&EPA
United States Industrial Environmental Research EPA-600' 7-78-1 14
Environmental Protection Laboratory June 1978
Agency Research Triangle Park NC 27711
An Electrostatic
Precipitator Backup
for Sampling
Systems
Interagency
Energy/Environment
R&D Program Report
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
EPA REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
for publication. Approval does not signify that the contents necessarily reflect
the views and policies of the Government, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-78-114
June 1978
An Electrostatic Precipitator Backup
for
Sampling Systems
by
P Vann Bush and Wallace B. Smith
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
Contract No. 68-02-2131
Program Element No. EHE624
EPA Project Officer: D. Bruce Harris
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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ACKNOWLEDGEMENTS
We gratefully acknowledge the assistance of Mr. Kenneth
Trowbridge and Mr. David Hussey in the design of the E.S.P.
Back-Up. The work of Mr. Richard Walker and Mr. Ray Wilson in
performing the collection efficiency measurements is appre-
ciated.
We also appreciate the assistance and guidance of our
Project Officer, Mr. D. Bruce Harris.
We acknowledge that the disc-cylinder electrode geometry
and method described in this report for ionization and particle
charging are developments of Air Pollution Systems, Inc., Kent,
Washington and the Electric Power Research Institute, Palo Alto,
California, and that we have been advised by Air Pollution Sys-
tems, Inc. that United States and Foreign patents have been ob-
tained and others applied for.*
We are not aware of the details of said patents or any
pending applications and provide no assurance that practice
of any of the systems or methods disclosed in this report do
not infringe either said patents or any other private rights.
*U.S. Patent No. 4093430
-------
INTRODUCTION
Filters used to collect fine particles in source sampling
trains are troublesome in several ways. A large pump is usually
required to pull the sample gas through a high efficiency filter,
and many tests are terminated prematurely because of the large
pressure drop that results as a dust cake builds up on the filter,
This problem is especially severe when the test objective is to
collect a large sample of submicron particles because the poros-
ity is less than for large particles. Contamination of a sample
with filter material or physical removal of the dust from the
filter can also be problems.
This report describes the program carried out to design and
evaluate the performance of an electrostatic collector to be used
as an alternative to filters as a fine particle collector. Poten-
tial advantages of an ESP are low pressure drop and high capacity.
Potential problems are unreliability and poor collection due to
back corona or lack of particle adhesivity.
In the following section the theory of operation of the elec-
trostatic collector is described, and the results of experimental
measurements of electrical characteristics and collection effi-
ciency are presented. Shop drawings of the system and its compo-
nents are included in the Appendix.
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SYSTEM DEVELOPMENT AND TESTING
DESIGN
The electrostatic precipitator back-up filter was designed
to be operated at a nominal sample flowrate of 6.5 ft3/min., at
a temperature of 205°C, and to achieve near 100% collection of
submicron particles. Since it is possible that there will be a
need to operate the collector in situ, a secondary requirement
was that the collector pass through a 4 inch diameter port. Fur-
thermore, the system was designed to be convenient to operate and
clean, and to require a minimum of operator training or attention,
Figure 1 is a schematic diagram illustrating the main fea-
tures of the system that was selected. The collector is of a
cylindrical geometry with the collection electrodes arranged
concentrically to allow a large surface area to be contained with-
in a relatively short outer cylinder. Disc and needle discharge
electrodes were designed and fabricated, but only the disc-cyl-
inder geometry was evaluated during this program. The system
shown in Figure 1 is mechanically rugged and the collection elec-
trode geometry is such that the flow is laminar at the design
flowrate; thus, it is a simple matter to calculate particle tra-
jectories and the electrode length required for 100% efficiency-
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SAMPLE IN
0.32cm
COLLECTION .
-7.0cm-
2.86cm-
t±r
t
CORONA DISC
ELECTRODE
HIGH VOLTAGE
ELECTRODES
8.4cm
20kV
2kV
Figure 1. Schematic of the electrostatic collector with
the disc discharge electrode installed.
