EPA-650/2-74-043
MAY 1974
Environmental Protection Technology Series
I
55
\
o
al
O
^^^^^^^^^^^^^^jls^^^^^^^^^^^^^^
-------
EPA-650/2-74-043
PROCEEDINGS: SYMPOSIUM ON THE USE
OF FABRIC FILTERS FOR THE CONTROL
OF SUBMICRON PARTICULATES
(APRIL 8-10, 1974,
BOSTON, MASSACHUSETTS)
by
Leonard M. Seale, Editor
GCA Technology Division
Burlington Road,
Bedford, Massachusetts 01730
Contract No. 68-02-1316 (Task 2)
ROAP No. 21ADL-34
Program Element No. 1AB012
EPA Task Officer: Dennis C. Drehmel
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
May 1974
-------
This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
-------
ABSTRACT
This document provides the papers presented at the Symposium on the Use
of Fabric Filters for the Control of Submicron Particulates, which was
jointly sponsored by the Environmental Protection Agency and the GCA/
Technology Division. The primary purpose of the symposium was to better
define the role of fabric filter systems for the control of fine particle
emissions.
The effectiveness of fabric filter systems for controlling particulate
emissions from industrial sources is well accepted in the pollution con-
trol field. However, the vast majority of available performance data
depict overall weight recoveries with only minimal information on the
capture efficiencies for particles in the ^1 micrometer size range.
Experts from Government, Industrial and University groups discussed the
theoretical and practical aspects of filtration and important related
areas such as particle behavior, fabric selection and system evaluation.
The technical presentations were aimed at describing the fine particulate
control potential of existing fabric filter systems for the benefit of
regulatory and user groups and suggesting to manufacturing and research
organizations those areas where performance levels most need improvement.
111
-------
CONTENTS
ABSTRACT iii
ACKNOWLEDGEMENTS ix
THE SIGNIFICANCE OF PARTICULATE EMISSIONS
John K. Burchard
Introduction 1
Health Effects 2
Particulate Emission Sources 4
Fine Particulate Control 6
Conclusions 8
EMISSION STANDARDS FOR PARTICULATES 9
George W. Walsh
PERFORMANCE AND COST COMPARISONS BETWEEN FABRIC
FILTERS AND ALTERNATE PARTICULATE CONTROL TECHNIQUES
J.D. McKenna, J.C. Mycock & W.O. Lipscomb
Introduction 15
Pilot Plant 16
Efficiency Versus Particle Size 25
Economics Versus Efficiency 30
Conclusion 46
References 47
TYPES OF FABRIC FILTER INSTALLATIONS 49
Robert E. Frey
COMPARISON OF FINE PARTICLE CAPTURE IN FIBER
STRUCTURES AND FILTER CAKES 57
Charles E. Billings
OPTIMIZING FILTRATION PARAMETERS
Even Bakke
Introduction 59
Theory of Operation and Description of Pulse-Jet Filter 60
Optimizing Parameters 67
Experimental Apparatus and Methods 69
Discussion of Results 71
Conclusions 83
Acknowledgements 84
References 84
-------
CONTENTS (continued)
ENGINEERING AND ECONOMIC CONSIDERATIONS IN FABRIC FILTRATION
Gordon L. Smith
Introduction 85
Engineering Considerations 86
Economic Considerations 89
Conclusion 94
References 94
COLLECTION EFFICIENCY AS A FUNCTION OF PARTICLE SIZE,
SHAPE, AND DENSITY: THEORY AND EXPERIENCE
Richard Dennis
Introduction 95
Fabric Filter Efficiency Characteristics 98
Recent Experimental Measurements 108
References 138
SOME EFFECTS OF ELECTROSTATIC CHARGES IN FABRIC FILTRATION
Edward R. Frederick
Introduction 141
Some Effects of Electrostatic Charges in Fabric Filtration 142
Electrostatic Properties of Fabrics 143
Electrostatic Properties of Particles 150
Fabric Prescription for Control of Fine Particles 152
Carrier Gas 153
Moisture Effects 153
Artificial Charging 154
Some Filtration Case Histories 156
Agglomeration by Grounding 160
References 160
DESIGNING A FILTER SYSTEM TO MEET SPECIFIED
EFFICIENCY AND EMISSIONS LEVELS 161
Richard L. Adams
LABORATORY GENERATION OF PARTICULATES WITH
EMPHASIS ON SUB-MICRON AEROSOLS
Benjamin Y. H. Liu
Introduction 169
The Vibrating Orifice Monodisperse Aerosol Generator 170
Generation of Sub-Micron Aerosol Standard by
Electrostatic Classification 175
References 178
vi
-------
CONTENTS (continued)
METHODS FOR DETERMINING PARTICULATE MASS AND SIZE
PROPERTIES: LABORATORY AND FIELD MEASUREMENTS
J.D. McCain
Introduction 179
Measurement Techniques as Used in Current Practice 183
Results 195
Conclusions 195
References 198
MOBILE FABRIC FILTER SYSTEM: DESIGN AND PRELIMINARY RESULTS
Robert R. Hall and Reed Cass
Introduction 201
Mobile Fabric Filter System Design 203
Preliminary Results of Field Tests 212
Assessment of Tests and Future Plans 229
References 231
EXTENDING FABRIC FILTER CAPABILITIES
James H. Turner
Introduction 233
Specific Areas for Improvement 238
General Comments 251
Summary 253
References 254
NEW FABRICS AND THEIR POTENTIAL APPLICATION
Lutz Bergmann
Surface Modification of Filter Media 261
Material to be Handled 264
Filter Equipment 267
"Over-Cleaning" or "Puffing" Effect 270
Fiberglass 271
Efficiency of New Media 272
Glamex 272
Needled Fabrics in Shaker and Reverse Air Baghouses 274
Status Summary of Different Industries 277
Summary 281
References 282
NEW KINDS OF FABRIC FILTRATION DEVICES
Melvin W. First
Introduction 283
New Devices 287
Summary 292
References 293
vii
-------
CONTENTS (continued)
NEEDED RESEARCH IN FABRIC FILTRATION
Knowlton J. Caplan
Recirculation 295
Mechanism of Seepage 299
Stack Sampling: Inlet vs. Outlet Conditions 299
Electrostatics 301
An "Underwriters Laboratory" 301
viii
-------
ACKNOWLEDGEMENTS
The editor, and General Chairman of the Symposium, wishes to acknowledge
the capable assistance of Dr. Dennis C. Drehmel of the Environmental
Protection Agency and Mr. Richard Dennis of the GCA/Technology Division
in formulating the symposium program and carrying out its implementation.
IX
-------
THE SIGNIFICANCE OF PARTICULATE EMISSIONS
John K. Burchard
Director, Control Systems Laboratory
Environmental Protection Agency
Research Triangle Park
North Carolina 27711
INTRODUCTION
Fine participates, those solid or liquid aerosols less than 3 microns
in diameter, are the subject of increasing concern as one of the major
air pollutants. Compared to coarse particulate their greater capacity
for obstructing light and their slow settling rate in the atmosphere
cause the limited visibility typical of air pollution haze and smog.
More importantly, fine particulates constitute a health hazard, since
they can bypass the body's respiratory filters and penetrate deep into
the lungs. Further, because these particles can act as transport ve-
hicles for gaseous pollutants, both adsorbed and reacted, the resultant
synergistic effects can be harmful to human health. These problems
associated with fine particulates are intensified by the tendency of
some metallic materials to be highly active, both chemically and
catalytically.
Emissions of fine particulates typically result from physical or chemi-
cal processes, which may include condensed gaseous products or products
of chemical reactions. High temperature processes such as metallurgical
-------
operations and combustion of fossil fuels are major sources. Metal-
lurgical operations are major producers of metal fumes unique to the
process, such as lead, zinc, copper or iron oxide, while combustion
processes produce fuel ash containing a wide spectrum of materials.
Combustion of residual oil, for example, produces vanadium, chromium,
nickel, iron, copper and other highly reactive and catalytic metals.
Some processes emit solid and liquid hydrocarbons such as organic con-
densibles, tars, and carbon particles capable of sorption of more
volatile constituents. Such processes include pyrolysis, incomplete
combustion, vaporization of lubricating or process oils, and chemical
operations related to the textile, refinery, petrochemical, and plas-
tics industries. Forest fires, as well as controlled agricultural
and slash burning, also are sources of fine particulates.
HEALTH EFFECTS
As is frequently the case with non-infectious pollutants and toxicants,
the health effects case against fine particulates is not absolutely
clear cut. First it must be remembered that fine particulates are not
a single pollutant but a large category of pollutants with a common set
of size, transport, and behavioral characteristics. Once dispersed,
fine particulates behave, depending upon their size, like something
between a coarse particle and a gas. They remain suspended and diffuse,
are subject to Brownian motion, follow fluid flow around obstacles, and
can penetrate deep into the respiratory system.
The moderate amount of information that is available concerning this
deposition of particles is based upon mathematical models and experi-
mental data. Particles larger than 5 microns are deposited in the
nasal cavity or nasopharynx, while increasing numbers of smaller par-
ticles are deposited in the lungs. Over 50 percent of the number of
-------
particles between 0.01 and 0.1 microns that penetrate into the pulmonary
compartment will be deposited there. This ability of particulates to
penetrate into the respiratory system and be captured, is principally
a function of their geometry and is relatively independent of the chemi-
cal properties of the particle.
On the other hand, the health effects of the fine particulates that
have penetrated the respiratory system and been captured, are almost
completely dependent on their chemical or toxic nature. It is, there-
fore, not possible to generalize on health effects; specific materials
must be considered. Here the data become sparse and it becomes neces-
sary to draw on our knowledge of toxic characteristics of specific
substances gained from other information sources, and on our under-
standing of physiological mechanisms that work to dispose of collected
materials.
The principal effect on health is through inhalation and direct attack
on the respiratory system. This may result in short term irritant
effects, or longer term damage such as silicosis, asbestosis, chronic
bronchitis, and emphysema. In all these cases the respiratory system
is directly impaired.
A second mechanism of adverse effects involves the respiratory system
indirectly as a significant route of entry for non-respiratory toxi-
cants. In this case, substances which are deposited in the respiratory
system are translocated to the gastro-intestinal system by muco-ciliary
transport and are swallowed. They may then exert a primary toxic effect
directly or be absorbed and translocated to other tissues to exert ad-
verse health effects.
Because of the present scarcity of knowledge concerning the health
effects of specific pollutants and combinations of pollutants, it will
take years to develop the data base necessary to quantify the exact
-------
dose-response characteristics of fine participates. Sufficient infor-
mation does exist, however, to conclude that fine particulates must be
controlled to fairly stringent levels if public health is to be properly
protected.
PARTICULATE EMISSION SOURCES
In 1971, under a project carried out by Midwest Research Institute,
EPA published a systems study of particulate pollutants covering mass
emissions from U. S. industry. It was estimated that gross particulate
emissions in the U. S. totaled 18 million tons per year from various
major industries. These emission figures, based on 1968 production
data, took into account the degree of application of control devices
and their average efficiency.
This study also included estimates of the mass and number of fine
particles emitted, although analysis of the particle size distribution
data then available, indicated that almost all had been obtained using
sampling and sizing procedures that are just not suitable for particles
smaller than about 2 microns. Accurate data on the fractional effi-
ciency of commercial control systems were either completely lacking or
too generalized in nature.
Since reliable particle-size distributions were not available in the
0.01-2 micron range, it was necessary to extrapolate from the available
data for larger particles. This extrapolation was the basis of esti-
mates for fine particulate emissions, both mass and number data, from
the major industrial sources. (Inadequacy of data made it impossible
to make these calculations for agricultural operations, forest products,
clay products, and primary non-ferrous metals, and consequently these
important sources of particulate were not included.)
-------
These mass and number estimates led to the following preliminary
priority ranking of major industrial sources of fine particulate:
1. Ferro-alloy furnaces.
2. Steel-making furnaces.
3. Coal-fired power plants.
4. Lime kilns.
5. Kraft pulp mill recovery furnaces.
6. Municipal incinerators.
7. Iron foundry cupolas.
8. Crushed stone plants.
9. Hot-mix asphalt plants.
10. Cement kilns.
During this study, it became obvious that the severe lack of sub-micron
particle size data was the result of not having adequate sampling, siz-
ing, and particle measurement techniques in the sub-micron range. As
a result, the Control Systems Laboratory of EPA set out to sponsor the
development of such capability. Emphasis was placed on inertial impac-
tors as the most practical approach. This development culminated in a
recent comparison of available inertial impactors in a series of 192
individual measurements on one power plant; this comparison showed that,
when properly used, inertial impactors are reliable for measurement
down to about 0.2 micron. They are now used routinely by EPA personnel
and contractors; more than 50 sets of particle size distribution data
have been generated in the last few months.
In an effort to extend our measurement capabilities to even smaller
particulates, Southern Research Institute, under EPA sponsorship,
recently used a series of diffusion batteries coupled with condensation
-------
nuclei counters, to provide concentration and size distributions by
number over the size range from about 0.01 to 0.3 micron.
FINE PARTICULAR CONTROL
We are already learning some interesting facts about control of fine
particulate using these new measurement techniques. Four tests on
high-efficiency electrostatic precipitators, three on utility boilers
and one on a paper mill recovery boiler, have shown fractional effi-
ciencies better than 90 percent (in some cases better than 98 percent)
all the way down to 0.1 micron. We have also found some high-energy
scrubbers that show good collection capability.
Of particular interest to this fabric filtration oriented group are
the results of testing two baghouses (both with reverse air cleaning)
installed on coal-burning sources. The first is installed on a utility
boiler burning a mixture of anthracite coal tailings and metallurgical
coke. Tests made under standard operating conditions showed greater
than 99 percent removal of all particulate down to 0.1 micron. This
baghouse has operated efficiently and relatively trouble-free for over
1 year with no bag failures.
The second unit is a pilot scale baghouse installed on a slip stream
of an industrial boiler burning bituminous coal and operated at a high
gas-to-cloth ratio. Initial testing again showed greater than 99 per-
cent efficiency down to 0.1 micron.
These tests indicate that certain currently available devices--
including fabric filters—can effectively control fine particulates
under the right conditions. However, it should be emphasized that
the range of applicability of conventional baghouses, precipitators,
-------
and scrubbers to control fine particulates is limited. Baghouses
cannot be used for high temperature clean-up, or in gaseous atmos-
pheres which degrade the fabric, and their size limits applicability
in many retrofit situations. Precipitators are most effective on
particulates of a fairly narrow range of electric resistivity; at
both higher and lower resistivities, control efficiency drops off.
Unfortunately, most low sulfur coals produce high resistivity fly
ash, so that switching to low sulfur western coals decreases the
sulfur oxides problem but increases the particulate control problem.
EPA is intensively studying all facets of the problem of fine par-
ticulates which, of course, is a very complicated one. This includes
characterization of the chemical composition and toxicology of par-
ticulates as a function of particle size and industrial source. As
would be expected, chemical composition varies dramatically depending
on source. For example, particulate emissions from an open-hearth
furnace have been found to be about 90 percent iron oxide, with the
remainder being other metallic oxides and compounds depending on the
source of ore and fluxes used. In contrast, particulate from a cement
plant was 40 percent calcium oxide, 20 percent silicon dioxide, 10 per-
cent iron oxide, and the remainder primarily other metallic oxides.
Fly ash from fossil fuel burning varies markedly in composition depend-
ing on the source of coal and degree and type of combustion. In addi-
tion to substantial quantities of oxides of silicon, aluminum, iron,
and calcium, as many as 30 to 40 additional elements are present in
trace to significant quantities. Most exist at constant levels in all
particle sizes, although some of the more toxic elements appear in in-
creasing concentrations with decreasing particle size.
The complexity of sources of fine particulate emissions, and the
physical and chemical characteristics of the particles, as well as
of the off-gases bearing them, complicate the development of adequate
-------
control technology. In addition to studying the application of
currently available control techniques, we are continuously evalua-
ting numerous new concepts and novel devices. In the long run, we
believe it will be necessary to develop a number of different tech-
niques for control of the wide diversity of sources, and wide variety
of types, of fine particulate.
CONCLUSIONS
You may wonder if we are making any real progress in our goal to
decrease air pollution. In the 4th Annual Report of the Council on
Environmental Quality published in September of 1973, the statement
is made that of the 10 major cities with detailed particulate infor-
mation available, six have shown a general trend toward improved
levels of particulate. These improvements are due to the use of less
polluting fuels, as well as the installation of control devices. How-
ever, the report goes on to state that, although there have been sig-
nificant improvements, a massive effort is still needed to meet air
quality standards. Many areas of the country have ambient levels
which still exceed the primary standards for the six "criteria" pollut-
ants; this situation is worse for particulates than for any other major
pollutant. Combined with the special problems of control of fine par-
ticulate, it is clear that EPA, industry, and the control equipment
manufacturers, working together, have a difficult and challenging task
to accomplish in this area. Improved fabric filtration will be an
important step toward the successful accomplishment of this task.
-------
EMISSION STANDARDS FOR PARTICIPATES
George W. Walsh
Assistant to the Director
Emission Standards and Engineering Division
Environmental Protection Agency
Research Triangle Park
North Carolina 27711
The Emission Standards and Engineering Division is responsible for the
promulgation of standards of performance under Section 111 and national
emission standards for hazardous pollutants under Section 112 of the
Clean Air Act. The Division also serves as a primary source of techni-
cal expertise in defining stationary source control measures for State
implementation plans. Different criteria are applied when defining
stationary source emission standards, depending on which Section of the
Clean Air Act is being implemented. For State implementation plans we
are, for example, generally concerned with reasonably available control
for existing sources. Under Section 112, the criteria is one of pro-
viding an ample margin of safety to protect public health. In any case,
a major deficiency in developing emission standards is the lack of pre-
dictive capability when baghouses are intended as the control device.
This is best illustrated by confining this discussion to standards of
performance for new stationary sources. For purposes of orientation,
let me briefly review this Section of the Clean Air Act.
As defined in the Act, standards of performance are restricted in
application to new facilities, and must "reflect the application of
-------
best system of emission reduction." The term "best system of emission
reduction" was chosen to emphasize the concept that pollution control
includes the selection of raw materials, the manufacturing process, and
performance of the control device. Therefore, the potential exists for
the application of new technology, provided one can demonstrate that
the standards are achievable. Cost is a factor to be considered, but a
cost/benefit analysis is not mandatory. According to Congress, the
costs must be reasonable in terms of the economics of the industry for
which standards are being set. To date, except for standards on sulfur
oxides from fuel combustion, consideration of raw materials has been
minimal. Indeed, when faced with issues on the impact of raw materials
on control system performance, we have generally tended toward a relax-
ation of the standards. This has occurred because relationships between
raw material characteristics and system performance are not generally
available.
Since standards of performance must reflect "the best system of emission
reduction," our data base is always limited. A purist philosophy leads
to the conclusion that the standards should be based on a single system.
The word "reflects," however, provides some relief and the "necessary
consideration of costs" provides increased maneuverability. Nonetheless,
we are clearly not attempting to define averages of the most probable
emission rates given the application of some generic control device to a
large number of sources.
With the goal being a definition of best technology, it should be clear
that highly detailed design and operating characteristics are important.
For example, fabric structure, bag length-to-diameter ratio or filter
spacing may affect performance in a subtle but significant manner.
Given the range of variables, it is an almost impossible task for our
engineers co identify which dei-ails are important:, without: ou«_n. aa
identification, however, we are constantly faced with the need to relax
our recommended standards. This, in turn, does not force technology to
any great degree. Some basic field investigations are needed, therefore,
10
-------
which would define which parameters are critical to performance and
which are important only from a manufacturers viewpoint.
The need to better delineate control system performance as a function of
raw material characteristics, process variables, and collector design
parameters can be better appreciated by examining the full standard-
setting process.
In addition to the collection and analysis of hard data to arrive at
some suitable recommended number, the standard-setting process involves
an examination of energy consumption, economics and environmental impact.
All of this information is then subject to review, both in and out of
EPA. As you can imagine, the review process is generally one of
destructive criticism. To off-set the questions raised, many of which
refer to conditions not experienced at test sites, more and better
defined correlations are needed between equipment design, process
variables, and raw material characteristics. For example, in setting
standards of performance for asphalt plants a major issue was the
influence of raw material size on baghouse efficiency. Three size
distributions come into play: (a) the size distribution of the sand
before the drier, (b) the size distribution of entrained particles
leaving the drier; and (c) the size distribution of the particulate
matter as it approaches the fabric itself. It is generally accepted
that significant differences can exist from one point to another. It
can also be theorized that the geometry of the baghouse is important in
determining what the filter actually sees. Some quantification in this
area would be extremely helpful so we can better define system capability
under a variety of conditions.
The problem of adequately defining baghouse efficiency still haunts us.
Many of the systems we investigate emit gas streams that follow a
cyclical pattern. Without some predictive capability our testing must
include the entire cycle. With such general data we are never sure what
11
-------
happens when the process is changed, when the filter cleaning cycle is
altered, or when testing time is different. Basic oxygen furnaces and
electric arc furnaces are good examples of this problem.
Increased knowledge about fabric filter performance is also important
if we are ever to be able to set standards on similar processes without
testing. Why, for example, can we not establish an emission standard
for lime kilns based on our knowledge of how to control cement kilns?
The answer, of course is that we cannot demonstrate that the standards
would be achievable. Therefore, each source represents a new problem
and the rate of standard setting is decreased.
Our ability to predict collector performance is very important to the
issuance of modifications. The definition of a new source in Section
111 of the Clean Air Act includes construction (which is obvious) and
modification (which is not so obvious). A modification is defined as
any "physical change or change in method of operation which increased
The question which must be answered is the impact of physical changes or
changes in method of operation on emissions from an existing collector.
If the emissions are predicted to increase, then the source will be
modified, and the standards of performance applied. This, in turn,
raises a second question of how to upgrade collector performance so that
emissions will not increase or to achieve the standard of performance.
As you can imagine, answers to these questions are important to both the
regulated and the regulator.
Although a great amount of effort has gone into particulate control,
and even though a majority of the standards of performance are concerned
with particulates, we have not achieved any great degree of expertise in
setting standards. We tend to rely on broad categories of equipment,
and have not yet really been able to project the difference between
12
-------
baghouse A and baghouse B. This ability, however, is of great impor-
tance when dealing with modifications to existing plants which are
already controlled. In such a situation we like to know how the exist-
ing equipment can be up-graded to comply with the standards of perfor-
mance. This knowledge will be indispensable in deciding what is and
what is not a modification and who should and who should not be subject
to Section 111 of the Act. Our ability to predict collector performance
must also be increased to better answer the many questions raised during
the standard setting process and to enable a transfer of performance
from one source category to another.
13
-------
PERFORMANCE AND COST COMPARISONS BETWEEN FABRIC FILTERS AND
ALTERNATE PARTICULATE CONTROL TECHNIQUES
J. D. McKenna, J. C. Mycock &W. 0. Lipscotnb
Enviro-Systems & Research, Inc.
P.O. Box 658
Roanoke, Virginia 24004
INTRODUCTION
A study was conducted to evaluate performance and cost comparisons of
fabric filters and alternate fine particulate control techniques. In
relating the removal of fine particulate to costs, due to the lack of
fractional efficiency data, it was found necessary to treat a specific
application in order to make the study manageable. The case chosen is
that of the coal fired industrial boiler since Enviro-Systems has a
pilot program in this application area.
Thus, as a preamble to the economic comparisons a brief discussion of
the pilot plant activity and related economics will be made. This dis-
cussion will be followed by a treatment of the particulate removal capa-
bilities of electrostatic precipitators, fabric filters and venturi
scrubbers. Then, on the basis of the specific application of interest,
a comparison of the capital, operating and annualized costs will be
made for these three conventional methods of dust removal. Presently
very little actual field data is available on the fine particulate
removal capability of the above mentioned control techniques. Therefore,
it is necessary to base the economic comparisons upon a severely limited
amount of existing fractional efficiency data combined with extrapola-
tions of performance at larger particle sizes as suggested by Craig.
15
-------
PILOT PLANT ACTIVITY
2 3
As previously reported, ' the Enviro-Systems & Research fabric filter
pilot plant was installed at Kerr Industries in Concord, North Carolina.
This effort was jointly sponsored by the Environmental Protection Agency,
Kerr Industries and Enviro-Systems & Research. A slip stream for the
pilot facility was installed on a Babcock and Wilcox boiler with a
design capacity of 60,000 Ib/hr. steam. Stack sampling conducted in
January, 1973, indicated emission rates of 130 Ib/hr. Gas volumes were
determined to be around 35,000 acfm per boiler, at a temperature of
355 F which gives a grain loading of 0.4 gr/acfm. Neither the slip
stream duct nor the baghouse was insulated. Orsat analysis indicated
9.5 percent C02, 10 percent 02> 0 percent CO and 80.5 percent N». Coal
analysis showed a sulfur content of about 0.6 percent. Analysis of
inlet flue gas to the pilot unit indicated S0» concentrations between
3 and 8 ppm and S0» concentrations between 240 and 370 ppm.
The pilot plant was capable of handling 11,000 acfm of flue gas when
operating at an air-to-cloth ratio of 6/1. The baghouse is divided into
four cells, each containing 54 bags. The bags are 5" in diameter by
8' 8" long giving 11.5 square feet per bag, 620 square feet per cell
and 2,480 square feet for the house. The bags are set into a tube
sheet located at the top of the house by two snap rings incorporated
into the bag itself. A spiral cage is set inside the bag to prevent
collapsing. The dirty gases enter one end of the unit, see Figure 1,
pass thru the tapered duct, into the classifier, then through the bags.
The classifier forces the dirty gases to change direction 90 , then
180 . This quick directional change forces the larger and heavier par-
ticles out of the flow so they fall directly into the hopper. Gases
are forced thru the fabric filter into the center of the bags, leaving
the particulate on the outer surface of the bags where it is removed
periodically during the cleaning cycle. The cleaned gases are drawn up
and out through the center of the filtering bag into a center exit plenum
via an open damper in the cell above the tube sheet. The bags are
16
-------
Figure 1. Baghouse flow and collection diagram
-------
cleaned one cell at a time by closing off the cell damper to the exit
plenum and at the same time opening a reverse air damper. As the solid
matter collects on the outside of the filter bag, it builds a cake or
crust which begins to restrict the flow of the gases. During the
cleaning cycle, clean air enters the cell thru the reverse air damper.
The clean air is forced down the filter bag, opposite to the normal
flow direction. Damper system and control panel arrangements allow
for variations in main gas volume, reverse air volume duration of clean-
ing and frequency of cleaning. The existance of four cells allows for
repetitive sequential testing of different bag types without the need
to change bags.
Inlet particle size and analysis was obtained by the use of Anderson
and Pilat (University of Washington) in-situ particle size analyzers.
The results of these tests are shown in Figure 2.
Outlet particle size analyses were conducted at two air-to-cloth ratios
of approximately 3 and 6 ft./min. These results are shown in Figure 3.
Raising the A/C from 3 to 6 increased the outlet loading from .03 mg/SCF
to .06 mg/SCF. Table 1 shows that even at the higher gas velocity the
overall efficiency was still greater than 99.5 percent.
Installed costs were determined for a fabric filter dust collector sized
for 70,000 ACFM at 250°F and using Nomex felt as the filter material.
It was assumed that the house and hopper would be insulated and the bags
would be continuously coated with lime to prevent filter media deteriora-
tion due to acid dew point excursions. Air-to-cloth ratios considered
were 4.3, 6.3 and 7.5. As shown in Figure 4, the corresponding costs
were found to be $164,000, $141,700 and $104,000 or on the basis of
$/ACFM, they are $2.34, $2.02 and $1.48 respectively. These estimates
were based on a bag price of $15.50 each.
Annual operating costs were also determined based on 25 percent bag re-
placement per year and a pressure drop of 5 inches of water. These were
18
-------
c
o
l_
o
"i
UJ
2
< 0.8
Q
LJ
_l
O
fe
<
Q.
0.6
0.4
0.2
O PILAT
A
ANDERSON - BOILER LOAD 90%
D PILAT- BOILER LOAD 40%
O.I
5 10 20 30 40 50 60 70
% LESS THAN SIZE INDICATED
80
90
Figure 2. Inlet particle size distribution
19
-------
N)
O
10
9
8
7
6
5
r
i
UJ 2
N
to
_
O
I-
< i.o
2 0.9
0.8
0.7
0.6
0.5
0.4
0.9
o
OUTLET PARTICLE SIZE
DISTRIBUTION NOMEX FELT
AIR/CLOTH= 3/|
AIR/CLOTH = 6/1
I
0.1 0.2 OS 1.0 2
5 10 20 SO 4O SO 6O 7O BO
% LESS THAN SIZE INDICATED
90 95
98 99
Figure 3. Outlet particle size as a function of air-to-cloth ratio
-------
ISO
CASE: Notnex Felt, Reverse
Air Cleaning,
Insulated Lime
Coated
o
"o
•o
IO
O
en
O
o
o
LU
100
50
5 6 7 8 9 K)
AIR-to-CLOTH RATIO, ACFM/ft2
II
Figure 4. Installed cost vs. air-to-cloth
ration, Nomex felt
21
-------
Table 1. DUST REMOVAL EFFICIENCIES
Filter Media Nomex Felt
Air-to-Cloth Ratio ~6
Particle Diameter
> 9.5
6
4
2.8
1.75
0.9
0.54
0.36
< 0.36
Total
Inlet Load
mg/scf
4.221
2.292
1.482
1.254
0.8893
0.6844
0.4179
0.1920
0.7062
12 . 140
Outlet Load
mg/scf
.0068
.0060
.0033
.0039
.0175
.0054
.0035
.0060
.0072
.0596
Remova 1
%
99.84
99.74
99.78
99.69
98.03
99.21
99.16
96.88
98.98
99.51
found to be $13,270, $11,620 and $10,770 for air-to-cloth ratios of
4.3, 6.3 and 7.5 respectively. The operating costs vs. air-to-cloth
ratio are shown in Figure 5.
Annualized costs were computed from installed and operating costs, see
Figure 6. These were $35,000, $30,460 and $24,600 for the same air-to-
cloth ratios.
Development of the operating and annualized costs for the foregoing
fabric filter case, as well as the subsequent cases for electrostatic
precipitators and wet scrubbers, employed the formulae published by
4
Edmisten and Bunyard.
Capital costs for the fabric filter and venturi scrubber were based upon
Envirp-Systems & Research selling prices as of March, 1974. Electro-
static precipitator capital costs were based upon two independent vendor
quotations obtained in March, 1974.
22
-------
20-
CO
S-
(O
co
o
to
+J
CO
o
CO
S-
O)
ex
o
03
10-
5-
CASE:
REVERSE AIR CLEANING
25% BAG REPLACEMENT/
YEAR, POWER COSTS -
$.0175/KWH, OPERATING
TIME - 6,240 HRS/YR,
A P - 5X H20
8 9 10
11
Air-to-Cloth (ACFM/Ft. )
Figure 5. Annual operating costs versus
air-to-cloth ratio for Nomex
felt
23
-------
40-
s-
fO
co
CD
o
s_
30 -
o
o
-o
Q)
20
6789
Air-to-Cloth (ACFM, Ft.2)
10 11
Figure 6. Annualized cost versus air-to-cloth ratio
for Notnex felt
24
-------
EFFICIENCY VERSUS PARTICLE SIZE
Fabric filters, electrostatic precipitators and high energy scrubbers
are capable of achieving greater than 99 percent overall efficiency for
removal of fly ash particulates. There is, however, a significant dif-
ference in removal efficiencies when considering fine particles; i.e.
those less than 2 microns.
In removal of submicron particulate by fabric filtration, mechanisms
different from those which control removal of larger particles dominate.
The removal of particles greater than one micron is considered to be
controlled by impaction and interception while in the submicron region
diffusion and electrostatic attraction are considered the important
factors.
The efficiency of the fabric filter vs. fly ash particle diameter at
air-to-cloth ratios of 3/1 and 6/1 is shown in Figure 7. This curve
shows highest efficiencies at either end of the particle size range.
This difference is explained readily by classical filtration theory;
that is, different filtering mechanisms are responsible for entrapment
of fine particles as opposed to large particles. Particles less than
0.36 microns were removed at efficiencies greater than 99.9 percent.
This is an extremely important consideration when attempting to control
hazardous particles in the respirable range. It is notable that both
diffusion mechanism controls in the range below about 0.2 microns and
that for this mechanism the efficiency is inversely proportional to the
gas velocity. As seen in Figure 7, the removal efficiency at the size
range below 0.36 microns shows a sharp decrease as the velocity in-
creases. Unfortunately, very little data is available at this time, and
therefore confirmation of Figure 7 is presently underway. Increasing
the air-to-cloth ratio from 3/1 to 6/1 increased the outlet loading, how-
ever, overall efficiency remained greater than 99.5 percent.
25
-------
o
I—I
o
LLJ
_l
O
I
,001
Key Air/Cloth
o
o
6/1
3/1
.0001 i i 1 1
.1 1.0
1 I 1 1 1 1 1 1 1
10
PARTICLE DIAMETER (MICRONS)
Figure 7. Fabric filtration collection efficiency versus
particle diameter
26
-------
The efficiency of an existing electrostatic precipitator is a function
of the precipitator size (collection area) as well as the charge on the
dust and the electric field of collection. The particle size and the
electrical resistivity of the dust, the gas composition and the temper-
ature, all influence the precipitator efficiency. Of these, for the
case considered, the most influential is the resistivity of the fly ash.
Q
The highest precipitator efficiency is obtained in the range of 1 X 10
to 1 X 10 ohm-cm. For the case considered here, the sulfur content of
the coal is very significant since the resistivity of low sulfur coals is
generally higher than the optimum range and therefore difficult to con-
trol. A problem encountered in this study is that sizing a precipitator
is difficult, for as Oglesby points out, for fly ash precipitators the
size required for 99 percent collection efficiency can vary from
2 2
150 ft. /1000 cfm to about 450 ft. /1000 cfm, depending upon the pro-
perties of the fly ash. As is known for the case of coal fly ash, the
composition varies greatly, depending upon a number of source and operat-
ing factors. Thus, generalization of the economics and performance
and electrostatic precipitators as applied to industrial boilers is a
more difficult problem than either fabric filters or venturi scrubbers,
applied to the same case.
The efficiency of electrostatic precipitators for fly ash particulates
as projected by Oglesby from the Deutsch equation is given in Figure 8.
As he notes: "Some of the limitations of using the Deutsch-Anderson
equation become apparent as one attempts to extrapolate to the required
size from lower efficiency tests that use constant precipitation rate
parameter." Figure 9 shows actual field data for electrostatic precip-
itator efficiency here are significantly higher than that in Figure 8;
it is still not as high as the fabric filter case. This curve does,
however, show an increase in efficiency for the smallest particle dia-
Q
meter. Craig also noted this and reported the existence of a bimodal
distribution. Similarly, he reported 99 percent for greater than 2 mi-
crons, down to 92 percent in the 0.2 to 0.5 micron range then increasing
to 95-97 percent in the 0.05 to 0.1 micron and then falling off again.
27
-------
I— 1.0
— 0.1
— 0.01
0.001
. .1
I I I
0.1
1.0 10
PARTICLE SIZE - MICRONS
100
Figure 8. Electrostatic precipitation fly ash
collection efficiency versus
particle diameter
28
-------
UJ
HH
O
LiJ
I
rO.2
• 0.1
.08
.06
.04
L .02
• .01
.006
.004
- .002
.001
. . . ..I
0.1 0.2 0.4 0.6 0.8 1.0 2
PARTICLE DIAMETER - MICRONS
8 10
Figure 9. Electrostatic precipitation field data collection
efficiency versus particle diameter
29
-------
For venturi scrubbers, the dust removal efficiency is mainly a function
of the energy consumed in the dust to liquid contact. Since this con-
tact is achieved by passing the liquid and dust laden gas through the
venturi throat or orifice where the velocity is increased dramatically,
the efficiency is primarily a function of the pressure drop across the
throat. While there is some difficulty in establishing a purely theo-
retical model, good empirical correllation of efficiency and pressure
drop exist and take into account particle size distribution and density.
For a given pressure drop and particle density, the smaller the particle
the lower the removal efficiency. Figure 10, Scrubber Efficiency vs.
Particle Diameter, shows efficiencies for fine particulates dropping
off to the order of 90-95 percent, even at high energy levels.
ECONOMICS VERSUS EFFICIENCY
As stated previously, in order to make economic comparisons between
particulate control techniques, a specific area of application was
chosen. That application is the coal fired industrial boiler. All
costs were based on facilities sized for a stoker fed industrial boiler
burning 0.6 percent sulfur coal. The gas volume chosen is 70,000 acfm
(two boiler basis) at a temperature of 250 F. An attempt will be made
to compare capital, operating and annualized costs and to discuss the
pertinent factors affecting each for the three methods of control being
considered.
Figures 11, 12 and 13 show capital, operating and annualized costs for
a fabric filter at two levels of efficiency, 99.5 and 99.75 percent,
based upon the assumption that the pilot plant data can be confirmed.
These efficiency levels correspond to air-to-cloth ratios of 6/1 and 3/1
respectively. To go from 99.5 to 99.75 percent efficiency, assuming
the relationships between air-to-cloth ratio and removal efficiency,
the size of the baghouse increases as well as the number of bags employed;
thus, capital cost increases accordingly. Annual operating costs were
30
-------
,— 1.0
M
O
M
w
o
H
H
W
i-J
8
I
0.1 0.2 0.4 0.6 1.0 2 4
PARTICLE DIAMETER (MICRONS)
Figure 10. Venturi scrubber collection efficiency
versus particle diameter
10
31
-------
—400
- 350
—300
- 250
—200
- 150
—TOO
- 50
I I
I I
DUST COLLECTOR EFFICIENCY
99.5 99.75
Figure 11. Fabric filter installed costs
32
-------
20 H
o
Q
co
o
X
o
O
CD
(O
s_
O)
Q.
O
(O
10
99.51 99.75
Dust Removal Efficiency, %
Figure 12. Fabric filtration operating costs
33
-------
150r
CO
on
oo
o
100
CO
CD
O
LU
50
99.75
99.5
REMOVAL EFFICIENCY
Figure 13. Fabric filtration annualized cost of control
34
-------
computed on the basis of 25 percent bag replacement per year and opera-
tion at a pressure drop of 5 inches of water. The cost of bag replace-
ment is the most significant factor in operating costs. Annualized
costs were based upon 15 year straight line depreciation.
Figures 14, 15 and 16 show capital, operating and annualized costs for
an electrostatic precipitator at different efficiency levels. The
levels of efficiency are a little different between cases but are still
useful for the sake of comparison. Installed or capital costs are
proportional to the size of the electrostatic precipitator which is in
turn dictated by the efficiency required. Annual operating costs were
based on operating 6,240 hours/year, power costs of $0.0176/kw-hr and
precipitator power required of 0.004 kw/ACFM. The cost of power is the
most significant factor here. Annualized costs were again based upon a
15 year depreciation period.
Figures 17, 18 and 19 show capital, operating and annualized costs for
a venturi scrubber at two levels of efficiency -- 97 and 99 percent.
These efficiency levels correspond to 20" and 60" of pressure w.g. res-
pectively. Fan size is obviously an important capital cost element in
attaining the higher efficiency. Both sizes are based on using 316
stainless steel due to the abrasive and corrosive nature of the source.
The cost of attaining higher efficiency is more than twice that of the
lower efficiency. Annual operating costs were based on operating
6,240 hours/year and power rates of $0.0176/kw-hr. Here the cost of
power dramatically increases operating cost as one goes to the higher
efficiency level. This is due to the large increase in pressure drop
required for high efficiency removal of fine particles. Annualized
costs, based upon 15 year straight line depreciation, also show the high
cost required to achieve 99 percent efficiency.
Installed, operating and annualized costs for the three particulate con-
trol methods are consolidated in Figures 20, 21 and 22. The same costs
are also presented in Table 2 for easier reference. The fabric filter
35
-------
co
OL
o
o
250
oo
co
200
150 t
98.0 98.5 99.0
DUST REMOVAL EFFICIENCY, %
Figure 14. Electrostatic precipitator installed cost
36
99.5
-------
20-
in
-------
60-
s_
to
o
o
ro
o
X
o
S-
50-
40-
30.
o
O
-o
(U
N
to
20-
10-
97.5 99.5
Dust Removal Efficiency, %
Figure 15. Electrostatic precipitator annualized cost of control
38
-------
co
P- 350
1—300
I- 250
co
o
I—200
co
CO
o
o
co
I- 150
I—100
h 90
95
96
97
98
99
DUST REMOVAL EFFICIENCY, %
Figure 17. Venturi scrubbing installed costs
39
-------
160
140
l/l
* 120
"o
O
CO
O
X
to
to
o
2.
o
ZJ
sz
100
80
60
40
20
97
98 99 100
Removal Efficiency, %
Figure 18. Venturi scrubbing stoker fly ash
operating costs
40
-------
150
co
co
o
X
o
O
O
o
o
UJ
-------
300
99.5
99.0
oo
a:
o
o
co
o
X
oo
oo
O
o
oo
200
98.0
99.75
97.0
99.5
100
Electrostatic
Precipitation
Wet
Scrubbing
Fabric
Filtration
Figure 20. Installed cost comparisons at varying efficiencies
42
-------
99.
6G
40U
20U
s
LU
Q.
o
10-
8
6
99.5
97.5
99.75
99.5
98.8
97.2
Electrostatic
Predpltator
Fabric
Filter
VentuM
Scrubber
Figure 21. Operating cost comparisons
for varying efficiencies
43
-------
r—200
L 150
s
PO
o
s
Lioo
L 50
99.75
99.5
99.5
97.5
97
Fabric
Filtration
Electrostatic
Precipitation
Venturl
Scrubber
Figure 22. Annualized cost for varying
efficiencies
44
-------
Table 2. CONTROL COSTS
Control method
Fabric filter
Capital
Operating
Annualized
Electrostatic
Precipitator
Capital
Operating
Annualized
Venturi scrubber
Capital
Operating
Annualized
97 %
147,500
35,315
54,933
97.5 %
200,000
7,084
33,684
98 %
202,300
99 %
250,000
316,000
94,941
136,969
99.1 %
99.5 %
140,000
12,700
35,000
279,200
8,155
45,289
99.75 %
200,000
15,750
has the lowest capital cost while achieving the highest efficiency. The
electrostatic precipitator is second best with respect to capital cost
and efficiency. Scrubbers would appear to be a poor choice due to both
high capital costs and lower efficiency.
Considering annual operating costs, the positions of the fabric filter
and the electrostatic precipitator are reversed. Electrostatic precipi-
tators cost approximately 25 percent less to operate at the 99.5 percent
efficiency level than fabric filters. Scrubbers again appear to be a
poor choice with high operating costs due to large power requirement.
The scrubber operating costs are two to three times those for either
electrostatic precipitators or fabric filters.
Looking at the overall picture given by annualized costs, we find fabric
filters roughly 20-25 percent less expensive to install. Here again the
fabric filter not only compares best with respect to annualized costs,
achieves higher overall efficiency, and as we have seen higher efficiency
for removal of fine particulates. The annualized cost of scrubbers only
45
-------
serve to demonstrate again their inability to compete with the other
control methods in the control of fine participates.
CONCLUSIONS
Generalizations of the cost of fine particulate removal are inherently
dangerous. It does appear, however, that of the conventional routes to
particulate control, only fabric filters and electrostatic precipitators
are suitable to fine particulate removal. Venturi scrubbing, because
of the high energy required, does not compete economically. The
particular case studied probably exaggerated the economic shortcomings
of the venturi since the scrubber needed to be stainless steel. No
consideration is given here to the application of steam ejector Venturis
which have demonstrated the ability to achieve higher efficiencies than
9
the conventional venturi scrubber. This and other high energy scrub-
bing techniques in the development stage have not been considered in
this paper due to the uncertain economics at this point in time.
