t
       k  = Boltzmann constant,  1.38 x 10     ergs/molecule  K
                                3
       N  = ion density,  ions/cm
       c  = ion velocity (root  mean square),  cm/sec.
       t  = time,  sec.
                         o
       T  = temperature,  K
             The  theoretical charges  acquired by particles of various  sizes
 for  a rod and  cylinder  precipitator  assembly are shown  in Table  2.8.
                                                             40
 These agree  approximately  with reported experimental  values.     Since
 particles acquire sufficient charge  to  precipitate  rapidly in a  parallel
 field condenser,  this method has formed the  basis for the design of sev-
                                             46 49  51
 eral  electrostatic  particle-size classifiers.   '  '    The migration
 velocity  of  a  charged spherical  particle,  V  ,  in the direction of  a col-
 lecting electrode can be obtained  from the following expression, assuming
 that  air  resistance  is  given by Stokes Law:
                                    Enpe/3« uf Dp                  (2.6)
where E = field strength in the collecting space (esu/cm) , and C  is the
slip correction factor.
            Effective electrical precipitation depends upon the particle
charge level (n ) and a strong collection field gradient (E) which, in
turn, is attained by providing a large value for the product N t, or a
                                                              o
reasonable number of ions (N ) over a realistic time to charge (t).
A practical constraint is that the charging time, t, must be less than
the gas residence time in the system as determined by the volume-flow


-------
                                TABLE  2.8
                 NUMBER OF UNIT CHARGES ACQUIRED BY PARTICLES
Particle
Diameter
|im
0.2
2.0
20.0
Field Charging
Exposure Time, Sec.^
0.01 0.1 1 a
0.7 2 2.4 2.5
72 200 244 250
7200 20,000 24,000 25,000
Diffusion Charging
Exposure Time, Sec.
0.01 0.1 1 10
3 7
70 110
1100 1500
11 15
150 190
1900 2300
  *
   From Lowe and  Lucas,  Ref.  50.
    Limiting Charge
    Note:   Calculated  from equations  (2.4)  and  (2.5)  under  the  following
           conditions  typical  of  a  rod  and  cylinder  assembly;T=300°K,  N
           5  x 10  ions/cm-^, E = 2 kV/cm,  in air  at  atmospheric  condi-
           tions,  at 40 kV with a discharge current  of  40 uA/ft.
relationships.  Practical  devices utilizing  electrical  precipitation must
take  into  consideration  the  physical and  chemical characteristics  of the
carrier gas, and  particle  deposition, adhesion, bounce, and reentraintnent
caused by  fluctuations in  the  flow  field.
            Acquisition  of maximum  theoretical charge gives rise to
strong attractive coulombic  and dipole forces which can be used to collect,
separate,  or classify particles.  The presence of electrostatic charges
is often a severe problem when handling particles less than 10 um having
relatively low mass, as  fine particles tend  to clump together.  This
makes it difficult to produce good  air dispersions for subsequent sedi-
mentation, prevents dry  screening of powders with electro-formed sieves,
and complicates particle removal from fabrics.  Undesirable charge ef-
fects may be counteracted by exposing the aerosol to emanations from a
nuclear radiation emitter to produce excess air ions for neutralization,
or by humidification which enhances charge leakage.   The latter process,
however,  will also accelerate particle clumping.
            Powdered materials which are good electrical conductors
(e.g.  carbonyl iron spheres)  can be grounded before  and during sieving


-------
to reduco charge effects.  Fugitive electrostatic charge accumulation in
powdered materials handling systems is one source of  ignition  Cor ex-
plosive reactions, as  indicated above.
             The presence of electrostatic charge on  a particle gives rise
to forces influencing  its aerodynamic behavior in an  electrical field and
classifiers have been  designed to take advantage of the fact that a charge
may be placed on a particle in proportion to its size.  Theories of
particle charging by diffusion and electrostatic field charging mechanisms
                           40                    46
have been reviewed by White   and Whitby and Liu.
             Measurements of particle electrostatic charge can be ob- :
tained experimentally  by methods illustrated schematically in Figure 2.10.
The Faraday ice pail method shown at  (a) consists of  an insulated filter
holder connected to ground through an electrometer and a bucking circuit
                                                         52 53
(Figure 2.11), and measures the total aerosol net charge.  '    From an
independent measure of particle numbers (as with an automatic light scat-
ter aerosol particle counter, particle size analyzer, or by counting
particles on a membrane filter sample), the average charge per particle
(+ or -) can be obtained.  Millikan's oil drop experiment (Figure 2.10)
provides a means for measurement of the motion of a particle in an elec-
trical field.  Individual particles are suspended by  an electrical field
in a parallel plate condenser and their motion observed by light scatter
in a microscope.  Differential velocities under the action of gravita-
tional and electrical  fields are used to compute individual particle
charges.  The method requires great patience on the part of the observer.
Other modifications of the motion of particles in electrical fields are
illustrated in Figure  2.10, c and d.  Stroboscopic photographs of
particles are taken in a defined field and serial tracks are used to com-
pute size (from falling velocity) and charge (transverse displacement).
             Charge spectrometers have been employed to sort out particles
by electrical mobility in a parallel field configuration,(Figure 2.10 e,f)
With an independent measure of size (as by automatic analyzers), the spectrum
of charges can be deduced.  Whitby and Liu have used the method in reverse
by diffusion charging the aerosol particles to a known level,  (0.01 < D   <
                                                                    ~  P
0.5 urn)  and measuring collection of species of known charge  at  various


-------
                        aaroaol
                         filter
                       luaulator
                                                                           •witch
   a.  Faraday Ice Fall  (after Maatere)
                                                  b.  Nllllkan'a method



jf
i...
Ix 1
^
. 1
^ f
     c.  Hoppar-halay vcthod


   aeroaol jet
                            electrometer
  e.  Charge Spectrometer
     Whitley-Liu,
     Megaw & Well*
     Kraener & Johnitone
     Sturtevant, CIT


i
-------
                                                                           I       (
                                                                                 s
                             T
      (a)
                CIRCUIT OF MASTERS
       * 50/jAuf
f . ELECTROMETER, HI LKG
                    V= VTVM
         Faraday /
         Cylindeiy
                 6V
                             Qp
                             /	*
                               VVWVAA»—i
i  LuciTfc C"LiND£R(29CM)
  WRAPPED WITH
  TEFLON

2  BRASS COLLAR (19 CM)

3  LUC'T SCREES

4  HIGH EFFICIENCY
  FILTER  PAPER

5. STATIC TAPS
                                                                  (C)
( b)
          EXPERIMENTAL DUPLICATION OF MASTEFgCIRGUIT
                                                                             - Faraday Cylinder for Net

                                                                     Aerosol Charge Measurement
      Figure 2.11.   Simple Method  for Measurements of Aerosol Net Charge (From Anderson

-------
 velocities to deduce the size distribution.  The field mill (Figure
 2.10,g) and the spectrometer method of Hinckley and Dalla Valle (Fig-
 ure 2.10,h) measure the spectral distribution of charge in a flowing
 aerosol cloud.  Other methods of charge measurement include deflection
 of a charged particle at high velocity in a magnetic field, charge
 transfer upon deposition, etc.  Although none of these methods is com-
 mercially available for field investigation of particle charges they are
 frequently adapted for laboratory measurements of charges in simulant
 aerosol clouds.  In conjunction with an independent measure of number-
 size distribution, the system of Masters is readily adaptable to stack
 sampling for field measurement.
              The subject of electrostatic effects in filtration, the
 production of charge in aerosols or on fibers to promote collection, and
 the use of electrets (the electrical analog of a permanent magnet) are
 discussed in subsequent sections.
              Particle charge measurement and charge phenomena in dispersed
 powders has been the subject of extensive research on powder dissemination
 properties during the past ten years.  A summary of the present state of
                                                       54
 knowledge is included in a recent treatment by Lapple.
              2.2.2.7  Adsorption.- Suspended solid and liquid particles
 are surrounded by a surface film of gas molecules held by unbalanced
 electrical or chemical valence forces arising from interactions with
 the surface or near-surface molecules.  Gas or vapor molecules may be
 adsorbed  in  proportion  to  their  concentration  in  the surrounding gas  phase
 up  to  saturation  of  one monomolecular  layer.  Additional  layers have
 been demonstrated  in  typical  aerosol  systems of interest.    The quantity
 of  adsorbable  vapcjr  on  particle  surfaces may be used to estimate surface
 area and  average  particle size under controlled conditions, and instru-
 mentation  for  powder  surface  area determination is commercially available.56'57
 Atomic or molecular configuration, electrical charge, and particle shape,
 structure, and  fissures affect surface adsorption.  Presence of adsorbed
 vapors on ambient aerosol particles will modify the surface characteristics
 of  particles such as charge, adhesion, and evaporation.  The kinetics and
mechanisms of adsorption of gases and vapors on solid surfaces are treated


-------
extensively  in recent  texts on  the  physical chemistry of  surfaces,   '
the solid-gas interface,   and  from the standpoint of technological  con-
                                                    61
siderations  in recent  symposia  on particle surfaces.
                                               59
            Adsorption is described  practically   in terms of the empir-
ical adsorption isotherm:

     Volume of gas adsorbed = f  (partial pressure of the  adsorbent).
                                                                      59
            Typical adsorption  isotherms are indicated in Figure 2.12.
Type I represents the  Langmuir type  isotherm in which the vapor is ad-
sorbed as a monolayer.  Since the capacity is surface limited, the amount
of adsorbed material rapidly becomes independent of its partial pressure
in the vapor phase.  If a multilayer prevails, a common occurrence in
the case of physical adsorption, the Type II curve is more representative
of the adsorption process.  Type III is a rather unique case where the
heat of adsorption appears to be less than the heat of vaporization of
the absorbate.  Both Type IV and Type V curves tend to reflect capillary
condensation phenomena since they level off before the saturation pressure,
p , is reached.   The volume of vapor adsorbed is a function of the vapor
pressure and adsorbent properties.   Typical values of surface area of
                                                        £ p
several commercial carbon blacks are shown in Table 2.9,     (Columns
4 and 5).  Estimates of surface area calculated from particle size meas-
                                                                     <
urements with the electron microscope (Column 3) are seen to be generally
Lower than area measured using a nitrogen adsorption method.   The wide
range of areas is typical of variations in methods of producing particulate
materials and of the spectrum of particle sizes.   Column 2 indicates the
                                                               3
bulk (or apparent) density of the powders,  0.1 < p < 0.75 gm/cm .   True
                                               3
density of amorphous carbon is 1.8 to 2.1 gm/cm ,  and of graphite is
          3
2.25 gm/cm .   The conversion to the powdered state produces a density of
the order of l/10th to l/3rd of the true density,  as indicated previously
(Tables 2.3 and 2.4).
             Powder porosity (void fraction,  e = 1 - Ct)  is higher with
                                               _2
finer particle sizes,   i.e. ,  e = 0.93 at D  =10   ^m (high color channel

-------
black) and e = 0.625 at D  = 0.5 ^m (MT thermal black).  Specific surface
                         P              o                       9
area also varies from 103m2/gm (D  = 10"  um) to about 1 to 10 m /gm
(0.5 um).
                                                                                 J
                 Vads
                         P
                       Vads
                                    "type!
          Figure 2.12.  Brunauer's Five Types of Adsorption
                        Isotherms (From Adamson, Ref. 59).
             In many technical systems of interest, behavior of small
particles on contact or at separation is observed to be a direct function
of adsorbed ambient moisture.  The role of gas adsorption on atmospheric
particulate is largely unstudied and poorly defined at present.  It is
recognized that these processes are influenced by many factors including
concentration, humidity, catalytic effects, and natural irradiation
levels.
       9,63
            2.2.2.8  Adhesion.- Contact of small solid aerosol particles
with solid surfaces often results in very strong bonding forces.  The
phenomena of attachment of solid particles to surfaces is called adhesion,
and the force applied to overcome the attachment is called the particle
               &
adhesion force.   Since adhesion is an important characteristic of small
particles (high surface to volume ratio) it must be considered in many
practical aerosol systems; e.g., generation or dissemination of powders,
the collection of particles from disperse systems, the control of pollu-

-------
                          TABLE  2.9

            PHYSICAL PROPERTIES  OF TYPICAL CARBON  BLACKS
                                                       *1
Carbon-black
type
A.





B.






C.




D.







E.



F.




Color and
Ink channel
blacks :
High color
Medium col.
Low color
Long flow
Rubber
grade chan-
nel blacks:
CC
HPC
MPC
EPC
Gas fur-
nace blacks:
SRF
HMF
FF
Oil fur-
nace blacks:
GPF
FEF
HAF
ISAF
SAF
CF
Thermal
blacks :
FT
WT
Other car-
bons:
Acetylene
black
Graphite
Bulk
Density
K/cc*2


0.1-0.2
0.2
0.2
0.2



0.35
0.35
0.35
0.35


0.45
0.42
0.46


0.40
0.33
0.35
0.33
0.35
0.32


0.50
0.75





Av.
Particle
diam, mu


10-14
18
30
30



25
26
28
30


70
50
40


55
40
28
24
20
19


185
520


43

Large
E.M.
Surface Area
*3 8q «/a*4


218-186
130
95
100



110
105
106
95


25
40
60


40
60
75
120
140
120


16
6


60

platelets
N2
Surface Are*
sa m/g*5


1000-860
400
110
350



225
140
120
100


25
35
75


25
40
75
130
140
220


16
6


64


*1
*2
*3
From Smith, Ref. 62

Bulk density as commonly supplied to industry.  Color black may also be
supplied in densifled pellet form.

Average value from electron micrographs for arithmetic mean diameter.
  Calculated assuming spheres  from electron  micrograph  surface average
  diameter.


  Calculated from  adsorption  in  nitrogen  at  -195°C by method of  Brunauer,
  Enmett, and Teller.

-------
  tiints,  the assessment  of quantity of suspended material,  and in the build-
  up of deposits  on filters and duct surfaces.   Although adhesion phenomena
  are commonly displayed and of great technical  importance,  the calculation
  of adhesion forces  in  any given situation may  be  difficult.   Their estima-
  tion must  include the  effects contributed by solid  and surface properties,
  gas constitutents,  interface  geometry,  and history  of the  environment
  or system  under study.
               Consider  two 50  (am solid spherical quartz particles  brought
                                                                        **
  into contact under  ambient conditions.   From classical considerations,
  the force  of gravitational attraction between  the two particles by virtue
  of their mutual masses is equal to (approx.) 2 x  10    dynes.   If the
  particles  are oriented vertically in the gravitational field of the earth,
  and the upper particle is held fixed (for example,  by a glass  fiber),  the
  gravitational force of the earth on the lower  particle is  (F  = mg)  of
                 -2                                           8
  order of 2 x 10    dynes, thus indicating that the  lower particle should
  fall away.   In  fact, the lower particle will probably not  fall,  but be
  held firmly to  the  upper one.   If a separating force  is applied to the
  two particles (e.g.  in a centrifuge,  or by a microbalance  technique),  the
  actual adhesion force  can be  estimated  to be of the order  of about 0.5
  dynes.  This additional  adhesive force  arises  as  a  consequence  of several
  adhesion mechanisms  that operate at and near the  interface between the
  two particles.   If  two  particles having a common  contact area  of  1 sq  pm
  were assumed to be  bonded intimately through chemical  or physical means,
  the force  required  to  overcome the molecular attraction would  be  approxi-
  mately  10  dynes.  This  corresponds to the mechanical  strength  of  the
                             '92.
  parent materials, of the order of 10 dynes/cm .  Thus, the  magnitude  of
  adhesion forces encountered in situations of interest  for  particulate
     *
      The study of adhesion in friction, lubrication, and wear of surfaces
in relative motion is not considered here.
     **                                                  ,2-8
       Newton's law of universal gravitation, FN = G mitiwr ,  G ~ 7 x 10
dyne cm /gm .   For two bodies of different  size, r^ » r^,  FN ~ r?, and
the attractive force is directly porportional to the size of the larger.

-------
control technology are usually less than those estimated from considera-
tion of the strength and areas of materials in contact, but apparently
somewhat greater than predicted classically.  The problem of the analysis
or estimation of adhesion forces in aerosol deposits in fabric filters is
more complex because the particles are usually packed in close juxtaposi-
tion to one another as well as attached to a substrate.  This leads to
additional forces arising from particle to particle interactions, and
also to considerations of the actual geometry of the deposit.
             A summary of the major effects in adhesion phenomena is pre-
sented in the discussion which follows with empirical relations and ob-
servations suitable for technical or engineering estimating purposes.
The following aspects are briefly treated:
             . Adhesion forces for individual particles including
               effects of particle size, relative humidity and
               time-dependency.
             . Effects of electrostatic charge on particle adhesion.
             . Effects of particle and surface shape.
             . Effects of surface roughness.
             . Adhesion phenomena in ensembles of particle deposits.
             (1)  Adhesion Forces for Individual Particles - According
          64
to Krupp's   unified analytical  treatment, the phenomenon of particle
adhesion to a surface proceeds as follows:
             "1)  At first, particle and substrate come into contact
                  at one point by a contact area of atomic dimensions.
              2)  By long-range  attraction forces between the two,  the
                  particle... is subject to a moment of force so that
                  several contacts are formed... between non-perfectly
                  smooth adherents.
              3)  By the interaction forces the ... area at these
                  contacts increases until the attractive forces and
                  the forces resisting the further deformation at the
                  interface are  in equilibrium.  An adhesive area of
                  finite size is formed between the adherents."
             The effectiveness of the contact between particle and sub-
strate also depends upon the magnitude of the force acting on the particle
at the instant of contact; i.e., the kinetic energy given up by the

-------
 particle  through  its  stopping distance,  or  by the  pressure  or  gravitational

 force  applied  by  the  particle to  the  substrate to  provide  intimate con-

 tact.

             Upon application of  a separating force,  separation takes

 place  as  follows:

             "1)   The external forces of separation exerted on...  the
                   particle...  cause a partial or complete recovery of
                   the deformation at  the interface.

               2)   Generally,  the  centre  of  attack  of  the separating
                   force will  not  be symmetrical to the  location and
                   strength  of the individual  contact  sites,  so that
                   the particle is subject to  a moment of force and
                   individual  contacts may become broken separately.

               3)   Finally,  the last contacts  are separated;  the particle
                   is  set  free.

                   The adhesive area,  ...  is  ... the area of the common
                   interface between the  adherents  effective during this
                   last step."

 The  adhesive force is defined as  that force applied perpendicular  to the

 center of gravity of  the  particle necessary to remove the particle from
                                                64
 the  substrate  in  a fixed  period of time.  Krupp   classifies the inter-

 action forces  as  follows:

             "Class I - long-range attractive interactions  resulting from:
             Van  der  Waals  forces,  electromagnetic  fluctuation phenomena
             between  the  elementary oscillators of  a  solid;  and electro-
             static double  layer  forces,  arising when two solids in con-
             tact  charge  each  other by electron transfer (differential
             surface  energy),  the contact potential difference  at  equili-
             brium being  of order 0.1 V  for conductors.

             Class  II  - short-range attractive  interactions resulting
             from  the  various  types of chemical bonds and hydrogen
             bonding mentioned  below,  and

             Class III -  interfacial  reactions such as diffusion, dis-
             solution, and  alloying."

             The exact analytical prediction of adhesion forces from

physical models is difficult because adhesion results from mechanisms

operative at the molecular  level  leading to chemical and non-chemical

bonding at the interface,  and  to  the establishment  of a  complex field  of

mechanical stresses, strains,  and deformation around the interface.



-------
These  interrelationships  change with  time and with  the application of  the
                                                               64
separating  force,  leading to  requirements Cor a kinetic model.    Forces
that produce an attractive  interaction Between solids in contact  include
those which provide  intermolecular cohesion  in solids, such as metallic,
covalent and ionic primary  chemical bonds, and secondary van der Waals
attractions having energies of 1 to 10 and 0.1 eV,  respectively.  Inter-
mediate energies arise  from hydrogen  bonds,  electronic charge  transfer
bonds, and  the electrostatic  double layer forces.
             As discussed  below, adhesion forces  in practical  systems  arc
also related to ambient relative humidity or absolute humidity  in the
surrounding gas, as  a consequence of  capillary condensation of moisture in
the interfacial space.
             Analytically  tractable geometries of technical interest in-
clude sphere-to-wall (half-space), sphere-to-sphere, and sphere-to-
cylinder contacts.   Present knowledge is limited to estimates of adhesion
                              3
for particles between 1 and 10  um.  The lower limit arises because all
real solid surfaces  contain numerous  irregularities (asperities) under
10 um such that particles of  the same order are effectively embedded in
a fissure or crack.  In the case of larger particles,  gravitational
forces are greater than the adhesive  forces, because the true contact
area, which defines the adhesive forces, is determined by the relatively
                                                                 ll I
                                                                 64
                                               64
small dimensions of the surface irregularities.     The theoretical bases
of particle adhesion have been systematically presented by Krupp.
Substantial reviews of the fundamental relationships, technical estima-
tions, and measurements of adhesion forces have been given by Corn
  o „•    66
and Zimon.
             The following experimental methods      have been used to
measure the adhesion force of single particles on surfaces, as indicated
in Figure 2.13:
             . Microbalance technique for particles that can be
               manipulated (diameter 75 um)
             . Pendulum method which requires that the gravitational
               force,  F ,  exceed the adhesive force, F

-------
  filament fused-end
  geometries
                        "a • kh
                        TT
    a.  nlcrobalance method
                                                                     I I I I I
                                                  h.  Pendulum method
                                                              F  5-. C  i  A  u  /2
                                                               .1   b «  p
       c.  Centrifuge method
                          67,70,71
                                  d.  Aerodyaamic method
                                                      68,70,71
                    	». F =(ir/b)D   ,  (2n.;)(fsln at)
                    T      a      P   P
                        oscillator
                "77/77"
             e.  Vibration method
                               68
                                                              F • mg
                                                          allow to break
                                                          under own
                                                          weight or in
                                                          moving fluid
                                                       .-'.  Belscher's method
                                                                          69
Figure  2.13.
Experimental  methods  for  measurement of particle
adhesion forces.

-------
             . Centrifuge method which requires that particle mass be
               known
             . Aerodynamic method which requires definition of velocity
               gradients surrounding the contact regions
             . Vibration method in which the particle interface is
               subjected to an alternating compressure force
The following discussions relate to adhesive forces determined by each of
the several methods.  A modification of the microbalance technique was
used by Beischer   who allowed coagulating threads of individual Fe.O-
particles of 0.5 (im diameter to break under their own weight.  The force
required was estimated from the size of the separated fragment.  His
work indicated that for submicron particles the adhesive force was of
                     -4
the order of 0.5 x 10   dynes.
             If several identical particles are dispersed separately upon
a substrate the force required to remove each one will not be the same.
Application of a fixed force will remove but a fraction of the particles.
Complete separation of all particles may require a significant increase
in the force field.  The probabilistic nature of this phenomenon is
                         64
presented in Figure 2.14,   in which the relative number of particles
                                                                  200
                                                                  100
                           10                    20
                      ADHESIVE FORCE, FQ, mlllldyntt
30
                                                                       IK
  Figure 2.14.  Adhesion of Spherical Fe Particles of 4 pro Diameter to
                Fe Substrate at Room Temperature in Air as a Function
                of Applied Force (from Bohme, et al., Ref. 67).

-------
remaining after  sequential  application  of  discrete  centrifugal forces is
shown.  Whereas  fifty  percent  of 4^ni iron particles were  removed at
                     _3
approximately 4  x  10  dynes,  98 percent  removal  required a force in
                 _2
excess of  3 x 10  dynes.  Thus the  actual  adhesion forces for  a large
number of  identical  particles  must be represented by  a  cumulative dis-
tribution  function of  the  form N = N(F ).   The  number of  particles re-
                                       Si
moved by the application of  a  given  force can be  represented by its
derivative, the  distribution density function
                                                                    (2.7)
where  N  is  the  initial  number  present  and  dN(F)  is  the  fraction of
        o
particles  whose  adhesion force  lies  between F + 1/2  dF,  with  n(F)  = 0 at
                 64
F  =  0  and  F  -» «>.     Since n(F)  is  highly skewed,  a logarithmic  probability
distribution function, or other two-parameter distribution function,  will
enable presentation  of the median  detaching force and  standard  deviation
in any given experiment,  e.g.,
                  dN     0.43
                  N
                      F   .  cr «/2n
                      a
                                   exp
                                          (log  F  -log  F)2n
a
           dF_      (2.8)
             a
                       66
 as  suggested  by  Zimon.                   .

             Loffler   '   describes the adhesive properties of small
granular quartz particles (5 to 15 um) collected on different fiber filters
 (50 um nylon, polyester, and glass fibers) with a logarithmic probability
distribution, Figure 2.15.  Median forces required to remove 50% of ad-
                                                      -3          -3
hering particles from 50 um nylon fiber ranged from 10   to 6 x 10
dynes, for 5 to 15 um particles, respectively.  The median force required
to separate 5 um irregular quartz particles ( ~ 1.0 m dyne) was less than
that reported by Krupp (Figure 2.14) for smoother 4 um spherical iron
particles (~4 m dyne).  The geometric standard deviation, (crF),  of the
forces representing Loffler's data (Figure 2.15) was about 4.0 for each

-------
* %
§ 95
9 90
in
w 70
£ 90
5 30
§ '°
I '
5 *.
u
-

•
•
•
T « 1 I
f 96 % Conf M«M« InNrvol




0».9.l^«^

"j^Xx^
85/i.m O9/M
-

1 1 II


s
//
'^
M,um
I

4
s^P1
>/





*
X



till

^^i
'/
lr



1

4
X




1 1







till
J'4 2 9 IO"S 2 9 B* 2 • 9 1C
                               FORCE  DYNES
                    Figure 2.15.  Distribution of adhesion
                                 forces^logarithmic-probability
                                 plotj for classified crushed
                                 quartz particles collected by
                                 filtration at V = 42 cm/sec on
                                 polyamide (nylon) fiber, 50 urn diam.,
                                 at •-  50% R.H.
                                (From Loeffler,  Ref.  71.).
of the size ranges studied.  The high degree of skewiess in the distribution
was consistent with Krupp's findings, Figure 2.14.   The median force
required to remove 50% of the particles represented by the data shown  in
Figures 2.14 and 2.15, leads to an approximate force-size relation given by
                                     10 D
                                  (2.9)
 where the coefficient has the dimension of surface energy,  dyne/cm, and
 particle diameter is expressed in cm.   For removal of nearly all particles,
               i                   2
 the  constant  would be of order 10 .   The effect of particle size on ad-
 hesion of glass  and quartz particles  on flat glass plates  at 95 percent
 relative humidity is shown in Figure  2.16, from the extensive measurements
         72
 of Corn.    Corn's results can be represented by the empirical equation
                                     102  D
                                 (2.10)
at  95  percent  R.H.  for  5  < D   < 200  urn.
       He also gives the relationship by
7U „ D  * 600 D
 H20  p        p
                                                                    (2.11)

-------
             10.0
          8   1.0
          c
         O

         

         UJ


         O
            0.10
             QOI
     I
                                        • PYREX-OPTICAL FLAT

                                        o QUARTZ-  "

                                        o QUARTZ -GLASS SLIDE



                                        T=25°C

                                        P = 760MM. HG.
               1.0   2      5   10   20    50  100 200  5001000


                              PARTICLE  SIZE, microns



    Figure  2.16.  Adhesion  of  Quartz and Pyrex Particles at 95% Relative

                 Humidity  (from Corn, Ref.  74).




where 7     is the surface  tension at a water-air interface,  for 7

        2                                             73          H2°
70  dynes/cm.  Similar results were obtained by Bradley for pairs  of
quartz spheres having diameter D.. and D_:
F  = 212
 a
                                                                   (2.12)
over the range 200 < (D) < 600 ^m, and for sodium tetraborate
over the range 500 < (D)  < 1700 urn.
                                                                   (2.13)
             As a first approximation,  particle  adhesion  force  is propor-


tional to particle size at  or near  100  percent relative humidity.  The

-------
                                                    2
coefficient of proportionality is of the order of 10  multiplied by a small
integer.  For particles less than 1 ^m, the coefficient may be as low as
1, as reported by Beischer.
             Forces arising from molecular interactions are modified in
practical aerosol, systems by the presence of substantial (unknown) amounts
of adsorbed or condensed molecular species from the constituents of the
gas phase.    The open region found at the point of immediate contact
between a perfect sphere and flat surface represents a microfissure of
small radius of curvature.  The capillary condensation of vapor at this
location in accordance with the Kelvin equation,   produces a minute pool
of liquid in the interface.  The theoretical force of adhesion due to
this film of liquid can be shown to be:
 Since the surface tension of water (7,, rt) is of the order 70 dynes /cm, the
                                      H20
 constant in equation 2.14 should be of the order 10 ,  and seldom greater
 than 500.  The effect of equilibrium relative humidity (surfaces pre-ex-
 posed before contact) on adhesion of quartz and glass  particles to glass
 plates is shown in Figure 2.17.   The adsorption isotherm for water vapor
 on quartz is also shown, together with earlier data of Bowden and Tabor
 for glass spheres on glass plates (also see Figure 2.12).  Effects of
 capillary condensation are reported to diminish below  about 60 percent
 R.H. ,   although Corn   and Larsen   both reported effects ascribed to
 moisture substantially below 60  percent R.H.  The effect of relative hum-
 idity on adhesion force can be estimated from a linear approximation to
 Corn's data in Figure 2.17 as:

                     F  = 102 D  < 0.5+4. 8 x 10"3 (7, R.H.)'           (2.15)
                      a        p  1

 which will be reasonably consistent for 50 < % R.H.  <  95% for particleswith
 D   > 20 urn.
  P
              Zimon   (p. 82) presents the following data (Figure 2.18,
 Zimon's Figure 1.2)  for effects  of particle size with  50 < % R.H.  < 65,
 for spherical glass  particles on steel surfaces:


-------
O
5
D
X
LJ
QL
100
90
80
70
60
50
40
30
20
 10
            0.001
              i
                 SATURATION IN 8/9
                 0.002    0.003   0.004   0.005
            WATER VAPOR ADSORPTION CURVE
                              FOR QUARTZ
.CORN, REF 74, QUARTZ TO PYREX
741 MM. HG. < P < 756 MM. HG.
24°C
-------
     Dp, urn	

     F  , dynes
      fl
     Calculated from Equa-
       tion 2.14:

     Measured, for 50%
       removed:
40-60
2.3
                                           20-30
1.2
           10-20
0.7
           5-10
0.3
2.1xlO"4    2.1xlO"3    6.1xlO"3  1.3xlO"2
Zimon concludes from these data that R.H. < 50% has no effect on adhesion.

Figure 2.18   also illustrates the effect of varying force on detachment

of particles of varying sizes, and the shapes of the cumulative force-

removal curves for different particle size classes in the same experi-
                        Of '       2      3
                             log ^det. unit* of g
   Figure 2-18.   Integral adhesion curves for spherical glass particles
   of different  diameters adhering to a steel surface of the 13th class
   of units  of g (a),  and in absolute measure (b).  1) dp = 80-100-  2)
   40-60;  3)  20-30; 4)  10-20;  5)  50-10/1, (from Zimon, Ref.  66)    '
nental system.   In a similar study  at  R.H. near  100  percent,  Zimon66 re-

ported:

-------
D , urn
F , dynes
3
Calculated from
Equation 2.14:
Measured
7o Removed
40-60
2.3
0.9
59
80-100
4.1
4.3
78
100-200
5.1
4.7
75
              Condensation effects  also depend upon particle  size  and fiber
 or substrate  surface  as  shown in Figure 2.19   a,  b,  c,  small  particles
 apparently being less affected by  humidity in these  studies.   '   '
 Both  polyamide  and  polyester  indicate  much  less  capillary  condensation
 effect  i
 glass) .
effect on adhesion, as expected from the contact angle (< 90  vs 0  for
              In  spite  of  apparent  differences  between results  presented
 by  various  investigators,  the  data are  in  reasonable agreement  in  confirm-
 ing the  relationship between adhesion and  particle size  and relative
 humidity.   Very  low humidity   air   would be expected to.  produce  relatively
 low adhesion,  from the above data.  Reduced humidity, however,  removes the
 adsorbed vapor layer between the particle  and  substrate.  This  leads  to
 closer approach  and hence  the  formation of stronger bonds between  the two
 materials.  Since the  electrostatic charge on  the particle or substrate
 is  less mobile or less shielded at  low humidity it may be a major  reason
 for increased adhesion at  low  relative humidities.
             Effects of capillary  condensation which depends upon  exposure
 time of the interface  to the vapor, require about one hour to approach the
 full effect.     Other  time-dependent effects include stress-strain and
 deformations of  the particle-substrate interface under the action of the
 adhesion force.  In general, adhesion forces tend to increase with time
with a time constant of the order of an hour or more.     Dust removal
 processes, therefore, should be more effective if applied as soon as pos-
 sible.  The magnitude of time effects on adhesion may be  of the order of
 25  percent or more.
             (2)   Effects of Electrostatic Charge on Particle Adhesion
Electrostatic charge on a particle might be expected to cause a marked


-------
10   20  30  40   50   60  70  80  90  CO  HO   iaO
          RELATIVE  HUMIDITY- %

(a)  From Corn,  Ref. 74, Pyrex-Pyrex.
                                                                          20  40  CO  00  00
                                                             (b)  From Lo'ffler, Ref.  70,  Granular Quartz
                                                                 on 50 utn Nylon Fiber
                                                                 8  5

                                                                 i
                                                                 5  *
                                                                 o
                                                                 <
                                                                 a
                                                                       MffTKLC SOF «0 4^ m OMMFTBI
                                                                                  0.4*
                                                                                       6LASS (SOAjf )
                                                                     0  W  40  tO   CO  100
                                                                           %  urumve
(c)  From Lbffler,  Ref.  71, Granular  Quartz
     on Indicated  Fibers.
             Figure 2.19.  Effect  of  Particle Size and  Relative Humidity  on Adhesion for

-------
increase in adhesion force.  To a  first approximation, the charge effect
on adhesion force should be given  by Coulomb attraction:
                                       QQ

                                        e
where K  is the permittivity of the intervening dielectric, and  Q    are
       e                                                         1,2
the charges separated by distance r.  To find the approximate affect of
particle charge on attractive force, consider a 10 micron particle attached
to a conducting plane surface, with an expected fugitive charge  of the
order 100 electrons  (from Figure 2.9).
                                                    22    -9
             The  image  force will be of the order (Q /4r )=10    dynes, or
substantially  less than the observed adhesion forces.  On the other hand,
if the particle were charged to its maximum possible amount, the charge
               4
would be 5 x 10   e.  The resulting image force of attraction upon deposi-
                                                      -4
tion on a grounded conductor would be approximately 10   dynes,  or the
same order of magnitude as found experimentally by Zimon, for low humidity
tests data.  Since the  image force increases with the square of  the charge,
and the charge, in turn, as with the square of the particle size, ad-
hesive forces arising as a consequence of particle charging will be larger
than those for uncharged particles.  This applies to practical situations
such as electrical precipitation, flocking, and xerography, where charges
are high.
             Electrical forces also arise as a consequence of the contact
potential from different electronic states of the contacting surfaces, in
the form of an electrical double layer.   The charge will depend upon the
particle electrical conduction properties and the state of the interface.
Glass particles (50 to 70 ^m) successively detached from a painted sur-
face   were found to produce residual charges as shown in Figure 2.20.
The higher charges were associated with a higher particle retention after
application of a given force.  The electrostatic charges upon separation
of 50 urn particles were 3000 < Q  < 12,000 e.   If a high voltage is ap-
plied to a conducting substrate, the particles can be caused to leave.
However,  no application of this observation to fabric filter cleaning has
been reported.

-------
                           36 r
                           30
                           24
                        o
                        g   '8
                        «T
                            12
                              5     10     15     20
                               % REMAINING N(R)/N0

Figure 2.20.   Adhesive Force as a  Function of the  Charge Determined  on Detaching
              Glass Spheres of Various  Diameters (by the Vibration Method  )  From
              a Painted Metal Surface.  1)  d = 50+5nm Fdet=3.6  •  1CT3 dyn-
              2)  d=70+5um Fdet=1.9 •  10'2  dyn (From Zimon, Ref.  66).

               (3)  Effects of Particle and Surface Shape  - As  shown by
  Zimon (Ref.  66, Chapter 10), the shape of particles in contact and the
  substrate geometry  influence the magnitude of the adhesion  force.  For
  two spheres  of diameters D. and  D« in contact, the adhesion  force  can
  be shown to  be as given above:
                              F  ~ 10
                               a —
D1+D2,
                             (2.17)
   If Dj_ = D2,
                                       2      2
                                   2  Dl    10 Dl
                            F  =  10  -=—  -  	-
                             a    U  2D.      2
                              (2.18)
   or one-half the  value  expected  for  dissimilar  sizes.  For the case of
   D2»

-------
                          F=
                           a -  L"  "I  IBD./D,
                                          1   2>
Therefore  as D? '••  .v,  the  force  between a  plane  and  sphere  becomes

                               F * 102 D1                           (2.20)
                                3       J.
                 72
as  found by Corn.   .   Note  that the force between two  equal  spheres  is
about one-half of  that  between  a sphere and  a plane.   For  the  case of
                                                                        { Q
a sphere resting in  liquid  on  a cylinder  (as on a filter fiber), Larsen
derived the adhesion  force  from geometric considerations as
                            Fa =  2n K^  7H 0 D                        (2.21)

where
                             A               i             i
                                                                    (2.22)
c
|y +
1 i 1
k,V/2 (k2+D1/2
d c
and
                           _ diameter of  liquid layer   2 P          ,2
                         c     diameter of particle     D

                             diameter of  fiber  _  f                _ _,.
                         d   diameter of  sphere   D

Larsen"s derivation for the sphere- cylinder case is presented in Appen-
dix 2.1.  An experimental verification of the Larsen equation is not pos-
sible without a measure of the size of the liquid pool layer which cannot
be readily measured experimentally.  On the other hand, knowledge of the
experimental separating force for fiber-sphere geometry would provide in-
formation on the interfacial pool size, contact angle, etc.  Corn studied
the adhesion forces between spherical quartz particles and Pyrex fibers
with a microbalance  -chnique at 90% R.H.  These data are shown in Fig-
ure 2.21 as percent or adhesion to a flat surface vs D,/D .  The probabil-
istic nature of adhesion forces discussed above for a group of particles
on a substrate is also evident in the data of Figure 2.21.   Here attach-


-------
 ment  and separation of a single  particle  to the  same  surface  resulted in
 a  distribution of forces.   This  observation supplements  the data presented
 previously by Krupp and by Zimon,  for  a single particle  attached several
 times to the  same surface  and  then separated.  Corn also plotted the
 Larsen equation (2.21)  in  Figure 2.21  for the fiber and  particle sizes
 of his experiment (0.2  < D../D  < 40; 50 < D < 250  urn)  in terms  of  the
                         —  r p —       —  p —
 fraction of adhesion to a  flat plate.   He found  agreement with  the  form
100
                          LO
                 DIAMETER FIBER (Df)/DIAMETER PARTICLE (D )
          Figure 2.21.  Effect of Fiber Size on Adhesion of Quartz
                        Particles to Pyrex Fibers (From Corn, Ref. 74).
 of the function within 10 percent.  It is evident that Larsen's equation
 (.2.21) is of limited usefulness without data on the third (unknown) para-
 meter, i.e., the diameter of the  interfacial liquid geometry.  Under the
 assumption that the pool diameter is small with respect to either the
 fiber or particle diameter such that K « 1, Larsen's equation can be
 satisfactorily approximated for computation by
                                                                    (2.25)

-------
Data calculated for this approximation are  indicated  by  squares  in  Fig-
ure 2.21.  For D  « Dc, the  leading term of the series  expansion
                P     f
(1 - D /D .) provides a reasonable estimate, as shown  in  the calculations.
Larsen's equation represents  essentially the solution to the  two-sphere
problem, but approximates the sphere-cylinder data  produced by Corn.
             Since irregular  shapes are less tractable,  resort to experi-
ment is required.  Data for various surfaces and particles are presented
in Table 2.10  (from Zimon, Ref.  66, p. 111).  The adhesive properties of
flat platelets (graphite, kish,  mica) which may have  many points of con-
tact, approach theoretical values.  These forms are less  frequently en-
countered than irregular, three-dimensional shapes.
             Engineering estimates of the force of  adhesion for many cases
of practical concern at or near  95 percent R.H. can be approximated by:

                  2-sphere; F  = 102 D D /(D.+D )                   (2.26)
                              cl        \. £   \.  £,
                                     2
                Sphere-plane; F  x 10  D                            (2.27)
                               a        p
              Sphere-cylinder; Fa = 102 D (1+D /Df)"1               (2.28)

Reduction in R.H. reduces adhesion as indicated approximately in Equation
1(2.15).  Adhesion for small particles (< 20 ^) may be little affected by
humidity (c.f. Figure 2.19).
             (4)  Effects of Surface Roughness on Particle Adhesion - Lar-
                             3
ger particles (200 < D  <  10  ^im) and most plane surfaces of technical
interest have surface irregularities (asperities) of  the order of a few
microns or less, depending upon manufacturing and finishing processes.   The
effect of surface roughness was studied by Corn and by Bowden and Tabor,
Table 2.11.   Corn's data were obtained with a Stylus  profilometer on optical
flats and glass slides.   Bowden and Table used polishing techniques with
various finishing compounds.  All data indicate a consistent reduction of
the predicted adhesion force as asperity size increases.   A physically
acceptable model (assuming a single contact) would be the force relation-
ship between two spheres with the apparent size of the smaller sphere
approximately equivalent to the size of the asperity  ( h «D ).   Therefore:
                                                             P

-------
                               TABLE  2.10

              ADHESIVE  FORCE OF PARTICLES DETERMINED BY VARIOUS
                    METHODS FOR VARIOUS AIR HUMIDITIES*
Substrate material
Pyrex





Glass


Glass


Plexiglas
Teflon
Brass

Starch


Gold



Particle
Material
Detachment of
particles
Quartz (fused
ends of fila-
ments)

Glass

Centrifuging
Glass
Sand
Coal
Glass
Sand
Coal
Centrifuging
Starch


Gold



•D LJ ni
p
individual

25
36
63
88
400
800
for 7p= 2%
50
50
50
50
50
50
for 7F= 50%
7-9
13-15
18-21
4
5-6
7
8
Vibration method for 7 =
F
Steel
Glass
40-60
Vibration method for 7 =
Steel




Glass




5-10
10-20
20-30

40-60
Fa,
for air
50-60%


0.28
0.3
0.6
0.88
22
30

0.37
0.76
0.55
1.44
0.65
0.90

0.2
0.2
0.2
0.07
0.09
0.1
0.16
2%
1.64
50%
1.33-10"!:
6.12-10";;
2.15-10,
— u
2.13-10
dynes
humidity
90%


0.37
0.55
0.76
1.38
-
-

1.83
0.06
0.94
1.97
1.28
2.85

-
-
-
-
-
-
-

-

-
-
_

-
Note 7  = fraction of particle numbers remaining attached after appli-
cation of indicated force.
*
 From Zimon, Ref.  66.

-------
                                    TABLE 2.11  EFFECTS OF SURFACE ROUGHNESS IN ADHESION
Plate
N>
I
 RMS  Depth   Force,
_  M-m	Dynes     7, Fa
           Pyrex flat 1
           Pyrex flat 2
                        0.21
                        0.29
Glass, 500 Carborundum  0.10
 Paper

Glass, 320 Carborundum  0.40
 Paper

Glass 150 Carborundum   10.0
 Paper
                                   0.27
                                   0.17
                                             7.3
                                             4.7
                       27
                       17
Microscope Slide        0.34      0.15       15

Pyrex flat 3            0.48      0.12       12

Glass, highly polished  0.015     9.2     -100
                                             79
                                             51
                                                       -~ 0
                                                                     h/   xlO
                                                                       up
                                                                       1 + 10 h/Cp    Remarks
0.42


0.58


0.68

0.78

0.006


0.04


0.15
1.42


1.58


1.68

1.78

1.006


1.04



1.15
From Corn, Refs.
72, 74

50 _m quartz
particles

957= RH
                                                                 From Bowden and
                                                                 Tabor

                                                                 Ref 75, cited by
                                                                 Corn

                                                                 Ref 73, 260 um
                                                                 glass particles
             *   From Corn,  Ref.  72,  or calculated from F  = 2?T*    D .
                                                         a       H-O  p

            **   % of F  =.27TJT     D  ,  theoretical value for perfect sphere and plane with capillary condensate.

-------
                                 ,, / h D  \
                               10   hTt  '
                         (2.29)
 i.e., condensation of moisture  at the radius of curvature  is determined
 by  the  size of  the pip.  Using  the same argument as  that  for the  sphere-
 sphere  case,  let D  = pip  (asperity) height and D  the  particle diameter:
                                     f  (1+C
                                           1 D
                             (2.30)
The  ratio  h/D  was multiplied by  100  (arbitrarily,  to  fit data  to  a  curve)
and  plotted vs  the reduction of adhesion  from  2n 7H20 D   as  shown  in
Figure 2.22.  A function  suitable  for  technical estimation of  the effect
of surface roughness at 95% R.H. is obtained from Figure  2.22  as
Ffl = 2« 7
 a       t
D  (1+lOTi/D )
 p          p
                                                                    (2.31)
where 50 < D  < 250 |im, and h is the mean asperity height, 0.01< h <10um.
Further work  is required to improve the  prediction of adhesion on surfaces
encountered in practical devices.  A contributing factor to surface
roughness in  practical systems  is the deposition of  fine fugitive dust
from the ambient atmospheric aerosol and from industrial processes.
    72
Corn   studied the effects of carbon coatings on his quartz particles
(to Pyrex flat).  Typical results of interest are:
   Vacuum evaporated carbon

   Acheson "Dag" colloidal
   graphite, dip and dry
D
P
(itn
36
36
79
51
7Q
R.H.

93
46
93
90
Aft
Measured
Force,
dynes
0.26
0.23
0.03
0.08
n m
2"VDP
dyne
1.5
	
3.3
2.1
__ _
a

17
--
2.4
3.8

l+102h/Dp

_ _
--
2.3
3.0

The average sized particle in the colloidal graphite was reported to be
     72
1 (.im.    The lower adhesion in this instance was attributed by Corn to

-------
                    o BOWDEN 8 TABOR
                    ° CORN, GL. FLATS
                    + CORN, GRAPHITE ON GL.
                    ~ 95% RH
                                         r-4
                       l+l02h/Dp
Figure 2.22.
Effect of  Surface Roughness on Particle Adhesion.
(From Corn, Ref.  72).

-------
surface roughness.  Using the data above, h/D  is approximately 1/79 or
                  2                          p
0.0126 and (1 + 10 h/D ) is of the order of 2.3.  Adhesion was observed
                      P
to be approximately 2.4 percent of that predicted from condensation ex-
pectations.  Selected data from Corn's experiments on surface coatings
which are  indicated on Figure 2.22, tend to support the computational
approximations given above.  The effect of the evaporated carbon coating
on reduction of adhesion may be related to the change in contact angle
for water  on a hydrophobic carbon surface (0 x 86  for graphite, to 180
for carbon black, vs 0  for glass).  '    The evaporated carbon film depth
in Corn's  study was probably at least a few 0.01 ^m.
             Additional experimental data on effects of surface roughness
are presented by Zimon (Ref. 66, p. 93 et seq.) but with only qualitative
estimates  of roughness height, Figure 2.23 a, b and c.  Three conditions
are proposed.  In the first, where the plane and sphere are ideally smooth,
the forces can be calculated from fundamental considerations.  In the
second case, where the substrate contains microscopic asperities generally
smaller than the particle dimension, the adhesion force is decreased.  In
the third  case, the roughness depth is of the order of the particle size
and the particle rests in a trough, leading to increased contact area and
greater adhesion.
             Surface coatings (i.e., paints) and viscous adhesive films
(e.g., oil or adhesive-coated high velocity filter fibers, impactor or
settlement plates for fallout catch) modify the adhesion phenomena des-
cribed above.  Penetration of particles into the viscous film will, in
general, be expected to produce adhesion forces in proportion to the vis-
       68
cosity.    Tacky or oily surfaces provide much larger adhesion forces,
depending  largely on the depth of the film,  and degree of particle pene-
tration.  Condensible components in the gas  may accumulate on fibrous
filter collections, leading to such high adhesion forces that simple
mechanical (shaking) or pneumatic (jet or pulse) cleaning mechanisms,
developed for dry dusts in non-c(ondensible gases,  are unable to provide
sufficient dislodging energy.   This does not imply that  such deposits  are
unfilterable (or uncollectable),  but only that cleaning  techniques com-

-------
             100

             80


          O  60
          w

          |  40


          #  20
                 v*
                ROUOM
                                -•»• SMOOTH
 (a)  Adhesion of spherical glass  particles
      40 + 5 n in diameter as a  function  of
      tho class of finish on cast  iron  surfaces
      for various detaching forces.   (1)  F^et=
      2.2 •  10'2; (2) 9.3 • 10-2;  (3) 22.4  •  10':
                                   dyn.
 (b)  Profilograms of steel surfaces  (magnification:
      horizontal, 1050X; vertical, 2000X).
(c)  Types of substrate roughness associated with
     the adhesion of particles.
  Figure 2.23.
Effects of Surface Roughness on
Particle Adhesion (from Zimon,
Ret". 66)

-------
monly employed arc inadequate to overcome the forces present.  Typical
combustion effluents in which condensation of moisture occurs (incinerator,
boiler plant) are generally not amenable to filtration with present
cloanable industrial filters for this reason.  Similar restrictions apply
to tarry deposits, but in stationary applications  recourse to continually
washed or wetted collecting surfaces can prevent buildup of thick vis-
cous deposits.  Attempts to use fibrous filters or electrical precipitators
on particulates from internal combustion engine exhaust with widely vary-
ing moisture content (cold to hot cycles) have generally been unsuccess-
ful because of condensation problems associated with the capture or dis-
                                                :                       '
posal of the particulate components.  The collection of non-wettable
carbon black particles in electrical precipitators is related to the ad-
hesion and moisture adsorption character of the deposit.  The particles
build up to large, fluffy aggregates and blow-off the collecting plates
(because of the low charge conduction and few adhesion points).  Adhesion
of carbon black aggregates to surfaces should be low. based on the rela-
tively low affinity of the carbon surface for water, (i.e, large contact
angle, > 90 , for the interface carbon-air-water).  These same aggregates
are subsequently recovered satisfactorily in filter collectors with
tightly woven fabrics where adsorbed moisture effects on particulate ac-
cumulation mechanisms are less significant.
             (5)  Adhesion Phenomena in Ensembles of Particle Deposits -
Adhesion phenomena in ensembles of particles are related to the size dis-
tribution of the components in the deposit, to the manner in which the
particles attach to each other and the substrate, and the relative ad-
hesive forces that occur.  Typical deposits of particles on fibers in
the early stages of filtration before formation of a complete cake are
discussed below.  Aggregates of particles build up outward from the fiber
surface and form bridges over interfiber openings.  As deposition con-
tinues, and particularly in repetitive filtration as in cleanable fabric
filters, fibers will presumably become dust coated.   On the limiting case,
extended fabric use with certain polydisperse aerosols may plug the fabric
to the point that effective filtration is no longer  possible.  There are
no reported microscopic observations of the history  of deposition,  aggre-
gate formation, and location of residual dust deposits in fabric filters
operated cyclically.   At the particle-fiber (fabric) level,  the deposit

-------
probably appears approximately  as  indicated  in  Figure  2.24a.   Removal
forces are applied  to  overcome  adhesion  and  clean  the  fabric  by either
vigorously shaking  the  fabric as  in  (b)  or by directing air  flow backward
through the  fabric, as  in  (c).   In either cleaning mode,  fabric flexure
and local deformations  occur, tending  to separate  adjoining  particles and
fibrils.  In the shaking mode,  a  reciprocating  motion  is  produced on the
fabric, causing an  acceleration of the fabric and  dust,  which at the1            __,
maximum displacement is
                              a =  2n2  b  a>2                          (2.32)       __
                              -2
where a = acceleration, cm sec
      6 = displacement, cm
      o> = cycle frequency,  sec

The force applied to the deposit  is

                            F = maxapZ  n D.  /6                    (2.33)       —

             The mass  set  in motion  is illustrated schematically in  Fig-        __
ure 2.24 (b) and is equal  to the  sum of  the masses of  the  individual
particles separated.  Points of prior  attachment are indicated  by small
arrows, each having an  adhesive force  overcome  by  the  inertial  force de-
veloped within the deposit.  The deposit is actually three-dimensional,
                                                                                 —'
but estimates of the mass  removed  for  typical accelerations produced in
shaker-type cleaning can be made  from  the schematic model.  If  particles
were 10 jam, and 10 attachments  were  involved, for  3g acceleration (typical)the   ~"
mass of the aggregate removed would  be approximately (F  £ 0.1  dyne  x 10
             -4                  6                     a
particles) 10   grams or about  10  particles  (i.e., ~  100 (jrn  spatial ex-        _
tent).  When the dust deposit is large in amount and extent,  gravitational
effects will be expected to produce  additional  forces.  The analysis  of
the forces arising on a granular deposit, from  gas  flow backward  through
the layer  as in Figure 2.24 (c), can  be approximated  for estimating
purposes as
                            F % n(3jt D  u V)  C2                    (2.34)

-------
             particle  deposit
                                                  Nindividual fibers
   (a) Schematic^ Particle Aggregate Development  in  Fabric  Filter  Cake
                                                              2     2
                                    	4cr—"I  F     = ma  * m(2x  5 w )
                                       ^—^  max



                                              m  = X KD1/6
                                y = & sin wt
   (b) Model of Shaking Method for Removal of Dust Cake in Fabric Filter
(c) Model of Reverse-air Cleaning Method for Removal of Dust Cake in Fabric

                                                                     Filter


     Figure 2.24 Schematic of Cake Build-Up and Removal in Fabric Filter.




-------
where n = the number of particles  in the  layer and C7  is a cake drag  fac-
                     /-                              ^
tor.  For the same  10  particles of 10 pm diameter as  calculated above,
flow of air backward through the deposit  at the same velocity  (~3  ft/min)
as the forward filtering flow  (after filter flow has been stopped) would
be estimated to produce a  force of the order of one dyne, or sufficient
to overcome adhesion forces at 10  particles.  These estimates  are  intended
to illustrate the type of  calculation that could be made if observations
or data were available on  deposit  geometry and aggregate structure in
fabric filters during deposition and cleaning.  Very few studies have
been made on the size of aggregates removed upon filter cleaning as a func-
tion of deposit geometry or cleaning energy.  None affords sufficient
data for a detailed analysis as suggested above, however.  Those observa-
                                          78                 79        68
tions that are available (e.g. E. Anderson,  Billings  et al.,   Larsen,
     80       81
Corn,    Taub,   indicate  that the material removed from filter surfaces
(by air flow, jet action,  or shock flow)  consists of aggregates of many
individual particles, of substantial aggregate size, and further,  that
aggregate size removed tends to be smaller upon application of greater
                  79
cleaning energies.    These studies suggest the use of a primary filter
screen as a particle agglomerator  prior"to secondary inertial collection,
followed by tertiary filtration.
             Air velocities required to remove single  particles (~20 urn)
from individual filter fibers are generally greater than 10 m/sec  (> 2000
ft/min).  Velocities of this order are used in reverse-jet cleaned
fabric filters, but are not typical of other commercial cleaning methods.
All practical methods utilize fabric flexure as well as an applied force.
The possibility of cleaning fabric filters by means of lair flow directed
along the dust cake (parallel to it) has been suggested in certain com-
mercial cleaning methods, but Zimon (Ref. 66,  p.  308)   indicates that flow
velocities of the order of 20 m/sec are required (but  without citation as
to source of experiments).   The discussion of fabric filter cleaning in
Zimon's comprehensive test is limited to 6 pages  (303  to 309) illustrating
the lack of knowledge on this subject.   Table  2.12 (from Zimon, Ref.  66,
p. 112) illustrates adhesive forces observed experimentally for separation

-------
                           TABLE 2.12




                 ADHESIVE FORCE OF A POWDER LAYER
Substrate Material
Steel



Steel







Steel


Glass




Magnesite




Particle
Material
Glass (spher-
ical particles)


Aluminum
oxide






Lime dust,
ordinary non-
wetting type
Magnesite




Magnesite




Dp, Mm
60-90
40-60
20-30
10-20
324
163
97
81
68
47
35
25
-


200r300
150-200
88-150
75-88
60-75
200-300
150-200
88-150
75-88
60-75
F, dynes 2
(referred to 1 cm )
1.1
21.7
208.0
370.0
37
42
67
90
85
103
143
223
520


39
56
83
116
169
29
40
60
89
103
From Zimon, Ref. 66, p. 112.

-------
of powder  layers of various materials  from  various materials  from  various
substrates.  No filter  fabrics were represented  in Zimon's Table 2.12.
    73
Corn   studied removal  forces for  individual quartz,  particles  from wool
felt  fabric and all-glass filter paper  (used in high-volume samplers) with
results as shown in Figure 2.25.  Cleaning methods in commercial fabric
filters are developed entirely empirically, without regard to fundamental
properties of the particle-fabric  system.
             The coefficient of friction of powdered  materials  sliding
on themselves is an index of particle  internal adhesion.  A technical
measurement of such property is easily  performed.  Typical values  for
                                                 134
several materials are given in Table 2.20a.  Carr   discusses technical
measures of powder adhesion indices by  simple spatula-dip and angle of
response techniques.  The relationship  of these simple tests to actual
adhesion forces at the  particle and aggregate level are not available.
             2.2.2.9  Summary of Particle Properties.- The above discus-
sions indicate that there are two major characteristic groups of prop-
erties of particulates  that affect the  development of a filter  cake on
a fabric and its subsequent removal:
             1.   Particle size, size distribution, structure, density,
                 and the shape (morphology) of aggregates affect col-
                 lection efficiency and pressure drop of the layer.
             2.   Particle charge,  electrical characteristics, adsorptive
                 properties (especially with respect  to ambient water
                 vapor) and adhesion affect the cleanability, in con-
                 junction with properties of the fabric substrate and
                 cleaning mechanisms 'and kinetics.     :
             Electrical charge and adhesion also affect aggregate mor-
phology,  collection efficiency,and pressure drop, as discussed below.
                                     •v
             These aerosol and powder  properties  are not simply measured
in fabric filter systems of practical  interest and little data are
available relating these properties to  filter performance,  i.e,  pressure
drop,  efficiency,  life, and costs.   Further investigations  of particle
properties in relation to filter performance would be required for cost-
benefit analysis.

-------
    «0
                    fill
              • - 96%  KCLATIVC  HUMIDITY
                                                   T« 25'C.
                                                   PS4760MM. HG.
                                                   Df ~ !7Um
                          20       30       40
                        PARTICLE    SIZE-  MICRONS
                  (01)  Adhetton of quarti particle, to 100% wool fell Jabrk.
             10   20   SO   40   SO  60  70  SO  90  KH)
                     PARTICLE   SIZE-  MICRONS
Figure  2.25.
Adhesion of Quartz Particles to Wool  Felt Fabric
and all-glass filter paper  (from Corn,  Ref. 73)

-------
            This treatment of aerosol technology covers only those areas
 judged to be significant for fabric, fibrous, and granular(filtration.
Appendix 2.2 contains references to further sources of information on  the
properties, characteristics, and occurrences of aerosols and the relation-
ships of aerosol technology to contemporary engineering and science.

 2.3   FABRIC FILTRATION  PROCESSES
      2.3.1 Introduction
           The  fabric filtration process is concerned with filtration  of
solid particles  (usually) by fibrous, woven fabric, and granular obstacles
placed in the flow stream.  A fabric is a porous flexible textile material
made by weaving  yarns (twisted fine fiber strands of many individual fila-
ments or staple  long fiber) or by felting a random array of fibers.  The
                                                                     82
appearance of typical woven filter fabrics is shown in Figure 2.26(a)
Yarns are composed of spun staple or continuous monofilament stranded
fibers.  Yarns are visible to the eye, as in any common woven textile
material, and are of order 100 to 500 microns in diameter.   At the rela-
tively low (20x) magnification of Figure 2.26(a), filter fabric appears
quite dense, with reflected light.   Illumination from below with trans-
mitted light indicates a regular pattern of openings between the yarns
(Figure 2.26(a), right-hand side).

            In the case  of spun yarns,  individual  fibrous filaments  (the
 basic textile elements  within the strands) occur within the  interyarn
                                                                   83
 (interstitial)  spaces,  as shown diagramatically  in  Figure 2.26(b).
Similar  projecting filaments, 5 to  10 |j.m diameter,  and 5 to  20  ^m  long
 serve as exterior obstacles for the initial capture of dust materials  in
 the interweave  spaces.  Figure 2.26(c)  illustrates  the appearance of cotton
 (sateen) filter  fabric  at high magnifi-.ation.  The  interstitial  filaments
 (individual cotton fibers) can be seen  projecting between the yarns.   It
 is evident from  the appearance of these woven fabrics that the majority
of gas flow during each filtration must pass between the tightly twisted
yarns, that is,  through the interstitial spaces.  If the optical density
•listributions in the photographs of typical fabrics in Figures  2.26(a)
and (c) are any  index of the local permeability of the fabric,  it would
appear that little flow could pass through the yarns.  Therefore the


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•
                                 SS NJi  f'BEKGLASS IW 3      FIBERGLASS Nfl >   fiPE«OLA$S N»'

                          i1./. •*':<. r.
                     (a}  Comparisons of  structural  characteristics of  fabrics  ^
                         (from Spaite and  Walsh, Ref.  82)
                                       Construction Details of Test Fabrics
Filter Fabric
Air Permeability (cfro/ft2 at U» H,O)
Weight (
-------
Figuri- 2.2b(b) Diagram  of  Plain-weave  Filter  Fabric.  (From Stairmand,
               Ref.  83)

               A.   Hairs,  5  to  10  u diameter
               B.   Main strands, 500 u diameter
Figure 2.26 (c) Typical Sateen-weave Cotton Filter Fabric, 60 x.
                (Engineered Fabrics Corp., Style 1210, plain finish,
                 15-20 permeability, 310 Mullen burst, 9.70 oz./sq. yd.
                 96 x 60 count.)
             Figure 2.26.   Typical  Woven Filter Fabrics  (Continued)

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 process  and  the  progress  of filtration at least  initially,  depends  upon
 the  characteristics  of the  interyarn spaces,  and not  necessarily on yarn
 count  or weave design.
           Fiberglass  yarns,  and  other man-made  continuous  filament  yarns
 probably present  few interstitial fibers  in the  woven form.  Therefore
 they are expected to be poorer  fabrics from the  standpoint  of reduced
 collection efficiency  and greater seepage of  deposits.  Lower flow  rates
 are  required for  satisfactory field  performance.  Deposits  that  build up
 in these fabrics  have  fewer locations  in  the  interstitial spaces  to  build
 bridges  on,  and are  probably more susceptible to pressure drop forces in
                           '                 t                            !
 deposit  collapse  and bleed  (seepage) in the effluent.  The  use of staple
 yarns, or bulked  or  texturized  glass yarns, with many projecting  filament
 ends would be expected  to improve the  collection efficiency and  stability
 of deposit at higher flow velocities.   Effects of interstitial fibers on
 improved dust-holding capacity  and pressure drop should also be  evident.
 Repeated filtration  and cleaning  cycles will  cause a  gradual accumulation
 of residual  deposit  in  the  interstices, between  yarns, and  later  in  the
 interfiber spaces.
           The initial  phase  of the  filtration process begins with the
 capture  of individual particles by single  fibers within the flow  field
 (presumably  in the interyarn  spaces).   Particles  that deposit on  fibers
 projecting into the  flow  then act as additional  obstacles for future
 capture  of particles.   A  deposit  accumulates  on  the individual fibers in
 the  form of  loose chain-like  aggregates projecting into the flow.
           Observations of  particle deposits  during initial stages of
 filtration before formation of a  complete  cake are illustrated in Figures
                  84-88
 2.27 through  2.31.        Table  2.13 summarizes microscopic observations on
 the  structure of  solid  particle deposits on fibers, including data per-
 taining  to certain of the figures  (2.28, 2.29, 2.30, 2.31).
           Figures 2.27(a)  and  (b) indicate the effect of time on particle
deposition and aggregate growth at constant loading (uniform 1.3 urn parti-
         3     3
cles, 10 p/cm ) and constant filtering velocity at the same  site  on
                          84
is« lated untreated tibers.      Structure of aggregates indicates that
deposition occurs primarily on previously deposited particles (filtering


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                              0 minutes
                             60 minutes
                            1.35 minutes


-v^l

Am _

                           220 minutes
                          300 minute;
                          420 minutes
Figure 2.27a.  Deposits of 1.305-micron Polystyrene Latex Spheres on 8.7-micron
Diameter Glass Fiber operated at 13.8 cm/sec at an approximate concentration of
1000 p/cm3.  Aerosol flow into photograph. (From Hillings, Ref. 84).

-------
                         U)  120  minuUs
                         (b)   220 minutes
   Figure 2.27b.  Same as 2.27a, but 9.7 micron diameter fiber and 29 cm/sec.
   (From Billings, Ref. 84).
efficiency  increases).  Higher velocities appear to produce more compact
                                         92
aggregates,  closer to the fiber.  Tomaides   has reported that projecting
aggregates  tend to bend in the flow and become detached at higher velocities,
when aggregate length exceeds about 10 particle diameters.  Figures 2.28
and 2.29(a) and (b) illustrate the formation of aggregate deposits on sub-
                                                                         85
micron glass fibers in filtration of methylene blue (filter test aerosol)   and
                              fifi
metallurgical fumes' (Mg  ZnO).   The tendency for aggregates to form on previously
deposited particles is evident in these figures.  Substantial amounts of
bare fiber remain in each case.  Figure 2.30 illustrates formation of
particle aggregates of oil orange dye (aerosol L-l) and NH.Br (S-l) on

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        Figure  2.28.  Electron micrograph of methylene blue particles
        caught  on  glass  fibres  (from Dorman, Ref 85).
                                                   87
glass fiber at velocities from 1 to 23 ft. per sec    (aerosol flow from
left to right).  Aerosol L-l formed long hairy filaments projecting up-
stream into the flow.  Note that the rear side of the fiber, away from
the flow, is completely bare, a typical observation.  Aerosol S-l formed
short chains and clumps at the lower velocity (75 ft/min) and a denser,
more uniform deposit at higher velocities (600 and 1400 ft/min).   The
morphology structure and density of aggregates formed during filtration
are a function of aerosol material and deposition velocity.  The deposit
structure, which influences the porosity and permeability of the filter
cake, has important consequences in the estimation of fabric filter pres-
sure drop.  As indicated previously, aerosol particles reaching the filter
face may also consist of aggregated structures (typical metal oxide fumes).
Figures 2.31a-d illustrate,  the formation of substantial accumulated de-
                        87
posits on filter fibers.   . These large deposits are presumably the imme-
diate precursors of the more or less continuous filter cake typical of
normal operating conditions.  Pressure drop; observed by Wright et al,  at
the stage of accumulation shown in Figure 2.31 ranged from 2 to 8 times

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~
c
c
Figure 2.29a.  Magnesium Oxide Fume on Glass Fiber Filter Paper
    V = 2.5 cm./sec            Magn x 6500,  (from Cheever,  Ref 86.)
                        Figure 2.29b.  Zinc Oxide Fume on Glass  Fiber  Filter  Paper
                            V= 2.5 cm/sec, Magn x 6500  (from  Cheever,  Ref.  86.)
                                                  2-85

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Aerosol:   L-l
Velocity:  Approximately
           3-10 ft./sec,
                      Aerosol:  S-l
                      Velocity: 1.5  ft./sec.
Aerosol:  S-l
Velocity: 10 ft./sec.
                      Aerosol:  S-l
                      Velocity: 23 ft./sec.
Figure 2.30.
Photomicrographs of Fiber 30G loaded in
observation chamber (Aerosol flow from
left to right).
(from Wright, et. al., Ref. 87)

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                           Upstream face
    Downstream face
-
                                        :   g m
                 Aerosol Flow into Photograph
Aerosol Flow out of Photograph
                     Figure  2.31a.   Photomicrographs  of Pads  Containing Aerosol
                                    Special Low Concentration Run;  30 ft./sec.
                                    (From Wright,  et.al.,  Ref.  87)

 o
                 Aerosol Flow into Photograph

                 Upstream face 1st pad
                 Velocity:  60 ft./sec.
                 Pressure drop ratio: 3
                 Loading:  0.160 cu.ft. aerosol
                           material/cu.ft. fiber
Aerosol  Flow out  of  Photograph

Downstream face 1st  pad
Velocity:   1 ft./sec.
Pressure drop ratio:  8
Loading:   0.130 cu.ft. aerosol
           material/cu.ft.  fiber
                    Figure  2.31b.  Photomicrographs  of  Pads  Containing Aerosol
                                   Various  Velocities and  Loadings.   (From
                                   Wright,  et.al., Ref.  87)

-------
 After loading aL  1  ft./:;rc,
                      Aft:.T clr.m -•) i r at ') ft./so
After clean air at  10  ft./sec
                     Aft
-------
"

                     After  loading at 1 ft./sec,
After clean air at 3 ft./sec,
C
 — •
                      After clean air at 10 ft./sec.        After clean air at 30 ft./sec
                      Figure 2.31d.  Photomicrographs of Pads Containing Aerosol.
                                     Same conditions as (c), but 8 times the initial
                                     load.  (From Wright, et.al., Ref. 87).

C


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                    TABLE 2.13

OBSERVATIONS OF THE STRUCTURE OF SOLID AEROSOL PARTICLE
                 DEPOSITS ON FIBERS
Investigator
Watson (89)


Leers (90)

Wright
et al. (87)




ro


Cheever(86)



Dorman(85)
Radushkevich
and Kolganov(91)
Fiber
Material
(Diameter)
Cellulose
Rubber
Glass
Cellulose-
Asbestos
Glass
(30 urn)

Glass
(30 urn)
Glass
(30 pm)
Glass
(30 p.m)
Glass
(0.5 urn)
Glass
(0.5 um)
Glass
Asbestos
(0.06 pm)
Particle
Material
(Diameter)
Meth. blue
NaCl
Garb. bl.
NaCl

Oil
Orange
(0.3 p-m)
NH.Br
4
(1.2 M-m)
NH Br
(1.2 urn)
NHBr
(1.2 urn)
MgO

ZnO

Meth. blue
Polystyr.
(0.25 urn)
Particle
Shape
Sph.
Cub.
Sph.
Cub.

Cryst.


Cub.

Cub.

Cub.

Cub.

Stellar
Cryst.
Sph.
Sph.

Deposit
Velocity
cm/ sec
_
-
-
10

200


45

300

700

2.5

2.5

-
0.5-25

Structure
Chains and clumps
Chains and clumps
Chains and clumps
Chains

Chains


Short chains

Short clumps

Unif. deposit

Chains and clumps

Needles and clumps

Chains
Chains of 2 or 3

Magnific.
4,700
2,300
10,000
100
(est)
100


100

100

100

19,000

12,000

4,000
-


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the initial clean value for the filter.  Such values for accumulation
pressure drop are common for high velocity roughing filter.'? (ventilation
air cleaning of atmospheric dust) and for high efficiency all-glass filter
media (high efficiency papers required for collecting radioactive aerosols,
biological sterilization of air in hospitals, production of pharmaceuticals,
and in clean-room air supply.  Typical pressure drop values for fabric fil-
ters are:
     Clean, new woven glass fabric ~ 0.06 to 0.02 inches water at 2 ft/min);
(permeability, 15 to 50 cfm/ft2 at 1/2 inch water),
     Residual drag, 4 inches of water at 2 ft/min.
           The pressure drop ratio: (terminal value in service prior to
cleaning/clean, new fabric value) is approximately 200 to 1, and ~100 to 1
after cleaning.   It is evident that the characteristic appearance of a  filter
fabric during service is associated with at least 10 times the amount of dust
shown in Figures  2.31a-d.  A more or less continuous deposit of accumulated
material forms having the appearance of a uniform cake.
           The only photomicrographs of the fabric filter cake formation
                                                        88
process  that have been located are shown in Figure 2.32.    The final de-
posit appears as an undulating layer having surface features reflecting the
underlying structures of the fabric, yarns, and projecting fibers.  The ob-
servations of Figure 2.32 were apparently obtained at relatively low magni-
fication (est. X 10).
           Removal of deposited aggregates was achieved in the study of
             87
Wright et al,   subjecting the system to higher air velocities.  Figures
2.31(c)  and (d) illustrate the effect of progressively higher blow-off
velocities.  Substantial cleaning effects occurred only at velocities of
the order of 10 meters/sec, which agree with data reported by Zimon and
others cited in Section 2.2.2,8(4). Wright tests also included effects of
vibration on deposit removal, and studies of efficiency, pressure drop,
and depth effects.
           Effects of fabric construction on the depth-distribution of
deposit  is considered below with respect to pressure drop.   Felted fabrics,
either wool or man-made fiber needle-punched felts, which consist of random
orientations of individual filaments having no consistent directional char-


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           jpigure 2.32.   Five-ounce cotton cloth,  the lower half
           partially plugged with silica dust.   The unplugged meshes
           in the upper  half average 0.2 to 0.4 mm.   (After Page)
           (From Drinker and Hatch, Ref. 88).

                                  2-92
                  '
                                                                                   -
acteristics, are more nearly typified by construction of the type shown
in Figure 2.31.  Phenomena of deposition illustrated in Figures 2.27 to
2.31 are exactly the processes which occur in felts used in fabric filters.
The actual deposit formed in a fabric filter has not been carefully obser-
ved or reported.
           To recapitulate, the process of initial filtration in a clean
fabric filter occurs through contact of individual particles with single
fibers, then subsequent contact of additional particles with those already
deposited, leading to the formation of long chain-like filamentous aggregates,
or short stubby clumps.  These aggregates continue to grow outward from the
fiber.  They form more or less continuous interconnections along the fiber,
until the fiber is totally covered with a large hairy loose-grained deposit.
Fiber junctions fill in, interfiber volume gradually decreases, and probably
a more or less continuous deposit or cake forms having an outward appearance
at its surface similar to the underlying surface contour.  As dust structures
become larger, fluid flow is constrained to smaller channels, and pressure
effects probably begin to modify, consolidate, and tend to bend, compress,
reorient, or compact the underlying structures.  Pressure drop rises during
accumulation (smaller channels or more obstacles in the flow field) causing
a decrease in total flow below some desired value (as a function of the pres-

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 sure-volume characteristics  of  the  blower  used  and  the  system under ventila-
 tion).   In  describing  the  filtration  processes, 'Starting  first with a  clean
 (unused)  filter  and  continuing  through  the final  step where  the approaching
 aerosol  is  filtered  by a dust cake  or layer,  one  can establish three distinct
 filter regimes.   These have  been  characterized by Borgwardt  and Durham
 as blocking or straining  (n=2),   depth  or  intermediate  filtration  (n = 1.5)
 and  finally cake filtration  (n  =0).  The n term cited parenthetically  for
 each  step is the exponent  for the resistance  change, AP,  in  the following
 differential equation
                         i    d (AP)  =  K  (AP)n                     (2.34a)
                             dV
 where dV represents  the incremental gas volume passing  through filter  and
 K a  dimensional  constant.  If the system flow is  nearly constant,  it is
 seen that the rate of  resistance  rise is most rapid and depends upon the
 instantaneous AP value during the blocking and depth filtration phases.
 Once  the dust cake has developed, however,  the pressure term drops out and
 the  classical cake filtration theory  applies.  A  detailed discussion of the
 latter process is presented  in Section  2.4.   The  accumulated dust  is removed
 from the fabric  by stopping  the flow  and vigorously shaking  the element (or
 by on-line  reverse-air or  jet cleaning, etc.).  Within  the limits  of avail-
 able  observations, dust structures removed are quite large.   However,  under
 usual cleaning circumstances, substantial  amounts of the dust  deposit  re-
 main within the  fabric interstitial (inter-yarn,  inter-fiber)  spaces.  Re-
 sidual pressure  drop after cleaning is  substantially greater  than  the  ori-
 ginal clean fabric value.  Complete cleaning  is possible, but  usually  is
 uneconomical in  terms  of power or time required, or because  of detrimental
 fabric deterioration.
            Upon  resumption of flow, after  cleaning, on the second filtra-
 tion cycle,  the  incoming dust particles presumably see a combination of
 some bare fiber  and a  great deal  of residual  deposited particulate aggre-
 gates.  The  process of  filtration then resumes by particle collection on
 fibers in some locations, and on  previously deposited particles and aggre-
 gates in other locations.  These processes have not been observed at the
microscopic  level, so  the discussion of cake mechanics is in the nature of

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 hypothesis or speculation.  The amount of flow will vary locally (over
 terrain  distances  of  the  order  of  a  few hundred microns)  depending  upon
 rhc  amount of collected dust  that  has  been  removed  by  the cleaning  action.
 Locations  where  large amounts of dust  have  been removed will  permit greater
 initial  flow.  Pin holes  permitting  high  flow  and dust penetration  or  leak-
 age  arc  commonly observed in  the initial  stages of  filter operation with
 nearly new woven fabric.   However, the high flow velocity through the  more
 open areas also  raises the local particle flux (concentration x velocity),
 causing  greater  deposition to occur  and eventually  providing  a self-closing
 tendency for  the more open pores.  Aggregates  again form  around fibers, and
 presumably a  more  or  less continuous dust cake is again formed during  depo-
 sition and accumulation,  as in  the initial  filtration  cycle.
            During  the initial stages of reconstruction of a continuous par-
 ticle  matrix,  efficiency  and  pressure  drop  rise rapidly,  while the  open loca-
 tions  are  accumulating a  deposit.  Filtration  proceeds presumably by the in-
 teraction  of  oncoming particles with those  already  deposited  in granular
 (or  cake)  filtration.  Large  pressure  drop  (or diminished flow) eventually
 becomes  unacceptable  in terms of the system requirements,  flow is stopped,
 and  the  fabric is  again shaken  to remove  a  portion  of  the  accumulated de-
 posit.   Substantial residual  deposited  material remains after cleaning,
usually  more  than  that remaining after  the  first cleaning  cycle.   In typical
 industrial  applications of  fabric filters,  repeated filtration and  cleaning
 cycles occur  over  time intervals of  a  few minutes to a few hours.   After
 some 24  hours  (but possibly as much  as ~100 hours)  the residual deposit
 after cleaning arrives at a more or  less constant value,  as governed by the
 parameters  of the  cleaning method (energy input), j Continued filtration be-
 yond 'this  priming  or  ageing period results in a relatively constant pres-
 sure drop  cycle  during use and after cleaning.   The fabric is essentially
 saturated  or at  equilibrium with the deposited residual material,  such that
 no more  can be added  than is  shaken out upon cleaning.   If an equilibrium
 residual deposit does not occur after many filtration and cleaning cycles,
 residual pressure drop may continue to rise, (slightly less material is
 shaken off at each cleaning than is added during filtration),  causing the
 condition known as blinding of the  fabric.  As  a result,  residual  pressure drop
 is   set by blower and system characteristics.   More cleaning is  then re-


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 quired  to  remove  the  residual  deponit  than  is  furnished by  the  original
 combination  of  cleaning  parameters  in  use.   Fabric  can be shaken more vigo-
 rously  (greater frequency, amplitude,  or duration)  if this  does not damage
 the  fabric.  Reverse  air can be used to augment  shaking, or, depending
 upon length  of  time in service, the fabric  may be replaced, or  laundered,
 to remove  more  of  the residual deposit.  Both  seeping and blinding in fa-
 bric filters are manifestations of the adhesion  between particles and sub-
 strate  fibrous  material  (especially man-made monofilament fibers).  There-
 fore, effective bridging of interyarn  gaps  is  difficult to maintain causing
 material seepage through the fabric during  much  or  all of the filtering cy-
 cle.  Plugging  or  blinding of the fabric is the  inverse problem, where ad-
 hesion  forces are  greater than removal forces  applied by the cleaning method.
           In fabrics that remain in satisfactory service for many cleaning
           4     7
 cycles  (10 ~ 10 ), yarns begin to deteriorate from the mechanical flexure
 and  relative motion of adjacent internal fibers  in  the presence, of dust as
 an abrasive.  Individual filaments break, become shorter, the fabric wears
 thinner at flexure points, and dust penetration  increases in these areas.
 Eventually a thin  spot or tear develops that cannot be repaired by the dust
 buildup during  filtration so that the  fabric no longer has sufficient in-
 tegrity for continued service.   The fabric  is then replaced, or occasionally
 repaired.
     2.3.2 Particle Capture in Fibrous_? Fabric, and Granular Filters
           Particle collection in filtration occurs as a result of one or
more of the following mechanisms:
     1.  inertial deposition as  a consequence of the relative velocity
        between the particle and the fluid as the fluid  streamlines
        separate to pass  an obstacle in the flow field (a fiber, or
        a previously deposited  particle);
     2.  diffusion to surfaces of obstacles as a result of Brownian motion;
     3.  direct  interception or  streamline  contact with a  surface in
        the flow,  arising as  a  consequence  of finite particle size;
     4.  gravitational  sedimentation;
     5.  electrical separation attributable to particle or obstacle
        charge,  and polarization and space charge effects.
Mechanisms (a)  (b) and (c)  are  illustrated in Figure 2.33.

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                      Fluid
                     ..streamline
                       A is trajectory of particle center
                      which  iusl touches stationary object.

                          INERTIAL IMPACTION
                      Fluid
                      streamline
Trajectory of particle center
misses object.
             B is trajectory of parties center and fluid streamline
            Darticle surface touches objectut point of closest approach

                         DIRECT INTERCEPTION
                        C is path of particle center
                   due to fluid motion and random diffusion

                               DIFFUSION
Figure 2.33.   Mechanisms  of Mechanical  Filtration

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           Theories of particle capture by these mechanisms are based on
the following assumptions:
     1. collecting obstacles situated in the flow are sufficiently
        far apart so that the fluid flow in the vicinity of a single
        obstacle can be represented by the flow near an isolated
        obstacle, i.e., flow interference effects from adjacent
        obstacles are neglected;
     2. the particles approaching a surface do not interact with or
        distort the flow to produce additional hydrodynamic lift or
        drag; and
      3. the particles always adhere on contact, i.e., bounce,
         surface migration, and reentrainment are neglected.
           The first assumption is required to define the fluid flow field
approaching the object.  While it is a reasonable assumption for certain very
open fibrous filter geometries, in all cases of interest in operating fabric
filters, deposits of large numbers of adjacent particles present on the sub-
strate completely dominate the flow field.  Collecting obstacles are close
together, and in the case of a deposited layer or cake formation^filtration
is presumed to be by deposition of a particle on a previously deposited
particle present in a large deposit aggregate.
           The assumption regarding particle-surface interaction and hydro-
dynamic lift or drag is always accepted, but recent studies on particles
in laminar shear flow indicate that a lift force may exist.
           The third assumption has been investigated with single obstacles
and test grids of parallel wires exposed to aerosol flow.   Not all particles
adhere on contact, and they may bounce,  roll,  migrate or become reentrained
at some later stage in the filtration cycle.   Larger aggregates formed dur-
ing filtration appear to bend in the flow to seek a more stable configura-
tion,  or pieces of an aggregate may break off  and become reentrained in the
flow to deposit further within the filter bed.   In continuously cleaned
filter fabrics, gradual migration of dust to the outlet side of the fabric
has been observed after many filtration and cleaning cycles.  Particles
deposited at a given velocity may be removed by a higher velocity, as in-
dicated in Figures 2.31(c) and (d).   Effects of adhesion of particles to
surfaces and to each other upon deposition have been discussed above.
For purposes of analysis,  it is possible only  to defino a  surface accommo-


-------
elation coct'l'icicnt, P, related to  the ratio of adhesion  force  to hydrody-
namic force.  Presumably 0 < fi < 1, where fi = 1  is  equivalent  to assumption
(3) and p = 0 implies elastic collisions.  Although f3 t  1  in all cases  for
isolated obstacles, it appears to  be close to one for the  total filter.
           The current theories of filtration have been  extensively deve-
loped over the past 30 years for bare fibers at  the start  of the process
and are briefly summarized in the  following discussion.  Effects of de-
posited material on subsequent performance of filters has  not been studied
to anywhere near the same extent.
     2.3.2.1 Inertial Impaction.- As fluid approaches an immersed obstacle,
elements of the fluid accelerate and diverge to  pass around the object.
A particle suspended in the fluid may not be able immediately to accommo-
date to the local fluid acceleration and a difference in velocity between
fluid and particle may develop.  Inertia tends to maintain the forward
motion of the particle while the diverging fluid tends to drag the particle
aside.   Subsequent motion of the particle is the resultant of the inertial
projection and the fluid drag.   From dimensional considerations, it can be
shown that the solutions to the equation of particle motion depend upon
the impaction group, defined as:                       ;
                             I = 2mV/D f                          (2.35)
where m is the particle mass and f is the resistance of the fluid to the
particle motion per unit of velocity.  For small spherical particles of
diameter D , the fluid resistance can be assumed to be given by Stokes1
approximation, so that:
                            f = 3* u. D /C                        (2.36)
                                    f  p  s
where n,. is the fluid viscosity and C  is the (Cunningham-Millikan) aerosol
       r                             s
particle slip correction factor, (Figure 2.34 indicates the variation of
                                                94 95
C  with the particle Knudsen Number Kn = 2X/D ).  '    The impaction para-
 s                                           p
meter for small spherical particles becomes:
                         I = C  p  D 2 V/9 u. V                   (2.37)
                              s  p  p       r  °
where p  is the particle density, V is the undisturbed stream velocity
       r
approaching the obstacle, and D  is the diameter of the collecting ob-


-------
                                      i      (
                                                                                                (      f
         IO2
              10'
                               PARTICLE DIAMETER, Dp ftm, FOR NTP AIR X

                                    10°                          ICT1
Ni
I
VO
•O
       in
      O
      cr
      2  10'
o:
oc
8
                                                   Cs= SLIP CORRECTION FACTOR

                                                     = ( 1+0.42 Knr' + l.67 Kn

                                                   2 FOR NTP AIR = 0.653 X IO"5 CM
                                       lO'1                          10°

                                     PARTICLE KNUDSEN NUMBER, Kn = 2X/Dp
                                                                                           IO1

-------
 stacle.   Tills parameter also represents the ratio of the distance a small
 particle will travel in a still fluid when projected with an initial velo-
 city of  V to the characteristic dimension of the obstacle, (stopping dis-
 tance/D ).
        o
            Several numerical and empirical solutions have been presented for
 inertial collection of particles by spheres or cylindrical fibers.   Most of
                                                       13 93
 these solutions have been discussed in recent reviews.   '     In general,
 the particle collection efficiency is a function of the impaction parameter
 and the Reynolds number based on obstacle size.
            Numerical solutions for the collection efficiency (i) ) of a
 spherical particle approaching a spherical collector are summarized in
             96
 Figure 2.35.     In the case of filtration through a granular layer  of
 deposited particles of the same size as the approaching aerosol particles
 (say 10 urn) at a filtering velocity of order 2 fpm (1 cm/sec),  (C  =1),
 (p  = 2), the impaction parameter is of order 1.   Impaction efficiency
 increases with an increase in velocity, if all other factors remain the
 same, according to Figure 2.35.  Impaction efficiency for  the collection
 of spherical  particles by cylindrical fibers  and flat  ribbons  are  shown
                97
 in Figure 2.36.
      2.3.2.2  Diffusion.-  The transport of suspended particles to  an object
 in the flow under the combined effects of diffusion and  fluid motion can be
 determined from solutions to the equation of convective diffusion.   From
 dimensional considerations, the solutions can be shown to  be a  function  of
 the Peclet number, defined as:

                             Pe = Do V/Dse                        (2'38)
 where D  is the object diameter, V is the undisturbed stream velocity,
 and D  * is the (Stokes-Einstein) particle diffusion coefficient.   The.
      S t
 Peclet number is the characteristic parameter for the relative  magnitude
 of the effects of convection and diffusion in particle transport.
           Several theoretical solutions have been proposed for  the efficiency
of cylindrical fibers for particles of vanishing size.  They arc of the form:
 * D   = kT C /3* \±, D .
    se       s     f  p

-------
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01 O.I 1-0 10 10
Impaction Parameter /2 =» 1/2 « /JoDDV/18>«fD
Figure 2.36.
Impaction  Efficiency for Spherical  Particles and Various Obstacles
in Potential  Flow,  After Langmuir and  Blodgett (from Perry et  al.
Ref. 97, p. 20-68).

-------
where 1/2 < n < 1.  It is generally accepted by specialists in mass transfer
that n = 2/3 is the correct dependence for diffusion alone.
           The solutions also depend upon the character of the fluid motion
as measured by Reynolds number based upon obstacle size:
                               Re  = D  V/v                         (2.40)
                                 oo                                  '
 where v is the kinematic viscosity of the fluid.  For most filters in
 which diffusion is an important mechanism of removal, Re < 10  .   The
 dependence upon Re is logarithmic, and its influence is slight over the
                -4          -1
 usual range (10   < Re < 10  ).
            Effects of diffusion in fabric filtration of particles through
 a granular layer of previously deposited particles will be of significance
 for particles less than about 1 um.   The analytical problem has not been
 solved specifically for granular media.   Diffusional collection improves
 at reduced filtration velocity for smaller particle sizes, since  the col-
 lection efficiency is inversely proportional to Peclet number.
      2.3.3.3 Direct Interception.- Particle capture arising from  diffusion
 or inertial impaction can be determined  by assuming the particle  is a mathe-
 matical point having the property of random molecular motion or inertia.
 If a particle of finite size passes  near an obstacle as a result  of (a)
 diffusion, or (b)  inertia,  or (c) because of fluid motion alone,  contact
 can occur if the path of the center  of the particle comes within  a distance
                                                  i      |                !
 of one particle radius (a ) of the surface.   The effect of finite particle
 size on capture is called direct interception.   Collection efficiency can
 be shown to be a function of the direct  interception group:
                                  R = D /D                          (2.41)
                                       p   o                              '
      It  is possible to treat the effect  of direct  interception as  a boun-
 dary condition in  the solutions  for  collection  efficiency by  diffusion and
 impaction.   The solutions  then contain the interception group as  an addi-
 tional  parameter.   If the particle passes near  a fiber  surface as  a result

-------
 of fluid motion alone,  fiber efficiency because of direct interception is:
  '                                TI ~ R2                             (2.42)

 for R < 1 and Re < 1.
      The above three mechanisms of filter efficiency have been extensively
 developed and analyzed  for fibrous filters with bare fibers  having no
 prior deposit.  Collection efficiency by the diffusional mechanism decreases
 with increasing particle size,  whereas impaction efficiency tends to increase.
 These effects lead to a minimum in the efficiency-particle size curve for
                            83
 fibrous filters.  Stairmand  has calculated the efficiency of a fabric fil-
 ter at the start of filtration  (using as a model the geometry shown in
 Figure 2.26(b) as a function of particle size,  with results as shown in
 Figure 2.37.   Stairmand1s discussion related to this calculation follows'
      "A method of calculating the collection efficiency of a fabric
      or fiber filter has been given in detail...,  though, as with elec-
      trostatic precipitators, the exercise serves  mainly to help in
      understanding the  mechanism of operation of fiber filters, rather
      than to  provide accurate design data.   A worked example is given
      ..., which show that a normal fiber filter exhibits a grade-
      efficiency curve with a pronounced "dip" at about 0.9 p., where both
      impingement and diffusional efficiencies are  lowest (Figure 2.37).
      The formation of an effective floe, or the use of suitable
      compressed felt in an appropriate design of filter^would obviate
      this difficulty."

      It should be emphasized  that  the Stairmand calculation illustrated
in  Figure  2.37 is based  on  the relationships presented above for a clean
filter at  the  start of filtration.  Operating efficiencies observed in
fabric filters are usually  greater  than 99.9 percent for most particles
of  interest, as will be  discussed below.  The use of Stairmand's curve
to  represent fabric filter  operating efficiency is entirely erroneous and
misleading, and should be discontinued (see for example Figure 20 of Ref.
98, lower right hand corner).  Stairmand1s curve is typical of performance
for a fairly open fibrous filter, and many experiments have shown a dip
in  filter efficiency caused by a crossover from primarily diffusional
separation to impaction  collection.  Most fabrics used for industrial gas
filtration are quite dense and will have a grade-efficiency curve higher
than shown in Figure 2.37.

-------
   Figure 2.37,
             PARTICLE SIZE, microns
Grade-efficiency Curve for Fibre Filter before Particles
Accumulate (from Stairmand, Ref. 83).
     2.3.2.4  Sieving or Straining.- Operating fabric filters contain
extensive deposits of granular particulate material in the form of a more
or less continuous layer.  Filtration is presumed to occur as a consequence
of the flow field between or around granules.  If the grains deposited are
of the same size as the incoming gas-borne dust particles, then the pores
between deposited grains are smaller than the grains themselves.  Particle
removal can then occur by direct interception of a surface of the incoming
particle with the surface of a deposited particle.  This model for filtra-
tion in a granular layer is usually referred to as sieving (or straining)
(particle size > pore size).   Since most dust dispersions of practical con-
cern contain particles distributed over a broad spectrum of sizes, pore
openings also vary widely in size.  Sieving is probably important for the
larger grains (or agglomerates), say greater than 10 or 20 |im.  For the
finer particles found in metallurgical fumes, carbon black, oil combustion
ash, etc., collection is probably achieved by the mechanisms of mechanical
filtration.   Sieving as a mechanism of collection implies an efficiency of
100 percent  for the granular layer, and this is never achieved in a practical

-------
 industrial gas filtration system.   Characteristics of granular packings
 are discussed below.  The difficulty in providing an analytical model for
 filtration in an operating fabric  filter is a consequence of lack of quan-
 titative observations on the characteristics of the deposited layer.
       2.3.2.5  Collection by Electrostatic Mechanisms.- The effects of
 electrostatic charge on particle collection in fibrous filters have been
 considered in several studies.   Electrostatic mechanisms in the capture
 of particles by granular filters have been discussed by Anderson.
 Analytical and experimental studies have not been attempted for the pro-
 cess of fabric filtration.   Measurement of particle and substrate  charge
 distribution and location is difficult and is not usually done as  part of
 the experimental determination  of  fabric filter efficiency.   A number of
 experimental studies have been  performed on the effects of particle
 charge,  object charge,  and impressed fields in filters (see Reference
 numbers  13,  49,  53,  93,  99-112).   In most cases,  substantial improvements
 in collection efficiency result  from the added effects of electrostatic
 charge.   Effects  of  electrostatic  charge on fabric  filter pressure  drop
 are  under  investigation.
       2.3.2.6 Sedimentation.- Sedimentation  of  particles as  a method of
 collection in fabric  filters  is  usually assumed  to  be  negligible.   A gen-
                                                          53  113
 eral  effect  of gravity on filtration has  been proposed as   '
                  t •  (cross  section  of collector  in  vertical  direction)
           g      nVo.(cross  section of  collector  normal  to  flow)
              = G  = Cs p   g D  2/18'Uf  Vo                             (2.43)

where n =  the  number of  aerosol  particles  per unit volume, and V  is  the
 terminal settling velocity of the particles.  For a spherical collector
 in a  fabric  filter deposit, G is the ratio of the terminal settling vel-
ocity to the  local stream velocity.  Table 2.14 indicates the terminal
settling velocity for particles likely to be of interest  in fabric fil-
        13
tration.
      For a 10 |_im particle of unit density (p  = 1) terminal settling velocity
is 0.3 cm/sec.  At a typical filtration velocity of 1 cm/sec (2 fpm), the esti-

-------
                          /     TABLE 2.14
       TERMINAL VELOCITIES AND DIFFUSION COEFFICIENTS OF RIGID SPHERES
         OF UNIT DENSITY IN AIR AT 760 mm Hg PRESSURE AND 20°C*
Diameter
u
0.1

0.2
0.4
1.0
2
4
10
20
40
100
V
cm/sec
8.71 x

2.27 x
6.85 x
3.40 x
1.29 x
5.00 x
3.03 x
1.20
4.71
24.7
10"5
-4
10
IO-4
io-3
io-2
io-2
io-1



D
cm^/sec
6.84

2.02
8.42
2.76
1.28
6.16
2.41



x IO"6
-6
x 10
x 10'7
x IO"7
x IO"7
x 10'8
x 10'8
--
--
--
        *
         From Green and Lane, Ref. 13.

mated collection efficiency  for a granular spherical obstacle in the flow
(10 nm), will be of order 0.3, and substantial separation by this mechanism
would be expected.  In a packed bed of spheres with upward aerosol flow, the
effect of gravity opposes collection, as illustrated in the lower part of
Figure 2.38, whereas collection with downward flow is improved by gravity
(upper part).  Effects of gravity on submicron aerosol filtration through
fiberglass, sand and lead shot (1500 (am) were studied by Thomas and
Yoder    '   '   , with typical results as shown in Figure 2.39.  It is appa-
rent that penetration was reduced (greater efficiency) for downflow, even
for aerosol particles less than 1 urn as used in these studies.  (Experimen-
tal verification of the effects of collection by diffusional and impaction
mechanisms at different particle sizes may also be observed in these data,
maximum penetration  or minimum efficiency occurring in the vicinity of
C.35 (im).  Effects of shear flow on particle motion in the vicinity of a
collector have been cited above.   Effects of gravity on particle flow and
stratification inside a filter bag are discussed below.

-------
                                Descending Particle
                                      U:  Air  velocity
                                      7  :  Particle  settling
                                          velocity

                                      V:  Net  resultant  particle
                                          velocity
                                Ascending  Particle
Figure 2.38.
Deposition of Particles in Ascending and
Descending Streams  (From Thomas and Yoder,
Ref. 114).
        so
       y-90
      ui
      y
        99 *
        99.3
so 	











J r
• ^
?-/«5 cm/Mr v
/n 	 Ij

O-SO 0-JO O-4O O-SO O*O O-?O
PQrtteH ro&us, ^





5 _ 5 •. , s 	
** ^ X ' *
**'


H ssi _ _ _
^ ^
o-ao o-io MX.
Figure 2.39.  Filtration of aerosols through lead shot
(From Thomas and Yoder, Ref. 114)

-------
      2.3.3.7   Other  Collecting  Mechanisms.-  Turbulent  deposition of parti-
 cles  on  obstacles  and  boundary  walls  (ductwork,  flues,  checkers, sampling
 lines) in  dust collection  systems  are  of  particular  importance  in system
 design.  Impaction by  the  randomly varying transverse  fluid velocity is
 the principal  mechanism, and  particle  adhesion,  particle  bounce, and reen-
 trainment  are  the  particular  problems  of  interest.   Collection  of particles
 by this  mechanism  in fabric filters operated at  low  velocity  is  probably
 negligible.  If current experiments on high  velocity (100 to  1000 fpm)
 cleanable  industrial filters  become of importance, recent analyses  of tur-
 bulent deposition  may  be of significance  in  interpretation of the process.
      Thermal forces  occur  on  an aerosol particle in  a temperature gradient
 in hot gases flowing over  cold  walls.  As such,these become important in
 dust  deposits  on heat  exchanger surfaces  in  system design for heat  transfer
 coefficients.   Thermal effects  are not normally  expected  in usual fabric
 filters, but have  been cited  as a  mechanism  in cool  granular  filtration  of
                                             116
 high  temperature gases by  Strauss  and  Thring.
      2.3.3  Measurements of Fabric Filter Collection Efficiency
            Measurements of collection efficiency in operating fabric filters
 are illustrated in Figure  2.40.  Typical values are  presented for inlet  and
 outlet concentrations  in screen and tube  type collectors  (intermittently
 cleaned, woven fabric, field  data), reverse  jet collectors (continuously
 cleaned  felts,  field data) a  pulse jet collector (continuously cleaned
 felt, laboratory results), and  a pulsed woolen fabric type.   Low  efficien-
 cies  reported  are  characteristic of low inlet concentrations with continu-
 ous cleaning,  usually accompanied by poor maintenance or  seeping  dusts.
            Dennis,  et al,    in discussing  their field measurements of
 efficiency in operating fabric filters (open  and  full circle  data in
 Figure 2.40) indicated that "Test results do not necessarily show optimum
or expected performance since many collectors were  poorly maintained or
operated in excess of design capacity."  Such tests reflect  the  degree of
maintenance rather than the inherent  performance  of the fabric filter.   In
no case was there any correlation observed between  efficiency and particle
size.   Dennis,  et al  concluded that "For  collectors   with no  obvious leak-

-------

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°*v
Ox
y
X.OREVERSE JET TYPE
• SCREEN MM) TUBE TVK
	 CONSTANT EFT. LIMES
-x

         K31          IO2          IO3          IO4
        OUTLET DUST LOADING, nricrograms per cu. meter
   O • Dennis,  Johnson,First  & Silverman,
         field tests,USAEC Report  No.NYO-1588(1953)
     X   Caplan & Mason field tests,USAEC  Report
         No.  WASH-149  (1954)

     D   Pulse-jet laboratory data; USAEC  Report
         No.  NYO-4816(1962),resuspended vaporized
       .  amorphous silica  powder
     D  Same;  resusponded fly  ash

     SI Caplan, Collection Efficiency of Reverse Jet Filters
        and Cloth arresters, Report to NAPCA (Sept., 1968),
        CS filter, pearl starch, 15 urn.

     ET Ground limestone, 4/xm.
Figure 2.40.  Operating Fabric  Filter  Efficiency.

-------
 ago the effluent dusts had 98 percent of  their mass represented by  parti-
 cles smaller  than 5 microns."  Collection efficiency in operating fabric
 filters is very dependent upon cleaning' mechanisms and cleaning cycles,
 as will be discussed below.
                                             118
             The data of Whitby  and Lundgren    are of some interest at      '
 this point.   They evaluated a unit-type filter (Torit model 64) upon which
 a dust cake of resuspended fly ash was deposited.  This cake was then tested
 with monodisperse test aerosols, uranine  and methylene blue dyes.
             Collection efficiency for uranine and methylene blue test
 aerosols with the clean fabric and for a  fly ash deposit are shown  in
 Figure 2.41.  Particle size efficiency of the clean cotton sateen fabric
 (at 7 fpm) was essentially as that predicted by Stairmand's analysis.
 Figure 2.37 illustrates the operation of  diffusional and inertial mechanisms.
 After loading with fly ash (N.B.S. Air Filter Test Dust, ~ 15 urn) the parti-
 cle size efficiency was observed to be greater than about 99.5 percent for
 all sizes greater than 0.08 (am (filtering velocity not stated, but  calculated
 to be approximately 5.2 fpm).  The collector was shaken (by foot pedal-operated
 rapping bar)  for 10 and 35 individual cycles.  The major reduction  in pres-
 sure drop and collection efficiency occurred with the initial 10 shakes.
 Collection efficiency at 10 shakes remained greater than 85 percent for all
 sizes.  An additional 25 shaking operations produced relatively less reduc-
 tion in pressure drop and efficiency (>75%).  This series of tests corre-
 sponds to only the first three cycles of operation of a cleanable industrial
 filter (estimated from the data presented in Ref.  118), and illustrates the
 initial steps in the accumulation of a residual dust deposit within the
 fabric.   The Whitby^-Lundgren tests are of value to illustrate start-up
 phenomena in  a practical fabric filter.   However, they probably do not
reflect fabric filter performance after an extended period of operation
                  2
through many (> 10 ) filtering and cleaning cycles, when the fabric has
achieved an equilibrium residual dust deposit.   The test procedure is arti-
ficial because collection of fly ash having few particles below about 1 urn
would not be likely to result in particles below this size in the effluent.

-------
 99.99
  99.9


^99.8
UJ

y  99
U.
u.
uj  98

<

2  95



S  90
UJ
z

z

tr
   80
 . 70
UJ


g]
   60


   50
>  40
i

i  30

   20
10
0.05
                                    *5.2 FPMl
                                    IN WATERJ
                    LJOADED WITH NBS FLY ASH
                   NEW CLEAN UNUSED SATEEN FABRIC
                                                 I
                                                         V&*6.2 FPM

                                                               IN WATER
                          V«5.7 FPM

                    Ap»2.8 IN WATER
                    AFTER 10 SHAKES
                    AFTER 35 SHAKES
                               V«7 FPM

                        Ap«0.75 IN WATER
               O.I                     0.5       1.0

                      PARTICLE SIZE OF TEST AEROSOL, microns
                                                                        50
  Figure  2.41.  Fractional Efficiency of Collector-N.B.S.  Fly Ash Layer on Cotton

               Sateen using Methylene Blue and Uranine Test Aerosols.  (From

               Whitby and Lundgren, Ref. 118).

