EFFECT OF PARTICLE ELECTROSTATIC CHARGE
ON FILTRATION BY FIBROUS FILTERS D . A .
L’niversity
LUNDGREN’AND of
K. T. W H l T B Y
Minnesota, Minneapolis, Minn.
The influence! of electrostatic charge on particle removal from a gas by a fibrous filter was quantitatively measured as a function of particle diameter, particle charge, filter fiber diameter, and air flow velocity. Experiments were conducted using homogeneous, monodispersed, solid spherical aerosol particles purposely charged, to !various degrees, by a special corona charger. Particle charge magnitude and uniformity were measured with an aerosol charge spectrometer. At certain experimental conditions, particle charges caused a tenfold increase in the single fiber efficiency which, in turn, led to a hundredfold decrease in filter penetration, A functional relationship was shown to exist between the single fiber efficiency and a dimensionless image force--drag force parameter. A method is presented for calculating the effect of particle charge on collection by a neutral filter for any given set of conditions.
the several forces \\hich cause collection of particles in a fibrous filter is the electrical. Although a number of investigators have reported on the effect of electric charge on the collection efficiency of fibrous filters, few quantitative data have appeared relating specific electrical forces to the single fiber collection efficiency of filters. This paper reports on an experimental investigation into the relationship between a filter’s single fiber collection efficiency and one of these electrical forces, the image force between a charged particle and a n uncharged fiber. Interest in the effect of particle charge arose from the difference in charge between natural and artificial aerosols. In field use, fibrous filters are relatively uncharged and gas-borne particles passing through the filter usually carry a relatively lou charge. I n the laboratory, however, these relatively uncharged filters are tested using specially prepared test aerosols \\ hich are usually assumed to be electrically neutral but may actually be highly charged. Conditions encountered in many fibrous filters are such that collection resulting from particle charge can be the primary mechanism for particle collection. This is especially true for particles in the size range of 0 1- to 1-micron diameter, as this is the particle size range for maximum neutral aerosol penetration. Particles larger than 1 micron are collected mainly by impaction, while particles smaller than 0.1 micron are collected principally by Brownian diffusion. When a particle or a collector, or both particle and collector, have a n electric chargc., there exists a n electrical force affecting the particle’s motion and hence its collection. Kraemer (4) in a basic study of electrostatic particle collection by a spherical collector categorized the electrical forces into the folloi\ing : MONG
A
Coulombic force between a charged particle and a charged collector. Force between a charged aerosol particle and its image in a collector. Force between a charged collector and its image in a n aerosol particle. Space charge repulsion of a particle by the homopolar charged aerosol of which it is a part. Force between a charged particle and a collector which has a charge induced by the charged aerosol surrounding the collector. (This force arises only when the collector is maintained a t a constant voltage.) Present address, Center for Air Environment Studies, The Pennsylvania State University, University Park, Pa.
Dawkins (7) extended Kraemer’s work to electrostatic collection by a cylindrical collector. Katanson (7) reviewed the work of others and presented equations for collection of aerosol particles by electrical forces. Goyer, Gruen, and LaMer ( 3 ) ,Rossano and Silverman ( B ) , and Silverman, Comers, and Anderson (9) experimentally investigated the effect of charge on filtration efficiency. Gillespie ( 2 ) presented a filtration equation which combines electrical and mechanical effects. Although both theoretical and experimental studies have been conducted to determine the electrical force deposition of aerosol particles, neither a satisfactory analytical solution nor experimental data were obtained for collection of a charged gas-borne particle by a single uncharged cylindrical collector. This study provides data on the relationship between a filter’s single fiber collection efficiency and a dimensionless image force-drag force parameter which involves particle diameter, particle charge, filter fiber diameter, and air flow velocity. Experimental