The case of the fabric filter chosen, Nomex felt with lime injection,
is yet unproven and if the more expensive Teflon bags are required the
electrostatic precipitator will look even better. On the other hand,
if a filter media innovation such as Gore-Tex allows for the use of
higher air-to-cloth ratios, a dramatic reduction in the capital and
annualized costs of employing fabric filtration can be realized. Since
the economics of the electrostatic precipitator turn out to be very .
sensitive to the resistivity of the dust one must employ caution when
generalizing on economic comparisons from this specific case.
Very little fine particulate data is available at this point; therefore,
the cost developments had to be based on a number of assumptions. Ideally,
in order to develop these economic comparisons an electrostatic pre-
cipitator and a venturi pilot plant should be run on the same slip stream
and the in-situ particle size measured on the outlet from each.
46
-------
We hope to answer some of the questions remaining with respect to
fabric filtration with an EPA sponsored pilot plant now getting underway
at Kerr Industries.
All of the foregoing economics will need updating shortly for two rea-
sons. Firstly, at the time of this writing the first major effort is
underway to collect fractional efficiency data on field installations
of all three conventional dust control techniques. Secondly, these
economics have been executed in a period of unusually high inflation
rates and therefore are subject to significant upward fluctuations in
price.
REFERENCES
1. Craig, A. B., Overview of the Fine Particulate Problem, Proceedings
of Symposium on Control of Fine Particulate Emissions from Industrial
Sources/Joint U.S. - U.S.S.R. Working Group Stationary Source Air
Pollution Control Technology, January 15-18, 1974.
2. McKenna, J. D., The Application of Fabric Filter Dust Collectors to
Coal Fired Boilers, Presented at the Fourth Annual Environmental
Engineering and Science Conference, Louisville, Kentucky, March 4,
5, 1974.
3. McKenna, J. D. and Weisberg, R. , A Pilot Scale Investigation of
Fabric Filtration as Applied to Coal Fired Industrial Boilers, Pro-
ceedings of the Fourth Annual Industrial Air Pollution Control Con-
ference, University of Tennessee, March 28-29, 1974, p. 271.
4. Edmisten, N. G. and Bunyard, F. L., A Systematic Procedure for
Determining the Cost of Controlling Particulate Emissions from
Industrial Sources, JAPCA, V20N7, July 1970, p. 446.
5. Billings, C. E., Wilder, J., Handbook of Fabric Filter Technology,
Volume 1, p. 2-98, PB 200 648. National Technical Information
Service, U. S. Department of Commerce.
6. Oglesby, S., Design Considerations for Electrostatic Precipitators,
Proceedings: Design, Operation and Maintenance of High Efficiency
Particulate Control Equipment Specialty Conference, The Greater St.
Louis Section, APCA, March 1973, p. 41.
47
-------
7. ibid, p. 44
8. Craig, A. B., op. cit.
9. McCain, J. D. and Smith, W. B., Lone Star Steel Steam-Hydro Air
Cleaning System Evaluation, Environmental Protection Technology
Series, EPA 650/2-74-028, April 1974.
10. Calvert, S., Goldshmid, J., Leith, D., and Jhaveri, N. , Feasibility
of Flux Force/Condensation for Fine Particulate Collection, National
Technical Information Service, PB-227-307, U. S. Department of
Commerce, October, 1973.
48
-------
TYPES OF FABRIC FILTER INSTALLATIONS
Robert E. Frey
Vice President
The Torit Corporation
P. 0. Box 3217
St. Paul, Minnesota 55165
The market for fabric filters is currently running at the rate of
approximately 90 million dollars per year, flange to flange, based
upon IGCI Statistics, which represents approximately 90 percent of
the total manufactured. This breaks down into approximately 45 mil-
lion of high energy cleaned collectors and 45 million of low energy
collectors. The definition of high energy means that the cleaning
is done with high energy and primarily consists of Pulse Jet con-
figurations such as depicted in Figure 1. The low energy methods of
cleaning are shaking and reverse flow. Figure 2 shows a shaker system
operating in a grinding booth application. While the Pulse Jet or
other similarly configured units make up the high energy portion, the
low energy portion is split approximately 50-50 between continuous
and intermittent collectors. The continuous portion includes approxi-
mately 15 million dollars of large structural baghouse work, while the
intermittent portion contains approximately 10 million dollars of in-
plant size collectors. The remainder are made up of medium size types.
Table 1 provides a basic classification scheme for Fabric Filters.
49
-------
-.
-
Figure 1. Large pulse-jet installation
-------
Figure 2. Torit tube house dust collectors serve huge grinding
booth inside Allis-Chalmers Plant
:
-------
Table 1. TYPES OF FABRIC FILTERS
1. Cleaning energy level
A. High
B. Low
C. None
2. Fabric
A. Felt
B. Woven
C. Other
3. Duty
A. Continuous
B. Intermittent
C. Fail safe
4. Service
A. Particulate removal
B. Gaseous control (adsorption)
C. Process
D. Non-process (nuisance dust)
5. Application factors
A. Temperature
B. Dust loading
C. Moisture level
D. Housing suction or pressure
E. Size
F. Efficiency
The first classification of cleaning energy level follows the IGCI
figures and breaks it down into high and low energy cleaning methods.
In addition, we add a third category of cleaning energy which is 0,
this applies to units that are designed for a disposable media situa-
tion. The second classification is fabric and the basic types are:
a) felt and b) woven. The felt media is normally connected with the
high energy cleaning system, whereas, the woven media or woven cloth
Is connected with the lower energy of cleaning. The felt material is
a true filter media and may be kept as clean as possible, hence the
application to the higher energy system, whereas, the woven fabric is
52
-------
only, in general, a site upon which the true filter media or the dust
cake builds. A third classification of fabric would be an "other"
category where the media is a non-woven disposable configuration.
Another important classification is the duty. Under this we have the
basic split between continuous and intermittent collectors. Figure 3
shows the performance of an intermittent collector versus time, compared
to a continuous automatic unit.
*s CONTINUOUS AUTOMATIC
a
~^->
CJ
«- SEE NOTE A
CLEANING CYCLE
TYP. 5 MIN.
OPERATING CYCLE
TYP.
\ HR. TO
4 HRS.
V
INTERMITTENT
TIME
Figure 3. Performance of an Intermittent collector versus time
compared to a Continuous Automatic Unit
Note A - Reduction of cfm during operating cycle is a function of the
air moving device performance curve.
The third basic type of duty would be considered a fail-safe configura-
tion, where a maintenance spare compartment is provided so that cleaning
and repairs will take place during normal operation, and the unit has
full 24-hour, 7 day a week availability under all conditions.
The fourth major classification of fabric filters is the service. There
are two categories of service and the first is for particulate removal
or for gaseous control by way of adsorption. This technique is well prov-
en in industries, such as, primary aluminum where alumina is used quite
successfully to adsorb a gaseous fluorine, which otherwise would go to
53
-------
the atmosphere. In addition, other gaseous control situations would
involve the use of activated carbon in a fluid bed or dry scrubber to
remove odors.
The second basic distinction under the service is between process and
non-process work. The process function of a fabric filter may be such
applications as venting of dryers, where the full product is handled or
perhaps the use as an air conveying receiver, again where product
collection is the primary function. The non-process applications are
the typical nuisance dust venting jobs where mechanical conveyors and
other dust producing sources are properly hooded and then ducted to a
fabric filter.
It is interesting to note that with today's emphasis on pollution control,
even non-process collectors must receive the kind of care and attention
that only process collectors received in years past, so that the plant
emissions are held to an absolute minimum. In effect, the non-process
nuisance dust collector is a license to operate the plant and must be
kept operating properly.
The final classification would be by application factors and these are
temperature, dust loading, moisture level, housing rating, size and
efficiency. The current maximum temperature limit for fabric filters is
in the range of 550 F with the use of fiberglass media, although work
is currently proceeding to develop higher temperature medias. It should
be pointed out that a significantly higher temperature media may not be
too practical, since fabric filters are sized on an actual cfm basis.
Therefore, the current practice of conditioning the gases to some
temperature below 550 F may, in fact, prove to be the most economical
solution for most applications rather than attempting to handle the
scream ac mucn nigner temperatures. exceptions to this, o± course, would
be cases where energy may be recovered following filtration at much
hotter gas temperatures, assuming the media to be available.
54
-------
The dust loading factor is one that is often misunderstood. The pros
and cons of using a mechanical collector or cyclone ahead of a fabric
filter are somewhat complex. In the case of an intermittent collector,
it is often necessary to use a mechanical collector prior to the bag
filter in order to keep the dust load with time to an absolute minimum
and enable the unit to run a reasonable number of hours before the shut-
down and cleaning occurs. However, it must be born in mind that a
mechanical collector will remove most coarse material leaving only the
fine material for the fabric collector. This may result in changed
performance, since fine material only is more difficult to filter. In
the case of Pulse Jet Collectors, there is a singular insensitivity to
dust load above a certain level. In other words, any amount of dust
capable of being carried by the air can be handled in the collector
without resorting to a primary separator.
The next major application factor would be the moisture level and here
it should be pointed out that fabric filters can be engineered to
operate at extremely high moisture levels up to the greater than 90 per-
cent moisture vapor range, as long as the proper engineering precautions
are taken. Included among these are insulation, addition of heat and
proper control so that the moisture is always kept in vapor form and
the bags will remain in good condition for filtration. If a standard
collector is put on a high moisture situation without proper design of
the system, the bags are quite likely to turn into architectural columns
and be totally useless in a short while.
Housing suction and/or pressure becomes a rather obvious classification
affecting the size and shape of the housing and, of course, fabric
filters can be designed to operate at pressure in excess of 200 psi and
quite commonly at vacuums of half an atmosphere or 15 inches of mercury.
The normal collector is designed and operated for typical suctions pro-
duced by industrial exhaust fans and this is in the range of plus or
minus 20 inches of water.
55
-------
The subject of fabric filter efficiency is generally an academic
problem, because properly applied fabric filters approach 100 percent
efficiency in most cases and will perform at highly satisfactory
efficiency levels, assuming the proper maintenance is performed.
Efficiency problems with fabric collectors are generally associated with
installations that are of inadequate cloth area to do the job properly.
This results in poor operation from a differential pressure standpoint
and may cause considerable seepage because of the higher than proper
filtration velocities.
56
-------
COMPARISON OF FINE PARTICLE CAPTURE IN FIBER
STRUCTURES AND FILTER CAKES
Charles E. Billings
Environmental Engineering
Science Consultant
740 Boylston Street
Chestnut Hill, Massachusetts 02167
PAPER NOT AVAILABLE AT TIME OF PUBLICATION
57
-------
OPTIMIZING FILTRATION PARAMETERS
Even Bakke
Vice President and Technical Director
Mikropul Division
U.S. Filter Corporation
10 Chatham Road
Summit, New Jersey 07901
INTRODUCTION
The pulse-jet type of dust filter was invented and developed by T. V.
Reinauer of MikroPul (formerly Pulverizing Machinery) and introduced
on the market in the late 1950's. Since then, pulse-jet collectors
have been applied to hundreds of applications and MikroPul now has more
than 40,000 installations all over the world. The basic patent on the
device has expired and there are now many manufacturers of the pulse-
jet type of collector. According to Industrial Gas Cleaning Institute
(IGCI) statistics, the sale of pulse-jet collectors is now approximately
the same in dollars as for the shaker type of collector.
The operating principle for the shaker type of collector is relatively
2
well-known and documented; however, this is not the case for the
pulse-jet type of collector. In this paper, the fundamental operating
principle of the pulse-jet collector will be explained with a series
of performance curves, since, in order to optimize the collector, every
detail of its performance must be fully understood.
The operating principle of the pulse-jet collector is based on the use
of an air ejector for dislodging dust from the bags. The air ejector
59
-------
or jet pump produces a short pulse of compressed air in the opposite
direction of the gas being filtered. Since the energy level of the
cleaning pulse is much higher than the energy used in shaker or reverse
air types of dust collectors, the filter media must inherently have a
high dust particle collection efficiency, and the deposit of dust on
the bags is not relied upon for producing high removal efficiency. The
filter media used for the bags is therefore needled felt, where the
fibers are held together in a random fashion on a scrim or open woven
matrix.
The pressure drop through the collector will depend upon the bag per-
meability, i.e., the filtration rate per square foot of filter media
at a certain pressure drop across the filter media. The consumption of
compressed air for cleaning off the collected dust indicates the clean-
ing energy required to operate the dust collector at a steady state.
THEORY OF OPERATION AND DESCRIPTION OF PULSE-JET FILTER
The principle of bag cleaning is based on the theory of the ejector or
jet pump. A primary jet of high velocity fluid is used to create a low
pressure zone and transfer momentum to the surrounding fluid, thereby
inducing a secondary flow which will mix with the primary fluid in a
constricted zone or Venturi placed some distance downstream of the source
of the primary jet. In the case of a pulse-jet collector, the jet of
air is directed upstream into the bags, against the normal flow of fil-
tering gas.
The jet must accomplish three things:
1. Stop the normal filtering flow.
2. Transmit a burst of air to the filtration media which
will physically give the media a vibratory shock.
60
-------
3. Create enough pressure in the bag to ensure a reversal
of the flow from the clean side to the contaminated
side of the bags.
Figure 1 shows a schematic of a typical pulse-jet collector. In this
unit, the contaminated air enters into the hopper or a bottom inlet.
The coarser particles are removed by the "knock-out" effect, while
the finer particles flow upward and collect on the outside of the bags.
The bags are prevented from collapsing by a cylindrical wire cage
(retainer) which is fastened to a tubesheet and collar-Venturi combina-
tion. The filtered gas then flows upward inside the bags, through the
Venturi, into the plenum and out through the exhaust duct. The bags
are cleaned row by row from a series of compressed air distribution
pipes.
A short burst of compressed air controlled by a solenoid (pilot) valve
is released by the diaphragm valve, and the air then flows into the
header or blow pipe. Small nozzles or orifices in the blow pipe direct
jets of air at critical conditions (sonic velocity) axially into the
upper bag openings and the jets induce secondary air from the plenum
as shown. To convert the energy of the high velocity jet into pressure
and facilitate the induction of secondary air, the jet flows through
the throat of a Venturi which is attached to the tubesheet and extends
into the bag. The burst travels down the bag and stops the normal flow
of air, transmitting a shock on the filter media and giving a back-wash
of air through it. The pressure developed in the bag depends on the
jet pump characteristics of the nozzle and Venturi configuration and on
the permeability of the bag to be cleaned, i.e., the combined permea-
bility of the dust collected on it and the filter media. The bag clean-
ing is done row by row while the unit is onstream. The other rows of
bags, still onstream, take the excess flow during the cleaning and the
bag pressure drop is therefore constant at all times.
Figure 2 shows the jet pump curve for the standard Venturi. The pressure
developed at no flow is the static no delivery of the ejector, which
61
-------
UPPER PLENUM
BLOW PIPE
EXHAUST OUTLET
TUBE SHEET
MR SUPPLY LINE
COMPRESSED AIR
SUPPLY AT
BLOW PIPE: 100 P.S.I.G.
/ INDUCED FLOW /.SOLENOID
VALVE
DIAPHRAGM VALVE
REMOTE
CYCLIC TIMER
MANOMETER.
INDUCED FLOW
COLLARS
VENTURI
NOZZLE
WIRE RETAINERS
Figure 1. Pulse jet schematic
MATERIAL DISCHARGE
-------
Co
30
25
20
o
X
c
in
ui
u
in
0.
15
10
3/4" DIAPHRAGM VALVE
STANDARD VENTURI
MAXIMUM CLEANING POWER
POINT (MCPP)
\
3000 ~
X
c«
2000
1000
100 150 200
JET PUMP FLOW (SCFM)
250
300
350
Figure 2. Jet pump and cleaning energy curves
-------
corresponds to blowing into a bag with zero permeability or into a
closed chamber. The dotted diagonal lines show the pressure drop curves
of operating bags with different permeabilities. The intercept of these
lines above the abscissa is the bag pressure drop under normal operation
which the air pulse must overcome before a flow from the clean side to
the contaminated side of the filter media can take place. The crossing
of the jet pump curve and the bag pressure drop curve is the operating
point during cleaning. A certain amount of primary and secondary clean-
ing flow is developed at a certain pressure build-up in the bag. Since
the primary air is constant, the entrainment ratio, i.e., the ratio of
secondary-to-primary air, increases with increasing total flow, i.e.,
increasing bag permeability.
When there is a continuous increase in the pressure drop across the bags,
as is the case during start-up, the operating point will move upward
on the jet pump curve until the steady state operating point has been
reached.
If the power of the cleaning pulse, i.e., the product of the jet pump
flow and the developed pressure, is considered, a point of maximum
cleaning energy can be established (as shown in Figures 2 and 3). From
these graphs, the maximum point can easily be picked out. If the bag
pressure drop curve is assumed to be linear with respect to gas flow
(constant permeability), any bag operating condition can be laid out on
the jet pump curve by a line from the steady state pressure drop on
the ordinate with a slope equal to bag pressure drop divided by the
filtering flow through the bag (the inverse of the bag permeability) .
If this line intersects the point of maximum cleaning, bag operation
should be most efficient.
A typical static pressure puise measured oy a nign j-teyuency response
pressure transducer connected to the bag is shown in the insert of
Figure 4. The peak pressure corresponds to the intercept point of the
jet pump curve and the bag pressure drop curve. The electrical on-time
64
-------
IJ1
1" DIAPHRAGM VALVE
LARGE VENTURI
MAXIMUM CLEANING POWER
POINT (MCPP)
-i 6000
- 5000
- 4000
- 3000
- 2000
- 100C
100
200 300 400 SCO 600
JET PUMP FLOW (SCFM)
700
X
o
Figure 3. Jet pump and cleaning energy curves
-------
RISE TIME
O
*
SB
CO
CO
co
CO
Pn
1-1
ELECTRICAL
ON-TIME
MECHANICAL
ON-TIME
PRESSURE TRACE OF BAG CLEANING PULSE
150
250
300
BAG FILTRATION FLOW (SCFM)
Figure 4. Venturi pressure loss curves and cleaning pulse trace
-------
is the on-time of the electronically controlled pilot valve; the mechan-
ical on-time is the on-time of the diaphragm valve. This on-time,
together with the valve cycle time (the time for firing all valves),
determines the air consumption needed for cleaning all bags. The rela-
tionship between electrical and mechanical on-time is dependent on the
drain characteristics of the main valve, the compressed air capacity,
volume of blow pipe, and losses downstream of the valve.
Also shown in Figure 4 are two Venturi pressure loss curves for the
standard Venturi and for the larger Venturi for high filter rate appli-
cations. The larger Venturi is used when the loss through the standard
Venturi becomes too high.
To describe the operation of a dust collector on a specific dust, two
curves are normally used. The first is a filter rate (flow rate/bag
area) versus grain loading curve at a given bag pressure drop. The
second is a filter rate vs. bag pressure drop at a given grain loading.
The first curve is most useful for product recovery or pneumatic con-
veying where the grain loading is a variable. In the case of air pollu-
tion control, the second type of curve is the most useful, since in
this case the loading is relatively constant and the filter rate and
bag pressure drop relationship is of greatest interest. However, both
curves are needed to give an overall picture of the dust collector per-
formance.
OPTIMIZING PARAMETERS
When the performance of a pulse-jet type of dust collector is to be
optimized, the following parameters are used as the gauge for success:
Maximum filter rate at minimum pressure drop
(i.e., minimize collector size and fan horsepower)
67
-------
Minimum outlet dust loading or maximum collection
efficiency
Maximum bag life
The following variables should be considered to reach the optimizing
objectives:
Jet pump characteristics
Dynamic response of cleaning valves
Cleaning energy and air consumption
Operating bag pressure drop
Bag size and baghouse configuration
Filter media
In an optimizing program, these parameters can usually be varied, but
there are many parameters that are fixed for a given application that
will have a significant influence on performance. Some of these are:
Grain loading
Particle size distribution
Dust characteristics (sticky, hydroscopic, corrosive, etc.)
Gas temperature and humidity
Corrosive components in gas
Electrostatic charges on the dust particles
By considering the variables one by one, their effect on performance
can be established. In order to obtain a relatively complete picture
of the effect of the different variables, one usually has to resort
to laboratory studies, since some of the variables are very difficult
to change on operating units. The variables most easily evaluated in
the field are the effects of cleaning energy, air consumption, and total
flow.
68
-------
EXPERIMENTAL APPARATUS AND METHODS
During 1972, an intensive pulse-jet test program took place in the
Research and Development Laboratory at MikroPul. The objective of the
program was to determine the effect of the different variables and
parameters mentioned above.
A schematic of the test setup is shown in Figure 5. The test unit was
a cylindrical unit with 28 bags, 4-1/2 inches in diameter and 8 feet long.
Each Venturi had a pressure tap in the throat area, and by connecting
to inclined manometers the flow through each bag could be measured. By
installing different bags with various filter media, their relative
performance in terms of filter rate could be established by measuring
the flow through each bag. The total flow through the unit could be
varied and measured. A sampling train was connected to a sampling probe
in the outlet duct for measurements of outlet concentrations of dust.
The dust was recycled by feeding it into a mixing box with a variable
speed rotary feeder. By changing the speed of the feeder, the inlet
loading could be adjusted. A fan connected to the box would disperse
the dust and air convey it through a duct back to the inlet. Air dis-
persing rings used in the conveying duct were installed to break up
dust agglomerations. Two types of inlets were tested, a radial hopper
(bottom) inlet, and a tangential top inlet. These two types of inlets
gave very different flow conditions inside the unit; the hopper inlet
provided some knock-out of the coarser particles and created very tur-
bulent motion in the hopper and a counterclockwise secondary motion
(see Figure 5) in the housing. The top tangential inlet gave downward
spiral motion of gas and dust, with the finer dust particles being
sucked into the bag area. The turbulence was less and the flow much
more orderly.
69
-------
f.It COMPRESSOR
FLOW
UTLET METER
ON TIME
OFF TIME
--4
O
TIMER
VENTURI
BLAST
GATE
PRESSURE
I
sQ
10,000
ACFM
MAX.
FAN
4
SAMPLING
TP&TM
FILTER
U~^
GAS VACUUM
METER SOURCE
MIXING
BOX
VARIABLE SPEED
JUST FEEDER
AIR CONVEYING FAN
Figure 5. Schematic of test set-up
-------
The cleaning energy was adjusted by changing air pressure, adjusting
solenoid valve on-time and off-time, changing pneumatic valves (changing
the flow coefficient of the valve), and changing the diameter of the
orifices in the blow pipes. Two types of Venturis were tried, the stan-
dard for lower filter rates, i.e., less than 15 cfm/ftS and the larger
for higher filter rates.
Two different dusts were used: magnesium silicate (talc), with a mean
particle size of 1.8 microns with 25 percent less than 1 micron, and
wood sander dust with a mean of 26 microns. These two dusts enabled
us to optimize the dust collector performance on very fine and rela-
tively coarse dusts.
2
Several types of felt media were tested in groups, with 15 oz/yd wool
felt as the base material. The average flow through each group was then
determined by reading the Venturi pressure and converting to flow with
the help of a calibration curve.
By maintaining a constant inlet grain loading (always 10 gr/scf), by
adjusting the dust feed rate and changing the flow through the unit, a
filter rate vs. pressure drop curve was generated. By reading the
Venturi throat pressure for each group of bags, their relative per-
formance was determined. Then, by maintaining a constant bag differen-
tial pressure (always 3.5" w.g.)5 and varying the inlet grain loading,
its effect on filter rate was measured.
DISCUSSION OF RESULTS
The first part of the program consisted of exploratory work on the fine
2
particle size dust (magnesium silicate or talc) with 15 oz/yd wool felt
2
bags having a permeability of approximately 35 cfm/ft at 0.5" w.g.
pressure drop. First the hopper inlet was evaluated with standard dia-
phragm valves of 3/4", using standard cleaning with 45 msec, electrical
on-time and a 60 second cycle time. The filter rate curves are shown
71
-------
o
in Figures 6 and 7. Figure 6 shows that a filter rate of 4.45 cfm/ft
was measured at 3.5" w.g. bag pressure drop with an inlet loading of
10 gr/scf. By changing the inlet to the top entry, the filter rate
2
increased to 5.20 cfm/ft . Then larger 1" pneumatic valves were in-
stalled which gave an increased air consumption from approximately
1 scf/burst for the 3/4" valve to 1.3 scf/burst for the 1" valve. The
supply pressure in both cases was 90 psig. The blow pipe orifice dia-
meter was maintained at 0.25", which gave an increased filter rate of
2
5.75 cfn/ft , still at 3.5" w.g. pressure drop, and 10 gr/scf inlet
loading. The blow tube orifice was then increased to 0.3285" (21/64"),
which increased the air consumption by a factor of 1.73, to approx-
2
imately 2.2 scf/burst and the filter rate increased to 7.2 cfm/ft .
Figure 6 shows that for this condition the filter rate was independent
of the grain loading between 0.3 gr/scf and 10.5 gr/scf. However, for
the other curves with top entry, there was more dependency of filter
rate on grain loading than for bottom (hopper) inlet, which is normally
the case.
Figure 7 shows the dependency of bag pressure drop on filter rate for an
inlet loading of 10 gr/scf. This figure shows that as the inlet was
changed from bottom to top and the air consumption increased, the curves
straightened out and one could go to higher bag differentials and
still get increases in flow. As illustrated by the curve for bottom
inlet, the curve eventually turns vertical and no gain in flow can be
achieved by increasing the pressure drop. In other words, if the unit
were undersized, increasing the fan speed would not result in an in-
crease in flow because the bags would be overloaded with dust.
Figure 8 shows the effect of increased filter rates on collection effi-
ciency. The curves show the relationship between collection efficiency
of talc and bag pressure drop for the different modifications made.
The inlet loading of dust was 10 gr/scf. For the hopper entry, no
material could be measured on the sampling filter paper after a two-hour
sampling period, and therefore was considered essentially 100 percent
72
-------
11
10
Pn
O 6
W e
c?
O
C5
a
I 6
O 4
15 oz. WOOL TELT
TALC 3 3.5" W.G.
100 MS ON-fIME, 1 MIN. CYCLE TIME
O HOPPER INLET. 3/4" VALVE
• TOP INLET, 3/4" VALVE
• TOP INLET. I" VALVE, 0.25" ORIFICE
INLET, 1" VALVE, 0.328" ORIFICE
A
V
1.0 2.0 3.0 4.0 5.0 6.0
FILTER RATE (CFM/SQ. FT.)
Figure 6. Inlet grain loading versus filter rate
7.0
e.c
-------
12
O
p^
53
15 02. WOOI. FELT
TALC @ 10 GR/SCF
O KOPPER IKLET, 3/4" VALVE, 100 MS ON-TIME
1 HIN. CYCLE TIME
• SAKE, TO9- INLET
Q SAME, 1" VALVE, 100 MS ON-TIME
| SAME, 38 MS ON-TIME, 1:43 MIN. CYCLE TIME
! ! I ! !
i. e ' &
FILTER RATE (CFM/SQ. FT.)
10
12
Figure 7. Bag pressure drop versus filter rate
74
-------
3C
c
•r-(
IX
o
s
D
IT.
Ui
$
CM
O
<
SO
12
10 ~
2 (>-
I
D
I
I
I
100 99.98 99.96 99.94 99.92
COLLECTION EFFICIENCY (%)
15 oz. WOOL FELT
TALC @ 10 GR/SCF
Q HOPPER INLET, 3/4" VALVE
0 TOP TANGENTIAL INLET, 3/4" VALVE
TOP TANGENTIAL INLET, 1" VALVE
J
99.90
Figure 8. Bag pressure drop versus dust collection efficiency
-------
efficient. However, as the inlet was moved to the top, the efficiency
was found to decrease to 99.9945 percent at 3.5" w.g. pressure drop.
Then as the valve size was increased to 1", giving higher air consump-
tion, the efficiency decreased to 99.98 percent at 3.5" w.g. pressure
drop and the relationship between efficiency and bag pressure drop was
found to be linear.
Finally, the electrical on-time (the time period when the solenoid
valve is open) was varied to study its effect on the filter rate. The
results are shown in Figure 9, where the filter rate is plotted against
the inlet grain loading. The latter was kept as constant as possible
and the pressure drop was maintained at 3.5" w.g. This figure shows
that the filter rate can be improved by increasing the valve on-time;
however, this can only be done up to a certain point, or also over-
cleaning will occur. This means that the filter media is kept in an
open state too long by the cleaning burst, the collected dust particles
penetrate deep into the felt, eventually work their way through the
felt, and are more difficult to clean out. As shown in Figure 9, the
electrical on-time could be increased to 120 msec, with increases in
flow, but after a short time operating at 120 msec., the flow would
decrease. However, by reducing the on-time to 100 msec., the filter
2
rate recovered to a maximum of 7.1 cfm/ft . The starting point was
2
6.4 cfm/ft for 40 msec, on-time and an increase of 11 percent in filter
rate was realized by adjusting the on-time.
If we relate the pressure drop vs. filter rate curves at a given inlet
grain loading, as shown in Figure 7, to the jet pump curve (Figure 2),
we can see how the dust collector is operating in relation to the maximum
cleaning energy point on the jet pump curve. Assuming a constant per-
meability at any given point of the curves of Figure 7, the permeability
lines can be plotted on Figure 2, and the bag operating point during
cleaning is then the intersection of the permeability line and the jet
pump curve. Using this method reveals the following.
76
-------
0.7 r
5
2 0.5
(9
Z
s
s
3
u
H
i
0.4
0.3
0.2
0.1
6.0
6.5
TALC 9 3.5" W.G.
1" VALVE @ 90 psig
TOP TANGENTIAL INLET
15 OZ. WOOL FELT
1 HIM. CYCLE TIME
(5)120 ms.
o
100 ms.
(2) 80 ms
(3) 90 ms.
7.0
FILTER RATE (CFH/SC-FT.)
I i
7.5
Figure 9. Inlet grain loading versus filter rate
for different electrical on-times
-------
As the slope of the filter rate curve gets steeper, the operating point
slides upward along the jet pump curve. After passing the maximum
cleaning point, the slope of the filter rate curve will become very
steep, eventually to the point where the slope is vertical (e.g., point
#4), and no gain in flow will take place by increasing the pressure
drop. Also, if the cleaning operating point is close to the maximum
cleaning energy point, the rate of change of the slope will be largest,
or this will be the point where the filter rate curve rapidly starts
to turn vertical. This can be illustrated by considering points 3, 5
and 7 on the jet pump and filter rate curves (Figures 2 and 7).
This analysis points to the fact that if the pulse-jet dust collector
is operated close to the maximum cleaning power point (MCPP), performance
will be at its best. The flow can still be increased without a serious
increase in pressure drop. From a purely practical point of view,
this finding has another important implication. If the operator of a
dust collector wants to know whether he can increase the capacity of
the dust collector, all he has to do is measure the total flow and
divide it by the number of bags, measure the bag pressure drop, take
the ratio of the pressure drop to the flow per bag, and plot it on the
jet pump curve as described above. If the operating point during clean-
ing is below the MCPP, he knows the flow can be increased. If the
point falls much beyond the MCPP, the collector is on the vertical part
of the filter rate curve and any attempt to increase the flow will cause
a very sharp increase in bag pressure drop.
The next phase of the program was to investigate the performance of the
same pulse-jet collector on a very high filter rate application with
different filter media. We wanted to operate the collector at filter
2
rates between 20 and 30 cfm/ft , and a wood sander dust was selected as
the material. However, in order to reach these high rates, larger
Venturis had to be used. Their characteristics in terms of jet pump
and pressure loss are shown in Figures 3 and 4 respectively.
78
-------
2
Five different bag materials were selected: 10 oz/yd polypropylene,
22 2
11 oz/yd polyester, 12 oz/yd rayon viscose, 16 oz/yd polyester, and
2
bags made of 15 oz/yd wool. All felts had a permeability range from
2
30 to 40 cfm/ft at 0.5" w.g. pressure drop.
The filter rate vs. inlet grain loading curves for the different bags
are shown in Figure 10 for a top inlet. There is a significant spread
2
in performance, with the 10 oz/yd polypropylene giving the highest
2 2
filter rate and the 16 oz/yd polyester and 15 oz/yd wool giving the
lowest filter rates. Generally, the lighter weight material gave higher
filter rates. Figure 11 shows the relationship between bag pressure
drop and filter rate with an inlet loading of 10 gr/scf. Again, the
slope of the curves can be related back to the operating points on the
jet pump curve. The corresponding numbers in Figure 11 relate back to
the numbers shown on the jet pump curve, Figure 3.
2
Figure 11 shows that the 12 oz/yd rayon viscose performed very poorly
at higher grain loading; the filter rate actually decreased when the
pressure was increased beyond 3.5" w.g. This indicates that the felt
is overloaded with dust and cleaning effectiveness deteriorates very
rapidly. The bag cleaning point is beyond the MCPP, as can be seen
by considering point #5 in Figures 11 and 3.
Finally, the top inlet was changed to a hopper inlet and inlet grain
loading vs. filter rate curves for the different bag materials were
developed. The results are shown in Figure 12. It is interesting to
note that the spread in performance with the hopper inlet is much less.
The reason for this is that very strong secondary motions exist in the
bag area when the hopper inlet is used which increase the loading of
the dust presented to the individual bags. Also, for the same reasons,
the slope of the curves in Figure 12 is much steeper than for the top
inlet (see Figure 10).
79
-------
TOP
30
25
20
WOOD SANDER DOST
3.5" W.G.
TANGENTIAL IHLBT
10 ox. POLYPROPYLENE
11 OZ. POLYESTER
12 OS. RAYOM VISCOSE
16 or. POLYESTER
15 OX. WOOL
00
o
10
10.0
15.0
20.0
25.0
30.0
FILTER RATE (OTM/SO. FT.<
Figure 10. Inlet grain loading versus filter rate
-------
12
10
•
o
*
5 8
s
< 4
HOOD SANTER OUST
10 GR/SCT
TOP TANGENTIAL XRLET
Q 10 OX. POLYfROPYLEME
4) 11 OX. POLYESTER
Q 12. OX. RAYON VISCOSE
g 16 or. POLYESTER
15 ox. WOOL
ID 15 20
FILTER RATE (CFH/SQ.FT.)
25
30
Figure 11. Bag pressure drop versus
filter rate
81
-------
15
10
6
o
z
z
w
O
WOOD SANDER OUST
3.5" W.G.
HOPPER INLET
Q 10 OZ. POLYPROPYLENE
Q 11 oz. POLYESTER
Ql2 OZ. RAYON VISCOSE
m 16 oz. POLYESTER
15 OZ. WOOL
10.0 15.0
FILTER RATE (CFM/SQ. FT.)
Figure 12. Inlet grain loading versus
filter rate
20.0
82
-------
The relative gain in filter rate when switching from hopper inlet to
2
top tangential inlet ranges from about 40 percent for the 15 oz/yd
2
wool to 86 percent for the 10 oz/yd polypropylene. The gain is
inversely proportional to the weight of the material.
CONCLUSIONS
The following conclusions can be drawn from this study.
Using magnesium silicate with a mean particle size of 1.8 microns, the
filter rate was increased by almost 62 percent after switching the inlet
from a hopper type to a top tangential type and increasing the air con-
sumption by a factor of 2.2. However, the outlet loading increased from
virtually zero for the hopper inlet to a significant value for the top
inlet with increased air consumption and bag pressure drop. Also, when
the air consumption was increased, the dust collector could still be
operated at a much higher pressure drop with significant increases in
flow.
By adjusting the electrical on-time of the compressed air burst, an
optimized on-time could be found. An increase of 11 percent in filter
rate was realized by adjusting the on-time.
It was also shown that the performance of the dust collector, i.e. the
relationship between filter rate and bag pressure drop, can be related
directly to the jet pump characteristics of the cleaning pulse. The
best performance is obtained when the bag permeability is such that the
operating point during cleaning is at the maximum cleaning energy point
on the jet pump curve. The same relationship was also found to be true
for the coarse dust operating at a high filter rate.
83
-------
When the different bags with varying weights and fibers were evaluated
with the coarse dust, it was found that the lighter the felt, the
higher the filter rate at a given pressure drop. It was also found
that the spread in filter rates was much larger for the top inlet than
for the hopper inlet.
Finally, it was found that the dust loading has more of an effect on
the filter rate with top inlet; the higher the loading, the lower the
filter rate. However, if the cleaning air consumption is increased,
the change in filter rate will become much less.
ACKNOWLEDGEMENTS
The author expresses thanks to Mr. Peter G. Ford for conducting the
tests in the laboratory, and to Dr. David Rimberg for many valuable
suggestions concerning this paper.
REFERENCES
1. Frey, R. E., and Reinauer, T. V., New Filter Rate Guide,
Air Engineering, April, 1964.
2. Billings, C. E. , and Wilder, J., Handbook or taoric
Technology, Vol. I, Fabric Filter Systems Study, NTIS
Publication PB 200-648 (December, 1970).
84
-------
ENGINEERING AND ECONOMIC CONSIDERATIONS IN FABRIC FILTRATION
Gordon L. Smith
Supervisor, Modular Fabric Systems
American Air Filter Co., Inc.
215 Central Avenue
Louisville, Kentucky 40201
INTRODUCTION
My presentation this afternoon will briefly review the basic engineer-
ing and economic factors relative to fabric filtration systems and then
focus on several areas of current interest.
Industries, many for the first time, are considering the fabric filter
or other pollution control device an integral part of the production
process; a part that must be considered in the initial planning of a
process, one that must be carefully selected and one that must be main-
tained.
As the design of fabric filter systems becomes more of a science than
an art, the equipment manufacturer is faced with an increasing sophisti-
cation on the part of the customer. No longer are air pollution control
systems given cursory, last second examination.They are now being eval-
uated by a more educated customer who is writing tighter and better
specifications as his experience increases.
85
-------
Certain of the economic considerations relative to fabric filter systems
have been affected more than others by the spiral of inflation over the
past four to five years. The most important of these considerations are
the costs of labor, space, and utilities. Although the cost of material
has risen significantly, particularly in the past six months, the steady
increase in the cost of labor and overhead during the past several years
has caused the most significant changes in the design of fabric filter
systems. After briefly summarizing the basic engineering and economic
factors, I would like to specifically discuss how the increase in labor
costs have affected the design of the filter system.
ENGINEERING CONSIDERATIONS
The basic engineering considerations can be broken down into two major
classifications. First, there are the properties of the process gas
stream and, secondly, the variables involving system design to control
the emissions from the process stream.
The properties of the gas stream which must be determined and evaluated
in the design of the control system are as follows:
1. Average volume and temperature of the gas stream and
fluctuations or peaks in the temperature or volume.
Ideally, the customer should provide a graph of volume
or temperature fluctuations during the process cycle.
A fabric filter system can often be designed at less
than the absolute peak volume and temperature condi-
tions, particularly when these peaks are of short
duration.
2. The constitutents of the gas stream which must be
evaluated include, but are not necessarily limited,
to the following:
86
-------
a. Particulate matter
(1) Chemical composition
(2) Particle size distribution
(3) Presence of unburned carbon particles
(4) Concentration-maximum and average values
(5) Particle density, both packed and bulk
(6) Particle angle of repose
(7) Particle electrical properties
b. Gases
(1) Chemical composition
(2) Acid dew points
(3) Water vapor
c. Special Properties of Gas Stream
(1) Toxic properties
(2) Explosive properties
(3) Corrosive properties
(4) Abrasive properties of particulate
Once the properties of the process gas stream are available and analyzed,
the following basic engineering factors are taken into consideration in
the design of the control system:
1. Space restrictions relative to the size of collector.
2. Method of cleaning the dust from the fabric tubes,
basically shaking, reverse air, or pulsing.
3. Suction or pressure system.
4. Collector construction, basically structural, panel
or modular.
87
-------
5. Type of fabric.
6. Gas to cloth ratio.
7. Gas cooling or pre-conditioning requirements.
8. Maintenance provisions and access:
a. To fabric tubes
b. To dampers
c. To material handling equipment
d. Other points of access such as sampling points
e. Miscellaneous maintenance equipment such as
vacuum cleaning provisions
9. Method of getting the particulate matter from the emis-
sion source to the fabric filter:
a. Hoods
b. Duct
c. Fans and motors
d. Associated equipment
10. Material handling equipment:
a. Screw conveyors
b. Air locks
c. Pneumatic conveying, storage, pelletizing, etc.
11. Effluent discharges:
a. Stacks
b. Monovent s
c. Louvers
12. Electrical controls:
88
-------
a. Cleaning cycle
b. Damper operation
c. Fan and motor controls
d. Other electrical equipment
ECONOMIC CONSIDERATIONS
The economic considerations in the design of fabric filtration systems
can best be analyzed by referring to the "pie" diagram from the 1969
GCA Survey, as presented in Figure 1. As shown in this figure, the
cost of the fabric filter itself is approximately 5 percent of the an-
nual cost of the total system. It is also interesting to note that the
largest piece of the "pie" is the labor portion. Since the GCA Survey
in 1969, the costs of all components of the total picture have increased
but their relation to each other has probably remained relatively con-
stant. If one area has increased in relation to the others, it is in
the labor segment of the total cost picture. The cost of labor can be
broken down into two major areas: fabrication and erection costs and
maintenance costs.
I would now like to focus on these two cost areas and describe how the
design of the fabric filter system has changed during the last five
years as a result of the increase in labor costs.
FABRICATION AND ERECTION COSTS
During the last several years we have witnessed a growing trend toward
shop rather than field fabrication. Shop fabrication has the following
advantages over field fabrication:
89
-------
FAN
DUCTS
67.
ELECTRIC
POWER
UST
DISPOSAL
OTHER
EQUIPMENT X 4°/
INSTALLED
GENERAL
MAINT.
COST OF CAPITAL
PLANT
OVERHEAD
FABRIC
REPLACE-
MENT
117.
REPLACE
MENT
FABRIC
PURCHASES
Figure 1. Fabric filter cost distribution
(from GCA survey, 1969)
90
-------
1. Non-union labor can often be used in the shop, whereas
field work normally requires union workers.
2. Productivity of workers in the shop is approximately
20 percent higher than productivity in the field for
similar work. This assumes good weather and work that
can be done equally as well in the field as in the
shop.
3. Weather conditions add to the cost of field labor as
compared to shop labor.
4. Bad working conditions in the field may require pre-
mium pay.
5. Often a per diem rate must be paid to field personnel
that travel over a certain distance from their homes.
As the cost of transportation, lodging and food has
increased significantly during the past year, the per
diem rates have also increased.
6. Tools are more readily available in the shop than in
the field.
For the above reasons, the basic fabric filter construction has changed
from the panel or structural designs of the 1960's to the modular de-
sign of the 1970's. A module is a pre-fabricated section of a fabric
filter system that is shop assembled to the maximum extent possible.
Normally a single module consists of the largest practical box that
can be economically shipped to the jobsite as a unit. Almost by defi-
nition a module must be difficult to ship if it is small enough so
that shipping is not a problem it probably will not be competitive.
Normally all internal components such as tube sheets or venturi sheets,
cleaning mechanisms and even fabric tubes are shop installed in the
module. Although the idea of modular construction is not new, the ever
increasing gap between shop and field labor makes the modular concept
more important every day.
91
-------
MAINTENANCE COSTS
The cost of maintenance, more than any other factor in fabric filter
design, is being more closely evaluated every day by the customer. As
the GCA Survey indicates, approximately 29 percent of the annual cost
is in maintenance labor. As a result of the increasing evaluation of
fabric filter maintenance, the equipment manufacturers have made more
changes in collector design over the past five years to cut maintenance
costs than for probably any other reason.
