AEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-028 July 1981
Project Summary
Effect of Parameters of
Filtration on Dust Cleaning
Fabrics
Jan R. Koscianowski, Lidia Koscianowski, Bronislaw Werynski, Eugeniusz
Szczwpankiewicz, and Stanislaw Bethke
This report summarizes 4 years of
laboratory- and large-scale tests of the
dust filtration process and the basic
filtration parameters determining
performance. Physical parameters
describing fabric and dust cake struc-
ture were defined and three basic dust
filtration mechanisms were consid-
ered: selective precipitation, inertia!
sedimentation, and diffusion. From
test results of two Polish polyester
fabrics filtering separated fly ash,
three-dimensional probabilistic
models of dust collection efficiency
and filtration resistance were intro-
duced. A general filtration model,
based on random field theory, was
also developed that included full
mathematical argumentation.
Electrical properties of dusts and
fabrics were examined to define the
influence of electrostatic phenomena
on the dust filtration process. This
examination led to a quantitative
determination of local electric fields in
the fabric and dust cake.
Conclusions from this work are:
• Air flow through a clean fabric
and the dust filtration process
are normal stochastic processes.
• Dust filtration is a specific dry
filtration process, differing from
the air filtration process or the
high-efficiency air filtration
process.
• Dust collection efficiency during
the dust filtration process de-
pends on three basic filtration
mechanisms related to the
aerosol particle size distribution.
• Woven filtration materials are
heterogeneous and anisotropic
media characterized by periodic
structure that, at high values of
face velocity, exhibit transition
region flow described by a resis-
tance function, K1.
Microscale electric fields formed by
triboelectrical charge exchange be-
tween fibers and dust particles signifi-
cantly influence the dust filtration
process.
This Project Summary was develop-
ed by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the <
same title (see Project Report ordering
information at back).
Introduction
For many years the dust filtration
process has used textile filtration media
in industrial dust collection devices; i.e.,
filter bags, pocket dust collectors, bag
houses, and pulse-jet dust collectors.
While all these dust collection devices
perform the same overall function,
there are considerable differences in'
their individual process mechanisms.
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The dust removal process is very effi-
cient and dust collection efficiency
usually exceeds 99 percent. This
property promoted the rapid use of
fabric filters, despite the fact that their
performance has only a weak
theoretical base. Different fabric dust
collectors were designed during a short
time and were optimized for:
• Longer life of the filtration fabrics
(mainly by application of less
severe regeneration systems).
• Automation.
• Reliability, low pressure drop, and
architecture.
As dust collection devices developed,
so did the production of improved filtra-
tion materials. Filtration materials pro-
duced from natural fibers (cotton and
wool) were gradually replaced with
synthetic fibers (i.e., polyester, poly-
amide, and polyacrylonitrile), which had
batter mechanical properties and
chemical resistance. The higher
temperature resistance of the synthetic
fibers led to greater industrial use of
fabric filters.
However, it is noteworthy that the
development of new raw fiber materials
with improved physicochemical prop-
erties did not lead to new production
methods for woven filtration materials.
The new filtration materials were
similar in structure to those produced
from the natural raw materials. This
situation may have been caused by
failure to communicate the importance
of fiber spatial structure to fabric pro-
ducers. Such requirements were diffi-
cult to define because of fragmentary
research and limited understanding of
the dust removal process. Moreover,
early research focussed primarily on
quantitative process data.
The situation is paradoxical because
contemporary industrial-scale filters
are nearly perfectly constructed and
successfully operate as highly auto-
mated systems. No systematic theoret-
ical base for the dust filtration process
exists, however, from which to direct
further research on industrial gas filtra-
tion with woven filtration materials and
to further optimize the process and the
filtration medium. Further improvement
in filter construction without compro-
mising the filtration process constitutes
progress in dust removal because it
does not jeopardize clean air. Knowing
what changes will interfere with dust
collection requires theoretical under-
standing of the dust filtration process,
so such a theoretical base must be
developed.
Theoretically, dust filtration is a multi-
parameter process differing from other
filtration processes. Some researchers
have tried to explain the peculiarities
observed with industrial dust collectors
by resorting to the classical filtration
theory and especially the single fiber
theory. This approach led to confusion
in research planning and in interpreting
results. Many papers are incomprehen-
sible because the authors did not know
what type of filtration process they were
studying.
At present scientists agree, however,
that the basic mechanisms of dust
particle precipitation from an aerosol
stream that determine the process
effectiveness in the classical theory of
filtration are also observed in dust filtra-
tion. However, their physical interpre-
tation is different because of differences
in the basic assumptions of the two
processes.
In the Phase I and II reports of this
project we proposed standardized
process nomenclature and defined our
initial assumptions. Our proposal was
based on a theoretical physical model of
dust filtration, which is presented in
Section IV of the Project Report. The
efficiency of dust collection, the primary
qualitative parameter of the process,
was examined as a function of filtration
parameters; i.e., face velocity and dust
flux. We also tried to correlate filtration
efficiency with some structural param-
eter of the filtration medium.
The preliminary mathematical model
of dust filtration, based on experiments
conducted in our Institute and further
analyzed at the Institute of Mathematics
of WSP Opole, is an integral part of this
report. Some experimental results ob-
tained in other projects are also
included in an attempt to make this
report as complete a description of
filtration as possible and hence as
useful as possible in planning further
research.
