vyEPA
United States Industrial Environmental Research EPA 600 7-80-042
Environmental Protection Laboratory March 1980
Agency Research Triangle Park NC 2771 1
Performance of a
High-velocity Pulse-jet
Filter, II
nteragency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
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EPA-600/7-80-042
March 1980
Performance of a High-velocity
Pulse-jet Filter, II
by
David Leith, M.J. Ellenbecker, M.W. First,
J.M. Price, Anthony Martin, and D.G. Gibson
Harvard School of Public Health
665 Huntington Avenue
Boston, Massachusetts 02115
Grant No. R804700
Program Element No. EHE624
EPA Project Officer: James H. Turner
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U b i IWiROIVMENTAL PROTECTION AGENCY
Of! >:<• ul Research and Dt-veiopnirni
Washington DC 20460
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ABSTRACT
High velocity pulse-jet filtration has distinct advantages over filtra-
tion at low velocities in that the equipment required to clean a gas stream
is reduced in size and first cost as filtration velocity increases. However,
high filtration velocity brings about a number of problems, many of which can
be dealt with using the information presented in this report.
Experiments have shown that penetration and pressure drop increase
markedly at high filtration velocities. The reasons for this involve failure
to remove dust effectively from the bags and redeposition of the dust that
is removed. The gas flow pattern within the filter housing should strongly
affect redeposition. Using a bottom inlet to the filter housinp was found to
increase redeposition, and caused pressure drop and penetration to increase
over values found for a top inlet filter. These increases were more pronounced
at high filtration velocities.
Fabric type was also found to affect filter performance considerably.
Penetration, pressure drop, dust deposited on the filter bags at equilibrium,
apparent specific resistance of the dust deposit (K2) were all studied as
functions of filtration velocity and fabric type. Bags made from untreated
polyester, polyester with singed surface, and ptfe laminate over polyester
were used.
This and other information was used to develop a theory based on Darcy's
law, which relates pressure in a pulse-jet cleaned fabric filter to char-
acteristics of the filter and dust. The theory allows prediction of pressure
drop under stable or variable operating conditions including dust concentra-
tion? filtration velocity, pulse pressure and pulse frequency, and can be
used to identify operating conditions which cause pressure drop to increase
without limit. Agreement between the model and experimental data is reason-
ably good.
Understanding of the interactions between particles and fibers is
important for understanding the performance of a filter comprised of fibers
made into a felt. The particle collection caracteristics of a stainless steel
fibrous material collecting resuspended fly ash were investigated at high
filtration velocities. Under these conditions collection effeciency by
impaction should have approached 100% for all but the smallest particles
present; however, contrary to classical theory, measured efficiency decreased
with increasing particle size and filtration velocity.. These results are
explained in terms of particle bounce from the fibers on impact, and sub-
sequent particle reentrainment. Particle-fiber behavior of the type noted .
here may play an important role in the penetration characteristics of high-
velocity pulse-jet filters.
ii
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CONTENTS
Abstract i:L
Pigures iv
Tables v
Acknowledgements vi
1. Introduction 1
2. Conclusions 2
3. Recommendations 4
4. Performance of Top and Bottom Inlet Pulse- 5
Jet Fabric Filter
5. Effect of Fabric Type on. Performance of
Pulse-Jet Fabric Filter 12
6. Theory for Pressure Drop in a Pulse-Jet
Cleaned Fabric Filter 28
7. High Velocity Fibrous Filtration ^8
iii
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FIGURES
Number Page
4-1 Schematic drawing of pulse-jet filter with
bottom inlet (left) and top inlet (right). 7
4-2 Pressure drop vs. superficial filtration ve-
locity, inlet location as parameter. 8
4-3 Overall mass penetration vs. superficial fil-
tration velocity, inlet location as parameter. 9
5-1 Untreated polyester (50x). 13
5-2 GoretexR (teflon-covered polyester) (225x). 13
5-3 GoretexR (800x). 14
5-4 Tery.texR (singed polyester) (50x). 14
5-5 Equipment for pulse-jet filter experiments. 16
5-6 Particle size distribution for fly ash. 17
5-7 Effect of velocity and fabric on pressure
drop. 20
5-8 Effect of velocity and fabric on dust penetra-
tion. 21
5-9 Effect of velocity and fabric on K2. 22
5-10 Effect of velocity and fabric on dust deposit
areal density. 23
6-1 Hypothetical relationship between fraction of
dust deposit removed from fabric, e, and area
specific separation force, FS/A. 30
6-2 Pressure drop, AP, vs. dust areal density added
during one filtration cycle, WQ. 36
6-3 Apparatus used to determine relationship be-
tween pulse pressure, P, and induced static
pressure, Pg. 38
6-4 Static pressure induced, Pg, vs. pulse pressure. 39
6-5 Equilibrium pressure drop, AP, vs. filtration
velocity, v. 41
iv
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Number Page
6-6 Equilibrium pressure drop, AP, vs. pulse
pressure, P. 43
6-7 Equilibrium pressure drop, AP, vs. areal dust
density added during one filtration cycle, WQ. 44
7-1 Theoretical single fiber impaction efficiency
vs. particle diameter at 1 and 10 m/s filtra-
tion velocities. 51
7-2 High velocity filtration apparatus. 52
7-3 Ply ash penetration vs. particle size at five
filtration velocities. 56
7-4 Dioctyl phthalate (DOP) penetration vs. par-
ticle size at six filtration velocities. 57
7-5 Theoretical and experimental single fiber im-
paction efficiencies vs. impaction parameter
for DOP and fly ash. 59
7-6 Ply ash adhesion probability vs. particle
kinetic energy. 6l
7-7 Comparison of adhesion probability data from
this study with those of other investigators. 62
TABLES
4-1 Filter characteristics. 6
5-1 Pulse-jet filter: experimental results. 19
6-1 Values of test variables used in.this study. 4o
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AC KNOWLEDGEMENTS
Buffalo Forge Co., Buffalo, NY, donated the Venturis, un-
treated polyester bags, and bag support cages used in this pro-
ject. The polyester bags with singed surface were donated by
P & S Textiles, Inc., Skaneateles Palls, NY; the polyester bags
with ptfe laminate surface were donated by W. L. Gore and
Associates, Inc., Elkton, MD. The contributions made by these
companies are gratefully acknowledged. Dr. James H. Turner
served as project officer for the grant that supported this work.
His assistance with all aspects of these studies is gratefully
acknowledged.
vi
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SECTION 1
INTRODUCTION
Pulse-jet cleaned filters comprise a substantial portion of
the fabric filter market in the United States. However, the fac-
tors that affect filter efficiency and pressure drop are not well
understood. Considerable improvement to filter performance may
result from a better understanding of the factors that affect per-
formance, and proper application of this knowledge.
This report describes research on the performance of pulse-
jet cleaned fabric filters operated at high filtration velocities.
This research is reported in four sections, each of which describes
the results of investigations into one aspect of pulse-jet filter
performance. The first section describes the effect of gas flow
pattern in the filter housing on pressure drop and penetration.
The second section discusses the effect of fabric type and filtra-
tion velocity on penetration, pressure drop, and fundamental fac-
tors found to determine penetration and pressure drop. The third
section presents a model for pressure drop in a pulse-jet filter,
which draws on understanding developed through the research de-
scribed in the first two sections, and previous research.
The final section of this report is concerned with the per-
formance of fiber filters operated at high filtration velocities.
An understanding of how fibers interact with particles at high ve-
locities is essential for understanding how nonwoven fabrics or
other structures made from fibers behave under high velocity con-
ditions .
Our approach in preparing this material has been to compose
each section so that it can be read and understood independently.
However, our hope is that the reader will read all the sections,
since they are all aimed at the same goal - a better understanding
of the factors controlling pulse-jet filter performance. Perspec-
tive gained by reading all report sections will allow best assimi-
lation of the information we have developed while carrying out
this research.
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SECTION 2
CONCLUSIONS
The performance of a pulse-Jet filter is adversely affected
by redeposition of dust on the filter bags following a cleaning
pulse. Both pressure drop and penetration increase with in-
creased redeposition, which increases with increasing filtration
velocity. High velocity filtration flow sweeps dust and agglom-
erates back to the bag surface more effectively than low velocity
flow.
Redeposition increases when a bottom inlet configuration is
used, presumably because the net upward gas flow within the filter
housing reduces the effective velocity with which particles and
agglomerates released by a cleaning pulse can settle to the dust
hopper. Redeposition is minimized with top inlet because the gas
flow is downward and dust fall toward the hopper is promoted. As
a result pressure drop and penetration are minimized. For a super-
.ficial filtration velocity of 125 mm/s, filter resistance dimin-
ished by a factor of four and penetration by a factor of two when
gas inlet was moved from bottom to top, all else remaining the same,
Experiments with bags made from untreated polyester, singed
polyester (Terytex), and Gore-Tex laminate (over polyester) showed
that pulse-jet filter performance with fly ash test dust is af-
fected considerably by fabric choice. The dust deposit on the bag
at pressure drop equilibrium was greatest for the untreated poly-
ester, less for the singed polyester, and considerably less for
the Gore-Tex.
At least two factors contribute to differences in equilibrium
dust deposit. First, dust adhesion/release characteristics of
the fabrics may vary, so that the amount of dust separated by a
cleaning pulse varies by fabric type. Second, the abilities of
the fabrics to induce particle agglomeration may vary, so that the
size of the agglomerates freed by a cleaning pulse varies by fab-
ric type. Both the amount of dust released and the size of the
agglomerates released affect fabric "cleanability" and the amount
of equilibrium dust deposit.
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The apparent value for specific resistance of the dust depos-
it, K2s was about the same for the untreated and singed polyester
bags, but significantly higher for the Gore-Tex bags. The dust
deposit may more closely approximate a true dust cake when col-
lected on the smooth surface of the Gore-Tex than when collected
throughout the depth of the other, more open fabrics. The appar-
ent specific resistance of the particle-to-particle dust cake
would presumably be much higher than that of a porous, particle-
on-fiber deposit.
Penetration and pressure drop increased for all fabrics as
filtration velocity increased. At a filtration velocity of 50
mm/Sjpenetration was highest for untreated bags, lowest for Gore-
Tex bags; pressure drop was highest for Gore-Tex bags and lowest
for untreated bags. At a velocity of 100 mm/s both penetration
and pressure drop were highest for Gore-Tex bags; both penetration
and pressure drop were lowest for singed bags at this velocity.
A theory for pressure drop in pulse-jet cleaned filters has
been developed. With simplifying assumptions, the theory can be
readily used to interpret pressure drop data or to predict the
equilibrium pressure drop of a pulse-jet cleaned filter operated
under any conditions. The theory also allows prediction of operat-
ing conditions under which filter operation will become unstable
and cause pressure drop to increase without limit.
Particle bounce strongly affects fly ash penetration through
mats of stainless steel fibers. Bounce probability increases
with both increasing particle size and increasing filtration velo-
city. Particle adhesion probability was found to be inversely
related to particle kinetic energy. These results help explain
why penetration is higher than might be expected through filters
made from fibers or arrays of fibers, particularly for large par-
ticles and high filtration velocities.
