EPA-600/2-76-041
February 1976
Environmental Protection Technology Series
EVALUATION OF
ELECTRIC FIELD FABRIC FILTRATION
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-76-041
February 1976
EVALUATION OF ELECTRIC FIELD
FABRIC FILTRATION
by. '
M-. P. Schrag and L. J. Shannon
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
Contract No. 68-02-1324, Task 12
ROAP No. 21ADL-029
Program Element No. 1AB012
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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CONTENTS
List of Figures iv
List of Tables v
Acknowledgements. . vi
Introduction.. , 1
Theoretical Aspects of Electrified Fabric Filters 2
External Electrostatic Fields 2
Internal Fields with Potential Applied to the Filter
Element 4
Electrets 4
Experimental Studies of Electrified Filters 7
External Fields 7
Internal Fields 9
Electrets 9
Conclusions 15
References 17
iii
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FIGURES
No.
1 Ziekman1a;model of a high-porosity filter system. ..... 5
2 Performance of "real" filter in the absence and presence
of external electric field 10
3 • Performance of "real" filter in the absence and presence
of external electric field 11
4 . Collection efficiency of wire grid filters with coal dust . 12
5 Collection efficiency of wire grid filters with quartz
dust 12
6 Penetration versus load curve for electret filler 14
iv
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TABLES
No. Page
1 Performance of "Real" Filters in the Absence and
Presence of External Electric Fields ... 8
v
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ACKNOWLEDGEMENTS
This report was prepared for EPA/IERL-RTP under Contract No. 68-02-1324.
The work was performed by Mr. M. P. Schrag, Head, Environmental Systems
Section, and Dr. L. J. Shannon, Assistant Director, Physical Sciences
Division.
vi
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INTRODUCTION
The work presented in this report was performed by Midwest Research Insti-
tute for the Industrial Environmental Research Laboratory-RTF as Task No.
12 on Contract No." 68-02-1324. The objective was to evaluate the use of
electric fields in fabric filters as a means of controlling fine particu-
late emissions, from industrial sources.
Particles are normally brought to the surface of a fiber by diffusion and
convection as well as by the effect of the flow field of the gas and re-
tained on the individual fibers comprising the filter, by surface forces.
The magnitude of electrical forces compared to other forces that can be
applied to particulates suggests that improvement in aerosol filtration
should occur when electric fields are present in fabric filters. The
static charges on most naturally occurring aerosols may also play a role
in the collection and retention of the particles.
The following sections of this report present a discussion of theoretical
studies of electrified filters, a review of experimental studies of model
filter systems, and conclusions.
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THEORETICAL ASPECTS OF ELECTRIFIED FABRIC FILTERS
External fields, internal fields, and electrets have been studied in con-
junction with fiber filter systems. Each of these possibilities is re-
viewed separately.
EXTERNAL ELECTROSTATIC FIELDS
If a filter is placed in an initially homogeneous electric field, the
fibers will be polarized to produce an inhomogeneous electric field near
the fiber surface. Neutral particles entering the filter will be polarized
by the external field and will therefore interact with the field around the
fiber. Any net charge on the particles will also interact with the local
field.
I/ 2/ 3/
Zebel,— Kazutaka and Koichi,—' and Rao, et al.,— among others have theo-
retically studied the problem of the deposition of aerosol particles from
an air stream in a homogeneous electrical field. A general theory for de-
position upon an isolated, uncharged cylinder in an electrical field when
the particles are charged or uncharged has been formulated by Zebel. The
isolated fiber used by Zebel gives a good representation of an individual
fiber in a model filter with widely and regularly spaced fibers. In such
a model filter, a uniform external electric field will produce uniform
polarization in fibers and particles alike, which is not the case with
densely packed filters.
Kazutaka and Koichi developed theoretical equations to estimate the col-
lection efficiency of an isolated fiber in an external field and then
utilized a logarithmic expression to relate single fiber efficiency to
the overall collection efficiency of a fiber filter.-' These authors
also developed a semi-empirical equation to account for the interference
effect of neighboring fibers in a random two dimensional arrangement of
fibers. Kazutaka and Koichi present calculations which show a very sig-
nificant increase in collection efficiency of single fibers in the pre-
sence of an electric field. =-'
Rao et al.— extended Zebel's theory by including the effect of the close-
ness of fibers on the deposition of charged particles by use of a three
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cylinder model. Rao et al. assumed potential flow in their model and
corrected the velocity and electrostatic potentials by the method of images
when the distance between the cylinders is small.
