xvEPA
United States Industrial Environmental Research EPA-600 7-78-141
Environmental Protection Laboratory July 1978
Agency Research Triangle Park NC 27711
EPA Fabric
Filtration Studies:
6. Influence of Dust
Properties on
Particle Penetration
Interagency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
EPA REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approver]
for publication. Mention of trade names or commercial products does not con
stitute endorsement or recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service. Springfield, Virginia 22161.
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EPA-600/7-78-141
July 1978
EPA Fabric Filtration Studies:
6. Influence of Dust Properties
on Particle Penetration
by
R.P. Donovan
Research Triangle Insitute
P.O. Box 12194
Research Triangle Park, North Carolina 27709
and
B.E. Daniel and J.H. Turner
Environmental Protection Agency
Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, North Carolina 27711
Program Element No. EHE624
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
This report examines the importance of dust properties in determining
dust penetration through a fabric filter. The major property considered
is the size distribution of the dust which is an important dust property
for dust penetration. Most other important variables of dust penetration
depend more on the dust/fabric combination than on the dust alone.
The report begins by reviewing dust penetration mechanisms and
relating them to dust and dust/fabric properties. These interactions
are illustrated using data generated in the EPA in-house laboratory as
well as data published in the open literature. Both shaker baghouse
data and pulse-jet baghouse data are used in an attempt to identify
commonality in dust penetration independent of fabric cleaning technique.
ii
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PREFACE
This report is the sixth in a series of reports, entitled EPA Fabric
Filtration Studies, which summarize the results of EPA laboratory testing of
new baghouse fabric materials and present the conclusions of specialized
research studies in fabric filtration. These tests have been carried out over
the past 5 years by EPA's Industrial Environmental Research Laboratory,
Research Triangle Park, N. C. and previously by predecessor agencies. The
purpose of these investigations was to evaluate the potential of various new
fabrics as baghouse filters and to Obtain data for use by the fabric filtra-
tion community.
The testing consisted of simulating a baghouse operation in a carefully
controlled laboratory setting that allowed measurement and comparison of bag
performance and endurance. The simulations discussed in this report cover
only a very narrow range of operating conditions: room temperature filtration
3 *
with an inlet dust loading of about 3 gr/ft .
Extreme caution should be used in extrapolating the results reported
here to the substantially different conditions that occur in field applications.
The usefulness of the present results is primarily either as an initial
screen of candidate fabrics for baghouse applications or as an exploration of
baghouse phenomena.
Author Donovan's efforts in this report are represented by EPA Contract
68-02-2612, Task 42 with Research Triangle Institute, Research Triangle
Park, N.C. 27709.
*EPA policy is to use metric units. For convenience and consistency with
existing baghouse conventions, British units are used in this report. Readers
more faimilar with the metric system may use the factors listed elsewhere in
this report to convert to that system.
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The EPA Fabric Filtration Studies series now consists of the following
reports:
1) Turner, J. H. Performance of Non-woven Nylon Filter Bags, EPA-
600/2-76-168a (NTIS PB 266271), December 1976.
2) Ramsey, G. H. et al. Performance of Non-woven Polyester Filter
Bags, EPA-600/2-76-168b (NTIS PB 258025), June 1976.
3) Donovan, R. P., B. E. Daniel, and 0. H. Turner. Performance of
Filter Bags Made from Expanded PTFE Laminate, EPA-600/2-76-168c
(NTIS PB 263132), December 1976.
4) Donovan, R. P., B. E. Daniel, and J. H. Turner. Bag Aging Effects,
EPA-600/7-77-095a (NTIS PB 271966), August 1977.
5) Daniel, B. E., R. P. Donovan, and J. H. Turner. Bag Cleaning
Technology High Temperature Tests, EPA-600/7-77-095b (NTIS
PB 274922), November 1977.
6) Donovan, R. P., B. E. Daniel, and J. H. Turner. The Influence of
Dust Properties on Particle Penetration (this report).
iv
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TABLE OF CONTENTS
Page
Abstract ii
Preface iii
List of Figures vi
List of Tables vi
List of Abbreviations and Symbols vii
Metric Conversion Factors vii
Sections
1 Introduction 1
2 Conclusions 3
3 Experimental Work 5
4 Results and Discussion 10
4.1 Size Distribution Effects 10
4.2 Effects of Other Dust Properties 23
4.2.1 Polyester Felt Fabric 23
4.2.2 Nomex Fabric 27
4.2.3 Summary 29
5 Relationship to Existing Fabric Filtration Literature 31
5.1 Fractionating Effects During Filtration 31
5.2 The Role of Dust Properties in Fabric Filtration 34
6 References 36
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LIST OF FIGURES
Number Page
1 Size distribution of inlet dusts 6
2 Stability of flyash size distribution 8
3 Flyash penetration in classical dust cake filtration 11
4 Section through felted filter media showing
distribution of dust [Ref.9] 12
5 Fraction of total flyash emitted, which is accountable
to indicated emission mechanism, versus deposit
thickness [Ref. 11 ] 14
6 Optical counter traces in outlet of pulse-jet baghouses 16
7 Size distribution of particles in the first minute
of various cycles following the termination of
dust feed [Ref .4] 18
8 Cross section of Gore Tex/Nomex fabric (215X) [Ref. 3] 20
9 Time dependence of flyash particles penetrating Gore
Tex/Nomex bags [Ref. 3] 21
10 Comparison of the size distribution of the delayed
component of particle penetration (solid curve) with
that of the previously fed inlet flyash [Ref. 12] 33
11 Dependence of pressure drop buildup across woven glass fabric
on dust properties (from Dennis [Ref. 12]) 35
LIST OF TABLES
2
1 Pulse-jet filtration using 16 oz/yd polyester felt fabric 25
2
2 Pulse-jet filtration using 16 oz/yd Nomex 28
3 Summary of trends in the pulse-jet filtration of
flyash and rock dust 30
VI
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LIST OF ABBREVIATIONS AND SYMBOLS
2
A = filtration area of fabric (ft )
o
C = mass outlet concentration (gr/1000 ft )
E = mass collection efficiency (percent)
2
K, = true value of specific cake resistance (in. H00/fpm)/(lb/ft )
2
Kp = measured value of specific cake resistance (in. H^O/fprn)/(Ib/ft )
APr = pressure drop across bag extrapolated to time zero of filtration
cycle (in. H20)
APT = pressure drop across bag at end of filtration cycle (in. HpO)
Sr = effective drag (in. H^O/fprn)
Sj = terminal drag (in.
A/C = air/cloth ratio (fpm)
METRIC CONVERSION FACTORS
Non-metric Times Yields Metric
ft 0.30 m
ft2 0.09 m2
ft3 28.3 t
gr 0.06 g •
in. 2.54 cm
Ib 0.45 kg
oz 28.3 g
yd2 0.84 m2
vii
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SECTION 1
INTRODUCTION
Physical properties of the particles making up a dust source influence
the rates at which the dust penetrates through a fabric filter. Previous
studies reported in this series [Refs. 1-5] and the 1977 annual report
[Ref. 6] have emphasized the importance of the fabric properties in the
dust/fabric interaction. This report emphasizes primarily the importance
of the dust properties.
