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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
iMrtUl CollMtor
IMt«i •IfUlMCT. eye I
Low rMlataaca oallular 0y«loMa
llffc «fflcU»cT
I»yl«§j»iat acrubbar (Doyla tjraa)
ialf-latuead
Voft4
Flutdlixl b«4 «crol*.r
Irrlgiod c«r|«t >cnibb«r
•l«atro«e«ct« ptmatfitmtor
Irrlfct«4 •l«ccro«tati«
icrubb«r lov
i«rubb«r
V«Dturl-»crubb«r •«
(UctroitMtc rr«clplotor
Vcrubb>r -
9hak«r-typ« fibrlc fllt«r (vm« fabric)
K>viro«-J
-------
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.
-------
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
-------
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)
-------
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)
-------
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.)
-------
., 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.)
-------
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.
-------
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.
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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,
-------
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
-------
o
-o
o
1.6e -Cloth Screen Dust Collector. (Pangborn
Corporation). (From Ref. 20.)
1.6f -Bag Filter (Dracco Corporation).
(From Ref. 20.)
-------
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
-------
§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.
-------
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.
-------
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.
-------
' • ^ ' 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
-------
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
-------
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
-------
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)
-------
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
-------
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.
-------
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.
-------
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
-------
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
-------
c
Figure 1.13 -Typical Fabric Filter Arrangement
(Courtesy of Wheelabrator Corp.)
O
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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,
-------
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.
-------
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
-------
•
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)
-------
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
-------
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
-------
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
-------
~
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
-------
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)
-------
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
-------
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
-
-------
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-
-------
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-
-------
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
-------
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-
-------
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.
-------
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
-------
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
-------
/ J < » 7 K)
*•
4
z
o
6 "•
d
8 oj
01
01
0
~T"f ••-r r t i i | - t -i- i t i ['•••! i i '_,j:;^ r » r i i -f -^
' /""""" •'"" " 1
pon-niftt _ , '.' -•-'
'LOU -» / ^\
/ .>s.v»fous ^
/ ••' "°"
/ , -1°"
IMA2Ma — /! ' ORA1INO
TKAJtclOm # /;i' TRAJICTOU -1
/' /
/ /
/• /
/•' /
/ /
/ /
/ /
/ /
1 I 1
I •' i
1 •'
" ///
-// /{/
Y ' / •/
• A / / 'j
'!•• •
- <7 / /
.J'S /
..--;.x/' . /
• ' - iy t ....J L_l_* — 1 — L/ 1 ' ' ' ' • • ' 1 * I - .11
or
at
OS
o«
O3
01
Ol
J 4 > 7 .6
u
^0.5
.4
0
•rl
^
O
^0.2
9
O
O.I
8
Intercepts
— Ribbon
Spntrt
or cylindtr: )
•^ /
i / .
i
^
,«<•
/
i
*y /
^
^
/
f
4
/
fv
K
^
/
f
/
p
>
/
/
?
^
/
>«
y
•(
?
&
^
4F/.
fr
^
x
y\
>«
^>
^
^
^
^f
X
— -
^
*
— •
?5
^
=5B
•
•
a
••
•i
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-
-------
IO1
l\J
4J
0)
C3
3
° !/"»£
n 10
u)
a.
UJ
(if
00
- -1
13^
-(
3
— <
/j
— | ^
IU
H
^
-«
-1
r/Jpf
*/
^
/
/
LJ D
/
/
*
/
s
/ c
x
/
o\o^X
/
0 //
vl . X
1*1 o\ox^
/X c^XJ «f
T ^^^^^ "T*
>X^ '
^ ~
•'t/
^
/
/ %
/
/
s
/
/
S n
X
X
r
s
/
•/'
/
/'
s
s
/' °
/ O
X
X
00 xx
O X
/•
%^/x
^
*^*x
D n*?X
^x
s
/
s
'•) /
&'
,'
.
f
/
'
/'
'
/
°*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.
-------
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)
-------
,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,
-------
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)
-------
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-
-------
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
-------
(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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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
for Coagulation, by Brownian Motion, Ch 2, Aerosol Sci. CND ed.