The single-cylinder model of Zebel significantly over estimates the de-
position of particles in low-porosity filters because of the neglect of
the interaction of neighboring fibers. The three-cylinder model proposed
by Rao et al. predicts that the deposition on a given fiber will decrease
as the porosity of the filter decreases—a result more in agreement with
experimental observations.
The preceding models all predict that electric fields should enhance the
collection efficiency of fabric filters. However, these models, as well
as those for conventional fabric filters, are of limited use in assessing
the probable performance of an actual filter in an industrial application.
The relatively limited utility results because the models do not accu-
rately represent the complex structure of a filter and incorporate the
assumption that the filters are sufficiently clean for no deposition to
take place on previously deposited particles. The latter assumption is a
serious limitation with regard to industrial fabric filter systems. A
layer of deposited particles, called the filter cake, forms part of the
filtering media in an industrial fabric filter. The filter cake can cause
important changes in the filtering characteristics. Deposited particles
do not, as a rule, distribute themselves evenly over the surface of the
fibers, but build-up chain aggregates which act themselves as very fine
fibers and may collect particles more effectively than the material of
which the filter is made.
Theoretical models of fabric filtration also fail to adequately incor-
porate important engineering parameters which markedly influence the
performance of fabric filters. The air-to-cloth ratio, cleaning mecha-
nism, temperature, humidity, weave pattern, fabric weight, gas flow rate
and filter fabric "surface" characteristics appear to be the most important
engineering parameters. The filter fabric "surface" is defined as that
region of the fabric which, in successive loading and cleaning cycles,
has a significant influence on deposition and removal characteristics.
Theoretical models of fabric filters also generally do not consider the
electrical, geometric, adhesive and mechanical properties of the filter
fabric "surface" despite the fact that the interface between the dust
and fabric "surface" is, perhaps, the most important facet of the entire
fabric filter system.
In summary, although theoretical models predict that external electrical
fields will enhance the collection efficiencies of fabric filters, it is
not clear whether the predicted increased performance can actually be
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achieved in industrial systems. Experimental investigations of actual
systems will be required to answer this question. Available experimental
data are analyzed in a later section of this report.
INTERNAL FIELDS WITH POTENTIAL-APPLIED TO THE FILTER ELEMENT
Walkenhorst has studied the filtration of dust by filter systems utilizing
internal electric fields with potential applied to the filter elements.-t/
The specific configuration investigated by Walkenhorst consisted of a
filter composed of an array of individual wires arranged in a lattice.
The filter consists of parallel thin wires with well-insulated surfaces
and the basic concept is that the deposition of particles in these filters
results from electrical forces. The polarity of the two sets of wires is
periodically reversed, with the period depending on the amount of dust
retained in the filter and the charge which it carries.
Walkenhorst discusses some of the theoretical aspects of these filters,
but does not develop a model--preferring to test the concept with experi-
mental models. The results of his experimental work is discussed in a
later section of this report.
ELECTRETS
Electrets are devices which have a "frozen-in" effective charge.
As a consequence, electrets are the electrostatic analog of permanent
magnets since an electrical field is produced without the application of
external potentials. This field can be stable for long periods of time.
If a filter is constructed from fibers of an electret, inhomogeneous
electric fields will occur around each fiber. It is then possible to ob-
tain electrical enhancement of aerosol filtration without applying external
fields or potentials.
Ziekman has investigated electrets, both theoretically and experimentally.—'
His theoretical work was based on a model of a high-porosity system shown
in Figure 1. The filter model consists of parallel cylinders at equal
distances (d). The electric field in the neighborhood of the cylinder
in the center is calculated by superposition of the components of the
electric field from the nine individual cylinders shown in Figure 1. Con-
tributions of other cylinders were neglected. The calculated electric
field is thought to be representative of the electric field in the neighbor-
hood of an arbitrarily chosen fiber in the filter. The flow field was com-
puted by the Kuwabara-Happel stream function which is based on a system
of parallel cylinders located at random, with the direction of the flow
being perpendicular to the axes of the cylinders'
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Figure 1. Ziekman's model of a high-porosity filter system
5
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- (rf/r)2 - 2 In (f/rf)}
I In 3 + K")
where K = 0.75 according to Kuwabara,-' and K = 0.50 according to Happel,-/
3 = 1 - e> where e is the porosity,
r = polar coordinate,
T£ = radius fiber,
v = undisturbed flow velocity (flow at infinity).
