xvEPA
Particulate
Control
Highlights
PARTICULATE
TECHNOLOGY
BRANCH
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
EPA-600/8-78-005d
June 1978
Research on Fabric
Filtration Technology
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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
environmental technology. Elimination of traditional grouping was consciously
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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 SPECIAL REPORTS series. This series is
reserved for reports which are intended to meet the technical information needs
of specifically targeted user groups. Reports in this series include Problem Orient-
ed Reports, Research Application Reports, and Executive Summary Documents.
Typical of these reports include state-of-the-art analyses, technology assess-
ments, reports on the results of major research and development efforts, design
manuals, and user manuals.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the contents necessarily
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/8-78-005d
June 1978
Particulate Control Highlights
Research on
Fabric Filtration Technology
by
R. Dennis and N.F. Surprenant
GCA Corporation
* Burlington Road
Bedford, Massachusetts C1730
Contract No. 68-02-2177
Program Element No. EHE624
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
Significant developments in fabric filtration technology are highlighted in this report. Selected
results of several field and laboratory studies performed over the last 10 years by or under the sponsor-
ship of the U.S. Environmental Protection Agency are reviewed so that the user may better assess the
capabilities and limitations of filtration equipment. Discussions are initiated with a background presenta-
tion of fabric filter design and operational concepts followed by a sampling of actual field performance
with various coal fly ash aerosols and a description of an operational, mobile pilot filter system that is
used to facilitate the selection of operating parameters, fabric type and method of fabric cleaning. Fabric
weave and constitutents are discussed with respect to their bearing on working temperatures, method of
cleaning, fabric life, pressure loss and dust retention properties. Attention is also called to pinhole or
pore leakage and its impact upon collection efficiency and effluent size properties. The pros and cons of
increasing air-to-cloth ratio (face velocities) to reduce fabric and other capital costs are compared with
the attendant disadvantages of increased power needs and higher emission rates. Both pilot and bench
scale tests show that effluent concentrations increase very rapidly with face velocity. Recently develop-
ed modeling concepts that provide realistic predictions of glass fiber performance with coal fly ash are
reviewed.
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CONTENTS
Background 1
The Filtration Process 1
Field Performance Measurements 4
Design and Field Evaluation of a Mobile Filter System 5
Fabric Structure and Filter Performance 5
Pinhole Leaks and Filter Effluents 6
Effect of Face Velocity on Coal Fly Ash Filtration With Glass Fabrics 6
Predicting Specific Resistance Coefficient, «2 8
Fabric Filter Cleaning - Dust Dislodging Forces 11
Fabric Filter Cleaning - Residual Dust and Adhesive Forces 12
Predicting Filter Performance 12
References 14
Other Reports Available 14
FIGURES
Figure
1 Schematic, development of dust cake 3
2 Schematic, single compartment operation in a multicompartment filter system 3
3 Variations in pinhole leaks due to fiber presence and pore size 7
4 Effect of fabric loading and face velocity on outlet
concentrations. Bench tests with coal fly ash and woven glass fabrics 9
5 Typical drag versus fabric loading curves for various levels of partial cleaning 10
6 Cleaned bag with inside illumination by fluorescent lamp 11
7 Fabric cleaning and distribution of adhesive (separation) forces
versus fabric loading and adhesive (separation) force - Coal fly ash 13
TABLES
Table
1 Measured and predicted performance for woven glass bags with coal fly ash 13
iii
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SECTION 1
RESEARCH ON FABRIC FILTRATION TECHNOLOGY
BACKGROUND
A rigorous state-of-the-art appraisal of air
and gas cleaning technology was initiated by the
U.S. Environmental Protection Agency in 1969.
As part of the overall program, GCA/Technology
Division undertook a fabric filter systems study
that culminated in the preparation of a Handbook
of Fabric Filter Technology, and equally im-
portant, in the identification of future research
needs. Since 1969, several laboratory and field
studies have been performed by GCA in which
the theoretical and applied aspects of filtration
have been examined. Considerable effort has
been directed towards the control of fly ash
emissions from stationary sources. This area be-
comes increasingly important with increased
coal combustion and stricter particulate
emission regulations. Despite the considerable
historical background, critical data necessary for
the optimum use of filtration technology were
not available in 1969. The research projects sum-
marized in this report represent part of an on-
going effort to fill in these gaps.
The control of fly ash emissions from coal-
fired boilers has until recently fallen within the
domain of electrostatic precipitators (ESP). Prior
to the development of high temperature glass
fabrics, precipitators and scrubbers afforded the
only practical means for hot gas cleaning. Con-
sequently, with the successful evolution of the
latter technologies there was little incentive to
experiment with untested fabric filters even with
the advent of special, high temperature fabrics.
The picture has changed, however, since the
promulgation of the 1971 New Source Perfor-
mance Standards (NSPS) by the U.S. Environ-
mental Protection Agency. A review of exper-
ience at many coal-fired steam-electric genera-
tors has revealed that the NSPS of 43 ng/Joule*
is not attained by existing ESP systems. Several
possible reasons for noncompliance have been
discussed in a recent EPA report"! along with
suggested corrective measures. High ash elec-
trical resistivity, which reduces ESP particle cap-
ture, is a major problem that will become more
severe as the need to burn low sulfur coal in-
creases. Therefore, the role of fabric filtration as
a practical control alternative has been carefully
studied.
With respect to SO2 removal, wet (alkaline)
scrubbing now affords a viable approach. In
most applications, however, precollection of
particulates by ESP or fabric filters is prerequi-
site to effective utilization of absorbing liquids
and to compliance with particulate emission
standards. Limited field applications and recent
laboratory research suggest that fabric filtration
will eventually play a larger role in the control of
coal fly ash emissions.
