<>EPA
United States Industrial Environmental Research EPA-600/7-79-108
Environmental Protection Laboratory April 1979
Agency Research Triangle Park NC 27711
Studies of Dust Cake
Formation and Structure
in Fabric Filtration:
Second Year
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.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-108
April 1979
Studies of Dust Cake Formation
and Structure in Fabric Filtration:
Second Year
by
Bernard Miller, George Lamb, Peter Costanza,
George Harriott, Janet Dunbar, and Michael Mokricki
Textile Research Institute
P.O. Box 625
Princeton, New Jersey 08540
Grant No. R804926
Program Element No. EHE624A
EPA Project Officer: James H. Turner
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ABSTRACT
This report describes progress toward the project objective:
to explain changes in the performance of fabric filters on the basis
of changes in the structure of the deposited dust cake. Whether im-
proved performance (that is, greater collection efficiency with lower
pressure drop) is obtained by imposition of electric fields, by mak-
ing the filter fabric out of fibers having modified geometries, or
by varying fabric construction, the improvement is associated with
deposition of the dust close to the upstream region of the filter
rather than deep within it. (In this work the dust cake is under-
stood to include dust that penetrates into the filter fabric.)
An electric field applied to a fabric filter is more effec-
tive in reducing pressure drop if the fabric has loose fibers at the
surface. When a dust cake is formed on such a fabric in the presence
of a field, photographs of the cake surface show heavy deposition on
protruding surface fibers. In contrast, after filtration without a
field these fibers are bare, and the mass of dust penetrating into
the fabric is greater. Thus the combination of applied field and
protruding fibers results in reduced penetration into the filter
and concomitant pressure drop reduction.
Without an imposed field, using fibers with deeply lobed ra-
ther than round cross sections results in improved performance, and
the associated increase in upstream capture is attributed to induced
localized fields due to the collection of the naturally charged
aerosol particles. Using the principles evolved in bur earlier
studies of the effects of fiber geometry on performance, layered
filters have been prepared having upstream and downstream layers
made of fibers differing in linear density which give better per-
formance than homogeneous fabrics. These fiber and fabric modifi-
cations that improve capture in the upstream fabric layers also
allow more effective cleaning (that is, less long-term dust reten-
tion) , which accounts for at least part of the reduction in pressure
drop.
In general, it appears that collection closer to the upstream
filter surface is associated with higher overall collection effi-
ciency, easier cleaning, and lower pressure drop.
This report is submitted in fulfillment of Grant No. R804926-2
by Textile Research Institute under the sponsorship of the U.S. En-
vironmental Protection Agency. This report covers a period from
December 20, 1977 to December 19, 1978, and work was completed in
December 1978.
11
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CONTENTS
Page
Abstract ii
Figures iv
Tables v
Acknowledgments vi
I. INTRODUCTION 1
II. SUMMARY AND CONCLUSIONS 2
III. EFFECTS OF EXTERNALLY-APPLIED ELECTRIC FIELDS 4
A. MICROPERMEABILITY SCANNER 4
B. MEASUREMENTS OF CAKE DEPTH 8
C. NEW EMBEDDING PROCEDURE FOR CAKE CROSS-SECTIONING. . . 12
D. PHOTOGRAPHIC ANALYSIS OF DUST CAKES 14
E. EFFECT OF FABRIC STRUCTURE 14
p. STUDIES OF DENDRITE FORMATION 21
G. EFFECT OF CORONA 23
IV. EFFECTS OF FIBER GEOMETRY 26
A. MECHANISM OF PRESSURE DROP REDUCTION WITH
TRILOBAL FIBER FILTERS 26
B. X AND Y-SHAPED FIBERS 26
C. COMPOSITE FILTERS 33
1. Initial Cycles 33
2. Conditioned Fabrics 34
V. REFERENCES 38
ill
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FIGURES
Number 2*2*.
1 Micropermeability scanner 5
2 Scan of permeability (expressed as pressure drop at
constant air flow) for dust cakes deposited at 0, 2,
4, and 6 kV/cm 7
3 Fractional collection on layers of electrified
layered filters 9
4 Capture efficiency of each layer of filters in
Figure 3 9
5 Upstream surfaces of layers of nonwoven fabrics of
Figure 3 used to filter flyash at 0 (left) and 2 (right)
kV/cm 1°
6 Upstream surfaces of layers of nonwoven fabrics of
Figure 3 used to filter flyash at 4 (left) and 6
(right) kV/cm H
7 Photomicrograph of cross section through commercial
needled PET filter fabric and dust cake 13
8 Cross section through dust-coated fabric prepared by
embedding procedure 15
9 Photographs of the surfaces of the dust-laden filters
in Figure 8 ^
10 Effect of fabric upstream surface structure on
pressure drop ratio 19
11 Cross section through the fabric used for Figure 9
but with no dust 20
12 Basic structural configurations for flyash deposits
(schematic) 22
13 Collection of charged particles on (a) charged and
(b) neutral surfaces (schematic) 22
14 Micrographs of flyash collected on trilobal polyester
fibers 22
15 Variation of calculated surface field with applied
voltage for wires of different diameters 24
16 Flyash retained by layered filters as conditioning
proceeds 27
17 SEM cross sections of trilobal fibers 29
18 SEM cross sections of acetate fibers 30
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Figures (con.)
Number Page
19 Comparison of pressure drop with fabrics made of
I, Y, and X-shaped acetate, trilobal PET (A), and
round PET Co) fibers 32
20 Comparison of penetration (1-efficiency) with fabrics
made of I, Y, and X-shaped acetate, trilobal PET (A),
and round PET (o) fibers ' 32
21 Performance curves for the fabrics in Figures 19
and 20 32
22 Changes in pressure drop during conditioning of
composite PET filters 35
23 Changes in pressure drop during the 20th cycle for
composite PET filters 35
24 Pressure drop across composite vinyon-bonded PET
filters having varying proportions of 3 and 6-denier
fibers in two layers 37
25 Penetration through the composite filters of Figure 24 37
26 Residual dust after cleaning the filters of Figure 24 37
TABLES
Number Pa3e
1 Efficiencies and Pressure Drop Changes Due to
Electric Field of 2 kV/cm Applied to Woven and
Nonwoven Glass and PET Fabrics 17
2 Properties of Felts Made from Acetate Fibers with
Various Cross Sections 31
3 Composite Filters Made of 3 and 6-Denier Trilobal
Polyester Fibers 33
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ACKNOWLEDGMENTS
The authors would like to express their appreciation to
Dr, James H. Turner of the Environmental Protection Agency for
his advice and encouragement throughout the period of this research.
They would also like to thank Mr. Harold W. Lambert and Mr. Harry
Buvel of TRI for their invaluable work on apparatus, Mr. John P.
Hession for his assistance with microscopy, and Dr. Harriet G.
Heilweil for her help with reports.
