ENGINEERING AND PROCESS DEVELOPMENT havior under conditions very far removed from those prevailing in this study, they were used t o compute some of the results of Tepe and Woods (11) (Table 11). For these measurements a circular tower 8.5 inches in diameter was employed, with very small plate spacings, and 0.1 1-inch perforations. The domnspouts were circular, 2 inches in diameter. The dispersed liquid was isobutyl alcohol, and the continuous liquid was water, both mutually saturated solutions, u i t h u = 2.5 dynes per cm. With two exceptions the data are predicted very well.
vdisp
= superficial velocity of dispersed liquid, based on Sc,
6
= differential operator = time, seconds
feet per hour
8
time for bubble formation, seconds
eb
=
pc PD
= density of continuous liquid, pounds per cubic foot = density of dispersed liquid, pounds per cubic foot
Ap
=
U
e
difference i n liquid densities, pounds per cubic foot
= interfacial tension, pounds per foot ( = dynes per cm.
6.85 x 10-5).
x
Literature Cited Nomenclature 4
CO
d
db
do gc *
h
hs = he = hoont = hdiaD
=
h~ hp ho h,
=
K Reo SO SO St
UD UO
(1) Adam, N. K., “Physics and Chemistry of Surfaces,” p. 11, Ox-
= orifice coefficient, dimensionless = diameter of bubble a t time e, feet = ultimate bubble diameter, feet = perforation diameter, feet = 32.2 Ib. mass feet/lb. force sec.2 = total thickness of coalesced dispersed
= = = = = = = =
= =
Voont =
liquid below top surface of plate, feet thickness of layer owing t o change of flow direction, feet thickness of layer owing t o contraction, feet thickness of layer owing to continuous liquid flow, feet thickness of layer owing to dispersed liquid flow, feet thickness of layer owing to expansion, feet thickness of layer owing t o friction in downspout, feet thickness of layer owing to orifice effect, feet thickness of layer owing to interfacial tension, feet contraction coefficient, dimensionless Reynolds number for flow in downspout, dimensionless cross-sectional area of downspout, square feet area of all perforations, square feet cross-sectional area of tower, square feet superficial velocity in downspout, based on SD,feet per second superficial velocity through perforations, based on SO, feet per second su erficial velocity of continuous liquid, based on St, feet per hour
ford. Eneland. Clarendon Press. 1938. (2) Eversole, W.G., Wagner, G. H., and Stackhouse, E., IND. ENO. CHEM., 33,1459 (1941). (3) Fujita, S., Tanizawa, E., and Chung-gyu, K., Chem. Eng. (Jaguan),17, hTo.3, 111 (1953). (4) Harkins, W. D.. in “Physical Methods of Organic Chemistry.” Vol. 1, A. Weissberper, ed., New York, In6rscience Publishers, 1945. ( 5 ) Hayworth, C. B., and Treybal, R. E., IND.ENG.CHEW,42, 1174 (1950). (6) Mayfield, F. D., and Church, W. L., Ibid., 44, 2253 (1952). (7) Mayfield, F. D., Church, W. L., Green, A. C., Lee, D. C., and Rassmussen, R. W., Ibid.,44,2238 (1952). (8) Perry, J. H., ed., “Chemical Engineers’ Handbook,” 3rd ed.. pp. 388-90, New York, McGraw-Hill Book Co., 1950. (9) Pyle, C., Colburn, A. P., and Duffey, H. R., IND. ENG.C H ~ M . , 42,1042 (1950). (10) Rouse, H., ed., “Engineering Hydraulics,” p. 422, New York, John Wiley & Sons, 1950. (11) Tepe, J. B., and Woods, W. K., Atomic Energy Commission, AECD-2864 (1943). RECEIYED for review May 12, 1953. ACCEPTEDJuly 31, 1953. An adaptation of the theses of R. J. Bussolari and Seymour Schiff, submitted in partial fulfillment of the requirements for the degree of M.Ch.E. at New York University.