THE FILTRATION PROCESS
Fabric filters, as a class, provide the highest
collection efficiencies of all particulate control
devices at the expense of a significant operating
pressure drop,~0.50 to 1.75 kPat relative to~0.1
kPa for an ESP and 3 to 10 kPa for low to high
energy wet scrubbers. When allowance is made
for total energy usage and capital cost factors,
however, annualized costs for fabric filters and
electrostatic precipitators are much closer than
suggested by their respective operating pressure
drops. Those fabrics designed for hot gas filtra-
tion usually display emission rates less than one
tenth of those for an ESP system with compar-
able inlet loadings. At ambient temperatures,
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emissions can be reduced by up to two orders of
magnitude by using woven napped fabrics. The
term "nap" refers to a mechanically induced,
loose fiber cover on the fabric surface that great-
ly enhances filter performance.
*43 ng/Joule = 0.1 lb/106 Btu.
tl kPa = 4 in. water.
The predominant particle capture mechanism
at high loadings, >0.5 g/m3, is sieving by the
dust layer that builds up on the fabric surface.2,3
Only during the first few minutes of filtration
with new or just cleaned fabrics, do the classical
collection mechanisms prevail; i.e., inertial im-
pact ion, diffusion and interception with or with-
out augmentation by secondary processes. An
ideal filter should function as a supporting sub-
strate for the dust layer which, without discon-
tinuities in the form of cracks or pinholes, con-
stitutes a nearly impenetrable layer for particles
of the same size making up the dust cake. Unfor-
tunately, normal variations in fabric structure
such as nonuniform pore size or an absence of
free fibers within the pores may lead to signif-
icant dust penetration. Only when the fabric
pores (interyarn openings) are spanned by an in-
tercepting fiber array is it possible to obtain
complete pore bridging and hence a solid dust
cake as shown in Figure 1.
High interstitial fiber counts are associated
with many fabrics woven from staple yarns (cot-
ton and numerous synthetics), especially those
that are napped. Because staple yarns are spun
from short, ~5 to 10 cm long fibers, many fibers
project from the yarns, thus providing extended
collection surface. When both warp and fill yarns
are spun from staple fibers, the discrete fiber
fraction far exceeds that afforded by the mix of
staple and multifilament yarns customarily
found in glass fabrics. Warp yarns; i.e., those
that extend lengthwise in a loom, are usually
aligned with the bag axis. The crossing yarns are
described as fill or woof yarns. Unfortunately,
most nonmineral fibers will fail at flue gas tem-
peratures because of physical and chemical
degradation.
However, twill-weave glass fabrics with
special surface lubricants to reduce yarn
abrasion have been used successfully in the
U.S.A.3,4,5 The axially-aligned warp yarns,
which are spun from continuous filaments, pro-
vide the tensile strength whereas the bulked fill
yarns furnish the extended fiber surface needed
for particle capture. Unfortunately, complete
pore bridging may not be obtained such that
some of the approaching aerosol escapes. Thus,
with glass fabrics, collection may be reduced to
the 99.9 percent range in contrast to the 99.999+
levels attainable at ambient temperatures with
napped and/or all staple fabrics.
Dust concentration; particle size, shape and
charge; and humidity influence fabric pressure
drop through their effects on the characterizing
specific resistance coefficient, K2, for the dust*
and dust cake release during cleaning.2 Increas-
ed filtration velocity or air-to-cloth ratio leads to
increased filter resistance and outlet concentra-
tion. Hence, fora given system, velocities must
not exceed some limiting value when using this
approach to reduce fabric and space require-
ments. 2|3
A typical filter system, Figure 2, consists of
many vertically aligned bags or tubes suspended
in several compartments through which the gas
flow is uniformly distributed. Compartments are
sequentially isolated from the system by control
valves to allow cleaning and maintenance with-
out system shutdown. Uniformity in operating
pressure drop and emissions increase with the
number of compartments. With too few compart-
ments, excessive differences in pressure loss
may cause flow fluctuations which would be un-
acceptable for most combustion and ventilation
processes.
Glass fabrics used for fly ash filtration are
usually cleaned by bag collapse and reverse flow
with occasional augmentation by gentle
mechanical shaking. Conversely, vegetable and
organic fiber fabrics can withstand vigorous
shaking or, in the case of felted media, high
energy pulse jet action. The latter process in-
*«2 is the proportionality constant in the equa-
tion which states that the increase in pressure
loss, P, across the filter is proportional to the
filtration velocity, V, and the change in the areal
density of the dust deposit, W; i.e., P = K2VW.
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DUST
YARN
BULKED FIBERS
UNUSED FABRIC
EARLY DUST BRIDGING OF FIBER SUBSTRATE
SUB SURFACE DUST CAKE DEVELOPMENT SURFACE OUST CAKE DEVELOPMEN1
Figure 1. Schematic, dust accumulation on woven glass fabrics.
REVERSE AIR INLET
SHAKING
(OPTIONAL)
VALVE CLOSED
CLEAN AIR
OUTLET
VALVE OPEN
TO ON-LINE
COMPARTMENTS
DUSTY AIR
INLET
VALVE OPEN
1-
SCREW CONVEYOR
FILTERING
BAGS INFLATED
CLEAN AIR
OUTLET
VALVE
CLOSED
CLEANING
COLLAPSE PREVENTED
BY INTERNAL RINGS
Figure 2. Schematic, single compartment operation in a multicompartment filter system.