17. C. S. Wang, A Mathematical Study of the Particle Size Distribution
of Coagulating Disperse Systems, Ph.D. Thesis, Calif. Inst. Tech. (1966)
18. W. E. Clark and K. J. Whitby, Concentration and Size Distribution
Measurements of Atmospheric Aerosols and a Test of the Theory of
Self-Preserving Size Distributions, J. Atmosph. Sic., 24, 677 (1967).
19. J. M. Beekmans, The Denisty of Aggregated Solid Aerosol Particles,
Annals Occ. Hyg.. ]_, 299 (1964)
20. H. R. Johnstone and G. A. Sehmel, The Agglomeration of Solid Aerosol
Particles, Engineering Experimental Station, University of Illinois,
U. S. Atomic Energy Commission Report No. COO-1018 (March, 1959)
21. W. B. Kunkel, Magnitude and Character of Errors Produced by Shape
Factors in Stokes' Law Estimates of Particle Radius, J. Appl. Physics.
12, 1056, (1948).
22. W. J. Megaw and R. D. Wiffen, The Diffusion of Chain Like Aggregates,
Jour, de Recherches Atmosph.. 2_, 69 (1966).
23. F. Stein, N. Esmen, and M. Corn, The Density of Uranine Aerosol Particles,
Am. Ind. Hyg. Assn. Jour. 27, 428 (1966).
24. G. A. Sehmel, The Density of Uranine Particles Produced by a Spinning
Disc Aerosol Generator, Am. Ind. Hyg. Assn. Jour., 28, 491 (1967).
25. M. E. McKnight and M. I. Tillery, On the Density of Uranine, Am. Ind.
Hyg. Assn. Jour.. 28, 498 (1967)
26. P. A. Benson, M. W. Nathans, A. Amos, and L. Leventhal, The Density
of Fallout Particles from an Air Burse, Health Physics. 13, 1331 (1967).
27. A. S. Foust, L. A. Wengel, C. W. Clump, L. Maus and L. B. Andersen,
Principles of Unit Operations, John Wiley and Sonsz Inc. New York, 1960.
29. 11. G. Dorsett, Jr., M. Jacobson, J. Nagy, and R. P. Williams, Laboratory
Equipment and Test Procedures for Evaluating Explosibility of Dusts^
U. S. Department of the Interior, Bureau of Mines Report of Investigaion
5624, (1960).
30. M. Jacobson, J. Nagy, A. R. Cooper, and F. J. Ball, Explosibility of
Agricultural Dusts, U. S. Department of the Interior, Bureau of Mines
Report of Investigation 5753, (1961).
-------
31. M. Jacobson, J. Nagy, and A. R. Cooper, Explosibility of Dusts Used
in the Plastics Industry, U. S. Department of the Interior, Bureau of
Mines Report of Investigation 5971 (1964).
32. M. Jacobson, A. R. Cooper and J. Nagy, Explosibility of Metal Powders.
U. S. Department of the Interior, Bureau of Mines Report of Investigaion
6597, (1965).
33. H. G. Dorsett, Jr., and J. Nagy, Dust Explosibility of Chemicals.
Drugs, Dyes, and Pesticides, U. S. Department of the Interior, Bureau
or Mines keport or investigation 7208 (1968).
34. H. G. Dorsett, Jr. and J. Nagy, Dust Explosibility of Chemical, Drugs,
Dyes, and Pesticides, U. S. Department of the Interior, Bureau
of Mines Report of Investigation 7132 (1968).
35. J. Nagy, A. R. Cooper, and H. G. Dorsett, Jr., Explosibility of
Miscellaneous Dusts. U. S. Department of the Interior, Bureau
of Mines Report of Investigation 7208 (1966).
36. I. Hartmann, Dust Explosions,in Marks' Mechanical Engineers Handbook,
T. Baumeister, Ed., McGraw-Hill Book Co., Inc. New York, p 7-43 (196J
37. R. D. Cadle, Particle Size Determination. Interscience, New York, 1953.
38. R. R. Irani and C. F. Callis, Particle Size Measurement, Interpretation.
and Application. J. Wiley & Sons, New York, 1963.
39. L. B. Loeb, The Basic Mechanisms of Static Electrification. Science
102:573 (1945); also see L. B. Loeb, Static Electrification, in
Progress in Dielectrics, Vol. 4, J. B. Birks, Ed., Heywood & Co.,
Ltd. (1962), Vol. 5, 1963.