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SECTION I
INTRODUCTION
The following report describes work that was done in the
second year of a three-year program and must, therefore, be read
as a continuation of the report issued a year earlier [4]. Simi-
larly/ some of the conclusions drawn are not final but relate to
studies which continue and will be discussed further in the final,
third-year report.
The main object of the work is to find explanations for
observed changes in -Filtration performance that result from cer-
tain modifications QJ. conventional fabric filtration technology.
One such modification is the use of fibers having different geo-
metric properties from those of fibers commonly used in filter
fabrics (cross-sectional shape, surface roughness, linear density,
etc.). Another is the application of electric fields to the fa-
brics. A third is the use of creped fabrics.
In each case some of the improvements in performance, which
have taken the form of increased capture efficiency, reduced pres-
sure drop, or both, appear to coincide with changes in dust cake
characteristics. Therefore, a primary goal of the present work
is the identification of features of the dust cake or of the fabric-
dust system which may explain the observed changes in performance.
A second goal is to use this information as a basis for the de-
sign of improved filter fabrics or systems.
The term "dust cake" tends to mislead because it conveys
the image of a dust layer in contact with but separate from the
filter fabric. In fact, there is considerable deposition within
the fabric, so that a study of dust cake structure is really a
study of the dust-laden surface layers of the fabric.
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SECTION II
SUMMARY AND CONCLUSIONS
Studies of the local permeability of dust cakes formed with
externally applied electric fields have shown that light and dark
bands observed in top-view photographs are not due to uneven depos-
ition across the fabric and therefore cannot be the basis for an
explanation of the reduced pressure drop produced by a field. On
the other hand, measurements of the depth of penetration of the dust
into the fabric indicate that the effect of the electric field is
to concentrate the dust accumulation in the upstream layers of the
fabric, allowing less to penetrate deeper inside.
Study of the vertical cross-sectional structure of the dust
cake as modified by an electric field was attempted by a photogra-
phic approach. An embedding procedure was developed which at first
appeared to preserve the cake intact and to allow cross-sectioning
of cake and filter fabric. It later became apparent, however, that
the embedding process disturbs the features which are of greatest
interest, namely the deposits on the surface fibers.
On the other hand, examination of top-view photographs of the
filter-cake surfaces has revealed features indicating that reduction
in pressure drop due to an electric field depends on fabric construc-
tion. Heavy deposition on fibers protruding from the fabric surface
is evident in the photographs, suggesting that the shift of the dust
deposit upstream is extreme. It has also been found that pressure
drop is reduced by the electric field more or less depending on whe-
ther or not the fabric has a surface layer of loose fibers.
The question of whether, in addition to the changes in the posi-
tion of the dust cake, there also occur microscopic changes at the
single fiber level when an electric field is applied has been ad-
dressed by photographic studies of dust captured on single fibers.
Initial results indicate that, whereas in the absence of an electric
field dendrites tend to form, a field causes formation of a more uni-
form, nondendritic deposit. This finding is consistent with the
lower pressure drop developed with a field.
Work was also continued on the effects of fabric or fiber geo-
metry on dust cake formation in the absence of fields. The effects
of fiber geometry were further examined by measuring dust deposition
in different layers of fabrics made of round or trilobal fibers.
The previous report presented data showing how when these fabrics
are new (i.e., at the first cycle), dust tends to be captured more
efficiently in the upstream layers. The amount of dust in the vari-
ous layers of the filters after a number of conditioning cycles has
now been measured, and the fabric made of trilobal fibers retains
less dust in all layers. It is assumed that by favoring capture in
the upstream layers, trilobal fibers also favor easier cleaning.
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This results in less long-term dust retention and explains the lower
pressure drops measured in the past with fabrics made of trilobal ra-
ther than round fibers.
Similar results were obtained with fabrics made of fibers with
"Y," "X,"or "I" cross sections which had deeper and/or more lobes
than the trilobal fibers used before. In each case, the performance,
gauged by both penetration and cake resistance, was improved over
the shallow-lobed trilobal fibers. Since this finding fits a theor-
etical prediction of better capture efficiency by many and deep-lobed
fibers in an electric field, the hypothesis is that the improved
performance is due to electric charges on the aerosol which when de-
posited in the fabric, generate an electric field.
Finally the stu ,y of composite fabrics has continued. It has
long been felt that better filters,filters that are durable as well
as effective and economical, could be produced by fairly simple modi-
fications of fabric construction. An example is forming the fabric
in dissimilar layers. Measurements on fabrics made of layers of rel-
atively fine and coarse fibers (3 and 6-denier) over a large number
(^50) of conditioning cycles gave several interesting results. As
conditioning progresses, the relative ranking of different fabrics
with respect to performance changes. The first result, therefore,
is that in studies of this type, it is important to insure complete
conditioning before drawing conclusions about relative performance.
The second result is that the performance of such composite fabrics
is quite different from what might be expected from knowledge of the
performance of the component layers. There is thus a good potential
for designing improved filter fabrics by this type of combination.
The third result is that the differences in performance appear to be
associated with the amount of dust retained in the fabric.
The work described in this report indicates that external appli-
cation of electric fields, use of lobed fibers, and use of composite
fabrics have a common effect: modifications that cause dust to be
deposited closer to the upstream surface of the fabric bring about
improvements in both efficiency and pressure drop.
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SECTION III
EFFECTS OF EXTERNALLY-APPLIED ELECTRIC FIELDS
The largest improvements in fabric filtration performance
have been obtained by the application of electric fields. Effects
on capture efficiency have been reported by a number of workers
11,2,3,], but few commercial filter units designed to take advan-
tage of such improved efficiency have appeared on the market. Lit-
tle has been reported regarding reduction in pressure drop accompa-
nying the application of electric fields, though this is perhaps
more important than increased efficiency since it has potential
for lowering the cost of filtration by fabrics. This may make
electrically stimulated fabric filtration (ESFF) commercially attrac-
tive where improvements in efficiency alone might not. The mechan-
isms whereby efficiencies increase in the presence of electric
fields have received a great deal of attention and are well under-
stood, at least for clean filters. The cause of a reduction in
pressure drop can only be conjectured at this time, though it is
obviously related to some change in the dust cake structure. The
exact nature of that change, however, is not known, and a better
understanding of it was the aim of the studies described in the
following section.
A. MICROPERMEABILITY SCANNER
In the preceding report [4] photographic evidence was given
of nonuniformity of distribution in dust cakes formed on patch fil-
ters under the influence of electric fields. In most cases there
appeared to be different amounts of dust deposited on or near the
electrode wires than between them, and this suggested a possible
mechanism for the pressure drop reduction effect with ESFF. If
the electric field causes an uneven dust deposit to form, then the
resistance to gas flow through the dust cake would be reduced,
since (by analogy with electric resistance) if R is the overall
resistance to flow and R^ is the resistance of an element of dust-
cake area, the relationship would be of the form
R = [Z j ,
where R becomes smaller with increasingly dissimilar R^.