-
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volves the discharge of brief, ~ 0.1 sec, pulses
of compressed air at about 90 psig (0.622 MPa)
into the bag outlet. The resulting rapid flexing of
the bag dislodges the surface dust layer. Except
for pulse jet systems, the isolation time for com-
partment cleaning varies from 2 to 5 minutes in-
cluding at least 1 minute with no cleaning action
so that dust may settle to the hopper.
In contrast to electrostatic precipitators, the
efficiency of fabric filters is expected to be inde-
pendent of the electrical resistivity of the ash.
There are potential problems, however, to be
avoided. For example, failure to maintain gas
temperature above its dewpoint can lead to con-
densation related difficulties; i.e., excessive
pressure drop requiring (a) boiler turndown un-
less a bypass capability is provided and (b) pos-
sible rupture of filter bags. Remedial measures
used or proposed to prevent condensation in-
clude complete system insulation supplemented
by internal heaters and continuous gas recircula-
tion during shutdown.2,3
Interstitial penetration of tar droplets and fine
particles during the break-in of new bags must
also be minimized. Injection of fly ash, lime-
stone or other mineral dusts during boiler start-
up and preheating periods will precoat fabrics
sufficiently to provide a cleanable, superficial
deposit.
Although no major difficulties are foreseen,
boiler scale-up from 50 to 500 MW or greater may
reveal some unanticipated problems. It should
be noted that reported bag lives of up to 41/z
years are not based upon the burning of typical
pulverized coal.5 On-line testing of new, fabric
filter controlled, coal-fired systems now under
construction will answer these questions.
Several research programs designed to improve
the quality of filtration are reviewed in the next
section.
FIELD PERFORMANCE MEASUREMENTS
Field measurements have been conducted4"6
to evaluate woven, glass fabrics used to control
dust emissions from coal-fired boilers and steel-
producing arc furnaces. Systems were tested at
the Sun bury Plant of the Pennsylvania Power and
Light Company^ using a fuel mixture of anthra-
cite and petroleum coke in 43 MW pulverized fuel
boilers and at the Nucla Plant of the Colorado
Ute Electric Association^ where a bituminous
coal was burned in 13 MW, spreader stoker
units. Similar evaluations were carried out on
fabric filters used to contain emissions from
30-ton, electric arc furnaces at the Marathon Le-
Tourneau Company in Longview, Texas.5
Standard EPA sampling methods were used
to estimate mass concentrations and particle
size properties. Based upon 31 tests at the Sun-
bury facility,5 the average mass emission rate
was 1.98 ng/J of coal fired which corresponds to
an average weight collection efficiency of 99.91
percent. Similarly, 22 tests at the Nucla Plant in-
dicated an average mass emission rate of 4.3
ng/J, equivalent to a collection efficiency of
99.84 percent.6 Currently, 43 ng/J is the allow-
able (NSPS)* emission rate for fossil fuel-fired
steam generators. No significant deviations from
the average emission rate were noted for
variations in firing rate, fuel sulfur content, or
fuel ash content.
Cascade impactors were used to determine
particle size properties up- and downstream of
the filters. These tests were supplemented by
condensation nuclei measurements so that the
fine particles in the effluent could be better char-
acterized. According to Sunbury tests, there was
no significant reduction in aerodynamic mass
median diameter, aMMD~7.5 ^m, as the fly ash
passed through the filter. On the other hand, an
apparent decrease, 6.5 to 5.5 ^m, aMMD was
observed at the Nucla installation. Subsequent
laboratory tests indicated that dust samples col-
lected immediately before and after the above
fabrics showed no significant size differences.3
The Nucla size reduction was attributed to an ap-
preciable loss of the larger particles, >15 jim
diameters, between the upstream sampling point
and the filter face by gravity and inertial separa-
tion. The similarity of up- and downstream size
properties is explained by the fact that 95
percent or more of the downstream aerosol is
composed of the upstream aerosol fraction that
*NSPS - New Source Performance Standards for
Particulate Emissions from Coal-Fired Boilers
with Firing Rate in Excess of 74.8x 10@ MJ/sec.
Promulgated by U.S. Environmental Protection
Agency. December 1971.
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has passed unchanged through pin holes or un-
blocked pores.
The disadvantage with field tests is that oper-
ating conditions cannot be readily varied nor can
special measurements be carried out except at
prohibitive costs. On the other hand, the impor-
tance of field tests, when supported by labora-
tory measurements, cannot be overstated.
The application of glass fabric filters at high
temperatures was also evaluated for electric
furnace operations. Limited measurements
indicated emissions in the range of 0.0032 to
0.0044 g/m3, well below the allowable EPA limit
of 0.012 g/m3.
DESIGN AND FIELD EVALUATION OF A
MOBILE FABRIC FILTER SYSTEM
The high efficiencies for fabric filters and the
prospect of stricter regulations have accelerated
filtration research. A major advantage of labora-
tory experiments is that the experimenter can
custom-design his system so that selected par-
ameters can be varied at will. Draemel.8 has re-
lated the performance of 123 fabrics to clean
fabric and test dust parameters. However, a not-
able disadvantage of most laboratory investiga-
tions is that the simulated aerosol seldom dup-
licates the field aerosol.
While field studies eliminate the problem of
aerosol simulation, it is rarely possible to alter
cleaning parameters, substitute different fabrics,
vary face velocities or institute other field
changes. As a means of providing versatility
while simultaneously working with real aerosols,
the Environmental Protection Agency contracted
with GCA/Technology Division to design, fabri-
cate and evaluate a mobile fabric filter system.9
By extracting a representative fraction of an in-
dustrial gas stream as the test aerosol, a practi-
cal means is provided to evaluate fabrics, clean-
ing methods and filter operating modes on a
pilot scale.