40. H. J..White, Industrial Electrostatic Precipitation. Addison-Wesley
Pub. Co., Inc., Reading, Massachusetts (1963).
41. Y. Boisdron and J. R. Brock, On the Stochastic Nature of the
Acquistion of Electrostatic Charge and Radioactivity by Aerosol
Particles, Atmos. Envir., 4:35 (1970),
42. W. B. Kunkel, The Static Electrification of Dust Particles on Dispersion
into a Cloud. J. Appl. Phys. 21:820 (1950).
43. J. M. Dallavalle, C. Orr, Jr., and B. L. Hinkle, The Aggregation of
Aerosols. 'Brit J. Appl. Supp. No. 3 S198 (1954).
44. J. H. Daniel and F. S. Brackett, Investigations of Small Airborne
Charged Particles by an Electrical Method. AMA Arch. Ind. Hyg. and
Occ. Med. 3:505 (1951).
45. W. J. Megaw and A. C. Wells, Production of Monodisperse Submicron
Aerosols of which Each Particle Carries a Specified Number of Electronic
Charges. Nature 224:689 (Nov. 15, 1969).
-------
46 K T. Whitby and B. Y. H. Liu, The Electrical Behavior of Aerosols,
in Aerosol Science. C. N. Davies, Ed., Ch III, Academic Press, 1966.
47. H. A. W. Schroter, The Applicability of Aerosols in Xerography, Staub
26, (8), 15 (1966).
48. T. T. Mercer, Aerosol Production and Characterization, Health Physics,
10, 873 (1964).
49. D. A. Lundgren and K. T. Whitby, Effect of Particle Electrostatic
Charge on Filtration by Fibrous Filters, Ind. Eng. Chem., Proc
Des. Dev.. 4, 345 (1965).
50. H. J. Lowe and D. H. Lucas, The Physics of Electrostatic Precipitation,
Suppl. No. 2.-Brit: J. Appl. Phvs* (London)(1952).
51. G. Linger and J. L. Radnick, Development and Preliminary Testing
of a Device for Electrostatic Classification of Submicron Airborne
Particles, J. Appl. Phvs. 32_:955 (1961).
52. J. I. Masters, An Aerosol Analyzer, Rev. Sci Instruments, 24, 5bb
(1953).
53. D. M. Anderson and L. Silverman, Mechanisms in Electrostatic Filtration
of Aerosols with Fixed and Fluidized Granules. Harvard Air Cleaning
Laboratory, U. S. Atomic Energy Commission Report No. NYO-4615,
(August, 1958).
54. C.E. Lapplc, Electrostatic Phenomena with Particulates, in Advances in
Chemical Engineering, Vol. 8, Edited byT.B. Drew, G.R. Cokolet, J.W. Hooper,
Jr., and T. Vermeulen, Academic Press, New York (1970).
55. B. M. Smith, J. Wagman and B. R. Fish, Interaction of Airborne Particles
with Gases, Env. Sci, Tech. 3:558 (1969)
56. S.'. J. Gregg and K. S. W. Sing, Adsorption^ Surface Area, and Porosity.
Academic Press, New York, 1967.
57. Aminco Adsorption, Aminco Sor-BET, American Instrument Co., Inc.,
8030 Georgia Ave., Silver Spring, Md.
58. A. W. Adamson, Physical Chemistry of Surfaces 1st Ed., Interscience,
(1960). /
59. A. W. Adamson, Physical Chemistry of Surfaces 2nd Ed.. Interscience
(1967).
60. E. A. Flood, Ed., The Solid-Gas Interface Vols. 1 and 2. Marcel
Dekker, New York (1967).
61. Significance of the Surface for Dust Technology, Staub, 28_ 1968
pp 1 - 87 (English Translation) CFSTI, DOC. Arlington, Va. 22151
-------
62. W. R. Smith, Carbon Black, in Petroleum Products Handbooks. B.
Gouthrie, ed, Sec 15, McCraw-Hill, CI, (1960).
63. M. Corn, T. L. Montgomery, and R. J. Reitz, Atmospheric Participates:
Specific Surface Areas and Densities, Science, 159 . 1350 (1968)
64. H. Krupp, Particle Adhesion Theory and Experiment In Advances
in Colloid and Interface Science - eds. I, pp 111-239, John
Wiley & Sons, Inc. New York 1967.
