EPA-600/2-76-204
July 1976
Environmental Protection Technology Series
                                       EFFICIENT  USE  OF
             FIBROUS  STRUCTURES  IN  FILTRATION
                                    Industrial Environmental Research Laboratory
                                          Office of Research and Development
                                         U.S. Environmental Protection Agency
                                   Research Triangle Park, North Carolina 27711

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               RESEARCH REPORTING SERIES

Research reports of the Office of Research and Development, U.S. Environmental
Protection  Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:

     1.    Environmental Health Effects Research
     2.    Environmental Protection Technology
     3.    Ecological Research
     4.    Environmental Monitoring
     5.    Socioeconomic Environmental Studies

This report has been  assigned to the ENVIRONMENTAL  PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new  or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
                     EPA REVIEW NOTICE

This report has been reviewed by the U. S. Environmental
Protection Agency, and approved for publication.   Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency,  nor does mention of trade
names or  commercial products  constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.

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                                   EPA-600/2 -76-204

                                   July 1976
            EFFICIENT  USE

      OF FIBROUS  STRUCTURES

            IN FILTRATION
                     by

          M.  Mohamed and E. Afify

        North Carolina State University
      Schools of Engineering and Textiles
             Raleigh, NC 27607
              Grant No. R801441
        Program Element No. EHE624
      EPA Project Officer:  J.H.  Turner

 Industrial Environmental Research Laboratory
   Office of Energy, Minerals, and Industry
      Research Triangle Park, NC 27711


                Prepared for

U.S. ENVIRONMENTAL PROTECTION AGENCY
      Office of Research and Development
            Washington, DC 20460

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                         CONTENTS




                                                              Page
 List of Figures                                                  iv




 List of Tables                                                 viii




 Acknowledgments                                                x




 Sections




 I       Conclusions and Recommendations                         1




 II      Introduction                                              3




 III      The Needle Punched Structure and Particle Collection       5




 IV      Theory                                                   8




 V      Fabric Parameters                                       32




 VI      Fabric Characteristics and Their Methods of Testing       34




 VII     Results and Discussions                                  48




 VIII    References                                             118




 IX      List of Publications                                     121




X      Nomenclature                                           122




XI     Appendices                                             126







                             iii

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                          FIGURES

 No.                                                           Page

 1     Needle-punched structure and particle collection             6

 2     Cross-sections  showing pore formation in needle
      punched fabric                                              7

 3     The needle punched fabric  -  The model                    20

 4     Nondimensional Darcy coefficient vs. solid fraction         24

 5     Nondimensional Darcy coefficient vs. solid fraction         26

 6     Drag force vs. solid fraction                               29

 7     Drag force vs. solid fraction                               30

 8     Apparatus                                                  36

 9     Centralized controls flyash apparatus                       37

 10    Filter holder                                               39

 11    Cumulative particle size mass distribution                  40

 12    The dust feeder                                            41

 13    The dust feeder                                            42

14    Temperature and humidity control system                   44

15    Aerosol penetration testing equipment                       45

16    Fabric density vs.  needling intensity (20 gauge)             49

                              iv

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                     FIGURES (continued)

No.                                                            Page

17    Fabric density vs.  needling intensity  (25 gauge)              50

18    Fabric density vs.  needling intensity  (32 gauge)              51

19    Air permeability vs. needling intensity (20 gauge)            53

20    Air permeability vs. needling intensity (25 gauge)            54

21    Air permeability vs. needling intensity (32 gauge)            55

22    Air permeability-thickness product vs. needling intensity
      (20 gauge)                                                   56

23    Air permeability-thickness product vs. needling intensity
      (25 gauge)                                                   57

24    Air permeability-thickness product vs. needling intensity
      (32 gauge)                                                   58

25    Bursting strength vs.  needling intensity (20 gauge)           59

26    Bursting strength vs.  needling intensity (25 gauge)           60

27    Bursting strength vs.  needling intensity (32 gauge)           61

28    Pressure dropper  unit thickness vs.  needling intensity
      (air velocity 90 ft/min) (20 gauge)                           62

29    Pressure drop per  unit thickness vs.  needling intensity
      (air velocity 90 ft/min) (25 gauge)                           63

30    Pressure drop per  unit thickness vs.  needling intensity
      (air velocity 90 ft/min) (32 gauge)                           64

3.1    Fabric density vs.  needling intensity  (random-laid)           66

32    Air permeability vs. needling intensity (random-laid)        67

33    Air permeability-thickness product vs. needling intensity
      (random-laid)                                               69

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                     FIGURES (continued)

 No.                                                           Page

 34    Bursting strength vs. needling intensity (random-laid)       70

 35    Pressure drop per unit thickness vs. needling intensity
       (air velocity 90 ft/min) (random-laid)   •                     71

 36    Fabric thickness vs. needling intensity effect of fiber          l"
       length                                                      73
                                                                    ? -
 37    Fabric weight vs. needling intensity effect of fiber
       length                                               ;  •     74

 38    Fabric density vs. needling intensity effect of fiber
       length                        .                              75
                                                      '  .  5
 39    Air permeability-thickness product vs. needling intensity
       (effect  of fiber length)                                       76

 40    Fabric bursting strength vs. needling intensity (effect
       of fiber length)                                    •          77

 41    Arrangement of needle and plates                            79

 42    Fabric weight vs. needling intensity (Cerex and Reemay
       spunbonded fabric only)                                     84

 43    Air permeability vs. needling intensity (Cerex and Reemay
       spunbonded fabric only)                                     85

 44    Ball burst vs. needling intensity (Cerex and Reemay
       spunbonded fabric only)                                     86
                                                   ,      >
 45    Fabric  thickness vs. needling intensity (spunbonded scrim)   88

 46    Fabric  weight vs. needling intensity (spunbonded scrim)     89

 47    Fabric  density vs.  needling intensity (spunbonded scrim)     90

48    Air permeability vs. needling intensity (spunbonded scrim)   91

49    Air permeability-thickness product vs. needling intensity
      (spunbonded scrim)                                          92

                              vi

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                     FIGURES (continued)

No.                                                             Page

50    Ball burst strength vs. needling intensity (spunbonded
      scrim)                                                     93

51    Bag appearance - A -  Commercial Bag
                       B -  Bag made from fabric 5              102

52    Nondimensional pressure gradient vs. needling intensity
      (Dacron 3 den. x 1.5 in.,  crossed-lapped)                 105

53    Percent penetration vs. velocity for 0. 5 |j.m                106

54    Percent penetration vs. velocity for 1.099 (am             107

55    Percent penetration vs. velocity for 2.02 )j.m               108

56    Dust cake on a needle  punched filter                       111

57    Effect of filtration time on cake formation                 112

58    Effect of flyash concentration on efficiency (368 punches/
      inch^, 25 gauge,  3.0 denier x  1^ in. Dacron) 10 minutes
      test duration, batch testing, air velocity  -  45 ft. /min.    113

59    Effect of filtration time on collection efficiency (245
      punches/inch^, 25 gauge needle, 3.0 denier x 1.5 in
      Dacron), batch testing,  air velocity -  45 ft/^nin.          115

60    Effect of filtration time on pressure drop (batch testing)    116

B-l   Effect of air velocity on pressure gradient                 130

B-2   Effect of air velocity on pressure drop                     131
                              VI1

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                           TABLES

 No.                                                           Page

 1    Batch Filtration Results (Effects of fiber orientation and
      needle size)                                                65

 2    Batch Filtration Performance (Effect of Fiber Length)        78

 3    Effect of Needle Penetration on Packing Density              80

 4    Effect of Needle Penetration on Air Permeability-
      Thickness Product                                          80

 5    Effect of Needle Penetration on Ball Bursting Strength        81

 6    Batch Filtration Results (Effect of Needle Penetration)        82

 7    Batch Filtration Performance of Spun-Bonded
      Sc.rimmed Fabrics                                          94

 8    Properties of Bag Fabrics                                   96

 9    Batch Filtration Performance of Bag Fabrics                98

 10    Baghouse Testing Results                                    99

 11    Fabric Properties Used For Verification of Theory         104

 12    Dorman  Parameters                                       109

 13    Effect of Humidity                                          117

A-l  Fabric Thickness  (Dacron  3 den. x 1.5 in.)               126

A-2  Fabric Weight (Dacron  3 den. x 1.5 in.)                   127

                              viii

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                     TABLES (continued)




No.                                                          Page




A-3   Fabric Packing Density (Dacron  3 den. x 1.5 in.)          127




A-4   Fabric Tenacity (25 gauge) (Dacron  3 den.  x 1.5 in.)       128




A-5   Fabric Elongation (25 gauge)  (Dacron  3 den. x 1.5 in.)     128
                              IX

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                    ACKNOWLEDGMENTS

The authors gratefully acknowledge the advice and cooperation of
Dr. J. H. Turner and the Staff of the Industrial Environmental Research
Laboratory, Environmental Research Center,  EPA,  Research Triangle
Park,  North Carolina^or conducting the bag testing experiments.

The assistance of Dr.  R. W. Work, Professor Emeritus of Textile
Chemistry, is highly appreciated.  The following graduate students
contributed  greatly to the experimental work of the project in the
course of their studies as research assistants:
            J. W. Vogler              V. W. Herran
            L.  L. Saleh                M.  A. Hassab
            M. Venkatesan             Scott Penn
            S. Sandukas                C. J.  Thornton

The authors also wish to express their appreciation to E. I. duPont de
Nemours and Company for providing the Reemay  samples, Monsanto
Textiles Company for providing the Cerex  samples,  Hercules,  Inc.
for manufacturing the bag fabrics at their ResearchCenter, Research
Triangle Park, North Carolina and The Torrington Company for
providing  the needles used in this project.

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                          SECTION I

           CONCLUSIONS AND RECOMMENDATIONS

Conclusions

The  structure-of noodle punched fabrics offers in addition to the high
efficiency characteristic of filter fabrics the advantage of low pressure
drop.  It also offers the advantage of using higher flow rates than are
normally used in filtration using woven fabrics, -which has economical
implications in baghouse applications.

The  results of the investigation of the various parameters and their
influence on the fabric properties  and filtration performance in batch
testing led to  the following conclusions:
         1.  Fiber length and orientation have no  significant effect on
            most of the fabric properties.  Random-laid webs give
            higher packing density and lower air permeability than
            cross-lapped webs.
         2.  Needle size and needle penetration are significant para-
            meters.   Large size needles produce undesirable fabrics
            for filtration.  Increasing needle penetration improves the
            fabric.   However, high level of needle penetration leads
            to fiber damage and deterioration in  filtration performance.
         3.  Needle punched fabrics without reinforcement lack
            strength and dimensional stability.  Spunbonded fabrics
            (Reemay   and Cerex  ) when used as scrim improve the
            needle punched fabric properties without sacrificing its
            filtration performance.
        4.  Needling  intensity is significant in affecting  fabric properties.
            High levels of needling intensity achieved in one passage of
            the fabric through the needling process results in fiber and
            scrim damage.  Needling with a small intensity and
            repeated  over a number  of passes and from  both sides
            improves fabric characteristics.

                                1

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          5.  Fabric weight per unit area and packing density are very
             important factors affecting the fabric characteristics.
             The air permeability-thickness product related more
             closely to the pressure drop for clean air than the air
             permeability alone.
          6.  Calendering needle punched fabrics damages the fabric
             structure and leads to a considerable increase in the
             pressure drop without a proportionate increase in efficiency.
          7.  Monodisperse particle testing indicates that the diffusion
             mechanism is not utilized effectively for the needle punched
             fabric samples tested in the  submicron range without
             cake formation.  In testing with hetrogeneous dust particles,
             the cake formed on needle punched fabrics differs
             distinctively from that formed on any other fibrous
             structure.  The deposition of dust on the fabric shows
             three-dimensional mounds around the pores.
          8.  NCSU developed needle punched uncalendered fabrics with
             nonwoven scrim, were  superior to commercial fabrics in
             some regimes of baghouse operation.   With  high levels of  .
             inlet loading and at high air-to-cloth ratios, more cleaning
             difficulties were encountered with NCSU fabrics than  the
             commercial one.

 Recommendations

 The present investigation concentrated only on one type of fiber and
 round cross-section.  It is felt that work is needed  to study the use of
 different fibers (such as Teflon, Nomex,  Nylon, etc.) with different
 fiber cross-sections and crimp in needle punched fabrics.

 Modification of the structure of needle punched  fabrics by various
 finishing treatments (such as shrinkage and resin applications) should
 improve  ':he filtration performance, especially  in the submicron range.
 This  seems to be an important area for continuing the present work.   It
 is also recommended to continue experiments to establish the durability
 of needle punched fabrics  under different testing conditions.  For  the
 purpose of economy it is suggested that a special miniature baghouse
 apparatus be built for such experiments.

 Optimizing filter specifications for best performance requires adequate
 information on tr. 3 relationship between filter parameters and air-dust
flow conditions.  A single performance value (rather than pressure drop
 and efficiency) is needed to simplify the optimization problem required
 for the design of filter fabrics.  This is particularly true for needle
 punched fabrics,  since the specific cake resistance used by many
 researchers does not apply due to the nonhornogenity of the  dust cake.
                                2

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                               SECTION II

                             INTRODUCTION
The need for high-efficiency filtration of particulate matter from dusty
atmospheres is ever increasing.  Fibrous structures  are widely used
as filter media.  Fabric filter technology has been growing  steadily and
plays an important role in air pollution control.  It  has been the general
practice to use woven fabrics in the majority of baghouse applications.
Woven fabrics are produced by interlacing two sets  of parallel rows of
yarns at right  angles in a square array.  In operation, dusty gas passes
through the filter normal to the  fabric surface.   Filtration has been
shown to take place over three phases [1].  At the start of the  filtration,
dust particles  deposit on individual fibers  and yarn  surfaces.   Additional
particles then  deposit and. accumulate on already deposited particles
forming  aggregate structures which project into  the gas stream.  As
deposition continues, openings between yarns become gradually filled
with aggregates which eventually form the dust cake.  Further accumu-
lation of dust particles on the cake continues and the resistance to the
gas,flow increases until removal of the cake takes place during the
cleaning cycle.  The collection efficiency of woven fabrics has been
found to be a function of the pore size distribution [2], Bleeding or
leakage of dust was found to be a function of the number of pores above
a critical size which is related to size properties of the dust being
filtered.

Structural properties of a fabric strongly affect its  filtration performance
and the fabric's interaction with a dust.  Fabrics designed for capturing
large particles,  will leave the air contaminated with small particles.
On the other hand, fabrics designed to collect fine particles must have
small pore size which results in high resistance to  the gas flow,  eco-
nomic considerations require minimum pressure drop through a filter,
and normally a balance has to be made between the  cost of cleaning and/
or replacing a clogged  filter and the power consumed in driving the flow.

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Recently, nonwoven fabrics have been enjoying increasing interest and
various structures have been looked at for many filteration applications.
Nonwoven fabrics are normally less expensive than, and can be made
as durable as, woven fabrics.  With most nonwoven structures, high
efficiency and low pressure drop in air filtration can be achieved.
Spunbonded fabrics were found to be more efficient, longer lasting and
cheaper than equivalent woven bags [3],   The performance of latex-
boncV-l nonwoven fabrics was studied [4J. The  results indicated that
with suitable choices of fiber properties it is possible  to improve
efficiency and air drag characteristics to a significant extent.  Another
nonwoven structure which  is becoming very widely used in filtration is
needle punched fabrics. Needle punched fabrics have excellent features
as a filter material.  High collection efficiency  at low  pressure drop
can be obtained.

This report deals mainly with the performance of needle punched fabrics
in batch and baghouse testing.

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                         SECTION III

THE NEEDLE PUNCHED STRUCTURE AND PARTICLE COLLECTION
The needle punched fabric is produced by penetrating barbed needles
into a fibrous mat.  The penetrating barbs transport surface fibers and
embed them  in the vertical direction, creating entanglements of fibers
inside the pores  (A Figure  1).  The fibers pulled by the needles exert
pressure on  the fibers surrounding the pore (B Figure 1) thus causing
an increase in packing density around the pores.   The pore is not a
free hole,  simply because of the fiber entanglements as  well as the
disruption which takes place in the fiber order when the  needles are
withdrawn from the fabric.   The number of punches per unit area  is
controlled by the distribution of needles on the needle board, the rate
of fabric feed and the  number of passages through the machine.  The
shape and size of the pores depend on the needle  shape,  size and
penetration.   Figure 2 is a micrograph showing the cross-s ection of
a needled fabric.

Uncalendered needle punched fabric does not act as a sieve, because
of the in-depth fiber entanglements in a venturi-like shape.  This
shape should offer a small  resistance to the air flow resulting  in
better flow characteristics across the filter.  Due to changes in the
packing density over the area of the fabric, the air streamlines will
be as shown  in Figure  1,  which leads to the deposition of large  particles
around the proes.  Small particles will follow the streamlines  through
the pores and will be collected by the fibers in the pore.  This gives
needle punched fabrics an advantage over woven fabrics  as far as
dust-loading  capacity  is concerned.  With needle  punched fabrics,
higher air-to-cloth ratio  could also be  used than  with woven fabrics,
which means that less fabric would be required to perform in a
filtration unit.

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                               AIR STREAMLINES
                                         PARTICLES
Figure I. Needle - punched  structure and  particle  collection

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Figure 2  Cross Sections Showing Pore Formation in Needle Punched Fabric

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                          SECTION IV

                           THEORY


 4. 1  Prediction of The Pressure Drop

 The prediction and knowledge of the  dependence of pressure drop on
 operating conditions and filter properties are essential for the
 development of needle punched fabrics for filteration.