The EPA mobile fabric filter system has the
following capabilities:
• Filtration can be conducted at cloth veloc-
ities as high as 6.1 m/min at pressure dif-
ferentials up to 5 kPaand gas temperatures
up to 290°C.
• The system can be cleaned by mechanical
shaking, pulse jet or low pressure reverse
flow with the capability to vary the frequen-
cy and intensity of cleaning.
• The unit can be operated as a single or
three-compartment system with automatic
controls to facilitate long term testing.
Design and performance features for the sys-
tem are described in a report prepared for the
U.S. Environmental Protection Agency.9 Field
tests were conducted at a secondary bronze
smelter, a hot mix asphalt plant and a coal-fired,
power station to appraise the system's capa-
bility. At the conclusion of a successful evalua-
tion period, the mobile system was delivered to
the Environmental Protection Agency for subse-
quent use in a large scale program in which a
mobile wet scrubber and electrostatic precip-
itator were also included.
FABRIC STRUCTURE AND
FILTER PERFORMANCE
The most efficient woven fabrics are those
whose yarns are spun from staple fibers where
many free fibers occupy the pore region. When
these fibers with diameters ranging from~5 to 30
^m are uniformly dispersed, they provide an ef-
fective substrate for dust layer growth (Figure 1).
Yarns spun solely from glass staple are char-
acteristically low in tensile strength.3 Therefore,
compromise weaves with multifilament warp
(axially aligned) yarns are used to provide the
necessary tensile strength. Unfortunately, this
approach 'diminishes the quantity of discrete
fiber collectors that enhance filter performance.
Microscopic observation of fabrics furnishes
valuable insights on probable field performance.3
In conjunction with thread counts, weave type
and yarn dimensions, one can estimate the
number and size of the pores which may vary ap-
preciably from one fabric to another.3 However,
the number of open pores may be drastically re-
duced (~50 percent) with some fabrics due to
yarn proximity, thus decreasing permeability.
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Although contributing significantly to tensile
strength, multifilament yarns are poor particle
collectors. Hence, a nonuniform distribution or
absence of intrapore fibers can cause pinhole
leaks shown in Figure 3. In Figure 3a, the pin-
hole, whose-dimensions were defined roughly by
the 100 urn yarn spacing, appeared in a 3/1 twill
weave glass fabric. Figure 3b shows a monofila-
ment screen with 200 urn openings in which
bridging is only 95 percent complete.
Once filter resistance increases to 0.75 kPa or
greater, a few pinholes can lead to excessive
dust penetration because their low resistance to
air flow leads to pore velocities 1000 times or
more greater than the average face velocity.
Thus, even if the total pinhole area in only 0.01
percent of the total filter area, 10 to 20 percent of
the total flow may pass through the pores. With
respect to Figure 3b, more than 98 percent of the
flow channeled through the 5 percent pinhole
area.3
Clean cloth permeability may be a poor index
of particle collection because resistance alone
may not reflect the presence of substrate fibers
nor the number and size distribution of the
pores. In addition to direct microscopic observa-
tions, simple tests with submicrometer dusts
provide excellent insights as to dust capture
potential. A modest improvement in efficiency
with atmospheric dust, 64 versus 40 percent for
cotton and glass fabrics, respectively, may sig-
nal a dramatic lowering in fly ash outlet concen-
tration; e.g., from 10-3 to 10-5 g/m3.3
Because temporarily or permanently un-
blocked pores are characteristically associated
with many fabrics including the glass media
used for fly ash collection,5,6 # /s important to
note the effect of these pore properties on filter
effluents.
PINHOLE LEAKS AND FILTER EFFLUENTS
It was pointed out that a disproportionately
large gas flow passes through the pinholes be-
cause of the latter's minimal resistance to gas
flow. Detailed measurements showed that the
mass of dust conveyed through the pinholes was
nearly proportional to the leak flow.3 This means
that few particles >15^m are separated from the
aerosol as it converges to accelerate through a
pinhole. On the other hand, the undisturbed dust
cake is nearly impenetrable due to its high effic-
iency sieving action. Thus, the size properties of
the up- and downstream particles are essentially
the same because 95 to 99 percent of the effluent
is composed of the unaltered upstream aerosol.
As indicated previously, some particles (~5 to
10 percent by weight) are inertially scavenged
from the pinhole flow as may be inferred from
the "anthill", Figures 3a, surrounding the pin-
hole. Conversely, comparative condensation
nuclei counts showed no separation of nuclei
class particles (0.0025 to 0.5 Mm) in passing
through the pores. As a corollary, tests indicated
that effluent nuclei concentrations were propor-
tional to the total effluent mass concentration.3
The significance of the above findings with
respect to coal fly ash-woven glass fabric sys-
tems is that inlet and outlet mass concentration
measurements coupled with effluent size deter-
minations are usually sufficient to describe filter
system performance. The observed differences
between up- and downstream size properties
often result from particle losses between the up-
stream sampling point and the filter and/or
sampling errors. For such systems, computed
fractional size efficiencies usualy depict erron-
eous statistics relative to true filter behavior.