65. M. Corn, Adhesion of Particles, in Aerosol Science. C. N. Davies
Ed., Ch. XI, Academic Press Inc. (London), 1966.
66. A. D. Zimon, Adhesion of Dust and Powder, Plenum Press, New York,
1969.
67. C. Boehme, et al, Adhesion Measurements Involving Small Particles
Trans. Instr. Chem Engrs. (London) 40. 252 (1962).
68. R. I. Larsen, The Adhesion and Removal of Particles Attached to
Air Filter Surfaces. Am. Tndust. Hyg. Assoc. Jour., 19, 265 (1958).
69. V. D. Beischer, Festigkeitsuntersuchungen an Aerosolsedimenten,
Roll. Zeit.. 89, 214 (1939).
70. F. Loffler, Investigating Adhesive Forces Between Solid Particles and
Fiber Surfaces, Staub, (English Translation) 26, 10 (June 1966).
71. F. L'dffler, The Adhesion of Dust Particles to Fibrous and Particulate
Surface. Staub, (English Translation) 28_, 32 (November 1968).
72. M. Corn, The Adhesion of Solid Particles to Solid Surfaces. Ph.D.
Thesis, Harvard University, 1961.
73. M. Corn, The Adhesion of Solid Particles to Solid Surfaces, 1. A Review,
Jour. Air Poll. Control Assoc.. 11 523 (1961)
-~ " ~ "" - -" *.i^M^H««_B - — -"I —• ^
74. M. Corn, The Adhesion of Solid Particles to Solid Surfaces, 11,
Jour. Air Poll. Control Assoc.. 11, 566 (1961).
75. F. P. Bowden and D. Tabor, The Friction and Lubication of Solids.
Clamendon Press (Oxford), 1950.
76. Ref. 59, p 364
77. J. T« Davies and E. K. Rideal^,Interfacial Phenomena, Academic Press,
New York, p 438, 1963
78. E. Anderson, Separation of Dust and Mist, in Chemical Engineers
Handbook. J. H. Perry, ed., McGraw-Hill Book Co., Inc. New York, 1934.
-------
79. C. E. Billings, L. Silverman, R. Dennis and L. Levenbaum, Shock Wave
Cleaning of Air Filters, Jour. Air Poll. Control Assoc.. J.O, 318, 1960.
80. M. Corn and L. Siverman, Removal of Solid Particles from a Solid Surface
by a Turbulent Air Stream, Am. Indust. Hyg. Assoc. Jour., 22, 337 (1961).
81. S. Taub, Carnegie -Mellon Univ., Personal Communication in connection
with aggregates resuspended from granular aerosol filters (1970).
82. P. W. Spaite and G. Walsh, Effect of Fabric Structure on Filter
Performance, Am. Indust. Hyg. Assoc. Jour. 241 357 (1963).
83. C. J. Stairmand, Design and Performance of Modern Gas Cleaning Equip-
ment, J. Inst. Fuels (London) 29, 58 (1956).
84. C. E. Billings, Effects of Particle Accumulation in Aerosol Filtration
W. M. Keck Lab. Env. Hlth Eng. Report, Calif. Inst. Techn. (1966).
85. R. G Dorman, Filter Materials in High Efficiency Air Filtration,
P. A. F. White and S. E. Smith, Eds7 Butterworths, London Ch 4. (1964).
86. C. L. Cheever, Recent Air Cleaning Developments at Argonne National
Laboratory, in Seventh AEC Air Cleaning Conf., USAEC Report No.
TID-7627 Book 1, March 1962.
87. T. E. Wright, R. J. Stasny, and C. E. Lapple, High Velocity Air Filters.
WADC Technical Report 55-457, Donaldson Co., Inc., October 1957.
88. P. Drinker and T. Hatch, Industrial Dust, 1st ed., McGraw-Hill Inc.
p 234, 1936.
89. J. H. L. Watson, Filmless Sample Mounting for the Electron Microscope,
Jour. Appl. Physics. 17. 121 (1946).
90. R. Leers, Die Abscheidung von Schwebstoffen in Faserfiltern, Staub,
50, 402 (1957).