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SECTION 3
RECOMMENDATIONS
Because pulse-Jet filters operate with lower pressure drop
and penetration if aerosol inlet is at the top of the filter hous-
ing, the inlet should be in this position and not at the filter
bottom. This recommendation is especially important under high
velocity filtration conditions. More work needs to be done to
promote dust deposit separation from the filter bag, to promote
separation of larger dust deposit agglomerates, and to promote
active agglomerate transfer to the dust hopper so that redeposi-
tion will be minimized.
Fabric structure clearly affects pressure drop and penetra-
tion of a pulse-Jet filter. The effects of fabric structure have
been quantified in terms of macroscopic filtration parameters such
as K2 and residual dust loading. Considerable work remains to be
done to incorporate these data in models such as the one described
in this report, to allow reliable interpretation and prediction of
filter penetration and pressure drop.
Penetration through pulse-jet cleaned filters increases to
levels that may be unacceptable at high filtration velocities.
Reasons for this may involve particle bounce or the failure of
dust to adhere to the clean side of the fabric during and after
cleaning. Considerable work remains to be done to relate the ad-
hesion information presented here for fly ash particles and fiber
mats to the situation found in an industrial pulse-jet filter.
However, we believe that an understanding of penetration through
pulse-Jet filters predicted on fault processes such as particle
bounce or failure of particles to adhere to fibers is more likely
to explain penetration characteristics of these filters than
theory based on classical particle collection mechanisms alone.
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SECTION 4
PERFORMANCE OP TOP AND BOTTOM INLET
PULSE-JET FABRIC FILTER
INTRODUCTION
High velocity fabric filtration has distinct advantages over
filtration at low velocities in that the equipment required to
clean a gas stream is reduced in size and first cost as filtra-
tion velocity increases. However, experiments on pulse-jet
cleaned filters have shown that penetration(l-4).and pressure
drop(5-8) increase markedly at high filtration velocities. The
reasons for this involve dust redeposition on the bags following
a cleaning pulse. At high filtration velocities, most of the dust
freed by a cleaning pulse is swept back to the bags rapidly, and
does not fall into the hopper. As a result, a thick dust deposit
develops on the bags and pressure drop increases(9). When a bag
snaps back and hits its supporting cage at the end of a cleaning
pulse, rapid deceleration of the fabric causes particles and ag-
glomerates to be loosened from the dust/fabric matrix and to seep
through to the cleaned gas stream when filtration resumes(3).
More seepage occurs at high velocity both because the thicker re-
deposited dust layer provides more particles which potentially can
seep through, and because high filtration velocity drives the bag
against its cage faster and causes it to hit with greater impact,
thereby loosening more dust from the deposit. If redeposition can
be minimized in a pulse-jet filter, reductions in pressure drop
and penetration should follow.
The gas flow pattern within the filter housing should strong-
ly influence redeposition. When the filter cake is freed from
the surface of a bag during a cleaning pulse, it must fall through
the filter housing to reach the dust hopper. The direction and
velocity of gas flowing within the housing should have a. strong
influence on whether this dust falls to the hopper or redeposits
on a bag. When the gas in the housing helps this freed dust reach
the hopper rather than hinders its transport there, redeposition
should be reduced and filter performance improved.
Commercial pulse-jet filters are available.with gas inlet
at the bottom or top of the filter housing. A bottom inlet is
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often preferred because it permits large particles to fall into
the dust hopper immediately on entry and avoids their collection
upon the filter bags. It is believed that this results in re-
duced bag wear and lower bag resistance.
EXPERIMENTS
The influence of gas motion within the filter housing on
filter performance was evaluated using a three bag filter (Figure
4-1) operated at superficial filtration velocities of 50, 75, 100,
and 125 mm/s with gas inlet at the top or bottom of the filter
housing. Data which describe the characteristics and operation
of the filter are given in Table 4-1.
_ _ TABLE 4-1. FILTER CHARACTERISTICS _
Bags: untreated polyester; 0.54 kg/m2; 144 mm dia., 2.44 m long;
150 mm/s at 124 Pa permeability
Dust: fly ash; count median dia. 0.3 um, geometric standard devia-
tion 2.7; inlet flux 9 x 10~5 kg/m2-s; density 2200 kg/m3
Pulses: pressure 6.8 atm; interval once/min-bag; valve time 75 ms
electrical, 240 ms mechanical
Upstream and downstream isokinetic dust samples were collected
on tared glass fiber papers and used to calculate mass penetration.
Replicate experiments were run in random order, except that all
experiments with top inlet were concluded before experiments with
bottom inlet were begun. The data presented are averages of the
replicates.
RESULTS
Figure 4-2 is a plot of filter bag resistance against super-
ficial filtration velocity for gas inlet located at the top or at
the bottom of the filter housing. Although at the lowest velocity
tested bag resistance was nearly the same for- bottom inlet and top
inlet, as velocity increased the resistance rose much more rapidly
for the bottom inlet configuration. At the highest filtration ve-
locity attained with a bottom inlet, resistance and fan energy con-
sumption were about four times higher than for the top inlet.
Figure 4-3 is a plot of dust penetration against superficial
velocity for gas inlets located at the top or bottom of the hous-
ing. Penetration was moderate at the lowest filtration velocity
tested. As velocity increased, penetration increased also; how-
ever, the increase was much more rapid for the filter with bottom
•inlet. At the highest filtration velocities tested, penetration
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Figure 4-1. Schematic drawing of pulse-jet filter with
bottom inlet (left) and top inlet (right).
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I
I
SO
75 IOO
F/LTtMr/ott uetocrrr, mm/s
Figure 4-2.
Pressure drop vs. superficial filtration
velocity, inlet location as parameter.
8
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0./2
SUP£/tr/CML f/LT^r/Off VeLOC/TY, mm/s
Figure 4-3.
Overall mass penetration vs. superficial filtra-
tion velocity, inlet location as parameter.
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became too high for many control applications with either inlet.
DISCUSSION
It will be obvious that gas inlet location has a strong in-
fluence on. filter resistance and dust penetration for pulse-Jet
cleaned filters. Figures 4-2 and 4-3 show that as filtration ve-
locity increases the top inlet filter performs considerably better
than the bottom inlet filter. This occurs because as the volu-
metric gas flowrate through the baghouse increases, both the velo-
city of the unfiltered gas Inside the filter housing, and the velo-
city of the gas through the filter bags also Increase.
High horizontal gas velocity through the bags draws particles
and agglomerates back to the bag surface Immediately after a
cleaning pulse and thereby causes a thick dust deposit, regardless
of the gas inlet location. However, high filtration velocity al-
so causes greater vertical gas velocity in the filter housing—
downward if gas inlet is at the housing top, and upward if it is
at the bottom. With top inlet, particles and agglomerates freed
by a cleaning pulse are drawn toward the dust hopper by the gas
flowing downward through the filter housing. With bottom inlet,
gas flows upward through the filter housing. This keeps dislodged
dust suspended inside the housing and permits easy redeposition
on the filter bags.
With bottom entry, the net upward gas velocity within the
Housing will be greatest at the housing bottom where the entire
gas stream must enter and flow upward to be filtered, and least
at the top, by which point all process gas will have been drawn
through the bags. Following a cleaning pulse dislodged dust must
fall through the housing bottom, where upward gas velocity is
highest. To reach the hopper, particles and agglomerates must
have a terminal settling velocity greater than that of the upward
moving gas. Clearly, when small particles or particles which do
not agglomerate readily are to be collected in a pulse-jet filter,
a gas inlet near the housing top is of substantial advantage com-
pared to a gas inlet near the bottom. The advantage becomes
greater as filtration velocity increases.
SUMMARY
The performance of a pulse-Jet filter is adversely affected
by redeposition of dust on the filter bags following a cleaning
pulse. Redeposition increases with increasing filtration velo-
city because high velocity filtration flow sweeps dust and agglom-
erates back to the bag surface. Redeposition also increases when
a bottom inlet configuration is used because the net upward gas
flow within the filter housing reduces the effective velocity with
which particles and aggomerates released by a cleaning pulse can
settle to the dust hopper. Redeposition is minimized with top in-
"let because the gas flow is downward and dust fall toward the
10
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hopper is promoted. As a result, pressure drop and penetration
are minimized. For a superficial filtration velocity of 125 mm/s,
filter resistance was observed to diminish by a factor of four
and penetration by a factor of two when gas inlet was moved from
bottom to top, all else remaining constant.
REFERENCES
1. Dennis, Richard and John Wilder. Fabric Filter Cleaning
Studies. EPA-650/2-75-009, Environmental Protection Agency
Office of Research and Development, Washington, 1975-
2. Mohamed, M. H., E. M. Afify and J. Vogler. Needle Punched
Fabrics in Filtration. Paper presented at Technical Sym-
posium on Nonwoven Product Technology, International Non-wo-
vens and Disposables Association, 10 E. 40th St., New York,
1974.
3. Leith, David and Melvin W. First. Performance of a Pulse-Jet
Filter at High Filtration Velocity I. Particle Collection.
J. Air Poll. Control Assoc., 27:534, 1977.
4. Leith, David and Melvin W. First. Performance of a Pulse-Jet
Filter at High Filtration Velocity III. Penetration by Fault
Processes. J. Air Poll. Control Assoc., 27:754, 1977.
5. Holland, C. R. and E. Rothwell. Model Studies of Fabric Dust
Filtration 1. Flow Characteristics of Dust Cakes Uniformly
Distributed on Filter Fabrics. Filtration and Separation, 14:
30, 1977.
6. Holland, C. R. and E. Rothwell. Model Studies of Fabric Dust
Filtration 2. A Study of the Phenomenon of Cake Collapse.
Filtration and Separation, 14:224, 1977.
7. Leith, David and Melvin W. First. Pressure Drop in a Pulse-
Jet Fabric Filter. Filtration and Separation, 14:473, 1977.
8. Bakke, Even. Optimizing Filtration Parameters. J. Air Poll.
Control Assoc., 24:1150, 1974.
9. Leith, David, Melvin W. First and Henry Feldman. Performance
of a Pulse-Jet Filter at High Filtration Velocity II. Filter
Cake Redeposition. J. Air Poll. Control Assoc., 27:636, 1977.
11
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SECTION 5
EFFECT OF FABRIC TYPE ON PERFORMANCE
OF PULSE-JET FABRIC FILTER
INTRODUCTION
In pulse-jet cleaning of bag filters, less than 155 of the
dust cake is typically removed by a single pulse(l). This is due
to failure.of the pulse to remove some of the dust cake and to
redeposition of removed dust. Some work has been done to mini-
mize redeposition of the dust(2,3). This report is concerned
with the use of alternative bag materials designed to increase
the ease of dust removal by a pulse. Three types of material
were studied: untreated needled-felt polyester; (Joretex®, a
polyester fabric covered with ptfe and Terytex®, a needled-
felt polyester whose outer surface has been singed to remove pro-
truding fiber ends. The nominal weight of all fabrics was 0.5^
kg/m2.