EFFECT OF FACE VELOCITY ON
COAL FLY ASH FILTRATION
WITH GLASS FABRICS
Operation of fabric filters at high air-to-cloth
ratios reduces space requirements and equip-
ment capital costs. However, increased velocity
also increases pressure loss and cleaning
frequency which will eventually override the ad-
vantage of reduced equipment and space costs.2
Less well understood is the relationship between
face velocity and outlet concentration. This dis-
cussion focuses on glass twill weaves and their
nonmineral counterparts, in which the warp
multifilament yarns provide the strength and the
bulked or staple fill yarns provide the actual
collection capability. Ordinary wool and syn-
thetic fiber felts, >380 g/m2, which are used at
high face velocities (~3.1m/min) are not the
subject of this review. Given a typical fabric,
some variability in pore size and intrapore fiber
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-
(a). Pinhole leak, filtration surface, showing
characteristic mound, substage lighting
(20X magnification).
(b). Massive pinhole leakage with monofilament
screen - without loose fibers.
Figure 3. Variations in pinhole leaks due to fiber presence and pore size.
-
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dispersion is expected. At moderate velocities
(0.6 to 0.9 m/min), maximum unblocked pore
areas seldom exceed 10"4 percent of the total
face area so that collection efficiencies fall in the
99.9 percent range.3 However, this pattern may
change radically when the air/cloth ratio is
increased.
Because pinhole flow at a fixed filter resis-
tance varies directly with pinhole area, it is es-
sential that pore bridging be completed as soon
as possible. With typical inlet loadings of ~2.3 to
7.0 g/m3, nearly complete bridging takes place
within a few minutes leaving only the larger
pores to be closed. The extent to which the re-
maining pores become blocked is velocity de-
pendent. At higher velocities, an equilibrium
may develop between the dust deposition and re-
entrainment rates such that certain larger pores
are never blocked, Figure 3. The effect of velpcity
(V) and fabric loading (W) on outlet concentra-
tion, Co, is indicated in Figure 4.
Outlet concentrations decrease rapidly dur-
ing the early loading phase, ~ minutes, followed
by an asymptotic decline to a lower limit that
ranges from 5 x 10-4 g/m3 at a face velocity of
0.39 m/min to 2 x 10'1 g/m3 at 3.35 m/min.
Thus, there is a 400 fold increase in minimum
concentration and a 25 times increase in average
outlet concentration as a result of the velocity in-
crease. 3 These measurements indicate that
emission levels may determine the maximum air-
to-cloth ratios.
PREDICTING SPECIFIC RESISTANCE
COEFFICIENT, K2
The permeability of a dust layer, usually ex-
pressed by the specific resistance coefficient,
K2, bears the same importance to filtration as
does dust electrical resistivity, n, to electro-
static precipitation. In filtration, high «2 values
mean high dust cake resistance and thus in-
creased fan power and more frequent fabric
cleaning. High electrical resistivities without
compensating measures can seriously reduce
particle collection. The successful modeling of
fabric filtration and electrostatic precipitation
requires that both dust properties, K2 and n, be
defined accurately.
Presently, it is difficult to predict the K2 value
despite an extensive literature on the subject.2
Problems arise because most theories derive
from overly simplified geometric concepts and
because the key variables are difficult to measure
accurately. Therefore, r\2 should be measured
directly to avoid serious estimating errors.3
However, because of the unexplicably broad
scatter in reported K2 values,2 recent data were
analyzed to explain inconsistencies. A true K2
value must be based on the ratio of the increase
in filter drag, AS, when the dust deposit and face
velocity are uniform over the filter surface,
K2 = AS/AW. Most field measurements do not
permit the direct computation of true K2 values
because fabric loadings are not uniformly dis-
tributed on individual bags nor in collector
compartments. The problem is illustrated in Fig-
ure 5 in which typical drag versus average load-
ing curves are shown for completely and partially
cleaned fabrics. Curve 1 provides the only true
estimate for K2. The shapes for Curves 2 through
4 reflect various degrees of flow apportionment
between cleaned and uncleaned surfaces that
depend upon their respective initial resistances.
Only when the filtration is performed over
lengthy periods without cleaning will such
curves converge to the same and correct slope
for the K2 value. In many commercial applica-
tions, the intervals between cleaning are too
brief for a uniform dust deposit to develop. Ad-
ditionally, there is seldom complete information
on the size properties of the particles in the dust
cake per se.
Because recent tests provided the required
data, the Kozeny-Carmen relationship was used
to predict K2 values for comparison with actual
measurements.2.3 The variables requiring defin-
ition were gas viscosity, ji; the specific surface
parameter for the particles in the dust cake, S0;
the discrete particle density, Pp and the dust
cake porosity, e. The Term So derives from the
mass size distribution obtained by cascade
impactor measurements; Pp, by pycnometer
measurement; and e from discrete particle den-
sity, pp, and the bulk density p of the dust, the
latter determined by light shaking of an open
container of the dust.
For several dust and fabric combinations (fly
ash, granite, and talc with woven glass and
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10
NOTE'SOLID LINES REPRESENT CURVE FIT
BY MATHEMATICAL MODEL3
0
"40 60 80 l(
FABRIC LOADING (W), g/m2
140
Figure 4. Effect of fabric loading and face velocity on outlet concentrations. Bench tests
with coal fly ash and woven glass fabrics.