-------
In other words, for Lho fly ash test aerosol with no particles  bi-low  1  nin,
collection efficiency starts at 90 percent even after 35  shakes  (> 99.8 at
> 2 urn). The efficiency for test aerosols below 1 p.m in a  fabric  filter  de-
posit of fly ash particles substantially greater than 1 micron  is  not a
true reflection of the filter capability.  Similar results on a  0.3 urn
                                       119
test aerosol were reported by Stairmand    from the data  of Skrebowski
          120
and Sutton    as shown in Table 2.15.
                                 TABLE 2.15
            EFFECT OF DEPOSITED DUST ON FILTRATION  EFFICIENCY*
Fabric
Lightweight plain cloth
(synthetic fibre)
Heavy raised-surface cloth
(synthetic fibre)
Heavy raised-surface cloth
(ttatural fibre)
**
Aerosol Efficiency
New clean
cloth
7o
2
24
39
After dust
deposition
%
65
75
82
After
cleaning by
blowback
"L
13
66
69
   * From  Stairmand, Ref.  119.
  ** Test  aerosol 0.3 Mm radioactive particles.

 Particle  size-efficiency  data  for operating fabric filters is discussed
 in Chapter 6.

 2.4  FLOW THROUGH POROUS  MEDIA
      2.4.1  Introduction
             The relationship between the pressure drop across the cloth
 and dust  cake, the velocity through the cloth and cake, the gas viscosity,
 etc.  involving fundamental filtration mechanics, is a basic part of  the
 economics of operating a  fabric filter system.  For practical purposes,
 the pressure-velocity relationship can be expressed as Darcy's law.   Al-
 though more sophisticated relationships can be extracted via fluid mechanics

-------
 theory  through  solutions  of  the  Navier-Stokos  equation,  there  is  little  ap-
 plication  for such  refinement  at the  present time.
             The  principle that pressure drop across a porous bed  is  pro-
 portional  to the flow  through  is basic to intragranular  flows.  Darcy's
        *
 equation   can be written  simply  as
                               Ap_ = W                            <
                               L     K
     which states that the pressure difference across the bed  depends
 on the  bed depth L, on the gas viscosity (if, and on the  permeability of  the
 bed, K, as well  as  on velocity,  V.  All variables are readily-obtained en-
 gineering parameters except  for  permeability K which is  the subject of a
 following  section.
             Note  that V in Equation 2.44, refers to the  superficial gas
 velocity through  the bed  and not  to the velocity through the individual
 pore structures.  V is equal to  the volumetric flow approaching the bed
 divided by the area of the bed i.e., by the cloth area.   V is the apparent
 or superficial velocity through  the bed;  the true velocity averaged across
a single passageway in the bed is greater by the factor  1/6 where e Is the
volumetric porosity of the bed.
            Darcy's law relating pressure drop and overall velocity applies
 to a wide variety of beds and  fluids with only a few restrictions.  The
 flow should be only slightly compressible or not at all,  i.e.  the pressure
                                 I
 drop across  the bed should be but a small percentage of the ambient pres-
 sure level.  The flow should be  steady, that is, there should be no sharp
pulses that could excessively compress the gas.  The viscosity of the gas
mixture should be Newtonian, and the flow rate low enough so that the re-
 sistance to  flow is determined by viscous and not inertial effects.  The
 test for viscous flow is  to compute Reynold's number for a typical depo-
 sited particle or fiber, whichever is larger:

                             Re = Pf V/Do                          (2.45)
 Darcy's equation is usually expressed in differential form.


-------
where (>  and |j  apply to the gas and V equals V/e,the average velocity
past the particle or fiber whose diameter is D .  As  long as the Reynold's
number for particle and fiber are both less than unity the resistance to
flow through the cake and cloth is mainly viscous and Darcy's law applies.
This will be the case in most fabric filtration.  In  the case of high Re
values, another term should be added to Darcy's equation to allow for in-
                                                  2
ertial effects, i.e. pressure loss a function of V  as well as v.
     2.4.2  Permeability of Rigid Media
            The permeability K for use in Darcy's equation is fairly well
understood for rigid porous beds, that is, beds whose passageways do not
bend or collapse during the flow.  (Unfortunately particulate beds and
cloths often do change during the flow and this will be discussed in the
next section).  Permeability is the openness of a material to the trans-
mission of fluid and is defined by Equation (2.44).  It is also experi-
mentally determined by measuring pressure drop across a fixed bed length
and at a given velocity.  Since there is an infinite number of different
dust cakes, cloths, packed towers, petroleum shales, ceramic clays, etc.,
one needs a way of predicting permeability based upon a knowledge of par-
tide sizes, shapes, and arrangements.
            From practical fabric filtration experience,  K's computed
from laboratory and field measurements on different dust  cakes vary by
a factor of 1000 or more with the larger permeabilities usually being
associated with the larger dust particles.  Photomicrographs presented
above suggest that smaller particles often pack together  in feathery
                                !                 I      .                 :
clusters while only the largest particles pack closelyilike marbles.    •
This indicates that there are apparently large differences in the porosity
of dust accumulations and that there are differences in cake structure and
or arrangement of adjacent particles.   These same observations are made in
many other kinds of porous beds.   Analysts have traditionally concentrated
on size,  porosity, and structure in trying to predict permeability.
            Although many permeability theories have been proposed,  none
have been widely accepted, partly because of the  difficulties in predicting
porosity and bed structure.   Perhaps the most widely used theory is  the
Kozeny-Carman equation,  which is sufficiently simple and  reliable to have

-------
 at  least  some utility in most kinds  of  porous  beds.   It  is  derived by ap-
 plying the concept of hydraulic  radius  to a capillary passageway through
 the bed.   The Brinkman equation  for  permeability is  less widely used but
 is  derived specifically for loose  assemblages  of spheres exerting a drag
 on  a flowing fluid.
      2.4.2.1  Kozeny-Carman Theory.- Consider  a  fluid flowing  slowly
 through a tube of diameter d and length L.   From Poiseuille's  law for
                                                                         2
 capillary flow, the pressure drop  across the tube will be Ap/L = nfV'/(d /32)
 where Vx  is the velocity averaged  across the passageway. If the tube is
                                                                 2
 replaced  by wide plates L long and spaced d apart, Ap/L  =» uV'Cd /1 2).
 The hydraulic  radii  m for these two  configurations,  defined  as
                         passageway cross  sectional area             (2.46)
                     m ~ passageway perimeter
                         (which  for  a  straight passageway  is  equal  to:
                         passageway  volume \
                         passageway  surface/
are d/4 and d/2 respectively.  When d is replaced by hydraulic radius in
                                                2        2
these two expressions, the denominators become m /2 and m /3 for the tube
and plates respectively.  By analogy with Darcy's law, Equation 2.44, the
                                                           2        2
permeabilities for these two configurations are precisely m /2 and m /3.
Since the configurations are quite, different in cross section, one might
suspect that almost any passageway configuration might be represented by
     2
K = m /k where k should be of the order of 2.5.
            If one had a material composed of many such passageways,
arranged in parallel, the overall permeability would clearly depend on
the number of passageways, that is, on the relative openness of the material
or the porosity e such that K = m e/k where m is the average hydraulic radius
lor all passageways now defined as
     — _ void volume for all passageways per unit of dust particle vol.
         surface area of all passageways per unit of dust particle vol
Here the denominator is simply S , a standard bed property defined as
                                o
the specific surface of the porous material.   The numerator is the volume
ratio (c/l-c).   Thus finally

-------
                             K =      c                             (2.47)
                                 kS2(l-e)2
                                   o
which is the Kozeny-Carman equation for permeability.   Note that although
simple in concept and form it contains the three ingredients sought from
fabric filtration experience; particle size (S  is closely related to size),
bed porosity c, and arrangement structure (chiefly k, but also S ).  Carman
in a related study determined a shape factor for a number of non-spherical
particles, as a means of predicting S  when particle size is known.  (Ref. 122,
page 334-5).  Just as K absorbed everything that was difficult to measure
when the Darcy equation was written, so now k absorbs the uncertainties
remaining in this expression for K, since e and S  are distinctly defined
and reasonably measurable.  Kozeny believed k should have a value of 2;
          1 O/
Carman   '    derived the value of 5 for k.  As one might expect, experi-
mentally it turns out to depend on the bed structure.  A number of efforts
to predict k by introducing such concepts as tortuosity have not been uni-
versally useful although these efforts may help to predict permeability in
materials of specific type, e.g., packed rings, cylinders perpendicular to
                                  125
the flow, etc.  Happel and Brenner    discuss several models designed to
evaluate k and indicate a significant dependency of k on porosity for
porosities above 0.7.  They say that theoretically k ranges from 1 to 70
for very porous beds depending on the type of particle and the model used.
Table 2.16 and Figure 2.42 illustrate the calculated variation of k with
porosity for spheres and cylinders.   In view of the great variation in fabric
filter dust ':akes it must be said that for present purpose;; k is not well
understood.  The value of 5 is probably a good starting point for making
estimates of permeability in beds of spherical particles.  Although k may
vary from this either way by a factor of 2, anticipating the amount of
variation in bed structures, this uncertainty in k is far smaller than the
range of 1000 in experimentally determined K values.  The Kozeny-Carman
theory cannot apply  to anisotropic materials unless k, c and S  are assigned
values that take direction into consideration.  K is a local property of the
  This derivation of Kozeny-Carman theory follows the important concepts.
  A more rigorous development can be found in  Scheidegger.


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                               TABLE 2.16
      THEORETICAL VALUES OF THE KOZENY CONSTANT (k) FROM DIFFERENT
              CELL MODELS (From Happel and Brenner, Ref. 125).
r-rnciional
Voiil
Volume, .
O.'W
().««)
O.KO
0.70
O.M>
: 0/0
0.4c
Flow
Parallel
to Cylinders
31.10
7.31
5.23
4.42
3.96
3,67
3.44
r-iow
Perpendicular
lo Cylinders
53.83
1 1 .03
7.46
6.19
.V62
5.38
5.28
Flow through
Random
Orientation
of Cylinders
46.25
9.79
6.72
5.60
5.07
4.97
446
Flow through
Assemblages
of Spheres*
71.63
11.34
7.22
5.79
5.11
4.74
4.54
 material which may well vary from point to point, as in a fabric for example.
 Thus in Equation 2.44 it is understood that either K is averaged over the
 entire distance L or else the material is homogeneous.   It is reported that
 K does not apply to high porosity materials presumably  because the theory
 is based on discrete passageways; thus Kozeny-Carman theory should be used
 with particular caution for e values X).7.

      2.4.2.2   Brinkman Theory.- Brinkman  addressed himself  to  the  permeability
 of a swarm of  spheres  not  quite touching  but so close together as  to  affect
 the fluid  drag on one  another.  That  is,  each particle  deforms the  flow field
 around  itself  and the  closer  the  particles are together the sharper the field
 deformation, and the larger the drag.  An array of particles close  together
 form a  more impermeable  front  than the same number of particles widely se-
 parated.   For  example, in  sedimentation,  a cloud of particles fall together
 as  a  large but  extremely low density body, i.e., the cloud falls more slowly
 than  a  single  particle having  density and size similar to the cloud consti-
 tuents  because  the drag is higher.
     By vector calculus Brinkman found the drag on one sphere among a swarm
of spheres to be
                                   /          -  2\
                                                                      (2.48)

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                    ,0
                                     ' Free surface model
                                   (perpendicular to cylinders)
                                 Free surface model
                                 (parallel to cylinders)
                       Sparrow and Loeffler
                       triangular array
                                                irrow and Loeffler
                                              square array
02     04      0.6     0.8
    e, Froctionol void volume
                                                           1.0
                                                                                       J
            Figure 2.42.  Comparison of Theories  for  Flow Relative to
                          Circular Cylinders.  (From Happel and Brenner,
                          Ref.  125).

 The  first term is simply the  Stokes drag on an  isolated sphere, but the
drag  is  increased by two other terms involving particle  radius r_ and the
swarm permeability, K, which was  then solved for as  follows.   Using n the
                                                                O
number of  particles per unit volume equal to (1  - e)/(4/3x  r   ),
the drag on a unit volume of cloud is:
                            n
                                       K
                                            nF,
                                             (2.49)
                                                                         (2.50)
 This expression of  Brinkman's for permeability must obviously be  less
 universal than the  Kozeny-Carman equation  since bed structure is  not
 taken into account.   However, the Brinkman model should apply to  near-
 spherical particles  in very loose, high porosity beds which is most
 convenient since  the Kozeny-Carman model fails at high porosities.  The
 Brinkman theory is  less widely quoted and  appears to have received  less
                       125
 experimental testing.      A comparison of  the  two theories can be made
                                    2-118

-------
   by replacing SQ (the ratio of particle surface to particle  volume)  by  3/r
   (for spheres barely touching)and setting k - 5  in the  Kozeny-Carman
   expression:
                     K-C:     K - rD2 /W3 \                         (2-51)
   Iht? curves are plotted in Figure 2.43.
        .2
 ~    10
CNJ
     10
 3?     o
 2    10
2
ac
ui
a.
UJ
I
UJ
c
       -i
     10
 5
 2

 5
 2
i

 5
 2

 S
 2
     10
                                                   K-C.
                                                               Br
        0.3
                 O4
                   0.5
0.6
0.7
0.8
0.9
1.0
                              POROSITY  (€)
       Figure 2.43.   Permeability-porosity Relationships  for Kozeny-Carman
                     and Brinkman Models.
  We gain confidence from Figure 2.43 in that despite the different approaches
  the two theories predict remarkably similar permeabilities.   The importance
  of porosity is evident; over the range of porosities expected in fabric fil-
  tered dust cakes the cake permeabilities may well vary by a  factor of 100
  due to porosity.
       2.4.2.3  Other Permeability Theories.- As already mentioned a groat
  deal of effort has been spent in trying to unify observations of permeability,

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            121
Scheidegger    discusses  the claims  for many of  these attempts under  the
groupings of empirical correlations, capillaric models  (e.g., Poiseuille's
law), hydraulic radius theories  (e.g., Kozeny-Carman),  drag theories  (e.g.,
Urinkman),  and statistical theories  of passageway arrangement and molecular
kinetics.
     2.4.3  Permeability of Changing Media
            Since K is a point property of the porous bed, the fact that
the filtration dust layer is growing thicker is no limitation to permea-
bility theory nor to Darcy's law.  This theory assumes  that one may state
the structural arrangement adopted by the particles as  they land on the cake
and hence determine the value of the permeability.  This value plus the
slowly changing layer thickness make it possible to determine the slowly
changing relationship between pressure drop and velocity at any time.
            A much more difficult situation arises when the dust structures
formed within the cake collapse because of higher pressures.   The dust at
the immediate surface of  the cake is under no mechanical stress, except
for a small momentum flux from the landing of later particles.  However,
as soon as particles land they exert a drag on the fluid which has to be
supported by a pressure from the particles underneath.   Likewise the under-
neath particles exert a drag on the particles supporting them plus the pres-
sure from all particles which they support.  This accumulation of particle-
to-particle pressure results in a maximum approximately at the bottom of
the cake where the compressive stress on the cake equals the pressure drop
across the dust cake (typically a few inches of water).   This compressive
stress is always higher than thestresses at the surface of the dust cake,
perhaps even including the stress during the initial landing shock.   As a
result some reorientation or compaction deep in the cake is apt to occur.
If it does, the resistance to flow through the collapsed layer rises,  in-
creasing the compressive stress on the cake below this  area.
            The equations governing this process  are as  follows.   The  com-
pressive stress p  at depth x in the cake will be
                                      x
                                                                  (2.52)

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whcro p   is any surface stress associated with the landing particles.
       CO
The pressure gradient comes directly from Darcy's law in differential
form:
                              d£ = - ^                           (2.53)
                              dx      K
where now we include provisions for a dependency of permeability on the
compressive stress:
                              K = K(pc)                            (2.54)
This latter dependency involves the rheology of the dust cake about which
very little is known at present.  One could hypothesize that a typical
dust cake may be fairly plastic as opposed to being elastic or viscous,
that is, it may not collapse at all until a certain threshold of compressive
stress is reached.

            A napped fabric may also exhibit an increasing flow resistance
as the pressure drop across it increases, that is, as the dust cake pres-
sure on the cloth surface increases.  One would surmise that this would be
basically an elastic process, resulting in a slow exponential increase in
pressure drop in addition to any other increases during the filtering cycle.
            Evidence for cloth or cake compression during filtration is seen
                                    1 9fi
from time to time.  J.P. Stevens Co.    plotted the pressure drop across
clean cloths as a function of filter velocity and found approximately ex-
ponential pressure increases in practically every case; velocities ranged
                               127
up to 20 FPM.   Borgwardt, et al    report that the resistance of a cake of
fly ash formed at 2.26 FPM was increased irreversibly about 207, by simply
slowly doubling the flow rate and then reducing it to its original value,
all the while using clean air.  At doubled air flow, the resultant pressure
drop across the cloth and cake was increased 3 to 4 times on again in-
creasing the flow, the resistance didn't change appreciably until the
previous velocity maximum was exceeded whereupon the resistance again began
to slowly increase.  No recovery of permeability was observed.   Earlier in
                                    128
the same laboratory, Stephan, et a\,    found similar phenomena, observing
607o changes in permeability when velocity was approximately doubled;  but
the interesting thing is that the changes appeared to happen in 4 or  5 dis-


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crete  steps as  if  the  collapse were not entirely plastic.   In  the  related
                                                                     129  130
field  of  liquid filtration  cake  collapse  is a well known phenomenon.    '
            Still  further evidence of  cake deformation due  to  excessive
drag is the familiar observation that  once a cake has formed it may  puncture,
especially with fairly coarse cloths and  fine dusts.  This  we  can  conceive
as a collapse of the cake very close to a high-velocity spot in the  cloth.
The result is that  the cake in that vicinity does not reform but is  pulled
right  through the  cloth.
            From observations of fabric filter equipment in use it seems
that there is too  often little regard  for excursions in pressure and
velocity  as long as they are brief.  Closing off one compartment raises  the drag
suddenly  on the dust cakes  in adjacent compartments; fan blades create
pressure  shocks considerable distances away.  Some cloths are  deliberately
cleaned by pressure pulses.  There is nevertheless some preliminary  evi-
dence  that 1070  or  more of the filtering pressure may be saved  in certain
cases  by  carefully preventing unnecessary collapse of the dust cake.
     2.4.4  Permeability in Non-uniform Beds
            To  obtain  overall permeability K of a cloth-dust cake combina-
tion in which the  local  permeability K  varies from point to point through
                                      X
the bed,  one must  in effect  add  the resistances, or the reciprocals of per-
meability, for  each successive layer in the bed.  From Equations 2.44, 2.52,
and 2.53, neglecting p   ;

                     K = LMfV = Lu V j       1	 i             (2.55)
                          Ap
or
                            C,  ^
                                                                   (2.56)
While a theoretical treatment may be able to express K  as a function of
x through the dust cake and cloth, more often K  will be estimated for
                                               JC
two or more layers of the laminate.  In this case

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                                                                    (2.57)
     2.4.5  Resistance vs Permeability
            Theoreticians customarily speak in terms of the permeability
of a porous bed rather than its reciprocal concept resistance.  On the other
hand, in fabric filtration practice the standard term is specific resistance
which has the commonly accepted units of inche;
The two are related by the following identity:
                                                                      2
which has the commonly accepted units of inches of water per FPM-lb/ft .
Specific resistance = _1	  /^W_i_ V*f W 1 V  "f   \  (2-58)
                      (permeability) \  W / \PermAp  / \PermMp (l-e)j
                                     >    '  \    r \ C '  \   I \ P     /
where p. is the gas viscosity, L is the cake or cloth thickness, W is the
cake or cloth weight per unit area of cloth, p  is  the apparent density
of the cake or cloth, p  is the density of the individual particles or
                       P
fibers, and e is the porosity.
            In this middle ground between theory and practice an example
                                                              _Q   O
of a conversion may be helpful.  If the permeability is  2 x 10   cm ,
viscosity is 2 x 10   poise, and p  is 30 Ibs/ft ,  then  the specific
                                  c
resistance can be computed as follows:
                                              -11    2
     1.  Convert the permeability to 2.09 x 10    ft
                                           -9          2
     2.  Convert the viscosity to 6.05 x 10   Ib min/ft
                                 3
     3.  Use density as 30 Ibs/ft
     4.  Compute Sp. Res. = 11.1/lb
                                 ft
         Since 1 J.b  equals 0.193 inch of water,
                 ft2
         the Sp. Res. = 2.14 inches of water
                        FPM - lb/ft2

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      2.4.6  Characteristic Geometric Properties of Porous Media
             Three types of porous material are of importance  in the
 analysis  of pressure drop of fabric filters:  woven and  felted  fibrous
 materials,and granular porous media cake (deposited particles).   The
 most  usual configuration is a textile fabric woven of multifilament
 or  spun staple yarns of natural (cotton, wool) or man-made fiber  (glass,
      R        R              R
 Orion ,  Dacron ,  nylon, Nomex , etc).  Typical plain, twill,  and  sateen
 weaves are illustrated schematically in Figure 2.44.  Basic appearance
 of  representative fabrics has been indicated previously  in Figure 2.26a,
 b,  c.
                                 .nr^cr.,    r
                                             :"aTi
                                            -TLnj-yru-v-i.-i
                                         i"  ~j"*J ,-j H  *--~- K

                                       jfc O J5 ^f^ ";^
                                       ^-r-3 'ixj .;; b.T:' bur 5
Warp
Direction
             Figure  2.44.   Typical  Filter Cloth Weaves,
            Filter  fabrics  woven with spun yarns will have many fibrils pro-
jecting into the  interyarn  spaces,so that the flow geometry in a new fabric
will be through a semi-fibrous  aperture.   Aperture size depends upon
weave design, yarn diameter, and thread count.   Characteristics of the
flow through the aperture will  depend upon the  quantity of interstitial
fibrils (interstitial packing density), their sizes,  the flow geometry
(weave dependent), and the  degree  to which flow occurs  between the

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 yarns  (within the aperture)  and through the tightly twisted fibers of
 the yarns themselves.   Fabrics with no interyarn fibrils are generally
 poorer media for gas filtration because of the inability of the dust to
 form bridges over the  pores.   A typical research model for this flow
 geometry is  a woven monofilament fabric or a woven wire screen, in which
 dust builds  up on the  wires  and then sloughs off as larger aggregates
 (depending upon weave  tightness).   (Sieves,  screens,  wire gauzes,  punched
 perforated metals,  or  sheets  or grids of metal plates are not usually
 employed in  gas filtration of fine  particles,  although they are widely
 used for size-separation of  powdered or granular materials.   They  are
 occasionally used as stilling screens for turbulence  reduction, or for
 subsequent gas flow profile modifications or control,  in fluid mechanics
 studies  and  as straighteners  in pipe flow or in electrostatic precipita-
 tors after inlet ductwork elbows to cause a  duct to flow full or with a
 more uniform velocity  profile.)
             Napped  woven fabric,  such as  cotton flannel is  produced by
 mechanically abrading  the surface of a woven fabric to separate and
 withdraw fibrils  of the  individual  yarns,  producing a soft,  furry  sur-
 face of  many fibers (of  order  1/8 inch long) projecting into the up-
 stream gas flow.  Napped fabric  provides  a greater  amount of fine  fiber
 for  better capture  of  dust particles.   The (permanent)  residual deposit
 maybe greater  in this  type of  fabric  with  certain methods of cleaning,
 which may  not  be  able  to separate the  interfiber deposits from  the
 fabric.  Napped  configurations such  as  cotton flannel  or napped  sateen
weave cotton are  intermediate  in filtration characteristics  between
woven and  felted  textile  fabrics.
             Natural wool  felt  and needle-punched felts  of man-made
 fibers (a barbed needle  is inserted and withdrawn from  a loose woven
 filament scritn to separate and withdraw longer fibrils) are  other common
 types of media used in dusty gas filtration.  Felt appears microscopically
as a dense mass of  randomly intertwined fibers, and is  quite similar
in many respects to fiber filters of paper, metal wools, slag or mineral

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 wool, glass batts, paper, and other non-oriented long filamentous fibrous
 media.  Felts require pneumatic pulse or reverse-jet cleaning for satis-
 factory pressure drop regulation and control.  They cannot be cleaned by
 mechanical shaking.  Their basic efficiency for particle capture is high.
             To be able to predict the pressure drop during operation of
 a fabric filter on a given dust, the filtration engineer requires some
 knowledge of several properties of the dust and fabric which are essen-
 tially physical geometric quantities.  (In addition, other characteris-
 tics of the dust-fabric system may be required such as, electrostatic
 charge properties, adhesion, humidity or condensation effects,  tempera-
 ture, etc, and the specific actions of the components of the dust
 collector associated with periodic cleaning.)  In practice,  the dust-fabric
 properties are estimated by the filter design engineer based on his
 experience with a previous application on the same or a similar dust.
 There is no substitute for this experience,  at present,  in the  develop-
 ment of the understanding of the mechanics of fabric filtration.
             The intrinsic or Darcy permeability of an homogeneous iso-
 tropic porous material in slow viscous flow (Re  <10) is a property
 defined by the Darcy equation (2.44)  as:
                 L uf V
             K = -ST-
 with dimension cnr (c.g.s),  where Ap  = pf g £h,  and h = cm fluid  flowing.
 Permeability (K)  depends upon the following geometric properties  of  the
 porous material:
             1)   Porosity,  void fraction
             2)   Specific surface
             3)   Pore size  distribution
             4)   Particle (grain) size
             5)   Pore structure
             6)   Shape factor
             7)   Granule  surface  roughness
 and  is presumably independent of the  fluid  flowing.
             Table 2.17 presents  a summary of  these  geometric properties
with methods  for measurement.  Typical permeability values for various

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                                 TABLE 2.17

            MEASUREMENTS OF CHARACTERISTIC GEOMETRICAL PROPERTIES
                                OF POROUS MEDIA
     Characteristic Property
            Remarks
 A.   Porosity
     1.   Direct Method
     2.   Optical Methods
     3.   Density Methods
     4.   Gas  Expansion Methods



     5.   Other

B.   Specific Surface

     1.   Optical Methods
    2.  Adsorption

    3.  Fluid Flow Methods
    4.  Other Methods
C.  Pore Size Distribution

    1.  Mercury Intrusion
    2.  Surface Adsorption
    3.  Capillary Condensation

    4.  X-ray Scattering
 (See Table 2.18 for typical values)

 Measure bulk volume of porous material
   and compact to remove all voids (for
   soft materials only, e.g., organic
   materials)

 Microscopic measurement of plane porosity
   of a random section, photograph,
   planimeter; also statistical
 e =»
     1 - a - 1 -
p /p  where p  = bulk
 a  p        a
   (packing)  density of a known
   volume  of  the  porous material ,  pp  •
   true  density of  the material, a «•
   fraction of  solids

 Direct  measurement of gas  contained  in
   pore  spaces  of known volume  of  porous
   medium, by evacuation  to lower  pressure

 Resistance to  flow using Kozeny-Carman model

 (See Table 2.22  for typical values)

Microscopic  measurement  of circumference
   of pores to  total area of random plane
   section, photomicrograph, planimeter;
   also  statistical

Adsorbed gases or vapors by Brumauer,
   Emmett, Teller (BET) method
Resistance to  flow  using Kozeny-Carman model
Thermal conduction, fluidization,  ionic
   adsorption, chemical reaction rate, heat
   of wetting,  isotopic exchange
Capillary pressure •» pore size

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D.
    5.  Optical Method

    6.  Other Methods
Particle (Grain) Size

1.  Sieve Method
    2.  Microscopic Method
    3.
    Centrifugal Aerosol
      Spectrometers
                         Table 2.17 (cont)

                               Microscopic, photograph

                               Surface adsorption, capillary condensation,
                                 x-ray scattering, sequential crushing
                                 and porosity measurement.
Serial sieving of sample of powder, average
  diameter retained on one sieve a yd^d-
  where d^ = sieve opening on retained screen,
  &2 = sieve opening on screen passed just
  above.  Suitable for granular materials
  down to •••325 woven mesh (44 Mm); for <44 uin
  use electoformed micromesh sieves down to
  < 5 Mm
Microscope, photograph, can use light optical
  for .*•! Mm < Dp < 44 Mm, electron microscope
  to ~0.005 Mm; sampling and specimen pre-
  paration technique important

Inertial classifier, Bahco  for > 5 Mm,
  starts with powder, disperses in air,
  subjects to centrifugal winnowing action,
  others  available for finer sizes, small
  powder volumes.
    4.
    5.
    Gravitational Spectrometer Gravitational classifier, Timbre11
    Other Methods              Sedimentation or elutriation in air or
                                 liquids, cascade impactors for segre-
                                 gation of air-borne cloud particles,
                                 light scatter instruments, etc.  (see
                                 Figure 2.2)
E.  Pore Structure

    1.  Theoretical Packings of
          Uniform Spheres
    2.  Packing of Natural
          Materials
T.  Shape Factors

G.  Granule Surface Roughness
                               e = 0.0931,  (hexagonal lattice)
                               e = 0.259 (rhombohedral or face-centered
                                    cubics  densest stable packing)  to
                               e = 0.875
                               Pore Size ~ grain size; shape,  non-uniform
                                 sizes present fill interstitial spaces,
                                 angularity promotes bridging
  Adapted from Scheidegger, Ref. 121.

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   granular and fibrous materials are indicated in Table 2.18  from measures
                             121
   of flow and pressure drop.     These values are of little value for
   analytical generalization without additional information on the geometry
   of the flow system.  Measured and calculated permeabilities specific
   to the requirements of porous media filtration of dust and fumes are
   treated below.
               2.4.6.1  Porosity.-  Consider a homogeneous isotropic filter
   of packing (bulk) density pfe (gm/cm ) composed of granular (or fibrous)
   material of density pm (gm/cm3).   The fraction of solids in the filter
   is defined as
               0!
Pb/Pm
   and the porosity,  or void fraction,  as
               c  » 1  - a
                                                                    (2.59)
                                                 (2.60)
                              TABLE 2.18
         TYPICAL VALUES OF PERMEABILITY FOR VARIOUS SUBSTANCES^
    Substance
         Permeability  range
        (Permeability  in  cm
Literature Reference
Berl saddles
Wire crimps
Black slate powder
Silica powder
Sand (loose beds)
Soils
Sandstone ("oil sand")
Limestone, dolomite
Brick
Bituminous concrete
Leather
Cork board
Hair felt
Fibre glass
Cigarette
Agar-Agar
1.3 x 1CT3 -3.9 x 10"3
3.8 x 10'5 -1.0 x 10"4
4.9 x 10"10-1.2 x lO'9
1.3 x 10-10-5.1 x 10-10
2.0 x 10-7 -1.8 x 10-6
2.9 x 10-9 -1.4 x 10'7
5.0 x 10-12-3.0 x 10-8
2.0 x 10'u-4.5 x 10'10
4.8 x 10-U-2.2 x 10-9
1.0 x KT9 -2.3 x 10-8
9.5 x 10-10-1.2 x 10'9
3«3 x 10-6 -1.5 x lO-5
8.3 x 10~6 -1.2 x 10"5
2.4 x 10=7 _5.i x io-7
1.1 x lO-5
2.0 x 10-10-4.4 x 10"9
Carman, 1938
Carman, 1938
Carman, 1938
Carman, 1938
Carman, 1938
Aronovici and Donnan, 1946
Muskat, 1937
Locke and Bliss, 1950
Stull and Johnson, 1940
McLaughlin and Goetz, 1956
Mitton, 1945
Brown and Bolt, 1942
Brown and Bolt, 1948
Wiggins et al., 1939
Brown and Bolt, 1942
Pallmann and Deuel, 1945
From Scheidegger, Ref. 121.

-------
             A typical value of clean porosity for a wool  felt  swatch
 (17.2 cm long x 12.4 cm wide x 0.25 cm thick) weighing 11.1017 gms
 (p  m 1.4 gm/cm3 at 75% R.H.) is thus e»l-a = l- (0.208/1.4) =•
   m
 1  - 0.148 = 0.842. This  is  a  reasonably  typical  value for porous fibrous
 media (e > 0.8).
            Since woven fabric is  not  homogeneous,  because of the presence
 of yarns,  interstitial hairy  voids, and  non-planar geometry, porosity
 is !not obtainable  from a simple  geometric model  as above for felts.
 Using a plain-weave wire screen  as  a  reasonably  well-behaved geometrical
                             131
 model of a fabric, Robertson     reported porosity  values (% open area)
 of the order of 50%  (range  25 to  65%)  for wire diameters in the range of
 75 jam to 0.2 cm.   Open area is defined  (for a square  or  plain weave) as:
                              (1  -  td)   per  square inch
(2.60)
 where t = threads per  inch and d = yarn diameter.   In a subsequent study
 of 45 textile fabrics, porosity  (% open area)  ranged  from 0.2% to 40.8%
                                          ''2
 for fabric weights from 13.8 to 2.5 oz/yd  ,  respectively (air flow
                                                                           f
 permeability at 1/2 in. water ranged  from 23.5 to  greater than 700 cfm/ft41
 for these same fabrics).  Figure 2.45 presents Robertson's data on air
                                                                          132
                    IOCOL-
                  E   \-   	'
                  tj   (_  AJz-05'XO
                  t iooj_
                    10
                            'will.
                      O.I
                                 I.O          IO
                                 OPELN AREA '/.
                                                     iOO
        Figure 2.45.  Air Flow Permeability vs Fabric Open Area
        (at 0.5 water pressure differential across fabric) (after
        Robertsonl31).
*                                                           7
 Style No. 1753; Gurley air flow  pi'.rmeability  25-30  (cfm/ft   at  1/2 in.  water
 pressure drop; 255 psi Mullin-Burst;  16-17 oz/yd2;  plain,  Engineered Fabrics
 Corporation.

-------
 flow permeability and open area (porosity) for four major weave designs
 (of bright,  continuous filament viscose rayon).   For a typical sateen
                                          .fc-jfc
 weave cotton fabric shown in Figure 2.26C  ,  with permeability 15-20
       2
 cfm/ft ,  the porosity is approximately 0.5% (using "other" curve, Fig-
 ure 2.45).   Further discussion of fabric weave design, material charac-
 teristics,  and air flow is given below and in Chapter 4.
             The porosity of granular powders  can be obtained from
 (e  = 1 -  a)  the bulk density (weight of a specified volume of the powder)
 and true  density of the material.   Typical values of interest are presented
               121      133         134
 in  Tables 2.19   ,  2.20    and 2.21   .   Porosity of usual granular
 packings  (e.g.  sand)  is  of order  40 to 50%.   The actual  porosities of
 dust deposits  on operating fabric  filters have never been measured.
 Based on  dust  structures presented in Figures  2.27 through 2.31 above,
 and the agglomerate particle densities of aggregated fine particles
 given in  Tables 2.3 and 2.4,  porosity of the  dust deposit on fabric
 filters will probably range from greater than  0.9 to of order 0.4,
 depending on particle size,  pressure drop,  drag  forces on the aggregates,
 deposition velocity,  and effects of adhesion,  humidity and electrostatic
 forces.
                                                                             135
             The effect of particle size  on porosity is shown in Figures 2.46
        1 Q/L
 and 2.47   .   (Also see effect of  porosity on  permeability,  Figure  2.42.)
 Particles below about 30 \ua tend to form much  bulkier  aggregated  structures,
 due presumably  to the varying interrelationships  of adhesion forces  and
 particle mass.   Search limitations  have  prevented development of  more
 of  these  data  from  the powder literature.   It  is  easily obtained  in
 practice  from measures of bulk density (true density assumed  known or
 easily  obtainable by  pyncnometer)  and  particle size.   No  data were found
 for  dusts on actual  fabric  filters  in  service.

            Additional  data and  information on packing  porosity is
presented below  in discussions on pore structure.
 **
  :                                                o
  Style No,  1210;  310 psi Mullen-Burst;  9.7 oz/yd^;  96 x 60;  sateen;
  plain finish;  Engineered Fabrics Corporation.

-------
             2.4.6.2  Specific Surface.- The Carman specific surface of

 a granular porous medium (or other configuration of the flow field) is

 defined as the surface area exposed to the fluid per unit volume of

 solid (not porous) material.

                surface area of solids in a given total volume
           S  =
            o
volume of solids in a given total volume
                surface area of one particle
                  volume of one particle
                            A
                            _E
                            V
                             P
                                                   (2.61)
             (2.61a)
                                TABLE 2.19
                    REPRESENTATIVE VALUES OF POROSITY FOR
                           VARIOUS SUBSTANCES*
          Substance
              Porosity  range
             (Porosity  in %)
Literature reference
Berl saddles                         68-83
Raschig rings                        56^65
Wire crimps  (5 ~)                    68-76
Black slate  powder                   57-66
Silica powder                        37-49
Silica grains  (grains only)          65.4
Catalyst  (Fischer-Tropsch,           44.8
  granules only)
Spherical packings, well shaken      36-43
Sand                                 37-50
Granular crushed rock                44-45
Soil                                 43-54
Sandstone ("oil sand")               8-38
Limestone, dolomite                  4-10
Coal                                 2-12
Brick                                12-34
Concrete (ordinary mixes)            2r7 -
Leather                              56-59
Fibre glass                         88-93
Cigarette filters                    17-49
Hot-compacted copper powder          9-34
                               Carman,  1938  (1 < D   < 5  cm)
                               Ballard  and Piret, 1950
                               Carman,  1938  (Dp  328  urn x 0.6  cm)
                               Carman,  1938  (2 < Dp< 80  urn)
                               Carman,  1938  (2 < Dp <80  urn)
                               Shapiro  and Kolthoff,  19413
                               Brotz and Spengler, 1950

                               Bernard  and Wilhelm,  1950
                               Carman,  1938  (Dp  - 200  Mm)
                               Bernard  and Wilhelm,  1950
                               Peerlkarnp, 1948
                               Muskat,  1937
                               Locke and Bliss,  1950
                               Bond et  al., 1950
                               Stull and Johnson, 1940
                               Verbeck, 1947
                               Mitton,  1945 .
                               Wiggins et al, 1939
                               Corte, 1955
                               Arthur, 1956
 From Scheidegger, Ref. 121

-------
                               TABLE 2.20

           APPARENT DENSITY AND POROSITY FOR SOME INDUSTRIAL

                   DUSTS COLLECTED IN FABRIC FILTERS**

Alfalfa aval
Aluai
Aliaaina
AluBlnuB
AeaKmluei chloride, uln.
Antlanny
Aabaetoe, ehred
Aahee, Hard Coal
Aahee. Soft Coal
Aaphalt, cruahed
AjBBoniuai eulfate
Bagaeee
Bakente, powdered
••king powder
Bauxite, cruahad
Beana, Mai ace.
Bantonlta
Bicarbonate of .Soda
BOaOMal
Bonai, ground, ntnui 1/8"
Boneblack
Bonechar
Borax, powdered
Bran
Braaa
Breweri grain, epant, dry
Stick
Calclua Carbide
Calcluai Carbonate
Carbon, aanrph., graph.
Carbon black, channal
Carbon black, furnace
Carborundum
Caaain
Cait Iron
Caul tic Soda
Calluloaa
Caaant, port land
CeMnt , cllnkar
Chalk, atnua 100 Mali
Charcoal
Ctnderi, Coal
Clay, dry
Coal, bltualnoua
Coal, anthracite
Coca, powdarad
Coconut , ahraddad
Coffee
Coke, bltualnoua
Coke, petroleum
Copra (dried coconut)
Cork, fine ground
Corn, cracked, ate.
Coraaaal
Gullet (broken giant)
Dlcalcluai phoapbatn
Doloailta
Ibonlte, cruahad
B(g powder
Ipaoai aalta
raldapar
rarroue aulphata
riah Mat
Hour
Flua duat, dry
Fluorepar
My aab
Pullara earth
Oalatlna, granulatnd
Olaae batch
SW
34
103
2)0
IM«.
94*
414
153
31
43
87
113
20
100*
80
158
82*
110*
137
75
100*
65
80
109
35
530
65
118
137
169
260
15
15
250
80
450
88
94
100
131
143
25
46
85*
87*
100*
70*
45*
48
83
110
45
30
70
BO
140*
144*
181
72
35*
162*
160
118
80*
50
235
200
85
»5*
MI
162*
BW
17
55
60
160
52
417
23
35
43
45
45
8
40
41
80
41
51
41
55
50
23
40
53
15
165
28
135
80
147
130
5
S
195
36
200
40*
80*
80
78
73
21
43
63*
50
55
35
22
25
30
<>0
22
15
50
40
100
43
100
59
16
45
70
60
40*
35
117
82
40
47
32
95
(
.51
.47
.76
-0-
.45
-0-
.84
-0-
-0-
.48
.60
.60
.60
.49
.50
.50
.54
.30
.27
.50
.65
.50
.51
.57
.69
.57
-0-
.56
.13
.50
.67
.67
.22
.55
.55
.55
.17
.20
.'0
.49
.16
.06
.26
.43
.45
.50
.51
.48
.64
.64
.51
.50
.28
.50
.28
.70
.45
.82
.45
.72
.56
.50
.50
.30
.50
.59
.53
.50
.50
.41
COM
N
N
VA
H
M
VA
H
VA
VA
VA
N
M
M
M
VA
N
VA
H
M
N
M
M
VA
N
M
H
A
A
A
M
M
H
VA
M
VA
H
M
VA-
VA
A
N
A
A
A
A
N
N
N
A
A
NCor
M
N
H
A
M
A
"l
H
H
A
A
M
M
M
A
VA
A
M
A

Clue, ground
Gluten Mai
Oralna, dUtlllery, dry
Graphite
Cypauai
IlMnlte ora
Iron ox Ida
Lead
Lead artenate
Lead oxlda
Lignite
Llaw, ground
Line , , hydratad
LlMttone
Litharge
Magnetite
Magnet luai
Hagnaaluni ehlorlda
Malt, dry
Manganaaa aulphata
Maple, bard
Marble
Marl
Mica, ground
Milk, dried, powdarad
Monal »et«l
Muriate of potaah
Kaphthalene flakaa
Oak
Oxatlc acid cryatala
Phoaphata rock
Phoiphata land
Porcelain
Quartl
Raaln
Rubber , ground
Rubber , hard
Rubber, aoft
Salt, rock
Salt, dry, coaree
Salt, dry, pulverised
Saltpeter
Sand
Sandltone
Sawduet
Shale, cruahad
Slag, furnace, granulated
Slate
Soap, chlpa, flakaa
Soap powder
Soapetona talc
Soap aah, light
Soda aah, heavy
Sodluai Nitrate
Sodluai Fhoiphata
Soybaana , Mai
Starch
Steal
Steal cblpa, cruahad
Sugar
Sulphur
Talc
Tanbark, ground
Tin
Tltanluai
Tobacco
Vanalcullta ora
White laad
Zinc Oxide

8V
M
80*
70*
132
145
312*
330
710
400
567
85
87
81
163
560
187
109
138
44
125
4
168
120
175
70
554
160
71
47
104
160
190
150
165
67
72
74
69
136
138
140*
138
150
144
35
175
132
172
30
50
175
74
134
134
94
90
96
447
417
10)
126
169
110
457
280
M
160
120
3*0

BH
Ml
40
30
40
75
140
100*
710
72*
180*
SO*
60
40
85
180*
187*
100*
33
26
70
43
96
80
15
35
550
77
45
15
60
80
95
75
100
35
23
59
55
45
50
75
80
100
95
12
87
62
85
1)
25
62
35
6)
70
45
4)
40
100
60
53
60
60
))
4)9
too
2)
M
74
3)

t
.50
.50
.57
.70
.48
.55
.70
-0-
.82
.68
.41
.31
.50
.48
.68
-0-
-0-
.76
.41
.44
-0-
.43
.33
.914
.50
•0-
52
.37
.68
.42
.50
.50
.50
.40
.48
.68
.20
.20
.67
.64
.48
.42
.67
.34
.66
.50
.53
.50
.M)
.M
.6)
.53
.M
.52
.52
.50
.)>
.M
.8*
.M
.52
.M
.30
-0-
.«•
.M
.to
.42
.Ml

OH
1
1
1
1

It
71


I
1
Ti

V
1
V(
VI
1
1
/
1
/
1
1
1
»
»
1
1
1
11
v/
1
VI
\







?
V

»

















V





X
1
1
1
k
I
k
I
k
k
L
I
I
k
k
k
k
k
k
1
k
1
I
k
1
1
1
1
1
1
1
i
k
1
k
1


































CODE:  VA-very abrasive  A-abrasive  M-midly abrasive  N-less abrasive

       SW-specific wt. lbs/ft3   BW-bulk wt. lbs/ft3  e-porosity.
*
 Estimated
**
  From Ref. 133.

-------
                                                       TABLE 2.21
                                                                                                                       j
        PARTICLE SIZE DISTRIBUTION,  BULK  DENSITY,  POROSITY,  AND  FLOWABILITY  OF SOME  TYPICAL POWDERS'
Solid**
Altaian
Coal
Limestone
Salt
Silica
Soda A*h
Sugar
Sulfur

Granules
Powder
Granules
Gran. Pwd.
Granule*
Gran. Pwd.
Powder
Granule*
Pod. Gran.-.
Gran.&Pwd.
Granule*
Gran. & Pwd.
Ponder
Powder
Powder
Pwd. (porous)
Granule*
Pwd. Gran.
Gran. & Pwd.
Granule*
Gran. Pwd.
Powder
Pwd. Gran.
Pwd. & Gran.
Pwd. (lumped)
Average
Particle
Size
Microns
840
<30
3000
<74
2000
<30
<30
250
70
30
80
60
<74
5
0.01
3.5
250
100
74
150
74
<74
220
74
<74
Mesh
Powder
I
0.0
100.0
5.0
80.0
1.0
80.0
100.0
0.0
25.0
65.0
0.0
60.0
100.0
100.0
100.0
100.0
1.0
20.0
60.0
0.5
72.0
100.0
8.5
52.0
100.0
Size
Granule
I
100.0
0.0
95.0
20.0
99.0
20.0
0.0
100.0
75.0
35.0
100.0
40.0
0.0
0.0
0.0
0.0
99.0
80.0
40.0
99.5
28.0
0.0
91.5
48.0
0.0
Bulk Density True Density
Loose Packed Working gm/cm
99
48
43
28
85
55
42
74
63
46
97
51
51
27
2.1
1.9
63
36
33
50
29
23
70
35
36
114
74
54
36
105
81
66
86
79
62
108
79
73
44
2.7
2.7
76
47
51
57
43
36
86
46
50
101 , ,,
57 2'65
46 145
30 1- 3
89
62 2.62
51
75
67 2.22
50
98
61
58 7 ftS
34 2'65
2.4
2.0
65
39 2.15
37
51
33 1.69
28
73
38 2.02
40
***
Loose
Powder
Porosity Flowabillty
(Void Fraction)
.40
.71
.53
.69
.48
.66
.74
.46
.54
.67
.41
.69
.69
.84
.9872
.9885
.53
.73
.75
.53
.72
.78
.45
.72
.71
Excellent
Poor
Good
Poor
Excellent
Poor
Poor
Good
Passable
Poor
Excellent
Poor
Poor
Very Poor
V. Very Poor
V. Very Poor
Excellent
Passable
Poor
Good
Poor
Very Poor
Pa* cable
Poor
Very Poor
Floodable
(or can be
fluidlzed)
No
Very
Ho
Very
No
Yes
Yes
No
No
Ye*




No
Ye*
Yes
Yes
Yes (dusty)
Yes (dusty)
No
No
Yes
No
Yes
Ye*


No
Possible
No
Proa Carr, taf.  134
• 1 - Bulk Density/True Density
 Carr daflna*:  particles > 200 mesh  (74 ua)  a* granular, free-flowing units  for average weight material of average M.W.  (Inorganic)
 of 25-40 lb/cu.  ft. -» :  particles  < 200 mesh a* "powder*", non-flowing.  The lighter a material, the coarser 1* it* powder; a

-------
        0.3
                 2   345  678910     20   3040   6080100
                  AVERAGE PARTICLE DIAMETER (micron*,suffoc* m«on)
Figure 2.46.
Bulking Properties of Various Powders,  Rol-
ler's Data,  from Dalla Valle, Ref.  135.
14
I i
?! •
u
i :
OKO
     Figure 2.47.   Bulkiness of Powders,  data by Shapiro
                    and Kolthoff on  Silica (Ref. 136).

-------
                                  2              3
For  spherical  particles, A  •» nD   and V  » nD  /6 and A /V  » 6/D ,
                   -        PP       PP         PPP
with dimension cm"1.   For  non-spherical particles, an empirical shape
factor  (greater than  1)  is derived,  as discussed  below.   Some represen-
                    So
tative  values  of S (x— =•  surface area of  the  pores of a unit bulk
                                                         121
volume  of  the  porous  medium)  are indicated in  Table 2.22   .

                               TABLE 2.22

      REPRESENTATIVE VALUES OF SPECIFIC SURFACE FOR VARIOUS  SUBSTANCES*
     Substance
  Specific surface range
(Specific surface in cm~l)
   Literature Reference
Berl  saddles
Raschig rings
Wire  crimps
Black slate powder
Silica powder
Catalyst  (Fischer-
 Tropsch, granules
 only)
Sand
Leather
Fibre glass
         3.9-7.7
         2.8-6.6
   2.9 x 10 -4.0 x 10
   7.0 x 103-8.9 x 103
   6.8 x 103-8.9 x 103
        5.6 x  10
   1.5  x 102-2.2 x  102
   1.2  x 104-1.6 x  104
   5.6  x 102~7.7 x  102
Carman, 1938
Ballard and Piret, 1950
Carman, 1938
Carman, 1938
Carman, 1938
Brotz and Spengler, 1950
Carman, 1938
Mitton, 1945
Wiggins et al., 1939
 From Scheidegger, Ref. 121
            For computational estimates of granular bed pressure drop
to be considered below, values of A /V  for spheres (•> 6/D ) are presented
in Table 2.23.  Note that the range of (A/V)2 is from 104 to 1010 for
particles ranging from 500 to 0.5 (Jon respectively.  A typical value for
                                     9             2
sand (from Table 2.22) is S  = 2 x 10/0.6 « 3 x 10  (for an assumed
particle size of 60-90 mesh sand  approximately 200 (im, as given in
Table 2.19 and from Carman, Ref. 123).  Note that the value for spheres
is 3 x 102, in Table 2.23.

            2.4.6.3  Pore size distribution.-  Pore size distribution
depends upon the nature and state of a given packing,  and is determined
from a mercury-injection method involving flow and pressure measures
and capillarity, or by bubbling pressure (bubble point  test).

-------
                               TABLE 2.23
                VALUES OF Ap/V  FOR SPHERICAL PARTICLES IN
                         FABRIC FILTER DUST CAKE
Particle Size
urn
500
300
200
100
50
30
20
10
5
3
2
1
0.5
*
So =
cm
5 x 10"2
3 x 10'2
2 x 10~2
10"2
5 x 10'3
3 x lO'3
2 x 10"3
io-3
5 x 10~4
3 x ID'4
2 x 10'4
1 x 10~4
5 x 10-5
(A/V) - 6 rtDp
A/V*
cm"!
1.2 x lO2
2 x IO2
3 x IO2
6 x IO2
1.2 x IO3
2 x IO3
3 x IO3
6 x IO3
1.2 x IO4
2 x IO4
3 x IO.4
6 x IO4
1.2 x IO5
2/«Dp3 - 6/Dp,
(A/V)2
cm"^
1.44 x IO4
4 x IO4
9 x IO4
: 3.6 x IO5
1.44 x IO6
4 x IO6
9 x IO6
3.6 x 10?
1.44 x IO8
4 x IO8
9 x IO8
3.6 x IO9
1.4 x IO10
cm'1
The true nature of the openings responsible for  fluid  flow  in woven  tex-
tile fabrics is not actually known.  Pore size distributions in  14 tight
cotton textile fabrics (6 water repellent) were measured by Wakeham  and
Spicer    using a mercury intrusion technique.  These  are given  in
Table 2.24.  Total free volume (V ) was obtained from

          V  = total volume of fabric - volume of fibers
          V  = area x thickness (at 1 psi) - wt. of sample/p  for cotton.

            Yarn to yarn spacing (interyarn) openings were  indistinguish-
able from internal yarn pores between individual (interfiber) cotton fibers
when y-y distance was less than about 50 urn.  Wakeham  and Spicer describe
the sizes of pores in terms of
           "... the arbitrary designation of 50 n as the upper
            limit of diameter of interfiber pores, as  distin-
            guished from interyarn openings.".

-------
                                                          TABLE  2.24a

                     PHYSICAL PROPERTIES  OF  FABRIC SAMPLES INVESTIGATED (DESIZED AND SANFORIZED)
•OBlnal yam no.
8**ple
•o.
Group I
1
2
3
4
5
Group II
6
7
8
Group III
9
10
11
12
13
14

Description

Plain weave
Oxford
Sateen
Herringbone twill
Modified herringbone tvill

2/1 twill, 7/8-ln. average cotton
2/1 twill, 1-ln. average cotton
2/1 twill, 7/8-ln. average cotton
(water-repellent treated wltb Bom
Oxford A
Oxford B
Oxford C
Oxford D
Oxford E
Oxford F

Warp

13/1
13/1
13/1
13/1
13/1

13/1
13/1
13/1
M)
80/2
80/3
80/3
60/3
60/3
30/3

Tilling

10/1
10/1
10/1
10/1
10/1

10/1
10/1
10/1

40/2
40/3
40/3
45/3
30/3
15/3
Thread count

Warp

82
85
84
82
82

82
82
82

166
136
159
142
140
82

Filling

52
54
71
64
63

S3
53
53

81
63
64
63
52
39
Weight
(0«./e,.
y«.)

9.2
9.7
11.2
10.6
10.6

9.8
9.3
9.4

6.6
8.3
9.0
9.5
10.0
13.3
thlek-
ne*a IB
i/ioeo
la.

20
22
30
23
24

24
23
23

13
16
17
18
20
25
Air
P*TBM-
•lllty*

2.M
1.41
5.10
2.19
2.17

5.83
5.70
7.07

1.57
1.58
0.80-
0.84
1.00
1.21
**•
rental

MM*

..
mm
ff
„
—

10

--

54
54
80
68
75
50
«•*!•
kllic*
**w
rue

^
~

"

..

>10.000

..

I.IK
•01
u
3*7
307
84*
oo
                     Air flow In cu. ft./sq. ft./Din.
                     Bydroatatlc pressure in cm. of water on the fabric.
                     Permeability In water flow in cc. through a 5 1/2-In. circle in 50
                     rroai Uakenir-i and Splcer, Ref. 137.
i In.
                                                             TABLE  2.24b

                                                       FABRIC  POROSITY DATA



Sample
no.
Group I
1
2
3
4
5
Group II
6
7
8
Group III
9
10
11
12
13
14




Description

Plain weave
Oxford
Sateen
Herringbone twill
Modified herringbone twill

2/1 twill, 7/8-ln. average cotton
2/1 twill, 1-ln. average cotton
2/1 twill, 7/8-ln. coarae cotton

Oxford A
Oxford |
Oxford C
Oxford D
Oxford I
Oxford r

Specific
volume of
fabric
(cc./g.)

1.69
1.70
2.04
1.70
1.65

1.93
1.92
1.94

1.55
1.57
1.45
1.47
1.54
1.46
%
Total
free
VOluBK
(cc./g.)

1.02
1.04
1.37
1.03
0.98

1.27
1.25
1.27

0.88
0.90
0.78
0.81
0.87
0.79

Inter fiber
pore
veluxK
(cc./g.)

0.503
0.600
•0.828
0.609
0.592

0.770
0.686
0.732

0.440
0.477
0.3M
0.386
0.420
0.518

Volixae of
intervarn
apacaa
(cc./g.)

0.52
0.44
0.54
0.42
0.39

0.50
0.57
0.54

0.44
0.43
0.40
0.42
0.45
0.28
Htdiai
dtamtei
Interfl
porei
(u)

9.(
10. <
14.1
10.1
10.

13.
12.
14.

9.
10.
8.
8.
9.
10.
>
• of
Lber
I


>
k
)
)
I











-------
                'Microscopic observations and measurements of
             the observable interfiber and interyarn spaces in
             fabrics  have supported the distinction between these
             two types  of pores.   When interyarn pores are
             appreciably smaller  than 50 |~t in diameter, they are
             no  longer  recognizable as interyarn openings be-
             cause  cotton fibers  20 n or so in diameter consti-
             tute the interyarn-pore walls.  In Table (2.24b),
             therefore,  the interfiber pore volume is that for
             all pore spaces in the fabric less than 50 n in
             diameter. "

                "The  difference between the total free volume
             In  the fabric and  the  interfiber pore volume has
             been designated the  volume of interyarn spaces.
             In  view  of the fact  that all fabrics studied in
             this investigation were tight fabrics with prac-
             tically  no observable interyarn pores through the
             fabric, the interyarn  volumes shown in Table (2.24b)
             may appear to be rather large.   This result is due
             to  the definition  of the total free volume in the
             fabric.  ...these total free volumes are based on a
             thickness  measurement with a pressure of 1 Ib/sq.
             in.  on the fabric.   At this pressure the fabric
             undergoes  only a slight compression or distortion,
             and the  total free volume,  therefore, includes
             interyarn  spaces which are due to the surface
             unevenness  of the  fabric (such as ridges and mounds)
             in  addition to the spaces between yarns and pores
             within.  The interyarn volume as given in Table  (2.24b)
             includes,  therefore,  interyarn "pores" parallel  to
             the fabric  surface as well  as those perpendicular
             to  it. "

             Considering  these  reservations,  pore area estimated  from

data in Table 2.24a  and  b,  yields  an average interyarn space of
        _2
2.2 x 10   cm or 220 urn.   This figure is  of  the same order or  magnitude

as discussed by Stairmand (see Figure 2.26).   Wakeham and  Spicer  indicate

no such pores visible  in the microscope.

             Average pore  sizes of  fibrous  felted materials,  papers, and

the like can be estimated from the porosity,  e,  (as  calculated above),

with a geometric model of the  fibrous medium and the fiber size,  as in

typical photomicrographs   (Figures  2.28 and  2.29,  for example).

             Pore sizes of granular  media  depend upon grain size,  shape,

and packing  arrangement.. The general assumption made for  relatively

-------
large grains  (e.g. sands)» 100 urn,  is  that  the pore  size  is  approxi-

mately equal  to granule size.

            Pore sizes for some typical fibrous, granular  and gauze
                                        138
filter materials are given in Table  2.25

            2.4.6.4  Particle (grain) Size.-  Particle size in granular

media affects porosity, surface area, and pore geometry.   Particle size
            „  JJ       .  .        ,       8,9,13,37,38,135  „   ..
measurement is discussed  in several  texts.                  In all cases

of practical concern, particles will be present over  a range of sizes.

Formation of a granular porous medium will involve grains  at the larger

end of the spectrum present, which may  form a loose network with inter-
grain openings filled in  to some certain extent with  the finer sizes.

            2.4.6.5  Pore Structure.-  The physical shape  and appearance

of pores in a granular porous medium of natural materials  is presumably
related to particle size  distribution, shape, and surface, as well as to

the degree of compaction  or compression applied.  The only cases analy-

tically tractable are spheres, for which certain broad conclusions can

be drawn to assist in interpretation of estimates in non-spherical cases
(similarly, cylindrical models are used in analysis of fibrous felted
porous media).  Pore geometry and structure in spherical (model) arrays

are illustrated in Figure 2.48.   These studies lead to the conclusions

that:

            (a) For any one mode of packing (for spheres all the
                same size) the porosity of the bed is independent
                of sphere size.

            (b) The porosity of  stable beds of uniform spheres
                varies  upward from about 0.259 (rhombohedral or
                face-centered-cubic configurations  shown in
                Figure  2.48c and d);  with uniform spheres,  simple
                cubic packing yields  e « 0.476.

            (c)  The densest packing of non-overlapping uniform
                circles  that can be achieved (0.9069)  on an area
                basis yields a porosity of e  = 0.0931  in a  hexa-
                gonal lattice.   °

            (d)  If  interstitial  spaces are partially closed by
                smaller  particles  touching at three points
                (stable)  (see  Figure  2.49)  the resulting  packing
                factor  is

-------
            TABLE 2.25
PORE SIZES OF VARIOUS FILTER MEDIA*
Material
a
•Mfe poioue •orMUiBi




Poroui Tefloa flltere




filter papara




Heatbraae end Cell, filter.



llltreflna and ultra Cella flltere




18-8 etelnleee-




•emu Flee tic Filter Co., Slee C**»,
Wt* York



Carl fchUlaker a (cluell Co., Beea*,
Nev lavaabira



Carl (chlelcber 4 (cbuell Co., leeaa.
•a* laBvablr*


Carl Icblaiehar a Sebuall Co., leene.
•M laavablre




Micro Metallic Corp., Brooklyn, Mev
Tork



•laghaai, tngland








































Plltroe, Inc., Sect lochaeter, lev Tork






Plltroe, Inc., Bast lochaeter, lev Tork

Berkefeld-Ptltar O.ei.b.I. . Celle, Otr-
•any
Prauanlts, 01 as imif karauUcba Filter
taboratorluei fur Flltretlon, Oaavar-
tellung, Dlalyee, ti traction, all ilia
Verlagagaaellscnaft, Ulpalg, 1933
IrllheU Icbuler C.s>.b.l. , (laanbarg.
deraany




Corning OUae Worke, Cora lag, lev
Tork




Multl-Httal Hire Clotb Co.. ••> Tort








•anejri Wire Clocb Oe. . Bsvark, Maw
Jeraey



•M OUee, tM., TtHUatf. gt* Jamay







1m


































VM« eUe detanlnad by baMllaaj areetnre.
*aai. — 	 * 	 ~^*» — 	 •&_*! *«ae

-------


        (a)
Figure 2.48(a)  Packing of Spheres: Illustration of "square"
                 and "triangular" holes.

   RhonMxdnl p*aUng of ^hm.

Figure 2.48(b)  Packing  of Spheres:  Hexagonal close-packed
                  (top) and cubic  close-packed (bottom).

           (From Kolthoff,  Ref.  139.)

-------
                           a - 1 - (1 - 0.9069)K                    (2.62)
                  and      a -* 1 as K »  1,  i.e., e -* 0.
              (e) Packing  of non-spherical  natural particles will
                  lead  to  bridging and increases porosity,  offset
                  partly by  the tendency  of smaller particles to
                  accumulate in larger interstitial spaces.
 No studies  of pore morphology have been reported for fabric filter
 deposits.
Figure 2.49.   Tricuspid  Interstices in the Osculatory Backing  of Finite
               Areas with  circles (from Ref.  140).
             2.4.6.6  Shape Factors.-  If particles  are not spherical,
 the  specific area-volume relationship is given as A/V » X 6/Dp, where
 the  particle shape  factor X is applied,  (1< X< 10).   Typical shape
 factors are given in Table 2.26 as  taken from Leva, et al.
             Leva's  discussion of his shape factor is  given below:
                       "In order to define eta •hap* factor. X, ... let
                       »B " average diameter of • particle of any arbitrary shape;
                       Dp * dla»eter of a sphere of equivalent  volume;
                      A  • surface area of a particle of arbitrary shape, and
                      A  • surface area of a sphere of equivalent volume.
                       2                                      2
            Then,, A •  00)  ,  where a Is an area shape factor, and A  - *D  .
                      *   	_.                          P    P

-------
                                        TABLE 2.26

           SHAPE  FACTOR, X,  FOR TYPICAL GRANULAR POROUS  PACKING MATERIALS
                                                        141
Shape
Spheres
Rounded Granules
Rounded Granules
Sharp Granules
Cylinders
Cylinders
Raschig Rings
Raschig Rings
Raschig Rings
Berl Saddles
Nominal Dimensions,
Inches
all
0.170
0.1875
0.165
0.180
5/16 x 3/8 in. D.
0.252
0.400
0.944
0.480
0.765
Shape
Factor
1.00
1.10
1.10
1.10
1.15
1.16
1.50
1.90
2.19
2.07
2.71
Remarks

Aloxite
MgO, Oman and Watson
Aloxite
Alundum
Tungsten sulfide catalyst
Pellet h/Dc - 0.833
Clay
Clay, Oman and Watson

Cylinders
Cylinders
Lessing Rings
0.247  x 0.236  in.  D.     1.145
1/2 x  5/16 in. D.        1.175
       0.953              2.67
Copper
Pellet h/Dc -  1.048

Copper
Pellet h/Dc =1.60
                     for the sphere of equivalent volume.

                               "By earlier definition, V was designated as the volume of
                     the particle.  Then,


                                     VP ' V ' I Dp3'
                    where y Is • volume shape factor.
                                             it
                                         Y-
                                     '  " Yp-6

                    for the sphere of equivalent volume.

-------
                       By definition let
                                       2
              Since
              solution for D yields
                                     - f
                             Dp - 1.211 Y 1/3 Dm                       (7)
              Substituting (7) Into (6) yields
                                       2
                                0.642QD
              Tor »ny particle,
              substituting this Into (8) and recalling that D yl/3 ™ V   • yields

                              X - 0.205 —jrr                          (9)
                                      P

             2.4.6.7  Granule Surface  Roughness.-  For particulate granu-
lar bed  systems in slow viscous flow  (Re   < 10), Leva et al and other
workers  have found no dependence of permeability on surface roughness
of the granules composing the bed.  This  is analogous to the situation
observed in flow through cylindtical  tubes in the so-called streamlined
range  (Re  .   <- 1000).  Surface roughness  is only significant in the
onset and character of tubulence developed at higher Reyholds numbers,
at least as far as is now known.  Since much of  the work in fabric fil-
tration  has been performed in the slow vicous flow regime, no studies
of effects of grain roughness are available.   As technological applica-
tion of  fabric, fiber, granular, or grid filtration devices goes to
higher gas velocities (V > 100 ft/mln), effects  of roughness may be of
significance in analyses of filter performance.   Surface roughness effects
at Re  greater than 10 are indicated below.
     2.4.7  Working Equations
             Pressure drop equations have  been extensively investigated
for various types of porous media.


-------
             For a single layer of a plain-weave rectangular mesh wire
 screen,  the (Lapple-duPont) pressure drop equation cited in Reference 97
 is:
                            u    1  -   f      V
                                ~J~T  '   2j
                                c   c                              (2.63)

 where  h  =  head loss  in cm fluid flowing
       c  =  screen discharge coefficient, dimensionjess
       c  =  fractional free projected  area of screen;  porosity,
           dimension less
       V  =  free stream velocity approaching screen,  cm/sec
                                             2
       g  =  gravitational  constant,  980  cm/sec
 Note the use  of V     =  V-   / e ,  in equations  (2.63)  and (2.64)  below.
                  pore    free
 For a  series  of screens,  the  overall head loss  is directly  proportional
 to the number of screens  and  is unaffected by spacing  or orientation.
 There  are  several other  studies on screen pressure drop  that have  not
 been included here,  primarily in wind  and water  tunnel flow systems.
 The screen discharge  coefficient is  a  function  of the  modified Screen
 Reynolds number
                                '  Ds  V pf/Mf
where D  = the aperture width, cm
       8                      3
      pf = fluid density gm/cm
      \j.f = fluid viscosity, gin/cm-sec
The (Lapple-duPont) discharge coefficients are given in Figure 2.50 and
are stated to predict the head loss to within +20%.  A value of C greater
than 1 implies pressure recovery downstream of the screen.
            Use  of plain weave wire screens as models  for  regular,  open
textile materials was studied by Robertson, who  found
                           c/  =      r 9»K~-ii/9                    (2.65)

-------
where C
      Q
      h
      g
     A
             discharge coefficient, dimensionless
                                              3
             volumetric flow rate per pore, cm/sec
             pressure drop, cm of fluid flowing
             as above
        ,   = upstream flow channel area, cm
                                         2
       A_   = pore projected open area, cm
       A   - A../A, - €
              110'
          U    2
          mi
          8
             10-'
                   I I I I Mill  I  I I I Illll  I  I I I Mill  I III Illll  I  I I H|)fI
                                                         10
                                                         D,Wf
        Zji \f\-Z   i  i 11 mil   i i i mill  i i i mill  i  i i null  i  i j mill   i i Mini
              ICT2     10"'      10'      10*      I03     I04      I08
                       MODIFIED REYNOLDS NUMBER NRt=
      Figure 2.50.   Screen Discharge Coefficients, Plain Rectangular-
                    Mesh Screens. .(From Perry, et al., Ref. 97).
The discharge  coefficient  is  a  function  of  screen Reynolds  number
                                                                 (2,66)
where D  = diameter, or width of square orifice.
Robertson's plot of C vs Re  is shown in Figure 2.51  (which confirms  the
1944 data of G. I. Taylor and R. M. Davies   ) for the screen configura-
tions shown in Table 2.24 and for plain-weave rayon fabric (yarn, 900
den.) of 25 x 25 ypi (15.6% open area).

-------
                     1.4
                     1.1
                     1.0
                     at
                    0.9
                     o*
                    at
                    oo
                                 to         no
                               *IYMOL03  MUMMff
                       Figure 2.51.  Screen and  fabric  data.
                                    (from Robertson, Ref.  131.).
                     132
            Robertson    extended his study to some 45 rayon  fabrics  using
the same equation, with values of the discharge coefficient as  indicated
in Figure 2.52 for a number of different fabrics.  Presumably,  Robert-
son's data can be extrapolated downward  (to Re  ~ 0.1) by reference to
                                              s
the Lapple-dePont data in Figure 2.50.  As will be shown below,  the
C~Re relationship is linear for porous media having Re < 10.  Note that
these correlations are essentially pore-related models (i.e.  a  capillary
                                            132
flow model is assumed).  Robertson indicates    that "... even  in the
tightest weaves tested the yarns can be considered to be impermeable  bar-
riers"... to flow, all the flow passing essentially through the  inter-yarn
spaces.
            For random felted fibrous media, the flow of clean gas can
be considered as a part of the general problem of flow through homo-
geneous isotropic, porous media.  At the usual velocities and ambient
pressures encountered in filtration, the gas can be considered  incom-
pressible.  The volume occupied by fibers is usually small (  <10%) in
contrast to granular porous media (  >30%).  The average distance be-
tween fibers is several times larger than the fiber diameter, again in
contrast to granular media where pore and particle size are approximately
equal.  The macroscopic flow is one-directional and fibers lie essen-
tially perpendicular to the flow.

-------
             »*i«aci
"'"••     a    x.  \
         ii\
              $  •
                                    •»,  —
o
  OA
         S

                                                  1.4
                                               8
                IO           lOO
              C'EYNCX.03 NUMBER.
                                         IOOO
                                                  00
•    e
8    g
                                                                                          H
                                                                                         IOOO
                                                              IO           lOO
                                                             REYNOLDS  NUMBER.

( O)  l'.xperimentui  porosity data for  pluin-weavr  '.  ' VI   Experimental porosity data for basket-weave
             fabric construction.                                fabric construction.
w i i ( : i i u 1 1 i i i i i 1 1 ij „ "i ' i, ' ' ' 'J
'-'. ;" j o «* *i* | ® „ *\:j J#*» ^'"~
iT" 1 * ta't^L^tt' *^
IW'CXfl," * ^a^LitT^^V '• ""

"4 i ^* '
T" 44 1™ ' * M • ""
v..^ ! <•
^f i i i i i!:! ; l 1 l l |lll .. .1 .:J__LJ._Lill
a
§
iy
u*


1"
I*
00

-


-
—

1 1 M 1 1 1
J 12
i %
• 4<
9

A
1 1 1 1 1 11
i i i i i n 1 1 i i i 1 1 1 1 1
• —
» _
, * • • _
• ».. «"
/%'^v/xx:
(
j
i 1 1 1 1 1 1 ii i 1 1 1 1 1 ' >i
                 .0            100
               eELYNOLDS NUMBER.
                                          IOOO
                                                             REYNOLDS NUMBEK.
         I'urosity data for plain, basket, and mock-Uno
                    fabrics.
                                                 (d)    Experimental tftfotity  dab for ttM-v/eaw
                Figure  2.52.
                               Discharge Coefficient  -  Reynolds  Number
                               Relationships  for  45 Fabrics  (from
                               (Robertson,  Ref. 132).

-------
            For steady  flow  through  fiber  filters under  consideration
here (Df  < 20 microns,  V   <  100 cm/sec), the Reynolds number based  on
fiber size is less than about one.   Therefore, Darcy's Law applies,
name ly :
                                                                 (2.44)
where k  is the intrinsic permeability of the medium, and  &p is the
pressure gradient.  For flow in fiber filters, it is usual to express
Darcy's Law in the form:
                                      2
                                                                (2.67)
where  A p is the resistance to flow or pressure drop across the medium,
a   is the fiber radius, and K ( a) is the theoretical resistance coe-
fficient.  The intrinsic permeability is related to this coefficient
     i      2
by k  =  a  /K (  
-------
                                TABLE 2.27

              EXPERIMENTAL  RESISTANCE COEFFICIENTS FOR FIBER FILTERS
Investigator
        Experimental
      Coefficient  (K  ')
                   o
 Remarks
   Sullivan


   Klasewitz
   et al.

   Davies

   First et al.
   Wong and
   Johnstone
   Chen
K«oc2/K"(l_
-------
                or
                u.
                o
                                                 10  *'  « • »> 100
                                  ApD2/
           Figure 2.53.  Correlation of Bed Density  (1 -e  , where e~
           Voidage) with a function of pressure drop Ap and superficial
           gas velocity: (ApD2/V JL/if), (from Strauss, Ref. 112).
                     Fluid  flow  through fibrous  material
   o Glass wool
   • Glass wool and copper wire
A Kapok
A Merino wool
   •f Glass (fibres perpendicular to   A Cotton wool
     flow)
   x Glass (fibres parallel to flow)  A Rayon
 Cotton wool
 Camel hair
' Down

'Glass wool
                                              3-n
                      k
                      g
                                             D
                                                         3-n
                                                                 (2.69)
where n =  flow factor,  = 1 for slow viscous flow at low granule
           (particle)  Reynolds  number < 1' and ••» 2 for 'Re» 1
   v  - - '          •             }
      k =  the Kozeny  -  Carman  constant.
For slow viscous  flow (the usual  case in fabric filtration) the equation
reduces to:
                L     g
     %2      '3
                                                                 (2.70)
where Ap = pounds per square  foot  pressure  drop.

-------
                                                   143
             This is identical to the Fair aid Hatch    equation, derived
 experimentally and later on dimensional grounds (with n m 1):
                                                                 (2.71)
              141
 Leva,  et.  al,     compare their equation to the form commonly used in
 chemical engineering for flow in terms of the usual Fanning friction
 factor,  f,
                         2£
                    L    p  g D        e3
                          f    P
                                     2
where G = Vpf = mass  flux,  pounds /ft -sec,  from which it  follows that
                      f = C(Dp G/n                               (2.73)

in slow viscous  flow, where  the constant C  is  experimentally derived.
The friction  factor<-Reynolds  number  (Re ) relationship  found by Leva,
     141                               ^
et al    is shown  in Figure  2.53a and  their smoothed  form in Figure
2.53b.  It is apparent  from  these correlations that the pressure drop
of a granular layer can be predicted to within a  factor of 2 at best,
unless the particles are smooth spheres.
            The k  in the Hatch equation is  related to the usual Fanning
friction factor by:
                              k =  2fXs2                          (2.74)
                                                  2
noting that f above in  the Fanning form contains  X  .   For slow viscous
flow (Re  < 10)  the Leva, et al,    form becomes:
                                       O           O
                   Ap_ _  200 G . ^£   Xs   .   (1-e)               (
                   L =    g     P               £

-------
                                                                        IT TBF
                                                              Hl.'lil1  T1
                                      • - Round lond
                                    _  X - Shorp tana
                                    r.  O- Gloti b«ad>
                                   [IT A - Alundum cylinders
                                      T- Cloy Rotchlg rlngi
                                       - Ltod $ho!
                                       - Mlxtd llm ( round land )
    • :  : fH-'n-'--"-^-- "l-i:!'F  ; :-)"i"j..
             ',—MODIKII'll)  KHICTION I''AC;TOU,S vs.  MUDI !• I Kl) KDVNOI.DS M:.MHKK.
                                                                              	1

              b .    MOOIKIKI)  KRICTION  FACTOIfS  VS.  MODI Kl K|>  |{KV N()|,I)S
                                         Nl'.MHKH.

Figure  2.53     Friction  Factor -  Refolds  Number Relationship for Granular
Beds (from Leva,  et  al.,  Ref.  141)

-------
 for shape factors X  = 1.00 for smooth spheres, to 1.50 for sharp sands.
                    s
 We note that the equivalent value for the Hatch equation with the Kozeny
                                2        2
 Carman constant « 5 and (A /V )  - 36 /D  ;

                          2f = 5 x 36 - 180

 which is well within the estimation accuracy of these correlations (for
   2
 X   of order 1).  The smoothed data of Leva, et al, (Figure 2.53b) are
 equivalent to the usual presentations in chemical engineering texts,
 which arise from similar studies and analyses (by Burke -Plummer, for
 example).  Note that the velocity used in the particle Reynolds' number
 is modified by the solids fraction ( 1 - e)  in the definition of pore
 velocity (Dupuit - Forscheimer assumption)  in some correlations.
             Effects of particle roughness are seen in the Leva,  et al,
 data at Reynolds numbers  (Re  ) greater than  about 10,  and arise  as a
 consequence of the actions  of surface asperities on the inertial motion
 of the fluid near the granule surface.   Surface roughness apparently
 has  no effect for Re < 10.
             Estimation of pressure drop in fabric filters depends upon
                            2  3
 the  porosity function (1  -e)  /e  ,  which is generally  unknown in  an oper-
 ating  system.  The general  relations for  granular packings of various
 geometries  are  shown by Leva,  et al.,  vs.  the granule /container  diameter
ratio.  For very  small  particle  sizes  (D   < SOiam) ,  surface  adhesion
forces dominate gravitational  forces and  the porosity  rises sharply as
shown previously  in Figures  2.46 and 2.47,  but  is not  observed  in  the
data of Leva, et  al,  for granules greater than  50 urn.   Porosity func-
             23             3
tions (1  -e ) /e  and (1 -e  )/€  are indicated  in Table 2.28 with  an
approximate range of particle  sizes likely  to be associated with the
indicated porosity, from the Roller data  (Figure 2.46).
            Porosity also depends upon compaction forces or vibrational
effects on the packing ^ and in  fabric filters  appears to be  a function
of the deposition velocity.  Aspects of compaction are discussed in
Appendix 2.3., in  terms  of pressures on beds  of particles and from  the
data of Leva., et  al, on "dumped, pounded" (vibrated) packings.

-------
                             TABLE  2.28
                POROSITY  FUNCTION  FOR  GRANULAR POROUS MEDIA
Porosity
0 . 90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25

Solids Fractions
a = 1-c
.10
.15
0.20
0.25
0.30'
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75

2
t-
0.81
0.72
0.64
3
0.73
0.62
0.51
0.56 0.42
0.49 0.34
0.42
0.36
0.30
0.25
0.20
0.16
0.12
0.28
1-e
3
.137
.24
0.39
0.60
0.88
1.27
0.22 : 0.85
0.17 2.70
3
*•
.014
.036
.078
.15
.26
.45
.74
1.22
0.12 4.0 , 2.0
0.09 | 6.0
0.06
9.4
0.041 15.1
0.09 0.03
0.06

0.02
25.9
48.0
3.3
5.8
9.8
18.1
36.0
I
1
Probable
Particle
size*
microns
1.0
1.5
1.5
2.0
2.5-3.0
3.5
5.0
8.0
10-12
20
25-30
> 30
> 30
> 30

Estimated from Figure 2.46.

-------
      2.4.8  Flow and Pressure Drop in Fabric Filters
             The quantity of air or process gas, dust concentration, and
 the specific flow-resistance properties of the particulate deposit deter-
 mine the amount of cloth area required for any desired value of operating
 pressure drop,.  Cloth area is generally selected to provide an operating
 pressure drop in the range of 3 to 4 inches of water, but some designs
 can operate substantially in excess of 10 inches of water.  Average fil-
 tration velocity (total air volume filtered/total cloth area), commonly
                                                                       2
 called air-to-cloth ratio, is generally in the range of 1 to 15 cfm/ft ,
 (i.e., 1 to 15 ft/min).  However, values in excess of 50 ft/min can be
 achieved at moderate pressure drop with certain cleaning devices.
             The resistance of clean fabric prior to filtration of dust
 is  determined by fabric design and construction, and is reported by fabric
 manufacturers as air flow permeability, equal to the air flow through the
                 2
 fabric in cfm/ft  at 0.5 inch water pressure drop (Appendix 2.4).  In gen-
 eral,  gas flow through fabrics is viscous at low velocities and pressure
 drop is  directly proportional to flow,

                            Ap  = B^V                             (2.75a)
 where  Ap = pressure drop across fabric,  inches  of water
       K, = resistance of the fabric,  inches  of  water per  ft/min
       V  = filtration  velocity,  ft/min.

             Based  on above  correlations for  Reynolds  number vs.  discharge
 coefficient for  square  and  rectangular  openings,  this  equation  assumes
 that slow viscous  flow obtains  (i.e., n »  1).  The  factor K, is
 related  to  either a discharge coefficient  (as defined above) or to a
 friction  factor  (or Kozeny-Carman  factor).  K, is a coefficient of resis-
 tance to  flow and>for the simpler weave  fabrics^can be calculated from
 fabric parameters as indicated above, (e.g., see Figure 2.52).  In
operating fabric filters, at usual dust loadings, the basic fabric resis-
tance is negligible ( <  10% of the total  Ap).
            Effects of dust deposition on pressure drop and collection
efficiency have been analyzed for various dilute cases in aerosol


-------
 minuLos; and the fluid viscosity, 

 for C. = grains/ft  , V =  ft/min, and  t  = min.  The  resistance  coefficient
 then has the dimensions:
                                         2
         K- =  inches of water/(lb dust/ft  cloth area)-(ft/min
               filtering velocity),
 where K~ = the specific dust-fabric filter resistance coefficient, pre-
 sumably calculable by  the  terms in brackets above.  The coefficient,
 which can also be derived  from observations on an operating fabric filter
 will be referred to below  as K~, the  experimentally determined value.
 It may be computed via  the nomograph  in Figure 2.54 which is equivalent
 to Equation 2.80 in relating K' to p(t) or vice versa.  Such determina-
 tions are required in designing a new fabric filter installation in
 order to predict the relationship between operating pressure drop, fil-
 tering velocity, time between cleaning cycles and required fabric area.
            Values of the  theoretical area per unit volume of dust for
 particle sizes of interest are shown  in columns 1 and 3 of Table 2.23
under the assumption that particles are spherical.  Experimental values
of A /V  for Equation 2.80 can be  estimated for a specific dust of in-
    P  P
terest  from gas adsorption data obtained with commercially available
 lui
  3.
             2
equipment (cm  area/gram) and the true density of the material (grams/
cm
   ),  (see Section 2.4.5, b(l) (22)).  Porosity or void volume in a
particular dust will have a range approximately as indicated in column
                                    3
5 of Table 2.28.  The term (1 -e)/e  in Equation 2.79 varies from about
0.4 to 48 as shown in column 5.  Void volume is proportional to the range
of particle sizes present in the dust, as indicated previously.  It is
also affected by the forces acting on the deposit producing consolidation
as a consequence of the drag caused by the gas flow through the layer.
Typical experimental values of void volume as a function of particle size
for sized powders are shown in Figure 2.46.  They are also readily

-------
                                                                      (In)

                                                                      000
                                                                      100
   100-
                               Bxplanatlon:
                                  Line A - working Una for Ap(t) and C^;
                                  Line B • working line for Lin* A end V;
                                  K  - working lin* for Lin* B and t.
Example #1.  Find 1(3 from observed values  ofAp(t),  Cl,  V,  and  C
  (a) Line a between the value for^p(S) and  C*  determines  a  point on Lin* A.
  (b) This point on Line A and the value of V determines a  point on Line  B.
  (c) This point on Line B and the value for  t determines Kj.

Example #2.  Determine Ap(t)  from values of Kif  Cj,  V  and t.
  (a)  The values for Kj and  t determine a point on  Line B.
  (b)  This point on Line B and the value  for V  detmmlne a point on Lin* A.
  (c)  This point on Lina A and tha valu*  of  Ci  determineAp(t).
                     Figure 2.5*.  Nomograph for  Kj[

-------
obtainable experimentally  from  the  weight  of  a  known dust  volume,  in
conjunction with a  particle  size  analysis.
            The relationship between  void  volume  (e)  and  particle size
shown in Figure 2.46 can be  approximated by a single point  (P)  in  Figure
2.55.  For example, spherical dusts with an average  size of 10  microns
typically deposit with a void volume  of 0.5 (line a).  This approximation,
together with Table 2.23 for the  dust area per  unit  volume,  enables  the
bracketed expression for K«  in Equation 2.80  to be evaluated from  par-
ticle size alone.  This is demonstrated by line (a)  where K-  is esti-
                                        2                   ^
mated to be 10 inches of water per  Ib/ft -FPM.  If on  the other hand
the dust is not spherical  or is not typically consolidated,  then point
(P) does not apply, and 1C is estimated from  particle  size  and a more
appropriate value of void  volume.  For example, line  (b) in Figure 2.55
for non-typical dust gives a value of K2 of 1.  As noted in the Figure,
the values of K? thus obtained should be scaled upward or downward,
depending on fluid viscosity and  particle specific gravity.
            The depth of the dust layer removed is an  important charac-
teristic of the deposit structure and can be used to calculate forces
in the cake during shaking or other methods of  cleaning.  Layer depth
may be computed simply by  Equation 2.76 from  deposit weight,  particle
density, and deposit porosity.  This has been done in  Table 2.29 for a
variety of filtration conditions  and particle sizes.   The data  indicates
that depth ranges from a few microns  (7) to a few millimeters (6)  for
usual dust concentrations  (1-30 gr./ft ), particle sizes (1-30 l(m) and
operating times (1-100 min.).  For a typical  pressure  drop  increment of
the order of two inches of water, the depth of  the dust layer ranges
from 100 to 2000 microns for operating velocities of  1-10 fpm.  In
accordance with the earlier  assumptions,- Kl is  seen  to vary  only with
particle size  in the cited  examples, 4 to 57 inches of water per
Ib/ft2-fpm.
     2.4.9  Analysis of the  Specific Dust-Fabric Filter Resistance
            Coefficient (K-)

            The value of the specific dust-fabric filter resistance coe-
                                                                     147
fficient has  been defined above by the Williams, Hatch, and GreenbUrg


-------
                  Particle Size
                  D,
                  300-
           Based on Kn <*
(Eqn.  2.78)
                where
                     k "  5
                     y -  1.8x10"   d  sec/cm
                    p  =  1 gm/cm3
                     ? =  6/D
Example (a)  Many spherical dusts  are deposited with  porosities  such  that  the
             lines connecting size and  porosity happen  to  pass  through point
             (P) .  For such dusts, point  (P) enables  an estimation of specific
             resistance or of porosity, based on  particle  size  alone.

Example (b)  A particle size of  lO^i and a porosity of 0.8  would  indicate a
             specific resistance of 1 in.  R-Q/Ulfft2 -FPM) ,  using the  above
             constants .
        Figure  2.55.   Specific  Rt-sistance  Determined by Particle Size and
                       Deposit Porosity.

-------
                                            TABLE  2.29

                       CALCULATED VALUES OF THE  SPECIFIC 01,'ST-FABRIC FILTER RESISTANCE
                  COEFFICIENT, K2', THE DEPTH  OF DEPOSIT, AND RESULTING PRESSURE DROP
1' . S . c
°P
gin/cm
Kr/ft3
V
ft/min
t
min

. ll.s/ft2
w

L
gm/cm i | cm
K '
u in/#ft-FPM
m
" 'V*1
in.
  1  ,,m  0.9
      0.9
 1   0.9
 1   0.9

 3   0.7

10   0.5

30   0.4
1
1

1

I
1
1
1
1
1
I
1

1
I
1
1
1
1
3
10
1
1
1
1
1 1
1 1

1 1
. 	
1 2
1 2
1 2
1 3
1 3
1 3
1 10
1 10

1 10
1 3
3 3
10 3
30 3
1 3
1 3
1 3
10 3
10 3
10 3
10 3
10°
io1
2
10
0
10
io1
io2
10°
io1
io2
10°
10°
2

io1
io1
io1
io1
io1
io1
io1
io1
io1
io1
IO1
l.'txlO"4
14xlO"4
-4
140x10
-4
2.8x10
28xlO"4
280xlO~4
4.2xlO"4
4.2xlO"4
420xlO"4
14xlO"4
140xlO"4
-4
1400x10
.'.2xlO~4
126x10""
420xlO"4
1260xlO"4
42xlO"4
42xlO"4
42xlO"4
420xlO"4
420xlO"4
420xlO"4 '
420xlO"4
7 x IO"5
70xlO"5
-5
700x10
-5
14x10
I40x!0"5
1400xlO"5
21xlO"5
210xlO'3
2100xlO"5
70xlO"5
700xtO"5
-5
7000x10
210xlO"5
630xlO"3
2100xlO"5
6300xlO"5
2iOxlO"5
210xlO"5
2\OxlO"5
2100xlO"5
2100xlO"3
2100xlO"5
2100xlO"5
7 x IO"4
70xlO"4
-4
700x10
-4
14x10
140xlO"4
1400xlO"4
21xlO"4
2lOxlO"4
2100xlO"4
70x!0"4
700x1 O"4
-4
4000x10
210xlO"4
630xlO"5
2lOOxlO"4
6300xlO"4
210xlO"4
70xlO"4
2lxlO"4
2100xlO"4
700xlO"4
420xlO"4
350xlO"4
7
70

700

14
140
1400
21
210
2100
70
700

7000
210
630
2100
6300
210
70
21
2100
700
420
350
58
58

58

58
58
58
58
58
58
58
58

58
57
57
57
57
57
57
57
57
41
16.8
4.4
. 008
.08

.82

.033
.33
3.3
.073
.73
7.3
.82
8.2

H.2
0. 73
I'.l'l
7.3
2.1-*
0.73
0.24
0.073
7.3
5.1
2. I
0.54
  Assumptions:  Kozai y constant k (Inks  )  =  5

               Air viscosity = 2.2 x IO"4  dynu sec/cm  (200°F)

-------
  Equation 2.79.  Other calculated values  of K2 are shown In Figure 2.56,
  as a function of  particle size, and parametric in porosity (€).
             ' Calculated values of K_ for  particle sizes of interest in
  fabric filtration which include the approximate variation of  £ with
  particle size below 30 wm, are given  in  Table 2.29.  These values may
  be compared directly to the original  data of Williams, Hatch  and Green-
  burg presented  in Table 2.30 •  The deviation of K' (measured)  from the
  calculated value  reflects the difficulty of measuring or establishing
  the effective particle size (distribution), shape, specific surface,
  and porosity and  the appropriate fabric  parameters in an operating
  fabric filter-dust deposit.  Although these parameters are measurable
  quantities, they  are not often reported  in the literature.
              In  an attempt to assess the  state of engineering  technology
  available to the  designer of fabric filtration devices, a large  amount
  of published and  unpublished data was retrieved, as tabulated in Appen-
  dix 2.5.  This  has been analyzed for K2'  in the following pages  as a
  function of:
                        0.001
                                                ^
                                  1.0     10.0      OO
                                   Particle diamtter, micron*
                             Approximate rang* of doto for voriom tan.
                             Williams, Hatch and GrMnburg
                             Minn 200-mnri cod duit, Mumfcrd, MorkMn
                             and Raven
                             Cellulose ocelot* dull (flocculated)
                             Pipe-line dust, Cap** 1 1
                             Zinc prf roaster fines
                             Tai: du'.i
                                                     woo
Figure 2.56.
Resistance  factors for dust layers.   Theoretical curves  given
are based on  a shape factor of 0.5  and a true particle specific
gravity of  2.0.  (From Perry,et al.,  Ref. 97).

-------
                             TABLE  2.30
         FILTER  RESISTANCE  COEFFICIENTS  K'  FOR CERTAIN INDUSTRIAL DUSTS
                         (Industrial  Cloth-type Air  Filters)
                                                                         a*
MATERIAL
                       Coarse
                                        PARTICLE SIZE
                                                 Medium
                  Fine
             <20 Mesh   <140 Mesh   <375 Mesh
                                             <90|.im
                                                            <20pm   <2p m
Granite        1.58       2.20
Foundry        0.62       1.58
Gypsum
Feldspar
Stone          0.96
Lamp Black
Zinc Oxide
Wood
Resin (cold)    -         0.62
Oats           1.58
Corp.          0.62
                                      3.78
                                      6.30
                                      6.30
                                     1.58
                                              6.30
6.30

9.60  11.80
3.78   8.80
              19.8

              18.9
              27.3
                                                           25.2
                                                                   47.2
                                                                   15. 7
Inches, water gage, per/pound dust per square foot cloth) per foot/minute
filtering velocity.
Flocculated material not dispersed, size actually larger.
Theoretical size of silica; no correction made for materials having other
values of p .
           P
From Williams, et al., Ref. 147.

                        particle size
                        particle type
                        filtering velocity
                        fabric surface effects
                        clean fabric permeability
                        other parameters,  (e.g.,  compression effects,
                             cleaning method,  electrical phenomena)

-------
                           2.4.9.1  Data for K,,- Approximately 600 reference sources
               dealing with fabric filtration were retrieved and reviewed for suffi-
"~              cient data to compute K».  About 10 percent of these articles were
  <             selected for further analysis.  A total of 31 sources had sufficient
s-              information to enable computation of Kl  for a variety of applications,
               dusts, fabrics, etc.  These data are summarized in Appendix 2.

                           In addition to data computed from literature reported values,
_              a questionnaire-interview survey was conducted (1969) among users of
               40 operating fabric filters across a broad range of industrial applica-
               tions for some 31 applications.  K^ values completed from the data furnished
               are provided in Table 2.31.
__                          2.4.9.2  Effect of Particle Size - Data presented in these
               two Tables has been plotted against reported or estimated particle size
               as shown in Figure 2.57.  Although methods of particle size determina-
               tion were not investigated during the present study, they would appear
               to have a strong bearing on the relationship of Kl to deposit properties.
               It must be emphasized that the data reported were obtained in many dif-
               ferent configurations,  extending from "square-foot" bench scale labora-
—              tory experiments (e.g.,  Williams,  Hatch,  and Greenberg data (Ref.  147)
               c.f. Table 2.30) through single bag tests,  (e.g.,  Durham data,  Ref.  149,
_              and  including  multi-compartmented  fabric  filcers  operating  in actual  field
               situations.  There  are  a multitude  of  factors  operating  among these data
               that  have  not  been  adequately  quantified.
                           The data for Kl vs D  shown in Figure 2.57 indicate wide
               spread.   The data also  tend to confirm the  finding of Williams,  et al.,
               (as  indicated  in Figure  2.56), regarding  a  bend in the curve  in the
               vicinity of 20 to 30 urn.   This finding is consistent with the increase
               in bulk  density for particles  below this  size  (Figures  2.46 and  2.47).
               Data of  Williams,  et al.,  are  regraphed as  enveloped curves in Figure
—             2.58 (from Figure  2.56)  to establish  the  regression line  form.   NAPCA-
               GCA  Fabric Filter  System Study data  (from Table 2.31)  are shown as  indi-
__             vidual data  points.   It  is  evident  from both  figures  that estimation  of
               particle  size  alone  is  inadequate  to  provide sufficient  information for

-------
                         TABLE 2.31

SPECIFIC DUST-FABRIC-FILTER RESISTANCE COEFFICIENTS FOR OPERATING
     COLLECTORS SURVEYED IN FABRIC FILTER SYSTEM STUDY (1969)
Dust
Carbon
black
Carbon
black

F62°3


Fe2°3
Fe2°3
Fe2°3
ZnO

ZnO,
PbC£
Fly Ash

PbO

Fe2°3
Fe 0
Dust
Loading
Operation gr/cu.ft.
Oil-furnace
Oil-furnace

Elec. furnace


Elec. furnace.
Elec. furnace
Elec. furnace
Brass smelter

Blast furnace

Oil-fired fee.

Smelter

Cupola
Cupola
14
26

(1.5)


1.5
0.8
0.3
8.1

1.2

0.01

2.3

« 1)
0.7
Filtering
Velocity
fpm
1.6
1.1

3.3


3.0
3.0
1.4
0.6

1.2

6

1.0

12.5
2.1
Operating
Drag
In H20/fpm
Residual
t Maximum
* 1
4.4
4.0

1.4


-
1.0
3.5
0.9

7.2

1.0

2

-
2.9
5.0
6.2

1.6


-
2.6
4.9
5.4

7.3

1.1

3

0.7
4.3
/*
K2
56
38

(3)


(10)
45
715
40

18.5

127

57

(10)
121
Particle Fabric
Size** Charac-
ura teristics
Material ,
Remarks
< 1
< 1

< 1


< 1
< 1
< 1
< 1

< 1

< 1

0.5-10

0.5-50
0.5-50
Glass, Sili-
cone
Glass, -
f. •»
Dacr.-Orl. ,
2.2tw.,
12.5 oz.
Orion, -
Orion, -
Dacron, -
Glass, -

Glass, tw. ,
9 oz.
Glass, 10 oz.

Dacron, 10 oz.

*
Nomex, -
Glass, -
Cleaning
Method
Reverse
Flow
Rev. flow
& sh.

Sh. & rev.
flow

Shake
Shake
Rev. flow
Sh.& rev.
flow
Shake

Rev. flow,
collapse
Sh.& rev.
flow
Pulse jet
Shake
Teinp.
°F
425
375

215


330
110
225
600

375

260

275

240

-------
                                                     TABLE 2.31  (continued)
K3
I
CT>
ID
Dust
Fly Ash
Fly Ash

PVA

CaS04
Cement
Cement

Lime


Fe203,
ZnO
Gypsum
wallbd
Flour
Resin,
fiber -
Cement
Dolomite
Cement
Dust
Loading
Operation gr/cu.ft.
Mun. incin.
Mun. incin.

Matl. handl-
ing
Dryer
Bagging
Kiln

Kiln


Elec . furnace

Trim saw

Milling
Matl. Handl.

Milling
Kiln
Clinker
0.3
0.5

0.05

< >
(10)
0.5

7.5


0.5

0.7

14
2.9

5
7
4
Filtering Operating
Velocity Drag,
fpm I* H25/fpm K'*
Residual
Maximum
2.5
4.5

10.4

7.5
2.5
1.5

2.3


1.9

3.4

8.6
2.7

1.8
3.3
2.5
1.3 1.7
0.9 1.1

0.1 (0.1+)

( ) ( )
(4) (6)
2.0 3.3

(2) (2+)


2,8 3.1

( ) ( )

0.2 0.6
(I) (1+)

1.0 1.6
2.9 5.0
1.8 2.6
180
50

25

0.4
(350)
12

8.8


66

9.2

(4.3)
(8)

70
670
9
Particle Fabric
Size** Charac-
yU.m teristics,
Material ,
Remarks
3
(10)

3

5
10
15

10


10

> 10

1-30
1-100

20
40
50
Glass, plain
Glass, -

Wool, felt

Dacron, felt
Cotton
Glass, tw.,
9 oz.
Glass, 14 oz.


Glass, -

Cotton, tw.

Wool, felt
Cotton, flannel

Cotton, 17oz.
Dacron, -
Glass , 3x1 tw. ,
Cleaning
Method
Collapse
Pulsed rev.
flow
Pulsed rev.
flow
Pulse jet
Rev. flow
Collapse

Rev. flow
& for.
pulse
Collapse

Shake

Rev. jet
Shake

Shake
Collapse
Collapse
Temp
°F
480
425

70

220
70
525

500


195

140

110
70

325
150
500
               cooler

-------
                                                   TABLE   2.31  (continued)
NJ
I
Dust

Kish

Hypo-
chlorite
Alum.
hydrate
Sinter
dust
Sand,
iron
scale
Dust Filtering Operating Particle Fabric
Loading Velocity Drag ^ Size ** Charac-
Operation gr/cu.ft. fpm In H^O/fpm K. ^m teristics
Residual Material,
Maximum

HM pour 0.4 2.5

Matl. Handl. 2.3 3.3

Natl. Handl. >103 1.0

Sinter disch. 1.9 2.3
crusher
Casting 6.7 5.0
clng

I , Remarks
2.4 4.2 230 80 Dacron", s ili-
cone, monofil
0.8 1.0 15 (50) Dynel*', -

0.8 1.1 0.2 100 Dacron, fe'tr,
18 oz.
1.5 2.9 12.5 (100) Glass, -

0.8 1.6 3 <200 Cotton, -


Cleaning Temp
Method °F

Shake 200

Shake 70

Pulse jet 70

Collapse 287

Rev. jet 70


        k f             2
        K_ = 7000 Ap/LV t, inches of water per pound of dust per square foot of fabric, per foot per minute
        filtering velocity.
      **
        Particle size as stated by user or estimated from process characteristics.   The terms in parenthesis

-------
                                                                                                         f        /
N3
I
                                  Plotted  numbers are
                                  filtering velocity
                                    (FPM)
                                                                                                                   ooo

-------
HJ
9
2
I02
5
U.
I '
u.
U.
§• 10'
3.
z
5" 5

2
10°
5
2
• », .. 	 	 ^
•MB __l ^L
1 -X
;___:_ 	
- . . \ \ *
^ \""~™TF"w^"^^"1^ * \ ^
\ \ -
• \. , « ^
. V 	 	 \ a
I \ \ '
NOTES: \ \
_ (a) Dotted lines represent envelope \ \ _
curves for Hatch 8 Williams data. \ \
(b) Individual points from NAPCA-GCA \ V
data. \ \ -
	 \. V
                                     I   2
10°  2     5   10'
      PARTICLE SIZE, microns
I02  2
I03
Figure 2.58.  Resistance Coefficient  (K2) vs BBrticle Size for operating
              Fabric Filters Surveyed in  1969.

-------
                                   (        I        I        (        {        /       f
                                                                            r        r
too
  10
 0.1
                  ial*
  c - Clinker, kiln, milling cement lime
  • • Rock, scone, chiseling, etc.
  i - Casting cleaning sand
            I   I  I  I I Mil      I    I  I  I  II I ll      l	I  I  I  I I  I I
                                                 I   1
    OJ
I
10
100
10,000

-------
an unambiguous estimate of Kl .  Bulk density data  on  the specific dusts

tested would be expected to improve the correlation.  Bulk density  from

Tables 2.19, 2.20 and 2.21 was used to estimate void  volume, and K_ was

computed  from Figure 2.55b.  The actual bulk density  that exists in the

dust-fabric deposit would be expected to improve the  correlation even
more, but will be affected by fabric fiber population (Figure 2.55).

These data are not available, but may be measurable under appropriate

experimental conditions.  Typical Kl values are given in Ref. 135.

For a first approximation of Kl from Figure 2.57,
                               2 ~  D 2                         (2.81)
                                     P

with D  in microns, for 30|_im  0.9 probably.

            0  Fly Ash  (Figure  2.61)  - Pulverized coal fly ash,  labora-
               tory redispersion and  resuspension of Cottrell precipi-
               tated  fly ash, freshly formed  fly ash,  and  fly ash  plus
               limestone additive for S03  control.   Fly ash particles
               tend to be  silicate cenospheres and  iron oxide spheres,
               of sizes  predominantly above 1 micron.   Cement,  lime,
               and dolomite  values from Figure 2.32  have been replotted
               with fly  ash  data.

            o   Irregular  Particles (Figure 2.62)  - Sinter  dust, ground
               mica platelets, mixed  fibrous material,  corn oats,  flour,
               wood dust, kish, paint, aluminum  flake.