Computer calculationt performed by Ziekman demonstrated clearly the in-
creased single filter particle collection efficiency in comparison to the
efficiency of nonelectrified fibers. Maximum collection efficiency was
predicted for highly charged particles. The collection efficiency enhance-
ment was also, predicted to be maximized in the lower Reynolds number re-
gion.
Ziekman1s theory can be used to calculate single fiber efficiencies by
calculating limiting particle trajectories which result in collection.
From the single fiber efficiencies, total filter efficiencies may be cal-
culated. It must be emphasized, however, that efficiencies calculated
in this way are initial values of the collection efficiency. Efficiencies
decrease as the filter becomes loaded with aerosol due to a continuous re-
duction of the filter's polarization by charge carrying aerosol particles'*
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EXPERIMENTAL STUDIES OF ELECTRIFIED FILTERS
Many investigators—*—*— have reported data from experiments utilizing
models which incorporated one or more wires or fibers, oriented perpen-
dicular to the gas stream. Various methods for charging either the
aerosol, the filter area, or both were used. Gas velocity (particle
velocity) and type and size of aerosol also varied. Typical examples of
such systems are those reported by Kirsch—' and Walkenhorst.—'
This experimentation has provided valuable reinforcement and refinement
of the basic theory as discussed previously. All studies indicate sub-
stantially reduced penetration (increased collection efficiency) for
filters with applied external fields. However, in almost every case,
the models were, quite simple, monodisperse areosols were used and aerosol
concentration was low. Exceptions are some parts of the work of Walkenhorst^
and Kirsch.—' The principal results of previous experimental work are re-
viewed next.
EXTERNAL FIELDS
Little data were found which directly compared differences in penetration
through a "real" fibrous filter operated with and without an applied
external field. Kirsch—' did investigate these effects directly using a
polymer fiber "real" filter. The Kirsch experiments used a fiber filter
with fiber diameter of 25 urn, a (volume fraction) = 0.029, H (thickness) =
1.1 cm. Gas velocities ranged from 4.4 cm/sec to 27.6 cm/sec. Field in-
tensity, when used, was varied from 0.91 to 11.5 Kv/cm. Monodisperse
aerosol diameters were 0.12 um to 3 urn, with standard deviation less than
/ C Q
1.05 in all cases. Particle concentrations were less than 10 and 10D/cm
for D ~ 3 um and < 1 um, respectively. The duration of the test runs
"did not exceed a few minutes so that secondary effects of particle de-
position could not influence the results."
Table 1 is a partial reproduction of the Kirsch data with additions. NQ
is inlet particle concentration, N^ is outlet particle concentration in
the absence of an electrical field, N2 is outlet concentration with appli-
cation of the electrical field. For fine particles (D = 0.12 um), re-
presented by experiments 33 through 36 inclusive, it can be seen that there
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Table 1. PERFORMANCE OF "REAL" FILTERS IN THE ABSENCE AND PRESENCE OF
EXTERNAL ELECTRIC FIELDS -/
Exp. U cm/sec r» Km NI/NQ ^'^ N2/N0* E, Kv/cm
29
29a
30
30a
31
3 la
32
32 a
33
33a
34
34a
35
35a
36
36a
4.4
4.4
10.0
10.0
18.8
18.8
26.6
26.6
6.4
6.4
14.6
14.6 '
21.0
21.0
27.6
27.6
1.50
1.50
1.33
1.33
1.33
1.33
1.1
1.1
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.77
0.77
0.83
0.83
0.86
0.86
0.87
0.87
0.87
0.87
0.92
0.92
0.93
0.93
0.94
0.94
0.55
0.08
0.17
0.001
0.27
0.0055
0.45
0.025
0.93
0,53
0.96
0.72
0.92
0.81
0.94
0.81
0.42
0.062
0.14
0.0008
0.23
0.0047
0.39
0.022
0.81
0.46
0.88
0.66
0.86
0.75
0.88
0.76
0.91
2.27
2.8
10.8
2.72
10.0
2.72
11.5
2.3
10.9
3.6
10.2
4.3
10.0
4.1
10.2
* Where N2/N0 = NJ^/NQ x N2/Nj
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is little increased collection due to imposition of low strength field.
The effect is less than 10% improvement at the four particle velocities
investigated. This is graphically illustrated in Figure 2. When a much
higher field intensity (2-4 times) is used, improvement is quite significant
at low particle velocities and decreases as high flow velocities are reached,
although at 27.6 cm/sec the effect is still noticeable (Figure 3).