Aerosol collection due to image force (the force between a charged particle and its electrical image in a neutral fiber) was measured using test aerosols of 0.1- and 1-micron diameter, Table 1. Variable
Range of Variables Investigated 1-Micron0.7 -MicronDiameter Diameter Particles Particles
Filter face velocity, v,, ft./min. 4 . 4 to 160 Aerosol median charge, Q,,, electrons /particle - 300 to +320 Aerosol concentration, N,, particles/cc. 30 to 120 Charged aerosol penetration, P (ratio) 0.005 to 0.71 Filter fiber diameter, D,, microns 10 to 43 Image force parameter, K,,, dimen1 x 10-4 to 3 x 1 0 - 2 sionless Single fiber efficiency, ‘le,dimensionless 0 . 0 1 to 0 . 3 Reynolds number, Re, dimensionless 0 . 0 1 to 1 . 0
VOL. 4
4.4 to 80 0 to +6
4 X l o 3 to 6 X 105 0.38 to 0.89 10 to 43 2
x
1 0 - 6 to 2
x
10-4
0.003 to 0 . 0 5 0.04 to 1 .O
NO. 4 O C T O B E R 1 9 6 5 345
purposely charged to various degrees; the ranges over which parameters were varied are listed in Table I. The required experimental equipment included : aerosol generators, aerosol charger, aerosol charge spectrometer, filter holder, filters, test duct, sampling system, and sample evaluation equipment. Aerosol Generation. Two different aerosol generators were used to generate test aerosols. Solid 1-micron mass median diameter (mmd) particles having a geometric standard deviation, u g , of 1.09 were generated with a spinning disk generator (77, 72) from 4 parts of methylene blue dye and 1 part of uranine dye dissolved in a denatured alcohol-demineralized water solution. Solid 0.1-micron mmd particles having a ug of 1.28 \yere generated with a British Collison atomizer followed by a special impactor ( 7 7 , 72) by nebulizing and evaporating a mixture of 1 part of methylene blue and 1 part of uranine dye dissolved in demineralized water. Aerosol samples for particle sizing by electron microscopy were collected using a low pressure impactor technique (77, 72). Particle Charger. Neutral aerosol particles were charged in a corona charger of the type developed by Langer ( 5 ) (Figure 1). In this device the aerosol is charged by passing it, in the form of a narrow filament contained within a clean air sheath, through the corona emanating from a 3-mil platinum wire. The resulting particle charges are listed in Table I1 together with the charge on the neutralized aerosol and the naturally occurring charge on the I-micron-diameter aerosol. Aerosol Charge Spectrometer. T h e aerosol median charge, Q,, and charge uniformity, or geometric standard deviation, uuQ,,were determined with a n aerosol charge spectrometer (6) similar to that developed by Langer (5). I n this instrument a thin filament of charged aerosol was injected into a laminar flow field between two parallel collector plates which were maintained at equal but opposite potentials, variable from 0 to f 10,000 volts. Charged particles were deflected in the electric field and precipitated onto the collector plates; par-
Particle Charge and Charge Uniformity Case 1 Case 2 Case 3 Case 4 D,, micron 1 .o 1 .o 1.0 0.1 electrons/particle f320 +I50 $90 t-6 uno,dimensionless 1.18 1.38 1.36 1.26 Table II.
up,
Naturally occurring charge on 1-micron-diameter particles Q, = -300 electrons/particle, uug = 1.5 Charge on neutralized I-micron particles 90% of particles had less than - 6 electrons/particle 10% of particles had more than -6 electrons/particle Charge on neutralized 0.1-micron-diameter particles About 257, of partlcles had -1 electron/particle About 30% of particles had +l electron/particle About 45y0 of particles had 0 electron/particle Table 111.