Semiempirical Equation of Electrostatic Precipitation ERIC A. WALKER, Pennsylvania Stafe College, Stafe College, Pa. JOHN E. COOLIDGE, Borg- Warner Central Research Laboratory, Bellwood,
w
T
HE design of electrostatic precipitators for the separation of dust particles must, even after more than 40 years of commercial experience, still be classed as an art. The majority of the large electrostatic precipitators for cleaning gas with heavy dust loadings have, in this county, been made by three companies. Each has gradually built up a body of design data as precipitators with larger capacities, higher dust loadings, shorter influence time, etc., have been designed and constructed. New ground has been broken by extrapolating from tested units and occasionally an uncertain but usually successful step has been taken from one type of aerosol t o another. European companies have concentrated on smaller and usually more efficient precipitators; and there have been some communication and transfer of design data between European and American manufacturers. However, t o step out and t r y t o precipitate entirely new aerosols at different dust loadings has always been fraught with danger; and, even when it has been thought t h a t conditions were November 1953
111.
well understood and the solution was straightforward, “queer resultsJ’have occasionally been obtained, requiring the use of inelegant procedures, much like using rubber gloves as a cure for leaking fountain pens. A thorough study of the field, going more into the basic aspects than is usually done by design engineers, and more into design and practice than the theoretical investigator usually touches, is now in order. Such a program has been undertaken at the Pennsylvania State College with four main objectives: (1) t o survey the available knowledge, both theoretical and operational; ( 2 ) t o collect operational d a t a on as many existing commercial installations as possible; (3) t o make a laboratory study of the more fundamental processes involved in electrostatic precipitation; and (4) t o study the possibility of extrapolating, from small scale laboratory operation t o full scale commercial precipitators. T h e results of the literature survey, along with some initial results of the laboratory study, were reported in a previous paper (6). The
INDUSTRIAL AND ENGINEERING CHEMISTRY
2411
ENGINEERING AND PROCESS DEVELOPMENT
HE ATE RS
e
€ L E C T R IC DUSTER
1
I
n
D I ST R I B U T I O N
I
,
I ' AR IFLOW
'
I
TEMP. a HUMIDITY MEASURE
INPUT SMOKEMETER
I
TEST PRECIP!TATOR
1
1
HAST INGS HOT WIR E AN€ M O M ETE R
I OUTPUT SAMPLING PIPE
8 SMOKEMETER W A T E R SPRAY
I ~
M I X I N G FAN
I
AIR J E T S
Figure 1.
Diagram of Test Setup
puipose of this paper is to present a more detailed discussion of the laboratory equipment and tests. Equipment includes Air-conditioning Unit and Test Precipitators
Air Conditioner. To obtain reproducible results in the study of the important parameters in electrostatic precipitation, an airconditioning unit was constructed t o control the temperature, humidity, velocity, and duqt loading of the sample aerosol. The mit consists of a metal bo.;. 8 X 5 X 3 feet, partitioned as shown in Figure 1 t o increase the time available for dust diffusion and t o provide control of the other parameters. The temperature is changed by the lamp bank arid other heating units shown in the first arid third sections. Thisprovidesalimited temperaturerange of from 20" t o 60" C. hut it could be extended bv the use of theimoinsulation aiound t h r coriditioncr 01 by the addition of
more heaters. However, the results of preliminary expeiiments within the 40' range available did not indicate the necessity of further extension until other and more important variables had been explored. The humidity of the air stream is increased by a water spray located in the second section. A 60% range of relative humidity is possible, although a decrease below that of the incoming air can be obtained only by raising the temperatuie There is some evidence that a more finely divided spray is desirable as well as an increased diffusion time. It is also apparent t h a t the humidity a t which the dust has been stored is an import a n t factor; if possible, the dust should be stored at the desired temperature and humidity for a considerable time before an experiment is started, t o ensure t h a t proper moisture equilibrium between hhe dust and air has been attained, and, therefore, the electiical resistance of the layers which form on the collecting plate has been stabilized. The relative humidity was