A primary physical property of a dust source is its size distribution.
Reports in the literature disagree as to the importance of this property.
Early workers argued that there should be a mimimum in the plot of
collection efficiency vs. particle size (conversely a maximum in particle
penetration vs. particle size), since large particles (3.5 ym and larger)
are efficiently removed by mechanisms such as sieving, interception, and
impaction and since small molecular-si zee1 particles (0.05 urn and smaller)
are captured because of diffusion processes. Between these extremes are
the intermediate respirable particulates (0.05 to 3.5 ym) which penetrate
the fabric more readily than the larger or smaller particles. Dust
penetration through a fabric filter should therefore vary according to
particle size distribution. And early measurements of fractional efficiency
seemed to support this prediction [Refs. 7-9].
More recent works [Refs. 10-12], however, describe dust/fabric
systems in which dust penetration is relatively independent of particle
size and, if anything, increases as particle size changes from the
respirable range to the larger range. Various mechanisms have been
postulated to explain this size-independent particle penetration.
Depending upon the dominant mechanism in a given dust/fabric system, the
effect of dust size distribution on penetration seems to vary.
Fabric cleaning technique may be an important parameter in understanding
particle penetration through a fabric filter [Ref. 13]. This report
emphasizes the unity of particle penetration, regardless of cleaning
1
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technique, rather than the differences: distinctions between the various
cleaning mechanisms are subordinated to the similarities. Fabric cleaning
technique is explicitly specified for all data cited, however. The
importance of cleaning parameters during pulse-jet operation has been
forcefully presented by Leith, et al. [Ref. 14].
This paper presents the results of further study of the dependence
of particle penetration on dust properties including: size distribution
(Section 4.1) and other properties (Section 4.2).
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SECTION 2
CONCLUSIONS
Particles penetrating through a fabric filter are classified as either
prompt or delayed. Prompt penetrating particles follow straight-through
trajectories without colliding with the fibers or dust cake of the fabric/dust
cake composite. Their transit time through the fabric filter is the same
as that of the gas flow in which they are entrained. These prompt penetrating
particles are fractionated in size according to classical fiber filtration
mechanisms: typically, they decrease in concentration as the dust cake grows.
Delayed penetrating particles are removed from the gas flow, no matter
how briefly, but subsequently become reentrained and are emitted from the
clean-air side of the fabric. Delayed penetration particles are further
subdivided into two subclasses: those with the same size distribution as the
inlet dust; and those which penetrate by some size-dependent mechanism.
The mechanisms by which particles penetrate a fabric filter vary in
importance from one dust/fabric system to another and can vary in importance
as a function of time during filtration. The dominant mechanism depends on
the dust, the fabric, their interaction, and time. Examples in this paper,
illustrating different penetrating mechanisms and behavior, are drawn from
data based on:
1) Flyash filtered by woven polyester, spunbonded polyster, Gore Tex/
Nomex, polyester felt, and Nomex fabrics (shake-cleaned baghouse).
2) Rock dust filtered by polyester felt and Nomex fabric (pulse-jet).
3) Other published work in the open or U.S. Government literature
(shake-cleaned, reverse-air-cleaned, and pulse-jet).
These data sources generally support the physical model that favors
delayed, non-fractionating particle penetration when the filtration is carried
out at high UlO fpm) air-to-cloth ratios (such as the pinhole plug mechanism
described by Leith et al., [Ref. 11]) and fractionating delayed particle
penetration when filtration is carried out at low (1-4 fpm) air-to-cloth
ratios (such as Dennis1 rear-face slough-off [Ref. 12]). Prompt
3
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straight-through processes dominate at the beginning of filtration cycles
in which the filtering action depends primarily upon dust cake filtration.
Fabric cleaning technique (pulse-jet vs. shake-cleaning or reverse-air-cleaning)
may also influence particle penetration [Refs. 13,14].
The size distribution of a dust source can therefore be an important
filtration property both because of its influence on particle penetration
mechanisms and also because of its effect on dust-cake growth and properties.
Other dust properties, such as affinity for water, and particle adhesion
and cohesion, could also have major influences on dust cake formation and
particle penetration. In this report, comparisons between rock dust and
flyash penetration of certain fabric filters show that the major filtration
performance parameters depend more on the properties of the dust/fabric system
than on the properties of the dust alone.
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SECTION 3
EXPERIMENTAL WORK
The apparatus used in gathering the data presented in this paper
consisted of both a shake-cleaned baghouse and a pulse-jet unit. The
dusty air source is the same for both units and the analytical taps in
the outlet, while not identically the same line, are similar. Outlet
air measurements included total mass over a 20 minute sampling period (the
Millipore filter technique), optical counter determination of outlet
particle size distribution, and impactor analysis of inlet particle size
distribution. The first two techniques are the same as has been used in
the work previously described [Refs. 1-5]. The impactor data have not
previously been reported in this series. They were gathered using either
a 7-stage MRI impactor or an Andersen impactor preceded by a 3-stage
cyclone. The purpose of the impactor data was to determine the size
distribution and density of dust particles in the inlet air. Originally
the plan was to sample the outlet dust with an impactor also but this plan
was abandoned because of the impractically lengthy sampling times and/or
system modifications required for statistically significant data. The
only direct size-distribution measurements of the outlet dust were carried
out with the Climet optical counter.
Figure 1 compares impactor analysis of the size distribution of two
inlet dusts with those determined by a Coulter counter. The two dusts
are power plant flyash (the same Detroit Edison flyash used in all
previous experimental work reported in this series) and asphalt plant
rock dust. The two analytical methods agree reasonably well for the
rock dust; less well for the flyash. Both sets of measurements, however,
show that the rock dust is composed of smaller particles than the
flyash--the cumulative percentage of particles smaller than any given
size is greater for the rock dust than for the flyash. This observation
will be recalled later to establish a difference between the penetration
of rock dust particles and the penetration of flyash particles.
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to
co
UJ
UJ
o Coulter Counter
o Andersen plus 3-stage cyclone
Rock dust 12/9/75
— Flyash 10/29/75
Figure 1. Size distribution of inlet dusts.
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The test dust recovered in the collection hopper of the baghouse is
reused over and over again. With some addition for losses, the same
flyash has been recycled through the baghouse and back to the dust feeder
hundreds of times over a period of years. That the size distribution of
this dust has not changed appreciably over the years (Figure 2) shows that
the particle penetration through all (or at least most) of the many fabrics
tested is essentially independent of size. This absence of fractionation
is not true for all flyash/fabric systems, however, and Figure 2 also
includes a size distribution curve for the dust collected in the hopper
when using spunbonded polyester to filter flyash [Ref. 2]. For this dust/
fabric system the filtration appears size-dependent, since the size
distribution of the collected particles shifts significantly to the larger
particles when compared with the size distribution of the inlet dust.
This shift says that a proportionally larger number of small particles
are penetrating the fabric; their absence in the collected dust gives rise
to the changed size distribution.
This shift is atypical: it was not observed for the flyash/fabric
systems to be reported here. The size distribution of the rock dust was
not checked before and after filtration but, as reported in Section 4.2,
other evidence suggests that the rock dust filtration would also be
size-dependent when filtered by the fabrics used in these experiments.