In order to check this hypothesis, an apparatus was devised
for measuring the permeability of the dust cake at a number of
small areas on a patch filter. Of several designs tried, the one
shown in Figure 1 was found to be the most satisfactory. After
the dust cake has formed, the dust- laden fabric is carefully
clamped between the perforated plates A and B. As shown in
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(a)
r.
A -H
B-
(b)
Manometer
«- Dust cake
Fabric
Air flow
Brass tube
A- Dust cake
B
Electrode wires
(d)
... .. Pressure
Needle regulator
valve
Filter mount
=5=
Manometer
Air
supply
Figure 1. Micropermeability scanner: a) side view of filter
mount; b) detail; c) top view; d) general layout.
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Figure la, the pins C and D have the double function of insuring
that the holes in the plates will be aligned, and also that, when
the plates are placed on the fabric, there will be no lateral
motion that could disturb the cake in the small areas to be
measured. As shown in the detail, Figure Ib, the hole on the
cake side is slightly larger than its counterpart on the downstream
side of the fabric. A brass tube, which has a sliding fit into
the cake-side hole, is placed in the hole and pressed against the
fabric, making a tight seal. A steady stream of air passes down
the tube and through the small area (o.6-mm diameter) of dust cake
within the seal. The pressure drop across the dust cake is mea-
sured by the manometer (Figure Id). The holes in plates A and B
are arranged so as to scan across the patch at positions on the
wire electrodes, and at 1/4, 1/2, and 3/4 of the distance between
them, as shown in Figure Ic.
A sample of polyester (PET) needled felt was fitted with
copper wire electrodes. The felt was a commercial filter fabric
(the patch was cut from a purchased bag) having a central scrim
and was composed of 9-denier polyester fibers compacted into a
felt structure by a needling process. It is listed in Table I
as the Carborundum PET nonwoven. The electrode wires were
threaded just below the upstream surface by means of a sewing
needle. Figure Ic shows how the electrodes were coupled to the
power supply in this and all subsequent measurements involving
filter patches fitted with electrodes. The filter was conditioned
in the patch filter apparatus with applied fields of 0, 2, 4, and
6 kV/cm. The last cycle was interrupted before cleaning and the
fabric lifted out with the dust cake. The cake was then scanned
by making pressure-drop measurements at holes across the filter,
according to the procedure outlined above. The flow of air in the
tube was adjusted to be equal to the face velocity during formation
of the dust cake. Consequently, the pressure drops measured could
be directly compared with the final pressure drop APf obtained with
the particular dust cake. The results are plotted in Figure 2.
The four curves are permeability scans for cakes formed at the four
electric field strengths. The pressure drops measured in each scan
can be seen to correspond well with the final pressure drops APf
measured as the cake was being formed, given 4n Figure 2.
The results in Figure 2 show no particular pattern of cake
permeability with respect to position relative to the electrodes.
Some fluctuations in cake permeability are seen, but these do not
coincide with the electrode position and are not introduced by the
field since they are present even when no voltage is applied.
Therefore, the lower pressure drop is the result of a general
increase in permeability of the entire cake. In fact, these
results may be taken as proof that the hypothesis of pressure-drop
reduction by unbalanced permeabilities is invalid. The patterns
of apparently uneven distribution seen previously [4] still remain
to be explained, but they appear to be irrelevant to the question
of pressure-drop reduction by ESFF.
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PRESSURE DROP
0 kV/cm
36.0 mm
4 kV/cm
• 29.5 mm
2 kV/cm
26.8mm
6 kV/cm
APj « 13.0 mm
4 8 12
HOLE POSITION ACROSS FILTER
15
Figure 2. Scan of permeability (expressed as pressure drop at
constant air flow) for dust cakes deposited at 0, 2, 4,
and 6 kV/cm. APC values are for the entire patch.
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B. MEASUREMENTS OF CAKE DEPTH
It was seen in the previous section that observed reductions
in pressure drop cannot be accounted for by nonuniform dust
distribution across the filter face. This implies that the amount
of dust per unit area of filter surface is likely to be fairly
uniform and that the patterns seen are due to differences in the
depth of penetration of the dust into the fabric. To verify this,
four sets of layered filters were made. Each set consisted of four
thin separable nonwovens made by pressing a card web in a press for
a given time and at a given pressure. The webs contained 15% of
a binde.r fiber (vinyon) . Copper wires, 150 ym in diameter, were
threaded into the upstream layers of each set in the usual configu-
ration (parallel wires, 15 mm apart/ alternate wires grounded),
and the sets used to filter a flyash aerosol. Only one 5-minute
cycle was run, and each set was run at a different voltage. After
a run the layers were separated and weighed. The total mass of
dust collected on an absolute filter downstream of the main filter
was also weighed. The fractions of dust captured by each layer and
the efficiency of each layer are plotted in Figures 3 and 4. Only
one set of these measurements was performed, so that each point in
Figures 3 and 4 was obtained from only one measurement. However,
the consistency of the trends within each curve and between curves
was taken as sufficient to make duplicate measurements unnecessary.
When a field is applied, a major shift upstream occurs in
the dust deposition. The fraction captured by the upstream layer
(layer number 1) increases from about 50 to nearly 90%, while that
captured on the three downstream layers decreases. As the field is
increased to 4 and to 6 kV/cm, the fraction collected on the down-
stream layers increases with a slight decrease in the upstream
layer that is not apparent in the log plot. The total penetration
(given in Fig. 3) decreases considerably as field strength increases,
with a sharp decline between 4 and 6 kV/cm. The calculated effi-
ciencies for each layer, plotted in Figure 4, show a sharp increase
for the two downstream layers at 6 kV/cm.
When the layers were separated after filtering, it was
found that the penetration into the downstream layers was not
uniform. Areas under the electrodes showed evidence of much greater
penetration (Figs. 5 and 6). The deposits under the electrodes
were visually more pronounced as field strength increased. This
greater deposition below the wires is assumed to account for the
measurements of greater fractional deposition on downstream layers
with increasing field strength. The electrodes were 150-ym copper
wires so that onset of corona discharge should have occurred below
the lowest field used, i.e., 2 kV/cm. The possible relationship
between these patterns of deposition and corona discharge will be
discussed in a later section.
The above results appear paradoxical, since they mean that
in the presence of an electric field the dust accumulation in a
fabric is shifted in an upstream direction. The greater mass
8
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FRACTION
COLLECTED
ON LAYER
1.0
O.I
0.01
0.001
CAPTURE EFFICIENCY
,or OF LAYER
0.9 -
I 2 3
LAYER NUMBER
Figure 3. Fractional collection on layers of
electrified layered filters, o - 0 kV/cm,
A - 2 kV/cm, • - 4 kV/cm, 0-6 kV/cm.