Figure 5-1 is a photomicrograph of an untreated polyester
bag, magnified 50 times. Although unused, this bag was slightly
dirty as particles can be seen adhering to the fibers in the
photograph. The average fiber diameter was 18 urn.
Figure 5-2 shows the outer surface of a Goretex bag magnified
225 times. Ptfe filaments can be seen, stretched over a matrix
of polyester fibers. A large portion of the photographed area is
blinded. Although this is a new bag, examination of two small
samples showed this type of blockage covered a large portion, per-
haps half the area, of the samples examined. Figure 5-3 is an
800X magnification of the same fabric, showing an area not affected
by the blockage. The mean fiber diameter is,20 um for the matrix
and less than 1 ym for the ptfe filaments.
Figure 5-4 is a photograph of the outer surface of a Terytex
bag, magnified 50 times. The melted bits of polyester, a result
of the singing process, are evident. The mean fiber diameter is
19 nun.
Each type of bag was operated at several filtration veloci-
ties, and the effects of velocity and fabric on pressure drop and
penetration were studied.
12
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Figure 5-1. Untreated polyester (50x).
Figure 5-2. GoretexR (teflon-covered polyester)(225x)
13
-------
Figure 5-3. GoretexK (800x).
Figure 5-4. TerytexK (singed polyester) (50x)
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EXPERIMENTS
The experimental apparatus is shown in Figure 5-5. Air
flow through the filter is induced by a blower connected to the
outlet duct. Filtration velocity is controlled by a blast gate
in line with the blower. The inlet and outlet ducts are circu-
lar, 8" diameter, and the filter bags are Ilk mm diameter x 2.44 m
long and are supported by internal metal cages. Fly ash is meas-
ured and fed by a turntable dust feeder and is entrained in the
air stream by an aspirator. The fly ash, density 2200 kg/m3 from
a coal-fired power plant, has a particle size distribution by
count given in Figure 5-6. The resulting aerosol is directed in-
to the inlet duct and passes a Stairmand disk near the entrance.
The Stairmand disk(4) provides mixing of the dust with the inlet
gas stream and causes a pressure drop, measured by manometer A,
which increases with the air flowrate. Manometer B measures the
total pressure drop across the dust deposit, bags, cages, and ven-
turis. The bags are cleaned by pulses of 6§0 kPa (100 psig) com-
pressed air. One of the 3 bags is pulsed every 20 seconds, so
that each bag is cleaned once per minute. Each pulse is 250 milli-
seconds in mechanical duration. The dust removed from the bags
falls into the hopper and is transferred via the rotary feeder in-
to the dust bin below. Dust penetration through the bags is deter-
mined by sampling isokinetically at the upstream and downstream
taps. Samples are drawn by a vacuum pump through MSA 1106-B paper
for mass samples or 0.2 um nuclepore paper for particle sizing.
Relative humidity is not controlled, but is measured using a sling
psychrometer. The recycle loop, shown in dashed lines, was in
place during the Goretex© experiments. It was used to test the ef-
fect of recirculation on baghouse air flow patterns. These results
are not discussed here.
Fifteen experiments were run with the untreated polyester
bags: three replications at each of the following filtration velo-
cities: 50, 75, 100, 125, and 150 mm/s. Equilibrium (constant pres-
sure drop across the bags) was not achieved in any of the 150 mm/s
runs. The Goretex bags were tested at 50, 100, and 150 mm/s in
a total of 24 experiments. Again, none of the 150 mm/s experiments
reached equilibrium. Eight experiments were completed with the
Terytex bags, at the following velocities: 50, 100, 150, 175, 200
and 225 mm/s. Equilibrium was reached in all the Terytex experi-
ments.
To determine the clean fabric resistance, Ki, a sample was cut
from a new bag of each of the three materials. Each fabric sample
was placed in an air stream at 5 known flowrates, and the pressure
drops were measured. The slope of a plot of pressure drop versus
air velocity is Kj_.
Dust deposit areal densities were determined (1) from bags equil
brated under normal filter operating conditions. After equilibrium
was reached,the bags were removed from their housing. Each bag was
15
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Figure 5-5. Equipment for pulse-Jet filter experiments.
-------
10 20 30 40 SO «0 70 do
0.«
U.7
II.(i
II. 4
0.)
0,]
0<1
Percent of pcrtlclti equal to or evallcr Chan •t*t«d •!<*, by count
Figure 5-6. Particle size distribution for fly ash.
-------
examined at 10 cm intervals along the bag for a total of 24 loca-
tions, by a beta gauge which measured the counts per minute of
beta radiation passing through the dust deposit. The density
of the dust deposit was then calculated.
RESULTS
Values of the dust deposit specific resistance, K2, were
calculated from the dust deposit areal density measurements using
the method developed by Ellenbecker (1) . This method avoids the
assumption of uniform dust deposit over all the bags. Each of
the 3 bags was divided into 24 segments and the areal density was
determined for each segment, using beta absorption and correcting
the results based on the actual weight of the dust deposit. The
value of I?2 was then calculated, by iteration, from the following
equation:
Ap = vA
-1
n A
Ai
I
1=1 Kl + K2wi
(5-1)
Data and calculated values of w, K^, and K2 are shown in Table 5-1.
Figure 5-7 is a plot of pressure drop versus velocity, and
shows that pressure drop Increases with velocity, and is higher
for Goretex than for the other fabrics. Figure 5-8, penetration
versus velocity, shows penetration increasing with velocity, Tery-
tex generally shows the lowest penetration. Figure 5-9 shows that
dust deposit specific resistance, K2, is not strongly dependent on
velocity but is dependent on fabric type. Figure 5-10 shows the
dust deposit density increasing with velocity for Goretex and Tery-
texa but remaining roughly constant for the untreated polyester,
It can also be seen that Goretex retained the least dust, Terytex
retained somewhat more, and untreated polyester had the highest dust
deposit areal density.
DISCUSSION
The relationships between filtration velocity and pressure
drop, penetration, apparent specific resistance of the dust deposit,
and. areal density of the dust deposit on bags made from untreated
polyester, Gore-tex, and Terytex are discussed in this section.
AltTiough the data will be discussed in terms of their dependence on
filtration velocity, as for example a plot of pressure drop against
velocity, the inference should not necessarily be made in all cases
that velocity changes alone cause the trends plotted. For example,
increased filtration velocity may increase redeposition of dust
freed by a cleaning pulse, leading to a dust deposit with higher
areal density. The apparent specific resistance of this deposit may
then be greater as the deposit more closely approximates a true
dust cake. The increases in pressure drop found with increased
filtration velocity may, therefore, be more the result of changes
18
-------
TABLE
E-JET
RESULTS
POLYESTER 50 mm/s '
Ap(Pa) Pt K (s ' w RH c± KI
(kg/m2) (%) (g/mj) (Pa-s/m)
240 j .016
220 S .02
240 ] .016
75 mm/s
100 mm/s
125 mm/s
GORETEX 50 mm/s
100 mm/s
TERYTEX 50 mm/s
100 mm/s
150 mm/s
175 mm/s
200 mm/s
225 mm/w
300
1470
420
630
1630
580
800
780
760
1957
1553
1425
2065
1996
1504
2095
2035
3605
4028
4077
5120
3979
4530
4963
4943
245
750
2110
610
1250
725
680
3525
.023
.028
.021
.032
.079
.037
.069
.06
.058
.005
.01
.014
.009
.005
.015
.008
.009
.046
.068
.068
.046
.065
.091
.03
.038
.015
.011
.024
.024
10,770
8,758
12,106
8,821
35,198
14,588
14,280
31,469
14,207
17,047
17,069
12,629
185,945
184,505
348,536
371,918
172,698
204,632
344,252
249,715
16,455
23,797
34,752
.046
.039
.072 '
.427 24,948
.3844
.4199
.3489
.3798
.5457
.3478
.4016
.5011
.3718
.3375
.3306
.4405
.1487
.1567
.1189
.1075
.2219
.2151
.1418
.1945
.2757
.3089
.4153
.6315
24
22
28
29
36
24
26
28
30
27
20
28
49
28
20
22
15
16
21
20
26
32
20
22
23
28
18
27
31
42
69
46
64
32
28
62
.97
.977
1.08
.644
.644
.72
.505
.433
.494
.326
.325
.378
.84
.957
.9
.885
.884
.92
.846
.77
.416
.402
.402
.372
.397
.41
.439
.407
.346
.317
.181
.244
.23
.208
.193
.125
85
\
187
\
64
0
/
5
/
(0
1
V
19
-------
CORETEX
.025
_L
J-
.1 .125 .15 .175
Superficial Filtration Velocity (m/s)
.2
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1
1,000 I
188:
700 g
600 u
500
400
2
a
300 £
200
100
Figure 5-7. Effect of velocity and fabric on pressure drop.
-------
I. -I- -W ,1, .rUr ,-U ffV™ ,rV
0.025 0.05 0.075 0.1 o.12i o.i5 n.i/i o.i
1 0
.07 k
.0* 3
•" I
•M
.03
.01
.01
00?
006
005
004
003
I
Superficial Flit rut Ion Velocity IN/.)
Figure 5-8. Effect of velocity and fabric on dust penetration
-------
: i 1 1 r 1 1 1 1 1 :
~
_
; * A
•
"*" ^ (to»tm
"A
• *
•
* O • Tvrytm
*~^^~* "D "
o e -
O Polywttr
A Corcltx
D T«ryt«x
r^ J= A 1 1 1 1 ' i
is
MX), 000
MO.OOO
400,000
100,000
200,000 ^
100,000 1
folorn >t
M.OOO j
M.OOO |
40,000 «
KI.OOO a
20,000 S
n
M
10,000 |
»,ooo r
t.ooo *
7,000 S
6,000 J
5,000
4,000
1,000
2,000
1,000
.1 .125 .15 .175 .2
Sup.rfli-Ul Klltrttlon V.loclty (M/ii)
.221
Figure 5-9- Effect of velocity and fabric on
-------
oo
O Polyester
A Ooretex
C3 Tcrytex
Goretex
Terytex
0.025 0.05 0.075 0.1 0.125 0.15 0.175 0.2 0.225
Superficial Filtration Velocity (M/s)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(i
Figure 5-10. Effect of velocity and fabric on dust deposit areal density,
-------
in the amount and characteristics of the dust deposit than the
result of increased velocity through a deposit of constant pro-
perties. These relationships are discussed in more detail else-
where (1).
Pressure Drop Versus Velocity
Figure 5-7 is a plot of pressure drop versus superficial
filtration velocity, with fabric type as parameter. It shows that
over the range of velocities tested, pressure drop across the Gore-
tex fabric was higher than that found across either the untreated
polyester or the Terytex fabrics. Although the resistance (K.,) of
the new Gore-tex fabric is more than twice that for either of the
other fabrics tested as shown in Table 5-1,, the comparatively
high pressure drop across the clean Gore-tex fabric alone does
not account for its high pressure drop in service; pressure drop
across the new fabric was generally less than 10% of that across
the conditioned fabric at equilibrium conditions. High pressure
drop across the Gore-tex fabric bags is due to the high value of
apparent specific resistance (K2) of the fly ash dust deposits
found on these fabrics. Reasons for the high K2 values found for
Gore-tex are discussed below.