91. L. V. Radushkevich and V. A. Kolganov, Study of Collection of Highly
Dispersed Aerosols in Aerosols: Physical Chemistry and Applications,
K. Spuruy, Ed., Publ. by Czechoslovak Acad. Sci., p 247, 1965.
92. M. Tomaides, University of Minnesota, Personal Communication, 1967.
93. N. A. Fuchs, The Mechanics of Aerosols, The Macmillian Co., New York
P. 231, 1964.T"~
94. N. A. Fuchs and I. B. Stechkina, Resistence of a Gaseous Medium to
the Motion of a Spherical[Particle of a Size Comparable to the Mean
Free Parts of the Gas Molecules, Trans. Faraday Soc., 58, 1949 (1962).
95. R. A. Gussman, On the Aerosol Particle Slip Correction Factor,
Jour. Appl. Meteorol.. 8, 999 (1969).
-------
96. H. Herne, The Classical Computation of the Aerodynamic Capture of
Particles by Spheres, In Aerodynamic Capture of Particles. E. G.
Richardson, (ED), Pergamon, New York (1960).
97. R. H. Perry, C. H. Chilton, and S. D. Kirkpartrick, Chemical Engineers
Handbook. McGraw Hill Book Co., Inc. New York (1963).
98. c. I).Sargent-, Dust Collection Equipment, Chemical Engineering, p 141,
(Jan. 27, 1969).
99. II. !•'. Kraemer and H. F. Johnstone, Collection of Aerosol Particles
in the Presence of Electrostatic Fields, Ind. Eng. Chem. 47, 2426
(1955).
100. T, Gillespie, The Role' of Electric Forees in the Filtration of Aerosols
by Fiber Filters, Jour. Colloid Sci.. W 299 (1955).
101. C. E. Billings, R. Dennis and L. Silverman, Performance of the Model
K Electro-Polar Filter. Harvard Air Cleaning Laboratory, U. S. Atomic
Eneigy Commission Report No. NYO-1592, (July 1954).
102. A. T. Rossano, Jr. and L. Silverman, Electrostatic Mechanisms in Fiber
Filtration of Aerosols, Harvard Air Cleaning Laboratory, U. S.
Atomic Energy Commission Report No. NYO-1594, (May 1955).
103. L. Silverman, E. W. Connors, Jr., and D. M. Anderson, Electrostatic
Mechanisms in Aerosol Filtration by Mechanically Charged Fabric
Media and Related Studies, Harvard Air Cleaning Laboratory,
U.S. Atomic Energy Commission Report No. NYO-4610, (Sept. 1956).
104. R. Dennis, E. Kristal, and L. Silverman, Evaluation of the Electro-
PL and Electro-Klean Dust Collectors, Harvard Air Cleaning Laboratory
U. S. AEG Report No. NYO-4614 (July 1958).
105. J. W. Thomas and E. J. Woodfin, Electrifield Fibrous Air Filters,
Applic.and Industry (AIEE) (Nov. 1959).
106. D. Sweitzer, Electrets, Literature Search No: 308, Jet Propulsion
Laboratory, California Institute Techn., National Aeronautics and
Space Administration Contract No. NASw6 (Feb. 1961).
107. R. D. Rivers, Operating Principles of Non-Ionizing Electrostatic
Air Filters, ASMRE J., 4 37 (Feb, 1962),
108. E. R. Frederick, How Dust Filter Selection Depends on Electrostatics,
Chemical Engineering, p. 107 (June 26, 1961).
109. W. Walkenborst and G. Zebel, tfber ein nenes Schwebstoffilter hoher
Abscheideleistung und geringen StrSmungswiderstandes, Staub. 24,
444 (1964).
110. G. Zebel, Deposition of Aerosol Flowing Past a Cylindrical Fiber
in a Uniform Electrical Field, Jour. Colloid Sci.. 20, 522 (1965)
-------
111. G. Zebel, Improving the Separation Efficiency of Fiber Filters by
Electrical Fields, Staub, (English Translation) 2<>, 18 (1966).
112. W. Strauss, Industrial Gas Cleaning, Pergamon Press, New York (1966).
113. JL W. Thomas and R. E. Yoder, Aerosol Size for Maximum Penetration
thru Fiberglass and Sand Filters, A._ M. A. Archives of Indust. Hlth,
13. 545 (1956).