Perhaps the most important maintenance cost associated with fabric fil-
ters is the cost to locate and change a broken tube. With most filter
designs in the past, (an exception is the pressure type structural bag-
house), this was a dirty, time-consuming and difficult or in some cases
impossible task.
As one of the more outspoken equipment manufacturers put it, in describ-
ing his tube access provisions, he tried to eliminate "sweaty trips
through the bowels of the baghouse to search out broken bags." This
is probably the best description of the task that I have seen. In some
cases, workers must be paid a premium to enter the fabric collector to
change tubes, particularly where a noxious or toxic dust is present.
Some of the design changes to provide better tube access are as follows:
1. Top access provisions are now standard on many types
of pulse units. Access to the tubes is accomplished
through doors in the roof and a broken tube can be
quickly identified and replaced. In some cases, en-
closed upper access areas rather than doors in the
collector roof are providing the ability to remove
tubes from pulse collectors during inclement weather.
2. Top access is also being provided in shaker and re-
verse air type units either in the form of a tilting
walkway near the top of the unit or the more preferred
upper access area with removable grating panels di-
rectly over the tube support members.
92
-------
3. Snap band type cuffs are now standard on most small
diameter bags for shaker and reverse air units. The
snap band arrangement not only allows for substan-
tially lower tube sheet costs but also for much
easier maintenance. Tubes are merely snapped into
the tubesheet without the need for tools to loosen
the hose clamp as in earlier designs. Any dust that
accumulates on the tube sheet floor can easily be
swept into the hopper. With the older collar type
design, the dust had to be vacuumed from between
each collar.
In addition to design changes that make fabric tubes easier to inspect
and replace, other areas were re-evaluated to minimize maintenance
costs including the following:
1. Stair, rather than ladder access is becoming more
prevalent, especially on the larger units. On one
job now being engineered by American Air Filter
where 34 foot tubes are being used to filter sev-
eral million cfm, the customer originally speci-
fied that an elevator be provided to get from
grade to tube sheet and tube suspension level.
2. Many specifications now call for training sessions
for maintenance personnel.
3. Specifications are also calling for "maintenance
compartments" in addition to cleaning compartments
that can be off-line for extended periods of time
without affecting overall system performance.
4. Five years ago, most shaker mechanisms were pro-
vided inside of the dust collector housing, es-
pecially on low temperature applications. The
trend since that time has been toward outside
mounted shaking mechanisms and today the "inside"
shaker is almost obsolete. It did not take the
customer long to realize that there is no such
thing as the "clean" side of a fabric filter.
One side of the tube is dirty and the other side
is very dirty. Thus motors and bearings have
been removed from the inside of the units and
placed outside protected from the dust and heat.
93
-------
CONCLUSION
In conclusion, I would like to say that the customer is becoming in-
creasingly aware that the cost of the equipment is a small percentage
of the total annual cost and has demanded and gotten changes in col-
lector design to minimize maintenance. Although much work remains to
be done, the philosophy of encouraging effective maintenance by design
is, and will continue to be, one of the most important considerations
in the planning of fabric filter systems.
REFERENCES
1. Billings, C. E. and Wilder, J. E. , Handbook of Fabric Filter
Technology. Volume 1. Fabric Filter Systems Study, GCA-TR-70-17-G
APTD-0690, Contract CPA-22-69-38, PB-200-048, December 1970.
(Department A, Clearinghouse, U. S. Department of Commerce, Spring-
field, Va. 22151).
94
-------
COLLECTION EFFICIENCY AS A FUNCTION OF PARTICLE SIZE,
SHAPE, AND DENSITY: THEORY AND EXPERIENCE
Richard Dennis
Principal Staff Scientist
Air Pollution Control Laboratory
GCA/Technology Division
Burlington Road
Bedford, Massachusetts 01730
INTRODUCTION
Increasing concern about the potential physiological hazards and visi-
bility problems associated with fine particle emissions from a multi-
plicity of stationary and mobile sources leads us to re-examine the
particle collection capabilities of fabric filter systems. Electro-
static precipitators and high energy wet scrubbers can provide high
efficiency collection for certain high temperature, wet and/or cor-
rosive gas streams that are difficult to clean with filtration equip-
ment. Fabric filters, however, afford the best means for the retention
of fine particles. In the present discussion, fine particles are con-
sidered to range from about 2 (im diameter down to nuclei sizes ~ 0.001
to 0.1 |j.m. The particles of concern here are those emitted from sta-
tionary sources such as fossil fueled heating or power plants and
numerous industrial operations wherein the uncontrolled emissions
frequently exceed permissible discharge levels based on mass emission
rates and/or plume opacity.
95
-------
The present primary and secondary ambient air quality standards, 75 and
3
60 [ig/m , respectively for the annual geometric means, are based solely
on the mass concentrations in the atmosphere although it is the particle
size properties of the suspended particulate that determine its environ-
mental impact. Except for being able to infer that the clean suburban
3
aerosol of 25 |ig/m following a rainfall is principally submicron, or,
conversely, that the 200 to 500 |ig/m concentrations occasionally en-
countered in heavily industrialized areas reflect a significant 5 to 10 |j.m
2
size fraction, one cannot predict from current ambient measurements the
true extent of potential particulate problems. Similar limitations are
encountered with respect to the size descriptions of particulate emis-
sions from stationary sources. In the absence of size parameters, it is
not possible to establish the meteorological dispersion patterns nor the
available particle surface for gas adsorption and chemical reactions.
With the exception of such highly toxic substance as asbestos, beryllium
or plutonium, the assessment of ambient concentrations or stack emissions
on a mass basis has heretofor been considered sufficient to determine the
potential hazards. For this reason, plus the fact that accurate size
determination measurements are difficult and costly to make, the perfor-
mance of most air and gas cleaning equipment is described in terms of
mass (or weight) collection efficiency. Exceptions to the above approach
include the use of stain efficiency tests for moderate efficiency filters
or low-voltage electrostatic precipitators designed to reduce the soiling
properties of ambient atmospheres and the use of special OOP aerosols,
~ 0.25 urn, to rate high efficiency, HEPA or AEG type particulate filters.
In this paper, we have analyzed the performance features of both indus-
trial and experimental fabric filter systems with respect to the concentra-
tion and particle size properties of the collector effluents. It is quite
simple to cite from a qualititative viewpoint those factors that should
influence significantly the collection characteristics of a fabric filter
system. Key factors should include the following:
96
-------
Dust Properties - concentration, size distribution, shape
factor, density, charge, chemical reactivity, surface
features.
Fabric Properties - woven, felted; yarn and fiber proper-
ties, i.e., density, twist, dimensions, staple, filament;
fabric density, thickness, porosity, tensile properties;
chemical composition, mineral, natural, synthetic, surface
treatments.
Operating Parameters - filter velocity, resistance to air
flow, gas temperature and composition, cleaning frequency.
Filter Cleaning Method - mechanical shaking, pulse jet,
reverse flow, reverse jet.
Critical Interdependencies - dust/fabric, dust/resist-
ance, dust/cleaning method, resistance/cleaning method,
cleaning method/service life.
When it becomes necessary, however, to predict quantitatively the per-
formance of a filter system or to establish design parameters for a
given field application the available guide posts are quite limited
with respect to any generalized approach. Considerable data have been
reported, however, based upon industrial experience and laboratory
studies that provide excellent support for limited applications.
In the following sections, we have reviewed the results of past and
present studies that aid in describing the basic relationships between
various fabric filter and particulate systems. Emphasis is directed
toward certain concepts that must be clearly understood before select-
ing or designing fabric filter equipment.
97
-------
FABRIC FILTER EFFICIENCY CHARACTERISTICS
FILTER LOADING AND EFFLUENT CONCENTRATION
Fabric filters are classically rated as 99 to > 99.99% efficient for the
average field application. At these levels, the incentive to examine
rigorously the effluent size properties is low when mass emission criteria
are the only concern. More important, in terms of materials recovery, one
is seldom impressed with the difference between 99.9 and 99.99% collection.
In the above case, however, there is a tenfold difference in the emission
levels and were the efficiency to increase to 99.999% a 100 times de-
crease in effluent concentration would result.
The above comparisons are based on a constant inlet loading. Another way
to evaluate efficiency ratings for filter systems is to apply a fixed ef-
ficiency rating to a variable inlet concentration for a specified dust.
If the efficiency were actually constant, for example, at 99%, the ef-
3
fluent concentrations for inlet loadings of 0.1 and 10 grains/ft, would
be 0.001 and 0.1 grains/ft. , respectively. In the latter case, the ef-
fluent concentration far exceeds permissible emission levels. Actually,
field and laboratory studies have indicated that it is the filter effluent
concentration rather than the efficiency that is more nearly constant for
a given fabric filter design and a specified aerosol.
The dependency of filter efficiency on inlet concentration levels for
otherwise similar systems has often been neglected for filters operating
at < 1% penetration. According to Figure 1, however, field measurements
indicate an inverse relationship between penetration and loading. On
the premise that equal gas volumes are filtered, these measurements sug-
gest strongly that the mass emission rates are essentially constant for
a given dust/fabric system irrespective of the inlet loading. It is in-
4
teresting to note that recent tests with a coal fly ash having size
properties similar to the foundry dust, ~ 5 urn MMD, fall on the regres-
sion line for the foundry dust. As far as the field measurements are
98
-------
1.0
0.1
0.01
0.001
0.01
COTTON SATEEN FABRIC,
MECHANICALLY SHAKEN
Q FOUNDRY DUSTS
A COAL FLYASH
O WOOL FELT TUBES, REVERSE
JET (BLOW RING) ABRASIVE
DUSTS -SiC2AI 0S,B4C
0.1 1.0
INLET CONCENTRATION , groins/ft8
10.0
Figure 1. Inlet concentration versus percent
weight penetration--ambient
temperature
99
-------
concerned, the general spread of experimental points reflects varia-
tions in filtration velocity, shaking intensity and quality of field
maintenance.
The same test series also indicates that the effluent size properties as
determined by light field microscopy are similar to those for low atmo-
spheric dust concentrations and only slightly dependent on inlet dust size
distributions. The above findings are qualitatively consistent with
filtration theory; i.e., only those particle diameters of the order of
1 urn or less should exhibit any singificant penetration. Despite the
strong correlation between penetration and inlet concentration, however,
the best estimates of outlet concentrations from Figure 1 or related
graphs are of "order of magnitude" quality until the pooled variables are
examined separately.
FILTER EFFICIENCY VS. PARTICLE SIZE
It has been the policy of some manufacturers of air and gas cleaning
equipment to provide fractional particle size efficiency ratings for de-
vices such as inertial separators, wet scrubbers and occasionally electro-
static precipitators. On the premise that the size properties of the inlet
aerosol components have been determined, it should be possible to estimate
both the mass concentration and size distribution of the collector ef-
fluent. Unfortunately, graded efficiency data for the above devices are
seldom usable for particle diameters below 2 p,m. Thus, their primary ap-
plication is constrained to coarse particulates in the 10 p.m mass median
diameter (HMD) range or larger.
The concept of fractional particle size efficiency is valid when the
characteristics of the inlet aerosol and the capture forces within the
collector do not change. For example, once steady-state conditions
relative to wall deposition and re-entrainment rates have been reached in
cyclones, scrubbers or electrostatic precipitators, the size properties
100
-------
of the effluent aerosol will be nearly constant. It has been observed,
however, that very large variations in inlet concentrations, ~ 100 times,
lead to increased cyclone efficiency, due mainly to agglomeration effects
in the peripheral zone. Under these circumstances, enhanced collection
of small (1 to 5 |_im) particles is attained by agglomerative attachment to
larger particles.
Various high porosity filters (bulk fiber and deep bed units) and even
conventional woven or felted fabrics tend to exhibit relatively constant
collection efficiencies for given particle sizes when inlet particle con-
centrations are low; e.g., order of ambient levels or less. After several
months, however, dust accumulation on the fibers and within the pore struc-
ture will lead to increased interstitial velocities and projecting de-
posited particles will provide additional and more effective collection
targets.
Under typical field conditions at dust loadings of the order of 1 grain/
3
ft. , a few minutes dust accumulation changes completely the internal and
superficial structure of the filter. During the course of a normal fil-
tration cycle (from the start of filtration following fabric cleaning to
termination of filtration immediately before cleaning), very great dif-
ferences occur in filter effluent concentrations and particle size dis-
tributions. Thus, there is no way that one can assign any specific
fractional particle size description to a fabric system. In the case of
highly repetitive processes in which the inlet dust concentrations and the
filtration cycles are constant, one can calculate average parameters to
describe adequately the effluent characteristics. For example, fractional
size parameters described by Harris and Drehmel are based upon relatively
lengthy (~ 2 hours) sampling periods of fabric filter effluent streams.
101
-------
Data published by Whitby et al give fractional size efficiencies for
very low (< atmospheric dust level) concentrations for organic dye par-
ticles when filtered through "loaded" and "just cleaned" sateen weave
cotton fabrics, Figure 2. Although Whitby's original presentation
defines very clearly the limitations of these data, out-of-context inter-
pretations have given the impression that fractional particle size ef-
ficiencies are readily available or calculable parameters. The experi-
enced individual will recognize that Whitby's values apply uniquely to
the specified dye aerosols, the dust deposits (fly ash or A.C. dust) on
the filters, and the filter structure itself. Furthermore, the tests are
not realistic since the inlet dust differs from that deposited previously
on the filter and the inlet concentrations are several orders of magnitude
lower than found for most industrial applications. Perhaps the most im-
portant conclusions to be drawn from Whitby's study are that a) fractional
particle efficiencies in the 0.05 to 0.5 ^m range are not strongly size
dependent, and b) fractional particle size efficiencies may vary from 85
to 99.57o depending upon the weight and size distribution of the dust de-
posited on the filters.
OUTLET CONCENTRATIONS VS. FABRIC PROPERTIES
Q
A recent report of EPA research examines in considerable detail the base
performance characteristics of more than 130 synthetic woven filter fabrics,
principally Dacron but also including some Dacron/Nylon weaves. A re-
suspended coal fly ash was the principal aerosol although limited tests
were performed with limestone and amorphous silica. The fabrics were light
2
weight, ~ 4 to 5 oz./yd. , with many variations in warp and fill counts,
mixes of multi-filament and staple fibers, permeability, and free area.
3
Inlet fly ash concentrations and filtration velocities were 3 grains/ft.
and 3 ft./min., respectively, while the cleaning was performed every 20
minutes by mechanical shaking. (4 cps for 2 minutes at 1-7/8 inch
amplitude.)
102
-------
99.99
99.9
99.8
99.5
- "
98
u 90
o
u.
ui 80
< 70
o
P 60
o
Ł 50
u.
40
30
20
CURVE DESCRIPTION
I LOADED, NBS FLY ASH
2 LOADED, A.C. COARSE DUST
3 10 SHAKES, A.C. COARSE DUST
4 10 SHAKES NBC FLY ASH
10
. . I
0.05
O.I
0.5
PARTICLE SIZE,
1.0
5.0
Figure 2. Fractional efficiencies, loaded and shaken
sateen weave cotton filter media, a.c.
coarse dust and NBS fly ash?
103
-------
The outlet concentrations determined during the evaluation of several
different fabric weaves have been plotted against the fabric free area in
Figure 3. Free area is defined as the fraction of open or "see through"
area with the line of sight normal to the fabric surface. In cases of high
thread counts and flattened yarns, the calculated free area may sometimes
have a negative sign, although zero was the lower limit used in EPA com-
putations. Although many fabrics were described by the same free areas,
there were often significant differences in thread count, yarn dimensions
and construction, weave, and particularly in the effective residual drag,
Se, observed during filtration tests. The latter variation in Se ap-
peared to contribute significantly to the data point spread at low free
areas.
Inspection of the regression line, Figure 3 , indicates that one should not
expect to obtain high level particulate removal if the fabric possesses a
high free area. Practically speaking, the broad lumping of filter struc-
tural parameters in Figure 3 does not permit a refined estimate of par-
ticulate emissions for the tighter fabric weaves. Furthermore, the dust
being filtered also plays a major role in describing the concentration/
free area relationship shown in Figure 3 .
OUTLET CONCENTRATION VS. EFFECTIVE DRAG
Outlet concentrations shown in Figure 3 were also correlated with effective
(residual) filter drag, the latter quantity based upon the filtration of
fly ash as described in the previous section. As indicated in Figure 4,
the data points associated with any drag level depict several fabric
weaves, types of yarns, pore sizes and free areas. Generally, the spread
in data points approximates that for the concentration/free area relation-
ship with the free area differences accounting for the large dispersion of
points at the higher drag levels.
Based upon the measurements presented in Figure 3 and 4, one can deduce
that the effective drag for specified dust and filter operating parameters
104
-------
IO
O
8
I.
UJ
o
§10'
o
h-
UJ «
_J 5
»-
O
- o
I
_L
I
0.001
0.01
FREE AREA
O.I
Figure 3. Outlet loading vs. free area. Woven Dacron nylon bags, fly ash filtration
at 3 grains/ft3 and 3 fpm 8
-------
,o 2
(A
"6
UJ
o
l.ld
LJ
O
O UNNAPPED FABRICS
(a) FREE AREA = 0.002
(b) FREE AREA =0.000
A NAPPED FABRICS
NOMINAL 6ft. x s.ein. BAGS
MECHANICALLY SHAKEN AT
AMPLITUDE, 4cps, FOR 2 MIN
EFFECTIVE DRAG { Se) in H20/FPM
Figure 4. Outlet loadings vs. effective (residual) fabric drag woven
dacron and nylon bags, fly ash filtration at 3 grains/ft3
and 3 fpm 8
106
-------
must depend upon many factors in addition to free area. It can also be
seen that the use of drag values alone for predicting outlet concentra-
tions is no more precise than the free area approach.
g
Table 1 presents data excerpted from EPA studies that indicate outlet
concentrations, C , and specific resistance coefficients, K, for several
e
Table 1. FILTRATION PERFORMANCE FOR TWILL WEAVE DACRON FABRICS WITH
VARIOUS TEST DUSTS
Fabric3
W 76 f
F 82 f
F.A. 0.001
P.D. 0-10
W 76 f
F 73 s
F.A. 0.0109
P.D. 17-35
W 76 s
F 82 s
F.A. 0.037
P.D. 50-60
W 59 s
F 54 s
F.A. 0.0198
P.D. ~ 100
Fly ash
HMD = 3.7 p.m
cb
e
8.0
84.0
106
261
Kc
8.5
11.0
3.2
1.1
Limestone
HMD = 18.5 |im
Cb
e
8.6
4.5
11.4
142
Kc
15.3
14.0
11.5
4.9
Amorphous
silica
HMD = 17 ^m
C b
e
2.9
2.9
1.5
272
Kc
43.5
33.2
27.2
18.4
i, F82 = warp and fill thread counts, f = continuous multifilament,
s = staple, F.A. = free area, P.D. = pore diameter, |im.
•L O O
C = outlet cone., grains/10 ft. .
"K = spec, resist, coef.
in. H20 (ft/min)
lb./ft2
107
-------
dust/fabric combinations. Fly ash tests, which are also described in
Figures 3 and 4 , show an increase in dust penetration as the filter free
area and pore diameter increase. Outlet concentrations increase rapidly
when the fabric pore diameter is of the order of 10 times the particle
diameter. It is assumed that bridging is only partially completed when
the ratio of pore to particle diameter is > 10 such that considerable dust
9
penetrates the filter. This concept appears quite reasonable when the
outlet concentrations for limestone and silica are examined for pore di-
ameters < 60 ^im. Comparison of the latter measurements also points out
that the characteristic size of the dust is not sufficient to estimate K
values which are seen to be much higher for the amporphus silica.
The principal reason for presenting the Table 1 data is to emphasize the
fact that the concept of a unique K value for a given dust or specified
particle size is usually incorrect. Because emission characteristics and
K values vary with respect to dust, fabric and dust/fabric combinations it
is difficult to predict performance without prior field or laboratory
tests. It is apparent, therefore, that more fundamental and applied mea-
surements programs are required.
RECENT EXPERIMENTAL MEASUREMENTS
UiSl' tfHOCEDURES
4
As part of a fabric filter cleaning mechanisms study, weight collection
efficiencies and particle number concentrations were determined for several
dust/fabric combinations and three common fabric cleaning procedures. The
detailed results of this study will be presented in a forthcoming report.
Although the background data given here are sufficient to describe the test
systems, they do not reflect the complexity of the measurements nor the
108
-------
numerous instrument problems. Additionally, it is not recommended that the
experimental findings be extrapolated to dust/fabric systems and operating
conditions differing significantly from those reported here.
Test Fabrics
The filter bags evaluated in this study were readily available and com-
monly used commercial products. Bags used in mechanical shaking systems
were sewn with a top loop for attachment to the shaker arm and a bottom
cuff for connection with the thimble plate. Felted tubes used with the
high pressure, pulse jet system were fabricated with a flat, closed bottom
and a top cuff for clamping to the interior supporting cage. Basic bag
specifications are given in Table 2.
Test Dusts
In this paper, test results are given for coal fly ash and commercial talc
dust only. The size properties of the resuspended dusts as determined by
Andersen impactor in the inlet air stream are given in Figure 5 . Accord-
ing to microscopic sizing of the dry powders when well dispersed in im-
mersion oil, the HMD value of the fly ash was lower, roughly 3 p.m. It was
concluded, therefore, that the 90 psig compressed air used in the ejector-
dispersor system was insufficient to break up all the agglomerates. Inlet
dust concentrations, unless otherwise indicated, were usually in the range
3
of 3.5 and 12 grains/ft. , respectively, for shaking and pulse jet systems.
Basic Testing Conditions
Air to cloth ratios for shaking and pulse jet tests as reported herein
3 2
were maintained constant at 3.5 and 8.5 ft. /min. per ft. of fabric.
Most measurements, except for woven fabric life tests, were performed with
3
single bags and a total system gas flow ranging from 25 to 44 ft. /min.
Gas temperature and relative humidity levels were held within the bounds
109
-------
Table 2. DESCRIPTION OF FABRICS USED IN PARTICIPATE EMISSION STUDIES
Filter
fabric
Cotton
Cotton
(napped)
Dacron^
DacronR
Dacron^-
Wool
Weight
oz./yd.2
10
10
10
10
18
16
Weave
Sateen
Sateen
Plain
1/3 Crowfoot
Felt, needled
Felted j
no scrim
(HCE treatment)
Yarn count
95 x 58
95 x 58
30 x 28
(staple)
71 x 51
(filament)
Permeability
13
13
55
33
35
30 - 40
Application
Shaking
Shaking
Shaking
Shaking or
reverse flow
Pulse jet
Pulse jet
Note: Woven fabrics - 10 ::t. x 6 in., 10 ft. x 4 in. and 5 ft. x 6 in. bags
F;jlted fabrics - t, ?.t. x 4.5 in. tubes
-------
10.0
5.0
cc
UJ
h-
LJ
Q
UJ
2.0
(T
<
CL
.0
0.5
i I I I T
T I I I I
I i
i i i i i
2 5 10 30 50 70
PERCENT MASS < STATED SIZE
90
Figure 5. Comparative size properties for resuspended fly ash (coal) and
talc by Andersen Impactor
111
-------
of 70 ± 2°F and 40 to 50% R.H. Filter bags cleaned by mechanical shaking
were operated for a 30 minute cycle with fly ash and a 20 minute cycle
with talc to maintain similar resistance increases.
Cleaning Procedures
The mechanical shaking motion consisted of an essentially horizontal, har-
monic displacement over a range of shaking amplitudes and frequencies of
4
1/2 to 2 in. and 4.3 to 11.4 cps. In a forthcoming report, it will be
shown that for a fly ash/sateen weave cotton system dust removal is a
2
strong function of the acceleration, ~ a , imparted to the shaker bag for
accelerations less than 4 g's. At greater g values, the dependency on
acceleration is weaker, roughly ~ a.*-'*-, with an indication that there is a
limiting acceleration beyond which no further useful cleaning is attained.
4 10
The same study program and prior tests reported by Walsh and Spaite also
point out that there are upper limits (~ 200) to the number of shakes
needed to obtain effective dust removal. A 45 second shaking period was
preceded and followed by a one minute settling interval while the filter
flow was shut off. Pulse jet testing described here is limited to one
basic cleaning system; i.e., 70 psig air pressure, a pulse frequency of
one pulse per minute, and a pulse duration of about 0.06 second. By means
of a supplementary damping tank, the wave form of the pressure pulse was
altered in some tests to reduce the rate of pressure decrease when the
pulse air was stopped.
Dust Concentration Measurements
Inlet dust concentrations were measured by one or a combination of dust
feeder delivery rate, hopper dust recovery, filter samples or Andersen
impactor measurements. Effluent concentrations from shaken bag systems
were usually determined by Bausch and Lomb (B&L) single particle light
scattering counter because of the very low concentrations. When filter
112
-------
*
performance was less effective (~ 99.9%) an RDM mass monitor was used to
determine the integrated mass concentration. The effluent gas stream from
pulse jet systems was sampled with the RDM and/or the B & L sampler de-
pending upon the purpose of the test. Even when effluent concentrations
-3 3
were in the 10 grains/ft, range, it was necessary to sample approxi-
mately 2 hours with the cascade impactors. Therefore, any calculated
fractional efficiency values represent only average performance. Addi-
tionally, to have any predictive capability, it is also necessary that
inlet loading conditions be constant or follow some fixed cyclical pattern.
There were both advantages and limitations to the sampling equipment cited
above. The Andersen impactor could be used for both up and downstream size
and concentration measurements (different sampling periods) when filter ef-
ficiencies were of the order of 99.9 to 99.99%. On the other hand, the
B & L device was, confined to downstream testing since extensive dilutions,
4
~ 10 times, would have been required to use it for upstream sampling.
Because of its high degree of sensitivity and rapid response time, the
B & L was a very useful device to track changes in particle size properties
and number concentrations during a filtration cycle. Although the computa-
tion of downstream mass concentrations from B &L measurements required
somewhat tenuous assumptions with respect to particle density and light
scattering properties, comparisons with concurrent RDM measurements usually
indicated agreement within a factor of 5. Although the B &L values were
recongized to be low in many cases, their principal function was to in-
dicate relative changes in concentration levels.
TEST RESULTS
A study of the factors that determine the overall effectiveness of various
fabric cleaning methods has shown that filter effluent properties (con-
centration and size distribution) for a single dust/fabric system can
*Single stage, cascade impactor with mass sensing by beta attenuation.
113
-------
undergo extreme changes. As pointed out previously such variations are
sometimes overlooked when filter systems operate at 99.5% or higher weight
collection efficiency. In view of prospects for more stringent emission
standards, however, it is essential that the filter effluent be character-
ized along with those parameters defining operational and power
requirement s.
The data presented in this paper involve only a small fraction of the tests
performed to identify and investigate the physical mechanisms responsible
for dust removal in shaken bag and pulse jet cleaning systems. The results
are considered to furnish a good index of single bag field performance
under the stated cleaning conditions and for similar fabrics and dusts
having the same basic properties of coal fly ash or industrial talc. It
should be remembered, however, that most large filter units operate as
multi-chamber systems with sequential compartment cleaning. In the case
of single compartment pulse jet systems, the tubes (or other filter medium
configurations) may be sequentially cleaned as individual or groups of
tubes. The net result is that the integrated effect on filter drag, total
gas velocity distribution, and particulate emissions for multi-chamber
units must be developed in accordance with procedures suggested by
11 12 13
Robinson et al, Walsh et al and Spaite et al. Analyses of the above
9
approaches and many other concepts by Billings and Wilder indicate that
the success of such predictions depends upon the availability of specific
performance information for the dust/fabric combination of interest. In
the absence of such base line data, the particle concentration and size
results described in this section may be used to predict relative but not
absolute changes for filter media and aerosols not included in the study.
The experimental results described in this section suggest that the par-
ticulate emissions from a filter can be attributed to some combination of
the following sources:
114
-------
a. Inlet dust that because of its small size, passes directly
through the filter, usually in progressively smaller
amounts as the filter pore structure becomes plugged.
b. Dust that migrates through the filter by successive
deposition and re-entrainment under the combined effects
of aerodynamic and mechanical (vibrational) forces. Such
dust penetration is often referred to as "seepage" in
commercial parlance. It may be more pronounced in the
case of multi-filament yarns, spherical or smooth sur-
faced regular particles, and in the absence of electro-
static or other forces enhancing adhesion or cohesion.
c. Dust dislodged from the shaken fabric during cleaning
that has penetrated to the clean air region. Resumption
of air flow flushes out the clean air side of the system,
often producing a visible puff of dust.
d. Dust loosened during the cleaning process whose bonding
to the fibers or interstitial dust structure is not suf-
ficiently strong to resist the combined dislodging forces
(aerodynamic and mechanical flexure) when system air flow
is resumed.
Although it appears difficult to weigh the relative importance of the
above sources, it is suspected that items Ł and d_ may account for a large
fraction of the total mass emission, probably in the form of a few large
particles, whereas items a and b_ are responsible for the discharge of
most of the submicron material.
Based upon the relationship between adhesive and/or cohesive forces and
particle size; it appears that particles collected singly on the filter
surfaces will most likely be dislodged in the form of agglomerates. One
could conceive of the extreme case where a freshly generated fume composed
of particles less than 0.5 um could very readily produce a filter effluent
composed of much coarser particles despite a high overall mass collection
efficiency.
115
-------
Mechanical Shaken Systems
Several experiments were performed with unmapped, cotton sateen bags
(10 ft. x 6 in.)> Table 2 to determine filtration parameters for a fly ash
aerosol. During these tests, filtration velocity and inlet dust concen-
3
tractions were held constant at about 3.0 ft./min. and 3.5 grains/ft. ,
respectively. The bags have been described as new (N) since each had ex-
4
perienced less than 10 individual shakes. They were essentially at
equilibrium, however, with respect to air flow/resistance data for re-
petitive filtration cycles. The main variables for the above tests were
shaking amplitude and frequency as shown in Table 3. The filtration
interval was 30 minutes and 360 shakes were used in all tests.
The effect of amplitude and frequency variations on outlet mass concentra-
tion (and 7o penetration) is shown in Figures 6 and 7 . Emissions de-
creased by as much as 5 orders of magnitude over the first 5 minutes of
filtration. The sensitivity limit of the B&L counter allowed for no
3
estimates of number concentrations less than 150 particles/ft, nor de-
-9 3
rived mass concentrations less than 10 grains/ft. . Approximately 907,
of the dust emission took place during the first minute of filtration and
the average concentration for the 30 minute filtration period (10 to
10~5 grains/ft. ) was about 30 times lower. Average dust emissions were
shown to increase significantly (order of 30 times) for the amplitude
range 1/2 to 2 in. but were essentially unchanged with respect to fre-
quency variations.
The filtration of ambient air* showed a more pronounced increase in outlet
concentration over the complete 30 minute cycle with 2 in. amplitude
shaking, Figure 8 . It is interesting to note that the average emission
for 30 minutes, however, was about the same as that for fly ash, Figure 9 .
Inlet loading defined as room air when dust feeder was turned off.
In fact, some fly ash deposited in the inlet piping was probably
resuspended when the air flow was resumed.
116
-------
Table 3. COLLECTION EFFICIENCY AND EFFLUENT CONCENTRATIONS FOR VARIOUS SHAKING SYSTEMS'
Shaking
system
cps
7.5
7.5
7.5
4.3
7.15
11.3
Ampl.
(in.)
2
1
1/2
1
1
1
Average effluent concentration - grains/ft-*
Fly ash filtrationb
First minute
3 x 10"4
3 x 10"5
1 x 10'6
1 x 10"4
5 x 10"5
5 x 10"5
30 Minutes
1 x 10"5
1 x 10"6
3 x 10"7
3 x 10'6
2 x 10~6
2 x 10'6
Ambient air filtration0
First minute
2 x 10"4
2 x 10"5
1 x 10"5
2 x 10"4
2 x 10"5
2 x 10"5
30 Minutes
2 x 10"5
1 x 10'6
3 x 10"7
1 x 10'6
1 x 10'6
1 x 10'6
Shaking
tension
(Ibs.)
10.7
3.5
4.8
7.5
Fabric
dust
holding -
(grains/ft )
200
300
410
420
290
200
a 4
360 shakes per cleaning cycle, sateen weave, unnapped cotton bags. New, < 10 shakes.
bFly ash, 3.5 grain/ft3.
"Ambient air
10"4 grains/ft3.
At resumption of filtration.
-------
1000
"b
X
ro
r
9
6?
a
§
UJ
o
100
10
O.I
0.01
0.001
10 ft. x 6 in. COTTON SATEEN
INLET LOADING, 3.5 groins/ ft3
NEW BAG, < I04 SHAKES
2 in
2 3
TIME , minutes
-2
10
lO'3
0>
u
e»
a
10
UJ
10"
I0"7
Figure 6. Penetration versus shaking amplitude at
constant frequency (7 cps)
118
-------
1000
(0
o
ro
•»-
v»
Ł
I
9*
Z
O
I-
<
O
O
UJ
100
10
O.I
0.01
0.001
10 ft. x 6 in. Cotton Soteen
INLET LOADING.3.5 groins/ft3
NEW BAG, < I04 SHAKES
I
I
2 3
TIME, minutes
10
-2
-3
10
,o-4
10
id r
10'
Figure 7. Penetration versus shaking frequency at
constant amplitude (1 in.)
c
d)
0)
Q.
UJ
Q.
Q
119
-------
1000
0.001
-2
10 ft.x6 in. Cotton Sateen
ROOM AIR FILTRATION
NEW BAG < I04 SHAKES
2 3
TIME, minutes
Figure 8. Atmospheric dust penetration versus
shaking amplitude at constant
frequency (7.5 cps)
120
-------
1000
o
„Ł 100
«^
^
s
o
^
9
z- 10
o
i
i •
UJ
O.I
0.01
FLY ASH 3.5 grains/ft8
AMBIENT™ io"Voint/ft*
DUST
NOTE-.
. MASS CONCENTRATION DERIVED
V\ FROM COUNT DATA OF FIGURE S
I
I
1234
FILTRATION TIME % minutes
-10
-t
I0-»g
cr
h-
Ul
10"
Figure 9. Decreased in outlet mass loadings
with increased filtration time
for low and high inlet concentra-
tions
121
-------
The above comparison applies to a shaking system using 360, 1 in. am-
plitude shakes at 7.2 cps. This finding appears to support field data
presented in Figure 1 that show nearly constant outlet loadings for a
fixed filter type (and dust) regardless of inlet load levels.
Figure 10 shows how particle number concentrations varied with respect to
filtration time based upon B&L measurements. Mass concentrations at
specified times were computed from these data by assuming a specific
gravity of one and using the arithmetic average of diameters for each size
/v
range. Although one does not expect this calculation process to be very
accurate, the outlet concentration and penetration values for the pilot
plant fly ash/sateen weave cotton system fall on the Figure 1 regression
line for foundry dust/sateen weave cotton measurements.
Inspection of Figure 10 indicates that the discharge of particles > 1 |j,m
is restricted to the first few minutes of filtration thus explaining the
very rapid decrease in outlet mass concentrations.
Accelerated shaking of cotton bags in conjunction with periodic fly ash
dust loading and 30 minute filtration tests was carried out with sateen
weave cotton bags to simulate probable performance changes over extended
periods of use. After 20 x 10 shakes, it was postulated that a bag had
seen the field equivalent of 3 to 5 years service. Average 1 and 30
minute outlet concentrations are shown in Table 4 for bags shaken at two
tension levels, one fairly taut at 3.1 Ibs. and the other installed at
near slack conditions, 1.3 Ibs. Surprisingly, the increases in average
emission levels were relatively small, roughly a one- to twofold increase
after 20 x 10 shakes. At the same time, the bags shaken at the higher
tension showed consistently higher (about 1 to 2 times) outlet concentra-
tions for both abbreviated and extended shaking.
—
0.3 to 0.5 (im range, d = 0.4 um
0.5 to 1.0 |im range, d = 0.75 p.m.
122
-------
i FLY ASH 3.5 groins/ft*
\ \ „ AMBIENT~IO"*groint/ft*
X DUST
\ ^ FILTER VELOCITY 3ft/min
\ V 7.2cos,360 SHAKES, I IN
1.0 2 3
FILTRATION TIME, minutes
Figure 10„ Variation in outlet loadings (number
and size basis) for shaken cotton
sateen bags (no nap - B and L size
measurements)
123
-------
Table 4. FLY ASH EFFLUENT CONCENTRATIONS VS. NUMBER OF SHAKING
CYCLES FOR 1 AND 30 MINUTE AVERAGING PERIODS,
UNNAPPED COTTON SATEEN 10 FT. X 6 IN. BAGS,
3.5 GRAINS/FT3 INLET LOADING, 3 FPM FILTER VELOCITY
Number ofb
shaking
cycles
6 x 106
10 x 106
15 x 106
20 x 106
Average effluent concentration - grains/ft3 x 10^ a
Taut bagc
First minute
750
750
500
900
30 Minutes
25
25
17
30
d
Loose bag
First minute
250
350
450
30 Minutes
8.7
8.3
12
15
Tleasurements made after loading filter to ~ 700 grains/ft , and then
cleaning.
Shaking cycle 8 cps, 1 in. ampl. 360 shakes.
°Static tension = 3.1 Ibs., shaking tension 6.5 Ibs.
Static tension = 1.3 Ibs., shaking tension = 4.5 Ibs.
Size distribution curves were constructed from B&L data of the type shown
in Figure 11. These data, Figure 11, show that the dust discharging from
the filters was composed of relatively coarse material during the earlier
phases of filtration. After 3 minutes, however, the outlet dust size
properties were reduced to approximately those of atmospheric dust as de-
termined by light field microscope.
The filtration characteristics of fly ash were also studied with napped
sateen weave cotton, plain weave Dacron and crowfoot weave Dacron,
3
Table 5. These comparisons were made at a 3.5 grains/ft, loading,
3 ft./min. filtration velocity, and a 30 minute filtering period. The
cleaning cycle consisted of 360, 1 in. amplitude shakes at 8 cps. Changes
in outlet concentration with time were again computed from B&L counter
124
-------
4.0
20
a:
LeJ
1.0
o
UJ
_J
o
1 0.5
0.2
NOTE' FLYASH FILTRATION,SATEEN WEAVE COTTON.
INLET CONCENTRATION, 3.5 grains/ft3
FILTER VELOCITY, 3ft./min
SHAKING CONDITIONS, 6 CpS,45sees,
\ in. AMPLITUDE
1 I 1
I
I I 1 I
I I
I
I I
0.5
10 30 50 70 90 95
PERCENT NUMBER < STATED SIZE
98 99 99.5
4J.
Figure .11. Changes in effluent size properties with filtering time for new (< 10 shakes) and old
(2 X 107) shakes) bags (Sizing by B and L optical counter)
-------
Table 5. FLY ASH FILTRATION CHARACTERISTICS FOR NEW (< 104 SHAKES) AND WELL-USED
(2 x 107 SHAKES) BAGS
Residual drag
in H20/Łpm
Effective
residual drag
in H20/fpm
Terminal drag
in H20/fpm
Dust collected0
per cycle
grains/ft2
Residual dust
grains/f t2
70 Dust removed**
by shaking
Fabric type3
Plain weave
Dacron
N
0.17
(0.35)
(0.81)
278
207
57
U
mm
0.30
0.73
255
113
69
Crowfoot
Dacron
N
(0.37)b
0.43
1.12
288
92
76
U
(0.02)
0.47
1.11
275
73
79
Napped
cotton sateen
N
(0.20)
0.23
0.82
295
449
40
U
(0.53)
0.67
1.17
312
336
48
Unnapped
cotton sateen
N
0.47
0.67
1.24
284
413
41
U
(0.60)
(0.73)
1.41
290
375
41
N>
10 ft. long x 6 in. diaci. bags, N = new, U = well used.
Parentheses indicates estimated values.
c 3
Inlet loading - 3.5 grains/ft. , filter velocity - 3 fpm, 30 min. filter cycle.
Cleaning cycle - 360 shakes, 1 in. amplitude, 8 cps.
-------
measurements. Reference to Figure 12 shows that measurable effluent con-
centrations for both types of Dacron media persisted throughout the 30
minute filtration interval. In contrast to sateen weave cotton, the 1
minute and 30 minute concentrations were not appreciably different (2 to
5 times) and the average outlet concentrations over the full 30 minute
filtering cycle were about 1000 times higher. In terms of weight collection
efficiency, the average values for the Dacron bags were about 99.8%.
Average outlet dust concentrations were compared with the filter residual
holdings at the resumption of filtration. The differences in residual dust
2
holdings (expressed as grains per ft. of fabric) resulted from experi-
mental variations in the shaking method, type of dust and type of fabric.
According to Figure 13, the amount of dust retained by the fabric matrix
after cleaning plays a significant role in determining dust retention. It
may be inferred that the pore sealing process is more nearly complete for
cotton media than for the Dacron weaves. It should also be noted that
whereas the differences in filter resistance indicated that fan power
requirements would be about 25% higher for cotton fabrics when treating
identical gas streams choice of the cotton would reduce particulate emis-
sions by roughly 1000 times. When emission data for similar talc filtra-
tion studies were adjusted for deposit bulk density (roughly 4.5 times
lower than fly ash) they too fell on the same regression line. Since the
residual dust holdings for talc were about 4 to 5 times lower than the fly
ash levels, it appears that talc is more readily dislodged from the fabrics
and also that a more permeable filter media should result. However, on
the premise that the higher specific volume of the talc requires 20 to
25 percent as much to fill a given pore, the dust retention properties
shown in Figure 13 appear reasonable. In effect, it is assumed that it is
the volume and not the mass of dust within the pores that controls the
emission characteristics.
127
-------
10.000
I I I I
PLAIN-WEAVE
OACRON
CROWFOOT
DACRON
NOTE-.
FLY ASH LOADING 3.5grolnt/fts
FILTER VELOCITY 3ft/min.
NAPPED COTTON
UNNAPPED
COTTON
8 12 16 20
FILTRATION TIME, minutes
Figure 12. Fly ash emissions for various filter media cleaned
by mechanical shaking; 30-minute filter cycle, new
bags, less than IcA shakes (based on optical
counter measurements)
128
-------
10'
*0
g I0<
"6
0
UJ
o
o
Ul
I-
o
UJ
o
a:
UJ
io
10"
0 CROWFOOT DACRON
X PLAIN WEAVE
DACRON
UNNAPPED COTTON
NAPPED COTTON
*• TALC
F = FLY ASH
200
400
600
RESIDUAL DUST, grains/ft.
8OO
Figure 13. Average outlet concentration vs. residual dust
holding
129
-------
The result of EPA studies summarized in Figure 4 show a similar relation-
ship between outlet concentration and effect drag. It should be noted,
2
however, that the effluent concentrations from the 10 oz./yd. media
(Dacron or cotton) are about 500 to 1,000 times less than those for the
2
4.5 to 5 oz./yd. fabric forming the basis for the Figure 4 data. One may
conclude, therefore, that fabric aereal density as well as filter drag,
free area, and residual dust holding must also be considered in fore-
casting filter behavior.
Pulse Jet Systems
The results of pulse jet cleaning studies cannot be extrapolated directly
to field applications because measurements were made with a single tube
system. As with any large compartmented units the resultant effluent from
several tubes undergoing sequential cleaning should be cleaner than that
from the most recently pulsed tube. A precise definition of the field
effluent depends upon the fraction of tubes cleaned at any one time, the
apportionment of gas flow among all the tubes in the system, and the effect
of the pulse jet parameters on particulate emissions.