Penetration Versus Velocity
Figure 5-8 is a plot of penetration versus superficial filtra-
tion velocity, with fabric type as parameter. The figure shows
that penetration increases with velocity for each of the fabrics
tested. Penetration through the untreated polyester and Gore-tex
fabrics is somewhat higher than that through the Terytex fabric at
all velocities tested.
Penetration through a pulse-jet filter may be related to impact
of the cleaned bags on their supporting cages at the end of a
cleaning pulse, as normal filtration resumes. Higher filtration
velocity will drive the bags back to their cages faster so they hit
with greater impact, thereby driving more dust through the bags and
aggravating seepage. Differences in penetration for bags of dif-
ferent types may be explained by differences in the ability of each
fabric to prevent dust from being driven through. These results sug-
gest that Terytex bags may be more effective in this regard than
either of the other fabrics tested. An additional factor which may
affect penetration is the magnitude of the dust areal density on
each bag. A bag with more dust on it may be likely to have more
penetration both because more dust is available to seep through
and because the impact with which the bag hits its cage increases
with increasing dust mass; a collision with greater impact may
aggravate seepage.
Apparent Specific Resistance, Kp, VersusJVelocity
Figure 5-9 is a plot of the apparent dust deposit specific re-
sistance, K2, against superficial filtration velocity, again with
24
-------
fabric type as parameter. The data indicate that apparent K2
has very little or no dependence on superficial filtration velo-
city. However, the data do show considerable differences in
apparent K2 with changes in fabric type. These differences may
be explained in terras of the fabric surface structure.
-The surface of the untreated polyester is very open as shown
in the photomicrograph, Figure 5-1- This fabric had no surface
treatment. The Terytex fabric is virtually identical to the un-
treated polyester except that fibers protruding from the felt
surface were singed as shown in the photomicrograph, Figure 5-^.
The surface of the>Terytex fabric is still open, but somewhat
smoother than that of the untreated polyester. Gore-tex surface
is much smoother than either of the other fabrics as shown in
photomicrographs 5-2 and 5-3. The ptfe "mesh" which covers the
surface of the polyester felt substrate for this fabric effectively
precludes dust penetration into the fabric.
As a consequence, the dust deposit that develops using Gore-
tex fabric most closely approximates a true dust cake in that
it lies on the surface of the fabric and does not effectively pene-
trate the fabric interstices. This dust deposit is likely to be
close-packed and compact. By comparison, the dust deposit on the
surface of the Terytex and untreated polyester bags is less con-
fined to the fabric surface, and will be based on fibers within
the felt, below the fabric surface, as well as on the surface of
the felt itself. The dust deposit within the felt is more likely
to form inter-fiber bridges which will increase the porosity of
the dust deposit which develops. A more open, porous dust deposit
will have a lower specific resistance. For these reasons, the re-
latively smooth surface of the Gore-tex fabric leads to a dust
deposit with a relatively high specific resistance, whereas the
relatively rough and open surface of the untreated polyester fab-
ric leads to a deposit with lower specific resistance. The Tery-
tex fabric, whose surface characteristics are between those of
the Gore-tex and untreated polyester, but closer to the latter,
has a dust deposit with apparent specific resistance between those
found on the other two fabrics, but closer to that of the untreated
polyester whose surface it more closely resembles.
If a sufficiently thick dust deposit developed on the surface
of even the most open, porous fabric, the specific resistance of
the dust on the outside surface of this deposit would be close to
that of a true dust cake. However, the surface of most felts used
in pulse-jet'filters, with the exception of Gore-tex, are so open
and porous that it seems unlikely a dust deposit sufficiently large
to approximate a cake, even at its surface, could-be tolerated be-
cause of its high pressure drop. The consequence of high pressure
rlrop would be a need for more effective cleaning to reduce the dust
deposit to a less'cake-like form. In this regard, pulse-jet cleaned
filters differ from shaker or reverse-air cleaned filters which use
woven bags, often with relatively smooth surface, and for which it
is often necessary to develop a true dust cake to assure high collec
tion efficiency.
25
-------
Areal Density of Dust Deposit Versus Velocity
Figure 5-10 displays areal density of the dust deposit
against superficial filtration velocity, again.for each of the
three fabrics tested. For the Gore-tex and Terytex fabrics,
dust deposit increases with increasing velocity. This may be
due to reduced effectiveness of the cleaning pulses as velocity
increases. The increased operating pressure drop found under
higher velocity conditions reduces the effective "reverse pres-
sure drop" across the bags generated by the cleaning pulses,
necessary to expand the bags outward and cause cleaning. An al-
ternative explanation is that under high velocity conditions more
freed dust redeposits at the end of a cleaning pulse rather than
falls to the dust hopper. This causes each pulse to be less ef-
fective at cleaning dust from the bag as velocity increases, al-
though the amount of dust momentarily freed from the bag may re-
main the same. The relative insensltivity of areal density to
filtration velocity for the untreated polyester felt is perplexing.
Additional data, taken at higher velocity than the data shown here,
do demonstrate an increase in areal dust denisty with velocity
for this velocity (1).
SUMMARY
The performance of a pulse-jet filter using bags made from
various materials, as determined by variations in dust penetration
and filter pressure drop, clearly changes with increasing filtra-
tion velocity. Many of these changes are related to differences
in the properties of the dust deposit such as its magnitude or
apparent specific resistance, and to differences in the charac-
teristics of the fabric Itself such as its surface properties or
clean-flow resistance. Although qualitative explanations of per-
formance data presented here can be made in terms of dust deposit
and fabric properties, more satisfactory, quantitative explanations
are not always possible at this time due to our incomplete under-
standing of the processes involved. The data presented and dis-
cussed here are for collection of fly ash. The conclusions reached
might or might not be similar if another dust were to be used, as
the relationships between dust properties and filter performance are,
at present, incompletely understood.
26
-------
NOMENCLATURE
A area of filter bag, square meters
A^ area of segment of filter bag, square meters
C-^ inlet concentration, grams/square meter
K]_ clean fabric resistance, pascal seconds/square meter
K2 dust deposit specific resistance, seconds"1
n number of filter bag segments
AP pressure drop across bag and dust deposit, pascals
Pj. penetration (decimal)
RH relative humidity, per cent
v superficial filtration velocity, meters/second
w dust deposit areal density, kilograms/square meter
•w arithmetic mean areal density, kilograms/square meter
REFERENCES
1. Ellenbecker, M.J. Pressure Drop in a Pulse-Jet Fabric Filter.
Sc.D. Thesis, Harvard School of Public Health, Boston, Ma.,
1979.
2. Leith, David, Melvin W. First and Dwight D. Gibson. Perfor-
mance of a Pulse-Jet Filter at High Filtration Velocities.
In: Symposium on the Transfer and Utilization of Particulate
Control Technology, Volume 2, EPA-600/7-79-944b, NTIS, Spring-
field, Va., 1979.
3. Leith, David, Melvin W. First and Dwight D. Gibson. Effect
of Modified Cleaning Pulses on Pulse-Jet Filter Performance.
Filtration and Separation, September-October 1978, pp. 400-406
4. Stairmand, C.J. Sampling Gas-Borne Particles. Engineering,
... .... ig]|1^
27
-------
SECTION 6
THEORY FOR PRESSURE DROP IN A PULSE-JET
CLEANED FABRIC FILTER
INTRODUCTION
Pressure drop through gas cleaning equipment is an important
component of system operating cost. For this reason, knowledge
of the factors that affect pressure drop, and methods for pressure
drop prediction are important. The characteristics of gas flow
through a porous medium, fundamental to modeling pressure drop in
a fabric filter, have been well studied(l-5). Much of this work
concerns flow through a particle deposit with known properties
such as pore size and porosity. With this information, dust de-
posit specific resistance, K2» the pressure drop per unit velocity
and per unit deposit thickness, can be predicted. Rudnick(5,6)
has shown that K2 can be estimated using an adaptation of the Hap-
pel unit cell model(7). This approach gives much better agreement
with data for the usual case where dust deposit porosity is great-
er than about 0.8 than does the Kozeny and Carman model frequently
used(8).
Before these theories can be applied to a practical fabric
filter, it is necessary to consider the pressure drop contribution
of the dust-conditioned fabric, and to account for the effect of
incomplete fabric cleaning. Some models propose that virtually
all dust is removed from a woven fabric cleaned by shaking or re-
verse air(9»10). However, experimental work by Dennis e_t al. (11,
12) demonstrated that virtually all dust is removed only from
some portions of woven fabrics cleaned by these methods, and that
other portions are apparently not cleaned at all. As cleaning in-
tensity increases, the ratio of cleaned to uncleaned areas in-
creases. Dennis et al. described a model which utilized these
ideas for predicting pressure drop in a filter using woven cloth
cleaned by shaking or reverse air(13).
The fabric characteristics and cleaning method for a pulse-
jet filter with felt bags cleaned on-line are considerably differ-
ent from those of a shaker cleaned filter, so that carryover of
the model proposed by Dennis et al. is inappropriate. For a filter
with woven bags cleaned off-line by shaking, more than half the
dust is usually removed(12). For a pulse-Jet filter with felt
28
-------
bags cleaned on-line, one percent or less of the dust on a bag
may be removed per pulse, and this percentage may decrease as
filtration velocity increases(l4-l6). For a shaker cleaned fil-
ter, dust separates from the fabric in patches(ll,13). For a
pulse-jet filter, the distribution of dust on the bags after clean-
ing is almost uniform(l^). Off-line shaker cleaning allows time
for the separated dust to fall to the hopper before filtration re-
sumes, whereas when a pulse-jet filter is cleaned on-line in a
fraction of a second, separated dust has little time to fall to
the hopper. Much o,f the separated dust may redeposit on neighbor-
ing bags or on the pulsed bag(l6). Bag spacing, dust type, pulse
characteristics, bag tension, housing design and gas flow pattern
within the housing may all affect dust redeposition in a pulse-jet
filter(12,17).
Empirical models(18,19) have been used to describe pressure
drop(Ap). One such model(19) developed for fly ash collected on
polyester bags for a range of'filtration velocities, v, pulse
pressures, P, and areal dust densities added during a filtration
cycle, WQ, is:
V2.34 0.45
Ap a T73% (6-1)
in which
W0 = civt
Here, c-^ is inlet dust concentration and t is time between pulses
to each bag.
This paper presents a theoretical treatment of pressure drop
in a pulse-jet filter. In the development, some assumptions must
be made which should be reexamined as data become available.