-------
            (        f
f        (
               IOOJO
Ln
                                                                            Fumes (O~ Zn, Brass, Copper refining
                                                                                   x : Electric furnace (steel)
                                                                                   D . Other
                                                                                                      1000
                                                                               10000
                                                                        IT1
                                                                        K2
                                                                   /

-------
 100.0
   10.0
 a.
o
                                            C = Kiln cement

                                            0=KUn lime, dolomite
                         D - Fly ash (fresh)
                             and limestone.
                             (not oil)
                '  I  I Hill
'   I   I  I I I III
'  I  I  Mill	\	1  I  Mill
                                                                              1000
                                                                  10,000
         Figure 2.61.  Resistance Coefficient (KjO  vs  Particle  Size for Fly Ash; slope =  -2
                        positioned by  eye-
             I       I
                                                                  (       I

-------
                                        f       f
           I       f
                      1       <
N>
I
                 100 r
                  O.I
                                                          M   M  MMM
                                                                        SLOPE
                                                             Irregular
                                                             Particles
               S  - sinter line
               M  - mica, ground
               f  - fibrous mixtures
               O  - corn, oats, flour, wood,
               K  - kish, paint
               A  - aluminum
                                              I   I	I I
      1  L  I  1 JJ 1
                                                                                                        II  I  I  I 111
                     0.1
10
100
1000
10,000
                                Figure  2.62.  Resistance Coefficient (K ') vs. Particle Size
                                               for  Irregular Particles;  slope = -2;  positioned

-------
             o  Miscellaneous Soft, or Collapsable Materials (Figure 2.63)-
                Talc,  ZnO,  carbon black, CaSO^, aluminum, amorphous
                silica fume.
 Inspection of Figures 2.59 through 2.63 indicates that correlations
 between Kl and D  with respect to gross particle morphology ar.e still
 fairly  scattered?  Based on the slope of a line of -2 through most of
 the  data in each of the Figures 2.59 through 2.63 (at D  = lO^m), we
 conclude that variations due to particle type on K'  can be approxi-
 mated in the following fashion:
             Type of Granule            K , /= Relative Factor in K*  s;
             —  —    •   -               sn \  -    ..... — "••-« --- ~ ------  ~'C~!
             Total Correlation         l,i.e., Kl =  10 /D
                                                2        p
             Crushed                   10
             Fumes                     0.05
             Ash   '                    4
             Irregular                 3
             Soft,  or  collapsable       0.2
 These correlating  factors  are applied to the straight line shown in
 Figure  2.57,  to reflect  the  different locations  of the (-2 slope) curves
 shown.  None  account  for effects  of  eon K0  at D  < 30 urn.
                                           2      P
           2.4.9.4   Effect  of Filtering Velocity.  - The velocity of depo-
 sition  (or filtering  velocity)  is  hypothesized  to have some effect on
 K2,  due  to the kinetic  energy a particle imparts  to  the  cake structure
 upon landing and also,  as  a  consequence of  the greater pressure forces
 set  up  in  the cake  at higher velocity.   Both tend to  produce compaction
 of the  layer and hence,  a  lower value of porosity.
           Effects  of  V  on  K_  are  shown in Figure  2.64  in the form  of  the
relative increase in K_ (K.^/OU)., , as a function of the relative in-
crease in V, (V2/V,).  It is evident from most of these data that in-
creases of velocity by factors of 2 or 3 tend to produce similar increases
in K~ .  Unfortunately, data are not sufficiently well-behaved to enable
the extraction of a single functional relationship.  (Pressure effects on
pake porosity are expected to be of the forme   =  e  e  i , p =» pressure,
k. = a constant, c.f. Appendix 2.3).  Fabric substrate compressibility
and particle cake compressibility are both anticipated to be parameters
in a relationship between K» and V.  No data were located on porosity of
filter dust cakes as a function of V or  p.  Observations by Borgwardt,
et al., of NAPCA    indicate in some studies that K- ~  V   .  These

-------
                                                 {        (        1,1        (       (        [
                                                   T - Talc
                                                   P - PVC, Plastics
                                                   2 - ZnO
                                                     - Carbon Blk
                                                   G - CaS04
                                                   A - A.'
                                                                   1000
10,000
Figure 2.63.   Resistance Coefficient (KjO v* Particle Site for  Soft Collapalble

-------
 findings, however, are not supported by  other data shown  in Figure  2.64
 where the exponents appear  to be nearer one for all  other data.
           Effects of  filtering velocity  on  the dust resistance  coeffi-
 cient are illustrated in Figures 2.65 and 2.66 for three  woven  fiber-
 glass fabrics of indicated design and properties, from Spaite and Walsh
     ft9
 data  .  Averaged values of K? computed  here from Figure  2.65 (at 400
          2
 grains/ft ) are as follows:
                                                        2
      V,fpm      (K2)i(inches water/pounds of dust per  ft   of fabric)-
                  (ft per rain, filtering  velocity)
                                           V2/V1
        2                3.5                 1              1
        4                8.8                 2              2.5
        6               19.3                 3              5.5
These  three  values are shown by asterisks in Figure 2.64.  Similar com-
putations of K«  and velocity ratios  for data shown in  Figure 2.66  (K- -
7000/dust permeability) are indicated by  the dashed lines  in Figure
2.64.  While there does seem to be a non-linear dependence of K. on  V
from these data, these relationships are  not monotonic at  higher V.  As
an approximate estimating figure  for the  effect of V on K. we suggest:

                              K   ~ V /V
                               v   V2/V1                         (2.81a)

(for V usually about 3 ft/min.) which implies K ^ V ,  until further  data
become available.
                                                       I
          2.4.9.5  Fabric Surface Effects. - Fabric effects on K2 are
readily apparent in experimental studies, but are not  easily quantified
at present (e.g., see data of Durham, Table  2.38,  item 3).  Characteristic
effects of fabric nature are indicated approximately in Figures 2.67 and
2.68.   For initial deposition of dusts on clean fabrics Davies   ,  in
discussing Figure 2.68, says "Fibrous dust filters  operating by sieve
action are rarely used, since thin,  sieve-like materials rapidly become
clogged and the filtration and air resistance characteristics are those
of the cake of solid that is built up,  rather than those of the clean

-------
3>
I
                                  \ VREL (A/^ ro*i°«
   Figure 2.64.  Effect t»f Filtering  Velocity on Resistance Coefficient






-------
   Construction Details of Test  Fabrics (also  see Figure 2.26).
FttUr Fmbrie
Air Pannaablllty (efm/(t> at H' HfO)
W4«ht 
-------
               Q.
               O
               ce
               £3
               £
                            BALLOON FABRIC
                            (Sieve type)
                         SWANSDOWN
                         (Nap type)
                      "0     20    40    60    80    100

                        DUST LOAD- GM./SQ. FT. FABRIC
Figure 2.67.   Effect of Dust Load  on Pressure D*op  of various Fabrics

              (from Billington and Saunders,  Ref. 171).
                    '0      250     500     750    1000

                 DUST LOAD- GRAMMES PER SQUARE METRE
                       ft Off **> ft
COTTON SHEET    NAP
                                               FELT
Figure 2.68.  Clogging of various Types of Filter Material (from Oavies,

             Ref. 163 adapted from Billington and Saunders R«f.  171).

-------
material.  Cloth, with a nap raised  on  the side  facing  the  flow  is
better, because some dust particles  are entangled before  reaching the
finer pores  of the base, and the  life of  the  filter  is  consequently
extended.  Better still are homogeneous fibrous  pads of felt-like con-
sistency, as  they can be made effective against  fine particles,  which
deposit in depth, even though the pores are much greater  in size than
the particles."
          The phenomenological description of  the formation of particle
deposits on  fibers and granules leading to a more or less continuous
cake in fabric filtration is apparently not yet  quantitative.  Indications
are however  that the interyarn spaces,  interfiber suaces, and fabric sur-
face  characteristics will all affect the formation and nature of the cake.
Although we  use  the word cake, and  the  concept of a  more  or less con-
 tinuous cake in  analyses for Recalculations  presented  above  indicate
such a cake  is probably  less than about  1 mm  thick for  usual  inlet dust
concentrations ( <10gr/f 3) and filtering velocities (  <10  fpm). In
continuous on-line cleaned collectors,  the cake  formed  between hydro-
dynamic cleaning actions ( <1 min)  is probably less  than  0.1 mm  (100 am
thick).
          Provision of a deep fibrous mat provides larger storage area
for dust, more fibrils for formation of a more open, less dense  cake,
lower pressure drop per unit of dust deposit,  and initially intrinsically
higher collection efficiency.  This  approach  to  aerosol filtration has
been termed defense in depth.
          Tightly woven yarn fabrics provide  less interstitial dust
storage capacity in the inter-yarn spaces.  The  deposit presumably forms
nearer the surface with somewhat higher pressure drop per unit of dust
deposited.   (Flat sieve-like geometries presumably result in highest
pressure drop if pores are effectively plugged by depositing particles).
          Monofilament weaves,  twisted continuous monofilament yarns
(glass and man-made polymer fibers)  with few projecting interyarn fibrils
should result in lower dust storage  capacity,  somewhat higher pressure
drop per unit of dust deposit,  and reduced efficiency at start of filtra-
tion,  during formation of a dust  collecting deposit,  but should be  rela-

-------
 tively cleanable by vibration or shaking.  Typical staple yarns with
 projecting fibrils in the interyarn spaces provide good initial effi-
 ciency, intermediate pressure drop, and intermediate cleaning ease.
 Felts and napped staple yarn fabrics present a large amount of indivi-
 dual fiber for filtration.  Therefore,  they should provide good dust
 retention and storage capability,  but may require more cleaning to re-
 move deposit.  Felts can only be used for industrial applications in
 conjunction with jet and pulsed air cleaning techniques.   Felts are not
 sufficiently strong to be shaken nor can they be effectively cleaned by
 this  method.   Use  of jet  or  pulse  air cleaning  techniques  (repetition
 rate  < 1  min,  on-line)  requires  felt  for  maintenance  of high cleaning
 efficiency.  Woven fabrics are  seldom employed  in jet  or pulse-cleaned
 devices because  of the  dust  bleed  problem.  Use  of reverse-jet  cleaning
 on woven  fabric  will periodically  (~once  per minute)  disturb dust  bridges
 in the  interyarn spaces,  causing dust penetration to  the clean  side of
 the media.  This can lead  to abrasion and wear  of the  reverse jet cleaning
 ring  (blow-tube),  cleaning ring carriage  drive  (gears, sprockets, bearings,
 etc.,) and probably  excessive wear  of the  fabric  exterior.   Separation of
 the ring  from  the  bag surface to avoid interface  wear  problems will re-
 duce  the  effectiveness  of  the jet cleaning action,  and raise costs  of
 power per unit of  dust removed  (or  per unit of air  treated by the filter).
 These general  observations are illustrated schematically in Figure  2.69,
 with  the probable  implications (hypotheses) of fabric  structure and mor-
 phology on performance as indicated.  Effects of  finishes on filaments,
 strands and yarns  on dust holding or  cake release characteristics are at
 present unknown, except qualitatively.  Probably  to a  first approxima-
 tion, finishes have no effect on cake build-up properties, but may have
 an important effect on residual dust deposit retained within interyarn
 and interfiber spaces, which  in turn modifies cake build-up from the
substrate.  Finishes are occasionally applied to retain material and
prevent seeping.  Finishes also are employed to improve deposit release
during cleaning.
          Technically, a plain weave wire screen should be a poor filter,
since dust passes easily through the openings upon start of filtration.

-------
Multifilament yarns  (long
  continuous fibers)

Interyarn and interfiber
spaces relatively  free
of projecting fiber
ends, or fibrils,

Interfiber spaces deter-
mined by filaments per
strand, strands per
yarn, and twists per
inch, and probably of
order of 10 ^m or less
(see Wakeham and Spicer,
Ref.  137).
Probable effects on performance

Particle layers primarily
particle to particle, slow
bridging of smooth open
pores, dust holding
capacity low.

Relatively high K« and
pressure drop, per unit of
dust deposited.

Efficiency lower after clean-
ing, pores open easily
under cleaning,

May require lower filtering
velocity to achieve accept-
able efficiency.

Relatively easily cleaned by
gentle reverse flow of air,
by collapse pucker, easy
cake release.

Interstitial dust deposit retained
less easily, except as modified
by finish and its adhesive char-
acter (i.e., silicone, graphites).
            Figure 2.69(a) Probable effects of Fabric structure on
                           K2, the specific dust-fabric filter
                           resistance coefficient, and performance
                           in service during filtration.

-------
             Spun Staple Yarns  (twisted
             shorter  fibers)

             Interyarn  and  interfiber
             spaces have many projecting
             fibers,  fiber  ends,  and
             fibrils.

             Interfiber space   determined
             by  yarn  and strands  as above;
             but substantially  modified by
             presence of many available fibrils
             for dust deposition. Yarns
             probably more  open,  more porous.
o
                                     Figure 2.69(b)
                                              2-187
                                                              Probable effects on
                                                              Performance
                                                              Particle layers primarily
                                                              particle to fiber, but soon
                                                              bridges form over pores, as
                                                              supported by hairy fibrous
                                                              substrate; dust holding
                                                              capacity intermediate.

                                                              Intermediate K«, intermediate
                                                              pressure drop per unit of dust,

                                                              Efficiency lower after clean-
                                                              ing, but pores open less after
                                                              cleaning.

                                                              Low to intermediate filtering
                                                              velocity.

                                                              Relatively easily cleaned by
                                                              shaking, not very cleanable
                                                              by collapse, gentle reverse

-------
Napped Fabric (Surface
filaments)

Interyarn and inter-
fiber spaces have
many projecting fibrils.

Interfiber spaces
determined by yarn,
strands as above but
probably less important
on effects as nap pro-
vides first order
influence on perform-
ance .

Intermediate in con-
struction and morphology
between woven staple
and felted fabrics.
Probable effects on performance

Particle layers primarily  particle
to fiber, gradually bridging  to
form more or less continuous
homogeneous deposit or cake,
dust holding capacity inter-
mediate to high.

Intermediate K2> intermediate
to lower pressure drop per
unit of dust deposited.

Efficiency fair to good after
cleaning, less easily opened
substrate.

Intermediate filtering velocity

Relatively easily cleaned by shaking,
but with some adhesive dusts may
blind or require gentle  reverse
flow of air, cannot be cleaned
effectively by collapse,

Interstitial dust deposit easily
retained.
                         Figure 2.69(c)

-------
                  Felted Fabric
                  (wool or needled man-
                  made)

                  Interfiber spaces always
                  adjacent to a free
                  fiber

                  Random orientation of
                  individual fibers,
                  many projecting ends
Probable effects on Performance

Particle layers primarily particle
to fiber, gradual penetration of
felt by migrating interstitial
deposit, no straight-through
passages -

Low K-, large dust holding
capacity.

Efficiency intrinsically high •

Filtering velocity high, when ag-
gressively cleaned.

Effective cleaning requires high
velocity reverse-jet or pulse-jet
mechanisms to control porosity,
permeability,and cake depth and
structure.

Interstitial dust deposits retained
to large extent, except migrate as
indicated above.  Basic character
of fiber surfaces different in wool
(scaly) and man-made (smooth) affects
dust deposit retention, except as
modified by finishes and treatments
e.g.  silicones,  resins for effective
dust control and reduction of seeping
in continuously cleaned designs.

c
                                          Figure 2.69(d)

-------
However, dust deposits accumulating  on grid wires  should  build  up  to some
more-or-less critical size.  After resuspension  as  relatively  large
chunks, they can be more easily collected by  secondary  and  tertiary
(e.g., inertial device followed by fabric) treatments.  Limited work on
filtering performance of screens as model filters has been  conducted by
Gallily172, Mercer173, Marshall174 and Corn80.
          The  same general statements should  be  true for  relatively
 large granular  filters  (  lOOO^m)  in service  on  fine particles.  Dust
bridges will be  formed  only with  difficulty,  except at  grain-grain con-
 tact areas, and  dust  deposits  are  detached  as large aggregates more
                                       Q 1
easily collected  by a secondary stage.    Granular filtration  devices
have been reviewed by AVCO   .  Other studies of aerosol  filtration per-
 formance of granular  beds  are  summarized  in Table 2.32.
          Effects  of  fabric character on K- are  illustrated in Figure
2.70 for the data  previously presented.   In summary,  estimated  numerical
values  for K_ appear  as follows:
          Fabric Surface Effect    Relative K0 Values (K   )
              '     '                	2	*—FS-*-
          Smooth,  unnapped             1
          Napped                        1/2
          Felted                        1/4
Fuchs  (Ref. 93, p.  231) summarized similar findings  as  follows:
          "Filters  in which  the specific surface area of  the fibres  is
          high have a low  value of (lOg) .  Fabrics with  a  nap, for
          example, have (K~) smaller  than similar ones without  a nap.
          Using the same aerosol, a glass fabric had  (K£) 10 times,
          and an Orion fabric with a  small nap A times, larger  than  a
          similar  fabric with a large nap.  It is possible  for  (I^)  to
          vary a thousandfold in different fabrics and  for  various
          aerosols."
          The implication  that the specific surface area  of available
fibers in the fabric should enter the coefficient K_ is apparent from
Fuchs1 comments and from the arguments presented in Figure 2.69.  It is
evident that surface area characterization of available filtration fibers
in fabrics  requires consideration in interpretation of K2 values.

-------
                                TABLE 2.32
       SUMMARY OF STUDIES  ON GRANULAR BEDS  USED  FOR AEROSOL FILTRATION
Author
Katz &
Macrae
Egleson
Ref.
(***)
(*)
Date
1947
1954
Granules
Charcoal
Coke
                D.M.  Anderson
53
1958
Sheet,
plastic
spheres
Thomas &
Yoder(l)
Thomas &
Yoder (2)
Yoder &
Empson
Strauss &
Thring
Cheever,
et al.
AVCO
Ducon Co.
Squires
& Graff
Schoenburg
Taub
*
G.C. Egleson, H.P. Simons,
113
114
115
116
(**)
175
(****)
APCA
paper
(175)
( 81)
L.J. Kane,
1956
1956
1958
1960
1966
1969
1969
1970
1969
1970
and A.E.


Sand
Crushed
refractory
Sand
Review

Sand
Review
Research on
filter with
granular beds
Sands, The Moving-bod Coke
Filter, U.S. Dept. of Interior, Bur. of Mines. Rpt. of Investigations 5033,
1954.
&&
  C.L. Cheever, C.R. McFee, J. Sedlet, and T.L. Duffy, ZPPR Roof Sand Filtra-
tion of Uranium, Plutonium, and Uranine Aerosols, Proc. 9th AEC Air Cleaning
Conf., Boston, Mass. (13-16 Sept. 1966).
^fffie
   S. Katz and D. Macrae, J. Phys.  Coll. Chem. 52, 695 (1948).
   A.M. Squires and R.A. Graff, City College, N.Y., Panel Bed Filters for
Simultaneous Removal of Fly Ash and Sulphur Dioxide; III. Reaction of Sulphur
Dioxide with Half-Calcined Dolomite, presented at the 63rd Annual APCA
Meeting, St. Louis, No. (June 14-18, 1970)
****
    Chem. and Eng. News, 57 (15 December 1969).

-------
£
b*
X

c
    100
     50
     20
     10
u
d
CO
•r-t
tn
Q.
to
              Smooth
              Fabrics
Napped
Fabrics
Felts
     Figure  2.70.
                   Changes  in Specific Resistance Due to
                   Fabric Surface.   (From published results
                   of  5  separate studies, in each of which
                   fabric surface was the primary variable.)
                   (See  also Appendix 2.6).

-------
           2.4.9.6   Clean Air-Flow Permeability.  -  The  air  flow permea-
bility  of  new clean unused  fabric furnished  by the manufacturer (as
      2
cfm/ft   at 1/2 inch of water  pressure  drop)  was  examined as  a  variable
in K-.   Data  relating K~ during  filtration and the original  clean cloth
permeability  are shown in Figure 2.71.  Although the data  are  not as  con-
sistent as desired, an approximate relationship  for estimating indicates
that K- chi
meability;
                                                2
that K- changes by a factor of 2 for a 50 cfm/ft  change in clean per-
           Initial Clean  Cloth Permeability            J£
                     _               ..	_               perm-
             re fm/ft @  k  inch water\
                         10                             1.3
                         20                             1.2
                         30                             1.1
                         40                             1.0
                         50                             0.9
                         60                             0.8
                         70                             0.7
                         80                             0.6
                         90                             0.5

           2.4.9.7  Other Effects. - Other possible physical  factors that
affect K2  include typical  pressure differentials  (Ap) encountered in a
cotnpartmented filter and the associated compression and decreased
porosity,  e.  Effects of cleaning modify porosity, residual  deposit,
deposit length, and  permeability, and electrostatic charge effects   , as
indicated  in Figure  2.72.
           Summarizing the  above effects, K. can be predicted approxi-
mately as  follows:
            -2

             K ,              Material                  K ,
              sn             —————                   sh
                             Crushed                    10
                             Fumes                     0.05
                             Ash                       4
                             Irregular                 3
                             Collapsible               0.2

-------
400
                                           Conclusion: Slope « as double Ho*
                                                     or greater I.e.,
                                                     K halves for a SO CPM
                                                     Increase, on average.
                                                     (Practically all lab
                                                     dat.i)
             10
20       30       40        50        60      70
CLEAN  PERMEABILITY(CFM/052H 0)
                Figure 2.71.   Influence of  new fabric air-flow permeability
                               on resist, coef.  (!(„')•

-------
   Comparison of efflclwiclM obtained by giving o filter fabric potltlvt and negative charges
B
   60
   50
   40
o
UJ
o
LU ™
a: 30
UJ
t
o 20
u
    10-
                 A = NO CHARGE
                 B = POSITIVE CHARGE
                 C = NEGATIVE CHARGE

                     I               2              3
             WEIGHT OF DUST (oz. per square foot)
 Figure  2,72.   Effect of  Fabric Charge on Pressure Drop.
       or
                   Smooth
                   Napped
                   Felts
                                               1
                                               1/2
                                               1/4
  K
   perm
                   Permeability
                   10
                   20
                   30
                   40
                   50
                   60
                   70
 perm
1.3
1.2
1.1
1.0
0.9
0.8
0.7
                                                         (2.81b)

-------
                              80                        0.6
                              90                        0.5
 from which (K9)  predicted - 1000 D    (K ,. K  .  K__ . K    ) .  For
              £                    p     sn    v    ra    perm
 example,  for fly ash of 20 /A m particle size at a filtering velocity of
           2
 4.5 ciWft , on a napped fabric of new permeability 30:
      (K2) predicted = j     . 4 . () . %  . 1.1   =8.2 inches water
 per (pound per square foot) (ft/min) .  (The value of K_ calculated from
 the Williams, et al., relationship at p  » 2 is K» = 4, Figure 2.55b).
           Values of (K-) predicted were plotted against K2 observed for
 all the  field data in Table 2.39, as shown in Figure 2.73.  With some
 reestimation of particle diameters in certain isolated instances, 80
 percent of the data lie within a band bounded by a factor of 5.  The
 estimation of friction factor for a packed bed (Figure 2.53, Leva
       141
 et al.   ) can only be obtained to within a factor of about 2.  Since
 diameters in these present data are estimates (or for unknown analyses),
 the spread is not surprising.  The data indicates that more detailed
 information is required on both particle-bed variables and fabric
 variables.
          For comparison with the  foregoing multi-factor prediction
method, K_ values were also predicted  by the Williams, et  al.  relation-
ship (Figure  2.55b) and also plotted against  the  observed  field data -
obtained K_ values.  The correlation,  however, was poorer  than that shown  in
Figure 2.73.  Still another attempt at prediction might include a  func-
tion of dust  deposit porosity as one factor in the multi-factor method.
   i                      ;
          The multi-factor prediction  method  for  (K») was  further  tested
by using all NAJPCA data for fly ash (or fly ash plus granular additive)
as shown in Figure 2.74.  Much of  the  data lie within a 5x band width
as found above, but no clear trend is  evident.  Since NA.PCA data were
obtained by the same group using similar experimental methods for  the
most part, the lack of improvement in  agreement between predicted and
observed apparently reflects the limitations of the approximation scheme.
          Further discussion of K_ and the effect of fabric, geometry,
and collector design factors is contained in Chapters 4 and 6.


-------
I      f       (
                                                      (      {
                         i       r      (       r      f      i

                              " .        <3;
  lOO.Otr
   10.0
I
    i.o
                 Envelope  of Data Points
     O.I
              I    I   I  I i  I I
                                                     I   I   1  I  I I I I I	I   I   I  I  I I I I
       O.I
                         1.0
10.0
100.0
1000
                                                    *( Predicted)
                                                               /

-------
'0  I
                            Ka(pred.)
   Figure 2.74.
Comparison of Specific Resistances, Predicted and
Observed, for NAPCA Data.

-------
 2-5   SYSTEM PRESSURE AND FLOW
      Sections  2.4.7  to 2.4.9  describe  the  relation  between  filtration
 velocity  and pressure drop  across  the  cloth.   By  bringing in  other  parts
 of the  fabric  filter system,  ducting,  fan,  etc. the relationship  between
 fan  pressure and  total flow can  be estimated.  This will be seen  to depend
 on factors  such as the length/diameter ratio of the bag, the  location  of
 the  gas inlet  to  the baghouse, reverse cleaning air,  etc.   For  a  first
 approximation, however,  the pressure drop  at the  fan can be estimated  at
 about twice  the pressure  drop across the cloth.   The total  flow will be the
 product of  the amount of  cloth being used, which  will usually change from
 time  to time in a multi-compartmented  fabric filter using automatic sequen-
 tial  compartment  cleaning,  and the average  filtration velcoity.   Velocity
 will  also change  with time  and be  different at different parts  of the
 baghouse, so that average velocity may differ considerably  from measured
 velocity at  any specific  location  in the filter.
      2.5.1   Flow  in  a Single  Bag
             After entering  the bag or  tube, the flow  continually  decreases
 as part of it filters  through the  cloth; or, if the  dust is collected
 on the outside of the  tube, the flow continually  increases  inside the  tube.
 The same principle applies  to frame-type filter elements unless they are
 Vee shaped.  In cases  of horizontal flow through  the  baghouse this  change
 in velocity  probably  doesn't  make much difference.   In cases of downward
 flow where the dirty  gases move slower and slower as  the bottom of  the
 tube approaches, more  large particles will tend to  fall into the hopper
without touching  the  cloth.    Furthermore, if the downward flow continues
 during the cleaning cycle it may hasten the fall of collected dust  into
 the hopper.   In cases of upward flow, all dust particles carried into
 the filtering  tube  reach the cloth, the largest  ones collecting
near the bottom of the tube, whereas only the smallest ones  reach the
 top where  the upward  velocity approaches zero.

-------
             The expected consequence of this elutriation would be a higher
 filtration resistance near the top of the  fabric collector so that the
 filtration velocity would drop off even more sharply in the upper region.
 In fact,  however,  the flow is usually fairly turbulent  in the entrance
 region and more cleaning energy is imparted to the  top  of the bag.   This
 can result in over-permeability of the top cloth and under-permeability
 of the bottom.   As a result of both efforts, the middle of the bag should
 probably  collect the heaviest loadings as  confirmed by  NAPCA in about'
 1960 during some tests.   These show differences of  a factor of three in
 cloth loading along the  bag.   One  concludes that since  the behavior
 of dust on the  cloth is  not generally predictable the longitudinal  flow
 in the bag is not  completely predictable either.  The velocity V  of
 flow entering a tube is  4L/D x V or entering an evelope,  2L/W x V where
 L  is distance from inlet to dead end,  D is  diameter,  and  W is entrance slot
 width.  Since V is usually 1  to 10 FPM,  the entrance velocity ranges
 from 100  to 1000 FPM.  Table  2.33  lists  the particle sizes that can be
 supported  by velocities  of this order.   To  a first  approximation, all
 larger particles stay  below the bag entrance and  all smaller  ones rise
 to  a point  of smaller  velocity.  Table  2.34  and Figure  2.75 indicate typical
 transport  velocities  required  to support and convey  common industrial  dusts.
 Since  all  are >1000 fpm, much  dust  of  larger sizes will be deposited in the
 hopper  of  an  operating fabric  filter.
            There  is a small,  often negligible  pressure loss  associated
with velocities of~100 fpm as  the,flow first  suddenly enters  the bag and ,
                                                                  2
 then moves  along the bag.   The  entrance  loss is on the order of  AVe
or  typically 0.01  inch of  water.  Fluid density,A  ,  is the sum of  gas
and particle cloud  densities, but except for  concentrations above about
        3
 50  gr/ft   (typical  of  pneumatic  conveying)  the  latter is negligible  since
                                                  3
normal  gas  densities.are on the  order of 500  gr/ft..  In  the  case of
 filtering outside  the  bag  the head  loss is even smaller.
            Flow moving through  a straight porous duct undergoes a  pressure
change due  to friction (viscous drag at the walls of  the duct), gravity,

-------
       Table  2.33.   Spherical Particle Sizes Transported by Indicated
                    Upward Air Velocities (Terminal Settling Velocities)
                                         (cm/sec)
X
0.01
0.02
0.05
0.1
0.2
0.5
1.0
2.0
5.0
10.0

1 	
1
.000033
.0007
.00018
.00037
. 00074
.0018
.0037
.0074
.018
.037
- Particle
•T
.OOOi:
.00026
.0007
: .0014
. 0028
.007
.014
.028
.07
.14
or Aggregate
3 10
.00(1,' .00,' »•
.0011)
.004
.008
.016
.04
.08
.16
0.4
0.8
.000
.016
.032
.064
.16
.32
.64
1.5
3.1
Diameter,
20
.
.
.
.
0
1
2
6
12
026
07
14
28
.7
.2
.5
.4
.5
°P
50
.15
0.4
0.8
1.6
3.9
7.6
14
35
60

\f*p)
100
.fiO
1.6
3.0
5.9
14
25
45
97
166

:?00
l :
2.)
5.5
11
19
42
70
114
223
352

'•>00
10
23
39
65
121
195
310
575
890

~ " ' i
UiOO
' \
51
83
130
250
390
625
1100
1730
f
The  density of a dust particle can be substantially lower  (<0.1)  than
the  true  density of the material from which it is made, as a consequence
of air  inclusions in aggregates - see Tables 2.3 and 2.4,  for  typical
particle  densities, Tables 2.19, 2.20 and 2.21 for typical dust bulk
densities,  and Figures 2.46 and 2.47 for the variation in  bulkiness
with particle size.
                       '00
   3OO      IOOO
DIAUCTfR OF MRTICLt
                                                3OOO
      Figure 2.75. Conveying Velocities  (from  Fan  Engineering,
      Buffalo Forge Co.)

-------
             Table  2.34(a). Conveying Velocities  for
I
ro
o
Material
	 	 	
Wood riuur — sander dust
Sawdust, light, dry
Sawdust, heavy, wet or green
Shavings, light, dry
Shavings, heavy, wet or green
Wood blocks, edgings, heavy, wet or green
Hogwasio
•*"*.'-,'• i -.-
urinomg dust
Foundry dust, tumbling barrel, shake-out
Sar.a blast dust 	
Burring lint, dry 	
Buffing lint, sticky
iletat turnings
Lead dust
/"• j-j. " '
Cotton 	
Cotton lint, flyings
Wool

-uteiint, flyings
..^;re Dicker stock, shredded bagging
fU'.c dust shaJter waste
Jute butts (conveying) —
ti iv. in
(/-'---d feed, 1/,-in. screen (conveying)
ur; — uust ...
c..-.~:..-v; ..e^ns
•_-•••'* "--•-
-"•-.". .->cr C\:S;
. .i.-. -•? ;r.ojcin;j powder
— - ..v...c .':.^.i.:..'.^ pOV.'ue;' CJUS1
L-\.:,.;,o cii.st ana surfacer chips —
Velocity
Fro:::
:500
2000
30uO
2000
. 3000
S500
3500
3000
3500
3500
2500
3000
4000
A /VAA
4000
2500
1500
3000
2800
30GO
3100
3600
2500
4500
2000
3000
30G&
200-1
"iOGO
'•':" :"•'";
3000
• — F.PJL
To
2000
3000
400C
' 3000
4000
4500
: 4500
4000
• 4500
. 4000
: 3000 •
4000
5000 :
'. 5000
3000 l
2000
4000
3000
3300
3400
4500
: 4000
5000
3000 :
3500
,-, ."* •-• •-.
-lUVU
2500
35HO
*>-.'••:
— .^/v^O
4000 :
     From Alden, Ref.  178.
       Table 2.34(b).  Velocities
                         Pneumatic

                  Material
  ;	
  j  Wood flour  .
  i  Sawdust  ....
    Hog  waste
  i  Pulp chips ...
  i  Tanbark,  dry .
    Tanbark,  leached, damp
    Cork, ground
    Metal turnings
    Cotton
    Wool
    Jute
    Hemp	
   Rags   	
    Cotton seed	
   Flour 	
   Oats	
   Barley   	
   Corn   	
   Wheat	
   Rye

   Coffee beans, stoned
   Coffee beans, unstoned
|j .Sugar
•\  Salt
;j  Coal, powdered

 j Ashes, clinkers, ground
 i Lime
 I Cement, Portland
; Sand
                                                                                                    for Low Pressure
                                                                                                    Conveying
                                                                                                          Velocity. F.r-.
From
4000
4000 ,
	 4500
	 4500
	 4500
p . : 5500
{*-'•*•• • • .... ^ -. • -v w
	 3500
5000
1 4000 '
' 4500 :
. 	 	 *f*JJ\.-
4300 '

	 j 4500 <
	 j 4500 1

j 4000 !
i 3500 •

	 _ ; 4500 ;
	 : 5000 :
	 i 5000 !
. . . 1 5000 i
'• » ,-» *i
	 i 5000 ;
	 | 3000 '
; 3500 ;
i * 1
	 I 5000 '
	 •• 5500
	 , , 4500 :
' 6000 :
5000 .
6000 '
6000 j
To
6000
6000
6500
7000
7000 i
7^0
i Ouu
5500
r*/\ f\f\
/ t'Uv
6000

6000 '
i
f* f\t\f\
bOOO
6000
/»XAA '
ooUU
C f\f\f\ '<
bOOO
/* f\f\f\
bOOO :
6000 !
r>50o
7000
7000
7000
:.;r>oo
sOOO :
60UO
7500
6000
H500
7000
9000

-------
and  change  of  velocity.   Comparing  points  1  and  2  along  the  tube,
        +/fvl  + Fr   =  P2  + /9fv2
where  F   is  the  frictional  drag,AL  is  the  difference  in  height  beween
the points,  and  P  is  the aerosol  density being  lifted through this
height.   In  the  gravity  term, normal dust loadings  are insignificant  but
500° filtration  can add  0.5 inches of water at the  top of a 40 foot bag
just from the added gas  buoyancy.   (Buoyancy doesn't contribute  to pressure
drop across  the  cloth, just across the  baghouse).   The friction  term  is
probably  negligible, since  even a  nonporous bag  having an L/D ratio of
40 and carrying  gas at 300  FPM would only have a  .05 inch pressure drop
                 yr
from end  to  end.    Furthermore, the  process of filtering  continually
reduces the  boundary layer  thickness (and related drag loss).  The
velocity-squared terms in Equation (2.82) are normally negligible.  Thus
an  analysis of  all terms in Equation (2.82) shows  that there is usually
not much  pressure difference between the ends of a  vertical or horizontal
filtering tube,  and likewise for   a  frame filter.   Abrasion and  turbulence
associated with  high entrance velocities are discussed in Chapter 8.
     2.5.2  Flow in a Single Compartment
            Air velocity within a hopper or other expanded inlet plenum
decreases since the product of average velocity and cross-sectional area
of a duct remains constant.  As the vertical component of velocity decreases,
all particles settle faster, in accordance with Table 2.41, so that in a
very spacious hopper considerable material may never reach the cloth.
Turbulence, however, which is present with or without baffles, retards this
settling and may even re-aerosolize some of the dust from the bottom of
                                     9
 In a non-porous pipe, A-f>= f(A.'-/p)(V /2g), where f is the pipe friction
 factor, a function of Re pipe and wall roughness, typically 0.05 + a
 factor of 2.  (Hunsacker and Rightmire, p. 126).


-------
 the hopper.  (Air in-leakage through hopper seams, valves, etc., also
 will resuspend part of the dust and increase the dust load to the fabric).
 Most particles caught in the accelerating flows rushing into the filter
 chamber cease to settle appreciably and eventually they reach the filter
 cloth.

              Some baghouse  inlets direct  the  flow  tangentially around
  the  inside of  the  hopper either to  reduce  the  loading  to  the cloth
  or to protect  the  cloth from very  large  particles.  The particles in
  a rotary  flow  are  thrust radially outward  with an acceleration  equal  to
  2
  V /R where R is the  radius  of  curvature.   Since acceleration is  of  the
  same order as  gravity, Table 2.41 gives  very rough estimates of  the
  velocity  of  outward  travel  of  the particles.   Whether  they move  outward
  quickly enough to  reach the vicinity of  the  wall  where they can  quietly
  settle depends on  their size,  on velocity, on  the inlet configuration,
  and  the degree of  turbulence.

              If clean air were  blown into a compartment where all filter
 elements had  uniform  permeability,  the  flow would  probably be essentially
 uniformly  distributed among the elements.   In actual practice, however,
 perfect distribution  is probably rare,  due  to some elements  receiving
 fine  particles  than others,  variability in cleaning  intensity,  and differences;
 in local operating  temperatures, etc.   The  resulting mal-distribution  of
 flow  which is generally hard to analyze can lead to variable bag  life.
            In  small  hoppers or .inlet plenums,  the flow may bend  abruptly
 as it enters  the filter entrance so  that  the  centrifugal effect described
 above may  reduce the  concentration of large particles reaching the cloth.
            Flow enters the  filtering portion of the compartment  at fairly
 high  velocity and,  except in tapering filter  elements, slows down as it
 approaches  the dead end.  On the clean  side of  the cloth,  however, the
 reverse happens; the  flow accumulates and accelerates toward  the compart-
ment exit.   As it does sofsmall pressure changes take place similar to
 *
  From Alden,  Ref.  178.

-------
 those described in 2.5.1.
             Flow suddenly  entering a large chamber from a small one, such
 as  from an inlet duct to the hopper, undergoes pressure increase
                                                            ,2/A
 velocity slows  down.   The increase  is  given by (p2 -  D-) =/f^l(
 For example,  with an  area ratio of  1/5 and  an inlet velocity of 3000 FPM,
 the  pressure  increase  would  approximate    0.2  inches  of water.   If the
 entrance  were very gradual  the  pressure  increase would be larger and given
          2 /   2  2 V
 by (^)/JfVi (l-A^/A- I.      In  the example,  the  increase would  be  0.6 inches;
 thus the  sudden  expansion causes a  flow-to-heat  energy "loss" equivalent
 to 0.4  inches of pressure.
      2.5.3   System  Flow
             The baghouse,  usually  consisting  of  a  group  of  compartments,
has an overall drag or  resistance  to  air  flow that depends  on  that
of each compartment.  Each compartment with its  lead-in  ducting  has  its
own flow resistance which,  as discussed above, depends on where  it  is
in the cleaning filtering  cycle. among other things.   Since  the compartments
are arranged  in parallel,  the resistances  combine  as  in  conventional
electrical circuitry:
                                1 - 1,  2,  ---- n                     (2.83)
                   Rt
where R  is the overall baghouse resistance for n compartments.  The  resis-
tance of any one compartment plus lead-in ducting is to a  first approximation
the ratio of the flow through the compartment to the pressure across  it.
The pressure across the compartment is approximately the same for all
compartments.  Thus equation (2.83) could be written:

                  1  "  J2t  = 1-  S Q*   '   i a l'  2>  	n        (2.84)
                                   <-

-------
 where  Q  is  the  total  flow through the baghouse andAp  is the
 pressure  across  it.    The pressure across  the primary fan is  greater
 than the  pressure  across  the baghouse because of friction in  the
 ducting which can  be  several inches of water for high duct velocities
 or  large  L/I) duct  ratios.   The pressure across the fan is the sum
                              +Apt                            (2<85)

where F,  D,  and  t  refer to  fan,  duct,  and  baghouse  respectively.
             The  pressure across  the  fan, however,  is  uniquely related to
the  flow  through the  fan by the  fan  manufacturer's  operating  curves.   The
fan  curve is different  for  each  fan  type,  see  Figure 2.76.  A  curve  for a
centrifugal  blower is given in Figure 2.76(b).  Although  one has the option
of operating a fan on a variable speed basis,  a  constant operating
speed is  the more  usual practice.
             The  dotted  line in Figure 2.76b  represents the relationship
between baghouse flow and baghouse-plus-ducting  pressure drop determined
as above.  This  curve depicts a  transient  condition since the drag  of
individual compartments  is  continually changing  (although at  a slow rate
in some systems).  The  slope of  the  curve  represents  1/R for  the baghouse-
duct combination.  In order for Ap to be given by both the fan curve
and equation (2.85), that is, for both curves  in Figure  2.76(b) to  apply
simultaneously, Q and Ap must be at  the intersection of  the curves.
In other words the flow  through  the  entire baghouse system is  determined
by the baghouse-ducting  flow resistance and by the fan curve.

-------
(a) Typical fan performance
    curves
                                                                       Centrifugal
                                                                       Fan Cum
(b) Gas flow-pressure system
    curves for  the  usual fabric
    filter system.
                 Figure 2.76.  Typical  Fan Curves.

-------
2.6   REFERENCES FOR CHAPTER 2

      1.  C.E. Lapple, Characteristics of  Particles  and  Particle Dispersoids,
          Journal of Stanford Research Institute,  5_,  95

      2.  M. W. First and P. Drinker, Concentrations of Particulates Found
          in Air, AMA Archives of Industrial Hygiene and Occupational
          Medicine.  5,  387, (1952).

      3.  W. T. Sproull,  Viscosity of Dusty Gas, Nature  (London) 190, 976
          (1961).

      4.  A. R. Kriebel,  Analysis of Normal Shock Waves in Particle Ladeu
          Gas, ASME  Paper No.  63-WA-13,  Trans ASME, J. Basic Eng.. (1964).

      5.  R. E. Singleton, Fluid Mechanics of Gas-Solid Particle Flow in
          Boundary Layers. Ph.D., Thesis, California Institute of Technology,
          (1964).

      6.  J. T. C. Liu, Problems in Particle-Fluid Mechanics, Ph.D. Thesis,
          California Institute of Technology (1964).

      7.  S. L. Soo, Fluid Dynamics of Multiphase Systems, Blaisdell (1967).

      8.  L. Silver-man, C. E.  Billings and M. W. First, Particle Size Measure-
          ment in Industrial Hygiene, U.  S. Atomic Energy Commission Mono-
          graph Series, (in press, 1970).

      9.  R. Dennis, ed., Handbook on Aerosols,  U.  S.  Atomic Energy Commission
          Monograph  (In Press,  1970)

     10.  K. T. Whitby  and B.  Y.  H.  Liu,  Dusts,  Encycl. Chem. Technology.
          Vol.  7,  p  429,  1965.

     11.  W. C. McCrone,  R.  G.  Draftz, and J. G. Delly, The Particle Atlas,
          Ann Arbor-Humphrey Science  Publishers, Inc.  (1965).

     12.   W. C. McCrone and  M. A.  Saltzenstein,  The Microscopic Identification
          of Atmospheric  Particulates, APCA Annual  Meeting Paper No.  61-10,
          APCA, Pittsburgh,  Pa.  (1961)

     13.   H. L. Green and W. R.  Lane, Particulate Clouds: Dusts, Smokes
          and Mists, 2nd  Ed.,  D.  van  Nostrand Co.,  Inc.,  New York (1964).

     14.   W. Stober, A. Berner,  and R. Blaschke, The Aerodynamic Diameter
          of Aggregates of Uniform Spheres,  J. Coll.  Interface Sci..  29,
          710 (1969).

     15.   R.  Whytlaw-Gray and  H.  S. Patterson, Smoke:  A Study of Aerial
          Disperse Systems,  E. Arnold and Co., (London),  1932.

-------
 16.   S.  K.  Friedlander and C.  S.  Wang,  The Self-Preserving Size Distribution
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158.  Pangborn Corp., Electric Arc Furnace Dust and  Fume  Control.  Bull.
      No. 932, Pagborn, Hagerstown, MD.

159.  M. W. First, Silverman, L., et al., Air Cleaning  Studies;  Progress
      Report, Tech Info Ser, Oak Ridge Tenn. NYO-1586,  Dec. 52

160.  R. H. Borgwardt, R. E. Harrington, P. W. Spaite,  Filtration
      Characteristics of Fly Ash, JAPCA. 387. 68.

161.  F. H. Valentin, Cloth Filtration of Fine Aerosols,  British Chem
      Eng.. "7, 268, 4 62.

162.  J. L. Lynch, Some Answers on Dust Control, Rock Products. May 67.

163.  C. N. Davies,     Separation of Airborne Dust and Particles,
      Proc. Inst. Mectu, Eng., I B, 185, 1952.

164.  J. S. M. Botterill, and J. M. Dallavalle, The Determination and
      Control of Industrial Dust. USPHS Bull 217, Wash. D. C. 135, Apr. 35.

165.  J« J« Bloomfield and J. M. Dallavalle, The Determination and Control
      of Industrial Dust, USPHS Bull 217, Washington, D.  C. 135, Apr.  35.

166.  Air Preheater Co., Inc., Evaln of Fabric Filter as  Chem. Contractor
      for Control of S02 From Flue Gas, Final Rpt-Part  1, for NAPCA
      Contract No. PH22-68-51, 28 August 1969.

167.  G. A. Dick, Underground Dust Control With Cloth Filters, Mln Cong.
      JNL, 31 August 1963.

168.  W. C. L. Hemeon, Plant and Process Ventilation, The Industrial Press,
      New York, 1963,

169.  A. L. Labbe, Donoso, J. J., Modern Baghouse Practice for the
      Recovery of Metallurgical Fumes, J. of Metals, Trans AIME 188.
      792, May 50.

170.  W. Licht, Removal of Particulate Matter From Gaseous Wastes-Filtra-
      tion, American Pt?troleum Inst., New York, 61.

171.  N. S. Billington and D. W. Saunders, Air Filtration, Jour. Institution
      of Heating and Ventilating Engineers. (London), 15, 46, (1947)
                                                i
172.  I. Gallily, On the Filtration of Aerosols by Filter Models of Various
      Porosities, Jour. Colloid Sci., 161,, (1957)

173.  T. T. Mercer, C. M.  Mills, and A. R. Gibb,  Collection Efficiencies
      of a Cascade Filtration Device Employing Wire Gauges, Annals
      occupational Hyg. 1, 301 (1960).

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174.  C. G. Marshall, Deposition of Aerosol Particles on Screens, Ph.D
      Thesis, Ohio State U, Piss. Abstr. Jji, 1948, (1965) Publ. No. 17,
      392.

175.  T. Schoenburg, Evaluation of Granular Bed Devices, Final Report,
      Contract No. PH-86-67-51, Phase III, AVATD-0107-69-RR, AVCO
      Applied Technology Division, Lowell, Mass. (June 1969).

176.  E. Butterworth, The Filtration of Dusts,  Manufacturing Chemist.
      66, January 1964, February 1964.

177.  R. H. Borgwardt, and J.  F. Durham, Factors Affecting the Perfor-
      mance of Fabric Filters, Paper 29c, Presented at 60th Annual
      Meeting, A.I.Ch.E, New York City  (29 November 1967),

178.  J.L. Alden, Design of Industrial Exhaust Systems,  pp  61,  168, The
      Industrial Press, New York, N.Y., 3rd Ed. (1959).

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                            CHAPTER  3
                      TYPES OF  FABRIC  FILTERS

 3.1   STANDARD  AVAILABLE  FABRIC  FILTER  EQUIPMENT                     3-3
      3.1.1   Configuration                                           3-3
      3.1.2   Cleaning Methods                                        3-4
      3.1.3   Filter  Fastening Techniques                             3-6
      3.1.4   Size                                                    3-7
      3.1.5   Typical Fabric Filter Equipment                         3-7
 3.2   FILTER  CONFIGURATIONS                                          3-20
      3.2.1   Panel vs. Tube Filters                                  3-23
      3.2.2   Upward  vs. Downward Flow                               3-24
      3.2.3   Inside  vs. Outside  Filtering                            3-26
      3.2.4   Length, Diameter and Length/Diameter Ratio              3-27
      3.2.5   Other Configurations                                    3-32
 3.3   CLEANING  MECHANISMS                                            3-32
      3.3.1   Shake                                                   3-35
      3.3.2   Reverse Flow - No Flexing                               3-39
      3.3.3   Reverse Flow with Collapse                              3-42
      3.3.4   Pulse Cleaning                                          3-43
      3.3.5   Reverse Jet Cleaning                                    3-47
      3.3.6   Vibration and Rapping Cleaning                          3-50
      3.3.7   Sonic Cleaning                                          3-51
      3.3.8   Manual  Cleaning                                         3-52
 3.4  CONSTRUCTION AND MATERIALS                                     3-53
     3.4.1   Housing                                                 3-53
     3.4.2   Hopper  and Disposal Equipment                           3-56
 3.5  EXTENSIONS OF FABRIC FILTRATION EQUIPMENT                      3-58
     3.5.1   Variations In Standard Design                           3-58
     3.5.2  Control of Gases and Odors                               3-60
     3.5.3  Control of Mists                                        3-60
     3.5.4  Ultrafiltration                                         3-62
3.6  REFERENCES                                                     3-62


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                              Chapter  3


                       TYPES  OF FABRIC FILTERS


     Approximately  50  U.S.  manufacturers  produce  over  100 fabric  filter

models,  each  of which  is  available1  over a range of  sizes.   The term "model"

is used  to  distinguish principally  between:  (a) the configuration of the

cloth  filter  element;  (b) the method of cleaning  the cloth;  and to some ex-

tent (c)  the  way  the cloth  is held  in  place.  These variations constitute

the main  differences between  the  available fabric filter  designs.   This

chapter briefly describes the models of fabric collectors  currently avail-

able and  in addition considers the  reasons for the  designs of  presently

available equipment; for  example, the  relationship  between the filter  bag

length/diameter ratio  and the performance  of the  collector as  a whole.

3.1  STANDARD AVAILABLE FABRIC FILTER  EQUIPMENT

     Following a  1969  mail  survey and  a series of subsequent technical

discussions,  the  data  presented in Appendix 3.1 was  assembled  listing
                                                     it
the fabric  filtration  equipment available  in the  U.S.   The equipment

is described  in the appendixed tables  across the  design characteristics

discussed below.

     3.1.1  Configuration

            Nearly every  fabric filter is  of one  of  two types, envelope

or cylindrical.   In the envelope  filter,  flat panels of cloth  are
     *  It should be emphasized that the following discussion and the
information presented in Appendix 3.1, is based upon the information
provided by the manufacturers included in the survey.  We attempted to
include all known manufacturers of fabric filter systems by including
all companies in the survey that were listed in the appropriate cate-
gories in the 1969 Product Listing in Environmental Science and Tech-
nology and the Journal of the Air Pollution Control Association.  In
addition, we cross checked the membership roster of the Industrial Gas
Cleaning Institute and other known sources of listings of fabric filter
manufacturers.  We are unable to fully evaluate the completeness of the
product information provided in the survey, but are confident that we.
have received a large percentage of the non-proprietary information
available from these companies.

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 stretched over a frame.  The panels are usually in pairs, or more rarely,
 pleated.  The cylindrical filter is either open at both ends or more
 frequently closed at one end in the shape of a bag.  Both envelope and
 cylindrical tube elements are nearly always vertical, so the accumu-
 lated dust can fall to a collecting point >for disposal.  Usually, many
 such elements are located side by side in the dust collector.
             As Figure 3.1 indicates, the configuration of the filter
 element is related to the flow of gases through the element; dirty
 gases usually enter the bottom of the compartment and flow upward.
 Also basic to the filter operation is whether the dust will collect on
 the inside or the outside of the filter element,  and this fixes the
 number and location of tube  sheets and plenums in the compartment.
 Consequently,  the configuration of the filter element determine^ the
 configuration of the entire  baghouse compartment.              •••'•$'
      3.1.2  Cleaning Methods
             A  second way to  distinguish between fabric filter models
.is  by the  manner in which the  cloth is cleaned.   The cleaning process
may be  entirely  manual,  or it  may  be initiated manually and  then com-
pleted  automatically.   Larger  systems  usually completely automatically
clean the  elements  by using  a  timer or pressure  limit switch.   The
most common  methods  are  by shaking  or  by  reversing  the  flow  through
the cloth  in some manner.  Shaking  methods  include  horizontal or  ver-
tical shaking  of one end  or  all  of  the filter element;  vibrating  or
rapping; fluttering  with  air currents;  snapping the  cloth with  a
pulse of compressed  air;and  sonic  cleaning.  Methods  of  flow reversal
include  forcing  the  dust  cake  off  the  cloth with back pressure; col-
lapsing the  cloth with associated  flexure and  cracking  of the dust
cake; snapping the cake  off  with a  pulse  of compressed air; and blowing
it  off with  a jet of air  through the cloth.  Many of the available
equipment models use a combination  of  cleaning techniques.  An example
is horizontal shake  with partial collapse.  The most common single
method is horizontal shake with about half the models using this clean-
ing  technique.   In comparison, sonic cleaning  is a relatively new
approach with more limited utility.


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                                     Envelope or Frame Type
                                     Up, Down, or Through  Fl'ow
 Normal
(Upward)
  Flow
                      — Cylindrical Typ«8
              Outside
             Filtering
                 "   ~k
                    ( j... .- . -|
 Inside
Filtering
  Vj

Down
Flow
                                              /  (Tube Type)
          Figure 3.1.  Configurations of Fabric Filters

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      3.1.3  Filter Fastening Techniques
             Associated with the filter configuration and cleaning
method is the way the filter element is held  in place.  An envelope
is  typically buttoned to  a  frame, part of which may be movable to
tighten the cloth.  The tubular filter is clamped to a thimble plate
at  one or both ends.  If  there is no flow through the other end, it is
hooked or buckled to a common cross bar or infrequently left free.
There  must be some way to tension the tube and  the adjustment is not
always easy.  If the filter is to be stationary during cleaning, it
.will be supported on one  side by wire mesh or spaced rings.  Appendix
3.1 does not show how the filter element is held  in place or supported,
but this can be estimated from consideration  of the configuration and
cleaning method.
             Figure 3.2 depicts some of the alternate techniques for in-
stalling the fabric collector in the collection system.
                 Intermittent
                 Cleaning
                 Intermittent
                 Reverie Flow
                 Cleaning (Con-
                 tinuous If
                 compartmented)
                                  Pressure
                                   Type
                               Suction
                               Type
                             (1)
                             (2)
                             (3)
                             -KK>
                             -cb-o
Continuous Com-
purtmented Re-
verse Flaw
Cleaning
                Continuous Pulse
                or Jet Cleaning
                             (4)
                                    ~°      ~°
           Figure  3.2.
              Key:   | \ |- Fabric filter
                   •(^V- Primary fan
                   -/"V - Auxiliary blower or compressor
        Types of Fabric Filter Systems  Depending
        on Cleaning Method.

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 For example,  intermittent equipment (Type 1)  may either be proceeded or
 followed  by the  primary fan and  requires  no other flow branches  in the
 system.   Reverse flow equipment  can be  operated with the fan after the
 collector to provide  the needed  suction,  (Type  2),  but more often an
 additional fan or additional ducts  to the separate  compartments  are
 used to achieve  the reverse flow (Types 3,4).   The  cleaning methods
 requiring higher pressure air use additional  blowers or compressors
 (Types 4,5).   In order of popularity based on the numbers  described in
 the  manufacturer's brochures,  Types  1 and 5 are most common while Types
 3 and 4 are less often installed.   Except for intermittent equipment,  '
 most collectors  can be installed in  any of the  ways  shown.
      3.1.4  Size
            While  filters  called "unit" models  come  in standard  sizes
 down to that  of  ordinary  vacuum  cleaners,  many  other models  range up-
 ward to whatever size  is  desired.  The  largest  known filter  has  about
            2
 1,000,000  ft   of cloth  and  is  located at  a Canadian  asbestos  plant.
               2
 Large (5,000  ft   or more) baghouses  are nearly  always  assemblies  of
 compartment modules for economy  in design,  shipment, assembly and main-
                                                              2
 tenance.   Individual module  sizes are usually a few  hundred  ft   of
 cloth, as Appendix 3.1  indicates, but this  appears to  depend  on  the
 market around which the manufacturer has  built  his line.  Also,  the
 choice of  number  of compartments, and hence  compartment size, is a
 factor in  the service  to be  provided by the baghouse system.  For
 example, the more compartments the steadier (more uniform) the total
 system flow may  be.
     3.1.5  Typical Fabric Filter Equipment
            To illustrate the variety of  the available equipment, the
 following examples have been selected and are described, based on bro-
chures received from the manufacturers.
            .  High temperature glass cloth baghouse
               Unit collector manually cleaned

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              .  Typical shake-type baghouse
                Pulsing flow baghouse
                Reverse flow envelope collector
                Reverse jet collector
                Reverse flow cylindrical collector
                Reverse pulse collector
                Ultrafiltration equipment.
 A brief discussion of these different design  approaches  is  presented,  because
 rcach design  approach demonstrates a type of fabric  filter used  extensively
 for air pollution control.
      3.1.5.1  High Temperature Glass Cloth Baghouse.- This equipment,
 as depicted  in Figure 3.3a, is designed to operate with glass cloth for
 nigh temperature applications and moderately  large gas flows.  A typical
 dust would be electric furnace fume (fine particles) filtered at a
 fairly low velocity.  Most dusts would be expected to release fairly
 easily from  the smooth glass cloth.  In order to preserve the cloth,
 as gentle a cleaning method as possible is used, including collapse
 or collapse plus gentle reverse flow air.   This collector might be
 located outdoors where space is less expensive, in which case the
 added ducting could provide advantageous cooling of the gases.
      3.1.5.2  Unit Collector Manually Cleaned.- The system presented
 in Figure 3.3b is typical of many relatively small unit dust collectors
 which range approximately from the size shown down to one-bag designs
            2
 of a few ft .  The construction is usually light,  as the unit is usually
 intended for indoor service adjacent to the dust source.  These units
 may contain an integral blower and frequently  are  used in combination
 with inertial precleaning equipment  such as a  cyclone.   The  equipment
 need not be at floor level,  but can  be  mounted under,  on a workbench,
 or overhead as long  as  the shaker and cleaning part  are  accessible.
 Since the  equipment  is  close  at hand and simple,  it  offers good  reli-
 ability.   It is  widely  used  on small production operations,  frequently
 for  nusiance dusts  from buffing,  grinding,  etc., or  for  bin  vent,
.material transfer point,  etc.,  applications.

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             GLASS CLOTH COLLECTOR
        AIR VOLUME RATING CHART (CFM)
  Length
   Tube

     14
     16
     19
     21
     24
     26
     29
     19
     21
    24
    26
    29
    24
    26
    29
    24
    26
    29
    24
    26
    29
 No.
Tubes
  32
  52
  64
 84
112
Net Area
 Sq. Ft.

  1320
  1560
  1800
  2040
  2280
  2520
  2760
  2995
  3350
 3705
 4095
 4485
 4560
 5040
 5520
 5985
 6615
 7245
 7980
 8820
 9660
 Air/Cloth Ratio
(FPM/Ft.2 Cloth)
1V2         2
1,980
2,340
2,700
3,060
3,420
3,780
4,140
4,493
5,025
5,558
6,143
6,728
6,840
7,560
8,280
8,970
9,923
10,868
11,970
13,230
14,490
2,640
3,120
3,600
4,080
4,560
5,040
5,520
5,990
6,700
7,410
8,190
8,970
9,120
10,080
11,040
11,970
13,230
14,490
15.960
17,640
19,320
                                                           ARR. §-*
                                                  QLASS CLOTH COLUCTOR-HOPMR
                                                     AND OAMPIR OPIRATION
                        SEQUENCE OF OPERATION
                        The tubes to be cleaned are isolated
                        from the rest of the system by means
                        of  valves and collapsed by creating
                        a negative pressure inside the tubas.
                        This negative pressure inside the
                        tubes is created by the collapse
                        exhauster.
                        The sequence of operation is:
                        1. Inlet valve closes, isolating section
                         to be cleaned.
                        2. Reverse air valve opens,  collapse
                         exhauster evacuates gas from tubes
                         and hopper,- after a short time in-
                         terval, reverse air valve closes.
                        3. Inlet valve opens momentarily, par-
                         tially reinflating glass tubes, and
                         then closes. In addition to opening
                         the tubes for removal of dust, this
                         action imparts an inertia! cleaning
                         action.
                        4. A null period of zero pressure fol-
                         lows the collapsed  phase, atlewing
                         the collected material to fall into
                         the hopper.
                        9. The inlet valve  opens, and glass
                         tubes are back into operation.
Manufacturer:
Model No:
Brochure No:
   Am.  Air Filter Co.
   Amertherm
   283A
Filter Element Type:  Bag
Diameter/Spacing:      11.5"
Length:                 14  to 29
Flow Direction:        Up
Dust Caught on the  in side.
Retainment:   Hung and clamped
Cleaning method:  reverse,  collapse,
                    flutter
Control method:   automatic
Power required:   valves only
                   Baghouse shape: cyclindrical, 2 or  4
                                     comp.
                   Filter elements per  compartment: 8  to 56
                   Baghouse size:  (45 ft  x 12  ft x 12) max
                   Cloth  Area:  1300 - 10,000ft2
                   Operation: Continuous
                   Accessibility:  Intermittent
                   Fabric types:  Glass
                   Normal air/cloth ratio: l%-2
                   Applications:  Hi temperature  fumes, dusts
                   Temperatures:  Hi temperature  fumes, dusts
                   Loadings:      moderate
                   Cleaning frequency:  moderate
                   Dust types:
                   Efficiency:
               Figure  3.3a.   High Temperature Glass  Cloth  Baghouse

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      EXPLODED VIEW
INLIT AIR VALVE
 nrmt ASSEMBLY
                                     RMMM AM VALVt
               Figure  3.3a. High Temperature Baghouse (continued)

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        FT-40
        for "In plant" location* to recirculate cleaned
        air ... Available totally enclosed for collide
        venting or outdoor locations.
         The Model FT-40 Filter was designed for use with the
         3000 Series DUSTKOP where the fiberglass filter is not
         usually employed such as in woodworking or other appli-
         cations involving large volumes of very fine dust. It can
         also be used with "N" units smaller than the 3000 Series
         where a very large area of filter is required.
         Dust  laden air enters the top chamber through an inlet
         that can be placed either on the end or on the side. The
         air then passes downward through forty 5" diameter
         cloth  filter  tubes which remove all remaining dust. At
         intervals, dust may be. in.-irui'pi'y T-akr;i from the inside
         of the tubes by use of the shaker  bar accessible  from
         either end  of the  unit, motorized shaker is available.
         Dust falls into the  undisturbed air of the lower chamber
         where it can be  removed when this area becomes about
         two-thirds full. Dust removal is from either end.
         The FT-40 has 40 filter tubes with a filtering area  of 383
         square  feet. Tubes are sealed top and bottom with the
         same rubber grommet seal used in the FILTERKOP at the
         right. It is shipped completely assembled; shipping weight
         697 pounds.
Manufacturer:  Aget Mfg.
Model No:   FT-40
Brochure No:   736-2
Co.
Filter Element Type: Tube
Diameter/spacing:  5"
Length:  (61)
L/D ratio:   14
Flow Direction:  Down
Dust caught on the inside.
Retainment: clamped top and bottom
Cleaning method:  shake
Control method:  manual
Power required:
Dust disposal: Manual
Baghouse  Shape:  Rectangular
Filter elements  per compartment: 40
Baghouse  size:  (8x2x4)
Cloth Area:  383  ft2
Operation:  Intermittent
Accessibility: Intermittent
Fabric Types: Low temp,  cloths
Normal air/cloth ratio:  any
Normal pressure  across  cloth:  any
Applications: low temperature  emission!
Temperatures: low temperature  emission*
Loadings:  low
Cleaning  frequency: infrequent
Dust types:  usually large, dry
Efficiency:
                  Figure  3.3b.   Unit  Collector Manually Cleaned

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      3.1.5.3  Typical Shaker Type Baghouse.- While equipment of the              ___
 type shovm in Figure 3.3c is available in all sizes, most of the larger
 baghouses are of the shaker type because of the relatively low initial
 investment.  The equipment is suitable for a wide range of conditions,
 although less so at high dust loading, because the baghouse compartment
 must be shut down during the cleaning cycle.  On the other hand, com-            """'
 partmentization enables entry for inspection and repair without stopping
 the system.  This equipment is similar to that in use for the last 50            —
 years or more and this long experience has resulted in a fairly good
 performance/cost ratio.  Shake cleaning,  however is not Suitable for some        _J
 fabrics and for some dusts.
      3.1.5.4  Pulsing Flow Baghouse.- Pulse cleaning equipment  of the type       .^
 depicted in Figure 3.3d has been introduced by many manufacturers as the
 most recent improvement in fabric filter  cleaning technology.   Systems  of
 nearly any size are made possible,  as with most of the larger types  of
 equipment,  by simply adding more sections onto the baghouse.  Various kinds
 of  pulsing  are  used,  including the  reverse compartmental pulse  shown here.
 Designs employing separate pulses in each bag are discussed  below.   Pulse
 cleaning is effective with little or no additional attrition to the  cloth        —
 compared with other cleaning methods.   Felted fabrics  are usually used  in
 pulse equipment.   For these  two  reasons the  air/cloth  ratio  in  pulse             __
 equipment is  typically two or three  times  that  of most  other equipment  and
 the  size of the  baghouse  is  porportionately  smaller.  The initial baghouse
 cost,  per CFM of  gas  cleaned,  should be less  than for other  equipment,  but
 this  is  somewhat  offset by the need  to  purchase and operate an  air compres-
 sor.  A clear advantage of pulse  equipment  is the  virtually continuous
 operation during  cleaning  because the  pulse  time  is so  brief.  This means
 either  a  smaller  baghouse  or  steadier operation,  or both, and an almost           —
 unlimited dust concentration  capability.
      3.1.5.5  Reverse Flow Envelope  Collector.- Flat-panel filter equip-          —'
ment  offers several advantages over bag and tube  type equipment and  is
widely used in certain industries.  For one thing  the flat arrangement,           _j
as shown  in Figure 3.3e, packs 20 to 40 percent more cloth in the same

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 c
  —
   -


           Manufacturer:
           Modal:
           Brochure No:
WheelabraCor
Dust Tube
565E
           Filter Element Type:  Bag
           DlaMter/epacing:     5V
           Length:               13%'
           L/D Ratio:            32
           Flow Direction:       Up
           Duct caught on the Inside
           ftatainsunt:  hung and snapped  In
           Cleaning method:  shake
           Control method:  automatic
           Power Required:  % - 4 hp
Baghouse shape:  Rect.
Filter Elements  per Compartment: 72-960
Baghouse size:  (22 x 10 x 50) max std.
Cloth Area:  200  -  12,000
Operation: Intermittent
Accessibility; Continuous
Fabric types:  Cotton or other
Normal air/cloth ratio: 3:1
Normal pressure  across cloth:
Applications:
Temperatures:
Loadings:  Moderate
Cleaning frequency: Moderate
Dust types:
Efficiency:

                                                                                               1  hum tahM^ At tt&

                                                                                               2
                                                                        throaghl
                                                                     eod of doth tub* doiad by iawty

                                                                     In recessed aMtal o*ll ptati~DY
                                                                                           -
                                                                                               3
                                                                                               • n rfi arlre^
                                                                                               •Tuumju / • .
                                                                                               ttondt* the filtering i
                                                                                               cloth Amah*. Tub** «w
                                                                                               able to doth need new* be'un-

                                                                                               shaking action does not subject
                                                                                               cloth to ttraia.
                                                                                              4

                                                                                                  One man  working alone on

                                                                                               no tool* required. The cloth i
                                                                                              can be replaced in groups as time
                                                                                              permits during off hours, canaiag
                                                                                              no interruption hi production ma
                                                                                              5

   from walkway on clean air

bt nWd* during operation on
many job*,
                                                                        by dw drculmr Aipt, fcriblt
   Danube dawiffn fire* mioi-
                                                                     7
8                                                                        A*
                                                                        iacfa
                                                                        «•*
   Maftatt *
           Orfan,
                                                                                              Q •» **+* \m
                                           Figure 3-3c.   Typical  Shake-Type  Baghouse
C

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                                Schematic diagram
                             showing flow of du*t and
                              air and arrangement of
                               filter cylinders in the
                                MIKROPULSHAKE
                                       Collector.
                                       HOW  IT OPERATES

                                 The fitter bags O  are clamped to collars Q on the
                                 tube sheet & and suspended from the bag support
                                 •ngle* 0 by lengths of chain fl>. A blow tube Q is
                                 located opposite the outlet duct • of each compart
                                 ment. Oust laden air enters the collector under pressure
                                 or vacuum and cleaning takes place  at follows:  The
                                 remote cyclic timer O actuates the sotenoM controlled
                                 valve a) causing it to open. A momentary pulse of high-
                                 pressure air (100PSI) from the air supply pipe Q flows
                                 through the  blow tube and into the outlet duct from
                                 each compartment. The result is an interruption of out-
                                 ward flow and reversal of flow during which the filter
                                 bags begin to collapse. In some cases, the single pulse
                                 will permit release of accumulated dust and the cleaned
                                 compartment is immediately back on stream. In other
                                 cases, the valve must be pulsed on a cycle basis, which
                                 manipulates or shakes the bags several times conaecu-
                                 tively. The pulse, or series of pulses,  occur in a wary
                                 short time, and the net down time of a compartment Is
                                 so small as to be ignored.
                                 The collector housing O Is dust tight and divided by
                                 the tube sheet Into two plenums. The lower section con-
                                 sists of a hopper and inlet with some dust disposal
                                 device.  The hopper can be large  and  service all com-
                                 partments, or it can be separated into more individual
                                 units as desired. The upper plenum houses and sup-
                                 ports the filter bags and provides exhaust outlets and
                                 pulsing valve system for continuous cleaning.

                                 A Manometer shows the pressure drop  across the fitter
                                bags and Indicates fitter performance.
Manufacturer:
Model  No  :
Brochure No.
Pulverizing  Machinery
Mikro-Pulshake
3M/2/68
Filter  Element  type:  Bag
Diameter/Spacing:     5V
Length:   10'
L/D Ratio:   23
Flow Direction:   Up
Dust caught  on  the inside.
Retainment:   hung, clamped
Cleaning method:   pulse,  collapse
Control method:    automatic
Power required:    6-16 SCFM @ 100  psi
Baghouse shape:   Rect.
Filter  elements  per compartment: 80
Compartment  size:   19  x 6  x 6
Cloth Area:  3200 - 8800 ft2
Operation:   Continuous
Accessibility:   Continuous
Fabric  Types:
Normal  air/cloth ratio;
Normal  pressure  across  cloth:
Applications:
Temperatures:   to  550
Loadings:       High
Cleaning frequency:  Frequent
Dust types:
Efficiency:
                        Figure 3-3d.  Pulsing Flow Baghouse

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c

   .
  •
   -,-
    -
 HOW
 DYNACLONE
 OPERATES
In drawing, Dynaclone is under suction from
the fan. Area in blue indicates normal filter-
ing of air from dust source through flat cloth
bags. As air passes into each bag, the dust is
left on the outside surface, and only clean,
filtered air (clear arrows) is drawn through fan.
Dynaclone continuously cleans the deposit-
ed dust from filter bags, one at a time. Bags
not being cleaned remain in operation. Ex'
haust fan keeps  the filter under negative
pressure. This causes air from dust source to
flow through the bag; it also draws air in from
outside atmosphere for cleaning  the bags.
Cleaning air (dark arrows) comes through
flexible hose to traveler, which moves back
and  forth across  open ends of filter bags.
Cleaning air gently forces dust from outside
surface of the bag. Dust drops directly into
hoppers at bottom of filter.
Cleaning air is then picked up at the main air
stream and filtered through the active bags.
 C
                        Manufacturer:
                        Model  :
                        Brochure  No:
                       W.  W.  Sly Co.
                       Dynaclone
                       104
                        Filter Element  Type: Envelope
                        Diameter/spacing:   2.1"
                        Length:   3.6'
                        L/D ratio:   20
                        Flow direction:   Horizontal
                        Dust caught on  the outside
                        Retainment:  Buttoned
                        Cleaning  method;   Reverse flow
                        Control method;  Automatic
                        Power required:
Baghouse  shape:   Rect.
Filter elements  per compartment: 34-400
Baghouse  Size:  (24 x 16 x 81)  max
Cloth area:   700 - 10,000 ft2
Operation:   Continuous
Accessibility:   Intermittent
Fabric Types: Cotton and Other
Normal air/cloth ratio:  2.
Normal pressure  across  cloth:
Applications:
Temperatures:  Low temperature
Loadings:       Moderate
Cleaning  frequency: moderate
Dust  types:
Efficiency:
                      Figure 3.3e.   Reverse Flow Envelope Collector

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dust  collector  volume.  The  air  entering  the  space between the  panels
is usually moving  slower  than the  air  entering  a  cylindrical  filter.
This  may  reduce fabric wear  if the dust is  abrasive.  Access  to each
filter panel  can be  easier  than  access to a large number  (50  to 100)
of baga packed  close together and  pane Is  can  be brushed down  if nec-
essary.   Disadvantages of having a moving carriage in the  collector
include wear  and maintenance needs.  The  panels are usually supported
by a  mesh which may  add to  the initial baghouse cost, but  extends cloth
life.  The life does appear  to be  longer  than that of cylindrical col-
lectors,  perhaps partly due  to differences  in applications.
      3.1.5.6  Reverse Jet Collector.-  Reverse jet equipment,  such as shown
in Figure' 3.3f , became  known in the 50's for its high  capacity in
          2                    3
CFM per ft  of  floor or per  ft  of collector  volume, and  also for the high
filtering efficiencies the  equipment achieved with felted  fabrics.  Opera-
tion  is continuous and can  be almost steady,  or the cleaning  carriage can
cycle only as needed.  The  equipment is frequently used where very high
efficiencies  are necessary,  as with  toxic or  radioactive  dusts.  The clean-
ing mechanism requires careful maintenance  to prevent undue wear of the
cloth tubes,  because the  carriage  mechanism can be seriously  damaged by
abrasive  dusts  if the cloth  is once  broken.   In some cases  the downward
flow  of gases entering this  equipment may be an advantage in  tending t:o
distribute particle  sizes uniformly  along the tube.
      3.1.5.7  Reverse Flow Cylindrical Collector.- A few models  of
cloth dust collectors are distinct departures from the typical,  as were
both  the  pulse and the reverse jet types until they became popular.
                                                '      .                 5
The recently introduced collector in Figure 3.3h is different.  It
combines several features of other collectors, notably reverse flow
plus reverse pulse cleaning, a traveling carriage  and flattened  filter
elements.   The result is  a fairly compact  collector capable of high
filtration velocities and good efficiency.  The  cylindrical housing
enables the  filter casing  to withstand  substantial gas pressures.

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    -
c
                                     Dust  laden air
                                     delivered to collector
                                     inlet by  Buffalo
                                     Industrial Fan
                               ,             Kr\ *M'--O .KM supply blower


                                       KMi-riial l)ln\v-riiiK farriane ili'ivc

                                      Hlou-rinn cMrna^t1 i'<>unttM'\voii;hts


                                                       Kelt filter


                                  Ho.n> iluiv  rioM-lubrii-.iioil ill ivo chains

                                                    Hlow-rinK raiTiitur


                              Hi-rfaluirateil slop) tramo arul panel housinu

                                               \Veldoil  slerl dusi
    Dust
collected
in ho|>|>er
»la
               ^l*33BSi'.' ff ?P* jvu^fcflp""™**" • ii
            •jt When  activated,  the blow-ring  assembly
            \f  traverses up and  down .  .  .  slightly indent-
                ing  the tube.  A high velocity air jet,  from
                a  slot in the  ring, penetrates the tube  wall
                .  .  . combines with the flexing action  to
                dislodge the dust accumulation.
                                                                        Manufacturer:   Buffalo Forge
                                                                        Model: Aeroturn B
                                                                        Brochure No:   AP 650

                                                                        Filter Element type: Tubular
                                                                        Diameter/spacing:   12"
                                                                        Length:  8-20'
                                                                        L/D ratio:   8-20
                                                                        Flow direction: down
                                                                        Dust caught on the inside
                                                                        Retslnnent: clamped top and
                                                                                    bottom
                                                                        Cleaning method:  Reverse  jet
                                                                        Control method: Automatic
                                                                        Power required:  5 tip blower plue
                                                                                         1/2 hp carriage
                       Baghouse shape: Rect.
                       Filter elements per compartment:  16
                       Compartment sice: (32 x 7 x 7)  max
                       Cloth: 400 - 945 ft2
                       Operation: continuous
                       Accessibility! continuous
                       Est. FOB Cost:
                       Fabric types: Felt, tight cloths
                       Normal air/cloth ratio: 8-24
                       Normal pressure across cloth:
                       Applications:
                       Temperstures: moderate
                       Loadings: moderate to high
                       Cleaning frequency: moderate
                       Dust types:
                       Efficiency:
                                                          Figure  3.3f.   Reverse  Jet  Baghouse

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                                                            -.•.««,-     ;   /»>../.'  ;

                                                           Dust laden air enters the collector hopper and travels up-
                                                           ward uniformly about the tubular filter bags In each zone.
                                                           The air penetrates the felted filter media, depositing the
                                                           dust on the outside of the begs. Clean, filtered air continues
                                                           upward Inside the bags and Into the clean air plenum from
                                                           which It Is exhausted.
                                                           To clean the filter bags,   the poppet valve on the zone to
                                                           be cleaned closes; this Isolates the zone. After the poppet
                                                           valve closes,   the solenoid controlling passage of com-
                                                           pressed air to the isolated zone opens for approximately
                                                           1/10 second, emitting a burst of compressed air into the
                                                           plenumchamber. ' • Thispulseofair,expandsrapidlydown
                                                           into the filter bags, sets up a shock wave which flexes the
                                                           felt.   Dislodged dust drops into the hopper, where 
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o
          Time diagrams illustrate the unique DUAL reverse air cleaning system of the Type "CS" dust filter
                                                                   DUBM AM 2.

                           "CS" dust  Alter  incorporate* two
                        Diagram 1 above show* path of air sup-
                          to manifold which rotates fran hank
                         tube*. Havana air from the pnasunr
                         to expand or "puff" ao that the filter
                                                                                       ntnino ouit numon
                                                       This diagram dhows path of compreaaad air shock wave. What
                                                       filter tube has been expanded by air from the pressure blower,
                                                       a burst of compressed air is released. This air sat* up a shock
                                                       wave which dislodges  accumulated particles from  the niter
                                                       fibers. Dislodged particles are swept away by the continuously
                                                       flowing air (as shown in Diagram 1) from the pressure blower
                                                       to the hopper located beneath the filter tubas.
~
        Manufacturer:  Carter-Day Co.
        Model Nol: "CS"
        Brochure Not L-1126R2
 Filter Element Type:  Flattened bag
 Diameter/spacing:  (2-5")
 Length: 2-8'
 L/D ratio:
 Flow direction:  up
 Dust caught  on the outside
 Retalnment:  suspended
 Cleaning method: reverse flow, pulse
 Control method:  automatic
 Power required:  1/4 hp plus 4 to 14 SCFM
                 
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      3.1.5.8  Reverse Pulse Collector.- Installations such as depicted
 In Figure 3.3J tire typical of the equipment now increasing most rapidly
 Jn application.   Hie pulse of compressed air is released at the top of
 the filter element and travels downward removing most of the deposited
 dust.   The pulse is usually quite sharp initially,  but attenuates as it
 travels,  losing  its effectiveness.   Thus,  the filter elements are
 limited to several feet.   The equipment shown is designed to entrain
 additional air with the  pulse, giving  not  only a shock front, but also
 a  brief reverse  flow.  The two effects apparently combine for added
 effectiveness.  The advantages of this kind  of equipment include  high
 air/cloth ratios and thus small plant  floor  occupancy,  very high  dust
 loading capability and high efficiency by  virtue of the felts used.
 The compressed air requirements may  cost as  much as the power to  achieve
 filtration,  although initial costs are generally fairly low.   This  kind
 of equipment comes in  practically all  sizes.

       3.1.5.9 Ultrafiltration Equipment.- Ultrafiltration equipment,  such
 as  shown  in Figure 3.3j,  achieves efficient  removal of low concentrations
 of submicron particles by an extension of  standard  fabric filtration tech-
 nology.   In this  process  a "filter aid" is used to  provide the initial  dust
 cake and  subsequently  to  provide good  cake release.   By adding a  means  of
 introducing the  filter aid as  a powdered or  fibrous  material  ahead  of the
 filter, several  manufacturers  have modified  their standard equipment lines
 to include  this  capability.  Ultrafiltration  is  further  discussed in Sec-
 tion 3.5.4.
 3.2  FILTER CONFIGURATIONS
     Filter  elements are  available in  numerous  shapes, styles  and sizes.
This range  of  choices  is  due to  the need for equipment to  serve totally
different applications and  the different relative evaluations of space,
power,  initial cost and maintenance time.  Filter cloth  is now arranged
in panels or envelopes, in  tubes and bags and in variations on these.
Looking ahead, it  is not  likely  that any radical new geometry will be
developed, although somewhat more cloth per cubic foot of baghouse      ;

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           Manufacturer:  Flex-Kleen (Research
                          Cottrell)
           Model Not  (several)
           Brochure No: T-7

           Filter Element Type: Bag
           Diameter/spacing: 5 3/4"
           Length: 18 to 84"
           L/D ratio: 3 to 15
           Flow direction: Up
           Dust caught on the outside.
           Retalnment: hung and clamped;
                       supported
           Cleaning method: reverse pulse
           Control method:
           Power required:
                 automatic
                 approx. 10 SCFM,   ?
                 100 psig per 500 ft
                 of fabric
Baghouse shape: Rect. or cycllndrical
Kilter elements per compartment:
            8 and up
Baghouse size: variable
Operation: continuous
Accessibility:  Intermittent
Fabric Types: various felts
Normal Air/cloth ratio: 6to 15
Normal pressure across cloth:
Applications:  High loadings continuous
               service
Temperatures: moderate
Loadings:  high, moderate
Dust types:
Efficiency:  good
O
                                                       FLEX-KLE6N filter bags are constricted
                                                       of quality materials to precise dimen-
                                                       sions with double stitched  french-fell
                                                       seams.  Felted materials  are used  for
                                                       their high filtering efficiency at high air-
                                                       cloth ratios.
                                                       EASY Tfc ASSEMBLE—Filter bags are
                                                       slipped over the bag cages with the top
                                                       2 inches turned in over the  top of the
                                                       cage to form a seal on the bag cup. The
                                                       bag-cage assembly is slipped over the
                                                       bag  cup  and clamped with a quick-act-
                                                       ing clamp.  A split ring on the cage en-
                                                       gages with a groove on Jfte bag cup to
                                                       form a positive lock.
                                Figure 3.3a.  Reverse Pulse Collector

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                                                                                                     /"N
HOW THE TOTALAIRE  FILTRATION
SYSTEM OPERATES
The basic operating unit of the Totalaire system is
a standard  Pangborn cloth tube type dust collector
which is adapted for ultra high efficiency air fil-
tration by coating the tubes with a special, inex-
pensive filter aid at the rate of .075 Ib/sq ft.  Even
the finest particles of atmospheric dust and tarry
materials are collected and entrapped in the result-
ing filter mat.  As these particles are fine and light,
they cause no holding problem and they actually
increase filtration efficiency. In a National Bureau
of Standards Spot Dust  Test, Totalaire  filtration
proved 94 7' efficient. By weight, the efficiency of
the unit will exceed 99%.  It  will clean atmos-
pheric air to below .04 mg per  100 cu ft with in-
take concentration of 9 to 12 mg per 1000 cu  ft.
  Filter aid material is supplied  in 30 and  50 Ib
bags. Under normal conditions one application will
last from 2 to 3 years.  On units operating under
suction, air moving  equipment  of the ventilating
system  is  used to feed the filter aid  through  the
hopper  sections. On units operating  under pres-
sure, it  is fed into the system  ahead of the fan.
Charging time  ranges from a few minutes on small
collectors up to an hour or two on larger units.
  Filter aid material adheres to the inside surfaces
of the tubes when fed into the  collector.  After a
few hours of operation, it can only be removed by
operating the  shaker mechanism.  Under normal
conditions, removal of the filter aid  material and
accumulated dust  is required only after 2 to 3
years  of   24-hour-a-day  operation.   Obviously
maintenance costs are extremely low.
 Manufacturer:  Pangborn
 Model:  Totalaire
 Brochure No:   931

 Fabric types: standard
 Normal air/cloth ratio;
 Normal pressure across cloth:
 Applications:
 Temperatures:  Ambient
 Loadings:       Very  low
 Cleaning frequency:   very seldom (years)
 Dust  types: Small particle
 Efficiency:
      48
to o
m m
ra mv
                                                      S«
   I
   i"
                 12   18   24   30   36  42

                        MONTHS
                                            46 —
  This graph nlmwn Ihn lypir/n/ <,/mrolin« limn rmfnrn ahnkndiiwn '
for a Totalnim unit undnr wirioi/s dust l,,,,rix. whim tha unit i»
started 
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              volume may be  obtained  by closer  folding,  pleating,  or packing.   Possibly
              new designs  that  simplify cloth replacement  will be  introduced and higher
"•"            entrance  and exit velocities will be  developed for those  baghouse systems
  i;            not limited  by particle abrasion.  At the  present time, however,  we
 J            must continue  to  examine filter configurations on the  basis  of:  (1)
              panels vs tubes or bags; (2) upward vs  downward flow;  (3)  inside  vs
  !,            outside collection; and (4)  length, diameter,  and length/diameter ratio.
 £'
 >                 3.2.1   Pane1 vs Tube FiIters
  !
  '                        About 20 percent of the collector  models available use flat
              panels, the  rest  using  either  tubes or  bags.   The advantages of one type
              over the  other seem to  depend  on  the  application, since both are  widely
              used.  One major  manufacturer  of panel  type  filters cites  these advan-
 ''            tages:
                                                                               3
                   (1)  The  flat filter  gets 20  to  40 percent more cloth per ft  of
 |                     collector volume.
 "                  (2)  The air is moving more slowly when it enters the panel, than
                       air at  cylindrical bag entrances, hence abrasion is  less.
                   (3)  A  set of panels  can be inspected more easily since both sides
                       of the  cloth  on  every panel can be seen.
                   (4)  A  single panel can be changed more easily than a single tube,
                       although it may  be easier to  completely rebag the  tube  col-
                       lector  once it  is cleaned out.
                   (5)  Dust getting  through any leak in the  cloth piles up  under the
                       panels where  it  is more easily removed than dust piled  around
                       the  lower ends of tubes.
                   (6)  There are more choices of inlet and  outlet locations on a
                       panel baghouse than on a bag  type,where inlet must be at  the
                       bottom and outlet must be at  the top  (or vice versa).
                  (7)  The panel may be ghaken or pulsed more uniformly than a bag
 »                     which is shaken  or pulsed from one end only.
                  (8)  In the event  fibrous materials are encountered,   the panels
                       can be brushed down on the dirty side, whereas the dirty
                       side of the bag may be inaccessible.   If the dust bridges
                       between the panels, every other one can be removed.

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 A manufacturer of bag or tube filter equipment might counter with the
 following opinions:
      (1)   Since bags can be very long,  more cloth can be installed or
           otherwise  serviced per ft^ of floor space.
      (2)   In most installations, abrasion is not a problem and when it
           is it can  often be alleviated by suitable fabric design.
      (3)   It is easier to repair a leaky bag in place without removing
           it than a  panel.
      (4)   A single tube can be changed  more easily than a single  panel.
      (5)   Dust piled around the base of the tube is often the quickest
           way to locate a leak.
      (6)   Unless the baghouse inlet  and outlet are located logically
           from the standpoint of air distribution and dust settling,
           expensive  baffles or plenums  are necessary.   Standard inlet
           and outlet locations reduce costs.
      (7)   Shaking a  baghouse takes  less power and is quieter  than
           shaking a  panel collector.
      (8)   The spaces between panels,  being smaller,  are more  apt  to clog
           with coarse fibrous matter or in the event of a high surge  of
           dust than  the relatively open bag or tube.
The views  of both manufacturers obviously  have merit in certain in-
stances,  and have to be considered against  the collector application  of
interest.
     3.2.2   Upward vs Downward  Flow
             Traditionally,  the  collector inlet was  located at  the  bottom
of the baghouse where the heaviest dust settled  immediately to  the
hopper, thereby minimizing  fabric abrasion  and extending  the period
between fabric cleanings.  Since the  compartment  or  entire baghouse was
shut down  for  cloth  cleaning, the dust never had  to  fall against an up-
ward gas  flow.  Furthermore, downflow required the use  of an extra  tube
sheet and perhaps even more expensive, the use of an extra bottom plenum
(see Figures 3.3b and 3.3f).  Thus, the upward flow design was  logical,
s imple and  leas t  expensive.

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                                                  •   x/
             The introduction of reverse jet cleaning, and with it con-,
 tinuous on-line operation, brought in the extra tube sheet and intro-
 duced downward flow to encourage the dust to fall during filtration.
 Otherwise,  the dust mi^ht collect again on the nearby cloth.   In fact,
 this  may happen even with downward flow,  especially near the  bottom of
 the tube where the downward air velocity  approaches zero.  The four
 reverse jet models available today all use downward flow.
             It would seem that the same principle of downward flow for
 continuous  filtering would apply to pulse cleaning as well, but in fact
 less  than 10 percent of the approximately 30 pulse type models use down-
 ward  flow.   The contradiction is partly explained by the theory that
 pulse cleaning removes the cake without breaking it up as much as the
 reverse jet does,  so the dust falls as larger pieces.  Also,  when the
 pulsed air  is directed downward, as it usually is,  it tends to carry
 the dust downward  although only briefly.
             Because the settling velocity and primary flow velocity are
 additive in downward flow,  all sizes  of particles  tend to travel farther
 in  downward flow than in upward flow before being  caught by the fabric.
 Particles in downward flow have some  probability of falling completely
 through the filter tube to the hopper without being caught.   This is
 true  of all particle sizes, whereas  in upward flow  only particles
 larger than a size determined by the  tube entrance  velocity can escape
 the fabric  (Section 3.2.4.2;  for most  filter  applications  this  size  is
 of  the order of  3|jm.)  Consequently,  it would appear  that  the choice of
 upward or downward flow might reduce  the  average cake weight, depending
 on  the particle  size distribution.  Dusts  predominately larger  than  the
 limiting size might  be  more suited  to upward  flow filtration.    Smaller
 dusts,if filtered  downward,might result In slightly  lighter cake weights.
            A consideration that probably  overrides the minimization of
 cake weight  is the distribution  of particle sizes along the filter
surface and  the effect  this has  on dust deposit permeability and subse-
quent  local  filtering velocities.  Both large and small particles would

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 be distributed more evenly over the length of the tube in the case of
 downward flow.  This should result in a more uniform use of the entire
 filtering surface than in upward flow.
             Another important consideration is that the upward flow col-
 lector is generally less  expensive, having one less tube sheet or plenum.
 It is  harder to adjust the tube tension with two tube sheets.   In addi-
 tion,  with downward flow  there is  dead  air in the hopper which increases
 danger of condensation.
             Some panel types  are distinctly upward or downward flows,
 in which case  the above considerations  apply,  even though the  panels
 may be small in dimension.  However,  most  panel-type  filters  tend to
 use horizontal flow,  that is,  the  inlet and outlet are approximately at
 the same height on the baghouse.
             These are  the considerations in selecting the direction of
 flow.   To summarize, although  the  upward flow  collector  is  slightly less
 expensive initially, the  downward  system should  give  slightly  the better
 filtering performance  and lower  power requirement.  However, the  data
 available are  insufficient  to  quantitatively confirm  this  conclusion.
     3.2.3   Inside  vs. Outside Filtering
            While  this is not  a  consideration  in most  panel filters, bag-
 type equipment  offers  the choice of filtering  on either  the inside  or  the
 outside  of the  cylinder as was shown  earlier in  Figure 3.1.  Of course,
reverse  jet equipment  provides inside filtering, as does all but one
model  of  downward  flow equipment.  About 60 percent of all upward flow
equipment is the  inside filtering  type.  Clearly, the choice has nothing
to do with the  fabric which can be sewn with either side out.  An impor-
tant advantage  to inside  filtering is being able to enter the filter
compartment for inspection and maintenance during operation.  Also, in-
side filtering does not usually require the use of supporting mesh which
can increase maintenance difficulties.  However, inside filtering tends
to involve more fabric flexure during cleaning.

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      3.2.4  Length, Diameter and Length/Diameter Ratio   /
             Most filter bags are 5 to 12 inches in diameter and 5 to 40
 times the diameter in length.  The choice of diameter and length does
 affect filter performance, but the dimensions are more traditionally
 determined by their effect on the initial cost of the bags and that of
 the bagliouse.
             3.2.4.1  Tube Diameter.- Filter cloth is usually woven in
 standard widths,and in general,  bag diameters are constrained by the
 available widths  of fabrics as woven.  One common size is approximately
 38  or 39 inches wide,  from which two 5  or 6 inch diameter bags can be
                   '     ;                                             i
 obtained, allowing for the necessary overlap at seams.  For certain
 applications, an  11*5  or  12 inch diameter glass fabric bag is the most
 economical size  from the  available 38 inch wide glass cloth.   A few bag-
 houses are designed for use with 7  or 8 inch diameter bags.   This size
 is  probably based  upon a  54 inch wide cloth from which two bags can be
 obtained from a single width.  Wool felts,  used in reverse jet baghouses,
 are generally either 9, 10 or  18 inches  in diameter.
             The diameter  of the  filter bags also influences  the size of
 the baghouse.  For  example,  about 1,750  square  feet of filtering area
 can be provided in  about  80 square feet  of floor area by using 6 inch
 diameter by 10 foot long  bags.   If 12  inch  diameter bags  were used in-
 stead, they would need  to be about 14  feet  long  to  provide  the same  fil-
 tering area in the  same floor  space,  though 12  inch diameter  bags  can
 easily be  obtained  20  feet  long  or more when  there  is  adequate  head  room.
 This  (12  in.  x 20 ft.) would result  in a baghouse having  about  2,500
                                                            2
 square feet  of filtering  area  in the same  floor  space  (80  ft  ).
            While using a smaller  diameter  increases  the  filter  capacity
 per unit  occupied floor area,  there are disadvantages associated with
 small bags.   Small  bags may bridge across,  particularly in cases of un-
usually coarse collected dust  or extreme surges  in dust loading.   If the
hopper plugs, any bag can become filled at  the bottom with collected

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 dust and the smaller bag is less easily emptied afterward.  Smaller bags
 require  a larger number of clampings,  tension adjustments and inspections
 for the  same cloth area, but they are  easier to handle during maintenance
 and repair.
             3.2.4.2  Length-Diameter Radio and Tube Entrance Velocity. -
 The air  (and dust)  velocity at the entrance of a cylindrical filter tube
 is  given by:
                         Ve - Vf x 4 (
For  example:     Filtration velocity =  3.0  FPM,  Vf
                 Length/Diameter ratio  =  25,  L/D
                 Entrance  velocity » 300  FPM,  Ve
This velocity  is typical of many collector  installations.
             In upward  flow,  the greater the entrance  velocity  the more
large particulate will be  carried into  the  tube,  (The largest  particle
that can be  lifted by  an air current of given velocity and viscosity  is
discussed  in Chapter 2).    Thus,  the greater  the entrance velocity, the
faster the dust deposit  increases.   The permeability  of  the deposit may
be expected to be diminished by the  larger  particles,  however, as dis-
cussed in Section 2.4.8.
            The upward velocity will decrease  along the  tubular filter
to zero.  Thus, the size of  particles that  can be lifted (in upward flow)
decreases along the tube.  This results in  a partial  distribution of par-
ticle  sizes and a consequent  variation  in deposit permeability and weight
along  the tube.  These variables  are affected  by the  entrance velocity
and  the size distribution  of  particles  entering  the collector.
            The cost of power required  as a consequence of entrance head
loss  is negligible, being  less  than  one dollar per KCFM-year at 300 FPM.
However, in the case of abrasive dust,   the bag may be scoured and excess-
ively worn near the entrance  tube sheet, (Attempts to define the poten-
tially abrasive entrance velocity have not been successful;  see Section
4.5.1).

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             Length/diameter  ratio  affects  the  swaying  stability  of the
 vertical  tube.   Bags  should  not  rub  together either  during  filtering  or
 during  the  cleaning cycle.   Thus,  the higher the  ratio the  greater the
 advisable bag separation, partly offsetting the floor-saving advantages
 of  long tubes.   Also,  the lengthwise seam  of the  bag will stretch  dif-
 ferently  than the rest  of the cloth, making the high length-diameter
 ratio bag bend as the  tension or dust load changes.
             The  short  stocky tube may shake more  easily than the thin
 longer bag,  but  the stresses at  the shaking end may  still be higher.
There is  no  accepted way to  generalize this at present, as  amplitude,
 frequency, dust  weight  and several cloth parameters  are involved.  Also,
the relation between  fabric  stressing and  fabric  life  is not clear.
             Length/diameter  ratios are usually between 5 and 40, but more
typically between 10 and 25.  In purchasing, the  choice of  length/diameter
ratio seems  to be mainly determined by whatever equipment is available,
and this  is  more dependent on other considerations.
             3.2.4.3  Tube Length.- In addition to its  relation to dia-
meter, tube  length is also limited in other ways:
             (1)  The tension along the length of the  bag increases as a
                  function of height, starting with whatever tension is
                  being applied  at the bottom.   It may be so large at the
                  top as to  pull holes in the cloth at the cuff seams or
                  otherwise  damage the fabric,  in cases of heavy dust de-
                  posits .
             (2)  The longer the bag, the more expensive it is per square
                  foot, because  of the difficulty of handling and sewing
                  a long bag.  The choice of long bags requires a more
                  elongated  dust collector for the same fabric area and
                  since elongated housings require more siding and stronger
                  structural members, the collector cost will generally in-
                  crease.
             (3)  Some cleaning mechanisms are length  limited,  notably air
                  pulse and  shaking, wherein sufficient, energy for the en-
                  tire  length of the tube must be applied at one end to the
                  detriment  of that end.   The energy requirement increases
                  faster than length, in general.
             (4)  Continuously cleaned dust collectors require the removed
                  dust to fall to the hopper during filtration.  Instead

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                  of falling, many particles must be caught again on the
                  fabric.  This results in relatively heavy deposits near
                  the bottom of the bags.  The longer the bags, the heav-
                  ier this deposit will be.  In a similar sense, intermit-
                  tently cleaned collectors require a pause after cleaning
                  to allow the dust to settle to the hopper.  The shorter
                  the bags, the sooner the collector compartment can be
                  returned to service.

             (5)  Different thermal expansions between cloth and baghouse
                  require latitude in the bag fastenings or shorter bags,
                  or both.  For example:

                                                    Resulting
                                    Diff. Length   Stress Change    Tension

                     Steel-Glass:   .00042 ft/ft       4.2 psi      2.5 Ibs
                                     of bag

             (6)  Very long bags may require two or more people to install
                  and inspection and other maintenance may also be difficult.

             (7)  The baghouse may be ceiling limited if it is to be in-
                  stalled indoors.

             (8)  One cl&irn is made that bag surface scouring increases,
                  due to sliding of the loosened deposit downward along
                  the bag surface.  Scouring may assist the cleaning pro-
                  cess, but it may also damage the fabric surface fibers.
                  It is not clear which in general, is the more important.

             A review of bag lengths in a number of specific filter in-

stallations has indicated the use of slightly longer bags where smaller
particles are involved.  There is no clear physical reason for this.

Recent air pollution control applications are chiefly concerned with small

particles.  There is also apparently a slight trend in the fabric filter

industry toward longer bags and tubes, which may be coincidental.

             As will be seen from a review of typical costs of fabric
filtration (Chapter 7), plant overhead and especially the cost of plant
  *Assuming the fabric expands as a solid; per 100°F.
                                 4
 **Assuming tensile modulus of 10  psi and no bag fastening latitude;
   per 100°F.
***Assuming a 6-inch diameter bag and .03-inch thickness; per 100 F.

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 floor space are very important factors.   It is entirely possible that
 two collectors may be intended for almost identical service,  the only
 difference being that one will be used where floor space is plentiful and
 free, and  the other where floor space will contribute as much as 10 per-
 cent to the annual cost of operation. One would prefer to select equip-
 ment with longer bags in the second case.
             3.2.4.4  Panel Geometry.- The average velocity entering a
 space (W) between two filter panels of distance L from entrance to closed
 end is  given by V  = V,. x 2 (L/W) .   For  example,  if the filtering velo-
 city is 2 FPM and L and W are 3 ft  and 1.5 inches respectively,  the
 average entrance velocity is 96 FPM.   It is generally less than for bag
 and tube  filters and consequently,  many  of the above comments  relating  to
 entrance  velocity do not apply.  Furthermore,  as  the flow is  often hori-
 zontal,  there may be more settling.   There should be less abrasion of
 the fabric  surface because of the  lower  velocities.
             In the normal interest  of saving floor and  plant  space,
 panels will  be designed as close  together as possible.   This  is  subject
 to  the  need  to inspect  and replace  the cloth;  clogging  of the  space
 between the  panels;  touching during  cleaning,  such that the dust cannot
 fall  or such  that  one panel abrades against  the next; entrance abrasion;
 .and structural requirements.  Most panel spacings  are between  one-half
 and three inches  and  equipment  using  oval bags  or  envelopes have  inside
 dimensions  on about  the same scale.
             Panel  length and height vary  from a few inches to a few feet.
 In  addition  to the above  limitations,  the manufacturer's  choices are
determined by  such things  as  the width of  the original bolt of cloth;
 the need to maintain tension over the  panel; the ease of  installation
and replacement of panels; and  thoroughness  of cleaning, which depends
on cleaning methods.

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      3.2.5   Other Configurations
             Panel type filters  vary from single flat fabric sheets  to
 oval envelopes  with only one end open,  really more like a bag than  a
 panel.   A standard item with Pangborn In the multi-tube Filter <• lament,
 two panels  sewn together at Intervals so that when Inflated six cylin-
 drical  filter elements result,  fastened together side by side.  This
 approach packs  more fabric in a given collector volume, but may increase
 maintenance costs, since a failure  of one tube requires maintenance or
 replacement involving  all six.   Also, tension adjustment of such  a  large
 filter  element  may be  slightly  more difficult.
             Reported in some of the early literature,  and perhaps still
 in common use in Europe,  is  the conical filter element which tapers from
 a  circular  bottom of ordinary diameter  to an apex at the top of the bag-
 'jouse.   This design should give nearly  constant upward velocity the
 entire  length of the element and ,therefore,  a more uniform particle size
 deposition.   The elements  can also  be packed more closely together  with-
 out  danger  of inter-abrasion, as long as  they are not  made too  long,
 except  that  space  must  be  provided  for  their maintenance just as with
 any  filter  element.  Tapered elements are  relatively expensive  to con-
 struct.  A  thin cone may bridge  across  the inside and  be difficult  to
 clean.   The  shake  dynamics of a  cone  are no  doubt substantially different
 than for a  cylinder.
            A few  other novel fabric  filter  configurations are discussed
 in Section 3.5
3.3  CLEANING MECHANISMS
     Except  for  studies of specific air pollution  control problems,  most
development toward better fabric filter equipment appears to go into
 improved methods of  removing the accumulated dust deposit from the
 fabric.   As a result, we have the variety of cleaning mechanisms already
mentioned in  this chapter.  Just as the dust particles cohere to one

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 another,  the  deposit  cake  adheres  to  the  surface  fibers  of  the  cloth,  or
 in the  case of deep or  napped  surfaces, among  the surface fibers.   The
 problem is to remove  the desired amount of  the deposit from the fabric
 quickly and uniformly without  either:
      •  removing too much of  the residual  deposit  which greatly  improves
        the collection efficiency at start-up on a subsequent filtering
        cycle,  for woven fabrics
      .  damaging the cloth, or  using too much power, either  of which can
        be a substantial part of operating cost
      •  excessively dispersing  the  removed dust particles, as these  would
        probably then  have  to be re-filtered.
     The standard cleaning methods are listed  in  Table 3.1  along with  a
 number  of superficial characteristics frequently,  although  not  always,
 associated with those methods.  More quantitative  discussion of some of
 these characteristics (efficiency, cost,  filtration velocity, etc.) can be
 found elsewhere in this handbook.  The cleaning principles  are  discussed
 here.
     The time required  for completion of  the cleaning process is an im-
 portant consideration in comparing cleaning methods.  This  time should
 be much shorter than  the time between cleaning periods, which as shown
 in Section 2.4  is determined by filtering velocity, dust loading, pres-
 sure drop tolerances  and the "K" value of the dust.  For a high dust
 loading, the cloth might have  to be cleaned very often - perhaps every
 30 seconds, for example.  For this, a manual cleaning process would be
 out of the question.  Likewise, a shaking process which might require
 1 minute or more just to accomplish the cleaning,  would mean that at
 least two thirds of the fabric in the system would be out of use at all
 times.  On the  other hand the reverse jet method accomplishes the
 cleaning quickly, but is limited in repetition by the slowness of the
 carriage.  Pulse cleaning is  very fast,  but  also has comparative dis-
advantages.  It seems  that  every cleaning  method in use has  at least
 one advantage  over each of  the others, and conversely.