Dennis et al.,— Silverman et al.,—' and Whitbyll' have investigated
electrically assisted filters which are commercially available for use
on atmospheric dusts. The general experiences reported in these refer-
ences are (1) electrification improves collection efficiency for very
light loadings of submicron aerosols; and (2) penetration of fine aerosols
is relatively high (40-50%), depending on the flow velocities and porosity
of the filter media.
INTERNAL FIELDS
Walkenhorst conducted experimental studies on filters consisting of arrays
of insulated wire 0.08 mm in diameter (0.5 mm apart). Potentials of 600-
1,000 volts were applied to the wires, with the polarity reversed at
frequent intervals to reduce the effect of the polarization of the collected
dusts. Typical results are shown in Figures 4 and 5 for coal and quartz
dusts at different relative humidities and gas stream velocities. Experi-
ments on coal dust were conducted at 34% and 95% relative humidity, 600
volts were applied to the filter, and the polarity was reversed every 10
sec (Figure 4). At a flow velocity of 10 cm/sec and low relative
humidity, the penetration of particles in the size range of 0.4-1.0 um was
5% or less. With increasing flow velocity, penetration increased signifi-
cantly reaching 65% at 0.4 um and 25% at 1.0 um for a velocity of 80 cm/sec.
Figure 5 presents the results for quartz dust. The results are similar
to those with coal dust, with collection efficiency somewhat less than
for coal dust.
The pressure drop at 10 cm/sec was less than 0.001 in. of water. Electric
power consumption would be expected to be very low.
ELECTRETS
Ziekman investigated filters formed from electret fibers which were
initially approximately 23 um in diameter. These filters showed a penetra-
tion for 0.72 um di-octylphtalate aerosols of less than 1% (compared to
80-90% for comparable "unelectrified" filters) at a pressure drop of less
than 0.04 in. of water and flow velocity of approximately 1.25 ft/sec.
However, the penetration of the filter increases with particulate loading,
as shown in Figure 6.
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1.0
0.9
0.8
0.7
Z
P. 0.5
0.4
0.3
0.2
0.1
0
I
Particle Diameter - 0.12//.m
Field Intensity Range - 2.3-4.3 Kv/cm
0= Ml/No - No Field
D= N2/N0 - Field
I
10 15 20
AEROSOL VELOCITY, cm/sec
25
30
Figure 2. Performance of "real" filter in the absence and presence
.of external electric field
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1.0
0.9
0.8
£ 0.7
z°'6
8 °-5
s
fc 0.4
0.3
0.2
0.1
0
Z
LLJ
Particle Diameter - 0. 12/im
Field Intensity Range - 10.0- 10.9 Kv/cm
0= NJ/NQ - No Field
D= N2/N0 - Field
I
I
I
10 15 20
AEROSOL VELOCITY, cm/sec
25
30
Figure 3. Performance of "real" filter in the absence and presence of
external electric field
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60
(U
Coal Dust
(1) a 10 cm/sec RH 34%
(2) • 10 cm/sec RH 95%
(3) o 10 cm/sec RH 95%
Field Not Reversed
(4) A 80 cm/sec RH 45%
i i i i
1.0 2.0 3.0 4.0 5.0
Particle Diameter, /zm
6.0
Figure 4. Collection efficiency of wire grid filters with coal dust
D-
c
•2
.—
1
Q
-------
10°,
10
rl
c
o
u>
V
Q_
10
,-2
10-3
10
J 1 i i i i i 11 i i i i i i i
_2
10'
rl
1 _1- I L 1 I I ll 1 1 I I I I I I
10'
Aerosol Load, mg
Figure 6. Pentration versus load for elecfret filter
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This Us .the result.of the cancellation of the electric field of the
electret by the collected polarized aerosol particles. The data shown
in Figure 6 were taken with a filter 3 cm in diameter. Consequently,
for a maximum penetration of 1%, at a particulate loading of 1 grain/ft ,
only about 0.2 ft-* of gas/ft of filter could be treated before regenera-
tion or replacement of the filter.
14
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CONCLUSIONS
Available theoretical and experimental information indicates that electric
fields can improve the collection efficiency of fabric filters. Hovever,
the lack of adequate data on filter systems suitable for industrial ap-
plications precludes a firm judgement on the potential usefulness of an
electrified filter for industrial applications. Some intuitive state-
ments of advantages and disadvantages can be proposed.
It may be possible to utilize a system composed of a conventional bag
filter with an electrification addition. The field would be applied
intermittently, e.g., just after cleaning, to maintain uniformly low
penetration during, the period of cake repair. The same combination
operated continuously might have a positive benefit in reducing penetra-
tion of fine particles. The system might also combat reentrainment of
collected particles, however deposited, due to electrostatic adhesion.
