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.
-------
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
-------
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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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)
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
tn
-------
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
-------
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
-------
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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