The pulse jet tests described in this section involved the following vari-
ations in cleaning parameters:
Pulse jet pressures - 40 to 100 psig (direct and damped)
Pulse duration - 0.06 second
Pulse interval - 1 minute
Direct pulses were the result of the direct venting of compressed air from
the reservoir tank to the bag exit region. Damped pulses were produced by
placing a dead end expansion tank in the line such that a more gradual de-
crease in pressure took place within the bag when the solenoid valve was
closed. In both cases, the rate of pressure rise and the maximum reverse
pressure were the same. All tests reported here were made with fly ash
130
-------
3
and wool or Dacron felt tubes at inlet dust loadings ~ 12 grains/ft, and
an inlet velocity of 8.5 ft./min.
Average outlet concentrations for fly ash filtration are shown in Figure 14
for direct and damped pulses at varying reservoir pressures.
Outlet concentrations for direct pulse systems were significantly higher
than those with damping. High speed photography showed that bag deflation
in the absence of damping took place with a sharp, snapping motion in the
direction of normal filter flow. Thus, both the return air flow and the
bag acceleration acted in concert to increase the penetration of any
agglomerates loosened by the initial pressure shock. Absolute outlet con-
centrations were about 1000 times greater than those determined for shaken
cotton bags.
The minimum operating resistance after cleaning varied inversely with
pulse jet reservoir pressure, Figure 15. The 20 percent higher resistance
levels associated with damped pulses are presumed to result from a higher
residual dust holding. The approximately fourfold reduction in outlet
concentrations, however, may represent an advantage despite the higher
minimum and average resistance values.
Changes in particle number and mass concentrations are shown in Figure 16
for typical fly ash filtration tests with the pulse jet system. It can be
seen that the same trends noted previously for mechanically shaken bags;
i.e., a rapid tailing off in the penetration of dust particles as the
filtration continued, was also exhibited by the pulse cleaning process.
Although the highest number concentrations were noted during the first
20% of the cycle, the initial mass concentration was seldom more than 2 to
5 times the average outlet concentration. These results suggest that a
brief, ~ 1 minute, filtering period is insufficient to permit any ex-
tensive pore blockage.
131
-------
(O
o c
»- 5
o»
UJ
o
13 o
O 2
CD
S
I I I
DACRON FELT
FLY ASH -10grains/ft3
VELOCITY-8.5 ft/min.
PULSE INTERVAL/I min.
PULSE DURATION 0,06 Sec.
0.06
0.05 p
cc
K
UJ
0.04^
0.03
UJ
0.02
0.01
UJ
o
10 40 60 80 100
INITIAL RESERVOIR PRESSURE, psig
Figure 14. Dust emissions for fly ash versus pulse intensity and pulse
wave form
132
-------
o
CJ
Ul
o
I
to 4
o:
ui
b
JZ 3
\
\
DAMPED PULSE
DIRECT PULSE
DACRON FELT, 4ft. x 4.5 in.
FLY ASH, 12 groins /ft.3 @ 8.5 fpm
PULSE DURATION 0.06 TO 0.15 sec.
PULSE INTERVAL 0.4 TO I min.
20 40 60 80 100
INITIAL RESERVOIR PRESSURE, psig
Figure 15. Pulse jet pressure vs. minimum bag resistance for
direct and damped pulses
133
-------
°x MASS
DACRON FELT
FLY ASH LOADING-12 grams/ft
VELOCITY 8.5ft/min
PULSE PRESSURE 70 psig
PULSE DURATION 0.06 Sec
10 —
0.2
0.4 0.6
TIME , minutes
0.8
— 10
1.0
Figure 16. Variations in particle number and mass
concentration versus diameter and time
134
-------
Because of the much higher effluent concentrations, it was seldom possible
to make B&L measurements for particle diameters less than 1 |im due to
choking in the fine particle channels of the B&L. Under these conditions,
erroneously low estimates of number concentration were expected. With a
few exceptions, however, there appeared to be a constant ratio (~ 5/1) be-
tween B&L and RDM or Andersen impactor mass measurements. As pointed out
previously, the relative and not the absolute values of the B&L measure-
ments were the primary considerations in this study.
Comparative outlet concentrations (number basis) are shown in Figure 17
for direct and damped pulsed systems and wool and Dacron felts.
In accordance with the higher outlet concentrations (mass basis) observed
for the direct pulse system, higher outlet concentrations were also in-
dicated for particle sizes greater than 3 |_tm, Figure 17. Presumably, it
is the large size fraction that accounts mainly for the increased pene-
tration. The limited data presented here show no significant differences
between Dacron and wool felt performance.
A comparison of up- and downstream Andersen impactor measurements in
Figure 18 shows that the average effluent aerosol is actually slightly
coarser than that entering the system. No rules of filtration are con-
tradicted by these results. The downstream particulate is composed
largely of agglomerated material loosened by the high energy pressure
pulse and driven to the clean air side of the felt by a combination of
fabric acceleration and normal air flow. It is emphasized that the ef-
fluent from a multi-tube system would probably show a finer downstream
particulate in almost all instances since the majority of the coarse
particles are associated only with the most recently pulsed element.
These data should not be interpreted to mean that effluent dust from pulse
jet collectors is always larger or the same size as that entering the col-
lector. Figure 18 information relates specifically to the dust/fabric
combination studied and the indicated operating and cleaning parameters.
135
-------
10'
10
UJ
a:
"0
10*
o
10
i i i i r
DIRECT PULSES
I I i r
DAMPED PULSES
j i
j i
0.3 0.5 1.0 2.0 3.0 5.0 10 0.5 1.0
PARTICLE DIAMETER, fun
2.0 3.0 5.0 10.0
Figure 17. Average outlet number concentrations for 1-minute pulse intervals -
pulse jetting at 70 psig for 0.06 sec
-------
10.0
5.0
E
dL
•»
o:
liJ
\-
UJ
5 2.0
u
o
»-
o:
1.0
NOTE:
CLEANING CYCLE,70psig
AIR, I PULSE/min/bag,
0.06 sec DURATION
DESCRIPTION
INLET DUST, LIGHT
FIELD MICROSCOPY
INLET DUST, ANDERSEN
IMPACTOR
OUTLET DUST, ANDERSEN
IMPACTOR
I I I I I I I I 1 I I I I I
I I
0.; 0.2 0.5 I 2 5 10 30 50 70
PERCENT MASS< STATED SIZE
90 95
Figure 18. Fly ash filtration which Dacron felt and pulse jet cleaning.
Size distribution for inlet and outlet dusts at weight
collection efficiency of 99.837«
-------
REFERENCES
1. Friedlander, S. K. Small Particles in Air Pose a Big Control
Problem. Environ. Sci. Technol. 7:1115, 1973.
2. Dennis, R. and J. E. Wilder. Factors in the Collection of Fine
Particulate Matter With Fabric Filters. Proceedings, Symposium
on Control of Fine-Particulate Emissions From Industrial Sources,
U.S. - U.S.S.R. Working Group, Stationary Source Air Pollution
Control Technology, San Francisco, California, January 15-18, 1974.
3. Dennis, R., G. A. Johnson, M. W. First, and L. Silverman. How
Dust Collectors Perform. Chem. Eng. 59:196, 1952.
4. Wilder, J. E. and R. Dennis. Fabric Filter Cleaning Mechanisms and
Kinetics Study. Contracts EHS-D-71-19 and 68-02-0268, GCA Corpora-
tion. Final Report in Preparation.
5. Johnson, G. A., S. K. Friedlander, and R. Dennis. Performance
Characteristics of Centrifugal Scrubbers. Chem. Eng. Progr.
51:176-188, April 1953.
6. Harris D. B. and D. C. Drehmel. Fractional Efficiency of Metal
Fume Control as Determined by Brink Impactor. Environmental Pro-
tection Agency, Research Triangle Park, North Carolina. (Pre-
sented at 66th Annual Meeting of the Air Pollution Control Asso-
ciation. Chicago, Illinois. June 24-28, 1973.)
7. Whitby, K. T. and D. A. Lundgren. Fractional Efficiency Charac-
teristics of a Torit Type Cloth Collector. Torit Manufacturing
Co., St. Paul, Minnesota, August 1961.
8. Drehmel, D. C. Relationship Between Fabric Structure and Filtra-
tion Performance in Dust Filtration. Control Systems Laboratory,
U.S. Environmental Protection Agency, Research Triangle Park,
North Carolina. Report Number EPA-R2-73-288. July 1973.
9. Billings, C. E. and J. E. Wilder. Handbook of Fabric Filter
Technology. Volume 1. Fabric Filter Systems Study. GCA/Technology
Division. Department A, Clearinghouse, U.S. Department of Commerce:
Springfield, Va. 22151. Report Number GCA-TR-70-17-G, APTD-0690,
Contract CPA-22-69-38, PB-200-648. December 1970.
10. Walsh, G. W. and P. W. Spaite. An Analysis of Mechanical Shaking
in Air Filtration. J. Air Poll. Central Assoc. 12:57, 1962.
11. Robinson, J. W., R. E. Harrington, and P. W. Spaite. A New Method
of Analysis for Multicompartmented Fabric Filtration. Atmos. Envir.
1:495, 1967.
138
-------
12. Walsh, G. W. and P. W. Spaite. Characterization of Industrial
Fabric Filters. (Presented at ASME Annual Winter Meeting.
December 1960.)
13. Spaite, P. W. and G. W. Walsh. Effect of Fabric Structure on
Filter Performance. Am. Ind. Hyg. Assoc. J. 24:357, 1963.
139
-------
SOME EFFECTS OF ELECTROSTATIC CHARGES IN FABRIC FILTRATION
Edward R. Frederick
Technical Operations Manager
Air Pollution Control Association
4400 Fifth Avenue
Pittsburgh, Pennsylvania 15213
INTRODUCTION
In filtration operations, electrostatic charges may have both good and bad
effects. The good features that result in outstanding collection effi-
ciency are believed to be attributable to particle-to-fabric attractions
and agglomeration. On the bad side, static charges and their forces of
attraction don't necessarily, stop at the end of the collection cycle and,
therefore, they often restrict cleanability. But, this too, is sometimes
desirable.
The charges generated, induced or whatever, on fabrics and undoubtedly on
particles too, have specific characteristics, the features of which are
dictated primarily by surface properties. These characteristics are po-
larity, intensity and rate of dissipation. Our experience indicates that
these characteristics of the charges on the fabric filter and on the par-
ticulates have a direct influence on one another and, most often, deter-
mine in large measure how the filter performs. Agglomeration, or a low
density condition of the cake that resembles agglomeration, is promoted
during the filtration process, according to our interpretations, by use of
a fabric with suitable electrostatic properties. In general, then, we
contend that effective balancing of static properties of fabric and
141
-------
particulate and also a suitable charge dissipation rate in the medium,
when such is needed, represent conditions considered necessary for at-
taining optimal filtration parameters.
In this review, special consideration is given to the electrical prop-
erties of fibers, of fabrics, of particles and of the gas that carries
the particulates. The three ways that moisture affects the filtration
process are noted and artificial charging, as a potentially new technique,
is also discussed briefly. Finally, a few case histories are offered to
indicate how charge balancing leads to optimum filtration performance.
SOME EFFECTS OF ELECTROSTATIC CHARGES IN FABRIC FILTRATION
It is a pleasure indeed for me to consider with you some of our impressions
concerning the effects of electrostatic charges in fabric filtration. Un-
like other mechanisms of the classical filtration theory that contribute to
the removal of particles by fabrics, electrostatics have received barely
minimal consideration. But now that better control of submicron particles
is demanded, the contribution and restrictive influence of electrostatic
charges on both the collecting fabric media and on the collected solids
must be understood and utilized fully. At the present time charge contri-
butions to, and restrictions on, the filtration process are vaguely under-
stood and rarely if ever applied knowingly in commercial practice.
Our experience from the evaluation of almost fifty different dusts by
both shaker and reverse air-jet experimental filtration support a theory
that emphasizes the importance of electrostatics for controlling operating
parameters. Accordingly, the electrostatic charge requirements of the
preferred fabric filter seem to be dictated by the electrostatic, as well
as the physical properties, of the collected particulate. Charge exchange,
or neutralization between the medium and some dusts, leads to agglomeration
that greatly enhances the efficiency of fabric filters. Inasmuch as
charge retention on the filter tends to influence particle-to-fabric ad-
hesion, the charge dissipation rate of the media and/or of the particulates
influence, and sometimes may be modified to optimize, overall performance.
142
-------
When conditions permit, the use of one kind of fabric filter in prefer-
ence to others will most often lead to better control of fine participates
and to greatly improved performance.
In this report, I hope to consider the characteristic features and prac-
tical utilization of electrostatics in fabric filtration. No attempt shall
be made to cover the theory of contact electrification. The basic static
properties of polarity, intensity and dissipation rate, as they apply to
fabrics, particulates and the carrier gases, however, will be reviewed
and related to filter performance. Comments will also be directed to
three ways that moisture influences the filtration process. Then, after
considering the relatively recent development of an operation that I refer
to as artificial electrification, a few case histories will be offered.
ELECTROSTATIC PROPERTIES OF FABRICS
Whenever two dissimilar materials (usually, at least one of which is an
insulator) are rubbed together, one becomes electropositive, the other
electronegative. This is the way particles and fabrics become charged.
This polarity difference that occurs between particulates and fibers is
generally conceded to offer the means by which fabrics draw particles out
of the gas stream to achieve very high collection efficiency; higher than
predicted by the classical filtration theory. Polarity variations among
fabrics reflect inherent fiber differences that may be demonstrated from
rubbing tests. The testing unit that we used is shown in Figures 1 and 2.
By the rubbing method, a triboelectric series may be obtained in which all
fabrics may be listed from those that are very electropositive like wool,
glass and nylon to those that are quite electronegative like the
polyolefins, and especially, "Teflon". It will be apparent that by re-
peated rubbing trials, the series may be expanded to include any number
of fabrics (refer to Table 1). A variety of materials, including par-
ticulates, may be located in the same series. Other factors being equal,
the greater the spread between materials in the series, the greater the
143
-------
Figure 1. AFC static generation and evaluation equipment--static charge
generation. 1. test fabric; 2. test fabric tensioning weight;
3. test fabric support frame; 4. test fabric frame tensioning
weight; 5. reference fabric (contacting test fabric); 6. ref-
erence wheel drive motor; 7. voltage probe (retracted)
144
-------
Figure 2. AFC static generation and evaluation equipment — static charge
measurement. 1. test fabric; 2. test fabric tensioning
weight; 3. test fabric support frame; 4. test fabric frame
tensioning weight; 5. reference fabric (removed from test
fabric); 6. reference wheel drive motor; 7. voltage probe in
measuring position
145
-------
Table 1. TRIBOELECTRIC SERIES FOR SOME PRODUCTION FABRICS
Positive
+25
+20
+15
+10
+5
-5
-10
-15
-20
Wool felt
Glass, filament, heat cleaned and silicone treated
Glass, spun, heat cleaned and silicone treated
Wool, woven felt, T-2
Nylon 66, spun
Nylon 66, spun, heat set
Nylon 6, spun
Cotton sateen
Orion 81, filament
Orion 42, needled fabrics
Arnel, filament
Dacron, filament
Dacron, filament, silicone treated
Dacron, filament, M-31
Dacron, combination filament and spun
Creslan, spun; Azoton, spun
Verel, regular, spun; Orion 81, spun (55200)
Dynel, spun
Orion 81, spun
Orion 42, spun
Dacron, needled
Dacron, spun; Orion 81, spun (79475)
Dacron, spun and heat set
Polypropylene 01, filament
Orion 39B, spun
Fibravyl, spun
Darvan, needled
Kodel
Polyethylene B, filament and spun
Negative
146
-------
interaction between the two materials. Table 2, the triboelectric series
arrangement for fabrics used in one (Co-A) of the test programs, further
emphasizes the fact that not all fabrics made from the same type of fiber
fall in the same location in the series.
The intensity of the electrostatic charge (together with any polarity dif-
ference) has a marked influence on collection efficiency and is a function
of surface (fiber, yarn or fabric) roughness as well as of the inherent
properties of the polymer. Other factors equal, the rougher the surface,
the higher the generated charge. Rough fibers like wool, rough yarns like
low twist spun yarns and high cover fabrics like those that are napped,
tend to develop higher charges than smooth surfaced materials such as those
made from smooth (melt extruded) continuous filaments fibers in low twist,
pressed, calendered or otherwise smooth fabrics.
Naturally, chemical as well as mechanical treatments can impart roughness
and influence the intensity of the generated static charge. The resin
treated wool that serves as an extremely efficient filter in respirators
is one example of how extreme polarity combined with roughness frictional
properties that increases charge intensity produces super high particle-to-
fiber attraction.
The rate of charge dissipation that so often affects cleanability criti-
cally, is a function primarily, but not exclusively, of fiber resistivity.
It will be evident from Table 3 that as the fabric's volume resistivity^
or surface resistivity decreases much below 10-*-" ohm cms or 10 ohms,
respectively, the rate of charge bleed-off increases significantly. '
Other differences among fibers, yarns and fabrics also contribute to dif-
ferences in the rate of charge decay. As fibers become longer and
smoother, as yarns become more compacted and smoother and as fabrics be-
come tighter, more compact and smoother, charges tend to bleed-off faster.
Charges are surface generated and here they remain until they flow-off
either internally (as in an electrical conductor) or along the surface or
directly to the surroundings. For effective dissipation, they must find a
route like that offered by conductive elements in the surrounding atmosphere
or, if these are absent, they may travel, usually relatively slowly, over
147
-------
Table 2. TRIBOELECTRIC SERIES OF FABRICS USED IN THE CO-A TEST PROGRAM
Positive (+)
Nylon 800
Dacron 2362 NS (90360)
Dacron S 484 (89979)
Orion 2339 (80889), OrLon 2420 (81332), Orion S 481 (93423)
Orion S 481 (900780), Orion S 481 NS (93423A), Orion S 481 NS
(90078B), Orion S 428 (90251B), Dacron 2362 (82924), Dacron
2362 (82386)
Dacron S 485 (89953), Orion 81 - 2459 (81660)
Dacron 2362, Dacron S 484 (93424), Dacron S 447 (90840)
Dacron S 483 (89952B)
Orion S 627, Orion 2339 NS (84651)
Darvan [B-831], Darvan S 547, Darvan S 456 (81214)
Darvan S 456 (90784), Darvan S 626 (16321)
Darvan S 456 NS (90784A)
Negative (-)
Note: The two underlined fabrics served as electrostatic references.
148
-------
Table 3. RESISTIVITY VS. ANTI-STATIC RATING
Volume
vertical
resistivity
(ohm cm)
>10U
1010 - 1011
109 - 1010
108 - 109
< 108
Surface
horizontal
resistivity0
( ohm)
> 1013
1012 - 1013
1011 - 1012
1010 - 1011
< 1010
Anti-static
rating
Nil
Poor
Moderate
Fairly good
Good
ASTM D257-61 Method.
Report of E. R. Frederick to Mellon Institute, December 1,
1964 - May 31, 1965.
'"The Electrical Resistance of Textile Materials as a
Measure of Their Anti-Static Properties," Wilson, D.,
J. Text. Inst., 54, T104, 1963.
the fiber surface. For short (spun) fibers, the path is short and the
charges tend to accumulate at the ends. They are likely to remain there
until they either bleed-off slowly on the surrounding gases or they may
build-up until they reach a point at which discharge occurs as they reach
the breakdown voltage of the surrounding gases.
The atmosphere surrounding the fibers always has an influence on the rate
of charge dissipation. But when the same low conductivity conditions
exist for both spun and filament yarns (of the same polymer), the filament
fabric displays a higher rate of charge loss. Similarly, a compacted
calendered fabric will lose its electrostatic charge faster than the same
fabric at the lower density without the smooth surface.
Treatments can have a profound influence on the performance of a filter
fabric. If the finish applied to the fiber is fugitive, as most organic
treatments tend to be in ordinary service, its influence can hardly be
149
-------
favorable. Initially, the finish exerts one kind of an effect but this
will change in service use, and finally disappear completely upon heating.
Among the potentially serious problems is that which may be introduced by
the fiber producer's finish. These fiber lubricants applied to minimize
fiber damage in processing, may not always be removed completely by the
fabric manufacturer. As a result, that finish which remains, like the
anti-static resin treatment that we studied, may be converted to the amine
derivative of the original finish. In this new form, the treatment pro-
duces a very strong electropositive influence before it finally burns-off
completely. A finish of this type, therefore, may cause the fiber to
exert three different static effects—anti-static at first, then strongly
electropositive and, finally, that which reflects the true properties of
the fabric. This is further support for a practice that would provide
electrical data for both the as-received and as-cleaned filter fabric.
ELECTROSTATIC PROPERTIES OF PARTICLES
The electrostatic properties of particles are more obscure than those of
filter fabrics. Presumably, as noted, particles may be located together
with fabrics in the same triboelectric series. Certainly, electrical
resistivity must have a primary influence on static properties and it
would also seem evident that particle-size--shape—roughness, etc., must
influence charge intensity and, to a lesser extent, charge retention.
That resistivity varies for different kinds of the same particulate and
also changes with temperature, will be evident from Figure 3.
A relatively crude test that exposes a number of fabric swatches to dust
tends to support the view that particles may be located with fabrics in
the T.E. series. Differences in the weight of dust picked-up by similarly
constructed fabrics made of different fibers serve to specify polarity
differences. The extent of differences appear to indicate a location for
the particulates in the triboelectric series with the test fabrics. Other
factors being equal, the fabrics most distant from the dust in the series
150
-------
10
13
250
Temperature (°C)
300 350 400
12
10
10n
E
JT
o
10
.>
'*-•
(/5
& 109
10'
2.0
1.8 1.6
1000/T (°K)
1.4
Figure 3. Typical log resistivity versus recip-
rocal absolute temperature plots for
ashes A, B, C, D, E, and F
attract the greatest quantity of dust. The method, with refinement, may
deserve further consideration.
Agglomeration is another feature of most dusts that appears to have a very
significant, if not a controlling influence on many filtration operations.
Our observations suggested that the many dusts that can agglomerate may be
agglomerated or at least produce an effect like agglomeration by contact
with a filter fabric of preferred polarity. The evidence indicated that
agglomeration or this effect that resembles agglomeration occurred with
varying ease but the condition could usually be assured in the formation
of the cake when a fabric was used that had high intensity of charge
opposite in polarity to that of the particulate (refer to Figure 4).
151
-------
Charged (+)
particles
Neutral
Agglomerated
+G-C9-
+
fiber surface
donating negative (-) charges
Figure 4. Particulate agglomeration through charge neutralization
FABRIC PRESCRIPTION FOR CONTROL OF FINE PARTICLES
The following comments summarize the electrostatic needs of fabrics for
filtering fine particulates efficiently, according to our interpretations
of experimental and commercial data.
When both the necessary electrical and physical data of the particulate are
known, fabric filter requirements may be specified for attaining optimal
overall collection efficiency. Accordingly, for:
A. - Very Fine, Non-Agglomerating Particulates, the most effective
filter medium is that which becomes highly charged to a
polarity opposite to that of the particles so that the forces
of attraction are maximized. In addition, to be sure that
minimum leakage occurs during successive filtration cycles,
some of the cake must be retained on the filter surface after
cleaning. This may be achieved with a fabric of very low
rate of charge decay.
B. - Very Fine. But Agglomerating Particulates. the most effective
filter medium is that which becomes charged, sometimes to a
very high intensity, to a polarity suitably opposite to that
of the particles to cause the charges on the particles to be
neutralized. Once neutralized, particle-to-particle contact
is no longer restricted and agglomeration may proceed on the
filter surface. Depending upon the ease in effecting
152
-------
agglomeration, the rate of charge dissipation from the
fabric may be high or low. A low rate is preferred when
a difficult-to-agglomerate dust is being collected, where-
as a high rate may be desired for its value in improving
cleanability when it does not interfere with the agglom-
eration process. For some dusts, agglomeration occurs so
easily that mere neutralization by rapid charge bleed-off
by contact with a grounding or anti-static surface, for
example, is adequate to cause this desirable change.
CARRIER GAS
The composition of the gas carrying the particulates most certainly can
have an influence on charge generation and on charge retention. A gas of
high resistivity favors high charge build-up and slow charge bleed-off.
The presence of conductive elements in the gas restricts charge retention
because, of course, these paths offer a means for any charge to be carried
away. It will be apparent that the conductive component of the gas may
take many forms. At least twice, in the course of our triboelectrification
studies, we related a failure to obtain even a small amount of charge in
the testing of fabrics to the presence of radiation products in the labora-
tory atmosphere.
MOISTURE EFFECTS
Our experience indicated that triboelectric charging and charge retention
is not restricted at room temperatures by values of relative humidity of
up to at least 35 percent. No doubt, the relative humidity at which charge
bleed-off becomes high falls off quite appreciably as the higher tempera-
ture of most industrial filtration processes is reached. At some rela-
tively low moisture content dependent upon the temperature, then, charge
bleed-off becomes very rapid.
153
-------
Excess moisture in the particulate carrying gas stream may have other
effects. Below the dewpoint, for example, moisture may cause the forma-
tion of a paste of the dust on the filter surface. The severity of the
pasting effect may depend upon other factors including the water solubility
of particulate or other components of the gas stream. The end effect of
the paste formation with its subsequent surface blinding of the filter,
however, is quite obvious.
When the components of the filter system are capable of reacting, moisture
often serves to promote the reaction. The chemical reaction in itself may
not pose a serious threat to the filter process, but the reaction products
can be a real problem if they form around and become locked within the
fibers to produce an unremovable type of plug. Repetition of this process,
of course, leads to complete binding of the filter.
ARTIFICIAL CHARGING
Artificial charging or the combination of principles akin to electrostatic
precipitation and fabric filtration has been receiving more and more con-
sideration as a means for improving collection efficiency. Whether or not
the technique has real merit for upgrading the filtration of very fine
particles is not known, but a positive effect might be postulated. Cer-
tainly, according to our experience of a decade ago, the other parameters
of fabric filtration were all upgraded by applying artificial charging.
In these trials, we compared the filtration performance of certain pre-
ferred fabric media with and without the influence of an applied electrical
field. Actually, a rather large number of variations with respect to volt-
age levels and current types (d.c. and a.c.) are possible but I shall con-
fine my remarks concerning our studies to the program that applied
7500 volts a.c. between a wire passing down the center of a three inch
diameter test bag and either the test bag itself or a metal screen cage
surrounding the test bag. It will be apparent that when the bag itself
served as one electrode either it was inherently electrically conductive
154
-------
(carbon fabric) or was made so by suitable (metallizing) treatment. Both
colloidal graphite and the electro-less metallizing (copper or nickel
coating) processes were used to impart electrical conductivity to certain
conventional filter fabrics.
To determine the influence of the conductive coating and artificial
charging, four separate filtration tests with the same type of particulate
were conducted. Comparative data were obtained for the normal filter
fabric, for the conductive finished fabric of exactly the same type, for
this latter fabric with a voltage impressed between a center wire and the
fabric and finally, for the normal fabric with voltage impressed between
a center wire and a screen cage surrounding the fabric during the col-
lection cycle. In order to permit variations in cleaning methods for the
latter test, the wire cage was hinged for removal during this part of the
process. Now, to avoid extending this portion of the review excessively,
let me provide a summary of some of our conclusions.
First, not all particulate collection operations are benefited in even a
minor way by artificial charging but there appears to be a tendency for
the moderate-to-low resistivity dusts (carbon, certain cement and zinc
oxides) to improve in their collection parameters by using a conductive
filter medium or especially by applying artificial charging during the
collection cycle. For example, by substituting a conductive fabric for
the same normal filter fabric in the collection of cement dust the amount
of dust collected increased by 100 percent as the process was continued to
the same pressure drop. At the same time, the amount of plug accumulated
in the fabric was reduced by 18 percent. With the electric field applied
to the metallized fabric, collection was further increased by 37 percent
with the plug continuing to show a reduction of at least 12 percent. Thus,
compared to the normal fabric, the artificially charged fabric showed an
increase in filtration capacity by 175 percent and a reduction in plugging
by 13 percent. The improvement by artificial charging between a center
wire and a cage surrounding the normal filter fabric was the same as that
achieved by electrification of the metallized fabric. Similar studies,
again with finished cement dust but with a carbon filter fabric with and
155
-------
without artificial charging, demonstrated that electrification doubled
cement dust collection and reduced plugging by 24 percent (refer to
Figure 5).
Within the last two years, apitron, a collection system offered by Pre-
cision Industries, is claimed to offer better filtration performance by
reason of artificial charging.
Professor Gaylord Penney, whom you will recognize as the inventor of the
two stage electrostatic precipitator, demonstrated over a year ago the
usefulness of artificial electrification in fabric filtration." Using
fly ash in filter tests without and with added electrification, he not
only demonstrated that the collection cycle could be more than doubled but
also that the nature of the deposited cake was responsible for this im-
provement by reason of its "pyramid-like" structure.
Professor T. Ariman of Notre Dame, has also shown from recent studies that
significant advantages may be realized by electrifying a fabric filter.
SOME FILTRATION CASE HISTORIES
TACONITE--REVERSE AIR-JET COLLECTOR
After a comprehensive study of the fabric requirements for collecting
taconite by the reverse air-jet or blow-ring method, a polyester (Dacron)
2
needled felt of 72 cfm/ft (at 0.5" wg) permeability was selected as the
preferred filter medium. Since repeated trials verified the reliability
of this choice, one of the test bags was washed clean of the collected
dust and treated with colloidal graphite to a 2 percent add-on. When this
fabric of the very same base fiber and same permeability was used or,
rather, was tried under the same conditions as before, leakage was so very
serious from start-up that the operation had to be terminated after just a
few minutes.
Our explanation for this change in filter performance was the significant
change in electrostatic properties. Originally, the charging properties
156
-------
CONDUCTIVE AND NON CONDUCTIVE FABRICS, W/WO ARTIFICIAL CHARGING CEMENT
(FINISHED) FILTRATION A/C =3.6 CFM/FT2, 90°F, DUST LOADING =
27-28 g./MIN CLEANING - ONE VACUUM FULLBACK ONLY
Test Conditions
Wiring cross section
Fabric
Observations
Dacron (866 B) with-
out & with Cu coat-
ing., permeability
(cfm/ft2 0.5 wg)
34.5 19
Dacron (866 B) with-
out Cu coating,per-
7500 VAC meability
\ fo 34.5 cfm/ft2 0.5wg
Filter Bag
WO/Cu
Filter Bag
W/Cu
7500 VAC
7500 VAC Dacron (866 B) with
Cu coating, per-
meability
19 cfm/ft2 0.5 wg
Dacron (866 B) with
Cu coatings per-
meability
19 cfm/ft2 0.5 wg
Filter Bag
W/Cu
Compared to fabric
without Cu collect-
ability up ~ 100%
Plug down 187o
Very low leakage
Compared to fabric
without Cu & with-
out electrification
Collectability up
~ 175%
Plug down 13%
Low leakage
Compared to fabric
without Cu & with-
out electrification
Collectability up
~ 175%
Plug down 13%
Very low leakage
Compared to fabric
with Cu & without
electrification
Collectability up
~ 37%
Plug up ~ 6%
Very low leakage
Figure 5. Effect of artificial electrification
157
-------
of the dacron fiber were suitable in polarity and intensity to cause the
taconite to agglomerate into relatively large particles on the filter
surface. When the same fabric was made conductive by means of the graphite
finish, the charges were no longer adequate for agglomerating taconite be-
cause they were carried away rapidly to ground over the conductive fabric
surface.
URANIUM OXIDE AND GRAPHITE—REVERSE AIR-JET COLLECTOR
Professor Caplan reported a similar problem but offered no explanation.
In the reverse air-jet filtration that he mentioned, the collection of a
uranium oxide-graphite mixture proceeded extremely well until the fabric
became impregnated and/or coated with the graphite component of the par-
ticulate. When this condition was reached, dust leakage occurred to a
very serious extent and the operation had to be discontinued.
DISPERSION GRADE PVC--SHAKER TYPE COLLECTOR
This extremely active (electrostatically) dust was to be collected by a
new type air-shake collector but this innovation was insignificant by
comparison with the exaggerated charge needed to cause agglomeration of
this PVC dust. After proving the need for extreme polarity, high intensity
and low dissipation rate, and then establishing the capability of a highly
positive wool-nylon fabric, this new filter fabric was adopted. The fabric
was a high grade, relatively lightweight woven fabric made on the woolen
system with 75 percent wool and 25 percent nylon at a permeability of
50 cfm/ft^ (at 0.5" wg). Repeated laboratory tests verified the effec-
tiveness of this new fabric. First commercial trials in an entirely new
collector using the air-shake cleaner system and supposedly the same type
of wool-nylon filter fabric were not so favorable. In fact, the pressure
drop was higher by far than we had predicted and leakage was quite serious.
Examination of the fabric indicated that the fabric supplied for the com-
mercial installation had a permeability of 80 cfm/ft^. This high degree
of openness explained the high leakage but what about the high pressure
drop? Examination of the fabric and laboratory collection trials re-
affirmed the high permeability, the high pressure drop and the excessive
leakage. Further study of the fabric revealed that the unsatisfactory
158
-------
fabric had two less picks per inch and much less cover fiber than the
preferred fabric. With this information, a new wool-nylon fabric of
55 cfm/ft permeability was provided in the filter bags of the commercial
baghouse. This new fabric, that met the original specifications, performed
favorably in the field as well as in the laboratory.
CALCINED CALCIUM SILICATE "MCE"--REVERSE AIR-JET COLLECTOR
A number of different needled fabrics examined as the filter media both
in the field and in the pilot plant for collecting this very fine dust
failed completely, mostly because they leaked so seriously. The tests
carried out on dacron bags, regardless of construction, had to be ter-
minated early in the program because dust leakage was excessive and equi-
librium could not be attained at reasonable air-to-cloth ratios. Orion
bags, on the other hand, at the same permeability (25 cfm/ft^ at 0.5" wg)
as the tested dacron fabrics, performed favorably without leaking even as
the air-to-cloth ratio was raised to 20 to 1. When these same orlon bags
were treated with a charge dissipating (anti-static) finish without
changing its permeability, leakage again became very serious. A darvan
needled fabric also functioned quite well but only below the 20 to 1 A to
C level. At the higher flow rate, leakage became excessive even though
the pressure drop did not rise significantly.
On the basis of the observations that both orlon and darvan fabrics were
much more electronegative than the dacron bags but that the more favorable
orlon material displayed a lower rate of charge loss than either dacron or
darvan and, of course, much lower than the antistatic finished orlon, we
believe:
The reverse jet collection of this "MCE" dust is favored by
an electronegative medium (or fabric of quite opposite po-
larity) with a low discharge rate so that a protective
"filter-aid" layer of the particulate is held on the surface
at all times.
159
-------
AGGLOMERATION BY GROUNDING
Professor CapIan mentioned another filtration anomaly that I believe might
be explained on a basis of electrostatic effects. He noted that a bag
collector failed and had to be replaced by another kind of control device
when an inactive screw-conveyer was removed from the duct carrying dust to
the baghouse.
I suggest that in the original process, the electrically grounding screw
with very considerable surface area caused the dust to dissipate its charge
and become agglomerated before entering the collector. When the screw was
removed and the large charge bleed-off contacting surface was no longer
available, the bags in the collector received fine, mostly non-agglomerated
particulate. Conceivably, if the "right" filter fabric had been used, ag-
glomeration could also have been accomplished to make the baghouse feasible.
REFERENCES
lo Frederick, E. R. Chem. Eng., 68:107, 1961.
2. Frederick, E. R. Amer. Dyest. Rep., 57:31, 1968.
3. ASTM D257-61 Method.
4. Frederick, E. R. Report to Mellon Institute, December 1, 1964=
May 31, 1965.
5. Wilson, D. J. Textile Inst., 54:T97, 1963.
6. Frederick, E. R. U.S. Patent No. 2,896,263 (To Albany Felt Company,
July 28, 1959).
7. APITRON Bull., American Precision Industries, Inc., 1973.
8. Penney, G. Private Communication, Carnegie-Mellon Univ., Pittsburgh,
Pa., January 18, 1973.
9. Monezunski, J. News Release, Univ. of Notre Dame, December 28, 1973.
10. Bickelhaupt, Roy E. Electrical Volume Conduction in Fly Ash. JAPCA,
24, No. 3, 253, March 1974.
160
-------
DESIGNING A FILTER SYSTEM TO MEET
SPECIFIED EFFICIENCY AND EMISSIONS LEVELS
Richard L. Adams
Director, Fabric Filter Products
Wheelabrator-Frye Inc.
930 Fort Duquesne Boulevard
Pittsburgh, Pennsylvania 15222
When I was requested to present this paper, my immediate reaction was
"Why? We know so little about the control of submicron particulate
by using fabric filters." Upon reflection, however, it seemed to me
to be a good idea that those of us deeply involved in fabric filter
technology sit down and discuss what we know and those things we need
to learn. As a representative of the manufacturing part of the indus-
try, I can state that we can supply very little practical field data
at this point in time to substantiate some of our feelings with regard
to control of submicron particulate.
It is a well-known fact that only in the past year have acceptable
methods become available to use in sampling submicron particulate.
Even today, the number of people qualified to conduct this type of
testing is very limited. Thus it is to be expected that good field
data is so limited at this point as to be practically meaningless.
However, we do have available to us good field data that tells us
generally that a fabric filter is highly efficient, in at least a
part of the submicron range.
161
-------
With these comments as a background, let's launch into a discussion
as to what we believe the place of the fabric filter and associated
systems may be in control of submicron particulate. First, I would
like to state unequivocally that it is my opinion that both woven and
felted fabric filters represent viable means for collection of sub-
micron particulate. In the course of this paper, I will present the
reasons for this belief. In any case, the fabric filter is only part
of the collection device. The system represents the other part of
the total collection equipment and it is impossible to divorce one
from the other when examining performance. It is well-known that in
certain metallurgical processes the effects of gas cooling on the
particulate characteristics are pronounced. I believe we should,
therefore, examine several of the industrial processes generally
handled by fabric filters where we know that submicron particulate
exists so as to determine the adequacy of present system designs in
this rather new area.
One of the major sources of submicron particulate must be the metal-
lurgical fume which is generally in the submicron size range. I would
like to examine one specific metallurgical fume generating process
with the thought that the principles discussed will be applicable
pretty well across-the-board to other metallurgical processes. For
purposes of discussion, I have selected the electric arc furnace as
a device which generate? relatively large amounts of submicron par-
ticulate. As most of you know, an arc furnace will generate between
20 and 50 pounds of fume per ton of steel melted. This fume ranges
between 70 and 90 percent less than 2 microns in particle size. A
detailed size analysis below two microns is not available but we feel
that most of this will fit our discussion category of submicron
particulate =
In the area of fume pickup, there are three generally employed systems.
As you know, there are the direct shell evacuation system, the furnace
162
-------
mounted side draft hood system, and finally, the canopy hood system.
I believe that the first two of these will be totally adequate to meet
any requirements for collection of submicron particulate. In fact,
if any particulate is missed by these two types of pickup it would
probably be the larger particles which are less subject to directional
change influences. If you will examine the principle of the side draft
hood, you will see that it depends upon horizontal vectors caused by
airflow to turn the fume emitted from the furnace electrodes into the
collecting hood. Certainly, submicron particulate should be much more
susceptible to this physical force than the larger particulate. The
same will be true of the direct shell evacuation system.
I have some doubts, however, about the adequacy of a canopy hood system
in this area. For the very reasons that the first two devices appear
to be unaffected, I feel that a canopy hood system, which depends upon
thermal drive to carry the fume into the collection hood located some
20 to 40 feet away, could be adversely affected in the submicron range
by building cross-drafts. Once again, the heavier particulate given
proper thermal drive in an upward direction will probably continue
unless cross-drafts are excessive. However, the submicron particulate
because of its extremely small size and relatively large surface area
to weight ratio will probably be subject to the influences of cross-
drafts to a much greater degree. I, therefore, have reservations about
the adequacy of a canopy hood system in this area, and I believe that
we must do field testing on existing installations which appear on the
surface to be more than adequate in order to determine the performance
in this new area. Unfortunately, this testing will be neither simple
nor inexpensive.
I feel that the comments given on the process, as described above, can
be carried pretty well across-the-board to most industrial processes.
The thought would be that those processes that are subject to very
local dust or fume pickup can be adequately ventilated even in the
163
-------
submicron particulate range without undue concern. On the other hand,
those processes that depend upon remote pickup will probably have to
be restudied in some industries in order to achieve greater control of
submicron particulate.
In those industries generally using fabric filters for ventilation of
various processes, I believe that the following processes currently
have system designs that are adequate to insure pickup of the submicron
particulate:
Carbon black furnaces
Cement and other kilns
Power and industrial boilers
Primary and secondary metals reverbatory furnaces
Secondary metal rotary furnaces
Secondary metal blast furnaces
Cement clinker coolers
Driers and rotary coolers
Some of the existing systems that I believe may be inadequate from a
submicron particulate pickup standpoint would be:
Electric arc furnace canopy hoods
Basic oxygen furnace charging and tapping
Kish collection and hot metal transfer
Asbestos milling
Material handling systems if involving submicron
particulate
Charging and tapping operations in the primary
and secondary nonferrous industry
164
-------
Each of these processes must be further studied to determine the
adequacy of present systems. As you can see, we believe that the
pickup systems, in the majority of cases where fabric filters are
employed, will be adequate in the submicron particulate area. Now
I believe we should study what happens after the material is picked
up and prior to the time that it enters the filter.
The most important feature of the system between the pickup point
and the fabric filter, as far as submicron particulate is concerned,
will be in those processes utilizing gas cooling. The cooling method
may have at least two effects in the behavior of the particulate once
it reaches the fabric filter. Generally, these effects will not be
found but they occur in a sufficient number of cases so that they
warrant some discussion.
In a limited number of metallurgical processes, and in a few other
applications, shock cooling of the gases is recommended. Shock cool-
ing seems to provide a different crystalline formation of some of the
particulate from that experienced when slow cooling by radiation or
other means is employed. This different crystalline formation ob-
viously affects the filtration characteristics of the particulate
once it reaches the fabric filter.
Secondly, the use of waterspray cooling provides a high moisture
content in the carrying gas stream to the fabric filter. While the
electrostatic phenomena occurring in a fabric filter are not well-
understood, it is obvious that the high moisture content and change
in conductivity of the conducting gas stream will have a major effect
in those processes where electrostatic phenomena do occur. I am sorry
to say that we know very little about either of these two areas of
fabric filtration but I believe that they may become increasingly
important as we continue to investigate the ability of fabric filters
to efficiently handle submicron particulate at reasonable pressure drops.
165
-------
Finally, we come to the fabric filter itself, and the selection of
the proper device to handle the process effluent picked up and pre-
pared for filtration by the system designed. Once again, we must
admit to inadequate knowledge in this new area of concern. Of pri-
mary importance will be the selection of either a woven fabric filter
or a felted fabric filter. Our previous experience would indicate
that both of these filters can operate efficiently on submicron par-
ticulate. We would have some concern in the area of felted fabric
filters, however, because of the blinding tendencies experienced when
handling certain types of metallurgical fume. Is the blinding char-
acteristic a function of only particle size or is it a function of
some other characteristics of metallurgical fume that would not exist
on other submicron particulate? Frankly, I believe it is a function
of other characteristics and I believe that the felted fabric filter
is able to operate successfully in the submicron particle size range.