Another possibility involves usage of a more porous filter media in an
electrified filter system to increase capacity, operate at lower pres-
sure drop, or allow control of difficult sources. The economics and
reliability of such a device is speculative.
Nearly all of the test work to date involved short-terra effects. The
test regimes were constructed to minimize or eliminate the other import-
ant filtration mechanisms. Further testing would be needed to document
any improvement in collection efficiency when these other mechanisms are
in operation, particularly with respect to fine particulates.
Nearly all tests utilized extremely low concentrations of aerosol. Con-
sequently, for a short test-run the deposition on the filter was light.
Little information is available on the incremental effect due to electro-
static benefication when a more normal filter cake is present.
No studies investigated cleaning mechanisms, possible difficulties in
cleaning, or penetration increase during cleaning. Cleaning of an electri-
fied filter or an electrostatically assisted system will probably be more
15
-------
difficult than merely reversing polarity or turning off the field. There
are some reservations, as well, when one considers introduction of an
electrified filter into industries which collect dust having an explosive.
or fire hazard.
16 .
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REFERENCES
1. Zebel, G., "Deposition of Aerosol Flowing Past a Cylindrical Fibre
in a Uniform Electric Field," J. Colloid Science. 20^ (6), 522
(1965).
2. Katzutaka, M. and tinoya Koichi, "Estimation of Collection Efficiency
of a Dielectric Fibrous Filter," Kagaku koguku. 33 (7), 88 (1969).
3. Rao, K. S., et al., "Collection of Dust by Fabric Filtration in an
Electrostatic Field," Department of Mechanical and Aerospace
Engineering, University of Notre Dame (EPA Grant No. AP-01303-01)
(1973)...
4. Walkenhorst, W., "Reflections and Research on the Filtration of Dust
from Gases with Special Considerations of Electrical Forces,"
J. Aerosol Science. 1 (3), 225 (1970).
5. Zieckman, P., "Aerosol Filtration by Electrified Fibrous Filter Mats,
III," Report No. 1970-16, Chemisch Laboratorium RVO-TNO,
Netherlands (1970).
6. Kuwabara, S., "The Forces Experienced by Randomly Distributed
Parallel Circular Cylinders or Spheres in Viscous Flow at Small
Reynolds Numbers," J. Phys. Soc. Japan, 14 (4), 527 (1959).
7. Happel, J., "Viscous Flow Relative to Arrays of Cylinders," Am. Inst.
Chem. Eng. J., 5, 174 (1959).
8. Kirsch, A. A., "The Influence of an External Electric Field on the
Deposition of Aerosols in Fibrous Filters," Aerosol Science. 3,
25 (1972).
9. Dennis, R., et al., "Evaluation of the Electro-PL and Electro-Klean
Dust Collectors," USAEC Report NYO 4614, Harvard University, July
1958.
17
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10. Silverman, L., et al., "Performance of the Model K Electro-Polar
Filter," USAEC Report NYO 1592, Harvard University, July 1954.
11. Whitby, K. T., "Evaluation of Air Cleaners for Occupied Spaces,"
Technical Report No. 14, Cooperative Research Project, University
of Minnesota Department of Mechanical Engineering and U.S. Public
Health Service.
18
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TECHNICAL REPORT DATA
(Please read Inunctions on the reverse before completing)
1. REPORT NO.
EPA-600/2-76-041
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Evaluation of Electric Field Fabric Filtration
5. REPORT DATE
February 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO
M. P. Schrag and L. J. Shannon
9. PERFORMING ORQANIZATION NAME AND ADDRESS
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1324, Task 12
12. SPONSORING AGENCY NAME AND ADDRESS
E PA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 6/74-1/75
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
EPA project officer for this report is B.C. Drehmel, 919/549-8411, Ext 2925.
is. ABSTRACT
reporl gives results of an evaluation of the potential usefulness of
electrified fabric filters for industrial gas cleaning. Available theoretical and
experimental information indicates that electric fields can improve the collection
efficiency of fabric filters. However, the lack of an adequate data base on filter
systems , representative of those used on industrial applications , precluded a firm
judgment on whether electrified fabric filters will be adaptable to and useful for the
control of emissions from industrial sources.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATi Field/Group
Air Pollution
Filters
Electrostatics
Industrial Fabrics
Industrial Plants
Air Pollution Control
Particulate
Fabric Filters
Gas Cleaning
Collection Efficiency
13B
20C
11E
3. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO.
OF F
25
PAGES
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form Z220-1 (9-73)
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