CaIc u lotions
Physical Properties of Test Filters
Special Felt
filter weight per unit face area, gram/sq. cm. fiber density, grams/cc. 1. filter thickness. cm. &, fiber volume fraction, w / l p t dimensionless D,, fiber diameter, microns u~,,fiber diameter, geometric standard deviation, dimensionless R, fiber relative electrical surface resistance, ohms Filter fiber cross-sectional shape Filter fiber length profile w,
pf,
Glass
0.0305 0.0395 0.0259 1.32 1.26 2.7 0.31 1.34 0.40 0.0745 0.0234 0.024 10.0 17.3 42.7 1.30
1.18
1.01-
1013
1012
102
e
4
w-
L , length of fiber per sq. cm. filter 9,800 area = 4 w / ~ p j D j ~ c m . / scm. q. LD,, projected fiber area per sq. cm. filter area, sq. cm./sq. cm. 17.0 Filter pressure drop at 80 ft./min. face velocity, inches of water 0.313
346
Urethane
ticles not collected on the plates were collected on a high efficiency afterfilter medium. The aerosol charge distribution was calculated for the known particle size from the measured particle mass distribution on the collector plates and afterfilter (72). Filter Holder. The filter medium was supported in either one of the two samplers shown in Figure 2. Air flow through each sampler could be varied while the total air flow was kept constant and equal to the particle charger air flow. A test filter was used in one sampler, while the other served for determining the inlet aerosol concentration. Test Filters. The three fibrous filters used as test media were: a ao-ounce, white, untreated, wool felt; an 80-poreper-inch expanded urethane foam; and a special filter made of silver-plated 10-micron-diameter glass fibers (70). These filters were of uniform thickness, fiber density, and fiber diameter. Physical properties of the three filters are tabulated in Table I11 together with pressure drop data. Sample Evaluation. Aerosol concentration before and after the test filter was determined by fluorometric measurement of the amount of dye collected on disks or high efficiency binderless glass paper or membrane filters. Test Procedure. The test equipment arrangement and air flow system are shown schematically in Figure 2. For tests involving neutral aerosols an aerosol charger was not used. Neither the charge neutralizer nor the aerosol charger was used for tests on naturally charged 1-micron aerosol. For each particle size and charge the particle charger was operated a t fixed air flow, voltage, current, and physical setting. Aerosol charge, Q, (based on particle mass median diameter), and charge geometric standard deviation, gBg, were measured with the aerosol charge spectrometer for each operating condition of the particle charger, for the neutralized aerosol, and for the naturally charged aerosol. Several filter efficiency tests were run on each filter for each test condition. I n a series of 10 tests, at a given test condition, the filter penetration varies from a low of 58.2Yo to a high of 62.1%. These values were typical of the reproducibility obtained for all test conditions. Tests were also run to determine any effect on filter efficiency produced by variables such as time, filter area, filter backing or support, and aerosol concentration. Aerosol losses in the filter holders and tubing, and in the charge spectrometer inlet tubing, were determined by washing out the filter holders or the tubing and evaluating the aerosol deposition fluorometrically. A correction was made for any losses which varied from about 1 to 5% of the incoming aerosol concentration.
e
4,000
12,200
17.0
12.2
0.156
0,346
l & E C PROCESS D E S I G N A N D DEVELOPMENT
Single Fiber Efficiency. Single fiber efficiency, 7, is defined as the ratio of flow stream area from which all particles are removed to the project fiber area, both areas taken perpendicular to the direction of free stream flow. Direct measurement of single fiber collection efficiency, as a function of each of the significant variables, is an ideal approach but is not practical for measurements on complete filters. Therefore, a total filter efficiency was measured and a single fiber efficiency calculated by assuming a simple filter model in which all fibers are of one diameter and are uniformly distributed in n layers perpendicular to the air flow direction. T h e fraction of aerosol removed by a unit length of fiber per unit flow area is V D ,where D, is the projected fiber diameter. Total fiber length, L , per unit filter face area was obtained from measured fiber diameter, fiber density, and filter weight per unit area (Table 111). Each of the n layers, where n is assumed to be large, contains L/n cm. of fiber. Aerosol penetration through each layer, E', can be written:
Assuming complete aerosol mixing after each of the n layers, the total filter penetration, P,can be represented by:
1.0
PLATINUM FOIL
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1
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1
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1
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2
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HIGHGOLTAGE
0 IO
CONNECTION
Figure 1 .
Aerosol particle charger
Figure filter
-
30 VELOCITY
IO00
300
100
FEET PER MINUTE
3. Measured filtration efficiency through felt test
FILTEREP AIR
I
FILTERED AIR FOR DILUTION OF AEROSOL
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1 1 1 1 1
1
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0.8
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1
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DOWNSTREAM (EFFLUENT) SAMPLER CHARGE NEUTRALIZER
0.4
0.2
SPECTROMETER
Figure 2. Schematic of test equipment arrangement and air flow system
0-
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