Shifts in the opposite direction (the dust becomes enriched in small
particles because the filtration process removes a proportionally
greater number of the large particles) are also common, especially when
the initial dust contains many large dust particles and has a mass median
diameter (D5Q) in excess of 20 to 30 ym. Many of these large dust particles
simply settle out in unrecoverable regions of the flow systems and are
lost to the dust recycling action. Cement dust, exhibiting a D5Q of
about 50 ym before filtration, lost a sufficient number of large particles
after 19 hours of operation in the high temperature baghouse [Ref. 6] to
have its D5Q value reduced to about 15 ym. This shift was the largest
observed; other checks of the cement dust size distribution showed
similar shifts with time but of reduced magnitude.
The observation is that it is possible for the outlet size distri-
bution of a dust source to remain the same as the inlet distribution
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Collection hopper,
~ , 5/16/74
Inlet dust / X Spunbonded polyester
5/16/74 -^_ / A ' bags
, ° 5/16/74 data
«> °- A/ / D inlet dust, 10/29/75
A Outlet dust (Mi Hi pore
sample), 1/15/73, woven
polyester bags
Figure 2. Stability of flyash size distribution.
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during filtration or to shift toward either larger or smaller particles,
depending on baghouse properties and the mechanisms of dust penetration.
A recycled dust can go either way because of the possibility of large
particle loss to the system (rather than penetration of the fabric).
That is, a baghouse divides an inlet dust into two fractions: the outlet
dust and the dust collected in the hopper. The dust quantity collected
in the hopper is generally about 2 to 3 orders of magnitude greater than
that passed through the fabric to the outlet. Consequently, fractiona-
tion during fabric filtration is more rapidly detected in the size
distribution of the outlet dust. Conversely, however, changes detected
in the hopper dust are major effects. Hopper dust analysis by a Coulter
counter is primarily what was carried out here (Figure 2, however, did
present a Coulter counter analysis of the dust collected on a Millipore
filter in the outlet). The outlet dust size distribution should show
simply the small difference between the inlet dust and the hopper dust.
For the original work reported here, some supplementary analyses of the
size distribution of the outlet dust were carried out with the optical
detector. The number data generated by the optical detector are not
directly comparable with the inlet dust size distributions generated by
either an impactor or the Coulter counter and no such comparisons are
made.
The shake-cleaned baghouse used in this work is described in prior
reports of this series [Refs. 1-5]. The pulse-jet is a commercially
available, nine-bag MikroPul binvent baghouse, described in the most
recent annual report [Ref. 6].
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SECTION 4
RESULTS AND DISCUSSION
This section is divided into two topics: size distribution effects
and other dust property effects. The first section draws only on data
collected using the classified flyash that has been the primary dust
used in the previously reported work [Refs. 1-5], The second section
compares rock dust filtration with flyash filtration under identical
laboratory conditions.
4.1 SIZE DISTRIBUTION EFFECTS
For a large class of dust/fabric systems, particle penetration with
time is of the form shown in Figure 3--the maximum in penetration occurs
immediately after the cleaning cycle (or very shortly into the new
filtration cycle). Most of the particles that penetrate through the
fabric do so in the first half of the filtration cycle. In this model
the primary filtration media is the dust cake itself; the fabric serves
mainly as an initiating and supporting lattice on which the dust cake
forms and to which it remains affixed, as shown in Figure 4. The high
rates of particle penetration occuring in the initial time period of
the filtration cycle correspond to the time period in which the dust
cake is of minimum thickness and is being rebuilt (repaired) following
the cleaning cycle. Partial destruction and removal of the dust cake is
necessary in order to recover tolerable pressure drop and drag so that
the optimum cleaning cycle is a compromise between the restoration of
low pressure drop and the loss of filtration efficiency.
Of particular interest is the time dependence of the size dis-
tribution that is reflected in the plots of Figure 3. During the first
half of the filtration cycle (zero to 10 min) the size distribution of
the outlet dust changes dramatically. Over the second half of the
filtration cycle, it remains much more constant. To take the Millipore
filter catch and analyze it in a Coulter counter or, alternatively, to
take impactor samples from the outlet would, of course, average these
time dependencies and give an average size distribution. In dust cake
10
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Filtration velocity = 4 fom ,
Inlet concentration = 3 9r/ft
Shake-cleaned baghouse
16.
14.
12.
10-
X 2-4
• 1-2
urn
A 0.5-1.0 um
o 0.3 - 0.5 um
a) 5 oz/yd woven polyester (3 x 1 twill,
continuing filament) [Ref.4].
o
o
o
t-H
h-
o;
A
- / \
*-* /"
a
a
a
i i
\
•V
\ D
V A \ 23^;
\\
\ <>•
\0 A-^:
1 1 **f— • — 4
23" Relative Humidity
— D
b) 6 oz/yd spunbonded polyester [Ref. 2]
28% Relative Humidity
2 4 6 8 10 12 14 16 18
TIME MEASURED FROM START OF NEW FILTRATION CYCLE, min.
Figure 3. Flyash penetration in classical dust cake filtration.
11
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FELTED MEDIA
APPROX. 3mm
TIME OF TRAVEL O.2-0»W«
Figure 4. Section through felted filter media showing distribution
of dust [Ref. 9].
1
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filtration the dust cake/filter composite is a dynamic structure: it
changes rapidly with time during the initial growth of the dust cake.
Its properties as a filter and the mechanisms of particle penetration
through it also change with dust cake growth [Ref. 11].
Penetration mechanisms and their dependence on dust cake thickness
have been described for penetration through a pulse-jet filter by Leith
et al. [Ref. 11] in terms of straight-through passages, seepage processes,
and pinhole plug bursts.
The straight-through component is that which penetrates without
ever being captured; seepage is that component which is captured one or
more times but is released after each capture and eventually works its
way through the filter; and pinhole plugs are those particles which are
captured and clustered about a pore and then released in a single
penetrating burst. Leith et al. state that both the seepage component
of penetrating particles and that attributable to pinhole plugs should
have the same size distribution as the inlet dust. To interpret their
experimental data, they assume that seepage, in keeping with its size
independence, is a queuing process in which the first particles to
arrive at the filter face are also the first particles to leave the rear
face of the filter--there is no way one particle can pass another in
traversing the bulk of the filter.
These mechanisms were investigated experimentally by Leith et al.
[Ref. 11] with chemically tagged flyash and quantitatively evaluated
for a specific flyash (from an electrostatic precipitator)/polyester
needled felt in a pulse-jet bag filter. Based on these physical concepts
and experimental measurements, they deduced a plot, reproduced in
Figure 5, to describe particle penetration mechanisms as a function of
dust cake thickness (hence, filtration time).
The data taken in our experiments cannot be analyzed quantitatively
as could Leith et al.'s data (Leith used chemically tagged flyash to
follow groups of flyash and particles through the filter). Some
qualitative comparisons can be made, however.