measured by a wet- and dry-bulb thermometer. The velocity of the air stream is controlled in this arrangement by a damper on the exhaust side of a fan placed in the output duct. The cleaned air is discharged to the outdoors Dust Loading and Measurement. The dust loading is produced by sifting the dust into the air stream over two small air sprays which increase the dust entrainment. From the section where this IS accomplished, the gas flows into a large fan-stirred mixing chamber t o provide a mole homogeneous aerosol. The output from the conditioning chambcr is taken from a 12-inch duct located a t appioximately the center and provided with a perforated baffle t o give a more uniform velocity distribution ovei the entiie cross section. I t was found easih- possible t o produce uniform loadings, from very light up t o 5 grains per cubic foot To obtain instantaneous recording of the dust loading, the light absoiption of the aerosol is measured by a phototube apparatus The theory and application of a photoelectric smokemetei have been completely described ( 2 ) . It can be shown t h a t the light absorption by a layer of aerocol is given by
I = Ioe-am
Figure 2.
2418
Model Precipitator
Theproportionality constant, a,isafunction of the length of dust column, particle size distribution, density, and opacity of the particles. Thus, a! is difficult to calculate and usually must be determined by experiment. In these tests t h e light meter was calibrated for a given dust b y a direct weight sampling method,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
ENGINEERING AND PROCESS DEVELOPMENT
.*
using a filter paper or a precipitator of very high effic'iency. It is necessary t o recalibrate for each new material and for each operating velocity. The input smokemeter is located in the 12-inch duct between the air conditioner and the test section. It consists of a light source and phototube with a current amplifier and a suitable pen recorder. The output smokemeter is located in a small 2-inch sampling tube directly behind precipitator unit. Because of the low dust loading and the short absorption path, a bridge-type system is used to get greater sensitivity. I n order t o prevent settling in the 2-inch sampling tube, a higher velocity is used than in the main chamber. Unfortunately, if the velocities become too high, centrifugal separation may occur. By empirical methods it was determined that a sampling tube velocity about twice that of the duct velocity gives mostreliable results for this type of aerosol.
0
0
0
0
2 0 m i l diam. S t e e l Wire a
0
0
b Figure 3.
Pg
"
0
0
C
Cross Section of Precipitator
0.
0
d Single-Stage
The precipitator efficiency is calculated by a comparison of the output smokemeter indications with and without the precipitator active, the input unit serving only t o indicate constancy of dust loading. This method accounts for mechanical settling of dust in the precipitator. Test Precipitators. The precipitator unit itself, shown in Figure 2, consists of 13 galvanized plates, 12 X 4 inches, spaced 1 inch apart. Four 20-mil wires for production of the corona are mounted equidistant between the plates, as shown in Figure 3. It is important to insulate both the plates and wires from ground in order to separate the true corona current from the current that leaks across the insulators. The latter is of no importance in these studies and, indeed, it may completely obscure the real issue. Negative wire corona was used in all tests except where otherwise indicated. When it is desired t o study two-stage operations, two of the wires in the above configuration can be replaced by a plate 1.75 inches long, as shown in Figure 4. This arrangement is not as flexible as is desirable, but i t does permit some preliminary experimentation on this type of operation. The power supply used in these experiments is well filtered and has very little ripple. The precipitator was located in the test section, as shown schematically in Figure 1, in such a position that a uniform velocity distribution was obtained across the active precipitator cross section. Method of Test. Relatively short-time experiments were used to prevent excessive build-up on the plates because of the small wire-plate spacing and heavy dust loads. An average test lasted 15 minutes, during which time six or seven readings were taken. Following each test, the unit was blown clean with compressed air, although for some experiments it was necessary t o wipe both the wires and plates clean. Equipment Considerations. Although all the above steps were taken t o measure and control the parameters believed im-