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Disc Discharge Electrode
The disc-cylinder design has been shown to be stable at very
high values of corona field and current, and at high gas veloci-
ties.1'2 The electric field (neglecting space charge) in the
disc-cylinder geometry is given by the equation:3
2V 1 I ln
b/r)1
(b/a) V I x In* (b/a) J
where V = applied voltage on the disc,
b = radius of the cylinder, and
a = radius of the disc.
At the cylinder, r = b and the equation becomes
F = 2V ,
^ '
This is the minimum value of the electric field. Using the values
b = 3.5 x 10~2m(1.38 inch), a = 1.43 x 10~2jn(.56 inch), and V =
20kV, we obtain E = 4.1 x 10 5 volts/m.
Experimental current-voltage characteristics for the disc-
cylinder electrode geometry at 200 °C and 7.5 SCFM show (Figure
2) that for V = 20kV, the current, i, is approximately 200 yA.
The ion concentration can be determined from the equation N =
j/eyE, where j = i/2iTbh = current density, h = width of the
1 Klemperer, H., and J. E. Sayers. "Design Aspects of an Electro-
static Precipitator for the Collection of Small Solids ahead of
the Air Heater." Transactions of the ASME, Feb., 1956. pp. 317-
326.
2 Tassicker, 0. J., and J. Schwab. "High-Intensity Ionizer for
Imp:
61.
s Improved ESP Performance." J. EPRI, June/July, 1977. pp. 56-
3 Pontius, D. H., et al. "Fine Particle Charging Development."
EPA-600/2-77-173, August, 1977. p. 125.
6
-------
I04
I03
z
LJ
CE
tr
o
102
Spark
Corona Disc Electrode _
10.
-20°C -i
-200°CJ
10 20
VOLTAGE , kV
30
Figure 2. Current-Voltage characteristics of
the ESP Collector at two tempera-
tures and a sampling rate of
7.5 SCFM.
-------
charging region on the cylinder wall, e = electronic charge, and
y = ion mobility. The average time of residence in the charging
region is expressed t = h/v, where v = average gas velocity.
Thus, the ion concentration-time product necessary for calculat-
ing particle charge is derived from
Nt = i/2TTbeyEv. (3)
A typical value of ion mobility is y = 2.2 x 10~'*m2/V-sec
and e = 1.602 x 10 19 coulombs. With a volume flowrate of y =
6.5 ft3/iain = 1.84 x 10 J m3/min, the average gas velocity, v, is
given by v = u/A, where A is the annular area through which the
gas passes. Therefore, using A = 3.206 x 10 3 m2, we obtain the
value v = 9.56 x 10"1 m/sec. Thus, the ion concentration-time
product is found to be Nt = 6.58 x 1013 sec/m3.
The mobility of particles having diameters between 0.5 ym
and 0.05 ym was determined from the equation
m = qC/6iran, (4)
where q = charge per particle, calculated from the sum of the
diffusion and field charging equations,
C = Cunningham slip correction factor,
a = particle radius, and
n = viscosity of the medium.
A plot of particle mobility versus particle diameter was made to
determine the diameter of the particle having the minimum mobility
(Figure 3). For a particle of diameter 0.2 ym, the charge per
particle in elementary units may be determined from theory to be
q = 2.76 x 10~18 coulombs. It is assumed that all particles of
equal diameter receive an equal charge although in fact there is
a distribution of charges. The value determined above represents
the mean of such a distribution where the particles are assumed
-------
u
0>
w
eg
O
6
- 4
03 3
o
UJ
_i
£
IT
<
Q.
' I ' I
E = 4.l x I05 V/m
Nt- 6.6 x I013 sec/m3
k= 5.0
T=20°C
I
I
I
.2 ,3 .4
PARTICLE DIAMETER , ,um
.5
Figure 3. Particle mobility vs. particle
diameter.