114. J0 W. Thomas and R. E. Yoder, Aerosol Penetration Through a Lead
Shot Column, Am. Archives Indust. Hlth. JJ, 550 (1956).
115. R. E. Yoder and F. M. Empson, The Effectiveness of Sand as a Filter
Medium, Am. Indust. Hyg. Assoc. Jour. 19, 107 (1958).
116. W. Strauss and M. W. Thring, Studies in High-Temperature Gas Cleaning,
Jour. Iron Steel Inst. 196, 62 (1960).
117. R. Dennis, G. A. Johnson, M. W. First, and L. Silverman, How Dust
Collectors Perform, Chemical Engineering, 59, 196 (1952).
118. K. T. Whitby and D. A. Lundgren, Fractional Efficiency Characteristics
of a Torit Unit-Type Cloth Collector. Torit Manufacturing Co. Tech.
Report (Aug. 1961).
119. C. J. Stairmand, Removal of Dust from Gases, In Gas Purification
Processes, G. Nonhebel (ed), Ch 12, p 494, G. Newnes, Ltd., London,
1964.
120. J. K. Skrebowski and B. W. Sutton, Development of a Radioactive
Aerosol for Testing Filter Fabrics, Brit, Chem. Eng, 6_, 12
(1961), reported in Ref. 119 above.
121. A. E. Scheidegger, The Physics of Flow Through Porous Media
The Macmillian Co., New York, 1960.
122. C. E. Lapple, Fluid and Particle Mechanics. University of Delaware,
Newark, Del. (1956).
123. P. C. Carman, The Determination of the Specific Surface of Powders,
I. Jour. Soc. Chem. Industry, 57., 225 (1938).
124. P. C. Carman, The Determination of the Specific Surface of Powders,
II, Jour. Soc. Chem. Industry, 58, 225 (1938).
•'i
125. J. Happel and H. Brenner, Low Reynolds Number Hydrodynamics, Prentice
Hall, Inc. Edgewood Clifts, New Jersey,,1965.
126. J. P. Stevens and Co., Inc. Selecting Fabrics for Filtration and
Pus t Co1lee t ion, plus Supplement N., J. A. Stevens and Co., Inc.
New York (1969).
-------
127. R. H. Borgwardt, R. E. Harrington, And P. W. Spaite, Filtration
Characteristics of Fly Ash, Jour. Air Poll. Control Assoc. 1.8 ^
387 (1968).
128. D. G. Stephan, G. W. Walsh, and R. A. Herrick, Concepts in Fabric
Air Filtration, Am. Indust. Hygiene Assoc. Jour. 21., 1 (1960).
129. J. M. Chalmers, L. R. Elledge and H. F. Porter, Filters,
Chemical Engineering, p. 191 (June 1955).
130. II. P. Grace, Resistance and Compressibility of Filter Cakes,
Chem. Eng. Progr. 49, 303 (1953). ,
131. A. F. Robertson, Air Porosity of Open-Weave Fabrics, Part I, Metallic "^
Meshes, Textile Res. Jour.. 2£, 838, (1950).
132 A. F. Robertson, Air Porosity of Open-Weave Fabrics, Part II, Metallic —-
' Meshes, Textile Res. Jour., 20, 844, (1950).
133. R. L. Stephenson and H. E. Nixon, Centrifugal Compressor Engineering.
Hoffman Industries Division, Clarkson Industries, Inc., 103 Fourth _j
Ave., New York, 1967 /
i
134. R. L. Carr, Jr., Properties of Solids, Chemical Engineering, p 8, i
(Oct. 13, 1969). -"
135. J. M. DallaValle, Micromerttics. 2nd Ed. Pitman Publ. Corp., j
New York, 1948. _J
136. I. Shapiro and I. M. Kolthoff, Jour. Physical and Colloid Chem. ;
52, 1020 (1948). _j
137. H. Wakeham and N. Spicer, Pore-Size Distribution in Textiles, A Study
of Windproof and Water-Resistant Cotton Fabrics, Textile Research
Jour. 12 703 (1949). -*
138. A. B. Gumming and F. B. Hutto, Jr., Filtration, in Technique of Organic f
Chemistry. Vol. Ill, Part I. Separation and Purification, A Weiss- _,
berger, ed., p 682, Interscience Publ. Inc., New York, 1956.