 4. 1. 1  Review of Literature

 There have been many theories and models used by different investigators
 to predict the pressure drop through fibrous filters  which can be
 grouped in the following two categories:
      1.  Idealized theoretical models approximating the actual situation
          to be amenable to the application of known physical and
          hydrodynamical concepts.
      2.  The variables affecting the system are grouped together from
          dimensional consideration to form empirical correlations
          with numerical constants determined experimentally through
          a simple curve fitting procedure.

 The  first of the two groups is preferred since the results can be  applied
 to a  wide range of actual untried conditions, providing the basic
 assumptions are valid.  The second  group is  simpler  but the results
 are limited in application,  and their validity for other than the tried
 conditions is questionable.

 Most of the theoretical models for sufficiently small flow through
fibrous  media are based on the well known Darcy's  equation.

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                       AP  =  - k n U AL                        (1)

Darcy's law implies that for sufficiently low flow through a porous
medium the pressure drop is caused only by viscous energy losses. The
validity of Darcy's equation for low Reynolds number Newtonian-flow
has been established through many experimental studies,  notably that
of Chen [5] for continuum flow and that of Stern, Zeller [6] for slip flow.

Among the well known theoretical models are the channel theory,  the
drag theory, the theory of tortuosity, and the non-Darcy approach.

The "Channel Theory"  or "Hydraulic Radius Theory"

The Channel theory has been reviewed by Chen [5] and Linkson [7],  This
theory considers fibrous beds  as a system of interconnected  channels
and the pressure drop through them is given by Darcy's law.  The
constant,  k,  in Darcy's relation is known as Darcy's drag coefficient.
The reciprocal of k is commonly defined as the permeability of the medium.
The dimension of the permeability is that of (length)  , this length  was
introduced by Kozeny [8] as the hydraulic radius.  Darcy's equation was
extended to partially include the porosity of the medium by defining an
average pore velocity "v =  —  and replacing U by V in Darcy's
expression [9 ]

                       AP  =  - k  pi — AL                      (2)
                                a    e
                                      •*
                              k
                 and    k  =  —                                 (3)
                              e
where k   =  constant
       a
        e  =  porosity

Blake and Kozeny [10] used the concept of the mean hydraulic radius  for
correlating data on flow through granular beds and came out  with the
following  relationship
                                        2
                       AP  =  - ^ \i U— AL                    (4)
                                       e
Where S  =  surface area per unit volume of porous media.   Carman [11]
modified Kozeny relationship to relate Darcy's drag coefficient to the
physical constants  of the bed by the so-called Kozeny-Carman equation

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                            k  So2  (1 - e)2
                       k  = — - ^ -                       (5)
                                  e
Where k  is a constant found out to be 5. 0 for granular beds of low
        v_*
porosity
         So  = Surface area per unit volume of solid material.

Sullivan [12] modified equation (5) to account for fiber orientations by
the following relation.

                            k,  So2  (1 - e)2
                       k  = -^ - - -                       (6)
                                k  e '
                                 e
Wlioi-v- k   -  shape factor having the same  value for all geometrically
              similar channels.
       k   =  an orientation factor, which has the value of 1 and 0. 5 for
        (_"»
              fiber parallel to the flow and for fibers perpendicular to the
              flow respectively.

The "Drag Theory"

Brinkman [13] proved that equations based  on the Channel theory are
not applicable to highly porousfmedia.  Since the porosities of ordinary
fibrous filters are higher than 75 percent,  the application of the Channel
theory is then in doubt.  The Drag theory treats the walls of the fibers
as obstacles to an otherwise  straight flow of the viscous fluid.  The
pressure drop across a unit thickness of the filter is  the total drag
force on the fibers per unit volume of the filter.  Iberall [14] used the
Drag theory to obtain the pressure drop through a fibrous filter having
an equipartition of the fibers in  the three perpendicular direction. For
fibers parallel to the direction of the superficial velocity, based on
E mo r sic ben [15J he estimated the  drag froce to be

                       F =  4 IT p. —                             (7)
                                    t»

For fibers perpendicular to the  direction of the superficial  velocity he
used Lamb's equation for the drag force of an isolated cylinder.


                                      U
                                 2(2-InRe)
                                10

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By adding the drag forces in the three perpendicular directions Iberall
obtained the following equation for the pressure drop.
                       AP  =  I6ll  (1 - e)  (4 - InRe)
                      ALU       2    e     2 - InRe              W
                               ME

Where d   =  effective fiber diameter.
        XL/

Iberall found  out that the experimental results were best fitted by the
expression

                       AP   =  9.4ii (1  - e)  (2.4 -  InRe)
                      ALU     ,2      e       2 -  InRe            (   '
                               dE

The difference between Iberall equation and the hydraulic radius theory
is that the permeability in Iberall equation is not only a function of the
filter properties but also the Reynolds number.  It  also should be noted
that equation  (8) of Lamb does  not take  the effect of the neighboring
fibers into consideration which results  in  underestimating the drag
force especially at low Reynolds  number.

Chen [5] accounted for the effect  of the  neighboring fibers by using
Wong's [16] equation:
                              Cd  .
F.    =  drag force per unit length of fiber with diameter d.

Cd   =  drag coefficient for fiber diameter d. in filter with fiber volume
   ai     ,    ,.                              i
         fraction ou

By summing up all the drag forces on the fibers in a unit volume of
filter as the pressure drop across unit thickness of filter he obtained

                                       (df)
                  AP  = ~ a  p U  Cd   - ^p-  AL
                        TT           OC  . , -. £
                                        (df ) S

Where Cd~  = drag coefficient of a fiber of average size (df)   in a filter
         ^"                                                 Cl« *r
              with fiber volume fraction a

Chen  used the Wheat [17] equation rather than the Lamb equation to
evaluate the coefficient Cd .
                                11

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                       Cd
 Ac
         v^ j     -L            -*•    *—'      *

                        A       (          n    i i  TT A T .
                                                                 (ID
"X 	 J-v
2 e


•e drop is
4
11 In(k
g
-0.5
In k a
g
given by:
"f a
-0.5. 1 - a
a . )





U U AL
(df)l

 Where (df)   =  surface average fiber diameter.
          s
      k , k   =  constants depend on the fiber orientations within the
                filter, and the manufacturing technique.
                                                    Cda
 Chen used his experimental results to plot the group — - — Re against Re.
 He found that C^q  Re calculated from data for one filter is independent
 of Re for Re   2   <1 ,  which confirms Darcy's law.  Wheat [17] noticed
 that filters containing fibers with diameters approaching the mean free
 path of the air molecules will  show lower pressure drop at a given flow
 than would be expected by Darcy's law because of the slip flow
 phenomenon.  He introduced the following relationship for Darcy's
 coefficient.
                       k  =   --              -                 (12)
Where k is a function of Cunningham slip correction factor given by the
following equation

                     k =  0.034 + 0.601 (C -  1)                  (12a)
where C is the Cunningham correction factor given by

                 C  =  1 + j (2.46 +.0.82 exp  (-0.44 d/X)}        (13)

where X = mean free path of the air molecules of value 6.45 x 10
           Cm at 20°C and atmospheric pressure
       d = fiber diameter
C can be approximated  depending on the value of d.
For d <0. 2 micron
                 C  -  1 +| {2.46 + 0.82 (1 - 0.44 d/X)}         (14)
                               12

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and for d > 0. 2 micron
                        C =  1 + 2.46 ~                          (15)
The  scattered data used to derive equation (12) make its validity rather
doubtful.

The  preceeding discussion shows that the goal of the studies  reviewed
was  to exclude as much as possible of the filter properties from Darcy's
drag coefficient by introducing relations in form of k  = k* f (filter
properties e, orientation,	etc.) where k* is a constant, to be
evaluated experimentally,  which  depends only on the manufacturing
technique- and has the same value for all similarly constructed filters.
There has also  been some attempts by different investigators to
evaluate1 theoretically Darcy's drag coefficient (k) using different
mathematical models,  such as the "Cell Model" and the "Brinkman
Model".

The  Cell Model
The Cell model or the free surface model has been developed and used
to predict  sedimentation by Kuwabara [1.8],   Kuwabara suggested for
the case of parallel circular  cylinders distributed  at random and homo-
geneously  in a viscous flow that there has to be an  envelope  or a free
surface around each cylinder at which both the vorticity and the  normal
component of the velocity vanish.  The imaginary free surface is assumed
to be coaxial with the circular cylinder and of radius "b".  The cross-
sectional area rrb  of the imaginary free  surface is equal to the  free
area corresponding to each cylinder, namely

                        nb2  =  1/n                               (16)

where n  =  number of solid cylinders per unit area.

Happel [19] extended the application of Kuwabara cell model to fibrous
filters.  The only difference  between Happel and Kuwabara's assumptions
is that Happel  assumed  that the  shear stresses, instead of the vorticity,
vanish at the free surface. He applied Navier-Stokes equation in
cylinderical coordinates in the  complete form for the fibers parallel
to the direction of  superficial velocity and obtained the following
expression for Darcy's  drag  coefficient
                                13

-------
                J      224     4    4    b
       I/k   ~  —- (4a b  -a  -  3b  + 4b  In—)                    (17)
         Z  '   Kb                            a

 It should be noted that in this case the inertia terms vanish because
 there  is no change of velocity along the fibers.  For fibers  normal to
 the flow, Happel neglected the inertia terms  by applying Navier-Stokes
 equation in the creeping form.  He obtained the following expression
 for Darcy's drag coefficient:
                         2             44
                  1     b      b    1 ,b_-a
                  k~  = T Inl -2 {~4	4)                    (18)
                   1                  b  + a

 The value  of "b" in equation (17) and (18) can be evaluated in terms of
 the fiber volume fraction ( a ) in  the following way.  From equation (16)
                       n A TT a  t          2
    but            a  =	—7	  = n TT a
                           A  *-
                          .f}.  \f
                                   2
                           —  - T13-
                           n     a
 substituting into equation  (16) one gets

                             i       2
                             I    Tra
                           TTb   =  	
                                    a


                             b  .  4=
                                  Va
where A  = cross section area of the filter
        t  = thickness of the filter
       n  - number of fibers per unit area.

It is clear that by decreasing the porosity,  a will increase and "b"
approaches "a" which means that the fibers are almost touching each
other.  For the above reason Happel solution fails at porosities smaller
than 0. 5.

Brinkman Model
Brinkman [20] stated that Darcy's drag coefficient for low velocities
depends only on the filter properties and its geometrical construction
                               14

-------
for continuum flow and also on the mean free path of air molecules for
slip flow.  This has been proved experimentally by Linkson e_t ad.  [7]
on a wide range of glass filters by showing a linear relationship
between the pressure drop per unit thickness and the superficial
velocity up to 2. 5 ft/sec.   The relation deviates gradually from the
linearity for Reynolds numbers higher than unity.

Since the pressure drop for small flow does  not depend on the Reynolds
number, Brinkman concluded that Stoke's law for creeping motion
applies for the flow through the medium.  In this case, the fiber
boundaries in the  vicinity  of any given fiber must affect the fluid motion
around that fiber  in such a way that inertial effects remain negligible
when compared with  viscous effects through  the entire region of flow.
The essence of Brinkman  hypothesis is that, on the average, the fluid
in the proximity of an obstacle imbedded in a porous medium experi-
ences a body damping force proportional to the velocity, in addition to
viscous and pressure forces.    The damping force accounts for the
influence of the neighboring objects on the flow.

The essential difference between the Brinkman model and the Cell model
is that Brinkman  model implies that the neighboring fibers  damp the
ensemble average microscopic flow near the central fiber precisely the
same way  the fibers  of the medium damp local flow through the medium
when averaged over all conceivable fiber arrangement.  While the
"Cell Model" account for neighboring fibers  influence by means of
microscopic envelope around the central fiber.  The characteristic
envelope size depends on the microscopic voids.

The validity of Brinkman hypothesis is  limited to conditions where the
neighboring fibers are distributed about the central fiber in approximately
the same way as they are  generally distributed in the medium.  The
hypothesis, therefore, breaks down when applied to media of sufficiently
low porosity because in this case the  effect of many solid boundaries in
the immediate proximity to the central  object cannot be well described
by a simple damping coefficient.

Brinkman  [20] initially used his hypothesis to investigate flow through a
swarm of spheres and hindered settling velocity.  The model was also
used by Debye and Bueche  [21] to predict certain hydrodynamic
properties of disolved polymer molecules.
                               15

-------
 Recently the Brinkman model was used by Spielman and Goren [22] to
 evaluate  Darcy's drag coefficient mathematically for fibrous filters of
 different geometrical constructions and fiber orientations namely:
      a.  Filters with fiber axes all lying in planes perpendicular to
          the direction of the superficial  velocity,  but having completely
          random angles in those planes.
      I).  Fiber axes all parallel to the direction of the superficial
          velocity.
      c.  Fiber axes all lying in planes parallel to the direction of the
          superficial velocity,  but having completely random angles in
          those planes.
      d.  Fiber axes completely randomly oriented in all directions.

 The pressure drop results of Spielman and Goren  showed better agreement
 with Duvies [23] Empirical results than those of Kuwabara and Happel
 using the Cell model.

 Theory of "Tortuosity"

 Clarenburg and Pickaar [24] used a pure theoretical approach to predict
 tho pressure drop through fibrous filters.  The  model used assumes the
 pores to  be small capillary tubes with log-normal distribution.   Starting
 with Poiseuilles law and using  the pore distribution,  they derived the
 following relationship.

                                             N   L  2
                        AP  =  11.4 n  AL— • -£ (~r- >             (19)
                                          e   T2   L
                                              —2
 where N    =  number of pores  on surface area  1
        £
         1  =  mean fiber length
       LV_  =  effective thickness
        E

 LE
—-— is known as Tortuosity Factor and given by
                                                          _ O

                                            . QZ4)2 0.389-        (20)
which is vaJid for (-; < 0. 94
                               16

-------
N  is given by
 P
                       N     N (N- 1)
                         P        TT       2

               nber of fi
            2d
Where N  = number of fibers in a slice of surface area 1  and thickness
The relationship was tested experimentally over a range of porosities
0. 88 - 0.96, for which slight difference between theory and experiment
were found in the high porosity range.  It has been found that the theory
of tortuosity leads to erroneous results for filter porosities  exceeding
0.94.

Non-Darcy Approach

The one  common assumption -which underlines the theories and correlations
so far,  is  the validity of Darcy's law which is only true if the flow
through the medium is  sufficiently low for the pressure drop to be
caused only by viscous energy losses.  As can be seen this assumption
implies that for a given fluid flowing through a particular medium, the
pressure drop is a linear function of the velocity, or simply
                                          TT
                                      = Y U
                                 AL    YU

Where y is a constant which is characteristic of the particular fluid and
medium.

Linkson et al. [7] showed that the pressure drop is a linear function of
the superficial velocity for Reynolds numbers up to unity after which a
deviation from the linearity gradually appeared.

Beavers and Sparrow [25] used a more  generalized equation for the
pressure drop through fibrous media which is represented by

                         AP                  2
                       -~-=k|jU  + TipU                   (21)
where- T]  ~  constant,
       k  =  Darcy's  drag  coefficient.
                               17

-------
 Equation (21) differs from Darcy's equation by the term (n p U  ) which
 represents the inertial effects at high Reynolds numbers.   The results
 of the experimental work of Beavers and Sparrow showed  that the
 constant r\  is independent of the superficial velocity U.

 Dimensional Analysis Approach

 Davies [23] used the dimensional analysis approach to derive a relation
 between the pressure drop through fibrous filters and their solid
 fraction (a) for fibers with axes perpendicular to the direction of the
 superficial velocity U.  From dimensional analysis, assuming Darcy's
 law of flow through porous media,  Davies suggested that a unique
 relation must exist between a,  the  packing density, and the dimension-
 less group.
 where A.  = area of filter
      a   = mean fiber radius.
       e

 Davies had difficulty in defining the fiber radius as the dispersal of the
 fiber material is rarely so perfect that the fibers act individually.  In
 fact, the effective fiber radius is usually greater than the actual radius
 measured  under a microscope.  Some tendency to clumping exists and
 the air prefers to flow through open interstices, avoiding the fine spaces
 within clumps.  Errors due to these causes are diminished by introducing
 an effective radius (ae ) determined empirically.  Measurements on
 actual pads made of wide variety of fibrous materials showed a  small
 "scatter" about the curve.

                        Aa2
                             =  '* a1'5  (1+56 a3)                (23)
Davies  stated that there is no appreciable dependence on the length of
fiber.   The left hand side  of equation (22) is known as the permeability
coefficient.

Davies  claimed that his expression is valid for fiber diameter  ranging
from 1.6 -  80 microns and filter porosities  ranging from 0.700 - 0.994.
Davies  equation does not show any dependence of the pressure  drop on
the structural arrangement of the fibers in  the filter which has been
shown by Chen [5] to be of importance.
                               18

-------
Davies also neglected the effect of the slip flow for the low range of
fiber diameter which has been pointed out by Wheat [17] to be of great
importance.
        and Clarcnburg [26] studied pressure drop through glass fiber
filters for fiber diameters in the order of the mean free path of the air
molecules for all fibers perpendicular to the direction of the superificial
velocity.   They found out a linear relation between Davies permeability
coefficient and the porosity function defined by (1 -  e)^'^ for a given
filter.  The ratio between the permeability coefficient and the porosity
function is known as  the resistance coefficient.   They plotted the
experimental results between the resistance coefficient and Cunningham
slip factor given by equations (13), (14), and (15), and came out with
the following relationship.