-------
to have a relative dielectric constant of 5.0 and are at a temp-
erature of 20°C. (Higher temperatures would give an increased
mean charge on the particles.) The mobility of the 0.2 ym dia-
meter particles is found to be m = 1.51 x 10 7m2/V-sec.
The maximum distance a particle must travel before it reaches
the collection electrode is just the plate separation, 0.32 cm
(.125 in). With the concentric cylinders closely spaced, the flow
is laminar (Reynolds number Re ~ 132) and the collection efficiency
can be calculated from the particle trajectory. The velocity
component downstream is U/AC, where u is the sample flowrate and
Ac is the open cross-sectional area of the collection zone (Ac =
3.175 x 10 3m2). The velocity component toward the collection
plate is the migration velocity, w, which is equal to the product
of the electric field and particle mobility. The length of the
collection plate required for 100% theoretical collection effi-
ciency is then
1 = —•- (5)
1 A w ' ^'
c
where d = 3.2 x 10 3m is the cylinder spacing. If we choose a
value of 6.25 x 105 volts/m for the collecting field (2 kV ap-
plied), the migration velocity for the 0.2 ym diameter particles
is 9.44 x 10~2m/sec, and 1 is equal to 3.27 cm. Thus, the 8.4 cm
collector plate length used in the final design is extremely con-
servative .
Needle Discharge Electrode
The sharp needle discharge electrode system was subjected to
the same type of analysis as the disc system described above.
The current-voltage characteristics for the needle-cylinder elec-
trode geometry are shown in Figure 4. An applied voltage of V =
16 kV corresponds to a total current of i = 200 yA. The ion
concentration-time product is determined from Equation 3 using
10
-------
I04
I03
H
UJ
(T
tr
o
I02
10
Spark
I
10 15 20
VOLTAGE, kV
25
30
35
Figure 4.
Current-Voltage Characteristics of the ESP Collector
with the Needle Discharge Electrode and at T = 200°C
and Flowrate =7.5 SCFM.
11
-------
the value of the average electric field strength, E = V/b =
4.6 x 105 volts/m, v = 8.86 x 10"1 m/sec, y = 2.2 x 10~" m2/V-sec,
and e = 1.602 x 10~19 coulombs. The resulting value is Nt =
6.33 x 1013 sec/m
3
The mobility of the 0.2 ym diameter particles is determined
from Equation 4 to be 1.57 x 10~7 m2/V-sec. This gives a migra-
tion velocity, w, of 9.8 x 10 2 m/sec. Thus, Equation 5 gives the
length of collection plate required for 100% theoretical collec-
tion efficiency to be 1 = 3.1 cm. Here again, the selection of
8.4 cm as the collector plate length in the final design is very
conservative.
The prototype ESP collector was fabricated from glass-filled
teflon and type 316 stainless steel. Figure 5 is an assembly
drawing of the ESP collector. Complete shop drawings are included
in this report as an appendix.
Performance
Figure 6 illustrates the experimental arrangement used to
evaluate the ESP collector in preliminary laboratory tests. A
lognormal aerosol of approximately 1 ym mass median diameter and
geometric standard deviation of 2.0 was generated by nebulizing
a 0.1% solution of fluorescein in 0.1N NHs. The aerosol was
pumped through the ESP collector with the disc discharge electrode
installed. The size dependent collection efficiency was determined
by measuring the particle size distribution at the ESP outlet, with
the power on and off, using a Thermosystems Model 3030 Electrical
Aerosol Analyzer. The results of these experiments are summarized
in Table I. It is clear from the table that the ESP is capable
of achieving the desired high collection efficiency for submicron
particles. Laboratory tests were not conducted with the needle
discharge electrode.
12
-------
H-
d
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DIFFUSIONAL DRYER
HIGH PRESSURE
AIR
REGULATOR
TO ELECTICAL AEROSOL
ANALYZER
VENT
Figure 6. Laboratory setup for evaluating the
electrostatic collector.