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                                                     TABLE 3.1

                                           COMPARISONS OF CLEANING METHODS1
10
-p-
Cleaning Uniformity
Method of
Cleaning
Shake
Rev. Flow,
no flex
Rev. Flow,
Collapse
Pulse-com-
partment
Pulse-bags

Reverse jet

Vibration,
rapping
Sonic assist
Manual flex-
ing
Ave.
Good

Ave.

Good

Ave.

V . Good

Good

Ave.
Good

Bag
Attrition
Ave.
Low

High

Low

Ave.

Ave . -Hi

Ave.

Low
High

Equipment Type
Ruggedness Fabric
Ave.
Good

Good

Good

Good

Low

Low

Low
--

Woven
Woven

Woven

Felt,
Woven
Felt,
Woven
Felt,
Woven
Woven

Woven
Felt,
Woven
Filter Apparatus Power
Velocity Cost Cost
Ave .
Ave.

Ave.

High

High

V.High

Ave.

Ave.
Ave.

Ave.
Ave.

Ave.

High

High

High

Ave.

Ave.
Low

Low
M.Low

M.Low

Med.

High

-High

M.Low

Med.
--

Dust Max. Sub-
Loading Temp.** Micron
Efficiency
Ave.
Ave.

Ave.

High

V.
High
High

Ave.

--
Low

High
High

High

Med.

Med.

Med.

Med.

High
Med.

Good
Good

Good

High

High

V. High

Good

Good
Good

        These value judgments do not permit comparison of performance aspects,  only of methods.

       **

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      3.3.1   Shake
             Bags are  most  commonly  shaken from the  upper  fastening.
 Several  combinations  of horizontal  and  vertical motion are  used.   The
 bags  may all be  fastened to a  common  framework which moves  horizontally.
 The. frame may have  slight  additional  upward  or downward swing,  depending
 on the linkage holding  the framework.   The framework may  instead  be
 oscillated  vertically.   Alternatively,  the bags may be attached in rows
 to a  rocking shaft.   In this case the location of the  point of  attach-
 ment  with respect to  the center  of  rotation  determines whether  the motion
 is predominantly horizontal or vertical.
             Rarely  are  the bottoms  of the  bags  shaken  instead of  the
 tops.  Panel filters  on the other hand, are  usually shaken  top  and
 bottom,  that is, the  entire filter  bank moves  from  a single drive  point.
 The shake amplitude is  usually designed into the equipment  and  may be
 anywhere  from a fraction of an inch to'a  few inches.   The frequency of
 shake is usually several cycles  per second and  can  often be adjusted to
 obtain the most suitable fabric  motions.  Most  shaking is essentially
 simple harmonic or sinusoidal  motion.  When  it  is,  the peak acceleration
may be easily computed;  it  is  usually from 1 to 10  g s.
             During the  shake,  the filtering  should  be  stopped.  Other-
wise, the dust will work through the cloth, reducing the efficiency and
 possibly damaging the cloth by internal abrasion.   In  some  equipment,
 the flow  is  slightly reversed  during shaking, both  to  prevent penetration
and to aid in cake removel.  Still more elaborate, but possibly more
effective, cleaning procedures involve a series of alternate flows and
shakes which  take time, but are  claimed to give a gentler net treatment
 to the cloth, plus more uniform and thorough cleaning.
             In a typical cycle, the inlet flow to the compartment is
first dampered off by an automatic timer and  valve mechanism.  If neces-
sary,  the outlet vent is also closed (Figure  3.4).  In the absence of an
air lock  between adjacent hoppers it may be necessary to close a damper

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0




*

1
•
'V
»
k
\
f
/ Jv
f/
               1_
              . t*,.«
               c*.s.  -•
                 • •
      ;"*A
      UvJ ^

Filtering —^
                                                                  Outlet
                                                                  Shut
                                               Cleaning
                           Remaining Compartments
                                                            Reverse  Flow
                                                              Option
                                    Time
              Figure 3.4.   Shake Cleaning Process and Associated
                           Pressure Cycles.

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 there to prevent the intrusion of dirty air from hoppers still operating.
 There should be zero forward pressure across the fabric during shaking,
 since otherwise dust will work through the fabric.   The timer starts the
 shaker motor and the bags are shaken horizontally at 1 to 5 cps with an
 amplitude of up to 2 inches; that is, the shaker imparts accelerations
                        i
 on the order of a few g  s.   Shaking continues for 10 to 100 cycles.
 Then the timer may start a small flow of clean reverse air using an
 auxiliary blower or a secondary duct-and-damper system for 10 to 20
 seconds.   The shaking may be repeated,  this time during the small re-
 verse flow.   Finally,  the cleaning  is stopped and after pausing to
 allow the dust to settle,  the inlet and outlet dampers are opened and
 the  compartment begins its  filtering again.   The entire cleaning cycle
 may  take  from 30 seconds  to a few minutes.   Some installations do not
 return the compartment on line  until the next one is ready to be cleaned,
 thereby achieving a fairly  steady overall flow through the baghouse
 system at  the expense  of  some over-capacity.
            The pressure  across  the compartment during the cleaning
 cycle  is  sketched in Figure  3.4.  If there  are only  a few compartments
 in the baghouse system, shutting  one down will increase the flow and
 pressure  drop across each of the  others,  the  amount  depending  on the
 fan  curve.  The pressure across  the baghouse  while the above  cleaning
 is performed  is also indicated  in Figure  3.4.   This  increase  in  filtering
 velocity  and  pressure across  the  fabric  in  the  other  compartments  can  be
 expected  to have  some effect, adverse but probably slight,  on  the  filter
 cakes  forming  in  those compartments.
            The mechanics of  cleaning by  shake  have apparently not been
 studied in any  detail.  The  cloth is  flexed to  some extent  and the cake
 is thereby cracked  or loosened from  the cloth  fibers.  Shaking produces
 inertial stresses at the cake-cloth  interface, both shear and tensile,
and if these exceed  the adhesive strength, then some of the dust cake
 falls  off.  It  is reported that a fine balance of bag tension, shake
 frequency and shake amplitude is necessary in order to obtain the "s"

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 shaped wave  in the moving  bag  giving  best results.   On the other hand,
 simple manual  shaking  equipment  seems to work with  a minimum of science,
 although  careless manual cleaning  is  hard on the  fabric.   The amount  of
 shake energy transmitted along the bag decreases  from a maximum at  the
 shaken end (usually  the top) to  a  smaller value as  the energy is absorbed
 along the bag.  A heavy residual dust cake,not removed by  the cleaning
 process (usually in  the case of  sticky fine  dusts),  changes  both the
 mass and  the flexibility of the  bag and must affect  the rate of energy
 absorption locally.  Further,  the  added residual  dust weight accumulates
 upward and increases the tension toward the  top of  the bag,  affecting
 the shape of the shake "wave"  for  better or  for worse.
             Shake equipment has  few limitations in application.  As
 noted above, the dust  loading  time cannot be too  short.  The dust should
 be fairly easily removable from  the cloth as it is easy to damage the
 cloth by  over-shaking.  This is  especially true of glass bags.   Although
 it was at first believed that  glass could not be  shaken, the  introduc-
 tion of certain glass finishes has made this possible on some dusts which
 separate  fairly easily from glass  fibers.  Shake  baghouses are  popular
 for both  very small systems (manual shake) and very  large  continuous
 sys 'cems.
             Two problems in particular  are often  reported with  shake
 equipment.   First, in an effort  to keep manufacturing  costs  down, a
 number of shaker mechanisms are underdesigned.   They wear and as
 they do,  they shake less effectively.   The bags load up or the operator
 intensifies  the shaking, and the mechanism then destroys itself unless
 given an  inordinate amount of maintenance.   Second,  as  the free end of
 the bag is shaken, the cloth must  flex  extensively at  the fastened end,
 resulting in fiber-fiber abrasion.   This may explain  the common occur-
 rence of bag failures within the first  couple of  feet of bag.  These
 entrance end bag failures  may also be related to dust deposit weight,
which is  often heavier at  the lower end of the bag and may result in a
higher rate of cleaning energy absorption in this  region.   Another
possible  factor is particle entrance abrasion.

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     3.3.2  Reverse Flow - No Flexing
            If the dust releases fairly easily from the fabric, a low-
presoure reversal of the flow may be enough to loosen the cake without
flexing or mechanical agitation.  To minimize flexural attrition of the
fabric, it is supported by a metal grid, mesh or rln^s, and is usually
kept under some tension (Figure 3.r>).  The support is usually on the
clean side of the tube or bag, although dirty-side support can help to
keep the sides of the bag or the panels sufficiently apart to allow the
cake to fall to the hopper.  Some filter equipment relies solely on low
pressure flow reversal, while other models use it in conjunction with
another method, shaking:for example, or use the much higher reverse
pressures of pulse or jet cleaning.
            There are several ways of accomplishing flow reversal.  In
addition to the standard dampers on each compartment,  each one can have
its own reversing fan.   A few models have a traveling apparatus that goes
from bag to bag or from panel to panel, blocking off the primary flow
                    Figure 3.5.  Reverse Flow Cleaning.

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and  introducing  some  air  in the  reverse  direction.with a secondary
blower  (Figures  3.3e  and  3.3g).   Perhaps a  simpler method is  to take
advantage  of  a suction  on the  dirty  side, or  a  relative pressure on the
clean side, without using another blower as sketched in Figure  3.6. If in
using this cleaning system it  becomes  necessary to increase the suction
or pressure being  used  for cleaning  by partially closing off  the inlet
or outlet  primary  duct, then to  avoid  the added power  consumption,  it
may  be  better to use  a  small secondary fan  or fans.
            Any  flow  volume reversed through  the filter must  be refil-
tered.  This  means that in addition  to taking cloth out of the  system
for  cleaning, this cleaning method increases  the total air flow in the
remainder  of  the system.   The  net increase  in air/cloth ratio is  nor-
mally 10 percent or less,  but  this may cause  a  large (perhaps 40  percent)
increase in power  consumption.   This is  partly  through a decrease  in
cake permeability.
           ' The  pressure  across  the  compartment  and baghouse will be
similar to that  sketched  in Figure 3.4.  However, some equipment users
believe repeated reverse  flows  introduced suddenly   to  "pop"
the cloth are beneficial,  in which case  the pressure-time  cycle will be
different.
            Reverse flow  equipment with a minimum amount  of cloth  flexing
finds the same wide range  of general applications as shaking equipment,
the one main requirement being that the dust release easily from the
fabric.  Thus, felts are not cleaned by  low pressure reverse flow.
            Disadvantages  of reverse flow equipment include the in-
creased filtration velocity  (or alternatively a proportionally larger  in-
stallation),  the costs of any additional fans, ducts  or dampers, and
when necessary the cost of a support grid.  This grid  tends to be a
nuisance when it comes to changing the bags  or panels and considerable
effort has gone into its design.   The grid or rings may chafe  against
the cloth unless the cloth is so tight it can't  move,  which introduces

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                                          Optional,  to  avoid
                                          temperature changes
                                                 (Pressure)
      F:  Compartments filtering

      R:  Compartment being cleaned by dampered
         control  from suction side of system.
Figure 3.6.   Schematic  for  Reverse  Flow Cleaning
             During Continuous  Filter Operation.

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tension problems,  or unless  the  cloth  is  fastened  to the  support,  as  for
example, rings  sewn to  the tube.   Even such hard  fibers as  glass  can
apparently  fail by chaffing.   Chaffing is  such  a problem  that  the  sup-
ports are occasionally  done  away with  at  the risk  of excessively  flexing
the cloth.
     3.3.3  Reverse Flow With  Collapse
            Even though cloth  flexure  is detrimental,  especially when
the fabric  is penetrated by  grit,  flexure  is frequently the preferred
means of cloth  cleaning.  It is sometimes  used  in  conjunction  with  other
methods.  The cleaning  method  excludes panel type  filters and  felts,
but cloth bags  having the dust cake  on the inside  are  often cleaned this
way.  The cylinder is often  reported as collapsing  for some reason  into
a cloverleaf pattern (see Figure 3.5).  The  collapse  is not 100 percent,
however, or the cake could not fall  to the hopper.  There should be just
enough reverse  flow to  crack and shear the cake until  it falls off.   It
may then be carried to  the hopper by the reverse air current,  depending
on collector design.
            With this kind of  cleaning  there  is an  optimum  tightness  of
the cloth, said to be somewhat between 25  and 100  Ibs  for a standard
11.5 inch diameter glass bag.  More tension  than this  is hard  on the
fastenings, or prevents the bag from collapsing enough.  Less  tension
results in too sharp a  cloth bend at the tube thimble where the cloth
is fastened (see Figure 3.5).  The tension of a suspended bag can be
automatically changed during the cleaning  cycle if necessary,  by adjust-
ing the position of the hanger frame.
            The collapse method of cleaning uses essentially the same
damper,  fan and ducting equipment as the method of reverse  flow without
collapse.   Just as  repeated shaking is standard practice,  the  bag or
tube may be collapsed more than once.  It may also be "fluttered" by
lightly and rapidly pulsing the reverse air to give a shaking  effect,

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at  some risk  of damage  to  the  cloth.  Such  flow changes may also  trans-
mit pressure  pulses  to  adjacent compartments and damage the cake  struc-
ture  there.
            Chief disadvantages of collapse cleaning are  flexural and
chaffing attrition of the  cloth, and some lack of control of cleaning
intensity.  For instance,  if the cake fails to come off sufficiently
and begins to stiffen the  cloth, there will be still less cleaning and
consequently, extra  flexing in adjacent areas and faster wear.  There is
little one can do to increase  cleaning intensity, although one can re-
peat  the process or change the rate of flexure.
      3.3.4  Pulse Cleaning
            This method attempts to overcome several of the difficulties
associated with other methods  of cleaning.  In this kind of equipment a
sharp pulse of compressed air  is released in the vicinity of the fabric
giving rise to some combination of shock, fabric deformation and  flow
reversal.  Depending on the design, and there are over 25 models  of
pulse cleaned equipment. The result is the removal of the dust deposit
without more  than a brief  interruption of the filtering flow.  The fabric
receives a minimum of flexural wear and the filter installation is
smaller because the fabric is  in use practically all the time.  Also,
for reasons on page 3-46,  most pulse equipment utilizes felt rather
than cloth.  With felt  the filtration velocity can be typically 3 to 4 times
that used in shake or reverse  flow equipment,  so the pulse filter in-
stallation is smaller.
            The main distinction of pulsed equipment is the brief clean-
ing time,  typically around one-tenth of a second.   The very low ratio of
cleaning time to filtering time makes pulsed equipment uniquely useful
at very high dust loadings, up to several hundred  grains per cubic foot
on  large particles.   Thus, this equipment is widely used for pneumatic
conveying  systems,  and it  also has  application at  moderate and low dust
concentrations.

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            The earliest designs of this kind, as shown in Figure 3.7,
use open top bags with a pulse nozzle in or just above the top of each
bag.  A timer controlled solenoid valve releases 60 to 120 psi air
through the nozzle for a small fraction of a second.  As the air pulse
moves down the bag, it may draw in other air and the combination tends
to bulge the bag.  In one variation the filtering is on the inside of
the bag and the dust is partly removed by reverse collapse, but then a
pulse similar to the one shown above snaps the bag open and completes
the dust removal.  Note that this forward pulse cleans without appa-
rently plugging the fabric.  A second variation in pulse cleaning design
is the compartment pulse, in which several bags or envelopes are served
by the same nozzle located in a plenum at one end of the group; or,  the
entire compartment is served by one or more nozzles.  For example, at
least one model injects a reverse air blast into the compartment inlet,
which temporarily stops the flow and reverses it,causing partial col-
lapse of the filter elements.
     Filtering
Pulse
Cleaning Wave
                    Figure 3.7.  Reverse Pulse Cleaning.

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           These variations in clean vs. dirty side pulsing and  inside
vs. outaidv'-the-bag pulsing, raise the question of the mechanism or
imichnniHtroi uf dust removal.  Sovrrnl imThaniBtiw nppt-ar poHHlhlt-  «lppiMi«l-
Ing on dt'
           1.   The  bag or  tube  stretches  au the air balloons through it,
               cracking and blowing off the cake,  (inside pulses only).
           2.   The  air pressure change, even after expansion and dilu-
               tion,  is still  long enough and intense enough to cause
               a flow reversal  which blows off the cake,  (clean side
               pulses only).
           3.   The  inertia of  the air mass pulls a vacuum behind it which
               pulls  off the cake, (dirty side pulses only).
           4.   The   shock  of the pulse accelerates the fabric away from
               the  dust cake,  even though the distance traveled by the
               fabric is very  small, (probably dirty side pulses only).
           5.   The  fabric  and  dust accelerate outward together, but only
               the  fabric  has  the elasticity to snap back into shape.
               Outside dust is  left behind; inside dust is thrown off
               at the end  of the snap-back, (inside pulses only).
 Among  the approximately 25 pulse-type collectors the pressure-time func-
 tion varies  considerably from a true shock to little more than a quiet
 reversal of  flow,  so it is entirely possible that several of these
 mechanisms apply.
              Efficiency of use of compressed air is important because the
 electric power needed to run the compressor can equal that for the pri-
 mary fan. Some models claim improved efficiency by using Venturis sur-
 rounding the pulse nozzles as was shown earlier in Figure 3.3.i.   These
 entrain a certain  amount of additional air which expands the bag,  causing
 the  collected dust to drop off the outer surface.   The entrainment may
 also set up  a shock wave which accelerates or flexes the filter cloth.

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             It  is expeciaily interesting that only one of the approxi-
 mately 25  available  pulse-cleaned models uses downward flow.   Apparently,
 the  dust is  able  to  fall through the  upward flow to the hopper despite
 upward velocities typically  3 to 4 times those in shaker and  reverse
 flow equipment.   Evidently the pulse-removed deposit falls  in large
 pieces, but  even  so,  a  large quantity of dust must be redeposited  on
 nhe  fabric.   Possibly,  these large pieces are beneficial in acting as
 filter aid,  but on the  other hand they increase the total deposit  on
 the  filter.
             The components of a pulse cleaning filter include  an air
 compressor,  a storage or surge tank,  piping,  solenoids  and  nozzles,
 and  some models use  Venturis and fabric support gridwork as well.  Few,
 if any, ducting dampers  are  needed along with their associated controls.
As there are no moving parts required,  the pulse method has an advantage
 in terms of  maintenance.
             Just  as  in shake cleaning,  in which all the cleaning energy
must be applied at one end of the  bag,  so in pulse cleaning the effect-
iveness of cleaning decreases  with length of bag.   This  is  evident in
the short bags typically used  in pulse  equipment,  rarely if ever over
ten  feet in  length.   Since there is little  fabric  motion, the  bags or
 tubes  can be packed slightly closer without  interchaffing.  Even so,
 large  installations still occupy more  floor space  than  types with  30- to
40-foot bags.
             Both  the  intensity and frequency  of pulse cleaning are
easily adjusted, but  there is  little control  of top to^bottom uniformity
of cleaning.  Woven cloths tend  to be over-cleaned by pulsing, resulting
in excessive  leakage  following cleaning, whereas felts are more easily
maintained at a satisfactory  level of residual dust deposit over the
entire bag length with excellent cleaning efficiency.

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            There  are  several  other  reasons  for using  felts  rather  than
woven  cloths,  although in  a  few rare cases tightly woven  fabric  is  used.
The  dimensional  properties of  felts  may make them more amenable  to  pulse
cleaning  involving "ballooning".   Bag inlet  velocities are high  in  pulse
equipment and  felts having softer  surfaces may withstand  more abrasion
than cloths.   Felts form a more porous dust  cake  having lower specific
resistance, so they can be cleaned less often than cloths for the same
pressure  build-up.  Equipment  using  felt  can be smaller because  higher
filtering velocities are possible.  Finally,  felts can be used even
though they are  initially  more expensive, because there is little mech-
anical wear of the fabric  in pulsed  equipment.
            Like other types of equipment pulsed  cleaning has its dis-
advantages.  It  is limited in  temperature to around 450 F at present,
because felt materials  are not  available  for higher temperature. High pulse
pressure  can damage the  fabric  by  over-stretching it.  Felts tend to
plug in depth  rather than  blind.   They may have to be  cleaned rather
than discarded because of  their greater value.  Felt,  compressed air,
power  and  the  compressor are all relatively  expensive  so  that economic
balances  are different  for pulse equipment.   As  a result, equipment
sizes  and  shapes are different.  Pulsed equipment may  be  best for some
applications and simply not economical for others.
     3.3.5  Reverse Jet Cleaning
            Introduced  in  the early  1950's,  this method of cleaning
became known for its high  filtering efficiency and high filtration  velocities,
It was the first method to give felted materials a good,  uniform cleaning.
It did this by using a small-volume jet of moderately pressurized air
spaced at moderate time intervals.  Figure 3.3f shows how this can be
done.  A carriage carrying the jet rings moves up and down the filter
tubes,  driven  originally by chains and sprockets,  or by cables con-
trolled by limit switches.   On the carriage a slotted ring close to the
fabric surrounds each filter tube.  Most of the jet air goes  through the
felt toward the dirty side, blowing the dust  out of the deep  felt surface.

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                                                                                J
             Since  the  four reverse jet models on the market are based on
 the  original Hersey patents,  the models are much the same.   One type
 uses adjustable  segmented rings  around the tubes,  another uses  a flat          —*
 plenum through which are  cut  holes about the same  diameter  as the tubes.
 The  traveling carriage is easily designed for either cylindrical or
 rectangular  baghouses. Tube  lengths  are in the  range of 6  to 30 feet.
                                                                                  i
 In all models the  flow is downward and outward.  For this reason,  the            j
 collector  can be open  sided,  but as reverse Jet  equipment is often used
 on toxic materials  because of its high efficiency,  health codes usually           j
 require that the filtered gases  be contained until  exhausted outdoors.
             There are  times when a very low dust concentration  must be
 filtered,  e.g.,  valuable  or toxic dusts.   At such  times  the reverse jet
 collector  is an  early  consideration,  not  only because of high efficiency,         i
 but  because  of high air capacity,  typically 10 to 30 FPM depending             -*
 mainly on  particle  size.   The time required for  cake buildup can be as
 long as a  month, depending on concentration.   To prevent useless running        ~->
 of the carriage  the limit switches may be over-ridden either manually
 or by a pressure switch.   Once the carriage starts  to move,  it  travels            '
 at its normal rate  of  a few feet per  minute.   It pauses  between trips
 for  cake buildup, thus lengthening the mechanical life of the equipment.          I
                                                                                _j
             Conversely with moderate  dust loadings,  the  carriage usually
 runs  continually.   On  higher  loadings  it  can be  speeded  up  to a limited
 extent if  necessary.   Note that  in continuous  operation  the  pressure
 drop  across  the compartment is not quite  steady, since compared with  the          j
midpoint of  the tubes  the  end of the  tube is  first  cleaned  too  soon and
                               i                  '
 then cleaned  late on the  next pass.   How  much  this  causes the primary            ,
 flow through  the system to fluctuate  depends  on  rate  of  pressure buildup,       —'
 rate  of carriage travel,  the  fan curva, etc.
            Reverse jet equipment  offers  control of both  intensity and          —i
 frequency  of  cleaning,  and even  offers  the possibility of adjusting the           ,
                                                                                  i
 intensity according to carriage  position  if necessary, although there           ^

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 I
 L
               is  no report of this  being  done.   The cleaning is practically uniform
 >              from top to bottom, the  more so since the deposit tends to be uniform
 '—             due to downward flow. For  these  reasons  the felt should last longer
   0            without plugging than with  any other method  of cleaning.  Unfortunately,
 <_             these advantages arc  partly offset by wear problems  in both the  felt
               and the moving  parts.
 *
 L.;                        Reverse jet  equipment is well suited  to  fine particles,
               expecially  if these cohere  well.   Otherwise,  there is  considerable re-
 [__             entrainment when the  deposit is removed and  the collector operates at a
               higher pressure drop. This equipment is  also suited to coarse particles,
 !              especially  non-abrasive  ones.
                           Regarding wear,  it is good design policy and almost  necessary
 !              to  have  as  much of the moving  mechanisms  as  possible outside  the com-
               partment.   Even the clean side of the compartment sooner or later gets
 ]              dusty,  if not from seeping  dust then from perforated fabric.   When this
 ****
              happens  the mechanism tends  to wear  and,  eventually, to stick and jump.
              The limit switches may become  fouled and  unless fail-safe the drive
               chains will break, cocking  the carriage and probably ruining  the fabric
               tubes.  Once  fouled it is very hard  to get the mechanics  back into the
 L.             original clean  and unworn condition.   Consequently,  the  frequency of
              maintenance increases.
 i
 L.                        In  a number  of  installations  a hard deposit  of  dust  and
               fiber  bits has  built  up  on  the jet rings  over a. period of time so close
 ^_             to  the cloth as  to cause abrasion.  This  deposit  can wear through the
               fabric unless removed.
 ^                        The air required to clean  the fabric  is usually supplied by
              a high-speed blower at pressures of  5  to  20 inches.  The volume required
              is roughly 5 percent  of the primary  filtering flow which increases the
*_
               filtration velocity only slightly.

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      3.3.6   Vibration and  Rapping Cleaning
             Although  several methods  of cleaning  involve motion and even
 fluttering  of  the  fabric,  higher  frequency  agitation with little cloth
 travel  can  also be effective.   In vibration and rapping, either trans-
 verse or  longitudinal elastic waves  travel  over the  fabric,  accelerating
 the  fabric  surface through displacements that  are usually small. The
 inertia of  the dust cake causes  stresses at  the cake-fabric interface
 which detach the dust cake from the  fabric.  Since it is easier to
 vibrate or  rap a compact group  of taut   filter elements, envelope filters
 are  the most commonly rapped type.
             This method of cleaning  is  especially successful with deposits
 of medium to large particles adhering relatively  loosely.   Since larger
 particles are  typically from low  or moderate temperature sources, high
 temperature  fabrics are not  usually used.  Woven  fabrics rather than
 felts are used because they  clean less  expensively and even  paper filter
material can sometimes be  used.
             The vibrating  mechanism should be  located outside  the housing
 to minimize  dusty  abrasion.  At least one end  of  the  frame supporting  the
 filter  envelopes is floating and  attached to an eccentric  arm  or vibra-
 tor.  If the cleaning  is to  be done manually,  a common rod may  protrude
 or the  housing may be  opened to strike  each filter frame.
             The compartment  is shut off during the vibrating or  rapping.
If continuous  filtering is necessary, several  compartments can be used
with sequential automatic  cleaning.  The length of the vibrating time
varies.  However,  as with  shaking, about 100 vibration cycles should be
sufficient.   Longer vibration is probably not as detrimental to  the
cloth as in shaking.  The power requirement is low and the cloth life
can be several years barring heat, chemical, or other problems.
            For larger particles,  for which this equipment is especially
suited,  the  filtration velocity can be higher than with other types  of  in-
termittent equipment.   Therefore,  vibration/rapping equipment is compact.

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                  With the  larger  particles,  higher  pressure  drops  across  the  cloth  can be
                  tolerated without  as much danger of blinding  the  cloth or  blowing  the
     __           cake through  the cloth.  For  this  reason  small,  light, unit-type inter-
                  mittent equipment  may  operate  safely without  attention for relatively
     i>             long periods, even without  instrumentation.   Eight hours is  a  typical
                  operating period between cleanings.  Even though  the  cost  of filtering
                  power at higher  pressure drops across  the small collector  is high,  it is
     fr"           frequently  only  a  small part of the plant process cost.
                              The  main disadvantage  of this kind of equipment  is probably
     *""           the  relatively few dusts for which it  can be  used.  If the equipment  is
                  located in  a confined  space it can be  noisy.
i
!                       3.3.7   Sonic Cleaning
                              Agitation frequencies still higher than those used  in vibration
I                  and  rapping have been  attempted with ultrasonic and sonic  cleaning methods.
'                  Although these frequencies are known to slightly  improve the preagglomera-
i     ^^
i                  tion  of a few fine dusts, they have not,  on the whole, been  very effective
!                  in fabric cleaning.  Lower sound frequencies  are used, however, with  success
]     *-            in a,  few installations.  The Fuller Company (Dracco) has installed sonicly
i
I                  cleaned equipment  in cement plants for over ten years.  They also used a
     ^_            successful  combination of reverse  flow-collapse plus sonic cleaning in
                                                                               2
                  Bethlehem Steel's  open hearth furnace baghouse about 1964-65.   Usually,
'                  sonic cleaning must be supplemented by some other method.
i                            The frequency band used( can be  estimated from the  sound,  des-
i                  cribed as low and mixed like a fog horn or railroad whistle.   The cloth
i     '
J                  motion is not apparent, but the vibration can be felt by touch.  At the
!                  Bethlehem installation the sound was generated by compressed air, with
i     —                                           "                               2
I     „             less  than 300 SCFM needed for 50 horns serving about 80,000  ft   of cloth.
                  The pressure required was not determined.   Following reverse flow and
i
     ~~            collapse the horns  gave three five-second blasts, after which the collapse
                  and horn blasts were repeated a second time.

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               In  all  reported  installations  of  sonic  cleaning  (cement  and
 open hearth  fumes), glass  cloth has  been used,  at  filtration velocities
 of  2 FPM  or  less.  However,  it was claimed  in 1959 that  use of sonic
 cleaning  would  reduce the  normal  cloth  pressure drop  of  4  inches  by 25
 to  50  percent,  raising the  possibility  of a  higher filtration  velocity.
              Sonic cleaning costs have  been reported as lying between
 those  of  reverse  nir  and shaking  systems.  The  compressed  air  equipment
 is  apparently  less than for  pulse cleaning equipment.  However, sonic
 cleaning  has never achieved  the popularity of most of the  other cleaning
 methods,  for reasons  which have not  been reported.
       3.3.8   Manual  Cleaning
              Any of  the automatic cleaning  cycles can be  initiated man-
 ually  at  convenient times, as between shifts.   An  even simpler method
 of  cleaning  is  to approach each bag  or  panel and remove the cake  by
 hand as in beating a  rug.  The method has no hardware to wear out and is
 as  reliable as  maintenance can be.   However, the  cloth itself generally
 wears  rapidly and endures  fewer cleaning  cycles than  other methods.
              This method  is only practiced on  smaller dust collectors
 having, at most, only a few filter elements.  They  can be brushed in
 place, or thumped to  knock the cake  loose.  Alternatively,  they can be
 removed and turned inside out for more thorough cleaning.
              When the more expensive fabrics blind or plug,  well before
 their  other qualities  are depleted,  they are reclaimed by various
 methods such as vacuum cleaning,  dry cleaning,  or  laundering.   An advan-
 tage here is that weak spots may ;be detected and repaired before a hole
 in  the fabric actually occurs.   A certain amount of skill goes  into re-
moving dust without damaging the  fabric.  Manual cleaning of  any large
 amount of ordinary cloth is not economical.

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 3.4  CONSTRUCTION AND MATERIALS
      Fabric filter manufacturers are rarely able  to guarantee  the per-
 formance of their equipment,  because performance  depends  so much  on how
 the equipment is  installed and used.  Any manufacturer  can, however,
 guarantee the quality of his  dust collector as  sold.  As  a  check  on
 quality, the manufacturer has standard specifications for the  production
 of the  equipment.  These specifications include a large amount of detail
 for construction  and  materials,  an example of which is  presented  in
 Appendix 3.2.
      3.4.1  Housing
             External  framework and configuration  design is  available  in
 three types,  the  choice  depending mainly on moisture condensation con-
 siderations.   The open-sided  pressure  design (inside filtering) is
 cheapest and used mainly below 135 F.   The closed pressure  type is  next,
 and followed by the slightly  more expensive closed  suction  installation.
 Above 160°F,  the  equipment is usually  closed and  insulated.  The  cost
 increases with the area  and thickness  of metal  used, which  depends  on
 the pressure the  walls must withstand.   Cost will also  reflect any neces-
 sary weather protection,  insulation,  etc.,  as well  as gas corrosiveness
 and temperature.
            Most  small unit collectors  are  assembled at the factory,
 usually  welded, while  larger  units may  be  either  assembled at  the
 factory  or  on location.  The  largest unit  that can presently be shipped
 by  railcar  is  approximately 10' x 40' x  12' high;  consequently many
 larger designs are assembled  from standard preassembled modules by
 either welding or bolting  on  location.  It  is difficult in field assembly
 to make  good  pressure or vacuum-tight seals between panels,  modules and
 flanges  and seal quality is a chief complaint among fabric filter users.
            To create a strong structure and provide good seals against
weather  and condensibles, under the large deformations  due to changes

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in  temperature and pressure, some variations  in construction are neces-
sary.  Several styles  of housing joints and sealants are  indicated  in
Figure 3.8.  For steel, one manufacturer uses galvanized  14 gauge sheet,
others use  10 gauge welded hot-rolled steel.  Much heavier material can
be  used, up  to one-quarter inch or more.  The open hearth FF installation
at  Bethlehem Steel used 3/16 inch for walls, roof and partitions, 1/4
inch for floor plates  and 5/16 inch for hoppers and inlet plenum (Fuller-
              2
Dracco design) .  In passing, let it be noted that competition forces
many manufacturers to  underdesign unless otherwise instructed (see
Section 8.3).
            Materials  other than steel may be used for housings; for
example, cement plant  collectors are often made of concrete and one
fabric filter manufacturer has a design for precast reinforced concrete
panels.  An aluminum company stated it feels that aluminum is definitely
the best all-round material to use.  Corrugated asbestos cement paneling
is  often used for exterior roofing and siding, with the interior walls
and partitions made of steel.  Various other composite panels can be
used.  Insulation can be sandwiched in the paneling or added to the
outside at factory or  later as needed.  The main precautions in selec-
tion of material are corrosion and changes in temperature that cause
thermal stresses in the seals and even in the bags.

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            fSealant)   !
        Vertical     ) !
         Siding  Joint)*
         Horizontal
          Siding  Joints
                                   mi
                    (Insulated)
                                                        U
[frfWTi
. y r|r
» B
Roof Joint

u



1
1
!
i *

Siding


J


tj
          Panel
           Stiffeners
i^^_

I
                        1
                      Main
                      Frame
i
Figure 3.8.  Examples of Some Styles of Fabric Filter Compartment  Joints,


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     3.4.2  Hopper and Disposal Equipment
            Hopper material  In usually similar to  that used  for  the hous-
ing with similar joints and  seals.  The  inside may be treated  to minimize
adhesion of the dust.  Hoppers are commonly available with either 45  or
60  slope depending on the need to conserve height or to provide adequate
sliding, as affected by the  plugging properties of the dust.
            Most hoppers are under a few inches of pressure  or vacuum.
Consequently they require some kind of valve at the bottom to  let the col-
lected dust out without admitting air to reentrain dust and  redeposit it.
Automatic filter equipment usually uses a rotary valve consisting of a
sealed paddle wheel as depicted in Figure 3.9.  These valves rotate at
about 5 RPM, and are typically 6 to 12 inches in diameter.  Thus they
handle around 2 CFM of collected material.  If the solids flow is higher,
the valve can be run faster or more or larger valves can be used.
            For low solids flow,and for many non-automatic installations
requiring periodic attention,sliding gates may be used.  Unless the hopper
is under practically zero pressure these gates are used only when the com-
partment is shut off.  When the hopper is under no pressure a gate may not
be necessary.      A stocking perhaps 6 feet long reaches from the hopper
bottom to the dust bin.  Likewise, the rotary valve or gate seldom dis-
charges with an open cascade of dust.  A stocking is used or more commonly
the solid flow enters a conveyor, usually screw-type.
            If for reasons of compartment cleaning or maintenance the com-
partments must be completely isolated then the rotary air lock valves are
located between the conveyor and the hoppers.  Otherwise it may be less
expensive to install the conveyor directly on the hoppers and use one valve
at the end of the conveyor.   The air slide, bucket, and belt methods  of
conveying powdered materials are normally used only for high solids  flow.
There are .however, times when the screw conveyor doesn't work as with
gummy dusts and those that lock up under pressure.

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\ /
J L
, i r
i i i




r, p




j
1






Chain
Drlva






4
^



c



);.


<

—





D;

r

— —
hj


i
                                                                      Full
                                                                      H«lf-skeleton
                                                                      Skeleton
    Rotary Valve
                                                    Manual  Sliding C«ten
   Trip Gatf

(Automatic or Mcnual)
     Dlgcharge Duck

(Knotted, Clamped, etc.)
            Valve #1
                  I
      Drive
                  L.
                              Valve #2, etc.
                        Screw Conveyors
                                                        Discharge
                                                                Rotary
                                                                Valve
  Figure 3.9 - Types of  Hopper  Discharge Equipment

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3.5  EXTENSIONS OF FABRIC FILTRATION  EQUIPMENT
     In addition  to  the  standard  FF designs  already  discussed,  from time
to time novel  filter  equipment arouses  interest  among  those  acquainted
with the  limitations  of  standard  equipment as indicated  earlier  in  Chap-
ter 1.  The  patent literature records a  fascinating  history  of design
variations,  of which  only a  small  percentage are now in  practice.   Be-
sides  these  efforts  to control dust,  fabric  filtration has demonstrated
other  capabilities including control of gases and perhaps odors, control of
mists, and ultrafiltration for very finu  particulates  in atmospheric dusts.
     3.5.1   Variations in Standard Fabric Filter Designs
     3.5.1.1   Bourdale Rotary Filter.- The filter media  surrounds a central
shaft  and rotates "until such time that  the  cake of  dust has such a  mass
that its centrifugal  force exceeds the  fan suction,  when it  flies off the
surface and  settles down into the hopper."   This is a compact unit in use
on a wide variety of  dusts up to  210  F.   It  is made  by Wheelabrator Alle-
vard of Paris.
     3.5.1.2   Tower Collector.- Two tiers of glass cloth bags are used, one
above  the other,  in a 60 foot or higher cylindrical  chamber.  The flow is
downward; cleaning is by sonics.  Although the brochure  of the company
that produces  this configuration  is not specific, the  bags are probably
butted end to  end on  an  intermediate  tube sheet to relieve the tension that
would  otherwise be extreme on a single 60 foot bag.
     3.5.1.3   Self-Emptying  Bag.- In one such design  up to 8 bags,  each
about  6 ft by  1.5 ft  dia., are held at the top by a guy cord during filter-
ing.   For cleaning the cord  is released and under ;the weight of the  dust,
the bag sinks  through the lower tube sheet, turning  itself inside out and
cracking off the cake.   In another design used in a copper mine in Zambia
the bags are held up  by  inflation pressure, and their top is guided  up
and down by a  taut wire through a grommet in the top center of the bag.
Dust dumped as the bag inverts itself is later washed away by a stream
                         Q
through the  inlet plenum.

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      3.5.1.4  Granular Bed Plus Screen.- This equipment uses pulses to
 clean a combination sand bed plus filtering screen,  as sketched in Fig-
 ure 3.10.  Large particles form a cake on the screen(s), and fine particles
           SCREEN
SAND
                     Filtering
                 Cleaning
           Figure 3.10.  Ducon Sand and Screen Filter Cleaned by
                         Back Flow.
are caught in the sand(s).  "The channels in the filter element are not
completely filled with sand.  Thus, during the cleaning cycle, short blasts
of high-pressure air expand the beds.  The air dislodges filter cake from
the outer screen and restructures  the granular material into a filtering
                                          9
bed essentially the same as the original."   The Ducon Co. says the device
can operate at filtering velocities of 30 to 40 fpm  at temperatures
well above 1000 F and on sticky dusts.
     3.5.1.5  Foreign Equipment.- Filtration and Separation, a British
publication,  regularly lists developments in dust control equipment in
Great Britain and Europe.  Much of this equipment is slightly different

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  from U. S. Designs  although  several U.S. manufacturers have  subsidiaries
 in  Europe, and  this  handbook  survey of available  equipment  styles  could
 be  extended to  include European  equipment.

     3.5.2  Control  of Gases  and Odors
            Several  experimental studies have shown that finely divided
 absorptive or adsorptive  granular materials aerosolized into  the gas stream
 entering the fabric  filter, can  effect the removal of certain gaseous com-
 ponents.  The combination of  good powder control with high  specific surface,
 and long, intimate gas-powder contact especially on the fabric, are advan-
 tages offered by fabric filtration equipment over most other  equipment.
 Most recently the removal of  up  to 98.4% of SO- by sodium bicarbonate has
 been demonstrated at a coal-fired power plant by Air Preheater Co. under
 NAPCA contract.  Fly ash  and  N02 are also removed at the same time.
 The optimum equipment design  for gaseous control would probably not be much
 different from  present filter designs, except for the system  for introducing
 and possibly recirculating the powdered material.  This addition is esti-
mated to be a small part  of the  fabric filter system cost.
            The addition  of particle aggregates to a filter cake should
 reduce power costs by reduction  of the cake porosity.   The partial recir-
culation of agglomerates  from the collector hopper seems attractive but
 there is no report of this practice.   It would appear that agglomerate
recirculation in conjunction with absorptive powder recirculation would be
attractive in numerous possible  gas and odor control applications.
            Similarly, although  as far as is known absorbing additive in
fabric filter equipment has not been used to control odors,  it is a likely
prospect.  Granular charcoal, catalysts,  or fritted impregnated material
could be introduced to and removed by the filter, regenerated in special
closely confined equipment, and then recirculated.
     3.5.3  Control of Mists
            It  is widely observed that the fabric in an ordinary fabric
filter collector must be kept dry or  it  will rapidly plug  with dampened  dust.

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  This may  be  reversible, however, as  shown by pilot plant experience at a
 Southwest  Portland Cement Company plant  some years ago using silicpnized glass
 fabric,  collapse without shake cleaning, and cement kiln effluent normally
 about 500°F:  "...During subsequent months of testing, the bags at times were
 saturated  with water  from condensation during normal variations of atmos-
 pheric and kiln conditions  (wet-process) and were even deliberately wetted.
 In  all cases, rising  temperature dried the bags and, at the next collapsing
 cycle, they discharged readily and returned to normal pressure drop.  In-
 spection of the fabric showed that no blinding had occurred, and the lack
 of  deterioration suggested  an entirely satisfactory life of at least one
     ,,11
 year."
            Other filter beds, more open than normal filter fabrics, are
 used for large particle demisting in a number of processes.  These are
 usually  self  draining, but  if they are of intermediate porosity and there
 is  danger  of  clogging they  can be kept open with water sprays.  One such
 study obtained effective removal of submicron TiO. particles plus around
                                                                  12
 90% removal of sulphuric acid mist, by irrigating synthetic felts.
 Filtration was at 70 to 165 FFM, the spray rate was around 10 gal. per KCFM,
 and felts  fairly similar to felted fabric filtration materials were used.
 The pressure  drops encountered were 8 to 10 inches of water and were nearly
 independent of filter velocity, possibly because the air permeability of
 the wet  felt  depended on the amount of water remaining in the felt.   Pro-
 jected initial costs of a complete 56,000 CFM system were cited as $2.40
 per CFM  plus minor installation costs.
            These examples of effective particulate control with fabric at
 conditions below dewpoint may point the way to solution of condensation
 problems in fabric filtration, enabling cooler and more economical operation
without  fabric blinding aid plugging.   There are many possibilities  for
 research and development in this area including the opportunity for  coin-
 cidental wet-process odor control.   Although the field is not a new  one by
 any means,  new synthetics and changing economics make it a viable  and
 inviting challenge.

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     3.5.4  Ultrafiltration

            The high efficiency made possible by filtering with a dust

cake is used to obtain very low effluent concentrations of toxic and radio-

active dusts, bacteriologicals, and atmospheric dust.  Very low concentra-

tions of liquid or tarry particles may also be controlled.  Aside from a system

for introducing the filter aid, the equipment is much the same as that for

any fabric filter system.  Since the fabric is cleaned only very infre-

quently, however, perhaps once a year or longer, the cleaning mechanism and
dust disposal equipment can be minimal.  A high fabric packing density can

be used without concern of chaffing.  Maintenance is low and the equipment is

available for operation with a number of filter aids.


3.6  REFERENCES FOR CHAPTER 3

     1.  Wheelabrator Corporation, Pustube Collectors.  Bulletin No.  565-E
         page 10 (1967).

     2.  Herrick, R.A., Olsen, J.W., and Ray, F. A.,  Oxygen Lanced Open
         Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
         Jnl. A.P.C.A. 16:  1, 7 (January 1966).

     3.  Sonics Clean Dust Collector Bags Soundly, Chemical Week.
         (14 November 1959).

     4.  Adams, R.  L., High Temperature Cloth Collectors, Chem.  Engg.
         Progress.  62;4, 66 (April 1966).

     5.  Bourdale Rotary Filter Dust Collector,  Filtration and Separation.
         170 (March 1969).

     6.  Fuller Company, Collecting Hot Dust and Noxious  Fumes with  Dracco
         Glass-Cloth Dust Collectors. Bulletin No.  DCB-1B (1967).

     7.  The Ducon  Company, Dueon Dust Control. Equipment. Bulletin No.
         F-9069, (1969).

     8.  Flannel Bags Key to  New Dust Filter,  Engg. and Mining Jnl.  166:12,
         98 (December 1965).

     9.  Chem.  and  Engg. News.  57 (15 December 1969).

    10.  Liu, H., and Chaffee,  R., Evaluation of Fabric Filter as  Chemical
         Contractor  for  Control  of S02  from Flue Gas, Final  Report to NAPCA,
         Contract PH22-68-51,  by Air "Preheater Co., Wellsville,  N. Y.
         (31 December 1969).

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11.   Hot Kiln Gases in Glass Cloth Bags, Pit and Quarry. 80,
     (October 1958).

12.   Morash, N.,  Krouse, M., and Vosscller, W. P., Removing Solid and
     Mist Particles from Exhaust Gases, Chem. Engg. Progress. 63.:3, 70
     (March 1967).

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                               CHAPTER 4
                            FABRIC SELECTION

4.1  INTRODUCTION                                                  4-3
4.2  MATERIALS                                                     4-3
     4.2.1  Natural Fibers                                         4-4
     4.2.2  Man-Made Fibers                                        4-7
            4.2.2.1  Acrylic Fibers                                4-7
            4.2.2.2  Modacrylic Fibers                             4-9
            4.2.2.3  Nylon, Polyamide                              4-10
            4.2.2.4  NomexR Nylon                                  4-12
            4.2.2.5  Olefin                                        4-12
            4.2.2.6  Polyester                                     4-15
            4.2.2.7  Teflon Fluorocarbon                           4-18
            4.2.2.8  Vinyon                                        4-20
            4.2.2.9  Glass                                         4-20
            4.2.2.10 Other Man-Made Fibers                         4-22
     4.2.3  Physical and Chemical Properties                       4-22
4.3  YARN PRODUCTION                                               4-28
4.4  YARN PROPERTIES                                               4-29
     4.4.1  Threads and Yarns                                      4-31
     4.4.2  Yarn Twist                                             4-31
     4.4.3  Yarn Number                                            4-32
     4.4.4  Simple and Plied Yarns                                 4-34
4.5  FABRIC PRODUCTION                                             4-35
     4.5.1  Weaving                                                4-35
     4.5.2  Felting and Needle Punching                            4-41
     4.5.3  Fiber Additives, Yarn Treatments,  and Fabric            4-42
            Finishes
4.6  FABRIC PHYSICAL CHARACTERISTICS                               4-46
     4.6.1  Abrasion Wear                                          4-46
            4.6.1.1  Sandblast or Surface Scour                    4-48

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            4.6.1.2  Internal Chafing                             4-50
            4.6.1.3  Internal Abrasion                            4-52
     4.6.2  Flexibility                                           4.52
     4.6.3  Strength                                              4-55
     4.6.4  Permeability                                          4-58
     4.6.5  Dust Deposit Release                                  4-59
4.7  AVAILABLE FABRICS                                            4-59
4.8  REFERENCES                                                   4-60

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                              CHAPTER 4
                           FABRIC SELECTION
4.1  INTRODUCTION
     Satisfactory performance of a fabric filter on a specific applica-
tion requires selection of a fiber material compatible with the gas-
particle environment, and a fabric design appropriate to dust collector
geometry and collector cleaning requirements.  Fiber, yarn, and fabric
parameters influence the ability of the fabric to collect dust at a, reas-
onable pressure drop.  !A brief summary of developments in fiber, fabric,
treatments, and applications has been presented in Chapter 1.  Man-made
fibers have been developed over the past 30 years with superior resistance
to physical and chemical environmental requirements.  While traditional
fabrics of cotton and wool are still employed for many fiber applications,
the impact of man-made fiber fabrics has extended the range of application
of fabric filters to a much broader gas cleaning market.  Continuing de-
velopments in fiber and fabric technology, as in dust collector design,
can be anticipated.
     The purpose of this chapter is to present information on fiber and
fabric properties important to their application for industrial gas
filtration.  In addition, pressure drop during fluid flow through fabrics
and dust deposits has been considered in Chapter 2; effects of fabric
structure on filter performance has been discussed in Chapter 6; and fabric
costs in dust collection have been considered in Chapter 7.   In what fol-
lows, a number of terms unique to the textile industry are used, and these
are defined in Appendix 4.1.
4.2  MATERIALS
     Production of textile fabrics begins with the basic structural  unic,
a single fiber.   Fibers are obtained from traditional natural vegetable
or animal sources, such as cotton (cellulose)  or wool (protein).   Fibers
are also produced from man-made or modified natural organic  polymeric

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materials through modern chemistry and chemical  engineering.   Industrial
filtration fibers are indicated in Table 4.1 by  generic group  manufacturer,
                2                                                      3
and Trade names.   A more complete index has been compiled by  Dembeck.
The internal molecular orientation and crystallite  structure of  the  fiber
determines its  basic physical  and chemical  properties.  End use  fiber
characteristics are substantially controlled by  additives and  treatments.
Within a generic group, polymers, co-polymers, and  homo-polymers may be
produced.  For  all these reasons, there are differences in physical  prop-
erties and chemical resistance for each manufacturer's product in each
      2
group.
     Fibers are produced as staple (relatively short  lengths (1  < length <
10 inches long), as monofilament (single long continuous filament L  » 10  in.)
and as multifilament (many continuous parallel filaments).  Natural  fibers
are used for industrial filtration staple.  Man-made  filtration  fiber is
used in both staple and filament lengths.  Cross sections of typical nat-
ural and man-made fibers are shown in Figure 4.1.   Fiber microscopy such
as this is a useful technique  in textile identification and filtration re-
search, as discussed in various references. '
     4.2.1  Natural Fibers
            Cotton and wool fibers are commonly used  in fabric filter dust
collectors.   Cotton fibers are elongated single hollow cells, often
flattened (Figure 4.1), with characteristic irregular lengthwise twists.
Cotton fibers from different varieties of plants vary in length from 1/2
to 2 inches, and in width from 12 to 25um.
                                t
            Sheep wool also varies with specie and with geographical loca-
tion.  The surface of a wool fiber is made up of flat scales  (cuticle)
which overlap like shingles.  These scales are spaced approximately 20 ^m
apart along the fiber.   They are very important in determining the mechanical
properties of wool fiber assemblages, and probably influence  dust collec-
tion and release properties as well.   Wool fibers are 10 to 70 urn in
diameter,  and 1 1/2 to 15 inches in length.  They are almost  uniformly
cylindrical  except for the ends which are tapered.

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                                        TABLE 4-1

MANUFACTURERS AND  TRADE NAMES  OF  INDUSTRIAL  FILTER FIBERS**
       ACETATE AND TRIACETATE
       Calanesa.-"Acetate".  "Arn.l"*
       DuPont--"Acele"*
       EaetsMn Chemlcal--"Estron"*
       r.M.C Corp., American Vlscoss Dlvlslon--"Avlsco"*
       ACRYLIC
       American Cyanamld--"Creslan"*
       Chsmstrand--"AcrHen"*
       Dow Cheulcal--"Zafran"*
       DuPont--"Orlon"*
       HOOACRYUC
       KastlMn Chenical--"Verel"*
       Union Carbide--"Dynel"*
       POLYAHIDBS
       Allied Cheolcal--"CaproUn 6"*
       American Enka--Enka nylon 6
       Cheast rind--Chens t rand 6.6
       DuPont--nylon 6.6 "Nomex"* nylon
       Nylon Induetrles--Celanese Nylon
       Firestone Synthetic Flb*r<--Flr«tton« nylon 6
       U.S. Rubb*r--nylon 6.6
       V«etr«--nylon 6.6 nylon 6
       POLTOTMS
       AMrlcaii Knka Corporation- -EnV«  Polyeiter
       B««untt  Plb«r«--"Vycron 2.5"*
       DuPont--"D»cron"*
       EMMn Chalc*l--"Kodel 2.4"*
       Flb«r bidu«crl« (Cel«nen)--"Portrel"*
       Ktotlon--"Cry*t*l Milt"*
       Vectr«--Poly««ter
       POLYBTHYLBNE
      Alun Folywr  Corpor«tlon--"R«evon"»
      American Th«m>pl«itlc PToduct«""A««rfll"*
       Dawbarn--"DLP"*
       Flraiton* Synthetic Ptb«n--Plre«ton« Polyethylene
       G.P. Chealcal>--"Gerfll"*
       Induatrlal  Pla»tlc--"Tuff-Llta-L"*
      Vectra--"Wynene"*
      Vogt Manufacturlng--"Vopl«ii"*
      POLYPROPYLENE
      Alamo Polymer Corporation--"Reevon"*
      American  TharoopUitlc Producte--"ABerfll"*
      Dawbarn--"DLP"*
      Flraatone Synthetic  Flbare--Plreatone Polypropylene
      Herculee  Powdero'o"Herculon11*
      Industrial  Ple»tlc--"TuMLlca-P"*
      Vectra --Polypropylene
      Voft Menutecturlng--"Voplei"*
      RAYOU VISCOSE                     ,
      Aaarlcan Enka--"3up.r«nka"*, Fiber 700"
      Beaualt Fibers--Beavmlt Rayon
      Celaaa«e--Celaneee Rayon, "Portlian"*
      Courtauldj North Anerlcen--"Plbro"*
      F.M.C. Corporation, Anarlcan Viscose Dlv.--"Avlsco"*,
         "R«ytl«x"*, "Super  Raytlex"*,  Fiber 40"*, "XL"*
      Industrial Rayon--"Lekroeet"*, "Tyron"*,  "VlUwyte"*
      SARAM
      Dov CheMlcal--"Rovana"*
      Firestone Synthetic Fibers--"Valon"*
      Vectre--Saren
      TBTLOW
      DuPont--"Teflon"*
      vniYOH
      F.N.C. Corporation, American Viscose Division--
         "Avlsco Vlnyon HH"*
      Rhodl* Aceta (France)—"Rhovyl"*
      Vogt Manufacturln(--Voplax Vlnyon
      CLASS
      FaeBO--Fa»co Fiber
      Johns ManvlUe--nber  Glass
      Owene-Cornlng-Flbercles Corp.--"Fiberflas"*
      Plttabursb Plata Claes--Flber Class
       CerssElc, Aabeatoa, Metal     i
      COTKM AMD WOOL
      Available as natural fibers
  *    Registered Trade  Mark
  **  from  J.P.   Stevens,  Inc.,  Ref.   2

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(a) Cotton  (X280)
 (A) Acrylic
      (AcrilanR) (X410)

(b) Wool  (X410)
(e)  Modacrylic
     (DynelR)  (X410)
(c) Polyacrylonitrile
     (Orion*)  (X380)

 (f) Polyamide  (Nylon)
      (X380)
(g) Polyester            (hj Vinyl (Vinyon)         (1)  Giant   (X150)
     (DacronR) (X380)         (X380)

    Figure 4.1.   Cross Sections of Filtration Fibers,  (From Harris, Ref.4 ,)

                                  L.f,


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      4.2.2  Man-Made  I-'ihers
            "Certain  (man-made)  fibers  offer  outstanding  resistance  to
 acids,  others  to  alkalies.  Most are  impervious  to  mildew.   Some....
 have  high temperature  resistance so that....operations  such  as  cooling
 of  exhaust  gases  can be  reduced  or eliminated.   Because of their  inherent
 toughness and  resistance to abrasion  and  other environmental conditions,
 many....offer  substantial overall savings  due to increased service life
 and reduced equipment  down-time.  The initial cost  of fabrics made from
 (man-made)  fibers may  be greater than fabrics made  from natural fibers.
 However, the increased service will make  them the most economical in  the
 long  run	A careful choice of filter or dust  collection fiber  for  each
                                                          2
 application can pay dividends in efficiency and  economy."
            While the  chemical processes used to manufacture the various
 polymeric materials used as filtration  fibers may differ,  '  '   the fibers
                                                  348
 themselves  are  made by somewhat  similar processes.  '  '   The liquid material
 is  spun (wet,  dry, or  extrusion)  to mono-  or  multifilament,  stretched to
 orient the  constituent molecules, then processed into yarn strands; and
 by  spinning, texturizing or through-tow and staple, to yarn.
                                                                    9 10
            4.2.2.1  Acrylic Fibers.- An acrylic fiber is designated  '
 as  "A manufactured fiber  in which the fiber-forming substance is any
 long chain	polymer composed of at least  857<> acrylonitrile  units	",
      CH
 (-CHo- V -)  ,  the remainder frequently being a  copolymer such as vinyli-
    L CN    n
 dene chloride  to provide  dye sites.   Process  steps in the production of
 acrylic fibers  are discussed in References 4  and 7.  Among other acrylics
 commonly used  in fabric  filtration are Orion  and Acrilanv   Both are some-
what dumbell-shaped in cross section (Figure  4.1) and their  surfaces are
 irregularly striated to  various degrees.  Acrylic fiber diameters are
 about 15 to 35 (am.

            Characteristics, properties,  and  available producer's forma
 are given in Table 4.2.   The acrylics offer a good  combination  of
            "....abrasion resistance and  resistance to heat  degrada-
            tion  under both wet  and dry conditions.  An outstanding

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                                 TABLE 4-2

         CHARACTERISTICS, PROPERTIES, AND FORMS OF ACRYLIC FIBER FOR

                           INDUSTRIAL FILTRATION*
  HEAT  RESISTANCE:

  Dry Heat;   Inferior  to  "Teflon"  and
  Nomex" nylon, which  are  outstanding
  in this  respect.   Inferior  to  poly-
  esters,  but  superior to  nylon  and
  natural  fibers.
  Moist Heat;  Rated below "Teflon"
  and "Nomex"  nylon, but considerably
  superior to  polyesters,  nylon, rayon
  and natural  fibers.

  CHEMICAL RESISTANCE:

  Acids:   Satisfactory resistance  to
  most mineral or organic  acids.
  Superior to  polyamides,  polyesters
  and cellulosic fibers but less resistant
  than other synthetics.
 Alkalies;  Inferior  to most fibers
  in this respect, except natural
  protein fibers, silk and wool.
 Oxidizing agents;  Fair  to good
 resistance to most oxidizing
 agents.   Superior to polyamides,
 polyethylene and natural protein
 fibers.
 Organic  Solvents;  Excellent
 resistance to most common organic
 solvents.  Superior to modacryl-
 ics polyethylene, polypropylene
 vinyon and protein fibers. .

 PHYSICAL PROPERTIES:
 Tenacity and Elongation:

 Filament;  Dry and wet tenacity
 ranges from 3.8 to to 4.8 g.p.d.
 Dry and  wet elongation ranges
 from  13  to 23.

 Staple;   Dry  and wet  tenacity
 ranges from  1.8 to  3.5 g.p.d.
 Dry and wet elongation ranges
 from 20 to 557,.

*from J.  P. Stevens & Co., Inc.  (Ref 2)
  Specific  Gravity;   1.12  to i.18
  Abrasion  Resistance;   Good,  but
  less  resistant  than polyamides,
  polyesters,  and polypropylene.
  Superior  to  "Teflon."

  FORMS AVAILABLE:
  DuPONT  "ORLON"

  Staple; 1.5  to  16 denier/filament.
  Cut   lengths 3/4" to 5%".   (Can be
  converted to spun yarns  of many
  sizes).
  Tow; 2  to 10 denier/filament.
  All of  the above are available  in
  bright, semi-dull and black.
  CHEMSTRAND "ACRILAN"
  Staple; 1 to 15 denier/filament.
  (Can be converted to spun  yarns
  of many sizes).
  Tow;  2 to 8 denier.  Available in
  high shrink form.
 AMERICAN CYANAMID "CRESLAN"
 Multifilament;  Deniers of 75,150
  and 200.  Semi-dull only.  Special
  industrial 166 denier—107. shrinkage
  in boiling water.
  Staple;   1.5 to 15 denier/filament.
 Bright and Semi-dull. Regular and
 high shrink.  (Can be converted to
 spun yarns of many sizes).        :
 FARBENFABRIKEN BAYER AG.  "DRALONG T"
 Multifilament;  200-800 denier, bright.
 Staple;  2-10  denier, white, dull and
 doped-dyed light green for identifica-
 tion purposes.   (Can be converted  to
 spun yarns of many sizes).
 RHODIACETA "CRYLOR"
 Multifilament;  Denier of  100 to  225.
 Staple;  Net yet .available  in U.S. Market
DOW CHEMICAL "ZEFRAN"
 Staple;  3  and 6 denier per filament
 cut 2 inches  (Can be converted to  spun
 yarns of many sizes).

-------
            characteristic is the ability of acrylics  to withstand
            a hot acid atmosphere, making this fiber a good choice
            in the filtration of exhaust gases.  Acrylic fabrics are
            used for dust collection in the manufacture of ferrous
            and non-ferrous metals, carbon black, cement, lime and
            fertilizers.

            "Other dry filtration or dust collection applications
            include the drying of raw  flour, sand and  coal, mining
            and ore dressing.  Wet filtration applications include
            the manufacture of dyestuffs, paint, varnish, solvents,
            storage batteries and mineral oil; also galvanizing
            and copper ipining.  Acrylic filters function well in
            the presence of steam, as  in heat exchangers.

            ". . . . .Acryl.lc fibers made  especially for industrial
            use, such as "Dralon T" and "Crylor," are  available
            as homo-polymers, composed of 100% acrylonitrile
            units.  The homo-polymers  offer a good hydrolytic
            resistance, and are recommended for temperatures of
            up to 284°F, while copolymers are recommended for
            temperatures of up to 248°F.  Dow Chemical Company's
            new acrylic fiber is a homo-polymer and will operate
            in the higher temperature  range recommended for homo-
            polymer acrylic fibers.  Wet performance of Dow's new
            acrylic is maintained to be superior to other acrylic
            fibers."2

                                                                       9
            4.2.2.2  Modacrylic Fibers.- Modacrylic fiber is designated

as "...a manufactured fiber in which the fiber-forming substance is any long

chain. .. polymer composed of less than  85%, but at least 357» by weight of
                                           P
acrylonytrile units..."  For example, Dynel  is a 40-60 acrylonitrile-

vinyl chloride mixture.  The cross sections of Dynel fibers resemble dumb-
                                        4
bells, and their surfaces appear uneven.

            Although few gas filtration applications use modacrylics, the

one most used for filtration and dust collection is manufactured by

Union Carbide Chemicals under the trade name 'Dynel'.   'Verel' (Eastman

Chemical Products)  is also an important available modacrylic fiber.

(Other producers are indicated in Ref.  3.)  "Modacrylics have good chemi-

cal and abrasion resistance generally,  offer excellent dimensional

stability and are unaffected by many acids and alkalies,  even at high

concentrations.   Water has no adverse effect on the fiber which retains

-------
 oyer 95% of its  strength when wet.   Dry and moist heat degrade modacrylic

 mere than the other fibers discussed in the brochure,  with the exception
            2
 of vinyon."   Characteristics and properties of modacrylic fiber are given
 in Table 4.3.
                                                                  9
             4.2.2.3  Nylon,  Polyamide.- Nylon fiber is designated  as....

 "a manufactured  fiber in which the  fiber-forming substance is  any long

 chain...polymer  having  recurring  amide  groups ...(-(J-NH-)...as  an integral

 part  of  the  polymer  chai.n."   Table  4.4  indicates  the basic  polymer in  the

 several  types of nylon  currently  available.   Nylon fibers  are  cylindrical,

 as  shown earlier in  Figure 4.1, with smooth  surfaces devoid  of markings.

 The  fibers  are uniform  in diameter  (10  ^m  and upward)  and  appear round  in

 cross  section.   Continuous filaments and staple  fibers  (1  to 5 in.  long)

 are  produced.

             "Three  types  of  nylon are available  for filtration:
             Nylon 6,6  (DuPont, Celanese and  Chemstrand), Nylon 6
             (Allied  Chemical,  Enka  and  Firestone)  and  'Nomex1
             nylon (DuPont).

             " 'Nomex' nylon  differs  considerably  from  the  other
             types, and  therefore  is  treated  separately.

             "Nylon  6,6  and nylon  6 have similar characteristics,
             except that  the  latter has  a lower melting  point and
             somewhat poorer  heat  resistance  at extreme  tempera-
             tures.   Possibly for  this reason, and  because  nylon
             6,6  was  introduced in this  country before nylon  6,
             the  former  is used more  widely in filtration.  Where
             elevated temperature  is  not a  factor, however, it
            would seem  that  the two  types would give comparable
             service.

             "Because of nylon's high abrasion resistance,  it is
            used in  filtration of abrasive dusts or wet abrasive
             solids at low temperatures.  Nylon's good elasticity
            makes it ideal for conditions where continuous flex-
             ing  takes place.  This fiber is  a good choice  in wet
             filtration  applications, at low  temperatures and in
            an alkaline atmosphere,  for example in dyestuffs.
            Fabrics of  nylon provide good cake discharge."2

The general characteristics  and properties of nylon fibers in use for

 filtration are indicated  in  Table 4.5.

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                                TABLE 4.3

          CHARACTERISTICS,  PROPERTIES AND FORM OF MODACRYLIC

                     FIBER  FOR INDUSTRIAL FILTRATION*
 HEAT  RESISTANCE:

 Dry Heat;   Shrinkage  starts  at  250 F
 but can be  heat stabilized at higher
 temperatures below the  stiffening
 state.  For prolonged use, 180°F
 should be considered maximum.
 Surpasses acetate, polyethylene,
 saran and vinyon in heat resistance,
 but inferior to other fibers.   This
 fiber will  not support  combustion.
 Moist Heat;  As with most fibers,
 moist heat  has more effect on the
 fiber than  dry heat.

 CHEMICAL RESISTANCE:

 Acids;  Little effect even at high
 concentrations for most mineral and
 organic acids, including aqua regia,
 chromic acid, nitric acid, phosphoric
 acid and sulfonic acid.  Can be used
 in the presence of moderate concen-
 trations of nitric and  sulfuric acids.
 Adversely affected by phenol and high
 concentrations of acetic acid.
Alkalies;  The fiber has good alkaline
 resistance under most conditions.
 Oxidizing Agents;   Excellent resistance
 to nearly all oxidizing agents.
Organic  Solvents;   Softens or
dissolves  in warm acetone and
some other ketones, otherwise
would be considered good for
organic  solvents.   Not  affected
by dry cleaning  solvents or gas-
oline.

PHYSICAL PROPERTIES:

Tenacity and Elongation;  Depending
on type  2.4 to 3.0  g.p.d. dry
and wet  tenacity.   Dry  and wet
elongation ranges from  32 to 39%.
Specific Gravity;   1.30 to 1.37.
Abrasion Resistance;  Good, but
inferior to polyamides, including
"Nomex"  nylon, polyesters and
polypropylene.   Superior to
"Teflon."

FORMS AVAILABLE:

Staple Only;

EASTMAN  ("VEREL") 3-24  denier.
(Can be  converted to spun yarns of
many sizes).
UNION CARBIDE ("DYNEL") 2-24 denier.
(Can be  converted to spun yarns of
many sizes).
 from J. P. Stevens & Co., Inc., Ref. 2

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                               TABLE  4.4
                    CHEMICAL COMPOSITION  OF  POLYAMIDES
          Nylon-4             Pyrrolidone
                5             Valerolacturn
                6             Caprolactum
                6T            Hexamethylene  terephthalamide  (aromatic)
                7             Heptanoamide
                8             Caprylamide
                9             Amino-nonanlc  acid
               11             Amino-undecanoic acid
               12             Lauryl  lactum  from butadiene
               66             Hexamethylene  adipamide
               68             Hexamethylene-diamine and suberic acid
              610             Hexamethylene  diamine and sebacic acid
             MXD-6            Metaxylylene adipamide (aromatic)
                          j^
            4.2.2.4  Nomex  Nylon.- Notnex Nylon is a proprietary aromatic -
polyamide linked structure developed  by DuPont for applications requiring
good dimensional stability and heat resistance.  "Nomex can be used at
temperatures at which other fibers melt.   Unlike glass, it is resistant
to fluorides and has good abrasion and flex  resistance.  It has a wide
range of filtration applications, including  the cement industry, carbon
black, non-ferrous metals and steel."^  Characteristics, properties, and
producer forms are indicated in Table 4.6.
                                                               it
            4.2.2.5  Olefin.- An olefin fiber is designated  as	 a
manufactured fiber in which the -fiber-forming substance is any long chain.
polymer composed of at least 85% by weight of ethylene, propylene, or
                      9
other olefin units..."  Polypropylene is  one olefin used for industrial
gas cleaning applications.
            "The production of polypropylene fiber varies among
            the manufacturers.... to achieve certain properties,
            such as dyeability,  light  stability,  heat sensitivity,
            and shape of the filament  cross section.   The pro-

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                                 TABLE 4.5

   CHARACTERISTICS, PROPERTIES, AND FORMS OF NYLON FIBER FOR FILTRATION*
 HEAT RESISTANCE:

 Dry Heat: Up to 250°F. nylon  has
 reasonably good dry heat resistance.
 It is superior to acetate, modacrylic
 and saran.  However, the other
 synthetics are superior to it.
 Moist Heat;  Nylon performs adequately
 in moist heat at temperatures ranging
 up to 225°F.  However, its high initial
 tensile strength and abrasion resistance
 will make it a preferred choice in a
 number applications.

 CHEMICAL RESISTANCE:

 Acids;  Most mineral acids cause
 degradation and partial decomposition.
 Soluble in formic acid.
 Alkalies;   Good resistance to alkalies
 under most conditions.   In this respect
 nylon is better than acrylics but
 not as good as the olefins.
 Oxidizing Agents;   High concentrations
 and temperatures  may cause complete
 degradation.
 Organic  Solvents:  Withstands common
 organic  solvents  very well.   Some
 phenolic compounds cause  solubility.

 PHYSICAL PROPERTIES:

Tenacity and Elongation:  Both nylon
 6 and nylon  6.6 are  produced  in  a wide
range  of strengths.
Filament;  Dry and wet  tenacity  ranges
from 4.0 to 9.2 g.p.d. with dry  and
wet elongation running  from 16  to 42%.
Staple;  Dry and wet  tenacity ranges
from 3.5 to 7.2 g.p.d. with dry  and
wet elongation running  from 16  to 50%.
 Specific  Gravity;   1.14.
 Abrasion  Resistance;  One  of nylon's
 outstanding  characteristics is
 abrasion  resistance and in this
 respect nylon  is superior to all
 other  fibers.

 FORMS  AVAILABLE:
 NYLON  6.6:
 Mpnpfilament;  7 to  30 denier
 Multlfilament; deniers of 20,30,40,
 50,60,20,80,90,100,140,200,260,400,
 420,520,630,780,800,840,1050,1260,
 1680,
 (NOTE: The underlined deniers are
 those  that are used most  often).
 Staple;   1.5 to 18  denier/filament.
 (Can be converted to spun yarns
 of many sizes).
 Tow;   3 to 18 denier/filament
Above  forms available in  bright,
 semi-dull, dull and black.
 NYLON  6:
Monofilament; 15 to 20 denier
Multifilament;  deniers of: 30/40/
 50/70/100/140/200/840/1050/1260/
 2100/2400/2500/3150/3360/4200/
 5000/7500/10,000/15,000
 Staple;   2.0 to 15  denier/filament.
 (Can be converted to spun yarns of
many sizes).
 from J. P. Stevens & Co., Inc. Ref. 2

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                                TABLE 4.6
CHARACTERISTICS, PROPERTIES, AND FORMS OF NOMEX
                              FILTRATION*
       NYLON FIBER FOR  INDUSTRIAL
 HEAT RESISTANCE:

 Dry Heat;  "Nomex" nylon does not melt,
 but at temperatures above 700°F degrad-
 ation sets in rapidly.  In dry heat,
 up to and including 450°F., this fiber
 may be used satisfactorily, as long as
 there is no acid dew point problem.
 Moist Heat;  Small amounts of water
 vapor at elevated temperatures, and
 in intimate contact with water or
 saturated steam, "Nomex" nylon exhibits
 a progressive loss in strength.  How-
 ever, it withstands these conditions
much better than Nylon 6.6 or many
 other fibers.

 CHEMICAL RESISTANCE:

Acids;  Withstands both mineral and
 organic acids much better than nylon
 6.6 or nylon 6 but not as well as
 polyesters and acrylics.
Alkalies;  Excellent resistance to
 alkalies at room temperature (better
 than polyesters and acrylics)
 but degraded by strong alkalies at
 elevated temperatures.
 Oxidizing Agents;  Like nylon 6.6
or nylon 6, "Nomex" is degraded by
oxidizing agents.
Organic Solvents;  Highly resistant
to most hydrocarbons and many
other organic solvents.

PHYSICAL PROPERTIES;

Tenacity and Elongation;  Dry
tenacity: 5.5 g.p.d.  Wet tenacity:
4.1 g.p.d. Dry elongation: 18%
Wet elongation: 15%.
Specific Gravity; 1.38
Abrasion Resistance;  Superior to
acrylic fibers and about equal
to polyesters and nylon 6.6.

FORMS AVAILABLE:

Multifilament;  Deniers of 100,200
and 1200.
Staple; 2 denier/filament (can
be converted to spun yarns of
many sizes).
Both filament yarn and staple
are available in natural (off-
while), international orange
and olive green.
* J. P. Stevens & Co., Inc., Ref. 2

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    '         duction of  polypropylene  fiber results  in a
             relatively  economical  and inexpensive  fiber be-
             cause  the basic  substance from which  it is  made
             is  propylene  gas--a  by-product of  petroleum dis-
             tillation.  The  most costly  aspect is  the manu-
             facturer's  initial  investment  in research and
             plant  establishment."8

             "Polypropylene combines the  virtues of  lightness,
             high strength and excellent  resistance  to most
             acids  and alkalies with the  important  added at-
             traction of low  cost.  Having  the  lowest  density
             of  any synthetic fiber in filtration and  dust
             collection, polypropylene offers the greatest
             cloth,yield per  pound  of  yarn  and  is one  of the
             most economical  synthetics.

             Polypropylene fibers are  manufactured under such
             trade  names as "herculon" (Hercules), "Reevon"
             (Alamo Polymer)  and  "Vectra" (National  Plastic
             Products).

             The sleekness of the fiber allows  for a fabric
             providing good cake  discharge  and  resistance to
             blinding.  Since its moisture  absorption  is  vir-
             tually nil, polypropylene is a good choice  for an
             application such as  dye production, where pigment
             changes  can be made  with  only  a  light wash  be-
             tween  batches.

             "Polypropylene has been used successfully at
             165°F  in the  greige, and  in  the heat set  stage
             at up  to 250°F."2

Characteristics, properties, and producer  forms are given in Table 4.7.

             4.2.2.6  Polyester.- A polyester fiber  is designated as
                                       t
             "...a  manufactured fiber  in which  the fiber-forming
             substance is  any long  chain...polymer composed of
             at least 85?0  by weight of an ester of dihydric
             alcohol  and terephthalic  acid  (p - HOOC - C,H,  -
             COOH)	"9                                  *

The general  characteristics of this material, which is a commonly used

dust collection fabric particularly in the cement industry, are presented
                          D
in Table 4.8.  The Dacron  polyester  fibers are round (see Figure 4.1),

but variation in shape of the spinnaret holes will affect the appearance

-------
                               TABLE 4.7

      CHARACTERISTICS, PROPERTIES, AND FORMS OF POLYPROPYLENE FIBER
                       FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE:

Dry Heat;  Polypropylene has the
lowest heat resistance of all the
synthetics except the modacrylics,
and loses tenacity in direct propor-
tion to increases in temperature.
It should be remembered, however,
that the fiber's very high initial
tenacity will leave a generous margin
for many applications.
Moist Heat;  Since polypropylene is
non-hygroscopic, its heat degradation
characteristics are essentially the
same under moist conditions as they
are under dry conditions.

CHEMICAL RESISTANCE:

Acids;  Very good resistance to both
mineral and organic acids.  Attacked
at high temperatures by nitric acid
and chlorasulfonic acid.
Alkalies;  Generally good, except
poor resistance to sodium and
potassium hydroxide at high tem-
peratures (above 200op) at high
concentrations.
Reducing Agents;  Good resistance to
most reducing agents.
Organic Solvents;  Good resistance to
most organic solvents.  Exceptions
are ketones, esters, aromatic and
aliphatic hydrocarbons at high
temperatures.   Soluble at  160UF
in chlorinated  hydrocarbons.

PHYSICAL PROPERTIES:

Tenacity and Elongation;
Filament;  Dry  and wet tenacity
4.8 to 8.5 g.p.d. with dry and
wet elongation  15 to 30%
Staple;  Dry and wet tenacity
ranges from 4.5 to 6.0 g.p.d.
with dry and wet elongation 15
35%.
Specific Gravity;  0.90 to 0.91
are of the lowest specific gravities
of any fiber.
Abras ion:  Re s istance;  Excellent
abrasion resistance wet and dry.

FORMS AVAILABLE:

Monofilament;   6 to 12 mils  round
and flat.
Multifilament;  165 to 4000 denier.
Staple;  1.5 to 15 denier. (Can
be converted to spun yarns of
many sizes)
Available in natural and solution
dyed colors.  ,
 from J. P.  Stevens & Co.  Inc., Ref. 2

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      TABLE 4.8.   CHARACTERISTICS,  PROPERTIES AND FORMS OF POLYESTER
                           FIBERS FOR INDUSTRIAL FILTRATION*
 HEAT RESISTENCE:

 Dry  Heat:  Polyesters  are  not  compar-
 able to "Teflon"  and  "Nomex"  nylon,
 which have usually high heat  degrad-
 ation resistance,  but they  are
 superior  to most  other synthetics
 in this respect,  though subject  to
 hydrolytic degradation under
 certain circumstances.  However,
 polyesters are used for filtra-
 tion and  dust collection  under
 conditions of wet  heat, particu-
 larly where the initial yarn
 strength  exceeds minimum  re-
 requirements sufficiently to
 allow for  hydrolysis.   Below
 350  F there is little  shrinkage
 of polyester fiber.

 CHEMICAL RESISTANCE:

 Acids;  Good resistance to most
 mineral and organic acids except
 high concentrations of nitric,
 sulfuric and carbolic  acids.
 Alkalies;  Good resistance to
 weak alkalies and moderate resis-
 tance  to strong alkalies  at low
 temperatures.  Strong  alkalies
 at high concentrations and temper-
 atures dissolve polyesters.
 Oxidizing  Agents; Good resistance
 to most oxidizing agents.
 Organic Solvents:  Excellent  to
most organic solvents, but
 unsuited for some phenolic
 compounds  and affected by
 cyclohexanone at 313°F.
PHYSICAL PROPERTIES:

Tenacity and Elongation:
Filament;  Dry and wet tenacity
ranges from 4.4 to 7.8 g.p.d.
Dry elongation from 10 to 2570.
Water has little effect on either
the strength or elongation of
polyester fibers.
Staple; Dry and wet tenacity ranges
from 2.2 to 5,5 g.p.d.  Dry and
wet elongation from 18-50%.
Specific Gravity; 1.35 - 1.38.
Abrasion Resistance;  Polyester
fibers have excellent abrasion
resistance ranking next to nylon
in this respect.

FORMS AVAILABLE:

Multifilatnent;  Deniers of 30, 70,
100, 140, 150, 220, 250, 420,
440, 840, 880, 1000, 1100, 1680,
and multiples of 1100 up to 16,500.
Staple;  1.5, 2.25, 3.0, 4.5, 6.0
8.0, 15.  (Can be converted to
spun yarns of many sizes).
Direct Spun Yarn;  From 4s/I to
30s/l.
*From J. P.  Stevens & Co.,  Inc., Ref. 2.

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and mechanical properties of the fiber, as will the extent to which the

fiber is drawn.  The usual diameters are 10 to 30 urn.  The following

quotation provides some additional information on this material.

            "Polyester fibers are manufactured under such trade
            names as "Dacron" (DuPont), "Fortrel" (Fiber Indus-
            tries /Celanese) , "Vycron"  (Beaunit), "Kodel" (East-
            man Chemical Products) and Enka Polyester (American
            Enka Corporation).  They can be woven into filter
            and dust collection fabrics affording very good
            resistance to chemicals, abrasion and dry heat
            degradation, plus excellent dimensional stability."^
                           P
            4.2.2.7  Teflon  Fluorocarbon.-  The characteristics, prop-

erties and forms of these fibers are summarized in Table 4.9 and described

in the following quotations.

            "Teflon - Dupont's trade name for their fluorocarbon
            fibers - is available in two forms.  The multi-fila-
            ment yarns are made from the homo-polymer polytetra-
            fluoroethylene (TFE).  The monofilament yarns are
            produced from the co-polymer of tetrafluoroethylene
            and hexafluoropropylene (FEP).   Both polymers are
            composed of long-chain molecules in which all of the
            available bonds are completely  saturated by
            fluorine.  These carbon-to-fluorine bonds are ex-
            tremely strong, resulting in fibers which are excep-
            tionally stable to both heat and chemicals.   Teflon
            fiber has no known solvents except certain perfluor-
            inated organic liquids at temperatures above 570°F.
            The fiber is inert to concentrated mineral acid,
            organic acid,  alkalies,  oxidizing agents and
            organic solvents at elevated temperatures.

            "The fiber remains flexible and non-brittle  from
            minus 100°F to plus 550°F.   Teflon withstands
            prolonged exposure at 450°-500°F without degra-
            dation.   Above this temperature some  decomposi-
            tion results but even at  500°-550°F decomposi-
            tion is slow.   The gaseous  decomposition products
            evolved at high temperatures are highly toxic
            and must be removed from the work areas  through
            adequate ventilation.

            "Teflon fibers have a very  low  coefficient of
            friction resulting in excellent cake  discharge.
            This fact,  coupled with  its chemical  inertness
            and resistance to dry and moist heat  degrada-

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      TABLE 4.9.   CHARACTERISTICS,  PROPERTIES AND FORMS  OF TEFLONR
                    FIBER FOR INDUSTRIAL FILTRATION*
 HEAT RESISTANCE:
            "Teflon" TFE can be used
                               o.
Dry Heat;
in continuous service up to 500"F.
"Teflon" FEP can be used in continuous
service up to 450 F.
Moist Heat;  Due to zero moisture
absorption, these fibers withstand
moist heat temperatures about as
well as dry temperature.

CHEMICAL RESISTANCE:
Acids; Inert
Alkalies: Inert
Oxidizing Agents; Inert
Organic Solvents;  Inert
The only substances known to react
with these fibers are alkali metals
fluorine gas at high pressure and
temperature, and chlorine trifluoride.
This is the most chemically resistant
fiber produced.

PHYSICAL PROPERTIES:

Tenacity and Elongation:
Filament and Staple; Dry and wet
tenacity for type TFE is 1.6 g.p.d
and for type EEP is 0.5 g.p.d.
Dry and wet elongation is 15%.
 Specific Gravity;  2.1
 Abrasion Resistance;  Teflon
 fibers are  inferior  in abrasion
 resistance  to the  acrylic, nylons
 and polyester fibers, but are
 superior to glass  fibers.

 FORMS AVAILABLE:
 "TEFLON" TFE
 Filament Natural Brown
 100/200/400/1200 denier.
 2400 up to  26,400  denier plied
 yarns
 Filament Bleached White
 225/450/1350 denier.
 2700 up to  29,700  denier plied yarns
 Staple Natural Brown
 6.67 denier/filament
 Staple Bleached White
 7.50 denier/filament
 Staple available in cut lengths
 of 0.5" up  to 5.0" in 0.5" incre-
ments (Can be converted to spun
yarns of many sizes).
 "TEFLON" FEP
Monofilaments (clear, colorless)
 3/5/8/11/16/20/32/50/60 mils.
 from J. P. Stevens & Co., Inc., Ref. 2

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             tion, makes  Teflon suitable  for  filtration and
             dust  collection  under  severe conditions.   Its
             major weakness  is  abrasion resistance  in  which
             Teflon  is  inferior to  all  other  synthetic
             fibers  except glass.   The  high price of Teflon
             fiber limits its use  in  the  filtration field.
             However, for uses  under  extreme  temperature
             conditions,  Teflon may prove to  be most
             economical in the  long run.   It  would  further
             be  expected  that as the  demand for Teflon in-
             creases and  production expands,  the price of
             Teflon  will  decline."2
                                                        9
             4.2.2.8 Vinyon.-  Vinyon fiber is designated   as  ..."a  manu-

factured  fiber  in which  the  fiber-forming substance is any long  chain...

polymer composed of at least 8570 by  weight of vinyl chloride  units  (-CH-  -

CHC.0-)1.1   The characteristics,  properties and producer forms of the  fiber

are presented in Table 4.10.   The  following  quotations summarize these

characteristics.

             "Characteristic  of vinyon  fibers is the dumbbell
             shape of their cross sections....Fibers with an
             occasional twist are observed.   Both continuous
             filaments  and stable fibers  (1-5 in.)  occur in
             trade,  the width of all  types being 16 to 18 u."^

             "Vinyons are made  of 100%  polyvinyl chloride by
             the French Societe Rhovyl, and as a copolymer
             of  polyvinyl chloride  and  polyvinyl acetate by
             the FMC Corp., American  Viscose  Division.
            Vinyons offer very good  resistance to  most
             chemicals, even  in highly  concentrated form.
            They can be  made into  fabrics providing extremely
            smooth  cake  discharge, and are used for filtra-
            tion of air  and  (numerous  liquids)."2
                                                       :
            4.2.2.9  Glass.- Glass fiber  is  a product  of fusion, a non-<
crystalline silicate analogous  to  other  fiber polymeric materials,  (-SiO,-)
Selected silica sands, limestone,   soda ash,   and borax  or other ingredients

are melted at about 2500 F and  the mixture is extruded through spinnarets.

The resulting filaments may be  drawn while still molten and later twisted

and plied into filament  yarn.   Or, the extruded glass may  be drawn and

broken by jets of compressed air into staple of lengths 8  to 15  inches.

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          TABLE 4.10.   CHARACTERISTICS,  PROPERTIES, AND FORMS  OF
                VINYON FIBER FOR INDUSTRIAL FILTRATION*
 HEAT  RESISTANCE:

 Dry Heat;  The maximum working
 temperature  for Rhovyl's  type
 "Clevyl T" is 350°F., provided
 the fabric is clamped to  prevent
 shrinkage.
 Moist Heat;  Due  to vinyon's low
 water absorption  the effects of
 moist heat are similar to those of
 dry heat.

 CHEMICAL RESISTANCE:

 Ac ids;  Excellent resistance at
 room temperature  to mineral acids,
 including hydrochloric, nitric,
 and sulphuric acids, aqua regia
 and organic  acids with the exception
 of carbolic  acid.
 Alkalies; Very good resistance to
 alkalies such as  potassium hydroxide,
 sodium hydoxide and ammonium
 hydroxide.
 Organic Solvents;  Dissolved by
 ketones, and partially dissolved
 or softened by esters and  ethers.
 Certain other organic solvents
 cause swelling at certain  temper-
 atures.  In general, this  fiber should
 not be considered for use with
most organic solvents other than
mineral oil, aliphatic hydro-
 carbons, alcohols and glycols.

 PHYSICAL PROPERTIES:

Tenacity;  Filament and Staple:
 "Clevyl T" (Dimensionally  stable
 to 212°F) Staple 1.7 to 2.0 g/d
 "Type 55" (557, shrinkage at 212°F)
Filament 2.7 to 3.0 g/d.
Abras ion Res is tance;  Not as resis-
tnat as polyesters or nylons, but
comparable to the acrylics.
Specific Gravity;  1.34-1.38

FORMS AVAILABLE:

RHOVYL STAPLE:
"Clevyl T" (Dimensionally Stable
to 212°F) 3.5/5/8/15 d/fil.
"Type 30" (30% shrinkage at 212°F)
3d/fil.
"Type 55" (55% shrinkage at 212°F)
 1.8/3.0/6.4 d/fil.
RHOVYL FILAMENT:
"Type 55" only: 75/100/200/400
800/1600 denier.
FMC VINYON H.H. STAPLE ONLY
Deniers of 1.5 to 5.5 staple
lengths of 1/2 inch to 2 inches
but not in all deniers. (Can
be converted to spun yarns of
many sizes)
 From J. P. Stevens & Co., Inc.,Ref. 2

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The  fibers  are  then  treated with a lubricant  which  is  of great  importance
 in the  durability  of the  eventual fabric.   Following drying,  the  fibers
are  processed much like the more conventional fibers.
             Glass  fiber photomicrographs are  shown  in  Figure  4.1.   The
fibers  are  perfectly round  with  very  smooth and  structureless surfaces.
Diameters range from 5 to 16 urn  for most textile fibers.  Characteristics
and  forms of glass fibers are given in Table  4.11.
             4.2.2.10 Other Man-Made  Fibers.- Other generic organic  fiber
materials that  could be used for industrial gas  filtration include  acetate
and  triacetate  (cellulose acetate), rayon  (regenerated cellulose),  manu-
factured rubber, saran (vinylidene chloride),  and vinal  (vinyl  alcohol).
They appear  to  offer no significant advantages for  industrial gas filtra-
tion in most instances, although saran has  some  application in  spray
tower mist elimination.   (Other  unclassified  fibers are  summarized  in
Ref.  3, pp.  218-221).
             Several  fiber materials and finishes are currently  available
in pilot plant  quantities having potential  for high temperature applica-
tions or control of  electrostatic effects  in  fabrics.  Some metals and
                                                                          3
ceramics having  potential filtration uses are summarized  in Appendix 4.2.
         R              R
Fiberfrax ,  and Brunsmet  ,  have  been laboratory  tested and found satis-
factory for  filtration at high temperatures ( >  1000 F), but require further
development  in  filtration systems.
     4.2.3  Physical  and  Chemical Properties
            Relative  physical and chemical properties of filter fiber
materials are summarized  in Tables 4.12,  4.13 and 4.14;.  Table 4.13
shows general quantitative  physical properties and a qualitative estimate
                                           2
of chemical, heat,  and abrasion  resistance.   Where both high and low
tenacity fibers are  produced, the range of strength and breaking elonga-
tion is given.   Since many  fibers are produced in both filament and staple
fibers, the general  physical properties are shown separately for each
form.  Fiber resistance to specific chemical compounds  is indicated in

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                               TABLE 4.11

            CHARACTERISTICS, PROPERTIES, AND FORMS OF GLASS FIBERS
                         FOR INDUSTRIAL FILTRATION*
HEAT RESISTANCE
CHEMICAL RESISTANCE
PHYSICAL PROPERTIES
FORMS AVAILABLE
Fabrics made from glass yarns actually gain in
strength as the temperature rises from room
temperature to 400°F.  From that point, strength
and flexibility decrease.

The recommended operating temperature is 500 F
with surge limits up to 600°F

Glass is resistant to acids of ndrmal strength
and under ordinary conditions.  It is attacked
by hydrofluoric, concentrated sulphuric, and hot
phosphoric acids.  Overall resistance to acids
is slightly above average.

Hot solutions of weak alkalies will also attack
glass.  Overall resistance to alkalies is poor.

Operating a glass baghouse at or below the dew
point can be particularly damaging if acid
anhydrides or metallic oxides are entrained in
the gas stream.  Fluorides and the oxides of sul-
fur are particularly damaging to glass.

Glass is considered to be incombustible because
it is completely inorganic.  In addition, it
has a low coefficient of linear expansion and
hence is dimensionally stable.  Although glass
has an extremely high tensile strength it has
poor flex-abrasion resistance.

Various chemical treatments to the glass fabric
improve the flex-abrasion characteristics of
glass bags.

Filament
Filament and Textured
Filament and Spun
      From Albany Felt Co., Ref. 11.

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          TABLE 4.12.   RELATIVE PROPERTIES OF MAN-MADE FIBERS*
 Resistance  to
 Acids"
 "Teflon"
 Polypropylene
 Vinyon
 Modacrylics
 Acrylics
 Polyesters
 "Nomex" nylon
 Nylon 6.6 &  6
 Rayon

 Resistance to
 Alkalies
 "Teflon"
 Polypropylene
 Vinyon
 Modacrylics
 Nylon 6.6 &  6
 "Nomex" nylon
 Polyesters
 Rayon
 Acrylics

 Resistance
 to Oxidizing and
 Reducing Agents
 "Teflon"
 Polypropylene
 "Nomex" nylon
 Modacrylics
 Polyesters
 Acrylics
 Nylon 6.6 &  6
 Vinyon
 Rayon
tensile
Strength
Nylon  6.6 &  6
Polyesters
Polypropylene
"Nomex" nylon
Rayon
Acrylics & Modacrylics
Vinyon
"Teflon"
Resistance to
Abrasion__(Wet_& DryJ
Nylon  6.6 &  6
Polypropylene/Polyesters
"Nomex" nylon
Acrylics & Modacrylics
"Teflon"
Rayon
Vinyon
           J-9.
Dry Heat
"Teflon"
"Nomex" nylon
Polyesters
Acrylics /Rayon
Nylon 6,6
Nylon 6
Polypropylene
Modacrylics
Vinyon

Resistance^ to
Moist Heat
"Teflon""
"Nomex" nylon
Acrylics
Nylon  6.6 &  6
Rayon
Polyesters
Polypropylene
Modacrylics
Vinyon
Max. Recommended
For Continuous
"Teflon"
"Nomex" nylon
Polyesters
Acrylics
(homopolymers)
Acrylics
Rayon
Nylon 6; 6
Nylon 6
Polypropylene
Modacrylics
Price Relationshi
Service
      ,o
   5001-
   450°*
   300°*
   284°

   275°
   275°
   250°*
   250°*
   225°
   180°
(Highest to Lowest)
STAPLE           FILAMENT
"Teflon"
"Nomex" nylon
Glass
Nylon 6.6 & 6
Polyesters
Acrylics
Modacrylics
Vinyon (Rhovyl)
Polypropylene
Viscose Rayon
  "Teflon"
  "Nomex" nylon
  Acrylics
  Modacrylics
  Polyesters
  Nylon 6.6 & 6
  Glass
  Vinyon (Rhovyl)
  Polypropylene
  Viscose Rayon
*  These fibers are subject to hydrolysis when exposed in hot, wet atmospheres
   in varying degrees.  Polyesters degrade to the greatest extent.  "Nomex"
   nylon next, then nylon 6, and nylon 6.6 least.
   The following is quoted from DuPont's NP-33, "Properties of 'Nomex1.11
   "At elevated temperatures, "Nomex" fiber in intimate contact with water
   or saturated steam exhibits a progressive loss in strength with water
   vapor.  (Nylon 6,6 completely deteriorates in less than 100 hours under
   the same conditions)"

** Based on prices per pound at time of publication (1965 est) - See Chapter 7.
   From J. P., Stevens and Co., Inc., Ref. 2.

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                                                                                                I  o
                                                     TABLE 4.13

                       SUMMARY OF PHYSICAL AND CHEMICAL PROPERTIES OF  INDUSTRIAL FILTER FIBERS
-O
I
///***// //
* / / / / / / / / /
FILAMENT i
Breaking Tenacity gpd (Dry, \\ '« ' " " « ^ "^i
B,eak,ng Tenacity gpd, We,, °] 3| 38 40 J° 79 * ' ^ 7 J
Breakmg E.onoa.ion « s , Dry, » « » 1< |3 « 18 ; J° ;
Breaking Elongalic. • , , We,, » « 23 '« Jf » '5 £ \
STAPLE
Breaking Tenacity gpd, Dry, {* *° ». 3° » » 38 ":»j
Breaking Tenacity gpd, We,, ?8 J« J« » J* *J ^ 4'° ! *| !
Breaking E,onga,,o,.^Dry, » » « » ; » | » « " ' «
Breaking Elongation -., We,, S S S » M » « '"'.£'

Specific Gravity 1 33 1 18 1.18 1 12 1 15 i 1.14 . 1.30 1 14 ; 1 38 • 1.38 '
Mai Recommended Operating f ; 175 275 275 275 284 .. 275 : 180 250 ! 450 300 i
i i i '• ' • i
Resistance to Abrasion G : G G G G G G E E ( E ;
Resistance to Dry Heat FjG.G G'GjGJFG :E!G!
:••?•• ' ' " : -H -..».-.. 	 , 	 , 	 ,-
Resistance to Moist Heal F ; G G G E-GiF G E'Fl
Resistance to Mineral Acids • p '• G G G'G^GG'P F G
Resistance to Organic Acids • P ' a G G G : G G F E G!
Resistance to Alkalies P|F F F.FiFiGG GiGi
Resistance to Omdizing Agents FG;GGG:GG F |Q Q|
Resistance to Solvents !F-E EE'EiE'GE E;Ei
f * J?
///////
S / / f f f 3
: i
44 4.6 ; 1 0 4.8 ! 1.5 ' '
78 ! 54 70 . 8.5 , 57 23
44 46 1.0 4.8 : 07 : •
7.8 • 5.4 70 8.5 36 23
10 19 . 10 15 9 15
25 25 80-30-30 30
19 19 20 15 l» "5
25 - 25 80 30 40 25

22 ' 25 53 45 15 i 0
40;55'S4 6.0 46 15
2.2-25 53 4507 10
40 5 5 5.4 6.0 3.6 , 1 5
18 24 . 32 15 9 15
50 : 45 40 35 , 30 25
18 : 24 32 15 14 15
50 45 40 ' 35 40 25

1.38 i 138 : 1-38 ' .92 . 90 ; 1.52 170
• • r- | :
300 I 350 j 300 ; 150 225 | 275 ; 1.60

	 • 	 1 ' ' !
E'EJE,G;E.,GG
GiGJG'FlG.GF
F 1 F 1 F F F • G i F
GjGJG-G'E'.PiG
	 1 	 j 	 r- -t ! !
GJG:G,G EiGjG
G G G:GjE|FjG
	 1 	 ! 	 " 1 :
G G | G . P ; G ; F F
E:EiE;GJ'GJGjG
/ f f f
/ / oj /
; i i
• 6 C? 60
30 73 j
• 6 07 39 j
30 47 1 i
•5'23 ;
•c.
•i '2 25 j
•C J
t
16 07' i 1
30 '
• 6 37 1
30 i ;
15 40 I
60 ^
15 40
60 i 1

210 1 34 254 . 1
500 350 ; 550 , 200
I | j—

F , G ' P | F .
'ill
E : P i E | F L
E P ' E : F ,
E E \ E j P
E . a: E| e|
E G P ! G |
« F E ! F j
E| F E j E i
// /
i i
i
: 3B i
i : ;
i -
i i
i 1T
,
i- -i- '
30 '• ' 10 !
19 { M7 i
J3 ! OB j
i.| ,6,
3 ! ! 25 1
7 i j 35 ;
3 i i 25 ;
7 J 50 i

50 ; 1 25 i 1 32 !
J25 ; 175 j 200 !
	 ,, 	 » 	 *
! i
. i J
G ; G j G
G F i F !
!
G F 1 F j
P ! F ' F
-4— j 	
GIF F
G ! P P
F i P P
j 	
EG F

-------
Table 4.14.   (Additional detailed  engineering  test  data on  fiber  proper-
                                   2  12
ties is presented in Appendix 4.3.  ').

            "It is possible  that a fiber  indicated  as being
            fair within a general  chemical class  (in Table
            4.13) may be rated as  good for a particular chem-
            ical within the  general  grouping.  Each fiber has
            its own strong and weak  points and must be evalu-
            ated on its own  merits."2

     The evaluation of the resistance of fibers to chemical reagents pre-

sented in Table 4.14 is based on liquid filtration requirements.  However,

the data presented in the table have direct applicability to dust filtra-

tion as the time,  temperature, chemical concentration,  type of polymer,

treatment of fabric, etc., all have their effect on final selection of a

filter or dust collection medium.  For these reasons,  tests should be made

under operating conditions,  as a composite table cannot cover all possible

conditions to which a filter or dust collection fabric  may be subjected.

     The maximum temperature, degrees Fahrenheit ( F), as enumerated in

Table 4.14 is the maximum temperature that should be employed normally

for dust collection or liquid filtration of dilute solutions.  A fiber

marked R (recommended) can be expected to usually withstand most conditions

reasonably well.  S (satisfactory)  fibers should be considered only for

low concentrations and moderate temperatures.   N (not  recommended) fibers

can be expected to give poor results under any conditions.

            "Chemical composition of synthetic fibers  greatly
            affect their chemical resistance.   This is  true
            within a generic family as well as from one type
            of fiber to another.   Chemical resistance  of a
            co-polymer fiber is usually lower  than that of a
            pure homo-polymer base  fiber.   The co-mono-mers
            used to form "links" are particularly sensitive
            to external effects....

            Using  acrylic  fibers as an example, in acidic  or
            oxidizing solutions,  in dry or humid  air be-
            tween  275°-350°F, the difference  in behavior  be-
            tween  homo- and co-polymers  becomes very important.
            The homo-polymer is superior.   The influence  of
            composition of the basic  acrylic  polymer has
            little effort  when considering organic  snlvpnts,
            weak acids, most mineral salt solutions and
            cleaning or bleaching agents.   In alkaline  media
            up to intermediate concentrations  and tempera-



-------
                                       TABLE 4.14

                           RESISTANCE OF FIBERS TO  CHEMICAL REAGENTS

BtaB. Twv. QapMi FaWriwn * n 3
laanjuu. *ooa
Aqua R«0i« N

Pho»pho«C JS j
Sulfunc IN i
Tmilf|rrT. agp, |
Acetic S
6«ruO»C N
Carbolic N
Formic N
Lactic S
Diane R
Salicylic N
Aoimoniitfn HydrovidB S
Catciwn HydronMM N
Potautum Hytfroiida N
Pobmnm Carbonate N
Sodnm Hydrouda N
MLTC
CateiuaiCMDrida 8
FavricCMorUa N
SodHMAcaMa N
SodMi BanzoaH R
SodwmBroaMa 8
SediMmCMonda R
SodwmCyankla S
SottwnlMraia S
SoawMSrffaii S
£ncCMorid» S
ajKMUjna 8

CMortoa S
FluoMW N
IT • i <
T8;JT5 7TJ
i *
R -R-nr
s s s
s s is




s s s










m?
sl


si




s










ni
j


5
11



S










•oaooiaaoitfo-xoaoD jiupw:r%it«o.uDii?ojsu!2aoi2is IISDOD
4-4—1- :.,..,::;']•; j

,isU|s:R . s s ..R:R!R;RI..«:S!«;

Ring's B'R'R'R-^ ~K~ R • 5 ; R | a ! R 1 R ' s '

,!•'. s ..M-.:,-. sVR'sls^J..:

n is i « ;P S:SiR,R s R R;R.N:R R N N
	 4 	 f 	 . 	 . 	 j 	 . 	 . 	 . 	 \ 	 , 	 . 	 »— , 	 ; 	 •
1 i i j I i i i ; 1 1 :


-M 	 !— I— I 	 tlii.il
.|.|.!.L.!.vJ.4i^r^

S!N!R S'NJR S!R:S SIN'N

! i i i i 1 j 1 i ; i 1 i I i
                                                                 * /-*/   '//
                                                           */// * // *•/ [I ff// /////./
                                                           //if IIffft / e r i///_//1 if


-------
             Cures,  both  types  of  acrylic  polymers  behave  in
             a  simiLnr  manner.   As alkaline  conditions  become
             more  severe  the  homo-polymer  again shows  super-
             iority.

