VVA XX*
CP
V
\
\
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle/ark NC 27711
Research and Development
EPA-600/S7-81 -027 May 1982
Project Summary
Performance of a High-Velocity
Pulse-Jet Filter, III
David Leith, Michael J. Ellenbecker, and Melvin W. First
Dust can pass straight through a
pulse-jet-cleaned fabric filter and can
also pass through by seepage. A model
is presented which describes penetra-
tion by each of these processes. Com-
parison of the model with data shows
that outlet mass flux from operating
filters can be accounted for by seepage
alone. Although insufficient informa-
tion is available to use the model for
penetration prediction on an absolute
scale, these conclusions suggest that
additional research on developing a
penetration model should emphasize
seepage of collected particles through
the filter rather than the process of
particle collection itself. Furthermore,
these results suggest the trends in out-
let flux that should occur with changes
in operating variables such as filtration
velocity, pulse pressure, and fabric
type.
The utility of the present model lies
in its ability to interpret penetration
characteristics of pulse-jet-cleaned
filters that previously could not be
explained effectively. The agreement
found between data and model predic-
tions for outlet flux over a range of
filtration velocities, aeral dust densi-
ties, and for two different fabrics lends
strong support to the validity of the
assumptions used in the model's deri-
vation. These results strongly suggest
that penetration models which do not
consider seepage as an important
penetration mechanism are seriously
flawed. Furthermore, these results
indicate that penetration models in-
tended to describe the fractional effi-
ciency characteristics of a pulse-jet-
cleaned filter must consider the ag-
glomeration characteristics and particle-
size-dependent release characteristics
of the fabric and dust deposit, rather
than relationships between particle
size and straight-through penetration
alone. These results suggest which
future research is likely to be produc-
tive, and which is not.
This Project Summary was develop-
ed by EPA 's Industrial Environmental
Research Laboratory. Research Trian-
gle Park. NC. to announce key findings
of the research pro/act that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
This report is the third in a series
dealing with performance of pulse-jet-
cleaned fabric filters. Although each
report can be read and understood inde-
pendently, they are all aimed at the
same goal—a better understanding of
the factors controlling pulse-jet-filter
performance. Maximum understanding
may be gained by reading all three
reports.
Although pulse-jet-cleaned filters
comprise a substantial portion of the
fabric filter market in the United States,
the factors that affect filter efficiency
and pressure drop are not yet well
understood. It is likely that filter per-
formance can be improved additionally
when a better understanding of the
factors that affect performance can be
applied to filter design and operation.
-------
This report describes studies of pulse-
jet-cleaned fabric filters operated at
filtration velocities ranging from conven-
tional to high. It consists of two parts:
first, a model for dust penetration
through pulse-jet-cleaned filters; and
second, an investigation of the forces
necessary to remove deposited dust
from the surface of a fabric.
Penetration Through e
Pulse-Jet-Cleaned Fabric Filter
In spite of many attempts to model
penetration through fabric filters (1-4),
there is no satisfactory way to predict
dust penetration through a pulse-jet-
cleaned fabric filter. This is not because
of insufficient interest in the problem.
Pulse-jet filters have captured a substan-
tial portion of the fabric filter market (5),
and the efficiency with which they
operate is of strong interest to regulatory
officials, to industrial users who must
meet emission regulations, and to equip-
ment manufacturers who supply these
filters with a performance guarantee.
Performance characteristics which
must be considered when modeling pen-
etration include: (1) particle collection
by clean fibers in a new fabric, (2)
particle collection by the dust deposit
accumulated on and in these fabrics,
and (3) retention of this dust so it does
not seep through the fabric during the
rather violent cleaning cycle.
An adaptation of clean fiber bed
theory to a study of particle penetration
through clean felt of the kind used in
pulse-jet-cleaned filters was undertaken
by Hampl and Rim berg (1). They found
that penetration of,0.35 to 1.1 /um
particles through clean, new industrial
felts ranged from 20 to 70% at typical
pulse-jet filtration velocities, penetra-
tions that are much higher than the 1 %
or less generally found for intermittently
cleaned industrial filters using fabrics
well conditioned with dust (6-8). Al-
though agreement is excellent between
penetration theory and data for clean,
new felt fabrics (1), the performance of
new felt in the laboratory is clearly dif-
ferent from that of a well conditioned
felt when used in an industrial pulse-jet
filter.