14
-------
Ul
TABLE I
Laboratory Determination of the
Collection Efficiency of the ESP Collector
With the Disc Discharge Electrode
Corona Voltage = 25 kV
Corona Current =200 yA
Collector Voltage 2 kV
Ammonium Fluorescein Aerosol
Particle
Size (ym)
Flowrate
(ACFM)
.898
2.97
4.65
5.82
0
.0133
0.0237
0.0422
0.
Collection
99
99
99
.86
-
.76
.34
99.93
99.77
100.00
99.88
100.00
100.00
99.99
100.00
99.
99.
100.
100.
0750
0
.133 0
.237 0.422
0.750
Efficiency (%)
97
93
00
00
100
99
100
100
.00 99
.95 99
.00 100
.00 99
.98 100.00
.50 99.62
.00 100.00
.96 100.00
100.00
100.00
99.60
99.85
-------
Tests were conducted to determine the efficiency of the ESP
collector when sampling redispersed fly ash of high resistivity.
The purpose of these tests was to investigate possible deleterious
effects due to back corona and heavy dust loading.
Figure 7 illustrates the experimental setup for the fly ash
tests. A sample of dust-laden gas was withdrawn from a duct down-
stream from a pilot-scale precipitator. Fly ash was injected
into the heated gas stream with a sandblaster. The precipitator
was not energized and thus acted as a settling chamber to effec-
tively reduce the mass median diameter of the sampled particles
without severely diminishing the number concentration of fine
particles. The extracted sample passed through a heated probe, a
cyclone, and into the ESP collector. A filter was located down-
stream of the ESP to collect any dust that penetrated it, for
determinations of the collection efficiency.
Two tests were run with the disc-cylinder electrode system,
each for six hours, with a dust loading of 1.88 g/m3 at the pre-
cipitator inlet. One test was run at 25°C, and the other at
171°C. After six hours' testing, there was no noticeable increase
in pressure drop across either of the filters. After each test,
the ESP collector was disassembled, immersed in a beaker contain-
ing distilled water, and agitated ultrasonically. For these
tests, the resultant suspension was filtered. The filters were
then dried and weighed to determine the amount of particulate
material collected.
The test conditions and results for the disc-cylinder system
fly ash experiments are presented in Table II. Again, the ESP
collector performed well, with efficiencies of 96 and 98% for the
two tests.
Experiments with the needle discharge electrode in the ESP
collector were not performed. The efficacy of the needle-cylinder
geometry in field applications has not been determined.
16
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DUCT WALL
NOZZLE
CONDENSER
COIL
HEATED
PROBE
-Rl
OVEN
ESP COLLECTOR
CYCLONE
""^FILTER
PUMP
Figure 7. Experimental setup for evaluation of the
ESP collector using redispersed flyash.
17
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TABLE II
Collection Efficiency of the ESP Collector
With the Disc Discharge Electrode
Test 1 Test 2
Corona Voltage 22 kV 19 kV
Corona Current 190 yA 200 yA
Collector Voltage 2 kV 2 kV
Flowrate 142 slpm 142 slpm
Temperature 25°C 171°C
Run Time 6 hours 6 hours
Dust Loading 1.88 g/m3 1.88 g/m3
Cyclone Catch 40 g 0.56 g
ESP Catch 0.435 g 0.704 g
Filter Catch 0.016 g 0.017 g
Efficiency of ESP 96.3% 97.6%
18
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Power Supply Package
The power supply package for the ESP collector has been de-
signed to provide essentially "turnkey" operation. All operating
parameters have been .pre-set in order to provide simple field op-
eration. The front panel controls consist of a power switch,
operating/fault displays, and screwdriver adjustments for collec-
tor voltage and ionizing current.