                The resistance  coefficient =  180 C~ "            (23)
and hence the pressure drop is given by


                 AP  = 180 C-2-5 ^AkH (1 _  e)3/2             (24)
                                    d
                                     e

where C  = Cunningham correction factor

4. 1.2 Mathematical Development of The Pressure Drop For Needle
       Punched Fabrics

As needle punched fabrics are usually highly porous, it can be concluded
from the  review that the channel theory and the  theory of tortuosity are
not suitable to  adopt.  A geometrical model based on the drag theory is
devised to describe the flow through the fabric.   The Brinkman model,
used by Spielman and Goren [22],  has been adopted since their results
were in better  agreement with Davies empirical equation than those of
Kuwabara and Happel using the cell model.

The  model can be described, as shown in Figure 3,  as:
      a.  The fibers  around the pores are assumed to lie in plane's
          perpendicular to the direction of flow,  with the fibers
          randomly distributed  in each plane.
      b.  The fibers  in the pores are assumed parallel In tin; direction
          of flow.
                               19

-------
THE NEEDLE PUNCHED FABRIC
THE MODEL
                     Figure   3.

-------
Referring to the geometrical model,  let a, be the volume of fibers per
unit volume of fabric of the fibers in the areas around the needle pores;
and 
-------
 while for continuum flow is
 /
'
                                        -          -
                        2     -         -          -                2
                                        *
       4rr M  U  =    (kLa )  -f (k  a) KL (k  a)   KQ (k  a)  S
                                                                   (30)
 whore- F   is the drag force per unit length of the fibers  of the first
          1 group K   K  are modified Bessel functions of zero and
            is first order.

 The total drag froce of a unit volume of the mat due to the first group
 of fibers (normal) is given by
                                             f-±
                        Fl =  FD  (ai/n a  >                     (31)

 For the second group  of fibers (all axes parallel to the  superficial
 velocity) there, is only one component  of velocity u   given by
                                                  Li Z
                               ±
                          K  (kf r)
     =  U[l- - r - ° - ; - r— r - T— ]           (32)
               K  (k^  a)+k  (1  +k  )" k^ aK. (kf a)
                o   
-------
Then
                  dp
                  dL  =
Knowing the values of k  and k   the numerical values of the functions
cp  and cp_ can be evaluated and the pressure gradient can be calculated
from equation (37) for the given medium and the fabric properties.

Determination of Darcy's Drag Coefficients k  and k
                                             J. " * " 7"' "J ~~~'~£*

To determine the coefficient k for the first group of fibers, consider
a unit volume of a hypothetical fabric having geometrical construction
similar to that of the fibers in the  unpunched areas with  solid fraction
a* .  Using equation (31) along with Darcy's equation gives
                                *
                              TT a

As the Knudsen number for the range of fiber diameters used in this
investigation is very small the flow is considered continuum.
Accordingly from equation (29)

                             (k? a)K   (k? a)     a*
    FT  = 4rr n U [| (k_aZ) + —	-	— ] -  (-^)             (39)
                               tr  n,*  i        "a
                               K  (k* a)

and

                             .1         i
          4a".          2    (k? a) K  (k* a)
    k   = -J^- [| (k a ) i 	,	 •       ]                      (40)
     1     a                  K  (kf a)
                                o   1
                                      >'"
This equation gives k  implicity with a  as a parameter and  can be
solved by iteration.  Newton's-Raphson method is  adopted to insure
rovergence and to save computational time.  The results are plotted
in Figure 4 which gives values of k  for different values of at.
                               23

-------
     6.0
CM
  O
 -  5.0
     4.0
 u_
 3
     3.0
     2.0
 5
     0
        0              0.08             0.16

                         SOLID  FRACTION  Q,

       Figure  4.  Nondimensional  Darcy  coefficient  vs.

                  solid   fraction
0.24
                          24

-------
To determine the coefficient k  for the  second group of fibers,  consider
a unit volume of a fabric having geometrical construction similar to
that of the fibers in the pores with solid fraction a^-  The total drag
force of a unit volume of the mat is given by

                       F2 -  FD  <~> =  ^k2
                                 2   TT a
From equation (34) and (41) k   can  be expressed as
                                L        L
                       2 oT    (k2 a) K.  (k* a)
                  k2  = -r- £~ - r-^— 3
                         a      K   (ki a)
                                 o  2
                                         *
To solve the above equation for k  with a~ as a parameter, an iteration
technique similar to that used in the evaluation of k,  is applied.  Figure 5
gives the values  of k  for different  values of cuf
                   -.*<       y$
Determination of a1 and  q From The Physical Properties of The Filter

The most essential parameters which describe the needle punched
filters are
      the solid fraction a,                  ->
      the needling intensity ^(punches /inch ),
      the filter thickness AL (inch),
      and the needle size D (inch).
                                       2     *
If the actual number of punches per inch  is N ,  the number of fibers
per pore is n, and the actual  diameter of the pore is  D ' .   Then the
volume of the punched pores per unit volume of the filter is equal to
ft   JjO     >|;
4"  D  .   N.  and,  the volume around the punches per unit volume of filter
               TT  #2     *
is equal to  1 - ~ D   "  N .
              .'-
By definition ou is the volume of fibers per unit volume of fabric having
geometrical construction similar to the pore construction of the actual
fabric.
                                 2
                          *     TT a   n
                              TT*2
                               25

-------
CVJ
 o
  CVJ
 ~ 0.6 r
 UJ
 o 0.5
 UJ
 o 04
 o u
   0.3
   0.2
   O.I
o

<
o
CO
 o
                   0.08
                                0.16
               SOLID   FRACTION   Q
0.24
    Figure  5,  Nondimensional  Darcy   coefficient
                vs.  solid  fraction
                    26

-------
                                      2
                              *    4 a  n

                       i,e-   <*2 =  — *2~                         (43)
                                    D


Similarly for a filter having a surface area A and thickness AL



                                     2          *
                  *  _  a A. • AL -  rra  n •  A • N  •  AL

                 ai              TT  *2    *
                        A.  • AL -    D   • N   • A.  • AL
                               2       *
                       a - TT a n •  N                             , . . x
Equation (43) and (44) relates the solid fractions a  and a_ to the actual

parameters D''~ , N' ,  a and n of the filter.



It has been observed that the actual parameters of the filter such as
  >'c       >'c
D''* and N' are different from the machine setting for the parameters D

and N.  This is mainly due to the overlapping of the  pores and the

relaxation of the fabric caused  by the punching process.



Determination of q1 and i\  For The A.ctual Filter
                   1      Lt


For a needle punched filter aj  is defined as the folume of fibers

perpendicular to the  flow per unit volume of the filter.  If the solid

fraction of the filter  is a,  A. is  the surface  area and  AL is the filter

thickness.

                                     2   *
                       q. A • AL - 'rra  N n A. • AL

                a
                  i             A •  AL
                              2    *
                 a   =  a - TTa  n N                                (45)
Similarly
—• — •                      2
                 a   =  TT a  n N
                  LJ
                                27

-------
 Prediction of The Pressure Drop

                                                                  >!<
 Based on the knowledge of. the actual parameters (Appendix C) a,  D , a.,
 n,  N  and AL, the value  of a|. Ctp ai and 02 can be calculated using
 equations (43), (44), (45), and *(46) respectively.   The Darcy's coefficients
                    *                                              >'<
 k,  and k? can be evaluated using Figures 4 and 5 for the values of aj
 and aX determined  previously.  Using equations (30) and  (34) which are
 presented graphically in Figures 6 and 7,  the corresponding values of
 cpi  and cp? can be obtained.  The nondimensional pressure drop can then
 be  determined using the  values of rpj, cp£,  04 and 0,3 in equation (37).

 4.2 Collection Efficiency

 The mechanisms  of collection that play an important role in filtration
 are the inertial, interception and diffusional.  Although a wealth of
 information  for the collection efficiency for each of the individual
 mechanisms is available, no satisfactory theoretical work exists that
 can generally quantify the share  carried by these mechanisms when
 they act collectively. This is due to the; fact that while most of the
 mechanisms are mainly  affected by such variables as flow rate, size
 and density of dust,  the filter characteristics and the properties of
 the medium,  their effectiveness  vary from one set of conditions to the
 other.

 There are numerous theories for particulate collection developed for
 fibers or fibrous mats which could be applied to most nonwoven
 fabrics.  In  the case of needle punched fabrics,  because of their unique
 structures,  these theories do not apply.  This is due to the in depth
 fiber orientation in the needle punched fabrics and the nonhomogcnity of
 their packing density.  Because of this it was felt necessary to  examine
 the role of the various  mechanisms of collection for needle punched
 fabrics.  To this effect,  the  semi-empirical  approach developed by
 Dorman [27] and used by Jonas et_ al_. [28] and Hampl and Rimberg [29],
to determine the collection efficiency -was  found to be best suited for
this investigation.

Dorman developed the following formula, which allows for the relative
contribution  of inertial, diffusion and interception mechanisms of
collection for particle size of 0. 3 microns:

      log P% = 2 -  (k   ALV2 + k  ALV~    + k AL)               (47)
                      R          D             1
                               28

-------
  )           0.08          0.16
           SOLID   FRACTION  0^
Figure  6.  Drag  force vs. solid  fraction
                         0.24
29

-------
   0.6
   0.5
 CVJ
   0.4
LJ
O


O  0.3
U.
   0.2
   O.I
             J	I
  0            0.08         0.16

             SOLID FRACTION

Figure  7.  Drag  force  vs. solid   fraction
                                             0.24
                    30

-------
where P% =  percentage penetration
       A I.'  =  filter thickness in cm
        V  =  face velocity in cm/sec.
       kj^ =  inertial impaction parameter in cm.~  sec
       kp =  diffusion parameter in cm!"
       k   =  interception parameter in

Jonas e_^ a]_.  [38] modified Dorman's equation by using a discrete velocity
for maximum aerosol penetration.  When penetration is maximized,
i_. e_. , differentiating equation (47) w.r.t. V and equating to zero:
                                         Q I -I
                           kD  =  3kRVm                         <48'

where V    = the velocity at maximum penetration.  Substituting for
        m
k   in equation (47) by its value in equation (48) gives:
                              2     S / 3   2/3
      2 - log P% =  k_ AL (V  + 3V    V     ) - kAL             (49)
                      K.             m             1
                              /*     A / ^    ? / -^
Plotting 2 -  log  P% against (V  + 3V     V"    ) yields a straight line
whose intercept gives the value of k and its slope  gives the value of k  .

For the purpsoe of this investigation, not only the  effect of variation in
velocity on penetration was investigated, but also the effect of particle
size.  This  sheds  more light on the role of the mechanisms of collection
involved and helps in the development of needle punched fabrics for
filtration. This is of value in determining the contribution of the fabric
structure in hag filtration,  in spite  of the fact that in bag filtration the
dust cake plays  a great role.
                               31

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                         SECTION V

                   FABRIC PARAMETERS
The important parameters affecting needle punched fabric properties
are:  Fiber properties,  fiber  orientation, needling intensity, direction
of needling,  needle  size, needle penetration,  needle  shape,  number of
barbs on the needle, the number of passages through the needling
process and fabric finishing.  For the purpose of this study, the
needling intensity was taken as the  independent variable and the effects
of changing most of the  other parameters is presented.

5. 1  Fibers Used
The fibers used for this investigation are Dacron  polyester.  Polyester
fibers gained general acceptance in filtration due to their excellent
abrasion and dry heat resistance and low cost.  In addition to these
properties, it has good resistance to mineral acids and alkalies.  Many
types of polyester fibers are available but the major difference between
these types that can be of significance in  filtration, as far as nonwoven
fabrics are concerned, is the specific gravity.  .Specific gravities of
1.22 and 1. 38 are available.  This means that fibers of the two types
having the  same denier cio not have the same diameter.  Most of the
work was done using 1. 5 inch staple x 3 denier fibers.  Experiments to
investigate  fiber length were carried out  and the results will be dis-
cussed.  Since it is well known that the finer the fibers the better  will
be the  filtration perforrmince, attempts were made to proHncn  fabrics
from 1. 5 denier fibois.  Difficulties were encountered due to fiber
damage and spreading which produced very weak fabrics.  For this
reason only the  3 denici results are presented.  All fibers used had
round c ross- section.
                              32

-------
 5.2  Fiber Orientation

 Since fiber orientation is known to be an important parameter in affecting
 the properties of nonwoven fabrics, two methods of producing fibrous
 batting  have been used, these are:  (a) Random-Laid; produced by air-
 laying of fibers .on a Rando-feeder Rando-webber.  To obtain the required
 weight a number of layers was  used,  (b)  Cross-Lapped; produced by
 laying a number of layers of a cross-lapped web superimposed over
 each other in the same direction.  Because of weight requirements, the
 angle of cross-lapping was 84 degrees to the direction of carding.

 5. 3  Needle Punching

 Needle  punching was carried  out on a James Hunter experimental fiber
 locker.  A wide range  of needling intensity was obtained by using different
 fabric feeds varying between  1/16 and 3/8 inch per stroke.  The fabrics
 were passed through the machines more than once, but in most cases the
 number of passages was kept down to two.  The range of needling intensity
 used was from 122 to  735 punches/inch  .  Most fabrics were produced
 by needling the total number of layers without prepunching, later
 experiments were carried out in which prepunching the web layers was
 done.  Three  needle sizes were used, large (20 gauge), medium (25
 gauge) and small (32 gauge).  All needles were 3. 5 inches long and had a
 triangular blade  with 9 barbs spaced 0.083 inch apart.  At the
 beginning,  needle penetration was kept constant at one barb going
 through the  fabric.  The effect  of needle penetration was later studied
 by increasing the penetration one barb for every experiment up to 4
 barbs penetration.  The number of needles used in the board  was kept
 at 575 and the machine speed was 185 strokes/minute.

 5. 5  Fabrics Produced

 Fabrics without scrim or support were first studied and then various
 types of scrim fabrics were used to provide strength  and dimensional
 stability.

 5. 5  Fabric Finishing

 Fabric finishing is a very important parameter in affecting the properties
 of a  fabric.  Shrinking,  calendering, resin and heat treatments, etc. ,
are possible finishing  techniques which can improve the performance of
needle punched fabric  in filtration.
                              33

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                         SECTION VI

   FABRIC CHARACTERISTICS AND THEIR METHODS OF
                          TESTING
Filter fabrics have to satisfy two basic  requirements; to have high
collection efficiency at low pressure drop and to endure the mech-
anical strains experienced in use.  The physical properties investi-
gated include fabric thickness,  weight,  density, air permeability,
bursting and tensile strengths.  Fabric  resistance to  the air flow for
clean filters was  measured for different rates of flow.  Pressure
drop and collection efficiency using flyash contaminated air were
measured.  Efficiency measurements using homogeneous size aerosol
were  also carried out.  The following represents the  relevant fabric
characteristics  and their methods of testing.

6. 1  Fabric Thickness

The thickness measurements were made using a compressomete r and
according to A.STM Standard Method D 1777.  The presser foot was
7 3/32 inches  in diameter and the pressure  applied was 0.005 psi (which
is the lower limit recommended for nonwovens by the ASTM standard).
The instrument  precision was ±0.001 inch.  Five measurements were
                        f-
made and the average was calculated.

6. Z  Fabric Weight

The measurements were made for each fabric according to the ASTM
Standard Method D 1910 and the average weight in oz. /yd? was
calculated.
                              34

-------
6. 3  Fabric Density

From the measurement of thickness and weight per unit area., the
density of the fabrics in g/cm3. was calculated.  Since packing density,
defined as the ratio between the volume of fibers to the volume of filter,
is normally used to characterize filter fabrics, the values of the packing
density were also calculated.

6.4  Air Permeability

The  air permeability was measured according to the ASTM Standard
Method D 737 using a Frazier Apparatus.  The pressure drop was
maintained constant  at 0. 5 inches of water.  Five measurements were
made and the average was calculated.  For most textile applications
the aii- permeability is measured in terms of the volume of air passing
through the fabric per minute and per  unit of area of fabric.  In this
case the fabric thickness is not considered important.  In the case  of
nonwovens, such as  used in this investigation, the fabric thickness
varies considerably  and had to be considered.   Therefore the values
of the modified permeability (air permeability-thickness product)
were calculated.

6. 5  Bursting Strength

Bursting  strength is an important property for filter fabrics.  The
bursting strength was measured on the Scott Tester with a bursting
attachment,  according to ASTM Standard Method D 231.  The diameter
of the hole was 1. 75 inch and the ball was  1 inch in diameter.  Ten
measurements were made and the average was calculated.

Since the  fabric weight was a variable, the bursting strength was cal-
culated by dividing the bursting load by the weight per unit area.
Only bursting strength rather  tha,n tensile  strength is considered
since it closely relates to the  deformations experienced in actual
applications.
6.7  Pressure Drop and Efficiency

To test the filters for pressure drop and collection efficiency using
flyash,  an apparatus has been designed and constructed.  It consists.
as shown in Figures  8 and 9 of an air duct, a dust feeder, a testing
section and a  temperature and humidity control system.  Efficiency
                              35

-------
oo
                    1. AIR  COOLER
                   2. ABSOLUTE  FILTER
                   3. SETTLING  CHAMBER
                   4. TEST FILTER
                   5. HIGH  EFFICIENCY FILTER
6. SURGE  TANKS
7. BLOWER
8. ROTAMETER
9. DUST FEEDER
10. HUMIDIFIER
                           Figure  8.  Apparatus

-------
Figure 9.  Centralized Controls
           Flyash Apparatus
             37

-------
 measurements using homogeneous size solid particulates were also
 carried out.  For this purpose another apparatus consisting of an
 aerosol generator, test section and photometer was used.