November 1953
portant, limitations produced a spread of results which required anumber of readings on asinglepoint to produce reliable results. A very important, but certainly anunexpected, consideration was the extent of the effect of the humidity of the gas; for although reference had been made to the effect of humidity in past literature, i t was of a qualitative nature. Articles which have appeared since the start of this program have confirmed the authors' observations (4, 6). It is not believed that humidity has any direct effect. I n fact, corona experiments in clean air at room temperature show the humidity t o be of little or no importance; but, it is believed that this parameter makes itself felt through its influence on the resistivity of the precipitated material. Another annoying parameter which is difficult t o control is the particle size distribution of the dust being studied. T h a t this is so is apparent from the fact t h a t precipitation itself, which involves an agglomeration process, will change the particle size. Therefore, dust caught from an aerosol on a slide or by a filter will not show the same particle size distribution as dust t h a t has been precipitated by a cyclone (centrifugal effect) or by an electrostatic precipitator. However, much effort was devoted to a careful determination of the physical characteristics of the aerosol while cognizant of the limitations mentioned above. Samples were taken by means of a high efficiency precipitator of the dust loading entering the precipitator at the various velocities of operation, and particle size measurements were made t o determine the range and average size of the particles, These measurements were carried out on an apparatus designed specifically t o give accurate sedimentation results with the small samples available. The samples were also examined under a microscope t o check the unitary nature of the particles. Spot checks were made of the dispersion during the course of the experiments, but it is felt t h a t variations in the distribution from one experiment to another contributed greatly t o the spread of results.
I"
0
k
0
-I
2 0 m i l diam. S t e e l Wire 0
8
-
b Figure 4.
*-
c
d
Cross Section of Two-Stage Precipitator
After many preliminary experiments with various materials, gypsum was chosen as the basic test precipitate because the method of dispersion best simulated its field operation, and, even more important, reproducible results could be obtained, A temporary goal was established to measure the dust remaining after precipitation within 2%-that is, the measurement should be reproducible within 1 2 % . With careful control, and attention t o all parameters, this could be accomplished. Spot checks have been made on the precipitation of other materials, mainly to observe their differences. These checks lead to the belief that with similar care equally good results can be obtained. Equation Has Been Derived for Efficiency of Electrostatic Precipitation
Tests included in this study on gypsum can be fitted to the following equation:
INDUSTRIAL AND ENGINEERING CHEMISTRY
2419
ENGINEERING AND PROCESS DEVELOPMENT E
= [l
- e--Kh5(V - V c ) ( L / v ) ]X
100
(1)
The precipitator efficiency is defined as the ratio of the difference between output dust loadings, with and without the precipitator active, t o t h a t without the precipitator active, and is expressed in per cent. The critical corona voltage is t h a t voltage which will initiate corona discharge a t the surface of the discharge wire of the precipitator. This is calculated for the particular precipitator configuration.
100
eo
that given by Equation 1 for these operat,ing conditions. The same trend is shown in the second configuration; but Equation I suggests a final efficiency of 3%, not 0, a variation within the experimental accuracy. This suggests that the cause of low efficiencies a t low relative humidities is the deposit of dry gypsum on the plates and wires, and is not inherent in the charging and transportation characteristics of the process. Since t’heoretical efficiency expressions describe only the charge and transport action, and not the collection process, they cannot cont’ain humidity considerations. An alternative interpret,ation, suggested by ’IV. Sproull of t,hc Western Precipitation Corp., is that the presence of the high resistivity dust on the collector plates results in a distortion of the electric field distribution sufficient to reduce the effective collection field or even possibly t,he charging action.
60
w 40
-‘ 30
Table 1.