Dust Removal from
Non-Woven Fabrics
The pulsed jets of compressed air
commonly used to clean non-woven
fabrics in fabric filters are inefficient at
removing the deposited dust (6,9). Meas-
urements on a pilot-scale pulse-jet
fabric filter using fly ash test dust
indicate that less than 1 % of the dust on
a bag is typically removed to the hopper
by a cleaning pulse (9,10).
Improving the effectiveness of pulse-
jet cleaning offers the potential for
dramatically improving pulse-jet filter
performance. Both pressure drop and,
to a leaser extent, collection efficiency
are adversely affected by the failure to
clean the fabric efficiently. The basic
processes controlling the removal of
dust from non-woven fabrics must be
understood if improvements in cleaning
efficiency are to be identified. This
.section of the full report describes the
results of an experimental program to
investigate factors affecting dust re-
moval from non-woven fabrics.
Dust is removed from a non-woven
fabric in two stages. First, the cleaning
pulse separates some fractions of the
dust from the fabric. Second, the sepa-
rated dust falls toward the hopper. Some
fraction of the removed dust actually
reaches the hopper, but some redeposits
on the cleaned bag or on adjacent bags.
The fraction of dust reaching the hopper,
£, is thus the product of two separate
processes:
where
a = fraction of dust removed from the
fabric by a cleaning pulse
/} = fraction of removed dust which falls
to the hopper.
As discussed above, the fraction of
dust reaching the hopper in actual
practice is very low; this can be caused
by a failure to remove dust from the
fabric, the redeposition of the removed
dust, or a combination of both factors.
Some investigators (5) have assumed
that redeposition predominates, (i.e.,
the cleaning pulse removes most of the
dust deposit) so that dust retention is
caused primarily by redeposition. No
experimental evidence has yet been
presented, however, to confirm or deny
this assumption.
It is important to determine the rela-
tive importance of inefficient dust re-
moval and redeposition to inefficient
cleaning, as system modifications to
improve performance (i.e., reduce dust
retention) could differ greatly depending
on which factor is more- important.
System modifications which would elim-
inate redeposition altogether (e.g.,
compartmentalization and air flow shut-
down) might have no effect on improving
dust removal from the fabric.
Factors affecting the removal of dust
from a non-woven fabric by a com-
pressed air pulse are discussed else-
where (11). The air pulse causes static
pressure to increase inside the bag; the
difference between this static pressure
and the operating pressure drop across
the bag during cleaning causes a force
which accelerates the fabric and dust
outward. The fabric is not stretched
tightly around the cage, and so can
accelerate radially outward to reach a
maximum velocity during cleaning, ve.
At some point after attaining velocity
vc the fabric approaches its full outward
expansion and decelerates; the dust
deposit, however, tends to continue
traveling radially outward. If the adhe-
sion force binding the dust to the fabric
is less than the peak deceleration force
caused when the fabric substrate slows
as the bag reaches full expansion, the
dust will separate from the fabric.
At least one other mechanism could
operate to remove deposited dust during
a cleaning pulse. Besides flexing the
fabric, the pulse causes air to pass in the
reverse direction through the fabric and
dust; this reverse air flow could reen-
train and remove deposited dust parti-
cles. L&ffler (12) has summarized the
available experimental data concerning
"blowoff" of particles from fibers. The
velocities needed to remove a deposited
particle are always much higher than
the velocity at which the particles were
deposited. Smaller particles are more
difficult to remove than larger particles
because of their larger adhesive/drag
force ratio. Even relatively large particles
need rather high velocities to blow them
off fibers. For example, Larsen (13)
found that a velocity of 20 m/s was
required to remove the first 16 yum
diameter glass sphere from an 830//m
glass fiber.
Conclusions
Dust can pass straight through a
pulse-jet-cleaned fabric filter and can
also pass through by collection and
subsequent seepage. A model is pre-
sented that describes penetration by
each of these processes. The model
reflects empirical data that show that
most dust loss from operating filters
takes place by seepage alone. Although
insufficient information is available to
use the model for penetration predic-
tions on an absolute basis, the results
may be used to indicate the penetration
-------
trends that will occur with changes in
such operating variables as filtration
velocity, cleaning pulse pressure, and
fabric type. These results indicate that
research should emphasize seepage of
collected particles through the filter
rather than the process of particle col-
lection itself.