DUST SEPARATION FROM FABRIC
First, consider the forces which bind dust to the fabric. If
a certain separation force per unit area, FS/A, were applied to
this dust deposit in the appropriate direction, a certain fraction,
e, would be separated. A hypothetical distribution of these unit
area forces is plotted in Figure 6-1 as e vs. FS/A. The exact
shape of this curve is unknown, but it should pass through the
origin as no dust will be separated if no force is applied. The
value of e should approach unity as an asymptote, because the in-
cremental effect of additional force is likely to diminish after
most of the dust has been removed. Similar curves were developed
by Dennis and Wilder (12) for woven fabrics cleaned by shaking.
They show filter capacity (dust mass removed by cleaning) vs. bag
acceleration during the cleaning process.
29
-------
D
UJ
S
U)
O
is
o
SEPARATION FORCE PER AREA, F$/A
Figure 6-1.
Hypothetical relationship between fraction of dust
deposit removed from fabric, e, and area specific
spearation force. F_/A.
O
-------
For pulse-jet .cleaned filters, experimental measurements de-
monstrate that e is less than a few percent(14-16). For lack of
better information, it may be assumed that the relationship be-
tween e and Fg/A is a simple proportionality:
e = kFs/A
(6-3)
Next, consider the interaction of separating forces with the
dust deposit and fabric. The net force per unit fabric area dur-
ing cleaning which drives the fabric and dust away from the cage
that supports them during filtration will be called the reverse
pressure drop, Apr; it is the difference between the static pres-
sure developed by the pulse within the bag, Ps, and the operating
pressure drop across the bag at the time the pulse begins, Ap:
Apr = PR-Ap
(6-4)
If the jet pulse develops a maximum pressure less than the pressure
drop across the bag (Ps
-------
drop interacts with the bag to remove dust; Kj^ expresses the ef-
fect of reverse fabric motion. Strangert(20) reports that in-
creased reverse pressure drop increases cleaning effectiveness.
Dust separated from the fabric must fall to the hopper to be
removed from the system. However, some separated dust may rede-
posit on the fabric before reaching the hopper, and cause actual
fractional separation to be less than that predicted by Equation
6-3. Redeposition may cause the effective value of constant K3~
in a full-size filter to depend on factors that affect redeposi-
tion, such as filtration velocity, housing design, or other vari-
ables. The present model does not account for the effect of re-
deposition; further work on this subject is planned.
AREAL DENSITY OF DUST DEPOSIT
Models for the pressure drop across a woven fabric generally
assume that total drop equals the sum of losses across the con-
ditioned fabric, K^v, and across the dust deposit, K
Ap = K^ + K2vw (6-9)
K-, is the resistance of the fabric after it has been operated for
some time and is well conditioned with dust, K2 is the specific
resistance of the dust deposit, and w is the areal density of the
dust deposit on the bags.
A mass balance for the dust mass per unit area on a bag can
be written for one filtration cycle, dN, which includes by defini-
tion exactly one cleaning pulse:
Additional areal density Decrease in areal density Accumulation
due to dust added to the - due to dust removed from = of areal den-
bag per cycle the bag per cycle sity on the
bag per cycle
»o - WE • m (6-10)
Substitution of Equations (6-8) and (6-9) into Equation (6-10)
gives:
This can be solved after the following substitutions are made:
a = -wQ (6-12)
b = K3PS - K-^v - K4 (6-13)
c = -K2K v (6
32
-------
q = (K3Ps - K1K3v -K^) - llWgK^v (6-15)
The solution to Equation (6-11) is:
in which
2cw + b - /q
P(N) =( ) exp (-N/q) (6-17)
2cw± + b - /q
Here w. is the initial areal density, the areal density of the
dust deposit at N = 0, the beginning of the first filtration cycle
when operating conditions are changed. Equation (6-16) gives the
areal density of the dust deposit on the bag at any time; that is,
at any number of filtration cycles, N, since operating conditions
are changed and cycle counting begins.
EQUILIBRIUM
If operating conditions are constant, Equation (6-16) can be
used to determine the value of areal dust density that the filter
will achieve at equilibrium (N-»- °°). In this case, Equation (6-17)
shows that F(N)+0 and Equation (6-16) becomes:
w = - * (6-18)
or
.v /(P -K,/K,-K,v)2 - 4wnv
± -- - § — I - 3 — ± - y
P -
w = _§
w '
-
2K2v
Substitution of Equation (6-19) into Equation (6-2) gives equili-
brium sure dro:
brium pressure drop:
P -K-./K-.+K-.v /(P -K11/K,-K1v)2 - 4wnvK9/K,
Ap = —— 2 —~——\ (6-20)
SUBSTITUTION
Experiments have established that under certain operating
conditions pressure drop across a pulse-jet filter does not stabi-
lize but increases without limit. Mathematically, this is equiva-
lent to a situation in which F(N) = 1, as can be seen by inspection
of Equation (6-16). Solution of Equation (6-17) for this condi-
tion gives the following criterion, which must be satisfied for
stable filter operation:
33
-------
2cw± + b + /q > 0 (6-21)
or
>0 (6-22)
The square root term within Equation (6-22) contains terms related
to the characteristics of the filter, KI through Kij, and to the
conditions under which it is operated, Ps, v, and WQ, and must be
real if stable filter operation is to be possible:
(Ps-K4/K3-K1v)2 - IIWQVK^ > 0 (6-23)
If Equation (6-23) is not satisfied, the characteristics of the
filter or its operating conditions must be changed.
The balance of Equation (6-22) contains variable wis the
areal density of the dust deposit at time zero. If this term is
sufficiently large, that is, if too much dust is on the bags, the
stability criterion given by Equation (6-22) will not be met and
pressure drop will increase without limit. However, for the same
filter and operating conditions, stability may be achieved if the
initial areal density is low enough. This has practical implica-
tions for fabric filter operation. Before operating conditions
such as filtration velocity, v, dust type, K2, or pulse pressure,
Ps, are changed, the bags should be pulsed repeatedly to remove
as much dust as possible and decrease w^. This will give the
best opportunity for the system to achieve stable operation under
the new operating conditions.
Equation (6-22) can be rewritten:
Ap± < ApL (6-24)
where
APi = KlV + K2vw± (6-25)
and
PS-K4/K +KIV /(PS-K4/K -
= - - - "
2 2 (6-25)
The actual operating pressure drop, Ap^, immediately after operat-
ing conditions change must not exceed a certain limiting pressure
drop, ApL, defined by Equation (6-26). If Ap± is higher than ApL,
then the system is unstable and pressure drop will increase with-
out limit. -If, however, Apj. is lower than this quantity, then the
system will be stable and approach pressure drop equilibrium.
-------
For example, a filter might have a stable, equilibrium pres-
sure drop of 1000 Pa under a particular set of operating condi-
tions. Pulse pressure might then be reduced, causing a reduction
in PS, and in ApL as predicted by Equation (6-26). In this case,
the Initial pressure drop under the new conditions Ap^ (here equal
to 1000 Pa) may now be greater than APL, and violate the stability
condition set by Equation (6-24). If this occurs, pressure drop
would rise from its initial value of 1000 Pa and increase without
limit. However, if the reduction in PS had been less, the value
for ApL calculated in Equation (6-24) would be greater and the
stability condition given in Equation (6-22) might still be satis-
fied. In that case, pressure drop would still rise from its ini-
tial value of 1000 Pa, but would eventually come to equilibrium.
Figure 6-2 is a plot of pressure drop, Ap, as given by Equa-
tion (6-20) against WQ, the dust areal density added during a fil-
tration cycle for typical operating conditions. The figure shows
that as more dust is fed to the bags between pulses, either by
increasing inlet dust concentration or by increasing the time be-
tween cleaning pulses, equilibrium pressure drop increases as well,
At the maximum value of WQ for which stable filter operation is
possible, the value of d(Ap)/dWQ becomes infinite; that is, a
slight increase in WQ causes equilibrium pressure drop to increase
at an infinite rate. This point is at the edge of the unstable
region defined by Equation (6-23).
Also shown in Figure 6-2 is a plot of ApL, the maximum ini-
tial pressure drop for which the filter will ultimately achieve
stable operation, given by Equation (6-26). Comparison of Equa-
tion (6-26) and Equation (6-20) for equilibrium pressure drop
shows they are identical except for the sign on the square root
term.
The arrows shown in Figure 6-2 represent the direction in
which pressure drop will proceed over time and pulses, starting
at any point on the figure. The arrows were determined by evaluat-
ing the derivative of areal density with respect to number of
pulses at various points. For all initial conditions leading to
pressure drops within the parabola shown in Figure 6-2, pressure
drop will decrease to reach equilibrium. For all initial condi-
tions leading to pressure drops outside the parabola, pressure
drop will increase either until it intersects the parabola and
reaches equilibrium, or else without limit.
COMPARISON WITH DATA
The static pressure increase within a bag during cleaning,
Ps, is related to the rate at which air is forced backwards
through the bag during cleaning. For a jet-pump, this flowrate
depends in part upon jet pressure. The relationship between pulse
pressure and static pressure developed, Ps, was determined for a
35
-------
8000 -
6000 -
O
S
ve 0.075 m/s
Ps=8280 Pa
K,«850Pas/m
EQUILIBRIUM
AP
4000 -
2000 -
0 0.01 0.02
DUST AREAL DENSITY ADDED DURING
ONE FILTRATION CYCLE, w_, kg/m*
Figure 6-2.
Pressure drop, AP, vs. dust areal density added
during one filtration cycle, WQ.
36
-------
6.4 mm (V) Jet discharging into a standard pulse-jet venturi
using the apparatus shown in Figure 6-3. Compressed air was in-
jected continuously and the no-flow static pressure across the
venturi, Pg, measured. Results of these tests for several pulse
pressures are given in Figure 6-4, and are described by:
P,(Pa) = 164 P(kPa)0'6 (6-2?)
a
Pulse pressure, P, in pounds per square inch is converted to kPa
through multiplication by 6.9.
Equation (6-20) for equilibrium pressure drop requires values
for operating variables w0, v, and Ps as well as for constants K^,
K2/K3, and Kh/K^. Data on the pressure drop characteristics of a
three bag pulse-Jet filter operated at four filtration velocities
from 50 to 125 mm/s have been reported and discussed in detail
elsewhere(14), and are summarized in Table 6-1. The test dust was
fly ash collected electrostatically at a coal-fired utility boiler.
Examination of Equations (6-7) and (6-8) shows that:
K^/I^ = v* wb/t± (6-28) .
Dennis and Wilder(12) determined that the maximum outward velocity,
v*, attained by one polyester felt bag laden with fly ash is 1.4
m/s, and that the time necessary to reach this velocity was about
5.5 milliseconds. If the clean fabric has an areal density of 0.54
kg/m2, the value of Kh/K, from Equation (6-28) is 137 Pa, less
than 2J8 of Ps from which it is subtracted in Equation (6-20) to
calculate equilibrium pressure drop. In this case, K^/K^ may per-
haps be neglected, making subsequent analysis much simpler. The
size of the error introduced by this assumption is not well estab-
lished, but may be small unless the filter is operated in an un-
stable or nearly unstable manner.