Efficiency vs. Apparent Resistivity
20 10
0
I
2 3 4 5 6 7 8 9 101112I314 h( V-Vc)a
Material Gypsum 15% R H Gypsum’ 35% R H Gypsum’ 60% R H Fly ash,’BO% R H Carbon black
Apparent Resistivity, Ohm-Cm. 2 X 1014 2 x 1012 2 X 10” 8 X 106 2 X 108
E5ciency 20 52 38 69 80
Precipitator and Conditions (Modo1 I precipitator,
i137750volts Fig. 3 , feet per second a.
Model 2 preripitator, Fig. 4 , h . 7750 volts ‘ 3 feet per second ( 0 0 % relative humidity f
~
V V
a. Gypsum, 5, 10, 18 microns v. V.
h. Figure 5.
Single-stage precipitator 2 to 4 grains per cu. foot 0.8 to 3 feet per second 7.75 to 9.2 kv. 76’ to 80’ F. 0.1 3 to 0.70 relative humidity Vc, 5.7 kv.
Fly ash Blast furnace Gypsum
90
79:
Because of the evident importance of the plate deposit, the apparent resistivity of gypsum was measured at different relative humidities along with that of fly ash, furnace material, and carbon black. The method used in these measurements was similar to
Semiempirical Equation for Electrostatic Precipitation
40
This result is shown on semilogarithmic coordinates in Figure 5 along with data points and range of variables of the experiments. The accuracy of the exponential representation by the method of least squares was found to be 3.5% for the constant K .
30
20
Experiments Are Adaptable to Exponential Form
IO
One of the comforting results of these experiments is their adaptability t o a n exponential form in agreement with the theoretical derivations of Deutsch and others (3). In addition, dust loading does not appear in the efficiency expression; again in agreement with theory. It is true that dust loading affects precipitator efficiency in field installations to some extent; and this effect can be produced in the laboratory experiments if one allows separation by settling or centrifugal effects, or if re-entrainment of precipitated particles is permitted. However, statistically the observed change in efficiency with loading is within the over-all experimental accuracy. Humidity. The presence of relative humidity, in the exponentis1 term of Equation 1,indicates that it is as important in determining the efficiency of gypsum precipitation as the applied voltage, the contact time, L/v, or the particle size, On the other hand, in tests conducted on materials such as fly ash, blast furnace dust, and carbon black, little or no effect was produced by relative humidity changes. In order to provide a basis to explain this difference, the following experimemts were conducted: Starting with a perfectly clean precipitator, the efficiency of gypsum precipitation was measured as a function of time. Typical results for lower humidities are shown in Figure 6. A similar effect was not present at higher humidities. It can be seen t h a t the efficiency of the single-stage unit of Figure 3. a,gradually decreased with increasing exposure, approaching a limiting value of 15%, 2420
8 X 106 8 X 106 1 X 10”
0 0 1 2 3 4 5 6 7 8 9 1 0 EXPOSURE TIME IN MINUTES Figure 6. Efficiency vs. Exposure Time
n
Shows marked effect of gypsum build-up on precipitator plates at low relative humidities. At 60% relative humidity this effect is not present
A O
c.