 6.7. L  Flyash Apparatus

 6. 7. 1. 1  Air  Duct and Testing Section

 The air duct is made of aluminum piping 4. 5 inch inside diameter.  An
 eight inch  diameter section is provided at the  upstream end of the
 duct to house  an  absolute prefilter to rid the incoming air from
 particulates larger than 0. 3 micron.  A 40 inch long calming section
 is provided before the test filter to insure uniform streamline flow.
 The air flow through the test section is governed by means of a gate
 valve and a by-pass valve located at the downstream end of the duct.

 The test filter is mounted in a filter holder,  as shown in Figure 10. It
 is designed with  adequate  sealing features to prevent any possible
 leakage  either from the atmosphere to the test section  or from
 upstream to downstream around the filter.  A. periphery blower having
 a capacity of  98. 8 CFM is used to induce  the flow of air through the
 duct.  The air flow is measured by means of three rotameters  manu-
 factured by Brooks to cover the range from 0.22 to 22 CFM with
 precision of ±1%. A micromanometer manufactured by Meriam and
 having a range of 0 - 10 inches of water with precision ± . 001 inch
 reading is used to measure the pressure drop across the filter. Two
 surge tanks in series are placed between  the rotametcr and the blower
 to eliminate fluctuation in the flow readings.

 6.7. 1.2  The  Dust Feeder

 The dust used for the collection efficiency tests was flyash having a
 classification as  shown in Figure 11.  A. modified version of the dust
 feeder previously used by Lockheed-Georgia Company [30] has been
 adopted.   A. complete assembly of the feeder is shown in Figure 12 and
 13.  It consists of a conical dust hopper and a mixing chamber.  The
 dust is fed from  the hopper into the chamber by an auger placed
 centrally in a pipe extended from the bottom, of the hopper.  The rate
 of dust feeding is controlled by adjusting the speed of rotation of the
 auger.

 A pressurized, housing around the dust pipe at the exit of the dust hopper
is  designed with  tangential air passages as shown in detail A of
                               38

-------
                        GASKET
Figure 10.  Rlter  holder
            39

-------
o
    0
        I I 1
                                      Illl III! III! IIIU I III
                      iiumiiiii 11  i i  1111
Hill
       0.01 0.1      I
10  20   40  60   80     95    99   99.9  9999
        PERCENT
        Figure  II.   Cumulative  particle  size  mass  distribution

-------
                SPEED
               REDUC
ER   MOTOR
  \M   IXXX
 DUST
AND AIR\\
   \
                AUGER
                 SECONDARY
                     AIR
 MIXING
 BLADES
          DETAIL (A)
                  MIXING
                  CHAMBER
                  XXXXXN
             3 I PRIMARY AIR
 \XVx\X\A\\xxxx\\\\H
  Figure 12. The  dust feeder
               41

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Figure 13.  Dust feeder
           42

-------
Figure 12. This generates a swirl action which helps in breaking up any
coagulated dust.   Mixing blades are also used to this effect in the
mixing chamber.  Means to control the swirl is also instrumented in
the design by controlling the air feed to the pressurized housing.  The
concentration and size of the particulates leaving the feeder is
regulated by mixing it with controllable bled air in a vertical pipe
(8 inch inside diameter and 6 feet long) before entering the main duct.

6. 7. 1. 3  Temperature and Humidity Control System

In order to provide for specified temperature and relative humidity
condition of the flowing stream, a temperature and  relative humidity
control system was designed and constructed as shown in Figure 14.

Air enters through an air cooling unit (1) where the temperature and
humidity of the air stream are reduced from that of room conditions.
An electric heater (2) is provided to raise the temperature of  the air
if required,  while a humidifer (5) is provided to increase the relative
humidity to the desired value.  The humidifer is a steam generator
working  under atmospheric conditions.  The level of the water is
controlled by a float system (6).  For safety purposes a low level
alarm is also included whenever the float valve fails.  The heating
element  is regulated by means of an electric  control system which
uses a humidity grid  sensor, placed in the air stream, to sense its
relative  humidity.

The  relative humidity of the system is controllable  in the range of 30 -
99% at 75°F.

6.7.2  Latex Aerosol Apparatus

The apparatus consists of an aerosol generator, a test section, and a
photometer as shown  in Figure 15.
                                            /

The aerosol generator is a Fluid Atomization Generator  Model 7300 .
It utilizes air-blast atomization and inertia impaction to produce a
monodisperse  aerosol.  It produces aerosol at rates of up to 1()9
particles/second with size range of 0.03 to 3.0 micron.

The test section consists of a  main duct  15/8 inch inside diameter
pipe at the end of which a filter holder is attached.
'Manufactured by Environmental Research Corporation, St. Paul, Minn.


                               43

-------
I. AIR CONDITIONER B AIR INTAKE    6. STEAM GENERATOR WATER
2. HEATING  ELEMENT
3. VAPORIZOR  INLET &  OUTLET
4. HUMIDITY SENSING ELEMENT
5. STEAM  GENERATOR
  LEVEL CONTROL 8 ALARM
7. HEATING  ELEMENT  CONTROLLER
8. HUMIDITY CONTROLLER
                         (	I
           Figure 14.  Temperature and humidity  control  system

-------
    COMPRESSED
    AIR
(45 psig,min)
          ATM.
                            AEROSOL  GENERATOR

                                           ABSOLUTE
                        135 psigj
 AIR
DRER
     BALL
     FLOWMETER
        A
                                              DILUTION
                                                AIR
                                               COLLISION
                                                  AIR
                    LATEX
                  EXTENSION
    BALL
 FLOWMETER!
    B
                         TEST
                         FILTER
                        HOLDER
            MAIN DUCT
              THREE-WAY
                VALVE
            1111 n
            CLIMET
          PHOTOMETER
                                                        ATM.
 FLOW METER  2
Figure  15.  Aerosol  penetration  testing  equipment
                               45

-------
 The photometer is a Climet CI-250 portable particle counter than can
 measure concentrations up to 10& particles/ft,  and particle sizes
 larger than 0. 5 micron.

 6. 7. 3  Test Procedure

 Experiments were made without dust to  determine the effects of flow
 rate,  needling  intensity and the various -fabric parameters  on the
 pressure drop  of the filters.  The  temperature and relative humidity
 of the air were kept at 75°F and 50% throughout these experiments and
 the flow rate was varied up to 200  feet/minute.

 During the  flyash experiments only one filtration cycle (starting with a
 clena filter) was used  and the air velocity was maintained constant at
 45 feet/minute. The efficiency of  collection was determined by
 measuring  the weight of flyash collected on the test  filter and the weight
 of flyash penetrated during the test.  Accordingly

 „ „   ,.    f,.  .                       weight of flyash collected
 Collection efficiency  =	—;	**-	—•*	
                        weight of flyash collected + weight of flyash penetrated

 The weight of the flyash collected was  obtained by weighing the test
 filter before  and after  the test.   The weight of the flyash penetrated was
 determined by passing the air downstream through a high efficiency
 filter capable of capturing particulates down to 0.3 micron with 100
 percent efficiency.  The difference in weight of this  filter before and
 after the test gave the  weight of flyash penetrated through the test filter.
 The weighing was done on a sensitive balance with precision of _+ 3 x lO^g.
 The test duration was  kept at 10 minutes and the pressure drop was
 recorded each minute.

 In the aerosol experiments air is supplied to the aerosol generator
 (Figure 15) at 100 psig and is reduced  to 35 psig by  valve (1).  After
 passing through a desicant air drier and an absolute filter,  the air
 flow branches into collision air and dilution air.   The collision air is
 always set at 9 liters/minute and the flow rate is  indicated on flow-
 meter (B).  The collision air passes through the atomizer and collides
with the latex suspension  which is  introduced  into its path.   The
 dilution  air is regulated by valve (5) to 67 liters/minute as indicated by
flowmeter (A).   This flow rate is sufficient to evaporate  the moisture
droplets in  the aerosol mixture. The two branches  then  combine and
form a mixture of air and latex particles.  The  aerosol then passes
through the ionizer where  charges  on the particles are neutralized.
                               46

-------
The aerosol is then passed through the main duct and the flow rate is
varied by using valves (2) and (4).  Any excess aerosol mixture is
exhausted and thus the aerosol generator remains at a steady rate
conditions while the system demand varies.  The particles are counted
by the Climet photometer before  and after the test filter by means of a
3-way valve (3) for various  velocities.  For low  velocities valve (2)  is
closed and the flow is controlled  solely by valve  (4) and the flow rate
is measured by flowmeter (2). For.high velocities  (above 8.  75 cm/sec)
valve (2) is adjusted to allow a fixed flowrate to pass as indicated by
flowmeter (1).  Additional flow passes through the photometer and is
controlled by valve (4) and measured with flow meter (2).  The filter
face velocity is found directly from the total flow from flowmeters (1)
and (2).
                               47

-------
                         SECTION VII

                 RESULTS AND DISCUSSIONS
In designing a filter fabric two major performance qualities have to be
considered.   First, the fabric performance as a filter in terms of
collection efficiency and pressure, drop.   Second, the fabric has to
possess certain mechanical properties to endure the stresses applied
during filtration and cleaning cycles.  The effects of the main fabric
parameters  on the properties  of needle punched fabrics and their
filtration performance are presented.

7. 1  Effect of Fiber Orientation

Fabrics were produced using random-laid and cross-lapped webs having
approximately the  same weight per unit area.  The various fabric
properties were measured according to the previously mentioned
procedure.  Fabric thickness  and weight per unit area values are given
in Tables A.-1 and  A-2. In general, there is a considerable reduction
in thickness and weight by increasing needling intensity.  Random-laid
fabrics had slightly lower thickness than cross-lapped fabrics.  This is
due to the low thickness of the original random-laid web caused by the
air pressure on the fibers.  Another important factor is the fiber
spreading which normally takes place during needling.

Figures  16 to 18 show the effect of fiber  orientation and needling
intensity for the three needle sizes used.   There were little differences
between the density values for the two  orientations, with the random-
laid slightly higher than the cross-lapped.  The density was increased
at a diminishing rate with needling  intensity,  as  expected.   Table A-3
gives  the  corresponding values for  the packing density.
                               48

-------
     5r
CM
I
o

^
10
 3  3
 co
 §
 u
    0
      0
A  RANDOM-LAID

O  CROSS  LAPPED
 200          400          600

 NEEDLING  INTENSITY (punches/inch2)
800
       Figure  16.  Fabric  density  vs.  needling  intensity (20 gauge)

-------
Ul
o
       OJ
        I
        O
        i  4
       to

        I
        S  3
CO
1
o
S
IS
           0
             0
                                           A RANDOM-LAID
                                           O CROSS  LAPPED
                   200          400          600
                   NEEDLING  INTENSITY (punches/inch2)
800
            Figure  17.  Fabric density  vs. needling  intensity (25 gauge)

-------
CM


 O
to
 e

 o»  3
 »^»


 m
     2
     o
      o
A  RANDOM-LAID

O  CROSS LAPPED
   200          400          600

    NEEDLING  INTENSITY (punches/inch2)
800
    Figure  18.  Fabric density vs. needling intensity (32  gauge)

-------
 Figures 19 to 21 give the air permeability results for the two fiber
 orientations.  It can be seen that, the air permeability relationship with
 needling intensity  shows a minimum.  At low needling intensities., the
 effect of increasing the packing density  dominates leading to the
 decrease; in air pcrmeability.  At high needling intensities, however,
 the effect  of punching is more pronounced and thus the increase in air
 permeability. It can also be seen that the effect of fiber orientation
 was not significant.   Figures 22 to 24 show the relationship between
 the modified  air permeability and needling intensity for  the two fiber
 orientations.  The effect of needling intensity is to reduce the air
 permeability-thickness product, even at high needling intensities,
 which is in agreement with the increase in fabric density.  This shows
 that the air permeability-thickness product is a more accurate
 representation for the resistance of the  needled fabric to the air flow
 than air permeability alone.

 Bursting strength  results are given in Figures 25 to 27.  It is clear  that
 bursting strength is  increased with needling intensity in a similar
 manner to the fabric density.

 The pressure drop was measured using  clean air at different flow rates
 and the results are given in Appendix B.  It was found that there were
 little differences in the values of the pressure  drop per  unit thickness
 at low levels  of flow rate to make any conclusions related to changes in
 fabric structure.   The results for flow rate of 90 feet/minute are
 presented  for the two fiber orientations  in Figures 28 to 30.  The
 pressure drop per unit thickness increases with needling intensity with
 a diminishing rate.  This is again similar to the change in fabric
 density with needling intensity.   It can be. seen that the effect of fiber
 orientation is very small.

 Filtration  results for the fabrics produced with the two  orientations
 and the throe- no cello sizes are given in Table  1.  During the experiments,
 considerable  variation in inlet concentration  was found to occur.
 This was duo lo the1 extreme difficulty experienced in controlling the rate
 of dust feed from the hopper to the mixing chamber.   For this reason
 the efficiency and pressure drop results are  accompanied by the
 concentration  values.  It can be  seen that the efficiency  and pressure
 drop changed  over a narrow range; however, on the average the random-
 laid fabrics gave higher  efficiency than  the cross-lapped fabrics.

 From the foregoing discussion it was concluded that little differences
existed between the fabrics produced with the two fiber  orientations.
In addition to  the slight advantages mentioned for the random-laid


                               52

-------
in
OJ
            300r
            200
             100
              0
                0
                                              A  RANDOM-LAID
                                              0  CROSS  LAPPED
J	I	I	I
      200         400          600         800

      NEEDLING  INTENSITY (punches/inch2)
               Figure 19.  Air  permeability vs. needling  intensity (20 gauge)

-------
4-
             300
            200
             100
         LX.


         tf
              0
                0
                    A  RANDOM-LAID
                    O  CROSS  LAPPED
  I	I
J	I
200         400         600         800
NEEDLING  INTENSITY (punches/inch2)
               Figure 20.  Air permeability vs.  needling  intensity (25 gauge)

-------
           300r
       CM
ai
' V
 £


 H
        LU
           200
            100
                                                              O
                                  A  RANCX)M-LAD

                                  o  CROSS  LAPPED
               0           200         400          600

                          NEEDLING  INTENSITY (punches/inch2)

              Figure 21.  Air permeability  vs.  needling intensity (32 gauge)
                                                          800

-------
    24
    16
    8
    0
      0
                                 A  RANDOM-LAD
                                 O  CROSS  LAPPED
 200          400          600
NEEDLING  INTENSITY (punches/inch )
800
Figure 22.  Air permeability-thickness  product  vs. needling  intensity (20 gauge)

-------
     24r
 *-
I
 fcr
     16
      8
      0
       0
                                  A  RANDOM-LAID

                                  O  CROSS LAPPED
800
                    200         400         600


 §               NEEDLING INTENSITY (punches/inch2)


Figure 23.  Air permeability-thickness product vs.  needling intensity (25 gauge)

-------
Ul
oo
            24
            16
             8
            0
              0
                                          A  RANDOM-LAID
                                          O  CROSS LAPPED
 200         400          600

NEEDLING INTENSITY (punches/inch2)
800
        Figure 24  Air permeability-thickness product vs.  needing intensity (32 gauge)

-------
    24
•&
*
     16
     8
     0
       0
             A RANDOM-LAD

             O CROSS  LAPPED
                                                   -O
200        400         600        800

NEEDLING  INTENSITY (punches/inch2)
      Figure  25.  Bursting  strength  vs. needling  intensity (20 gouge)

-------
24
16
 8
 0
             A RANDOM-LAID
             o CROSS  LAPPED
  0          200        400         600        800
             NEEDLING  INTENSITY (punches/inch2)
 Figure 26.  Bursting  strength  vs.  needling  intensity (25 gouge)

-------
5f
    24r
     8
     0
                       A RANDOM-LAID
                       O CROSS LAPPED
                                                      800
             200         400         600
            NEEDLING  INTENSITY (punches/inch2)

Rgure 27  Bursting strength vs. needling intensity (32 gauge)

-------
     1.0
-  0.8
 _CM 0.6
 o
0.4
    0.2
      0
       0
                  A RANDOM - LAID
                  O CROSS  LAPPED
                200          400          600
                NEEDLING  INTENSITY (punches/Inch )
800
    Figure 28.  Pressure  drop per unit  thickness  vs.  needling  intensity
                 (air  velocity 90  ft/min) (20 gauge)

-------
     I.Or


 2  0.8

 O
 ^ 0.6
 to

 J  0.4
Q.U
    0.2
  0
    0
                                   A  RANDOM-LAID
                                   0  CROSS LAPPED
                    200          400           600
                    NEEDLING  INTENSITY (punches/inch2)
800
Figure 29.  Pressure drop per unit  thickness vs.  needling  intensity
               (air velocity 90 ft/min) (25 gauge)

-------
      1,0
 _   0.8
 -5
 .£
 Ss.   0.6
     0.4
     02 -
o
       0
        0
    A  RANDOM-LAID
    O  CROSS  LAPPED
             O
200           400           600
NEEDLING  INTENSITY (punches/inch2)
800
    Figure  30.  Pressure  drop  per unit  thickness  vs. needling  intensity
               (air velocity  90  ft/min)(32 gauge)

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      Table  1.  BATCH FILTRATION RESULTS
                (Effects of fiber orientation and needle size)
Needling
Fiber Needle Intensity i
Orientation Size 2 gr./ft?
Punches /in.