The power supply package consists of two major subsystems
(see Figure 8): the collector voltage power supply with associ-
ated display, and the ionizer power supply with associated control
and monitor circuits. The display circuits for the collector
voltage consists of awindow comparator which monitors the output
of a voltage divider. Whenever the monitored voltage is outside
the pre-set window, the pulse stretching circuit (a monostable
multivibrator with a one-second output pulse) turns on the fault
display. This display will remain on as long as the collector
voltage is outside the pre-set window. The ionizer power supply
consists of a Spellman voltage programmable modular supply and
control circuitry. A control voltage is derived by integrating
the difference between a reference voltage and the voltage drop
across a buffered resistance in the ground return line. This
converts the power supply into an adjustable constant current
source. The display circuits for the ionizer are essentially
identical to those of the collector voltage supply with the ex-
ception that they monitor the voltage drop across the buffered
resistance.
The last item of interest is the "ground fault detection and
high voltage inhibit" circuit. Whenever the current in the ground
return line drops below a low pre-set value (nominally 12 yA) for
a period of time greater than 100 milliseconds, the outputs of
both power supply modules are inhibited. This insures that if
19
-------
Figure 8. Power Supply Package Circuitry.
20
-------
either the external ionizer connection or ground return line con-
nection is broken, no high voltage hazard will exist.
Please note that at the time of this report, no extensive
testing had been performed on this power supply package. There-
fore, some modifications may be made at a future date.
Power Supply Specifications
Ionizer Current: Constant 15 yA to 1 mA (upper limit dependent
;>^~x0,', ••... on electrode geometry)
adjustable
Collector Voltage: 1.8 KV to 3.0 KV adjustable
| /--'V . ,„-
Operating Temperature: Control & Display Circuits -25°C to + 85°C
High Voltage Supplies: 0°C to +70°C
Power Requirements: 105 to 120 VAC, 6A(max.)
21
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SUMMARY
The ESP collector is a highly efficient collector of sub-
micron particles. When set to 200 yA on the corona disc electrode
and 2 kV on the collector (both well below breakdown values), no
further adjustments are necessary for proper operation. The power
supply developed for the ESP collector facilitates correct opera-
tion. Since there is a potential for degraded performance due to
back corona if the collected particles are of high resistivity,
it is suggested that the collector be routinely used with a back-
up filter following it in the sampling train. If experience has
shown the system to operate reliably at a particular source, the
filter can be eliminated.
After the sample is collected, the ESP is disassembled, im-
mersed in a suitable liquid, and agitated ultrasonically. The
wash can be filtered or evaporated to dryness, depending on the
nature of the dust and the objectives of the test.
The electrostatic collector prototype developed and tested
in this research effort fulfills the design criteria: near 100%
collection of submicron particles when operated at a nominal
sample flowrate of 6.5 ft3/min and a temperature of 200°C, sized
to fit through a 4 inch diameter port for in situ operation, con-
venient to operate, and clean.
22
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APPENDIX
SHOP DRAWINGS OF THE ESP COLLECTOR
23
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-78-114
2.
3. RECIPIENT'S ACCESSION NO.
. TITLE AND SUBTITLE
An Electrostatic Precipitator Backup for Sampling
Systems
5. REPORT DATE
June 1978
6. PERFORMING ORGANIZATION CODE
. AUTHOR(S)
P. Vann Bush and Wallace B. Smith
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
68-02-2131
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND F
Final; 10/77-4/78
PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES IERL-RTP project officer is D. Bruce Harris, Mail Drop 62, 919/
541-2557.
16. ABSTRACT
The report describes a program carried out to design and evaluate the
performance of an electrostatic collector to be used as an alternative to filters as a
fine particle collector. Potential advantages of an electrostatic precipitator are low
pressure drop and high capacity. Potential problems are unreliability and poor col-
lection due to back-corona or lack of particle adhesivity.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Dust
Sampling
Electrostatic Precipitation
Air Pollution Control
Stationary Sources
Particulates
13B
11G
14B
13H
13. DISTRIBUTION STATEMEN1
Unlimited
19. SECURITY CLASS (ThisReport)'
Unclassified
21. NO. OF PAGES
35
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
35
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