O.I -
LAYER NUMBEP
Figure 4. Capture efficiency of each layer of
filters in Figure 3. o - 0 kV/cm, A - 2 kV/cm,
• - 4 kV/cm, Q-6 kV/cm.
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LAYER 1
LAYER 2
LAYER 3
LAYER
Figure 5. Upstream surfaces of layers of nonwoven fabrics
of Figure 3 used to filter flyash at 0 (left) and 2 (right) kV/cm.
10
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LAYER 1
LAYER 2
LAYER 3
LAYER
Figure 6. Upstream surfaces of layers of nonwoven fabrics
of Figure 3 used to filter flyash at 4 (left) and 6 (right) kV/cm.
11
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concentrated in the first layer would be expected to lead to
greater pressure drop. In fact, the opposite occurs/ and we must
conclude that cake formation in the uppermost layer must also be
modified by the presence of the field. The mass per unit area of
layer number 1 was about 62 g/m2. Assuming a fiber volume fraction
of 0.1 and a polyester density of 1.38, the thickness of the top
layers was 0.45 mm. The average fiber-to-fiber distance s is
given by s = (2ita2/a)%, where a = fiber radius and a = volume
fraction. For the 3-denier fibers in these felts, s was 55 ym
and the thickness of the upstream layer was thus equivalent to
about eight layers of fibers. It was felt that the resolution
of the method was not sufficient to examine in greater detail the
events occurring in such small depths of fabric. Therefore, a
different method was tried as described in the following section.
C. NEW EMBEDDING PROCEDURE FOR CAKE' CROSS-SECTIONING
It has been reported previously [4] that microscopical
examination of dust-cake formation by scanning electron microscopy
CSEM) is not informative, for the reasons that only the cake
surface can be examined and that the electron beam appears to disturb
the cake structure by removing particles. To avoid these diffi-
culties, various embedding and sectioning techniques were tried.
After some unsuccessful attempts, a procedure was found that appears
promising.
A problem associated with flooding a fabric filter bearing
a dust cake with embedding fluid is the disturbance of the cake by
the fluid. In some cases the dust cake can actually be caused to
float off the fabric. The new procedure consists simply of extremely
gradual addition of embedding fluid to the dish in which the fabric
sample lies, so that the fluid seeps upward towards the cake by
capillary action. A small piece (about 1 square inch) of the
fabric with the dust cake on it is carefully cut from a patch and
laid in the bottom of an aluminum dish, cake side up. The embedding
fluid is added to the bottom of the dish, a few millimeters from
the edge of the fabric, a drop about every 15 s. Addition of fluid
is stopped when the sample gives the appearance of being only wetted,
rather than submerged. Figure 7 shows, however, that even at this
stage fabric interstices are filled thoroughly. The embedding fluid
was "Epon 812," manufactured by the Shell Chemical Co.
Figure 7 is a composite optical photomicrograph of a section
through a commercial needled PET sample embedded in this fashion.
The section exposes a view into the thickness of the fabric. -The
dust cake is the dense area at the top of the photograph, and th6
air flow would be from the top downwards. It is interesting to
note that dust tends to lodge on the upstream side of fibers, sup-
porting the assumption that the dust was not displaced by the medium.
The photo also gives a simple illustration of the interaction of
fabric and dust. At the upstream surface the heavy dust accumula-
tion clearly presents few paths for dust penetration. Further
12
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mm
. p
4*W
Figure 7. Photomicrograph of cross section
through commercial needled PET filter
fabric and dust cake.
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downstream dust can pass freely between fibers and is only cap-
tured when it impacts on one- Particles are seen throughout the
filter, but their number gradually decreases as depth into the filter
increases. The importance of cake formation, permeability, and
stability is somewhat better understood after examination of the
photo.
D. PHOTOGRAPHIC ANALYSIS OF DUST CAKES
The method described in the previous section was now used
to determine the special features of the cake structure formed in
the presence of an electric field that would account for the lower
pressure drop usually found. Two identical patches of commercial
needled PET fabric were used. One was fitted with electrodes.
Both were used to filter a 5-g/m3 flyash aerosol at a face velocity
of 6 cm/s (12 ft/min) with a 9-kV potential applied to the elec-
trodes in the one patch. The pressure drop rose from 2 to 18 mm
HaO without a field. With the field it rose to 3 mm. This strong
effect is frequently obtained with clean fabrics (i.e., fabrics
that have not been "conditioned" by repeated filtering and cleaning).
Afterwards, small pieces of fabric were cut from each patch and
embedded as described above. Micrographs of cross sections from
each sample are shown in Figure 8. The detail of the cake structure
is disappointing since, in both cases, the cake appears as a
compact structure. There is no obvious difference between the two
cakes and nothing that can be identified with lower pressure drop
due to ESFF.
In a different approach, the cakes were photographed with a
camera fitted with a close-up lens. Photos obtained in this fashion
are shown in Figure 9. Here substantial differences can be seen at
once. In the absence of an external field, the dust deposits in a
smooth compact cake. Fibers protruding from the fabric surface are
free of dust. With the field applied, a great deal of deposition
occurs on these protruding fibers, and the entire appearance of the
cake is changed.
The contrast between Figures 8 and 9 suggests that the
embedding procedure is not suitable for examination of the up-
stream boundary of the dust cake. The differences between Figures
9a and 9b are not apparent in Figures 8a and 8b.
E. EFFECT OF FABRIC STRUCTURE
Measurements of the change in performance obtained by
electrical stimulation of various fabrics indicate a strong depen-
dence of the effect on fabric structure. Table I gives results
obtained with woven and nonwoven (felted) fabrics made of glass
and polyester fibers. It can be seen that on application of an
electric field, a reduction in pressure drop is obtained only with
the nonwoven felts. The three woven fabrics (made of untextured
14
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v f
•
'•
Figure 8. Cross section through dust-coated fabric prepared
by embedding procedure,
a) .Dust collected with no field applied.
b) Dust collected with 6-kV/cm field. Peak on surface
occurs over electrode.
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Figure 9. Photographs of the surfaces of the dust-laden filters
in Figure 8.
d) Dust collected with no field applied.
b) Dust collected with 6-kV/cm field.
16
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TABLE' I. EFFICIENCIES AND PRESSURE DROP CHANGES DUE TO- ELECTRIC FIELD OF
2 kV/cm APPLIED TO WOVEN AND NONWOVEN GLASS: AND' PET FABRICS
Producer
Style No.
Type
Material
wt. Cg/m2)
Wt. (oz/yd2)
AP (f ield)/AP (no field)
Burlington
486/38
Woven (2x2
Twill)
Glass
519
15.3
41/42
Cri swell
445-57/-DC2T
Woven (3x1
Twill)
Glass-
36£
10. -8
92/83
Owens-
Corning*
Non woven
Glass
473
12.9
28/49
Milliken*
—
Woven Fila.