We know that the woven fabric filter has a history of successful
operation in handling particulate in this size range. Woven type
fabrics have been employed for years in handling particulate as fine
as carbon black, which ranges down to 300 angstroms in particle size,
and in ventilating many metallurgical processes where most of the fume
generated is submicron. I am sure that we not know all of the asso-
ciated phenomena but I believe agglomeration plays a great part in the
ability of the fabric filter to operate in this range, particularly in
handling materials such as carbon black. I believe that our limited
knowledge would suggest the selection of a woven fabric filter in most
metallurgical processes for technical reasons with the choice being
left open between a woven fabric filter and a felted fabric filter
with economics being the dictating factor on other processes even
though they involve submicron particulate.
Once the type of filter is selected, I am sure that we all agree that
cloth construction can be critical, particularly in the ability of the
166
-------
fabric filter to operate efficiently in the area under discussion.
There are other papers on the program that will cover this subject
in-depth and, therefore, I will not attempt any discussion in this
area.
I do believe, however, that we should briefly discuss the relation-
ship between the type of fabric and the method of cloth cleaning in
a woven fabric collector. The two generally available types of cloth
cleaning are, of course, reverse air and mechanical shaking. We feel
that filament type fabrics generally lend themselves to reverse air
cleaning while staple or combination type fabrics lend themselves to
shaker type cleaning. While there appears to be little difference in
the efficiency of either fabric construction as related to an individual
process, we have found that in some cases the staple or combination type
fabrics do allow operation at lower pressure drops particularly where
fine particulate is involved.
We would not expect any significant variation in efficiency due to
different fabric composition. Fabric selection will generally be
dictated by temperature and chemical considerations. In a few iso-
lated cases, we have found variations in pressure drop characteristics
that are probably due to the little understood electrostatic effect.
As indicated previously, I believe that these electrostatic effects
need to have substantially more study as we enter into the problems
of submicron filtration.
In summary, I believe it is fair to state that many of the existing
system designs used in conjunction with fabric filters will be adequate
to meet the requirements of submicron particulate capture. Further,
I believe that fabric filters as we know them today can adequately
handle submicron particulate in most cases, and finally, I feel that
now that we have the tools of submicron particulate testing available
to us, we will be able to rapidly progress our level of knowledge in
this area.
167
-------
LABORATORY GENERATION OF PARTICIPATES
WITH EMPHASIS ON SUB - MICRON AEROSOLS
Benjamin Y. H. Liu
Professor of Mechanical Engineering
University of Minnesota
Minneapolis, Minnesota 55455
INTRODUCTION
Aerosol generation is an important part of any laboratory research
program involving aerosols. The calibration of aerosol measuring or
sampling instruments, the testing of particulate control devices and
the fundamental study of aerosols all require aerosol particles with
prescribed physical or chemical properties.
During the past few years considerable advances have been made in
methods and techniques for the generation of monodisperse aerosols.
In particular, methods have been developed for generating monodisperse
aerosols of a known size and concentration, aerosols which can be used
as primary standards in the field of aerosol physics and technology.
Such monodisperse aerosol standards can now be generated from 0.01 p.m
to over 50 |j.m in particle diameter at concentration levels up to 10
particles/cc in certain size ranges. In addition, aerosols can be
generated from a variety of solid and liquid materials, and the par-
ticle size can be calculated from the operating conditions of the
aerosol generators directly to a high degree of accuracy (1 percent
169
-------
or better). Thus the need to measure the size of the generated par-
ticles by the tedious and often inaccurate optical or electron micro-
scope methods has largely been eliminated.
In this paper we will briefly review these recent advances in aerosol
generation techniques, focusing on work done in our Laboratory.
Two approaches have been used for generating monodisperse aerosol
standards, one involving the controlled disintegration of a liquid
jet by a vibrating orifice, and the other the pneumatic atomization
of a liquid and the electrostatic classification of the polydisperse
aerosol. The first method is applicable to larger particles over
0.5 um in diameter while the latter method is more suited for genera-
ting smaller particles below 0.5 um.
THE VIBRATING ORIFICE MONODISPERSE AEROSOL GENERATOR
Figure 1 is a schematic diagram of the vibrating orifice monodisperse
aerosol generator described by Berglund and Liu (1973). The generator
consists of a droplet generation and dispersion system and an aerosol
dilution and transport system. A Krypton-85 radioactive neutralizer
of 10 millicurie activity is placed within the generator drying column
to neutralize the particle electrostatic charge incurred during the
droplet generation process.
In the vibrating orifice generator, uniform liquid droplets are gener-
ated by forcing a liquid through a small (5 to 20 \JM diameter), vibra-
ting orifice by a syringe pump at a predetermined rate of 0- cc/sec.
The orifice is vibrated by a piezoelectric ceramic at a predetermined
frequency of f Hz supplied by a signal generator. Within an appro-
priate frequency range, the liquid jet is broken up into uniform
170
-------
AEROSOL
ROTOMETER
ABSOLUTE
FILTER
PRESSURE
REGULATOR
AEROSOL
GENERATOR
HIGH
PRESSURE
AIR
SIGNAL
GENERATOR
MEMBRANE
FILTER
INFUSION
PUMP
COVER
HOLDER
VARIABLE
TRANSFORMER
BLOWER
DIFFERENTIAL
PRESSURE GAGE
DISPERSED
DROPLETS
DISPERSION
ORIFICE
POROUS
PLATE
ELECTRICAL
SIGNAL
Figure 1. Vibrating orifice monodisperse aerosol generator
Left: schematic of system
Right: generator head
-------
droplets. Since each cycle of the disturbance produces precisely one
droplet, the individual droplet volume is equal to
(1)
The uniform droplet stream is then injected axially along the center
of a turbulent air jet to randomize the particle motion and to prevent
particle collision and coalescence. The dispersed droplets are then
mixed with a much larger volume of filtered dry air to evaporate the
solvent and to obtain a stable aerosol of a particle diameter
Dp = (6 Qx C/it f)1/3 (2)
where C is the volumetric concentration of the non-volatile aerosol
material in the liquid solution.
Equation (2) shows that the diameter of aerosol particles produced by
the vibrating orifice principle can be calculated from the liquid flow
rate, Q1 , the frequency, f, and the solution concentration, C. Since
these quantities can be easily measured to a high degree of accuracy,
the particle diameter can also be calculated to a high degree of
accuracy by means of Equation (2). However, when dilute solutions
(small C) are used, the non-volatile impurity in the solvent must be
taken into account in the calculation of particle diameter. Experi-
ments have shown that with proper precautions, the particle diameter,
D , can be calculated by means of Equation (2) to a considerably higher
P
degree of accuracy than can be measured by the conventional microscope
techniques.
Table 1 is a summary of the operating conditions of the generator for
three orifice sizes, and Figure 2 shows some typical particles produced
by the generator.
172
-------
Table 1. CHARACTERISTICS OF THE VIBRATING ORIFICE
MONODISPERSE AEROSOL GENERATOR
Diameter
of liquid
orifice
(urn)
5
10
20
Nominal
frequency
(kHz)
450
225
60
Droplet
diameter
(tini)a
15
25
40
Particle
diameter
range
(um)b
0.6 - 15
1.0 - 25
1.8 - 40
Nominal
concentration
(par tide s/cc)c
273
137
36
Continuously adjustable over an approximate 25 percent range by
varying the frequency
Obtainable by the solvent evaporation technique
«
Theoretical concentration based on the nominal aerosol output of
100 liters per minute
Theoretically the vibrating orifice generator can also produce
aerosols of a known particle concentration. The theoretical aerosol
concentration is given by
Nth = f/Qa
(3)
where Q is air flow rate.
3
The actual aerosol concentration at the generator output is less than
the theoretical concentration due to particle loss in the drying and
neutralization chambers and in the transport system. However, the
operation of the generator is sufficiently stable so that after these
losses are determined the aerosol concentration at the generator out-
put is known. Figure 3 (Liu, Berglund and Agarwal, 1974; shows the
measured aerosol concentration at the generator output expressed as
a percentage of the theoretical concentration and as a function of
particle size. The actual concentration is seen to be about 81 per-
cent (+ 5 percent) of the theoretical concentration for particles
173
-------
,
o
o
o
O O
O P
o
o
o
O O
o o
(a)
(b)
(c)
Figure 2. Monodisperse particles produced by the vibrating orifice generator
(a) 3.7 urn diameter solid methylene blue particles
(b) 9.5 micron diameter liquid DOP (di-octyl phthalate) particles
collected on an oil-phobic slide
(c) solid sodium chloride particles of 27.4 cubic micron volume
-------
smaller than 6 um diameter, and the actual concentration decreases
steadily with increasing particle size. This particular feature of
the generator is very convenient for determining the sampling effi-
ciencies of aerosol measuring and sampling devices. For instance,
to determine the absolute sampling efficiency of an optical particle
counter, it is necessary only to apply the aerosol to the counter and
to compare the counts registered by the counter with counts calculated
from the known aerosol concentration and the sampling flow rate of the
counter.
i.o
O .8
.4
.2
i i
O MASS METHOD
• COUNT METHOD
I
2 5 10 20
PARTICLE DIAMETER, p.m
40
Figure 3. Output aerosol condensation of the
vibrating-orifice aerosol generator
as a function of particle diameter
(Liu, Berglund and Agarwal, 1974)
GENERATION OF SUB-MICRON AEROSOL STANDARD BY ELECTROSTATIC CLASSIFICATION
For generating sub-micron aerosol standards below 0.5 urn, the system
shown in Figure 4 has been developed (Liu and Pui, 1974). The device
175
-------
produces uniform (relative standard deviation 0.04 - 0.08) particles
at concentration levels up to 10 particles/cc by pneumatic atomiza-
tion and electrostatic classification. The system consists of a
Collision atomizer, a Krypton-85 radioactive neutralizer, a diffusion
dryer, a differential mobility analyzer, and an electrometer current
sensor.
BBS
Figure 4. Apparatus for generating sub-micron
aerosol standards (Liu and Pui , 1974)
In the apparatus a polydisperse aerosol is produced by the Collision
atomizer. The aerosol particles are then brought to a state of charge
equilibrium with bipolar ions produced by the ionizing beta radiation
from the Kr-85 source. Below a size of about 0.075 jim diameter, most
of the particles are either electrically neutral or carry one elemen-
tary unit of charge with an electrical mobility of
Z = 300 eC/3rt
P
D cm /volt- sec.
P
176
-------
where e = 4.8 x 10 esu is the elementary unit of charge, C is the
dimensionless slip correction, (j, is the gaseous viscosity, and D (cm)
is the particle diameter. These singly charged particles can then be
separated into monodisperse fractions by means of the differential
mobility analyzer.
The differential mobility analyzer shown in Figure 4 is in the form
of a cylindrical condenser with concentric electrodes. The inner
electrode is held at a high voltage and the outer tube is grounded.
Under a given set of operating conditions, charged particles in the
aerosol stream flowing along the outer tube are deflected through the
inner clean air core. If these particles have the appropriate elec-
trical mobility, they would arrive at the exit slit near the lower
end of the center electrode and be swept out by the air stream flowing
through the slit. The electrical mobility of the particles is given
by
Zp = [Qo ' <1/2>
-------
particles in the aerosol stream is small, particularly for small
particle sizes, the absolute concentration of the aerosol can be
measured by measuring the total current associated with the aerosol
particles,
I = qe e N (6)
where q (cc/sec) is the aerosol flow rate and N (particle/cc) is the
aerosol concentration. The electrometer current sensor shown in
Figure 4 is used to measure this current from which the aerosol con-
centration, N, can be calculated by means of Equation (6).
This generator has been used to provide an absolute concentration
standard for calibrating condensation nuclei counters. The particle
diameter accuracy is about + 2 percent and concentration accuracy,
+ 5 percent. These accuracies can be further improved if necessary.
By the use of a fluidized coal dust feeder, in place of the atomizer
shown in Figure 4, monodisperse coal particles having the same degree
of monodispersity have been generated (Liu, Marple, Whitby, and
Barsic, 1974) .4
REFERENCES
1. Berglund, R. N. and B. Y. H. Liu. Generation of Monodisperse
Aerosol Standards. Env. Sci. Tech. 7:147-153, 1973.
2. Liu, B. Y. H., J. K. Agarwal, and R. N. Berglund. Experimental
Studies of Optical Particle Counters. To be published in Atm.
Env., 1974.
3. Liu, B. Y. H. and D. Y. H. Pui. A Sub-micron Aerosol Standard
and the Primary, Absolute Calibration of the Condensation Nuclei
Counter. J. Colloid & Interface Science. 47:155-171, 1974.
4. Liu, B. Y. H., V. A. Marple, K. T. Whitby, and N. J. Barsic.
Size Distribution Measurement of Airborne Coal Dust by Optical
Particle Counters. To be published in A.I.H.A.J., 1974.
178
-------
METHODS FOR DETERMINING PARTICIPATE MASS AND SIZE PROPERTIES;
LABORATORY AND FIELD MEASUREMENTS
J. D. McCain
Research Physicist
Southern Research Institute
2000 9th Ave. South
Birmingham, Alabama 35205
INTRODUCTION
In order to determine the fractional efficiency of particulate control
devices, measurements must be made of the size and concentration of
particles suspended in the flue gas at both the inlet and outlet of
the devices. In making these measurements, one encounters a wide variety
of testing conditions. Flue gas temperature, pressure, moisture content,
and the physical properties of the particulate vary widely from device
to device. Concentrations vary by orders of magnitude from inlet to
outlet, and from one control device to another. Because of this com-
plexity and the limited range of testing devices, more than one instru-
ment is required to give complete information on the particle size dis-
tribution, even at a single site.
Currently, practical particle sizing techniques for making fractional
efficiency measurements fall into three categories: inertial, diffu-
sional, and optical. Of these, only the optical and, to a certain extent,
the diffusional methods offer the capability of real time, continuous
monitoring. An electrical particle counter being developed at the
179
-------
University of Minnesota is a promising candidate device for real time
data acquisition for particles in the 0.01 to 0.5 um range. Also, an
instack "Beta-Tape" impactor being developed for the Environmental Pro-
tection Agency by GCA/Technology Division may provide very nearly real
time data for particles in the 0.3 um size range.
INERTIAL METHODS
Impactors, impingers, cyclones, and centrifuges have been used for many
years for determinations of particle size distributions. Because of its
compact arrangement, the cascade impactor has generally been found to be
the most suitable inertial device for mass distribution measurements of
pollution emission sources. ' In most cases, the impactors can
be inserted directly into the duct or flue, thus eliminating many con-
densation and sample loss problems which occur when probes are used.
DIFFUSIONAL METHODS
A diffusion battery consists of a number of long, narrow, parallel
channels, or a cluster of small bore parallel tubes. Systematic varia-
tions in length and number of channels (or tubes) and in the aerosol
flow rate are used as a means of measuring the number of particles in a
selected size range. It is assumed that once a particle diffuses to the
wall of a diffusion battery, it will adhere and thus be removed from
the samole eas stream. Because particle diffusivities increase with
decreasing particle size, the fraction of an influent aerosol that
penetrates a battery will depend on the particle size distribution
of the aerosol. The penetration of the battery can be measured with
a condensation-nuclei (CN) counter. Figure la shows a typical diffusion
battery geometry used in the work described in this paper, and Figure Ib
shows the penetration characteristics of this battery under typical
operating conditions.
180
-------
Figure la. Parallel plate diffusion battery - the
batteries have 12 or 100 channels,
0.1 x 10 x 48 cm
90
80
*1 70
g
"o 60
o! 5O
40
30
20
10
0.01
O.I
Particle Diameter,
Figure Ib. Penetration curves for monodisperse aerosols
(100 channels, 0.1 x 10 x 48 cm)
181
-------
OPTICAL METHODS
Optical/Electron Microscopy
One method for determining the concentration and size distribution of
particles in a process gas stream is to collect the particles on a suit-
able medium (i.e., a membrane filter, electron microscope carbon film
substrate, etc.) by filtration, electrostatic deposition or some other
suitable technique. This sample can subsequently be examined at the
microscope and by manual or automated counting techniques arrive at a
size distribution based on some characteristic dimension of the par-
ticles. In practice, this method tends to be slow. Furthermore, the
surface density of the particles on the collection medium must be low
in order to avoid overlapping particles, thus, the total gas volume
sampled may be small and may not be representative of the total process
gas stream.
Photoelectric Particle Counters
Photoelectric or optical particle counters function on the principle
of light scattering. Each particle in a continuous flowing sample
stream is passed through a small illuminated volume. Light scattered
by the particle is imaged on the surface of a photodetector during the
time of the particle is illuminated. The intensity of the scattered
light is a function of particle size, shape, and index of refraction.
Photoelectric particle counters will give reliable information if the
concentration of particles is such that the probability of illuminating
more than one particle at a time is low. Typically, this restriction
2
places an upper limit of about 300 particles/cm for instruments pro-
viding size information down to diameters of 0.3 \aa, the practical lowest
sensitivity for optical sizing. Thus, optical particle counters are
effective in the same size regime as inertial sizing devices. Disadvan-
tages are the necessity to dilute the sample to number concentrations
3
less than 300/cm and
refraction and shape.
3
less than 300/cm and the dependence of the calibration upon the index of
182
-------
MEASUREMENT TECHNIQUES AS USED IN CURRENT PRACTICE
IMPACTORS
Table 1 shows some characteristics of several commercially available
cascade impactors. It is usually impractical to use the same impactor
at the inlet and outlet of a pollution control device for efficiency
measurements because of the difference in particle concentration. For
example, if a sampling time of thirty minutes is adequate at the inlet
of a control device with a collection efficiency of 99 percent, approx-
imately 3000 minutes (two days), of sampling would be required at the
outlet for the same amount of sample to be collected if the same sam-
pling flow rate is used. Although the flow rates of impactors can be
varied somewhat, they cannot be adjusted enough to compensate for this
difference without causing undesirably large shifts in instrumental
calibration and other problems. For example, extremely high flow rates
result in particle bounce and in scouring of impacted particles from the
lower stages of the impactor where the jet velocities become extremely
high. On the other hand, using a high flow rate for inlet sampling
may necessitate short sampling times which can result in atypical sam-
ples being obtained as a result of momentary fluctuations in the particle
concentration or size distribution within the duct.
The actual extraction of a size fractionated sample from a gas stream
using cascade impactors is a well established procedure at this
12358
time. ' ' ' ' The following paragraphs deal mainly with unexpected
or non-ideal behavior that has been encountered in field applications,
Wall Losses
Particles are lost within an impactor by diffusion and impaction of the
9 10
walls and jets. Lundgren and Gussman ejt al_ have shown that the losses
per stage can amount to as much material as is collected by the stage.
Measurements of mass concentrations taken with impactors in the course of
183
-------
Table 1. SIZE FRACTIONATING POINTS OF SOME COMMERCIAL CASCADE
IMPACTORS FOR UNIT DENSITY SPHERES
Stage
Cyc
0
1
2
3
4
5
6
7
8
Modified
Brink
(0.85 LPM)
18.0 urn
11.0
6.29
3.74
2.59
1.41
0.93
0.56
Andersen
Mark III
(14 LPM)
14.0 urn
8.71
5.92
4.00
2.58
1.29
0.80
0.51
U. of W.
(Pilat)
(14 LPM)
39.0 urn
15.0
6.5
3.1
1.65
0.80
0.49
C • R.« C •
Tag
(14 LPM)
11.1 \aa
7.7
5.5
4.0
2.8
2.0
1.3
0.9
0.6
field work performed by Southern Research Institute have consistently
produced results about 70 percent as large as those obtained by the
non-sizing, standard techniques. Laboratory work is now underway at SRI
to determine the magnitude of interstage losses in the various cascade
impactors commonly used for stack sampling.
Reentrainment
All the particles that strike a collection stage do not stick. A tech-
nique used to enhance the retention of particles on the original impac-
tion site is to coat the impaction substrates with a suitable viscous
grease. One approach that was found to be satisfactory was to make a
10 to 15 percent suspension or solution in benzene of high vacuum sili-
cone grease or certain ethylene glycol compounds used in chromatographic
columns. After placing an appropriate quantity of this suspension on the
collection surface and allowing the benzene to evaporate, the coated
184
-------
substrates are baked for about an hour at 400 F and desiccated until
actually used. As yet, no coating suitable for use at temperatures over
400 F has been found; however, some hi
to offer promise for this application.
400 F has been found; however, some high temperature lubricants appear
Even with greased substrates, significant scouring and loss of material
occurs on the last stages if the jet velocities are too high. Experi-
mentally, we have found a value of about 65 m/sec to be the maximum
velocity usable without reentrainment or grease erosion using greased
substrates, and about 35 m/sec using ungreased substrates. In effect,
these phenomena place an upper limit on the flow rates at which an im-
pactor may be operated to obtain a valid particle size distribution.
One model of the Andersen Stack Sampler uses glass fiber filters as
impaction substrates. These filters seem to be a satisfactory alterna-
tive to a greased substrates in minimizing particle bounce and reentrain-
ment. Glass fiber substrates have also been used with the Brink im-
pactor at high temperatures.
Weighing Accuracy
The maximum stage loadings for any of the currently available impactors
are about 10 mg or less, depending on the impactor stage, the material
being collected, and the operating conditions of the impactor. If the
sample collected at the most heavily loaded stage can be, at most, only
10 mg, some stages will collect samples of only a few tens or hundreds
of micrograms. In order to maintain reasonable accuracy in the results,
a weighing accuracy and sensitivity of about 10 to 30 micrograms is
necessary.
Techniques which have been used to minimize problems in weighing to
the required accuracy are: to reduce the tare weight of the collection
stages by using lightweight inserts made of aluminum foil, stainless
steel shim stock, or glass fiber filter material. In instances in which
185
-------
the concentration of large particles is high, as compared to the concen-
tration of fine particles, two techniques are commonly used to prevent
overloading of the first stages of the impactor before obtaining ade-
quate samples on the last stages. These techniques are to purposely
bias the sample against large particles by pointing the nozzle down-
stream or to use cyclone precollectors to remove large particles before
the sample enters the impactor. The use of lightweight substrates and
cyclone precollectors is becoming more common. This combination gives
the maximum weighing accuracy and still permits near isokinetic sampling
to be used, allowing one to obtain some information on the concentra-
tions of large particles. The capture efficiency of the precollector
cyclone must be taken into account when doing data analysis; however,
or the results will not be representative of the true aerosol for large
particle sizes.
Isokinetic Sampling
A selection of nozzles having different bores is available for each im-
pactor so that at a single volumetic flow rate, several velocities may
be achieved at the inlet of the impactor. Once a sampling velocity is
chosen, however, it cannot be changed during a test because the frac-
tionation points of each collection stage would change. Since the
impactor flow rate is fixed during a test, any velocity fluctuations in
the process gas stream will introduce errors in the sampling of the
larger particles. Traverses of a duct are best obtained by suitably
averaging several single point samples.
Particle Size Calculations
The Stokes diameter may be used to describe a particle size. This is
the diameter of a sphere having the same density and terminal velocity
as the particle, and frequently provides a reasonable approximation of
the true physical dimensions of the particles. In the presentation of
much cascade impactor data, an aerodynamic diameter is used. This is
186
-------
the diameter of a sphere of unit density which has the same aerodynamic
behavior as the particle; that is, the particle behaves in the impactor
in the same way as would a water droplet of the indicated size.
Stage collection efficiencies for cascade impactors are calculated using
4
equations developed by Rantz and Wong. The effective stage fractiona-
tion points (D,-Q) may be determined for unit density spheres (as was
done in Table 1) and corrected for density as required, or approximate
physical diameters calculated for spheres having the estimated density
of the aerosol particles, depending upon the intended use of the data.
An "average" density can be calculated from true particle volume-weight
data taken with a helium pycnometer for a representative sample. If
the chemical composition of the particles is known, the bulk density
may be used. In some cases, the aerodynamic diameter may be the only
information needed and density is not a factor.
The validity of size information based on an average density depends
upon the uniformity of the density from particle to particle. A mixture
of particles having very different densities could cause large errors in
attempts at estimating true diameters of the particles collected on the
various stages.
The results of a typical set of measurements based on 20 samples is
shown in Figure 2. This figure presents mean mass concentration in equal
logarithmic size intervals centered on the indicated sizes and probable
error bands.
OPTICAL AND DIFFUSIONAL METHODS
Dilution
Because of the concentration limits for operating optical counters
o c
(300 particles/cm ) and condensation nuclei (CN) counters (10 particles/
3
cm ), problems with condensation in the sampling lines, and losses due
187
-------
10
u.
o
s
O.I
0.01
O.I
I
PARTICLE DIAMETER, jun
10
Figure 2. Mass size distribution for a. coal fired power
plant burning western coal as measured with
cascade impactors
188
-------
to agglomeration during the measurement process, it is necessary to
dry and dilute the sample aerosol before it reaches these devices.
Figure 3 shows schematically the testing configuration for optical and
diffusional sizing. The sample is introduced at the apex of a perforated
cone and clean dry dilution air is pumped through the perforations,
creating a highly turbulent mixing zone. In verifying the performance
of the diluter in the laboratory using test aerosols, it was found that
calculated and measured dilution factors agreed to within the uncertainty
in measuring the sample and dilution air flow rates. The concentration
was found to be uniform for sampling points from wall to wall across the
body of the diluter.
Plugging of the sample metering orifice can be a problem, even when
condensation does not occur. To prevent this, a cyclone precollector
with a D,.,, of about 2 p.m is used to eliminate large particles. Typical
sample flow rates are from 0.1 1pm to 5 1pm with the cyclone flow rate
maintained at about 14 1pm.
Diffusional losses in the probes and sample lines used in our work are
estimated to be about 98 percent at 0.001 ^im diameter, 25 percent at
0.005 |_im diameter, and 12 percent of the 0.01 jam diameter particles.
Strom has shown that particles having diameters less than about 2 urn
are not lost by impaction or settling for a wide range of conditions.
Thus, losses in the sampling lines used in this work are probably not
significant for particles having diameters between 0.005 p.m and 2 p.m.
Optical Sizing
A Climet Particle Analyzer, Model No. CI-201, is the primary instrument
used in our field work. Other manufacturers of similar instruments are
Royco and Bausch and Lomb. These instruments use light scattering by
single particles to obtain particle size information. A view volume is
189
-------
Flowmeters
Cyclone Pump
Process
Exhaust
Line
Neutralizer
Flowmeter
Particulate
Sample Line
Diffusional Dryer
(Optional)
Charge
Neutralizer Pressure
Balancing
Line
Recirculated
Clean Dilution
Air
Filter
Pump
Bleed
Figure 3. Optical and diffusional sizing system
190
-------
located at the focus of an illuminator optical system, and the photo-
multiplier is used to detect scattered light from particles as they
pass through the view volume. The amplitude of the scattered light
pulses is related to the particle size and the rate at which the pulses
occur is related to the particle concentration. Operated in the con-
figuration shown in Figure 3, with a cyclone precollector, such a
counter has an effective upper limit of about 2 p.m for sizing. Also, the
counters have inherent lower limits of about 0.3 p.m. Thus, the counters
respond to a limited size range, but give information in an important
regime, that where data from the diffusional and impactor measurements
converge.
Optical particle counter responses are affected by the index of refrac-
tion, and shape of the particulate. Some generalization may be made
13
from Hodkinson1s review.
When using white light as a source of illumination, the response to an
assembly of randomly oriented, identical, nonspherical particles will
be the same as that for the spherical particles of equal mean volume
whose polydispersity resembles the "orientation polydispersity" of these
particles. The major effect would be more apparent polydispersity than
actually exists.
If the scattered light is collected in some small angle about the forward
direction, and white light is used, the dependence of the amplitude
upon index of refraction can be minimized. For particles greater than
2 [j.m diameter, the amplitude of the forward scattered light pulse is
almost independent of the index of refraction and is proportional to
the cross-sectional area of the particles. For particles less than 2 p.m
diameter, however, there is no simple relationship between particle size
and amplitude, and the index of refraction is more important. Berglund
found that measurements deviated from theory by as much as a factor of
two in the size range of 0.4 p.m to 1 nm and in some cases the same
14
response was obtained for several particle sizes.
191
-------
Figure 4a shows our laboratory calibration for polystyrene latex spheres
having an index of refraction of 1.6. Calibration shifts caused by
refractive index changes can be made from a theoretical basis; however,
because of the uncertainty in making a theoretical correction for index
of refraction (n), we generally correlate the optical data with sedi-
mentation data, a method which is independent of refractive index.
Referring again to Figure 1, if the diffusion batteries are laid on their
sides, so that the long dimension of the slots is horizontal, the most
important mechanism for the removal of large particles will be sedimen-
tation, with rather high efficiencies being obtained for micron-sized
particles. Concentrations of particles producing particular optical
responses entering and exiting the sedimentation chambers can be measured
with the optical particle counter and Stokes diameters based on sedi-
mentation losses can be calculated for the particles producing the par-
ticular scattered light intensity. A typical set of correlations ob-
tained in this manner at a coal fired power plant is shown in Figure 4b.
Diffusional Sizing
Fuchs has reviewed diffusion battery sizing work up to 1956, while
16 17 18 19
Sinclair, Breslin et al, Twomey, and Sansone and Weyel have
reported more recent work, both experimental and theoretical.
Diffusion measurements are less dependent upon the aerosol parameters
than the other techniques discussed and perhaps are on a firmer basis
theoretically.
Disadvantages are the bulk of the diffusion batteries and peculiar pro-
blems introduced by particular testing situations. For example, on
one occasion, when testing emissions with a high gaseous SCL content,
particles were actually "created" within the diffusion batteries by
oxidation of S0? to SO- and subsequent formation of macro-molecular
192
-------
Voltage Discriminator Level, V
o o — rc
ro bi b c
>
*S
f
/
/
/
'
'
^
X
y
/
f
/
/
PANEL METC
Ola. C
-03 0
0.5 1.
1.0 1.
3.0 3
>
:R
rue
.45
0
6
.0
0.3 0.5 1.0 2.0
Particle Diameter,
3.0
Figure 4a. Calibration curve for climet optical par-
ticle counter. Polystyrene latex (PSL)
spheres were used as standards
Sedimentation Diameter, urn
0 -
en b
• 12 CHAN
A 100 CHAI
NEL BA
4NEL B
PTERK
ATTE
' DA'
»Y [
9
t
rA
>ATV
/
1
I
t
/
1
/
\
/I
jf
7
O.I 0.5 J.O
Indicated Optical Diameter (equlv. PSL dia.). um
Figure 4b. Correlation of optical and sedimentation
diameters
193
-------
clusters of sulfuric acid molecules. As in the case of optical sizing,
dilution and drying of the sample air is necessary to prevent coagulation,
growth of hygroscopic particles, and water condensation.
The geometry chosen for our experimental work was that of parallel
plates (see Figure 1), partly because of ease of fabrication and availa-
bility of suitable materials, but also because sedimentation can be
ignored if the slots are vertical, while additional information can be
gained through settling if the slots are horizontal. The mathematical
expression for penetration (n/nQ) of a rectangular slot or parallel
plate diffusion battery by a monodisperse aerosol was given in series
20
form by Gormley. The coefficients were calculated and tabulated by
Twomey.
The data reduction technique used in our work was suggested by Sinclair.
A nomograph is prepared using the penetration for each diffusion battery
geometry and flow rate and a large number of monodisperse particle
sizes. Comparing this nomograph with experimental penetrations, one
calculates the particle size distribution using a "graphical stripping"
process.
By using several diffusion batteries, each at several flow rates, a
large number of data points can be obtained. The inlet and outlet con-
centrations may be measured with a single CN counter, or in cases where
the concentrations fluctuate rapidly, the inlet and outlet concentra-
tions can be monitored continuously using two CN counters. Flow pulsa-
tions caused by the cyclic processes in the CN counters were minimized
by using anti-pulsation devices as described by Sinclair.
Diffusional sizing is independent of density, index of refraction, and
to a large extent, shape. Further, if the response of the CN counter
is linear, diffusional sizing and control device efficiencies derived
therefrom are independent of errors in calibration of the CN counter.
194
-------
RESULTS
To date, the methods described in the previous sections have been used
to determine fractional efficiencies of four full scale and six pilot
scale control devices. The particulate sources included coal fired
boilers burning both eastern (moderate to high sulfur) and western
(low sulfur) coals, an open hearth furnace, and S0~ absorbers installed
on a sulfite pulp mill and a coal fired power boiler.
Typical size distributions of the influent to some control devices are
shown on a mass basis in Figure 5 which includes data from a coal fired
boiler and the open hearth furnace. Figure 6 shows size distributions
for these same sources over the range from 0.01 p.m to 1.0 p.m as determ-
ined by optical and diffusional methods together with control device
outlet data for a scrubber installed on the open hearth furnace, and
an electrostatic precipitator installed on the coal fired boiler.
Transformation from number distribution to mass distributions up to a
limiting size of 1 |_im was done for data from one of the coal fired
boilers. Comparison of the mass distribution thus obtained with the
mass distribution obtained with impactors showed a reasonable agreement
(+ 50 percent) on a cumulative mass basis.
CONCLUSIONS
Although no single particle size measuring device was found suitable in
both the fine and ultrafine particle size regimes, the combination of
impactors, optical counters, and diffusion batteries with condensation
nuclei counters has been successfully used to measure particle size
distributions and fractional efficiencies from 0.005 p.m to 10 |j.m dia-
meter. It is believed that this combination represents a viable package
which will continue to be useful in making measurements of this type.
195
-------
IU,ULHJ
I,OOO
0
TJ
to
o>
E 100
10c
-
-
• /
D
' A
: V
/
r
1 1 1 1
J**r°-
A
/
/
i i i
7
/
/
-1
).l 1 10 1C
Particle Diameter,
A Coal fired boiler (Precipitator A)
D Open hearth process (steam-hydro scrubber)
Figure 5. Typical inlet mass distribution
data from pollution sources
196
-------
10'
10
10'
I0a
to
I I04
a io3
s
C
o
o
o
o 10'
IOV
10" -
10"
Open Hearth Process-,
Steam-Hydro Scrubber
Coal-Fired Boiler-,
Electrostatic Precipitator
Mill
0.01
O.I 1.0
Particle Diameter,ym
Inlet
Outlet
i 11
10.0
Figure 6. Typical inlet and outlet cumulative number
distribution data from pollution sources
197
-------
REFERENCES
1. May, K. R. The Cascade Impactor: An Instrument for Sampling Coarse
Aerosols. J. Set. Instr. 22, October 1945.
2. Andersen, A. A. New Sampler for the Collection, Sizing, and Enumera-
tion of Viable Airborne Particles. J. of Bacteriology. 76, 1958.
3. Brink, J. A., Jr. Cascade Impactor for Adiabatic Measurements. Ind.
and Eng. Chem. 44, June 1952.
4. Rantz, W. E. and J. B. Wong. Impaction of Dust and Smoke Particles.
Ind. and Eng. Chem. 50, April 1958.
5. Pilat, M. J., D. S. Ensor, and J. C. Busch. Cascade Impactor for
Sizing Particulates in Emission Sources. Am. Ind. Hygiene Assoc. J.
32, August 1971.
6. Liu, B. Y. H. , K. T. Whitby, and D. Y. H. Pui. A Portable Electrical
Aerosol Analyzer for Size Distribution Measurement of Submicron
Aerosols. (Paper No. 73-283 presented at the 66th Annual Meeting of
the Air Pollution Control Association. June 1973.)
7. Pilat, M. J. University of Washington, Department of Civil Engin-
eering. Private communication.
8. Cohen, J. J. and D. N. Montan. Theoretical Considerations, Design,
and Evaluation of a Cascade Impactor. Am. Ind. Hygiene Assoc. J.
March-April 1967.
9. Lundgren, D. A. An Aerosol Sampler for Determination of Particle
Concentration as a Function of Size and Time. J. Air Pol. Con.
Assoc. 17, April 1967.
10. Gussman, R. A., A. M. Sacca, and N. M. McMahon. Design and Calibra-
tion of a High Volume Cascade Impactor. J. Air Pol. Con. Assoc.
23, September 1973.
11. Strom, L. Transmission Efficiency of Aerosol Sampling Lines.
Atmos. Env. 6, 1972.
12. Haberl, J. B. and S. J. Fusco. Condensation Nuclei Counters:
Theory and Principles of Operation. General Electric Technical
Information Series, No. 70-POD 12. 1970.
13. Hodkinson, J, R. The Optical Measurement of Aerosols. In: Aerosol
Science, Davies, C. N. (ed.). Academic Press, 1966.
198
-------
14. Berglund, R. N. Basic Aerosol Standards and Optical Measurements
of Aerosol Particles. Mech. Eng. Dept., U. of Minnesota. (Ph.D.
dissertation. 1972.)
15. Fuchs, N. A. The Mechanics of Aerosols. New York, The McMillan
Co., 1964.
16. Sinclair, D. A Portable Diffusion Battery. Am. Inst. Hygiene
Assoc. J. November 1972.
17. Breslin, A. J., S. F. Guggenheim, and A. C. George. Compact High-
Efficiency Diffusion Batteries. Staub-Reinhalt.Luft. 31, August
1971.
18. Twomey, S. The Determination of Aerosol Size Distributions from
Diffusional Decay Measurements. J. F. I. February 1963.
19. Sansone, E. B. and D. A. Weyel. A Note on the Penetration of a
Circular Tube by an Aerosol with a Log-normal Size Distribution.
Aerosol Science. 2, 1971.
20. Gormley, P. G. and M. Kennedy. Proc. Roy. Irish Acad. 52A, 1949.
21. Fuchs, N. A., I. B. Stechkina, and V. I. Starasselskii. On the
Determination of Particle Size Distribution of Polydisperse Aerosols
by the Diffusion Method. Brit. J. Appl. Phys. 13:280-281, 1962.
22. Liu, B. Y. H. University of Minnesota, Mechanical Engineering
Department. Private communication.
199
-------
MOBILE FABRIC FILTER SYSTEM: DESIGN AND PRELIMINARY RESULTS
Robert R. Hall, Chemical Engineer
Reed W. Cass, Mechanical Engineer
Air Pollution Control Laboratory
GCA/Technology Division
Burlington Road
Bedford, Massachusetts 01730
INTRODUCTION
The high efficiency capabilities of fabric filters and the increasing
need to control fine particulate emissions has resulted in an intensive
effort to learn more about the parameters determining the performance of
filter systems. While considerable data are available on the overall
mass efficiency of fabric filters, only limited data are available on
their collection capabilities for fine particulates. Fabric filters are
therefore the subject of many laboratory and field studies, some with
special emphasis on fractional size efficiencies.
One advantage of laboratory experiments is that the experimenter can cus-
tom design the total system so that the parameters under study may be
conveniently and systematically varied while those not being studied can
be held constant. Draemel was able to evaluate 123 fabrics with various
dusts while studying the relationship between clean cloth fabric structural
parameters, dust parameters and filter performance. Laboratory situations
are well suited to wide ranging studies of the fundamental effects and
interrelationships of fabric filter parameters. However, a major disad-
vantage of laboratory studies is that the test aerosol seldom, if ever,
201
-------
duplicates the real industrial aerosol. The basic techniques for dust
dispersion are compressed air dispersion and venturi mixing. These methods
may fail to generate a fine particle distribution comparable to representa-
tive industrial sources because of incomplete dust redispersion. Many fine
particulates encountered in the field result from condensation processes
such as the zinc oxide fume from a secondary brass foundry. Although this
operation could be simulated by boiling zinc in a chamber with sufficient
oxygen to form zinc oxide, this technique would be difficult and not en-
tirely satisfactory. In addition to duplicating the particle size and con-
centration properties of an actual field emission, one would also desire
to duplicate the other aerosol properties such as chemical composition,
density, shape and surface characteristics, and electric charge properties,
as well as the temperature, humidity, and contaminants present in the
gaseous stream. Problems in extrapolating laboratory performance to field
performance are often encountered, since differences in aerosol properties
such as those mentioned above are very common.
Field studies of operating industrial fabric filters do not, of course,
present the problem of dust generation. The Environmental Protection
Agency has and is now sponsoring field tests that will provide important
data on fabric filter performance during normal operations. However, it
is generally not possible, or at least not practical, to vary the clean-
ing parameters, change the fabric, vary the filtration velocity or make
other changes in the operation of an industrial fabric filter. In order
to study the effects of fabric filter parameters when filtering an actual
industrial effluent stream, it is necessary to vary these parameters in
the field. In an attempt to provide this needed information, the En-
vironmental Protection Agency has contracted with GCA/Technology Division
to design, fabricate and operate a mobile fabric filter system.* The
design characteristics of this apparatus and some preliminary field data
collected with this system are summarized in this paper.
*Contract Number EPA 68-02-1075.
202
-------
MOBILE FABRIC FILTER SYSTEM DESIGN
GENERAL DESCRIPTION
The mobile fabric filter system is designed for the purpose of deter-
mining the effects of dust properties, fabric media, cleaning parameters
and other operating parameters on fabric filter performance. Specifically,
the mobile fabric filter system has the following capabilities. Filtra-
tion can be conducted at cloth velocities as high as 20 fpm with a pres-
sure differential up to 20 in. of water, and at gas temperatures up to
550°F. The mobile system can be adapted to cleaning by mechanical shaking,
pulse jet or low pressure- reverse flow. Cleaning parameters can be varied
easily over broad ranges. One to seven filter bags of any fabric media,
4 to 10 feet long and up to 12 in. in diameter may be used. Automatic in-
struments and controls permit 24-hour operation of the mobile filter
system.
The system is transported to field sites on a 1-1/2 ton stake truck having
a body platform 12 feet long and 7 feet wide. Although the equipment will
be operated on the truck in most cases, it can be removed from the truck
with a small truck-mounted crane and operated at locations not accessible
to the truck. The heavier components of the system are the primary fan
(about 400 pounds), the compressor (also about 400 pounds), the five sec-
tions of the filter housing (total weight about 600 pounds), and the con-
trol console that weighs approximately 200 pounds. Figures 1, 2, and 3
are schematic diagrams of the mobile fabric filter system.
SPECIFIC DESIGN FEATURES
In order to minimize any possible short- or long-term corrosion problems
caused by test aerosols or climate factors, the five section filter
housing was constructed of stainless steel. Section 1 consists of a
203
-------
AUTOMATIC COKTROL LOOP
BYPASS
RETURN
TEST
INLET
5
s
s
FLOW RATE SENSOR
BLOWER
K,0.
/H Ł
r
MANIFOLD
AUTOMATIC
CONTROL
VALVE
N.C.
N.C.g
f
REVERSE
AIR FLOW
BAG
COMPARTMENT
TEMPERATURE
SENSOR*
HOPPER
n
REVERSE
AIR
CONTROL
RECORDER
FILTER Ap SENSOR
N.O. = NORMALLY OPEN
N.C. = NORMALLY CLOSED
S = SAMPLING LOCATION
figure 1. Reverse flow system control arrangement
-------
AUTOMATIC CONTROL LOOP
CO
o
TEST
INLET
ASS
JRN •*-
Ik
^
s
I
1
s
^
s
j
>
•
V
S
s
s
s
S
s
s
s
\
s
V
s
s
N
J;
H/
r
t
A
C
\i
BLOWER
A
1 ^^^
\T^""^7 ^V
/ ^^^^j|
\^/
LirroMATic
»NT80L
ALVE
"RV PAQQ \7AT\?
S/
/v
N.O.
K A f
lYi (
W V
I —
r N.C.
^
kf/il
r_
S
SHAKER
MECHANISM
BAG
COMPARTMENT
k HOPPER y
FLOW RATE SENSOR
..... .1 , ,_
1 ^^^ l\\Vv
o / \\Y\\
<>_ TIMER / \\^ \
SHAKE ( J
CYCLE \ y
CONTROL I -^
\^ _
TEMPERATURE
SENSOR
FILTER A p SENSOR
M n - MHDMATTV OPFM
N.O.
