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Experimental Study of Aerosol Filtration by Fibrous FiltersK. W. Leea; B. Y. H. Liuaa Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota,

Minneapolis, MN

First published on: 23 December 1981

To cite this Article Lee, K. W. and Liu, B. Y. H.(1981) 'Experimental Study of Aerosol Filtration by Fibrous Filters', AerosolScience and Technology, 1: 1, 35 46, First published on: 23 December 1981 (iFirst)

To link to this Article: DOI: 10.1080/02786828208958577URL: http://dx.doi.org/10.1080/02786828208958577

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Experimental Study of Aerosol Filtrationby Fibrous Filters*K . W . Lee? and B . Y . H. LiuParticle Technology Laboratory, Mechanical Engineering Department, University of Minnesota,Minneapolis, MN 55455

Submicron aerosol filtration by fibrous filters has been diameter range. Filter solidity has ranged from 0.0086studied experimentally employing a filter efficiency to 0.42. Filtration velocity has been varied between 1measuring technique based on the use of moderately and 300 cm/sec. The results of the measurement havemonodisperse aerosols and an electrical aerosol de- been compared quantitatively with the available the-tector. Using this technique, the filtration efficiencies of ories. It has been found that theories taking into accountfilters made of uniformly sized fibers have been the interference effect of neighboring fibers are inmeasured by the use of particles in the 0.03.5-1.3 pm reasonable agreement with the experiments.

N O M E N C L A T U R Edr ag coefficientconstantfiber diameter (cm)particle diameter (cm or pm)constantefficiency of filter matefficiency due to filter holderefficiency due to filter plus filter holderfilter mat thickness (cm)pressure drop across filter (in. H,O o rdyn /cm2)Peclet num ber= rD ,/Dinterception parameter, diameter ratio ofparticle to fiber. o r dimensionless particleradius= D , l D ,-

* This paper ia based on the thesis of K . W. Lee m partialfulfillment of the requirements for the P h. D . Degree at the Universityof Minnesota.

t Present address: Bat te l le Memoria l Inst~tute ,ColumbusLabora tor ies , Columbus. OH 4320 1.

Reynolds number=PBUD /Pface velocity, undisturbed air velocity(cm /sec)average air velocity inside filter (cm/sec)=U,/(1 - )filter soliditysingle fiber efficiencyair viscosity (poise)stream function param eter= c/(l - a)@]air density (g/cm3)dimensionless fiber drag param eter= [R/( 1- )4n]dimensionless fiber dra g param eter=C, Re12

I N T R O D U C T I O NAmong various ways to remove particles fromair, filtration using fibrous filters is relativelyinexpensive and simple to implement, yet itprovides a most efficient means for collectingsubmicron particles. Because of the increasing

Aerosol Science and Technology 1: 35 46 (1982)1982 Elsevier Science Publishing Co., Inc.

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36 K . W . LeeandB. Y . H. Liu

need to protect human health and valuabledevices from exposure to fine particles, filtrationis becoming more important.The important mechanical mechanisms caus-ing particle deposition in fibrous filters arediffusion, interception, and inertial impaction. Inaddition, electrostatic forces and gravitationalsettling may contribute to particle collection in afibrous medium.During the past 10 years, considerable prog-ress has been made in air filtration studies.Improved theories have been developed usingmore reliable and exact flow fields (Spielman

and Goren, 1968; Stechkina et al., 1969;Dawson, 1969; Harrop and Stenhouse, 1969;Yeh and Liu, 1974).The corresponding progressin experimental studies, however, has been slowprimarily because of a lack of suitable means formeasuring filter efficiencies over the broad rangeof conditions that are encountered in practice.In this study a new technique based on certiannew instrumentation developments at theParticle Technology Laboratory, University ofMinnesota, was used for measuring fibrous filterefficiencies. This technique enables filter ef-ficiencies to be measured quickly and accuratelyover a wide range of conditions. Filters withsolidity ranging from 0.0086 to 0.42 and madefrom uniformly sized Dacron fibers were tested.The measurements were made for di-octyle

FIGURE 1. Schematic diagram of filter efficiencymeasurement system.

phthalate (DOP) particles in a wide size rangeand for filtration velocities up to 300 cm/sec.The results of the filter efficiency measurementsare given as a function of particle size andfiltration velocity and as a function of filtersolidity. A series of comparisons between theexperimental results and existing theories aremade.