139. I.M. Kolthoff, et al., manuscript on the compressibility of powders, I
submitted for publication as an ASD Technical Documentary report —'
(December, 1962).
I
140. H. H. Klaus-Becken,VonSchmeling and N. W. Tschaegl, Osculatory
Packing of Finite Areas with Circles, Nature, (London)225, 1119 (1970).
141. M. Leva, M. Weintraub, M. Grummer, M. Pollchick, and H. H. Storch, f
Fluid Flow Through Packed and Fluidized Systems, Bureau of Mines —J
Bulletin 504. U. S. Gov't Pringtlng fltTice, Uash. 1951.
142. G. I. Taylor and R. M. Davies, The Aerodynamics of Porous Sheets, __,
Reports and Memoranda of the Aeronautical Research Council,
No. 2237, 1944, from Collected Works of G.I. Taylor, Vol. III.
-------
143. G. M. Fair and L. P. Hatch, The Streamline Flow of Water Through
Sand, Jour. Am. Water Works Assoc.. 25, 1551 (1933).
144. L. V. Radushkevich, The Kinetics of Formation and Growth of Aggregates
on Solid Obstructions from the Flow of Colloidal Particles, Colloid
Journal, 2_6 235 (1964) (In Russian).
145. N. Kimura and K. Bnoya . Chem. Eng. (Tokyo) 2£
166 (1965); abridged ed. in English J3, 193, (1965), cited in
K. ttnoya and C. Orr, Jr., Source Control by Filtration in
Air Pollution, V.III, A.C. Stern, Ed., Ch. 44, p 419, Academic Press,
New York, 1968.
146. C. N. Davies, The Clogging of Fibrous Aerosol Filters, Aerosol
Science, ],, 35, (1970).
147. C. E. Williams, T. Hatch, and L. Greenburg, Determination of Cloth
Area for Industrial Air Filters, Heating, Piping, and Air Conditioningt
12, 259 (1940). '
148. J. W. Robinson, R. E. Harrington, and P. W. Spaite, A New Method
For Analysis of Multicompartmented Fabric Filt., Atm. Environ 1
499, July 67.
149. J. F. Durham, Filtration Char, of F. F. Media, Unpub., Feb. 69.
150. C. A. Snyder, R. T. Pring, Design Considerations in Filtration of Hot
Gases, Ind s Engg Chem 47, 960, May 55
151. Egorova, L. G., Sakhiev, A. So, Bassel, A. B., and Kosareva, N. S.
Use of Bag Filters for Removing Fine Metal Particles from Air
Suspension, Soviet Powder Metallurgy and Metal Ceramics, 33, 9,
774. Sept. 65.
152. H. Kohn, Dust Elimination by Fabric Filtr, Tonindustrie-Zeitung .89
97, 1965.
153. H. Kohn, Theory and Practice of Dust Elimination Through Fabric Filters,
Staub, _21, 9, Sept. 61
154. R. A. Gussman, Billings, C. E., and Silverman, L. Open Hearth Stack
Gas Cleaning Studies, Semi Ann. Rpt. to Am. Iron and Steel Institute.
SA-17, August 62.
155. D. J. Robertson, Filtration of Copper Smelter Gases of H.B.M. + S.
Co. Ltd., Canadian Mining and Metallurgical Bulletin. 326 , May 60.
156. L. J. Kane, Chidester, G. E. and Takach and Shale, Porous Stainless
Steel Filters for Removing Dust from Hot Gases^ Report of Investigations
5842 Department Interior, Bureau Mines, Feb. 61
157. jr. p. Anderson, Furnace Fume Collector, Foundry, 152, September 55.
-------
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).
-------
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).
-------
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
-------
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.
-------
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.
-------
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
-------
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.
-------
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
-------
. 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.
-------
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
-------
EXPLODED VIEW
INLIT AIR VALVE
nrmt ASSEMBLY
RMMM AM VALVt
Figure 3.3a. High Temperature Baghouse (continued)
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
-
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
-------
-.•.««,- ; /»>../.' ;
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
-------
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
-------
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 ;
-------
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
-------
/"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
-------
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.
-------
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.
-------
• 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
-------
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.
-------
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
-------
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).
-------
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
-------
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.
-------
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.