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DESCRIPTION
MAXIMUM POSSIBLE CLEANING
HIGHLY EFFICIENT CLEANING
AVERAGE CLEANING RANGE
(MECHANICAL SHAKING)
AVERAGE CLEANING RANGE
COLLAPSE WITH REVERSE
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0.5 W,
0.75WT
W,
AVERAGE FABRIC LOADING,W
Figure 5. Typical drag versus fabric loading curves for various levels of partial cleaning
10
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napped, sateen weave cotton) the above input
parameters (in conjunction with a modified
Kozeny-Carman constant of 2.5 instead of 5.0)
gave fair predictions of K2, *50 percent of meas-
ured values.3 Although the above accuracy is
hardly sufficient for design purposes, it betters
the 10-fold ranges often found in the literature.2
Aside from uncertainties in size measurements,
it is important to note that small, ~70 percent,
errors in estimating porosity and particle density
can lead to 50 percent errors in K2-2.3
Gas velocity was shown to exert a significant
effect on K2 as reported by previous investiga-
tors.2 For fly ash/glass fabric combinations, K2
was observed to increase as the square root of
the face velocity over the range 1.3 to 3.5
m/min.3 This behavior is attributed to the fact
that increased particle momentum at higher
velocities creates a less porous cake. One must
differentiate between this effect and that of grad-
ual cake and/or fabric compression that occurs
with dusts that deposit initially as highly porous
structures.
FABRIC FILTER CLEANING -
DUST DISLODGING FORCES
Fabric filtration is effective only when the
filter can be cleaned periodically and economic-
ally without impairing collection efficiency or
disturbing the system gas flow. Although fabric
filters have been used for many years, the clean-
ing process has only recently been examined
quantitatively.7 Highlights of recent studies on
filter cleaning by (a) mechanical shaking, (b) re-
verse flow or (c) combinations of (a) and (b) are
discussed below.
In a simple shaking system, the oscillation of
the shaker arm alternately accelerates and de-
celerates the dust laden bag surfaces. The re-
sulting tensile and/or shearing forces exerted at
the fabric/dust layer interface, if greater than
local adhesive forces, will remove slabs or flakes
of dust from the fabric as shown in Figure 6. A
fluorescent tube within the bag reveals clearly
the dust dislodgement sites.
The separating force (assuming that tensile
and shear forces are roughly equivalent) can be
estimated from the dust loading, W, and the
average acceleration a imparted to the dust laden
fabric.3 The acceleration is computed from
shaker arm amplitude (half-stroke) and shaking
frequency. Field and laboratory tests have in-
dicated that average acceleration must be at
least 3 g's to impart the shaking motion over the
entire bag7 Low frequencies, <4 cps, and small
amplitudes, <1 cm, generate acceleration forces
appreciably less than that attainable in a gravity
field7
Bag collapse accompanied by a clean, re-
verse air flow (usually less than the face velocity)
is a preferred method of cleaning glass fabrics
because it avoids the stresses caused by
mechanical shaking. Here the cleaning principle
is the same as that for shaking except that the
dislodging force is now defined by the product
W. g rather than W-a. The flexing rate and the
bag curvature after collapse, which may also play
important roles in dust dislodgement, require
further study.
Figure 6. Cleaned bag with illumination from
inside by fluorescent tube.
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FABRIC FILTER CLEANING -
RESIDUAL DUST AND ADHESIVE FORCES
Although dust removal forces may be approx-
imated for shaking or reverse flow systems, de-
termination of the actual amount and location of
the separated dust requires information on
dust/fabric adhesion. Rough estimates of ad-
hesion have been proposed for selected fly ashes
and twill weave glass and sateen weave cotton
fabrics.2,3 These measurements, however, pro-
vide insights as to (a) how dust separates from a
fabric and (b) how adhesive forces are probably
distributed.
First, because fly ash usually forms low
porosity (<0.7) deposits, the cohesive forces
within the dust cake because of multiple particle
contacts far exceed the adhesive forces between
widely spaced yarns and the interface particles.
Therefore, dust separates at the dust/fabric
interface where the bonds are weakest. The
cleaned region beneath the dislodged dust al-
ways displays the same residual dust holding,
WR, and the same cleaned cloth drag, SR. Re-
sidual dust on glass fabrics averages about 50
g/m2, whereas cotton sateen retains more, 75
g/m2, because of increased fiber cover. In both
cases, the residual dust is found mainly within
the bulked, loosened fibers and rarely on the
smooth surfaces of multifilament yarns.
The unique properties of the cleaned fabric,
Figure 6, allow one to determine the gas flow
distribution with respect to location and time
once a filter compartment is returned to service
after cleaning.^ It is only necessary that the frac-
tion of cleaned bag area (ac) be determined. An
empirical approach has been proposed which al-
lows ac to be estimated from the fabric dust
loading before cleaning, Wp, or the separation
force, FS, when the dust loading is acted upon
by gravity or shaking acceleration; i.e., Wg or
Wa.3 Since the adhesive force, FA, is just ex-
ceeded by the separating force at the instant of
dust dislodgement, Figure 7 also furnishes a
rough measure of adhesive force.
Despite the data scatter, the description of
dust separating forces in terms of the products,
W-g or W'3 appears as a rational means for es-
timating the amount of cleaning accomplished
by reverse flow and mechanical shaking. The
principal limitation to this approach is that each
dust/fabric combination possesses its unique
adhesion properties as suggested by glass and
cotton fabric data, Figure 7. Thus, until an im-
proved theory is developed, it will be necessary
to determine ac by special laboratory studies or
by detailed analyses of field measurements.
PREDICTING FILTER PERFORMANCE
The adaptability of glass fabrics to fly ash
filtration suggests their use where low sulfur
coal and/or high ash resistivity preclude ef-
ficient electrostatic precipitation. Until recently,
however, there were no means short of pilot
plant testing for predicting operating and perfor-
mance parameters for a specified dust/fabric
application in a prototype system.