If Kij/K3 is neglected, then Equation (6-20) for equilibrium
pressure drop involves as constants only K^ and the ratio I^/K^.
Lacking other data, the resistance of the conditioned fabric,
Kn, might be approximated from the inverse of clean fabric Frasier
permeability, F: ',
K^Pa-s/m) = 24^500 (6_29)
F (cfm/ft at % inch of water)
The consequence of this assumption is to incorporate all pressure
drop beyond that across the clean fabric into the term for resist-
ance across the dust deposit. Although this approximation may not
be realistic for conditioned fabrics which retain an appreciable
amount of dust, it does allow the analysis to proceed when data on
the actual resistance of the conditioned fabric are unavailable.
37
-------
COMPRESSED AIR
TO
ELECTRICAL
CONTROL
PRESSURE
GAUGE
PULSE
VALVE
VENTURI
THROAT
MANOMETER
FOR
MEASURING
Figure 6-3. Apparatus used to determine relationship between
pulse pressure, P, and induced static pressure, P
.
s
38
-------
Q
CO
UJ
8000
O
>
U4 —
^ Z 4000
UJ UJ
CO
co o
£ | 2000
U Q
1000
1 1 1 1 1 1 1 1
164 P(kPa)
1 1 I 1 1 i 1 I i
100 200 400 800
PULSE PRESSURE, P, kPa
Figure 6-4. Static pressure induced, P , vs. pulse pressure.
s
-------
Table 6-1. EXPERIMENTAL CONDITIONS FOR PULSE-JET FILTER TESTS
Number:
Size:
Fabric:
Weight:
Permeability:
Kn :
Pulses
Dust
Frequency
Pulse Pressure, P:
Induced Pressure, P,
Type:
Count Median Diameter:
Geometric Standard
Deviation:
WQ:
Pressure Drop
mm dia. and 2.44 m long
polyester felt, no surface treatment
0.54 kg/m2 (16 oz/yd2)
30 cfm/ft2 at \ inch of water
850 Pa-s/m (new fabric)
once per minute to each bag
690 kPa (100 psig)
8280 Pa (33 inches H20)
Fly ash precipitated from coal-fired
utility boiler
0.30 urn
2.7
3.1 x 10~3
kg/m'
Velocity, mm/s
50
75
100
125
Average:
AP, Pa
260
390
615
790
K?/K3, Pa/s
1.13 x 1010
1.11 x 1010
1.31 x 1010
1.32 x 1010
1.17 x 10
10
Kjj/K3> Pa
assumed equal
to zero, all
conditions
0
If KT is known or assumed, and if equilibrium pressure drop
is known for one set of stable operating conditions, then I^/Kg
can be determined and pressure drop under any other stable operat-
ing conditions can be found using Equation (6-20). Values of
K2/K^ were determined in this way for pressure drop at each velo-
city listed in Table 6-1. The values of K2/Ko were found to be
relatively constant over the range of filtration velocities con-
sidered.
Figure 6-5 is a plot of pressure drop against filtration ve-
locity for the data in Table 6-1. The line is based on Equation
(6-20), with K2/K^ equal to its average value for these data, and
reflects well the trend of pressure drop vs. velocity given by the
40
-------
1000
800
L 600
O
at
O
to
to
IU
OC
a.
ca
tu
400
200
I
I
THEORY
I
I
I
I
0 0.05 0.10 0.15
FILTRATION VELOCITY, m/s
Figure 6-5. Equilibrium pressure drop, AP, vs.
filtration velocity, v.
-------
data. This analysis gives no indication whether agreement between
theory and data will be as good for variations in variables other
than filtration velocity.
The constants listed in Table 6-1 are specific for the appara-
tus and experimental conditions used in their determination.
Changes in pulse pressure or changes in amount of dust fed to the
bags between pulses were not made in these experiments. However,
Equation (6-20) can be used to estimate the effect on pressure •
drop of variations in these variables. Results are plotted in
Figures 6-6 and 6-7, pressure drop vs. pulse pressure and WQ, re-
spectively. In Figure 6-6, pulse pressure converted to Ps using
Equation (6-27) before the theoretical pressure drop line was
plotted.
Equation (6-1) relates pressure drop to these variables em-
pirically and was established for fly ash collected on polyester
felt bags. However, the dust and bags used to develop the con-
stants used in the theoretical analysis were different. The
slopes of the pressure drop vs. pulse pressure and pressure drop
vs. WQ relationships predicted by the empirical equation are also
given in Figures 6-6 and 6-7. The lines given by the theory and
the empirical results are in general agreement. A more complex
test of the theory presented here awaits development of more data.
SUMMARY
A theory for pressure drop in pulse-jet cleaned filters has
been presented. Assumptions made are that total pressure drop
is the sum of drops across the conditioned fabric and the dust
deposit, that the fraction of dust separated from a bag depends
upon the separation force applied, and that this force can be
found from impulse and momentum considerations as the bag and dust
deposit move away from their supporting cage during a cleaning
pulse.
The theory allows prediction of operating conditions under
which filter operation will become unstable and cause pressure
drop to increase without limit. With simplifying assumptions, the
theory can be used to interpret pressure drop data or to predict
the equilibrium pressure drop of a pulse-jet cleaned filter
operated under any operating conditions.
NOMENCLATURE
A fabric area, m2
a see equation (6-12)
b see equation (6r-13)
c see equation (6-l4)
c-j^ dust inlet concentration, kg/m^
F Frasian permeability (cfm/ft2 @ 1/2" HpO)
P(H) see equation (6-17)
F_ force acting to separate dust deposit from fabric, N
-------
700
600
EMPIRICAL
EQUATION
o
oc
O
CO
(O
IU
500
400
§ 30°
IU
THEORETICAL
EQUATION
v = 0.075 m/s
w0= 0.0031 kg/m*
Kj'850 Pas/m
K2/ 1
200
1. 17
1
Pa/$
1
400 600 800
PULSE PRESSURE. P, kPa
1000
Figure 6-6.
Equilibrium pressure drop, AP,
vs. pulse pressure, P.
-------
1000
800
i
o
DC
400
1 200
1
v = 0.075 m/$
P$= 8280 Pa
Kf 850 Pa t/m
K2/K3« I.l7x|0 Pa/s
EMPIRICAL
EQUATION
THEORETICAL
EQUATION
0.002 O004 0.006
DUST AREAL DENSITY ADDED DURING
ONE FILTRATION CYCLE, w.. kg/m*
o
0.008
Fieure 6-7. Equilibrium pressure drop, AP, vs. areal dust
density added during one filtration cycle, WQ.
-------
KI fabric resistance, Pa-s/m
K2 specific resistance of dust deposit, s""1
Kg constant, see equation (6-8)
Kjj constant, see equation (6-8)
k proportionality constant, see equation (6-3)
N number of filtration cycles
P pulse pressure, kPa
Ps maximum static pressure developed inside bag as result of
cleaning pulse, Pa
q see equation (6-15)
t time between pulses to each bag, s
t^ time during which APr acts, s
t2 time during which Ps acts, s
v superficial filtration velocity, m/s
v* maximum velocity achieved by fabric and dust during clean-
ing, m/s 2
w areal density of dust deposit, kg/m
wb areal density of fabric, kg/m2
w^ dust deposit areal density at N=0, when operating condi-
tions change
WQ dust areal density added during one filtration cycle, kg/m2
Ap pressure drop across bag and dust deposit, Pa
ApA pressure drop at N=0, when operating conditions change, Pa
APL minimum Ap^ if stable operation is to be attained, Pa
Apr difference between static pressure inside and outside the
bag during cleaning, Ps-Ap, Pa
e fraction of the dust deposit removed from a fabric
REFERENCES
1. Carman, P. C. Fluid Flow Through Granular Beds. Trans. Inst.
Chem. Eng., 15:150, 1937-
2. Williams, C., T. Hatch and L. Greenburg. Determination of
Cloth Area for Industrial Filters. Heat. Piping Air Cond.,
12(4):259, 19^0.
3- Stephan, D. G., G. W. Walsh and R. A. Herrick. Concepts in
Fabric Air Filtration. Am. Ind. Hyg. Assoc. J., 21(1):1,
I960.
4. Pich, J. Pressure Characteristics of Fibrous Aerosol Filters.
J. Colloid Interface Sci., 37(4):912, 1971.
5. Rudnick, S. N. Fundamental Factors Governing Specific Re-
sistance of Filter Dust Cakes. Sc.D. Thesis, Harvard School
of Public Health, Boston, Ma.,.1978.
-------
6. Rudnick, S. N. and M. W. First. Specific Resistance (K2) of
Filter Dust Cakes: Comparison of Theory and Experiments. 3rd
Symposium on Fabric Filters for Particle Collection.
EPA-600/7-78-087, NTIS, Springfield, Va., 1978.
7. Happel, J. Viscous Flow in Multiparticle Systems: Slow Motion
of Fluids Relative to Beds of Spherical Particles. AIChE J.,
4(2):1969 1958.
8. Billings, C.E. and J. Wilder. Handbook of Fabric Filter Tech-
nology. Volume I. Fabric Filter Systems Study. NTIS Number
PB-200-648, Springfield, Va., 1970.
9- Davis, W. T., P. J. LaRosa and K. E. Noll. The Generation
and Evaluation of Fabric Filter Performance Curves from Pilot
Plant Data. Filtr. Sep., 13(6):555, 1976.
10. Robinson, J. W., R. E. Harrington and P. W. Spaite.• A New
Method for Analysis of Multicompartmented Fabric Filtration.
Atmos. Environ., 1(4):499, 1967.
11. Dennis, R., R. W. Cass and R. R. Hall. Dust Dislodgement.from
Woven Fabrics Versus Filter Performance. J. Air Poll. Control
Assoc., 28(1):47, 1978.
12. Dennis, R. and J. Wilder. Fabric Filter Cleaning Studies.
EPA-650/2-75-009, NTIS, Springfield, Va., 1975.
13. Dennis, R., R. W. Cass, D. W. Cooper, R. R. Hall, V. Hampl,
H. A. Klemm, J. E. Langley and R. W. Stern. Filtration Model
for Coal Fly Ash with Glass Fabrics. EPA-600/7-77-084, NTIS,
Springfield, Va. , 1977-
14. Ellenbecker, M. J. Pressure Drop in a Pulse-Jet Fabric Filter.
Sc.D. Thesis, Harvard School of Public Health, Boston, Ma.,
1979.
15. Ellenbecker, M. J. and D. Leith. Pressure Drop in a High Ve-
locity Pulse-Jet Fabric Filter. Paper 78-62.6 presented at
the 71st Annual Meeting of the Air Pollution Control Associa-
tion, Houston, Tx., 1978.
16. Leith, D., M. W. First and H. Feldman. Performance of a
Pulse-Jet Filter at High Filtration Velocity II. Filter Cake
Redeposition. J. Air Poll. Control Assoc., 27(7):636, 1977.
17. Leith, D., D. D. Gibson, and M. W. First. Performance of Top
and Bottom Inlet Pulse-Jet Fabric Filters. J. Air Poll. Con-
trol Assoc., 28(7):696, 1978.