20
t-» \/
gauge


25
Random-
Laid gauge


32
•~J 4^
gauge


*\ ^\
20
gauge



25
Cross- „„„«<»
T , gauge
Lapped



32
gauge


122
245
368
490
735
122
245
368
490
735
122
245
368
490
735
122
245
368
490
735
122
245
368
490
735
122
245
368
490
735

1.42
2.39
1.62
2.46
1,87
2.07
1.54
1.69
1.79
2.00
1.77
2.06
2.06
1.28
1.92
2.39
2.40
1.69
1.72
1.89
1.39
1.82
1.69
1.80
1.76
1.72
2. 10
1.77
1.62
1.85
AP
c
Inc he s
H_0
2
0. 085
0.080
0.081
0.080
0,080
0. 120
0. 105
0.090
0.080
0,070
0. 100
0. 100
0.090
0. 110
0,090
0.080
0.095
0.060
0.070
0.068
0.095
0,075
0.075
0.090
0,085
0.105
0.090
0.070
0.090
0.080
AP£
Inches
H00
2
0.665
0.890
0.900
0.940
0.880
0.755
0.705
0.800
0.740
0.795
0.615
0.880
0.925
0.645
0,865
0.835
0.850
0.590
0.600
0.610
0.815
0.785
0.800
0.830
0.810
0.850
1.070
0.890
0.825
0.790
Efficiency
%

97. 18
97.66
97.59
97.68
97.33
93.60
98.48
97.23
97.93
98.00
97.91
98.25
98.45
97.42
97.81
97.91
97.92
96.69
97.44
97.30
96.98
97.80
97.99
98. 17
97.90 ,
98.14
97.95
97.74
98.15
97.95
Air Velocity  = 45 ft. /min.
/\p  -  Pressure drop at the end of 10 minute duration tests.
AP  -  Pressure drop for clean filter
 C°  -  Inlet concentration
Needle penetration  -  0.25 inches

                            65

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 fabric::, high production speeds in manufacturing the webs led to the
 decision in favor of using random-laid webs throughout the rest of this
 investigation.

 7.2 Effect of Needle Size

 In general, needle size  is an important parameter  affecting the properties
 of needled fabrics.       Large needles normally reorient  a  large
 number of fibers into the pores, however, the larger the needle the
 more fiber disruption and fiber damage take place.  In this investigation,
 it was necessary, to study the  effect of needle size on the performance
 of needled fabrics in filtration.

 Figure 31  shows the effect of needle size on fabric density.   The 25
 gauge  needle gave the highest fabric density.   It may be expected that
 the 20 gauge needle should give the highest density, but because of
 fiber disruption which occurs during needle withdrawal from  the fabric   '
 the packing is reduced.

 The effect of needle size on air permeability is presented in  Figures  32
 and 33.  The air permeability-thickness product shows clearly the effect
 of needle size.  The fabrics made by the 25 gauge  needles gave the
 lowest values which is in good  agreement with the  density results.

 Figure 34  shows the effect of needle size on the fabric bursting strength
 and again the results agree with the explanation mentioned earlier for
 the fiber damage caused by the 20 gauge needle. It is. clear that the
 25 gauge needle gave the highest fabric strength.

 The pressure drop per unit thickness results  shown in  Figure 35 also
 show that the 25 gauge needle gave  the highest pressure drop per unit
 thickness as a result of the high packing density.  However,  the flyash
 test results given in Table 1 indicate that the  pressure drop (AP ) for
 the fabrics made by the  25 gauge needles did not differ appreciably
 from those of the other  fabrics.  The efficiency values show  a slight
 advantage  for the 25 gauge fabrics.  Based on these results the decision
 was made to eliminate the  20 gauge needle and to limit the range of
 needling intensity to 122-490 punches /inch  .  During the study of the
 other parameters the 25 gauge  needle was mostly used.

 7. 3  Effect of Fiber Length
To study the effects of fiber length on the performance of needle
                                                                 •7.
                                                                  2 J
punched fabrics, Dacron® type 54, 3 denier fiber of length 1-|, 2, 2,\
                              66

-------
    5r
CM
l

o
    3
 c/5
 o
    0
020 GAUGE


A 25 GAUGE


Q 32 GAUGE
     0
        200          400          600


        NEEDLING INTENSITY  (punches/inch2)
800
 Figure  31. Fabric  density  vs. needling  intensity (random-laid)

-------
o
00
            300
       CM
            200
            100
                      I	I
O  20  GAUGE

A  25  GAUGE

0  32  GAUGE
J	I
               0           200        400          600

                          NEEDLING  INTENSITY (punches/inch2)

          Figure 32.  Air  permeability vs.  needling intensity (random-laid)
                   800

-------
   24r
    16
     8
     0
                                         O  20 GAUGE
                                         A  25 GAUGE
                                         ti  32 GAUGE
      0
200
400
600
                  NEEDLING  INTENSITY (punches/inch2)
800
Figure 33.  Air permeability-thickness  product  vs.  needling  intensity (random-laid)

-------
24
•R

g
16
 8
 0
            O  20  GAUGE
            *  25  GAUGE
            Q  32  GAUGE
                                                  800
                 200         400        600
                NEEDLING  INTENSITY (punches/Inch2)
   Figure 34.  Bursting  strength  vs. needling intensity (random -laid)

-------
     1.0 r
-r  0.8
    0.6

 £  0.4
    0.2
      0
       0
O  20 GAUGE
A  25 GAUGE
Q  32 GAUGE
       200          400          600
        NEEDLING  INTENSITY (punches/inch2)
800
     Figure 35.   Pressure drop per  unit  thickness  vs.  needling  intensity
                 (air velocity  90  ft/min) (random-laid)

-------
 and 3 inches, were used.   The results are presented in Figures 36 to
 40.  Generally there was no significant change in all the fabric properties
 due to fiber length.  The reason for the high bursting strength shown in
 Figure 40 for the 3-inch fabric is the fact that the sample size is 2^ -
 inch in diameter.  This means that the structure  does not play any role
 and the strength is derived from the long fibers in the sample.

 The results of the batch filtration experiments also showed no appreciable
 effect for fiber length on the pressure drop or efficiency of collection as
 shown in Table 2.  Accordingly,  the l^r-inch fiber  was  chosen for its
 easy handling on the Rando-Feeder Rando-Webber Machine.

 7. 4 Effect of Needle Penetration
 To study the effect of needle penetration it is useful to present the  defi-
 nition of penetration [31]. Figure 41 shows  a schematic drawing for the
 arrangement of needles and the guide plates.  The penetration is given
 by the length of the  needle protruding below the top surface of the
 bottom guide plate.   During this investigation four levels of needle
 penetration were used ranging from 0.25 to 0. 5 inches with a step  of
 one barb penetration of 0. 083 inch, in batch testing.

 Table 3 gives the results of fabric packing density for different needle
 penetrations.   The packing density was increased with the  increase in
 needle  penetration as expected.  The air  permeability-thickness  product
 values  are given in  Table 4.  This shows that the air  permeability-
 thickness product is reduced with increasing the needle penetration due
 to the increase in packing density.  Table 5 gives the results of the
 fabric ball bursting  strength which increases with penetration up to a
 certain level.  At high needle penetration the bursting strength
 decreased indicating an increased level of fiber damage.   Table  6 gives
 the filtration results for the various needle penetrations and on average
 a 0. 333 inch   penetration (p2) gives the  best results for  efficiency
 andAPf-  In making the comparison,  it is clear that  penetrations PI
 and p^ had disadvantages; fabrics with p, had low packing  density and
bursting strength, whereas,  those with p . had high packing density and
low bursting strength.   The decision was to  use penetration p • since
this setting resulted in a low level of needle damage  during punching.
Therefore  a]l the fabrics presented in the remainder  of this report
were  produced with  needle penetration setting p   (0.333 inch).
                               72

-------

12

1
CO
ffl 8
o
X
o
a:
CO
i2 4

n
O
$
O
A
6
13 0
o
6
8

0 iHa11 FIBER
Q 2 " FIBER
A 2'^ "FIBER
O 3 " FIBER
i i i i i
     0             200           400
        NEEDLING   INTENSITY (punches/inch2)
Figure  36. Fabric  thickness vs. needling intensity
           effect  of fiber  length
                 73

-------
    I2r
(M
 5.  8
 UJ

 o
 §5  4
 iS
     0
      o
                         o

                         A
                         O
O lite" FIBER
D 2" FIBER
A 2te" FIBER
O 3" FIBER
     200
400
         NEEDLING  INTENSITY (punches/inch2)
 Figure 37.  Fabric  weight  vs.  needling  intensity
            effect  of fiber  length
                 74

-------
Q
-2.  4
 to
  5  3
                                    o
                                    A
                            O     " FIBER
                            Q  2" RBER
                            A  2te" FBER
                            O  3" FIBER
       0       100    200    300     400    500
            NEEDLING INTENSITY (punches/inch2)

Figure 38.  Fabric  density  vs. needling  intensity
           effect  of  fiber  length
                   75

-------
 E

CM1
    10
 v>   8
 CO
 LJ
 o
=!  4
GD  H
<
LJ



1  2
CL  t-

tr
    0
                    G


                    O
O  11/211 FIBER

A  2" FIBER

0  2 Ifc" FIBER

   3" FIBER
                            B
      0      100    200    300    400    500


         NEEDLING  INTENSITY (punches/inch2)


Figure 39. Air permeability-thickness product vs.

           needling intensity

          (effect  of fiber length)
                  76

-------
T  12
*
Kr
     8
           D
           O
4 '
                     a
                     A
                     o
                                   0
                    O
                O
                                   o
                          o        e
                           O  I  " FIBER
                           CD 2 "FIBER
                           A 2^" FIBER
                           O 3 " RBER
       0      100    200    300    400    500
          NEEDLING  INTENSITY (punches/inch2)
 Figure 40.  Fabric  bursting strength  vs. needling
            intensity
           (effect of fiber  length)
                  77

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                   Table 2.  BATCH FILTRATION PERFORMANCE

                                   (Effect of Fiber Length)*
Needling
Fiber Intensity
Length , ..2
Punches /in.
122
IM 245
2 368
490
122
2"
368
490
122
?i,, 245
2 368
490
122
•an 245
368
490
c
gr. /ft.
2.45
2.25
2.00
2.38
2.03
1.89
2.27
1.81
2.02
1. 80
2. 16
1.83
1.76
1.74
2.04
1.66
AP
Inches
H2°
0. 100
0.055
0.085
0.090
0. 120
0. 115
0.090
0.088
0. 110
0. 100
0.090
0. 100
0. 105
.095
.110
.090
APf
Inche s
H2°
0.96
1.18
0.73
0.85
1.05
0.94
1.01
1.05
.98
1.06
0.92
0. 93
0.84
0. 74
1.04
1.79

Efficiency
97.88
97.96
98. 14
98.27
98.02
98.54
98. 17
98.83
98. 33
98.2
98.10
98.03
98.04
98.3
98.44
97.62
Air velocity  = 45 ft/min.

Needle penetration =  0. 333 inch

*
 Random laid webs
                              78

-------
 KXXXXXXl/JKXXXXX?
                         NEEDLE BOARD

                         TOP GUIDE  PLATE
BOTTOM  GUIDE
PLATE
Figure 41. Arrangement  of needle  and  plates
             79

-------
Table 3.  EFFECT OF NEEDLE  PENETRATION ON PACKING DENSITY
Needling
Intensity
Punches /in.
122
245
368
490
Packing Density %
Pl
1.4
2.0
.2.4
2.9
P2
1.9
2.4
2,8
3. .2
P3
2. 0
2.6
2.9
3.3
P4
3.0
2.9
3.4
3.5 .
       Table 4.  EFFECT OF NEEDLE PENETRATION ON AIR
              PERMEABILITY-THICKNESS PRODUCT
Needling
Intensity
Punches /in.
122
246
368
490
Air Permeability- Thickness
Product (ftf /ft? min. )
Pl
16. 5
13.0
12.4
11.4
P2
8.8
6.1
4.9
4.2
P3
7.1
5.2
5.2
4.5
P4
4.6
4.0
3.9
3.9
Random-laid,  25-gauge,  Dacron®  1-|" x 3. 0 denier
p   =  0.250", p2  =  0.333", p  =  0.416", p  =  0.500"
                         80

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    Table  5.  EFFECT OF NEEDLE PENETRATION ON BALL
                          BURSTING STRENGTH
Needling
Intensity
Punches /in.
122
245
368
490
Ball Bursting Strength
Ibs. /oz. /sq. yd.
Pl
5.0
6.8
10.0
13.4
P2 P3 P4
5.5 8.7 7.9
9.0 11.4 10.4
12.0 11.9 10.6
14.2 14.4 11.3
Random-laid, 25-gauge, Dacrort  l|" x 3. 0 denier
PI  =  0.250", p2 =  0.333", p3  = 0.416", p4  =  0.500"
                            81

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    Table 6.  BATCH FILTRATION RESULTS

              (Effect of Needle Penetration)
Needling
Needle- Intensity
Penetration _ , ,.2
Punches /in.
122
245
p 368
490
122
245
p 368
490
122
245
p 368
490
122
245
p 368
490
c
i
gr. /ft?
2.07
1.54
1.69
1.79
1.85
1.63
1.64
1.50
1. 62
1.61
1.68
1.77
1.66
1. 84
1.71
1.77
APC
Ihc he s
H2°
0. 120
0. 105
0.090
0.080
0. 140
0. 110
0. 125
0. 130
0. 140
0. 130
0. 120
0. 080
0.080
0.090
0. 120
0.080
APf
Inches
H2°
0.755
0.705
0. 800
0.740
0. 820
0.735
0.730
0.700
0.825
0.770
0. 875
0. 870
0. 730
0.835
0.785
0.870

Efficiency
rri
%
98.60
98.48
97.28
97.93
98.42
98.26
98.42
98.32
97.61
98. 32
98.30
97.67
98.23
98.34
98.49
97.67
Air velocity  = 45 ft/min.

Random-laid, 25-gauge, Dacron  1 ^" x 3.0 denier
= 0.250
= 0.333
                                =  0.416",  p  =  0.500"
                                            T:
                             82

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7. 5  Effect of Scrim Material

Needle punched fabrics without scrims were used in the earlier part of
this  investigation to study the effects of the fabric parameters without
the complications of the effect of the scrim material.   Tensile  strength
and elongation data for a series  of these fabrics are given in Tables A-4
and A.-5  respectively.  The data indicated that these fabrics had low
strength arid, high elongation .making  them dimensionally unstable.  This
is not suitable for bag filtration, since changes in the dimensions of
the bag affect  the filtration performance.  It is also doubtful that such
fabric  will stand the strain of filtration and cleaning cycles for a reason-
able lifetime, although some  commercial fabrics are made without scrims.

Woven scrim fabrics are normally used  to reinforce needle punched
fabrics in applications other  than filtration.  In these  applications the
major  goal is strength and dimensional stability.  In filtration, however,
the use of closely woven fabrics as scrim material  leads to an  increase
in the pressure drop.   If open woven scrims are  used there is very
little gain in dimensional stability.  This was supported by preliminary
experiments.

Examination of the various nonwoven structures indicated that spun-
bonded fabric have considerable strength and dimensional stability to
be used as a scrim  material. Two spunbonded fabrics were selected,
one in which the filaments are nylon  (Cerex  ) and the other made from
polyester (Reemay  ).     Fabric weight is very important in the scrim
application.   Light weight fabrics do not give the desired effect while
heavy fabrics lead to considerable needle damage and to problems
during manufacturing.  Experiments carried out to determine the
best fabric weight indicated  that 1. 0 and 1. 5 oz. /yd. were the most
suitable.

To study the effect of needling on the scrim itself, experiments were
carried out on the Reemay and Cerex fabrics using  25 gauge needles
and the results are  presented in Figures 42 to 44,   It can be seen
from Figure 42 that needling, in general, had very little effect on the
fabric  weight but the Cerex 1. 5  oz. /yd? fabric showed more change than
the corresponding weight of the  Reemay.  This may be attributed to the
movement of the filaments in Reemay which also reduces the level of
m?cello damage.  Figure 43 showed the changes in air permeability for
the two cases; and it is clear that the increase in needling  results in
an appreciable increase in the air permeability of the Cerex fabrics.
This can also be related to the high  packing of the Cerex which leads to
                              83

-------
CM
                   O 1.0 OZ/YD2REEMAY
                   A I.50Z/YD2REEMAY
1  O O O § o
o O


     0      100    200    300    400    500
        NEEDLING  INTENSITY (punches/inch2)
     Figure  42.  Fabric  weight  vs.  needling
     Intensity (Cerex and Reemay  spunbonded
     fabric  only)
               84

-------
  1200
  1000
£200
O  1.0 OZ/YD* REEMAY
A  1.5 OZ/YD2 REEMAY
O  1.0 OZ/YD2 CEREX
CD  1.5 OZ/YD2 CEREX
      0     100   200   300   400   500
    NEEDLING  INTENSITY (punches/inch2)
 Figure 43. Air permeability  vs. needling intensity
 (Cerex and Reemay  spunbonded  fabric only)
               85

-------
          A
O 1.0 OZ/YD* REEMAY
A 1.5 OZ/YD2 REEMAY
O 1.0 OZ/YD2 CEREX
a 1.5 OZ/YD2 GEREX
   0     100   200   300   400   500
  NEEDLING  INTENSITY (punches/inch2)
Figure 44. Ball burst vs. needling  intensity
(Cerex and Reemoy spunbonded, fabric only)

-------
high levels of filament damage with the increase in needling intensity.
The  bursting strength curves also show an appreciable reduction for
the Cerex with needling as indicated by Figure 44.