N.C. = NORMALLY CLOSED
S = SAMPLING LOCATION
Figure 2. Mechanical shaking system control arrangement
-------
AUTOMATIC CONTROL LOOP
ho
o
BLOWER
AUTOMATIC
CONTROL
VALVE
n
ON-OFF
VALVE
f
FLOW RATE SENSOR
AIR COMPRESSOR
COMPRESSED AIR
DISTRIBUTION
BAG
COMPARTMENT
TEMPERATURE
SENSOR •-
HOPPER
L-oo-
PULSE
JET
CYCLE
CONTROL
TIMER
RECORDER
O
FILTER A p SENSOR
S - SAMPLING LOCATION
Figure 3. Pulse jet cleaning system control arrangement
-------
hopper with a rotary dust discharge valve, designed to operate at 550 F.
Section 2 is a 4-foot section that is presently used for pulse jet clean-
ing. The third section is a 6-foot section that is now used for shake
cleaning. Both the 6- and 4-foot sections can be assembled in series so
that bags up to 10 feet long may be used. Separate top sections have been
built for shake cleaning and pulse jet cleaning. Reverse flow cleaning
can be employed with either of the two top sections cited above.
Gas flows ranging from 26 to 280 cfm, as determined by cloth velocity, bag
size, and bag number, must be carried by the mobile system ducting. Duct
velocities are maintained between 2000 and 4000 fpm to minimize dust set-
tlement and to prevent excessive pressure drop. Thin-walled, stainless
"if
steel pipe joined by Morris couplings is used.
The primary fan for the mobile fabric filter is a Chicago Turbo-Pressure
Blower*" capable of supplying 21 in. of water suction at 550°F. At am-
bient conditions it can supply 42 in. of water at a flow of 420 cfm at
5 horsepower. Although the fan is designed primarily for clean air use it
can be and has been used briefly on dusty air.
Automatic valves for diverting the gas flow when cleaning by shaking or
reverse flow were fabricated by GCA (see Figures 1 and 2). These modified
gate valves, which provide tight sealing during shutoff and relatively fast
response, are corrosion resistant and are able to operate at high and
varying temperatures. These valves have a tested leak rate of less than
0.01 cfm at 20 in. of water pressure differential. A schematic diagram
of the modified gate valve is shown in Figure 4.
The system pressure, flow, temperature, time controlling and recording
instruments are located in a single control console. Flow through the
fabric filter is indicated by the differential pressure across a Stairmand
*Morris Coupling and Clamp Company, 2240 West 15th Street, Erie, Pa. 16512.
+Chicago Blower Corporation, 1675 Glen Ellyn Road, Glendale Heights,
Illinois 60137.
207
-------
(•) Moaser 3 Gate Valve
(b) Asbestos stem
packing
(c) Stainless gate
(d) 3" IPS iron nipple,
(GCA)
(e) Stainless valve
seat (GCA)
The valve seats are cut from stainless mixing bowls, of suitably
shallow curvature and springiness.
The seat is welded to the nipple. Adjustment of the nipple affects
the degree of sealing.
(b)
, ^3-15 psig
External \S v &
(f) 3 way solenoid
(g) Johnson Service piston
(h) Return spring
(i) Connector (GCA)
(J) Bracket (GCA)
(k) 3-1/2" p.D. pipe connectors
(GCA)
Figure 4. Gate valve
208
-------
disk. The above pressure differential and that across the fabric filter
ju
are displayed on Magnehelic gauges, Bailey indicating gauges and also
recorded on a dual channel recorder. A pneumatic controller permits auto-
matic operation at either constant flow or constant filter pressure drop.
Filtration temperature is maintained above the gas stream dewpoint but is
not allowed to exceed the temperature limit of the fabric. Fiber glass
insulation and three heating tapes are used to maintain appropriate gas
temperatures. A thermocouple temperature recorder with an on-off con-
troller and adjustable high and low setpoints is used to activate the
heating tapes. The temperature controller and one of the previously men-
tioned automatic valves can be used for dilution cooling, if necessary.
Automatic timing of the system operating and cleaning cycles is provided.
Five timers and two stepping switches control the system when using the
mechanical shaker. The first timer (T^) with a range of 3 to 60 minutes
controls the filtration time. At the end of the filtering interval, timer
TI causes the bypass valve to open, the valves isolating the filter housing
to close, and timer T2 to start. The second timer, T2, provides a delay
time for the isolation valves to close, engages through a stepping switch
the bag or bags to be shake cleaned and initiates the shaking. Timer 13
or 14 then controls the length of the shaking cycle. 13 has a range of 1
to 10 seconds for short shaking cycles and T^ has a range of 11 to 150
seconds for longer cycles. After the shaking has ceased, timer 15 allows
a delay of 0.25 to 5 minutes for the dust to settle and then repeats the
filtering cycle as controlled by timer T^. Although reverse flow opera-
tion uses the same timers, timers 13 and T^ operate the reverse flow fan
in lieu of the shaker motor. A different control system consisting of two
timers and a stepping switch is used to operate the pulse jet cleaning
system. The first of these timers sets the interval between pulses, 0.25
to 5 minutes while the second timer sets the length of the pulse 0.01 to
99 seconds. The stepping switch selects the bag to be cleaned.
*Bailey Meter Company, A Subsidiary of Babcock and Wilcox, U.S.A.,
Wickliffe, Ohio 44092.
209
-------
Electric power is distributed to all components of the mobile fabric filter
system through circuit breakers and receptacles located at the rear of the
control console.
An apparatus designed to shake the upper end of a row of up to three bags
was constructed. Shaking motion is adjustable with regard to amplitude
which may be varied between 3/8 and 3 inches. A 1/2-horsepower permanent
magnet DC motor and a solid state speed controller is used to select and
regulate shaking frequency over a working range of 2 to 15 cps. Fre-
quency is measured through a microswitch on the motor which controls a
counter mounted on the control console. Pneumatic clutches permit shaking
of one bag at a time. This option was included to simulate operation of a
3-compartment system, as well as special series filtration tests. The bag
suspension points can be adjusted vertically in order to adjust the bag
tension. Bags may be attached at the top either by a loop or a cap ar-
rangement. Removable windows at the top and bottom of the filter housing
permit easy access to the bags and quick changing of filter media. The
amplitude and frequency range for the shaking apparatus was chosen to
encompass that usually encountered in the field and also to provide the
necessary energy transmission to clean the fabric. With minor changes,
the present frequency and amplitude range can be increased if test con-
ditions require it.
An apparatus designed to clean up to seven bags by high pressure reverse
pulse jet was constructed using standard commercial valves, venturi bag
cages and bags purchased from Mikropul. A 5-gallon pressure fa^V ••'s
mounted immediately on top of the pulse jet unit in order to maintain
supply air pressure during pulsing. Compressed air is provided by a com-
pressor which can furnish 8.3 cfm (free air delivery) at 90 to 125 psig.
A digital solid-state timer is used to set the exact length of the signal
that activates the solenoid valve that controls the pulse. The amount of
air that is delivered per pulse for various pulse durations and reservoir
*Mikropul Division, U.S. Filter Company, Chatham Road, Summit, N.J. 07901.
210
-------
pressure was determined so that the economics of a particular cleaning
method could be evaluated.
The low pressure reverse flow cleaning fan is designed to supply air at a
minimum back pressure of at least 2 inches of water to a filter system
having a cloth area of about 30 square feet. Based on an estimated mini-
mum residual drag of 0.5 in. water per fpm, this pressure will be accom-
plished by a flow of up to 120 cfm. For high temperature operation, es-
pecially when the aerosol contains appreciable amounts of water vapor,
the filter bags should not be cooled during reverse flow. Therefore, a
heat exchanger will be used to heat the reverse flow air when necessary.
An orifice meter, a damper valve, and the previously mentioned automatic
valves are the other components used during reverse flow operation.
A variety of sampling and measurement techniques are needed to evaluate
filter efficiency on a mass and/or number basis. Inlet mass concentration
can be determined by conventional sampling with glass fiber papers or by
weighing the dust collected in the hopper dust valve over some selected
averaging period. Downstream mass concentrations, usually over longer
averaging periods, can be determined with glass fiber filter papers
provided that the concentration is not too low. At very low effluent con-
centrations, a B and L* light scattering particle counter can be used to
obtain a rough estimates of both mass and number concentrations. Particle
size concentrations before and after the fabric filter are currently
determined by impactor measurements. A Brink impactor operated at 0.03
to 0.1 cfm is generally used before the fabric filter while the higher
flowrate, 0.5 to 0.8 cfm, Andersen^ impactor is used on the downstream
side. Particle size distribution by number can be determined for the
effluent with a B and L light scattering particle counter. In addition,
*
Bausch and Lomb Incorporated, 820 Linden Avenue, Rochester, N.Y. 14625.
Brink Impactor--Monsanto Enviro-Chem Systems Inc., 800 North Lindbergh
Blvd., St. Louis, Missouri 63166.
^Andersen 2000 Inc., P.O. Box 20769, Atlanta, Georgia 30320.
211
-------
preparations are underway to measure fine particulate size distributions
with a diffusion battery and condensation nuclei counter.
PRELIMINARY RESULTS OF FIELD TESTS
PROCESS DESCRIPTION
The mobile fabric filter system was operated recently at a secondary brass
foundry in Massachusetts. Emissions from these plants are principally
zinc oxide and lead oxide fumes with a particle size range of 0.03 to 0.3
microns. Since the typical brass foundry melts 50 tons/day and the
emission factor for an uncontrolled reverberatory furnace is 70 Ib/ton,
such an operation would be expected to emit 1.8 tons/day. Some secondary
brass operations have emission control equipment consisting of wet scrub-
bers and electrostatic precipitators whose performance has not always been
satisfactory. Because of the small particle size, fabric filters are the
most frequently employed collectors for solids emissions from secondary
brass processes. Regulations for new secondary brass refineries will re-
quire control efficiencies of approximately 99 percent. Therefore, it is
most important that the capabilities for controlling fine particulates by
fabric filter be investigated at these installations. The plant at which
the mobile fabric filter tests were performed now employs a shaker-type
fabric filter.
The brass ingot manufacturer uses an oil-fired, cylindrical reverberatory
furnace to melt 45,000 pounds of scrap metal per 7-hour melting cycle.
Each cycle consists of the following steps:
Step 1. About 30 percent of the total scrap metal is charged
to the furnace and heated for 140 minutes.
212
-------
Step 2. The oil burner is shut down and 23 percent of the
total scrap metal is charged. Charging takes about
15 to 20 minutes.
Step 3. After charging, oil firing is resumed and the fur-
nace is heated for 70 to 85 minutes.
Steps 2 and 3 are repeated three times before the melting process is ter-
minated. Since most of the emissions occur during the melting cycle, the
fabric filter system operation was altered during the charging cycle so
that no cleaning took place. The step was taken to prevent overcleaning,
and hence excessive dust penetration, during the charging period when dust
emissions were relatively low.
During these field tests, the mobile fabric filter system was operated on
the truck. The plant test effluent was extracted approximately iso-
kinetically at 70 to 110 cfm through a 3.3-inch nozzle from the main duct
about 45 feet upstream of the plant fabric filter. Lightweight 2.5-inch
OD stainless steel tubing was used for the remainder of the necessary
piping leading to the mobile unit. Fiber glass insulation and 50 feet of
460-volt heating tape were used to maintain the gas temperature between
250 and 390°F at the mobile fabric filter inlet.
PULSE JET CLEANING
Pulse jet cleaning was used during the first series of tests when the
refinery was producing a brass alloy of the following composition:
Material Weight %
Copper 77-79
Zinc 9.75-14.5
Lead 6.0-8.0
Nickel 0.0-1.0
Iron 0.0-0.4
213
-------
Particulate Sampling Procedures
Particulate concentrations entering the mobile fabric filter were deter-
mined by glass fiber filters and by the weight of dust collected in the
fabric filter hopper. During charging, the inlet mass concentration was
less than 0.05 grain/scf, while during each heating cycle the concentra-
tion varied between 1 grain/scf at the start and 2 to 2.5 grains/scf at
the end of the cycle. Average inlet dust concentration, exclusive of the
charging period, was 1.7 grain/scf. Three Brink impactor measurements
were made, each on a different day, to determine the inlet particle size
distribution. The results, assuming spherical particles and a density of
o
1 gm/cm , show a mass median diameter of 0.5 |j.m. Results of the Brink
impactor measurements were in good agreement. Figure 5 shows the inlet
particle size distribution.
System Operating and Cleaning Parameters
Nomex, 16 oz. per square yard, felt bags were used for the pulse cleaning
tests. The test conditions and filter characteristics are shown in
Table 1. Three bags, each 4-1/2 inches in diameter and 4 feet long were
used. The gas flow direction was in the conventional "outside to inside"
direction used commercially with wire cage supported felt tubes. The
filter velocity was maintained at an average of 8 fpm so that the total
flow was 110 cfm for a cloth area of 13.9 ft^. Average filtration tem-
perature was 320°F. The fabric filter was run for a complete 7-hour cycle
before the collection of performance data was undertaken.
The pulse jet cleaning parameters are listed in Table 2. The interval
between pulses was 40 seconds for the first two tests and 20 seconds for
the third test. Since the cleaning was continuous and three bags were
being used, the total time to cycle through all the bags was 120 seconds
for the first two tests and 60 seconds for the last test. Pulse duration
was a nominal 0.1 seconds, as set on an electronic timer for all tests.
The above time duration actually designates the electrical "open time"
214
-------
10.0
Ul
5.0
1
uj 2.0
UJ
<
U
i-
at.
1.0
0.5
0.2
J I
I
I
I
I l I l I
I
I
I
J I
2 5 10 30 50 70 90
PERCENTAGE OF MASS < STATED SIZE
95 98 99
Figure 5. Inlet particle size distribution for zinc oxide fumes,
from brass refining furnace by Brink cascade impactor
-------
Table 1. SYSTEM COMPONENTS AND MAJOR OPERATING PARAMETERS FOR ZINC
OXIDE FILTRATION BY MOBILE PULSE JET FILTER
Filter material
Bag dimensions
Number of bags
Support
Filtration velocity
Total filter area
Total gas flow
Inlet dust concentration, average
Inlet particle size
Test duration
o
Nomex felt, 16 oz/yd^
4-1/2 in. diameter, 4 ft long
3
Wire cages
6 ft/min
13.9 ft2
110 ft3/min, STP
1.7 grain/ft3, STP
0.5 ^m, mass median diameter
7 hour s
Table 2. PULSE JET CLEANING PARAMETERS, FOR ZINC OXIDE FILTRATION
Interval between pulses, sec
Cycle time, sec
Pulse duration, sec
Pulse supply pressure, psig
Compressed air requirement,
ft3/min (STP)/1000 ft2 filter area
Test 1
40
120
0.1
60-80a
40b
Test 2
40
120
0.1
80
40
Test 3
20
60
0.1
80
80b
fSupply pressure increased to 80 psig for last half of test.
Equivalent to 0.37 ft3/pulse.
216
-------
and not the effective open time of the valve whose motion lags signifi-
cantly electrical start-stop signals. Pulse supply pressure was 80 psig
except for the first half of Test no. 1 when the pressure was 60 psig.
The amount of compressed air used by a pulse jet cleaning fabric filter,
which is an important economic consideration, was estimated to be 40 scfm
per 1000 square feet of filtration area for the first two tests and
80 scfm/1000 square feet during the last test.
Experiment Results
Fabric filter performance for each of the three pulse cleaning tests (7
hours long) is summarized in Table 3. The filter pressure drop shown in
Table 3 is the average for each test with the peak pressure drop in paren-
thesis. During Test 3, in which the filter bags were cleaned twice as
often, the average pressure drop across the filter was only 5 inches of
water as compared to 9 and 10 inches of water for the first two. tests. How-
ever, another result of the increased cleaning frequency was doubling of the
percent penetration. Percent penetration Was 0.081 and 0.078 percent for
Table 3. SUMMARY OF PERFORMANCE PARAMETERS FOR ZINC OXIDE FILTRATION
Inlet dust concentration, grains/ft3, STP
Inlet particle size, [j.m
Average filter pressure drop, in. water
Outlet dust concentration, grains/ft3, STP
STP
Penetration, 7,
Efficiency, %
Test 1
1.7
0.5
10 (14)a
0.0014
0.081
99.919
Test 2
1.7
0.5
9 (13)a
0.0013
0.078
99.926
Test 3
1.6
0.5
5 (6)a
0.0028
0.15
99.85
Values in parens indicate peak pressure drop.
217
-------
the first two tests but increased to 0.15 percent during the last test.
Corresponding efficiencies were 99.919 and 99.926 percent and for the
third test 99.85 percent. Tests 1 and 2 are approximate replicates de-
spite the lower pulse supply pressure at the start of Test 1. Effluent
concentration (and source strength) were approximately doubled as a result
of the increased cleaning frequency.
Two Andersen impactor samples were taken at the fabric filter outlet in an
effort to determine effluent size properties. Insufficient material was
collected on any of the impactor stages, however, to determine an accurate
fractional efficiency even though the impactors were run 1 to 2 hours.
Most of the exiting fume was collected on the Andersen back-up filter in-
dicating that the outlet aerosol was smaller than the inlet aerosol. The
failure to collect a significant amount of material on the Andersen im-
pactor was the result of high filter efficiency (~ 99.9 percent) and the
small inlet particle size, mass median diameter 0.5 p.m and 85 percent by
weight less than 1 ^m, see Figure 5.
Because fabric filter pressure drop is a very important design considera-
tion, graphs of filter pressure drop versus time are presented in Fig-
ures 6 through 8 for Tests 1 through 3. Figure 6 shows the instantaneous
pressure drop across the filter versus time during Test 1. After 2-1/2
hours the furnace oil burner was shut down for charging during which in-
terval the fume loading to the fabric filter was very low. A few pulses
lowered the pressure drop to about 4 inches of water. At this time, the
pulse cleaning unit was shut off until the furnace was re-ignited as evi-
denced by a rise in the filter pressure drop. The pulse cleaning system
was shut, down during charging operations so that the bags would not be
excessively overcleaned. The perturbatipns in pressure drop reflect
variations in the inlet dust concentration.
218
-------
(J
5
H
b.
15
10
Charging Interval
TIME, HOURS
Figure 6. Pressure-time relationship for brass fume filtration with
pulse jet cleaning, Test 1
I
15
10
5
2 3
TIME, HOURS
Figure 7. Pressure-time relationship for brass fume filtration with
pulse jet cleaning, comparison of Tests 1 and 2
219
-------
g
I
15
10
5
3 4
TIME, HOURS
Figure 8. Pressure-time relationship for brass fume filtration with
pulse jet cleaning, comparison of Tests 1 and 3
Figure 7 compares the pressure drop versus time relationships for Test 1
(solid line) and Test 2 (the dotted line). The filter pressure was lower
during the first half of Test 2 because the pulse supply pressure was
80 psig compared to 60 psig during the first half of Test 1. After 5-1/2
hours of Test 2 there was a sudden rise in the pressure drop because the
pulse jet cleaning system had been prematurely shut down. Generally, the
variations in filter pressure drop throughout Tests 1 and 2 are similar.
In Figure 8, the pressure drop versus time results for Tests 1 (solid line)
and 3 (dotted line) are compared. The lower pressure drop during Test 3
reflects the doubled pulse rate. The transient high pressure drop at the
start of Test 3 was caused by a stepping switch malfunction.
The pulse jet cleaning tests suggest that the fine metal oxide fume from
a secondary brass smelter can be efficiently filtered (~ 99.8 percent)
with Nomex felt bags at a cloth velocity of 8 ft/min. By using 80 scfm of
compressed air per 1000 square feet of filter area, the average filter
pressure drop was maintained near 5 inches of water. It should be noted
that the reported compressed air consumption for many industrial processes
is often lower, ranging from 5 to 30 ft3/min, STP.7'8' In the present
220
-------
situation, however, time did not allow investigation of the feasibility
of changing pulse jet parameters so as to reduce compressed air volumes
(or delivery costs). The filter bags showed no signs of plugging or
binding after 28 hours in use. The life span of Nomex bags in this type
of operation, of course, cannot be determined from short period tests.
MECHANICAL SHAKE CLEANING
Several field measurements were also performed using a conventional me-
chanical shaking approach with woven Nomex bags, the latter conforming in
fabric properties to those used in the present refinery fume control
system. The objective of these tests was to simulate the operation of the
plant fabric filter system so that by varying cleaning parameters on the
mobile system the performance of the plant system might be improved.
Refinery personnel have had problems with excessive pressure drops across
the plant fabric filter that have reduced the system ventilation capability.
Shake cleaning parameters cannot be varied conveniently on the plant
fabric filter without major mechanical modifications.
Nomex filament fabric is used in the plant system with a design filtration
velocity of 3 ft/min. A horizontal shaking motion with the bags spring ten-
sioned at about 10 pounds, a shaking amplitude of 1/4 to 1/2 inch and a
shaking frequency of 3 cps constituted the main cleaning parameters. The
filter house consists of 4-compartments with each chamber on-line for 20
minutes, cleaned for 1 minute, and then shut down for 1 minute to allow the
dust to settle. Rough Pitotstatic tube measurements indicated that the
actual filtration velocity at 10 inches of water pressure drop across the
filter was only 1.5 to 2 ft/min.
System Operating and Cleaning Parameters
The Nomex multifilament fabric used in the mobile system was 3x1 twill
with a thread count of 98 x 79 and a weight of 4.5 ounces/square yard.
Clean cloth permeability was 18 ft^/min/ft^ at 1/2 inches of water
221
-------
pressure drop. The mobile system bags were 6 feet long and 5-9/16 inches
in diameter while the plant system used bags 8-1/2 feet long and 5-1/2
inches in diameter. Because the bag motion in the mobile system follows
a slightly arcing path in contrast to an essentially horizontal path for the
plant system, the bag tension on the mobile system was set lower than the
plant filter. For the initial simulation of the plant system, the am-
plitude was 3/8 inch, the shaking frequency was 3 cps, and the filtration
velocity was 3 fpm. The filtering and cleaning cycle per compartment was
the same as that for the plant system except that a 15-second pause im-
mediately before cleaning, allowed the automatic valves to move. At an
amplitude of 3/8 inch and a frequency of 3 cps the maximum acceleration
of the shaker arm was estimated to be 0.35 g. Based upon prior GCA
tests, which indicated poor cleaning for g levels below 4 to 5, effec-
tive cleaning was not expected for either the plant or mobile filter
system.
Filtration conditions for Tests 4 and 6 differed from those of the pulse
jet cleaning tests because the brass melts contained less zinc, 4.5 to
6.0 percent versus 9.75 and 14.5 for the earlier pulse cleaning studies.
Detailed data on brass compositions are presented in Table 4.
Experimental Results
The average inlet dust concentration during Tests 4 and 6 was 0.65 and
0.54 grains/ft3, STP compared to 1.6 grains/ft3, STP during the pulse jet
cleaning tests, mainly because of the reduced zinc content. During me-
chanical shake cleaning Tests 5 and 7, the same type of brass was produced
as that during the pulse jet cleaning tests. However, the average inlet
o
dust concentration was only 0.90 to 0.92 grains/ft , STP as compared to
3
1.6 grains/ft , STP for the pulse jet cleaning tests. Table 5 summarizes
key aerosol data for Tests 4 through 7.
*1 g = 32.2 ft/sec6.
222
-------
Table 4. BRASS COMPOSITIONS DURING MECHANICAL SHAKING TESTS
Test number
Brass type
Composition
Copper
Tin
Lead
Zinc
Nickel
Iron
4 and
1
4.4 -
4.0 -
4.5 -
0.5 -
0.0 -
6
6.0
5.7
6.0
1.0
0.25
5 and
2
2.0 -
6.0 -
9.75 -
0.0 -
0.0 -
7
3.25
8.0
14.5
1.0
0.4
Table 5. AEROSOL PARAMETERS FOR MECHANICAL SHAKING TESTS
Test number
Brass type
Inlet dust concentration, grains/scf
Inlet particles size,3 microns
Inlet gas temperature, °F
Gas dew point, °F
4
1
0.65
0.5
240
50-60
5
2
0.92
0.5
270
50-60
6
1
0.54
0.5
270
50-60
7
2
0.90
0.5
270
50-60
223
-------
The first test, Test 4, was designed to simulate the operation of the plant
fabric filter. Therefore, the initial cleaning conditions consisted of a
3/8 in. shaking amplitude, 3 cps shaking frequency, and a bag tensioning
of 5 Ibs with a 3 ft/min filtration velocity. As expected, very little
cleaning was accomplished during the first two hours of this test. Ac-
cording to the pressure drop-time curve of Figure 9, the resistance rose
to 14 in. water and even after shaking was only slightly reduced to 13 in.
water. After 3 hours, the shaking frequency was increased to 5 cps which
increased the shaker arm acceleration to 0.93 g. However, cleaning was
still not significantly improved. After 4 hours, the shaking frequency
was increased to 6 cps (a shaker arm acceleration of 1.33 g) but the
cleaning remained unsatisfactory. At the end of the test, the filtration
velocity had fallen to 2.3 ft/min but the pressure drop was near 16 in. of
water. It is emphasized here that the mobile system flow would probably
have decreased to much lower levels than the 2.3 ft/min cited above had a
conventional exhauster been used, ~ 10 in. water static pressure. Because
of the need for flexibility in the mobile system, a special high static,
thin scroll centrifugal fan was selected that minimized flow variations.
Further evidence of poor cleaning was the small amount of dust, 50 to
100 grams, collected in the hopper during the entire 7-hour test. At the
end of the test, the bags were shaken by hand and about 640 grams of dust
were collected. After being shaken by hand, the pressure drop across the
bags was reduced to only 1 in. of water at 3 ft/min velocity.
In an attempt to improve collector performance, shaking amplitude was in-
creased to 7/8 in. during Test 6 and the bag tension adjusted to about
1 Ib. The shaking frequency was maintained at 6 cps for the first hour of
filtration followed by an increase to 8 cps for the remainder of the test.
The alloy composition and fume inlet concentration were about the same as
those noted for Test 4. The energy imparted to the shaking bags as defined
by shaker arm acceleration was 3.2 and 5.7 g's, respectively, for the two
frequency levels.
224
-------
N3
3.5
3.0
2.5
2.0
1.0
20
15
Time, Hours
Figure 9. Pressure-time relationship for brass fume filtration with mechanical shaking, Test 4
-------
In Figure 10, the graphs of filter resistance and filtration velocity
versus time show a marked improvement for the higher shaking amplitude
(dashed line) relative to the system performance obtained under Test 4
conditions (solid line). After 2 hours of filtration, the pressure drop
across the bags was 9 in. of water instead of 14 in. as in the previous
test. Filter pressure drop after cleaning was reduced by 3 in. of water
compared to less than 1 in. for Test 4. Through most of Test 6, filter
pressure drop was maintained between 8.6 and 10.6 in. o'f water while, at
its termination, the pressure drop was 11 in. of water at a velocity of
3 ft/min compared to 16 in. of water at 2.3 ft/min in the previous test.
A net twenty to fifty grams of dust were dislodged after each cleaning
cycle instead of a total of 50 to 100 grams for the entire test. Addi-
tionally, only 100 grams of dust were collected by hand shaking at the end
of Test 6 compared to 640 grams in Test 4. It was concluded that increasing
the shaking frequency and amplitude (both contributing to higher bag accel-
eration) reduced the filter pressure drop and increased the filtration
capacity.
Average fume penetration was 0.08 percent during Test 4 and 0.16 percent
during Test 6. The results are consistent with theory which indicates an
inverse relation between dust penetration and filter dust holding. One
can also infer that the more intense acceleration and particularly the in-
crease in shaking amplitude would enlarge the filter pores thus allowing
more dust to penetrate the fabric structure.
Tests 5 and 7, Figure 11, reflect mobile filter system operation at the
higher fume loadings (see Table 5) obtained with the high zinc alloys.
Test 5 was run at a filtration velocity of 3 ft/min, a shaking amplitude
of 7/8 in. and a shaking frequency of 6 cps. The pressure drop character-
istics were only slightly higher than those for Test 6 despite the lower
shaker arm acceleration of 3.2 g:s and the higher inlet dust concentration
of 0.9 grains/ft , STP. Tests 5 and 7 were the same except for the lower
226
-------
Ni
ro
3.5
3 1.0
20
15
10
Time, Hours
Figure 10. Pressure-time relationship for brass fume filtration with mechanical shaking, comparison
of tests 4 and 6
-------
00
3.5
3.0
2.5
2.0
1.0
20
15
10
J
Time, hours
Figure 11. Pressure-time relationship for brass fume filtration with mechanical shaking, comparison
of tests 5 and 7
-------
filtration velocity during Test 7. During Test 7 the filtration velocity
was 2.5 ft/min for the first half and 2.0 ft/rain for the remaining time.
The resultant lower pressure drop is clearly shown in Figure 11. The lower
penetration measured during Test 7 is attributed to the fact that residual
dust holding should be greater with less vigorous shaking, 3.2 vs. 5.7 g's,
and the lower filtration velocity should permit enhanced diffusional
capture of fine particles. The specific dust fabric resistance coefficient
in. H 0/ft/min
K2 ( 5 ) as calculated for various periods throughout the me-
Ib ft
chanical shake cleaning tests ranged from 150 to 300. Results of the me-
chanical shake cleaning tests are summarized in Table 6.
Low pressure reverse flow cleaning was also briefly tested. However, the
test constituted a shake-down trial and the data collected did not warrant
analysis.
ASSESSMENT OF TESTS AND FUTURE PLANS
It is premature at this time to make comparisons of the cost effectiveness
of the fabric filtration techniques investigated in the field with the
mobile filter system. With respect to regulations citing an upper limit of
o
0.022 grains/ft STP for emissions from secondary brass smelting operations,
both the pulse jet and mechanical shaking systems produced acceptable ef-
fluents (0.0014 to 0.0029 and 0.0003 to 0.0012 grains/ft3 STP, respec-
tively). On the average, the above performance for pulse jet and mechani-
cal shaking system indicates that these levels are only 13 and 1.3 per-
cent of the allowable values. Because it was not possible to explore
several alternative combinations of cleaning parameters within the time
frame of these tests, no cost estimates have been presented.
229
-------
Table 6. SUMMARY OF MECHANICAL SHAKE CLEANING TESTS WITH ZINC OXIDE FUME
Test number
Cleaning parameters3
Amplitude, in.
Frequency, cps
Tension, pounds
Filtration velocity, fpm
Filter pressure drop, in. water
Average penetration, percent
o
Average outlet dust concentration, grains/ftj (STP)
4
3/8
3-6
5
3-2.3
0.08
0.0005
5
7/8
6
2
3
6
7/8
8
1
3
(See Figures 9, 10, 11)
0.14
0.0012
0.16
0.0087
7
7/8
6
1
2.5,2.0
0.05
0.0003
N)
OJ
O
same cleaning cycle was used for all tests. Filtration for 20 minutes, a 15-second pause, a
1-minute shake followed by a 1-minute pause before filtration was resumed.
-------
As a matter of practical concern, the apparent feasibility of using
mechanical shaking based upon mobile system tests may be countermanded
by the structural features of some full-scale commercial equipment. For
example, separate evaluations of the smelter filter system indicated
that shaking amplitudes in excess of 1/2 in. caused massive vibration and
swaying of the bag house structure at low frequencies, ~ 2 to 4 cps.
These field tests have demonstrated that a mobile filter system having the
flexibility to apply several methods of cleaning and to vary operating
parameters, as needed, can provide useful design data.
It is planned to use the mobile unit to test fabric filter performance on
a number of other industrial particulate emission sources. The next test
series will study hot mix asphalt plants which may be very significant
sources of fine particulates. *•*• Most asphalt plants use a cyclone to
collect coarse dust emitted from the rotary drier although some use wet
scrubbers or fabric filters to collect particulates passing through the
cyclone. Because the particulate concentration is high, pulse jet cleaners
are the usual choice for asphalt plant fabric filters.
Other proposed field evaluations involve operating the mobile system at a
coal-fired electric power plant and possibly at a municipal incinerator or
iron foundry. Following these tests, the Environmental Protection Agency
will take delivery of the system and use it in a large scale testing pro-
gram incorporating other mobile particulate removal systems such as wet
scrubbers and electrostatic precipitators.
REFERENCES
1. Draemel, D. C. Relationship Between Fabric Structure and Filtration
Performance in Dust Filtration, Office of Research and Monitoring,
Environmental Protection Agency, EPA-R2-73-288, July 1973.
231
-------
2. Hammond, W. F., Nance, J. T., and Spencer, E. F. Secondary Brass
and Bronze Melting Process, Air Pollution Engineering Manual,
Danielson, J. A. (ed.). Environmental Protection Agency, Research
Triangle Park, North Carolina, Publication No. AP-40, May 1973.
3. Background Information for Proposed New Source Performance Stan-
dards. Environmental Protection Agency, Research Triangle Park,
North Carolina, Publication No. APTD-1352A, June 1973.
4o Compilation of Air Pollutant Emission Factors. Environmental Pro-
tection Agency, Research Triangle Park, North Carolina, Publication
No. AP-42, April 1973.
5. Air Pollution Aspects of Brass and Bronze Smelting and Refining
Inducting. Brass and Bronze Ingot Institute and National Air
i Pollution Control Administration, National Air Pollution Control
Administration, Raleigh, North Carolina, Publication No. AP-58,
November 1969.
6. EPA Proposed Control Standards for Seven New Stationary Sources of
Pollution. Federal Register, Vol. 38, No. Ill, Washington, D. C.,
June 11, 1973.
7. Pulseflo, Western Precipitation Division/Joy Manufacturing Company,
Los Angeles, California, Catalog No. PF-100.
8. The Mikro Pulsaire Dust Collector, Mikro Pul Division, United States
Filter Corporation, Summit, New Jersey, Bulletin PC-3.
9. Amerpulse Continuous—Cleaning, Pulse Jet Dust Collector, American
Air Filter Company, Louisville, Kentucky, Bulletin No. DC-301.
10. Wilder, J. E. and Dennis, R. Fabric Filter Mechanisms and Kinetics
Study. Contracts EHS-D-71-19 and 68-02-0268, GCA Corporation (Final
Report in Preparation).
11. Particulate Pollutant System Study. Volume II, Fine Particulates.
Contract No. 0*^22-69-104, Midwest Research Institute, August 1971.
232
-------
EXTENDING FABRIC FILTER CAPABILITIES
James H. Turner
Chief Engineer
Particulate Technology Section
Control Systems Laboratory
Environmental Protection Agency
Research Triangle Park
North Carolina 27711
INTRODUCTION
At present we can readily filter particles at temperatures up to about
550 F. We can filter them in harsh atmospheres and we can filter them
through the same bag for up to 10 years, although the average is more
nearly 1 or 2. Fabric filters have often been touted as 99.9 percent
efficient as particle collectors, but there haven't been very many mea-
surements made of the particle size efficiency on operating baghouses.
EPA has some data, presented in Figures 1, 2 and 3, that imply good
efficiency down to about a tenth of a micrometer, but which should be
hedged about with all kinds of restrictions and reservations. The
c
Although it is EPA's policy to use the metric system for quantitative
descriptions, English units are used in this report in order to avoid
confusion. Readers who are more accustomed to metric units may use the
following conversions: multiply °F by 5/9 (°F-32) to obtain °C.; mul-
tiply ft/min by 0.508 to obtain cm/sec; multiply lb/106 BTU by 0.00180
to obtain kg/106 cal.
233
-------
190
99
I
o
to
2 97
o
o
o
I
94
EPA CONTROL SYSTEMS LABORATORY
SINGLE-POINT IMPACTOR DATA.
(SHAKE CYCLE NOT INCLUDED)
OVERALL MASS KFFICIENCY = 99.7%
1.0
2.0
PARTICLE SIZE, ym
3.0
4.0
Figure 1. Baghouse performance
-------
100
S3
Co
Ln
w
to
W
(D
•a
n>
H
l-h
O
O
O
(D
I
(0
rt
i-l
H-
SO
a-
O
IT)
l-l
99
H-
00
l-i
n>
N3 ^
Ł. 98
>-
o
g "
LU
O
O
96
95
94
I
BELOW 0.36vim (FILTER CATCH)
EPA CONTROL SYSTEMS
LABORATORY SINGLE-
POINT IMPACTOR DATA
(NOT VERIFIED)
• NORMAL A/C RATIO a 3:1
• HIGH A/C RATIO * 6:1
OVERALL MASS EFFICIENCY AT NORMAL A/C RATIO =99.76%
OVERALL MASS EFFICIENCY AT HIGH A/C RATIO =99.51%
1.0
2.0
PARTICLE SIZE, ym
3.0
4.0
-------
100
BELOW 0.11pm (FILTER CATCH)
-•
•8
03
90
U)
O
c
0)
(D
D)
3
O
(D
C
rt
cr
O
o
UJ
o
o
a
o
o
QL
a.
-------
streams may not be representative; there may have been particle bounce
through the impactors; sampling conditions may have affected results;
but the graphs do imply good efficiency in the test range.
REASONS FOR EXTENDING CAPABILITIES
What are the reasons for wanting to extend fabric filter capabilities?
The National Academy of Engineering has cited fine particulates as being
potentially harmful to health, as contributing to visibility problems
and as contributing to weather modification, and EPA continues to do
work which supports such a contention. It's not just fabric filter cap-
abilities that need extending; it's also electrostatic precipitators
(ESP's), scrubbers and any other good collection devices, and the ex-
tension isn't necessarily in general performance capabilities. Improve-
ments need to be made in those areas in which collection efficiency is
poor, and also in those applications in which control is not now exer-
cised, or is exercised poorly. Referring back to the National Academy
of Engineering report of 2 years ago: about 75 percent of crushed stone
operations and about 38 percent of stoker fired industrial boilers were
not controlled. Those two sources were assumed responsible for almost
40 percent of total mass emissions from major industry. In light of
state and federal regulations on particulates, that looks like a lot
of business for dust collector manufacturers, assuming the manufacturers
are ready with working equipment for such sources.
If one talks specifically about fine particulates, the sources of primary
importance get changed around. Crushed stone operations drop down on the
list while industrial boilers stay near the top along with other com-
bustion sources (utilities and municipal incinerators), iron and steel
plants, ferroalloy plants and primary nonferrous metallurgy.
What has to be done to a fabric filter to make it capable of controlling
any given one of these important sources, if it doesn't already control
sufficiently well?
237
-------
SPECIFIC AREAS FOR IMPROVEMENT
In some cases, perhaps not too many, the filter (not the baghouse) needs
to be more efficient. The entire baghouse may require attention in
regard to leaks, flimsy construction, lack of insulation and poor main-
tainability. If one must install control equipment at government be-
hest, then the cost of buying and running such equipment should have no
undue impact on the purchasers"s wallet or on his process. For fabric
filters this last requirement may lead to such areas as high temperature
filtration and high velocity filtration.
HIGH TEMPERATURE PROCESSES
Since all of the leading fine particle producers involve high temperature
processes one can start with extending high temperature capabilities.
2
In a filtration paper written a number of years ago, the range of 550
to 750 F was given as a practical maximum for filtration based on power
requirements, gas viscosity and type of cooling. Filtration at tem-
peratures above 750 F would be dictated by special process requirements.
The paper did not, however, talk about changes in air-to-cloth ratio
3
with temperature. Nor, with one exception, does the writer remember
seeing that subject explicitly dealth with. It would seem that bag-
house vendors have taken that paper very much to heart, since the upper
limit of commercial filtration has hovered around 550 F for about 25
years (albeit with increasing bag life and performance over that period).
Apparently the paper did not have as much impact on any electrostatic
precipitator manufacturers who may have read it, since they have sold
precipitators in the 700 F range and have worked on pilot units that go
o 4
up to 1700 F. Perhaps this statement is unfair to fabric filter manu-
facturers, since ESP design and operating problems appear less severe
than those for filtration, and there is also a very high cost associated
with development of new fibers and fabrics. There is one pilot baghouse
238
-------
that operated at temperatures up to 900 F, and the Germans have intro-
duced a filter fabric, Pyrotex T, touted as good for continuous opera-
tion at about 660 F or at short bursts to 750 F. In this country Globe-
Albany claims fabrics good to 600 F.
In order to go to higher temperatures, we are probably talking about
inorganic fibers. There are already glass fiber filter fabrics, but
some people have said that 600 F is the upper limit for glass because
its strength drops off above that temperature. Other people are saying
that, provided a suitable finish could be found for the glass, it should
be useable at up to 1200 or 1300 F which is still a few hundred degrees
below the softening point. Protective finishes used up to the present
i>
have been siloxanes, siloxanes plus graphite, siloxanes plus Teflon
and in some cases siloxanes plus graphite plus Teflon. Figures 4, 5, 6
and 7 are photomicrographs of glass fabrics coated with graphite/sili-
cones and with Teflon, and in used and unused condition.
In order to protect glass to higher temperatures, one might use com-
pounds such as polycarboranesiloxanes. Olin Corporation makes such a
compound which is supposed to be stable to 800 or 900 F, is resistant
to most chemicals except strong bases and some organic solvents, and
bonds to glass. A disadvantage to the material is its price: as a
gas chromatography stationary phase it sells for $3000-4000 per pound,
which is even more expensive than some fluorinated compounds. Given the
market, Olin could probably produce specially tailored compounds at
much lower prices.
Ceramic or metal fibers have been looked at in the past and may come
up again. The Pyrotex T fiber mentioned previously was stated to be a
mineral fiber (presumably asbestos). Another pair of fabrics has been
9
introduced by 3M Company. These have alumina-boria-silica or zirconia-
silica fibers as their basis. Maximum extended use temperatures are
given as about 2200 and 1800°F, respectively, and one can even choose
239
-------
3
-
:•
Figure 4. Clean glass fiber fabric, graphite-silicone treated. Treated
fiber diameter approximately 7 |im
-------
{
/D\
Figure 5. Clean glass fiber fabric, Teflon treated,
fiber diameter approximately 7 |im
Treated
-------
Figure 6. Dirty glass fiber fabric (cement dust),
graphite-silicone treated. Treated
fiber diameter approximately 7 [im
242
-------
r -
I
I -
/r>\
Figure 7. Dirty glass fiber fabric (cement dust), Teflon treated. Treated
fiber diameter approximately 7 [j.m
-------
whatever color he desires. The cost of the fibers is on the order of
$25 per pound with projections of fabric prices at about $10 per pound.
Carbon and graphite fabrics have been mentioned, but they start oxidizing
significantly near 600 F, and their price is high. Boron nitride would
seem to be a good candidate material, but is expensive and may have other
drawbacks as a filter fabric.
There are metal fiber felts such as stainless steel good up to about
850 F and sintered chrome-nickel mats for use to about 1100 F. Elec-
trical conductivity of these materials is high, so that they can provide
good protection against static discharge and might well affect filtra-
tion or cleaning. The Brunswick Corporation also makes a static con-
trol yarn containing 8 |im stainless steel fiber blended into nylon or
other yarns .
If one doesn't wish to pay the price for high temperature fabrics, there
is the option of paying for cooling the gas stream by radiation, water
injection, dilution with air or conventional heat exchange. One way or
another it's going to cost money to process a high temperature stream,
and it appears that if a good high temperature fabric is available,
there will be a market for it.
CHEMICAL RESISTANCE
Combustion effluents always present the hazard of acid attack, but there
are ways to get around the problem. For applications above about
o R R
275 F one can use fabrics based on glass, Teflon or Nomex , and if
there's very much S02/S0., present, and enough water vapor and a high
enough temperature, Nomex in its present form will have short life.
DuPont is purported to have an acid resistant Nomex in the works, which
will be an improvement for organic fabrics. For inorganic materials,
Pyrotex T is supposed to have good acid resistance and excellent alkali
resistance and the 3M ceramic fibers are advertised as being "essentially
chemical resistant".