Research Objectives
The basic objectives of the program
financed by EPA and conducted by the
Institute of Cement Building Materials
in Opole were:
• To describe the effects of fabric
structural parameters on the
pressure drops associated with
gas flow through the clean fabric.
• To describe the effects of
structural parameters of both the
fabric and the dust cake on pres-
sure drops during the filtration
process.
• To describe the functional depen-
dence between dust collection
efficiencies and the variables of
the dust filtration process.
• To test, by mathematical model-
ing, fabric structures with the best
filtration properties.
Total program research includes:
• Laboratory testing, including test-
ing of dust and fabrics.
• Large-scale testing.
• Auxiliary studies.
• Application of mathematical
methods, including modeling.
Conclusions *
Both experimental testing and theo-"
retical analyses formed the basis for the
conclusions reached during this project.
Dust Filtration Process
• Three types of dust filtration occur
during laboratory testing:
• Filtration Type I: filtration begin-
ning with the virgin fabric and
ending at the initiation of the first
regeneration cycle.
• Filtration Type II: filtration by the
fabric and accumulating dust but
before the equilibrium state is
reached (the quantity of dust col-
lected during the filtration cycle
exceeds that removed during the
subsequent regeneration cycle).
• Filtration Type III: steady state fil-
tration by the fabric and its dust
cake characterized by stable pre-
and post-regeneration conditions
(the quantity of dust removed dur-
ing regeneration is approximately
equal to that collected during the
filtration cycle—the large-scale
test conditions representative of
industrial dust filtration).
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Dust collection efficiency and fil-
tration resistance, apart from the
type of filtration, are determined
by initial aerosol state param-
eters, filtration parameters, and
structural parameters of the dust-
fabric system.
Dust collection efficiency and fil-
tration resistance can be ade-
quately described by probabilistic
mathematical models over the
range of the empirical data.
Mean dust collection efficiency
during a filtration cycle (Filtration
Types I and III) depends on the
time required to build a dust cake
and its thickness.
The collection efficiency of the
dust cake cannot be compared to
the collection efficiency of the
filtration layer (the fabric) because
of the physically different
filtration media.
Consideration of the effect of the
particle size distribution of the
aerosol on the dust collection effi-
ciency permits a correlation of the
dust-fabric system efficiency with
important, initial aerosol state
parameters.
• The main mechanisms that deter-
mine filtration process efficiency
in the dust cake, are selective
precipitation, inertia! sedimenta-
tion, and diffusion.
• Dust collection efficiency and
filtration resistance depend on the
pore size distribution of the dust-
fabric system.
• The pore size distribution function
is a .characteristic of each fabric
dust cake system and has a log-
normal distribution.
True Filtration Processes
• The dust collection efficiency for
Filtration Process Type I depends
on the degree of dust pulveriza-
tion, the dust concentration, the
shape of the aerosol particles, the
face velocity (qg) and the area!
dust loading of the filter (L0).
Area! mass density of the dust in
or on a dust-covered filter (the
terminal dust load, L0) is a reliable
parameter for estimating filtration
effects.
• Increases in qg and the degree of
dust pulverization decrease dust
collection efficiency and increase
filtration resistance.
• Increases in dust concentration
and Lo both increase dust collec-
tion efficiency.
• Dust collection efficiency for Fil-
tration Process Type III depends
on qg and the a real mass density
of the dust cake, Lp.
• Increasing qg decreases efficiency
and increases filtration resis-
tance.
• Increasing Lp increases efficiency
and shifts process performance
toward higher filtration resistance
• The functional dependence of
dust collection efficiency on qg is
determined by the kind of fabric
included in the dust-fabric
system.
• Both Filtration Processes Type I
and Type III are characterized by
dust cake defects, which influ-
ence dust collection efficiency
and filtration resistance.
• The formation of dust cake defects
is related to disturbed equilibrium
between the dust particles com-
prising the dust cake, resulting in
the dislocation of the dust parti-
cles and the formation of ducts/
canals after a certain pressure
drop is reached.
Recommendations
The results of this project point to the
need for further investigation in the
following areas:
• Development of the relationship
between the porosity of woven
materials and their free area (FA)
and basket free area (BFA) distri-
bution, and empirical verification
of this relationship over a wide
range of materials.
• The relationship between the
pulverization state of suspended
dust and the specific area of the
dust layer formed from it, includ-
ing the role of particle shape and
the hydraulic properties of the
layer.
• Empirical verification of the role of
fabric properties in the process of
filtration through a dust layer.
Investigation of these areas would help
perfect, qualitatively and quantitatively,
mathematical models of the dust filtra-
tion process.
Jan R. Koscianowski, Lid/a Koscianowski, Bronislaw Werynski, Eugeniusz
Szczwpankiewicz, and Stanislaw Bethke are with the Institute of Industry of
Cement Building Materials, 452-641 Opole. Oswiecimska Str. 21, Poland.
J. H. Turner and L. S. Hovis are the EPA Project Officers (see below).
The complete report, entitled "Effect of Parameters of Filtration on Dust
Cleaning Fabrics," (Order No. PB 81-188 542; Cost: $32.OO, 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 Officers can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
, US GOVERNMENT PRINTING OFFICE 1«1 -757-OU/717Z
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