Figure 3 shows that the time of maximum particle penetration varies
with particle size: the smaller particles peak later in the filtration
cycle than the larger. In Figure 3a, for example, peak penetration of
the 1-2 pm particles occurs 2 min into the new filtration cycle; peak
13
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PULSE-JET
CLEANING
o
»—I
CO
CO
o
HH
I—
O
PINHOLE
PLUGS
DEPOSIT THICKNESS,micrometers
Figure 5. ' Fraction of total flyash emitted, which is accountable to in-
dicated emission mechanism, versus deposit thickness [Ref. 11]
14
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penetration of 0.5-1.0 pm particles occurs at 5 min; and peak penetration
of 0.3-0.5 ym particles occurs at about 7 min. This observation suggests
that the regrowing dust cake does not trap the smaller particles as
efficiently as the larger particles and that, in the initial phase of
the filtration cycle, the fabric/dust cake composite may actually lose
particles that were previously trapped but which have been dislodged by
the shake-cleaning, the forward air flow through the fabric freshly
stripped of dust cake, or a combination of the two.
Similar observations apply to pulse-jet cleaning as well (the rock
dust/polyester felt data in Figure 6). The 0.3 ym particle pulse peaks
later in the filtration cycle (the filtration cycle is defined as
beginning with the cleaning pulse, .indicated by the solid arrows in
Figure 6; the pulse peaks at the broken arrows) than the 0.5 ym particle
pulse (which peaks later than the 1 urn particle pulse). The time scale
for the traces of particle concentration shown in Figure 6 differs greatly
from that of Figure 3: the time between pulses (Figure 6) is on the
order of 60 sees. The same general time dependence of particle penetration
seems to take place, however, in both cycles—at least for the rock dust/
polyester felt system.
The pinhole plug mechanism of particle penetration described by
Leith et al. [Ref. 11] predicts an initial increase in particle pene-
tration with dust cake thickness (Figure 5). Particle penetration
attributable to pinhole plugs then peaks at some intermediate value of
dust cake thickness, showing a time dependence similar to that of the
particle penetration curves of Figure 3. The differentiation according
to particle size evident in Figure 3 is not, however, accounted for by
pinhole plugs—at least not by the Leith description in which the particle
size distribution of the pinhole plug is assumed to be that of the
incoming dust.
That one size range of penetrating particles decreases with time
is not unusual—Leith's straight-through mechanism predicts this (Figure
5). What is not accounted for is the concurrent increase in the con-
centration of a smaller particle range. (In Figure 3, for example,
the concentration of 1-2 pm particles decreases between minutes 2 and
4; yet, in this same time interval, the concentration of 0.3-0.5 pm
15
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Rock Dust/Polyester Felt
EPA Test Flyash*/Polyester Felt
I
»
Pulse
Peak
Pulse
Start
Time
Inlet Loading: 11.91 gr/ft3
1.4651 gr/1000 fr
Inlet Loading: 12.04
gr/ft3
Co:
2.0418 gr/1000 fr
Rel . Humidity: 50
E: 99.988?. JJ
>
?.
r3
•..•^
/
r/
>
^s
X "H
5
U-
Rel. Humidity:
E: 99.983%
i 1
4
Pulse
1,739,480 particles/ft3 6,149,039 particles/ft3
a) 0.3 IIITI particle traces
ii
*From Detroit Edison
1 ,383,853 particles/ft 4,644,581 partlcles/ff
b) 0.5 urn particle traces
1,553,040 particles/ftj 1,447,977 particles/fr
c) 1.0 urn particle traces
Figure 6. Optical counter traces in outlet of pulse-jet baghouses.
16
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particles increases.) The conclusion would seem to be that different
mechanisms of penetration dominate different size ranges or that the
dominant mechanism of penetration has size and time dependence—perhaps
similar to that shown for the straight-through mechanism in Figure 5.
A mechanism of delayed particle penetration in addition to pinhole
plugs seems required. This component of delayed particle penetration is
defined here simply as that which depends on size—it is a fractionating
form of delayed particle penetration, complementing the non-fractionating
mechanism of pinhole plugs. The physical description of seepage, given
by Leith [Ref. 11], might well cover this mode of particle penetration if
seepage is viewed as a particle diffusion or permeation through the fabric-
a size-dependent process rather than size-independent as presented by
Leith.
Regardless of the physical processes, the data presented in Figure
3 reflect a size-dependent penetration. Figure 7 shows additional optical
counter data taken with the flyash/woven polyester system, but after the
dust feed had been stopped. These data were collected in the shake-
cleaned baghouse, filtering flyash with woven polyester fabric filters
(with same experimental conditions as in Figure 3a). For these specific
data, the dust feed was stopped after a standard cycle and the succeeding
cycles (labeled No. 6 and succeeding) were run the same as before except
for no dust feed. The particle count presented in Figure 7 is that
accumulated during the first minute of the filtration cycles given. Since
no dust feed was used, the emitted dust must be delayed penetration of
the dust particles previously fed (air flow through the bag continues
but with ambient dust load only—total particle concentration estimated
to be less than 10 /ft ). The property of the delayed particles emitted
that is emphasized here is the change in their size distribution with
time. Large particles dominate the first dust-off cycle but immediately
decrease in concentration as the smaller sizes increase. This behavior
is similar to that observed during dust-on filtration (Figure 3). The
significant difference here is that the Figure 7 data are for the delayed
component of particle penetration.
17
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10
ro
Co
-------
The classification of penetrating particles that is consistent with
these experimental observations is therefore slightly altered from
Leith's:
1) Prompt penetration (same as Leith's straight-through).
2) Delayed penetration.
a) With same size distribution as the inlet dust (same
as Leith's pinhole plugs).
b) With size distribtuion differing from the inlet
(modified from Leith's description of seepage; similar
to Dennis1 rear-face slough-off [Ref. 12]).
Further support of the dust cake model of filtration comes from
fabrics constructed so as to minimize the role and presence of dust
cake. Such a fabric is the Gore Tex/Nomex laminate [Ref. 3] which consists
of a fine fibriHated film of PTFE (polytetrafluoroethylene) supported
by a Nomex backing. Figure 8 shows a cross section of Gore Tex/Nomex
at 215X. The wispy web-like network on the left is the PTFE film. The
much coarser Nomex fibers are on the right and dominate the bulk of the
fabric. In a sense the PTFE film simulates a dust cake but is a controlled,
permanently affixed film that remains in place throughout the filtering
and cleaning cycles—unlike the conventional dust cake which must be
continually rebuilt. Flyash does not adhere to the PTFE surface very
well; consequently, very little conventional dust cake builds up. Particle
penetration during the filtration cycle is quite different under these
conditions than under those previously described. Figure 9 is a plot of
flyash penetration through shake-cleaned Gore Tex/Nomex fabric. Very
little lessening of penetration is evident at A/C = 4 fpm. Penetration
actually increases with time for A/C = 9. At both air-to-cloth ratios
the size distribution of the outlet particles remains constant with time--
unlike the behavior depicted in Figure 3 for a shake-cleaned woven
fabric. These observations are consistent with sieving as the primary
mechanism of filtration, increased penetration, and peaks at A/C = 9
being due to pressure-induced stretching and pulsing.
A comparison of the penetration curves shown in Figures 3 and 9
illustrates the importance of particle size distribution with respect
to the properties of the fabric filter. The dust used to gather the data
19
-------
c
Figure 8. Cross section of Gore Tex/Nomex fabric (215X) [Ref. 3]
-------
o
o
Inlet dust loading: 3 gr/ft3
Shake-cleaned baghouse
4
3
2
1
0
a. A/C 4 fpm
D
V
,O— Q—O— D-O—D—D
V
i i
! 1
i i
024
6 8 10 12 14 16 18
b. A/C 9 fpm
oo
-------
for both figures was the same and presumably had the same size distribution.