-------
18. Dennis, R. and L. Silverman. Fabric Filter Cleaning by Inter-
mittent Reverse Air Pulse. ASHRAE J., 4(3):43, 1962.
19- Leith, D. and M. W. First. Pressure Drop in a Pulse-Jet Fab-
ric Filter. Filtr. Sep., l4(5):473, 1977.
20. Strangert, S. Predicting Performance of Bag Filters. Filtr.
Sep., 15(1):42, 1978.
-------
SECTION 7
HIGH VELOCITY FIBROUS FILTRATION
INTRODUCTION
Mats of fibrous materials have been used for many years to
collect particles from gas streams(l). Several different collec-
tion mechanisms, including diffusion, impaction, and interception,
can contribute to the overall collection efficiency of fibrous
mats. These collection mechanisms are well understood, and the
theories describing them can be used to accurately predict collec-
tion efficiency over a wide range of filter and operating charac-
teristics.
The theory and experiments described here are concerned with
a set of filtration conditions which are outside the range of "nor-
mal" filtration. The predominant characteristic of this filtra-
tion regime is the very high filtration velocities used - in the
range of 1-10 m/s (200-2,000 fpm). These filtration velocities,
when combined with the tested filter materials (randomly oriented
8 urn stainless steel fibers) and dust (resuspended fly ash), re-
sult in particle collection characteristics which cannot be de-
scribed satisfactorily by classical filtration theory.
This combination of particle sizes, fiber diameter, and fil-
tration velocities results in particle collection predominantly
by impaction, even for very small particles. Just as importantly,
the kinetic energy of large particles and the drag forces on col-
lected agglomerates are sufficiently large so that particle bounce
and reentrainment can be expected.
This research was undertaken to investigate fly ash collec-
tion characteristics in the high inertia regime. In particular,
the research is aimed at determining combinations of fiber beds
and filtration conditions where small particles are effectively
collected by impaction while large particles bounce through or
are reentrained. Since the large particles which would penetrate
such a filter can be collected by other simple devices (e.g., cy-
clones), this penetration characteristic could possibly form the
basis for a two-stage, high efficiency collection device.
-------
THEORY
Particle Collection Mechanisms
Introduction—
Filtration theory attempts to predict overall particle re-
moval in a filter based on an understanding of the interaction of
particles with a single filter element. This element can be an
isolated fiber or previously collected dust particles.
The single element efficiency, ns, is defined as the number
of particles collected by a filter element divided by the number
of particles whose centers pass through the element's projected
area as the particles approach it(2). The major mechanisms by
which particles collect in a filter element are inertial impac-
tion, diffusion, interception, gravity settling, and electrostatic
deposition(2).
Inertial Impaction—
As the gas flows past a collector element, the streamlines
curve around the collector. Near the collector surface, inertia
will cause particles to deviate from the streamlines. Particles
with sufficient inertia can cross streamlines and hit the collect-
or. Inertial impaction has been found to be proportional to the
dimensionless impaction parameter(2), fy:
(7-D
where: Pp = particle density (kg/m3)
dp = particle diameter (m)
v = superficial filtration velocity (m/s)
'Cc = Cunningham slip correction factor
y = gas viscosity (Pa-s)
d.j, = fiber diameter (m)
Much experimental and theoretical work has been devoted to
determining the relationship between the impaction parameter, i{j,
and the single fiber collection efficiency from impaction, nj.
One widely accepted empirical formula was derived from experiment
and theory by Landahl and Herrmann(3). It was developed for a
.fiber Reynold's Number of 10 and can be expressed as(2):
+ 0.22
(7-2)
Typical Calculations—
Theoretical impaction collection efficiencies can be calcu-
lated for values of the variables used in this study, which are
-------
summarized in Table 7-1. All experiments were conducted using
8 ym diameter fibers. Figure 7-1 plots single fiber collection
efficiency nj as a function of particle size for the lowest ve-
locity (1 m/s) and the highest velocity (10 m/s) used here. Note
that a significant fraction of even very small particles will im-
pact a fiber at these velocities.
Other Collection Mechanisms —
Since the fibers used in this study were very small (8 ym
diameter), interception might be thought to be important. How--
ever, velocities were sufficiently high so that impaction always
completely overshadowed interception. The theoretical single
fiber collection efficiency from impaction and interception com-
bined was calculated assuming that the mechanisms are independent
but that a particle caught by one mechanism cannot be caught by
the other (4); the combined collection efficiency never exceeded
the single fiber collection efficiency from impaction alone by
more than 1%. Likewise, particle collection from diffusion was
negligible under the conditions considered.
Overall Collection Efficiency
Dormant 5) gives the overall collection efficiency for a fil-
ter of thickness L and solidity a as:
where ns is the combined single fiber collection efficiency from
all effects. In this study, impaction dominates all other collec-
tion mechanisms so that
ns 2? ni (7-4)
Equation(7-3)is difficult to apply in this case, because at the
high velocities used here the filter mats compress and L changes
in a manner which is difficult to measure and/or predict. However,
Equation (7-3) can be modified by making use of the definition of
solidity, a, defined as the ratio of fiber volume to total filter
volume:
V,. nu/p
0 =
V LA
(7-5)
T
mf
a = —JTT (7-6)
where pf is the filter density and mf and A are the filter mass
and cross-sectional area, respectively. Solving Equation (7-6)
50
-------
01
«/
O.5
5.0 /co
Figure 7-1. Theoretical single fiber impaction efficiency vs. particle
diameter at 1 and 10 m/s filtration velocities.
-------
for filter thickness L and substituting into Equation (7-3):
-4m -n
"-7)
The overall theoretical filter collection efficiency can thus
be calculated from easily measured filter bed parameters (m,., p^,
df, A), the theoretical single fiber collection efficiency from"
impaction (nj), and filter porosity (1-a). Porosity changes with
filtration velocity; however, the porosity of a filter with no air-
flow can be measured using pycnometry, and changes in porosity with
filtration velocity can be estimated from the slope of the meas-
ured pressure drop versus velocity curve.
EXPERIMENTAL APPARATUS
Apparatus
Figure 7-2 is a diagram of the experimental high velocity fil-
tration system. It contains a 30 cm square filter holder by
which fibrous materials can be packed to depths of up to 15 cm.
A movable 4-mesh screen upstream and a stationary 4-mesh screen
downstream provide rigid support at high filtration velocities.
A downstream centrifugal blower draws the test aerosol through the
20 cm round entry duct, the filter holder, and the Venturi meter
which measures the airflow rate. A Dwyer photohelic gauge is used
with a motor driven blastgate to maintain a constant airflow rate.
A National Bureau of Standards (NBS) dust feeder (6) generates
the fly ash aerosol at mass rates from 3 to 125 g/min. The test
dust is an electrostatically precipitated fly ash with a count
median diameter (CMD) of 0.14 ym and a geometric standard deviation
(GSD) of 1.9- A polydisperse (CMD = 0.7 ym, GSD = 2.05) dioctyl
phthalate (DOP) aerosol can also be generated using a DeVilbiss
nebulizer (The DeVilbiss Co., Somerset, PA). The test aerosol and
inlet air are thoroughly, mixed before entering the filled section
by a Stairmand disk(7) located at the inlet duct.
Simultaneous up and downstream isokinetic samples are taken
to determine aerosol penetration of the filter medium. Microscopic
and gravimetric analyses of the sample filters determine particle
size distribution of the fly ash and downstream of the filtering
section as well as overall collection efficiency. Eight stage An-
derson impactors (Anderson-2000 Inc., Atlanta, Georgia) are also
up and downstream for particle sizing. DOP penetration is meas-
ured using a Particle Measuring Systems, Inc., laser scattering
aerosol spectrometer (PMS Model CSASP-100).
Procedures
Upon packing a filter material into the filter housing and
52
-------
vn
uo
EXHAUST
-PHOTOHELIC
MANOMETER
EbXUSK
BLOWER
SCREEN
FIBROUS
FILTER
MEDIA
DISR
Figure 7-2. High velocity filtration apparatus.
-------
setting the airflow rate, test aerosol is generated and directed
to the filter. Inlet dust concentration is held constant and
after the filter reaches maximum pressure drop (about 7-5 kPa)
the filter is removed for inspection and cleaning. During the
dust loading, pressure drop across the filter is recorded and
aerosol samples are taken at regular intervals. Cleaning of the
filter material can be done by blowing compressed air through the
upstream side of the filter, striking the filter and filter frame
against a rigid surface, or shaking the filter manually. After
cleaning, the filter is placed in the filter housing and retested
at original flow rate and dust loading.
EXPERIMENTS
The fractional penetration of fly ash through filter mats of
8 urn stainless steel fibers was measured at five filtration velo-
cities: 1.5, 2.9, 3.2, 6.9 and 8.0 m/s. The characteristics of
the fly ash and filter material are given in Table 7-1.- At the
lower velocities the filtration time between cleanings was long
enough so that adequate Anderson impactor samples could be col-
lected in one cleaning cycle, while at the higher velocities mul-
tiple cycles were needed to obtain sufficient impactor deposits.
TABLE 7-1. VALUES OP TEST VARIABLES USED IN THIS STUDY
Dust
Type resuspended fly ash
Count median diameter 0.14 ym
Geometric standard deviation 1.9
Particle density (pp) 2.2 x 103 kg/m3
Filter
Fiber type 310 stainless steel
Fiber diameter (df) 8 jim
Fiber density (pf) 7-92 x 103 kg/m3
Filter cross-sectional area(A) 0.093 m2
Filter mass, mf 0.032 kg
Filtration velocity (V) 1-10 m/s
The fractional penetration of the polydisperse DOP aerosol
was also measured over the same velocity range. The DOP concen-
tration was extremely low, so that the filter exhibited no loading
effects during the DOP tests.
-------
RESULTS
Fractional penetration curves for fly ash collected at five
different filtration velocities are presented in Figure 7-3. In-
creased penetration with both velocity and particle size is evi-
dent.
Similar measurements were made with the liquid DOP aerosol
at six velocities; the fractional penetration curves for these
experiments are shown in Figure 7-4- Shown also for comparison in
Figure 7-4 are the 1.5 and 8.0 m/s fly ash penetration curves from
Figure 7-3. In contrast to the fly ash, the DOP aerosol penetra-
tion follows the trends predicted by classical impaction theory,
decreasing with both velocity and particle size.
Dust concentration downstream of filters loaded with fly ash
using clean inlet air were also measured and found to be insigni-
ficant at these velocities.
DISCUSSION
Particle Bounce
Equation (7-7) is a theoretical formula which predicts overall
filter collection efficiency (n) from the single fiber collection
efficiency (nx) and gross filter parameters (mf, Pf, df, A, 1-ct) .
However, Equation (7-7.) can also be used to calculate experimental
single fiber collection efficiencies (nfi) from experimental fil-
ter collection efficiency data (nf)- Such numbers are more use-
ful than overall collection efficiencies in comparing results to
theory because they are independent of the particular filter con-
figuration tested and should only be a function of the particle-
fiber impaction parameter (Equation 7-2).