             Under weak or  intermediate  conditions  of  chemi-
             cal concentration  and/or  temperature,  generic
             family  only  will need to  be considered.  As
             severity of  conditions  increase,  then  special
             fibers  within  a  generic type  become  important
             considerations.  "Tailor-made"  fibers  are  being
             manufactured within a generic family.   "Nomex"
             nylon will withstand  450°F  whereas nylon  6 or
             6,6,  the common  types of  nylon, are  useful up
             to 250°F."
             "In the acrylics,  "Crylor"  and  "Dralon T"  can
             be used at higher  temperatures  than  most other
             acrylics because of their higher  acrylonitrile
             content.   All  manufacturers of  synthetic  fibers
             are striving to  produce better  fibers  for  fil-
             tration and  dust collection."

4.3  YARN PRODUCTION

     The standard definition of a yarn  is as  follows:

             "Yarn: a generic term for a continuous  strand
             of textile fibers,  filaments or material in
             a  form  suitable  for knitting, weaving,  or
             otherwise  intertwining to form  a  textile
             fabric.  Yarn  occurs  in the following  forms:

             a.  A number of  fibers twisted  together
             b.  A number of  filaments laid  together without
                twist
             c.  A number of  filaments laid  together with
                more or  less twist
             d.  A single filament,...monofilament
             e.  A narrow strip  of material  such  as paper
                cellophane, or  metal  foil...

             Varieties  include single  yarn,  plied yarn,
             cabled yarn, cord,   thread,  fancy yarn."

     Yarns formed of short staple fibers are called spun yarns.  Yarns

may also be  formed of a  twisted bundle  of individual fine monofilaments
(typically several hundred, each monofilament of order 10 ^m diameter), in

which case the yarn is called mult ifilament  or filament.   Single or mono-

filament yarns (e.g. as  in monofilament fishing  line,  5 to 10 Ib. test)
are not as widely used in  fabrics  for industrial gas filtration as multi-


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      Multifilament  yarns  are  made  directly from individual  monofilaments,
 frequently  as  they  are  extruded,  in which  case  the  desired  number of spin-
 narets  is used.   Following varying amounts of thermal  and mechanical
 molecular crystallite orientation,  a finish lubricant  may be  added and  a
 slight  twist is  given the yarn  to  maintain multifilament order.   This  low-
 twist basic yarn is then  coverted  to the specific yarn form required for
 the  desired fabric  design either by the fiber producer, by  an intermediate
 processor, or  by the fabric manufacturer.   Man-made  fiber staple  may be cut
 frommultifilament yarn.   Staple yarns  are  made  by the  fiber yarn  or fabric
 manufacturer by  parallelizing the  short fibers  and  twisting them  together
 for  the desired  strength.
      The method  or  "system" of yarn production  is determined  by the type
 of fabric desired for dust collection.  The major fabric types  are pre-
 sented  in Table  4.15 based on information  provided by  Albany  Felt Company.
      A variation of the woolen system  is the  worsted system,  in which the
 fibers are given additional parallelizing  treatment, or combed, before
 being spun.  As  a result  the  worsted yarn  is  smoother  and of  finer quality
 (Figure 4.2).  The  type of spinning machine as well as the  amount  of
 combining influences the  fiber orientation, and  thus has an influence on
 the  amount of  free  fiber  available  for dust filtration.
4.4  YARN PROPERTIES
     Yarns of  staple fibers can be  made with  various fiber  lengths and
various fiber  diameters.  The diameter of  the yarn itself can range from
relatively fine  to  comparatively heavy and  thick.  The amount of  twist
 is also variable.  All yarns  composed of staple  fibers must possess
sufficient twist to hold  the  fibers  in place.  The amount of  twist  in
staple yarns also depend upon staple length,  shorter staple requiring
higher twist for adequate weaving strength.
     Filament yarns are smooth and  even unless they have been deliberately
formed in an uneven manner for novelty effects.   They may be  thick  in
diameter and heavy, gossamer  sheer  and light, or of any intermediate
weight and diameter.

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                                    TABLE 4.IS

                                 MAJOR FABRIC SYSTEMS *
Woolen System Spun Fabrics

     Thli  terminology relates  to « manufacturing system and  not  Just wool flb«ra.
Synthetic  can be  woven on the  'woolen system1.

     Woolen  System Spun Fabrics  are made  with soft,  lofty, open, heterogeneous,
low  twlat  yarns,  which result  In a fabric with many  small pores  for high capacity
and  efficiency.   This type  fabric Is Ideal  for vary  flna dust up to and Including
coarse grades ....

     This  type construction was  designed  primarily for  the Shaker type baghousa.

Cotton System-Spun Fabrics

     This  terminology relates  to a manufacturing system and  not  Just cotton fib-
ers.   Synthetic  fibers can  be  woven on the  cotton system.

     In this type fabric, the  yarns are smaller,  tighter and of  high twlat.  Be-
cause  of this,  the yarn count  is Increased  to prevent duat leakage and the result-
ing  fabric is generally lighter  in weight and offers less capacity than a woolen
system spun  fabric.
     This  type fabric could be used for a Shaker type baghouse.

Filament Fabrics
     Filament filtration fabrics are characterized by slick  yarns without any pro-
truding fiber ends along the yarn and therefore  the  surface  of the fabric Is very
smooth .

     Since this type  fabric Is completely without a  surface  "cover" or "nap", the
yarns  are  packed  quite closely together,  even tighter than a cotton system fabric
of equal permeability.
     This  results  In  a fabric  which has excellent release characteristics, but
limited  capacity.
     This  type construction was  designed  for the  Reverse-Air type baghouse.

"Combo"  Fabrics (Spun- filament)
     This  type fabric Incorporates  some of  the advantages of both the woolen system
fabric and the Filament  fabric.   In this  unique  design,  the  filament yarna ere
manufactured  in the lengthwise or warp direction  of the  bag whereas the woolen
system spun  yarns  are manufactured  in the crosswise or filling direction.

     The "combo"  fabrics  are a so-called  one sided construction whereby the fila-
ment yarna are predominant  on  one side, giving the Inherent advantage of cake re-
lease.  The  spun yarns are  used  to  add strength and to cover the Inters tlcea of
the yarn and  thereby  reduce pore  sice.                                      /
     This  fabric was  designed  primarily for Shaker baghouses, however,  it  also has
limited use  in Reverse Air  Collectors.
     The "combo" fabric offers a  good  balance of capacity and cleanabllity where
plugging problems are encountered.

Needled Fabrics
     This type fabric la manufactured by mechanically interlocking  layere  of fibers
by inserting a multitude of reciprocating barbed  needles. Needled  fabrics should
uae a woven base fabric (sometimes referred  to aa a scrim) for strength and  stability.

     This type fabric offers excellent capacity and wear with the reverae-Jet and
pulse jet type collectors.       '
  From Albany Felt Company, Ref .  11

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Figure 4.2.  Diagram of Fiber Lay- a. Woolen Yarn  (note, Fibers Usually  Lay
             in Much More Random Manner); b. Worsted Yarn  (Ideal  Fiber Lay).
             (From Joseph, Ref. 14).
      4.4.1   Threads  and  Yarns
             Thread  and yarn are  basically similar.   Yarn  is  the  term usu-
 ally applied when the assemblage of  fibers is  used  in the  construction
 of  a fabric.   Thread is  the product  used  to join pieces of fabric  together
 in  the  construction  of textile products.   Thread is  frequently of  plied
 construction.   It is fine,  even,  and strong.   Several types  of thread are
 available on the  market:   there  are  simple ply threads, cord threads,
 elastic  threads for  special use,  monofilament  threads of  the man-made
 fibers,  and  mult ifilament  threads.
      4.4.2   Yarn  Twist
             As  the staple  fibers  or  filament fibers  are formed into  yarns,
 a certain amount  of  twist  is added to hold the fibers together.  The
 amount  of twist is measured by the number of turns per inch.  The more
turns per inch the stronger the yarn  becomes,  up to a  point.  Beyond  the
optimum,which varies depending upon  fiber  content, staple  length, size
of the yarn and appearance  desired,  the yarn will become somewhat brittle
and tend to break easily.  According  to Gurley   yarns with  low twist
have  less than 5  turns per  inch; medium twist  yarns from 5 to 20 turns
per inch; and high twist yarns from  20 to  30 turns per inch.  It has  been
suggested that optimum ranges are 3 to 6  turns per inch for  filament  fiber
yarns and 10 to 20 turns per inch for staple fiber yarns.

-------
            The direction of twist is also  important.  Yarns may be
twisted either with a right-hand twist or a left-hand twist.  The right-
hand twist is called a Z twist, while the left-hand twist  is an S twist,
as in Figure 4.3.  Various effects can be obtained by combinations of
yarns ot: different twist direction, and the  durability of  yarns may be
increased by efficient plying of S and Z twist single yarns.
     4.4.3  Yarn Number
            Yarn number is a measure of linear density.   Direct yarn num-
ber is the mass per unit length of yarn; indirect yarn number is the length
per unit mass of yarn.   Yarn number is frequently called yarn count in the
indirect system.  To some extent the yarn number is an indication of the
diameter when yarns of the same fiber content are compared.
          Figure 4.3.   Diagram of S and Z Twist in Yarn.  (From
                       Joseph, Ref. 14).
            Over the years various methods of determining yarn number
have  been developed.  Cotton yarns have been numbered by determining the
weight  in pounds of 840 yard hanks, or, aiore frequently, the number of
840 yard hanks required to weigh one pound.  For example, if 840 yards
weigh one pound, the yarn number is Is; if it requires 30 such hanks to
weigh one pound, the yarn is a 30s.  A heavy yarn would be Is; a medium
yarn  is considered a 30s, while a very fine yarn might be a 160s.

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            Woolen system yarn is measured by  the number of  300-yard hanks
per pound while worsted system yarn  is measured by the number of 560-yard
hanks per pound.  Man-made fiber yarns are usually measured  using the denier
                                                                           *
system.  The denier is equal to the  weight in  grams of 9000  meters of yarn.
            Recently the textile industry has  considered among other systems
the Tex numbering system for all fibers.  A kilometer of yarn is weighed in
grams and the weight becomes the yarn number.  In this system the larger the
number the heavier the yarn and, conversely, the smaller the number the
finer the yarn.  Note that:

                         Tex = 	Wt in grams	
                               1000  meters of  yarn               ^   '

and
                             9 Tex = 1 denier                    (4.2)

The denier or Tex may be used to determine monofilament fiber size and
approximate yarn diameter from the appropriate fiber material density.
                                        j^
            Consider for example a Nomex  nylon yarn stable  fiber (see
Table 4.6, for forms available) of 2 denier/filament (2/9 tex).  The
diameter of this fiber is obtained as follows.  Since
                               f\
                           T D   L p. = Wt in  grams,             (4.3a)


           J Df   (9 x 10   x 10  cm)(1.38 gm/cm3) = 2 grams       (4.3b)

from which
                               Df =  14.3um

The fiber density, Pf, was obtained  from Table 4.13 (also see Appendix
4.3).
     *
      This system dates back to early Roman history where a coin, the
"Denier" (~ 0.5 gm) was used as a medium for buying and selling silk.

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                                            p
            A  100 denier multifilament Nomex  nylon  yarn  (Table 4.6) would
be composed of approximately 50-2 denier  filament ends.   The yarn dia-
meter would be of the order

                            (1-0 I Df2 N- f Dy2                 (4.4)

where 1-e = a. = the  packed  fraction, depending upon  twist and yarn tight-
            ness
      Df  = the individual  filament diameter
      N   = the number of filaments in the yarn
      D   = the yarn diameter
       y
from which (assuming 1-e = 0.6), D  & 14.3 v30 % SO^m, or approximately
the thickness of a human hair.
            Breaking tenacity  (c.f. Table 4.13 or Appendix 4.3) in grams/
denier can be converted to stress from the fiber size and breaking weight
                                P
applied.  For the 2 denier Nomex  nylon staple fiber above, breaking ten-
acity (dry) is 5.3 grams/denier, or 10.6 grams.  Stress at rupture is
thus determined as follows:

                           Stress % Force/area
Stress = 10.6 x 980/(14.3 x 10* )  n/4 = 6.5 x 10  dynes/cm  or about
9.5 x 104 psi.
     4.4.4  Simple and Plied Yarns
            Yarns  that are even in size,  have an equal number of turns
per inch throughout and are relatively smooth, are called simple yarns.
A simple, single yarn is the simplest assemblage of  fibers suitable for
operations such as weaving and knitting.   These yarns may be made from
any of the fibers  and by any of the basic systems.
            A simple-ply yarn  is composed of two or more simple-single
yarns plied or twisted together.   In naming a ply yarn the number of
singles  used precedes the word "ply."  For example,  if two singles  are
used,  it would be  called 2 ply.  Typical  plied and cord yarn are shown in
Figure 4.4.

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  a-

                 4.5  FABRIC PRODUCTION
                      4.5.1  Weaving
                             The production of a flexible textile  fabric  for use  in  filtra-
                 tion systems involves  weaving;  rarely are completely non-woven materials
                 such as paper used. Most felts used in filtration are first woven, then
                 given further treatment.   Figure 4.5 shows the major weaves used in non-
                 felted filter fabrics.  Woven fabrics are formed  by (interlacing  yarns at
                 right angles on a loom, after which the raw or "greige"  fabric may  be
                 further treated.   While there are many patterns of interlacing,  the fabrics
                 in most common useage  in  gaseous filtration are classed  generally as twill
                 and sateen (satin). Plain weave fabric is also sometimes used.   The engi-
                 neering technology of  these and other weaves is discussed in detail else-
                      16                                          '         8
                 where   and the following quotation from Potter and Corbman  summarizes
                 the fabric production  process:
                   (a) Photograph of Cord,  Composed
                       of Four Plies, Each  Ply Com-
                       posed of 7 Single Yarns.
(b)  Four Ply Yarn,  Slightly Mag-
     nified,  Showing Fibers Sep-
     arated in One of the  Single
     Yarns Forming the Ply.
.
                          Figure 4.4.  Plied Yarns.  (From Joseph, Ref. 14),

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               (a)   Plain Weave,  Showing Loose or Open Construction
                    and Close Construction, Such as Muslin.
                                       (b)  Twill Weave.  "This drawing shows
                                            a three-shaft twill-two warp yarns
                                            in each repeat.  This is a right-hand,
                                            filling-faced twill because the
                                            diagonal moves from the upper right
                                            down to the lower left, and more
                                            filling than warp appears on the
                                            face of the fabric.  It is also re-
                                            ferred to as  a one up and two down
                                            twill (1) because the warp goes over
                                            one and under two filling yarns.""
(i)  Warp Floats are
    Seen Interlacing
    Every Eighth Filling.
(ii)  Warp Floats  are
     Seen Interlacing
     Every Fifth  Filling.
(iii)  Filling Floats
      are Seen Interlacing
      Every Fifth Warp.
    (c)  Satin Weave,  Showing (i & ii )  Warp-Face and (iii)  Filling-Face
        Construction.
      Figure 4.5.   Weaving Styles for Filtration Fabrics.  (From Ref.  8).

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 PREPARATION FOR WEAVING
 "In the  weaving operation,  the  lengthwise  yarns  that
 form the basic  structure  o£  the fabric  are called  the
 warp.  The  crosswise  yarns  are  the  filling,  also re-
 ferred to as the weft or  the woof.   The filling  yarns
 undergo  little  strain in  the weaving process.  In  pre-
 paring them for weaving,  it  is  necessary only  to spin
 them to  the desired size  and give   them the  amount  of
 twist required  for  the type  of  fabric  for  which  they
 will be  used.

 Yarns intended  for  the warp  must  pass  through  such
 operations  as spooling, warping,  and slashing  to pre-
 pare them to withstand the  strain of the weaving
 process.  These operations  do not improve  the  quality
 of  the yarn.  In spooling,  the  yarn is  wound on  larger
 spools,  or  cones, which are  placed  on  a rack called a
 creel.   From this rack, the  yarns are wound  on a warp
 beam, which is  similar to a  huge  spool.  An  uninter-
 rupted length of hundreds of warp yarns  results, all
 lying parallel  to one another.  These  yarns  are  un-
 wound to be put through a starch bath  called slashing,
 or  sizing.   The slasher machine covers  every yarn with
 a starch coating to prevent  chafing or  breaking  during
 the  weaving process.   The sized yarns  are  passed over
 large steam-heated copper cylinders  that remove  the
 moisture  and set the  size.   They are then  wound  on  a
 final warp  beam and are ready for the  loom.

 TWILL WEAVE
 A distinct  design in  the  form of diagonals  is  charac-
 teristic  of the second basic weave,  called the twill.
 Changes  in  the  direction of  the diagonal lines pro-
 duce variations,  such as the herringbone,  corkscrew,
 entwining,  and  fancy  twills	The  values of  the
 twill weave include its strength and drapability.
 The  diagonally  arranged interlacings of  the  warp
 and  filling provide greater  pliability  and resili-
 ence than the plain weave.   Also, twill  fabrics  are
 frequently  more  tightly woven and will  not get dirty
 as  quickly  as the plain weave,  though twills are
more difficult  to clean when they do get soiled.
The yarns" are usually  closely battened, making an
especially  durable fabric.   Twill weaves are there-
 fore commonly used	where strong  construction
 is essential.

 In the twill weave,  the filling yarn interlaces
more than one warp yarn but never more than  four,
as strength would be  sacrificed by so doing.  On
each successive  line,   or pick,   the filling yarn
moves the design one  step to the right or to the
left, thus  forming the diagonal.  Whichever  the

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 direction of the diagonal on the face of the
 fabric,  the design runs in the opposite direction
 on the reverse side.

 When the direction of the diagonal starts from the .
 uppcT left-hand side  of the fabric and moves down
 toward the lower right, it is called a left-hand
 twill.  When the direction of the diagonal starts
 from the upper right-hand side of the fabric and
 moves down toward the lower left, it is called a
 right-hand twill.  Although there is no advantage
 of one over the other,  the direction of the diagonal
 can aid  in the recognition of the face of the fabric.

 The steepness  of the  diagonal can indicate strength
 and durability in the fabric.  In order to obtain
 a  steep  twill, more warp yarns must be used than
 filling  yarns.  And since warp yarns have a higher
 twist and are  stronger  than filling yarns, the
 steeper  the twist the stronger the fabric is
 likely to be.

 Twill weaves are named  according to the number of
 harnesses required to make the design.   A three-
 of four-harness twill is frequently used.   The
 word "shaft" may be substituted for "harness,"
 as in three-shaft or  four-shaft twill.

 Twill weaves are also classified as even or
 uneven according to the number of warp  and fil-
 ling yarns  that are visible  on the  face of the
 fabric.   The even twill,  for example,  shows  an
 equal number of warp  and filling yarns  in the re-
 curring  design,  such  as two  over and two under.
 This  pattern makes  what is called a four-shaft
 twill, and  it  requires  four  harnesses.

Most  twill  weaves  are uneven.   An uneven twill
may  show more  warp  than filling yarns  in the re-
 curring  design;  this  is  called  a warp-face  twill.
 If more  filling  yarns than warp  yarns show on
the  face, the  weave is  called  a  filling-face  twill.
Warp-face twills are generally  stronger  than fil-
 ling  face twills because the  stronger warp yarns
on the face  of  the  fabric can  take more  abrasion
and wear.  Warp-face twills  generally have much
more  warp than filling  yarns;  consequently,  such
 fabrics  hold their  shape better  and  drape better
due to the warp's greater twist  and  resilience.
Twills are  described  in  terms  of  the interlacing
of the warp  yarns over  and under  the filling  yarns.
An uneven four-shaft  twill,  for  example,  that  has
three warp  yarns riding  over  one  filling yarn  is
referred to  as  a three up and one down,  or   ly.
On the other hand,  a  three-shaft  twill  that  has
one warp yarn  riding over two  filling yarns  is
referred to  as  a one up  and  two  down, or  "z-


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SATIN WEAVE
In the basic construction, the satin weave  is
similar  to the  twill weave but generally uses
from five to as many as twelve harnesses, pro-
ducing a five-  to twelve-shaft construction.  It
differs  in appearance  from the twill weave  be-
cause the diagonal of  the satin weave is not
visible; it is  purposely interrupted in order to
contribute to the flat, smooth, lustrous surface
desired.  There is no  visible design on the face
of the fabric because  the yarns that are to be
thrown to the surface  are greater in number and
finer in count  than the yarns that form the reverse
of the fabric.  The satin weave may have a warp-
or filling-face construction.
Warp-Face Satin Weave.  Warp-face satin is woven
so that the warp may be seen on the surface of
the fabric.  For example, in a five-shaft con-
struction, the warp may pass over four filling
yarns and under one; in a twelve-shaft construc-
tion, the warp may pass over eleven filling yarns
and under one.  Since  the warp lies on the sur-
face and interlaces only one filling at a time,
the lengths of warp between the filling are called
floats.  These  floats  lie compactly on the surface
with very little interruption from the yarns going
at right angles to them.  Reflection of light on
the floats gives satin fabric its primary charac-
teristic of luster,  which appears in the direction
of the warp.

The long floats found  in the satin weave might be
considered a disadvantage because they represent
a minimum of interlacings, and therefore a poten-
tial weakness in the fabric.   Furthermore, to
increase the smoothness and luster of the fabric,
the yarns are given a minimum of twist and are
therefore relatively weak.  The longer the float,
the greater the chance that the surface of the
fabric will snag, roughen, and show signs of
wear	
Satin-weave fabrics  drape well because the weave
is heavier than the  twill weave,  which, in turn,
is heavier than the  plain weave.   More harnesses
are used for satin weave, thus compressing a
greater amount of fine yarn into a given space
of cloth.  This compactness gives the fabric more
body as well as less porosity	
Filling-Face Satin Weave.  The filling-face
satin weave is also  called the sateen weave;
however,  this sometimes causes confusion because

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some  cotton  and  rayon  fabrics  are  also  identified
a.s .sali'c'ii.   In thi.s  construction,  the-  Tilling
yarn  lies on the  surface of  the  fabric  as  it
pauses  regularly  over  and under  the warp yarns.
For instance, a  filling yarn may pass over four
warp  yarns and under one.  The floats are  con-
sequently made up of the filling yarns, and the
luster  appears in the  filling  direction.

"On the conventional loom, the warp yarns
that  are to  run  lengthwise in  the  fabric are
wound on a cylinder  called the warp beam,  which
is at the back of the  loom.  The warp also ex-
tends to a cylinder  called the cloth beam, which
is at the front of the loom  and  on which the
fabric  is rolled  as  it is constructed.  Sup-
ported  on the loom frame between these  two
cylinders, the warp  yarns are ready to  be  inter-
laced by the  filling yarns that  run in  the width
of the  cloth, thus producing the woven  fabric.

In any  type  of weaving, four operations are
fundamental.  They are performed in sequence
and are constantly repeated	The essential
parts of the  loom are:  warp beam, cloth beam,
harness or heddle frame, shuttle, and reed.  These
parts perform the following operations.

        Shedding—raising warp yarns by
        means of the harness or heddle
        frame
        Picking—inserting filling
        yarns by means of the shuttle
        Battening—pushing filling
        yarns firmly in place by
        means of  the reed

        Taking up and letting off-
        winding the  finished fabric
        on the cloth beam and re-
        leasing more of the warp
        from  the warp beam.

CLASSIFICATION OF WEAVES
The manner in which groups of warp yarns are
raised by the harnesses to permit the inser-
tion  of the  filling yarn determines the pattern
of the weave, and in large measure the  kind of
fabric  produced.  Weave patterns can create var-
ying  degrees of durability in fabrics,  adding
to their usefulness and also to their appear-
ance.    In a simple weave construction,  consisting

-------
            of the filling going under one warp and over the
            next, two harnesses are needed:  one to lift the
            odd-numbered warp yarns, and a second to lift the
            even-numbered warp yarns.  More than two harnesses
            are required for advanced weaves, and as many as
            forty for figured weaves	

            PLAIN WEAVE
            The plain weave is sometimes referred to as the
            tabby, home-spun, or taffeta weave.  It is the
            simplest type of construction and is consequently
            inexpensive to produce.  On the loom, the plain
            weave requires only two harnesses.  Each filling
            yarn goes alternately under and over the warp yarns
            across the width of the fabric.  On its return, the
            yarn alternates the pattern of interlacing.  If
            the yarns are close together, the plain weave has
            a high thread count, and the fabric is therefore
            firm and will wear well."8

     4.5.2  Felting and Needle Punching

            The felts used in fabric filtration in their early stages of

production are also woven, but subsequent steps completely change the

character of the material from that of a woven fabric.   While felts can

be made simply by matting fibers together and by other non-woven

methods  (see below), the use of a woven base fabric called a scrim greatly

increases the strength and stability of the fabric.

            The production of a felt depends on whether its fibers are
naturally binding.  Because woolen fibers are scaley, and also shrink
when exposed to heat and moisture, a woolen scrim shrinks when mechani-

cally worked in warm water in the presence of certain lubricants and

chemicals.  The identity of the separate yarns tends to be replaced by

a more homogeneous character.  The material becomes felt-like in density,

stiffer and thicker.  Minor amounts of synthetic fibers added to the

woolen yarns can modify the properties of the felt thus produced.

            To further increase the homogeneous character of the felt
surface, it may be napped.  This has always required the use of teasels,
woody, thistle-like parts for a weed plant, the barbs of which pluck
fibers from the surface of the felt.  When enough nap has been raised in
this way, it may be singed or otherwise trimmed to the desired thickness.

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             Need It1  punching is  a method of combining two or more layers          —'
 of liber  into a  1'elt-likc fabric.   Usually one layer is a scrim for
 strength,  while  the  other(s)  may consist of fibers of almost any descrip-        _
 tion or combination.   Thus considerable control over separate properties
 of the  finished  material is possible.   For example,  the scrim may con-
 tribute the  desired  dimensional stability while the  top layer'contributes
 the  ideal  properties  for dust control.   The surface  layer might be 100%
 dacron, the  scrim 1007o nylon, for example.
             The  technique used  in needle punching is to prepare a scrim            '
 and  a batting separately.  The batting may be formed by carding, by air-
 lay  of  fibers, or by  other random web-forming equipment.   The batting
 is generally of  the  same order  of  weight or lighter  than the scrim.               •—'
 It is unrolled or otherwise spread over the scrim.   Needles having for-
 ward barbs are punched from the batting side into or through the scrim,          _j
 and  the batting  fibers thus laced  into  the  scrim remain behind  when the
 needles are  withdrawn.   Production is at the rate of about 10 FPM.
 Variations in the needling process include  needle angle,  number of
 repetitions,  two-sided  needling, etc.   When a  shrinkable  scrim  is used,
 the  needled  material may later  be  felted  in various ways  to  produce  a
 still more dense  and uniform  material.                  :
                                                                                  _j
             Non-woven  production methods  include  resin  bonding, wet  bond-
 ing  (paper-like materials), spun bonding  (while  the  fibers  are  still
 tacky from their  extrusion  stage), heat  bonding,  chemical  bonding, spray          "^
                                                                n  1 f
 bonding, and  stitch bonding (a  sort of  knitting within  a matt).  '
 Nearly all fibers used  in fabric filtration can be used  in non-woven              —'
 fabrics.  Because non-woven fabrics can  often  be  produced more  rapdily
 than by weaving,   it appears that filtration fabric or even  filter ele-            _,
ments might eventually be produced by non-woven methods.
     4.5.3  Fiber Additives, Yarn Treatment, and  Fabric Finishes                  _J
            Natural fibers are  produced with an outer molecular  film; cot-
 ton  cellulose fibers are  covered with a wax-like  adhesive; and wool  pro-          __
 tein fibers contain oils, fats, and waxes from glandular secretions dur-
 ing  growth.  Both may contain other agricultural  chemicals added to  pro-           '
 tect or preserve  the fibersto aid in the harvesting process, e.g., in-


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secticides, defoliants, etc.  Wool will have much of  the grease removed
in a washing, scouring or solvent extraction process.  Additional oil
(animal, vegetable, or mineral) may be provided as a  lubricant for  spin-
ning.  The production of fabrics from fibers and yarns Involves many
chemical, physical and mechanical processes.  ProporLlca of  the fiber,
yarn, and fabric are continually modified  through treatment  and additives
to meet the requirements of production machinery.
            Each additive or  treatment may result in  some residual  mater-
ial attached to individual fibers that ultimately affects its strength
in service or its ability to  act as a substrate for filtration.  The
potential combination of additives, treatment or finishes is nearly
limitless.  Prediction of fabric performance in service  (pressure drop,
efficiency, life) on a given  application is presently empirical through
service testing, and thus the science of fabric treatments for improved
performance is relatively undeveloped.  Furthermore the addition of
materials to improve filtration performance, particularly life in service
is often proprietary.  Many techniques are relatively new, and not  subject
to competitive or comparative testing procedures.   The entire field of
the role of finishes and additives in gas  filtration requires further
analysis before analytical generalizations can be produced for optimiza-
tion studies.
            Each of the generic man-made fiber materials  discussed above
is a complex chemical structure with varying physical properties and
surface characteristics.

            These polymeric materials frequently contain  one or more of
the following additives:
            •   Plasticizers reduce  flow viscosity or temperature
               in melt spinning the  resin,  such as  DOP in PVC;
               and are also employed to improve low temperature
               flexibility.
               Solvents  are used in  wet  spinning as  with  acrylic
               plastics  or rayons;  and in coating,  adhesives, etc.
            .   Organic peroxides are used as  polymerization  initia-
               tors  or for cross-linking reactions  in thermoplastic
               materials  to transform them into thermosets.

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               Antioxidants are added to reduce oxldative deter-
               ioration during manufacture, processing or storage,
               and to provide heat protection especially among
               vinyls.

These agents may bt; required in varying amounts.

               Flow-control agents may be required to control melt
               viscosity and transformation of the polymer to a
               stable form for end-use.
               Colorants, pigments, and delustrants include powered,
               colloidally dispersed or dissolved materials added to
               provide color or reduce brightness.

            ./Flame retardants are employed to provide fire protection
               for flammable polymers.

               Stabilizers are used to import thermal stability or
               mechanical protection during processing and to protect
               the mix against changes induced by other additives,
               e.g. by neutralization of contaminants, residues,
               or impurities.

               Ultraviolet absorbers are added to reduce UV absorption
               by the polymer or to quench molecular reactions, in
               order to limit physical degradation of the plastic ex-
               posed to sunlight.
            .  Antistatic agents may be applied as a coating to ex-
               ternal fiber surfaces or added internally; the agent
               acts as a hygroscopic material to assist the charge to
               leak away (make the fiber electrically conductive) and
               may modify the charging process, reverse the sign of
               the charge, or promote dissociation of ionic material
               present on the fiber.
               Other additives, filters, and processing aids may be
               employed as viscosity depressants, parting agents,
               emulsifiers, coupling agents, internal or external
               lubricants, or adherents.

            Many compounds ,are used in varying amounts to assist in manu-
facturing and processing operations and to provide stability,  compatibility,

and other desirable end-use properties.   Since filter fiber is a small

proportion of the total annual market for man-made fiber textiles, mater-
ials used in production and processing are developed,  produced, and used

primarily for the larger market applications,  rather than for fabric fil-

tration objectives.   Each producer's fiber,  while generically the same

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(i.e., same basic polymer), will be produced differently and will have
different additives which may affect dust particle-fiber attachment, de-
posit formation, and stability.  The reduction of these phenomena to
engineering design parameters in filtration will be difficult and complex.
            The yarn may be treated as well as the fiber.  Yarn treatments
now include surface addition of lubricants, antistatic agents, and var-
ious mechanical operations such as attenuation or stretching, heat set-
ting, and bulking or texturizing.  Warp yarns are subjected to greater
strain during weaving and may be sized or coated to prevent chafing and
breakage.
            Fabric finishing includes those processes to improve appear-
ance or serviceability of the fabric after leaving the textile machine
(greige  goods).  Cotton and wool fabrics are usually cleaned  (washed or
scoured) and bleached, and may be chemically treated to provide water-
proofing, mothproofing, mildewproofing, or fireproofing.  Mechanical •
finishes applied to cotton and wool include singeing, napping, shearing,
felting, or shrinking.  Synthetic fabrics are usually heat set to re-
lax internal stresses in the yarns set up during weaving.  This causes
shrinkage, although it enhances dimensional stability for subsequent
exposures to temperatures below the setting temperature.  Additional mois-
ture protection (water repellents) and antistatic agents may be applied.
            Glass fiber fabrics are subjected to high temperatures
(~10  F) to relieve yarn twisting and weaving stresses and to heat clean
the weaving size.  Glass fabrics are usually lubricated with silicones,
graphite and other proprietary finish agents to reduce fiber-fiber abra-
sion resistance during filter cleaning.  Graphite finishes of the order
of 5% by weight have been indicated to provide extended service life of
glass fiber fabrics at temperatures < 500 F, as illustrated in Figure 4.6.
Largely as a result of improved finishes, the use of glass fiber fabrics
for applications at temperature > 275 F has increased greatly.  Competi-
tive fabrics now include NomexR (< 425°F) TeflonR (< 450°F),  and newer
                         R           R
man-made fibers (Brunsmet , Fiberfrax , and Polyimides).  Improved lubri-
cants and finishes for glass fiber fabrics to provide longer life at
higher temperatures (> 600 F) are under continuing development.

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M
*
VI
Ol
u
(0
3
T3
C
W




800

700


600


500



400



300


200
100
1 1 i i i
O Legend:
\ & - Commercial ulieone fim*
~ 1 O - Commercial silicon* plut
\ tjrjphite finish
\
\
\
.\
\
\
\
\
\
\

\
0\
\ 0
*v^ 1 — "1^2* ~
i i i i i
3.5

3.0


2.5


2.0



1.5



1.0


0.5
0.0
                                                               CO
                                                               01
                                                               X
                                                               01
                                                               CO
                                                               C
                                                               O
                   200
300  400  500  600   700  800
            Figure 4.6.   Effect of Graphite on Glass Fabric Tempera
                         ture Endurance.  (From Ref.  18).
4.6  FABRIC PHYSICAL CHARACTERISTICS
     4.b.l  Abrasion Wear
            The mechanical abrasion of fabric is only one of several kinds
of wear, other kinds including chemical and thermal degradation, handling,
etc.  As Table 4.16 indicates, a fabric could conceivably experience many
kinds of wear at one time or another during its lifetime of manufacturing
and use. Most of these types of wear are terms from popular usage and con-
sequently are ill defined.  For present purposes the term "abrasion" will
be defined and used as an eroding away of fabric fibers or fiber surface
material, through moving contact between (a) the fiber and dusjt particles
or (b) adjacent fibers.

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                                TABLE 4.16
                 SOME  TYPES OF MECHANICAL WEAR IN FABRICS
                 Fatigue                    Migration
                 Sliding                    Slippage
                 Rubbing                    Breaking
                 Galling                    Cutting
                 Scoring                    Grinding
                 Snagging                   Plowing
                 Picking ;                   Penetrating
                 Flexing :                   Rupturing
                Welding                    Shearing
                Compression                Catching
                Plucking                   Bending
                Fretting                   Polishing
                Fracture                   Chaffing

            Material science describes the forms of abrasion as either two-
body or three-body abrasion.  Fiber-to-fiber rubbing or fiber-to-particle
collision (sand blasting) are two-body systems.  Interstitial dust particles,
caught and chaffing under pressure between adjacent fibers or yarns, repre-
sents three-body abrasion.  The mechanics of the two processes are somewhat
related, however, and involve the relative sizes of the bodies, the hard-
nesses or relative hardnesses, the pressure between the bodies, the coef-
ficient of friction and the nature of the motion between them.  The laws
of material abrasion are nevertheless only partially determined because of
other effects which are more difficult to evaluate such as humidity, rate
of oxidation of freshly exposed surfaces, smoothnesses of particles, any
accumulation of freshly abraded particles, etc.
            The laws of material abrasion are undeveloped, and fabrics,
being of complex geometry and having extremely varying stresses and strains,
are especially difficult to analyze.   Neither can fabrics be analyzed in
detail for abrasion during their fabric filtration performance, as abrasion

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 hi  usually reported  simply as "holes" in the filter element (Section 8.3).
 Kv«-n  tliouKh  i-t  appears  that abrasion represents up to 25% of fabric failures
 llu-rc- JM insufficient data relating to particle hardness, coefficient of
 I rl.(-tlon, ft cetera, to establish the rules of filtration fabric abrasion.
             Manufacturers of filter fabrics have little technology for
 the abrasion resistance of their fabrics, although they do measure the
 resistance of fabrics to abrasion by standard testing procedures.   Typi-
 cally a swatch of the fabric is rubbed back and forth over a rough surface,
 sometimes in the presence of grit,until the fabric becomes perforated.  The
 fabric abrasion resistance is rated according to the time required for per-
          19
 foration.    Other methods include sandblasting, and flexure or tension
                20
 during rubbing.     Thus it appears that fabric manufacturers as well as
 users rely on an evaluation of the destruction of the fabric without attemp-
 ting  to determine the basic cause or causes of failure.  Considerably more
                                                21 22              23
 analytical approaches have been taken by Backer  '   and Hamburger  , as
                    10 24           16
 reviewed by Kaswell   '    and Hearle
       4.6.1.1   Sandblast or Surface Scour.- Particles entering a fabric col-
  lector  filter  element  are apt  to strike the fabric surface with sufficient
 velocity to abrade  and remove  portions of fibers,  eventually perforating the
  fabric.   This  is  analogous to  the process called sandblasting, and although
  sandblasting  involves  perpendicular impact while the fabric is struck at a
                                                          25
  shallow angle,  the  mechanics should be related.   Raleigh   studied perpen-
 dicular sandblasting.   He assumed that the rate of wear is related to the
 maximum strain  produced in a surface by an impact.   This strain is propor-
  tional  to
                                 -. V N/'jJ
                                      Y
  where V is  the  impact  velocity,  p the density and  Y the tensile modules
  of the  impacted material.   To  the extent  that this  strain  exceeds  the
  tensile strain  limit of the material,  bits of the  material  will chip  out.
*
  Another test that is frequently used is to abrade the fabric against it-
  self with a rolling crease.  Cycles to failure are an indication of re-
  sistance to flexure and two-body abrasion.

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 KvidenLly  this applies better  to hard, brittle materials than to plastic
 ones, where,  the mechanism  is more probably plowing than chipping.
     Zcnz  and Othmer   reported a general agreement that 90  sandblast
 crrosion is  proportional to
                                 -LV3

 where L is sand loading or concentration, although the exponent was  reported
 to vary from study  to study between  3 and 6.  The exponent of 3 can  be  shown
 to be consistent with the  assumptions that wear  rate  (depth/time)  is propor-
 tional to  both the  particle impact rate  (number/time) and the average parti-
 cle kinetic  energy.  In the above case it is assumed  that the mechanism is
 plastic plowing, rather than brittle chipping.
     Fabric  abrasion using sandblast equipment has been studied experimen-
              27
 tally.  Parker   found with approximately 60 mesh granules that harder  abra-
 sive (circa  silicon carbide hardnesses) and higher blast pressures shortened
 fabric life; for example:

                  Pressure (psi):    30    40    50    60
                  Life (seconds):   100    54    35    23
Unfortunately Parker did not state the rate of sand flow which may or
may not have varied with blast pressure.
     Generally Parker found the rate of removal of fiber was constant over
the life of the fabric, although it slightly accelerated during the life of
                   2                   24
heavier (8-12 oz/yd ) fabrics.  Kaswell   also indicated an approximately
linear relationship between fabric strength loss and the time of operation
of the abrasion equipment.
     Types of surface abrasion test machines other than sandblast are more
widely accepted, and their data may be related to filter element scouring.
       24
Kaswell   reviewed these methods and a large number of studies of fabric
surface abrasion made chiefly by the garment industry.  He credits Backer
             22
and Tanenhaus   with an examination of fourteen fabric properties that re-
late to abrasion resistance:

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     Geometric Aspects
        1. Area o£ contact between  fabric and abradant
        2. Local pressures developing on specific yarn points
        3. Threads per inch
        A. Knuckle height
        5. Yarn size
        6. Fabric thickness
        7. Yarn crimp
        8. Float (knuckle) length
        9. Yarn conesiveness
       10. Congressional resilience
       11. Fabric tightness
       12. Cover factor
     Abrasion Aspects
        1. Direction of abrasion
        2. Magnitude and direction of tensions developed during abrasion
     In conclusion it appears that fabric surface scouring is related to
dust loading and impact velocity, and to particle sharpness and hardness.
Fabric designed with a dense surface (level; few knuckles) should retain
its strength longer. Nap, pile or sacrificial surface fibers will extend
the life of the fabric   A soft fiber material, flexible fibers, and re-
silient yarn and fabric will also reduce the rate of surface fiber degradation.

     4.6.1.2  Internal Chaffing.- The rubbing together of two fibers as the
fabric is flexed is also two-body abrasion.   Glass fibers are notoriously
weak in this respect.  Originally glass fabric had almost no durability;
crossed glass fibers apparently became nicked due to chaffing or due to
dissolution at the point of contact and then snapped easily.   However when
silicone and other lubricants were added, their life extended almost in-
definitely in comparison.
     When a yarn is bent its elementary fibers slip past one another, un-
less they are bonded together or under a large lateral pressure from twist-
ing or weaving.   In the latter cases the yarn is beam-like in rigidity and
liable to rupture in tensile failure if bent.   More normally,  fibers slip
to some extent as the yarn is bent; and also adjacent yarns tend to slip
as the fabric is flexed.   Thus the shaking or collapse cleaning of a filter
fabric is associated with  myriad tensile stresses and fiber slippages with
chaffing.

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     To a first approximation, the strain associated with a bending fabric
is  (ft = •« R) where t is the fabric thickness and R the radius of bend.
Also approximately, the amount of slippage (distance) between fibers of
diameter D  in a yarn undergoing bending strain 6 Is 1 ~ £D. , or
                                J ~ ^f                            (4.1)
                                     R
A precept of two-body abrasion is that the volume of material removed during
                          28
a rubbing of distance 1 is
                                        ;
where k is a wear, coefficient, L is the: loading between bodies, and H the
hardness of the body being abraded,  (v and L may alternatively be expressed
per unit area).  It appears that k depends on several aspects of the materials
and their geometry, but is frequently of order unity.
               29
     Rabinowicz   discusses four common types of two-body wear, any of which
may apply during the fabric deformations active in dust collectors:

             •  Adhesive wear.   One  surface  adheres  so strongly  to
               a  second because of  atomic bonds at  their contact
               that  on sliding, a portion of  one  is  torn loose.
             •  Gouging wear. A proturberance  on one  surface
               plows a groove  in the second.   It  appears that k
               (wear coefficient) may be of the order of the
                                           28
               protuberance size in microns.
             .  Corrosive wear.  Corrosive films that  normally tend
               to resist further corrosion  may wear  away, exposing
               fresh surface to corrosive attack.
             •  Fatigue wear. Tensile stresses  trailing the sliding
               body  may crack  the supporting  surface,  leading to
               chipping of fragments from the  supporting surface.
     It appears from these brief reviews that chaffing inside  the fabric
may be reduced by minimizing  the:

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             .  number  of fabric deformations (cleaning cycles)
             .  degree  (radius)  of deformation (cleaning intensity)
             •  tightness of the fabric  weave
             •  softness  of the  fiber  material
             •  fabric  thickness
             .  fiber diameter
             •  fiber roughness
             •  interfiber friction coefficient.
      Table  4.17  lists frictional properties for  several  fibers.  These  affect
 both  the  rate  of wear between  fibers and  the stiffness of  the yarns,  to be
 discussed below.
      4.6.1.3   Internal  Abrasion.- Many dust particles are  abrasive  in con-
 tact  with the  fibers  ordinarily used in filtration  fabrics.  Since  most
 industrial  dusts and  fumes include some particle of sizes  substantially
 smaller than the fiber  diameters (10-30um)  there is great  likelihood  of
 penetration between adjacent yarns and adjacent fibers.  Here the fine
 particles may  be trapped  by interfiber pressures, and as the fabric flexes,
 they  cut  and gouge the  fibers.   This is three-body  abrasive wear.   This
 wear  also follows Eqn. .(2) as  far as can be  determined, except that k may
                                                       28
 be an order of magnitude  smaller  than  for two-body wear
      The  evidence for wear of  a  fibrous structure due to imbedded grit  is
 scant, despite the claims  of carpet  cleaning services and the manufacturers
 of laundry detergents.  Soiling  of carpets and garments has received much
                               24
 attention according to Kaswell    but primarily from the standpoint of color
 changes rather than mechanical degradation.  Apparently dust filtration
 fabrics have not been tested for  degradation due to  imbedded particles.  While
 reports from filter operation  indicate a general weakening of the fabric
with  time, thermal degradation,  surface scouring and other mechanisms are
more  readily shown to be the causes than imbedded particulates.
     4.6.2  Flexibility
            The flexibility of a filtration fabric is important  for  at
least two reasons:  Removal of the dust deposit may be improved by flexing

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I
Ln
U)
                                                              TABLE 4.17
                                                   FIBER FRICTIONAL PROPERTIES*
THE COEFFICIENTS OF FRICTION OF VARIOUS AIR-DRY TEXTILE
FIBERS AND FILAMENTS IN COMMERCIAL CONDITION
Rubbing Surfaces

Nylon/nylon
Very fine
6 denier
27 denier
Bristle
Viscose rayon/viscose
rayon
Acetate rayon/acetate
rayon
Cotton/cotton
--from sewing thread
--from cotton wool
Wool/wool
from tops

commercially scoured

coarse, "clean"

Coefficient of
a
Friction


0.14
0.15
0.23
0.6 (0.5-0.8)

0.19

0.29

0.29
0.57

u2 = 0.38
Hi = 0.24
u2 = 0.49
Ul = 0.20
u2 = 0.42
ul =0.25
Fiber Daimeter(u)


18
28
62
500 approx.

30 (variable)

41 (variable)


18 (variable)

20

18

26

STATIC COEFFICIENTS OF FRICTION OF FIBERS
(Average Values)
Wool
With-scale
Anti-scale
Wool in water
With-scale
Anti-scale
Cotton
Jute
Viscose rayon .
Acetate
Nylon
Saran
Terylene

Steel









	 0.11
	 0.14

	 0.15
	 0.32
	 0.22
	 0.46
	 0.43
	 0.56
	 0.47
	 0.55
	 C.58

. 0.29









           In  the  case  of wool  (il  is  the  coefficient  of  friction in the direction of the scales (root-to-tip)
           and n2  is the coefficient  of friction in the direction against the scales (tip-to-root).



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of the fabric substrate and conversely, such flexure may cause  i'abric  de-
gradation as noted above.  Thus, fabric flexibility may be both necessary
and harmful.  Since the. role of flexibility in fabric filtration  is not
well established, only the principles of fabric flexibility are stated here.
A single fiber flexes beam-like according to the well-established

                                                       (4.3)
        24                          •>         /
relation                          ,3         4
                                        _     _
                             y ~ 48YI '    ~ 64
where t,  is the span length and y is the center span deflection under
       s
center load W.  Y is the elastic modulus and  I the moment of inertia of
the fiber which as expressed above is specifically for a circular fiber.
            A parallel group of N fibers with zero friction between them
would deflect 1/N as much, under the same  load conditions as the single
fiber.  On tho other hand with high friction the bundle would deflect as
                                                            2
a  single beam, and the deflection would be approximately 1/N  as much as
ior the single fiber.  A yarn of parallel  fibers, having intermediate
                                           2
friction, would deflect between 1/N and 1/N   as much as given by Eqn. 4
for only one fiber.
            A twisted yarn deflects somewhat more easily, but the exact
expression for deflection is highly involved.  The amount of inter-fiber
friction in a yarn is proportional to the  coefficient of friction and to
the pressure between fibers.  This pressure results from' twisting, and it:
usually changes with bending.  One simple measure of the amount of friction
in a yarn is the amount of permanent set it acquires when bent; a perfectly
frictionless yarn would normally recover elastical-ly.
            When woven, each yarn contributes its own stiffness to the
fabric.   The yarn stiffness will generally be increased however by in-
creased inter-fiber pressures from bending the yarns and packing them
together.   Thus a section of fabric N' yarns wide will generally be more

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 than N'  time as  stiff as N' single yarns.  Extenuating factors in calcu-
 lating fabric stiffness are slippage vs. friction between the yarns, the
 thickness of the fabric, and the position and the straightness of yarn
 through  the fabric.
            Fabric flexibility as a property of the fabric can be designed
 via any  of the above factors.  Other design variations include texturized
 (teased) yarns,  lubricants or bonding agents, fibers of several diameters
 or lengths, and a variety of finishing treatments including napping and
 calendering.  Many fabrics are designed to be more flexible in one direc-
 tion than in the other.   This opens the possibility in filtration fabrics
 of using relatively delicate fibers or yarns in one direction for collecting
 the dust deposit, and using more flexibly durable yarn in the other direc-
 tion for removing the deposit.
            Flexing of filtration fabric is of necessity associated with
 interfiber tensions and frictional chaffing.   Any grit present may cut
 the fibers during chaffing (Section b).  In addition all fibers undergo
 some molecular fatigue wj Li. repeated flexing.
     4.6.3  Strength
            Although fabric breaking strength is not usually an important
 parameter in filtration, per se, it is frequently specified because it  is
one check on the quality of yarns and fibers in the fabric.  Strength is
also one indication of the aging a fabric has undergone.  Filtration fabric
 rarely tears in use unless the filter element has been sewn in such a way as
 to concentrate the tension, i.e. at a cuff seam.  Tensile modulus is more
closely related to the performance of the fabric since it determines the
distribution of tension over the filter element and thus the distribution
of cleaning energy in mechanical cleaning.

            Fabric strength is related to fiber strength or denier, defined
 as the weight in grams of 9000 meters.   Appendix 4.3 lists the denier
 strengths (i.e.  weights) and other fiber mechanical properties of a variety
of fibers.  Strengths and moduli are also indicated in Figure 4.7.

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                                Tvpicul Mlri'MH Mlriiin curve  for n  textile liber. (Dillmi,
                              .1  II.. Inil.  I.UK  Clinn  44: 21 IT),  lil.V.M
                                                                              I  FiMrflM
                                                                              t  Collo* SO/1
                                                                              1  Ctlwctl
                                                                              4  H T
                                                                              5  Canon it/I
                                                                                Sow Mon»n N
                                                                             20  V««wi NOMU IWIiflMlMM
                                                                             ti
                                                                             (2  own.
                                                                             25  C«M Report No. M, Office of the QoarUrmMUr
General.
  The immediate elastic recovery is determined by extrapolating to sero load the initial straight-line portion of the recovery
                                                                        lieally defined M: 100 -  (ii
  Delayed recovery is the recovery which take* place slowly and is ma
ivcovery (%) - permanent aet (%)).
  Permanent a*t ia mtaeursH after retnoving the •train and allowing the sample to recover for
                       Figure  4.7.  Stress  Strain Curves  for  Fibers*

 * From Harris,   Kef.  4

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             'flu- strength of a yarn can obviously not be greater than the
 summed strength of its fibers and actually the yarn strength is practi-
 cally always less.  This is for two reasons: first, the central-most fibers
 in the yarn are nearly straight and tighten first; hence they break before
 the outer spiraling fibers reacli full load.  Second, part of the tension
 in the outer fibers goes into pressure directed into the yarn rather than
 into longitudinal tension; hence a tightly twisted yarn may be a very weak
 one in tension.  In the-case of staple fibers which must be twisted to ob-
 tain yarn strength, there is a twist giving maximum yarn strength; this
 maximum depends on fiber properties number of fibers, etc.
             If a typical fiber in the yarn is examined, it will lie at a
 twist angle  to the yarn axis as depicted in the following sketch :

 The stresses  (cr) in yarn  (y) and fiber  (f) are related by
                                        2
                             ,)  = n   cos  •!>                           (4. A)
Thus the strength of the fiber contributes less strength to the yarn.
Fiber and yarn strains are also related by a similar but more complex
relationship, since the yarn has a widely ranging fiber packing depending
                                  *
on fiber geometry within the yarn.   Consequently even though the fiber
tensile modulus is determined (See Figure 4.7 and Appendix 4.3), the ex-
pression for the yarn's tensile modulus, that is, the ratio of yarn stress
to yarn strain, is not algebraically simple.
            The process of weaving further modifies the contributions of
yarn strength and tensile modulus to the fabric.  The yarn is generally
weakened by being bent around orthogonal yarns, depending on its twist and
the sharpness of the bend.   The tensile modulus of the fabric is generally
increased by the weaving process over that of the unwoven yarns, depending
* Loose twisted yarns may elongate considerably before the fibers become
  tightened, in analogy to the members of a folding gate. Small elastic
  strains in homogeneous compressible media can be described by Poisson's
  ratio, but this approach is misleading in fibrous structure applications
  where the geometric deformations are often large.


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 on  the  tightness  of  the weave.  Consequently  the  fabric  is  less  resilient  in
 tension than  a  parallel assemblage  of  the  same number of fibers, that is,  it
 absorbs  less  strain  energy before rupture.  Kaswell   cites the  importance of a
 fabric's elongation-recovery properties  in numerous  industrial  applications.
            Strength and modulus arc further complicated by dependencies on
 humidity, mechanical fatigue, and of course the manufacturer's processing
 and finishing variables.  Backer and other at MIT and elsewhere  '   have
 done much to relate  fabric and fiber physical properties, and much of this
 developing science may  be useful in obtaining improved filtration fabrics.
     4.6.4  Permeability
            The resistance to flow of gases through porous materials has
 been discussed  in Chapter 2.  The permeability of filtration fabrics is so
 decreased by the residual dust deposit that the permeability of the clean
 fabric appears  to have little to do with its use.
            The objective in fabric design is to maintain a highly permeable
 residual dust and fabric combination, while yet passing a minimum amount of
 dust.  Toward this end the pores through a fabric  must be closely controlled.
 They must not exceed a certain bridging diameter.   (Section 2.3.2).  If the
 pores are too small they will cither plug or pass  too little gas, which dis-
 tributes itself according to the square of the pore diameter.   In an ideal
 filtration fabric probably all the fabric pores should be the same size,
 the size depending on dust properties, etc.
            Pore size and thus fabric permeability are dependent on cloth
                          24
design structure.  Kaswell   indicates reductions  in permeability with in-
creasing pick count and also with pick diameter as would be expected with
 the decreasing pore sizes.  Filling twist was found to have a greater effect
on fabric porosity and permeability for a given fabric than any other con-
structional variable; the tighter the filling yarn the more permeable the
 fabric.   This would be expected for any fabric passing more air between than
through the yarns.
            Pore size must not be visualized as  simply the  distance between
crossed cylinders, however.   For all but  filament  yarns  there  will  be nume-

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 rous  Cibor  ends  protruding  into the pores, and  in the extreme ease  (napped
 or  felted materials)  the pores will be primarily between fibers rather than
 between yarns.   In  such cases the uniformity of spacing is equally  important,
 but the means of achieving  it are less dependent on weave.  Yarn texturizing
 and post-weave surfacing treatments contribute much to the permeability of
 the clean fabric, and perhaps also to the residual dust deposit permeability.
     4.6.5  Dust Deposit Release
            The ability of  the fabric to release the deposited dust will
 dopond on the mode  and intensity of cleaning and also on the adhesive.
 character of the fabric.  Cleaning and fabric adhesion are discussed in
 Chapters 2  and 6.   The way  in which fabric construction relates to  deposit
 release has not  been  determined, but is presumed to depend partly on the
 electrical  resistances of selected fibers.  Resistance is seen to depend
 on  humidity, which  is independently known to have a marked effect on fil-
 tration fabric performance  (Section 2.2.2)
 4.7  AVAILABLE FABRICS
     There  are at least 50  U.S. manufacturers of dust filtration fabrics
 or  filter elements  for dust collectors.  The list of manufacturers  in Appen-
 dix 4.4 resulted from a 1969 survey of nearly 200 companies believed to have
 interests in the kind of filtration fabrics used in dust and fume collectors.
     The list is representative of U.S. fabric manufacturers and suppliers,
 but undoubtedly does not include all such firms, and may not accurately
 represent the interests of  every firm listed.  For example a firm although
 not specifically mentioning a fabric product in its brochures is often able
 to supply the product on short notice.   Other firms prefer to specialize in
 certain fibers, filter element types, etc.
     Chapter 7 discsses the purchase costs of filter elements, filter fabric,
 and typical fibers  in some  detail.   As indicated in Chapter 1, the market
 for dry filtration  fabrics  is estimated at $15 to 30 million annually,  about
half the fabric going into new collection equipment and half replacing fabric
which has worn out.

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      The  distribution  of  fabric manufacturers,  while  not  analyzed  in detail,

 may  be  assumed  similar to the  distribution  of  filter  equipment  manufacturers.

 That is,  the  typical fabric manufacturer  has one  half to  one  million dollars

 in sales,  and the  largest manufacturers have sales  of several million

 annually.

 4.8  REFERENCES FOR CHAPTER 4

 1.   J.  J.  Press, Man-made Textile Encyclopedia, Textile Book  Publishers,
      Inc.,  Div., Interscience Press  Inc.,  New York (1959).

 2.   J.  P.  Stevens  & Co.,  Inc., Selecting  Fabrics  for  Filtration and  Dust
      Collection. Bulletin,  New  York  (1961  est).

 3.   A.  A.  Dembeck, Guide  to Man-Made Textile Fibers and Textured Yarns
      of  the World,  3rd  Ed., The United Piece Dye Works, New York, (1969).

 4.   M.  Harris, Ed., Handbook of Textile Fibers, Textile Book  Publ. Inc.
      (Interscience) New York (1954).

 5.   A.  N.  J. Heyn, Fiber  Microscopy; A Textbook and Laboratory  Manual,
      Interscience Publishing, New York, 1954.

 6.   J.  M.  Preston, Modern  Textile Microscopy, Emmott  & Co., Ltd., London,
      1933.

 7.   H.  Bunn, Ind.  Eng. Chem.. 44: 2128 (1952).

 8.   M.  D.  Potter and B. P. Corbman, Textiles; Fiber to Fabric,  4th Ed.,
      McGraw-Hill Book Co.,  New York  (1967).

 9.   P.  L.  85-897,  85th Congress-Second Session, Textlie Fiber Products
      Identification Act. 15 U.  S. Code 70, 72 Statutes 1717 (1960).

10.   E.  R. Kaswell, Wellington Sears Handbook of Industrial Textiles,
     Wellington Sears Co.,  Inc., N. Y. (1963).

11.   Albany Felt Company, Dry Filtration Manual, Technical Bulletin,
      Industrial Fabrics Div., Albany, New York., (1968).

12.  W.   E. Morton and J. N. S.  Hearle, Physical Properties of  Textile
      Fibers, The Textile Institute, Butterworths, London,  (1962).

13.  American Society for Testing and Materials, Standards on  Textile
     Materials, Part 24, p. 39  (1965).

14,  M.  L. Joseph,  Introductory Textile Science, Holt,  Reinhart & Winston,
     Inc.,  (1966).

-------
15.  M.H. Curley, Man-Made  Textile  Encyclopedia,  New  York:  Textile B<«ok
     Publishers,  p.  229,  (1959).

16.  J.W.S. Ik-arlr,  P.  Grosberg, and  S.  Hacker, Structural  Mechanics  of
     Kiln-fa, Yarns,  and  Fabrics, Vol.  1, Wiley-Intcrscience,  N.Y., (1969).

1.7.  Modern i'lastics  Encyclopedia,  45:  14A,  (1968).

18.  T.W. Spalte, J.K.  Hagan, and W.F.  Todd  "A Protective  Finish  for  Glass-
     Kibcr Fabrics",  Chcm.  Eng'g. Prog.  59:4, 54  (April, 1963).

I1).  J.A. Sal vatorr.  A  Study of. the Influence of  Selected Paramo tors  on
     tin- Work to Abrade  a Fabric Using  a Modified Touting Device,  M.S.
     Thesis, Lowell  Technological Inst., Lowell,  Mass.  (1968).

20.  E.H. Allard, Wet and Dry Abrasion  Resistance of  Substrates as
     Measured by the  Stoll  Abrader  with  the.  Flex  Element, K.  S. Thesis,
     Lowell Technological Inst., Lowell, Mass. (1969).

21.  S. Backer, "The  Relationship between the Structural Geometry  of  a
     Textile Fabric  and  its Physical Properties;  II.  The Mechanism of
     Fabric Abrasion, Textile Research  Jnl.. Q, 453  (1951).

22.  S. Backer and S.J.  Tenenhaus, "The. Relationship  between  the Structural
     Geometry of a Textile  Fabric and its Physical Properties; III. Textile
     Geometry and Abrasion  Resistance", Textile Research Jnl., 21, 635  (1951)

23.  W.J. Hamburger,  "Mechanics of Abrasion  of Textile Materials", Textile
     Research Jnl. .  15_,  169 (1945).

24.  E.R. Kaswell, Textile  Fibers, Yarns, and Fabrics, Reinhold Publ. Co.,
     N.Y., (1953).

25.  Lord Rayleigh, "The Sand Blast", Nature. 9_3, 188 (1914).

26.  F.A. Zenz and D.F.  Othmer, Fluidization and Fluid-Particle Systems,
     Reinhold Chem. Engg. Series, Reinhold Publ.  Co., N.Y., (1960).
                            ;                      :      i
27.  E.P. Parker, Abrasion Resistance of Substrates as Measured by the
     Sand Blast Method, MS. Thesis, Lowell Technological Inst., Lowell,
     Mass. (1967).

28.  E. Rabinowicz, Department of Mechanical Engineering, Massachusetts
     Inst. of Technology, Cambridge, Mass., Unpublished Notes, (1964).

29.  E. Rabinowicz, "Wear", Scientific American.  206:2.  127 (1962).

30.  W.D. Freeston, M.M. Platt, and M.M. Schoppe,  "Mechanics of Elastic
     Performance of Textile Materials; Part XVIT:  Stress-Strain Response
     of Fabrics under 2-Dimensionai Loading", Textile Research Jnl...
     37.: 11,  948 (1967).

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                               CHAPTER 5
               ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS

TABLE OF CONTENTS
5.1  DESCRIPTION OF PROCESS EFFLUENT TO BE FILTERED                 5-4
     5.1.1  Gas Flow                                                5-5
     5.1.2  Gas Properties                                          5-6
     5.1.3  Dust Flow                                               5-6
     5.1.4  Dust Properties                                         5-6
     5.1.5  Variability in Aerosol Composition                      5-7
     5.1.6  Emission Requirements                                   5-7
5.2  DUST COLLECTOR DESIGN                  •                        5-8
     5.2.1  Pressure Drop                                           5-8
     5.2.2  Air/Cloth Ratio                                         5-9
     5.2.3  Cleaning Mechanism and Fabric                           5-11
     5.2.4  Cloth Area                                              5-12
     5.2.5  Cloth Life                                              5-13
     5.2.6  Housing Configuration                                   5-13
            5.2.6.1  Number of Compartments                         5-13
            5.2.6.2  Fabric Arrangement                             5-14
            5.2.6.3  Compartment Structure                          5-14
     5.2.7  Capital Cost Estimates                                  5-15
5.3  FAN AND DUCTING DESIGN                                         5-16
     5.3.1  Ducting Layout                                          5-16
     5.3.2  Ducting Costs                                           5-17
     5.3.3  System Pressure Drop                                    5-18
     5.3.4  Fan Selection                                           5-19
     5.3.5  Minimizing Fan and Ducting Costs                        5-22
5.4  PERIPHERAL EQUIPMENT, INSTRUMENTS, AND CONTROLS                 5-22
     5.4.1  Particulate Pre-Conditioning Equipment                   5-22
     5.4.2  Gas Pre-Conditioning Equipment                          5-23
     5.4.3  Instrumentation                        '                 5-25

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TABLE OF CONTENTS (Continued)

     5.4.4  Control Equipment                                       5-25
     5.4.5  Dust Disposal Equipment                                 5-26
5.5  FINAL SYSTEM DESIGN                                            5-26
5.6  PROCUREMENT AND RESPONSIBILITY                                 5-28
5.7  REFERENCES FOR CHAPTER 5                                       5-30

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                               CHAPTER 5
              ENGINEERING DESIGN OF FABRIC FILTER SYSTEMS

      The design of a  fabric  filter system is  similar  to most  engineering
 design assignments in that there is no single rigorous approach  to the
 solution of the problem at hand.  Many parameters must be  identified  and
 their inter-relationships understood.   Placing an important variable  in
 proper perspective whether it  be,  for  example,  the  fabric  cost or  gas
 stream dew  point,  represents a key step in the design process.   The pri-
 mary  purpose  of this  Chapter is to highlight  the many factors that must
 be considered in designing the overall system.   Since many of these inter-
 relationships are  discussed in greater detail in other sections, a very
 general procedure  for selecting and/or designing the  system components
 is presented  here,  with the emphasis placed upon the  pitfalls and  expec-
 ted problem areas.
      The  overall performance of the  filter system will only be as  good
 as that  of  the  poorest  functioning  component.   Therefore,  only those
 well  experienced with fabric filter equipment  should  assume the  respon-
 sibility  for  system design, especially  of new equipment and new  appli-
 cations.  The replacement of existing  equipment  is somewhat easier  since
 one does  have some' practical guidelines.  In  general, the  first  decision
 that  one must make  is whether  to do the job in-house, or to contract  for
 a "turn-key"  job in which the  entire system is  provided by an outside
 firm.   In either case,  the best  available consultation should be sought
 and the purchaser  should retain  the responsibility for complete descrip-
 tion  of the process effluent to be  filtered.  Once the above groundwork
 is established, the experienced engineer is presented with a tractable
 design problem which  he can undertake  in the conventional iterative
manner and which involves the  following main design steps:
      1.  Define the effluent -- mass flow rate, dust properties,  gas
         properties,  and process variations with  time.
     2.  Approximate  the collector design
     3.  Approximate  the fan and ducting needs.
     4.  Based on  (2)  and (3)  select peripheral equipment,  instruments,
         and controls.


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     5.  Repeat  (2) and  (3)  to minimize  the  estimated  total  system cost.          ,-
     6.  Review  for alternative  effluent control methods;  budget;  and
         procure.                                                                 '
The  first  three  steps  can  be carried  out approximately without  the cost
data for individual system components by using  the  guidelines set  forth
in this handbook.  In  later  stages, specific apparatus must  be  evalu-
ated,  in conjunction with  equipment supplier support,  to arrive at real-
istic  estimates  of initial costs.                                                 '•'
     Operating,  maintenance  and  overhead costs  are  also essential  con-
siderations in preparing design  specifications. Even the accounting
practices  of the purchasing  company may  decide  the  relative  attractive-
ness of one system over another, e.g., the depreciation of a low first           ~'
cost,  higher maintenance system  versus that  of  a higher capital  cost in-
stallation.  Total annual  cost is probably the  best criterion to use (see        _,
Chapter 7).  Other criteria  of performance that are difficult to assign
a cost to, but which affect  the  overall  quality of  the system,  include
                                                                                 ..'•y
the  emission level of the equipment, its  reliability, simplicity  of
operation, etc.  Although  the latter  factors are difficult to quantity
the  design engineer must take them into  account.
5.1  DESCRIPTION OF PROCESS  EFFLUENT TO  BE FILTERED
     The definition of the problem, which is common to any engineering
design effort, constitutes the basis  for its effective solution.   In the
case of fabric filtration, the crux of the problem is  to define  as  com-
pletely as possible the process  effluent properties.  The minimum  infor-
mation required  is listed  in Table 5.1.  So  important are these  data             ~
that,  in almost  all cases, preliminary stack samplingishould be  perfor-
med and, in some instances,  tho  entire operation should be simulated on  -        _^
a bench or pilot scale with  experimental dust generation and filtration
facilities.
     If a process is characterized by variations in gas flow and/or gas
particulate composition, the equipment must operate at  peak loads with-         	,
out media plugging,  as well as at reduced flows  where condensation may
occur.   Any potential  future increase in effluent loading should be con-

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sidered in estimating design capacity, since an initial overdesign is

often less costly than subsequent reconstruction.

     It appears highly advisable to prepare a brief summation as out-

lined below in which the key properties of the effluent and the control

requirements are Listed.  Most fabric filter manufacturers submit a

similar questionnaire to potential customers prior to quoting on new

equipment.


                                TABLE 5.1

                  EFFLUENT AND FILTERING REQUIREMENTS
  1.  Process Effluent

      (a) Gas flow:
           Average:
           Maximum:
          Temperature:
          Water Content:
          Other Constituents:

      (b) Dust flow:
           Average:
           Maximum:
          Six.e distribution:
          Size: <1   <5  <20 -50 <80 <95
          %:   __   _  	  __  _  _
          Particle density:    »
          Bulk density:
          Est. range of K2:
          Other properties:

                         3.  Exhaus t
  2.   System

       (a) Preferred  location  -
          (in)  (out)side

       (b) Space  limitations,  if  any

       (c) Ambient weather
           Range:
           Temperature:
          Snow, water, wind loads

       (d) Weight requirements

<9Q  um (e) Cost considerations
(a)
(b)
(c)
(d)
Particulate level:
Gaseous req'ts:
Visibility req'ts
Preferred exhaust location:
(in) (out) side
Distance from collector:
       5.1.1  Gas Flow

              Determine the volume of gas emitted by the dust generating

  process prior to any corrective adjustments of temperature or dew point.

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Should  there  be  temporal  variations,  the  maximum,  minimum,  and  average  flows
should  be  estimated.   The cost  of the fabric  filter  system  will be  approx-
imately proportional  to the  volume of gas emitted  by the  process.   There-
fore, it is imperative that  one establish what  gas volumes  will obtain
when practicable  process  changes and  closure  or  all  unnecessary vents
of  leak points are  considered.   One should anticipate, and be  prepared  to
defend, process changes that  constitute minor  expenses  relative  to the
savings achieved  through  reduced gas  handling capacity.
     5.1.2  Gas Properties
            Determine  the temperature and pressure of  the carrier gas
stream  and estimate its approximate water content.   Identify  any abnormal
gaseous constituents such as acid vapors,  toxic  and/or corrosive fumes,
combustible or explosive  materials, condensibles,  etc.  Determine whether
composition and/or concentration vary significantly  with  time,  particu-
larly during  process start-up or shut-down operations.
     5.1.3  Dust Flow
            Determine  the weight  (mass) rate  of  dust or fume  generation
by  the  process, again making certain  that  the quantity has  been minimized
as much  as possible by process  adjustment.  Variable load conditions,
particularly  peak values,  must  be  considered  in  determining filter capa-
city if  overloading or plugging  is  to be  avoided.  Standard procedures
for measurement of effluent properties are described in several  test
manuals  (for  example,  see  reference 1).
     5.1.4  Dust Properties
            The better the system designer understands the properties of
the dust particles (see Chapter 2), the easier his task of designing the
filtration system becomes.  Minimal information for developing a functional
system, however,  must include a characterization of the particles in
terms of mean or median diameter and,if possible, the distribution of
sizes.   A knowledge of effective densities for discrete and bulk par-
tides  as well as an estimated permeability (K») for the dust  is also
useful   in establishing filtering conditions.

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             The  value  i>£  (K?), which  is  thought  to  reflc-ct  tlie  integrated
 effoctfl  of  particle dimensions;  e.g.,  length  to  diameter  ratio,  cohesive-
 ness between particles, dust  cake rigidity, and  spatial arrangement  of
 particles in the dust  cake, is a valuable design tool.  On  the  other
 hand,  individual measurement  of  the factors which presumably determine
 the K- are  not as yet  sufficiently understood to make their quantifica-
 tion possible.   Additional dust  properties for which no strict  quantita-
 tive definition  is currently made, but which  constitute important  in-
 puts for system  design are: the  softness or stickiness of the dust as a
 function of temperature or humidity; abrasiveness;  agglomerating charac-
 teristics;  "seeping" tendency; adhesion of the dust cake  to the  fabric.
     5.1.5   Variability in Aerosol Composition
             Allowance  should be made for the  fact that.even without  inten-
 tional modification of the gas temperature or the particle  size  proper-
 ties, there  may  be radiation cooling, moisture leakage into the gas
 stream, agglomeration  of  the smaller particles and/or sedimentation  of
 the larger  particles,  or  other changes during transit through the system.
 Therefore,  one must attempt to define the aerosol as it enters  the filter
 unit.
     5.1.6   Emission Requirements
             The  degree of particulate control which must be attained
with the overall  filter system should be determined early in the design
 process.  This will usually be stated as a maximum tolerable weight
emission rate rather than as a system efficiency.  The requirements may
also specify other factors that must be considered,  e.g.,  toxic gases,
odors,  or visibility of particulate or steam plumes.

             It must also  be decided whether the  filtered effluent can
 be discharged directly to the outside environment (with the attendant
 problems of heat loss, make-up ventilation and visible exhaust)  or re-
 leased within the building.  In  the latter case,  the problems of heat,
 materials toxicity, nuisance and/or hazard in the event of  filter rup-
 ture, and noise  take on added importance because  of confinement.

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 5.2  DUST COLLECTOR DESIGN
     The design  criteria applying  to  the  collector  itself will,  in  turn,
 affect  those  for other system components.  During the early design  phase,
 however, it may  not be possible  to predict the  interrelationships be-
 tween design  criteria and costs.   Therefore, until  realistic trade-offs
 can be  established among collector size,  fan requirements, and hoods and
 ducting, cost estimation within  approximately + 50  percent is acceptable.
 As a subsequent  aid to cost optimization, it may be helpful to construct
 a table by which one can estimate  how the collector cost will increase
 with respect  to  an increase, say 10 percent, in any significant  variable:

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Will add this much to the
     INITIAL COLLECTOR COST: ($)	
     ANNUAL SYSTEM COSTS: ($)   	

 The effects of  selected  variables on  the annual operating and maintenance
 costs may also  be estimated,as  Indicated, since the collector design has
 a  strong affect on  the annual costs (Chapter 7).
      5.2.1  Pressure Drop
             Estimate the average pressure differential across the filter
 media and deposited dust layer  during normal operating conditions.
 Although the value  selected may be somewhat arbitrary, several practical
 considerations,  e.g., collector strength under pressure or vacuum, fan
 power requirements, and dust cake mechanics, point to a few inches of
 water as the optimum pressure drop.  Lacking any better design guides,

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3  inches  of water  is acceptable as a  typical value.  On  the  other  hand,
the use of high velocity  filtration,  felted fabrics, or  the  presence  of
a  sticky  or low porosity  dust cake often require  that  optimum pressure
drop be of the order of 8 to  10 inches  of water.  Pressure loss  through
the collector alone, exclusive of the media drop, is usually small com-
pared to  that of the loaded fabric.  The pressure loss associated  with
the duct, hood and stack  system will probably be  in the  same range as
that for  the collector with fabric.
            The pressure  drop through the combined fabric and dust layer
can be treated as an independent variable in the sense that  the  design
engineer  can exercise considerable control over the cleaning mechanism.
By increasing the intensity and/or frequency of cleaning it  is possible
in some cases to reduce pressure drop to levels approaching  those  of  the
clean fabrics.  If this concept is carried too far, however, the collec-
tion efiiciency may be lowered, the fabric itself damaged, and the power
costs for driving the cleaning mechanism increased to prohibitive  levels.
Thus, the selection of the optimum operating pressure loss becomes a
matter of trade-offs based upon engineering judgement and field  trials.
Since the final operating pressure loss may not necessarily  conform to
the original design point, it is not practical to over-refine the  pre-
liminary estimates of average and peak pressure drops.

      5.2.2  Air/Cloth Ratio
                                                   2
             This ratio (CFM of air filtered per ft  of cloth filter area)
 is very important in determining collector performance.   The ratio (or
 its equivalent,  filtering velocity)  is discussed throughout this handbook
 in connection with dust  deposit  characteristics,  collector configurations,
 collector efficiency and  pressure drop and maintenance requirements,  to
 mention some  principle effects  related to air/cloth ratio.   Ratios in
 current use  range from less  than 1:1 to more  than 20:1.   The choice de-
 pends  on cleaning method  and  fabric,  and on characteristics  of the par-
 ticles .
            There is  no precisely determinable ratio  for a given applica-
 tion as  the choice also depends  on  estimates  and  trade-offs, such as


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 initial collector cost and recurring power costs.  Consequently, there is
 no precise analytical method for determining the best air/cloth ratio,
 although Chapters 2 and 6 indicate several approaches to the construction
 of analytical models.  Instead, it is customary to select ratios based
 on similar previous experience, that is, ratios that have been.proven on
 similar combinations of cleaning method, fabric, and dust.
            Each dust collector manufacturer has guidelines for the selec-
 tion of air/cloth ratio, based on his experience with a variety of appli-
 cations.  These guidelines vary from manufacturer to manufacturer,  largely
 as a result of differences in equipment.  Four such guidelines are sum-
marized in Appendix 5.1 for a shaking bag collector,  a glass cloth col-
 lector employing principally reverse flow plus flexural cleaning,  a
reverse jet collector, and a reverse pulse collector.  These are typical
 of the guidelines that manufacturers have made publicly   available.
Normally, these guidelines should enable estimates to within at least
25 percent of the optimum design ratio.   In unusual cases,and for  a more
exact estimate, consultation with an experienced manufacturer is advis-
able.  Frequently, in new applications  a pilot study  has been used  to
determine the best air/cloth ratio.   Such studies  can be misleading,
 however, unless they accurately model the proposed equipment and use a suit-
 able aerosol.
            There is an optimum air/cloth ratio for each set of filter
 system design parameters: i.e.  dust to be filtered, configuration of  filter
system, cleaning mechanism to be employed, fiber material to be utilized and
configuration of the fiber media.  However, for a given set of the above
parameters, the total system cost versus air/cloth ratio relationship is
rather flat near the optimum ratio.   Thus there is a tendency to minimize
initial costs by selecting an air/cloth  ratio toward  the high end  of  the
range.   On the other hand it is frequently reported that, with lengthy
operation at two different filtration velocities,  the lower of the  two
filtration velocities results in lower  operational costs.  Thus one must
carefully weigh the traditionally cited  advantages of lowered air/cloth
ratio (i.e. lower power costs,  decreased maintenance  and higher collection
efficiency) against the larger  initial  capital costs  associated with


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 increased  collector size and  the penalty for  its  space  occupancy.   Further,
 as  pointed out  earlier in Chapters  1  and 2, it  is anticipated  that  with
 advancing  fiber,  fabric media and cleaning  technology the  optimum
 range  of air/cloth ratios,  for any  set  of design  and operating parameters,
 will tend  to  increase.
     5.2.3 Cleaning Mechanism and  Fabric
            The selection of  the cleaning mechanism arid the  filter  fabric
 are best made together, since both  items ate  closely related (Section
 3.3).   For example,  felted  fabrics  are  almost exclusively  cleaned by
 pulse  or reverse-jet air, whereas most  woven  fabrics are cleaned by
 other  means.  Of  the relatively few choices of  fiber media (approximately
 eight,  see Section 4.2),  most will  be eliminated  for reasons such as
 poor temperature  and/or corrosion resistance  or excessive  cost.  Of the
 several cleaning  mechanisms used in filtration  systems,  only two or
 three  will meet the specific  requirements for a given installation: i.e.,
 high,  low  or moderate dust  loadings;  continuous or intermittent operation;
 ease of removal of dust from  the fabric;  small  floor area; minimal  pressure
 drop;  high efficiency;  etc.
            By  a  process  of elimination,  therefore, a review of past
 successful filtering performance will usually show that  only a few
 cleaning mechanism - fabric combinations  are  compatible  and  sufficiently
 attractive to warrant economic  evaluation.    The  time required for
 cleaning also determines  the  choice of  cleaning mechanism.   This time
 should  be  a small  fraction of  the time  required for dust deposition,
 since  otherwise too  large a fraction  of  the fabric will  be out of ser-
 vice for cleaning  at any given  time.  It  is common with  shake cleaning
 equipment,  for  example, to have  a cleaning-to-deposition time ratio of
 the order  of 0.1 or  less.  Applying this  criterion, having a ten compart-
     *It is not intended here to exclude or impugn the merits of a
hitherto untried combination.  The design approaches set forth in this
chapter, however, are based upon successful precedent or at least qual-
itative findings which lend some substance to cost estimates.  If it
is desired, or is absolutely necessary to consider some novel combination,
one should view the problem as a research and development effort and not
as a routine fabrication process.  The path that should be followed will  .
be more apparent once preliminary cost estimates have been prepared,
based upon conventional design approaches.

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 ment  baghouse  would  mean that  one  compartment  is  out  of service at all
 times.   Therefore,  the  choice  of cleaning  mechanism affects  system size
 as well  as  fabric  life,  maintenance,  etc.
      5.2.4   Cloth Area
             The  amount  of  fabric in actual use  at  a given  time  is  found
 by dividing  the  estimated  total  flow  entering  the  collector  by  the sel-
 ected air/cloth  ratio.   The  volume flow rate of the effluent entering
 the collector  will  not  necessarily be the  same  as  that  discharging from
 the generating process,  owing  to temperature changes, the  added volume
 of vaporized cooling water,  and  dilution air which may  be  added deli-
 berately for cooling purposes  or accidentally by air  leakage.   The latter
 factor may contribute to a significant  flow increase  in systems operating
 under large  negative pressures.  In filter applications  involving  a vary-
 ing flow, some judgement is  required  to decide  whether  to  size  the
 equipment for  the  peak  flow, the average flow,  or  for some intermediate
 point.   It  is  again necessary  to seek a compromise between the  increased
 cost  of  larger equipment and the increased cleaning cost,  and the  possible
 risk  of  fabric damage associated with short term,  high  pressure drops.
             Except  for  certain systems  that are operated intermittently,
 e.g., a  few  hours on line  followed by cleaning  only during down time,
 most  filtration  units will require reserve fabric  capacity to allow for
 off-line cleaning,  inspection  and maintenance.  Since it is  common prac-
 tice  to  isolate  temporarily defective  filter units  until it  is  convenient
 to replace them, additional reserve capacity may be required.   The  total
 fabric area  to be installed can  be estimated by multiplying  the  area to
 be in actual use at  any  time by  the term

                             (1 + TR+F)                        (5.1)

where (T^) is  the time ratio (cleaning  to  deposition) discussed  in  the
        K
 preceding section and (F) the fraction  of  the fabric area expected  to be
 out of service at any time due  to replacement,   inspection,  or mainten-
ance operations.   Judicious timing  of the above procedures  in relation
 to peak flow periods may, however,  reduce  the multiplying factor.

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             Costs for Hie installed fabric media as  a function of unit
 size may be estimated from data given in Section 7.3.
      5.2.5  Cloth Life
             Fabric deterioration often results  from  the  combined  assault
 of several factors (Chapter 8),  rather than from any single  effect such
 as thermal erosion,  mechanical  stress through repeated  flexure, chemical
 attack,  abrasion,  etc.  All possible  modes  of failure should be con-
 sidered  during  the preliminary  design phase.  Again,  previous experience,
 especially that relating  to similar fabric-cleaning-dust applications,
 may  be the best and  only  guide.   Extrapolating  from  experience, one might
 estimate  that the  reduction in  fiber  life  through  thermal erosion might
 double for a 20 F  rise  in temperature,  or  that  the mechanical attri-
 tion rate might double  when the  frequency  of cleaning is doubled.
             Generally,  it should  be possible to estimate fabric life
 within a  factor of two  in situations  where  no direct  experience can be
 cited.   If performance  data are  available,  estimated  reliabilities may be
 upgraded  to perhaps  + 20  percent, which  is  the  order  of dependability
 of the best pilot  plant data.  Having established  a  reasonable estimate
 of fabric life,  one  can then reach an annual cost  figure for fabric media.
      5.2.6  Housing  Configuration
             5.2.6.1  Number  of Compartments. -  Selecting the  number of
 separate  compartments for a fabric filter  installation is a  relatively
 easy  decision.   The  basic information required  is  the allowable variation
 in gas flow with respect  to  process or plant ventilation, the  availabi-
 lity  of sizes of commercial  units (compartments  or filter house modules)
 and the expected frequency  of maintenance.  In  small  collectors,  indivi-
 dual  compartments may contain as  little as  100  square feet of  fabric sur-
 face, although  collectors as large as approximately 50,000 CFM capacity
may also have only one compartment.  Multiple compartments of  almost any
 size may be chosen,  subject  to availability.  With the exception of re-
verse jet  and pulse  jet units, at lease one compartment will be out of
service during  the cleaning  cycle.  It may also be necessary to provide

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                                                                                            .1
 additional compartments  for  emergency,  extended maintenance,  or unexpec-
 ted  increases  in process  effluent.
             5.2.6.2   Fabric  Arrangement.  -  Fabric  filter  media  in panel,
 tube,  and  bag  form represent the  most  commonly used  industrial  configura-
 tions.  Although the  selection of filter  geometry  may  occassionally  be
 a matter of preference,  the  type  of fabric  and the cleaning method
 usually dictate  the configuration.   Panel filters, for example,  cannot
 be cleaned by  flexure, and reverse  jet  cleaning requires  open ended
 tubes.  Other  important  considerations  are  discussed in Section 3.3.
             Panels,  tubes,  or bags  are commercially  available over a
 broad range of dimensions as discussed in Section  3.1. Once  the require-
 ment for  fabric surface  per  compartment is  determined, it remains to
 decide what combination  of filter length, diameter,  and spacing will be
 the  least  expensive.   This  is too often interpreted  as a  maximum filter
 packing and a  rather  compact filter housing,  i.e., a container  with  no
 dimension  much greater than  twice the  smallest dimension.  Closely packed
 filter elements  tend  to wear against one  another,  and  make inspection and
 maintenance difficult (Section 8.3).  Compact  housings incur  a  greater
 cost  for plant floor  space than taller  units.   Recently,  one  manufacturer
 has  introduced super-long bags  (—60 ft), partly to  conserve  floor
      (3)
 space   .   Compact housings  may,  however, be more  suitable in outside
 locations  where  land  use  costs  less  than  the erection  of  tall structures.
             Other considerations  in  selecting  the  fabric configuration  are
 downward vs. upward flow  and  inside  vs. outside  filtering.  The ease and
 the  frequency  of fabric replacement, and  the uniformity of flow distri-
 bution and  associated danger ;of condensation in  stagnant air  pockets are
 also considerations.
             5.2.6.3  Compartment Structure.  - Although many kinds  of
 fabric collectors can, in principle, be operated either under negative
 or positive  pressure,  the larger units  are often custom designed for one
condition or the other to minimize costs.   Least expensive is  the instal-
 lation needing no housing at all,  in which the particulate is  collected
on the interior surfaces  of a positive  pressure bag system. The danger
of cooling  below the  dew  point is  an important consideration,  particu-


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 larly in a suction housing where infiltration leakage  often occurs  unless
 a more expensive gas-tight design is  adopted.   Thermal excursions of  the
 collector during startup and shutdown can effect  the sealing character-
 istics of critical gasketed connections,  depending  on  the  structural
 materials.   Interconnection of  compartments  via ducting or hoppers  and
 their isolation, especially during the cleaning cycle,  are important
 considerations (Section 3.4).   The generalized  rules for designing  the
 compartment structure  are not well defined.   Construction  materials are
 likewise not generally prescribed. The most  common housing material  is
 steel, as discussed in Chapter  3.   The choice of  material  depends on  the
 nature of the dust and gas mixture and their  flammability,  corrosiveness
 etc.   Lacking a more specific guideline,  one would  do well to follow  pre-
 cedent in selecting materials of construction,  metal gages and dimensions.
      5.2.7   Capital Cost Estimates
             A detailed discussion of  initial and  annual cost  factors  is
 presented in Chapter 7.   The cost  of  the  baghouse,  including  its amorti-
 zation schedule  and plant overhead costs,should  now be  estimated based
 upon  the tentative equipment selections.   The designer  should now devote
 primary attention to initial installation costs with emphasis upon how
 these initial costs will contribute to the total  annual  cost  of the fabric
 filter system operation,  in accordance with specific company costing  pro-
 cedures.  At this  time,  sufficient  information  has  been acquired to  com-
 plete the working  estimate guide at the beginning of Section 5.2.
             Some of  the  design decisions  cited  above may have been based
upon  very limited data.  Consequently, one should review the compilation of
preliminary  estimates,  particularly those reflecting choice of fabric,
cleaning mechanism, and average pressure drop through the  filter media.
The objective should be to describe a  collector that will be within ~10
percent costwise of the optimum design.   It is not advisable, however, to
finalize the design until other components of the  overall system have
been  selected and evaluated with respect to their  possible influence on
collector design.

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 5.3   FAN AND DUCTING DESIGN
      The design and selection of filter system components, other than the
 collector discussed previously,  also require some iterative
 approaches during the planning phase.  Fortunately,  the alternatives are
 fewer, and in most cases,  the design features are simpler.   The main
 trade-off area is between the pressure drop through  the ducting and stack           i
 systems  (an inverse function of  duct diameter)  and the fan size and cost.          —'
 In many  cases, the pressure drop through the fabric  filter unit alone
 will  largely determine fan size.                                                   _j
      5.3.1  Ducting Layout                                                          ,
             The positioning and  amount of ducting are  determined mainly
 by the expected locations  of effluent source,  collector,  fan,  and venting
 point.   Ordinarily,  minimum duct lengths and diameters will be used un-            —'
 less  the ducting is  also  intended to provide radiant cooling of the gases
 prior  to filtration,  or to function as settlement chambers to remove the           _J
 larger particles.   The first step in design is  to establish the locations
 and lengths  of main,  branch and  riser ducts showing types  of junctions               '
 and bends.
             The distance  from the dust collector  to both the dust  source            !
 and the  point  of discharge  of the filtered  effluent, must  be considered
 with regard  to duct  costs  and space  availability.  The  cost of plant floor           i
 space may be a reason  for  locating  the collector  outside the plant,  other
 possible reasons being ceiling limitations  and  safety.  On the other hand,           i
 the outside  collector must  usually be  protected against weather and insu-          ~
 lated against  temperature changes.  Also, not to  be overlooked in  locat-
 ing the collector are space  clearances  for,  and accessibility to; the nee-           _J
essary ducting;  crane or hoist requirements, especially for fan repair;
 the weight of  the collector;  the  feasibility of removal and disposal of
 the collected  dust; and plant  insurance  costs.
            In  laying out the duct plan, the locations of  cleanout  doors,            '
                                                                                    •••*«••
dampers,  dilution valves and any  auxiliary  cleaning equipment,  i.e., dry
inertial collectors, scrubbers, or cooling  towers should be  taken into               '

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 consideration.   Whenever possible, all ducting should be located so as
 to be easily installed and maintained.
             It  is important to use as little horizontal ducting as  pos-
 sible and to select diameters which allow for complete transport of the
 dust load to the collector.  Sharp bends and abrupt changes in dimensions
 should also be  avoided to prevent dust accumulation.  Since the above
 approach suggests high velocity systems, it should be recognized that
 power costs also increase with higher duct velocities and that abrasive
 dusts can cause rapid duct erosion, particularly in bends.   Useful  velo-
 city guidelines have been set forth in Table 2.42(b) and in various engineer-
 ing manuals to  aid in this aspect of system design.
             Duct sizes through the system are estimated by  the recommen-
 ded transport velocities and the calculated volume flow rate through each
 section.   The latter value is computed from the  source flow,  corrected
 for any added volume attributable to dilution air,  and adjusted to  the
 temperature and pressure conditions at the location of interest in  the
 duct system.  In cases of fluctuating flow,  it is  recommended that  the
 ducting be sized for some intermediate flow between average and peak
 loads.
            The ducting  sections  located downstream of the  collector may
 be  sized  somewhat larger,  owing  to absence of settlable  particulate  in
 the  gas  stream.   The final dimensions  should  represent a balance between
 the  increased cost  for larger duct size  and  the  lower pressure  drop
 (power  cost).
     5.3.2  Ducting  Costs
            Standard  steel  ductwork  is normally used, unless  there is
danger  of  corrosion,  abrasion, adhesion, high  temperature,  thermal dis-
 tortion, need of  insulation,  or unusual  surges in pressure  or in tempera-
ture.  Aluminum, glass, plywood, etc., can sometimes be used as substitutes
for steel.  While the  duct material may be suggested by  the collector man-
ufacturer, the ducting is normally made up and installed  locally.
            Cost estimates  or quotations are readily obtained as soon as
diameter,  length, and guage specifications are available  (Section 7.1.3).


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 One  must  consider the coats of dampers,  ports,  Tee sections,  elbows,
 flanges  or other special  components,  keeping in mind the fact that fit-
 ting or  adapting the ducting to the above parts may involve more cost
 than that for assembling  straight vertical or horizontal runs.
       5.3.3  System Pressure Drop
              The total system pressure drop is  that of the combined  losses
 in the duct and  the fabric  filter unit.   Ducting losses vary  approximately
 as the square of the gas  velocity and can be readily calculated  by standard
 formulas  or from tabular  or graphical data (see Section 2.5).  Pre-
 sure drops for non-linear shapes  (elbows,  Tee's,  reducers,  etc.)  are
 usually expressed in equivalent length of straight duct of the same  dia-
 meter.  In conventional practice,  one traces the  largest branch  (usually
 from the  most remote source of dust generation),  noting the temperature
 and  gas flow through each succeeding  section up to the collector inlet.
 A similar procedure is followed from  the  collector exit to the point
 where  the filtered  effluent is discharged  to the  inside or outside atmos-
 phere.  Ordinarily,  the total  pressure drop associated with the  ducting
 alone  will be in the range  of  3 to  6  inches of  water,  although other
 values may apply in some  circumstances.
              One then  adds  to  the estimated pressure drop  through  the
 ductwork,  the collector pressure drop  and  that  of  any  other component of the
 system, e.g.,  a  centrifugal  collector.  The  result will  be  the net pre-
 sure to be  supplied  by the  fan  when ambient  pressures  are the same at
 system inlet  and  outlet.  Should the ambient pressure  at the system  in-
 let  exceed  that  at  the outlet,  the net pressure requirement for the  fan  ,
 is decreased  by  this difference.*  The opposite applies  if  the pressure
gradient  is reversed.
*The kinetic energy or velocity pressure retained by the air leaving
the fan may be deducted from the fan static pressure requirement, except
when the air is charged to a large stagnant space, e.g., exhausting the
fan outdoors.  The kinetic deduction is frequently overlooked, however,
on the justification that by omitting it, a safety factor is added.  See
the excellent discussion on fan selection in "Fan Engineering" by the
Buffalo Forge Company(6).

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     5.3.4  Fan Selection

            Of the types of fans commonly used in industrial ventilation,

centrifugal fans are most often chosen for the primary flow in fabric

filtration systems.  These are described below and in Figure 5.1:


           1. "Forward-curved blade types;  A multi-bladed, "Squirrel Cage"
               wheel in which the leading edges of the fan blades curve toward
               the direction of rotation.  These fans have low space require-
               ments, low tip speeds and are quiet in operation.  They are
               usually used against low to moderate static pressures such as
               encountered in heating and air conditioning work.  Not recom-
               mended for dusts or fumes that would adhere to the short curved
               blades, causing unbalance and making cleaning difficult.

           2.  Straight or Radial-blade (paddle wheel,  long shaving wheel):
               The "workhorse" for most exhaust system applications, they are
               used for systems handling materials likely to clog the fan
               wheel as the name indicates.  Such fans  usually have a medium
               tip speed and a medium noise factor and are used for buffing
               exhaust,  woodworking exhaust or for applications where a heavy
               dust load passes through the fan.

           3.  Backward blade type;   The type in which  the fan blades are in-
               clined in a direction opposite to the fan rotation.  This type
               usually has a high tip speed,  provides high fan efficiency and
               has non-overloading characteristics.   Except in case of direct
               driven arrangements,  non-over-loading feature is over emphasized
               in exhaust ventilation work as the exhaust system acts as a
               load limiting orifice to make  overloading  of any exhauster motor
               from variations  in system conditions  improbable.  Blade shape is
               conducive to buildup  of material and  fans  in this  group should
               be used only on  clean air containing  no  condensible fumes or
               vapors."(7)

         The  first consideration  in  fan selection is which of  the numer-

ous fan types  is best suited to the estimated air flow and pressure  re-

quirements.  Reference  to manufacturers' data depicting  the performance
of various types over a range of speeds and delivery volumes will estab-

lish this.  Another consideration in selecting the fan type is whether

it is to be used on the clean side of the filter or on the dirty side

where maintenance will probably be higher.  (See also the discussion of
types of collector construction, Section 5.2.6.)

         Although several fan sizes of a specific type  can meet the de-
sired pressure-volume requirement by changing the speed,  only one size

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  \	
BACKWARD CURVED BLADES
i
I
             VOLUME - CFM
STRAIGHT OR RADIAL BLADES
             VOLUME-
FORWARD CURVED  BLADES
             VOLUME-CFM
         Figure  5.1.  Some Fabric Filter System Centrifugal

                     Fans.  (From Ref. 7).

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will  perform at hits  peak  efficiency.  Therefore, there may be no parti-
cular economy in  selecting a smaller size unit of lower cost, since opera-
tion  at  a higher  speed will lend  to proportiately greater power consumption.
On  the other hand,  a  slight oversizing of the fan will result in lower
fan speeds  and probably  less maintenance without a serious drop in ef-
fiency.   Extra fan capacity is a safety factor against future process
expansion or unexpected  peak flow requirements which the fan must be capable
of  handling.   Another consideration in fan selection is the reliability re-
quirement:   Can a fan outage be tolerated, or should more than one fan be
installed?
          The process  of fan selection  from  the  several trial  sizes  in-
 volves the  following  steps.  First, choose  a maximum rated  capacity,
 roughly  20  percent  more than  the  required capacity.  Determine  from  the
 table or fan curve  the  power and  speed requirements  at the  required  pres-
               *
 sure  and flow, and from these data, estimate the installed fan and  motor
 costs and maintenance costs.   Using an acceptable amortization  rate, per-
haps  ten years and  the  local cost of power,  compute  the total annual cost
of  operating this particular fan  size.  Note that the  cost  of the drive,
mountings,  and motor  may easily double the cost of the fan alone.
         Note that  one  is  limited to available motor speeds if a direct
drive is to be used.  Direct drives consume  less power, need  less main-
tenance, and present  no  belt slippage problems.  On  the other hand, many
small and moderate  sized fans  are belt driven, both  for reasons of initial
cost  and to permit  future  speed adjustments.
         This initial cost estimate for fan  and associated equipment is
probably adequate for the  early phase of the system design process.  To
minimize fan costs, however, the  above computation should be repeated for
slightly larger or  smaller fans until a minimum cost is reached.  It may
become apparent before undertaking this step that a change in fan style
*Fan test data is usually based on standard temperature and pressure,
i.e., standard air density.  If the fan is to handle non-standard
density gas, different curves will be used.

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 or size should be considered, due to revisions in collector design pres-
 sure drop and in ducting requirements.  Having determined the fan arrange-
 ment of minimum cost, again consider whether there is a sufficient mar-
 gin of capacity and of reliability.
      5.3.5  Minimizing Fan and Ducting Costs
             Preliminary estimates of ducting requirements and costs should
 be reviewed.  It may be determined, for example,  that a small increase in
 duct diameter,  say 5 percent, will allow use of a smaller fan with a net
 lowering of overall cost; that is, the lower operating and capital costs
 for the fan assembly overrides the increased initial costs for the duct-
 ing.   In this example,  the decrease in pressure drop through the duct
 system would be of the  order  of 20 percent  based  upon the inverse fourth-
 power relationship between pressure loss and duct diameter.   Having es-
 tablished  a revised value for duct pressure drop,  new criteria are avail-
 able  for resizing the fan and motor drive.   If  the combined  cost of the
 duct  and fan system decreases appreciably as a  result of  this  reassess-
 ment,  the  process should  be repeated until  a minimum point is  identified.
             It  also is  recommended that  the above  cost  analysis  be exam-
 ined  in terms of the  key  variables affecting the  initial  and annual  costs
 for the combined fan-ducting  system.  Again,  a  tabular  array of  the  system
 cost  increase,as a  function of change  in the major design variables,con-
 stitutes a  useful working  tool.
 5.4   PERIPHERAL  EQUIPMENT, INSTRUMENTS,  AND CONTROLS
      In earlier  sections  of this chapter, a  preliminary design approach
has been outlined for the main components of  the fabric filter system.
As  the  few  remaining system parts  are  less  dependent upon  the primary
system  dimensions,  they should  now be selected so that  the overall fabric
filter  system design can be finalized (Section 5.5).
      5.4.1  Particulate Pre-Conditioning Equipment
             The pre-filter particulate treatment process, if any, may be
 designed for:  (a) the enhancement of particulate  agglomeration,  thereby
 precipitating out of the  gas  flow some material prior to  deposition on
 the fabric and/or improving the resistance  properties of  the dust cake,


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 (b)  the prc-separation of coarse particles by gravity sedimentation or
 inertial techniques,  or (c)  the injection of additives to produce a dust
 cake sufficiently dense to provide high efficiency collection of low con-
 centrations of fine material,  or alternatively additives to produce a
 more porous dust cake with lower resistance.
             Use of any of the  above techniques may require reappraisal
 of preliminary cost estimates  for the fabric filter system for the fol-
 lowing  reasons (see Section  2.4.8 for details):
             (a)  Change of deposit permeability and hence cycle time,
                  due  to alteration of particle size distribution.
             (b)  Change of inlet loading.
             (c)  Change of cake removability or residual permeability.
             (d)  Change in properties of the collected material such as
                  bulk density  and total volume.
             (e)  Changes  in  gas stream properties.
 One  must balance the  economic  advantages of changes in dust  properties
 against  the  costs  of  installing and operating  the  pre-conditioning equip-
 ment.  Of course,  if  the  treatment process  is  used  to elevate  collection
 efficiency  to  levels  satisfying pollution  control  regulations,  the equip-
 ment costs  become  a necessary  investment.
      5.4.2   Gas Pre-Conditioning  Equipment
             Hot process effluents  are usually  partly  cooled  before  fil-
 tering to reduce the  flow volume  which,  in  turn, decreases the  filter
 fabric area  requirement.   Lowering  the  gas  temperature  toward  the  dew
 point often  extends fabric life and may  permit the  use  of  less  expensive
 or more durable  materials.   Prior  to  entering  the collector, the process
effluent may also be  altered through  combustion, absorption, chemical
reactions or humidification, any  of which may  influence  the  design  of
other fabric filter system components.  Temperature conditioning, a com-
mon  treatment,  is usually accomplished by the methods described in
Table 5.2.   In  other  situations,  it may prove more  economical to filter
the gas essentially at process effluent temperature,or even  at  increased
 temperature, provided that the  temperatures do not  exceed the upper  limit
 for  the fabric.


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                               Table 5.2
                  METHODS OF TEMPERATURE CONDITIONING
 Radiatton-Conyoction Coo1ing  (long, uninsulated inlet ducts)

         Advantages:   Lowest  flow volume of the three methods
                       Smoothing or damping of flow, temperature, pressure
                          or other surges or peaks in the process effluent
                          stream
                       Saving  of heat (building space heating).
      Disadvantages:
Cost  of  extensive  ducting
Space requirements of  ducting
Possibility  of  duct plugging by sedimentation.
 Evaporation (by water injection well ahead of the filter)

         Advantages:
      Disadvantages:
Low  installation cost, even with  automatic controls
Capability  of  close and rapid control of  temperature
Capability  of  partial dust removal and/or gas control
   via  scrubbing.

Danger  of incomplete evaporation  and consequent
   wetting  of  the filter or chemical attack of the
   fabric or filter
Increased danger of exceeding the dewpoint and
   increased possiblity of chemical attack
Increased steam plume visibility, a hazard near highways
Possible increase in volume filtered.
 Dilution (by adding ambient air to the process effluent stream)
         Advantages:
      Disadvantages:
Lowest installation cost, especially at very high
   initial temperatures.