(2x1 Twill)
PET
231
&. 8
53/55
Carborundum
918-404 9-45S~3'-0517
Nonwoven
PET
546
16. ±
25/>47/
(AP in ram of water)
%E(field)/%EXno field) 70u. 2/57.0 86.7/&9..6
95.7/99.2
Experimental- fabric
-------
yarn) give no reduction. On the other hand, it is interesting to
note that all fabrics show increased capture efficiencies when the
electric field is applied. It is conceivable, therefore, that the
mechanisms whereby penetration and pressure drop are reduced are
not the same.
Further evidence of the importance of fabric structure to
the response to electrostatic stimulation is given by results of
an experiment in which a flyash aerosol was filtered through a
glass fabric iided by the application of an electric field of
average strength 6 kV/cm. The glass fabric was woven from a
combination of textured and untextured yarn. (Texturing means
treating a yarn to impart a kind of permanent wave to each component
filament. In an untextured yarn the filaments are straight and
closely packed. Texturing separates the filaments and increases
the effective volume of the yarn by increasing the void volume
between filaments.) The weave was such that one face of the fabric
was entirely covered with textured yarn. This was the face designed
by the manufacturer to be exposed to the incoming dirty gas. The
other face was roughly half covered by untextured yarn. Two samples
of this fabric were fitted with electrodes, in one attached to the
textured face, in the other to the face having some untextured yarns
exposed. Tne electrodes were coupled to a power supply as shown in
Figure Ic. Figure 10 shows pressure drop ratios tPDR) obtained with
the two arrangements. With no field applied, the pressure
drop was virtually equal at 36 mm H20 with either face upstream.
When an average field of 6 kV/cm was applied, however, the pressure
drop was reduced 67% to 12 mm H20 with the textured face upstream,
but only 40% to 22 mm H20 with the partially untextured face
upstream.
From these facts, the hypothesis can be proposed that in
order to increase pressure drop reductions, the fabric must have an
upstream layer of low fiber volume fraction. The electric field
causes the cake to form preferentially in this low density region
by capture on single fibers, rather than further downstream in the
denser portion of the fabric where the cake forms by bridging.
Where the low density region is absent, as in untextured filament
fabrics, no such changes in cake structure can be induced by the
field, and there are practically no changes in pressure drop.
The manner in which accumulation of dust in the low density
surface region may accomplish pressure drop reduction may be gauged
from the photo in Figure 9b and from Figure 11, which is a cross
section of the fabric shown in Figure 9 but with no dust. The dust
in Figure 9b forms heavy encrustations on the surface fibers visible
in Figure 9a. These encrustations look like cylinders with
diameters that range from 0.25 to 0.5 mm. The broken-line circle
in Figure 11 represents the cross section of one such 0.25-mm
cylinder to scale, as if it had formed on the protruding fiber
visible at the top of the photograph. This procedure is a construc-
tion of the kind of cross section that would have been obtained in
18
-------
PRESSURE DROP
RATIO
i.O r
0.8 -
0.6 -
Fine yarn* on fac*
0.41-
Bulky yarn* on fact
(Normal filtering surface)
10 2030 40506070 80 90 OO
CYCLE NUMBER
Figure 10. Effect of fabric upstream surface structure on
pressure drop ratio. Woven glass bag. Applied field:
6 kV/cm.
19
-------
v
v,t,t, "•
.
3 '
"'
*
I
G «- ° C
o .
Figure 11. Cross section through the fabric used for Figure 9
but with no dust. Broken circle indicates diameter of dust
encrustation on surface fiber as in Figure 9b. Horizontal
band indicates equivalent thickness of dust layer.
-------
Figure 8a if the cross-sectioning technique had been successful.
Isolated fibers that protrude above the fabric appear to have a
remarkable ability to collect dust in an electric field. The dust
cake photographed in Figure 9b had an areal density of 9xlO~3g/cm2.
The bulk density of the dust was approximately 1, so this mass is
equivalent to a layer 90 um thick. This is also drawn in Figure 11
as a horizontal band. It is interesting to note that if a parallel
array of cylindrical accumulations 0.25 mm in diameter spaced 0.4
mm apart were formed on the surface, all of the deposited dust
would be located in these cylinders. Inspection of Figure 9b shows
that structures of this type occur with almost this frequency.
The pressure drop increase due to the cake formed in Figure 9a was
18 mm of water; that due to the cake in Figure 9b was only 1 mm of
water. These observations provide a clearer picture of the changes
in dust cake structure that account for the reduction in pressure
drop with ESFF. Previous accounts of these changes have offered
only qualitative observations such as that in the presence of a
field the cake was "more porous".
F. STUDIES OF DENDRITE FORMATION
A separate approach to explain the mechanism of pressure
drop reduction by ESFF was the study of the microstructure of dust
cake formations. By this is meant the structure of the dust
collection at the single fiber level, as opposed to the structure
of the cake as revealed, for example, by the weighing of filter
layers described in a previous section. It appeared possible that
the observed effect on pressure drop might be a result of changes
in the dust cake microstructure as well as in the macrostructure.
Basic structural configurations for the flyash deposits may
be postulated. Two extreme cases are shown in Figure 12. In case
Ca) all the particles stack on top of each other, forming a den-
drite at a specific location on the fiber. The opposing possibil-
ity is illustrated by diagram (b), where the particles coat the
fiber surface uniformly before they build the next layer of the
filter cake. Theoretically these two cases ought to give markedly
different filtration performance. Butra J5J quite reasonably sug-
gests that a flyash deposit like (a) will collect particles much
more efficiently than the uniform coat (b). It is interesting to
note that the flyash used at TRI has a particle size distribution
such that the submicron particles, because of their sheer numbers
C\>105/cm3) / could build more extensive dendrites than the larger^
particles. If the cake develops by surface coating, however, then
the small particles become less dominant in the collection process,
but may still be important if they act as a glue and enhance
adhesion between larger particles.
As for the pressure drop, structure (b} should have less drag,
and consequently, a filter whose fibers become coated uniformly with
particles should have a lower pressure drop than one whose fibers
sprout dendritic growths. This argument applies to collection of
particles on fibers Cinitial filtration stage) or to the capture of
21
-------
Dendrite Height ~ nr
(0)
Coating Height x nr~
(b)
Figure 12. Basic structural configurations Figure 13. Collection of charged particl*
for flyash deposits (schematic). (a) charged and (b) neutral surfaces
(schematic).
Figure 14. Micrographs of flyash collected on trilobal poly-
ester fibers (a) with no applied electric field and (b)
with a 4.2-kV/cm field.
22
-------
pa.rti.cles by particles previously collected by fibers (later
filtration stages). Of course these structures represent extremes
and the actual structures are most likely combinations of the two.