Although pulse-jet cleaning has ad-
vantages over other fabric filter cleaning
systems, an inability to discharge a
large fraction of the deposited dust to
the dust hopper can have a seriously
adverse effect on system pressure drop.
Bench-scale tests indicate that pulse-
jet cleaning may become very inefficient
for removing deposited fly ash from the
surface of polyester felt. Experiments
over a wide range of fabric cleaning
conditions measured dust removal effi-
ciencies from 2 to 36%. Tests under
conditions thought to be typical of a full-
scale system resulted in a dust removal
efficiency of about 12%.
The fraction of dust removed from a
felt fabric during cleaning was found to
be closely correlated with the kinetic
energy imparted to the dust deposit. If
cleaning energy could be transferred to
he dust deposit more efficiently, either
}y improving the pulse-jet or applying
* Jifferent cleaning methods, non-woven
abric filters could be operated at higher
filtration velocities or at lower pressure
drops.
References
1. Hampl, V. and D.Rimberg. Aerosol
Penetration of Felt Filter Media.
Presented at Annual Conference
of Gesellschaft fur Aerosol-For-
schung E.V. (Association for
Aerosol Research), Bad Soden,
Germany, October 16, 1974.
2. Cooper, D. W. and V. Hampl. Fabric
Filter Performance Model. In: Con-
ference on Particulate Collection
Problems in Converting to Low
Sulfur Coals. EPA-600/7-76-016
(NTIS, No. PB 260 498) 1976, pp.
149-185.
3. Fraser, M. D. and G. J. Foley, A
Predictive Performance Model for
Fabric Filter Systems. 1. Intermit-
tently Cleaned Single Compart-
ment Systems. Paper 74-99 pre-
sented at 67th Annual Meeting of
Air Pollution Control Association,
Denver, Colorado, 1974.
4. Dennis, R., R. W. Cass, D. W.
Cooper, R. K. Hall, V. Hampl, H. A.
Klemm, J. E. Langley and R. W.
Stern. Filtration Model for Coal Fly
Ash with Glass Fabrics. EPA-
600/7-77-084 (NTIS, No. PB 276
489)1977.
5. Frey, R. E. Types of Fabric Instal-
lations. J. Air Poll. Control Assoc.
24:1148,1974.
6. Dennis, R. and J. Wilder. Fabric
Filter Cleaning Studies. EPA-
650/2-75-009 (NTIS, No. PB 240
372)1975.
7. Leith.D. and M.W. First. Perform-
ance of a Pulse-Jet Filter at High
Filtration Velocity. 1. Particle Col-
lection. J. Air Poll. Control Assoc.
27:534,1977.
8. Dennis, R. Collection Efficiency as
a Function of Particle Size, Shape,
and Density: Theory and Experi-
ence. J. Air Poll. Control Assoc.,
24:1156.1974.
9. Ellenbecker, M. J. and D. Leith.
Dust Deposit Profiles in a High
Velocity Pulse-Jet Fabric Filter. J.
Air Poll. Control Assoc., 29(12):
1236,1979.
10. Ellenbecker, M. J. Pressure Drop
in a Pulse-Jet Fabric Filter. Sc.D.
Thesis, Harvard School of Public
Health, 1979.
11. Leith, D. and M. J. Ellenbecker.
Theory for Pressure Drop in a
Pulse-Jet Fabric Filter. Atmos-
pheric Environment, 14:845,1980.
12. L8ff ler, F. Collection of Particles by
Fiber Filters. In: Air Pollution Con-
trol, Part 1, W. Strauss, ed. John
Wiley and Sons, Inc., New York,
1971.
13. Larsen, R. I. The Adhesion and
Removal of Particles Attached to
Air Filter Surfaces. Am. Ind. Hyg.
Assoc. J., 19(4):265, 1958.
•fr US GOVERNMENT PRINTING OFFICE; 1982— 559-017/0734
D. Leith. M. J. Ellenbecker, and M. W. First an with Harvard School of Public
Health. Boston. MA 021 IS.
Louis S. Hovis is the EPA Project Officer (see below).
The complete report, entitled "Performance of a High-Velocity Pulse-Jet Filter,
III." (Order No. PB 82-196 361; Cost: $7.50. subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
-------
United States Center for Environmental Research pees paj(j
Environmental Protection Information Environmental
Agency Cincinnati OH 45268 Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
PS 0000329
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 6060«
------- |