United States
                  Environmental Protection
                                                       Industrial Environmental Research
                                                       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
                   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).

                                                         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
                                                         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

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
 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.

  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

  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.
  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
 5.  Frey, R. E. Types of Fabric Instal-
     lations. J. Air Poll. Control Assoc.
 6.  Dennis, R. and J. Wilder. Fabric
     Filter Cleaning Studies. EPA-
     650/2-75-009 (NTIS, No. PB 240
 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.
 8.  Dennis, R. Collection Efficiency as
     a Function of Particle Size, Shape,
     and Density: Theory and Experi-
     ence. J. Air  Poll. Control Assoc.,
 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):
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,
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

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