It may be asked next, what conditions favor the growth of one
structure over the other? Since the high collection efficiencies we
have observed strongly suggest that the particles have some electric
charge, it is logical to consider the collection of charged particles
on charged and neutral surfaces. Figure 13 illustrates these
situations. The small solid circles represent particle positions,
the small dashed circles represent possible future positions of the
particles, the dotted lines denote particle trajectories, and the
large circles stand for fibers or larger flyash particles. In case
(a) the collector is charged-either naturally, or by applying an
external field,-and t^e particles are more strongly attracted or
repelled by the collector surface than by each other, hence producing
a uniform coat. In case (b), however, the collector is' neutral, and
the particle-particle electrical interaction dominates, giving a
dendritic growth. Hence, the application of an electric field is
expected to yield a Ibwer pressure drop, since the cake microstructure
will be more compact (ignoring, for the moment, macroscopic distri-
bution effects). However, it does not follow from this analysis
that the field should give a poorer efficiency. Even though the
field will suppress dendrite growth, it also adds a force which
drives additional particles to the collector, the net effect being
to increase collection.
In order to verify these postulates, the structure of cakes
deposited on individual fibers arranged in a regular, parallel array
in the presence or absence of a field was investigated microscopi-
cally. The arrays were made of 120-ym fibers spread roughly 3 fiber
diameters apart, the average interfiber distance in a nonwoven fabric
filter. They were exposed to the flyash for 330 s and observed from
the near stagnation point. From the results, shown in Figure .14, it
is clear that the cake produced in the absence of an electric field
is more dendritic and fluffy than the deposit formed with a field.
Once more, the field does not entirely eliminate dendrites, but
serves to stunt their growth. This should produce a lower drag and
may be a factor in the reduction in pressure drop with ESFF.
G. EFFECT OF CORONA
It is often observed that the wire electrodes used to produce
the electric field are bare, and the regions of fabric immediately
surrounding the wires are clean on the side of the filter facing
the flow, while dark bands mark the positions of the wires on the
otherwise clean downstream face. In the experiments using layered
filters, these effects were photographed (Figs. 5 and 6). It
appears, therefore, that the particles in the region of the wires
are more difficult to capture and penetrate farther into the filter.
This effect suggests that the aerosol is charged in the vicinity
of the wires, probably by an ion cloud surrounding the wire.
23
-------
The field in a vacuum at the surface of two parallel cylin-
drical conductors of diameter D whose centers are placed a distance
S apart and maintained at a potential difference V is:
E
V KS/D)2-!]
surface (S-D) In {(S/D) +
The variation of surface field calculated from this equation with
voltage for different size wires separated by 15 mm (the separation
most often used in our experiments) is shown in Figure 15. It
should be emphasized that this derivation neglects the effect of
fibers and particles on the field and should be used with caution
when applied to electrodes in a patch. When the surface field is
approximately 30 kV/cm, corona, or the ionization of air in the
vicinity of the wire, occurs. The convective velocity of the ions
due to the bulk flow is of the order of 10"1 m/s, but the field-
induced velocity is roughly 10** m/s, and so the ions travel instantly
towards the opposite electrode. Although the ion cloud may not
reach the other electrode due to interference from the fibers and
cake and collisions with air molecules, it does charge the fibers
and aerosol near the wire to the same sign, and the two repel each
other, resulting in no collection and clean fabric regions.
FIELD AT SURFACE
OF WIRE (kV)
IO
2 4 6 B 10
APPLIED VOLTAGE (kV)
12
Figure 15. Variation of calculated surface field with applied
voltage for wires of different diameters.
24
-------
After passing through, the region of like-charged fibers, the
aerosol is caught. However, once the particles form a layer or two
on the fibers, the collection decreases since particles passing near
the now coated and hence charged fibers will be repelled as they
were from the clean charged fibers near the electrodes. The parti-
cles must then pass to fibers deeper in the bed to be captured,
resulting in deeper penetration under the wires.
25
-------
SECTION IV
EFFECTS OF FIBER GEOMETRY
A. MECHANISM OF PRESSURE DROP REDUCTION WITH TRILOBAL FIBER FILTERS
In the previous report [4], it was concluded likely that
trilobal fibers enhance capture, not only because of reduced inter-
jiber distance, but also by the presence of localized electric
fields. Calculations indicated that in the presence of electric
fields, lobed fibers result in greater collection efficiencies than
round fibers. If the aerosol contains charged particles, their
deposition will set up localized electric fields near the upstream
fabric surface. In the presence of these fields, capture of
subsequent particles will be more efficient on lobed fibers.
Further studies have yielded data consistent with this theory, and
have indicated how this mechanism may also be responsible for the
..oduced pressure drop obtained with trilobal fiber filters.
The layered filter technique was used (see p. 7) to measure
penetration and single fiber efficiency for new trilobal and round
fiber filters. Now, similar data have been obtained with conditioned
filters. Measurements of flyash retained before cleaning have been
made at the first, twentieth, and fortieth cycles, and plotted in
Figure 16. It can be seen that at the first cycle upstream layers
j'n the trilobal filter collect slightly more than those in the round
fabrics. However, as conditioning progresses a reversal takes place,
and the round fabric is seen to retain more dust in all layers.
This is equivalent to saying that the dust is cleaned more easily
from the trilobal fabric, probably because the dust cake is formed
closer to the upstream surface in this fabric. This lower dust
retention is accompanied by a lower pressure drop. There is thus a
plausible connection between lower penetration and lower pressure
drop when trilobal fibers are used instead of round.
B. X AND Y-SHAPED FIBERS
Progress in the study of effects of fiber cross-sectional
shape on filtration performance is limited in part by the lack of
appropriate fiber samples. Thus, in studies of the effects of the
number of lobes in the cross section, the validity of the results
has, in the past, been diminished because no tetralobal fibers were
available, and the pentalobal fibers used had very shallow lobes.
Moreover, no series of fibers with a range of lobe depths has to
date been available. Samples of fibers with cross-sectional shapes
different from those used previously have now been obtained. These
include nominal X, Y, and I shapes, with linear densities of 3,5,
and 8 denier. The fibers were of crimped acetate supplied by the
Celanese Fibers Company.
26
-------
FLYASH RETAINED
BEFORE CLEANING (q)
All Loyers
Remaining
Layers
10 2O 30
CYCLES
40
Figure 16. Flyash retained by layered filters as conditioning
proceeds.
27
-------
Cross sections of these fibers are shown in Figures 17 and
18. Figures 17b, c, and d are the 3-, 5-, and 8-denier Y fibers.
These have deeper lobes than the PET trilobal fiber used hitherto
whose cross section is shown for comparison in Figure 17a. The
cross-sectional shape of a fiber can be described by an equation in
polar coordinates for a curve of the same shape. In the equation
p = . 1 + e cos m ,
p is the radius vector, m the number of lobes, <|> the vectorial angle,
and e is a measure of lobe depth. The diagram below shows an
example. When e = 0, the cross section is circular. Whereas the
trilobal PET fibers had an e value of about 0.25, the Y-shaped
acetate fibers have e values of about 0.6. Figure 18 shows cross
sections of I and X fibers. These shapes are generally more
irregular than the Y, and e values are more difficult to calculate.