Substantial increase in total filtering volume
Automatic control of both temperature and filtering
   velocity is not possible
Uncontrollable intake of ambient moisture, dust, etc
   without prior conditioning of the dilution air.
* See also Ref. 8.

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             Somewhat related to the three cooling methods outlined in
 Table 5.2  are the special systems for emergency protection of the equip-
 ment.  These include:C02 devices that automatically release high-pressure
 gas to cool the effluent, should its temperature exceed a critical pre-
 set limit; fail-safe dampers held in position by set-point melting links;
 burst-out  panels to prevent duct or collector rupture  due to pressure or
 vacuum excursions;  and various types of alarm systems.  Such equipment
 can be reliable and relatively inexpensive compared to the cost  of re-
 placing a  collector system.  Rapid-cool equipment can  also serve in shor-
 tening the time required to take equipment out of service.  Heating equip-
 ment may facilitate start-ups by preventing condensation.
      5.4.3  Instrumentation
             Instrumentation which provides a continuous record and/or
 direct display  of the  important  factors describing the overall filtration
 process is an essential  part of  a good  fabric filter system.  The cost is
 relatively low  (~ a few  percent  of the  total system investment)  and the
 judicious  selection of instrument type  and function may permit the use of
 less expensive  fabrics or materials  of  construction, e.g.,  ordinary sheet
 steel rather  than a special alloy.   A discussion of recommended  instru-
 mentation  is  presented in Section 7.1.5.   The  measurement  of  those system
 characteristics  that affect its  overall function,  e.g.,  temperatures,
 pressures  (absolute and  differential),  primary gas  and  dilution  air  flow
 rates,  water  rates,  dew  point  levels, and  possibly  continuous monitoring
 of stack oftluont concentrations, all enhance  the probability of succes-
 ful  system  operation.  Any  or all these devices  can be  designed  to alert
 the  operator  through appropriate warning systems  (horn, alarm bell,  flash-
 ing  lights) when abnormal conditions arise.
     5.4.4  Control Equipment
            Solenoid valves, damper actuators, timers,  etc., associated
xtfith the filter need not be considered here, since they are usually  in-
cluded  in  the basic  filter  price.  Auxiliary blowers such as those for
reverse cleaning may or may not be included and air compressors for pulse

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 cleaning arc seldom included as part of the filtration unit.   Special
 ducting  dampers  that must open and close rapidly or provide high leak
 tight  integrity  (^  0.1 percent leakage  under adverse temperature,  pres-
 sure and corrosion  conditions) should be specified in the  system design.
     5.4.5   Dust Disposal Equipment
             The  physical  and/or chemical properties of the dust  collected
 in  the hopper require appraisal at this  point.   Although the  disposal tech-
 nique  is usually independent of the primary system operation,  some  pre-
 treatment of the process  effluent  may contribute significantly to  the
 ease of  handling of the collected  dust.   For example,  the  injection of
 an  inert mineral dust might  reduce the  tackiness of some resinous mater-
 ials to  the  point where bridging problems  in hoppers  or plugging in screw
 conveyors could  be  reduced,  and the overall system economics  improved.
             The  final selection of hopper  design,  hopper outlet valving,
 vibration equipment  (if needed), screw conveyors,  chutes,  pneumatic  con-
 veyors,  and  other ancillary  equipment will  depend  upon several factors.
 These  may include the angle  of repose of  the  dust,  its bulk density  and
 the volume and/or weight  to  be  handled.  Another  factor is the tendency
 of  the dust  to flow  freely or  to bridge, agglomerate or become sticky
 under mechanical stresses  (stirring, rapid motion,  vibration) or under
 changes  in temperature or humidity.  One must consider whether the hopper
 is  to  operate under  positive or negative pressure, and also what labor
 requirements are necessary for dust handling, i.e., whether the dust is
 to be reprocessed, sold or dumped as waste.  Several types  of disposal
equipment are discussed in Section 3.4.2.
5.5  FINAL SYSTEM DESIGN
     Having established preliminary design features for the individual
                                      V
parts of  the system,  it is now necessary to integrate these into a com-
patible system.  This process calls for  more engineering judgement  and
experience than that required for the preliminary design phase.  In addi-
tion,  the final planning should be geared to management's  philosophy on
the relative importance of initial costs and total annual  costs.

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      Summary guideline  tables  outlined  in previous  sections  should  be  re-
 examined  to determine whether  overall costs  can be  lowered by  adjustments
 in any  of the  principal variables.  Should any  appreciable differences  in
 costing be  observed, design  optimization  procedures should be  repeated  at
 this  time.   Since  the basic  types of equipment  required are  now apparent,
 appropriate  manufacturers' equipment lines should be reviewed  for prepar-
 ation of  specifications.  It is highly  advisable to consider standard
 items when  possible.  Quality, expected service life, ease of  repair and
 delivery  times, as well as initial cost will be evaluated.   Where possible,
 preliminary  cost estimates or  final quotations  may now be obtained.
      Before  fixing the design  specifications, a final search should be made for
unique  or extenuating factors  that could  possibly influence  performance
 or total costs, such as safety features,  noise, general plant  ventilation,
 insurance, relocation of system components, tax rebates, etc.  One should
also re-examine future needs with respect to plant expansion or process
changes.  If there remain some questionable design areas,or  if certain
design aspects are controversial, it may be advantageous to  seek the pro-
 fessional opinion of some competent outside person(s) or agency.  Other-
wise, final design and cost estimates can be submitted for approval and
procurement.

      No further design or cost changes  of significance are expected.
 However,  it  may be possible  to trim the overall costs a few  percent by
                               A;.
 the  following  analytical procedure.  Let  the total annual cost (C) be
 expressed as the sum of: power  costs (P);  all costs related to  fabric life
 including materials and labor  (F); annual distributed initial  costs in-
 cluding interest, taxes and any other item that is directly  related to
 initial installed cost (I); and fixed costs not considered elsewhere (X)

                          C=P+F+I+X                     (5.2)

Select a key design parameter, such as  filter velocity (V),   and introduce
it into each of the terms of this equation where applicable.    In each
term, assign to the parameter its appropriate exponent (Table 5.4)."

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 Power,  for example, is approximately proportional to V .  This means that
 if a slightly different velocity (V) were chosen,  the revised estimate
                                                          2
 of power cost (P1) would be approximated by P' = P (V'/V) .   Substitution
 of the  design parameter into the other terms of the equation yields the
 following expression for a revised total cost (C1)

              C-  = P(V'/V)2 + F(V'/V)~°-5 + ICV/V)"1 + X         (5.3)
 That is,  the result of changing from the design velocity previously
 selected  to a new value can be estimated by the expanded equation.   Cost
 factors P,  F, I,  and X have already been determined by careful design pro-
 cedures.   To estimate the effect of a 5 percent increase in filter  velo-
 city,  the ratio (V'/V = 1.05)  is used,  and  (C1) is  readily evaluated.
      By few repetitions of this process,  the velocity  that will give the
 minimum total cost may be estimated.   If this new estimate is  appreciably
 different from the original design value, a re-examination of  the system
 design is called  for.
      By this process, the effect of minor changes in any design parameter
 may  be estimated.   For parameters  having both positive  and  negative
 exponents  in Table 5.3 (filter  velocity, Ave.  Cloth Ap,  etc.)  an optimum
 design value may  be  estimated.   Note, however,  that  the  parameter exponents
 listed in Table 5.3  should  be  accepted  as estimates  only,  owing to  the
 limited field  and  laboratory data  available  from which  they derive.  With
 respect to  trends,  i.e.,  direct  or  inverse  relationships,  and weak or strong
 dependency  of  a given cost  category,  the exponents  cited  in Table 5.3 may
 be interpreted as  tentative typical values.  These  exponents will not
 apply  uniformly to all  systems.  One  is also not constrained to  use  the
 precise grouping of  cost  factors cited here.
 5.6  PROCUREMENT AND RESPONSIBILITY
     The  final responsibility for the design,  fabrication, procurement
 of materials, approval  of specifications, performance testing,  mainten-
 ance, and the overall performance of the fabric filter system should fall
within one department and preferably with one individual of decision


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                                Table 5.3
                  APPROXIMATE OPTIMIZING EXPONENTS OF COSTS'
PARAMETER
Filter Velocity
Temperature
Particle Size
Ave. CLoth Ap
Cleaning Intensity
Flow Volume
Loading
Humidity
POWER
+2
+1
--
+1
-1
+1
+ .5
+--
CLOTH-RELATED
-.5
-1-2.75
-2
--
+1
+1
+1
+--
INITIAL
-1
+1.75
-1
-1
-1
+1
+.5
- —
 Note:   Fixed costs (X)  are invariant by definition.
 *
  May vary from above,  values  according to circumstances.
 insufficient information.
Blanks indicate
 making capability.   Although  several  departments  or  divisions  within a
 company may  contribute  their  services,  any  joint  sharing  of  responsibi-
 lity  should  be  avoided  to minimize  oversights,  schedule conflicts,  or
 other committee type problems.
      Following  receipt  of quotations  for system components and installa-
 tion  costs,  one should  verify compliance with specifications,  including
 the warranty aspects  of all components.  This is  equally  important when
 a  turn-key package has  been purchased.  Although  there will  be occasions
 when  completely defensible design changes may be  proposed by equipment
 suppliers, one  should not expect any extreme deviations in dollars or
 design with  a carefully executed program.
     Fixing  the  responsibility for on-line  fabric filter system perfor-
mance cannot be  overstressed, especially with smaller installations.
Frequently, a manufacturer is blamed for faulty equipment operation,
whereas the problems have actually arisen because of user abuse and neg-
 lect.   This has  led to dissatisfaction and experimentation in some plants,

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sometimes  leading  to  the  purchase  of  several  fabric  ftlt.er  designs  to

handle one  type  of process effluent.  This makes  maintenance  routines

unnecessarily complicated.  When dissatisfaction  has  resulted in purchase

from several different manufacturers, one cannot  expect  to  receive  inter-

ested customer service from any of  them.

     As a  final  point, few if any manufacturers will  guarantee  the  per-

formance of fabric filter equipment,  because  its  function is  highly sen-

sitive to  process effluent changes  as well as to  the  quality  of the in-

stallation job and subsequent maintenance.  All of these are  difficult
to document.  Perhaps one of the best procurement policies is to request

the manufacturer to provide and guarantee installation and startup as

part of a package.  He should also provide a set of guidelines  for

operational routine and maintenance.at no cost,  and he may provide

training for the men who will use the new system.

5.7  REFERENCES FOR CHAPTER 5

     1.   American Society of Mechanical Engineers, Determining  the Properties
         of Fine Particulate Matter: Power Test  Codes. PTC-28, New York (1965),

     2.   The Buffalo Forge Company, Guide to Shaking Bag Aeroturn Selection.
         Buffalo, New York,  September 1968.

     3.   The Fuller Company,  Collecting Hot  Dust and Noxious Fumes with
         DraccoR Glass-Cloth Dust Collectors,  Bulletin DCS-IB (July 1967).

     4.   R. Frey and  T.  Reinauer,  New Filter Rate  Guide,  Air Engineering.
         (30 April 1964).

     5.   F. A.  Zenz and  D.  F.  Othmer,  Fluidization and Fluid-Particle
         Systems. Reinhold Pub.  Co.  (I960).

     6.   The Buffalo  Forge Company,  Fan  Engineering.  6th  Edn., Buffalo,  NY,
         (1961).

     7.   American Conference  of  Governmental  Industrial Hygienists,  Indus-
         t;rial Ventilation,  10th Edn., Lansing,  Michigan  (1968).

     8.   P.  W. Spaite, D.  G.  Stephan and  A.  H. Rose,  High Temperature Fabric.
         Filtration of Industrial  Gases,  J.A.P.C.A.  11,  (5)  243, (May 1901).

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                              CHAPTER 6
                        FABRIC FILTER PERFORMANCE

6.1  INTRODUCTION                                                  6'3
6.2  LABORATORY PERFORMANCE OF CLEANED EQUIPMENT                   f-9
     6.2.1  Bench Scale Performance                                6-9
            d.2.1.1  Pressure Drop - Time Relationship             6-9
            6.2.1.2  Dust Collection Efficiency                    6-10
     6.2.2  Single Bag Performance, Pilot Scale Tests              6-13
            6.2.2.1  Pressure Drop - Time Relationship:             6-13
                     Basic Concepts                                6-13
                     Liquid Filtration Analogy                     6-17
            6.2.2.2  Dust Profiles                                 6-20
            6.2.2.3  Analysis of Mechanical Shaking                6-30
                     Residual Drag                                 6-33
                     Filter Capacity                               6-38
            6.2.2.4  Effects of Fabric Structure                   6-49
            6.2.2.5  Effects of Humidity                           6-62
            6.2.2.6  Effects of Velocity                           6-6*
                     Hopper Fallout                                6-6V
                     Particle Size Stratification                  6-70
                     Deposit Consolidation                         6-72
                     Particle Penetration                          6-74
     6.2.3  Single Compartment Performance                         6-75
            6.2.3.1  Shake-Type Collector                          6-76
                     Light Dust Loading                            6-76
                     Filter Aid                                    6-78
                     Periodic Shaking                              6-80
                     Normal Dust Loading                           6-81
            6.2.3.2  Other Single Compartment  Studies               6-83
                     Hopper Fallout                                6-83
                     Particle Size Stratification                  6-84
                     Deposit Consolidation                         6-84

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6.3  LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT             6-85
     6.3.1  Basic Pressure Drop Equations                             6-85
     6.3.2  Performance of a Multicompartment Collector               6-90
            6.3.2.1  Studies with Light Dust Loadings                 6-91
                     Basic Media Performance                          6-91
                     Further Studies of Filter Aids                   6-92
            6.3.2.2  Studies with Heavy Dust Loadings                 6-94
     '               Frequency and Number of Raps                     6-94
                     Reverse Flow Air                                 6-96
                     Inlet Dust Loading                        '       6-96
                     Comparison of 5 Fabrics                          6-9'J
6.4  LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLEC-     6-99
     TORS
     6.4.1  Reverse-Jet Filter (Mersey Type)                          6-99
     6.4.2  Pulse-Jet Collector                                       6-101
6.5  FIELD PERFORMANCE                                                6-101
6.6  REFERENCES                                                       6-105

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                               CHAPTER  6
                       FABRIC  FILTER  PERFORMANCE
 6.1   INTRODUCTION
      The  determination  of  fabric  filter performance includes the specifi-
 cation  of pressure  drop, collection efficiency, component life, and costs
 during  operation, in  terms of dust, gas, and fabric parameters.  Pressure
 drop  with a given dust, gas, and  fabric is determined by collector filter-
 ing velocity, by the  method or mechanisms of cleaning, and by the amount
 of cleaning.  These same factors  simultaneously influence efficiency, life,
 and operating cost.   Every fabric filter system in service can, in principle,
 he described by one or  more analytical statements relating pressure drop to
 dust  and  gas flow rates, dust-fabric resistance, and cleaning mechanism
 operating parameters.   This Chapter considers pressure drop performance
 and its variation.  Data are also presented on fabric filter efficiency
 as effected by operating parameters.  Efficiency, cost, and life have not
 been  reduced to analytical relationships as has pressure drop and, as a
 consequence, overall  optimization of performance across the several vari-
 ables has not been  reported.  Supplementary cost data are contained in
 Chapter 7, and maintenance, service life, and failure modes of components,
 principally fabric, are discussed in Chapter 8.
      There, are approximately 100 fabric filter models commercially avail-
 able  in the U.S.  (Chapter 3), each available in a range of sizes.   These
 differ principally  in the type and arrangement of fabric used and in the
method of cleaning  the fabric to control operating pressure drop.   The unit
 operation of all these models can be represented, as indicated in Figure
 6.1, by a system into which the particles and gas flow, and to which power
 is applied to produce external observable changes in gas and particle flow
rates.  (Additional related system inputs include costs and additional out-
puts  include component life considerations.)  With proper quantitative
statements about the input parameters shown on the left, the output vari-
ables may be estimated with reasonable assurance.   Not all the factors
indicated are equally important, and in specific instances many may be
relatively unimportant to the total performance of the system.

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                                         POWER
                         I
FLUID FLOW               -
  Temperature            I
  Density                I
  Viscosity              .
  Moisture               I
  Chemical Species       I
    Inert
    Acid
    Alkaline
    Corrosive
    Condensible
  Flow Rate, Average
  Rate of Change of Flow
  Flow History
PARTICLE FLOW
  Concentration
  Particle Size
  Size Distribution
  Shape
  Density & Packing Char.
    Bulk Density on Fabric
    Density Changes with Pres
  Surface Characteristics
    Moisture Adsorption
    Adhes ion
    Electrostatic Properties
    Oxidation
    Sintering or Bonding
  Chemical. Species
    Inert
    Ac id
    Alkaline
    Corrosive
                                      FABRIC  FILTER
                                         S YSTEM
I
V	
                                               FLUID FLOW
                                             PARTICLE FLOW
          PRESSURE DROP
         COLLECTION EFFICIENCY
          POWER EFFICIENCY
          COSTS
             Capital
             Operating
             Maintenance
             Repair
          COMPONENT LIFE AND  FAILURE ELEMENTS
             Operating History
             Fabric
             Seams
             Supports
             Mechanisms
          ANCILLARIES
             Dust  Handling System
             Control
             Other Systems
                 Figure 6.1.  Parameters Controlling and/or Describing The
                              Performance of Fabric Filter Systems.

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     TheHe external system parameters are determined by the functioning
of the  internal components of the  fabric filter, as illustrated  in
Figure  6.2.  Each of the components listed can exist over a range of
sizes,  shapes, operating variation, and arrangements within the  collector.
Table 6.1 presents a summary of the effects on performance of  fabric  fil-
ter  internal component configuration and operational functioning.
     This chapter discusses the relationships of the external  performance
of fabric filters to the form and  functions of the internal components,
insofar as this information is available.  Performance relationships  are
highly  dependent on properties of  the particulate material described  in
Chapter 2.  Chapter 4 contains fabric characteristics  which,  in conjunc-
tion with the deposited dust, also influence performance.  Details of the
internal configurations of alternate collector systems are discussed  in
Chapters 3 and 5.
     Laboratory testing of fabric  filters under controlled conditions has
provided much basic knowledge and handbook data for collector  design  and
interpretation of performance.  Laboratory or pilot plant research and
development studies require a simulant dust for the process to be modeled;
dust feeding and redispersion apparatus; flow, pressure, temperature,
and humidity measuring instrumentation; appropriate duct work; stack
sampling equipment; and other special equipment for measurement  and
analysis of performance parameters.  Most major fabric filter manufacturers
have development laboratories available for trials on specific dusts,
including particle size measurement instrumentation, one or more commercial
filters set up for test, fabric evaluation test rigs at bench scale, dust
generation apparatus, etc.  Data developed from these sovirces  is usually
proprietary.   However, summary charts, tables or nomographs are often pre-
sented to aid in selection of appropriate fabric area for specific filter
applications  (Section 5.2).
     Discussed below are the data available on filter performance in
terms of internal component configuration and function and input para-
meters.   The  basic types of collectors for which comprehensive labora-
tory data are available include:

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                  INTERNAL COMPONENT CONFIGURATION AND
                             CHARACTERISTICS
1.  Fabric
    A.  Size of Fabric Element, L.D.
    B.  Fabric Design, Weave, Arr.
    C.  Fabric Structure, Nap
    D.  Strand Structure, Arr.
    E.  Yarn, Fiber, Fibril Size, Arr,
    F.  Fabric Materials, Service-
        ability
    G.  Manufacture, Finishes,
        Lubricants
    H.  Permeability; clean;
        in-service
    I.  Fabric Weight
    J.  Fabrication, Sewing

2.  Collector
    A.  Compartments
    B.  Headers,  Dust Inlets
    C.  Fabric Attachments
    D.  Construction Materials
    E.  Hoppers,  Dust Outlets
Cleaning Mechanisms
A.  Mechanical
B.  Gasdynamic
C.  Combined, Sequencing
D.  Coupling Characteristics

Performance Re lat ionsh ip.s
A.  Local Pressure Drop
B.  Local, Efficiency
C.  Residual Drag, Local.
D.  Deposit Drag, Local
E.  Local Velocity
F.  Local Particle Concen-
    tration
G.  Changes in above as re-
    lated to internal con-
    figurations, external
    system inputs, and
    microscopic  processes.
              Figure 6.2.   Fabric  Filter Internal  Component  Parameters.

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                               TABLK  (). I

            EFFECTS  ON PERFORMANCE  OF FABRIC  FILTER  INTERNAL
                         COMPONENT  CONFIGURATION
 Component
   Typical
 Configurations
                                            Effects,  Remarks
 Housing
Dusty
Gas
Inlet
Class-
ifier
Dust
Hopper
 Rectangular,
 cubical,
 cylindrical
Top,  bottom,
tangential
Rectangular
chamber, one
module without
cell plate or
other internal
components
Inverted
pyramidal,
conical, see
Appendix 6.1
 On outside  filtering  fabric  configura-
 tions,  the  gas  approach  velocity  is  de-
 termined  by housing size,  relative to
 space occupied  by  filter medium;  housing
 acts as settling chamber in  this  case, to
 reduce  dust concentration  reaching fabric,
 especially  for  the larger  size  particles;
 and may amount  to > 50%  reduction in dust
 load to fabric  at C   > 5 gr/ft3.

 Inlet at  bottom, hopper, or  on  side  usually
 followed  by baffle to prevent direct jet
 of dusty  gas on fabric;  inertial  effect of
 baffle  reduces  the dust  concentration
 reaching  the fabric,  particularly for larger
 particles (> 50 urn);  tangential inlet en-
 hances  this effect; top  inlet designs with
 inside  filtering fabric  configuration tend
 to have less stratification  of  particle
 size on fabric; bottom inlet designs have
 larger  particles depositing  nearer bag in-
 let, produces wear, scour, and  abrasion
 problem on  the  fabric with some dusts; in
 large multicompartmented designs, inlet
 header must be  properly  proportioned for
 flow-pressure balance.

 Classifier  reduces dust  load to fabric by
 settling and inertial turn effect, espec-
ially for the larger more abrasive particles;
 usual large empty chamber adds  little or no
 increased pressure drop; same function can
 be  accomplished with  inertial skimmer or
 cyclone in  smaller space but with greater
 pressure drop requirement.
 In  conjunction with bottom inlet  location
 and inlet baffle,  hopper acts as  inertial
 skimmer or gravity settling chamber;  sized
hopper relative to gas flow determines
 particles  inertially  removed; dust build-up
 in hopper can reduce  effective flow  to
bottom entry bags,  hopper dust flow

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                          TAHLK  6.I  (Continued)
Contponont
    Typical
 Configurations
                                             Effects,  Remarks
Cleaned
03 S
Outlet
 Fahri c
 Arrange-
 ment
Fabric
Arrange-
ment and
Suspens ion
Tube
Sheets,
Thimbles
Cleaning
Mechanisms
Side, top
  Bag, tented
  top, open
  bottom
  Sleeve, open
  both ends
  Sleeve, open
  top, over
  wire frame
  Multibag
  Envelope


  S leeves-clamp,
  spring insert
  Closed ends-
  hook,  buckle,
  cap
  Envelope-
  button, hook
  (Numerous)
 problems, bridging, etc., associated with
 high humidity  (>  70%) and greater  adhesion
 (see Ch. 2); recent studies on hopper  de-
 sign and dust  flow  indicated  in Appendix
 6.1; bin vent  and silo  filter applications
 may discharge  directly  to storage  without
 hopper; hopper must be  air-tight when  under
 suction to  prevent  redispersion of collected
 dust to fabric.

 No effect of configuration on performance
 except insofar as outlet gas  flow  related
 to back flow air utilized for cleaning;
 large multicompartment  units  require careful
 design of outlet header to assure  even  in-
 let flow distribution.
 Fabric cotifiguration and utilization de-
 termined local dusty gas velocity  which in
 turn controls  Local dust-fabric resistance;
 fabric arrangement  relative to cleaning
 method and mechanism determines both resid-
 ual drag and effective  drag; too much  fabric-
 crowded into housing becomes difficult to
 service, thereby limiting maintenance and
 producing poor performance,  i.e. dust  leaks
 and accelerated deterioration of fabric and
 other components.
 Tension and tension changes  affect cleaning
 distribution, and indirectly, fabric por-
 osity.   Installation and ease of maintenance.
Wear often results at stress points.
Spacing of elements affects wear, aero-
dynamics and sedimentation, and maintenance
ease.  Thimbles can cause wear.
See Sections 3.3, 8.8.

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      . Single compartment intermittently cleaned designs
      . Multicompartment periodically cleaned designs
      . Continuously cleaned designs
 (Gradations between available commercial designs tend to make these cate-
 gories a matter of degree.)
     Most laboratory tests are directed primarily at pressure drop,
 and at ways to control, pressure drop within reasonable  limits by
 changes in operation, usually changes in cleaning.  Less data on the
 field performance of fabric filters in specific applications are avail-
 able, due to the usually limited range of variation of  parameters.  Per-
 formance from such pilot plant or full scale tests is discussed in a
 final section of this chapter.
 6.2  LABORATORY PERFORMANCE OF INTERMITTENTLY CLEANED COLLECTORS
      Intermittently cleaned collectors are designed and operated on a
 relatively light duty cycle.  Sufficient fabric area is furnished, in
 conjunction with low dust concentration, so that the pressure drop in-
 creases slowly over a time period in the order of hours.  The collector
 fan (exhauster) is shut off and some form of shaking or vibration is used
 to clean the fabric.  Typically, the filter may operate on its dust
 source over a full shift, and be shut down for cleaning when the produc-
 tion machinery is idle.  Most intermittently cleaned collectors operate
 with a filtering velocity in the vicinity of 2 to 3 fpm with dust
 loadings less than about a grain per cubic foot.  They  are usually em-
 ployed for relatively coarse dusts on dust control, venting, or nuisance
 applications.
     6.2.1  Bench Scale Performance
            6.2.1.1  Pressure Drop-Time Relationship.- Specific dust-fabric
 filter resistance coefficients (K«') discussed in Chapter 2 have been ob-
                                 ^                                  2
 tained in many cases with bench scale apparatus on the scale of 1 ft
          1 2
 of fabric.  '   The pressure drop increase for a stated operating time
may be estimated (see Chapter 2)  by the following equation:

                        & Ap(t) = KZ' cVt/7000                  (6-O


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where C. -  inlet dust concentration  (grains pur  ft'), V =  filtering  vel-
ocity (ft/min) and t - time  (min) .   Experimental values of K«/ determined
l>y Williams et al.,  Tor a constant  filtering velocity and a  single
(cotton) fabric-, were presented earlier  in Table 2.37.  They  were shown
to be a function of particle material and approximate particle size,  in
accordance with the expected influence of these  parameters on deposit  por-
osity.
                                              2
             Later studies by Snyder and Pring   demonstrated  that K  '  is
a function of dust properties and type of yarn or fabric (amount of  free
fiber surface available to the dust  deposit), as shown in Tables 6.2a  and
b.  Their data indicated that K,,' is a resistance coefficient specific for
the dust and fabric.  The quantitative separation of thfe effects of  fabric
from the dust properties in  the specific resistance coefficient was  not
attempted.  Effects of dust  particle size and shape, and fabric yarn or
nap on  the resulting pressure drop increment are shown in Figures 6.3a, b
and c.  Snyder and Pring's pressure  drop - deposit curves are seen to  be
non-linear with a marked upward curvature evident in several  instances.
Typical K»' values ranged from 234 for freshly-formed MgO fume on napped
     D
Orion   to 7 for petroleum coke fines on spun staple Orion napped both  sides
Napping resulted in about a  10% reduction in K«', but usually resulted in
a higher residual dust deposit after shaking.
            6.2.1.2  Dust Collection Efficiency.- Collection  efficiency
was found to range from 84°X,'  for MgO  fume to greater than 99.47 lor petro-
leum coke.  However, the Snyder and  Pring^ data are of limited value  as
they do not represent typical industrial usage.  Fabrics were not utilized
for long periods to achieve  equilibrium  priming  or aging, and cleaning
parameters were not quantified.  The principal value of their study  was
to direct attention to the effects of free available fiber on K,.', and
to the  non-linear variation  of K2' with deposit weight.   They also pro-
vided experimental validation of the predicted effect of «as viscosity
on pressure drop.

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                              TABLE 6.2a
               PROPERTIES OF VARIOUS ORLON FILTER FABRICS'

                       Dust:  fine petroleum coke
                    Results:  average of 5 cycles
Dust Residual
Cloth Weight Perme- Loading Dust, Collect-
Oz./Sq. ability Grams/ Grams/ ing
Yd. New Used Sq.Ft. Sq.Ft. Efficiency
Napped filament 3.9 35 10 29.00 4.18 99.91
Orion, 3/1 twill
Knit Orion, 7.6 85 58 15.40 4.58 99.70
napped
Orion spun 7.5 60 33 18.21 1.48 95.19
staple
Orion spun 9.0 100 50 19.95 2.15 99.64
Filtra-
tion
Constant ,
K '
K2
17.1
12.5
15.0
7.4
staple, napped
both sides
Orion spun fiber-
stock, 3/2 twill
4.9
110   62
17.96
0.99
93.42
13.0
      From Snyder and Pring, Ref. 2.
                              TABLE 6.2b

             FILTRATION CHARACTERISTICS OF NAPPED AND UNNAPPED
                            SIDES OF ORLON*

              Orion:  1 oz. napped filament, 76 x 72 count, 3/1 twill
            Results:  average of 4 runs
              Fume:   freshly generated magnesium oxide




Napped side
Unnapped side

Used
Perme-
ability3
4.2
8.1
Dust
Loading
Grams/
Sq.Ft.b
12.0
13.8

Residual
Dust ,
Grams0
15
4.5

Collection
Efficiency
%
89.4
84.0

K '
K2

234
251
      Permeability after shaking,  cu.  ft/min./sq.  ft.  at 0.5 inch w.g.
      Corresponds  to weight of dust removed by shaking,  grams/sq.ft.
     c
      Dust remaining on cloth after shaking.
t
 From Snyder  and Pring,  Ref.  2)

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o
 CN
re •
.5*
                                          Figure  6.3a.  Values of Ap vs.
                                          Deposit Weight (LTV) in filter-
                                          ing  fine  petroleum coke dust.
                                          Figure 6.3b.  Values  of Ap vs.
                                          LTV covering dust  generated in
                                          abrasive biasing of steel  paint
                                          drums on high twist,  unnapped
                                          Orion with an extremely  low fiber
                                          surface area per square  foot and
                                          fiberstock Orion.
     eo-~ 400—soa~ *6o—
     COO  2000  MOO  4000  SOOO
TOO	600  BOO
LTV GRAINS /fT*
Figure  6.3c.   Effect of particle
size and  shape on filter resistance
of cotton sateen cloth. Silica gel
dust, 43% less  than  10 microns;
Scale dust, 27, less  than 10 microns;
Limestone  dust,  32%  less than 48
microns.

(From Snyder and Pring,  Ref.  2).

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      6.2.2  Single Bag  Performance, Pilot  Scale .Testa
             6.2.2.1  Pressure Drop-Time  Relationship
             Basic Concepts.  - When textile fabric is used  as  a filter for
dusty gas in the form of  a bag, tube, or envelope, and then cleaned repe-
titively, an equilibrium  pressure drop-time behavior is observed.  After an
                                                                            2
initial  priming or aging  period (which.may range from < 10 hours to > 10
hours for certain dusts and  fabrics), the  pressure time response appears as
illustrated in Figure 6.4.   The pressure-time trace will then be repeated
cyclically, provided that the gas and dust flow rates are  maintained con-
stant and if the cleaning energy patterns,  durations, etc., are applied
uniformly.   The pressure-time curve depends upon the gas viscosity, particle
properties, (size, shape,  surface phenomena),  fabric properties, (fiber,
yarn,  weave, finish, nap)  and operating  characteristics of the cleaning
mechanisms.  If the flow  of  dust to the  fabric  filter is stopped at any
point, and  a pressure-flow curve is determined  using cle^an air,  it will be
found  that
                               Ap - S
   (I}"
                                                                     (6.2)
                    n
                    10
                  »
                  *  9
                  I
                  w
                  e  a
                  °~
                 §  4
Du*t:
tutlcli 111* Mi'-
Du«t Coocutratlo*
a - gralna/ft*
Fabric Warn  cc,  vt, AT?,
    |: Typa a«?Utudt (raq. duratioa
                                        Incrtuad pr«iur« loi> ciuM
                                        1>)F lncrt.it u K,' .t hljhtr
                                        vtloclty     2
                                2.1 x 5 - A.9 In.HjO. calculattd prtilurt drop
                                            •t iqual dtulcy, at
                                            hlghtr flov ratt
                        - Kfftctlvt prtnurt drop
                        -Rttldual pr«iure drop
           v • 3.0 fpm
                              "' '
                                      ?0
                                                 32  36
                                                        40  44
                                     TIMl, minutet a V, lbi/ft
    Figure 6.4.   Pressure Response  in Constant Flow  Rate  Gas Filtration
                  (Constant Gas and  Particle Flux).  (From  Hnt^wardl and
                  Durham, Ref. 3).

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where n -- 1 as long as viscous flow obtains  (for a granule Reynolds
number < 10).  The property S or "drag" is a variable function of
fabric, flow, and dust parameters as described  in Chapter 2;  it  is
analogous to the resistance in a purely resistive electrical  cir-
cuit described by Ohm's Law, i.e., Voltage = Resistance x Current.
                                            4 5
            The drag, S, may thus be defined '  as
                                                                     (6.3)
for small Reynolds numbers, which is the case of most fabric filtration.
S is a property of the  filter and applies to any filter area.  If S varies
from area to area, due  to variations in permeability for example, then
the overall effective drag S  is given by the following, again analogous
to the electrical case  of resistances in parallel:

                                                       j:            <6'4>
where the a. are the incremental component filter areas and the S. are
the local drags.
            The use of the drag concept allows direct comparison of filter
media from one filtering situation to another, regardless of filtering
                      6"
velocity or unit size.
            The Darcy permeability, K  , which defines the intrinsic per-
meability of the filter medium in terms of its structural form, is analogous
to electrical conductivity.  It is related to the specific resistance, K9 ,
of the filter medium by the relation
                        K      (l-e)pp   S ' (l-c)pp                   -

The ratio, W/S, provides another measure of cake permeability in terms of
the density of filter deposit per unit area, W and the resistance to air-
flow per unit velocity or drag S.  The latter description of permeability
is merely the reciprical of the previously cited specific resistance,  K«.
It should be noted that the permeability is an intensive property

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 (independent of mass) whereas  the  drag,  S,  is  an extensive  property (mass
 dependent) that relates filter resistance  to  flow rate.  While  the drag
 will always increase as additional material  is  deposited on the  filter,
 permeability may  increase,  decrease,  or remain  constant over the same
 period, depending upon  flow,  fabric,  and  dust interactions.
            Iltil izing concepts of filter drag and areal density  of deposit,
 the typical pressure-time response during equilibrium operation  of a
 fabric filter element shown in Figure 6.4 can be represented as  a drag-
density relationship as presented in Figure 6.5.  Several typical  features
                           FILTERED OUST MOSS, VMgrtlm/ft*)
              Figure 6.5. Schematic Representation of Basic
                          Performance Parameters for Fabric
                          Filters.

 of performance  are  illustrated.   The  total  drag of  the  filter medium in
 service  is  the  sum  of  the  drag  produced  by  the  fabric with its irreduc-
 ible residual deposit  plus the  drag of the  dust deposit added during
 the filtration  cycle.
             All  practical  cleaning methods  so far developed result  in
 some nonuniformity  in  the  effectiveness  of  cleaning obtained in different
 areas of a  filter cloth.   Because of  this,  when filtration is resumed on
 a recently  cleaned  cloth area,  the rate  of  change of drag  of the medium
 will vary in the early part of  the filtering cycle.  The curve in Fig-

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urc (>.r>  indicates that cleaning  leaves a nonhomogeneous and discontinuous
surface-  that is subject to rapid increases  in resistance when a new de-
posit is first being formed.  Curve segment A represents the increase
during this part of the cycle.  After the discontinuities have been
largely  eliminated by preferential flow of  the dust-laden gas through  low-
resistance areas, a relatively uniform deposit has been formed.  Sub-
sequent  deposition of dust generally results in a linear increase  in re-
sistance to fiow as dust accumulates, curve segment B.  Efficiency of  the
filter is lowest during the early part of the cycle while the discontin-
uities are being repaired.
            This relationship will hold for any dust-fabric combination
but the  shape of the curve may be significantly altered by differences
in the cleaning or the nature of the fabric.  Highly effective cleaning
will lower the residual drag value, S , and poor cleaning will increase
it.  For that part of the cycle represented by curve segment B where the
homogeneous deposit is formed, the specific properties of the dust tend
to control  the rate of increase of resistance.   The permeability, K, of
the dust-fabric combination will be reflected by the slope of the curve.
These general relationships relating to pressure loss through the medium
in a filtration process apply whether the area  being considered is a
single bag or many bags in a single compartment, so long as all  of the
area is  put in service at one time.
            Because of the dominant importance  of pressure drop in
equipment of this type, the drag of hhe filter  medium must be given
primary consideration.   Filter drag,  however,  is but  a single element  of
the total pressure drop that determines fan and power requirements.  In
practice, the total pressure drop to  be overcome may,  because of  duct
losses,  be  twice that attributable  to the filter medium alone.  The total
system will contain sections with gas in turbulent  as  well  as viscous  flow,
so that  total pressure  drop will vary with  flow exponentially by  some
power between 1.0 and 2.0 instead of  directly with  flow as  it does  for
the Ap across the medium alone.
            The residual drag,  S ,  is determined by  the properties  of
the dust and the fabric,in conjunction with the operation  of  the  cleaning


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mechanism.  Cleaning energy  input, as distributed through accolor.ition,
frequency, and duration, do fines the amount of dusL removed down to  some
irreducible minimum value.   The  effective drag, S ,  is defined as that
value of  the draj; obtained  from  extrapolation of the  linear portion of
the S-W curve to zero W, at  the  ordinate.  For practical purposes,  the
filtration cycle appears as:

                             S(W) = |£ = Se + W/K                      (6.6)

where S   is some function of S  , fabric, and dust parameters.  The actual
shape of  the S-W curve  in region A has not been presented; instead  it
is usually approximated by this  equation.
            In terms of the  specific dust-fabric filter resistance co-
efficient ,
                                 K2' = 7000/K                          (6.7)
                                              2
providing W is calculated as grains of dust/ft  of fabric.  Then  the
pressure-time relation  is generally of the form

                           Ap(t) = SgV + K2' C^V2 t/7000              (6.8)

when the  S-W curve is linear from the extrapolated value of S  .   The  form
of the S-W relation is not always linear, and seems  to depend on  the amount
of fiber  available at the fabric surface for dust holding, as will be dis-
cussed below.
            Liquid Filtration Analogy.- Because investigations of liquid
filter performance have been treated in much greater detail in the tech-
nical and engineering literature, it is of some value to examine  their
applicability to fabric filtration.  A typical liquid filter utilizing a
fabric, as applied in the Chemical Process Industry, operates at constant
pressure drop and the flow rate  (or total flow volume) is allowed to de-
crease until a fixed volume has been treated.   The filter is then stopped,
drained, and the resulting cake  (deposit) is removed by manual or semi-
automatic means,  which may involve the physical removal of the fabric
system from the main flow housing.   In rotary pressure or vacuum filters,
the deposit and removal steps are continuous and automatic.   In the manu-


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        nl paper Liu' rake formed from a slurry of wood  fibers  is contin-
                                                                        •7
nous I y withdrawn as product.  The use of. rapid sand  filters  (2-'J gpni/l'L")

for water purification (in Sanitary Engineering) involves operation of  a

fixed hod of granules as a filter for fine participates until  the deposit

storage capacity of the bed is exceeded, whereupon the  filter  is then

backwashed.

            In each of these areas there are extensive  investigations and

analyses of the filtration process under a condition of variable deposit

and variable flow or pressure drop.  However, the following reasons

preclude the direct utilization of these data to the processes of fabric

filtration of gases, even when the same filtration substrates are employed;

            1.   The adhesion forces between small particles at a
                fibrous or granular substrate are several orders of
                magnitude smaller in liquids.  Adhesion forces for
                particles in air or gases are typically of the order
                of 1 dyne (Chapter 2,  Table 2.10) for sizes in the
                range of 1 to 100 urn,  and are dependent upon humidity,
                capillary condensation, and surface effects such as
                contamination, roughness, electrostatic charge,
                etc.  Table 6.3 indicates that adhesion forces in
                liquids are. typically  less than 10"^ dynes.  They
                depend upon van der Waals forces operai ive at
                molecular dimensions (~- 10~8 Cm).

            2.   Pressure drop in liquid filtration is generally
                much greater than in fabric filtration of air
                and gases.   Typical terminal pressure drop in
                fabric filtration is less than 15 inches of
                water, i.e.,  less than 1/2 psig.   Pressures
                used in liquid filtration in the chemical pro-
                cess industries are generally greater than
                10 psi (rotary vacuum filter) and may exceed
                100 psi (plate or frame type).   Rapid sand fil-
                ters typically backwash at 3 to 5 psi.

            3.   The combination of lower adhesion forces and
                greater pressure drop  are believed to have sub-
                stantial effects on the characteristic of the
                deposit in liquid filters as contrasted to de-
                posits likely to occur in fabric filtration of
                dusty gas.   In liquid  filtration through fab-
                rics,  one typically observes a deep cake of
                order of several millimeters thick,  much
                thicker than the fabric media.   Calculated
                values of deposit thickness in fabric filters

-------
                   (Table 2.36) are generally less than 1 milli-
                   meter.  Measurements of deposit thickness in I'abric
                   filtration have not been reported.

                   The irreducible residual deposit pressure drop
                   in fabric filters, measured by Sg or Sr as shown
                   in Figure 6.5 is a major portion of the total oper-
                   ating pressure drop in every cycle.  In the case
                   of liquid filtration, Ruth (see Appendix 6.2)
                   and other investigators have shown that the
                   pressure drop of the filter medium at the be-
                   ginning of the filter cycle is of little sig-
                   nificance in the resistance of tho medium nflor
                                     T
                   a cake has formed.
                               TABLE 6.3

               ADHESION OF VARIOUS PARTICLES TO SUBSTRATES OF
                        VARIOUS MATERIALS IN WATER

                         (for F    = 7 x 10"5 dyn)*
Substrate Material
Glass

Steel

Bronze
Glass
Paraffin
Particle
Material
Quartz
Graphite
Quartz
Graphite
Graphite
Quartz
Quartz
V"
11.0
7.5
11. 0
7.0
7.0
5.0
5.0
•v 7 **
V /0
0
20
33
40
40
76***
0***
      *
       (From Zimon, Ref. 8).

        7  = fraction of particle numbers remaining attached after
        application of  indicated force.

         For F,   = 1.6 x 10   dyn.
              det

             The structure of the deposit formed at the surface of a

fabric dust filter is markedly influenced by relative humidity and ad-

hesion, and electrical characteristics of the dust and fabric.  The aggre-

gates formed are probably very filamentous and chainlike for fine part-

icles, although more compact, similar to the liquid filtration case, for

larger dust particles.  For these reasons, deposit formation in dust



-------
filtration probably differs from that  in liquid  filtration.   Liquid  fil-
tration concepts such as complete pore blockage, standard  blocking,  cake
filtration, and the like, are very  likely to be  inappropriate models for
gas filtration phenomena.  While analogies between  liquid  and gas  filtra-
tion are attractive to consider, especially the  concept of cake  formation
in the latter case, they require further investigation before their  utility
can be evaluated.  A summary of pertinent studies in  the field of  liquid
filtration applied to fabric filter technology is given in Ret".  3.
            6.2.2.2  Dust Profiles.-  Pressure-time  data shown in Figure
6.4 and S-W curves given in Figure  6.5 represent a  macroscopic average
over the total fabric surface for a single cylindrical filter tube (or
                                             9
for many bags in parallel).  Stephan,  et al.,  have investigated local
interactions of dust, gas, fabric,  and cleaning  parameters at various
locations on a single bag.  Local mass deposited (areal dust  density)
was measured with a I'-gaging mass probe and local flow rate with a
velocity probe.  These two instruments are shown in Figure 6.6a  and  b,
respectively.  The improved mass probe containing a fi-source  directed
toward an end window G-M tube is held adjacent to the filter  bag.  The
resulting signal is interpreted through calibration as mass per  unit
area.  The filter velocity probe measures air flowing through a  section
                                                       2
of the bag.  Both devices measure an area of 1 to 2 in. .   Clean  unused
                                         22               2
fabric has an areal density of ~500 gr/ft , 10 oz/yd , or 0.07 Ib/ft  ),
so that the dust deposits encountered in practice are 1 to 10 times  the
                                                                      2
weight of the fabric.  Residual dust weights are typically <  0.1 Ib/ft .
            Typical results obtained with the mass  and velocity  probes
on a 9-inch diameter by 60 inch long bag are shown  in Figure  6.7.  De-
posited mass is greatest at or near the center of the bag height (~ 800
     2
gr/ft ).  Filtering velocity is lowest in this same region ( < 1 fpm).
Figure 6.8 illustrates the variation of filtering velocity with dust de-
posit density over the bag.   Values of the local relative  specific dust-
fabric filter resistance coefficient (K') have been calculated  (assuming
a uniform 1 in.  H_0 pressure drop over the total bag) as  shown in Figure
6.9.   Dust resistance is lowest at the bottom (probably as  a consequence-

-------

                                           End window
                                           G-M tube
                                           detector
                                   Personnel
                                   dosimeter
                                    Figure 6.6a. Improved Mass Probe,
   -

                                1.83 in.
                                Air Flow
                                Aperture

                                                                       Haystings-Raydi
                                                                       thermal anemone
                                                                       flowmeter
                                                         I   Cellular  foam rubber
                                                            baffle
c
                                   Figure 6.6b.  Filter Gas Velocity Probe.


-------
                   600   700   800



                 WALL DENSITY \q,t. tl ? )
                                      HI.TCR VtLDCiry
        Figure 6.7.  Corresponding Mass and Filter Velocity

                     Profiles. (From Stephan, et al., Ref.  9)
U
o
                200       400         600        800


                         LOCAL WALL DENSITY, groins/ ft2
    Figure  6.8.  Variation of Filtering Velocity with Dust

                 Deposit Density, from Figure 6.7.

-------
                    00
                    01
                    00
                    n)
                    01
                   TD
                    3
Operating cow) it tow:
9 x 60 In. b«|, cotton i«t**n,
fly «ih c««t duic
                                                  7000 x 1 In. HjO
                                                     W~x~V
                         
-------
 100

  90

  eo

  TO

  «>
t:  30

<  20

    IO

     0
                             o.a
                                                      _L
'o,n
                                                                         I
          I
           30  4O    60  80 100
                                          ZOO   300 400   600 800 K>00     2000  3OOO
                                  Log Duit H»i (Cr«ln>/

                          Log Dust Drag (In.  HjO/fpn)
 2.0    3.0 4.0
             Flgur. 6.10b.   Developmont of Duel: Drag Prof 111 Through • nitration Plxlod
             rfrom R«f.  A)


Hot«« to Fltur*  6.10
Pllt.r tub.. v.r.  6 In.  In dl«a«t«r by 63  In.  long fabricated  of  atandanl cotton aatun  (National
Flltar Madia Styla Ho. 74, 96 narp x 60 fin, 9.7 ot/yd?) having a ratad Frailar Poro.lty of
about 15 ft3/mtn.   In tarn of raalitanc.  and  paraaablllty, tha fabric would ba ratal  at Sg «
0.033 In H20/«pm and K - 14,300 (gralna/ft2)/(ln H20/£pm).   Shaking action vaa vartlcal  at
tha cop. Kith a  2-1/4 In. anplltud. at a fraquancy of 6.79  cpa for a duration of ona alouta.
Taata vara conduct.d at avaraga flltar valocltlaa ranging fron 0.75 to 7.5 fp», and  duat coocan-
tratlona In tha  rang. 1
-------
develops which  Is maintained  for  the remainder of  the  filtration cyclo.
Hoth S and W profiles  tend  to become nearly  flat at high mass deposit
values.  Effects of particle elutriation and of velocity on dust-fabric
resistance (K~' ~ V) are no doubt factors in the departure of the pro-
Tiles  from flatness.  The variations in profile slopes near top and
bottom of the  filters  appeared  in all experimental runs, indicating that
even when starting, with a perfectly  flat profile,  dissimilarities develop
along  the bag  length near the top and bottom of the bags.  This investi-
gation indicated that  filtration does not occur uniformly over the length
of a filter bag.  Different quantities of dust are collected and local
filter velocities vary by factors greater than 2 in the initial part of
the filtering  cycle at different  locations on the  tube.  The correspond-
                                                        4
ing values of  local permeability vary in space and time.   These varia-
tions  reflect  the differences in cake removal by the pattern of cleaning
energy applied, the relative adhesion of different fractions of dust as
a function of  cleaning energy applied, and the differences in dust struc-
ture over the  length of the tube.
            The approach to profile  linearity is expected since areas of
low resistance will handle higher than average air volumes, and vice
versa.  Consequently more dust  is deposited on the low resistance areas,
less on the areas of high resistance, and a self-balancing system exists.
                                                                  2
It is  likely that the  period 0,1 •- 0,2 (i.e., addition of 50 gr/ft )
corresponds approximately to the period of deposit repair and non-linear
K«'.  Values for the effective  average resistance  coefficient for the
total bag structure, (K' = 7000/K = 7000 S/W) are shown in Figure 6.11
for each of the increments, 0,1 -+ 0,n.  The resistance coefficient in-
creases rapidly during addition of the first increment of dust, then falls
to a relatively constant value  through much of the remainder of the cycle.
The terminal value is somewhat higher, possibly because of dust deposit
compaction.   Dust permeability  profiles for another test series are
shown in Figure 6.12.   Average K«/ increased from 2 to 12.7 during the
test.   Effects of particle stratification are apparent at the top and
bottom of the bag,  at the various intervals during filtration.
            General conclusions drawn from these studies are:


-------
BltSI
•* «J
*J O
t-1  (VI
•^ X
u C
U>  -
a y
•o c
  01
O -H
•H O
O *M
£8
CO O
         1.
        0
l-'igure 6.11
                   09
                             10

                        At - Ap
                        in. H20
     1.5        20         2.!

bAp, Dust  Deposit Pressure Drop,
               Variation of Average  Specific  Dust-Fabric Resistance Co
               efficient During  a Filtration  Cycle.
            1.  The local specific dust-fabric  filter resistance
                coefficient is a variable with  respect to both
                location on a filter tube,  and  time  '
                dust deposited.

            2.  The dust mass and drag profiles reflect  interac-
                tions of particle size, flow, and structure
                during deposit formation.
            3.  The average specific dust-fabric filter  re-
                sistance coefficient tends  to rise during the
                filtration cycle.
            Usiag the same techniques of mass  and  velocity measurement,
 the deposit characteristics may be  studied during the  process of cake
 removal.  Effects of shaking on dust mass and drag profiles  are illus-
 trated in Figures 6.13 a and b.  Figure 6.13a shows a  mass profile just
 before cleaning, and after one and  20 minutes of  shaking.  In this par-

-------
                                                 Ave.  Av«. AP.   W, ,
                                                  K   JC9, "H90 gr/ft*
                                              0,1 "5577   2.4 To?  1060
                                              0,2  991   7.1  1.0
                                              0,3  671  10.4  2.0
                                              0,1)  553  12.7  3.0  2290
           200  900 400  600 800 1000    2000 3000 40OO 6000   10/300
                 DUST MASS PERMEABILITY (GRAINS / FT2)/(IN. H20 / f pm)
       Figure 6.12.  Decrease in Dust Mass Permeability  through  a  nitration
                     Period  (from Stephan ,et. al., Ref.  4)
 ticular  case,  89.5 percent of the dust deposit was removed by one minute
 cf  shaking and 97.2 percent by a 20-tninute shaking period.  The reductions
 in  drag  are illustrated in Figure 6.13b.   The first minute of shaking re-
 duced  the  pressure drop by 89.7 percent and the next 19 minutes reduced it
 by  only  an additional  2.8  percent.   However,  dust removal itself is of
 secondary  importance to resistance  decrease,  which determines the pressure
 drop after shaking.  Before shaking,  the  cake had an average resistance of
                    2
 7.4 inch H20/(lb/ft -fpm)  and this  value  was  slightly decreased to 7.2 by
 one minute of  shaking.   After 20 minutes  of shaking,  specific resistance
was increased  to 20.  Continued shaking may reduce dust mass proportionally
more than  resistance.   Reduction in effective resistance is  more meaning-
 ful than quantity of dust  removed in  determining  optimum lengths of clean-
 ing periods.

-------
                                          MIN CLI ANIN'.
                                            ITRIOOI
                                       I.I   J  .  1  1_J
                                  0   4(1  bO m loo  I'll'1  4illt 600
                                  OUSI MASS -graint / ft2 t
           Figure 6.13a.  Fiffect  of Cleaning on  Residual Dust Mass
                          Profiles (From Stephan,  et al., Ref. 4).

  100

  so
  80


  60 -
              a
              § 30h
              \~
              _l
              < ZO

                10

                 0
                          r
                       B  (20 MIN. CLEANING
                       °-'       PERIOD)
                    EFFECTIVE
                              * 0.973 in
u ^0 _    (I MINI S^-O.IOO
        (20 MIN.) sn,« 0.073
    I	.1	L	
            .Ol    .02    04 .C6 .06 .1
                                           S (I MIN. CLEANING
                                                   PERIOD)
M0.r -
                          .    .        .  .
                          DUST MASS Drag
                                                   A  .6 .a i.o
                                                              2.0
                              (Inches H.O/fpm)
          Figure  6.13h.   Kflucl of Cleaning on Residual  Dra^ I'ro-
                          f Mi's (From  Stephan, et al . ,  Re L'.  4).
             Additional  data on development  of residual dust  mas.; pro-
files during cleaning are  shown in Figure  6.14.   An essentially uniform
terminal  dust mass of 1650 grains per square  foot was reached  under the
filtration conditions given in the Figure.  The  filters were then shaken
for 5 seconds (= "1st shake," consisting of 34 strokes) and  the residual
profile was measured.   Approximately 8870 of the  terminal dust  was removed
during this first period.   Successive incremental shaking periods were
then conducted,  and profiles  were measured.   Only 3"/, of the  terminal
mass was  removed in the  second period.  The profiles were progressively
flattened  with further  shaking.

-------
                                          isi THROUGH 7tn SHAKE, EACH 5 SEC
                                          8m THROUGH nth SHAKE, EACH 2 WIN
                                          i;ih SHAKE, 5 MIN
                                          Ulh SHAKE, IOMIN
                                             TERMINAL Lf  30>r> H?0
                                             AVG F,,.TER VELOCITY oiii/m."-
                                             IV.ET DUST CONC , 75grom»';. (I
                               150    200     250
                              DUST MASS, groms/tq ft
                                                   JOO
                                                         J50
                                                                ••00
      Figure 6.14.  Residual Dust Mass  Variation with Cleaning Dura-
                    tion.  WT =  1650  gr/ft  .   Shake,  6.75 cps. (From
                    Stophan and Walsh,  Ref.  10.)

             Stephan and Walsh also discuss  aging,  equilibrium residual

profiles,  filtering velocity, and particle  penetration into the fabric.

They  emphasize  that a non-uniform residual  profile exists at the start

of  every  filtration cycle.  Filtration never  occurs uniformly over the

total  filter surface.  Different filter velocities,  amounts of dust col-

lected, and deposit structure occur at various  locations.   They conclude

that data  from  laboratory bench-scale determinations  of  dust-fabric

resistance  coefficients cannot  be equated for design  purposes  to the

effective or  average  resistance coefficients of the  same  dust  on full-
;ized filter  tubes.

            One practical implication of the Stephan-Walsh  study:

            "....concerns the differences  in local cleaning
            intensities which exist  between very short clean-
            ing periods and appreciably longer ones.... Filter
            cleaning  follows  the  law of diminishing returns  -
            i.e., each  shaking  stroke contributes  less and
            less, with  the degree of cleaning approaching
            asymptotically some  limiting value for a given
            set of cleaning conditions."

-------
             "This.... suggests...the hypothesis that  apprec-                       ^
             iably  shorter  cleaning periods  than are  now common;v
             in  use  may  be  employed without  proportionately
             reducing  the length  of the  subsequent  filtration
             periods.  Such a  situation  would  have  two distinct
             advantages:
             1.   "Rate of filter  wear, which will always                            —•
                 be  related in some way  to the total  number
                 of  cleaning strokes experienced by the
                 filter, could potentially be  reduced.                              _
             2.   With  the use  of  shorter cleaning periods
                 at  more frequent  intervals, increased
                 filter  ratios -  i.e., smaller equipment                            ""*
                 size  -  might  well  be possible." ^
This hypothesis  is  supported  by  the recent  successful development of pulse-        —
jet filters, which  are  continuously cleaned on-line  by a short (< 1 sec)
burst of high pressure  compressed  air (100  psi) every 30 seconds or so.            	
Higher filtration velocities  are used (~10  fpm), and  fabric  life appears
to be at least as long as  in  many  shaken designs.
                                                                                   •^
            6.2.2.3  Analysis of Mechanical Shaking  - Mechanical shaking of
a single bag or  compartment is discussed below.  Analysis  of multi-compart-
                                                                                   —s
ment shaking with and without simultaneous reverse air flow, and reverse-
jet and pulse-jet cleaning are considered in  following sections.
             The  fabric  is  cleaned  periodically to remove deposited material
by  any of  several  possible means (Section 3.3), of which mechanical shaking
                                                                                   _/
is  one of  the more  common.  A minimum pressure differential  at the start
of  the filtration  cycle can be achieved by  vigorous  cleaning to  keep aver-
age residual accumulation  and drag small  (Curve A, Figure  6.15).  However,         ~"
too much energy  applied to the fabric to remove accumulated  dust shortens
fabric life and  reduces collection efficiency (Curve  B).   There  is, thus,           _j
an economic optimum to  be  determined (Curve C), which depends  on many pro-
perties of dust, fabric and cleaning.
            Fabric  cleaning can be  generalized to considerations of force
applied to the deposited powder  layer as a consequence of:   (a) motion
induced in the fabric,  or  (b)  interaction of  the motion of cleaning gas
with the deposit and fabric.  Removal forces  produced on the deposit are
                                                                                   _/
resisted by adhesion  (particle-to-fabric) and  cohesion (particle-to-


-------
                              InCrnalCy and Duration of Clewing

          Figure 6.15.  Cost Analysis in  Fabric Filter  Cleaning


particle) forces within the deposit  matrix.   In principal,  the force app-
lied to the deposit can be determined from consideration of the  energy
applied during cleaning.  In practice, cleaning forces  are  produced by a
configuration having  fixed frequency and  amplitude,  or  other characteristic
patterns of application, and deposit mass variations.   The  point of opti-
mum cleaning is produced by field adjustments of  one  or more of  these
variables.
            A comprehensive analysis of mechanical shaking  in fabric fil-
tration has been reported by Walsh and Spaite  .  They  studied the  effects
ol shaker amplitude,  (c ) shaker frequency (u>) and duration (number of
                       9
strokes, u>t) on residual drag (S ) and filter capacity  (weight of dust col-
lected during the filtering cycle, W without exceeding  some terminal fil-
ter drag, S ).   Tube  geometry, fabrics, shaking direction,  etc., were held

-------
constant..  The  pilot scale test unit  is  shown in Figure 6.16.   bags were
installed with  zero tension and zero  slack.   The range of variables inves-
tigated  included  V = 3 fpm, 0.05 <  e  <  2  in. ,  5.2 < cu < 56 cps,  and 5
                                  """  S """*             ~~
< o)t < 700 strokes.   Maximum acceleration  produced by the simple  harmonic
motion is:
                                =  (2 in>  c                         (6.9)
where -    -  amplitude of the motion,  1/2  the  peak-to-peak stroke  length.
The muxJmum occurs at the limits of  the  shako motion.  The range oJ'
accelerations  studied was 0.72 <. ir v.  15.5 g's (10  < y physical  limits as illustrated in Figure 6.17.  Shaking
            r
I  ISLE
3  JNH
3
                                   . Bi.C*£«S
                       &)  5 OJST FEECE"
                          £ MiC
» SHAKING MCOivSv
0 :uST CATCH |;NS
  •N KACC*
1 1 OJST :ATCH iiNS C.J"-
                                               i2  HOPPER OX"
                                               .s  :U*T CATCH
                                             Fly /isli  test  dust;
                                             
-------
          10
       i
       i
       i
       ut
       O
       a!
       2°'
                                 V»- LIMIT OF PR* i ICAL
                                     AMPLITUDES («MO)
«'•
                        09 g'«
        0 01
          0 I
        * u«i,h », S|i«lt» t) produce an initial  reduction in residual  drag,
S .  A point is soon reached when increased duration produces little
further  reduction in residual drag (S   =  S  .  ).  The initial cleaning
                                      r    mm

-------
                JUT ciiwmoHi;:
                Ml.TUS i;nTWN 4ATKKN MhKIC
                KI.Y ASII rrsr imsi, t..1 m rtii).  • I.H. cm
                MI.TKK Vr.U*:\T> -  I-" 11'»>   K
                              I INCH
                              I// INCH
I *x
\ x^
1
,\
1 ^~0
I |
11
^-.?S
	 A--
1
                                        W'iLV!.1! IJ
                                                  /.inlllh |n I, In/
.M>»IU in./.In.;
I.I»H>'' In./nln.'
                                                  jo ,,,.t. ,,h,ik.-
                                                       'T~
               20    4O    6O   aO    tOO    I2O   I4O    ISO
                                     I IWIMBtH OF STROKES),
                                                     180   200   210
       Fi.j';uro b. L8.   Effect of Cleaning Duration on Residual
                      filter Drag for Several  Shaking Conditions,
                      (From Walsh and Spaite,  Ref.  1.1.)
strokes  accomplish most of  the  cleaning.  A practical  limit exists beyond
which  increased shaking duration did not substantially decrease residual
drag.     This limit, indicated  at N  in the figure,  ...  "was approximately
                                     s
reached  when oscillatory motion was developed over  the entire filter tube.
Physically,  N  was the number of strokes required to produce significant
              s
discontinuities in the dust mass structure so that  ...  drag was at a prac-
tical  minimum because of low resistance areas."     Maximum capacity was
achieved with a greater number  of strokes.   In either  case,  15 seconds of
shaking  was  sufficient to achieve an effective minimum drag and maximum
capacity.  Commercial fabric filters typically shake for  30 seconds or
longer.

             The effect of shaker acceleration on minimum  residu; I  dv.ig
(S     at N ) is shown in Figures 6.19a and b.   These data indicate that
v min     s
the minimum  drag produced (the  nearly horizontal portion  of the curves
in Figure 6.18) is a function of the acceleration, as
                       mm
                             (7
                                     (in.  H20/fpm)
              (6.10)
for 0.7 v.,v< 6 g's.   For i» 6 g's,  S  .   -^constant, independent  of
        ~~~  •*-*                •        m i. n

-------
         6
         o
         c
         i
        to
        o
        <
        s
           o a
         u. o 6
           0 4

                        111 : i u •. - COT i UN i*Ti t N r«b».c
                        ,,i'M  I.: uTHUH 0 'I > «S»i
                        I .LTI N vllOCi'T - 1 0 torn
                        rfRMINAl. FiLTf'NORAC-     -^ ?.0m
      Figure   19a.
o
 6, S   • constant, (0.19 for
                                          fly alls  on sateen weave cotton)
                                          0.7 _. O < 6 g's
                                                      I
                 1.0         235          10

                       SHAKER ACCELERATION  (da's)
                                                             20
  Figure 6.19b.  Minimum Residual  Drag as  a Function of  Shaker  Acceleration.

-------
             The  form  of  the  S  -cut:  function appears  to he relatively in-
dependent of  the  type of dust  (Figure  6.20),  within the  limits  of  the
data available.   Properties  of the dust  (particle size distribution,
shape, adhesion  forces,  density) and characteristics  of  the  fabric  (free
available fiber,  weave,  yarn,  fiber material)  as well as filtration
parameters will affect the S -tot values.   Hard-to-clean  dusts,  or  napped
fabrics, for  example, will require greater shaking  duration,  but the
shape of the  drag-duration curve would be.  expected  to be similar to those
shown in Figures  6.18 and 6.20.
            The path  followed  by residual  drag (S   <*  S .  ), as a function
of cleaning duration,was analyzed  using  the data of Figure 6.18 and found
to be represented approximately by:
                   s*. ' s      •}              i
                    t	r     3      .     x   1
                   S  - S  .  =4  los  (au)t)  ' 4                (6.11)
                    t    mm                                   v     '

where S  is the terminal drag prior to shaking, S  is the residual  drag
produced by the number of cleaning strokes (< N ), and S  .  is the  mini-
mum drag obtainable with a given  input acceleration (Equation  6.10).
Data points taken from Figure 6.18 (for cot < N ) are shown in  Figure 6.21,
                                           ~~  S
            The number of strokes  (tut) required to reach minimum resi-
            >  . )  is a functioi
            mm
Figure 6.22.  For these data,
dual drag (S .  ) is a function of shaker amplitude (e ) as indicated in             J
                          Ns '  (a*>s =                        (6.12)
where (  is the shaker amplitude (1/2 the peak-to-peak total motion of
       S
the shaker) .
            In summary, residual drag appeared to be determined by
(number of strokes x peak acceleration), Equation 6.11.  However, resi-
dual drags smaller than a certain limit were not obtainable, the limit.
depending on acceleration, Equation 6.10.  At that acceleration, N
strokes (Equation 6.12) were required to reach the limiting drag.


-------
Figure 6.20.
             CLEANING DURATION, UK, STROKES

        Effect of Dust on Residual-Drag-Cleaning  Dura-
        tion (S -ot) Relationship.  (From Walsh and Spaite , Ref. 11)
                          -H I tin A I Draft ••[!»
Figure 6.21.  Effect of Acceleration and Shaking Dura-
              tion on Residual Drag.

-------
          W
        01   3.0
          0
         4-1 CO
           01
         ^ jC
         tfl U I 0
         01 C
         CM M
         
-------
I.IOO
"U 1.000
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z
< BOO
(C
S3
£ 600
o
2
3 *°°

-------
6.13) for a  given  input  acceleration.   Data points taken from Figure fa.23
(for o>t < N  )  are  shown  in Figure 6.25.
        —  w
                            02    04    0.6    0.8    10
                             Q— FILTER CAWCITY RATIO
             Figure  6.25.  Effect  of  Acceleration and Shaking Dura-
                           tion  on Residual  Deposit.
            The required  number  of  strokes  to achieve N  is given
approximately by

                            «-  ^w* I7s                    (6.15)

for 2e  > 1 inch.  Smaller amplitudes  (2e  < 1 inch)  appear to arrive at
      S                                   S
N  sooner than estimated  by Equation  6.15.
 S
            In summary, Equation 6.13  indicates the maximum amount of
dust  (W   .  that can be removed  from the  fabric by a  given acceleration
input.  Equation 6.15 indicates  (approximately) the number of  shaker
strokes (N ) required to  reach the  maximum dust removal.   If the re-
          w
quired number of strokes  to reach maximum dust removal  is  not  achieved
during the shaking cycle  (cot < N )  an  (
                             —   w
removed (W)  is given by Equation 6.14.
during the shaking cycle (cot < N ) an estimate of  the  amount  of  dust
                             —  w

-------
            Thf.sr results arc indicated in Figure 6.26.  Data i'rom
i:tirvc C of Figures 6.18 and 6.23 have been plotted to indicate uhr
dust mass relationship through a single filtering and cleaning cycle.
These represent equilibrium conditions at constant filtering velocity
and dust concentration during the filtering cycle.  The relationship
i'or drag produced by added dust mass has been discussed above and in
Chapter 2.  For many dusts and fabric combinations, this is
               SOO = l^li - !iZ + s  - K2 ci vt + se           (6.16)
                        V     7000         7000
                    2                              3
 where  W  is  in gr/ft  of  fabric  and C.  is  in gr/ft  of  gas.   The  interval
 .shown  at A  in Figure  6.26  represents  the  process  of deposit  repair  when
 the  rate of change  of drag (resistance) is  a decreasing function of dust
 deposit.  The interval S^-S   is approximately 0.2 in.H.O/fpm.  Vnlues
 pf S -S  are discussed below, for  various dust, fabric,  and  cleaning
 combinations.   Tho  corresponding value of W at the  point where the
 linear part  of  the  S-W filtering curve actually starts is approximately
 100  grains/ft  .  The  filtering  cycle  (path  A & B) can be approximately
 represented  by

                         K ' C,  Vt
            Upon reaching the terminal drag  (S ), tho dusty gas flow is
stopped and shaking begins.  Drag is reduced  (path C) according to the
shaking mechanism inputs, as given in equations  6.10, 6.11 and 6.12.
(summarized in Table 6.4).

            Equations  6.9   through  6.17 define the loop shown in Figure
6.26, approximately.   For an assumed inlet loading of 5 gr/ft  and a
                                     2
filtering velocity of  3 fpm (15 gr/ft -min),  the pressure drop-time
history of the filtering and cleaning cycles  appear approximately as
shown in Figure 6.27  (from  Figure 6.26).  Equations and data presented
above provide an estimate of the transient conditions in filtering and

-------
     1.0-
   0-75
B
a.
O
 CM
33
CO

M

o

o>
    0.5-
         Irreducible
         Residual
         Dust Mass
         Held in
         Fabric In-
         terstices
         as a Func-
         tion of
         Fabric
         Dust Prop-
         erties
         Amount ob-
         tained tron
         shake -
         cleaned
         filter tube
         weight
  0.25-
                                Number
                                of
                                Clean-
                              ing Strotes
                       15 sec shake
                                                 Shaking Conditions
                                                 21=1 inch
                                                 a = 1.44g
                                                 a, = 5.25 cps
                                                 Curve C, Figs.  W
                                                  6.21, 6.26   -max
         New fabric
                   0
       Equilibrium
       Res idual
       Interstitial
       Dust Mass
 200        400

Filtered Dust Mass,  W,  gr/ft
600
  2
                                                                 800
                   Figure  6.26.  Fabric Filter Performance
                                With Intermittent Mechanical
                                Shaking (15 sec shake).

-------
                                TABLE 6.4
           SUMMARY OF CLEANING EQUATIONS FOR FABRIC FILTER DRAG
            AND DUST DEPOSIT DURING SHAKING WITHOUT AIR FLOW
6.9
6.10

6.11
              a .
                - (2
              S .   =      ;  0.72 < « < 6, For a > 6, S .  -» Constant (0.19)
                     a 1/2        ~   ~                rain
            S  - S
                   .
                  mm
                                            S  - S
                                                   .
                                             t    mm
6.12
                   (cot)  =
6.13

6.14
              W      810 (l-e"a)
               max
              W                       W
              ^  -f logfaeut)- f ;   jj£-  < 1
               max                     max
6.15
                 = (a*)w«     ;   2 es > 1 inch
                            s
cleaning cycles  for various values of  filter operating and cleaning
parameters.  As  an illustration of their utility, Figure  6.28 has been
constructed for  a cleaning time of 5 seconds, keeping all other para-
meters constant  (taken from the data shown in Figures 6.18 and 6.23).
Much less dust is filtered prior to reaching terminal drag.  The corres-
ponding pressure drop history is shown in Figure 6.29.  The number of
cleaning strokes used per hour for the 5 second cycle is 75, (Figure 6.28),
as compared to 108 per hour for the 15 second shake cycle, (Figure 6.26).
The amount of dust collected is 900 gr/ft -hr with either cycle.

            In contrast, a typical pulse-jet collector operated under
                                                           -I
similar conditions of inlet concentration (fly ash, 5 gr/ft ) at a fil-
tering velocity  of 8 fpm would clean each bag approximately 88 times per
hour, and each cleaning cycle  (0.16 sec duration, 100 psi jet air) would
                             2
remove approximately 27 gr/ft  of dust.  Equilibrium pressure drop for

-------
                                                1} tec
                                                •hake   •
                                                cycle (80
                                                •trokei)
                                   Number of Cleaning Strokei/Hr
                                                108
                             20        50
                            OmtATIM TME,
       Figure 6.27.  Filter Preieure Drop Hletory (Coniteat Velocity ead Duet Concentr«tloo).
this collector  would bi1 effectively constant at 4.0  in. H_0.   Amount of
                                 2
dust removed  would be 2400 gr/ft -hr.  Short-cycle on-linp cleaning has

been developed  rapidly by filter manufacturers during  the past few years.

The number of cleaning operations per hour in these  designs  is approxi-

mately equal  to or less than the number of cycles required in shaking

designs, so fabric life is probably at least as long.   Most  use felted

fabric having inherently high collection efficiency, so that short-cycle

cleaning does not  adversely affect average collection.   Dust handling

capacity is much higher per unit fabric area with on-line cleaning.

            Walsh  and Spaite   discussed the effects of the  duration of
cleaning cycle  on  filter capacityin terms of practical  applications of

the research  results presented above.  They indicate:

            "...that the influence of factors affecting the  transmission
            of  motion may be directly related to N , and it  has been
            shown  that longer shaking amplitudes might  be desirable
            since  they favor the transmission of motion.   A  definitive
            conclusion cannot be reached, however, until  the influence
            of  amplitude, on filter wear is known."

-------
      1.0
   •O.75
rt
(X
O
 CN
PC
    0.50
t>n
   0.25
                                  m
             New Fabric.
                                           5  sec.  shake
                                           Shaking conditions
                                           2e=l  inch
                                           c*=1.44g
                                           W=5.25cps
                                           Curve C.Figs.  6.21 &6.26
                                            I
                                                200
                                      i
                                     400
             Equilibrium
             Residual  DnsL  Mass
                     Filtered  Dust Mass,  W,  gr/Et
                                                 2
         Figure  6.28.
Fabric Filter Performance with  Intermittent
Mechanical Shaking (5 sec).
            "Another application of the data is related to reducing
            filter wear for a given shake by minimizing the total number
            of strokes applied over a period of time.  The cleaning dura-
            tion which will achieve this effect can be found by drawing
            a line from the origin and tangent to the curve of a "capa-
            city vs duration" relationship, as shown in Figure 6.30,,
            The duration at the point of tangency will allow the most
            dust to be filtered per shaking stroke, such that the total

-------
  o
  C
  •H
  Q.
  O
  t-l
  •a
  D
  v.
  -U
  J-,
  PL,
          3.0
          2.0-
                Data  1'rom
                Figure b.31
               Assume C.=5gr/ft
                     V = 3 fpm
Stop
main
dusty
gas
t: 1 ow >
                               3
                    Number of cleaning  strokes/hr
                           25 x 3 =  75
                            I
                            10

                      Operating  time,  rain.
       20
         0'
Kigurt- h.29.  filter Pressure Drop History (Constant Velocity
              and Dust Concentration.
       number of strokes utilized  over  a  period  of  time  (say  one
       week), will be significantly less than  the  tptuJ number usod
       with a duration of Nw strokes.   The  total  amount  of  dust
       collected over this  period  and maximum  filter  drag  for each
       cy.-le would be the same  in  both  cases.  For  the illustration
       shown, I he fotnl number  of  strokes required  for operation  at
       this optimum condition For a 40-hour interval would  ho  approxi-
       mately one-third the number required if N  strokes wore used
       for each cleaning period".„.(also see a similar comparison in
       Figures 6.26 and 6.28 using 2/3 the number of strokes  in going
       to N  vs N , respectively).
       "In practice,  operation unde?  this condition probably  would
       produce a residual dust  ...(deposit)... less uniform than  that
       after Nw strokes,  so that significant aging  (gradual increase
       in residual drag with time) may complicate operation in this

-------
 region...The extent of such nonuniformities can be seen by
 considering local velocities of filtration as shown in Figure
 6.31.  These data were taken while determining the curve of
 Figure 6.30 and indicate the degree of nonuniformity that
 existed after several cleaning durations.   For instance, the
 profile after cleaning with 28 strokes shows velocities on
 the order of five to six fpm through the upper portions of
 the filter, while the profile after cleaning with 688 strokes
 shows a uniform velocity of three fpm over the entire filter
 surface.  The former situation indicates that the shaking
 motion was effective in reducing local filter drag only at the
 top of the bag; hence, velocities there are higher than aver-
 age.   Repeated operation with this shake would result in a
 gradual build-up of drag at the bottom of  the filter and a
 gradual increase in the effective; drag of  unit.  This type of
 drag  Increase would not occur when operating with a duration
 of 688 strokes because the  entire filter area would be affec-
 ted by the shaking  motion".„.(also see Section 6.2.2.2 above
 for residual profile uniformity with duration of shaking).

 "Figure 6.19a,  which contains  several curves representing
 residual drag as a  function of acceleration and terminal drag,
 has certain practical implications.   These data indicate that
 in the region below the optimum acceleration,  residual drag
 was dependent on terminal conditions (see  equation 6.11),
 while in the region above the  optimum acceleration it was
 practically constant (see equation 6.12).   In other words,
 changes  in conditions  such as  increased  cycle  times or in-
 creased  dust concentrations, which would increase the terminal
 drag  of  a  unit,  will increase  residual drag  if the  accelera-
 tion  of  the  shake is  below optimum for the  system.   Also,  if
 a  unit  is  operated  on a  time-cycle and the acceleration is
 below optimum,  both  residual and  terminal  drags will increase
 from  cycle  to  cycle1'until the  drag reduction during cleaning
 equals  the  drag  increase  during  filtration.  This effect  will
 be  in addition  to any  other normal increases associated with
 an approach  to  equilibrium.  Assuming  that cleaning  duration
 is  in  excess  of  N ,  a  reduction in residual  drag  can be
 accomplished  only by  increasing acceleration.   This  type  of
 phenomena may explain  the results  reported by  Lemke,  et  al.,
 (12)  (ZnO   from hot  dip  galvanizing kettles),  wherein the
 residual drag of two  pilot-plant  filter units,  operating  on
 a  timed cycle,  increased  gradually over a period  of  time.
 The increase was of  such  an extent that vigorous hand  shaking
was periodically required to maintain  reasonably  low filter
 pressure differentials.  The vigorous  shaking by hand,  then,
was comparable  to increasing the acceleration  of the shaker.
 It would seem possible,  therefore, that a dependence  of res-
 idual drag on terminal conditions may be a general  indicator
 of  insufficient acceleration, and that the  region of opera-
 tion  (i.e., above or below the optimum acceleration) may be
determined by varying  filtration times an^.  monitoring  the
effect of such changes on residual drag."•'•I



-------
   000
   eoo
z
4
a
4  400
a
             A-MAKlMUM
             /  AVtHACC
/
                SLOPE
                            I     I     I     I
                            TtuwWiLWWO • i OiN M,0/t»«
        IDURATION FOR MAXIMUM CAPACITY PER SHAKING STROKE
     *    I     I     I     I     I     I     I     I     II
         100
              ZOO  300   400   500  6OO   700   800   900
                 CLEANING DURATION (NUMBER or STROKES)
                                                      1000 MOO
          DETERMINATION OF DURATION FOR MAXIMUM CAPACITY
                        PER SHAKING STROKE
    Figure (>..}0.
       Determination of  Duration for Maximum
       Capacity Per Shaking Stroke.  (From Walsh
       ;


' / '
/
/
*

PROFILE A
DURATION
OF
28
STROKES


1 1 1
1
-

—




/
f\
} '
(
\

1
'PROFILE B
DURATION
OF
344
STROKES


1 1 1
1 /I 1 1




PROFILE C
DURATION
OF
; 688
STROKES


I/ 1 1 1
                         t      :   »  «  e
                          FILTER VELOCITY 
-------
  Walsh and Spaite presented the following conclusions:
             "1.  A well-defined limit will exist beyond which increased
                 shaking with a given shaking stroke will not significantly
                 contribute to further cleaning (see equations 6.11  and
                 6.14).

             2.  Filter capacity and residual filter drag are a  function
                 of shaker amplitude and the square of shaker frequency
                 (maximum acceleration of bag cap), but  again a  well-
                 defined limit exists beyond which increases  in  these
                 factors will not result in further cleaning  (see equa-
                 tions  6.10 and 6.13).

             3.  Shaker amplitude has an influence on the number of clean-
                 ing strokes required for a given degree of cleaning.
                 This influence is independent of its effect  on  the maxi-
                 mum acceleration of the shaking  stroke  (see  equations
                 6.12 and 6.15).
             4.  A relative measure of the influence of  factors  affecting
                 the transmission of shaking motion can  be obtained by
                 considering cleaning durations required to produce mini-
                 mum filter drag.

             5.  For any given shaking arrangement there will exist a
                 cleaning duration which will produce the least  bag wear over
                 a period of time." 11

             There is a definite requirement for  study of the effects of

these same shaker variables  on fabric wear  and bag life for  optimization

of cleaning  costs.

             6.2.2.4 Effects  of Fabric Structure. - There are more than a

dozen generic  types of man-made fiber materials  that can be  used in fabric

for gas filtration,  in addition to cotton and  wool.   Fiber,  strand,  twist,

yarn, yarn count,  weave,  and  finish parameters are discussed in Chapter 4.
                                        2
The initial  studies of Snyder  and  Pring   indicated an effect of fiber

material, fabric  weave,  and mechanical  finish  on  filtration  performance

(See Figures 6.3a,  b,  and  c, and Tables  6.2a and  b).  More recent  inves-

tigations of the  effects  of  fabric  structure in pilot scale  systems  have
                                   13                11
been presented by  Spaite and Walsh    (two bag unit,   Figure  6.19),
      14                       15
Durham   and Borgwardt, et al.    Other data have  been  presented in

Chapter 2.

                            13                                           R
            Spaite  and Walsh    studied the performance  of three Fiberglas
              £
and two Dacron  fabrics, woven  from continuous filament yarns (specifica-

tions and photographs of test  fabrics are given in Figure 2.26a).



-------
            "Basic performance curves  for  the Fiberglas   fabrics  when
            operated at a nominal velocity  of 2  fpm are  shown  in  Figure
            6.32a.  Each set of test bags were exposed over  the entire
            range of nominal velocities  investigated  (2  fpm  to 10 fpm),
            and two runs were made at  low velocity to determine the
            extent of changes in the residual medium during  the tests.
            The lower curve for each fabric was  obtained  after new bags
            were operated at 2 fpm for 48 hours.  The upper  curve for
            each pair shows results of a repeat  run made  at  2  fpm under
            conditions that were identical, except that  the  cloth had
            been exposed for one week  to the higher filtration velocities,
            The higher resistance after exposure is a reflection  of  the
            effect of dust penetration into the  fabric,  or "aging".^

            Figure 6.32a shows an increased effect of aging  on the more
              n
open Fiberglas  fabric No. 1, as compared to No. 3:

            "...the tighter weave, No. 3, exhibited a higher resistance
            to flow when it accumulated a semi-permanent  residual dust
            mass.   Much of the advantage in low residual  drag  shown
            initially by the open fabric, No. 1,  was cancelled out by the
            effect of aging; ...dust cake permeability was affected,   but
            little by .aging.

             The importance of changes in filter drag and filter  resis-
            tance  coefficient is most evident when their  combined effect
            is shown as total resistance to flow.  In Figure 6.32b, the
            increase in filter pressure differential that occurs during
            filtration is  shown as a function of the mass of dust fil-
            tered.   These  curves represent the pressure increase during
            operation at velocities  of 2 fpm, 4  fpm,  and 6 fpm on the
            tightest (No.  3)  and loosest (No. 1)  FiberglasR fabrics.
            The filtration capacity  of the fabrics,  in terms of the mass
            of dust than can be  collected before  a given differential
            pressure is exceeded,  depends on the  fabric structure  most
            strongly at high filter  velocities.   At higher velocities
            this dependency becomes  greater as the mass deposited  becomes
            greater;  at lower velocities  it becomes  less  significant  as
            the total mass  increases.
            The effect of  fabric  structure and nominal velocity  on the
            specific dust-fabric  filter resistance  coefficient  is  shown
            in (the  upper  pnrLion  of)  Figure 6.33.   In every  instance
            the dust  deposit  creates  an increase  in  1'he  specific resis-
            tance coefficient  as  filtering  velocity  is  initially in-
            creased.

            These  changes  in the  resistance coefficient  are  substantial
            over the range  of velocities  that would  exist in multicompart-
            mented  units  operating at  nominal velocities  of 2 to 3 feet
            per minute.  Thus,  they  are an important  consideration in any
            analysis  of flow and resistance patterns  in such  an installa-


-------
                                        WET GROUND MICA TFST OUST
                                        NOMINAL VELOCITY  (3) 2 Ipm
                                        INLET CONCENTRATION (5) 5 GRAINS/FT
                       Dust Deposit Density, W  (gr/ft^)
 Figure  6.32a.  Effect of Fiberglass Fabric Fill  Count  Variation
                                          WET GROUND MICA TEST OUST

                                           —	FIBCRCI ASS N«3
                                                 flBfHGLASS N« I
                      Dust Deposit Density, W (gr/ft2)
 Figure  6.32b.  Effect of Fiberglass Fabric Construction
                                         WET OAOUNO MICA TEST OUST
                                         NOMMAL VELOCITY (g) t «»m
                                         I^LET COWCENTWTK5N ® S ORA(HS/FTS
                 0     100    200    100   400    SOO    tOO    TflO    *X
                       Dust Deposit Density, '.-.'  (gr/ft^>

Figure 6.32c.  Effect of DacronR Fabric Fill  Count Variation.
                (From  Spaite and Walsh,  Ref. 13)

-------
                     •o
                     4O
                     JO
                    « 20
                   1  -
                   I
                   U

                   i
                               (bottOB f««<). mlct
                               duit) J u»,
                               D R O.SM*
                                P
             FIBERCUS
             (top feed,
             fly uh.
             I) tin
                                           COTTON SATEEN
                                           (Cop (<«d, fly 
-------
                                         n
  The basic performance curves for Dacron , shown in Figure
 6.32c are similar to those obtained with FiberglasR.  The.
 filter drag at the end of the period of deposit repair was
 again found to be higher for the tighter weave fabric,
 Dacron  B.  The accelerated increase in initial drag for both
 Fiberglas  and DacronR indicates that such an effect is char-
 acteristic of the continuous-filament yarns.

  The effect of velocity on the specific resistance coefficient
 for the mica dust deposited on Dacron  followed the same gen-
 eral pattern shown for Fiberglas^ in Figure 6.33 (upper
 portion).

  The increases in pressure differential across the filtering
 medium during the filtering cycle are essentially the same
 for both Dacron^ and Fiberglas^.:  The previously discussed
 similarity in the relationships for the individual components
 of  the pressure loss (i.e., loss across the initial dust
 fabric medium and loss through the deposit...) are reflected
 here in curves that  are so nearly identical to those shown
 for Fiberglas^ in Figure 6.32b that it is unnecessary to show
 both sets.

  A  second measure of performance that must be considered when
 fabric structure  and velocity are varied is the amount of dust
 discharged through the fabric.  Most of this discharge occurs
 in  the early stages  of filtration.

  Figure 6.34 shows the manner in which dust discharge varied
 as  a function of  nominal velocity and Fiberglas^ structure.
 Each filtering cycle was terminated at a drag level of 1.5
 in. H20/fpm.   Because of this,  some cycles were ended before
 they could progress,  substantially beyond the region of for-
 mation or repair  of  the uniform filtering medium.   Such tests
 are  designated by circles  on  the curves.

  A most  interesting  aspect  of  Figure  6.34 is  the  curve  which
 shows  dust discharged per  cycle  as  a  function of  velocity  for
 FiberglasR No.  1.  The  data points  numbered  1 through 7 repre-
 sent  the  results  of  7  consecutive  tests  of  this  fabric,  each
 made  after 24  hours  of  operation at  test  velocity.

  The  efficiency was  poor  in early  tests,  but  showed  a marked
 improvement, associated with the aging  of this  loosely  woven
 fabric.   The interim runs at low velocitiep  (points  5 and  7)
 indicate  that  the  improvement  in efficiency  tended to be per-
manent.   During Run  7 the dust discharge  changed  from 11.8
grains/sq.ft.  for  the second cycle  to  10.9 grains/sq. ft.  for
 the fifty-second cycle.  Subjective estimates  of  filter effi-
ciency support these data: before aging,  a number of  pinholes
and high velocity  air jets were  in evidence  over the  surface
of the fabric; after aging, these  largely disappeared.  During
the tests, the efficiency of the fabric changed from  85.5
percent for Run J  to 96.1 percent for Run 7'.*3

-------
             The curves i.n Figure 6.34 show that dust discharge was approx-

 imately  proportional to velocity and inversely proportional to Tilling

 yarn count.
             "Conditions that developed in the filter enclosure were simi-
             lar to those observed in industrial fabric filters.  The bags
             developed a slight dust layer on the exterior surface, and
             some dust collected on the floor of the clean air side.  The
             weight of this material was not included in the dust discharge
             data.  The dust discharge data are probably higher than would
             be encountered in actual use because of the intensive cleaning
             employed for these tests.
                            •p
              For the Dacron  fabrics, the effect of nominal velocity on
             dust discharge per cycle is shown in Figure 6.34b.  A general
             linear relation is again shown, but the relationship is dif-
             ferent in that the Fiberglas* fabrics allowed greater dust
             discharges at low velocities and Dacron^ generally showed a
             lower rate of increase in dust discharge with velocity.
             These differences might be attributable to a more even dis-
             tribution of air flow through the DacronR fabric.  An
             indication of the distribution of areas available fui.  high
             air flow is  obtained by considering light transmission
             through the  fabrics (Figure 2.26a).

              The most striking point in the comparison of the two  cloth
             types is  that the two materials with nearly equal clean-cloth
             permeability  values (Fiberglas^ No.  1 - permeability  15.84
             and Dacron^ B -  permeability  14.62)  have different  performance
             characteristics.   The difference  is  reflected in dust  mass  vs.
             filter drag curves (Figure  6.32),  and  the  efficiency curves,
             (Figure 6.34).  The comparisons  demonstrate the  impossibility
             of predicting  performance  of  an untried fabric  from permeabi-
             lity data and  known performance data for a  second  fabric,  even
             when the  same  dust is  involved.  Thus,  cloth  permeability
             per se seems  to be of  less  importance  to these  filtration
             parameters than the manner  in which  the fabric structure is
             changed to alter  permeability1^^

             The effect of  filtering velocity on  the specific dust-fabric
filter resistance  coefficient  was also investigated by  Borgwardt, et al

Their flyash data  are shown in Figure 6.33  (lower portion).  They found
that:

                              K2' ~ V372                        (6.18)
for 1 < V < 4 fpm.  The earlier data of Spaite and Walsh (Figure 6.33,
upper portion, mica dust 5-10  urn D  x 0.5|im thick) are less well-

-------
                               N*l
                             WIT MOUNO MIC* TC8T OUST
                             TCNMNM. OHM $ 1.9 IN. M,0 / fpm
                             ( ) INDICATE EXKMMENTM. WOW
                             0 MOttATC K.EEOMO THMUOHOUT CYCLE
                                    FIKMLAM N»2
             r      4      *      *     10     it     i4     it
             Filtration Velocity, V  (fpm)

Figure 6.34a.  Effect  of Nominal Velocity  on Dust
                Discharge for Fiberglass Fabrics.
Irt
I
       40

     ~ li
     UJ
     d >o
             »«T GROUND MCA TEST OUST
             TERMINAL DRAG $ I S IN H.O / Ipm
           • INDICATE BLEEDING THROUGHOUT CTCLE
                                      OACRON A
        o     r     «     t      <     >o     ie     <4     K
             Filtration Velocity, V  (fpm)

Figure 6.34b.  Effect  of Nominal Velocity  on  Dust
                 Discharge for  Dacron^  Fabric.
                 (From Spaite and Walsh, Ref.  13)

-------
 behaved, but indicate that K- ~ Vn for n > 0 and 2 < V < 8  fpm.   In each
 of these studies fabric, dust, and velocity all affect K~.   (See  also
 Section 2.4.8.4).
             Figures 6.32(a) and (c) can be used to determine the  unknown
 interval (S  - S ).  The nonlinear portion of the S-W curves occurs during
            e    r                                      2
 the period of dust deposition less than about 100 gr/ft .  Approximate
 values of the interval (S  - S J are ~ 0.2 in.H 0/fpm.
                                  14
             Durham and colleagues   have extended the above investigations
 to nine additional woven fabrics indicated in Table 6.5.  Six different
                                    R        R                      R
 fiber materials were tested:  Nomex , Teflon ,  Polypropylene, Orion -
                   R         R
 Acrylic (Microtain ),  Dacron  and Cotton.  Yarn configurations included
 continuous  multifilament warp and  fill,  filament  warp  and  spun fill,  spun
                                 T>
 warp  and  fill,  and  napped  (Dacron  )  woolen system yarns-warp and  fill.
 Weaves  and  thread  counts were different  for  each  fabric, as  was  fabric
                                                         3
 weight  and  clean air-flow  permeability.   Three grains/ft   of flyash test
 dust  (  ~  10 (am  D )  was  filtered  at  4  fpm (constant mass  flux 1.7 x 10
       2         P
 Ibs/ft  min)  in all tests  (top entry,  single 5.5  in. diam. x 66.6  in.
 long  bag, under suction).   Cleaning was  accomplished with a  mechanical
 reciprocating shaker attached to a  flexible  connection at  the bottom  of
 the bag (2 e = 1.75  in.; co  = 5.6  cps;  duration, 0.5,  1, 3, and 5 min;  cot
 =  170,  340, 103, 1.7 x  103  strokes; ,1 =  2.85 g's).   (See equations 6.10
 through 6.15  for effects of shaking parameters on minimum residual drag
 and maximum dust capacity  per  filtering  cycle).  Filter drag-deposit
 density (S-W)   curves are  shown  in Figures 6.35a, b, and c,  and 6.36.
 Fabrics have been grouped qualitatively by yarn free available fiber
 area, according  to  the classification  hypothesis indicated in Figures
 2.69 a-d  (Chapter 2).  Four classes of fabric were identified according
 to a qualitative estimate  of  free available fiber:
            (a) Continuous  multifilament warp and fill, Figure 6.35a
                         j^
                 1.  Nomex
                2.  TeflonR
                3.  Polypropylene
* W is dust deposited on the fabric, as calculated from dust fed and
hopper fallout (see discussion below).


-------
                                                       TABLE 6.5
           Fabric Filter Media Specifications  and Performance  with Constant Particle  Flux
                                               (1.7  x 10"3lbs/ft2-min)
Fabric
••bar
1

2

3

4


5


6

7

1

9

	 T5» —
Conpoelttoo
•0.1'

Teflon*

Poly-
propylene
*-.»


D«ro.»


Cotton

Orion*

ee
acrylic

Dae too*

Tarn
Fl leant
»arp & fill
FUnent
•arp & fill
filament
varp 6 fill
fllanent
varp;apun
fill
Pliant
uerp:»pun
fill
Spun varp
6 fill
Spun varp
4 fill
Spun varp
4 fill
Woolen
ayet.
Heave Count ,
threads /In.
3*1 96*78
tvlll
3,1 76r.il
tvlll
3*1 74*73
tvlll
3*1 95x58
tvlll

3*1 77*81
' tvlll

Sateen 97*63
SH-U
)*2 85x77
tvlll
2*2 39x35
tvlll
2*2 41*39
tvlll
Clean
Air Flov
FeraeeMllty
15-20

20-40

10-20

20-25


20-30


15-20

15-25

to

55-65

Clean
Drag
0.028

0.017

0.033

0.022


0.020


0.028

0.025

0.008

0.008

Fiber
Denalty,
../c.5
1.38

2.1

0.9

1.38


1.3*


1.50

1.14

1.14

1.38

fabric
Veltbt
o»/ys*
4.5

8.6

4.3

5.4


5.8


9.5

5.7

9.8

12.5

Teredeal
drag.S,
la.ayvfpaj
1.5

1.0

1.5

1.0


1.0


1.0

1.0

0.7

1.0

Drag.S ,
In.HjO'fi
0.40

0.15

0.21

0.22


0.05


0.41

0.20

0.05

0.09

tlve
" J"»- 1.
In.BjO/fpn
0.99

0.54

0.92

0.55


0.41


0.56

0.32

0.25

0.20—

t-s
0.59

0.39

0.71

0.33


0.36


0.15

0.12

0.20

0.11

Terminal
Bepotlt,
' It. /ft2
0.045

0.055

0.095

0.060


0.075


0.077

0.12

0.10

0.205

Storage
Capacity.
"t««
0.030

0.055

0.06)

0.060


0.075


0.077

0.12

0.14

0.205

Specific KiUtance
Coefficient,
(IWf?2)-'
11.5

8.7

6.2

7.8


7.7


5.8

5.8

4.8

2.4~*

»|lpiMl.a»e
0/1000 cfv. '
0.85

0.51

0.68

0.47


0.39


0.39

0.33

0.23

0.17

 • - cfWft  at O.S la. EjO. eatuved linear Of - V.
 e* - Ubaay felt Hlcroula*
*** - latiaated from linear portion of S-W curve up to U < 0.1 lbe/ft ,  then t^' Increaalag to 8.5 at V - 0.2 lee/ft
 •ote: n« eah teat duet. 10 u« dlam., C, - 3 graln»/ftT, » - 4 fp., cleaning tin. 180 aec. 5.5 1«. «la». « 66.6 In. lea* bag*.

-------
             (b)  Filament warp and spun fill, Figure 6.35b                         ^

                 4.   Nomex
                 _   _      R
                 5.   Dacron
             (c)   Spun warp and  fill,  Figure 6.35c

                  6.  Cotton Sateen

                  7.  OrlonR                                                       -'
                                        T>
                  8.  Acrylic (Microtain )

             (d)   Napped,  Figure  6.36                                               _,
                            T>
                  9.  Dacron
                                                                                     i
             Effects  of  increasing  amounts  of free available fiber should          —<

include reduction in K/ and increased  amount of  dust  storage capacity

(W /S ).  More free  fiber  (A -* D)  should also reduce  the  amount  of dust           ^

deposit released  for a  given shaking cycle,   An  increase  in o)t on class A

fabrics should produce  little additional dust removal  (AW  small as   0) .                                /

            o  Dust  storage capacity  (Wt/St)  increases  with  amount of free
               available fiber (except for  polypropylene,  which is higher
               than  expected).  Capacity varies  from 0.03  to 0.2  Ibs dust/
               ft  of fabric per  unit  drag.                                        ""*

                                                                                   i

-------
   1.5
e
•a
o

c
00
n)
£
0)
c5
   0.5
                   1. NomexR Filament
                      warp and fill
     \3. Polypropy-
        lene Fila-
        ment , warp
        and  fill
    Te.flonR  Filament
    warp and fill
V .t^   .

  t  30 sec
0 ° O L80 sec
  0  300 sec
    cleaning time
                                           1.5 '•
     0
                                                40 NomexR
                                                  Filament warp,
                                                                       DacronK
                                                                       Filament
                                                                       warP.  Spun
                                                                       till
                                                                4-   )0 HIM'

                                                              OO- 180 sec
                                                                0 300 sec
                                                                 cleaning  tinu1
                                                    _L
          .02   .04   .06   .08   .10   .12
                                                    .02   .04   .06   .08   .10
                                                                                 T
    (a) Dust Deposit Density, T7,  lbs/ft2      (b) Dust  Deposit  Density, T7,  Ibs/ff
                       1.5
                    a
                   •H
                    s
                                             7.  Spun OrlonR

                              6. Cotton  Sateen
                                                8, Spun
                                                   Acrylic
                                                 30 sec
                                                180 sec
                                            Q° 300 sec
                                               cleaning time
                                              i	t    i   i
                         0    .02   .04   .06   .08   .10   .12   .14

                         (c) Dust Deposit Density, ft,  lbs/ft2

                 Figure 6.35.  Effect of Fabric on Filter Performance
                               (From Durham, Ref. 14).

-------
s   1.0
W

c
•H

c/1

u  °5
(0
                                       9.  Napped Dacron^
                                                        +  60 sec
                                                        o 180 sec
                                                        o 300 sec
                                                         cleaning time
                                                            '
                                                                  1
           Q02  Q04  0.06  0.08   O.I2  0.14   0.16  0.18   0.20  0.22  0.24
                                                 _        2
                            Dust Deposit Density W, Ibs/ft

                                                          P
        Figure 6.36.  Effect of Shaking Duration on Dacron  Fabric
                      Filter Performance (From Durham, Ref. 14).
             o  Amount of dust removed by an increase in shaking duration
                (cot)  is relatively lower with filament fabric (No. 1).
                Duration increase from 30 to 300 sec (170 to 1.7 x 103
                strokes) produced a 0.005 lb/ft^ increase in dust holding
                capacity.  Spun yarns (Nos.  5,  6, 3) tend to release dust
                more  slowly,  an increase in duration from 30 to 300 sec.
                producing a 0.015 lb/ft^ increase in capacity.  Napped
                fabric (No. 9)  shows the largest effect of_increased
                shaking duration, 60-300 sec. producing   W of 0.03 Ib/ft
                At_an inlet flux of 1.7  x 10"3  Ibs.  dust/ft2-min, these
                AW values correspond to an increase in operating time
                between cleaning cycles  of (Col. 4):
                Fabric
                Filament
                Fil./Spun
                Spun
                Spun
                Napped
No.
 1
 5
 6
 8
 9
W
                                    30-300
Time of
W
                          30-300
Time to
0.005
0.015
0.016
0.014
0.03
(60-300
2.9
8.8
9. it
8.2
17.6
sec)
min





                                                                         '180
                              26.5 min
                              44.0
                              45.0
                              59.0
                             121.0

-------
                                            V
             o  Increasing cleaning Duration from 30 sec. to 5 min. on fil-
                ament fabric (No. 1) produces about 3 min. longer filter-
                ing time in a 26 min. cycle, or no net improvement.  The
                filtering cycle is lengthened less than the amount required
                for the additional shaking.
             o  Increasing cleaning duration from 30 sec. to 5 min. on
                spun yarn fabrics (Nos. 5, 6, 8) produced about a 9 min.
                longer filtering cycle in a 45 to 60 min. cycle, or about
                15 to 20 percent longer filtering cycle.
             o  Increasing cleaning duration from 1 min. to 5 min. on the
                napped fabric (No. 9) produced an 18 min. longer filtering
                cycle (15 percent increase in filter operating time be-
                tween cleanings).
             Dust characteristics will modify the quantitative observations
 above,  but general trends shown are expected to be the same.  For most
 dusts,  increased cleaning duration beyond 30 sec. or 1 min. will probably
 not produce a net increase in filtering time greater than about 10 to 15
 percent.   The effect of increased cleaning time on reduction of fabric
 life  is assumed  to be  linear.   A five  to  ten times  increase  in cleaning
 duration  (30  or  60 sec  to 5 min) will  probably  reduce  fabric life  by  a
 similar factor.   Assume  fabric  life  is  of order 5 x  10   shaking  cycles.
 Cleaning  for  30  sec uses  170 cycles  (5.6  cps).   Let  the  filtering  cycle
 be  45 min long followed by a 30 sec  shake and 30 sec dwell  for settling.
 Then every  46 min will require  170 shake  cycles.  Fabric  life  is esti-
 mated as  (5 x  10   x 46/170 x 60) 2.8 years.  If the cleaning duration is
 extended  to 3 min (6 x, or 10   shaking  cycles per cleaning cycle)  the
 filtering cycle may be extended about  10  percent  or 49.5 min.  The com-
 plete cycle will  consist  of 49.5 min of filtering followed by  3 min shake
 and 30 sec  dwell  for settling,  or 53 min.  Fabric life may be  estimated
          f         O
 as  (5 x 10  x 53/10 x 60) 0.55 years,  or  about  l/6th of the life  obtained
with shorter cleaning duration.  Pulse jet collectors cleaned  on-line
 every 30 sec  (120  cycles/hr) would be expected  to have an average  fabric
 life of order 5.2  years, under 'the above assumptions.  None of these esti-
mates include effects of factors tending to reduce fabric life during fil-
 tration, such as scour and abrasion, temperature, condensation, acid  or
alkaline deterioration and other gas or particle factors.
                    14                         —
             Durham   observed anomalies in S-W curves for different
shaking times for  tests at different parts of the year.  For example, in


-------
Figure 6.35a, Fabric No. 1, it was found that data produced for 3 min.
shaking did not agree with data for 30 sec and 5 min shaking obtained
several months later.  Similar anomalies are evident in data for fabric
                                R                       ~
Nos. 6 (cotton) and 8 (Microtain ).  The shift in the S-W performance-
curves arc positive (Nos. 1 and 8, to the right, higher W  per units)
and negative (No. 6, smaller W /S ).