The method of fabric cleaning was the same. The mass median diameter of
the dust was small compared to the pores of the woven polyester (Figure 3)
but large with respect to the openings of the Gore Tex/Nomex (Figure 8).
The difference in the time dependence of particle penetration for these
two dust/fabric systems is dramatic.
An additional key difference is the stability of the size distribution
with time that characterizes the outlet dust penetrating the Gore Tex/
Nomex (Figure 9). This observation indicates that the concentration
ratios of the three particle size ranges are relatively constant through-
out the filtration period, including many periods when pulses of particles
are emitted. This time behavior is quite different from that displayed
by the optical counter data illustrated in Figure 3 and is an essential
part of the evidence used to distinguish between the dust cake filtration
reflected in Figure 3 and the sieving (or perhaps pinhole plug) penetra-
tion reflected in Figure 9.
These optical counter data cover only a small mass portion of the
complete size spectrum of the inlet dust. They do not, therefore, allow
size distribution comparisons between inlet and outlet dusts. They do,
however, allow comparisons between different outlet dusts. For example,
comparisons of Figures 3 and 9 show that the Gore Tex/Nomex fabric allows
more penetration of the small particles once a quasi-steady state is
reached (time measured from start of filtration cycle > 10 min). This
observation does not necessarily mean that the polyester fabrics are
better filters for fine particulates.
When the fine particulate penetration is averaged over the entire
filtration cycle, the polyesters, including the spunbonded, may not be
better, for fine particles, for this particular flyash source. The data in
Figures 3 and 9 do show that the fine particle penetration behavior is quite
different for polyester fabrics and Gore Tex/Nomex and that changes in the
dust properties, such as size distribution, might well affect one system
differently than the other. Size distribution directly affects penetration
by sieving. It influences dust cake filtration both directly and
indirectly. The indirect dependence is through the dependence of the
22
-------
growth of the dust cake and its properties on the size distribution of
the dust (among other variables).
Recognizing the dependence of dust cake growth on the particle size
distribution of the dust alerts one to the dangers of studying respirable
particle behavior by using a dust source, such as flyash, in which the
respirable particles are in the minority by mass (though not by number).
Dust cake growth depends on the full spectrum of particle sizes in any
dust. Without the large particles, different dust cake structure and
growth rates result but, in dust cake filtration, the dust cake captures
the small particles. Concluding that polyester fabrics would be more
effective in filtering small particles could be misleading because, from
Figures 3 and 9, the steady state small particles concentrations in the
outlet are less than for the polyester fabrics. If the dust source were
a classified flyash, containing nothing but 2 ym particles or smaller,
fine particulate penetration through the polyester fabrics could increase
much more than it would through the Gore Tex/Nomex because the reduced
dust cake growth on the polyester fabrics could adversely affect their
ability to collect fine particulates. Gore Tex/Nomex fabric, on the
other hand, does not depend on dust cake growth for filtration efficiency
and would therefore be unaffected.
4.2 EFFECTS OF OTHER DUST PROPERTIES
4.2.1 Polyester Felt Fabric
Changing dusts but keeping the fabric the same can also influence
particle penetration. Compare the differences in flyash penetration
through polyester felt with those of rock dust through the same fabric
(Figure 6). Both dust sources have approximately log-normal size
distributions: the D™ of the rock dust is about 2.25 pro and that of
flyash, about 4.0 urn (see Figures 1 and 2). Much more fine (0.3 - 0.5 vm)
flyash penetrates the polyester felt fabric than rock dust, even though
Coulter counter and Andersen impactor analyses show that the concentration
of this size particle in the inlet is much higher for the rock dust than
the flyash (see Figures 1 and 2).
23
-------
The rate of dust cake buildup is much more rapid for the rock
dust/polyester felt system than for the flyash/polyester felt system,
thereby reestablishing high efficiency filtration more rapidly following
a cleaning pulse. This behavior is consistent: specific cake resistance,
i
K2, is significantly higher for the rock dust/polyester felt system.
Obviously from Figure 6, the concentration of penetrating particles
lessens more rapidly following cleaning when filtering rock dust than
flyash. More rapid buildup of the dust cake would be expected to produce
higher pressure drops; indeed, the rock dust/polyester felt system
exhibits much higher pressure drops and drags than the flyash/polyester
felt system (Table 1).
That the dust types interact differently with the polyester felt
can also be seen by noting the humidity dependence of the outlet con-
centration and the collection efficiency. With flyash, the highest
outlet concentration occurs at the lowest humidity for all inlet grain
loadings. As relative humidity increases from 50 to 70 percent, however,
the results are mixed—down for one of the grain loadings, up for the
other two.
The dependence of collection efficiency on relative humidity listed
in Table 1 for the flyash/polyester felt system is consistent with that
found by Durham and Harrington [Ref. 15] who reported that various
shake-cleaned flyash/woven fabric systems showed dramatic increases in
collection efficiency as the relative humidity increased from 20 to 60
percent. Other dust/woven fabric systems investigated by Durham and
Harrington did not show this sensitivity to relative humidity: Durham
and Harrington interpreted this observation to be evidence illustrating
the importance of dust properties.
Pressure drops generally increase with humidity, but not always.
It's a mixed picture here, too, as it was in the work reported by Durham
and Harrington [Ref. 15],
For rock dust filtration, humidity dependencies are much clearer.
Outlet concentration always decreases with increasing humidity and so do
the pressure drops. The rock dust/polyester system is better in two
24
-------
Table 1. PULSE-JET FILTRATION USING 16 OZ/YD^ POLYESTER FELT FABRIC
(9 bags; total cloth area, 41.94 ft2; A/C, 6 fpm; total flow, 250 cfm)
a) Flyash (90 psi pulse j 60 sec interval)
Inlet
Loading
gr/ft3
4.07
3.92
4.16
7.05
6.91
7.05
12.10
12.04
12.22
4.15
3.81
3.99
6.91
6.97
7.04
11.94
11.91
11.85
Relative
Humidity
%
25
50
70
25
50
70
25
50
70
20
50
70
20
50
70
20
50
70
Outlet
Concentration
gr/ft3
1.0443
0.7975
0.8364
1.6106
1.0445
1.8314
2.6107
2.0418
1.5508
b)
3.8263
2.4970
1.7109
4.7966
1 . 7534
0.9144
6.3747
1.4651
0.7481
Efficiency
%
99.9744
99.9797
99.9799
99.9772
99.9849
99.9741
99.9785
99.9830
99.9874
Fuck Dust
99.9078
99.9345
99.9572
99.9307
99.9749
99.9871
99.9466
99.9877
99.9937
Penetration
%
0.0256
0.0203
0.0201
0.0228
0.0151
0.0259
0.0215
0.017Q
0.0126
(90 psi pulse
0.0922
0.0655
0.0428
0.0693
0.0251
0.0129
0.0534
0.0123
0.0063
Bag AP
In.