244
-------
Glass fabrics seem to be popular for combustion effluents, but become
suspect for effluents containing fluorides. Since glass fabrics cannot
survive without a lubricant on the individual fibers, it would seem that
not the glass but the finish is what must really be resistant to fluoride
attack, or that the fabric must be protected from contact with the
fluorides. One interesting way of doing so is to put a sorbent or
reactant material on the bag periodically while in service. Wheelabrator
used this method over a dozen years ago, and has recently used such a
12
system for a primary aluminum pot line. They don't say whether or not
they're using glass bags, but Teller does in a similar system on a
13
secondary aluminum smelter. In a like manner Nomex can be protected
from acid attack, to some extent, by pre-coating with lime/limestone.
At lower temperatures there are several fabrics that display good resis-
tance to a variety of chemical environments. Increases in resistance
for these traditional fabrics would probably have to come from the
fiber manufacturers.
FILTRATION EFFICIENCY
Until recently people have been saying that for most applications bag-
houses are 99 percent efficient and we don't even have to worry about
efficiency. First of all that 99 percent is on an overall mass basis
and the remaining 1 percent by mass can, for very small particles, make
up a very large part of the total number of particles. Remember we said
it was the little ones that we have become especially interested in
removing. Although there is at present no federal standard for fine
particulates (0.01 to 3 |jjn) there may well be one in the future. New
Mexico has written such a standard into its laws for coal burning equip-
ment: no more than 0.02 lbs/10 BTU of particulates less than 2 ^m.
In talking about removal efficiency for fine particulates, one needs
either to talk about number efficiency, or else an equivalent, mass
efficiency over small ranges of particle sizes. Many stack measurements
245
-------
have been made with cascade impactors, which makes it easier to use mass
efficiency by size. While there have been quite a number of laboratory
filtration investigations concerned with size efficiency, there hasn't
been much published on size efficiency of operating baghouses, or on
actual size distributions and grain loadings by size into and out of
baghouses. Six months ago many people thought that full scale collectors,
precipitators and scrubbers as well as baghouses, tailed off in effi-
ciency in the low or submicron range. EPA and contractor testing has
shown, however, that for the small number of units tested, the efficiency
can increase for sizes below about 0.5 (im for good collectors.
Tables 1 and 2 show the results of some of these tests. Theory predicts
this behavior for collection targets: good collection by inertial im-
paction for large particles, possible good collection by diffusion for
small particles and a no-man's land between about 0.1 and 1 (am. For
ESP's there are changes in charging mechanisms that help cause the dip
in efficiency.
There are several implications from this testing. One is that no more
work needs to be done on improving baghouse efficiency, which is an
oversimplification, since the test data apply to limited measurements
made under difficult conditions for specific dust/fabric combinations,
and high, uniform efficiencies are desired. Another is that if a bag-
house isn't efficient on a particular application there is either poor
maintenance (leaks), an awful lot of particles in the no-man's land
region, improper air-to-cloth ratio or else other phenomena which con-
tribute to poor efficiency, such as electrostatic hindrance. Main-
tenance and maintainability are responsibilities of both the user and
the designer. There have been baghouses that didn't even have a simple
U-tube manometer for the operator to tell pressure drop, and maintenance
programs that consisted of looking for broken bags when the plant
manager thought the stack looked too dirty when he drove into the plant
in the morning. That sort of system and program won't remain acceptable;
the product of the tin bender's art must evolve into a better designed
246
-------
Table 1. HIGH EFFICIENCY SCRUBBERS'
Source
Utility boiler
(Bituminous coal)
Ferro-Alloy furnace
Open hearth steel
furnace
Scrubber type
3- stage
turbulent
contactor
2-phase
Venturl
Steam ejector
Venturi
Particulate present, 7.
< 3 pun
60
60
95
< 1 nm
20
15
70
Mass
efficiency, 7.
99+
93
99.9
Fractional efficiency, 7.
1 urn
98
95
99+
0.5 urn
96
90
99+
0.1 urn
94
80
75a,b
0.05 M.m
70a'b
"Revised from Reference 16.
Ni
The data Indicate a minimum at about 0.05
to about 0.01 \m.
and increasing efficiency for smaller sizes at least
Table 2. HIGH EFFICIENCY ELECTROSTATIC PRECIPITATORS'
Source
Utility boiler
(Bituminous coal)
Utility boiler
(Bituminous coal)
Utility boiler
(Western coal)
Kraft recovery
boiler
Particulate present, %
< 3 urn
25
5
15
90
< 1 pirn
10
2
3
60
Mass
efficiency, %
99.6
99.6
98
99+
Fractional efficiency, %
1 (im
99+
95
96
99
0.5 pirn
98
92
91
99
0.1 p.m
98
98
98
96a
0.05 nm
99+
94*
99+
Revised from Reference 16.
-------
piece of equipment, and routine, adequate maintenance must be performed
by the user. For the companies and designers who have not already per-
formed this evolution there are helpful hints in the Handbook of Fabric
Filter Technology and the proceedings from the APCA specialty meetings
in St. Louis and Buffalo last year. '
For the case of large amounts of particles in the 0.1 to 1 p.m region it
would seem that efficiency could be increased by agglomeration, or by
changing filtration conditions to enhance either impactive or diffusional
collection. Agglomeration might be promoted by adding moisture to the
system; by increasing residence time prior to entering the baghouse,
perhaps as part of a cooling system; by adding chemicals or other dusts
21
to the gas stream; or by using sonics or by increasing turbulence,
although these last two methods may use too much energy to be effect-
ive. Impaction may be aided by increasing velocity through the
filter, decreasing the gas temperature, or making the path through the
filter more tortuous. Diffusion should be helped by decreasing velocity,
increasing temperature, making a more tortuous path through the filter
and according to equations in Strauss, also by using thicker filters
with smaller diameter fibers and lower void volume.
ELECTROSTATICS
It seems that in most of the papers one reads about filtration efficiency
or what makes filters tick, there is a reference to E. R. Frederick's
22
paper on electrostatics. Then there is a comment on how important
electrostatic effects are in fabric filtration, followed by comments
about how little we really know about electrostatic effects, and how we
really should pay more attention to the subject. At that point the sub-
ject is usually terminated.
There are several ways that electrostatic augmentation might be looked
21
at. Midwest Research Institute has been investigating use of electrical
248
-------
fields as aids for conventional control devices. They recommend doing
experimental work with both internal and external applied fields and
with naturally occurring fields such as are found with electrets.
23
Frederick has also given recent information on the subject.
FABRIC STRUCTURE
In information available to the public, EPA has reported on effects of
24
fabric structure on filtration performance. Undoubtedly, there is a
wealth of proprietary information on the same subject, and the writer
would be interested in hearing from anyone in the audience who could
confirm or deny EPA's experience. The major finding was probably con-
firmation of Tomaides1 communication that particles can form a rather
sturdy "bridge" about 10 particle diameters long. In other words if
the average fabric pore is more than 10 times the average particle
diameter, filtration efficiency will be relatively poor. Another
example would be work done by Textile Research Institute which shows
that trilobal fibers give higher filtration efficiency at lower pressure
25
drop than do round fibers. In any case it appears that there is still
much to be learned about the effects of fabric surface and construction
on filtration performance.
CLEANING
One other aspect of efficiency is cleaning. More than half of the bag-
houses now being sold are supposed to be pulse jet units, which can
19
bleed fine dusts on cleaning. The popularity of this type of baghouse
has been based on high air-to-cloth ratios and lack of moving parts in
the baghouse; but it may be that for control of fine particulates the
air-to-cloth ratios will have to be reduced, or else new fabrics or dif-
ferent cleaning conditions will have to be found. Penetration imme-
diately after cleaning isn't limited to pulse jet baghouses so that
revision of cleaning methods may be required for shake, and reverse air
baghouses too. GCA Corporation has done work on filter cleaning
249
-------
mechanisms and kinetics which should provide a basis for modifying
f\ r
cleaning to reduce penetration of fine particulates. It is apparent
that, at least for single bags, control of the cake on the bag at the
beginning of a filtration period could yield large dividends in stopping
fines. An alternate to cake control might be momentary recycling of the
effluent from a freshly cleaned compartment to the rest of the baghouse
until cake repair has been completed in the cleaned compartment. EPA
will evaluate such series filtration in a mobile filter used on field
sources. It may even be possible, in some cases, to eliminate cleaning
as a separate step. The writer knows of one instance in which the
reverse cleaning air to a baghouse was stopped, but the filtration con-
tinued. Pressure drop increased a relatively small amount and then
leveled out and didn't change. Apparently there was enough vibration in
the baghouse and the cake was fluffy enough, so that the outer surface
of the cake continued to slough off without destroying the cake next to
the bag, and the filtration continued without the need for a separate
cleaning step.
COSTS
The last topic before making some general comments and summarizing is
costs. Ignoring Barnum's dictum about lack of birth control among
suckers, let's assume that fabric filter systems aren't going to be
sold for a given application unless they can do the job properly and
cost less than competing control methods. In many areas it's coin* f-o
have to be cost that sells a baghouse, because ESP's and scrubbers have
turned out to be very efficient on fine particulates in given applica-
tions .
Starting with capital costs, what can be done to reduce the installed
price or a oagnouse and still maintain quality? Perhaps the largest
single item is air-to-cloth ratio. If one could use ratios of 100:1 or
1000:1 baghouses would be in a much more competitive position. There are
problems with bleeding of fines and/or rapid blinding of the fabric when
250
-------
relatively small increases are used; however, some work has been done
on high velocity filters, which shows the expected efficiency decrease
as ratios increase to about 100 fpm, but then there is a turnaround,
27
and efficiency at 1000 fpm is better than at 5 fpm. Additional work
has been done at Harvard under Dr. Melvin First, and there is at least
one company which is trying to commercialize a cleanable, high velocity
28
filter, and has test data at up to 2500 fpm.
Decreased operating costs are going to come from reduced maintenance
requirements, but since lack of maintenance has long been a problem it
would seem that smaller units and increased bag life are needed. Smaller
units means we are again talking about higher air-to-cloth ratios, and
longer bag life means either less strenuous cleaning or longer wearing
fabrics. An example of the latter might be spunbondeds, which are po-
tentially more efficient, longer lasting and cheaper than an equivalent
29 2
woven bag. One spunbonded nylon bag tested at EPA (2.4 oz/yd ) sur-
vived 40 million shakes with an efficiency change from 99.5 percent at
the start of testing to 98.8 percent at the end. Part way through the
testing a repair of the bag was made with room temperature vulcanizing
silicone rubber.
GENERAL COMMENTS
Fabric filtration has been around for a long time; it's practically an
ancient art. If one wants to build a baghouse he often depends on com-
pany files rather than design equations to come up with a set of specifi-
cations. Referring to the proceedings from last October's APCA Buffalo
19
meeting, (in which there are many excellent papers), there is one
striking lack. There are very few numbers. There are some cost figures
and some temperature capabilities, but not many numbers one could use in
design equations. On reflection this lack shouldn't be surprising, since
there's also not much in the way of design equations for fabric filters,
251
-------
and that's the point. If one wants to design an ESP he can at least
dust off the Deutsch equation, or its variants, and play around with it;
calculations can be made of drift velocities and corona gradients and
degrees of charging. Scrubbers aren't as far along, perhaps, but with
baghouses the choices are especially limited. Since control of par-
ticulates in general, and fine particulates in particular, will call for
extending control devices into unfamiliar regions and applications, it
seems difficult for the fabric filtration industry to remain competitive
if it can't analyze and design for potential applications mathematically
as well as verbally. Regardless of the varied and understandable reasons
for lack of analysis and design equations, it seems that the industry
needs to enlist someone who can formulate and publish such equations.
There is the caution that those who do the work must have an extensive
background in fabric filtration. The subject is too complicated for
rapid analysis by the neophyte.
There are similarities among the three major particulate collectors
that might be taken advantage of for design and analysis. Each has a
collecting chamber, each has an array of collecting surfaces, each is
concerned with placing the collecting surface in the path of particulates
(or influencing the path of the particulates toward the collecting sur-
face), each has a means for regenerating the collecting surface, and
each is vitally concerned with the interaction between collector sur-
face and particles either as individuals or as aggregates. The change
in magnitude of collecting forces with time between particle and col-
lection surface is probably greatest for fabric filters and of lesser
or secondary importance for ESP's and scrubbers. Fabric filters have
one aspect with both good and bad connotations: the variety of collect-
ing surface materials and shapes. Scrubbers generally use one material,
water, in several shapes, drops, sheets or bubbles; ESP's are limited
to a few metallic compositions for straight wires and for several shapes
of flat plates; but fabric filters have a bewildering variety of collector
materials, each of which can be formed in an equally bewildering variety
252
-------
of shapes. Bringing mathematical sense and order to the interactions
between particulates and fabrics is certainly an awesome task, but it
is one that needs doing.
SUMMARY
Reasons for needing better collection of fine particulates (health,
weather, visibility) have been given, and areas of R&D which may prove
fruitful in extending fabric filter capabilities have been suggested.
These areas include extension to sources which are the major producers
of fine particulates, mostly high temperature sources; more chemically
resistant and more durable fabrics; more efficient collection of fine
particulates through better maintenance, better cleaning or better
fabrics; and major reductions in filter system capital and operating
costs. Fabric filtration is at a disadvantage because of lack of
analytical and design expressions, and work certainly needs to be done
here. The question remains as to who will do all this R&D work. The
National Academy of Engineers recommended a tenfold increase in govern-
ment spending in the general area of R&D in particulate control, but
that was 2 years ago and the government, at least as represented by
EPA, has not increased spending to that level. Universities rarely
generate much of their own R&D money, and they've been falling on even
harder times lately. That seems to leave one segment to get the work
done: industry.
Many of the topics discussed in this paper may sound either contra-
dictory, too costly or perhaps just pain ridiculous, but the idea has
been to point out areas which are in some fashion documented and which
may eventually lead to extended capacilities.
253
-------
REFERENCES
1. Abatement of Particulate Emissions from Stationary Sources.
National Academy of Engineering. Washington, D. C. Report
No. COAPC-5, NTIS No. PB 211-961. 1972.
2. Spaite, P. W. , D. G. Stephan, and A. H. Rose, Jr. High Tempera-
ture Fabric Filtration of Industrial Gases. J.A.P.C.A. 11:243-
47, 58, May 1961.
3. Frey, R. F. and T. V. Reinauer. New Filter Rate Guide. Air
Engineering. 30, April 1964.
4. Oglesby, S. Jr. and G. B. Nichols. A Manual of Electrostatic
Precipitator Technology, Part II - Application Areas. Southern
Research Institute. Report No. NTIS No. PB 196-381. 1970.
p. 841.
5. Veazie, F. M. and W. H. Kielmeyer. Feasibility of Fabric Filter
as Gas-Solid Contactor to Control Gaseous Pollutants. Owens-
Corning Fiberglas Corporation. Report No. NTIS No. PB 195-884.
1970.
6. Dietrich, H. and H. C. Gurtler. New Textile for High Temperature
Filtration of Dust & Gases. (Paper presented at the Filtration
Society Conference. London. September 1973.)
7. Finch, R. W. A New Chromatographic Phase Stable to 500 F. North
Haven, Connecticut. Analabs Research Notes, 10, No. 3. July 1970.
p. 1-12.
8. Finch, R. W. Olin-Matheson Corporation, New Haven, Connecticut.
Private communication. March 1974.
9. Introducing M.G. Ceramic Fibers. 3M Company, St Paul, Tech-
nical Bulletin. 1974.
10. Brunslon Static-Control Yarn Information Kit. Brunswick Corpora-
tion. Needham Heights, Mass.
11. Impregnated Fabrics Collect Fluoride Fumes. Engineering & Mining J.
160:112, May 1959.
12. Squires, B. J. Fabric Filter Plants for Cleaning Gases from Non-
Ferrous Metal Furnaces. (Paper presented at the Filtration Society
Conference. London. September 1973.)
254
-------
13. Francis, F. J. Secondary Aluminum Smelter Air Pollution Control
Using a Chromatographic Coated Baghouse. (Paper No. 72-79 pre-
sented at the 1972 APCA Annual Meeting. Miami. June 1972.)
14. McKenna, J. D. The Application of Fabric Filter Dust Collectors
to Coal-Fired Boilers. (Paper presented at the Fourth Annual
Environmental Engineering & Science Conference. Louisville.
March 1974.)
15. Billings, C. E. and J. Wilder. Fabric Filter Systems Study.
Vol. I: Handbook of Fabric Filter Technology. GCA Corporation.
Report No. NTIS No. PB 200-648. 1970. p. 2-83.
16. Craig, A. B. Overview of the Fine Particulate Problem. Proceed-
ings: Symposium on Control of Fine Particulate Emissions from
Industrial Sources, Joint US-USSR Working Group, Stationary Source
Air Pollution Control Technology. Will be available through NTIS.
1974.
17. Strauss, W. Industrial Gas Cleaning. New York, Pergamon, 1966.
p. 230, 217, 228.
18. Design, Operation, and Maintenance of High Efficiency Particulate
Control Equipment. APCA Proceedings: APCA Specialty Conference,
St. Louis. March 1973.
19. The User ft Fabric Filtration Equipment. APCA Proceedings: APCA
Specialty Conference, Buffalo. October 1973. p. 114.
20. Durham, J. F. and R. E. Harrington. Influence of Relative Humidity
on Filtration Resistance and Efficiency. Filtration & Separation.
p. 389-392, July/August 1971.
21. Control Technology for Fine Particulate Emissions. Midwest Re-
search Institute. Will be available through NTIS. 1974.
22. Frederick, E. R. How Dust Filter Selection Depends on Electro-
statics. Chemical Engineering, p. 107-114, June 26, 1961.
23. Frederick, E. R. Some Effects of Electrostatic Charges in Fabric
Filtration. (Paper presented at the Symposium on the Use of
Fabric Filters for the Control of Submicron Particulates. Boston.
April 8-10, 1974.) Will be available through NTIS.
24. Draemel, D. C. Relationship Between Fabric Structure and Filtra-
tion Performance in Dust Filtration. U.S. Environmental Protec-
tion Agency. Report No. NTIS No. PB 222-237. 1973.
255
-------
25. Miller, B. , G. Lamb, and P. Costanza. Influence of Fiber Charac-
teristics on Particulate Filtration. (Paper presented at the
1974 TAPPI Annual Meeting. Miami. January 1974.)
26. Dennis, R. and J. Wilder. Fabric Filter Cleaning Mechanisms and
Kinetics Study. GCA Corporation. Will be available through NTIS.
1974.
27. Stern, S. C., H. W. Zeller, and A. I. Schekman. The Aerosol
Efficiency and Pressure Drop of a Fibrous Filter at Reduced
Pressures. J. Colloid Sci. 15:546, 1960.
28. Collins, J. 0., Jr. Johns-Manville, Denver, Colorado. Private
communication. March 1974.
29. Turner, J. H. Performance of Non-Woven Nylon Filter Bags.
(Paper No. 73-300 presented at the 1973 APCA Annual Meeting.
Chicago. June 1973.)
256
-------
NEW FABRICS AND THEIR POTENTIAL APPLICATION
Lutz Bergmann
Manager, Filtration Development
Globe Albany Corporation
1400 Clinton Street
Buffalo, New York 14240
Excitement in fabric filtration? Entirely new fabrics for future re-
quirements? Although there are radical new innovations there is no
"rotary engine" concept in fabric filtration, but there are some devel-
opments with quite encouraging prospects. Two areas of major concern to
equipment manufacturers as well as to end users are: improved effi-
ciency and cleanability of filter fabrics. However, this paper will
deal basically with high temperature filtration 250 F. and higher.
Approximately 1/3 of the particulate emissions from industrial sources
evolve from processes 900 F. or higher. The utilization of fabric fil-
tration systems in these environments typically requires gas stream
cooling by either radiative/convective means, air dilution or water
systems cooling. Even in cases where cooling systems are employed,
temperature surges sometimes occur and may cause damage to the operating
elements of the filtration system, particularly to the fabric. There-
fore, the demand for significant high-temperature resistance fibers in
fabric filtration is certain.
The growth and development of the fabric filter has paralleled the ex-
panding technology of synthetic fibers. The traditional barriers to
257
-------
baghouse developments as related to fabrics have been temperature limi-
tations and chemical resistance of fibers, dimensional stability and
flex strength. These limitations have often dictated the design limi-
tation of the system. Significant improvements, according to Culhane*
have been made possible with cotton and wool being replaced with man-
made fibers and glass.
The characteristics of most sub-micron particles to agglomerate have
resulted in the ability of filter fabrics to remove 99.9 percent/wgt.
of the sub-micron solid particles. This order of fine particulate
efficiency is being achieved in separating:
1. Carbon Black from process reactors.
2. Silicone Dioxide in .02 - .05 micron range from
submerged arc furnaces.
3. 100 percent of particles below 5 micron and
95 percent below 2 micron in ferrous fume emissions.
4. Fume dust from leaded glass melting furnaces either
80 percent of the particulate are below 2 micron and
have poor agglomeration tendencies.-*-
Therefore, in designing an efficient economical fabric filter, one must
weigh carefully the strengths and weaknesses inherent in each of the
available fabrics to arrive at a system which has filter bags which
do the job. To discuss all of these requirements at this time would be
impractical. The development of new high-temperature fibers is being
accelerated e.g. NOW 100 from DuPont would be such a candidate.
It is generally accepted that pollution control equipment, specifically
fabric filters and electrostatic precipitators, are least efficient in
removing particles in the critical 0.1-1 micron size range. Table 1
summarizes the separating mechanisms to particles of various sizes.
In each case, there are circumstances in which a minimum exists in the
curve of efficiency vs. particle size for constant flow velocity. In
the case of the fabric filter, very small particles (smaller than 0.1
micron) are efficiently removed by Brownian motion and the finer the
258
-------
Table 1. PARTICLE SIZE RANGE FOR SEPARATING MECHANISMS
Force
Particle size range, u
1. Gravitational settling
(for large size particles)
2. Physical or direct interception
(For intermediate size particles)
3. Inertial impaction
(for smaller size particles)
4. Diffusion of Brownian motion
(for very fine particles)
5. Electrostatic forces present on either
particle or fiber or both
> 1
> 1
> 1
< 0.01 - 0.2
> 0.01
easier their removal in the diffusion range. Figure 1 depicts the
major particulate removal mechanisms operative in the case of a single
fiber. Larger particles (larger than 1 micron) are collected by im-
paction and interception and their removal efficiency increases with
particle size. The particle size most difficult to collect is in the
2
range between removal by Brownian motion and by impaction. These
fine dust particles actually don't obey filtration laws.
This theoretical result has been experimentally verified for deep bed
fiber filters and for clean cloth filters in laboratory studies. How-
ever, in an EPA sponsored review, Billings and Wilder, 1970, noted that
field performance data for the efficiency of fabric filters as a frac-
tion of particle size are lacking. A somewhat similar situation exists
for electrostatic precipitators commonly used for fly ash removal from
power plant emissions. Particles larger than about 1 micron have high
mobility because they are highly charged. Particles smaller than a few
tenths of a micron can achieve moderate mobility even with a small charge
because of aerodynamic slip (least charge ability). (A minimum in effi-
ciency usually occurs in the transition range between 0.1 and 1.0 micron,
more specifically 0.4 - 0.6 micron).
259
-------
INKRTIAL IMPACTION
N>
&
O
INTERCEPTION
GRAVITY
ig'iri^ 1. Mechanisms for particle removal by a fiber
-------
Table 2 summarizes the factors affecting filtration efficiency in needled
felts. As can be determined by review of this table we see that the
decrease in efficiency due to an increase in fiber diameter is due to
the reduction and flow tortuosity per unit weight or mass of filter.
Needled felt efficiency, due to the diffusion phenomena, is reduced by
increases in face velocity, particle density and fiber diameter. The
one consistent fact, as far as the stated filter variables are concerned,
is that one should use the finest filter diameters available to achieve
maximum filtration efficiency.
Air permeability as discussed herein represents an approximation of the
starting condition of an air filtration fabric. Generally, rapid changes
in flow take place, the rate probably dependent upon fabric structure
and the filtrate characteristics, until a new approximately steady state
is reached (permeability equilibrium). Thus, while initial fabric
structure and air permeability constitute only part of the spectrum of
fabric performance characteristics, they undoubtedly are fundamental
to the entire filtration operation, including the type and rate of cake
build-up. Air permeability under working conditions - with dust layer-
will remain basically between 20 percent and 50 percent of the initial
throughput at differential pressures basically between 3" and 6" W. G.
The air permeability at this stage is referred to the permeability
equilibrium. It is the target in fabric design to keep this permeability
equilibrium as high as possible.
SURFACE MODIFICATION OF FILTER MEDIA
The capacity, cleanability and efficiency of fabric filters are greatly
influenced by surface properties. Surface modifications have been
shown to have an especially marked influence on the performance of
3
needled fabrics. Fine particles readily penetrate ordinary felted
fabrics and this often leads to serious plugging condition with
261
-------
Table 2. FACTORS AFFECTING FILTRATION EFFICIENCY IN NEEDLED FELTS
Efficiency due to:
Ni
CT>
Ni
Inci ease in:
Fact velocity
Particle size
Particle density
Fiber diameter
Decraase in
fibe r diameter
ravity
creased
creased
creased
effect
effect
Sieving
No effect
Increased
No effect
Decreased
Increased
Inertial
impaction
Increased
Increased
Increased
Decreased
Increased
Direct
interception
No effect
Increased
No effect
Decreased
Increased
Diffusion
Decreased
Decreased
Decreased
Decreased
Increased
significantly
-------
substantial reduction in air permeability. New developments in surface
alteration through controlled heat treatment tend to inhibit serious
penetration of particles by encouraging the formation-of a primary pro-
tective dust layer on the surface with only minimal penetration into
the fabric. As a result, permeability at equilibrium remains high and,
therefore, better overall filter capacity is realized. In many applica-
tions, in fact, the improved surface promotes formation of a more porous
dust layer.
Surface modifications of this type have been applied successfully to
polyester, acrylic, polyolefin and high temperature (Nomex) aramide
felts, but with each fabric system the modifying method differs. Needled
felts, altered in this way, have already demonstrated their special
effectiveness in collecting the emissions from asphalt, cement and cer-
tain chemical plants. The fabric is also being evaluated in a number
of other applications.
Aggregates on the fibers collect some particulates by impingement and
continue to grow in size, eventually bridging the spaces between fibers,
as depicted in Figure 2. This formation of the dust cake within the
fabric takes place over some little depth and having once formed, col-
lection of particles either takes place on the outer surface of this
dust cake, due to the self-filtering action of the cake, or on some of
the fibers between the fabric surface and the cake surface. The dust
cake rapidly builds back to the surface of the fabric and beyond it.
It will be noted that, in general, as the particle size decreases, the
acceptable filtering velocity decreases. This is a reflection of the
conditions within the media itself. In operation, the outermost fibers
of the felt act as the base for the retention of the first particles to
be filtered, until sufficient bridges have been established to support
the filter cake which then serves to "sieve" further particles from the
air flow.
263
-------
SCRIM.
PELTED MEDIA APPRO*. 3mm.
TIME OF TRAVEL 02-0 Ssec.
Figure 2. Section through felted filter
media showing distribution of
dust
When one considers the extension of such theories to needled felted
fabrics, the limitation of these theoretical developments to practical
situations are obvious, due to the three dimensionality of the system
and the fact that the dust particles are almost entirely irregularly
shaped. In the absence of an adequate theoretical treatment, or detailed
understanding of the mechanism of separation principle of fabric filters,
manufacturers have gradually built up a "know how" of satisfactory use
of these filters.
MATERIAL TO BE HANDLED
The shape and structure of a particle will influence its collection,
its interaction with the fabric, and its behavior as an element in a
264
-------
granular deposited layer or cake. Figure 3 presents an artists concep-
tion of the major shapes of airborne particulates and summarizes the
major sources of each particle shape. Characterization of the size,
shape and structure for most particles of concern requires costly and
sophisticated analysis and little has been done to relate properties
of the particulate system to behavior in a fabric filter deposit or to
effects on filter performance.
Most dry dust from manufacturing operations involving product handling,
venting, and the related processes, consist of highly aggregated systems
of single particles. Since they are often compacted so that their
envelope shapes are approximately spherical, their aerodynamic behavior
can be predicted adequately from spherical models. Most analytical
treatments are based on resistance forces arising from spherical shape.
Irregular shapes will experience greater resistance forces which counter
the gravitational force and lead to reduced settling velocity.
Sub-micron fume is molecular in size and the ability of the baghouse to
remove it is due to the filter cake formed by proportional number of
light agglomerated particles. The finer particles form a tighter
filter cake with higher density, resulting in a more compressive forma-
tion in comparison with a cake formed from coarser dust particles.
With trends towards more highly rated chemical plants and the increasing
likelihood of handling thermally liable materials with unusual properties,
filter manufacturers must be continually vigilant for the "problem
material".
Typical organic solids fall mainly.into the category of materials which
have been subjected to a drying process and that the intermediate or
final product is separated as a filter cake from usually an aqueous
suspension. There are current trends to use fluidized bed or spray
dryers. These have the advantage of considerably reduced settle times
265
-------
SPHERICAL
SMOKES, POLLEN, FLY ASH
CUBICAL
SALT CRYSTALS
IRREGULAR
CUBICAL
MINERAL DUSTS
FLAKES
MINERALS, GRAPHITE,
EPIDERMIS
ACICULAR,
SPINY
ZINC OXIDE, AMMONIUM
SULPHATE
FIBROUS
LINT, PLANT FIBERS,
ASBESTOS, MAN-MADE FIBERS
CONDENSATION
FLOGS
AGGLOMERATES
CARBON SMOKE, COAGULATED
METAL OXIDE FUME (E.G.
IRON OXIDE)
Figure 3. Major shapes of airborne particles
266
-------
although a high velocity circulating or moving stream of air is essen-
tial for operation if much larger volumes of dust loaded air must be
handled in classical counter current dryers.
In the asphalt industry, for example, one of the most important considera-
tions for any plant is the raw material to be handled and filtered.
Recent studies show that particle size distribution down to the sub-micron
size exhibit great variations, particularly with regard to the percentage
by weight of particles in that low micron range, as demonstrated in
Figure 4. It is not the coarse aggregates which cause major problems,
but mineral dust. The "filler" usually consists of fine ground particles,
crushed rock, limestone, hydrated lime, Portland cement, clay, Basalt,
slag, sandstone, or other non-plastic mineral matter.
In some aggregate dust, 5 percent of the minus 74 micron material is
below 5 microns, in the others, considerably higher amount of this micron
range is common. Further complications arise in that each hot mixed
asphalt plant produces a number of different asphalt mixes in which
coarse aggregate containing very low amounts of dust are combined with
fine aggregates in an infinite range of combinations at a moisture
content ranging from 1 to 12 percent.
The selected filtering velocity varies from application to application
depending upon the filtration characteristics of the dust and fume
particulate. The filterability of any given quantity of particulate
depends upon the particle size distribution, shape, surface properties,
and electrostatic forces.
FILTER EQUIPMENT
The most rapidly growing type of fabric filter in recent years is the
high ratio, pulse-jet or cage-type filter. The percentage of the total
267
-------
PARTICLE DIA. (UICRONS)
m!H!!«!HU»m«H«ii!H!!!H!i»!i!!!!!iB!!lIil!l!iiii"i.'iniiiii niiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i
PARTICLE DIA. (MICRONS)
Figure 4. Particle size distribution of emissions
for an asphalt plant
268
-------
PARTICLE DIA (MICRONS)
PARTICLE DIA. (MICRONS)
Figure 4 (continued). Particle size distribution of
emissions for an asphalt
plant
269
-------
filter market represented by this type of system is estimated to be as
follows:
1969 - 5 percent
1970 - 10 percent
1971 - 40 percent
1972 - 50 percent
1973 - 55 percent
There are today approximately 40 manufacturers in the United States
which offer this type filter with probably 50 different filter designs,
but all utilizing needled fabrics and working at A/C ratios basically
between 5:1 and 10:1. The cleaning device applied might have the most
significant influence on performance.
"OVER-CLEANING" OR "PUFFING" EFFECT
The "over-cleaning" or "puffing" effect is observed with high energy/
low air volume cleaning even if this method of bag regeneration has
certainly many advantages as compared to reverse air and mechanical
shaking type cleaning. The very abrupt full air shock destroys to
some extent the primary dust layer. If the dust contains very many
sub-micron particles, this layer has to be re-built each time after
cleaning resulting in insufficient collection until this is accom-
plished. This phenomenon can be improved by lowering the cleaning
pressure, but onlv at the expense of high di fferenHsi prp««"i»-e-
juiereiort:, rlexibtlxty in solving an erriciency prooiem with lowering
the cleaning pressure is limited.
In this connection3 it should be mentioned that there are certainly
several unknown factors in fabric filtration which should be the sub~
iect of more research and development work, particularIv rp.lat-pd t-o
this type of filter. Some of these areas are: venturi design, length
to diameter ratio, diameter influence on cleaning efficiency, venturi
placed inside or outside of bags, double wall bag configuration, cake
270
-------
density effect on cleanability, air permeability of fabric during
operation, pressure drop and particle size, particle shape, gas veloc-
ity and distribution, and relationship of cleaning cycle to inlet
loading, to name a few.
FIBERGLASS
Glass fibers are unique compared with the wide range of fibrous mater-
ials used commercially for filtration purposes. They differ from
naturally-occurring cellulose fibers and other man-made fibers in that
they are circular in section, straight and of uniform diameter, and
can be made far finer. They have a considerably higher density than
cellulose and most man-made fibers and a far wider, useful temperature
range. In addition, glass fibers do not suffer a change of form when
prepared for processing, retaining their original cylindrical shape
with none of the swelling or fibrillation associated with natural
... 5
fibers.
The all important factor controlling filtration characteristics of
4
the flass fiber medium is the diameter of the fiber.
Loeffler reported that the collection efficiency of fiber filters
is evidently vitally effected by a bouncing effect which occurs when
the particles strike the fibers at speeds much below 1 meter per sec-
ond and which reduces the collection efficiency.
The higher the filtration rate at which the particles had been fil-
tered onto the fibers, the higher was the blow-off speed (compress-
ivity) cake density.
271
-------
EFFICIENCY OF NEW MEDIA
The collection efficiency of needled fabrics is no longer determined
simply on a basis of weight and permeability. In addition.to surface
variations, fiber blends and inclusion of superfine fibers offers a
new dimension to needled felts. As a direct result, collection effi-
ciency has been improved. The inclusion of very fine glass fibers
with Nomex in the needled felts increases the fiber surface area sig-
nificantly. This allows more particles to be retained and causes more
fine particles to be held at or near the surface.
GLAMEX
For pulse-jet filters particularly, fabric requirements are becoming
more and more critical. Accordingly, new developments in felt-like
fabrics have been and are being made with a degree of production
sophistication not attained before. The fine glass - Nomex (Glamex) -
fabric, for example, has effectively replaced some 14 02./sq. yd.
Nomex felt in several asphalt plants due to its ability to effec-
tively control sub-micron particles. Other very successful appli-
cations of this combination fiber type of felt have been shown for
controlling lightweight aggregate, silica dust, carbon black and
/. o
cement ciimcer all at nigh efficiency and at elevated temperature.^'
Man-made fibers in Nomex, polyester, polypropylene and others for
filtration purposes are commercially available down to 2 denier or
1.5 denier. Denier is related to the fineness of the single fiber
and its manufacturing is limited therein. (The lower the denier
number tne riner cne Ti.oe.TC.)
272
-------
Glass, a mineral fiber, is available in much finer deniers so that
more fiber surface faces the dust and a larger portion of fine dust
particles settles down rather than penetrating through a fabric.
In fabric filtration the fiber diameter is the denominator, hence,
a decrease in fiber diameter will cause the parameter to increase
yielding higher efficiency by diffusion. It is known that the face
velocity in fabric filtration systems - the velocity at which the
gas passes through the filter fabric - determines if fine dust is
collected by impaction or interception rather than by diffusion.
Tests have indicated and proved that the increase of fiber surface
utilizing fine fibers influences fabric efficiency dramatically.
Efficiency of filter fabrics is influenced mainly by the ability of
the filter media to attract the very fine particles. The specific
gravity of these fine particles is very nearly equal to the molecular
weight of the carrier gas or air. These very fine particles settle
down due to the assistance of reflection on the fiber surface and
become arrested by capilliary adhesion forces. Such movement is
characteristic of small particles carried at low velocity.
Diffusional contact, therefore, is favored when small particles move
at low velocity against large filter surface. A very simplified
analysis: the human nose acting as a duct has at its entrance a
network of hairs acting as a filter to remove any foreign material
from the air stream before it reaches our lungs at a very low velocity.
Glamex has proven its ability in applications besides the asphalt in-
dustry; on Clinker Coolers in cement industries and spray dry applications
in chemical processing. Difficulties occur because dust with a low ten-
dency to agglomerate causes efficiency problems, particularly on high
ratio filters.
273
-------
It has been determined that the pressure, duration and cycle of clean-
ing have some influence on efficiency. Glamex has proved to be less
sensitive to cleaning pressure.
Glamex has been in the field for four years and will go into other
areas where the high ratio filter unit has proven its suitability.
Glamex, as a new filtration concept, is certainly the answer in many
areas where efficiency is a major problem.
The first generation Glamex fabrics use a woven glass scrim with a
combination glass/Nomex blend or web. Needless to say, in terms of
strength and, more specifically, flex characteristic, any glass scrim
is inferior to 100 percent woven Nomex scrim. It is too early to
make any prediction as to what time factor is involved with regard
to bag life. Our results to date are quite encouraging. We have
also developed a second generation Glamex fabrics which use the old
spun Nomex scrim and the fine glass fibers only in the web. With
reference to a special test of the Bureau of Mines, applied to res-
pirator felts, silica dust is used in the 0.4 to 0.6 micron range.
Efficiency tests have indicated that this second generation Glamex
fabric is twice as efficient as a 100 percent needled Nomex fabric.
NEEDLED FABRICS IN SHAKER AND REVERSE AIR BAGHOUSES
Shaking will remove a dust cake which has built up on the fabric
surface but it has little effect on the dust cake within the fabric.
Reverse cleaning will remove some of this material. Thus, when the
reverse flow dislodges, particles within the depth of the media are
subject to impingement on fibers near the surface. The nearer the
dust is to the surface the greater the likelihood of being removed
by reverse flow.
274
-------
Needled fabrics traditionally used on cage-type collectors are
also being considered in shaker and reverse air baghouses in the
United States today. Since the mid 60's experience in Europe with
needled fabrics in these collectors has been quite encouraging.
The basic baghouse design differs compared with this collector type
in the U. S.
Bag sizes are 7 1/2 inches or 8 1/2 inches diameter and 8 feet or 11 feet
long providing a length to diameter ratio of roughly 16, resulting in
a relatively low entrance velocity in filter bags. In addition, most
of these bags contain four to six spreader rings and operate at A/C
ratios ranging between 4 - 6:1 versus 2.5 - 3:1 in most comparable
U. S. shaker and reverse air filters.
Needled fabrics perform far better in large diameter bags than in
smaller 4-inch or 5-inch diameter tubes. The stiffness of the felt,
compared with woven fabrics, may cause mechanical wear problems in
the bottom cuff area. In order to evaluate the problems, several
companies designed their bags with woven boots. However, a flexible
surface modified needled fabric has been our main objective for this
application.
Is the needled fabric a better substitute for woven fabrics in these
collectors? The answer has to be proven in the field. Results so
far are very encouraging. The obvious advantages are:
Improved efficiency
Increased capacity
Better cleanability
Improved economics
Suitable at elevated
temperatures up to 400 F.
275
-------
Due to the in depth filtration properties of needled fabrics one may
consider higher A/C ratios when designing shaker or reverse air sys-
tems that used the needled bag instead of the woven fabric. Economi-
cally, therefore, such units consist of fewer compartments and, there-
fore, less hardware. Due to the needled felt structure, the efficiency
is considerably better than an equivalent permeability. The air per-
meability of needled fabrics is generally higher, compared with most
woven fabrics used in these collectors. A special surface modifica-
tion provides a better cake release. Fine particles readily penetrate
ordinary felts, resulting in serious blinding and high differential
pressure. The needled fabric with a heat-treated surface inhibits
serious penetration of particles by forming a primary dust layer
rather on the surface. All of these advantages should lead to in-
creased acceptance of needled fabrics in shaker and reverse air
baghouses.
A large cement company in Canada, with a total of 24,000 filter bags
operating a reverse air baghouse, converted more than 12,000 bags to
needled fabrics. Better efficiency and lower differential pressures
have been the main advantages. Several other trials are being con-
ducted, particularly in industries where capacity is a main problem
with existing fabric filters. Instead of adding additional compart-
ments, the unit can be converted to needled fabrics, with the same
number of bags but capable of handling a considerably greater amount
of air. In some cases, duct work and fans have been enlarged to
handle the additional air flow.
276
-------
STATUS SUMMARY OF DIFFERENT INDUSTRIES
ASPHALT PLANTS
This industry employs approximately 800 to 900 fabric filters today,
and it is anticipated that in 1974 another 500 to 600 units will be
installed. While there are a number of needled felts which can be
used for asphalt plant applications, as enumerated in Table 3, the
industry is utilizing basically needled 14 oz. Nomex felt. The fil-
ter unit is a cage-type high ratio unit with an average of 5.5 - 6:1
A/C ratio. Temperature generally measures between 240 F and 270 F
with temperature surges up to 375 F. Inlet loadings amount to 20 to
30 gr./cu.ft. One most important consideration from an efficiency
standpoint is the amount of sub-micron particles. This industry has
to meet the federal code of 0.04 gr./cu.ft. This becomes difficult
to obtain on a consistent basis due to very fine raw materials, at
least in some geographical areas. Special fabrics, i.e. GLAMEX and
style S-2283NR (Glass/Nomex in the web-Nomex scrim) are doing a
superior job in filtering these fines from the stream.
CEMENT INDUSTRY
For more than 15 years reverse air glass baghouses were very well
known in kiln operations. Within the last 2 years, Clinker Coolers
have been furnished with high ratio units. The average Clinker
Cooler operation handles 120,000 cfm with an air to cloth ratio of
5:1 using a 16 oz. needled Nomex filter bag, and has expected bag life
from 2 to 3 years. To date, 50 fabric filter units are in operation
or under construction in Clinker Cooler filtration.
CARBON BLACK
This industry represents the largest user of glass fabrics used in
reverse air filters. Several new treatments for flass fabrics, with
277
-------
Table 3. COMPARISON OF DIFFERENT NEEDLED NOMEX FABRICS FOR ASPHALT PLANTS
NJ
^J
oo
Fiber
Weight (oz /sq. yd.)
Air Perinea dlity
Air Permea illity after
dry heat exposure
(3 hrs. it 400°F)
Scrim Type
Scrim Coun : (W & F)
Scrim Weig it
(02./sq. yd.)
MIT Flex ( Jarp)
Mullen Bur it
(lbs./in:h)
Strength
(lbs./ir:h - W & F)
Nome <
14.1
25 - 35
30 - 40
Woolen
23 x 23
4.C
300.C.OO
30(
150 x 225
5
A
83% Nomex
17% Glass
14
25 - 35
30 - 40
Woolen
23 x 23
4.0
300,000
425
196 x 270
B
Nomex
14.1
35 - 42
50 - 60
Cotton
19 x 20
3
200,000
620
250 x 480
B
Nomex
14
38 - 44
50 - 60
Cotton
18 x 18
3.4
259,160
560
175 x 230
B
Nomex
13.2
38 - 45
50 - 60
Cotton
19 x 17
3
60,446
590
300 x 394
C
Nomex
14
32 - 41
50 - 60
Cotton
20 x 20
3.1
53,977
400
272 x 390
D
Nomex
13.7
35 - 42
50 - 60
Cotton
19 x 33
9.7
50,000
590
159 x 356
E
Nomex
13.6
31 - 34
50 - 60
Cotton
12 x 14
1.8
17,669
470
220 x 330
F
Nomex
15
31 - 44
50 - 60
Cotton
20 x 20
3.1
56,427
600
260 x 360
-------
an improved acid and flex abrasion resistance, are under trial. In
many plants the third generation treatment (silicone graphite, teflon)
has been replaced with superior treatments such as Q70 (Globe Albany
Corporation) or Teflon B (Du Pont) or a new treatment GAT-13 which
has been developed by Albany International Corporation.