-------
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
-------
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.
-------
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.
**
-------
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
-------
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.
-------
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"
-------
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.
-------
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.
-------
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
-------
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.
-------
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,
-------
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.
-------
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.
-------
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.
-------
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.
-------
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.
-------
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 ^
-------
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.
-------
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.
-------
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.
-------
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.
-------
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
-------
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.
-------
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,
-------
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.
-------
\ /
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
-------
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.
-------
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
-------
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.
-------
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.
-------
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).
-------
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).
-------
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
-------
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
-------
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
-------
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.
-------
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
-------
(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,
-------
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
-------
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.
-------
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
-------
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-
-------
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
-------
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
-------
' 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
-------
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.
-------
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-
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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.
-------
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-
-------
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.
-------
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.
-------
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
-------
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-
-------
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
-------
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.
-------
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-
-------
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.
-------
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
-------
(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.
-------
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.
-------
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
-------
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.
-------
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:
-------
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.
-------
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:
-------
. 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
-------
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).
-------
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
-------
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.
-------
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
-------
'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.
-------
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-
-------
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.
-------
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.
-------
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-
-------
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.
-------
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.
-------
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.
-------
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:
^
•rl
8
r~*
t>
M
0)
4J
r-l
•rl
A 10% increase in this variable: V
0)
M
s
cd
4)
(X
0)
H
T
0)
M
3
M
0)
£
U
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ri a
,a o
P
01
M
C cd
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O 01
Qu O«
si
DT
H
01
4J
Q)
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Q
Ol
r-l
U
•H
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a
D
P
r-l
g
r-l
PM
a
01
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4-1
H
Q
$
•rl
1
Q
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tance
w
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8.
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K2
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,
-------
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
-------
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
-------
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.
-------
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.
-------
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
-------
.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-
-------
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 '
-------
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).
-------
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).
-------
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
-------
\
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).
-------
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.
-------
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,
-------
(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.
-------
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.
-------
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
-------
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.
-------
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)."
-------
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
-------
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,
-------
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).
-------
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.
-------
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.
-------
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:
-------
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.
-------
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
-------
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.
-------
. 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
-------
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.
-------
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)
-------
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).
-------
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).
-------
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
-------
(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-
-------
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
-------
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-
-------
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
U-
v>
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»
14
t-t
~t*
o
CSl
«!.«
*
a
£'••
I"
«i.t
3
<0
S'-o
M
*.•
.•
<
.4
Q/C? • OUST LOADINt IN Oft*
• FILTCftlNa PERIOD
• SMAKIN9 PCNIOO
- acrM/so.Ft or COTTON •*• ,
TALC DUST (NYTAL 500) /
a>
•?;
/
f
>
!/
r
>
/
f
^
4
(
w1
O
«v
7
/
/
j
/
/
/
T
IMS
v»...
PE«
.... ,
H
>
/
/^
CUtIC FOC
SETTUN
CMKN)
/
*/
/
/
^
/
r
>T
0
V..
-------
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
-------
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
-------
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)
-------
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,
-------
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,
-------
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:
-------
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.
-------
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)
-------
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
-------
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).
-------
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
-------
' 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.
-------
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 $5 per foot for a 100-foot system of
6" 18 gauge ducting including soldered joints and about 3 gates, installed.
Table 7.6 also lists the costs of typical duct fittings be-
fore installation. The cost of a fitting depends, of course, on the
-------
TABLE 7.6
APPROXIMATE DUCT COSTS
(a) Installed Duct
Systems
($ 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.
-------
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.
-------
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
-------
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.
-------
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
-------
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
-------
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.
-------
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
-------
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.
-------
(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
-------
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.)
-------
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.
-------
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.
-------
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).
-------
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.)
-------
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
-------
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).
\
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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.
-------
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.
-------
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.
-------
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
-------
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
-------
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.
-------
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
-------
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.
-------
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.
-------
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
-------
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.
Fabric wear due to flexing may be a problem; see the comments _-/
under "Reverse Flow Plus Collapse" below.
8.2.10.2 Reverse Flow - The reverse current of air has two functions, \^
that of pushing the dust deposit off the cloth, and usually that o£ flushing
J
-------
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).
-------
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.
-------
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.
-------
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
-------
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
-------
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.
-------
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).
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