Nonwoven felt patches were made from each of these fibers,
and the fabric properties are listed in Table 2. The performance
of each felt was measured on the patch apparatus using a flyash
aerosol with a concentration of 3.3g/m3. The aerosol was neither
precharged nor neutralized, and no electrodes were placed in the
fabrics. Measurements of pressure drop and penetration were made
after 20 conditioning cycles. These are plotted in Figures 19 and
20. It is interesting to note that felts made with these deep-lobe
fibers have better performance than those made with the PET trilobal
fibers. Thus, in Figure 19 the pressure drop for Y fibers is
28
-------
Figure 17. SEM cross sections of trilobal fibers: a) 3-den
PET, shallow lobe; b) 3-den Y, acetate; c) 5-den Y,
acetate; d) 8-den Y, acetate.
-------
.
Figure 18. SEM cross sections of acetate fibers:
b) 5-den X; c) 8-den I.
30
a) 3-den I;
-------
significantly lower than that for the PET trilobal. In Figure 20
the penetration is lower for the Y fibers than for the PET tri-
lobal. The position in this performance scale taken by the X and I
fibers is less clear since, while they produce higher pressure drops,
they also give lower penetrations. It is not known how much the
differences are due to the different chemical compositions of the
fibers. However, cellulosic fibers should have a higher electrical
conductivity. This tends to dissipate electric charges and to lead
to poorer performance. The differences seen in Figures 19-21 are
therefore unlikely to be due to the different compositions of the
fibers.
TABLE 2. PROPERTIES OF FELTS MADE FROM ACETATE FIBERS
WITH VARIOUS CROSS SECTIONS
Fiber
cross-sectional
shape
I
Y
X
Y
I
Y
Trilobal (PET)
Round (PET)
Fiber
linear density
(den)
2.62
2.67
6.03
5.31
8.24
8.35
3
3
Fabric
weight
(kg/m2)
0.257
0.255
0.256
0.258
0.257
0.256
0.251
0.253
Fabric
density
(g/cm3 )
0.142
0.134
0.150
0.151
0.146
0.147
0.134
0.138
So-called "performance curves" have been plotted in the past
16] to allow for the trade-off between pressure drop and penetration
as fiber linear density changes. These curves are developed by
plotting penetration vs. pressure drop (or some equivalent parameter),
as in Figure 21, and ideal performance is represented by a point at
the origin. Performance points for felts made of fibers with a
particular cross-sectional shape but of different linear densities
will fall on a characteristic curve. The distance of the curves from
the origin can be taken as a measure of the performance associated
with that fiber. This criterion applied to the results in Figure
21 shows that, in fact, the Y fibers produce a curve which is closer
to the origin than the PET trilobal fibers. The points in Figure 21
represent an incomplete collection, which will be filled in the
future. In addition, the performance curves show,that the Y fibers
make for slightly better performance than the I fibers. From the
cross sections shown in Figure 18 it would appear that the X fibers
should give performance very similar to that of I fibers since they
all approximate irregular tetralobal shapes. The results in Figure
21, however, show that the X fibers are slightly better, though it
is impossible to identify a feature of their cross section to which
this better performance may be attributed.
31
-------
PENETRATION
.004
IOO
9O
80
70
60
50
A—TRILOBAL PET
0«-ROUND PET
.003
ACETATE
I-SHAPED
Y-SHAPED
X-SHAPED
.002-
i - 8-
10
LINEAR DENSITY (dm)
jOOl
Figure 19. Comparison of pressure drop with
fabrics made of I, Y, and X-shaped acetate,
trilobal PET (A), and round PET (o) fibers.
-ROUND PET
A—TRILOBAL
PET
ACETATE
Y-SHAPED
I-SHAPED
X-SHAPED
LINEAR DENSITY (dm)
10
Figure 20. Comparison of penetration (1 - effi-
ciency) with fabrics made of I, Y, and X-
shaped acetate, trilobal PET (A), and round
PET (o) fibers.
TRILOBAL\
\
ACETATE
I-SHAPED
Y-SHAPED
X-SHAPED
ROUND PET
20-
.OOI
.002 003
PENETRATION
.005
Figure 21. Performance curves for the fabrics
in Figures 19 and 20.
32
-------
A previous report [4] gave results of calculations of single
fiber efficiencies in electric fields for fibers with different
lobe depths. The results indicated a strong direct dependence of
capture efficiency on lobe depth. . These results have suggested the
possibility that the better performance of lobed-fiber filters may
be due to the better capture behavior of these fibers in localized
electric fields due to deposited charged aerosol particles. No
experimental verification of the hypothesis has been made, but the
better performance with deep-lobed fibers reported above appears
to support it.
C. COMPOSITE FILTERS
1. Initial Cycles
The last report [4] described measurements on filters made
by pressing together layers of vinyon-bonded nonwovens made with
different fibers. The fibers used at that time were 3-denier
trilobal and 3-denier round cross section polyester. It was found
that the upstream layers appeared to control the behavior of the
filter.
Measurements of this type were repeated with layers differing
in fiber fineness, namely with 3 and 6-denier trilobal polyester.
Table 3 describes the combinations used; just as in the first study,
two filters had all layers composed of either 3 or 6-denier fibers,
the other two had a top layer of one fiber and the remaining 5
layers of the other. Trial conditions were identical with the first
study. At the twentieth cycle of conditioning, the trials were
terminated and measurements recorded.
TABLE 3. COMPOSITE FILTERS MADE OF 3 AND 6-DENIER
TRILOBAL POLYESTER FIBERS
Upstream Downstream
layer layer
All 6 denier
(0.25 kg/m2)
All 3 denier
(0.25 kg/m2)
6 denier 3 denier
(0.04 kg/m2) (0.21 kg/m2)
3 denier 6 denier
(0.04 kg/m2) (0.21 kg/m2)
33
-------
Figure 22 shows an unexpected trend in pressure drop behavior
for the four fabrics during conditioning. In the first 10 cycles,
the upstream layers dominate the behavior, just as in the study of
cross-sectional shape. Thereafter, a reversal in the trend appears,
with pressure drop now being determined by the downstream fibers.
A similar crossover occurs in the pressure drop rise within one
cycle (the 20th) shown in Figure 23.
These phenomena are assumed to be the result of dust seepage
past the upstream layer, so that as the dust deposit advances,
responses characteristic of the downstream layers appear. Since
these data showed evidence that behavior had not yet stabilized, it
was difficult to make any additional conclusions concerning fine/
coarse composites. A new trial was initiated.