RH.,
%
Volts Gypsum, grains/ CU. foot Precipitator
6
28
52
18 7070
7750
2.6, 18-micron OV. size Fig. 3, a
17 4, 3 feet/second Fig. 3, b
that suggested by H. J. K h i t e (4). Csing the apparatus shown in Figure 7 , the measurement is not linearly dependent on the thickness of deposit and, for gypsum, the initial water content of the material is an important factor. Typical results of the resistivity are measurements shown in Table I, along with corresponding efficiency measurements on the single-stage and two-stage precipitators under the conditions indicated. Thus thesemeasurements lead t o the conclusion that the build-up of a high resistivity
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
ENGINEERING AND PROCESS DEVELOPMENT
ir
dust such as dry gypsum on the collector plates reduces the overall efficiency of precipitation, but that if means are provided t o raise the conductivity (in this case by raising the relative humidity), the efficiency can be restored. In general, these results agree with those presented by Schmidt (4), although i t is difficult t o make a comparison because of the sharp dependence on the method and conditions of measurement. Voltage. Some theoretical considerations of the role of the electric field in precipitator action suggest t h a t it should enter the efficiency expression as a squared quantity because of its effect on both particle charging and particle transportation ( 6 ) . The results of these investigations, however, indicate that such is not the case. All the tests conducted with gypsum and other materials showed an exponential dependence on the quantity (V V,)-the electric field being t o a first approximation a direct function of this voltage difference. It would appear that the accepted theoretical expressions are not applicable in this installation. Calculation of Critical Corona Voltage. The electric field intensity a t the surface of a wire mounted equidistant between parallel plates can be shown t o be
Corona Current. Measurements of the net corona current, excluding leakage currents, were made for all tests. Unfortunately, no correlation between precipitator action and corona current was possible, as was shown in a previous paper (6). Subsequent measurements showed that, except under extremely weak conditions of corona discharge, the charging action is very rapid and reaches a n early saturation. Thus, corona current can undergo great variations without affecting the charging process. Under these conditions, corona current will not appear explicitly in the efficiency expression. NUMBER OF WIRES
IO0
-
E=----- V 4d r In -3r
80
k2 60 I
>.
y- 40 0
k W
20
where r = wire radius; d = wire-to-plate spacing
0
0
Peek’s empirical formula (1) for the critical corona gradient is
c + I;“
E, = 30 M 1
=
0 30 30 X 0.61 X 1A 4 0 m
- , kv./cm.
Figure
- 53 kv./cm. -
or Ti, = 53 X 0.0254 X
@ TRAIN
r----
[I:In:::--
S600/1
THIOKNLSS INDICATOR
ION
Figure 7.
Model I .Resistivator
Preliminary resistivity measurements discussed were made with this apparatus
November 1953
0
5
8.
Effect of Precipitator Length on Efficiency 7750 volts g:Fcdative
humidity
3 feet per second 3 grains per cu. foot Precipitator, Fig. 3
Particle Size, Velocity, and Length. Variations in the effective length of the precipitator were obtained by removal of the discharge wires. A plot of the variation of precipitator efficiency with effective length is shown in Figure 8; again a n exponential relation is obtained. The initial tests with gypsum on the effect of air velocity on precipitator efficiency did not give the results expected from theoretical considerations. However, from particle size measurements of the gypsum dispersion a t the velocities of operation, i t was discovered that the average particle size varied directly with the air velocity. Typical results of these measurements are shown in Figure 9. Thus, the effects of velocitywere being maskedby an accompanying change in particle size distribution. However, theoretical considerations and additional tests on a dispersion which did not show this particle size variation indicated the individual role of
Ve = 5.7 kv.
OLAR
2 . 3
NO.OF WIRES
where M is the irregularity factor. For the laboratory electrostatic precipitator this factor was assumed t o be 0.61 because of the extreme irregularities caused by the collection of dust on the fine wire. Therefore, for the test precipitator the critical gradient
E,
1
Table II.
Efficiency vs. Wire-Plate Configurations
Figure 3 b 100 20 Figure 4’ d 10 20 Figure 4’ o 50 45 Figure 4’ b 300 75 Figure 4’ a 200 75 Figure 3: a 450 50 Although commeroial precipitator e5aiencies normally run much higher than observed in these experiments, it was preferable to operate at low effioienoies in order to produce a wide spread of differences.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2421
ENGINEERING AND PROCESS DEVELOPMENT Table 111.
Precipitator Efficiency vs. Applied Voltage Characteristics Blast furnace material
7750 peak volts 3 feet per seoond 18% relative humidity
Polarity of Wire Positive Negative Negative
K W
z
Wave Shape Flat d.c. Half wave Flat d.o.