Needle   punched fabrics were produced using two layers of web
4.8 oz. /yd? one on each side of the scrim.   The fabrics were tested
for all physical properties and filtration performance. Figure  45 shows
the changes in  thickness with needling intensity.  The lowest  fabric
thickness was obtained with the 1.5 oz. /yd? Cerex scrim.  Cerex in
general gives lower thickness than Reemay which is an indication of
higher packing density due to good locking of the fibers into the Cerex.
Figure 46 shows little change in fabric weight per unit area with
needling intensity and minor differences between Reemay and Cerex
scrim fabrics.  In both cases, it was observed that the level  of fiber
spreading was  greatly reduced as compared to the no  scrim situation.
Cerex scrim fabric,  as mentioned earlier,  gave higher fabric density
as shown in Figure 47.   In spite of this high density, Cerex scrim
fabric had higher air permeability than the  Reemay fabrics as shown in
Figure 48.  The effect of needling intensity was not appreciable in both
cases which indicated that the role played by the scrim is more
effective than the needling intensity.  The air permeability-thickness
product curves are shown in Figure 49.  Although needling the Cerex
fabric by itself gives lower bursting strength than the corresponding
Reemay fabric, it can be seen from Figure 50 that the opposite is true
when needling is done with fibrous webs.  The presence of the fibers
protected the filaments of the scrim from damage.

Samples of all  these fabrics  were batch tested for filtration performance
using flyash.  Table 7 gives  the data for the flyash concentration and
the results of the final pressure drop (after 10 minutes) and the collection
efficiency.  It can be seen that, in general, the Cerex scrim  fabrics
gave slightly higher pressure drop than the corresponding Reemay
scrim fabrics.  This can be  related to  the high packing density obtained
with the Cerex scrim as discussed earlier.  The efficiency results
indicate that there was no significant difference between Cerex and
Reemay scrim fabrics.  However, the  heavier the scrim fabric the
higher the efficiency.

7. 6  Bag Fabrics

In all previous experiments,  the fabrics were  made of web layers with-
out prepunching each layer.   The  layers were  assembled and punched
only from one side  and with minimum number  of passages through the
needling process.   This  was done to study the  effects of needling


                              87

-------
£20
6
en
LU
   16
o  8
a:
m
   0
O 1.0 OZ/YD* REEMAY
A 1.5 OZ/YD2 REEMAY
O 1.0 OZ/YD2CEREX
  1.5 OZ/YD* CEREX
    0     100   200   300   400   500
                                    2,
    NEEDLING  INTENSITY (punches/inchi )
  Figure 45.  Fabric thickness vs. needling
intensity ( spunbonded  scrim)
              88

-------
   12
                D   a
   10
CVJ
 T3
   Q
 IM O
 O
 h-
 £
   6
 —- T"
 (T
 CD
 
-------
•Q
x
•S" 4
Q

o
  0
                           o
O 1.0 OZ/YD2 REEMAY
A 1.5 OZ/YD2 REEMAY
OI.O OZ/YD2CEREX
QI.5 OZ/YD2 CEREX
    0     100   200   300   400   500
                                  2
     NEEDLING  INTENSITY (punches /inch)
  Figure 47  Fabric  density  vs.  needling
            intensity (spunbonded scrim)
             90

-------
 -^250
CVI
ro
 £200
   150
 oo

 2 100
 tr
 LU
 QL


 (T
50
     0
O 1.0  OZ/YD2 REEMAY

A 1.5  OZ/YD2 REEMAY

O \.0  OZ/YD2 CEREX

D 1.5  OZ/YD2 CEREX
      0     100   200   300   400   500

      NEEDLING INTENSITY  (punches/intensity)

     Figure 48. Air  permeability  vs. needling

              intensity (spunbonded  scrim)
               91

-------
 o

 o
 o
 ctr
 o.
 O
    12
 =J  8
 CD
 UJ


 1  4
 UJ
 a.
 <  0
o 1.0 OZ/YD REEMAY

A 1.5 OZ/YD2REEMAY

OI.O OZ/YD2CEREX
  1.5 OZ/YD^CEREX
     0     100   200   300   400   500

      NEEDLING   INTENSITY (punches/inch2)

  Figure  49. Air  permeability - thickness

product   vs. needling  intensity (spunbonded

scrim)

-------
(A
A
u
•••
CM
•5.
8 16 r Ol.O OZ/YD2 REEMAY
±
BALL BURST STRENGl
3 4*> 00 £5
A 1.5 OZ/YD2 REEMAY
Ol.O OZ/YD2 CEREX
Q 1.5 OZ/YD2 CEREX
p^ ^n^
i i i i i
    0     100   200   300   400   500
    NEEDLfNG  INTENSITY (punches/inch2)
Figure 50. Ball  burst strength vs.  needling
           intensity (spunbonded  scrim)
               93

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         Table 7.  BATCH FILTRATION PERFORMANCE OF
                       SPUNBONDED SCRIMMED FABRICS

Scrim
Fabric

Reemay
1.0 oz. /yd.


Cerex „
1.0 oz. /yd.


Re e may
1. 5 oz. /yd.
f

Ccrex ?
1. 5 oz. /yd.

Needling
Intensity
Punches /in
122
245
368
490
122
245
368
490
122
245
368
490
122
245
368
490
c
2 gr. /ft?
1
1
1
1
1
1
1
1
2
1
2
1
1
1
2
1
.44
.61
.64
.66
.86
.56
.46
.64
.05
.65
.20
.41
.71
.60
.13
.80
A
p
r
Inche s
H2°
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
105
122
115
120
110
102
122
143
120
130
120
110
115
145
133
094
A
•pf
Inche s
H2°
0.
0.
0.
0.
0.
0.
0.
1.
0.
0.
1.
0.
1.
0.
1.
0.
55
64
77
62
96
73
94
5
83
73
17
85
05
95
22
83


Efficiency
97.
98.
97.
98.
97.
97.
97.
97.
98.
98.
98.
98.
98.
98.
98.
= 98.
68
32
77
28
46
55
39
63
32
22
35
10
51
52
63
40
Air velocity  =  45 ft/min.
Needle penetration  = 0. 333 inch
Random-laid, 25 gauge, Dacron^  li" x 3. 0 denier
p   -  0.250'
= 0.333
p   =  0.416",
                                             =  0.500"
                             94

-------
intensity and the other fabric parameters to avoid complicating the
fabric structure.  It is well known that prepunching the individual layers
before assembly increases the packing density of each layer and hence
the packing density of the final fabric.  For this reason it was decided
to prepunch the  layers in preparation for the  bag fabrics which produces
fabrics with multi-layer structures. In baghouse applications with
needle punched fabrics,  cleaning is normally done by using pulse jet in
the opposite direction to the  flow.  If needling is done from one side
only, the face side of the fabric should be identified and has to be used
as the collecting surface.  The backside of the fabric will have fibers
that are not securely held in the fabric structure and may cause
problems in use.  Needling from both sides was found to yield better
consolidated fabrics than needling from one side only with the same
total number of punches  per  square inch.  Fabrics produced by needling
from both  sides will not require identification of a particular side
which makes fabric manufacturing less troublesome.  It is also
speculated that  such fabrics  will make the cleaning process  easier.  To
increase the strength of the fabric and the interlocking between the
layers and the scrim, punching was done over four or six passes through
the needle  punching machine at 122 punches per square inch each pass.
In the manufacture of these bag fabrics, the webs were
passed through the machine several times.   In manufacturing line this
could be  achieved by using multiple heads needling consecurtively  or
simultaneously from both  sides in one pass to increase productivity
and reduce handling and cost.

Bag fabrics were manufactured in the laboratories of Hercules
Incorporated, Research Triangle  Park, North Carolina.   Four layers
of random-laid webs, 4. 0 oz./yd~  each made from 3.0 denier x l-|-inch
Dacron® staple,  were used in every fabric.  The layers were prepunched
with 25 gauge needles at 122 punches/inch^.  Four fabrics were made
without scrim and four with Cerex 1. 5 oz. /yd" scrim with two layers
on each side. The assembled layers were  punched with 25 or 32 gauge
needles at a speed of 500 punches/minute.

Table 8 gives the properties of the eight fabrics.  It can be  seen that
the fabrics without scrim  suffered badly from fiber spreading as
indicated by the reduction in fabric weight which was also observed
during manufacturing.  Needling over six passes with the 25 gauge
needle reduced the fabric  strength as compared to that of the fabrics
with four passes. This  effect was not pronounced in the case of the 32
gauge fabrics.   The bursting strength results also show that the 25
gauge needle caused more damage than the 32 gauge needle.
                              9.5

-------
              Table 8.  PROPERTIES OF BAG FABRICS
Air Ball
Thickness Weight'
Fabric °
(mCheS) (oz./yd?)
Scrirn^ (25 gauge)
4 x 122
#
Scrim (25 gauge)
6 x 122
*.t.
Scrirn^ (32 gauge)
4 x 122
Scrim* (32 gauge)
6 x 122
No Scrim (25 gauge) '
4 x 122
No Scrim (25 gauge) ^
6 x 122
No Scrim (32 gauge)
4 x 122
No Scrim (32 gauge)
6 x 122
Permeability Bursting
/JM.3 icj?- • \ Load
(ft. /ft. mm. ) (lbg)
206.3 153.


217.9 105.


89.0 220.

84.2 225.

208.6 118.

208.4 86.

150.4 191.

144.1 187.

4


0


0

6

6

8

6

4

Scrim used was Cerex 1. 5 oz. /yd.
4 x 122 means 4 passes 122 punches/inch2 each (2 from each side)
6 x 122 means 6 passes 122 punches/inch2 each (3 from each side)
                          96

-------
Table 9 gives the  results of the filtration performance of the bag fabrics
in batch testing according to the procedure described under 6. 7.  The
scrim fabrics gave higher pressure drop and efficiency than the no
scrim fabrics.  Needling six times resulted in higher pressure  drop
than needling four times although there was no significant effect on
efficiency.  In spite of the large differences in the air permeability of
the fabrics made with the 25 gauge and 32 gauge needles, little
differences were detected in pressure drop and efficiency.

Bags of 4-2-inch diameter and 48 inches length were manufactured from
the eight fabrics and were tested in a pulse-jet baghouse at the
Industrial Environmental Research Laboratory, Environmental
Research Center,  EPA, Research Triangle Park, North Carolina.
Table 10 gives details of the testing conditions  and results for pressure
drop and efficiency for six fabrics.  Bags from the two 25 gauge fabrics
without scrim, were  not tested since they were apparently damaged due
to fiber spreading.  For the four fabrics (1 to 4) the results indicated
high efficiency and low pressure drop for inlet dust loading range  from
0. 5 to 12 grains/ftr and air-to-cloth ratio  up to 9/1.  In comparison
with a commercial needle  felt fabric (tested under the same conditions
at EPA) the four fabrics with scrim were superior for the above
conditions.   The no scrim fabrics tested showed high efficiency only at
low air-to-cloth ratio (6/1).  At higher air-to-cloth ratios, the
efficiency was low which can be  related to  the low dimensional stability
of these fabrics.  When fabrics 1 and 4 were tested at high air-to-cloth
ratios (40/1  and 33. 6/1 respectively) low efficiency and low pressure
drop were  reported  even at low inlet dust loading.  This could be due
to either the high  pulse pressure expanding the fabric or due to the
tight fit on the wire  cage reported which might have resulted in  fabric
stretch which could  lead to fabric damage.   Figure 51 shows the
appearance of one of the bags tested (fabric 5) at high air-to-cloth ratio
as compared to the commercial  felt bag.  The bad appearance is due to
the removal  of fibers from the fabric surface by handling which can be
overcome by finishing treatments.  Treatments such as calendering,
resin application or shrinking could increase fabric stability and
durability.  However, these treatments would lead to an increase  in the
pressure drop.  The effect of calendering was investigated and the results
will be discussed  later. Work is needed in the areas of shrinking and
resin treatments to  determine their effect  on filtration performance of
needle punched fabrics.
                               97

-------
        Table 9.  BATCH FILTRATION PERFORMANCE OF BAG
                                    FABRICS
r AP
C . c
Fabric i „ , .
,-3 Inches
8 ' H20
Scrim (25 gauge)
4x 122 '7
Scrim (25 gauge)
/ m I.ol U. c5
6 x 122
Scrim (32 gauge)
4x 122 UbV "'^
Scrim (32 gauge) Q24
6 x 122
NO 5=^(25 gauge, ^ „ 0^?
No Scrim (25 gauge)
6 x 122
No Scrim (32 gauge) ^^ Q_ 12
No Scrim (32 gauge)
, , 00 ^ . vy u. uv
6 x 122
Commercial Needle 1 „_ n 40
Felt (15.27 oz/yd2)
APf
, Efficiency
Inches _ '
H20
1.70 99.30
3.07 99.35
•1.61 99-23
2.70 99.20
1.02 98.01
1.12 97.93
1.52 98.85
1.67 98.86
5.20 99.70
Air velocity  =  45 ft/min.
Random-laid, Dacron   li" x 3.0 denier
Needle penetration = 0. 333 inch
Scrim used was Cerex  1. 5 oz/yd.
                             98

-------
                               Table 10.  BAGHOUSE TESTING RESULTS
vO
Ci Inlet
Fabric Loading
Grains /ft



1






2





3


25 gauge
Scrim
4 x 122




32 gauge
Scrim
4x 122



25 gauge
Scrim
6 x 122




0.5
3.0
6.0
12.0
3.0
0.5

0.5
3.0
6.0
12.0
3.0

0.5
3.0
6.0
12.0
3.0
Co Pulse Pulse
Grains/ Pressure Interval
3 loooft3 Psi§ Sec-

.2668
.3134
.0820
4.4587
11.2842
90.9600

.1833
,3954
. 3665
.8125
1.8960

.3761
.1109
.1279
.5931
2.6612

50
60
60
80
85
90

50
60
60
60
75

50
60
60
60
65

140
140
140
f~
100
60
140

140
140
140
140
140

140
140
140
140
140
Air /cloth ^Pf
Ratio Inches
ft. /min. HO
k*

6/1
6/1
6/1
6/1
9/1
40/1

6/1
6/1
6/1
6/1
9/1

6/1
6/1
6/1
6/1
9/1

0. 625
0.99
1.51
4.22
4.23
3.80

0.73
1.23
1.72
2.. 62
3.62

0.62
0.91
1.11
1.67
3. 18
Efficiency
%

99.961
99. 989
99.999
99.961
99.656
82.267

99.967
99.987
99.994
99.993
99.937

99.93C
99.996
99.998
99.995
99.912
Run
Time
Hours

24
20
21
21
12
16

25
21
12
15
11

23
22
12
13
12

-------
                        Table 10  (continued).  BAGHOUSE TESTING RESULTS
o
o
C. Inlet C _ .
i o Pulse
Fabric Loading Grains/ Pressure
Grains/ft3 1000ft3 Psig



4





5





6


32 gauge
Scrim 0. 5
6 x 122 3.0
6.0
12.0
3.0
0.5
32 gauge
No Scrim 0. 5
6 x 122 3.0
6.0
12.0
3.0
32 gauge
No Scrim -0.5
4x 122 3.0
6.0
12.0
3.0
. 1575
.0563
.0842
. 1051
23.7140
83. 1800

.4774
. .3129
.4019
.4543
343.9600

.4082
. 3874
. 5353
.6172
57.6599
60
60
60
60
85
90

50
60
60
60
85

50
60
60
60
80
Pulse
Inte rval
Sec.
140
140
140
140
45
110

140
140
140
140
140

140
140
140
140
140
Air /cloth
Ratio
ft. /min.
6/1
6/1
6/1
6/1
9/1
33.6/1

6/1
6/1
6/1
6/1
9/1

6/1
6/1
6/1
6/1
9/1
APf
Inches
H2°
0.68
1.21
1.69
2.60
6.42
4.85

0. 60
0.90
1.10
1.73
4.30

0. 57
0.90
1.11
•1.55
3.. 47
Efficiency
%
99.972
99.998
99.999
99.991
99.210
85.619

99.926
99.989
99.993
99. 996
88.663

99.932
99.987
99.991
99.995
98.0781
Run
Time
Hours
25
21
13
13
12
12

29
21
12
11
15

25
21
12
10
10

-------
                    Table 10 (continued).  BAGHOUSE TESTING RESULTS
C Inlet C
1 0
Fabric Loading Grains/
Grains /ft3 1000 ft3
Commercial
Dacron
Felt
16 oz /sq. yd.




0.5
3.0
6.0
12. 0
3.0
0.5

0. 1511
1.7471
9, 8174
46. 3768

2.8594
Pulse
Pressure
Psig

55
55
55
80

90
Pulse
Interval
Sec.

140
140
140
108

85
Air /cloth
Ratio
ft. /min.

6/1
6/1
6/1
6/1

32.6/1
^Pf
Inches
H20

0. 60
1.73
2.92
7.55

5.84
_,,. . Run
Efficiency „,
Time
/o TT
Hours

99.9698
99.9412
99.8339
99. 6080

99.4500

12
75
24
14

17
RH  =  50%
Temp  = 70° F
Ap at end of filter cycle
= Outlet concentration
Tests for the NCSU and commercial felt fabrics with 3.0 grains/ft,  inlet concentration and
air /cloth ratio resulted in blinding and the tests were terminated.

-------
                    -.
Figure 51.   Bag Appearance
            A -  Commercial Bag
            B -  Bag Made From Fabric 5
                  102

-------
7.7  Experimental Verification of The Theory

Fabric samples were tested for various properties which were needed
for verification of the pressure drop theory.  The data of these
fabrics given in Table 11 was used to calculate theoretical values
of the pressure drop.  Pressure drop measurements were carried out
without dust and the results are given in Figure 52.   It is seen that the
theoretical  predication is in good agreement with the experimental
values.  The dependence of this approach on counting the number of
pores per unit area and  the number of fibers reoriented in the pores,
makes this  process time consuming.  It is felt that more work is
needed to relate the number of  fibers in the pore to the other fabric
parameters which is a very large undertaking.