H20
0.71
1.00
1.11
1.03
1.39
1.38
1.68
1.81
1.61
j US sec i
7.39
4.90
3.80
9.34
6.41
5.38
11.13
8.38
6.97
Slope
K2
3.3853
3.5186
3.0782
2.5155
2.8499
2.5155
2.4428
2.6190
1.9345
iterval)
24.3825
30.3497
21.7069
20.1393
26.3637
25.5725
19.6949
22.0510
21.5009
Effective
Drag
0.1067
0.1550
0.1742
0.1559
0.2150
0.2142
0.2550
0.2750
0.2484
1.1675
0.7433
0.5775
1.4683
0.9517
0.7817
1 . 7050
1.2300
0.9984
Residual
Drag
0.1183
0.1667
0.1850
0.1709
0.2317
0.2292
0.2800
0.3017
0.2684
1.2317
0.8167
0.6325
1.5567
1.0684
0.8958
1.8542
1 . 3967
1.1617
Run
Time
hrs.
8.0
7.3
8.2
8.3
8.3
7.8
7.5
7.8
9.4
7.3
8.4
8.0
7.0
6.5
7.5
7.5
7.7
7.8
Sample
Time
ml n .
45
45
45
45
45
45
45
45
45
30
30
30
30
30
45
30
30
60
-------
ways at higher humidity: collection efficiencies are higher, and
pressure drops across the bag (and the associated drags) are lower.
Note, however, that the absolute values of the pressure drops and drags
are still much higher than those of the flyash/polyester felt system,
regardless of humidity effects.
With rock dust, the lowest outlet concentration occurs at the
combination of the highest inlet grain loading and the highest relative
humidity. Significantly less dust penetrates the bag under these con-
ditions than when the inlet grain loading is, say a third as much at the
same relative humidity. Lowering the humidity at constant inlet grain
loading increases particle penetration even more.
This behavior is plausible if the effect of humidity is to enhance
particle agglomeration of the rock dust (perhaps through Kelvin
condensation), before the filter and at the filter surface. Agglomeration
would cause the dust cake to consist more of discrete clumps or clusters
of deposits around which the air streamlines pass. This clustering
pattern is one which minimizes increases in air resistance per unit of
dust collected—hence the observed low pressure drops at high relative
humidity.
Because of their small size (with respect to the fiber dimensions)
and non-aerodynamic shape, the growing clumps become even more efficient
sites for removal of dust and dust agglomerates by impaction than the
initial fibers of the fabric—hence the observed high collection eficiencies
at high relative humidity.
If the effect of high relative humidity is to enhance particle
agglomeration and if this property depends only on the dust, then the
data in Table 1 suggest that the properties of rock dust (because of the
observed greater sensitivity of their filtration parameters to relative
humidity) are more humidity dependent than those of flyash. That the
primary interaction of relative humidity is through particle agglomeration
is not proven however. Data presented in Ref. 15 show that the effect
of relative humidity varies considerably from fabric to fabric when
filtering flyash. Those observations, plus those presented in Section
4.2.2, show that knowing only the dust type is not sufficient information
26
-------
for predicting the effects of relative humidity on filtration performance.
Humidity dependence during fabric filtration varies with the dust/fabric
system rather than either the dust or the fabric alone. Any mechanism
purporting to explain the humidity interaction must depend on the
properties of the dust/fabric system.
4.2.2 Nomex Fabric
A second pair of dust/fabric systems was used to explore these
effects—a rock dust/Nomex system and a flyash/Nomex system. Nomex is a
commercial nylon fabric used in baghouses; it ranks at or near the top
of varous published triboelectric series classifying fabrics according
to electrostatic behavior [Ref. 16].
Measurements similar to those listed in Table 1 are reported in
Table 2 for the Nomex systems. As with the polyester felt, the dependence
of the flyash outlet concentration (C ) on relative humidity is mixed: the
highest humidity always corresponds to the lowest CQ, but the highest CQ
occurs at 50 percent relative humidity, the mid-range value of humidity.
Pressure drops at any given inlet loading are lowest when the humidity is
highest, but the intermediate humidities have mixed C with respect to the
lowest humidities.
The rock dust/Nomex data show an even stronger lowering effect of high
relative humidity on outlet concentration than observed with the rock dust/
polyester felt system.
The difference in C between 50 and 70 percent relative humidity is at
least 1 order of magnitude at the highest inlet loading. The CQ decreases
by over 2 orders of magnitude as the relative humidity increases from 50 to
70 percent. The lowest values of C observed in these series (Tables 1 and
2) were measured at 70 percent relative humidity in the rock dust/Nomex
system, regardless of inlet loading. Again, somewhat unexpectedly, the
C decreased as the inlet grain loading increased. For the rock dust/
Nomex system at fixed inlet loading, however, the pressure drops increased
with increasing relative humidity (unlike for the rock dust/polyester felt
system). The ranges of pressure drops observed with the rock dust/Nomex
system are not dramatically higher than those of the flyash/Nomex system.
27
-------
TABLE 2. PULSE-JET FILTRATION USING 16 OZ/YD2 NOMEX
(9 bags; total cloth area, 41.94 ft3; A/C, 6 fpm; total flow, 250 cfm)
to
00
a) Flyash (90 psi pulse_j 60 sec interval)
Inlet
Loading
gr/ft3
4.18
3.83
3.98
6.91
6.97
7.16
12.34
12.22
12.22
3.76
3.78
3.93
6.70
6.73
6.97
12.82
11.76
11.67
Relative
Humidity
25
50
70
25
50
70
25
50
70
20
50
70
20
50
70
20
50
70
Outlet
Concentration
qr/ft3
0.4676
1.3482
0.5845
0.9469
1.0911
0.5105
1.0498
1.0832
Efficiency
99.9888
99.9648
99.9853
99.9864
99.9844
99.9929
99.9915
99.9912
Penetration
0.0112
0.0352
0.0147
0.0136
0.0156
0.0071
0.0085
0.0088
Baa AP
In.
H20
1.21
1.77
1.46
2.18
2.34
1.65
5.25
3.04
Slope
K2
4.0046
5.1482
3.9589
5.2723
5.3669
2.8893
5.5060
4.6750
0.4286 99.9965 0.0034 2.12 3.2241
b) Rock Dust (90_psi_pulse,_60_sec_interval_)_
11.2023
2.9887
0.2156
9.1372
4.6173
0.0805
6.5532
3.2029
0.0130
99.7025
99.9210
99.9946
99.8636
99.9315
99.9989
99.9493
99.9526
99.9999
0.2975
0.0790
0.0054
0.1364
0.0685
0.0011
0.0507
0.0474
0.0001
1.92
2.18
3.23
3.44
3.23
6.02
5.33
5.50
8.72
21.7154
27.7707
22.7992
22.0636
19.1631
17.7955
15.8288
17.7607
19.5351
Effective
Drag
0.1867
0.2950
0.2300
0.3325
0.3600
0.2575
0.8192
0.4575
0.3200
0.2509
0.2967
0.4625
0.4484
0.4292
0.8983
0.7158
0.7400
1.2667
Residual
Drag
0.2009
0.2950
0.2433
0.3634
0.3917
0.2750
0.8767
0.5059
0.3533
0.3200
0.3633
0.5383
0.5734
0.5383
1.0033
0.8875
0.9167
1.4525
Run
Time
hrs.