2. Conditioned Fabrics
A second study of fine/coarse composites was carried out
using the following series of samples:
Upstream / Downstream
0% 6d 100% 3d
17% 6d 83% 3d
50% 6d 50% 3d
83% 6d 17% 3d
0% 3d 100% 6d
17% 3d 83% 6d
50% 3d 50% 6d
83% 3d 17% 6d
The samples were vinyon-bonded at a higher temperature .than had
previously been used (I2l°c instead of 99°C), thereby achieving a
more stable fabric density. Measurements were made after a longer
conditioning than reported previously (30 to 40 cycles instead of
20 cycles). Each data point is the average of from 2 to 5 measure-
ments using replicate samples or vacuum-cleaned samples. The above
steps were intended to further increase the reliability of the
results. Experimental conditions were as follows:
Fabric weight : 0.25 kg/m2(7.3 oz/yd2)
Fabric density : 0.130 g/cm3
Fiber type : PET
Face velocity : 60 mm/s (12 ft/min)
Inlet concentration : 3.5 g/m3
Cycle time : 5. min
Pressure in reverse air cleaning : 209 kPa (30 psi)
Measurements were made not only of the penetration and the
pressure drop of these filters-, but also of the residual dust after
cleaning (expressed as a fraction of the base filter mass). Figures
24, 25, and 26 show these measurements as a function of composition
34
-------
.JQ All 3 Denier
6 Denier,
upstrean
-A 3 Den upttrenra
3 All 6 Denier
5 10 15
NO. OF CYCLES
20
Figure 22. Changes in pressure drop during conditioning of
composite PET filters.
AP DURING 20 TH CYCLE
(mm HgO)
ALL 3 DENIER
* 6 DEN UPSTREAM
3 DEN UPSTREAM
ALL 6 DENIER
I 234567
CAKE DEPOSITION (mg/cm2)
Figure 23. Changes in pressure drop during the 20th cycle
for composite PET filters.
35
-------
by weight (% 6d/% 3d) of the two-layer composite filters. In
these graphs, percentage of 6-denier fibers upstream increases to
the right, whereas percentage of 3-denier fibers upstream increases
to the left. It can be seen now that a composite with 3-denier
fibers upstream allows less penetration and also exhibits a lower
pressure drop than one with 6-denier fibers upstream or one with
100% 3-denier fibers. The 17% 3d upstream/83% 6d downstream
composite appears to be optimum.
These effects may be explainable by the correlation between
the curves of Figures 24 and 25 with those of Figure 26. It
appears generally that penetration and pressure drop are directly
related to the amount of dust retained after cleaning. Finer
(3-denier) fibers upstream prevent dust from lodging deeper into
the filter where it cannot be blown back. This in turn keeps the
penetration lower, and since unremovable dust increases resis-
tance, also keeps the pressure drop lower. The slightly upward
turn of the pressure drop curve at 100% 6d may also be due to
increased residual dust. Although 17% 3d/83% 6d appears to be
optimum, further studies with commercial-weight filters should
be conducted to verify these findings.
36
-------
AP(mmH201
120 H
PENETRATION (X)
0.8
O/OO 17/83
50/90 83/17 100/0
X8d/X3d
0/WO 17/83 50/50 83/17 DO/0
X6d/X3d
Figure 24. Pressure drop across composite Figure 25. Penetration through, the coitposite
vinyon-bonded PET filters having varying filters of Figure 24. Solid circles are
proportions of 3 and 6-denier fibers in points for all 3 or all 6-denier filters.
two layers.
(CLEANING
(Xofwt fabric)
300-
CMOO 17/83 50/50 83/17 100/0
X6d/X3d
Figure 26. Residual dust after cleaning
the filters of Figure 24.
37
-------
SECTION V
REFERENCES
1. Endres, H. A.., and W. T. van Orman, Electrostatic Properties
of Rubber and Plastics, SPE Journal 26 (Feb. 1953).
2. Silverman, L. et al.f Performance of a Model K Electro-Polar
Filter, USAEC Report NYO 1592, Harvard University, July 15,
1954.
3. Thomas, J. W. and E. J. Woodfin, Electrified Fibrous Air
Filters, AIEE Journal, 276-278 (Nov. 1959).
4. Miller, B., G. Lamb, P. Costanza, D. O'Meara, and J. Dunbar,
Studies of Dust Cake Formation and Structure in Fabric Filtra-
tion, EPA 600/7-78-095, June 1978.
5. Butra, S., Aerosol Particle Deposition in Fibrous Media with
Dendritic Pattern. Comparison Between Theory and Experiment,
Master's Thesis, Department of Chemical Engineering, University
of Houston, March 1978.
6. Miller, B., G. Lamb., P. Costanza, and J. Craig, Nonwoven Fabric
Filters for Particulate Removal in the Respirable Dust Range,
EPA 600/7-77-115, October 1977.
38
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/7-79-108
3. RECIPIENT'S ACCESSION NO.
TITLE AND SUBTITLE
Studies of Dust Cake Formation and Structure in
Fabric Filtration: Second Year
5. REPORT DATE
April 1979
6. PERFORMING ORGANIZATION CODE
Bernard Miller, George Lamb,Peter Cos-
tanza,George Harriott,Janet Dunbar, and Michael
Mnkricki
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Textile Research Institute
P.O. Box 625
Princeton, New Jersey 08540
10. PROGRAM ELEMENT NO.
EHE624A
11. CONTRACT/GRANT NO.
Grant R804926
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT
12/77 - 12/78
14. SPONSORING AGENCY CODE
EPA/600/13
^.SUPPLEMENTARY NOTES
pr0ject officer & James H. Turner, MD-61, 919/541-
2925. Report EPA-600/7-78-095 covers the first year's work.
16. ABSTRACT
The report describes experiments to improve fabric filter efficiency and
pressure drop by use of electric fields near the filter surface. Modified fiber geo-
metries and fabric construction are also investigated. Tests with patch filters
showed pressure drops reduced to about 13 mm H2O from about 36 mm H2O upon
the application of a 6 kV/cm electric field. Total fractional particle penetration was
reduced to about 0.001 from 0.170 under the influence of the same field. The electric
field was more effective when applied to filters having loose fibers at the surface.
Deeply lobed fibers produced filters with higher efficiency, lower pressure drop,
and better cleanability than filters made from round fibers. The effects were attri-
buted to induced localized fields at the lobed surfaces. The fields were produced
from collection of naturally charged particles. Fabric structure that promotes par-
ticle collection near the upstream surface of the filter gave the best performance.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Group
Pollution
Gas Filters
Fabrics
Dust
Caking
Electric Fields
Fibers
Shape
Pollution Control
Stationary Sources
Fabric Filters
Particulate
13B
13K
HE
11G
07A,13H
20C
12A
~8. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
45
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
39
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