To establish the role of the collection mechanisms,  tests were carried
out using latex spheres having diameters  of 0. 5, 1.099 and 2.02
microns and face  velocities ranging from 1. 1 to 15.75 cm. /sec.  in ten
intervals.   For each velocity, the particle count before and after the
test filter was repeated  three times and the average  was  used to
determine the penetration.   The data presented in Figures 53 to 55 give
the particle penetration  as  a function of the face velocity.  From these
curves the velocity at which maximum penetration occurs was used in
equations (48) and (49) to determine the Dorman parameters for the
fabrics designated A. to E in Table 12.  From this table it is noticed
that the values of krj for all the  filters tested were much  smaller than
those of  kj^ and. kj parameters.  This is mainly due to the very low
particle  loading (no cake formation) and the use of monodisperse  latex
particles which indicates little  contribution from the  diffusion mechanism.
This may render the fabrics tested unsuitable for use in the submicron region.
It is anticipated that modifications of the fabric structure by post
treatments, as  mentioned earlier, would improve the performance of
these fabrics in the submicron  range.

7.8  Mechanics of Cake Formation

Each of  the three collection mechanisms  functions efficiently over
limited ranges  of particle  size, gas velocity, packing density and other
gas/fabric properties.   The inertial mechanism is highly efficient when
large particles and gas  velocities are used.   The interception mechanism
is independent of gas velocity and is more influenced by the increase in
size of the  particles, while the  diffusion mechanism dominates when
submicron  particles and low gas velocity conditions  prevail.
                              103

-------
  Table 11.  FABRIC PROPERTIES USED FOR VERIFICATION OF THEORY
Needle
Size

20
LtJ \J
gauge


25
gauge


32
gauge
Needling
Intensity
2
Punches /inch
122
245
368
490
122
245
368
490
122
245
368
490
Actual
No. of Pores
Per Inch2
N*
120
170
215
235
115
160
210
250
110
135
185
230
Mean Pore
Diameter
*
D mm
0. 501
0.590
0.730
0.796
0.457
0.478
0.511
0.580
0.427
0.435
0.460
0.470
No. of Fiber
Per Pore
n
133
134
103
151
62
83
82
139
162
130
81
102
Fabric
Thickness
AL mm
26.2
18.2
17.5
14.5
16.2
12. 0
9.5
8.2
22.5
17.5
15.3
12. 6
Packing
Density
%
1.0
1.3
1. 0
1. 5
1.6
2. 1
2.5
2.7
1.2
1.5
1.7
1.9
Needle Dimension
  20 gauge,  d = 0.924mm
  25 gauge,  d = 1.178mm
  32 gauge,  d = 1.409 mm

(Dae r on  3 denier x if-inch,  Cross-Lapped)

-------
 ro
 'a
  x
CM
       15
      10
  25 GAUGE
EXPERIMENTAL

  25 GAUGE
THEORETICAL


  32 GAUGE
EXPERIMENTAL
  32 GAUGE
THEORETICAL
  20 GAUGE
EXPERIMENTAL

  20 GAUGE
THEORETICAL
        0   100  200  300  400  500
           NEEDLING INTENSITY  (punches/inch2)


   Figure 52.  Nondimensional  pressure  gradient vs.
              needling intensity (Dacron  3 den. x 1.5 in.,
              crossed - lapped)
                   105

-------
    100
2
!5
or
LU
80
    60
       0
             o
                                           * A    See
                                           o B   Table  12
                 5             10
                     VELOCITY (cm/sec)
15
 Figure  53.  Percent  penetration  vs.  velocity  for  0.5/^

-------
o
-j
             100
          LU
              80
          hk.

          tt
60
               0
                 0
                                o
A  A
o  B
*  C
Q  D
o  E
                                       See  Table  12
                  5             10

                     VELOCITY (cm/sec)
                             15
            Rgure  54.   Percent  penetration  vs.  velocity  for  1.099/x.m

-------
o
00
             80
         UJ
             60
         UJ
             40
             0
                             5            10

                              VELOCITY (cm/sec)
                                                     o
15
       A  &


       C  ^   See
       D  Q Table 12

       E  o
           Figure  55.   Percent   penetration  vs.   velocity  for  2.02/^

-------
              Table 12.  DQRMAN PARAMETERS

A




Fabric

4 x 122
32 gauge
Cerex 1. 5
Scrim
Particle
Size
|am


2
oz./yd.


0.
1.
2.


50
099
02

*I
(Interception)
cm- 1

0.
0.
0.


070
133
404

kR kD
(Impaction) (Diffusion)
cm-3 . sec^x 10"5 cm- 1/3, sec'2/3

2
16
65


.30
.30
.60


0.
0.
0.


0095
0411
0990

(185 punche s /min . )
B



6 x 122
32 gauge
Cerex 1. 5
Scrim

2
oz./yd.

0.
1.
2.

50
099
02

0.
0.
0.

050
102
402

9
13
64

.04
.70
.30

0.
0.
0.

0710
0720
2190

(185 punches /min. )
C


4 x 122
32 gauge
Cerex 1. 5

2
oz./yd.
0.
1.
2.
50
099
02

0.
1.
*
373
373

74
151
*
.09
.36

0.
0.
*
0141
3682
D
  Scrim
(500 punches/min. )

4 x 122
25 gauge        2
Cerex  1. 5 oz./yd.
  Scrim
(500 punches/min. )
0.50
1.099
2.02
*
0. 128
0.223
*
16. 15
43.39
*
0.0014
0. 1694
4 x 122
25 gauge
No Scrim
(500 punches /min. )
0.50
1.099
2.02

*
0.239
0.853

..i-
'i-
44. M
110.87

*
0.0083
0.0298

*
 Flat penetration curves were obtained leading to no V
 the parameters.
                                                    value to calculate
                              109

-------
 The cake formed on needle punched uncalendered filters differs distinc-
 tively from that  formed on any other filter media.  Normally the developed
 cake over the surface of a filter is homogeneous and has a uniform
 thickness, whereas in needle punched filters the dust forms
 distinct three-dimensional mounds around the pores  as shown in
 Figure 56. This is mainly due to the variation  in the packing  density
 over the surface of the filter caused by the needling process.  The
 tendency of the flow to follow the least resistance path causes the
 large particles to depart from the streamlines by inertia and deposit
 around the pores,  and causes the small particles to ride the stream-
 lines and deposit in the  pores as was explained  earlier.  The increase
 in packing density around the pores allow the inertial and interception
 mechanisms to be effectively employed.  The increase in the surface
 area of collection, due to the orientation of the  fibers in the pores
 parallel to the flow direction,  gives a better chance for the collection of
 small particles  in the pores.  The  characteristics of the cake formed
 will also reduce  the time  rate of pressure rise  across the filter as
 mostly the small size particles are deposited in the pore  area.
 Figure  57  shows the reduction in the number of pores due  to their being
 progressively plugged when the test time  was increased from  five
 minutes to 20 minutes.

 A. commercial needle  felt bag fabric having 15.  3 oz.  /yd. weight (which
 is believed to be calendered) was batch tested and the cake formed was
 homogeneous  similar  to that obtained with woven fabrics.   It is worthy
 of mention that with inlet  concentration (C-) of 1. 03 gr. /ft. the
 efficiency was 99.7% and  the pressure drop (APf) 5.2 inches of water.
 When a needle punched fabric was calendered similar results  were
 obtained.  This indicates that calendering needle punched fabrics
 destroys the structure and give high levels of pressure drop.

 7. 9  Effect of Dust Concentration On Efficiency In Batch Testing

 Figure  58  shows  the effect of concentration on efficiency for a needle
punched fabric.   The figure  indicates that the efficiency increases with
concentration up to  1.6 gr. /ft? after which the  efficiency does not
 increase substantially.  However,  it must be remembered that although
this trend remains the same for different conditions, yet the level of
concentrations after which the efficiency levels off may vary with test
conditions.
                              110

-------



           "* »..
                 -*  ' v—• />

                    *•• •     I '-
                                   -<*'
                                     ,
                                     -! *
                                        $
                                        ?4 *
Figure 56.   Dust Cake on a Needle Punched Filter
                       in

-------
         Figure 57.  Effect of Filtration Time on Cake  Formation

                     Air Velocity  =  45 ft/min.
                     Flyash Concentration:  1.54  gr./ft.^
i j
             »
                       '•Vi
       rafevi&j&te
       ;M$^'*O
       ' :w *'»'*" t? "%'lw
              5  minutes
                              1
m
                                            :
                    10 minutes
                                                          f.
                                                      /

              15  mi nutes
                    20 minutes

-------
COLLECTION EFFICENCY (%)
~ IND 4^ g> oo c
3 o o O o c

4
S
/



1
X
X
X
X
t



x>o— •
P




-O— O-









-o 	




•o-




                  1.0
2.0
               CONCENTRATION (gr./ft.3)
3.0
Figure  58.  Effect  of  flyash  concentration  on
            efficiency(368 punches/inch2,  25
            gauge , 3.0  denier * |!fe in. Dacron)
            10 minutes  test  duration,  Batch
            Testing, Air  Velocity - 45  ftVmin.
                      113

-------
 7. n Effect of Filtration Time On Efficiency and Pressure Drop

 Filtration time is an important factor in the determination of the
 optimum collection efficiency.  Long filtration time is prohibitive
 because of the high pressure drop developed across the filter.  While
 short filtration time will yield low efficiency values.  The effect of
 filtration time on collection efficiency is shown in Figure 59.  It is seen
 that for the needle punched filters tested the variation of efficiency
 with time is not significant after 10 minutes, which was used in all the
 batch tests.

 The effect of filtration time on the pressure drop is given in Figure 60.
 It is noticed that the time rate of pressure drop increases due to the
 increase in flow resistance with the deposition of flyash on the  surface
 of the filter.  The  same figure shows the comparison between the
 pressure drop-time curves for woven and needle  punched filter fabrics.
 The comparison illustrates the  effectiveness of needle punched filters
 in reducing the time rate  of pressure rise in support of the hypothesis
 explained earlier.

 7. 11 Effect of Humidity

 The filtration results previously reported were for air at relative
humidity of about 50%.  A limited experiment was carried out to
determine the effect of humidity on filtration performance.   The results
arc given in  Table 13 and it is seen that the pressure drop APc  increases
with the increase in relative  humidity.   There  is  also a slight increase
in efficiency but this may not be significant to draw any conclusion.  At
relative humidity about 90%,  the control on the flyash concentration
was very difficult.
                               114

-------
     80
     60
    40
     20
                  5         10
                   TIME  (minutes)
15
20
Figure  59.  Effect  of  filtration  time  on  collection
            efficiency  (245 punches / inch2 ,  25
            gauge  needle , 3.0  denier x 1.5  in.
            Dacron ) f Batch  Testing,
            Air Velocity  = 45  ft/min.
                   115

-------
    10
    8
^- -^
3-.
-* COMMERCIAL NEEDLE PUNCHED  -
_ o TEST  FILTER (NEEDLE PUNCHED)*.
 o COMMERCIAL WOVEN
-  4
or
    o
      o
               4      6
             TIME (minutes)
8
10
 Figure  60.  Effect  of  filtration  time  on pressure  drop
               (Batch  Testing)
*(245 punches/inch2, 25 gauge , 3.0 denier x 1.5 Dacron)
            Air Velocity = 45  ft./min.
                   116

-------
             Table  13.  EFFECT OF HUMIDITY
RH %
30
50
70
C
gr.
4.
4.
5.
'/ft?
11
17
95
APC
Inches H C
0.075
0.075
0.070
APf
) Inches HO
1.90
2.35
3.60
Efficiency
99.05
99.42
99.44
Fabric with Reemay  1. 5 oz0 /yd. scrim
245 punches/inch ,  25 gauge,  10.6 oz./yd.
                             117

-------
                        SECTION VIII

                        REFERENCES

 1.  Billings, C. E. and Wilder, J. , "Handbook of Fabric Filter
    Technology".  Volume 1,  1-37,  38, Contract No. CPA-22-69-38,
    EPA, 1970.

 2.  Draemel,  D. C. , "Relation Between Fabric Structure and
    Filtration Performance in Dust Filtration", Environmental
    Protection Technology Series, Report No.  EPA-R1-73-288,
    July 1973.

 3.  Turner, J. H. , "Performance of Nonwoven Nylon Filter Bags",
    Paper No.  73-300, A PC A Annual Meeting,  June 1973.

 4.  Miller,  B. , Lamb,  G. E. R. , and Costanza, P.,  "Influence of
    Fiber Characteristics on Particulate  Filtration", Environmental
    Protection Technology Series, Report No.  EPA-650/1-75-002,
    January 1975.

 5.  Chen, C. Y.,  "Filtration of Aerosols by Fibrous Media",  Chemical
    Review, 55, 3, 595,  1951.

 6.  Stern, S. C. ,  Zeller, H. W. , and Schekman, A. I., "The Aerosol
    Efficiency and Pressure Drop of a Fibrous Filter at Reduced
    Pressures", J. Colloid Sci. ,  15, 6, 546, I960.

 7.  Linkson, P. B., Caffin, D. A., and B rough, J. , "Pressure Drop
    Across Fibrous Filters",  Chemical and Process Engineering,
    p.  68, December 1970.

 8.  Kozney, J. , Wasserkraft U.  Wasserwirtech, 22, 67, 68,  1927.
    (In German).

9.  Scheidegger, A. E. , "The  Physics of Flow Through Porous Media",
    University of Toronot Press,  P.  137, Third Edition, 1974.
                             118

-------
10.  Blake,  F. E. , "The Resistance of Packing to Fluid Flow", Trans.
     Am.  I.  Chem. E.,  14, 415,  1922.

11.  Camran,  P. C. , "Fluid Flow Through Granular Beds",  Trans.
     Inst.  Chem. Engrs., 15,  150,  1937.

12.  Sullivan,  R. R. , and Hertel, K. L. ,  "The Flow of Air Through
     Porous Media",  Journal of Applied Physics,  11,  761,  1940.

13.  Brinkman,  H. C., "On the Permeability of Media Consisting of
     Closely Packed Porous Particles", Applied Science Research,
     A-l,  27,  1949.

14.  Iberall, A. S. , "Permeability of Glass Wool and Other Highly
     Porous Media",  Journal Research National Bureau of  Standards,
     45, 398,  1950.

15.  Emersleben, O.  , Z. Physik. ,  Bd.  26,  601,  1925 (In German).

16.  Wong,  J.  B. ,  Ph.D. Thesis in Chemical Engineering, University
     of Illinois,  1954.

17.  Wheat,  J. A. , "The Air Flow Resistance  of Glass Fibre Filter
     Paper", Can.  J. Chem. Eng. , 41,  67,  1963.

18.  Kwualbara, S. ,  "The Forces Wxperienced by Randomly  Distributed
     Parallel Circular Cylinders or Spheres in Viscous Flow at Small
     Reynolds  Numbers", J.  Phys.  Soc. Japan, 14(4), 527, 1959.

19-  Happel, J. , "Viscous Flow Relative to Arrays of Cylinders",
     Am.  Inst. Chem. Eng. J. , 5, 174, 1959.

20.  Brinkman,  H. C. , "A Calculation  of the Viscous Force Exerted
     By a  Flowing Fluid  on a Dense Swarm of Particles", Appl. Sci.
     'Res.  , Al,  27, 1947a.

21.  Debye,  P., and Bueche,  A. M. , "Intrinsic Viscosity,  Diffusion
     and Sedimentation Rate of Polymers in Solution",  J. Chem. Phys. ,
     16, 573,  1948.

72.  Spielman, L. and Goren,  S. L. , "Model For  Predicting Pressure
     Drop and  Filtration Efficiency in Fibrous  Media",  Environmental
     Science and Technology,  2(4), 279, 1968.
                             119

-------
 23.  Davies, C. N. ,  "The Separation of Airborne Dust and Particles",
     Proc.  Inst. Mech. Engrs.  (London) Bl,  185, 1952.

 24.  Clarenburg, L.  A. ,  and Pickaar,  H.  W. , "Aerosol Filters - I -
     Theory of the Pressure Drop Across Multi Component Glass
     Fibre Filters",  Chem. Eng.  Sci. , 23, 773, 1968.

 25.  Beavers, G. S.  and Sparrow, E. M. , "Non-Darcy Flow Through
     Fibrous Porous  Media", J. Appl.  Mech. , Paper  No.  69-APM-
     CC,  1969.

 26.  Werner,  R.  M.  and Clarenberg, L. A.,  "Aerosol Filters", Ind.
     Eng. Chem. Proc. Des. Dev. 4(3), 288, 1965.

 27.  Dorman,  R. G. , "Filtration", in Aerosol Science" (edited by
     C. N. Davies) Academic Press, London  and New York,  p. 192,
     1966.

 28.  Jonas,  L. A. , "Aerosol Filtration by Fibrous Filter Mats",
     Environmental Science and Technology,  6,  9, 821, 1972.

 29.  Hampl, V. and Rimberg, D. , "Aerosol Penetration of Felt Filters",
     Presented at Annual  Conference of Association for Aerosol
     Research, October 16, 1974 in  Bad Soden,  West Germany.

 30.  Lockheed-Georgia Company,  Marietta, Georgia,  "Velocity of
     Particulate in Laminar and Turlulent Gas Flow by Holographic
     Techniques",  Contract EHSD 71-34, October 1971.

 31.  Hearle, J. W.  S., Sultan, M. A.  I., and Choudhari,  T.  N. ,
     "A Study of Needle Fabrics:  Part  II Effects of the Needling
     Process", J.  Text.  Inst., 59, 2,  103, 1968.