8.4
7.7
8.5
8.0
6.8
8.1
7.5
6.0
6.7
5.0
6.0
6.7
3.0
6.5
7.0
3.7
5.5
7.0
Sample
Time
min.
30
30
30
30
30
30
30
30
30
30
30
45
30
30
45
30
30
45
a) Except the 3.78 inlet loading data (row 2) for which the pulse interval was 45 sees.
-------
Specific cake resistance during the filtering of rock dust continues to be
much higher than when filtering flyash.
4.2.3 Summary
Table 3 summarizes trends for the two dusts and the two fabrics.
The key dust-related effect is probably the difference in humidity
dependence reflected in the CQ trends of flyash and rock dust in both
systems. The C of rock dust decreases with increasing relative humidity
regardless of the fabric, while flyash CQ does not. That this trend occurs
in both fabric systems suggests that it could be a dust effect. That it
is so much more pronounced in the rock dust/polyester felt system shows
that it also .depends on the dust/fabric system rather than on the dust alone.
Mixed dust/fabric dependencies—those that depend on both the dust
and the fabric—dominate Table 3. They can be identified by a conflicting
trend when switching either fabric or filter. For example, with felted
polyester, flyash CQ increases with increasing inlet concentrations, but
rock dust CQ decreases. With Nomex fabric, both CQ's decrease with in-
creasing inlet loading.
Alternatively the mixed dependence could be a conflicting trend that
occurs for the one dust and the two fabrics but not for the other dust and
the same two fabrics. An example of this mixed dependence is humidity-
dependence of pressure drop. Using rock dust, the trends differ for the
Nomex fabric and the polyester felt; using flyash as the dust, the trends
are similar. Leith et al. [Ref. 14] speculate that the stronger particle-
to-particle and particle-to-fiber bonds brought about by higher relative
humidity could cause pressure drop to either decrease or increase with
increasing relative humidity, depending on whether the increased particle-
to-fiber bond strength dominates (and causes higher areal dust cake
loadings and higher pressure drops) or the particle-to-particle bond
strength dominates (and causes a more porous dust cake structure and
reduced pressure drops).
i
The trends in 1C reflect only small changes and do not justify any
general qualitative conclusions, especially in view of the effective
i
nature of the slope measurements that constitute K^.
29
-------
Table 3. SUMMARY OF TRENDS IN THE PULSE"JET FILTRATION OF FLYASH
AND ROCK DUST
With Respect to Increasing:
FLYASH:
Inlet Loading
(constant humidity.
Humidity
(constant inlet loading)
ROCK DUST:
Inlet Loading
(constant humidity)
Hurr.idity
(constant inlet loading)
Polyester Felt
(fr
Co
!a
^ v
X> a
t
3m Table
AP
1
Y^
1
t
l)t
s^
|a
S^
Nomex
-------
SECTION 5
RELATIONSHIP TO EXISTING FABRIC FILTRATION LITERATURE
5.1 FRACTIONATING EFFECTS DURING FILTRATION
As discussed in Section 1, fabric filtration literature seems to con-
tain conflicting conclusions regarding the fractionating effects of fabric
filtration. Data presented here are evidence that fabric filtration can be
either fractionating or non-fractionating, depending on the dominant
mechanism of particle penetration. Knowing whether fractionation occurs
or not is very useful in understanding the physics of particle penetration.
For example, if the size distribution of the outlet dust is identical to
that of the inlet, the straight-through processes of particle penetration
are not dominant, but rather some non-fractionating mechanism such as pin-
hole plugs [Ref.11].
Because of the experimental decisions made early in the work and
their consequent limitations, as discussed in Section 3, few direct
measurements of the size distribution of the outlet dust were made.
Indirect evidence (the size distribution of dust in the collection hopper
during specific experiments and the size distribution of the feed dust
sampled over long periods of time) was presented to support the conclusion
that both fractionating and non-fractionating filtration exists. Data in
the literature also support this position. For example, non-fractionating
filtration has been analyzed by Leith et al. [Ref.ll]. As mentioned in
Section 4, Leith et al. assign non-fractionating properties to their two
delayed components of particle penetration (seepage and pinhole plugs).
Leith's dissertation [Ref.17] documents the many experimental measurements
and analyses supporting the non-fractionating property of his filtration
work.
31
-------
Also from contemporary literature, however, are the data plotted in
Figure 10. These curves show the size distribution of the inlet flyash
to a fibrous glass filter (the dashed curve) and the outlet dust from that
dust filter after the inlet gas was switched to unloaded ambient air. The
dust in the outlet under these latter conditions is presumably primarily
delayed dust penetrating the fabric—Dennis calls it rear-face slough-off
[Ref.12]. Since the data in Figure 10 clearly show this dust to be of a
different size distribution than the inlet flyash, it cannot be penetrating
by the seepage/pinhole plug mechanisms of Leith et a!. An alternative
explanation is that seepage and pinhole plugs may fractionate by size after
all.
Highly significant in this experimental work is the air-to-cloth ratio
at which the filtration is carried out. In all of Leith's work for pulse-
jet filters [Refs.ll, 17] the A/C is 10 fpm or higher. His analysis predicts
pinhole plug penetration to increase with velocity. Dennis' data in Figure
10 for woven cloth filters cleaned by hand-shaking is based on filtration
at about 2 fpm, a more common operating value in field installations.
The implied correlation between face velocity and penetration mechanism
appears in the Gore Tex/Nomex data of Figure 9. At the higher A/C (Figure
9b) one can almost see the pinhole plugs bursting through the fabric and
producing the pulses of particles reflected in the data. Little evidence
exists for such bursts at the lower A/C of 4 fpm (Figure 9a).
Optical counter analysis of the outlet dusts penetrating through the
Gore Tex/Nomex fabric is inconclusive. The results given in Reference 3
show a small dependence of size distribution on air-to-cloth ratio: the
smallest particles (0.3 - 0.5 ym) fall from 66 percent of the total number
measured at A/C = 4 fpm to 57 percent at A/C = 10 fpm. Similar small A/C
dependencies of the size distribution of outlet dusts were found by McKenna
et al. [Ref.7] in their filtration of coal-fired boiler flyash with Gore
Tex/Nomex. McKenna's analysis of the outlet dust showed classical
fractionation effects—particle penetration increased as particle size
decreased below 10 ym, on down to about 0.5 ym. Below 0.5 ym, particle
penetration decreased sharply.
32
-------
10
CtL
UJ
-------
The conclusion is not firm: fractionating effects during fabric
filtration occur sometimes, but not always. Evidence from the literature
supports the view that the different, seemingly conflicting conclusions
regarding fractionation during filtration could well be consistent, being
simply different extremes of a complex interaction. Air-to-cloth ratio
is an important variable in determining the dominant penetration mechanism.
5.2 THE ROLE OF DUST PROPERTIES IN FABRIC FILTRATION
Some of the dependencies reported in Sections 4.1 and 4.2 are neither
unique nor original. Figure 11, reproduced from Dennis et al. [Ref.12],
shows the influence of size distribution on the filtration performance of
woven glass bags. The two granite dusts differ only in size distribution.