Solving Equation (7-7) for n'i»
irpf.d,.A(l-a)ln(l-nl)
(7-8)
and substituting the filter parameters from Table 7-1 gives:
n»i = -o.l45(l-ct)ln(l-nf) (7-9)
where the solidity, a, is a function of filtration velocity. As
discussed earlier, changes in a with velocity can be estimated
from the clean filter pressure drop curve; in any case, the quan-
tity 1-a is always close to unity in these tests so that changes
in a have little effect on the calculation of n'i-
The empirical formula of Landahl and Herrmann relating impac
tion parameter (ij>) and nz (Equation 7-2) is plotted in Figure 7-5
55
-------
/00.0
90.0
6O.O\-
4O.O
r
«Jf 00
&0
f.0
4.0
2.3
6.0 /O.O /S.0 /0,0
ll II
Figure 7-3. Fly ash penetration vs. particle size
at five filtration velocities.
-------
• AO " " "
V /.ff " WASH
V AO " "
O./
.6 /.O Z.O 4.O 6.OAO/O.O 2O.O
Figure 7-4. Dioctyl phthalate (DOP) penetration vs.
particle size at six filtration velocities,
57
-------
Also plotted are the experimentally determined single fiber col-
lection efficiencies for the lower test velocity (1.5 m/s,
a = 0.0085) and the highest test velocity (8.0 m/s, a = 0.0285).
The values for intermediate velocities are similar and have been
omitted for simplicity. As shown in Figure 7-5, the experimental
and theoretical single fiber collection efficiencies are in agree-
ment only for the smallest Anderson impactor fly ash size category,
when the fiber impaction parameter is approximately equal to 1.
As impaction parameter increases the experimental n'i decreases;
when m equals 0.99 (^ - 100), n'j equals only 0.2 for the lowest
velocity and 0.05 for the highest velocity.
Experimental single fiber collection efficiency data for the
liquid DOP aerosol are also shown in Figure 7-5- In contrast to
the fly ash data, the DOP n'i values closely fit the Landahl and
Herrmann curve. These data strongly suggest that fly ash par-
ticles do impact the fibers but bounce through the filter at high
velocities. Indeed, for particles larger than 4 urn the data indi-
cate that approximately 80/5 of the particle-fiber collisions re-
sult in particle bounce at 1.5 m/s and that 95% result in particle
bounce at 8 m/s.
Adhesion Probability
The amount of particle bounce for different velocities and
particle sizes is indicated by the particle adhesion parameter, h,
which is defined as the probability of a particle adhering to a
fiber after colliding with it(8). Theoretically, the probability
of particle adhesion can be calculated from a consideration of the
energies involved. A particle will not adhere to a fiber upon
collision if the kinetic energy of the particle on rebound is
greater than the energy of attraction (primarily due to London van
der Waals forces) (9). Lb'ffler(8) and Dahneke(9) present the en-
ergy balance equation and the procedures for calculating critical
conditions necessary for bounce to begin. As pointed out by
Dahneke(9) and Esmen, et al.(10), the equations are difficult to
solve for a polydisperse, irregular dust such as fly ash and a
multilayer fiber filter, so that adhesion probability cannot be
predicted as a function of particle size and velocity. However,
experimentally measured adhesion probability should show a corre-
lation with particle kinetic energy.
The adhesion probability can be obtained from experimental
data by taking the ratio of the experimentally measured single
fiber collection efficiency to the theoretical single fiber col-
lection efficiency. Since impaction predominates in this study,
t
n T
h = —- (7-10)
nl
58
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ui
vo
a/
o
CU/tVE
m n.r ASH, /.
• Ftr 4s/t, ff,o nys
A 00P.4LL HEL06/r/£S
\
/o-\
Figure 7-5. Theoretical and experimental single fiber impaction effi-
ciencies vs. impaction parameter for DOP and fly ash.
-------
The adhesion probabilities calculated for all of the data
points in Figure 7-3 are plotted against particle kinetic energy
in Figure 7-6. The expected decrease in adhesion probability with
increasing kinetic energy is evident. The data fall on a straight
line (log-log plot) at low energies (high adhesion probability)
but exhibit increasing scatter at higher energies (low adhesion
probability).
The spread in data at high energies may be caused by the
complex particle impaction and rebound pattern which would exist
in this regime. High energy particles, after bouncing, will ac-
celerate until reaching gas velocity; during acceleration they
may impact another fiber and rebound again or they may adhere,
depending on the velocity at impact. A very complex collection
pattern can thus be expected for a polydisperse dust in a multi-
layer filter for particles with large kinetic energy. However,
this mechanism cannot account for the very low adhesion probabili-
ties measured on two experiments (v = 5-2 m/s and 8.0 m/s) when
the particle kinetic energy exceeded 10~13 J. Additional data
will be collected in an attempt to clarify these results.
Relative humidity can have a great effect on particle adhe-
sion(ll), and thus could have contributed to the differences in
adhesion probabilities measured in different experiments. Rela-
tive humidity ranged from 20-505? during the experiments, but the
adhesion data could not be correlated with the relative humidity
data.
A regression line was fit to the data of Figure 7-6 and is
plotted in Figure 7-7. Also plotted are data of Lbffler(8) and
Esmen, et al.(10). Loffler's data were collected for 1.8-10.0 urn
diameter NaCl particles collected on 20 urn diameter polyamide fi-
ber filters while Esmen, et al., impacted 4.4 and 8.8 ym diameter
uranine spheres on a flat brass substrate. Considering the differ-
ent particles and substrates used, the agreement among the various
experiments is good. Figure 7-7 indicates a region of particle
inertial energy where particle adhesion can be expected to be
very good (EK > 10~13j). in the intermediate energy regime,
(10-15j < EK < 10-13j), particle adhesion probability will be a
strong function of the particular particle and filter used.
Practical Design Implications
The penetration characteristics reported here suggest that a
novel, compact, high efficiency particle collector may be feasible.
The device would operate at high gas velocities, and would collect
particles over a wide size range by impaction. It would consist
of two stages: 1) a fibrous media filter, which would efficiently
collect small particles but allow some particles to bounce through
or reentrain; and 2) a cyclone, which can efficiently collect large
particles. When operated in series, the two devices should collect
all particle sizes with high efficiency.
60
-------
cr\
/.o
8"
!
i
v
o
0 &* "
A «/ "
1-12
•-1 1 »-1 0
x/tir/o £#£**r,
Figure 7-6. Fly ash adhesion probability vs. particle kinetic energy.
-------
At
IX)
A
o
/771 TV
MKT srwr
V
A
- O
i -16
/o
i-l S
Figure 7-7. Comparison of adhesion probability data from
this study with those of other investigators.
-------
Such a device would have many desirable properties. Our ex-
periments indicate that the pressure drop across a clean fibrous
bed, when operated at high velocities, (e.g., 1.5 m/s), is about
the same as across a cyclone: 0.5-1 kPa. The total pressure
drop across the device would thus be fairly low - on the order of
2 kPa. The device would be extremely simple in design and easy
to operate; the only area where moving parts may be needed is in
the filter cleaning system, and that would depend on the cleaning
method chosen. The device, by operating at high gas velocities,
would be very compact. It would collect all types of aerosols
(solid and liquid) and would use relatively small amounts of en-
ergy.
The design and testing of such a two-stage device is now in
the planning stage. An important design consideration now being
addressed is the selection of an optimum fibrous material. The
ideal material would have a fractional penetration similar to the
stainless steel fiber beds but would be cheaper and easier to
clean in situ. Materials such as felts and sintered fiber mats
are being tested in preparation for the construction of a pilot
two-stage device.
SUMMARY AND CONCLUSIONS
Particle bounce has been shown to play an important role in
the collection of fly ash by mats of stainless steel fibers. The
penetration of fly ash particles through 8 ym fiber filters in the
1-10 m/s velocity range increased with both particle size and ve-
locity; since impaction was'the controlling collection mechanism,
these results run contrary to classical filtration theory.
Particle adhesion probability was found to be inversely pro-
portional to particle kinetic energy. This result is in general
agreement with previous adhesion studies, which were performed
with monodisperse aerosols under controlled laboratory conditions.
This study demonstrates that the particle adhesion properties of
a polydisperse dust (fly ash), under conditions similar to those
encountered by practical filters, can also be related to the par-
ticle kinetic energy.
These results have important implications for the operation
of fibrous filters at high velocity, and may help explain cases
where penetration is higher than anticipated. These experiments
also suggest the possible use of a novel two-stage inertial col-
lection device to .take advantage of the high velocity particle
penetration characteristics noted here.
NOMENCLATURE
?
A filter cross-sectional area, m
Cc Cunningham slip correction factor
df fiber diameter, m
63
-------
dp particle diameter, m
Eg particle kinetic energy
h adhesion probability
L filter thickness, m
nif filter mass, kg
v superficial filtration velocity, m/s
Vf total fiber volume, m?
V
-------
11. Corn, M. The Adhesion of Solid Particles to Solid Surfaces,
I. A Review. J. Air Poll. Control Assoc., 11(11);523-528,
1961.
65
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-80-042
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Performance of a High-velocity Pulse-jet
Filter, H
5. REPORT DATE
March 1980
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
David Leith, M. J.Ellenbecker, M.W.First,
J.M. Price. Anthony Martin, and D. G. Gibson
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Harvard School of Public Health
665 Huntington Avenue
Boston, Massachusetts 02115
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
Grant R804700
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; 9/76-9/79
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES jj;RL.RTp prOject officer is James H. Turner, Mail Drop 61,
919/541-2925. EPA-600/7-78-131 includes related work.
is. ABSTRACT
repOrt gives results of a. study of the performance of a high-velocity
pulse-jet filter. Such filtration has distinct advantages over low-velocity filtration
in that the equipment required to clean a gas stream is reduced in size and initial
cost as velocity increases. Although high filtration velocity causes a number of pro-
blems , many of them are dealt with in the report. Location of the gas inlet to the
filter was found to affect penetration and pressure drop; both were higher for inlets
near the bottom of the filter housing. Fabric type was also found to affect perfor-
mance by affecting the amount and characteristics of the dust deposit accumulated.
Fabric surface properties help explain the nature of this deposit. These ideas and
others were used to develop a mathematical model for pressure drop in a pulse-jet
cleaned filter. The model can be used to predict pressure drop under stable or
variable operating conditions , and to predict operating conditions that cause unstable
filter operation. An understanding of particle/fiber interactions is essential to
understanding the collection characteristics of a felt fabric. Under certain condi-
tions, particles bounce on impact with fibers. An adhesion probability was deter-
mined and found to depend on incident particle kinetic energy.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c, COSATI Field/Group
Pollution Felts
Filtration Adhesion
Pulsation
Jets
Fabrics
Mathematical Models
Pollution Control
Stationary Sources
Fabric Filters
Pulse-jet Filters
13B
07D
14B
20D
HE
12A
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
72
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 222O-1 (9-73)
66
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