32.  Adley,  F.  E.  and Anderson,  D. E., "The Effect  of Holes on the
     Performance Characteristics of High-Efficiency Filters",
     Presented at the  Eighth AEC  Air Cleaning Seminar, Oak Ridge
     National Laboratory, Oak Ridge, Tennessee, October 25,  1963.
                             120

-------
                        SECTION IX

                  LIST OF PUBLICATIONS

1.  Mohamed, Mansour H. ,  Afify, El Sayed M. and Vogler, John W.
    "Needle  Punched Fabrics In Filtration", Book of papers of the
    Second Technical Symposium of the International Nonwovens and
    Disposables Association (INDA), pages 17-47, March 1974.

2.  Saleh, L. L. , "Pressure Drop Through Nonwoven Needle  Punched
    Fibrous  Filters",  M.S.  Thesis  in Mechanical Engineering, North
    Carolina State University, Raleigh, North Carolina,  May  1974.

3.  Vogler,  J. W. ,  II, Walsh, W. K. and Mohamed,  M.  H. , "Electron
    Beam Curing of Binders For Nonwoven Filter Fabrics", Tappi,
    Vol.  58, No. 9:125-128, September 1975.

4.  Afify, E. M. and Mohamed, M. H. ,  "Collection Efficiency and
    Pressure Drop of  Needle Punched Filters", Transactions  of the
    ASME, Journal of Engineering for Industry,  Vol.  98, No.  2:675-
    608,  May 1976.
                            121

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                           SECTION X
                       NOMENCLATURE
A       =  cross-section area of the filter
a        =  fiber radius
A       =  mean fiber radius
  e
 A * T-l *            J-
A , B   =  constant
b        =  free  surface  radius
C       =  Cunningham correction factor
Cd .     =  drag coefficient for fiber diameter d. in filter with volume
            fraction a.
                     i
Cd      =  drag coefficient of a fiber of average size (df)    in a filter
            with fiber volume fraction a
C.       =  inlet concentration
  i
d        =  fiber diameter
  *
D        =  average pore diameter
d        =  effective fiber diameter
 IL
F        =  drag force
F        =  drag force per unit length of fiber with diameter d
  i                                                           i
(elf)      =  average fiber diameter
   av
(df)      =  surface average fiber diameter
   s
f(  )      =  function of
F        =  drag force per unit volume of filter having fibers normal to
            the superficial velocity
F9       =  drag  force per unit volume of fiber having fibers parallel to
            the superficial velocity
                              122

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F        =  drag force per unit volume of filter due to fibers normal to
           the superficial velocity

F        =  drag force per unit volume of filter due to fibers parallel to
           the superficial velocity

F        =  drag force per unit length of fibers with axes normal to the
   1       superficial velocity

F        =  drag force per unit length of fibers with axes parallel to the
   2       superficial velocity

g,( ), g9( ), fi ( ), f7( )  = function of
 1     L     1     L
k        =  Darcy's coefficient

k  , k ,  k  k  , k  ,  k ,  k   k  =  constant
     g   f   e   d   c   b   a
k        =  inertial impaction parameter
 R
k        =  diffusion parameter

k        =  interception  parameter

k        =  function of Cunningham slip correction factor

K  , K    =  modified Bessel functions of zero and first order respectively
 o   1
k        =  Darcy's drag coefficient for  fibers normal to the direction
           of flow

k        =  Darcy's drag coefficient for  fibers parallel to the direction
           of flow
  i
K        =  Knudson number
_n
1        =  mean fiber length

I,        =  effective  thickness
—-—      = tortuosity factor
 .l_j
n        = number of solid cylinders per unit area
 •A.
 •'I'*
n        = average number of fibers per pore
 *                                             .    2
N        = average actual  number of pores per inch

NT        = number of pores on surface area  1
 P                    *
p        = needle penetration

P%      = percentage  particle penetration
                              123

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 r        =  radial coordinate

 R       =  Reynolds number
  c
 S        =  surface area per unit volume of porous media

 S        =  surface area per unit volume of solid material
 o
 t        =  thickness  of the filter

 U       =  superficial velocity
 u        =  vectorial velocity

 u        =  velocity normal to the fiber

 u        =  velocity parallel to the fiber

 u        =  radial component  of the normal  velocity

 u        =  angular component of the normal velocity
  1 o
 V       =  face velocity of contaminated air

 V       =  velocity at maximum particle penetration

 X  ,X   =  non-dimensional factors

 a        =  volume fraction or packing density of porous medium, _i. e_.,
            volume of solids per unit volume of the porous medium

 a.       =  volume of fibers normal to the thickness direction of the
            filter per  unit volume of filter

 Ct7       -  volume of fibers parallel to the  thickness direction of the
            filter per  unit volume of filter
 -t*
 ff-
 a        =  solid fraction of filter with fibers all normal to the
            superficial velocity
 'i1*
 a        =  solid fraction of filter with fibers all parallel to the
            superficial velocity

AP      =  pressure drop

AP      =  pressure drop for clean filter

 "\P      =  pressure drop at the end of filtration test

 AL      =  filter thickness

 e        =  porosity

\,  r>      =  constant
                               124

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9        =  angular coordinate
X        =  gas molecules mean free path
p.        =  viscosity
p        =  density of gaseous medium
VP , VP  =  pressure gradient for fibers normal and parallel to the
           superficial velocity
cp ( )     =  function of
A        =  stream, function
                              125

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                        SECTION XI


                       APPENDIX A


                   FABRIC PROPERTIES


              Table A-l.  FABRIC THICKNESS
                 (Dacron   3 den. x 1. 5 in. )
Needling
Intensity
Punches /in.
122
184
245
368
490
735
Fabric Thickness (mm)
Random -Laid*
20
gauge
22.5
22. 3
17.3
20.0
13.0
12.5
25
gauge
16.6
11.6
10.7
8.5
7. 5
5.9
32
gauge
24.1
18.9
17.9
17.5
14.2
9.8
Cross -Lapped**
20
gauge
28.0
21.8
21.5
15.6
15.7
10.4
25
gauge
17.8
14.3
11.6
9-3
8.2
6.9
32
gauge
22. 1
20.8
20.0
15.6
13.9
12.4
*                             L
  Five layers of web 2. 52 oz/yd  each.
-!,>(.                           7
i- T*                         t  £i
  Four layers of web  3.0 oz/yd  each.
                             126

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           Table A-2.  FABRIC WEIGHT
             (Dacron   3 den. x 1.5 in. )
Needling
Intensity
Punches /ii
122
184
245
368
490
735
Fabric Weight (oz. /yd?)
Random- Laid
2 20 25 32
i. gauge gauge gauge
9.7 9.8 11.5
10.5 8.6 10.7
8.9 8.9 11.0
10.0 8.3 11.7
7.4 8.9 10.5
8.3 7.7 7.7
Cross -Lapped

20 25 32
gauge gauge gauge
10.3 10.8
9.6 10.5
9.7 9.6
8.6 8.6
8.2 8.5
6.9 7.7
10.7
10.6
10.8
9.9
9.1
9.2
Table A- 3. FABRIC PACKING DENSITY
'(Dacron' 3 den. x 1. 5 in. )
Needling
Intensity
Punches /i]
122
184
245
368
490
Packing
Random- Laid
20 25 32
i. gauge gauge gauge
1.1 1.4 1.2
1.1 1.8 1.4
1.3 2.0 1.5
1.2 2.4 1.6
1.4 2.9 1.8
Density %
Cross -Lapped


20 25 32
gauge gauge gauge
.9 1.5
1.1 1.8
1.1 2.0
1.3 2.3
1.3 2.5
1.2 -
1.2
1.3
1.6
1.6
735
1.6
3.2
1.9
1.6
2.7
1.8
                       127

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               Table A-4.   FABRIC TENACITY
            (25 gauge) (Dacron  3 den. x  1. 5 in. )
Needling
Intensity
Punches /in.
122
184
245
368
490
735

Needling
Intensity
2
Punches /in.
122
184
245
368
490
735
Tenacity (gf/tex)
Random- Laid
II
.07
.10
.12
.20
.31
.75
Table A-
(25 gauge
_L
.08
.08
.11
.19
.30
.52
5. FABRIC
) (Dacron
45°
.08
.10
. 13
.23
.31
.62
Cross -Lapped
||
.03
.04
.05
. 12
.17
.44
1
.11
. 12
. 15
.20
.22
.41
45°
.04
.06
.09
. 15
.23
.56
ELONGATION
3 den. x 1. 5 in. )
Elongation %
Random - Laid
1
83.2
81.9
88.9
97.6
90.6
101. 5
J_
86.2
86.4
85.3
95.2
116.7
103.4
45°
77.0
87.1
94.1
101.4
104.6
96.5
Cross -Lapped
II
131.8
145.1
159.9
189.7
159.5
155.3
_L
65.2
66.0
66.9
71.7
79.8
84.7
45°
109.5
118.2
128.2
143.2
128. 3
143. 3
11 Along machine direction.

i  Perpendicular to machine direction.


                             128

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                        APPENDIX B

       EFFECT OF FLOW RATE ON PRESSURE DROP

The results of the effect of flow rate for the clean filters tested are
presented in Figures B-l and B-2.  In Figure B-l it is noticed that the
pressure gradient AP/AL for needle punched filters varies linearly with
the flow rate up to 180 ft. /min.  In this range the flow is considered
viscous and follows Darcy's law.  For rates of flow above  this range,
which correspond to Reynold's Number (based on fiber diameter)
larger than unity, the linear relationship ceases to exit. Adley and
Anderson [32] found that the pressure drop of filters having holes  to be
nonlinear with air velocity.  His explanation was based on  the fact that
the flow through the holes follows the poiseiulle flow (i.e_.  Ap a  V^)
whereas the pressure drop through the remainder of the filter is
proportional to the  velocity.  The fact that needle punched  filters  show
a linear relationship between the pressure drop and air velocity is an
indication that needle punched filters can be treated macroscopic ally
as homogeneous filters.  Thus the pores of a needle punched filter
cannot be considered as holes.

In Figure B-2 comparison is made between two clean filters having
approximately the same weight per unit area; a cotton woven fabric
and a needle punched fabric.  It is seen that the needle punched filter
offers  considerably less resistance  to the flow than the woven one.
                             129

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I
I-
   0
              Needling  Intensity (punches/inch  )
     0
45       90       135

  AIR  VELOCITY (ft/min.)
180     225
 Rgure  B-l.  Effect  of  air  velocity  on  pressure
              gradient
                       130

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tn

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                        APPENDIX C

The pressure drop theory was based on the measurement of certain
fabric parameters, as given in Table 11.  The procedure for measuring
some of these parameters  is given in the following:

C. 1 Actual Number of Pores Per Unit Area
Because of the difficulty to count the pores in a needle punched fabric,
the following method was used.  A chemically bonded nonwoven fabric
was passed through the needle punching machine with a web of red
Dacron fiber glued to it.  Using a pick-glass, the positions where the
red fibers showed on the back of the nonwoven fabric, in an area of
one square inch, were counted.   Five  counts were made for each
needling intensity and the average was plotted against the calculated
needling intensity.  The measured number of pores  was found to be
less than the calculated number.  At high needling intensity the rate of
increase of pores was  reduced indicating the high probability that the
pores may have been punched more  than once.  The effect of needle
size on the number of pores does not seem to be significant between
the 32  and 25 gauge needles.  The large  needle  (20 gauge) gave less
pores than the other two.

C.2 Pore Diameter

Fabric cross sections  prepared by the method explained later were
used.  The diameter of the pore was measured on the Projectina Screen.
Ten measurements were taken for every fabric and the average values
of pore diameter are given in Table 11.  It can be seen that the larger
the needle the larger is the  pore diameter.  It can also be seen that the
higher the needling intensity,  the larger is the pore diameter with
every needle.   This is due to  the increased packing density which reduces
the disruption of the fibers in the pore as the needling intensity is
increased.
                              132

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C. 3  Number of Fibers In The Pores

Fabrics made with red tracer layer on top were examined in this
investigation using exactly the same conditions of the pore diameter
experiment.  Representative punches were cut away from the fabric
using small scissors and tweezers. The white fibers in the plane of
the fabric were  removed.  The red fibers were then separated and
evenly distributed in mineral oil on a slide and examined on the
Projectina. Each end was counted and tagged with ink so it was  counted
once.  Assuming that the number of fibers equals half the number of
fiber ends, the number of fibers was calculated 10 times for each
fabric.  The average numbers are  also given in Table 11.  This
experiment is a very tedious one and the variation in the  number of
fibers between pores is so great which makes it difficult to obtain
statistically significant result.  However, the experiment gives  rough
average for the  number of fibers.  The average number of fibers per
pore for all fabrics was 112.  The  results indicate that the  number of
fibers was higher with the  large  and small needles than with the  medium
needle. There was no particular pattern as  far as the effect of needling
intensity.

C.4  Method of Preparing Cross-Se'ctions

Fabric samples were mounted in a Dow  epoxy resin mixture of both
hard resin (D.E.R.   332)  and soft resin (D.E.R.  732).  Many resin
proportion ranges and curing times were experimented with before the
best properties  were attained to  allow microtoming of thin cross-
sections.   The best mixture  of resins  was three parts soft D.E.R.
732 to one part hard D.E. R.  332  with 15% by weight curing agent
(D-126, diethylene triamine).  A gel time of three hours  at room
temperature and curing time of 30  minutes at 55° C oven temperature
was  used.
              Formulation:    65% D.E.R.   732
                              22% D.E.R.   332
                              13% D-126

This particular  mixture provided the proper consistency for micro-
toming thin sections as thin as 200 microns.  This thickness is ample
to provide information about the  fabric structure.  The thin sections
were mounted in mineral oil for  microscopic examination and photo-
micrography.
                             133

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Better contrast was attained by using a top tracer layer of red fibers
(same  fiber type). For better contrast in photomicrography,  mono-
cromatic light was  used  for illumination.  The photomicrographs were
produced with a projection microscope  (Projectina) which used
transmitted light source and is equipped with a Polaroid   Land
Camera  —  (Graphic  ).
                              134

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                                 TECHNICAL REPORT DATA
                           (Please read Instructions on the reverse before completing)
 1. REPORT NO.
  EPA-600/2-76-204
                                                        3. RECIPIENT'S ACCESSION NO.
 4. TITLE AND SUBTITLE
 Efficient Use of Fibrous Structures in Filtration
                                                       5. REPORT DATE
                                                        July 1976
                                                        6. PERFORMING ORGANIZATION CODE
 7. AUTHOR(S)

 M.  Mohamed and E. Afify
                                                       8. PERFORMING ORGANIZATION REPORT NO.
 9. PERFORMING ORGANIZATION NAME AND ADDRESS
 SForth Carolina State University
 Schools of Engineering and Textiles
 Raleigh, NC  27607
                                                       10. PROGRAM ELEMENT NO.

                                                       EHE624
                                                       11. CONTRACT/GRANT NO.
                                                       Grant R801441
 12. SPONSORING AGENCY NAME AND ADDRESS
 EPA, Office of Research and Development
 Industrial Environmental Research Laboratory
 Research Triangle Park, NC 27711
                                                       13. TYPE OF REPORT AND PERIOD COVERED
                                                       Final: 6/72-6/76        	
                                                       14. SPONSORING AGENCY CODE

                                                        EPA-ORD
 15. SUPPLEMENTARY NOTES pr0ject officer for this report is J.H. Turner, Mail Drop 61,
 Ext 2925.
i6. ABSTRACT Tne repOrt gjves results of Si project to develop fibrous structures for air
filtration which are economical and efficient,  and have low pressure drop. The struc-
ture of needle punched fabrics showed excellent characteristics as filter media. Fun-
damental studies were carried out to investigate the effect of different needled fabric
parameters on their filtration performance and  mechanical properties. High efficiency
Levels were obtained  at relatively low pressure drop, compared to woven fabrics.
Fabric parameters studied were: needling intensity, fiber orientation and length,
needle size and penetration, scrim  material,  fabric weight, and number of passages
:hrough the needling process.  Spunbonded scrims were used to improve the strength
and dimensional stability of needle punched fabrics without sacrificing air permea-
bility. The pressure  drop for clean filters was predicted theoretically. Based on
literature review of existing theories , the Brinkman model was used. Analytical
study of the roles played by the various mechanisms of collection (using Dorman's
theory) showed that the diffusion mechanism is not fully utilized in the developed
needle punched fabrics. Based on the fundamental studies, fabrics were developed
n which Cerex 1. 5 oz/sq yd was used as scrim  and punching was done on stages and
"rom both sides.  These fabrics have been evaluated in batch filter testing as well as
                              fQlTCl Suerior  J" ™an  i*g«Pfta fn pnmmp-rmql fahrira
          application
 7.
                              KEY WORDS AND DOCUMENT ANALYSIS
                 DESCRIPTORS
                                           b.IDENTIFIERS/OPEN ENDED TERMS
                                                                   c.  cos AT I Field/Group
 Air Pollution
 Air Filters
 Fabrics
 Dust
                                           Air Pollution Control
                                           Stationary Sources
                                           Fibrous Structures
                                           Particulate
                                           Fabric Filters
                                           Needled Fabrics
                                               louses
13B
13K
HE
11G
 8. DISTRIBUTION STATEMENT

 Unlimited
                                           19. SECURITY CLASS (ThisReport)
                                           Unclassified
                                                                     21. NO. OF PAGES
   145
                                           20. SECURITY CLASS (Thispage)
                                           Unclassified
                                                                    22. PRICE
EPA Form 2220-1 (9-73)
                                        135

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