EXPERIMENTAL APPARATUSAND TECHNIQUEFigure 1 is a schematic diagram of the systemused for filter efficiency measurement. Thesystem is comprised of a condensation aerosolgenerator, a filter holder, and an electricalaerosol detector. The different parts of thesystem will be described separately.Aerosol Generation and Detec tion

E X C E S S P I N C H C L A M P S

The condensation aerosol generator previouslyreported was used to produce monodisperseaerosols (Liu and Lee, 1975). In this generator alow vapor pressure substance such as D O P isdissolved in a volatile solvent, such as alcohol,and the solution is atomized. The polydisperseaerosols initially produced are then heated andvaporzied. The vapor subsequently cools andcondenses to form a monodisperse aerosol. Theaerosol size is easily varied by varying theconcentration of D OP in the solution. Thismethod of aerosol generation had been usedpreviously (Liu et al. 1966; Tomaides et al. 1971).However, certain important improvements were

AEROSOL+ l i ELECTRICALAEROSOLCONDENSATION

- DETECTOR-

A E R O S O LG E N E R A T O R( 1 ) (2 ) *5)(4) FILTERH O L D E R

ROTAMETER

-Kr -85 NEUTRALIZER

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Experimental Study of Aerosol Filtration 37

made; the output of the present generator is verystable with a concentration fluctuation of lessthan 2.5:/, over a 1 hr period. In filter efficiencytests it is necessary to measure the aerosolconcentrations upstream and downstream ofthe filter as a function of particle size. Since it isdesirable that generator output remain constantduring a series of these time delays, the cap-ability of producing a highly stable output is animportant requirement for an aerosol generatorin filter efficiency measurements.

In order to avoid unwanted electrostaticeffects, electrical charges on the aerosols wereneutralized by exposing the aerosols to a cloudof bipolar ions produced by a radioactivesource. The 2 mCi 85kryptonsource is placedinside an aluminum tube and the aerosol ispassed through the tube. The particles are thusbrought to a state of Boltzmann charge equilib-rium. The size distribution of aerosols producedby the generator will be discussed later.To determine the aerosol concentrations up-stream and downstream of the filter, an electricalaerosol detector similar to the unit now com-mercially available (Model 3010, Thermo-Systems, Inc., St. Paul, MN 55108) was used.This instrument is a simplified version of thepreviously reported electrical aerosol analyzer(Liu and Pui, 1975); its two essential parts are adiffusion charger and an electrometer currentsensor. The aerosol is detected by first exposingthe particles to unipolar ions in the diffusioncharger and then measuring the charge on theparticles with the electrometer current sensor.Adetailed description of the instrument has al-ready been published (Liu and Lee, 1975).One of the main advantages of using theelectrical aerosol detector in the filter efficiencytests is that it allows the particle concentrationto be measured over a wide range of flow rates.The operating pressure can also be varied over awide range of values. In filtration efficiencystudies only the relative aerosol concentrationsupstream and downstream of the filter need tobe measured, and the absolute aerosol concen-tration need not be known. The electricalaerosol detector is particularly suited to thispurpose because it is basically a relative concen-

tration measuring device, and the measurementcan be made easily, quickly, and with goodaccuracy.

Filters TestedTwo different types of filters were used in thisstudy. These filters were made specifically forthis study and had fibers that were very uniformin size. They were intended as ideal filters so thatthe measurement results could be comparedwith the theories.The first type of filter, which will be denotedDacron filter A, is a filter consisting of uniform11.0 pm diameter Dacron fibers loosely packedtogether to form a filter mat. By compressing thefibers, the filter solidity can be changed. The filtersolidity was varied by compression between0.0086 and 0.299.The mean fiber diameter of 11.0 pm wasdetermined by an optical microscope equippedwith image-splitting eyepiece attachment thathad been first calibrated against a stage micro-meter. The geometrical standard deviation ofthe fiber was found to be 1.06.The homogeneity of the filter was confirmedas follows. First, several batches of filters withthe same nominal solidity and dimensions wereprepared and the pressure drop across each filterwas measured as a function of velocity. Theresults were compared. All of the filters werefound to have nearly identical pressure dropcharacteristics. It was concluded that the filterwas the same.The filter dimensions used in thisstudy are listed in Table 1.The second filter, Dacron filter B, was also aspecially prepared filter made of uniformDacron fibers of 12.9 pm in diameter. Thegeometric standard deviation of the fibers wasfound to be 1.06. The particular filter containsabout 10% by volume of small "fibrids" thatwere used to make the fibers adhere to oneanother to form a high density mat. The proper-ties of this filter are shown in Table 1. The actualsolidity of Dacron filter B was varied between0.0963 and 0.42. The filter thickness ranged from0.1 to 0.037 cm. The filter structure is not ashomogeneous as Dacron filter A.