              6.2.2.5  Effects of Humidity - Durham and Harrington
have shown that the apparent anomalies in performance just described
probably result from changes in ambient moisture content (Relative
Humidity, R. H.).    Relative humidity was controlled between 20 «nd 60
percent R.n. In the same experimental configuration described above. ^
              Resuspended fly ash was used as a test dust, and the  11 fab-
rics enumerated in Table 6.6 were tested.  Increasing relative humidity
generally reduced overall resistance of the filter.  Three responses were
calculated to identify the system resistance: effective drag, S , specific
                         /                                     e
resistance coefficient, K2, and terminal drag, S .  (Figures 6.5 and 6.35
show typical values of these three parameters.)
              Table 6.7 presents results of the effects of relative humi-
dity on K™, S  and S .  The specific dust-fabric filter resistance coeffi-
cient was significantly reduced by increasing the relative humidity.  Effec-
tive drag exhibited no particular trend.  In most instances, terminal drag
was reduced with increasing relative humidity.  The specific resistance
Coefficient determines the rate of increase in resistance during the linear
portion of the filtering cycle to achieve any given value of terminal drag.
Thus, reduction in K« offers potential for increased bag life and reduced
maintenance (i.e., less frequent shaking required).  Figure 6.37 illustrates
the effect of R.H. (20 < R.H. < 60 percent) on K_ for three acrylic fabrics
and for polypropylene.  An increase in R.H. from 20 percent to 60 percent
reduced K' by a factor of order 2.

-------
ON
U)
                                                           TABLE  6.6

                                                  FABRIC  CHARACTERISTICS

                                       (From Durham and Harrington, Reference  16)
Fiber
Composition*
Nylon
Creslan
R
Dae r on
Polypropylene
Crylon
R
Dralon
OrlonR
Cotton sateen
Glass filament
Glass Combination
Glass texturized
Type
Warp
CF
CF
CF
CF
CF
CF
CF
S
CF
CF
CF
Yarn**
Fill
CF
CF
CF
CF
CF
CF
CF
S
CF
S
T
Thread Count ,
Yarn Denier threads/in;
Warp Fill Warp Fill
210 210 74
200 200 80
250 250 76
210 210 81
200 200 77
200 200 76
200 200 76
95
54
48
46
68
76
66
69
63
71
62
58
56
22
24
Fabric
Weave Thickness,
Pattern Mils
2 x
3 x
3 x
3 x
3 x
3 x
3 x
2
1
1
1
1
1
1
Twill
Twill
Twill
Twill
Twill
Twill
Twill
Satin
3 x
2 x
3 x
1
2
1
Twill
Twill
Twill
9
10
9
12
10
9
8
24
9
24
16
.4
.8
.1
.4
.2
.8
.7
.1
.6
.6
.5
Fabric
Weight
oz/yd
4
4
3
4
5
4
4
10
9
16
14
.1
.0
.9
.6
.1
.4
.3


.5

                 R                                R                           R
        * Creslan  acrylic, Amer. Cyanamid; Dacron  polyester, Du Pont; Crylon  acrylic, Crylon S.A.(Fr);

          DralonR, Farberfabriken Bayer (W.Ger.); Orion  acrylic, DuPont.



-------
                                             TABLE 6.7
   EFFECT OF RELATIVE HUMIDITY ON SPECIFIC RESISTANCE COEFFICIENT, EFFECTIVE DRAG, AND  TERMINAL  DRAG
                                  (From Durham and Harrington, Reference  16
                Specific dust-fabric filter
                resistance in.  H.O/ft-min
                      lb/ft2    L
                                        Effective  drag,
                                        in. H 0/ft-min
                                          Terminal drag,
                                          in. H 0/ft-min
Description*
    Relative humidity,  %
 20   30   40   50   60
           Relative humidity,7=
  20    30    40    50    60
  20
                                                                                    Relative humidity,  7=
                                                                                      30    40    50    60
       R
Nylon

Creslan

Dacron

Polypropylene
      Ti
Crylon

T,  -,  R
Dralon
     t>
Orion

Cotton

Filament glass

Combination
   glass

Texturized
   glass
11.2  7.6  6.4  3.9  2.6

 9.9  9.6  7.8  5.6  4.0

 8.4  7.7  7.2  4.2  3.0

 9.6  7.8  7.8  5.0  3.6

 6.4  3.8  4.2  4.2  2.3

 9.3  5.2  4.2  4.1  2.3

 6.8  6.5  4.3  3.8  3.4

 8.0  7.5  7.3  5.8  5.4

 5.7  5.6_ 6.5  6.2  5.3


 5.1  4.3  4.4  3.7  3.2
0.24   0.23   0.32  0.32  0.30

0.12   0.14   0.23  0.30  0.29

0.69   0.61   0.63  0.60  0.62

1.05   0.98   0.99  0.94  0.95

0.12   0.12   0.18  0.21  0.32

0.45   0.40   0.38  0.37  0.32

0.78   0.82   0.74  0.59  0.61

0.46   0.48   0.45  0.45  0.49

0.53   0.55   0.48  0.63  0.68


0.25   0.24   0.20  0.18  0.20
                  7.7  7.0  6.4  5.5  4.3  |  0.49   0.45   0.40  0.41  0.42
                                           J_
0.60   0.48  0.55  0.45  0.38

0.45   0.48  0.50  0.48  0.40

0.98   0.88  0.88  0.73  0.75

1.37   1.25  1.25  1.10  1.08

0.34   0.33  0.36  0.36  0.30

0.76   0.68  0.56  0.50  0.41

1.00   1.03  0.90  0.76  0.73

0.74   0.73  0.66  0.53  0.74

0.73   0.75  0.73  0.85  0.88


0.43   0.38  0.35  0.32  0.30


0.75   0.70  0.58  0.58  0.58
* See Table  6.6  for generic fiber description and manufacturer.

** Mass  flux 1.7 x 10"3  Ibs/ft2-min,  for  20 min. filtering  cycle, W   = 0.034  lbs/ft'
                     L

-------
a
01
u
•l-l
<4-l
41
U
c
id
•H C-J
VM  4J
•H  VM


-------
                                               TABLE 6.8
                EFFECT OF RELATIVE HUMIDITY ON OUTLET DUST CONCENTRATION AND  EFFICIENCY
                              (From Durham and Harrington, Reference  16)
Outlet dust concentration, grains/1000 ff3
Bag
Description*
Nylon
-, R
Creslan
R
Dacron
Polypropylene
CrylorR
Dralon
Orion
Cotton
Filament glass
Combination glass
Texturized glass
20
130
168
34
36
148
26
12
0.04
148.1
10.4
63.8
Relative humidity, °L
30 40 50 60
148
177
32
32
89
24
7.5
0.2
135.9
10.3
40.2
61
100
13.1
35
56
17
6.9
0
106.4
2.2
19.9
4.4
37
1.9
7.0
13
0.8
3.9
0
25.4
0.1
6.0
0.02
3.1
0.7
2.7
1.3
0.6
0.8
0
9.1
0.1
1.1
95
94
98
98
95
99
99
99
95
99
97
20
.62
.47
.86
.80
.12
.11
.59
.99+
.00
.65
.84
Efficiency, weight 7=
Relative humidity, 7=
30 40 50
95.02
94.02
98.95
98.96
97.29
99.23
99.75
99.99+
95.31
99.66
98.66
98.02
96.35
99.12
98.85
98.14
99.43
99.78
99.99+
96.55
99.92
99.31
99.86
98.78
99.94
99.78
99.56
99.98
99.87
99.99+
99.18
99.99+
99.80
60
99.99+
99.90
99.98
99.91
99.96
99.99+
99.97
99.99+
99.71
99.99+
99.96
Note: Inlet dust concentration C. =3.0 gr/ftj, fly ash, 4.0 urn median- diameter.
      * See Table 6.6 for generic fiber description and manufacturer.
      (From Durham and Harrington, Ref. 16).
        I ._    L ..

-------
              probably tlve major factor in producing uniformly high effi-
              ciency.  Figure 6.38 illustrates the effects of relative hum-
              idity on the outlet dust concentrations for Crylor^, CreslanR,
              Orion**, and  polypropylene fabrics.

              Relative  humidity also significantly affects the outlet par-
              ticle concentration at various times during the filter cycle.
              Figure 6.39 indicates an initial outlet particle concentra-
              tion of about 1.5 x 1C)6 particles per cubic foot at the begin-
              ing of a 20-minute filter cycle.  As the filter cycle proceeds,
              the particle concentration drops off much faster as the rela-
              tive humidity increases.  Curves similar to those in Figure
              6.39 were obtained with all fabrics except cotton sateen.
              Relative humidity affects the rate at which the interstitial
              openings of the filter medium are bridged, the structure of
              the deposit, and the filtration characteristics of the system.
              The net effect of increasing the relative humidity of the
              carrier gas was to improve the efficiency of the system."16
o  I8°
 <4-
 g  160

 I
 2  140
 Q
 2  120
   100
 o
 o
    80|-
 UJ
 O
40|-


20

 0
           O   Creslan  Acrylic
           •   CrylorR Acrylic
               Polypropylene
                      Acrylic
                 10
                       20          30        40

                           RELATIVE HUMIDITY, %
50
60
         Figure 6.38.   Effect  of  Relative Humidity  on Outlet Dust Con-
                       centration.
                       (From Durham and Harrington, Rof.  16).

-------
10'
                                                 RELAXIVE  HIIMIDITY, 7»
                                                       D  20
                                                       O  30
                                                       A  40
                                                       •  50
                                                       •  60
   0
Figure 6.39.
              8
   10     12     14
FILTER TIME, min.
16
20   22
Relationship of Particle Concentration and Filter Time at
Various Relative Humidities. (From Ref. 16)

-------
              These results are consistent with observations of increased
particle  adhesion  force at higher R.H.  (c.f. Figures 2.17 and 2.19, and
Equation  2.15).  The  increased force  should tend to produce a more open,
porous, and  filamentous deposit having  lower K' and better retention  (less
particle  migration and bleed).  Humidity may also affect the size of  the
particles collected on the filter.
              It should be noted that the above effects were obtained only
with  fly  ash, while humidity had no apparent effect on resistance or col-
lection efficiency when using cement dust, pulverized limestone, or amor-
phous silica under identical test conditions.  Clearly, further research
is needed to determine what aspects of  fabric and particles affect, through
humidity, the resistance and collection efficiency of a filter system.
              6.2.2.6  Effects of Velocity - Several aspects of filter per-
formance  have been observed to depend upon total gas flow rate or average
filtering velocity.  These include:
              o  Increase in specific resistance with velocity;
              o  Amount of dust depositing directly in the storage
                 hopper before reaching the filter (hopper fallout);
              o  Particle size stratification of dust passing to the
                 fabric because of fallout, and within the bag, as
                 velocity (flow) decreases throughout the length of
                 the bag;
              o  Deposit reorieptation or consolidation if velocity
                 increases during filtering cycle (deposit collapse);
              o  Particle penetration through thin or open areas on
                 the fabric surface (deposit puncture).
              Hopper Fallout - Hopper fallout has been observed during
single bag and single compartment studies.  A fallout factor,  7, has been
defined  as:
                _ amount of dust deposited in hopper,  W - W _
                  amount of dust entering collector inlet,  W  '  or
                      w                                               (6.19)
              7 = 1 - ~
                      W

-------
 That is,  / i-s  the fraction of the total dust entering the inlet of a
 fabric filter deposited directly in the hoppers, without being filtered
                                                                         14
 by the fabric.   Hopper fallout is reported in the fabric study of Durham
 above, (but  without 7).  Values of 7 depend upon dust particle size and
 shape, as  related  to  hopper,  inlet;, baffle, flow, and collector configura-
 i. ion parameters.  Typical values Tor y  from fabric I'll Lei: cumpai LiueuL
 studies (discussed below, see Figure 6.50) are 0.1 - 0.25 over a flow
                                                 3
 range of  four for  either top  or bottom  bag inlet .
             Particle  Size Stratification.  - Velocity also affects dust
 resistance by altering the particle size distribution in the  deposit.
 The  bulk  flow of gas  and dust is parallel  to the filter  surface and de-
 creases in velocity from a maximum at the  entrance of the bag to zero vel-
 ocity at  the other end.  Filter arrangement also produces changes in par-
 ticle size distribution by inertial separation and settling of larger par-
 ticles as  fallout.  During bottom feed  operation,  a greater portion of
 larger particles enter the bag as the gas  throughput is  increased (i.e.,
 dust fallout decreases).   Thus,  the average particle size of  the fallout
 varies with  filtration rate,  as  does  the average particle size of the
 filter deposit.  Since the resistance coefficient  is sensitive to the por-
 osity of the cake  and  the surface-to-volume ratio  of the  particles,  as
 given by the Kozeny equation,  size  stratification  will contribute to
 changes in the resistance of  the dust cake.
             The  effect  of velocity  on particle  size  statification along
 the  length of a  single  bag  is  shown in Figures  6.40  and 6.41.   In these
 tests, made  on a single  filter bag, samples  of  the  dust cake were  taken
 from several different  vertical  positions  along  the  bag.  Figure  6.40  in-
dicates the  effect of velocity on median particle  diameter deposited  at
 four  different elevations  (of a  60  inch  bag  height).  Just above  the  bot-
tom  inlet  (2  in.),  median particle  size  varies  from  8 to  15 ^m, as aver-
age  filtration velocity is varied from 2.5  to 8.5  fpm (approximate upward
velocity at  bag  inlet,  109 to 327 fpm).   Just below  the top, most  of  the
dusty gas  has been filtered, upward velocities are  low, and median par-
ticle size varies from 5  to 13  um as average filtering velocity varied
from 2.5 to 8.5 fpm.   The deposit at  the top is composed of finer particles
(higher K~).   Figure 6.41 shows  the effect of filtering velocity on par-
ticle size distribution at the center of the bag.  Higher gas  flow supports


-------
                           POSITION ON BAG. Inch*! from bottom
                              2   •
                              16   .
                              27   *
                              58   «•
                                                        10  n
Figure 6.40.
               Effect of Filtration  Rate on Particle  Size in Deposit  at
               four  bag altitudes  (Single bag filter  unit, bottom feed)
               (From Bongwardt and Durham, Ref. 3).
                                                       7.4 fp«
                                                       6.1 fpn
                                                       3.7 fpi"  A
                                                       2.5 fp»  «
                          10
                                  IS       20       25
                                  FAHXCU SIZE, «ietOM
                                                          30
                                                                  35
                                                                           40
Figure  6.41.
              Effect of Filtration  Rate on Particle  Size Distribution in
              deposit at Center of  Bag (Single Bag Test Unit, Bottom Feed)
              (From Borgwardt and Durham, Ref. 3).

-------
 larger particles,  the median particle size increasing from about 6um at
 2.5 fpm to about 14 urn at 7.4 fpm.   These figures show that elutriation
 of the particles has  occurred inside the bags and that the degree of elu-
 triation depends upon rate of gas flow.   Changes in particle size distri-
 bution between different positions  on the filter surface result in the
 formation of non-uniform filter cakes.   These phenomena indicate that the
 specific resistance coefficient will depend upon filtration rate, through
 effects  on particle size.
             Deposit Consolidation.  - Deposit consolidation (collapse)
 occurs when forces  produced on and  in the granular matrix exceed frictional
 and adhesive forces supporting the  particle aggregate structures. Forces
 tending  to produce  consolidation arise from static pressure differentials
 across the deposit  (1 in. H_0 = 0.036 psi = 5.2  psf),  viscous  drag of the
 fluid  on the particles,  kinetic energy imparted  by depositing  particles,
 flow pulsations, or physical contact with the  filter.   If these  forces
 are greater than interfacial particle-particle-fiber  forces, the  structure
 can shift  to a more compact,  stable  orientation.   Permeability  is reduced,
 and pressure drop rises  .
             Deposit consolidation and collapse has  been  observed  experi-
                                                                       4
mentally on both bench-scale  apparatus and  in a  pilot-scale  2-bag unit.
 In  experiments with fly  ash  on  bench-scale  equipment, cake  collapse was
produced by  gradually  increasing air  flow through a dust  cake supported
on  a flat  circular  filter  1.5  inches  in diameter.  Deformation of the
matrix occurs in steps over appreciable intervals of filter velocity.
Deposit  thicknesses were measured with a microscope having a graduated
micrometer  fine adjustment.  Irreversible compressions up to 50 percent
                                                4
of  the original deposit  thickness were observed.    (Actual deposit thick-
ness was not reported.)
             In an experiment with a pilot-scale 2-bag unit, filters were
                                               2
loaded to an average dust mass of 950 grains/ft  at an average filter
velocity of 3.4 fpm as shown in Figure 6.42.  The resultant pressure  dif-
                                                                2
ferential was 3.4 in.  H~0 (effective permeability 950 (grains/ft )/

-------
I
-si
UJ
i rtuuu
E
o.
t:
x^eoo
z
V}
Z cnn
'ERMEABILITYJJGRA!
A C
o <
o <
«- 200
en
(O
2
Q <
1 1 J II 1
** '* ** **** ^2^1st COLLAPSE, A P=b-9 IN. H20
\ 2. nd COLLAPSED pr 8.7 IN. H20
~~ ^>- J^^(t<^3rd COLLAPSE, A P=I04 IN. H20
•^B" O CT O w*^ H2U
_ \.,xAP=l5.8 1N.H20 _
NOTE:
DUST MASS CONSTANT AT 950 GRAINS/ft2,
CAKE DEPOSITED AT 3.4 fpm
KEY:
m m INCREASING Ap
o 	 o DECREASING A P
1 II 1 1 1
324 6 8 10 12 U
                                     AVERAGE  FILTER VELOCITY (fpm)
                          Figure 6,42.  Successive Deposit Collapse Observed On Pilot-

-------
 (in. II  0/fpm)  ).   Filter  velocity was  lowered  to  about  0.75  fpm and  then         __,
 raised  to  5.9  fpm during  which  time  permeability  remained  constant at  950
 (K' =7.2).  As  filter  velocity was  gradually  increased above  5.9 fpm,
 however, permeability decreased rapidly  to  785  (K_  =  9.9)  at V = 6.5 fpm.
 From 6.5 to 7.2  fpm, the  permeability  of 785 was  maintained.   Between  7.2          >
 and 7.6 fpm a  second collapse occurred,  further reducing permeability  to         ~'
 745 (K' =  10.1),  a value  which  was maintained up  to 8.1 fpm.   At this  point,
 filter velocity  was reduced  to  5  fpm to  substantiate  that  a new matrix            —';
 structure  of lower permeability had  been created.   Two  more distinct col-
 lapses were observed as filter  velocity  was ultimately  raised  to 9.8 fpm,         _,
 and after  each of these,  filter velocity was lowered  and then  raised, to
 demonstrate that  a permanent consolidated structure had been produced in
 each instance.  After the final collapse, permeability  had been  reduced  to
 585 (K' =  11.9),  or to  about 60 percent  of  the permeability of  the uncol-           \
 lapsed deposit.   Filter drag was  increased from 1.0 in.   HoO/fpm  for  the uncol-
 lapsed cake to 1.62 for the final collapsed matrix, or by more than  60 percent.     ,
The same effect has been  observed in single compartment  and multicompartment      ^
 filters, as discussed below.  These effects also depend  upon R.H. in the gas,
and fabric parameters.                                                             ^J
             Particle Penetration - Woven fabrics arc susceptible to pin-
hole flow  leakage, a common phenomena with many types of porous media.
Deposit puncture,  which occurs  when small pinholes  are opened through the
matrix, should be  more prevalent on filament yarns  or worn staple fabrics.
Local resistance  is lowered and high flow occurs through the opening.
Local disintegration of the filter deposit is a more severe manifestation
of the application of external  forces that cause deposit collapse.   The           •~f
puncture may be self-repairing,  since increased flow through the area
brings more dust to the vicinity of the puncture.   Thus, inertial deposi-         _j
tion mechanisms are enhanced by the higher velocity.  If the holes  are
                                                                                    ;
too large,  however, penetration will continue.                                       !
             Deposit puncture has been confirmed experimentally as  shown
in Figure 6.43.  On a bench-scale flat filter paper it could not be  in-            _j
duced even at very high pressure differentials.  However, when  a cotton
sateen filter fabric was used as the support,  puncture occurred as  shown
in Figure 6.43.  It is  seen that the unpunctured cake  had a drag of  1.0

-------
    •$.
       2.0
       1.5
       1.0
      OS
NOTE: ORIGINAL DUST MASS APPROXIMATELY
      1,000 groins/ft.2, CAKE DEPOSITED AT
      5.3 fpm
            BEFORE
                           •—• INCREASING AP
                           o—oOECREASING AP
                     AFTER PUNCTURE
                                  , FABRIC RESISTANCE
                  10
            20       30       40
             FILTER VELOCITY, fpm
50
61
          Figure 6.43.  Deposit Puncture Observed on Bench-Scale
                       Filter.  (From Ref. 4).
in. H-O/fpm.  As filter  velocity was raised above 10 fpm,  drag began to
decrease sharply and continued to decrease, approaching a limit  erf
approximately 0.47.   Reduction of filter velocity to 10 fpm verified the
constant resistance  of the punctured deposit.   The pinhole punctures crea-
ted were easily visible  by microscopic examination of the  fabric against
a bright light.
              An example of pinhole healing is described by Stephan, et.
   4
al.   The air velocity through the perforated  area dropped from approxi-
mately 100 fpm to about  2.3 fpm during a few minutes of normal filtering.

     6.2.3  Single Compartment Performance
            The laboratory performance of single compartment intermit-
tently cleaned fabric filters is effectively represented by single bag
performance tests discussed above.  Differences are associated with scal-
ing and in distribution  of flow to each of the several  bags.

-------
             6.2.3.1  Shake-Type Collector - Billings, et. al.,   reported         -"
                                                             p
 laboratory performance data for a small Wheelabrator Dustube  (2 #35A)
                             2                      3
 cloth tube collector (500 ft  of fabric, nominal 10  cfm capacity).  This         _j
 equipment is representative of several commercial collectors.
                                                                                     i
             The test unit consisted of two identical steel chambers, each         —I
 containing 32-5" x 70" cloth tubes.  In normal operation, one chamber fil-
 tered the dusty air while the second was cleaned by mechanical shaking of         _^
 the cloth tubes.  Operational time between cleaning cycles varied with dust
 loading and desired pressure drop characteristics.   Tubes were usually
 shaken when they attained a pressure drop of 2 to 4 in.  H^O.
             As the number of compartments increases, different sections             !
                                                                                   —^
 may be shaken sequentially, but the proportion of bags not in operation
 decreases so that large multi-chambered units operate at nearly constant            j
           ,                                                                        —i
 pressure drop.

             The objectives of the test program were as follows:                    _J
             1.  To evaluate the resistance-efficiency characteristics
                 with a variety of aerosols at different  dust load-
                 ings and filtration rates.                                        ~J
             2.  To compare laboratory results with  those obtained
                 in the field, since only field results indicate
                 service life and the effect of maintenance pro-
                 cedures on filter performance.
             3.  To investigate the feasibility of this type  of
                 unit for removal of low concentrations (e.g. toxic
                 materials such as Be, radioactive dusts, etc.)
                 from air and gas streams by means of augmentation                   j
                 with filter aids.                                                 —'

            Efficiency and pressure drop tests were  conducted in three               j
operating modes:  (a) performance of woven cotton medium with light dust
loadings and no bag shaking (i.e., basic cloth performance),  (b)  per-                '•
                                                                                   _j
formance of asbestos-flocked bags with light loadings and no shaking, and
(c) performance with higher dust loadings and frequent, periodic  shaking.
            Light Dust Loadings.- Tests with dust loadings in the range
0.04 to 0.1 gr/1000 cu. ft. (atmospheric dust) illustrated initial fil-
                                 6-76

-------
ter performance and provided basic  information on cloth characteristics.
Figure 6.44  illustrates the efficiency and pressure drop  increase  as  new
cotton bags slowly acquired a deposit of atmospheric dust.  These results
were obtained during a period of 484 hours of operation on atmospheric
dust, but are presumed to illustrate what takes place during a  few  min-
utes when higher dust loadings are  being filtered.  Both efficiency and
   100
                                              FILTRATION RATE 3CFM/SPFT
                                              SATEEN WEAVE COTTONBAM
                                              KM X 68 THREADS PER INCH
                             i.s       a
                   •MAINS OF OUST PCM SOU Aft E FOOT Of CLOTH
Figure 6.44.  Efficiency and Pressure Drop; New Cotton  Bags With Atmo-
              spheric Dust,  (prom Billings, et. al, Ref. 17).

pressure drop rise as  a deposit forms but the rate of increase fails off
after  only  a  few grains of dust have been deposited.   (From observation
of  industrial operations  it is  known that this increase would continue
for1  several weeks  with  the very low dust  loadings  found in ambient air.)
Over the period  of this experiment  penetration decreased 3 fold  (from
27  percent  to 9  percent) while  pressure drop  doubled  (from 0.09  to 0.18
in.H20).
                                                                          o
                                                                            CM
                                                                           (X
                                                                           o
                                                                           £
                                                                           0)
CO
CO

-------
            Filler Aid. - Figure 6.44  indicates that  i.L  takes many hours
of operation (at dust  loadings less than 0.1 gr/1000 cu.ft.) to  increase          ^J
filter efficiency to 90 percent.  For recovery of atmospheric dust,and
                                                                                   I
chemically toxic  or radioactive dusts, a high initial  efficiency is re-           j
quired.   This can be  achieved with deep beds (up to 5  feet) of  fibers or
granular solids, or with glass fiber  paper filters which are discarded             ;
when their resistance  becomes excessive.  However, operations with  a
fabric filter on low dust concentrations can be simplified and mechanized          ;
                                                                                   j
by using a filter aid.                                                            —'
            Asbestos floats (fibers too small for other use) were dis-             j
persed and filtered onto clean cloth of the same type as above.  Table
6.9 indicates the effectiveness of this treatment in increasing the
collection efficiency  of oil smoke (Diol 55, mass median diameter 1.2 urn).        -J
Efficiency was measured optically.
                                                                                  —•
                                TABLE 6.9
        EFFECTIVENESS OF FILTER AIDS FOR LOW PARTICULATE LOADINGS                  '•
Quantity of
Asbestos on
Filter Cloth
gr.V sq.ft.
0
32
50
82
154
182
194
Pressure Drop

(at 3 cfm/sq.ft.)
in H20
0.098
0.135
0.160
0.170
0.187
0.202
0.212
Smoke
Efficiency

per cent
20.3
44.9
52.7
84.7
96.4
99.4
99.9
            This shows a very substantial gain in efficiency) 20.3 to              i
99.9 percent) for a modest pressure drop increase (0.098 to 0.212 in.
H~0) .   Since bag filters are usually designed for resistances of 3 to              J
4 in.  H»0 before shaking, flocked bags can be operated for many months
on low loadings before shaking of the bags and renewal of the flock.
                                                                                   i

-------
             Table 6.10 indicates how efficiency decreased as the filter
 aid was removed.  Some asbestos fibers were held permanently by the
 fabric; hence, after shaking, only a small smount of additional flock
 restored the high efficiency characteristics of the filter.

                               TABLE 6.10
      REDUCTION IN EFFICIENCY OF ASBESTOS FLOCKED BAGS DURING SHAKING
Time of Bag
Shaking
:Minutes
ob
0.5
1.0
2.0
3.0
5.0
7.0
10 0
15.0
20.0
30.0
40.0
.Pressure Drop
(at 3 cfm/sq. ft)
in. H20
0.212
0.208
0.204
0.196
0.182
0.180
0.176
0.171
0.167
0.166
0.166
Smoke
Efficiency3
percent
99.9
97.7
97.0
96.0
94.0
93.3
90.0
89 .0
88.4
85.3
83.0
83.0
 a.   1.2  micron droplets  and  optical  penetrometer
 b.   Flocked with  194  grains  of  asbestos per  square  foot of cloth
 (rrom Billings, et.al, Ref.  17).
Table 6.11 lists the efficiency of plain and asbestos flocked cotton bags
for a variety of other aerosols and demonstrates the wide application
of the technique.
            The quantity of asbestos required to produce high filtration
efficiency is dependent,to a large extent, on how uniformly the fibers are
dispersed on the cloth.  With the flocking apparatus used a  resistance
rise of only 0.1 to 0.2 in. H^O was adequate.

            Commercial systems lor high efficiency performance with filter
aids have been discussed  in Chapter 3 (see Figure 3.31).  Wheelabrator
                              *
now uses a preliminary precoat ,  prior to addition of asbestos or other
       *U.S. Patent No. 3,041, 808.

-------
                               TABLE  6.11
             EFFICIENCY OF BAG COLLECTOR FOR VARIOUS AEROSOLS*
"— •• • ' " 	 ' "• '
Aerosol
Atmospheric dust
Uranium trioxide
microsphcres
Copper sulfate
microspheres
Aluminum chloride
fume
Ammonium bi-
fluoride fume
Ammonium bi-
fluoride fume
Talc

Loading
gr./lOOO
ft3.
0.025
0.0079

0.86
166

15

15

0.1 -
6.0C

Median
Size by
Count
microns
0.5
0.8

0.9
0.6

0.5

0.5

1.4

Pressure
Drop
(at 3 cfm/sq.
in. H20
0.212
0.212

0.236
0.244

0.244

0.11

0.157 -
3.54

Bag
Treatment
ft2.)
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
Asbestos
flocked
None

None

Weight
Efficiency
per cent
99. Ob
99.9

99.1
99.2

99.7

74.0

99.9+
a. Sateen weave cotton bags, filtration velocity 3 cfm/sq.ft.
b. 85 percent efficient by discoloration test, 68.2 percent efficient
   by particle count
c. Grains per cubic foot
(From Billings, et. al., Ref.  17).
fibrous filter aid, to promote release of the filter aid and any tarry
atmospheric constituents.  The specific resistance coefficient for fil-
                                                                    2
tration of atmospheric dust on filter aid is 16 (in. H20/fpm)/(lb/ft ).
            Periodic Shaking.- Figure 6.45 indicates variations in pres-
sure drop during normal filtering and shaking operation.  The cycle
includes a 15 minute filtration period, 2.5 minutes for shaking, and a
2.5 minute off-period for dust settling.  There are some variations in
maximum and minimum pressures attained.  For these thoroughly aged cotton
_J

-------
t»
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t-t
~t*
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«!.«
*
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«i.t
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*.•
.•
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Q/C? • OUST LOADINt IN Oft*
	 • FILTCftlNa PERIOD
	 • SMAKIN9 PCNIOO
- acrM/so.Ft or COTTON •*• ,
TALC DUST (NYTAL 500) /




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-------
     10  20
             30   40  50  60  70  SO  90  KX> HO  120  ISO 140  ISO ISO  170
                            FR.TMATION TIME.- MINUTES
                                                                     2OO
Figure 6.46.  Effect of Dust Loading on Rate of Filter Pressure  Drop
              Increase.  (From Billings, et. al., Ref. 17).
greater than 99.9  percent  in  all  cases.   Pressure  drop,  when filtering
         3                     2
O.L gr/ft  of talc at  3  cfm/ft  ,  increased  from 0.3  to 0.7 in.  HLO in
                         2
30 minutes.  At 1.0 gr/ft  , the pressure  drop  increased  to 1.5  in H-O at
the end of 30 minutes.
            The outlet dust loading  increased  somewhat with inlet load-
ing, but collection efficiency remained above  99.9 percent.  These data
indicate that while there are some differences  in  specific cases, the
general outlet dust concentrations from this type  of collector  using
                                                              3
cotton sateen fabric will probably be  less  than 0.1  gr/1000 ft   (200 vg/
Data presented earlier in Table 6.8  for cotton  sateen and fly ash tend to
confirm this general observation.    Field  performance data (Table 6.23)
have indicated similar outlet loadings.   However,  outlet loadings up to
              3          3
180 gr/1000 ft  (0.4 gm/m ) were  observed with  man-made  filament  yarns.
Relative humidity also affects filter  efficiency as  discussed above.
                                                                         3
                                  6-82
                                                                                   J

-------
                               TABLE 6.12
      EFFICIENCY AND PRESSURE DROP OF "DUSTUBE" FILTER AT VARIOUS
                     LOADINGS OF "MICRONIZED" TALCa
Inlet
Loading
gr./cu.ft.
0.1
1.0
2.0
Pressure Drop

in. H20
0.3 to 0.7
0.3 to 1.5
0.5 to 3.0
Outlet
Loading ~
gr./lOOO ft .
0.012
0.034
0.070
Weight
Penetration
percent
0.012
0.0034
0.0035
Weight
Efficiency
percent
99.988
99.9966
99.9965
a.  Mass median diameter 2.5 microns, geometric standard deviation 1.6,
b.  Pressure drop increased from lower to higher value in 30 minutes.
c.  Air to cloth ratio 3 cfm/ft.^; sateen weave cotton bags.
    (From Billings, et. al., Ref. 17).
            6.2.3.2  Other Single Compartment Studies.- The data discussed
below were obtained on multi-compartmented collectors operated in parallel
mode, (no compartment off-line for cleaning) such that effects studied
relate primarily to single compartment performance.  The 3-compartment
                                                          3 18
pilot fabric filter system used for most of these studies, '   is shown
in Figure 6.47.  Each compartment contained 8 cotton sateen bags 60 in. long
x 5 in. diameter.
            Hopper Fallout.- Hopper fallout, 7, was defined in Equation
6.21 as the fraction of dust entering the inlet of a filter collector
that does not reach the fabric.  Figure 6.48 shows values of 7 when fil-
tering upward and downward, at several gas volume through-puts.  Average
                                                2
air to cloth ratio ranged from 1.3 to 4.5 cfm/ft  .  in bottom entry, up-
flow operation:
            . more material was carried into the  filter bags as
              velocity increased, and
            . The fallout varied from 27% at 1.3  fpm to 8% at 4.5 fpm.
In top-entry, down-flow operation:
            . the fraction falling into the hopper increased as
              velocity increased, and
            . the fallout varied from 8% at 1.3 fpm to 30% at 4.5 fpm.


-------



         Figure  6.47.
Three Compartment Baghouse (From Robinson,
et. al.,  Ref.  18).
Lacking internal configuration dimensions, an analysis of flow velocities,
particle sizes, settling velocities and stratification has not been made.
            Particle Size Stratification.- A particle size distribution
of fly-ash from hopper and filter deposits is shown in Figure 6.49 ob-
tained from a 30,000 cfm prototype 4-compartment field unit operated on a
                             18
pulverized coal-fired boiler.    Hopper fallout varied from 8 to 16%.
Fallout sizes were considerably larger than those reaching the fabric.
            Deposit Consolidation.- The effect of filtering velocity on
the specific resistance coefficient has been reviewed above (see Figure
6.42).  Further experimental data on the three compartment collector
operated in parallel mode is shown in Figure 6.50.   Dust was deposited
at low velocity (point a), the dust feed stopped, and the air flow in-
creased without further dust deposition.  At points b and c the gas flow
                                   6-84
                                                                                     o
                                                                                  J
                                                                                  u
                                                                                  -
                                                             i
                                                           — •
                                                              9

-------
  §
  u,
  o
  •-C
  u.
Ab
10
35
30
i . -1
2C

15

K)
',
0
^ ( i I I I ' i '
Top Feed = top inlet, downflow, dust deposition inside
_ sleeve
Bottom feed = bottom inlet, upflow, dust deposit on in-
side of sleeve
TOP FEED DOWN
BOTTOM FEED UP FLOW . FLOW
. 9
+v* /
2^
/• ^^^ ^
9/. 4 ^*^
^^^ >,
»x^» •»•
of
•
-
-
H
\
-i

-
3-comp. collect
(8)5 Y. 60in. cotton
~ sateen bags/comp. *"
(collector fabric area,
. J 1 1 l l i ' 1 .
) .'00 100 600 800

100
  Figure 6.48.
               TOTAL  GAS  FLOW_,  cfm
Fly Ash Fallout vs. Gas Throughput For Top and Bottom
Feed (Parallel Flow, Constant Rate Filtration) (From
Borgwardt and Durham, Ref. 3).
was decreased  toward  zero  and  raised  again.  The  test  shows  that  as  flow
is decreased the filter permeability  remains constant, but when flow  is
increased the  permeability decreases.  The apparent compacting effect
may be due to  a displacement of particles from the deposit at higher  gas
flow and redeposition deeper in the filter matrix.
6.3  LABORATORY PERFORMANCE OF MULTICOMPARTMENT EQUIPMENT
     6.3.1  Basic Pressure Drop Equations
            In discussing the performance of single compartment filters,
the parallel functioning of the separate filter elements was compared to

-------
                                                               1000
                            MRTCLC SIZE, MtorMw


Figure  6.49.  Particle Size  Distribution of Fly  Ash (from
               Robinson,  et al,  Ref.  18)
  10
                  HOW INCREASING

                  FLOW DECREASING „
                            •>..—.—. •
                              (a)
                                                  _L
                            2           3           4

                        AVERAGE GAS FILTER VELOCITY (»). fpn
 Figure 6.50.   Dust Cake Compression (Pilot  Baghouse,  Parallel
                Flow Operation)  (From Borgwardt and Durham, Ref.  3)

-------
 that of electrical resistances in parallel.  The same  analogy applies  to
 two or more compartments operating side by aide with  the  same differen-
 tial pressure driving force.   The equations  are essentially unchanged if
 the compartments are all shut down for cleaning at  the  same time.   If
 however,  the compartments are taken off line one at a time  for cleaning,
 they operate at different cycle phasings,  and the equations must  account
 for this.

             When a just-cleaned compartment  is  returned to  servi.ce, its
 drag will  be less  than that of the  other compartments  (Figure  6.51).
 Consequently the flow through the  compartment will  be greater  (Figure  6.52),
 This means  that  the  rate of deposition of  dust  in this compartment will
 be  greater  than in the other  compartments, and  the  compartment drag will
 increase more  rapidly.   The compartment will  rapidly approach  a level  of
 operation  similar  to the average  of  the other compartments.
             The  flow through  the  overall system,  which also  increases
 when the cleaned compartment  is  first  returned  to service,  gradually de-
 creases as  the  drag  in the separate  compartments  increases.  When the
 next  compartment is  removed from  the system  for cleaning, the overall
 flow will decrease again.  Thus  the  overall  flow  follows a cyclic pattern.
             The  variation  in  overall flow  depends on the characteristic
 curve of the fan driving the  system  and other system flow resistance
 characteristics.  As  a  consequence of  overall flow variation, the dif-
 ferential pressure across  the  separate compartments will also undergo at
 least some variation.   This is  indicated schematically and described in
 Figure 6.53.
             Velocity variations may occur not only from compartment to
 compartment, but also  from filter to filter and along  the length of a
 filter, as discussed above in single bag performance analysis.  Therefore
 the use of nominal velocity to describe overall filtration is arbitrary,
since for a given nominal velocity many combinations of internal flow
distribution may exist.

-------
raw
•0
i
fc •«
3 4O
£
= «
A
II 1 1 1
1
•
">•*-•
- 1 5
J±t|-
	 tn fc 	
M
1 1 1 1

0.0         IX)        1.5
   FILTER DRAG (INCHES
                                                       8.0
Figure 6.51.   Instantaneous Filter Drag  Profile  for Six Compart'
                ment Baghouse.  (From Walsh and Spaite,  Ref.  5).
            10
          £ •
                   "i	T—r
                        —r
                         AVERAGE VELOCITY : 3jO 
-------
      c\j
    CO
    UJ
    X
    z
    UJ
    tt:
    UJ
    u.
    u.
    o
    UJ
    o:
    :D
    co
    CO
    UJ
    a:
    a_
                           TIME
Figure 6.53.  Schematic Pressure Differential Curve for
              Multicompartment Baghouse)(From Ref. 5).
Explanation:   Time t.
               Time
               Time t
A compartment has just returned on-line
with a lowered drag; the system flow is
higher than usual, and the collector
pressure drop is lower than usual.
The next compartment has jus t been removed
for cleaning, and total cloth area is
less than usual. Thus filtering velocity
is slightly increased, and pressure diff.
has also increased. System flow has decreased.
The compartment has been cleaned, and is
returned on line.  The system returns to the
same state as at Time tft, and recycles.

-------
            Residual  conditions  are  those which  exist  in a  particular             ^
compartment as  it  is  put on  stream after cleaning,  and  terminal  conditions
are  those which exist  in a compartment  just  before  it  is taken out  of             _j
service  for cleaning.  The residual  drag (set  by cleaning method and dust-
fabric parameters) and terminal  drag( set by fan and dust-fabric resistance
characteristics) establish overall limits for a particular installation;
therefore they  are critical  operating parameters.   Variations in filter            :
drag,  in establishing differences  in filter  velocities,  also influence
dust penetration  into the  filter,  filter blinding,  and  the  structure of
the  dust cake.  Drag  itself  is affected by these changes.  Some  of  the
equations describing  multicompartment system dynamics  are developed in
Appendix 6.3, and  are of a form  perhaps best studied by computer tech-            ->
niques.
     6.3.2  Performance of a Multicompartmented  Collector                        ^
            Billings,  et al.   have  reported a laboratory performance of           j
a  commercial multicompartment collector cleaned  automatically on a  time           ~~*
       *
cycle.   The test  unit consisted of  four compartments,  each  containing             (
                                                    2
eight  bags 8 in.  in diameter and 6 ft.  long  (100 ft  of  cloth/compartment),       .^"
as shown in Figure 6.54.  Dusty  air  entered  the  bottom  of es<-h section,
passing  upward  and through the fabric tubes.   Once  every 5.2 minutes each
compartment was shut  off from the  fan by means of a damper,  vented  to
atmosphere, and the bags cleaned.  Dust removal  was accomplished by               |
lifting  the whole assembly about 1-1/2" and  dropping suddenly.
                                                                                   t
            In  addition, outside air was drawn into the  compartment by the
negative pressure of  the hopper  and  inlet plenum, and passed through the
bags in  a reverse direction  to help remove dislodged dust.   Reverse air
passed down the inside of the bags and  through the inlet header  to  the
other compartments.  This cycle was then repeated on another section.
Sateen weave cotton bags were normally used  in this equipment at  a  fil-           —'
                         2
tration  rate of 10 CFM/ft .
            Performance characteristics investigated on  this collector            V
were the effects on pressure drop and efficiency of changes in reverse             ,
flow air, amount and frequency of rapping,  dust  loading, air velocity,             __,
and aerosol particle material.   Performance of five different bag'mater-
ials was also compared.

-------
 c
   -



                                                              OUTLET,
                                                              TO FAN
                                                                 A.  BACK FLOW
                                                                     AIR OPENING
                                                                 B.  SHAKING
                                                                     MECHANISM
                                                                 C.  AIR VALVE IN BACK-
                                                                     FLOW POSITION
                                                                 D.  8-6' BAGS PER
                                                                     SECTION

                  Figure 6.54.  Cloth Tube  Filter  Cleaned by Mechanical Rapping and
                               Back Flow Air.   (From BilUa**,  «t.  «!.,  fcef.  17).
                   An adaption of the British Simon Suction Filter manufactured  in  the
               U.S. by Entolater Div.,  Safety Car Heating and Lighting Co.,  New  Haven,
               Connecticut.
   .-
c

                          6.3.2.1  Studies With Light Dust Loadings
            Basic Media Performance.- The combination of rapping bags,  as  an
integral unit, to minimize distortion and the use of reverse air flow per-
mits the choice of several bag materials other than sateen weave cotton
cloth.  Such fabrics are:  (1) wool felt with its inherently higher effi-
ciency but less porous structure which requires reverse air for adequate
cleaning and (2) woven glass cloth capable of withstanding high temperature
but having poorer flexing characteristics.  Several bag materials were  tested
including heavy and light wool felt, woven glass cloth lubricated with  a sil-


-------
                          R                           17
 iconc1,  woven  napped  Orion ,  and  sateen weave  cotton.    The  basic  pressure
 clrop-efIficiency  characteristics  of  these  fabrics were evaluated on  light
                                  3    3
 dust  loadings  (0.01  to  0.1 gr./lO  ft ).  No  bag cleaning was  employed
 during  these  tests.
            The  pressure  drop and efficiency  of various  fabrics is  shown
 in Table  6.13  for atmospheric dust  and copper sulfate microspheres.  On
 atmospheric aerosol  at  10 cfm/sq. ft., the  fabrics may be rated in  order
                                  p
 of increasing  efficiency  as: Orion  .cotton,  light wool, woven glass, and
 heavy wool.
            The  last, two  columns  of Table 6.13 show  the  inlet  loading and
 weight  efficiency determined with copper  sulfate microspheres  for each
 of the  fabrics shown in the  first column.   The clean cloth  pressure drop
 for each  fabric  is related to copper  sulfate  efficiency.  The  relationship
 between weight collection efficiency  and  resistance for  these  five  fabrics
 is *
                          Wt. eff.  =  98.5 (Ap)°-33,                 (6.20)
 for Ap  inches of water, at a filtration velocity of 10 fpm with a
                                                                          19
 copper  sulfate particle size of  2.7 microns(MMD).  Billington  and Saunders
 report  the exponent  of pressure  drop  as 0.43  for 13 air conditioning filters,
 using a stain density measure of  efficiency with particles  in  the same size
 range.  They also mention an exponent of 0.1  found by Bigg  for weight effi-
 ciency  of fabric filters  using particles in the range 25 to 100 microns.
            Further  Studies of Filter Aids.  - To extend the knowledge of
performance of filter aids to other fabrics and higher filtration velocities
                                              R
 than those described in Section 6.2.2.1, Orion , cotton, heavy wool felt,
and woven glass were flocked with asbestos floats and tested with copper
 sulfate aerosols at  a filtration velocity of  10 fpm.   Bags were not shaken
during  the flocking  process,  nor during the testing with copper sulfate.
The results of these tests are given  in Table 6.14.  It is possible to in-
crease  the efficiency of  these materials to over 98 percent by the appli-
cation of asbestos floats as  a filter aid.
                                 2
            The use  of 200 gr/ft.  of asbestos floats had been suggested
as an optimum amount in the study described in a previous section.  To


-------
                              TABLE 6.13

                      FABRIC COMPARISONS WITH LIGHT LOADINGS3
I'abr it-




Light Wool
Felt
Orion,
Napped
Cotton,
Sateen
Weave
Heavy Wool
Felt
Woven Glass
Clean
Fabric
Pressure
Drop in H?0

0.07

0.10

. 0.29


0.34

0.56
Atmospheric
Inlet Loading
gr./lOOO
cu.ft.

0.184

0.130
K
0.045


0.220

0.058
Dust
Weight
Efficiency
percent

74.8

60.4
/>
35. 5°


85.4

81.9
Copper
Inlet
Loading
gr./lOOO
cu. ft.
0.89

0.81

1.02


1.01

0.90
Sul fate
Weight
Efficiency
percent

41.1

45.6

63.7


71.3
.
81.1
'Filtering velocity 10 cfm/sq. ft.

 Believed to be experimental error.
£
 Probably nearer 60 percent.

(From Billings, at. al.,  Ref. 17).
                               TABLE 6.14


        FABRIC COMPARISONS USING ASBESTOS FLOATS AS A FILTER AID3
Fabric

Orion
Cotton
Dense Wool
Woven Glass
Pressure Drop"
in H20
Initial
0.16
0.36
0.36
0.81
Final
0.95
1.43
0.90
2.20
Copper Sulfate Weight
Efficiency
per
Initial
45.6
63.7
71.3
81.1

Final
98.56
98.95
99.28
99.60
3200 gr./ft2
  Filtering velocity - 10 fpm

 (From Billings, et. al., Ref. 17),

-------
 confirm this  on a different aerosol and at a different filtration rate,
 glass  bags  were flocked in small increments, and the efficiency was evalu-
 ated after  each step with copper sulfate microspheres (Table 6.15).  Each
 increment of  filter  aid caused  an increase in pressure drop and a decrease
 in  amount penetrating.   Effluent loadings at a velocity equal to 3 fpm were
 not appreciably different from  those at 10 fpm.

            Test  7 shows  the increase in penetration when  the unit was
 shaken five times, i.e.  five complete cycles or  20  strokes per compartment.
 About  60 percent  of  the original material,  by weight,  was  recovered from
 tVie hopper  at  this point.   The  remaining asbestos on the bags then amounted
                   2
 to  about 80 gr./ft .  Penetration and pressure drop  at this  time were dif-
 ferent than during loading due  to the uneven removal of the  flock and chan-
 neling of the  flow through less  flocked areas.   Use  of asbestos  floats  as
 a filter aid permits  control of  penetration and  pressure drop.   The primary
 aerosol particle  concentration  cannot be too high or frequent  replacement
 of  the filter  aid will  be  necessary (see Figure  3.31).  The  use  of glass
 bags with asbestos flock  can substantially  increase  collection efficiency
 for high temperature  work  in such applications as the  final  gas  cleaning
 stage  in incineration processes.   Field trials will  be required  to estimate
 service life of the  combination.   Present applications  of  filter  aids (1970)
 also include use  of  granular reactants  for  recovery  of  gaseous contaminants.

            6.3.2.2   Studies With Heavy Dust Loadings
            Frequency and Number of Raps.- The standard cleaning cycle
consisted of four raps per section,per 5.2 minutes, in conjunction with
entering reverse flow air.  Table 6.16 shows the variation of pressure
crop and penetration  as  the number of raps i's decreased.  Efficiency in-
creased as  pressure drop increased witn decreasing number of cleaning strokes,

            An investigation of  the effects  of the frequency of  rapping was
 made.   This was done  by reducing the  cycle  time  and  the number of raps,  in
 direct proportion, to keep  the number of raps  per section  constant  over any

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                                                                        f
                                         TABLE 6.15

        EFFECTIVENESS  OF FILTER AIDS  FOR LIGHT LOADINGS  OF COPPER SULFATE MICROSPHERES'
Test
No.
1
2
3
4
4a
5
5a
6
6a
7
Filtration
Velocity
cfm/ft.2
10
10
10
10
3
10
3
10
3
10
Cumulative
Filter Aid
on Fabric
gr./ft.2
0
25
50
100
100
150
150
200
200
b
Pressure Drop Copper Sulfate
in H«0 Loading
gr./lOOO ft.

0.86
1.56
1.76
1.80
0.63
2.00
0.66
2.20
0.72
1.46
Inlet
0.92
0.79
0.87
0.88
1.41
0.88
1.68
0.80
1.62
0.80
Outlet
0.130
0.024
0.019
0.012
0.007
0.008
0.008
0.003
0.003
0.026
Weight
Penetration
percent
18.9
3.1
2.2
1.4
0.5
0.9
0.5
0.3
0.2
3.4
Weight
Ef f icienc
percent
81.1
96.9
97.8
98.6
99.5
99.1
99.5
99.7
99.8
96.6
a.  Woven glass bags lubricated with silicone, no fabric cleaning during tests, except prior
    to Test 7.

b.  Recovered 60% from bags by rapping, five cycles = 20 strokes.

-------
                              TABLE 6.16
        EFFECT OF DECREASING THE NUMBER OF RAPS IN THE STANDARD
                     CLEANING CYCLE ON I^RESSURE DROP
                              AND EFFICIENCY
No. Raps/
Section in
5.2 min.
cycle
4
'}
2
1
Average
Pressure Drop
in. H«0

3.2
3.6
4.5
5.7
Dust
Inlet
gr./ft3

0.85
0.60
0.60
0.91
Loading
Outlet
gr./lOOO ft3

1.68
0.94
0.63
0.49
Weight
Penetration
percent

0.20
0.16
0.10
0.054
Weight
Efficiency
percent

99.80
99.84
99.90
99.946
 a.   Sateen weave cotton bags;  back flow air 100 cfm;  total air volume
     3000 cfm;  fly ash test dust.
     (From Billings, et. al., RcL.  1.7).
 long  time  interval.   Data presented  in Table  6.17  indicate  that  several  raps
 followed by  a  long pause  produced lower pressure drop  than  short cycling.  The
 rapping operation  appeared to  be  more  effective in removing dust than the
 reverse  air,  the  velocity of  which  was about  1 fpm,  (see below).
             Reverse  Flow  Air.-  The amount  of  reverse flow air  that  enters  a
 compartment  to aid in cleaning is often controllable.   The  above equipment was
 modified to  study  the effect of reverse air volume.  Results in  Table. 6.18
 indicate a 6-fold  increase  in  filter penetration when air volume was  in-
 creased 5-fold.  At  the same time, pressure drop decreased  about 50%.

             Inlet Dust Loading - The average pressure drop increases
with inlet dust loading as  shown  in Figure 6.55.  With fly ash, the relative
rate of pressure drop rise with dust loading decreases at higher loadings,
regardless of filtration velocity.  Talc produces  relatively higher pressure
drop at 2000 cfm and  the rate of rise with dust load is higher than with
fly ash.   These curves are  similar to performance  data for continuously
on-line cleaned reverse-jet and pulse-jet collectors.

-------
                                 TABLE 6.17

       KFFKCT OF VARIATION IN CLEANING CYCLE FREQUENCY ON PRESSURE DROP
                         OF A MULTICOMPARTMENT COLLECTOR*
Cycle No. of
Raps/Section
1
2
3
4
No. of
Minutes
1
2
3
4
Average Pressure Drop
in H20
Comp . I
5.8
5.4
5.0
4.8
Comp. IV
6.0
5.8
5.5
5.4
       *Sateen weave  cotton  bags,  back  flow air  100  cfm,  total  air
       volume 3000 cfm or 10  cfm/ft2,  inlet loading 0.65 gr./ft3,
       fly  ash  test  dust.   (From  Billings,  et.  al.,  Ref.  17).
                               TABLE  6.18

 EFFECT OF CHANGES  IN  REVERSE AIR VOLUME ON  PRESSURE  DROP AND  PENETRATION*
Amount of
Reverse Air
cfm
51
f.Q
85
120
180
250

Average
in H90
3.1
9 Q
2.6
2.6
2.3
2.2
Dust
Inlet
gr/cu.ft
1.10
i in
1.10
0.94
0.88
0.94
Loading
Outlet
gr/1000 cu.ft.
1.3
4.3
4.1
4.6
7.2
Weight
Penetration
Percent
0.12
0.39
0.44
0.52
0.77
Weight
Efficiency
percent
99.88
99.61
99.56
99.48
99.23
 *Sateen weave cotton bags; air volume 2000 cfm; 4/5 cleaning cycle; fly
  ash aerosol; 1 fr/ft-1.    (From Billings, et. al. , Ref. 17).
             Table 6.19 shows the penetration of fly ash as the inlet
                                                  3
dust loading is increased from 0.40 to 13.0 gr./ft .  Individual effluent

concentrations of each compartment are given in Table 6.20.  Compartment I

contributed almost twice as much to the effluent as the others, probably

due to uneven distribution of flow in the unit.

-------
o
 CM
CX
2
Q
CO
CO
01
0)
60
Cfl
H
01
                                    ENTOLETER UNIT

                              CURVE AEROSOL OUANTITV  RF AM
                               A  FLY ASH  3000 CFM  120 CFM
                               8   •  •   200O •   63 •

                               C   -  "   tOOO •   20 •
                               D    TALC   200O •   230 •
            Figure  6.55.
   INLET  DUST LQADING-GR./FT3


Variation of  Filter with Inlet  Dust Concentration

(From  Billings, et. al., Ref.  17).
                                 TABLE 6.19

       EFFECTS OF INLET  DUST CONCENTRATION  ON FLY ASH PENETRATION3
Dust
Inlet
gr . /cu. ft .
0.40
2.90
5.10
13.00
Loading
Outlet .,
gr./10J ft .
0.52
1.50
2.40
3.80
Weight
Penetration
percent
0.130
0.052
0.048
0.029
Weight
Efficiency
percent
99.870
99.948
99.952
99.971
 a. sateen weave, cotton bags;  back flow air  100  cfm;  total air volume
    1000  cfm or 3.3 cfm/sq.ft  ;  4/5 cleaning cycle.

    (From Billings, et. al., Ref.  17).

-------
            Compar J son of five fabrics.- The basic performance of  five  fabrics
 iltiM-ing light loadings of atmospheric dust and copper sulfate has been dis-
cussed above.  These fabrics were also used to filter heavy dust loadings to
obtain comparative performance data.  The pressure drop and penetration for
each fabric, used with the standard cleaning cycle, are given in Table  6.21
for fly ash and talc dusts.  Fly ash required a lower pressure drop on  all
fabrics.  Fly ash penetration was also greater.  Fabric order is similar in
                             n
each series, except for Orion  fabric, which had the highest penetration on
fly ash and near the lowest on talc.  This shift in order of penetration for
     D
Orion  may have been caused by humidity or electrostatic charge effects.
6.4  LABORATORY PERFORMANCE OF CONTINUOUS ON-LINE CLEANED COLLECTORS
     6 . A . 1  Reverse Jet Filter (Horsey Type)
            Billings et al   reported on laboratory performance tests of a
reverse- jet continuously on-line cleaned fabric filter (see Figure 3.3F).
Pressure drop and efficiency were found to vary with inlet dust loading,
filtering, velocity and cleaning ring operation parameters.  Empirical
pressure drop performance equations were derived for test dust simulants.
            For resuspended vaporized silica (particle size, order of 1 (Jim),
the equilibrium operating pressure drop (in. H^0)was found to be:


        Ap = fo.31V(M + 1.5)°'19 + 800Q - Vi  x 0.9s] ( RJoV°<18     (6.21)
             L                        1000          J \100/
where    V = filtration velocity, 7 to 20 fpm
                                                2
         M = dust flux to fabric, 1 to 87 gr./ft  - min, (=^7)
       V .  = velocity of reverse- jet air, 2000 to 8000 fpm
       RJO = percent of time reverse- jet operates, 6 to 100%
g
            The pressure drop equation for resuspended fly ash
       16, cr  >3) was found to be:
           S
        Ap =(b.047V(M + 14)°'35 + 4200 - Vrj x 0.70~| (W\"°>25       (6.22)
            L                                     J
                                  1000              UOO
for 10 < V < 30 fpm, 1 < M < 76 fr/ft2 -min, 2200 < V   < 4200 fpm and
7 < RJO < 100%.

-------
                               TABLE 6.20

    INDIVIDUAL COMPARTMENT EFFLUENT CONCENTRATIONS WHEN FILTERING AN
    INLET DUST CONCENTRATION OF 1.1 GRAINS PER CUBIC FOOT OF FLY ASH3
Compartment
(I farthest from
II
III
(IV nearest fan)
Overall
Dust Loading
Outlet ~
gr./lO ft .
fan) 2.2
1.3
1.2
1.2
1.2
Weight
Penetration
percent
0.19
0.12
0.11
0.11
0.11
Weight
Efficiency
percent
99.81
99.88
99.89
99.89
99.89
Pressure
drop
in H20
3.4
3.5
3.5
3.6

a. sateon weave cotton bags; back flow air 100 cfm; total air volume
   2000 cfm or 6.7 cfm/sq.ft.; 4/5 cleaning cycle.
                               TABLE 6.21
      COMPARISON OF FIVE FABRICS FILTERING HEAVY DUST LOADINGS
Fabric
Cotton
Light Wool
Dense Wool
Orion
Woven Glass
Orion
Cotton
Dense Wool
Light Wool
Woven Glass
Operating
Pressure Drop
in Ho^
5.5 A. Talc3
5.0
5.6
5.5
5.9
B. Fly Ash
2.4
3.0
2.7
2.6
4.7
Weight
Penetration
gr./10Jft .
0.990
0.740
0.340
0.330
b 0.063
5.60
1.40
0.30
0.26
0.12
Outlet
Loading-,
gr./10Jft .
99.9010
99.9260
99.9660
99.9670
99.9937
99.440
99.860
99.970
99.974
99.988
   Mass median diameter 2.5 microns;  geometric standard deviation 1.6;
   filtration velocity 5 cfm/sq.ft.;  inlet loading 1.0 gr./ft.;
   4/5 cleaning cycle, 4-Compartment  Unit.
b.  Mass median diameter 16 microns;  geometric standard deviation >3;
   filtration velocity 10 cfm/sq.  ft.;  inlet loading 1.0 gr./ft  .,
   4/5 cleaning cycle, 4-Compartment  Unit.

    (From Billings, et. al., Ref.  17).

-------
             Pressure drop for fine talc (M'  = 2.5 |am, cr  = 1.6) was
                                            o     x _      o
 Lound to be:
Ap -  fo.l6V(M + 1.3)°')0 + 2000 - Vl
      L
                                      1000
0.8~| /RJO\
   -1 \100/
                                                            -0.14
                                                                      (6.23)
 for 7 <. V < 20 fpm,  1 < M <, 70 er/ft  -min, 2000 < V   < 8000 fpm, and
 10 < RJO < 100%.
 Average inlet and outlet dust concentration for 8 test aerosols are given
 in Table 6.22.   Efficiency varied with amount and magnitude of cleaning
      6.4 2  Pulse Jet Collector
                                 20
             Dennis and Silverman   reported on laboratory test of a
               o
 Micro-Pulsaire  pulsejet continuously on-line cleaned collector (See
 Figure 3.3H).
      Equilibrium pressure drop for resuspended fly ash was given by:
           Ap = 0.14 +
                0.48 (Ci)
                                  0.23
                                 0.5
                                                                    (6.24)
                         te\  t1
                        l\100/
             For resuspended vaporized silica, pressure drop was found
 to be
              Ap = 0.14
                    1.75 (Ci)
                                     0.27
                            WO
                                   ).5
                   (6.25)
for 0.06 < Ci < 1 gr/ft , and other parameters same as above.  Weight col-
lection efficiencies were reported to be greater than 99.9% on all tests.
6.5  FIELD PERFORMANCE
     Pressure drop and efficiency for several fabric filters tested in the
                                   21
field were reported by Dennis et al  .   Data for intermittently cleaned
tube and screen type collectors is shown in Table 6.23.  Average operating
efficiency was found to be greater than 99% in all tests.  Field test re-
sults for reverse-jet collectors are shown in Table 6.24.  Efficiency for

-------
                                  TABLE  6.22

     AVERAGE  INLET AND  OUTLET  DUST  CONCENTRATIONS  FOR A VARIETY OF AEROSOLS

                        TESTED  ON  THE  REVERSE-JET COLLECTOR3
Aerosol
Atmospheric Dust
it M
ii ii
Copper Sulfatc Microsphcres
ii ii n
it n n
Uranium Trioxide Microsph.
ti n ti
n it n
Talc
n
n
n
Vaporized Silica
n n
M n
n n
y n
U ii
M n
Fly Ash
n n
n n
M n
n n
V "
Alundum
Calcium Carbonate
n n
n n
Filtration
Velocity
ft/min
25
17
8
25
17
8
25
17
8
8
11
8
8
20
28
17
22
10
10
10
10
29
20
10
10
10
17
25
25
25
Average Dust Loading
gr./cu.
Inlet
3.2x10";?
2.1x10"?
3.3x10°
ISOxlO'c
250xlO~
470x10
640x10";!
770x10°
1800x10
0.14
0.16
2.0
8.7
0.06
0.09
0.22
0.87
1.07
1.21
1.26
0.37
2.1
4.0
6.8
11.2
13.4
2.0
0.0055
0.033
0.25
ft.
Outlet
0.6x10";?
0.2x10°
0.2x10
4.9x10";?
3.7x10";?
31x10
27x10"^
68x10°
120x10°
1.2x10";?
1.2x10°
3.9x10°
6.0x10
2.6x10",
4.9x10°
4.4x10°
5.8x10°
1.0x10°
41x10°
1.8x10"
8.6x10";?
153x10°
63x10°
86x10°
44x10°
61x10
13. 8x1 O"5
e
11.1x10 ^
17.3x10°
48xlO-5
Reverse-Jet
Velocity
fptn
b
b
b
b
b
b
b
b
b
2000
1620
2000
2000
2000
2000
4800
2000
4000
4000C
4000
5000C
4000
4000
4000
2000
3000
3000
b
b
1450
a. Single 18"x60" bag; AmericanPelt Co., #51002, 19 oz./sq,
b. No reverse-jet operation
c. Three 6"x60" bags; same as above.
(From Billings et al., Ref. 17).
yd.

-------
                                                               TABU 6.23
                                     FIEU) TEST RESULTS FOR CLOTH SCREENS) AND TUBE  (T) COLLECTORS CLEANED BY
                                                                     MECHANICAL SHAKING*



o»
l~>
o





Test
and
Unit
13(T)b
16(T)
16a(T)d
17 (T)
18(S)°

19 (S)
20(S)
21 (S)
22(S)
23(S)
24 (S)

25(S)
Operation
Founding
Found ing
Founding
Founding
Granite
chipping
Founding
Truing &
shaping
grinding
wheels &
sticks
Rubber
Material
DUST DESCRIPTION
Inlet Outlet
Loading Median Size
Grains per Microns

Iron Scale
Si02
Bronze Scale
SiO,
Bronze Scale
SIO,
Iron Scale
sio2
Granite

Iron Scale
SiO,
Ai6
SiO,



Talc
Cu. Ft.
0.68
0.44
0.595
0.13
0.032

0.39
0.19
0.15
0.10
1.33
0.88

4.3
Mass
2.6
105
3.8

6.2
48
4.3
4.4
4.0
3.2

~
Count
0.63
0.82
0.71
0.46

0.47
0.56
0.51
0.57
0.45
0.62

1.3
Loading
Grains per
Cu. Ft.
0.000015
0.000069
0.000048
0.000013
0.000028

0.00063
0.000013
0.00011
0.000074
0.00003
0.0025

0.0064
Median Size
Microns
Mass Count
0.57
-
1.1 0.41

3.4 0.37
0.87 0.43
0.96 0.43
0.59 0.38
0.46


Air
Flow
Rate
rs.i.p.)
c£*a
7,000
560
1,850
2,000
7,020

18,200
13,900
32,000
32,000
4,520
32,000

7,000
CFM per
Sc. Ft.
Cloth
2.5
0.8
2.6
2.2
2.3

1.5
1.3
4.1
3.2
1.4
3.2


Pressure
Loss
Across
Collector
in. Water
1.5
5.0
0.3
2.2
3.0

1.9
1.2
2.9
4.1
1.7
3.3


Collection
Efficiency
by Weight
Per Cent
99.99
99.98
99.99
99.99
99.91

99.65
99.99
99.93
99.93
99.99
99.72

99.85
Manu-
fac-
turer
American
Wheela-
brator and
Equipment
Corporation

Sly
Corporation

Pangborn
Corporation




*STP » 70°F and 760 am Hg
 Cloth tubes
°Cloth envelopes over «etal screens
 TJnit 16 following replacement of worn-out bags

-------
 I
I—
o
                                                                            TABLE 6.24


                                                  FIELD TEST RESULTS FOR CLOTH BAG COLLECTORS CLEANED BY REVERSE-JET AIR
Test
and
Unit

26
27
28
29
30
31
32
33
34
35
35a
35b
36

Operation

Truing and
shaping
grinding
wheels and
sticks,
etc.

Metal
polishing
Crushing &
grinding
Found ing
Drying

Material

A/ -0
£. J




B,C
Polishing
rouge
BeO

BeO;Si02
Tapioca
Starch
DfST
Inlet
Loading
Grains per
Cu. Ft
0.130
2.260
0.650
3.10
3.65
2.44
0.075
0.0094
0.0048
0.14
0.029
0.0054
3.17

DESCRIPTION
Outlet
Median Size
Microns
Mass
18.0
7.2
8.5
10.0
3.3
-
2.3
9.8
1.2
-
-
-
7.7

Count
0.52
0.70
0.59
0.52
0.54
-
0.48
0.55
0.43
-
-
-
4.8

Loading
Grains per
Cu. Ft.
0.001
0.0009
0.0008
0.0009
0.0025
0.0011
0.0021
0.00022
0.00012
0.00052
0.00057
0.000022
0.00054

Median Size
Microns
Mass
0.59
1.1
0.75
0.72
2.0
0.74
1.8
1.2
0.56
-
-
-
1.1

Count
0.42
0.43
0.41
0.44
0.45
0.43
0.43
0.42
0.43
-
-
-
0.43

Air
Flow-
Rate
f C f p 1
' S • 1 . r . '
cfra
18,800
930
1,230
9,500
8,000
12,000
2,200
6,000
5,000
9,600
9,600
9,600
2,130

CFM
per
S,, Ct-
c . r L •
Cloth
17.0
5.5
7.2
32.0
26.0
53.0
46.0
32.0
27.0
12.0
12.0
12.0
11.3

Pressure
Loss Across
Collector
in. Water
4.0
3.5
3.5
3.2
2.9
3.1
3.6
6.4
3.6
2.4
2.4
2.4
4.4

Collection
Efficiency
by Weight
Per Cent

99.23
99.96
99.88
99.97
99.93
99.95
97.20
97.62
97.51
99.63
98.03
99.59
99.99

             STP - 70F and 760 mm Hg

           Note:  All units manufactured by Turner and Haws Company ("Aeroturn")•



-------
                               3                   /
inlet dust loadings ^ 0.1 gr/ft  was found to be 97) to 98%, with filtering
velocities in the range of 25 to 45 fpm.  These latter data have been attri-
buted to poor maintenance and possible bag wear, but indicate typical field
values under as-found maintenance conditions.
     The literature gives pressure drop and efficiency data for a number
 of fabric filter installations, along with at least partial descriptions
of the dust, gas, fabrics used, and operative detail.  These data are sum-
marized insofar as reported in Appendix 6.4.  Due to wide variation in con-
ditions of filtration, the data have not been analyzed.  The data is indi-
cative of fabric filter field performances, particularly those of new and
unusual filter installations as these are the ones most frequently reported
in the literature.
     The efficiency of fabric filters in collecting particles of various
sizes has not been reported.   Preliminary laboratory tests on particle size
efficiency using monodispersed aerosols have been discussed in Chapter 2.
 6.4  REFERENCES FOR CHAPTER 6.

 1.  C. E. Williams, T. Hatch, and L. Greenburg, Determination of Cloth
    Area  for Industrial Air Filters, Heating, Piping, and Air Condition-
    ing,  U, 259  (1940).
2.  C. A. Snyder and R. T. Pring, Design Considerations in Filtration of
    Hot Cases, Industrial and Engineering Chemistry 47, 960 (195'i).
3.  R. H. Borgwardt and J. T. Durham, Factors Affecting the Performance
    of Fabric Filters.NAPCA. PHS, USDHEW, AICHE, Annual Meeting Paper
    (Nov. 1967).
4.  D. G. Stephan, G. W. Walsh, and R. A. Herrick, Concepts in Fabric
    Air Filtration, Am. Industrial Hygiene Assoc. Jour., 21, 1 (1960).
5.  G. W. Walsh and P. W. Spaite, Characterization of Industrial Fabric
    Filters,ASME Winter Ann. Meeting (Dec. 1960).
6.  R. A. Herrick, Theory and Application of Filter Drag to Baphouse
    Evaluation, Air Engineering, p.  18 (May 1968).
7.  K. J. Caplan, ed., Air Pollution Manual, Part II...Control Equipment,
    American Industrial Hygiene Association, Detroit,  Mich., Section 5.3
    (1.968).
B.  A. D. Zimon, Adhesion of Dust and Powder, Plenum Press, N.Y., 116 (1969).


-------
                         6.4 REFERENCES  (Continued)
  <>.   I).  C..  Stophan,  P.  T.  Bohnsl.av, R. A. HerricU, G. W. Walsh, and
      A.  II.  RO.JO ,  Jr., A New Technique for Fabric Filter Evaluation,
      Am.  Industrial  Hygiene Assoc. Jour. 19.   276 (1958).

 10.   D.  G.  Stephan and  C',.W. Walsh, Residual Dust Profiles in Air Filtra-
      tion,  Ind. Eng.  Chem.  _52, 999 (1960).

 11.   G.  W.  Walsh  and P.  W.  Spaite, An Analysis of Mechanical Shaking in
      Air Filtration,  J.  Air Pollution Control Assoc.  12. 57 (1962).

 12.   E.  E.  Lemke,  ot  al.,  Air Pollution Control Measure for Hot-Dip
      Galvanizing  Kettles,  J. Air Pollution  Control Assoc. 10,  70 (I960).

 13.   P.  W.  Spaite  and G-W.  Walsh,  Effect of Fabric Structure on Filter
      Performance,  Am. Indust. Hygiene Assoc.  J. 24,  357 (1963).

 14.   J.  Durham, Filtration  Characteristics  of Fabric  Filter Media,  NAPCA,
      PUS, USDHEW,  internal  Technical  Report (Feb.  1969).

 1'3.   R.  H.  Borgwardt, R.  E.  Harrington,  and P.  W.  Spaite, Filtration
      Characteristics  of  Fly Ash  from  a Pulverized  Coal-Firec' Powe,:
      Plant,  J.  Air Pollution Control  Assoc.  18, (1968).

 16.   J.  R.  Durham, and  R. E.  Harrington,  Influence of Relative  Humidity
      on  Filtration Resistance and  Efficiency.  NAPCA,  PHS, USDHEW, AICHE
      63rd Annual Meeting  paper,  Chicago," 111.  (November  1970).

 17.   C.  E.  Billings,  M. W.  First,  R.  Dennis,  and L. Silverman,  Laboratory
      Performance of Fabric  Dust  and Fume  Collectors,  USAEC  Report No.
      NYO-1590-R, Harvard Air  Cleaning  Laboratory,  Boston, Mass., Rev.  Ed.
      (Jan.  1961).

 18.   J. W. Robinson,  R. E. Harrington, and P. W. Spaite, A New Method  of
      Analysis for Multicompartmented Fabric Filtration, Atmos.  Envir.  1
      495 (1967).                                        	  ~

 19.   N.S. Billington  and D. W. Saunders, Air Filtration,  Jour.  Institution
      of Heating and Ventilating Engineers,  (London),  15,  46,  (1947)

20.  R. Dennis and L. Silverman,  Fabric Filter Cleaning by Intermittent
     Reverse Air Pulse,  ASHRAEJ,    43  (1962).

21.  R. Dennis, G. A. Johnson, M. W. First and  L.  Silverman, How Dust
     Collectors Perform, Chem. Engg.»  196,  (February  1952)

-------
                               CHAPTER 7
                               ECONOMICS

7.1  INTRODUCTION                                                    7-3
7.2  INITIAL COSTS                                                   7-8
     7.2.1  Initial Filter Cost                                      7-11
     7.2.2  Initial Fan, Compressor Cost                             7-17
     7.2.3  Duct Costs                                               7-20
     7.2.4  Dust Disposal Equipment                                  7-22
     7.2.5  Instrumentation                                          7-23
     7.2.6  Planning and Design Costs                                7-27
     7.2.7  Foundations and Installation                             7-27
     7.2.8  Start-Up Period                                          7-28

7.3  OPERATING AND MAINTENANCE COSTS                                 7-29
     7.3.1  Power Costs                                              7-31
     7.3.2  Labor Costs                                              7-32
     7.3.3  Plant Overhead Costs                                     7-36
     7.3.4  Collection System Returns                                7-38

7.4  CLOTH AND BAG COSTS                                             7-39
7.5  ACCOUNTING COMPARISONS OF COSTS                                 7-44
     7.5.1  Annual Distribution Method                              7-45
     7.5.2  Anticipated Cost Summation Method                       7-46

7.6  ECONOMY IN FABRIC FILTER OPERATION                             7-48
7.7  REFERENCES FOR CHAPTER 7                                       7-48

-------
                           \
                           \  CHAPTER 7
                              ECONOMICS
7.1  INTRODUCTION
     Whether  fabric  filtration represents the optimum gas cleaning method
for a given set of performance specifications depends upon a detailed
cost analysis.  The  available data citing cost experience, however, are
limited and,  in many cases, oriented towards specific operations.  There-
fore, unless  the design engineer is confronted with a very similar and
proven fabric filter application, he should prepare a written specifica-
tion and seek expert professional opinion.  All cost factors associated
with the design, fabrication and installation of the fabric filter system
should be determined as accurately as possible before selecting the dust
control system.  In  those cases where the gas treatment equipment will be
of modest size or where it represents but a very small fraction of a large
capital investment,  one can justify a less rigorous approach.
     The intent of this chapter is to provide the design engineer with
practical guidelines for estimating the cost of various fabric filtration
approaches.  A concept underlying this chapter is that several trade-
offs must be considered between size of components and operating para-
meters in order to optimize costs.  For example, if low cost performance
and high efficiency were the only criteria,  one could never have too
much cloth area for  treatment of a fixed gas volume (Figure 7.1).  There
will be some point, however, when the initial costs for the larger fil-
tration system will override the lower recurring costs of power, mainten-
ance, etc.
     The cost data assembled here have been compiled from several sources;
e.g., equipment users, manufacturers of filter systems and ancillary
parts, and open literature surveys by many investigating groups.  Pur-
chase costs cited for past years have been converted to their 1969 equiv-
alents in accordance with the Marshall and Stevens Index, Table 7.1.
     Several  partial cost analyses have appeared  in recent years,  includ-
                      f\ f\              i
ing those by  Stairmand  '  and Stephan,  that relate the  collection efficiency


-------
                             TABLE  7.1

      MARSHALL AND STEVENS  INDEX FOR UPDATING EQUIPMENT COSTS*

1945
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
Example :
M&S
Index
106
170
181
181
183
186
192
209
226
232
236
Equipment
$1460 in
1969
Factor
2.64
1.65
1.61
1.61
1.53
1.51
1.46
1.34
1.24
1.21
1.19
costing $1000
(early) 1969.

1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
in 1955
M&S
Index
239
238
238
239
242
245
253
263
273
285
(296)
is estimated
1969
Factor
1.17
1.18
1.18
1.17
1.15
1.14
1.11
1.06
1.03
1.00
(0.95)
to cost
See Ref.  1,  pg.  26-8; and Chemical  Engineering. recent  issues,  last page,
                         Sli« ol collector (It" of fabric)
      Figure 7.1.  Trade-off in Costs  Due to Collector Size,  for
                   Fixed Gas Flow.

-------
of various types of particulate collectors )to control costs.   In Fig-
ure 7.2, for example,the efficiency is compared to control costs as a
function of particle size.  Although the costs quoted, which reflect
British experience, are about 50 percent lower than those reported in
this chapter, the relative  impact of particle size upon cost is directly
applicable.  Stephan's data, summarized in Table 7.2, gives purchase costs
and power requirements for  a spectrum of participate col led ors i-tuvtivr
on various particle sizes.
     In detailed cost analyses presented in this section, several appli-
cation-related variables will be shown to have an important bearing on
cost.  Therefore, in referring to rather generalized summations of the
types shown in Figure 7.2 and Table 7.2, one must keep in mind that
individual component costs, frequency of replacing parts, amount of main-
tenance, and cost accounting techniques may vary widely from one instal-
lation to another.  A reasonable and useful cost estimate can only be
prepared by examining in turn each contributing cost factor.  Figure 7.3
indicates several important aspects typical of most fabric filter systems:
                                                 1.50
                                                      fr
                                                      M
                                                      CO
                                                      0)
                                                 1.00 T
- •  .50
                                                      u
                                                      g
                                                  0
                   I  5 10 M M 60  80 90 95  98 99 99 5    99 9
                     Collection Efficiency (%)
               Figure  7.2.  Gas Cleaning Costs  for Dusts
                            of various Particle Sizes ,  for
                            300,, 000 CFM and 8000 hrs/year.
                            (From Stairmand, Ref. 2.)  (1968)

-------
                       TABLE 7.2
    APPROXIMATE CHARACTERISTICS OF DUST AND  MIST
                  COLLECTION  EQUIPMENT1
iqulpmrat Type
A.




1.










C.







D.








B.








r.














Settling
Chamber*
1. Simple
2. Multiple
tray
IMF till Sep-
arators
1. Baffle
chamber

2. Orifice
Impact Ion
3. Louver type

4. Gas re-
versal
Cyc lone*
1. SlngU


2. Multiple

3. Machsnl-
cal
Filters
1. Tubular


2. Reverse
J«t
3. Envelope


Electric*!
Preclplta-
tor»
1. One-itege


2. Two-stage


Scrubber*
1. 8pr«y
tower
2. Jet


3. Venturi

4. Cyclonic

J. Inert 1*1
6. Packed

7. Mechani-
cal
Purcha** Smallest Pressure Power
Cost* Particle Drop Uaad*** „_ .
(»/c£«) Collected (in H20) KV . «~«r"
(micron*)** (1000 cfa'
0.1 40


0.2-0.7 10



0.1 20


0.1-0.4 2

0.1-0.4 10

0.1 40


0.1-0.2 15


0.3-0.7 5

0.2-0.7 5


0.3-2.3 
-------
                                     Fan &
                                     Ducts
                                      67
Electric
 Power
      Dust
    Disposal
        77.
              Equipment
              Installed
                157,,
 General
 MaInt.
                   Cost of Capital
                         117,
                              Plant
                             Overhead
    Fabric
    Replace-
      ment
   Replace
   ment
Fabric Pur
chases
   107,
  Typical  Installed Cost,  $2.38/cfm

  Typical  Annual Cost,  $1.05/cfm-year


  Figure  7.3.   Fabric  Kilter Annual Cost Distribution.
                (From GCA Survey,  1969).
The cost of the collector is only a few percent of the total
cost; therefore, the collector cost taken alone is a poor
criterion to use in selecting the system.

Fabric replacement (labor plus material) represents about 1/5
of the total annual cost; therefore more serviceable fabric
may lead to substantial savings.

Fabric replacement cost is about 4 times greater than collector
cost; therefore the collector should be very carefully designed
to promote fabric life.

Labor costs are nearly 1/3 of the total annual cost, and are
twice as much as the initial costs.  Therefore the equipment
should be designed to need a minimum of attention time.

-------
      . Direct operating costs are half the total annual cost; therefore
       even after the system is bought and installed, substantial sav-
       ings may result from careful use.
     An itemization of the principal costs associated with the installa-
tion and use of a fabric system and "typical" values for each cost are
given in Table 7.3.  These costs and their normal apportionment as de-
picted in the "pie" diagram, Figure 7.3, are discussed in detail in the
following sections.
     Two basic cost categories, initial and annual, must be considered.
The initial cost includes the planning effort, site preparation, in-
stallation and other activities required to place the system into rou-
tine, effective operation.  The annual costs include not only those for
power, labor  and replacement parts but also various plant overhead
charges and a certain amortized fraction of the initial cost.   Methods
of combining these separate costs are discussed in Section 7.5.
7.2  INITIAL COSTS
     The cost of purchasing,  installing,  and placing the fabric  filter
system in routine service will  usually include the following items:
                                                         Reference
          Cost Item                 Typical Cost Range    Section
                                         ($/CFM)                  "
    Basic  filter  assembly  with           0.3 - 10.            7.2.1
      filter  media
    Fan,  blower,  compressor              0.05-1.              7.2.2
    Ducting  ($/ft)                       2-50                7.2.3
    Dust disposal equipment                --                 7.2.4
    Instrumentation                      0.01-0.1            7.2.5
    Planning  and design                  -0.1-               7.2.6
    Foundations and  installation         0.1-5.              7.2.7
    Start-up,  training, and shake-       -0.1-              7.2.8
     down period
     A more  detailed  check list  of  equipment  and  services  considered  as
 capital  cost items  is given in Appendix  7.1.  One  should also  prepare  a

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                          TABLE 7.3

                TYPICAL, COSTS OF FABRIC FILTRATION
 I.  Installed Cost - $2.38 per CFM

        Planning and       $0.10
          design
        F.O.B. baghouse     0.80
        Freight             0.05
        Fan and motor       0.25
        Ducting             0.65
        Disposal equip-     0.10
          ment
        Instrumentation     0.05
        Foundation and      0.28
         installation
         labor
        Start-up            0.10

              Total:      $2.38
 II.   Annual Cost - 0.77 per CFM per year

        Electric power     $  0.12    (
        Labor                 0.30    \  ;     .,
        Plant overhead        0.25--  \ "   '        .     A, ,
        01   L.     t-           n in    ( Insurance, etc.:. 045
        Cloth purchases       0.10    ^

              Total:       $  0.77
III.   Total Cost of Operation - $1.05 per CFM per year

        Annual cost        $  0.77
        15 yr.  amortization   0.16
          of the installed
          cost
        Interest on the       0.12
          unamortized
          portion of in-
          stalled cost,
          at 10%
               Total:       $  1.05
   (see text for explanation and variations)

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      detailed flow (lliiv.ram so that no vital components will be overlooked
      during the costing process,  Most ot the items listed above represent ex-
      penditures of funds that would be charged directly to the filter system.
      Certain costs, however, may not be charged to the system, but incorporated
      In the general plant overhead.
           The total initial investment is amortized over a period of time,
      usually 10 to 1.5 years, that is consistent with the expected equipment
      life.*  Since n fixed amount of capital is tied to the filter installation,
      thus preventing investment elsewhere, one must charge this loss of use
      of the capital to the project annually as interest.  Both amortization
      and interest distributions are discussed in Section 7.5.  They are com-
      bined with the annual operating and maintenance costs to establish the
      effective yearly cost of owning and operating the filter system.
           As a rough rule of thumb, the total initial investment cost includ-
      ing material and labor will average about $2.50 per CFM of gas filtering
      capacity, with an expected range of $1 to $7 from one installation to
      another depending on severity of the problems.  This overall average cost
      is based mainly on the results of a survey of 40 fabric filter installa-
      tions for which the ranges in size and cost are shown in Figure 7.4.
10
100
1000
     10
Ifl5
                                                                    10
                                      SIZES, ft  of fabric
                            ^ • >
                              r
10
         100
                          TOTAL INSTALLED COST,  $/CFM
             Figure 7.4.   Filter Installation Cost Data.   (Arrows Indicate
                          Median Installations,  ~5000 ft2 and ~$2.40/CFM.)
      This period varies; power plants anticipate a 30 to 35 year life
for most plant equipment, while 5 years may be used in corrosive chemical plants,

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The  breakdown of these costs, which has been given previously  in Table  7.3
and  Figure  7.3, is discussed  in  the following sections.  The average
Ligure  is apportioned more or less evenly among F.O.B. basic filter cost,
fan  and ducting costs, and other costs associated with the purchase and
installation of the system.
      Initial fabric filter system costs for many diverse applications are
listed  in Appendix 7.1.   Although these data are not always specific
;ts to what  equipment and  labor costs were  included,  tho  data should
onablo  cost estimates  to within  50 percent of actual,  cost  by the
experienced designer.
     While  the scale factor from F.O.B. collector cost up to installed
system cost is typically about three (Table 7.3),this value is obviously
highly dependent on the individual system.  It will be shown below that
both  the basic filter cost and the costs of ducting and  installation can
be quite different depending on the application.  Examples of  initial
cost variations in the Iron and Steel industry provided by an  experienced
contractor  in the air pollution control equipment field are shown in
Table 7.4.
     7.2.1  Initial Filter Cost
            The filter assembly as shipped by the manufacturer  normally
includes hopper,  cleaning mechanism with associated fans or motors,  and
one set of filter elements.  The equipment may or may not be assembled,
and it may not necessarily include supporting structures, paint or protec-
tive coatings,  and insulation.  Thus  "the initial filter cost"  is not
entirely meaningful until all these factors are  defined clearly.   Since
users of filtration equipment often fail to specify in their publications
the above details  on their initial  costs,  the data presented in this
survey are subject to  unavoidable uncertainties  in some areas.
            Initial costs are indicated in Figure 7.5 for four  common
types of fabric filter  dust collectors;  reverse  jet,  pulse  jet, mechan-
ically cleaned  multi-compartment  bag  collectors,  and  single compartment,

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                             TABLE 7.4
                 ESTIMATES OF CAPITAL COST BREAKDOWN''
    1.  Matt-rial breakdown for fabric filter systems (percent):

                                      BOF
Sinter Plant
  Windbox
Electric  Open   Shop Electric
Furnace   Hearth    Furnace
4
4
71
10
3
5
1
2
37
32
6
5
7
7
2
:«5
34
7
7
9
2
2
25
42
4
9
8
7
2
17
27
10
10
7
1
        Foundat ions
        Ductwork and
           stack
        Collector
        Fan and motor
        Structural
        Electrical
        Water treatment
           and piping
        Controls             2         443         5


    2.  Labor/Material ratio for various components of particulate con-
        trol systems(at $5/hr):

        Collector            0.35
        Fan, motor and       0.15
          starter
        Stack                1.00
        Ductwork             1.00
        Steel                0.30
        Foundations          1.30
        Electrical           1.50
         From Barnes, Ref.  25.



intermittently cleaned envelope collectors.  These costs are as the col-
lector is normally shipped by the manufacturer, with freight paid to the

shipping point (free on board, or F.O.B.).  Costs are given both as dol-
lars per square foot of fabric and as dollars per CFM of filtering capacity,

assuming whenever necessary a typical air/cloth ratio (filtration vel-
ocity).  These costs are based upon a survey of filter users, manufac-
turers' data, and the technical literature.  Certain generalizations may

be made from the information presented in Figure 7.5:

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10*
                      10*
 10   KX>    O    KT   10"    K
         Reverse Jet Equip-
         ment (15 fpm assumed)
                                           10'
                                           10*
                                          I01
                                                  I	I	I	1	I
      10    K»   O*   »4    I0»    10* "*
K>    100    10"   10*   10*    10*   won*
      Pulse Cleaned Equipment
         (10  fpm assumed)
 Note:
    (a)  All equipment without primary exhausters, but with secondary
         fans, blowers, etc.  used for cleaning.
    (b)  Symbol  (x)  indicates data points estimated using  the assumed
         aiWcloth ratio.
         Symbol  (T)  indicates high temperature equipment.
         Symbol  (0)  indicates miscellaneous data points.
    (c)  Numerals indicate data source:
         (1)  Kane,  American Air Filter Co., Ref. 26.
         (2)  Wright,  Dracco-Fuller Co., Ref. 27.
         (3)  Chetn.  Engineering, (9 Feb. 1970).
         (4)  Dennis,  Ref. 28.  Compressors and cleaning air fans
              included.
         (5)  US PHS,  Ref. 29.
         (6)  A manufacturer's price list.
    (d)  All prices  are updated, via Table 7.1, to reflect 1969
         price levels.

          Figure 7.5.   Initial Fabric Filter Costs - 1969  Basis;
                        F.O.B.

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                                         10
                                         10
                                         10
                                        10
                                        10
    10   100
                   10'
10
                                                    WO
                        10*
                                                  \
   DO
    Compartmented Bag-Type
    Equipment (3 fpm assumed)
                              CFM
                                          10
  100    10*
                     •0*
Unit Envelope-Type Equipment
   (3 fpm assumed)
Note:
   (a)  All equipment  without primary exhausters, but with secondary
        fans, blowers,  etc.  used for cleaning.
   (b)  Symbol  (x)  indicates data points estimated using  the  assumed
        ai*r/cloth  ratio.
        Symbol  (T)  indicates high temperature equipment.
        Symbol  (0)  indicates miscellaneous data points.
   (c)  Numerals  indicate  data source:
        (1)  Kane,  American  Air Filter Co., Ref. 26.
        (2)  Wright, Dracco-Fuller Co., Ref. 27.
        (3)  Chem.  Engineering. (9 Feb. 1970).
        (4)  Dennis, Ref.  28.   Compressors and cleaning air fans
             included.
        (5)  US PHS, Ref.  29.
        (6)  A manufacturer's  price list.
   (d)  All prices  are  updated, via Table 7.1, to reflect  1969
        price levels.
                     Figure 7.5.  (Continued)

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             1.   Reverse  jet  and  pulse  jet  collectors  are  about  twice
                 as  expensive as  other  types  in terms  of  fabric  area,
                 but are  comparable  on  the  basis  of  gas filtering
                 capacity.   Intermittently  cleaned equipment  is  least
                 expensive  because  it is  designed for  easier  dust  con-
                 trol  applications  and  interruptable operations.

             2.   Costs per  unit capacity  increase sharply  below  about
                 3000  CFM,  and approach levels  as low  as $.50 to $1.00
                 per CFM  in large installations.  For  a preliminary
                 estimate,  a  cost of $.80 per CFM of filtering capacity
                 might be considered as typical,  and the figure  of $2.
                 per ft2 of fabric would  represent a large number  of
                 collectors to within 50  percent  of  the true  value.

             3.   Although cost data  from  a  single source falls on  one
                 curve, costs  from different sources appear to vary by
                 as  much as a  factor of 3 for various  reasons  cited
                 previously.   This variation in cost is especially true
                 for bag-type  equipment,  which  is designed for a wide
                 variety of temperature and particulate applications.

             Part of the data  in Figure 7.5 is  shown in Figure 7.6 for

better comparison.  This was  published about 1965 by  a prominent  manufac-
      27
turer.

            The  type  of construction of  the fabric  filter assembly is

particularly important for bag-type units, and  for large equipment the

unit cost depends more on construction features  than  on size.  This var-

iability factor  is  shown in Table 7.5  for several collector configurations

designed primarily  for high temperature  applications.   These cost esti-
     30
mates   include  the baghouse,complete with the bag cleaning accessories,

and materials handling equipment to accommodate dust  removal from collec-
tion hoppers.  They do not include  the primary air moving equipment nor
any costs for field installation, piping, ducting,  or electrical wiring

or insulation.

            Except  for preliminary cost estimating,  prospective  purchasers

of filter equipment are advised to consult  reputable manufacturers for

accurate costs.   Purchasers are advised to  specify  quality of material

and construction, and to examine critically any  bargain prices.    Because

the trade skills involved  in  producing filter  equipment are  fairly modest,

the field is crowded.  Manufacturers are often compelled to seek  economies

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MCCHANICAL
tasRi
3- OOMIKNT WTICIINCV
                                LEGEND
                                             CiNTNIfUML
                                  6- HMH (NIMY-VtMTUMI
                                  NMHMC
                                  7-MTWWTTCNT
                                                JCT -
                                               DUTY
                                   AOTO*UTIC MUBURE TYW
            IOOO
 O.H  0.60  a 78
        COST/CFM
                                                                                J
Figure  7.6.   Cost Per CFM of 12 Different  Dust Collector
              Designs Compared to the Total Volume Handled
              Per Minute.  (To Convert to  1969 Estimated  Price
              Levels, Multiply Scale by Marshall and Stevens
              Index 1.14.)  (From Wright,  Ref.  27).

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L
                                        TABLE 7.5
L
                   APPROXIMATE COST FOR BAG-HOUSES OF THE INDICATED CONSTRUCTION
                                        PRICE RANGE
                                     $/SQ.  FT.  OF CLOTH
                    WITHOUT INTERNAL PARTITIONS:
                    Open pressure unit       $1.20 to 1.40
                    Closed pressure unit     1.55 to 1.70
                    All  welded  unit          1.85 to 2.00
L                 WITH  INTERNAL PARTITIONS:
                   Open  pressure unit        1.35  to 1.50
!                   Closed  pressure  unit      1.80  to 1.85
L,                 All welded  unit           1.95  to 2.20
L                   *
                    Based  on  11  1/2-in.  diameter  x  30.-ft.  long  fiberglas  filter
                    bags and  a minimum of  40,000  sq.  ft.  of  cloth  area.   Cloth
                    cleaning  by  repressuring.   (From  Adams,  Ref.  30}  1969 cost  basis,
               that are sometimes not  in  the best  long-term  interests of  the  buyer.  A
               dollar saved  initially  may cost several dollars  in subsequent  operating
i               and maintenance costs.  Furthermore, equipment that  is well designed  for
               one application may be  a poor selection elsewhere.   Rather than invest in
               cheap equipment, it is  advisable to consider,if  necessary, the  redesign or
               modification of good quality standard stocked equipment.  Although the
               purchase cost may be doubled, the long-term costs may be greatly reduced.
               It should be noted that about 15 percent of the  operational problems en-
               countered by fabric filter users are the result  of equipment inadequacies.
                    7.2.2  Initial Fan, Compressor Costs
                           The purchase cost of fans or blowers suitable for fabric fil-
               ter equipment, which includes motors, drives, and starters, varies in-
               versely with system capacity as indicated in Figure 7.7.  Purchase cost
               also increases with the design pressure to be delivered by the fan although
               this is not readily apparent in the figure because oC the limited data
               from any one source.  Cost is also determined by the style of the fan and

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                                                         ' Si-SO/CFM




                                          '  Ol?! '"anufacturer-
                                                 ,  several  <  Pr°duct
                                                     verai sizes  ft"
                                                                * *^ •
                                                        10
Figure 7.7.
             Fan and Blower Costs, Including Motor, Starter, etc.

             1969 Basis.   Fan Design Pressure Indicated.

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 its  design speed.   Generally,  fans  delivering  10,000 CFM or more will  cost
 between $.10 and  .20 per  CFM,  fans  delivering  about 1000 CFM will  cost
 $.30 to .55 per CFM,  and  the cost of  smaller fans  increases sharply.
             On the  largest  applications,  the fan(s) may  be custom  fabri-
 cated.   The gas properties  and type of  service affect  the trade-off be-
 tween initial  cost  and  annual  costs?  e.g.,  a smaller fan running at higher
 speed will  be  slightly  leas efficient,  and  will  probably require more
 maintenance than a  larger fan  operating at  less  than rated speed.  This is
 especially  true if  the  fan  is  located on  the inlet side  of the  filter,
 where slower speeds  are desirable if  the  dust:  is abrasive or gummy.  Fan
 maintenance, which  can  be a big problem in  some  installations, may justify
 a larger  initial size (and  cost).   It may be desirable to oversize the fan
 merely  to allow for  unforeseeable expansions of  the collector or increased
 process  effluents.   In  addition, it may be  advisable to  oversize the fan
 on collectors where  the overall pressure  differential  fluctuates for any
 reason,  to  minimize  reduction  of gas  flow during periods  of high resistance.
             Fan and  motor costs are approximately equal.  A starter for
 a 10 HP motor  costs  about $75,  and  the  drive cost should  not exceed $50.
 Fan-motor sets are  around 60%  efficient,  such that one air HP, e.g.
 1000 CFM capacity at  6.35 in.  water represents about 1.6  electrical HP.
 Installation labor cost which  is normally about 157= of materials cost and
 foundation  costs is not included above.
            The cost of an air compressor for pulse-cleaned equipment,
 pneumatically operated  instrumentation, and dampers in some filter houses,
 is normally  included with the  initial fabric filter cost  (see Section 7.1.1
 for  pulse cleaned equipment).  Otherwise a separate compressor must be
 purchased at considerable expense,  since the existing supply of shop air
 is often inadequate with respect to volume or pressure, or is  not con-
sistently available.  For pulse cleaned collectors, the amount of com-
pressed air varies according to the cleaning mechanism design.   A
typical design value is 2 SCFM per  1000 CFM of  gas filtered,  with delivery
pressures ranging between 75 and 100  psig.  The following estimates have
been  provided by compressor manufacturers  specifically for continuous
operation with dust collectors:


-------
I'll IS I'll
Air,
SCFM
135
100
10
Supp ly
Pr 1-8 Hiiro ,
psig
100
100
100
Motor Power,
IIP
25
25
3
Total Cost,
$ F.O.B.
2700
2500
700
Approx. Initial
Cost, $ per CFM
filtered
.04
.05
.14
 The cost  is  estimated to increase  with the 0.28 power  of the  volume re-
 quired.     Field  installation is estimated at  0.60 times the  cost  of the
 compressor,  and the  ratio of installation labor to total materials cost
 is  estimated at 0.27.
      7.2.3   Duct  Costs
             The cost  of  ducting depends  on the  gauge and diameter  as well
 as  on the length  and  the material.   It also depends on the  complexity  of
 the system,  i.e.,  the  number of elbows,  Tees,  transition sections,  etc.,
 and the amount  of fitting needed to  adapt  the  ducting  to existing  flanges.
 The ducting  may require  clean-out  or inspection ports,  dampers, or explo-
 sion panels.  The  cost of installing the ducting will  also  depend  on
 location accessibility,  as well as the method  of joining and  sealing the
 duct components.
                                                                 2
             One manufacturer of small  filter units  (up  to 1200 ft  ) has
 reported his  experience  in installing  ducting  systems which for present
                                                   32
 purposes would  be  considered smaller  than  typical.    He  indicated  an
 average cost  for  installed ducting of  $1.75 to  1.85 per  pound of sheet
 metal, depending on gage  and diameter.   This included both  fittings and
 straight piping on his medium and large  jobs, but  the cost was more for
 his  smallest  jobs.  For  example, a connecting duct assembly drop for a
 single lilter unit of this size averaged about  $53. (1969 cost basis).
In the Chicago area one duct  branch including headers  cost about  $250.
Using the  figure $1.85 per pound,  Table 7.6 gives the  estimated installed
cost per foot for several gages and diameters:  typically it  is around
$10 per foot.  In support of  this  Table,  a Boston sheet metal  contractor
independently estimated a cost of  $5  per foot for a 100-foot system of
6" 18 gauge  ducting including soldered joints and about 3 gates,  installed.
            Table 7.6 also lists  the  costs of typical duct fittings be-
fore installation.  The cost  of a  fitting depends,  of course,  on  the

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                              TABLE 7.6

                        APPROXIMATE DUCT COSTS

(a) Installed Duct
Systems
($ per foot)
Metal Gage
(U.S. Std.
6
14
16
18
20
22
24
26
30

Thickness
) (inches)
.203
.078
.062
.050
.0375
.0312
.025
.0188
.0125
(b) Duct Fitting
Duct Diameter (inches)
6 11 24 48
108. 216.
20.75 41.50 83.
8.25 16.50 33.00 66.
6.60 13.20 26.40 53.
4.90 9.80 19.60 39.
4.10 8.20 16.40 31.
3.30 6.60 13.30
2.50 5.00 10.00
1.65 3.30
**
Costs
($ per item, not installed)
Description
Straight pipe,
Same, with self
seal
Reducer
Flange
Tee, 45°
Gauge
per foot 20
24
-locking 24
26
20
20
20
Diameter (inches)
6" 12" 24"
2.40 3.90 6.50
.56 .95 2.97
1.50 2.80
1.18 2.24 7.60
$18. (12x6) $36. (24x12)
$23. (12") $35(24")
$33(12x12x8) $50(24x24x12)
 1969  cost  basis  of $1.85 per pound for galvanized  sheet  steel,  riveted

 and soldered,  including a nominal number  of  fittings.

T«
  1969 Boston area prices; in lots of 10 fittings each.

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 amount of work required to make it; typically the fitting costs 2 to 4
 times as much as the same length of straight pipe.
                                            31
             Installation labor is estimated   to be 85 to 100% of material
 cost; that is, the cost of the installed ducting is about half material
 and half labor.  This rule of thumb varies with material and difficulty
 of installation,  of course.
             Materials other  than galvanized steel are sometimes used.
 Estimates for shop fabricated, field erected ducting of aluminum,  gal-
 vanized steel, and stainless steel are $5.42,  8.00,  and 15.12 per  foot,
 respectively,  (no sizes or gauges  given).     These costs  are further esti-
 mated to increase With the 0.55 power  of the size.   These estimates  pre-
 sumably apply  to  the same  ducting  requirement  and therefore  should repre-
                                                          jf
 sent  thicker aluminum than steel,  for  structural reasons.
             There is little  in the filtration  literature  on  actual duct
 costs,  but  a survey of  nine  large  filter installations  averaging 50,000
 CFM indicated  the costs  of the associated  ducting to be $.60 per CFM
 (range,  $.25 to $1.00).  This  included  applications  of  both  high and low
 temperature  filtration,  and  both short  and  long  reaches.   In Iron  and
 Steel  Industry applications  the system  ductwork  cost is estimated  at
                                               2S
 around  30% of  the total  initial material costs.
      7.2.4   Dust  Disposal  Equipment
             The purchase of  dust handling and disposal equipment must be
 compared  against  doing  the job manually, but even so the  equipment is
 justified by the  high cost of  labor  on  all  but the smallest dust flow
 rates.  The  disposal equipment typically includes a  rotary valve and
 drive,  one or  more  dust  conveyors with  drives, and a collection bin  of
 some kind.   This excludes the hopper,  which is  usually provided as  part
 of  the  basic collector.
            A  good  quality 8"  dia. rotary air lock valve metering  on the
 order  of  1 CFM costs $400  to $500  including motor and drive.  Motor  and
     *
      Aluminum sheet goes by Brown and Sharpe gauge numbers, while steel
goes by U.S. Standard gauge numbers.  For approximately the same thickness,
use two B&S numbers lower than the U.S. Standard number.