Despite many past attempts to develop filtra-
tion models,2.3 failure to define the true nature
of a cleaned fabric surface usually led to poor
results when such models were applied to non-
replicate systems. Recent studies have indicated
that many conventional filtration processes can
be modeled if the following factors are definable:
• The amount of dust on the filter before
cleaning, Wp, and its terminal drag, ST.
• The fraction of cleaned area, ac, exposed
by the cleaning action and its characteristic
residual drag, SR, and fabric loading, Wp.
• The K2 value for the dust (preferably deter-
mined by experiment) and the relationship
between K2 and the filtration velocity, V.
• The relationship between the method and
intensity of cleaning and the fraction of
cleaned area produced.
• The relationship between outlet concentra-
tion and face velocity, fabric loading, inlet
concentration, and specific dust/fabric
combination.
Integration of the above data into an iterative
calculating procedure for sequentially-cleaned,
multicompartmented baghouses describe close-
ly the performance of real filter systems. 3 In view
of the numerous mathematical functions con-
stituting the model, the reader is referred to the
12
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original report for details on its design and ap-
plications.3 it is emphasized, however, that the
basic building blocks for the predictive equa-
tions are the well accepted filter drag versus
fabric loading relationships. What the model
does is to integrate the performances of individ-
ual filter elements operating in parallel where re-
sistance, velocity, dust penetration and K2 vary
with respect to time and location. Based upon
validation tests using field data for the Sunbury
and Nucla operations, its use as a diagnostic
tool showed very encouraging results, Table 1.3
ADHESIVE (SEPARATION) FORCE, dynti/cm2
10 50 100 ZOO
200
500 1,000 2,000
FABRIC LOADING, Wp-g/m2
Table 1. MEASURED AND PREDICTED PERFORMANCE FOR
WOVEN GLASS BAQS WITH COAL FLY ASH
Percent penetration
Measured* Predicted*
Nucla, Colorado
Sunbury, Pennsylvania
0.21 0.19
(1.52)t
0.15 0.20
Resistance-kPA
Nucla, Colorado
Average, cleaning and filtering
During cleaning only
Maximum just before cleaning
Minimum just after cleaning
Sunbury, Pennsylvania
Average, cleaning and filtering
During cleaning only
Maximum just before cleaning
Minimum just after cleaning
Measured
1.03
1.7
1.16
0.85
0.64
0.71
0.71
0.56
Predicted
0.97
1.52
1.16
0.72
0.62
0.66
0.66
0.57
'Averaged over cleaning and filtering cycles.
touring cleaning cycle only.
Figure 7. Fabric cleaning and distribution of
adhesive (separation) forces versus
fabric loading and adhesive (separ-
ating) force - Coal fly ash.
13
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REFERENCES
1. Oglesby, S. J., and G. Nichols. Particulate Control Highlights: Research in Electrostatic Precipitator
Technology. U.S. Environmental Protection Agency, Research Triangle Park, North Carolina. EPA-
600/8-77-020a. December 1977.
2. Billings, C.E., and J. E. Wilder. Handbook of Fabric Filter Technology, Volume I, Fabric Filter Sys-
tems Study, 1970. U.S. Environmental Protection Agency, Control Systems Laboratory, Research
Triangle Park, North Carolina. EPA-APTD 0690 (NTIS No. PB-200-648). December 1970.
3. Dennis, R., et. al. Filtration Model for Coal Fly Ash with Glass Fabrics. U.S. Environmental Protec-
tion Agency, Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina.
EPA-600/7-77-084. August 1977.
4. Bradway, R. M., and R. W, Cass. Fractional Efficiency of a Utility Boiler Baghouse - Nucla Generating
Plant. U.S. Environmental Protection Agency, Control Systems Laboratory, Research Triangle Park,
North Carolina. EPA-600/12-75-013a (NTIS No. PB-246-64/AS). August 1975.
5. Cass, R. W., and R. M. Bradway. Fractional Efficiency of a Utility Boiler Baghouse: Sunbury Steam-
Electric Station. U.S. Environmental Protection Agency, Control Systems Laboratory, Research Tri-
angle Park, North Carolina. EPA-600/2-76-077a (NTIS No. PB-253-943/AS). March 1976.
6. Cass, R. W., and J. E. Langley. Fractional Efficiency of an Electric Arc Furnace Baghouse. U.S. En-
vironmental Protection Agency, Industrial Environmental Research Laboratory, Research Triangle
Park, North Carolina. EPA-600/7-77-023. March 1977.
7. Dennis, R., and J. E. Wilder. Fabric Filter Cleaning Studies. U.S. Environmental Protection Agency,
Control Systems Laboratory, Research Triangle Park, North Carolina. EPA-650/2-75-009 (NTIS No.
PB-240-372/3G1). January 1975.
8. Draemel, D. C. Relationship between Fabric Structure and Filtration Performance in Dust Filtration.
Control Systems Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North
Carolina. EPA-R2-73-288 (NTIS No. PB-222-237). July 1973.
9. Hall, R. R., and R. Dennis. Mobile Fabric Filter System. Environmental Protection Agency, Control
Systems Laboratory, Research Triangle Park, North Carolina. Report No. EPA-650/2-75-059 (NTIS No.
PB-246-287/AS). July 1975.
OTHER REPORTS AVAILABLE
In addition to the reports listed above, the following have recently been issued by the EPA. Copies of
all EPA reports can be obtained from the National Technical Information Service, U.S. Department of
Commerce, Springfield, VA 22161.