Current, Ira. 20 450 440
Efficiency 20 i 4 57 i 4 78 i 4
I-
z
W
V
CK
W
a
v~locity.thus fixing the effect of particle size in the initial gypsum measurements. Frictional Charging Does Not Affect Measurements
Voltage Effects. Some tests weie made on the effect of voltage wave shape and polarity of the discharge wires, as is indicated in Table 111. These tests weie peiformed with the two-stage precipitator, using blast furnace dust. The large difference in the current drawn in the positive and negative operation (20 and 430 pa.) suggests the difference in efficiency is caused by insufficient charging action in the positive wire case. Unfortunately, arc-overs prevented increasing the operating voltage of the positive wire to increase the charging current. The reduction of operating efficiency with the unfiltered negative supply voltage is probahly caused by the sharp reduction in effective collecting field in the parallel plate section, because the charging current is approximatelr the same but the collection voltage is decreased by a factor of l / ~ . Acknowledgment
Two-Stage Operation. Tests conducted with the two-stage configuration of Figure 4 showed an increase in operating efficiency over the four-wire single-stage unit, with a decrease in current consumption. This indicates that collection, and not charging, is the limiting action in units with this short influence time. Table I1 shows some other representative results of measurements on various configurations. The precipitation observed with the plate section alone shows the extent of frictional electrification of the dust dispersion. The extent that the frictional charging affected measurements was investigated thoroughly. These studies, along with corona charging studies, showed that the frictional charging present in the system did not affect measurements of the action of the precipitation of dust by electrostatic means with a corona discharge. This conclusion is based on two main observations: 1. The gypsum was charged approximately equally positive and negative-that is, the over-all aerosol was neutral. 2. The charging action in the corona discharge is normally very rapid and limits the precipitation only in extremely weak conditions of corona space charge, so that the charge condition of the dust before entering the precipitator does not affect the precipitation process. Thus, the equation represents only the effects of the corona discharge precipitator action-it does not include frictional effects. Therefore, a t a voltage of V = V , an efficiency of zero may be expected from the corona charging electrostatic precipitator. The fact that a t V = V , some removal is obtained indicates that in the absence of a corona discharge frictional electricity produces some precipitation. However, this fact does not invalidate the expression for the action of corona charging electrostatic precipitation.
This study was made under a research grant by the Koppers Co., Inc , to which the authors express their appreciation. The authors are indebted to G. J. Schulz, who assisted in all laboratory experiments. Nomenclature lo = initial light intensity 1 = light intensity emerging from dust-laden column C( = proportionality constant m = dust loading E = precipitator efficiency for gypeum, per cent (precipitator of Figure 3 ) K = constant, for this prcripitator = 0.345 i 0.011 h = relative humidity, decimal equivalent a = average particle radius, microns V = applied voltage, kilovolts (negative wire corona) V , = critical corona voltage, kilovolts (calculated) L = precipitator length. feet t = air stream velocity, feet per second References (1) Cobine, J. D., "Gaseous Conductors," 1st ed., p. 254, New York, McGraw-Hill Book Co.. 1941. ( 2 ) Coolidge, J. E., and Schula. G. J., Instruments,24,534, 544,57880 (1951). (3) Deutsch, W., Ann. Phvs., 68,335 (1922). (4) Schmidt, W. A., IXD. ENG.CHEM.,41, 2428 (1949). ( 5 ) Walker, E. A . , and Coolidge, J. E., Heating, Piping,Azr Conditioning,23, No. 3, 107-11 (3Iarch 1951). (6) White, H. J., Robeits, I,. 31 , and Hedberg. C. W., Mech. Enp., 72, 873-80 (1950). RECEIVED foi rexiew M a y 26, 1932 ~ L C E P T E D .June 24, 1953.
END OF ENGINEERING AND PROCESS DEVELOPMENT SECTION
2422
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Vol. 45, No. I1