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drive1 onch cost about $75 depending on size and detail.  It may be pos-
sible to drive the valve from the conveyor shaft and eliminate the need
of a separate motor.
            The most common type of dust conveyor in use is the screw con-
veyor consisting of a 6" to 15" diameter covered trough, a screw of 10
to 16 gauge carbon steel, end plates, motor and motor mount, drive, and
drive guard.  The total cost is on the order of $40  per foot, depending
on design.  The trough and screw alone cost approximately $25 per foot
(9" diameter, 14 gauge screw, 12 gauge trough, 2" screw shaft diameter,
30 ft. long).  The cost of conveyor is about $450 including motor.
            Normal power requirements are l/15th HP per foot of conveyor,
but nof all dusts flow this freely.  A number of dusts become sticky as
the temperature drops, or pack together due to vibration or mechanical
churning; consequently many problems arise with screw conveyor plugging
and breakdage, and other types of dust conveying equipment are sometimes
used.  Table 7.7 gives cost estimates for several such conveying methods.
The costs do not agree with the above estimates, and the data in
Table 7.7 is suspect for other reasons, but it may be useful in making
comparisons.
     7.2.5  Instrumentation
            The effective utilization of any fabric filter system,regard-
less of size, cannot be obtained unless certain basic measuring devices
are incorporated.  Optimum collector performance depends upon the observa-
tion or control of several major variables such as gas temperature,
system pressure, fabric pressure drop, gas flow rate, humidity, particu-
late properties (mass concentration and size), dust levels in hoppers,
etc.  The above listing of variables does not necessarily represent their
relative order of importance.  In a system where accidental condensation
of moisture could produce rapid fabric blinding the sensing of dewpoint
might take priority.  High temperature filtration, on the other hand, re-
quires that the temperature not exceed levels beyond the tolerance of the
fabric.  In applications characterized by high and possibly variable dust
loadings, the maintenance of gas handling capacity depends upon the meas-
urement of fabric pressure loss.  The number of separate sensing devices

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                              TABLE  7.7

                   REPORTED  ESTIMATES OF CONVEYOR COSTS'
                          (per foot of length)
Type, Size Cost Size /ov

Belt 18 in. wide 460
24 550
36 640
42 720
48 770
Bucket (height)
30 tons/hr. ,,
t r\ • r- • \ 4- £j
(8 in.x 5 in.)
75 tons/hr. ,._
(14 in.x 7 in.)
120 tons/hr. _.«
(15 in.x 8 in.) ^
Roller, 12 in. wide 7
15 8
18 9
20 10
Screw, 6 in. dia 235
12 280
14 300
16 310
Vibrating, 12 80
in. wide
18 115
24 125
36 135
*Frotn Ref. 31.
Exponent
0.65
0.65
0.65
0.65
0.65

0.65

0.83
0.83
0.90
0.90
0.90
0.90
0.90
0.80
0.75
0.60
0.80

0.80
0.90
0.90

0.33
0.33
0,28
0.28
0.28

0.44

0.44
0.44
0.33
0.33
0.29
0.29
0.25
0.25
0.25
0.25
0.28

0.28
0.26
0.26

(1) Although reported as $/ft, some of these costs appear more reason-
able as total costs
(2) Cost increases with
(3) Installation costs
. 1969 basis.
equipment length

raised to this
include foundations, electrical,

powe r .
paint and fie]
     labor, and add about 65% to purchase costs.
(4)  Total installation labor is about 30% of total material cost.

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 justified  and  their  degree  of  sophistication are related to  the size
 and/or  the capital investment  associated with  the  fabric filter system.
 Regardless of  system size,  pressure measuring  devices  in some  form should
 bo used  and  are usually  supplied  by the manufacturer.  Simple  draft gages
 provide  a  measure oJ: fabric  resistance  (and magnitude  of dust  loading),
 and give the static  pressures  at  various points within the system.  These,
 in turn, can be related  to  gas  flow rates and  power consumption.
             In a small system,  observation of  key  pressures  permits manual
 adjustment of  gas flows  and  actuation of fabric cleaning equipment,
 either manual  or motor driven.  Similarly, reference to visual display of.
 temperature  indicates whether  dilution air dampers or  pre-cooling sprays
 should be  adjusted.   In  the  large  fabric filter system, however, re-
 liance upon  manual control  becomes highly impractical  and the  sensing
 systems  are  coupled  directly to control locations  for  automatic operation.
 A well  instrumented  system  not  only protects the investment  and decreases
 the chance of  malfunction,  but  also enables the user to diagnose and
 rectify  many of the  simpler  operating problems without resort  to outside
 professional consultation.   In  any case, outside assistance  can be employed
 more effectively when the operating characteristics of  the system are
 defined  and  understood by the user.
            Table 7.8 gives representative  catalog costs  of  instrumenta-
 tion types commonly used in fabric filter systems.   Relatively more
 expensive equipment  is recommended for long-term rugged use,  high  tem-
 peratures,  abrasive conditions, corrosive gases,  etc.   The  additional
cost of installing and adjusting the typical instrument is  estimated  to
be of the same order as the instrument price.
            The instrument indicators  can be grouped on a  central  control
panel,  the cost of the panel running $200 to $500 per  foot  of panel de-
pending on the density of the instruments.   This  cost  includes wiring  and
pneumatic piping but excludes instrument cost.   The cost  of installed
 instrumentation will vary from $.01 per  CFM for a large baghouse  to roughly
$.10 for a 1000 CFM unit.
            Standard instruments should  be  used whenever  possible  for  re-
liability and ease of replacement  parts.  Standardization will minimize

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                                    TABLE 7.8
                  TYPICAL INSTRUMENTATION CATALOG COSTS - 1969
Variable, type
Temperature
      Flue gas thermometer          x
      Dial-type thermometer         x
      Recording thermometer         x
      Sensing switches
      Sensor 4- Rustrak recorder     x
      Thermocouple bridge + amplifier

Pressure
      Draft gauge, inclined         x
      U-tube monomcter              x
      Sensing switches, min-max
      Sensing switches, differential
Flow
      Hand-held tube-type kit       x
      Low velocity mass flow,
              electronic            x
      Time record clock             x
      Motor-type valve or damper
              controller            x
          Purpose:
   A: For indicating
   B: For recording
   C: For controlling
                                        B
x
x
X
X
X
X
    X

    X
                       Catalog Cost
45 -
30 -
25 +
20
200
200
 30
 15
 30
 40
275

178
 80

125
                              70
                              100
                               70
                               60
Humidity
      In-line industrial hygrometer x
      Recording hygrometer (Temp,humid.)
                                    x   x
      Transducer, electronic            x
      Dew point sensor              x

Mechanical Sensing
      Lever-arm, mercury switch     x   x

Particle variables                   •«.
      Continuous size monitor
      Fabric hole detector          x   x
Mass Concentration meter            x   x
                         110

                         325
                         350
                         15
                         100

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maintenance costs which will be one of the factors traded off against the
initial costs.
     7.2.6  Planning and Design Costs
            A certain amount of time goes into planning the dust collect-
ing system, principally in the form of calculations and drafting but
also as meetings, procurement, travel, etc.  For the experienced user
of fabric filter equipment this may not amount to much, especially if he
uses so many small collectors that he keeps them in stock.  For inex-
perienced users and those designing collection systems for new applica-
tions, the time may range from 10 to 1000 man-hours or more.  Large
systems justify careful planning, as do those that must perform with high
reliability, and those that involve a large risk of fabric damage.
            It is customary to charge small amounts of staff time to an
overhead account, which means the cost will not be reckoned against the
filter system.  The cost of larger planning programs may or may not be
added to the other costs of obtaining and installing the system.  Never-
theless, the costs should be estimated and included in all comparisons of
alternative dust control systems, unless the planning is to be furnished
free by an equipment supplier.
            Procedures for designing a filter system are discussed in
Chapter 5.  Engineering of this kind will cost approximately $15 per
hour, including the normal overhead costs of maintaining an engineering
staff.  Thus a typical planning cost for a good sized system may be $1500,
or on the order of $.10 per CFM of system capacity.
     7.2.7  Foundation and Installation
            A fabric filter system may be installed outside the main plant
building when no space is available within the main structure or when a
cost analysis shows that the outside location is on the whole less expen-
sive.  Although the cost of outside site preparation is discussed at
                         1 31
length in the literature, '   the basis for cost breakdown is sometimes
cbscure, particularly between direct and indirect costs.
            Since fabric filter units weigh from 0.25 to 1 ton per 1000
CFM capacity not including fans,  motors and ducting, they require substan-


-------
Lial foundations and supporting members.  These may account for 4 percent         _^
ol" the installation for material costs and an additional 5 percent for
associated labor (Table 7.4).
            Many fabric filter installations, even though themselves mod-
orntoly well scaled against weather, still are best housed for easier
maintenance in bad weather.  A separate light weight building to house
                                                     2
the equipment is estimated to cost $1.50 to $5 per ft  of floor area
based on reported costs for single-story garage and maintenance shop                ^
shells.  This includes foundation, structural frame, outside siding, in-
side partitions, and paint.    The filter may alternatively be located            —     I
indoors, where it requires typically 1 sq. ft of floor space for each
50 CFM capacity.  For this space a substantial annual charge is normally          _
made (Section 7.2..3a).  Outside or inside, the cost of installation, at
about $5 per hour wages plus 507* overhead, amounts to 30 or 40% of the
initial equipment cost on  large equipment.  It may be, however, 2 or 3
times higher in special cases.  Itemized installation costs are discussed
in preceding sections and shown in Table 7.4.                                     ~"J     j
            A frequent consideration is whether it is best to purchase
the filter assembled or to assemble it on site.  The net cost to the
purchaser will probably be about the same; the cost of assembly at either
location is estimated at around 10% of the total on-line cost.   Factory           —
assembly is usually preferred from the standpoint of job quality unless                 !
unusual skills are available at the site.   Railroads, however,  have size          _
                                                  *                                     !
limits of approximately 9.5 x 12 (high) x 40 feet.    The cost of painting
including protective coatings is estimated at 57, of the assembled cost,                 j
and the place at which the painting is done makes little difference to                  j
the purchaser.                                                                           ;
     7.2.8  Start-Up Period                                                             j
                                     •   '                                                i
            Although not usually reckoned as part of the baghouse instal-            •   !
lation cost,  there is in fact nearly always a period following  installa-
                                                                                     r
      *
      Freight  rates  for  steel  vary with  the  amount  shipped.  Minimum car-
 load  rate  in the Boston  area requires  at  least 40,000  Ibs.

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 tion  when personnel  are  becoming  accustomed  to  the  equipment  and unfore-
 seen  operating  problems  are  being corrected.  This  period  ranges from  a
 lew hours to  over  a  year.  Start-up  cost  is  herein  treated  as an initial
 rather  than as  an  annual  cost  for convenience,  even though  it may  extend
 into  the  useful life of  the  equipment.
      Analysis of 78  problems reported by  the users  of 26 fabric filters
 of many types and  sizes  indicated that at least 20  percent  of the  problems
 were  related  to becoming accustomed  to the equipment or to  faults  in the
 installation.   Half  of these start-up problems were connected with the
 filter itself,  such  as poor seals  between panels, bags located too close
 together, or  failure of  the shaker mechanism due to weak design.   Other
 start-up  problems  are cited in Chapter 8.  Whether  from inadequate tlesign
 or untrained use,,  problems do often  arise and they  do cost  the purchaser
 money.  Furthermore  the installation of the equipment may require a tempo-
 rary  plant shutdown, which may be  considered a start-up cost.
      It is estimated that the start-up costs may on the average add 50% to
 the normal maintenance cost for the  first year, that is, amount to about
 $0.10 per CFM.  Start-up cost may be much higher, or on the other hand the
 purchase of equipment known to be of superior quality, and  the use of well
 trained or experienced personnel,  can much reduce the learning costs.
 7.3   OPERATING AND MAINTENANCE COSTS
      The costs  of  operating the fabric filter system from day to day and
 keeping it in good working condition include:
                 \
Cost  Item                              Typical Cost Range        Section
Powe»-
  Fan power for filtering              $  .10 - .25 per CFM-yr    7.2.1
  High pressure air for cleaning           0 - .25     "
Labor
  Fabric replacement                      .02 - .20     "         7.2.2
  General maintenance                     .02 - .20     "
  Dust Disposal                           .01 - .15     "
Plant Overhead
  Space, heat, lights, insurance etc.    .05 - .50     "         7.2.3

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 Operating and maintenance costs considered to be "typical" are indicated
 in Tablo 7.3 and Figure 7.3, where together they make up $0.67 per CFM-year,
 or about 2/3rds of the overall yearly filter cost.  In addition there are
 the costs of purchasing replacement fabric which are discussed separately
 in Section 7.3, and various incidental costs discussed in the following
 pages.  To these are added a yearly amortized portion of the initial costs
 of the equipment, installation, and capitol costs (Section 7.1) as dis-
 cussed in more detail in Section 7.4.
      Operational and maintenance cost  data were available for about 50
                                                                       *
 fabric filter installations via both the literature and the GCA Survey .
 These varied in size from 330 to 820,000 CFM,   The mean operating and
 maintenance cost as reported, including power,  labor and fabric replacement
                       &
 but excluding overhead , was $0.52 per CFM-year.  Only 6 installations re-
 ported O&M costs in excess of $1.  per  CFM-year  as indicated in Figure 7.8.
 Overhead is estimated at typically an  additional $0.025 per CFM-yr.,
 (Section 7.2.3).
                                                         10.
           Figure  7.8.   Total  Operating and  Maintenance  Cost.
                        (As  reported,  including  power,  labor,  and
                        fabric replacement,  but  excluding  overhead)
                        (From  GCA  Survey,  1969).
     The  following indicates the difference in apparent costs of two
hypothetical plants, one of which meticulously includes overhead while
the other uses only direct costs:
* It cannot be determined precisely what indirect costs are in the figures
  reported in the literature.   Engineers are inclined to report only direct
  expenditures, while others may include the costs of plant space, employee
  fringe benefits, tax rebates for expendible items etc.   GCA surveyed direct
  costs by asking for task labor requirements in hours, prices as billed,  etc.
                                   7-30

-------
      Power
      Labor
      Space
      Heat
      Other overhead
      Fabric (see  Section 7.4)
Annual O&M Costs ($/CFM-yr)
    Direct Cost Only
    $      .12
           .21
           .18
Including Overhead
$       .12
        .38
        .05
        .06
        .04
        .09 (Tax rebate)
                 Total:      $      .51                        $       .74
 Each  of  these  costs  varies  widely  from  installation  to  installation.
      7.3.1   Power Costs
             The  cost of power for  pumping  the gas  through the filter is
 perhaps  the  most easily determined of all  the system costs; it is  given
 simply by
                      Power Cost = CQp/E
                      where C = cost of electricity
                            Q = volume  flow rate
                            j5 = average fan pressure
                            E = fan-motor  efficiency
      The cost  of electric power is typically $.012 per kilowatt hour,
 depending on location and the type of industry.   The power conversion
 efficiency of  fan-motor units is usually taken as 60%.  The average pres-
 sure  at the  fan  is the only indeterminate  factor and even this is partly
 determined by  the filter operator's control of the fabric cleaning process.
 The average  fan  pressure is typically 5 to 10 inches of water (See Figure
7.7)  and the portion contributed by the filter alone, indicated in Figure 7.9,
is typically 3 to 6 inches.   Power cost for fan, blower, or compressor oper-
ation may be determined using Figure 7.10,  which is based on the above equation
using a 60% electrical to air power conversion efficiency.

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             (a) lie fore Filter Cleaning  •
                                        I  ttl  I J
                _
           oTi                     iTo      *            io7
                            (in. H20/FPM)   •
             (b) After Filter Cleaning
           0.1
  ltll.1.1  -1.     .
    i.oVio7
.  H90/FPM)
                            (in. nn'
            Figure 7.9.  Typical Filter Pressure Drops.
                         (Assuming an average filter velocity of
                          3 FPM, median pressure drops  are 7.5 and
                          4.5 inches of water, or 6 inches average.
                          (From GCA Survey,  1969.)
Figure 7.10 is based on full time operation; therefore, if for some reason
the filter system only operates 3 hours per day the actual power  cost  would
be one eighth as much.
      7.3.2  Labor Costs
             Labor skill requirements range from supervisory and  instrument
repair categories down to unskilled labor for such tasks as dust  disposal,
and these wages currently range approximately from $2.50 to around  $6.00  per
hour.  Considering the typical  labor overhead of employee  benefits, medical
coverage, administration etc.  the net cost of average  labor to the  company
can be estimated at almost twice the actual wage.
             Among about 30 fabric filter installations reporting labor data,
about .047 man-hours were required per'CFM-year,  equivalent to about $0.21
per CFM-year in wages.   Costs  for specific filter  installations are listed
in Appendix 7.2.1.   These costs were reported to  be  distributed as  in  Table 7.9
among several tasks common to most filter installations.   The task  labor dis-
tributions are further  shown in Figure 7.11.   Fabric replacement is indicated

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          50
          10'
Inches of
   Water,

 Fan  or
 Blower
 Diff.
 Pressure
            10                    100
                     Cost, $/ye«r per 1000 CFM
                   1000
        100 i
   Pounds
    per
   sq.ia 10  .',
    (psig)

    Fan or  •
   Compressor
    Pressure
3.0 c/kwh
  Power Cost
                                 10                 100
                     Cost, $/year per CFM at Pressure

          Figure 7.10.  Air Power Coats.  (Based on a 60% fan-motor
                         power efficiency and full-time operation.)

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                               TABLE  7.9
                 REPORTED LABOR  DISTRIBUTION  COSTS
Re
'X.
No. Reporting:
No. Practicing:
Mean Practice:
Median Practice:
Bag
>p lac ing
47
27
26
.084
$2.00/bag
General.
Maintenance
39
26
26
.083
$.045/CFM-yr
Dust Bag
Disposal &
13
27
13
.073
$2.00/ton
Cleaning
Repairing
1
27
4
Var.
—
Total
100
(30)
(30)
207**
	
(Units)
%
Count
Count
$/CFM-yr
	
* GCA Survey data. Reported wages, before overhead.

** The average of  combined task costs was $.207/CFM-yr; the combined task
   average cost is $.24/CFM-yr.
    A.  Replacement Labor, $/year          D.  Other Maintenance Labor,
    B.  Replacement Labor, $/bag      A        $/KCFM-year

    C.  Other Maintenance Labor, $/year    E-  Disposal Costs, $/year

                      F.  Disposal Costs, $/ton
Figure 7.11.  Reported Labor Costs, GCA Fabric Filter Systems Survey,
              1969, (Wages, before Overhead)

-------
 to  absorb  almost half  the  labor;  on  the average  it  takes about half an
 hour  to  replace a  filter element  including preparation  time,  filter clean-
 out time etc.  Fabric  lasts about one year on  the average  (Section 8.3).
      Fabric  filters  typically  require about 10 hours of attention per KCFM
 per year for such  things as inspection of the  fabric, instrument adjustment,
 fan servicing, etc., excluding the other labor categories  shown and excluding
 unscheduled  repairs.   (Unscheduled repairs are so variable that they were
 not made a part of the survey,  but may perhaps be estimated at 1 or 2 hours
 per KCFM-year).
      The purchase  of dust  handling and disposal  equipment  trades off
 against  doing the  job manually.   Among 13 fabric filter installations
 reporting dust disposal costs  the average direct cost was about $2. per
 ton with variations  from $12.  per ton down.  These companies had already
 installed a  reasonable amount  of  disposal equipment, and the $2./ton figure
 represents the manual balance  of  the task.  Most of these companies did
 the work themselves, but about  a  third of them contracted the work out to
 a local  trucking firm.
      Fabric  cleaning and repairing is apparently practiced by only a
 small percentage of filter users, usually those with expensive fabric
 that  tends to bind or plug frequently.  One installation periodically sent
 the bags out to be dry cleaned.   Another found that ordinary cotton bags
 could be patched in place quickly and inexpensively.  (See Chapter 8).
      These reported costs could be adjusted in two ways although as given
here  they have not been adjusted.  First, as mentioned above the cost of
 labor overhead could be included  as a factor of from 1.5 to 2, except for
labor contracted outside the company.  Second,  these costs could be ad-
justed for the fraction of the year the equipment was in actual use.   For
example,200 hours a year spent in replacing fabric on a filter used only
8 hours per day might be interpreted as equivalent to 600 hours a year  on
a full-time filter.  However,  there is no assurance that the fabric life
would remain the same, as if the  fabric were being damaged by condensation
due to frequent shut-downs  the fabric life might actually increase if  the
operation went to full-time.   Lifewise most other labor requirements  cannot


-------
be  .scaled with assurance.  A  scaling  factor may  if  desired  be  obtained  from      _j
Appendix 7.2, where approximately  two-thirds  of  the  installations  operated
                                                                                  i  .
full-time and the other  third operated an average of 227.  of the  time, giving
an  average operation of  757« of  the  time and a scale  factor  of  1.33.
     7.3.3  Plant Overhead Costs                                                  ^
            Just as there is an overhead cost associated  with  the  use of               i
manpower, there is a cost associated  with the use of the  plant.  This can         __,   :
include a variety oi" charges — Rentals, utilities, administration costs,                .
taxes, etc., and the overhead cost  factor depends to a  large extent on              ;   ]
                                                                                  jraj   |
the company's accounting practices.   Some of  these costs  may go  on the                 i
books against the. filter system whether or not they  involve actual cash
expenditures, while other costs may never be  compared with  the filter oper-       "~*   j
ation even though they are directly related.   The cost  of space  occupied by
the filter is apt to be  one of  the  latter.  The  following presents a brief        —•'   '
summary of typical plant overhead costs per CFM-year:

                                      Direct  Expense     Actual Expense               '
                   Space             $    0                $      .055
                                                                                  *-**
                   Heat                   .15                      .15
                   Insurance, taxes       .045                    .045             _
                   Other                Var.                    Var.
                           Total:     $  .195+              $   .25+
          Variances  in these costs are discussed briefly below.
            7.3.3.1  Space - The filter housing and its peripheral equip-
ment should be considered as renting the plant floor space occupied, as
they prevent the space being used for something more profitable.  In certain
circumstances the upper room space may also have value.  On the other hand
equipment located in unused buildings or outdoors may not incur a space
                                                                          2
cost.  Plant space varies in value, but a typical rental value is $2.75 ft
year (1969).  Rental cost may not include heat, light, water, sewer, taxes,
etc.

-------
                     The filter capacity packed into one square  foot  of
       K base area depends on the filter height,  the  filtration velocity and
 MM- duaiiing mechanism, and consequently the packing varies  widely from
 limtollntlon to installation.  Figure 7.12 shows  the capacity packing for
                     IM
                     140
                      110
                     100
                      •0
                      •o
                      40
                      to
Apparent Upper /o
Limit
Practiced
                                     i   o  .
                            10    100   IOOO  DOOO  K)OOOO  10
                                MOW UMM.ITY (CfM)
        Figure  7.12.  Plant  Floor Area Required per Filter Capacity.
                      (Based on Collector Dimensions plus 3 ft.
                      Perimeter Clearance.)   GCA Survey Data.
30 installations  surveyed.   It appears there is an upper packing limit
in practice;  for  example,  few installations of 10,000 CFM capacity would
                     2
exceed 80 CFM per ft  although many would occupy the same amount of plant
floor area and  filter  less  air.  As several maintenance problems appear when
the fabric is packed too  close together (Section 5.2.6) or when the air/
cloth ratio  is  made  too large (Section 4.2.2) there may be good reasons
to avoid the  apparent  packing limit indicated in the figure.
                     Picking from the figure a typical capacity of 50 CFM/ft2
of floor, and the  rental cost  of $2.75/ft2-year, one obtains a typical
space cost of $0.55  per CFM-year, which is  one  of the overhead costs.

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              7.3.3.2  Heat - Some baghouses exhaust unwanted heat to the
  outdoors,  but  others  waste heat, particularly in winter months.   In the
  northern half  of the  U.  S., fuel usage costs  approximately $.40  per CFM         ~
  per year when  the air is exhausted  outdoors.   This is  an annual  average
  based  on a 154 hour week and  an  8 to 9 month  heating season.   Over the          	
  U. S.  an average heat cost may be $0.15 per CFM-year.   Other cost consider-
  ations are the exhausting of  air previously air conditioned,  and the cost
                                                                                  -^*
  or value of providing alternative ventilation by means other than the filter
  system.
              7.3.3.3  Insurance and  Property Taxes - These combined are
  estimated  to cost about  1.870  of  the installed cost of  the baghouse, or
                                                                                  N»>
  about  $.045 per CFM per  year.
              7.3.3.4  Lights -  Lighting should be installed around the
                                                                                  ->
  baghouse,  but  the annual cost  is probably  negligible.
              7.3.3.5  Supply Inventory  - Normally at least one  set of bags
  is kept on hand at  all times,  and spare fan parts etc. may also  be stocked.
 Annual inventory costs amount  to approximately 10% of  the inventory invest-
                                                                                  _/
 ments.
      7.3.4  Collection  System Returns                                                  i
             	                                                 I
             Offsetting  the  filter system annual  costs in  some cases is  an
 actual profit  from  operating the  system.   For  example, particle -collection         •   j
                                                                                       i
 may be a basic  part of the  manufacturing process,  or the  dust  collected          ""'
                                                                                       l
 may have a  high sale value  as  in many  non-ferrous  metals  refining applications.       i
             The  return on investment  for a  fabric  filter  installed  to  control    *"   I
 a community air  pollution  factor  is frequently difficult  to asses.   In-               j
 stallations which eliminate dust damage claims and  expensive and  time-consuming  —'   j
 complaints  or litigation will  represent a direct  economic return  to the
user in terms of  reduction of management problems.                                ^ •  i
             Since many dust  induced diseases have an etiology requiring              _,- \
several years exposure, and  are non-specific, the utilization of fabric           .^   :
filtration for health protection purposes usually makes excellent  economical           ;
sense.   Other direct benefits from the reduction of visible but otherwise

-------
 innocuous dusts include improved plant  housekeeping  and  reduced maintenance
 on plant machinery,  improved visibility,  increased employee morale,  improved
 product  quality,  etc.
 7.4  CLOTH AND BAG COSTS
      Bags fitted  for a specific  model of  fabric  filter are available  from
 a  number of suppliers, usually including  the manufacturer of  the  filter.
 The prices depend very much on the  cloth  used; that  is,  on the kind  of  fiber,
 the weave and  weight of cloth,  and  on specific treatments given the  cloth,
 yarn,  or fiber during manufacture to protect the fabric  in use from  fire,
 rot,  mildew, abrasion and  so on.  Table 7.10 indicates costs  for  a number of
 fairly typical bags  as purchased a  set  at a time.  Also  given for comparison
 are some fabric and  fiber  costs.  A typical bag,  ready to be  installed  by
 the purchaser's maintenance crew, costs $10; however, bag prices  range  between
 $1 and $100.   A breakdown  of the cost of  a typical bag in Table 7.11  indicates
 that  the bag cost increase accrues  in making the  fabric, and  the  rest in
 making the bag from  the fabric.
      As  basic  as  the purchase price of  the bag is, it is only one factor in
 the fabric-related costs of operating the filter  system.  Even more important
 is the fabric  life which depends  on many  things  as discussed  in other sec-
 tions  of this  volume.   By  maximizing the  length  of time  the fabric remains
 useful,  the fabric costs and related replacement  labor costs  measured in
 dollars  per year  are minimized.   Figure 7.13 indicates the results of a
 survey of about 40 filter  installations.  As indicated, most  fabric lasted
 about  one. year, with a resultant median cost of about $0.10 per CFM-year
                                      ft
 and a  mean cost of $0.18 per CFM-year.    Cotton and  glass were the most
 widely used fabrics  reported in  the survey.
     Associated with the installation of  every bag is a labor cost, typi-
cally  between $1.50  and  $2 per bag;  this  is discussed in Section 7.2.2,
     Fourteen fibers used in  fabric filtration are listed in Table 7.12
 (See also Section 4.2.1).   The approximate price of  each fiber relative
to cotton is given along with its temperature limitations.   Figure 7.14
rolato.s temperature and cost for some  of the  common fibers  and shows  why
glass is  a popular fiber despite its mechanical  durability  limitations.


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                                  TABLE 7.10

                   TYPICAL FILTRATION FABRIC COSTS
Basic
Material
Cotton
Wool
OrlonR
Dacron
Nomcx
Nylon
Fiberglass
TeflonR
i>
Fiberfrax
n
Brunsmet
Fiber
Cost/lb.
.40
(Wide Var.)
.66-. 80
1.40
2.50-6.00
1.00
.60
--
20.00
-^40.00
Woven Fabric
Cost/Yd
.41
1.77
1.01
— 'V
1.04
--
.70
.98-1.68
8.00
(8 oz)
38.00
--
Felted Fabric
Cost/Yd
--
3.97
4.82
4.82
11.50
--
N.A.
36.70(23 oz)
29.20(19 oz)
~'
--
*
Selected Retail Bags
Cost Lengths x Diameter
13.60
1.50
50. F
35. F
5.
3.
30.
22.
13.70
2.80
25. F
50.
27.
—
27.
20.
16.50
10.
9.63
2.80
75.
._
--
21' x 9"
5' x 5"
14' x 7"
7' x 12"
9.5' x 6"
13.5' x 5"
30' x 10"
25' x 11.5"
22. 5T x 12"
9' x 6"
5' x 5"
25« x 11.5"
20' x 8"
—
20' x 8"
25' x 12"
25' x 11.5"
22.5' x 8"
12' x 5"
6' x 7"
25' x 11.5"
—
--
* Note: Bag costs and fabric costs are not related, as
        separate sources (References 9, 10,11, and GCA
(F) Felted fabrics
(R) Registered Trademark
the information is from
Survey, 1969).

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                          TABLE 7.11

                    COSTS OF TYPICAL  BAG

Bag purchase cost:
Sizo:
2
Cost per ft :
Fabric cost:
Fabric cost per bag:
Fiber cost:
«- Fiber cost per bag:
Fiber-to-fabric factor:
Fabric-to-bag factor:
Typical Bag
$ 10.
20 ft2 ,
$ .50
$ 2. /yard
$ 4.40
$ .75/lb
$ 1.10
~ 4x
~ 2x
Range Available
$ 1- 100
1 - 100 ft2
$ 0.15 - 4.00
$ 0.40 - 40. /yard
$ 0.25.- 25.
$ 0.40 and up/lb
(wide variation)
.. ..
~0.5x - 5. Ox
Types of fabric used:
            Cotton                      33
            Glass                       33
            Polyester  (e.g. DACRONR)    15
            Acrylic  (e.g. ORLONR)        5
            Wool                        5
            Other types                 9
               100
Fabric Life, months
                                             1000
                     i. . * t
        .01                    0.10
                 Fabric  Costs,  $/CFM-
                                      1.00
          Figure 7.13.
       Fabric Usage Reported and Costs.
       (Costs as reported, before tax credit.)
       (From GCA Survey, 1969, and miscellaneous
        literature data.)


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                                TABLE 7.12




            FIBER, TEMPERATURE RANGE, AND RELATIVE COST
Type (Typical Name)
Cotton —
Rayon, acetate —
Wool
p
Acrylic (OrlonR)
(Dynel )
Vinyls 	
Polyester (Dacron )
Polyethylene, ^
polyolefin (Polyfain )
Saran
Polyamide (Nylon^
- modified (Nomex )
Polypropylene 	
Poly-TFE (TeflonR)
Glass 	
Asbestos 	
Ceramic (Fibref rax' )
Recommended
Temp. Range:
Max. Contin.
225

250
275
240
—
325


250
500

500
600

2800
(160-190)
210
(180-235)
(200-275)
(150-180)
250
(250-280)
200
160
200
425

(225-450)
(450-550)
500
2300
Approximate
Cost
1
1.1
2.75
2.75
3.2
2,7
2.8
2.
2.5
2.5
8
1.75
30.
5.5
3.8
— 75
Metal
(Brunsmet  )
-100

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    0


    6


    4


    2


  20
I'8
u  16
   14


   12



   10


   8


   6


   4


   2


   0
                                     Poly TFE
                                     Glatt
             100     200      300     400


                        UPPER  TEMPERATURE
                           500
600   700
        Figure 7.14.
Approximate Temperature Capability/Cost Rela-

tionship for Filtration Fabric Materials.

(Costs Relative to Cotton.)

-------
 7.5  ACCOUNTING COMPARISONS  OF  COSTS

     As  discussed  in  earlier parts  of  this  Chapter,  fabric  filtration

 costs  fall  into two classes,  initial or  one-time  costs,  and periodic  costs.

 The  variations  in  each  from  installation to installation can be many-fold

 and  each installation must be separately analyzed.   Furthermore,  as noted

 earlier,  certain costs,  notably overhead items  but also  others, may or

 may  not  appear  as  costs  of the  filter  system even though they are related.

 In evaluating the  costs  of fabric filtration one  should  consider  how  the

 cost figures are going  to be  used in order  to decide which  of the hidden

 costs  to include,  if  any.

     First  costs (Section 7.1)  may  include:

          Planning and  preparation  for installation
          Equipment and  parts purchasing
          Installation
          Start-up and  training

     Periodic costs (Sections 7.2,  7.3)  may include;

          Replacement fabric
          Power
          Labor  and associated  labor overhead
          Plant  overhead, taxes
          Planned  replacement parts
          Unexpected  repairs
          Deductions  or  costs of such extenuations as
          Sale  value  of  dust  collected
          Effectiveness, reliability, convenience, etc.
          Capital  costs

     First  and  periodic  costs may be combined in  either  of  two ways, as

 an annualized distribution of first costs plus the annual charges, or as

 a summation of  future periodic  costs plus the first costs.  The first

 treatment is the simpler; it  is adequate  for most cost estimating, and

has been  Implied in earlier parts of this Chapter.  The  second is more
*
 In order to compare one particulate control system with another, one
should attempt to reduce all criteria of performance to the same denominator,
such as dollars per CFM-year, or dollars per ton, etc.  There is no
completely acceptable way of doing this.  One may use the data of Stair-
mand (e.g. Figure 7.2) to assign a dollar value to a given collection
efficiency, but this must be modified in terms of the plant emission
requirement.



-------
precise;  it  is  preferred by cost accountants because  it enables a more
accurate  comparison of alternative proposals.  Each is discussed below
with examples.
     7.5.1  Annual Distribution Method
            To  estimate the total average yearly cost of owning and using
the filter system, one can simply divide the total initial cost by the
expected  life of the system, and add the result to the expected annual costs
of operation.   This is called amortizing the investment using straight-line
depreciation; in this method the book value of the initial investment de-
creases steadily through the life of the system.   Every year a fraction of
the investment  is written off the book value, and accepted as a filter system
cost.
            Since money is tied up in the investment,  and since the same
money could have been used elsewhere to generate profit,  the investor suffers
a loss of interest income as another cost.   Using an interest rate of 10% and
$1000 invested, the first year title interest cost is $100.   The second year,since
the investment has depreciated, say l/15th in anticipation of a 15 year life,
the remaining investment is $933, and the interest cost is $93.33.   For the
average year the equipment will be worth $500 and the  interest cost will be
$50; thus over the 15 year period a total of $750 will be acknowledged on
the books as interest cost.  This total interest  cost  may be annually distri-
buted for convenience as simply $50 per year; it  is equivalent to simply
(Investment) x  (Interest rate) f- (2)  per year.   Note that even though the
interest may not be an actual cash outlay,unless  the money was borrowed,
it is attributable to the filter system.

      Example;
      Total  initial investment:   $1,000 including materials,  labor,  freight,etc.
      Cost of investment  capital:   8%
      Anticipated equipment life:   12 years
      Anticipated yearly  costs:   $325 per year  including power, maintenance,
                                 fabric,  plant  space etc.

-------
      Straight investment depreciation:  $  83.33 per year
      Capital (interest) cost:
                        $1000 x .08 * 2 -   40.00      "
      Yearly O&M costs:                    325.00      "
      Total estimated cost of owning and
         operating the fabric filter system:  $ 447.33 per year
                                                or $5,380 Total.
 Likewise the rate of depreciation need not be constant; other acceptable
 methods include faster-than-straight line depreciation and slower-than-straight
 line depreciation.  Plant accounting policies may prefer any one of several
 commonly used depreciation schedules, and income tax laws accept any of
 several schedules.  Unless the design engineer has a specific reason for
                                                                        36
 using another method, the straight-line depreciation method is simplest
      7.5.2  Anticipated Cost Summation Method
             Alternatively the anticipated future costs of using  the equip-
 ment may be back-computed to the present time and summarized, in terms  of
 present dollars.
      Example based on the system just discussed:
             Total initial investment:      $1,000.
             Capital (interest) cost:
                $40./yr x 12 yrs              480.
             O&M costs:
                $325./yr x 12 yrs            3.000.
             Total cost of owning and
                operating the system:      $ 5,380.  or $447.33  per year.
 This is of course equivalent to the annual distribution method,  and equally
 useful  for estimating;  but it is  not  as accurate  as  the following method.

     An important  principle  in  finance  is  that future expenditures are
 greater than their present value.   For  example, suppose a cost of $404 is
 anticipated at a time one year  from now.  By investing $372 at the present
 time at  a return of 8% the cost can be met when the time comes.  Thus the
 future  expenditure is said to have a "present worth" of $372.   All such
 anticipated future expenditures including interest costs can be summarized
 in present dollars.  The preceding example may now be treated in the following
manner:

-------
                      Approx.
   Year   Book Value    Interest    O&M   Total  Yearly Cost
             Present  Value
             of Costs at  SI
0 $
1
2
3
4
5
6
7
8
9
10
11
12
Total:
1000
917
833
750
667
583
500
417
333
250
167
83
0
- __ _ _ _
$ 77 $ 325 $
70 "
63 "
57
50 "
43 "
37 "
30 "
23 "
17 "
10 "
3 "
$ 480 $ 3900 $
Total initial investment:
Present value of future costs:
___
404
395
388
382
375
368
362
355
348
342
335
328
4380
$1,000.
2,794.
_ - _
$ 372
339
309
281
255
232
211
191
174
157
143
130
$ 2794

             Present value of system
              lifetime expenses:
$ 3,794.
According to this example, $3794 invested now would pay for the same
equipment and its use as would the $5380 estimated by the previous methods.
The smaller figure in this case represents more accurately the expected
costs to the company, and this accounting method (or still more refined
methods) should be used whenever their increased complexity is justified.
         This method should be used whenever two or more systems having
different initial/annual cost ratios or different lives are to be compared.
Consider for example two systems, the one described above and an alternative'.
system:

-------
       lixamplt
System No. 1
Total initial investment: $
Capita] (interest) cost:
87,, 12 yrs
O&M costs: (for 12 yrs)
IJasos for comparison:
I) Initial costs only:
2) Annual distribution:
3) Present value of future costs
1000
480
3900

1000
5380
3794
System No. 2
$ 600
288
4512

600
5400
3660
Hardly anyone comparing two different systems would look only at their
initial costs.  Using the method of annual distribution of initial costs          —'
as a criterion of comparison would, in the above example, lead to the wrong
decision, as the more detailed present value method shows the second system       ^
will cost the company less.
7.6  ECONOMY IN FABRIC FILTER OPERATION                                           _,
     Chapter 5 discussed ways to minimize costs in designing fabric filter
systems, and Chapter 8 will discuss guidelines for operating and maintaining      _/
the system.  Operating personnel should be aware that even though they may
never see the electric bill or the invoices for the replacement fabric,  they
can do much to affect day-to-day costs.   This applies to the personnel run-
ning the dust generating process as well as those close to the filter system.       i
Temperatures can be kept down, minimum cleaning can be exercised, minimum air
can be filtered, etc.  Many ways to keep costs down are discussed in other         ,
parts of this handbook.                                                            —'

7.7  REFERENCES FOR  CHAPTER 7                                                      :
                                                                                  —'
1.  R.H. Perry, C.H. Chitton, Kirkpatrick, Perry's Chem. Eng. Handbook.
    McGraw Hill, 4th Ed.
2.  C. J. Stairmand, Some Industrial Problems of Aerosol Pollution, Proc.
    Rov. Society. 307:209 (1968).
                                                                                   i
3.  C. J. Stairmand, Selection of Gas Cleaning Equipment. A. Study of Basis        ~*
    Concepts, Filtration Society Conference on Dust Control, Olympia,
    London, Sept. 22-25 (1 Sept. 1969).
                                                                                  ~j
4.  D.C. Stephan, Dust Collector Review. Am. Foundrymen 's Sot-. Annual
    Meeting, Philadelphia, Pa. (May 1960).
                                                                                   t'

-------
 5.   R. L.  Adams,  Application of Baghouses  to Elec.  Furnace Fume Control,
      JAPCA  14:8. 299  (August 1964).

 6.   F.A. Bagwell,  L.F.  Cox and  E.A.  Pirsh,  Design and Operating Experience
      with a Filterhouse  Installed on  an Oil  Fired Boiler,  JAPCA 19;3.  149
      (March 1969).

 7.   C. J.  Lewis and  B.B.  Crocker, The  Lime  Industry:  Problem of Airborne
      Dust,  JAPCA 19:1. 31  (January 1969).   .

 8.   Organization  for Economic Cooperation and Development, Air Pollution
      in the Iron and  Steel Industry,  O.E.C.D., Great Britian(1963)

 9.   W. B.  Harris  and M. G.  Mason, Operating Economics of  Air Cleaning
      Equipment Utilizing the Reverse  Jet Principle,  Ind. & Eng.  Chem.  47:12,
      2423 (December 1955).

10.   B. P.  Harrison,  Baghouse Cleans  500° -  Cement Kiln Gases, Air Engin-
      eering.  14 (March 1963).

11.   R. J.  Plass,  Comparison of  the Cottrell Electrostatic Precipitator
      and the  Silicone Glass  Bag  Filter,  Proc.  Seminar  on Electrostatic
      Precipitation, Penn State U., University Park,  Pa.  (December 1960).

12.   U. S.  Senate,  Com.  on P.W.,  Hearings Before  the Subcommittee on Air
      and Water Pollution (18 May 1967).

13.   J A. Fife, and R. H.  Boyer, What Price  Incineration Air Pollution
      Control?. Metcalf and  Eddy,  Inc.,  Boston,  Mass,  (circa 1967).

14.   J. H. Weber, The Impact of  Air Pollution Laws on  the  Small  Foundry,
      62nd Annual Meeting,  JAPCA.  (June  1969).

15.  W. Muhlrad, Baghouse  Dust Collection of Brown Smoke from an Oxygen
      Converter, Stahlund Eisen 82:22, 1579 (October  1962).

16.   J. L.  Smith and  H. A.   Snell, Selecting  Dust  Collectors,  Chem. Engg.
      Prog.  64:1 (January 1968).

17.   R. A. Gussman, C. E. Billings, and  L. Silverman,  Open  Hearth Stack
      Gas Cleaning Studies.  Semi-Annual Report  to  American  Iron and Steel
      Institute SA-17  (August  1962).

18.  A. Little and B.  W.  Sutton,  Industrial Air Cleaners -  A Study of  Cost
     and Efficiency,  Filtration and Separation. 109  (March  1967).

19.  J. H. D. Hargrave and A.  F.  Snowball, Recovery of Fume  and Dust from
     Metallurgical Gases at Trail, B.C., Can. Min. and Met.  Bull.. 366,
      (June 1959).

20.  T. Killman,  Dust  and Fume Control for Electric Furnaces,  Texas Steel
     Co.,  Fort Worth,  Texas  (May, 1969).

-------
21.  National Asphalt Pavement Association, Guide for Air Pollution Con-
     trol of Hot Mix Asphalt Plants, Nat'l. Asphalt Pavement Assoc.,
     Riverdale, Md., Information Series 17.

22.  R.L. Chamberlin, and G. Hoodie, What Price Industrial Gas Cleaning,
     Proc. Intern. Clean Air Cong,. , (London) 133, (1966)

23.  H. R. Crabaugh, A. H. Rose, and R. L. Chass, Dust and Fumes from Gray
     Iron Cupolas-Control In L.A. County, Air Repair 4:3, 125 (Nov. 1954).

24.  R.L. Chamberlin, and P.B. Crommelin, Economic Aspects of Air Pollution
     Control for the World's Heavy Industries, presented at the First
     World Congress on Air Pollution, Buenos Aires, Argentina (November 14-
     21, 1965).

25.  T.M. Barnes, A Cost Analysis of Air Pollution Control in the Inte-
     grated Iron and Steel Industry. Battelle Memorial Institute for
     NAPCA, Fed. Clearinghouse No. PB 184576 (15 May 1969).

26.  J. M. Kane, Operation,  Application and Effectiveness of Dust Collec-
     tion Equipment. American Air Filter Co., Bull No. 270 P2, Louisville,
     Ky. (1952).

27.  R. J. Wright, Select Carefully - Dust Collectors  Fit Different Needs,
     Plant Engineering, 8 (circa 1965).

28.  R. Dennis, E. Kristal,  G.  Peters, and L.  Silverman,  Laboratory
     Performance of the Mikto-Pulsaire Collector,  Air  Cleaning Lab.,
     Harvard University, Boston, Mass., Contract No. AT(30)-1:841 (June 1962).

29.  PHS, USDHEW, Control Techniques for Particulate Air Pollutants.
     NAPCA No. AP-51, Washington, B.C. (January 1969).

30.  R.L. Adams, High Temperature Cloth Collectors, Chem. Eng. Progress 62:4,
     66 (April 1966).

31.  Chemical Engineering. 76:6, 132 (24 March 1970).

32.  F.R. Chase, Application of Self-Contained Dust Collectors.  The Torit
     Corporation, St. Paul,  Minn. (1963).

33.  M.S. Peters, Plant Design and Economics for Chemical Engineers,
     McGraw-Hill Book Co., Inc., New York (1958).
                          \

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                               CHAPTER 8
                       OPERATION AND MAINTENANCE

TABLE OF CONTENTS
8.1  OPERATION OF BAGHOUSE SYSTEM                                    8-4
     8.1.1  Start-up                                                 8-4
     8.1.2  Routine Operation                                        8-5
            8.1.2.1  Use of Instruments                              8-5
            8.1.2.2  Flow Variation                                  8-6
            8.1.2.3  Cleaning Cycle                                  8-6
            8.1.2.4  Changes in Operation                            8-7
     8.1.3  Shut-downs                                               8-7
     8.1.4  Safety                                                   8-8
8.2  MAINTENANCE OF BAGHOUSE SYSTEM                                  8-9
     8.2.1  Hoods and Collection Points                              8-10
     8.2.2  Inlet Ducting                                            8-11
     8.2.3  Blast Gate and Flow Control                              8-11
     8.2.4  Fans                                                     8-12
     8.2.5  Entrance Baffles                                         8-13
     8.2.6  Hoppers                                                  8-13
     8.2.7  Bag Retainment                                           8-14
            8.2.7.1  Thimble Sheets                                  8-14
            8.2.7.2  Fastening                                       8-14
            8.2.7.3  Tension                                         8-15
     8.2.8  Filter Elements                                          8-16
            8.2.8.1  Spare Stock                                     8-16
            8.2.8.2  Installation                                    8-16
            8.2.8.3  Inspection                                      8-17
            8.2.8.4  Salvage of Filter Elements                      8-18
     8.2.9  Collector Housing                                        8-19
     8.2.10 Specific Cleaning Mechanisms                             8-19
            8.2.10.1  Shake                                          8-20
            8.2.10.2  Reverse Flow                                   8-20
            8.2.10.3  Reverse Flow Plus Collapse                     8-21
            8.2.10.4  Pulse                                          8-21
            8.2.10.5  Reverse Jet                                    8-22

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                               CHAPTER 8

                   OPERATION AND MAINTENANCE (Continued)

     8.2.11  Rotary Valves and Conveyors                              8-22
     8.2.12  Instrumentation                                          8-23
8.3  ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION PROBLEMS          8-24
     8.3.1  Types and Frequency of Problems Reported                  8-25
     8.3.2  Specific Applications Reporting Problem Types             8-28
     8.3.3  Literature Survey of Maintenance Problems                 8-29
8.4  REFERENCES FOR CHAPTER 8                                         8-33

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                              Chapter  8
                        OPERATION AND  MAINTENANCE

     A  good  fabric  filter  system can last  15  years  and  longer,  providing
 it has  the continuing  interest  of its  personnel.  Those dust  control
 systems  located  in  the  center of plant activity, and  those  systems  col-
 lecting  a valuable  dust, are the ones  that usually  receive  the  most atten-
 tion and need  the least supervision.   On the  other  hand the filter  house on
 the roof, out  back,  in  a cold place, etc.  and the filter that seems to
 contribute nothing  but  trouble  will  not be popular  and  will require extra
 effort.
     A  few principles for  successful operation apply  to any fabric  filter
 system:
     1.  Reduce  operating  and maintenance  costs by  selecting  the most
         suitable equipment in  the first place.  Study  the  operating,
         the instrumentation, and the  maintenance manuals before pur-
         chasing.   Get  equipment of  adequate  quality.
     2.  Follow  the manuals.  Know what is in them, and why.
     3.  Know what  is entering  the filter  system.
     4.  Treat the  fabric with  care  at all times.
     5.  Keep  the flow  into the  filter as  low as possible,  limited
         only by the danger of  reaching the dewpoint.   If there is
         one single way of minimizing  operating and maintenance costs
         this  is it, according  to the  reports  of many filter  users.
Complete guidelines for the operation  and maintenance of specific filter
models are available from most  fabric  filter  manufacturers.   This chapter
discusses upkeep problems and practices common to most  equipment.   Be-
cause both operating and maintenance personnel are  involved in system up-
keep, these practices are separated in two sections, each of which may be
used as a general guide.  The last section discusses fabric filter equip-
ment problems reported  in a survey of  about 50 different fabric filter
installations.  The problems are  analyzed for  common causes.

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 8.1  01'K RAT ION  OF  BAGHOUSE SYSTEM
     Like  any new  cqviipment,  the  filter  system has  a  start-up period which
 can  last a few  minutes  or many months, depending  in part  on how well the
 system was planned.  Afterward the  system functions with  much less  atten-
 tion.  It  is important  to remember, however,  that any change  in the input
 conditions will make at least some  difference in  the  filter's operation and
 require a  period of special  attention.
     8.1.1 Start-up
            When the new equipment  is started for the first time, the  fan
 should be  checked  for correct direction  of rotation and speed.  The duct-
 ing, collector  housing,  etc.  should be checked for  leaks.   Gas  flows and
 pressures  should be checked  against the  design specifications.  Instruments
 should then be  checked  for correct  reading and calibration  adjustments made
 as necessary.   Control  mechanisms, and especially all  fail-safe  devices,
 should be  checked  for operability.
            At  the first start-up of the  system, and  also whenever  new
 bags have  been  installed by  the maintenance crew, the bags  should be checked
 after  a few hours of operation for  correct tension, leaks,  and  expected
 pressure differential.   Initial temperature changes or the  cleaning cycle
 can  pull loose  or burst  a bag.  It  is wise to  record  at least  the basic
 instrument readings during this start-up  period on  new bags,  for ready
 reference  and comparison during later start-ups.
            It  is not always  specified, but it is generally a good  idea to
 start  a new set of bags  at a  filter velocity  lower  than that  to be  normally
 used.  The reason for this is that new bags have a  low flow resistance, and
 if the fan is run at normal  speed with no  other flow  resistance in  the sys-
 tern, the filter velocity will at  first be  quite high.  As a result  the effi-
 ciency will tend to be low and dust particles  may be  driven into and plug
 the cloth.   In contrast, a low initial velocity will  allow  the  first dust
particles  to stay closer to the surface of the cloth, bridging  over the pores
 in the cloth and leaving the rest of it much more permeable.  Then  the velo-
city can be gradually increased until, after a small  percentage of  the normal
dust cake has formed,  the normal  filtering velocity can be  utilized.


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            In addition, when removal of the dust cake must be unusually
complete during every cleaning cycle, it may be worthwhile to begin fil-
tration at a lowered velocity at every cycle.  This can be done manually
by gradually adjusting a blast gate, or if the procedure must be done
frequently  or if precise control is needed, it can be accomplished via
automatic control equipment.  Some filter equipment is specifically de-
signed for this.
            During any start-up, transients in the dust generating pro-
cess and surges to the filter house are probable and ought to be anticipated.
Unexpected temperature, pressure, or moisture has often badly damaged a
new installation. In particular, running almost any indoor air or combus-
tion gases into a cold filter can cause condensation on thji_j*alls and
cloth, leading to blinding and corrosion.  Condensation'in the filter-
house may void the manufacturer's guarantee.  It can be avoided by pre-
heating the filter or the gas.
     8.1.2  Routine Operation
            Day-to-day use of a filter system requires frequent observa-
tion and occasional adjustments in order to determine and adhere to the
best overall compromise between performance, bag life, power costs, etc.
            8.1.2.1  Use of Instruments - A single monometer used across the
filter cloth can provide a wealth ot information.  It indicates the
permeability of the cloth, tells how heavy the dust deposit is before
cleaning, how complete the cleaning is, and whether the cloth is starting
to plug or blind.  It tells what surges in velocity the dust deposit is
undergoing, and whether there is any flow through the cloth during the
cleaning cycle.
            It is a good idea to post a list or a recording of the normal
differential pressures through one filtration cycle, as a means to quick
detection of later trouble.  A high differential may mean:
                     •an increase of air flow
                     •the beginning of blinding of the cloth
                     •hoppers so full as to block off the bags
                     •condensation in the cloth
                     •cleaning mechanism inoperative


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while a low differential pressure may mean:
                      •the  fan  is slowing down, perhaps due to
                           belt slippage or fan motor problems
                      'broken or undamped bags
                      •plugged  inlet ducting or valves closed
                      •leakage  between sections of the filterhouse
The reading of the manometer should be easily visible from the plant
operating floor, and one man should have responsibility for checking it.
On an expensive system an  alarm should sound if certain pressure dif-
ferential tolerances are exceeded.
            Other instrumentation can be nearly as valuable.  If it has
been installed it is probably  there for a good reason and it should be
used.  Transients frequently occur in pressure, flow rate, temperature,
and humidity.  Operating personnel should know at once when an indication
is excessive and what the  causes may be.  A trouble-shooting manual, such
as shown in Appendix 8-1,  should be kept handy to the filterhouse.
            8.1.2.2  Flow.  Variations - With multiple or variable dust pick-up
points there will be variations in flow rate and filtering velocity.  All
the branches of the inlet  ducting may be open or some may be shut off,
depending on plant activity.  Also while one collector compartment is
down for its cleaning cycle, another may be down for bag changes or still
another for inspection.  These system changes affect the flow through
the filter.  Too much flow through too few bags amounts to an overload
or too high a filter velocity, leading to inefficient filtering, plugging
of the cloth, loosening or bursting of the bags, or unsatisfactory build-
ing ventilation.  Too low  a flow is a frequent cause of condensation.  When
flow variations are anticipated, it is wise to have at least manual means
of flow control, and automatic control equipment is often justified.

            8.1.2.3 Cleaning Cycle - As the cloth ages, adjustments in the
cleaning cycle may be advisable either in the amount or the intensity of
cleaning or in the length of the cleaning cycle.   One tries to use as
little cleaning as possible so as to prolong the life of the fabric; but
one has to use enough cleaning to keep the differential pressure at an

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 economic  level.   There is  In principle  a point  of  optimization,  although
 this  may  be difficult  to locate  in practice.  Operating  at  Che  point  ot
 minimum cleaning is  indicated by & gradual  build-up  of differential  pres-
 sure,  perhaps  over  a period  as long as  a few  days.   Then before  the  pres-
 sure  gets too  high,  a  slight increase  in cleaning  action (frequency  or
 intensity or duration,  as  appropriate)  should reverse the trend  in pres-
 sure.   After a few  cycles  or a few hours the  pressure should  reach a suf-
 ficiently low  level  to reduce the  cleaning  action.   If this gradual  fluc-
 tuation in differential  pressure is not  observed,  it may mean that the
 fabric  is being  overc leaned,  because the only way  to be  sure  the lilter
 operation is near the  point  of minimum  cleaning is to continually operate
 around  it.   Changes  in- process or  fabric condition will  undoubtedly  cause
 the cleaning requirements  to shift  from week  to week, requiring  a continual
 hunting for them.  Normally  the  process  will  be sufficiently  stable  through
 automatic control so that  day to day observation and adjustment  of the
 cleaning  cycle can be  made manually by  a skilled person  in a  few moments.
 This  procedure of hunting  for the minimum cleaning requirement is normally
 worthwhile  on  all except possibly  the smallest  filter systems.
             8.1.2.4  Changes in  Operation - The fiber is  carefully designed
 to operate  with  a certain  flow rate, particle size and type,  etc., and
 any changes  in these conditions, expected to exceed ten percent or so,
 should  be analyzed for their  effect  on  the overall filter operation.  This
 should  be an economic  analysis by an engineer skilled in  fabric  filter
 systems.   If large changes are indicated or a large  installation invest-
 ment  is involved, a  short  pilot  study using,if  necessary, borrowed  pilot
 filter  equipment  may be justified.

     8.1.3   Shut-downs
             The main precaution  in shutting down the filter system is pre-
 vention of moisture  in the filterhouse.  Condensation can appear  through
 the cooling  of gases containing moisture, particularly combustion gases,
 if they are  not completely purged from the filter system and replaced
with drier air before the filter cools  down.  This can also happen with

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 air  at  ambient  moisture  levels  if  the  filter  is  in  a  colder  location.   It
 can  happen  if weather  leaks  into  the collector  particularly  during  ex-
 tended  shut-down periods.  To  prevent  condensation  some  plants  purge  their
 systems  carefully on shut-down  and then seal  the system  off  completely.
 Others  continue a flow of  warm  air through  the  filter during the  shutdown,
 which also  helps prevent condensation  when  they  start up again.   A  par-
 tially  sealed-off filter system can be lightly  pressurized with reasonably
 dry  air  to  exclude seepage of damp air.
             In  addition  to general maintenance during shutdowns,  other
 reported practices ^include cleaning corners and  crevices  of  any dust
 accumulations which  might  solidify during a prolonged shutdown; removal
 of any material which  might  catch  fire on contact with air;and removal of
 the  bags for storage under more suitable conditions.
     8.1.4  Safety
            The preceding  portions of  this chapter -mention a number of
 precautions against  system damage.  There are of course  any  number of
 possible accidents.  Baghouse structures and ducting  are usually ade-
 quate to prevent metal fatigue  or  panel collapse during normal pressure
 excursions.  It may, however, be less  expensive  to  include explosion
 panels than to  design  the  entire system to withstand  a large  surge of
 pressure.  A surprisingly  large number of dusts  are flammable and some
 are  spontaneously combustible.  (A good test is  to  ignite a  small conical
 pile of the dust and observe the rate  and amount of burning.)  The danger
 of fire in a high velocity air  system  containing cloth is apparent.  Some
 plants find that their ventilating hoods collect a surprising quantity of
 combustible material like  lunch bags and candy wrappers.   Instrumentation
 responding quickly to  sudden temperature changes, such as an automatic
 cooling system,  may  be justified in some installations.
            Operating  personnel should be accustomed  to think "Hazard."'
whenever a fabric  filter is being  used on abrasive dust or toxic fumes
 or gases.   (Many dusts have some degree of abrasiveness which can make
 trouble in other  plant equipment unless controlled.)  Safety codes in
 such cases will  not  normally permit the recirculation of  the  filtered air


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to the building'but  if the air  is recirculated, any bag failure can be
serious.   If the exhaust  is to  the outdoors then consideration has to be
given to providing make-up air  for ventilation to the building, as no
sizable exhaust rate can  be made up by leaks through doors and windows.
At times the closing of windows or doors may cut down the air flow through
the building so much that the dust pick-up hoods fail to ventilate ade-
quately.   In certain weather the makeup air inlet may entrain some of the
outdoor exhausted gases unless  precautions are taken.
            The disposal  of collected dust can be a problem.  If the dust
is poured  or stirred in the open or if there are leaks in the disposal
equipment  some of the material will re-aerosolize; it does not take much
dust escape (~0.1 percent.') to offset the remarkably high efficiency
of the fabric filter.  The procedures needed for disposal of radioactive
dust can use up the permissible exposure quotas of many men unless care-
fully planned.
8.2  MAINTENANCE OF BAGHOUSE SYSTEM
     There is a wide tendency to regard the dust collector as a piece of
trivial equipment until something goes wrong with it, whereupon it becomes
despised as a troublemaker.  Planned preventive maintenance is a better
policy, the amount depending on how much the plant has to lose in case of
a system failure.  A few companies successfully contract out the dust col-
lector maintenance, some contracting the entire job and others only the
supervisory portion, leaving the routine maintenance to in-house men.

            Timing of maintenance is important.  A supervisor or mainten-
ance forman may spend two hours every day looking over a large system,
because to let things go any longer than this is,  in his experience,  in-
viting trouble.  For example, one leaky bag can rapidly destroy the ad-
jacent bags unless detected.   Other procedures may be best done once  a
month, or every six months, depending on what they are.
            Maintenance procedures will be based on the operating and
maintenance manuals furnished with most fabric filter equipment,  and  later
they will be supplemented by experience.   Appendix 8.1 lists examples of

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maintenance procedures and troubleshooting lists extracted from Heveral such
manufacturers' manuals.  Records of bag changes as well as non-routine
maintenance may prove invaluable later in pinpointing high-cost parts of
the collector system and for projecting costs of any proposed similar
equipment.
            In the following paragraphs maintenance practices are sugges-
ted for the various parts of the system.  While these are extracted from
reports of over 100 specific installations, some comments regarding equip-
ment apply generally.  The one factor most responsible for high maintenance
costs seems to be excessive filter velocities.   Another factor is loca-
tion; centrally located equipment receives more attention than isolated,
inaccessible equipment and as a result often gives better performance.
Similarly a single large system has fewer servicing points than a number
of small dust collectors and will cbst less to maintain, other things
being equal.

            The system components should all be readily accessible.  Lad-
ders, walkways, and cranes necessary for maintenance should be provided
at the time of installation.  Lighting outside and if possible inside
the filterhouse should be installed.  If the equipment is outdoors it
should be especially well protected, and the hoppers, disposal equipment
and fans should be enclosed to make servicing easier during bad weather.
Wherever there is possibility of dust accumulation or plugging there
should be access doors or cleaning ports.  Doors, valves, bag clamps and
tension adjustments should all be serviceable by hand with a minimum of
tools, to save time.

     8.2.1  Hoods and Collection Points
            Check periodically for ill advised changes such as holes cut
in the hoods, more hoods added, ducts blocked off,  or dust intakes moved
away from the dust source.   Be sure any temperature sensor located in
the gas stream is where it will pick up a temperature truly representative
of the mixed gas.

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      8.2.2   Inlet Ducting

             This is a moderately large problem area  Cor several reasons:

              (a)  Abrasion.  Flat, heavy 'plates installed at bends may
                  increase elbow life.  Alternatively soft, rubber-like
                  material may out-last metal.

              (b)  Corrosion.  Insulate to reduce condensation.  Ducting
                  made of plywood may be compatible with mildly corrosive
                  gases.  Long ducts, such as "hairpin" cooling systems,
                  are particularly subject to corrosion unless condensa-
                  tion is avoided.

              (c)  Sticking.  Some dusts may "paint, out" on the inside of
                  the ducts.  If this is due to dampness or due to thermal
                  precipitation from hot gas to cold duct, insulation
                  should help.  An increase in pressure drop from end to
                  end is an indication of plugging.  While vanes in duct
                  turns can minimize pressure losses at the turns, they
                  may collect material.

             (d)  Settling.  Install clean-out doors or air lances in
                  long horizontal runs if there is any danger of settling.
                  Frequent plugging may require a Vee-shaped duct with a
                  conveyor in the bottom of it, or a higher air velocity
                  (i.e., smaller duct) at the expense of an increased pres-
                  sure drop.  If possible keep the. bottom of the ducting
                  straight; when tapers are necessary, put them in the
                  sides or top.

             (e)  Temperature surges. Peaks in gas temperature will not
                  be reduced or  dampened appreciably by short lengths of
                  ducting.  However, the removal of duct insulation or
                  the addition of a heat absorber like brick work or a
                  steam boiler in the duct system will help to dampen
                  temperature excursions.   Otherwise a high-temperature
                  by-pass of the filterhouse or some other variety of
                  fail-safe equipment may be needed.

      8.2.3  Blast Gate and Flow Control

             Problems with flow control equipment are frequently reported.

Any valve which may have abraded, plugged, become moved, etc. should be

checked periodically.  Especially the blast gate should have a positive lock

against vibrational moving.  It  should be accessible, not in a duct high

overhead, as it may have to be adjusted frequently.

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             Filter compartment inlet dampers are a high maintenance item
and spare parts should be stocked.  An indication of a bad damper seal is
a differential pressure across the cloth when there is supposed to be no             ~
flow through the compartment.  This is given as a common reason for short
bag life in shake-type systems.  The valves on the clean side of the bag-            —
house, if any, may also malfunction if the bags have been allowed to leak
extensively and the valves are fouled.  Either air cylinders or motor
driven actuators for dampers and valves may be used, depending partly on
the required speeds of opening and closing.  Motor driven actuators, al-
though slower, may be slightly more reliable.
             Being able to isolate a compartment from the rest of the
system without shutting the system down can be invaluable to baghousc
maintenance.  However, even the best valves may not be acceptable in all              '
cases; for example if there is danger of carbon monoxide, internal main-
tenance may be prohibited except during plant shutdowns.                               :
      8.2.4  Fans                                                                    ~"
             Fans and blowers are reported to be a large  problem area,
particularly those located on the dirty side of the baghouse where material
can accumulate on the vanes and throw off the balance.  Corrosion and
abrasion can also cause trouble.  It is wise to anticipate and prepare for            ~~
some fan maintenance in order to avoid long, expensive shutdowns.   For
example, the fan should have access doors, electric disconnects,  and                 —/
crane facilities for both ease and safety.  Fan maintenance ia roduced
by installing rugged equipment, and a large slow fan will probably need
less attention than higher speed equipment.  If one type  of fan blade
does not seem to last very well, another type may be better (see Section 5.3).
Having two or more fans on the system may enable maintenance of one of  them
without shutting down the system completely.  Furthermore, seal maintenance
problems on the filterhouse may be reduced by having fans both before and            -'
after the baghouse, enabling operating the house at nearly zero gauge pres-
sure.  Condensation and corrosion in the fan may be alleviated with duct and         _w
fan insulation; most fans come provided with drains in anticipation that
water will sooner or later get into the fan housing.

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              The  fan  should  be checked  for direction of  rotation  periodically,
 as  even  a  fan running backwards moves some air.  Air Clow and  Can speed  should
 be  measured,  not  just estimated,  checking belt wear and  adjusting the belt
 tension  as necessary.  These checks can be combined with routine  lubrication
 procedures.
              Vibration noise probably means the rotor  is out of balance,
 and/or the bearings are  going bad.  Abrasive or sticky material may easily
 change the fan blade  weight,  and  the faster the fan speed, the more serious
 this  is.   Sometimes a relatively  small  adjustment in speed can temporarily
 alleviate  a vibration until  such  time as the fan can be  shut down for repair.
 One remedy for accumulation  of material on the blades  includes sand-blasting.
 This  is  a  matter  of hours, if the rotor has to be removed, but abrasive
 blasting systems  have been installed in the fan housing  to expedite the  job.
      8.2.5   Entrance  Baffles
              Good baffling can reduce maintenance in the filterhouse by
 helping  to distribute  the gas flow and the dust load more evenly  to each
 compartment and to each bag.  A baffle can protect the. fabric nearest the
 compartment inlet from direct impact of abrasive particles, or it can pro-
 tect  the first couple  of feet of each filter element from the same thing.
 Used  skillfully,  baffles can  either direct the largest dust particles
 downward to the hopper or upward to the filter surfaces, thus in effect
 controlling the flow  resistance of the dust deposit.  For any of these
 purposes it might conceivably be advantageous to use baffles that are manu-
 ally adjustable.  On  the other hand, baffles sometimes have to be removed
when they  contribute to maintenance problems by accumulating dust or abrad-
 ing too  rapidly.
      8.2.6  Hoppers
             Bridging and backup in hoppers should be anticipated whenever
 the dust is expected to have a high angle of repose (a steep angle of  slide
when piled), a. low aggregate density,  or a high particle length/diameter
 ratio.  Steep-sided and/or wide-bottomed hoppers will presumably have  already
been installed.  Further assistance may  be obtained by antifriction coating

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the hopper surfaces, or by air-pulsed rubber liners in the hoppers.  Vibra-
tors  are often used to loosen the flow and if used, heavy low-frequency vi-
bration may be preferable.  However, some dusts tend to lock up when vibrated,
especially when the dust is already deep in the hopper.  If surges in dust
load are causing back-up, then over-running of the rotary valves will help.
Moisture introduced through a leaking rotary valve in a suction system can
cause sticking on the hopper walls.  If condensation is occurring in the
hopper, insulation should help.
             Sledge hammering is reported to be one method of freeing a
clogged hopper.  A better method is the installation of poke holes in the
side of the hopper; these can be designed in the original equipment if
the need is anticipated.  Any hopper will get better attention if it is
housed against inclement weather.
      8.2.7  Bag Retainment
             8.2.7.1  Thimble Sheets - One fairly common complaint is that the
bags are packed together so closely as to abrade against each other or
against the baghouse.  Close bags can neither be inspected adequately
nor installed nor adjusted easily.  If the bags are more than about three-
deep beside the walkway, or two-deep for large bags, the man cannot reach
from the walkway to do his job.  Hence damage often results as he misplaces
his feet.  In the event of high maintenance resulting from overcrowded bags,
some baghouse users take out part of the bags and block off the thimbles.
Unfortunately this overloads the rest of the bags.   It would be better to
take out the old thimble sheet and seal in a better designed one.
             The thimbles themselves are ideally seamless and blunt-lipped
to minimize stress points and chafe on the cloth.
             8.2.7.2 Fastening - The method of fastening and terisioning bags is
one distinction between equipment by different manufacturers,  and there are
many variations.  Generally, a minimum of tools should be needed;  clamps
should be both quick and finger-adjustable.   By the same token, both hands
should be free for the work; that is, the man should not have  to hang onto

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 a  ladder or  loan far over  as  he  works  on  the  bag.   Fastenings  should  be
 secure,  because  as  a taut  bag is collapsed  it becomes  tighter  and  the  fasten-
 ing  tends to slip.   Also large changes in baghouse  temperature can lead to
 differential thermal expansion,  bag  tension changes, and possible  slippage.
 Sometimes -a  bag  pulls completely loose from the  thimble, as  indicated  by a
 dusty  exhaust  and an unusually low manometer  pressure.
              8.2.7.3  Tens ion -  At present  there is no general rule  for how much
 tension  a bag  should have  for best overall  performance.  A very  slack  bag
 can  fold over  at the lower cuff  and  bridge  across or wear rapidly.  Too
 much tightness will  damage the cloth and  work against  the fastenings.
 Because  of the seam,  as the tube is  tightened it may arc, banana-like,
 enough to touch  the  next tube.   Whatever  tension the bag has when  it is
 installed will be increased by the weight of  the. dust  cake, especially at
 the  top  of the bag.   This can  be an  increase  of several times  the bag weight
 with some dusts.  Circumferential tension as  from inflation may  further cause
 the  tube  to  tighten  lengthwise.   The fabric properties may change as the  bag
 ages, and this may also change the tension.    For all these reasons the tension
 of the cloth needs to be checked  from time to time, especially a few hours
 after installation of the cloth.
             Correct  tension is mainly a matter of filter dimensions and
 cleaning mechanism.   Shake cleaning in particular seems to require a
 unique combination of tension, shake frequency, and bag properties for best
 results.  The manufacturer's recommendation should be followed until there
 is more pertinent experience to  go on.   One rule for obtaining adequate
 tension is the two finger method; when two extended fingers are slipped
over the uninflated,  flattened tube,  it is claimed that the wrist should
 just be able to rotate 90 degrees.  This amounts to about one-half inch of
 slack, not a   precise  amount but a very ready test.   Bags that clean by
collapsing may be under the best  tension when they take on a cloverleaf
pattern as they collapse.   Correct tension is reported  as being from 25 to
 100 pounds for glass bags of one-foot diameter, enough  to keep them suffi-
 ciently open during cleaning to let the dust fall to the hopper.

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      8.2.8  Filter Elements
                   *
             In most filter systems the biggest part of the maintenance
program is related to fabric upkeep.  The cloth has to be inspected regularly,
and it is replaced either on a preventive basis or on short notice as trouble
occurs.  Some filter users are able to salvage some of the removed filter
elements, while others find salvage does not pay.  While other parts of the
filter system may be maintained by the standard skills found in most plants,
filter elements need special handling at all times.  Glass fabric,for example,
is especially fragile and ca,n be ruined by kneeling on it or dragging a tool
across it.
             8.2.8.1  Spare Stock - At least a few replacement and.usually a
complete set of filter elements should be kept on hand, the quantity de-
pending on the expected bag life, the risk, and the delivery time.  The
margin of safety and risk of shutdown are balanced against the costs of
storage and inventory, which are annually about 10 percent of the purchase
cost of the fabric stored.  The spare filter elements should be labeled to
indicate type and quality, clearly enough to avoid any possible confusion
with other sizes, cloths, or manufacturers, and kept well separated from
used filter elements.  Elements arc stored safely against mildew, larvae,
crushing, etc. usually on a first-in, first-out inventory system.
             Filter elements can be purchased either from the equipment
manufacturer or from a firm that specializes in sewing various types of
filter elements (Section 4.6).   On the one hand, it is a good idea to avoid
splitting the responsibility for filter system performance between the equip-
ment manufacturer and a separate bag supplier.   On the other hand, most filter
users feel vulnerable if they are committed to getting all their stock from
one source.   At least one filter user solves this problem by buying cloth and
sewing his own bags.   Generally, the experienced firm supplies the most re-
liable products.  Inexpensive filter elements often turn out to require a
lot of maintenance and be more expensive in the long run.
              .8.2.8.2 Installation - In many filter houses the elements  can be
installed by one manj but in the interest of a  short down-time a crew of
two or three men usually do the job.   (Not all  types of equipment have to

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bo shut down for bag replacement; sec Section 5*2.)  Practices differ be-
tween keeping old and new elements in separate compartments and mixing them
together, which is customary when accidents happen only to a small percen-
tage or when there is no problem about down-time.  Keeping them separate
is better preventive maintenance, although it is more expensive since some
cloth will be discarded before its life is up.  Keeping them separate is
preferred when filterhouse conditions are noxious, or when entrance to the
compartment takes a lot of preparation time.  In either method records should
be kept showing date of maintenance, a description of the cloth installed,
and the location of the change by thimble number or compartment number.
             8.2.8.3 Inspection - External maintenance inspection of the filter
house and system is usually performed daily, while the filter elements
themselves are typically inspected once a week to once a month.  The appearance
of the air exhausted from the filter is not always a reliable indicator
of element condition; the inside of the filterhouse should also be seen.
             Any dust on the floor of the clean side of the filterhouse
indicates faulty operation, and the location on the floor is often a quick
indication of which element has failed.  More often, however, locating the
hole in the fabric is a difficult, time consuming job, sometimes because
the hole is hidden by other elements.  Fortunately experience often shows
that most of the holes occur in certain repetitive areas of the bags.
Holes (or more important, thin places about to become holes) may sometimes
be located by running a fluorescent light tube or flashlight through the
filter element, perhaps while it is still in the filterhouse.   Sometimes
on squeezing the bag or tube a puff of dust will show the location of
the hole.
             It is important to repair holes as soon as possible after
they develop, of course, especially when abrasive dust is being filtered,
because a hole in one element can quickly cut a hole through the adjacent
fabric.   If there isn't time to repair or change the perforated bag it
should be tied off until such time.   The same applies to a seam failure;
use whatever quick remedy will permit continuing the operation  until a
downtime can be scheduled.

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              The inspection procedure at an 80,000 CFM 500°F installation
                                                             2
 has been described in the literature and may be of interest:  .
       "At present an inspection of the bags while the baghouse
       is operating requires a supervisor, the operator,  and
       three mechanics.   The operator locks out one compartment
       at the panel board, one mechanic closes the tipping gate
       valve at the bottom of the hopper, and the other two
       mechanics open the top and bottom inspection doors.   The
       supervisor enters the bottom of the compartment, inspects
       the bags ?..;<•• general appearance, and any defective bags
       are tied off or capped at the cell plate.   The procedure
       is reversed to put the compartment back in service,  and
       the next compartment is inspected.  All 10 compartments
       may be inspected and serviced in this manner in less than
       four hours.  Inspections are made about once a month.   Bags
       are replaced during furnace rebuilds, when the baghouse is cool."

                8.2.8.4   Salvage  of  Kilter Elements  -  liags  can be removed
 for cleaning and  repair  if most  of  the  cloth  is  still  sufficiently valuable.
 Patches can be  sewn  on,  or  sometimes  applied  with quick-setting  glue
 without removing  the bag from  the baghouse.   Thermosetting and pressure
 sensitive adhesives have been  used  to  repair  glass  bags.  Of course that
 portion of the  cloth surface patched  is  lost  for filtration, and the
 surrounding cloth takes  a slightly  higher  load.  Bags with holes all in
 one end can have  a new end sewn  on.
             There are a number of ways ot cleaning a removed bag,  if this
 is judged to be worthwhile.  Dry methods include turning it inside  out and
 beating or brushing it,  tumbling it in a  drum, vacuum cleaning  it,  and using
 jets of compressed air on it.  These techniques may damage the cloth,  es-
 pecially glass cloth.  For example, cloths plugged to the point  of  rigidity
may be actually broken by bending them.  Wet cleaning methods may be more
 practicable when  a cloth has become blinded or plugged well before  its me-
 chanical life is  up.  In rare cases wet'cleaning of the bags inside  the
 filterhouse may succeed.  Felts have been dry cleaned successfully.  Un-
 specified cloths were reported to withstand 6 to 8 washings before they had
 to be thrown away.  Consulting the fabric supplier or manufacturer might
well salvage a large cloth investment that has met with a partial accident.

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             It  is n°°d practice to  install  salvaged  fabric in separate
compartments from new fabric in order to get full service from the. new
compartments.
      8.2.9  Collector Housing
             The importance of accessibility has already been discussed:
a central location, inclement weather protection, ladders, walks, and
lights are all conducive to good maintenance and will reduce maintenance
expenses.  Also the interior of the collector should be readily accessible,
if possible during operation.  High temperatures or noxious dusts may make
collector entrance hazardous or unpleasant and thus expensive.  Respirators,
coveralls, gloves, etc. if needed for entrance, should be kept close, by for
emergency repairs.
             Faculty seals are a frequent complaint in collector housings,
especially when the equipment has been assembled by an unexperienced crew.
Also, seals tend to be weakened by weather, heat and age.  Seals should be
checked every six months to a year; one technique involves placing one man
inside the compartment at night with a light and another man outside.  While
large leaks may be indicated by a manometer that does not zero when it is
supposed to, minor leaks as along a. thermocouple wire which can admit water
to begin a corrosion problem are harder to find.  If necessary, the filter-
house panels and flange connections can be dismantled, cleaned up, and
reassembled, probably using both cemented gaskets and bolts.   Seals around
the doors to the filu>rhouse must not be overlooked.
             The doors should be openable by hand without need of special
tools; they should be large enough for easy entrance, fabric maintenance,
and periodic cleaning of the dust which inevitably penetrates the cloth to
the clean side of the compartment.  Glass panels in the doors may be worth-
while for observing the filter elements in operation.
      8.2.10  Specific Cleaning Mechanisms
              Each of the standard mechanisms (see Section 3.3 and below)
requires some maintenance procedures not shared by the others.  Generally

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the more elaborate the cleaning apparatus the more maintenance ot all types
needed.  One should use a minimum of cleaning energy, if the cloth fails
to clean adequately then a small temporary increase in the cleaning process
may bring the pressure drop back into line.  The process of blinding tends
to be an accelerating one, because as the free area of the cloth decreases
the fan pulls harder in attempting to maintain the flow.  This increased
pressure drop in an already marginal situation must hasten the blinding
process.  For the maintenance man, this means being alert and ready to adjust
the cleaning procedure as necessary.  At the same time, however, all changes
in cleaning procedure should be fully cleared before they are. put into prac-
tice, because the cleaning process has far-reaching effects and a large part
of the filter system expense is related to the cleaning action.                    ~

         8.2.10.1 Shake - Any wear in the shaking machinery results in a
                                                                                   "*••
lessening of the shaking action, and unless the trouble is recognized for
what it is, the shaking intensity may be stepped up until the mechanism            __
destroys itself.  Regular lubrication and avoidance of gritty dust in the
mechanism can best be achieved by having most of the shaker mechanism out-
                                                                                   «••*•
side the filterhouse.
              If the cloth is not cleaning, check the shaker rack to be sure
                                                                                   *w
it is moving. (For doing this while the rest of the filter system is down,
a jog button located near the door of the compartment is useful.)  A small
adjustment of shake amplitude or frequency may markedly change the propa-          *"
gation of the shake wave along the cloth tube and improve the cleaning.  If         <
the cloth still won't clean sufficiently with a safe amount of shaking, it         —•
may be necessary to reduce the filtration velocity or alter the particulate
characteristics for a few hours.  The velocity should, of course be abso-          ^
lutely zero or even negative duringrthe shake part of the cleaning cycle.
                                                                                   
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Llie dust  into  the hopper.   The  rate of  flow  (or  the back pressure) and the
timing need adjustment  from time  to time to  keep the residual drag at an
economical lovel.
               8.2.10.3  Reverse  Flow Plus Collapse  - Whenever  the  cloth  is  flexed
as part of the cleaning process,  and wherever  it flexes, there, is apt to bo
a high rate of wear  indicated by  a thinning  and  eventual perforation of the
cloth.  This is especially  common near  the thimbles where the flexing is
3-dimensional.  A decrease  of air flow  or an increase in cloth tension will
help to reduce flexing.  Frequently cloth flexure  is reduced by installing
rings inside the tube,  sometimes  sewing them into  the tube.  In the cases
of reverse pulse and panel  filters, wire grids are generally used to back
up the cloth.  While these  do reduce flexure wear, they may also introduce
frictional wear between the  cloth and the ring or grid.
               Note that any type  of mechanical wear of the fabric--abrasion,
flexure,  or tensile wear--will  be amplified  by adverse environmental con-
ditions.  The  molecular structure of both synthetic and natural fibers
may be damaged by high  temperatures, moisture, and/or chemical conditions,
thereby weakening the fibers and  making them more susceptible to mechanical
wear.  Thus in a particular  case, lowering the filtration temperature may
alleviate a flexural wear problem.  The converse can also apply.   For example,
fibers are available for filtering at very high temperatures (nearly 2000 F)
but at present there are no  practicable ways of cleaning them without ex-
cessive fiber  flexure.  In  other  words, reducing the mechanical demands on
a fabric may make it more tolerant of tough  environmental conditions.  Fa-
brics with different finishes, weaves,  etc.  may also be put on trial to cir-
cumvent a particularly  troublesome mechanical or environmental problem.
               8.2.10.4  Pulse -  As there are  almost no moving parts in the pulse
cleaned apparatus, hardware maintenance is  certainly reduced compared to
other cleaning methods.   However, the excessive use of cleaning air pressure
may balloon the bags so much as to weaken them by overstretching.   If the
fabric is being damaged and cleaning cannot be diminished,  one must try
another fabric or attempt to reduce the adhesiveness of the dust  (Section 2.2).

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               8.2.10.5  Reverse  Jet  - As  there  is a  fair amount of mechanism
within the clean side of the baghouse, any penetration of the bags by dust
results in a wearing of the mechanism, gradual or rapid, depending on dust
abrasivencss.  Beyond a certain point, unless the equipment is completely
overhauled the mechanism will not only destroy itself but the bags as well.
Thus this method of cleaning while popular in some applications is at other
times associated with fairly high maintenance.
              Sometimes in reverse jet equipment a hard residue of felt
fiber and dust builds up on the cleaning rings so near the bags as to
begin to rub.  This accretion can usually be sanded off, but may be prevented
by antifriction coatings or rings made of stainless steel.   The cleaning
rings must stay smooth and level and at the prescribed distance from the
cloth.   The cloth tubes must be kept taut.   Equipment operability can
usually be prolonged by changing from continuous tn intermittent cleaning,
in which the carriage pauses for a period of time between trips.   Here
again is the principle of using a minimum of cleaning.
      8.2.11  Rotary Valves and Conveyors
              These can be high maintenance items if the dust packs to-
gether or adheres to the paddle wheel or other surfaces.  The intake of
moisture due to faulty seals is a frequent cause of sticking.  Ordinarily
a good quality valve will give long service if it gets lubrication and a
check of its seals and clearances from time to time'.  Anti-adhesion coat-
ings may be used to some advantage on the wheel blades.   A glass window
in the valve (kept clean) is a popular method of telling whether the valve
is discharging normally.  Kraus  lists a number of engineering and main-
tenance practices for rotary valves used in pneumatic conveying.
                                       *-\
              Smeary dust material in a screw conveyor can directly cause
repeated plug-ups and breakdowns.   Converting to a pneumatic conveying
system is not always an improvement, depending on the dust.   Possibly dif-
ferent conditions of dust temperature or moisture will change the properties
of the dust; otherwise it may be necessary to use separate collection bins
instead of conveying to a central point.

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              Air  leaks  in  the  rotary valve and  sometimes air  leaking along
the conveyor  from  one  compartment  to another can re-aerosolize dust  and
overload  the  filter.   Indications  of this arc non-zeroing manometers and
imusuu I .flit rr  ilr.a(.',s .
      8. 2.. I2  Instrumentation.
              The  operability of fail-safe mechanisms and automatic  control
instrumentation is very  important  to the safety  of the filter  cloth, and
it is usually up to a  maintenance,  man to be sure these are in working order.
He may advise the  installation  of  more or better instruments,  since  most
manufacturers supply a minimum  of  instrumentation with their equipment in
the interest  of economy.  Good  instrumentation often pays for  itself,
however.
              One  thing  to  check after equipment installation is the loca-
tion ot all sensing instruments because a small  difference in location can
be serious.   For example, the wrong temperature  may be measured if the gases
are not well  mixed at  the sensor location; or, a high gas velocity may give
an error  in the static pressure sensed at one side of a differential mano-
meter.  All instruments  should be  calibrated after installation, and re-
checked monthly for sensor  location, leaks (manometers)., sticking, legibility,
etc.
              A central  panel for most or all instrument readout,as opposed
to scattered  instruments,has been estimated to reduce instrument mainte-
nance costs by  as much as 50 percent, as well as making operation of the
collector system more convenient. One should record instrument readings
over  one normal  operating cycle for use in checking and troubleshooting
later in the  life of the equipment.  The record should be posted beside
each  instrument.

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8.3  ANALYSIS OF FABRIC FILTRATION SYSTEM OPERATION             ;
     During a 1969 survey of about 50 fabric filter installations in a
variety of industries the people most familiar with the day-to-day
upkeep of these filter systems were personally interviewed.  These people
wore asked to assist with about 130 questions relating to the equipment
and its performance.  The questions included:
     - What are your principal causes of fabric failure?
     - Have you tried other fabrics, and why are you using
         the present one?
     - Do you receive any complaints regarding the quality of
         your filtered effluent?
     - Do you have problems associated with fabric blinding?
     - What, if any, are the major difficulties with with your filter
         system?
     - What aspects of performance or operation could be improved,
         based on your experience?
     - What suggestions would you make for improvements in design
         or manufacture?
     - What do you see as being the principle requirements for
         research or development?
From the answers to these questions, nearly 100 different suggestions
for possible research and development investigation were obtained, as
discussed in another volume of the contract documentation.  The surveyed
installations also reported a total of 112 operating problems which are
summarized in this section.
     As considerable care was taken to distribute the survey across a
spectrum of filter applications, the operating problems reported here
may with some confidence be considered typical of most filter users.
The list does not include all possible problems because of the limited
size of the survey.  There is furthermore a wide variation in problems
encountered from application to application, and from installation to
installation.

-------
      Appendix 8-2 lists 43 installations  which  reported operational

 problems,  together with the applications   data  suspected of  being related

 to  the  problems,   These are:

      -  dust  source
      -  particle  size
      -  air/cloth ratio
      -  maximum differential pressure
      -  temperature
      -  fabric material
      -  cleaning  method
      -  particle  abrasiveness

 There are  often  other circumstances that  also contribute to  filter oper-
 ating problems.

      In addition to the 1969  survey of  fabric filter users,  the fabric
 filtration literature reports numerous  examples of operational and main-
 tenance problems.

      8.3.1  Types  and Frequency of Problems Reported

             Table  8.1 lists  23 types of operational problems encountered

 by  the  installations surveyed.  They are  grouped into 5  causality  cate-
 gories  as  follows:

      1.  Fabric-dust interactions     9 types of    60 problems reported
                                       problems

      2.  Filter  element difficulties  2    "         6      "

      3.  Filter  element-hardware      5    "        12      "
           interactions

     4.   Collector  design problems    5    "        27      "

     5.   System design problems       2    "         7	"	
                                     23 types of     112 problems reported
                                     problems
The largest category of problems reported was the first:   60 different
fabric-dust related problems were reported.  Of these the most frequent

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                              TABLE 8.1

             TYPES AND  FREQUENCY OF PROBLEMS REPORTED
 1.  Fabric  - dust  deposit  interactions             Frequency

    a.   interstitial  deposit related                   8
         abrasion,  wear

    b.   flexure wear  failure                           10

    c.   seeping                                         4

    d.   blinding                                       14

    e.   burning, heat                                  6

    f.   holes, pinholes,  shot holes                     6

    g.   hydroscopicity                                  4

    h.   condensation                                    5

    i.   deposited  dust  hardens,  cake                    3
         tears, cracks bag                             	
                                         Subtotal       60

 2.  Fabrication failures  not. particularly related to dust interaction,
    mechanical

    a.   seams, sewing                                  2

    b.   tears at top                                    4

                                         Subtotal        6

 3.  Design  or maintenance failures related  to  tensioning, supports, rings,
    collars, or cleaning  device  interactions

    a.   chafe on housing  or other bags                 3

    b.   tensioning, bags  too loose                     1

    c.   cage, wire, ring  abrasion,                     5
         wear  (also dust related),
         support mechanism interact


(continued)



-------
                        TABLE 8.1 (Continued)
    d.  cleaning carriage bag wear                     1

    e.  seals around cloth-metal collars               2

                                       Subtotal       T2

4.  Collector design problems, internal, external mechanisms, incl,
    auxiliaries, (ex. pipes, hoods)

    a.  unable to enter collector to service or        4
        maintain during operation

    b.  hole detection problems or performance         3
        effluent monitor

    c.  hopper dust sticking, holdup, screw            6
        conveyors plug

    d.  internal mechanism wear                        4

    e.  external mechanism wear, timer, shaker,       10
        fan, bearings, doors, seals, wall failure

                                       Subtotal       27

5.  Dust collecting system design problems, external to collector
    (incl. pipes, hoods)

    a.  piping, elbows, abrade                         4

    b.  hood inlet control poor                        3

                                        Subtotal       7

                                        Total:        112

-------
type was blinding:   14 problems of blinding of the fabric by the dust
were reported, as detailed  in Table 8.1.  Over 25 percent of all problems
reported were from blinding, flexure wear, and interstitial abrasion.
            From this brief analysis one is able to anticipate that most
fabric filter maintenance will center on interactions between the fabric
and the dust.  The dust may not collect in a convenient way on the fabric;
the dust may not remove easily from the fabric; the dust may damage the
fabric in some way.  Being  thus forewarned, maintenance labor may possi-
bly be minimized, for example, by running pilot tests before selecting
the fabric.
            Appendix 8.3 compiles the data in Table 8*1 and Appendix 8.2
to indicate the kinds of application in which each problem type occurs.
For example ,in Appendix 8»3 one may observe that "fabric wear apparently
related to interstitial abrasion" was reported in:

     - 6 installations with particle sizes over 10 microns, out of
         7 reporting
     - 5 installations with temperatures under 200 , out of 8 reporting
     - 2 installations with very abrasive dust, out of 6 reporting
     - 4 installations with cotton fabric, out of 8 reporting, etc.

While this breakout  does not necessarily demonstrate cause-and-effect
relationships, the table may be useful in troubleshooting operational
problems, as it enables comparison with applications reporting similar
problems.
     8.3.2  Specific Applications Reporting Problem Types.
            Appendix 8.2 lists the problems reported by each of the  various
installations surveyed.  These installations are grouped by industrial
category, enabling for example, the observation that all of the (four)
combustion applications reported only problems related to fabric-dust

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interactions.  Iron and steel applications also reported fabric-dust
problems, notably abrasion from fabric support hardware.  Again, this
analysis does not necessarily demonstrate cause and effect relationships;
instead, the analysis is presented for comparison with the reader's ope-
rational experience.
    8.3.3  Literature Survey of Maintenance Problems
           A review of about 500 documents published during the last 10
to 15 years yielded a large number of comments related to problems asso-
ciated with operating and maintaining fabric filter systems.  These in-
cluded comments based on experience with a single filter system, as well
as comments from users of many systems and from manufacturers usually with
many years of experience.   Much of this information has already been woven
into the previous sections of this Chapter and is not repeated here.  Some
of the literature, however, is so valuable in amount of detail, or in de-
scription of a specific installation, or in insight and experience, that
it is indexed here in Table 8.2.  Most of these reports are from the Iron
and Steel Industry and the Nonferrous Metals Industry.
           Three cases in point are described in Appendix 8.4.  These are
fabric filter applications of sinter plant  discharge,  oil combustion,
and copper smelting.  The original accounts make interesting reading, as
they demonstrate the range of problems associated with  the development
of new filter applications.  These accounts are typical of the better
literature describing filter system operation and maintenance.

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oo

o
                                                    TABLE  8.2

                                 MAINTENANCE  PROBLEMS AND  PRACTICES  REPORTED IN THE
                                            FABRIC  FILTRATION  LITERATURE
Industrv* Reference
1 4,5
5 6
5 2
5 7
7 8
7 9
7 10
7 11
7 12
7 13
8 14
8 15
8 3
8 16
8 17

8 18
8 19
Bust/Source
Oil combustion
Carbon Black
Soap, etc.
Cleansers
Cement Plant
Cement
Cement
Glass Furnace
Various applies.
Coal
Cupola,
Foundry
Cupola
Open hearth
General (Steel)
Electric Steel
Furnace
Sinter strand
General (Steel)
Fabric
Glass
Glass
-
Cotton
Glass
Var
-
Var
-
Var
-
-
Glass
-
-

Glass
Glass
Problem**
lb,3b,3c
G,le
Misc.
Id
3b,li
G
G
lh,li,ld
lh,lb
le,lh
G
4e
3a,3c ,4c
Misc.
le

la,3b,ld
4c , 4e , Ih
Remarks
System and procedures development
for new application
General
Pneumatic conveying equipment
Also chemical attack reported
General
General discussion of fabrics etc.
General
Several applications discussed
Panel discussion
General
General manual
General
System and procedures development
Misc. applications at Bethlehem Steel
plants ; Nui^erous problems
Labor and material cost analysis for
one year
Discussion of system upkeep
Misc. applications at Jones & Laugh 1 in
Fluoride attack.
                                                                         L_

-------
                        TABLE  8.2  (Continued)
                                                                                                 f
Industry
9

9

9

9
9

9
9

9
oo 10
t
U)
G*
G :

G

G

G
G
G
G
Reference
20

21

22,23

24
25

26
27

28
29

30
31

32

33

34
35
36
1
Dust /Source
Uranium plants

Copper Smelter

Secondary Cu
Refining
Arsenic salts
Zinc Roaster

Smelter
Cu Smelting;
ZnO
Zinc galvanize
Air filtration

General
General

General

General

General
General
General
General
Fabric
Wool

OrlonR
Glass
Glass +

-
Glass

TeritalR
OrlonR

Cotton
Various

-
-

-

-

-
Felts
-
(Cotton)
Problem
Misc.

lb,3a

Misc.

lb,3d
li.la.ld,
2a
-
1-, 3-

Id
-

Id
G

G

Misc.

Misc.
3d
G
Misc.
Remarks
Numerous problems on 18 R.jet
collectors; costs
Detailed experience with several
installations
General discussion of system
experience
Russian plant experience
System and procedures development

Designed for low maintenance
Routine inspection described

Occasional manual cleaning required
Maintenance economics of low efficiency
filter equipment
Residual cake profiles
General discussion by system manu-
facturer
Inspection schedule, disposal
methods discussed
Abrasion, adhesion, bridging etc.
discussed
Filter sleeve maintenance and storage
Reverse jet equipment
Unit sized filters
General discussion by system manu-
37
General
facturer

Troubleshooting checklist, general

-------
                                            TABLE 8.2  (Continued)
Industry
G
G
Reference
38
39
Dust/Source Fabric
General
General
Problem
G
Remarks
General upkeep of all types of
collectors
World-wide survey of maintenance
                                                                      costs reported
 * Industry Key:

      1 - Combustion Processes
      2 - Food and Feed
      3 - Pulp and Paper
      4 - Inorganic Chemicals
      5 - Organic Chemicals
      6 - Petroleum Refining
      7 - Nonmetallic Minerals
      8 - Iron and Steel, Foundry
      9 - Non-ferrous Metals
     10 - Miscellaneous
      G - General discussion

** Problem lb, 3c, etc: For explanation, see Table 8.1.

G = General discussion of system upkeep experience.


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8.4  REFERENCES FOR CHAPTER 8

     1.  R. L. Clement, Selection, Application, and Maintenance of Cloth
         Dust Filters, Plant Engineering, pg. 24, circa 1965.

     2.  R. A. Herrick, J.W.  Olsen, and F.A. Ray, Oxygen-Lanced Open
         Hearth Furnace Fume Cleaning with a Glass Fabric Baghouse,
         JAPCA 16:1, 7 Jan. 1966.

     3.  M. N. Kraus, Pneumatic Conveying, I + II, Chem. Engg., 167
         (Apr.. 12, 1965) and 149 (May 10, 1965).

     4.  D.N. Felgar and W.E.  Ballard, First Years Experience with Full-
         Scale Filterhouse at  Alamitos Generating Station, Electric World,
         Circa 1966.

     5.  F.A. Bagwell, L.F. Cox and E.A.  Pirsh, Design and Operating
         Experience with a Filterhouse Installed on an Oil Fired Boiler,
         JAPCA 19:3, 149 (March 1969).

     6.  I. Drogin, Carbon Black, JAPCA 18:4, 216 (April 1968).

     7.  Dust Collection at Oakite, Anonymous,  Soap and Chemical Specialties,
         157 (June 1965).

     8.  R.E. Doherty, Current Status and Future Prospects-Cement Mill Air
         Pollution Control, Third Nat'l.  Conf.  on Air Pollution,Proc., PHS,
         USDHEW (December 1966).

     9.  R.F. O'Mara,  and C.R.  Flodin, Filters  and Filter Media for the
         Cement Industry,  JAPCA 9:2. 96 (August 1959).

    10.  G. Funke, Operation and Maintenance of Filter  Collectors, Staub
         (English Translation)  £6:4, 25 (April  1966).

    11.  A.B. Netzley and J.L.  McGinnity,  Glass Manufacture,  Air Pollution
         Engineering Manual, USPHS 999-AP-40, Cincinnati (1967).

    1-2.  Anonymous, Panel Probes  Dust Collection Problems,  Rock  Products,
         76 (January 1965).

    13.  H.E. Soderberg,  Considerations in the  Selection of Dust Collectors
         for Coal, Mining Congress  J.,  62  (October 1964).

    14.  American Foundrymen's  Society, Foundry Air Pollution Control
         Manual,  Des Plaines,  111.,  2nd Ed.  (1967).

    15.  R.W. Mcllvaine, Air Pollution Equipment  for  Foundry  Cupolas,
         JAPCA 17:8.  540  (August  1967).

-------
 16.   H. M. Chapman,  Experience with  Selected Air  Pollution  Control  In-
      stallation at Bethlehem Steel,  JAPCA  13:12,  604  (December  1963).

 17.   T. Killman, Dust and Fume Control  for Electric Furnaces. Texas
      Steel Co., Fort Worth" Texas  (May 1969)""     	"

 18.   T. A. Young, Gary Steel Works Experience with Dust Control  at No.  3
      Sinter Plant, Blast Furnace  and Steel plant, 1057  (December 1968).

 19.   S. Vadja, Blue  Ribbon Steel with Blue Skies, I&S Engr., 71  (August 1968).   ~

 20.   W. B. Harris and M. G. Mason, Operating Economics of Air Cleaning             :
      Equipment Utilizing the Reverse Jet Principle, Ind. &  Eng.  Chem.            —''
      42:12, 2423 (December 1955).

 21.   D. J. Robertson, Filtration of Copper Smelter Gases at Hudson Bay           _
      Mining and Smelting Co. Ltd., Can.  Min. and Met. Bull.. 326 (May 1960).

 22.   R, H. Graves, Discussion of Baghouse Design and Operation,  Nat'i.             i
      Assoc. of Secondary Materials Industries, Inc., Pittsburgh, Pa.             "^
      (June 1967).

 23.   Anonymous, Bag Life Extended at Copper Refinery, Air Enge (18 Jan. 1969).   J

 24.   V.G.  Matsak,   Smoke Purification Equipment, in USSR Literature, on
      Air Pollution Related Disease. USPHS TT60-21475 141 (1960).                   i
      	'	' —- 	-H J-L-LJi-- -   --!'_- - /  _—1- -LT.J—                                               ^J

 25.   L.P.  Landucci, and R.E.  Eyre,  Pilot Plant Filtration of Zinc
      Suspension Roaster Gases, Can. Min. and Met.  Bull.. 703 (October 1962).       i
                                                                                  —j
26.   Anonymous, Bag Filter for Smelting Plant is Fully Automatic.Filtration
      and Separation.  326 (July 1966).
                                                                                  —i
27.   H.E.  Schwartz, L.E.  Kalian,   and A. Stein,   Controlling Atmospheric
      Contamination in Smelting and Refining of Copper-base Alloys,
      Air Repair £: , 5 (May 1955) .                                                  •

28.   G. Thomas, Zinc-Galvanizing Equipment, Air Pollution Engineering
      Manual.  USPHS No.  999-AP-40, Cincinnati (1967).                               j
                                                                                  —/
29.   D.G.  Hill,  An Evaluation of the Costs of Air Filters,  Filtration
      and Separation,  297 (July 1967).

30.   D.G.  Stephan  and G.W.Walsh, Residual Dust Profiles in Air Filtration,       *""'
      Ind.  and Eng.  Chem. 52:  12,  999 (December 1960).
                          •"•                                                         I

31.   R.L. Adams,  How to Maintain Cloth-Type Collectors, Air Engineering,         —'
      20 (May 1963).

-------
32.   W.O. Vcdcler,  Filters Aid At-Source  Control  of Dusts,  Chem.  Eng..
      140 (May  1951).

33.   J.M. Kane,  Guideposts Tell How  to Select Dust Collecting Equipment,
      Plant Eng,, , (November 1954).

34.   R. Ashman,  Filtration Fabric Selection, Power and Works  Eng..
      53 (February 1959).

35.   K.J. Caplan,  Current Applications of the Reverse Jet  Filter Principle,
      A.M.A. Archives. Ind. Hyg. Assoc.. 21; 200 (March 1960).

36.   F.R. Chase,  Application of_S_elf-Contained Dust Collectors,  The
      Torit Corp., St. Paul, Minn." (1963)7	" """	""

37.   S. Levine, What You Should Know About Dust Collectors, Rock  Products,
      53 (April 1965).

38.   P. Staubung,  Operation Maintenance of Dust Removal Equipment,
      Wasser Luft Betr. (German) 12, 215 ( April 1968).

39.    R.L.  Chamberlin and P,B. Crommelin,    Economic Aspects of Air
      Pollution Control for ^he.World's Heavy Industries^ presented at
      the First World Congress on Air Pollution, Buenos Aires, Argentina,
      (November 14-21, 1965).

-------