APPLIED FILTRATION RESEARCH
1. Daniel, B. E., R. P. Donovan and J. H.Turner. EPA Fabric Filtration Studies: Bag Cleaning Technology
(High Temperature Tests). EPA-600/7-77-095b, U.S. Environmental Protection Agency, Research
Triangle Park, N.C., 1977. 41 pp.
14
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2. Leith, D. H., and M. W. First. Filter Cake Redeposition in a Pulse-Jet Filter. EPA-600/7-77-022, U.S.
Environmental Protection Agency, Research Triangle Park, N.C., 1977. 39 pp.
3. Leith, D.H., S. N. Rudnick and M. W. First. High-Velocity, High-Efficiency Aerosol Filtration. EPA-
600/2-76-020, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1977. 188 pp.
4. Schrag, M. P., and L. J. Shannon. Evaluation of Electric Field Fabric Filtration. EPA-600/2-76-041,
U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1976. 25 pp.
FABRIC RELATED STUDIES
1. Miller, B., G. Lamb, P. Costanza and J. Craig. Nonwoven Fabric Filters for Particulate Removal in
Respirable Dust Range. EPA-600/7-77-115, U.S. Environmental Protection Agency, Research Triangle
Park, N.C., 1977. 62 pp.
2. Mohamed, M., and E. Afify. Efficient Use of Fibrous Structures in Filtration. EPA-60072-76-204, U.S.
Environmental Protection Agency, Research Triangle Park, N.C., 1976. 145 pp.
3. Ramsey, G.H., R. P. Donovan and J. H. Turner. EPA Fabric Filtration Studies: 2. Performance of
Non-Woven Polyester Filter Bags. EPA-600/2-76-168b. U.S. Environmental Protection Agency, Re-
search Triangle Park, N.C., 1976. 37 pp.
4. Turner, J. H. EPA Fabric Filtration Studies: 1. Performance of Non-Woven Nylon filter Bags.
EPA-600/2-76-168a, U.S. Environmental Protection Agency, Research Park, N.C., 1976.
37pp.
FLY ASH FILTRATION
1. McKenna, J. D., J. C. Mycock and W. O. Lipscomb. Applying Fabric Filtration to Coal-Fired Industrial
Boilers (A Pilot Scale Investigation). EPA-650/2-74-058a, U.S. Environmental Protection Agency,
Research Triangle Park, N.C., 1975. 203 pp.
2. Szabo, M. F., and R. W. Gerstle. Operation and Maintenance of Particulate Control Devices on Coal-
Fired Utility Boilers. EPA-600/2-77-129. U.S. Environmental Protection Agency, Research Triangle
Park, N.C., 1977. 378 pp.
GENERAL
1. Turner, J.H. EPA Research in Fabric Filtration: Annual Report on IERL-RTP Inhouse Program.
EPA-600/7-77-042, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1977. 38 pp.
HIGH TEMPERATURE CERAMIC FILTER
1. Ciliberti, D.F. High Temperature Particulate Control with Ceramic Filters. EPA-600/2-77-207, U.S.
Environmental Protection Agency, Research Triangle Park, N.C., 1977. 171 pp.
2. Poe, G. G., R. M. Evans, W. S. Bonnett and L. R. Waterland. Evaluation of Ceramic Filters for High-
Temperature/High-Pressure Fine Particulate Control. EPA-600/2-77-056, U.S. Environmental Protec-
tion Agency, Research Triangle Park, N.C., 1977. 52 pp.
15
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TECHNICAL REPORT DATA
(Please read Inslruetions on the reverse before completing)
i. REPORT i\iO.
EPA-600/8-78-005d
2.
4. TITLE AMD SUBTITLE
Participate Control Highlights: Research on Fabric
Filtration Technology
6. PERFORMING ORGANIZATION CODE
!. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
June 1978
7. AUTHORIS)
R. Dennis and N.F. Surprenant
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
GCA Corporation
Burlington Road
Bedford, Massachusetts 01730
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-2177
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
Special: 11/77-5/78
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES IERL-RTP project officer is Dennis C. Drehmel, Mail Drop 61,
919/541-2925.
i6. ABSTRACT
repOrt highlights significant developments in fabric filtration technol-
ogy. It reviews results of several field and laboratory studies performed over the
last 10 years , by or under the sponsorship of the EPA, so that the reader may be
better able to assess filtration equipment capabilities and limitations. A background
of fabric filter design and operational concepts is followed by a sampling of actual
field performance with various coal fly ash aerosols and a description of an oper-
ational mobile pilot filter system that is used to facilitate the selection of operating
parameters , fabric type , and method of fabric cleaning. Fabric weave and consti-
tuents are discussed with respect to their bearing on working temperatures , method
of cleaning, fabric life, pressure loss, and dust retention. Attention is also called
to the impact of pinhole or pore leakage on collection efficiency and effluent size
properties. The pros and cons of increasing air-to-cloth ratio (face velocities) to
reduce fabric and other capital costs are compared with attendant disadvantages
of increased power needs and higher emission rates. Pilot and bench scale tests
show that effluent concentrations increase very rapidly with face velocity. Recently
developed modeling concepts that provide realistic predictions of glass fiber per-
formance with coal fly ash are reviewed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
C. COSATI l-'icld/Cjroup
Pollution
Dust Control
Gas Filters
Fabrics
Aerosols
Fly Ash
Coal
Glass Fibers
Pollution Control
Stationary Sources
Particulate
Fabric Filters
13B
13K
11E
07D
21B
2 ID
11B
3. DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Report)
Unclassified
Unlimited
21. NO. OF PAGES
20
20 SECURITY CLASS (This pagej
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
16
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