The other curves also reflect performance differences, possibly due in
part to differences in size distribution but perhaps also due in part to
other dust property differences, since the flyashes are from different
sources. Regardless of the interpretation of the exact mechanism, these
data are included to demonstrate the Importance of dust properties in
typical or simulated field installations.
The importance of dust properties, particularly with respect to
humidity effects, has been recognized and documented previously [Refs.5,
18]. For many dusts, but not all, higher relative humidity has been shown
to produce higher collection efficiency at lower pressure drop—like the
data shown in Table 2 for the rock dust/Nomex system. Such humidity
dependence, while not fully understood, is commonplace and reproducible
and would seem to possess the potential for providing more competitive
fabric filtration.
34
-------
1200
1000
CVJ
800
t—
co
5 60°
or
CO
400
200
0
Dust
— •— Fine Rhyolite
Coarse Rhyolite
Lignite Flyash
•* GCA Flyash
1 r
A/C: 0.61 m/min
(2 fpm)
granite dust
•
• •
200
400
600
800
1000
1200
AVERAGE FABRIC LOADING, Q/m'
Figure 11
Dependence of pressure drop buildup across woven glass fabric
on dust properties (from Dennis [Ref.12]).
35
-------
SECTION 6
REFERENCES
1. Turner, J. H. EPA Fabric Filtration Studies: 1. Performance of
Non-woven Nylon Filter Bags, EPA-600/2-76-168a (NTIS No. PB 266271/AS),
December 1976.
2. Ramsey, G.H., R. P. Donovan, B. E. Daniel, and J. H. Turner. EPA
Fabric Filtration Studies: 2. Performance of Non-woven Polyester
Filter Bags, EPA-600/2-76-168b (NTIS No. PB 258025/AS), June 1976.
3. Donovan, R. P., B. E. Daniel, and J. H. Turner. EPA Fabric Filtration
Studies: 3. Performance of Filter Bags Made from Expanded PTFE
Laminate, EPA-600/2-76-168c (NTIS No. PB 263132/AS), December 1976.
4. Donovan, R. P., B. E. Daniel, and J. H. Turner. EPA Fabric Filtration
Studies: 4. Bag Aging Effects, EPA-600/7-77-095a (NTIS No. PB
271966/AS), August 1977.
5. Daniel, B. E., R. P. Donovan, and J. H. Turner. EPA Fabric Filtration
Studies: 5. Bag Cleaning Technology (High Temperature Tests),
EPA-600/7-77-095b (NTIS No. PB 274922/AS), November 1977.
6. Turner, J. H. EPA Research in Fabric Filtration: Annual Report on
IERL-RTP In-house Program, EPA-600/7-77-042 (NTIS No. PB 267441/AS),
May 1977.
7. McKenna, 0. D., J. C. Mycock, and W. 0. Lipscomb. Applying Fabric
Filtration to Coal-fired Industrial Boilers, A Pilot Scale Investi-
gation, EPA-650/2-74-058a (NTIS No. PB 245186/AS), August 1975.
McKenna, J. D., J. C. Mycock, and W. 0. Lipscomb. "Performance and
Cost Comparisons Between Fabric Filters and Alternate Particulate
Control Techniques," J. Air Poll. Cont. Assn. 24, No.12, December
1974, pp. 1144-1148.
8. Turner, J. H. "Extending Fabric Filter Capabilities," J. Air Poll.
Cont. Assn. 2^, No.12, December 1974, pp. 1182-1187.
9. Bergman, L. "New Fabrics and Their Potential Application," J. Air
Poll. Cont. Assn. 24, No.12, December 1974, pp. 1187-1192.
36
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10. Dennis, R., and J. Wilder. Fabric Filter Cleaning Studies, EPA-
650/2-75-009 (NTIS No. PB 240372/AS), January 1975.
11. Leith, D., S. N. Rudnick, and M. W. First. High-Velocity, High-
Efficiency Aerosol Filtration, EPA-600/2-76-020 (NTIS No. PB
249457/AS), January 1976.
Leith, D., and M. W. First. "Performance of a Pulse-Jet Filter at
High Filtration Velocity: 1. Particle Collection," J. Air. Poll.
Cont. Assn. 27_, No.6, June 1977, pp. 534-539.
12. Dennis, R., et al. Filtration Model for Coal Flyash with Glass
Fabrics, EPA-600/7-77-084 (NTIS No. PB 276489/AS), August 1977.
13. Leith, D. H. Private letter communication, February 1978.
14. Leith, D. H., M. W. Firsthand D. D. Gibson. "Effect of Modified
Cleaning Pulses on Pulse Jet Filter Performance," presented at
Third Symposium on Fabric Filters for Particle Collection. Tucson,
AZ, December 5-6, 1977.
15. Durham, J. F., and R. E. Harrington. "Influence of Relative Humidity
on Filtration Resistance and Efficiency of Fabric Dust Filters,"
Filtration and Separation 8. July/August 1971, pp. 389-398.
16. Frederick, E. R. "Some Effects of Electrostatic Charges in Fabric
Filtration," J. Air Poll. Cont. Assn. 24, No.12, December 1974, pp.
1164-1168. —
17. Leith, D.H. "Particle Collection 1n a Pulse Jet Fabric Filter."
Unpublished thesis submitted for Sc.D. Degree to Harvard School of
Public Health, April 15, 1975. (Available from Countway Library of
Medicine, 10 Shattuck St., Boston, MA 02115.)
18. Billings, C. E., and J. Wilder. Handbook of Fabric Filter Technology,
Vol. I Fabric Filter Systems Study, EPA No. APTD 0690 (NTIS No. PB
200648), December 1970.
37
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-78-141
2.
3. RECIPIENT'S ACCESSION NO.
FABRIC FILTRATION STUDIES:
6. Influence of Dust Properties on Particle Penetra-
tion
5. REPORT DATE
July 1978
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
R. P.Donovan (RTI), B. E. Daniel, and J. H.Turner
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
EHE624
See block 12.
11. CONTRACT/GRANT NO.
NA--Inhouse Report
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 (
Task Final; 4/74-12/77
COVERED
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES TERL-RTP project officer for this inhouse report is J.H. Turner
Mail Drop 61, 919/541-2925. Previous related reports are in the EPA-600/2-76-168
and EPA-600/7-77-095 series.
is. ABSTRACT
report examines the importance of dust properties in determining dust
penetration through a fabric filter. The major property considered is the size distri-
bution of the dust, which is an important dust property for dust penetration. Most
other important variables of dust penetration depend more on the dust/fabric com-
bination than on the dust alone. The report reviews dust penetration mechanisms
and relates them to dust and dust/fabric properties. It illustrates these interactions,
using data both generated in EPA's inhouse laboratory and published in the open
literature. Shaker and pulse- jet baghouse data are used in an attempt to identify
commonality in dust penetration independent of fabric cleaning technique.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Filtration
Fabrics
Dust
Penetration
Properties
Particle Size Distribution
Cleaning
Air Pollution Control
Stationary Sources
Fabric Filtration
Particulate
Baghouses
Shake Cleaning
Pulse-let Cleaning
13B
07D
11E
11G
13H
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
45
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
38
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