The importance of fabrics to this industry is related to process fil-
ters which are part of the manufacturing equipment. Recently, also
100 percent woven Teflon is under test in this industry with the major
objective of obtaining longer life. Efficiency remains questionable.
CUPOLA IN FOUNDRY
The Harsell cupola emission control system is well established and
more than 70 installations are working very successfully. This unit
is designed for an A/C ratio of 1.9:1, with inlet loadings between
1 and 2 gr./cu.ft., operating at about 550 F. Bag life is between
3 and 5 years. Bag sizes of 22 feet 6 inches by 11 1/2 inches diameter,
provide a length to diameter ratio of only 23.5, avoiding extremely
high entrance velocities which would abrade the bottom of the bag.
MUNICIPAL INCINERATION
This is a relatively new field for fabric filters. However, a few
installations are under test. One filter manufactured by Combustion
Equipment Associates came on stream last fall. This unit cleans by
reverse air principle and handles 180,000 acfm at 500 F. A/C ratio
is 2:1, inlet loading is less than 0.5 gr./cu.ft. with an acid dew-
point 240 F to 280 F and a designed differential pressure of 2 inches
to 3 inches W.G. Bag size is 14 feet by 5-1/2 inches. Some minor prob-
lems have been experienced with condensation but, generally, this in-
stallation performed satisfactorily. Another approach is to utilize
the high ratio cage-type unit. One installation will come on stream
during 1974 furnished with needled teflon bags.
279
-------
GLASS FURNACES
4
Due to government regulations, specifically covering leaded glass fur-
naces, glass manufacturers are being required to install pollution
control equipment. The baghouse, specifically the reverse air bag-
house with glass bags, has been under trial for several years. A
modular-type baghouse is under consideration to assure that parts
of this equipment can be maintained while the rest of the unit is
operating at temperatures ranging from 400°F to 500°F. Inlet loading
is very low and the dust is extremely fine, consisting of 2/3 particu-
late matter and 1/3 fume. SCL and S03 as well as fluorides and chlor-
ides are of major concern to bag life. Today we know of approximately
five to ten units operating in the United States and in Europe.
POWER GENERATING
Two major areas for fabric filters, representing significant potential,
are: electric utilities and industrial boilers today use electro-
static precipitators as an interesting field for fabric filtration.
Several reverse air baghouses are in operation. The largest unit
contains 5,056 bags at Pennsylvania Power and Light Company in
Sunbury, Pennsylvania. This unit has been in operation for approxi-
mately 1 year and is operating very satisfactorily.
Another installation in Colorado has been on stream since last fall
and uses glass bags at a relatively high A/C ratio (3:1) and the re-
verse air method of cleaning. According to the consulting company
this unit is doing well. Several other baghouses are under considera-
tion or construction. However, the fabric for fabric filters is
limited due to acceptability, maintenance, and liability of the cur-
rently available fibers. A huge baghouse handling 600,000 to 700,000
acfm will be considered for 1976 in Nebraska.
280
-------
The most promising area for fabric filtration certainly is industrial
boilers. The industrial process industry has to install pollution
control equipment on their steam generating boilers. Roughly 12 to 15
installations are operating today. Several of them are pilot units.
In this area, reverse air filters with glass bags as well as high
ratio units are under consideration. One plant started with roughly
1600 needled Nomex bags and lime injection early this year. There
is only limited experience available so far, however, it looks very
promising. A new acid resistant fiber from Du Pont will probably be
available on a production basis during 1975 or 1976. This fiber will
allegedly withstand acid environment in industrial boiler applications
(S02 and S0_). Due to discontinuous operation conditions, the acid
dewpoint is of major concern to the fabric. The most promising prog-
ress will depend on the fiber availability. With this new fiber,
cage-type filter units are the most likely candidates for this inter-
esting and large filtration field.
SUMMARY
Looking at fabric filtration and specifically at fabrics and their
ability to handle fine dusts, including sub-micron particulate and
fumes, any new development will be measured in relationship to effi-
ciency. The types of fiber available will determine if this fiber can
be manufactured into a woven or needled fabric. This will be a deter-
mining factor as to the type of equipment used.
Glass fiber is not available today in a 100 percent needled fabric;
this certainly would have merits in pulse-jet units if the life would
be sufficient. This fabric is under development.
281
-------
Viewing several market studies, it is safe to say that fabric filters,
like small cars, are coming. They are, most likely, becoming the num-
ber 1 type of pollution control device used by industry. If the trend
to more stringent outlet regulations continues, it is safe to say fab-
ric filters in new equipment will surpass electrostatic precipitators.
It is anticipated that this will happen in the next 2 to 3 years.
REFERENCES
1. Culhane, F. R. Fabric Filters Abate Air Emissions. Env. Sci. &
Tech. 8, February 1974.
2. Friedlander, S. K. Small Particles In Air Pose A Big Control
Problem. Env. Sci. & Tech. 7(13), December 1973.
3. Butterworth, G. A. M. and M. Platt. Filter Fabric Selection and
Design: Consideration of Air Permeability and Fiber Character-
istics. (Presented at the Fabric Filter Symposium. Charleston,
South Carolina. March 1971.)
4. Farrow, R. M. The Filtration Characteristics of Glass Fiber
Media. Filtration and Separation. November/December 1966.
5. Loeffler, F. Flow-off of Particles Collected on Filter Fabrics.
Filtration and Separation. November/December 1972.
6. Rothwell, E. and P. Swift. The Integration of Fabric Air Filters
in Chemical Plants - Some Problems with Reactive Dusts. (Presented
at ACHEMA '73. Frankfurt. June 1973.)
7. Glastonbury, J. R. The Application of Fibre and Fabric Media to
Gas Filtration. (Presented at Clean Air Conference. Sydney,
Australia.)
8. Bergmann, L. High Temperature Fabric Filtration: American Ex-
perience And Innovations. (Paper presented at Filtration Con-
ference. London. September 25-27, 1973.)
9. Billings, C. E. and J. Wilder. Fabric Filter Systems Study.
Vol. I: Handbook of Fabric Filter Technology. GCA Corporation.
Report No. NTIS No. PB 200-648. 1970. p. 2-83.
282
-------
NEW KINDS OF FABRIC FILTRATION DEVICES
Melvin W. First
Professor of Environmental Health Engineering
Harvard University
School of Public Health
665 Huntington Avenue
Boston, Massachusetts 02115
INTRODUCTION
Industrial fabric filters have been around for an extraordinarily long
time in this era of rapid technological innovation and equally rapid
obsolescence of mechanical devices. Has this occurred because the fab-
ric filter, like the wheel, is a basic process and has been such a
successful invention from the very beginning that all that has remained
to be done can be classified as mere improvement through mechanization,
optimization, and reduction in costs? It would be easy for me to con-
clude that this is, indeed, the case because. I consider the fabric filter
to be the preeminent air cleaning device for aerosol particles of all
sizes and for the entire gamut of industrial dust loadings. I do recog-
nize, nonetheless that the horsedrawn carriage had already reached a
high state of technological development by the time it was suddenly and
totally displaced by the automobile, except for certain ceremonial pur-
poses in London. If we are now teetering on the receding edge of the
age of the fabric dust collector and, tomorrow, we can expect a revolu-
tion in the methods we will use for the future to remove particles from
aerosols, this revelation is hidden from me and I am only able to report
283
-------
to you new developments that promise to make the fabric filter we are
thoroughly familiar with more versatile and more reliable.
Significant improvements to the industrial fabric filter began in
earnest about 1950 with the invention and commercial introduction of
the reverse-jet filter by Harry Hersey. His primary objective was to
eliminate the visible puff of dust that was emitted from shaken bags
each time they were returned to filtration service after cleaning and
thereby to reduce gross dust emissions from filter houses. Even though
he failed in his primary purpose (reverse-jet filters did not prove to
be more efficient than shaken bags), he did produce a radically new
method of fabric cleaning and came up with a filter design that was
capable of operating indefinitely at almost constant air flow resis-
tance. In addition, by continuous fabric cleaning, he was able to in-
crease air-to-cloth ratios of fabric filters many-fold, and thereby,
to reduce significantly the size and weight of filter houses.
During the 1950"s, many new synthetic fibers and fabrics became avail-
able that increased the ability of filter houses to withstand the de-
structive effects of corrosive chemicals and elevated temperatures.
Woven glass fabrics lubricated with Teflon, silicone oils, and graphite
permit routine filter operations at temperatures up to 500 F. Orion
and polypropylene fabrics are capable of resisting many types of severe
chemical attack for years on end.
A third major development occurred at the conclusion of the 1950's that
has had a major influence on industrial filter design up to the present
time. I refer, of course, to the introduction of the pulse-jet filter
concept that, like the Hersey reverse-jet, permits continuous cleaning,
filter operation at uniform air flow resistance, and high air-to-cloth
ratios (though not as high as reverse-jet cleaning) and does all of
these things with almost a total absence of mechanical devices. This
simplicity of design has been an outstanding characteristic of pulse-
jet fabric collectors and has been an important factor in maintaining
the attractive price of this design.
284
-------
It is relevant to point out that the development of the pulse-jet con-
cept could not have taken place without the introduction of synthetic
fiber needled felts during the 1950's. That is to say, a single im-
portant innovation is likely to trigger others of even greater value.
This idea is by no means unique to fabric filtration.
This very brief historical review brings us up to the present day and
to the topic of my talk, new kinds of fabric filtration devices. Many
innovations that may hold some promise of improving the performance of
fabric filters have been described at conferences such as this and have
been the subject of innumerable patents over the past several years.
Indeed, by reading patents one gets the impression that every American
is entitled at birth to at least one patent to hang on the living room
wall beside the college diploma. Fortunately or unfortunately, depend-
ing on whether you hold a patent or merely wish to subvert one, almost
all patents covering air cleaning equipment are easy to get around by
using slightly different mechanical devices to apply the same prin-
ciples by other means. As a consequence, I suspect that one of the
important factors that has greatly impeded innovation and the develop-
ment of new devices for fabric filtration has been the near impossibil-
ity of obtaining patents that can be enforced; as distinct from the
ease of obtaining patent papers on almost anything.
A further difficulty in trying to describe to you new kinds of fabric
filtration devices is the concept of "new"; obviously, innovation dif-
fuses through different societies at different rates depending on their
degree of technological advancement and the urgency from improvement.
As an example, at a September, 1973, Filtration Society Conference in
London having the theme "What's New in Dust Control and Air Cleaning,"
the reverse jet filter was characterized as "the newer generation of
fabric filters." This suggests that "new" is a highly relative term
and I shall take full advantage of this ambiguity during the remainder
of my presentation.
285
-------
But first, I would like to remind you why the fabric filter is important
enough to be the subject of specialty conferences at frequent intervals
here and abroad. The 1972 report of the Ad Hoc Panel on Abatement of
2
Particulate Emissions from Stationary Sources pointed out that "it may
not be sufficient to evaluate the performance of pollution-control equip-
ment on the basis of mass-emissions alone....Special emphasis needs to
be placed on the amount of material in the fine-particle size range and
on its chemical nature." Fabric filter efficiency is insensitive to
particle size below about two or three micrometers (unlike many other
high efficiency collectors, such as the high energy Venturi scrubber)
and, therefore, has special application to the many processes, such as
metal refining, that generate large quantities of submicrometer par-
ticles. This important characteristic of fabric filters has been re-
3 4
ported by Dennis, Beach, and by other investigators, and was referred
to by Turner in this Symposium. Additional developments that favor in-
creased applications of fabric filters are new federal opacity regula-
tions for many widely dispersed industries and the passage of state
regulations, as in Maryland, that prohibit visible emissions from all
stacks. It will be obvious from this that if "order of magnitude"
improvements in retention of small particles from stationary sources
is required, the industrial fabric filter must be looked upon as the
most important control device for many large and important industries.
Satisfactory application of fabric filters to a larger variety of in-
dustries depends upon innovative modifications that are responsive to
special industry problems such as high heat, condensing moisture, cor-
rosive gases, and sticky or liquid particles.
286
-------
NEW DEVICES
FILTER PRECOATS AND FILTER AIDS
The use of filter aids for high efficiency hydrosol filtration by fab-
rics has a long and well documented history in chemical engineering
publications. The use of filter aids for aerosol filtration is not
new, having been investigated and documented since 1952 at the HACL,
but it has been applied only sparingly since then. The original ap-
plication of a filter aid to an industrial fabric filter was for the
purpose of greatly increasing small particle retention from aerosols
containing loadings of especially toxic dusts, such as beryllium,
3
equivalent to those found in the atmosphere (i.e., 100 |ig/m ). About
10 years later, a filter precoat of calcined dolamite on glass fabrics
was used at a Southern California Edison Company plant to trap con-
densed SO, droplets (H9SO.) formed during the burning of high sulfur-
7
containing residual oils. This flue gas treatment was successful in
correcting stack opacity violations that had occurred because of the
emission of 30 ppm SO,. This is the first recorded use of successful
fabric filtration for collecting aerosol droplets. The filter aid not
only provided for high efficiency collection of the submicrometer con-
densed sulfuric acid droplets, but was effective in neutralizing the
viscous liquid to avoid plugging the filter cloth and corroding the
filter housings and shaking mechanisms.
This is contrary to usual practices in the power industry where every
effort is made to avoid lowering the flue gas temperature below the
acid dew point to protect air preheaters, induced draft fans, ducts,
and stack from destructive corrosion. With the sudden rekindling of
interest in fabric filtration by the electric power industry to achieve
clean stacks when burning pulverized coal, the emphasis has been on
high temperature, corrosion resistant fabrics that can operate success-
fully above the acid dew point, usually in the vicinity of 300 F.
287
-------
Glass and Teflon fabrics have been considered for this service. But
is this the correct concept considering the current concern about sul-
fate in the atmosphere?
A more recent application of fabric filters for liquid droplets has
g
been reported by Guilloud in the aluminum industry where precoated
fabrics are used for the collection of oil mists. The nature of the
filter aid used for this purpose was not mentioned. The general ap-
plicability of this technique for collecting submicrometer aerosol
droplets at very high efficiency is obvious and only requires the dis-
covery of a suitable filter aid in each case, as well as a simple method
of dispersing it in the airstream to be filtered.
Fabric precoats may be used for purposes other than particulate filtra-
tion. Adsorbent and reactant coatings can extend the application of
fabric filters to gas treatment as well as to particle removal. This
use of fabric filters gas treatment was documented in the Filter Hand-
book in the following passage:
"The fact that good control of high specific surface
powder and contact time are attainable with fabric
filter systems suggests advantages over other collec-
tor types. Most recently, the removal of up to 98.4
percent of S02 by sodium bicarbonate powder on a fab-
ric filter has been demonstrated at a coal-fired power
plant in a joint APCO-Air Preheater Co. study. Fly
ash and some N0_ are removed at the same time."
Although there appears to have been very little use of this concept up
to now for treatment of gas mixtures, as distinct from aerosols, the
potential is obvious. It seems possible to use finely divided activated
charcoal for gas adsorption on a fabric support, discharging the precoat
to a desorption stage when it becomes saturated, and recoating the fab-
ric surfaces with a fresh charge of activated charcoal. Such a dynamic
system is especially attractive for control of malodors as the concen-
trations are likely to be in the part-per-billion range and renewal of
the carbon coating need not be made at frequent intervals. In this
288
-------
case, it would be convenient to ship the spent charcoal to the manufac-
turer for renewal rather than attempting to do it at the place of use.
Shaking the spent charcoal into a collection hopper is certainly simpler,
more convenient, and less costly than changing large numbers of canisters
and trays. It has also been recorded that mineral dust filter precoat-
ings, such as pulverized limestone, are effective in suppressing the
incendiary tendencies of pyrophoric metal dusts, such as lead-copper
alloys.
NEW FABRICS
The importance of needled felts of synthetic fibers for the development
of the pulse-jet fabric filter has already been noted. Recently experi-
ments have been conducted to alter the surface characteristics of needled
felts to improve cleaning characteristics. These fabrics have often been
described as "frosted" and the treatment consists of heating the dust
collecting side until the surface fibers melt together to form a smooth,
hard finish that is supposed to prevent dust penetration and assist
cake removal. Insufficient experience has been accumulated with these
surface-modified felts to arrive at any sound conclusions regarding
their usefulness. On theoretical grounds, it is hard to see how a less
porous fabric surface can assist filtration or dust removal; and on
the basis of limited field experience, experiment seems to confirm
theory in this instance.
Multiple fabrics were utilized for filtering electric arc furnace fumes
as far back as 20 years ago. The intent was to construct a fabric
that permitted filtration in depth, i.e., to collect the bulk of coarse
particles on the surface but provide for high efficiency collection of
a small percentage of fine particles in deeper layers. This arrange-
ment permits high efficiency dust collection with minimum pressure drop
and imparts important physical properties, such as strength and porosity,
to the composite fabric. In this case, also, little use has been made
289
-------
of this innovative development until recently. Bergmann ' has de-
scribed a number of multi-media fabrics compounded for strength, heat,
corrosion resistance, and for good filtration characteristics. For
example, needled felts prepared from blends of Nomex and glass fibers
have been used in the asphalt concrete industry where intermittent op-
eration, high temperature (with brief peaks to 500 F), high moisture
content, small particle size, and heavy dust loadings represent a
severe exposure for filter bags.
Other industries have special fabric requirements and many of these
have been described by Bergmann. Of special interest is a high tem-
perature (660-750 F) needled fabric prepared from mineral fibers on a
metal scrim core. Fabrics woven from yarns prepared by twisting Fiber-
frax fibers around fine stainless steel monofilaments were prepared by
Carborundum Corp. 20 years ago. These fabrics are capable of with-
standing temperatures of 1500 F for prolonged periods. Fabrics woven
from silica fibers are capable of withstanding still higher temper-
atures. Many other heat resistant fibers are available for fabric
manufacture. The difficulty in raising filtration temperature does
not lie with the fabrics but rather with the filter housings and clean-
ing mechanisms. There is little hope that conventional fabric filter
designs can withstand temperatures in excess of those currently used
for glass fabrics (550 F) without radical redesign of the fabric hold-
ing structures. At present there does not appear to be a demand for
what would prove to be an especially costly modification of a standard
design, except for cleaning high temperature gases in preparation for
additional high temperature processing such as catalysis.
AEROSOLS CONTAINING LIQUID PARTICLES
The principal deficiency of conventional fabric filters is their suscepti-
bility to failure when the aerosol contains liquids. The presence of
water vapor and sulfuric acid can be handled satisfactorily by
290
-------
heat-insulating the housing and maintaining the gas temperature sub-
stantially above the dew point. Modern heat-resisting and acid-
resisting fabrics make this procedure routine.
Tar-like materials and other substances cannot be filtered satisfactorily
by this technique. In addition, it is often highly desirable to remove
offensive liquid substances such as sulfuric acid and oil mists by fil-
tration after cooling and condensation.
A response to this requirement has been the appearance of fabric fil-
ters especially designed to collect solid and liquid particles and to
be cleaned by washing. One design, not yet generally available com-
mercially, utilizes a spinning cylindrical filter in such a fashion
that centrifugal force tends to strip the dust cake from the outside
surface of the cloth-covered cylinder. In addition, water or any suit-
able solvent can be sprayed inside the cylinder and allowed to flow
radically outward through the fabric by centrifugal force, washing the
cloth clean in the process.
It seems likely then that limitation of bag filters to dry dusts may
no longer be necessary when equipment is available that will permit
continuous filtration through wet fabrics.
OPERATIONS
A final series of developments of considerable importance for good per-
formance of fabric filters is in the area of instrumentation and math-
ematical modeling. Good instrumentation of filter houses has always
been considered a desirable practice that was largely ignored by pur-
chasing and operating personnel. This is changing, and well-instrumented
new units are beginning to be seen with reasonable frequency. This is,
of course, a prudent policy to protect the considerable investment that
is inherent in the installed cost of a unit of industrial size. But
291
-------
beyond this, it reflects the need to demonstrate by records adherence
to new, rigorous emission standards, including low stack opacity regu-
lations.
Predictive models for fabric filtration have a long way to go before
they describe perfectly the interrelated behavior of the several static
and dynamic systems that comprise a functioning fabric filter. They
are, nevertheless, proving to be immensely useful for studies that have
more limited objectives. You have already heard about several of these
from previous speakers. In the Harvard Air Cleaning Laboratory we are,
with EPA financial support, modeling filter cake parameters and basic
factors of high velocity fabric filtration with the objective of devel-
oping the theoretical factors and experimental confirmation that will
permit the design of industrial fabric filters having an order of mag-
nitude greater air volume capacity without degrading capture efficiency.
This desirable objective can be accomplished only with the infusion of
an order of magnitude greater energy input to the fabric filter, but
with Venturi scrubbers operating at 80 inches w.g. in the steel indus-
try, this no longer seems to be a formidable barrier. Of greater con-
cern, is the construction of fabrics and fabric supports capable of
withstanding these forces. It is too early in these studies to re-
port to you any results or to make any predictions.
SUMMARY
Industrial fabric filtration, the sleeping giant, is stirring. Devices
and concepts only 20 years old are being rediscovered and, more impor-
tant, being committed to commercial practice. It appears that we are
about to experience a new period of innovation and development similar
to the one that occurred during the 1950's. With the use of predictive
modeling techniques and a greatly improved ability, through the liberal
292
-------
use of modern instruments, to measure with accuracy transitory phe-
nomena, we should have improved concepts of how fabric filters func-
tion and therby acquire the tools for improving their performance.
REFERENCES
1. Squires, B. J. Fabric Filter Plants for Cleaning Gases from
Non-Ferrous Metal Furnaces. (Preprint presented at Filtration
Society's Conference on What's New in Dust Control and Air
Cleaning. London. September 25-27, 1973.) p. 24.
2. NRG - NAE Ad Hoc Panel on Abatement of Particulate Emissions
from Stationary Sources. Environmental Protection Agency,
Washington, D. C. R2-72-042, COPAC-5. July 1972. p. 3.
3. Dennis, R. Fabric Filtration Technology, Review of New
Developments. GCA/Technology Division. Bedford, Mass.
Unpublished Report. December 1973.
4. Beach, G. H. The Stack Test - Final Proof of Non-Pollution.
Air Pollution Control Association, Pittsburgh, Pennsylvania.
(Proceedings at The User and Fabric Filtration Equipment
Specialty Conference. Buffalo, New York. October 14-16,
1973.) p. 35.
5. Air Programs: Standards of Performance for New Stationary
Sources. Government Printing Office. Federal Register,
Vol. 39, No. 47, Part II. March 8, 1974. p. 9308.
6. First, M. W. et al. Air Cleaning Studies. Harvard Univer-
sity. Progress Report NYO-1581. April 21, 1952. p. 41.
7. Gosselin, A. E. The Bag Filterhouse for Oil-Fired Power
Plants. JAPCA. 15:179, 1965.
8. Guilloud, R. L. Fabric Filters in the Aluminum Industry.
Air Pollution Control Association, Pittsburgh, Pennsylvania.
(Proceedings at The User and Fabric Filtration Equipment
Specialty Conference. Buffalo, New York. October 14-16,
1973.) p. 146.
9. Fabric Filter Systems Study, Vol. IV. GCA Corporation.
Bedford, Mass. Final Report. December 1970. p. 3-32.
293
-------
10. Bergmann, L. Fabric Filtration - Today and Tomorrow. Air
Pollution Control Association, Pittsburgh, Pennsylvania.
(Proceedings at The User and Fabric Filtration Equipment
Specialty Conference. Buffalo, New York. October 14-16,
1973.) p. 85.
11. Silverman, L. and R. A. Davidson. Electric Furnace Ferro-
silicon Fume Collection. Journal of Metals, p. 1327,
December 1955.
12. Bergmann, L. High Temperature Fabric Filtration: American
Experience and Innovations. (Preprint presented at Filtration
Society's Conference on What's New in Dust Control and Air
Cleaning. London. September 25-27, 1973.) p. 29.
294
-------
NEEDED RESEARCH IN FABRIC FILTRATION
Knowlton J. Caplan
University Health Services
University of Minnesota
Minneapolis, Minnesota 55455
The description of needed research in this field will consist of two
new problems or areas of endeavor, only briefly described and not in-
tended to be a research protocol; and a summary of the needs that have
been implied by the reports of previous speakers.
RECIRCULATION
One of the new needs derives from a development in a related field which
does affect fabric filter utilization. That has to do with the recir-
culation of cleaned air from industrial exhaust systems back into the
workroom. The Ventilation Committee of the American Conference of
Government Industrial Hygienists publishes a Manual of Recommended
Practice, which is revised every two years; and which is cited by ref-
erence in the standards of the Occupational Safety and Health Adminis-
tration, U. S. Department of Labor. In the latest revision, the subject
of "recirculation" is covered in quite some detail. The Ventilation
Committee had been pondering what action to take in this area for two
years before the energy crisis hit the newspapers last fall.
295
-------
Air exhausted from the industrial work place for reasons of health
hazard control, of course, draws replacement air into the building.
If that replacement air is heated, cooled, tempered, or otherwise con-
ditioned, then that becomes what we call a make-up air system. Exhaust
air systems are energy-expensive. Historically, we have been deliber-
ately wasting the heat content energy of the air exhausted from the
building. The recirculation of that air, of course, would result in
almost 100 percent recovery of the thermal energy involved. Tradi-
tionally industrial hygienists have not permitted the recirculation
of cleaned workroom air if the contaminants involved were other than
nuisance; that is, if they had any real toxicity or other potentially
harmful effect. Even though the air-cleaning system involved may have
been basically adequate to clean up the air so that it could be recir-
culated, problems of misoperation or poor maintenance are very common
with such equipment and no one wanted to undertake the risk of delib-
erately dosing the workroom air with excessive concentrations of toxic
materials. With an abundant and cheap supply of energy, the easy way
out was to forbid the recirculation of exhaust system air regardless
of the cleaning system used if the contaminant was toxic. With the in-
creasing cost of energy and the scarcity of fuel, it becomes economic
to consider greater capital investment and greater expenditure of en-
gineering and technical effort to design such recirculation systems so
that they will be safe and can be operated without risking the health
of the people involved. This has led the committee to propose a formula
as follows:
CR = i(TLV-Co) x J x i
R
where C = concentration of contaminant in exit air from the collector
before cleaning, any consistent units
TLV = threshold limit value ot contaminant
C = concentration of contaminant in worker's breathing zone
with local exhaust discharged outside
296
-------
Q = total ventilation flow through affected space, cfm
Q = recirculated air flow, cfm
K.
K = an "effectiveness of mixing" factor, usually varying
from 3 to 10
In the typical circumstances to which this equation would apply, the
probable range of C will be from 0.1 to 10 milligrams per cubic meter,
depending on the properties of the substance and the ratio of recircu-
lated air to total ventilation.
Two ways are proposed in which such recirculation would be permitted.
One would be to have a second air-cleaning device in series with the
the primary device, a back-up system, if you will, which is adequate
to protect the workers in case the primary device is not properly op-
erated and maintained. An example would be a dust-producing operation
controlled by a local exhaust system with a good, properly-applied fab-
ric filter cleaning the air for recirculation. The air discharged from
the fabric filter would be clean enough "as is" to be recirculated to
the workroom. However, a back-up filter would be required. This would
be perhaps a stationary filter, or a renewable filter, not one that was
self-cleaning. If there was excessive leakage through the primary fil-
ter for whatever reason, the back-up filter would provide the safety
factor and would prevent excessive air contamination. Such filters of
adequate efficiency are available. The problem is that such filters
are not rated in terms that are usable in this application. They are
rated on the National Bureau of Standards dust spot efficient test--
which is an "optical dirtying power" kind of a test—or on the DOP test
for ultrafilters. The loading characteristics or the proper kind of
efficiency data is not available to design the back-up filter system
in usable and practical terms related to the expected life and the
practicality of the whole situation.
Another alternative system, somewhat less desirable as a safety pro-
vision, would be to install some kind of a monitoring device in the
297
-------
cleaned air stream. If excessive concentrations occurred, the system
would be shut down or the discharge would be bypassed to the outdoors.
The highly-sophisticated, expensive systems currently available for
monitoring stack emissions from new sources are not justified or even
practical for this application.
In the first place, the concentration ranges needed are lower. Sec-
ondly, the cost is much too high to be justified for many such systems
where the total system cost is in the range of $10,000 to $50,000. Last
but not least is a practical operating problem; in a factory where the
relatively simple machinery represented by the fabric filter is not
adequately operated and maintained, who is going to operate and main-
tain the delicate, sophisticated monitoring apparatus currently pro-
posed for stack monitoring? The need is for a monitor that is relatively
simple, relatively rugged, does not need a high degree of precision but
needs a high degree of dependability. A cumulative device operating
over a time span of several hours would be suitable for some applica-
tions .
It is my opinion that our society and our economy is going to find it
more advantageous in the future to devote the capital expense and the
technology necessary to enable conservation of energy by recirculation.
In order to accomplish this, we need:
(a) efficiency and loading data for high-efficiency filters
on different aerosols and in different terms than cur-
rently available; and/or
(b) relatively simple, relatively rugged monitoring devices
for lower concentrations (but longer time spans) than
currently contemplated for stack emission monitoring.
298
-------
MECHANISM OF SEEPAGE
Several research needs have been implied during the course of this con-
ference. One is the need for an in-depth investigation of the mechanism
of bleeding, or seepage (as it is termed), through the fabric filter.
We've heard from Mr. Frye, Dr. Bakke and myself, stories describing ob-
servations about this phenomenon. Mr. Dennis has reported that about
the same size distribution was found on both the inlet and outlet sides
of the fabric filter and that, for a given aerosol, there is a rela-
tively constant emission rate regardless of the loading to the fabric
filter. None of this is what one would expect as a result of the ap-
plication of classical filtration theory to the behavior of fabric fil-
ters. I disagree with the approach that the classical theory is
applicable. It may be theoretically valid, but the range of parameters
that it evolved from is so different from the application of fabric fil-
ters; it is so far removed from the single-fiber/single-particle theory;
that the quantitative difference is so great as to amount to a qualita-
tive difference. The phenomenon of bleeding and seeping is not explain-
able from the classical filtration theory, and obviously we need to
understand it in order to be able to prevent it or reduce it in a pre-
dictable way.
STACK SAMPLING: INLET VS. OUTLET CONDITIONS
Another direction needing more effort is that concerning instack sam-
pling in order to dispense with the perennial problem of probe losses.
Mr. Lilienfeld yesterday, in his announcement of the work GCA is doing in
this area, indicated some progress. There remains, however, a major
problem not being attacked by that research; the solution of the con-
flicting needs for variable flow rate capability for isokinetic sam-
pling, and constant flow rate for aerodynamic sizing by impaction
299
-------
techniques. Aerodynamic particle size is generally more useful than
other size descriptions; and since it is obtained directly by impaction
methods, that aspect of the technique should be retained. This then
leaves the problem of obtaining isokinetic conditions.
There is a temptation to derogate the need for isokinetic sampling on
the clean side of high-efficieny equipment on the assumption that the
particles are so fine that anisokinetic conditions create negligible
error. But is this assumption really valid? There are several indica-
tions (see "Seepage" above) that it may not be.
Another solution sometimes preferred is to change intake nozzles for
every point in the sampling traverse. In my opinion this alternative
is to be summarily rejected as a long-term solution. It is viable only
for cost-is-no-object research investigations, and then only because no
better solution to the problem currently exists.
This leaves, then, a continuing need for providing both the variable
flow rate for isokinetic sampling and the constant flow rate for impac-
tion sizing. One step that may ease the problem is to abandon the ap-
proach which strives to achieve one "universal" technique and/or
apparatus for both inlet and outlet sampling. (Although outlet sam-
pling may be all that is needed for emission standards or emission
monitoring, there are many and important needs for data on both inlet
and outlet in terms of control technology.) Perhaps we should recog-
nize that the thousand-fold range in concentrations and perhaps in
particle size, inlet to outlet, is too great a span to expect of a
single technique or a single apparatus. Perhaps a different technique
and apparatus could be designed for inlet sampling, hopefully using the
same basic separating and sizing mechanisms; and on the basis of theoryj
research, and practical experience, the inlet sample couia oe icepi. re-
latable to the outlet sample.
300
-------
ELECTROSTATICS
Mr. Frederick gave very interesting descriptions of phenomena related
to electrostatics. There is no doubt that more knowledge is needed,
badly needed, in this area. I would like to add to the thoughts that
he expressed, the request that we extend this type of investigation
into electrostatics as a cause of dust explosions in filter systems.
Such explosions typically are attributed to electrostatics. A dust
fire or explosion from electrostatics presumably would occur only if
the breakdown voltage of the air or gas had been reached; such voltages
are greater than those described relative to electrostatic filtration
properties of the materials and fabrics by several orders of magnitude.
The lay approach to this question seems to me to border on the ridic-
ulous. For example, in some industries the collector manufacturers
are required to sew a ground wire in the vertical seam of the bag, and
this is enough to satisfy the insurance company. For typical fabric
filter dimensions, there is a linear distance of from one to several
feet from the far side of the bag to the seam and the ground wire;
and with a high dielectric filter fabric and a high dielectric dust,
the concept of that ground wire effectively leaking off the electro-
static charges seems remote. I sometimes doubt that dust explosions
in fabric filters--which are real enough—are truly caused by the elec-
trostatic charges so frequently blamed. I don't believe this has been
proven one way or the other. It needs more and better investigation,
on a higher technical plane, than it has received in the past.
AN "UNDERWRITERS LABORATORY"
I wish to propose a new approach in the field of air pollution control
for all types of control devices, not just fabric filters but including
fabric filters. It is based on the presumption that the field of control
301
-------
equipment has reached the point where it is about time it "grew up."
The expenditures of time and effort wasted in unsuccessful attempts to
secure applicability and operability of air pollution control devices
is getting to be significant. We heard a plea from Mr. Walsh that the
basic generalizations were not sufficiently quantitative, that we lack
predictability; (and predictability is one definition of engineering).
Walsh reports that we cannot transfer or translate the data that we get
from one process application to another; that we need some way to get
adequate predictive capability. I couldn't agree more with what he had
to say along these lines.
The idea that I am about to suggest is not fundamentally new. It has
been used in other fields, and I suggest its adaptation and application
to the field of air pollution control equipment. What is this idea?
It is the idea of an "underwriter's laboratory" if you will, for air
pollution control equipment.
One can set up a laboratory in which a number of different artificially-
generated and dispersed contaminants could be tested against a manufac-
turer's model of a given line of his equipment. One could choose a
series of aerosols, inexpensive so that new material is used under
quality control for particle size and other properties. A series of
materials could be chosen which, although not truly representative of
any individual field application situation, would nevertheless present
the filter or other control device with a spectrum of challenges. For
example, the series could include ground limestone, obtainable in a
number of grades of fineness; fly ash, which has been widely used for
research; redispersed carbon black, freshly-generated iron oxide fume
from an arc or from the destruction of iron carbonyl; etc. A whole
spectrum of materials could be generated. The tested device would be
a manufacturer's model, the smallest in a line of actual or proposed
devices. It would be subjected to a battery of tests having to do
mostly with performance parameters--pressure drop, power consumption,
302
-------
water consumption, efficiency, fractional efficiency; whatever is
applicable, so that each individual commercial model could be given
some kind of a laboratory classification. Presumably the laboratory
would not be able to operate a test long enough to really develop any
long-range maintenance information. The laboratory would produce a
class rating which could be correlated with the extensive field test-
ing that is going on and that is what would result in a degree of
predictability. One would be able to say, for example, that an elec-
tric arc steel melting furnace requires a "Class 2A" collector; that
a smelter bag house needs a "Class 4B" collector; that a foundry
sand-handling system needs a "Class 1Z" collector; etc. A comprehen-
sive rating system eventually could be developed.
There are 200 people present at this meeting. There were about 30,000
significant pieces of air pollution control equipment bought in 1973.
Excluding the big million dollar jobs, most of it was bought by cus-
tomers who are relatively unsophisticated and shouldn't have to become
highly sophisticated in the black art of predicting the performance of
control equipment. "I have here in my hand," (as the rabble-rouser
would say) for example, an advertisement of a magic wet collector.
I'll call it the wet sponge collector, which is advertised to collect
99.9 percent of anything and everything with a pressure drop of not
more than 4 inches. The advertising literature list of customers reads
like the Fortune 500. At some place along the line this kind of un-
bridled exaggeration has got to stop.
The Harvard Air Cleaning Laboratory, for quite a few years, performed
tests somewhat along these lines for the Atomic Energy Commission, and
the EPA has contractors performing similar test programs on occasional
pieces of equipment today. But this is not the way to do it. In the
first place, the government shouldn't have to do it. In the second
place, the government has all kinds of restrictions and budgetary and
political problems in this kind of an operation. Government-issued
303
-------
brand-name class ratings for commercial equipment would create all
kinds of repercussions. On the other hand, it would seem that the
Industrial Gas Cleaning Institute and the reputable manufacturers,
who stand to lose from either unethical competition or by mispurchase
and misapplication of their own equipment, would be in favor of such
an impartial rating system. The Air Pollution Control Association;
the American Society of Heating, Refrigerating and Air-Conditioning
Engineers; American Society of Mechanical Engineers; the American
Society of Chemical Engineers; the EPA; many industrial trade asso-
ciations should be interested in this kind of an impartial, dependable
rating system for the application of pollution control equipment.
The first objection to the plan is that truism which we've heard
several times this week: that the laboratory does not attain the
same dispersions, the same contaminants under the same conditions,
as is found in the field. This problem is quite real. If one is
going to test a piece of equipment on open-hearth fume, one has a
choice of building an open-hearth furnace in the laboratory or moving
the test equipment to some existing open-hearth furnace; and the
choice there is obvious. But the field testing will be done, is
being done, and if we have a simultaneous, well-planned, broad spec-
trum challenge presented to equipment in the laboratory, I am quite
confident that good correlations for good predictability on a class
basis can be achieved. The charlatanism and the witchcraft will be
reduced. The customer, especially the small unsophisticated customer
who is not large enough to support a strong technical staff will get
more for his money. The enforcement activity and the plans approval
activity will be simplified. There will be less dependence on the
individual expertise, personal experience and opinion of the individual
engineer who is reviewing plans at the state and local level for permit
application, and we will have something to back up his decisions.
304
-------
Another long-range benefit that would be achieved is the reduction of
the routine stack testing that really should be accomplished for spe-
cific compliance with state plans in the absence of a rating system.
The total market for air pollution control equipment for 1973 was
$320 million, arid the average sale was probably on the order of
$10,000 to $20,000. That means 30,000 units that really should have
been tested last year. A good stack test costs at least $2,000 and
more often about $5,000; it is neither logical nor equitable to ex-
pect an acceptance test costing $5,000 to be performed on equipment
averaging $10,000 to $20,000 in cost. The whole situation seems to
be ripe for a certification laboratory-type rating plan.
Also this laboratory should not directly engage in research and devel-
opment work, and certainly not in the same premises or with the same
staff. The Harvard Air Cleaning Laboratory used to mix up the two
functions to a degree, but that was the nature of their contract.
They would at least vary the adjustable operating features of a given
piece of equipment, and usually with the manufacturer's blessing, be-
cause he got some information out of it also. I propose that the
certification laboratory, the "underwriter's laboratory," not do this
type of thing. The whole field has advanced since the day when Harvard
used to do that kind of work. Most of the reputable manufacturers have
their own lab and their own development facilities. Let them do their
own development work or let them hire a consultant to work with them;
the certification lab would be strictly a rating laboratory.
Of course, there would need to be some kind of a sizable financial
investment to get started. It should be started at a viable rather
than minimal level. It has to be done right, and on a large enough
scale to be useful, or it will not get the support of the industrial
community and the professional community. I feel that if it is done
right, it will get that support and that once it has proven its worth,
305
-------
the laboratory could and should become self-supporting through service
charges and fees (again just copying the underwriter's laboratory idea).
The application of this idea should get rid of a lot of uncertainty in
our technology. At the present time we have many literature-reported
field tests which are not comparable because they were done by different
methods and omit or perhaps even conceal, for competitive reasons, im-
portant parameters. A lot of tests are made which are not reported in
the literature and are used for competitive purposes; the quality of
such tests varies from excellent to horrible. There is quite a bit of
word-of-mouth reporting of acceptability within the user industries,
which is usually based on the lack of maintenance difficulties rather
than upon emission control effectiveness. Last but not least are these
unsubstantiated but also irrefutable advertising claims by the less
reputable members of the vendor community. Thus the proposed lab would
help rid us of all this wasted effort and wasted money.
When a "polluter" spends his money and gets something that doesn't
work, not only is it an economic hardship on him, but the community
continues to have the air pollution. It is usually some several years
until all the arguments and lawsuits are finished and the dust settles,
if you'll forgive the pun, and he goes on to the second round of trying
to fix his problems. A simplified analogy can be made with the DOP
(dioctylphalate) test for HEPA filters. HEPA filters are almost never
used for filtering DOP out of the air. They're used for filtering
ordinary atmospheric dust, dust which nay be carrying microbiological
infection, radioactive particles, etc. The fact that the application
of the filter is not to the substance on which it is tested does not
seem to hinder in any significant way the usability of that kind of
test, specifications, or rating. The comparison is, admittedly, a
vast oversimplification relative to air pollution control equipment.
I think that the difficulties can be overcome and the long-range ad-
vantages are worth the very serious consideration of both the industrial
and professional groups involved.
306
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-650/2-74-043
2.
3. RECIPIENT'S ACCESSIOK-NO.
4. TITLE AND SUBTITLE
Proceedings: Symposium on the Use of Fabric Filters
for the Control of Submicron Part iculates
(April 8-10f 1974f Boston. Massachusetts)
5. REPORT DATE
May 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Leonard M. Seale, Editor
8. PERFORMING ORGANIZATION REPORT NO,
9. PERFORMING ORGANIZATION NAME AND ADDRESS
GCA/Technology Division
Burlington Road
Bedford, Massachusetts 01730
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-34
11. CONTRACT/GRANT NO.
68-02-1316 (Task 2)
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, N. C. 27711
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
proc€|edings document presentations made during the Symposium which
was aimed at describing the fine particulate control potential of existing fabric filter
systems for the benefit of regulatory and user groups and suggesting to manufacturing
and research organizations those areas where performance levels most need impro-
ving. The primary purpose of the Symposium was to better define the role of fabric
filter systems for the control of fine particle emissions. Experts from Government,
Industry, and University groups discussed the theoretical and practical aspects of
filtration and important related areas such as particle behavior , fabric selection ,
and system evaluation. The effectiveness of fabric filter systems for controlling
particulate emissions from industrial sources is well accepted in the pollution
control field. However , the vast majority of available performance data depict over-
all weight recoveries with only minimal information on the capture efficiencies for
particles in the equal to or less than 1 micrometer size range.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Filtration -
Fabrics
Aerosols '
Cost Effectiveness
Particle Size
Particle
Particle
Air Pollution Control
Stationary Sources
Fine Particulates
Fabric Filters
Emission Standards
Particle Behavior
13B
07D
HE
13. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
306
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
------- |