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38 K . W. Lee and B . Y . H. Liu

T AB L E 1. Dime nsions and Pressure D rop C haracter is t ics of Expe r ime ntal Fi l tersFilter Dimensionless FiberSolidity thickness fiber drag param eter diameter

Fil ter 01 (cm) Re C D , / ~ (ium)Dacro n fi l ter A

Dacron fi l ter B

Filter HolderTh ree different filter holders were used, dep end-ing upo n the particular filter being studied or th eflow rate used. For Dacron filter A the holderdescribed by Yeh (1972) was used. T his par-ticular holde r can hold filters with a thickness ofup to abo ut 3 cm. However, some modificationswere necessary w hich will be described later. Fo rDacron filter B a 25-mm-diam Millipore filterholder (ModelXX 5002500) an d a 3.5-in.-diamSierra filter holder (Model 710) were used.

FIGURE 2. Schematic diagrams of (a) the con-ventional filter holder, and (b) the present filterholder design.

Fil te r Suppor t Sc ree nsFilter Holder Outer Housing1 ,

Fi l ter F(a )

Modifications t o the filter holders were m ad eto avoid the uncertainty in filtration velocitycaused by a po or ho lder design. Th e problem isillustrated in Figure 2a. It is seen that the flowcan ex pand radially as it passes thro ugh the filtermedium, causing a reduction in the filtrationvelocity. This problem is particularly seriouswhen the dimension L in Figure 2a is not smallin comparison with R , - , . This problem wasnoted by Da ws on (1969) in com parin g histheory with the experimental results of Wongand Johnstone (1953);he found he had t o make acorrection in the indicated face velocity. Therewas nearly a factor of 2 uncertainty in thefiltration efficiency caused by the uncertainty inflow.

Filter HolderInner HousingI /

7 b e r s \(b) Fi lt e r Suppor t Sc re en s

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Experimental Study of A erosol Filtration 39

To eliminate this uncertainty, the holderdesign shown in Figure 2b was adopted. A filterholding ring was inserted in the holder, therebyeliminating the uncertainty in flow. In actualuse, the filter fibers were first packed into thisring and the two supporting screens were at-tached to both sides of the ring with screws.The entire assembly was then inserted into theholder housing. Rings of different thickness hadto be used to create filter mats of dfie ren t thick-ness. This new filter holder design had theadded advantage over the conventional arrange-ment of allowing the filter thickness to be deter-mined very accurately.

Filter Efficiency MeasurementIn generating monodisperse aerosols with thegenerator described previously, a series of stan -dard solutions was prepared to produce aer-osols of the desired particle size. A total of 11different solutions were used to produce aero-sols 0.035- 1.3 pm diameter. Table 2 summarizesthe particle sizes chosen, the solution concentra-tions used, and the geometrical standard devi-ation of the aerosol produced.The op erating procedure for each experimentconsists of first placing the filter medium in the

TABLE 2. List of Particle Sizes Usedin the Eff iciency Measuremen t and t he DO PConcen t ra t ions Used fo r Genera t ingth e Aerosols

NominalDOP Nominal geometrical

concentrat ion mean part icle standard(vol. %) size (pm ) deviation up

desired filter holder, bringing the aerosol genera-tor to a steady operating condition, and thenmeasuring the aerosol concen trations upstreamand downstream of the filter with the electricalaerosol detector. The last step was accomplishedby directing the aerosol into the electricalaerosol detector either through the filter orthrough a bypass. The ratio of the downstreamto upstream aerosol concentration was thentaken as the filter penetration P, from which thefilter efficiency E= 1-P was then calculated.Owing to particle loss in the electrical aerosoldetec tor, it was necessary to restrict the flow ra tethrough the electrical aerosol detector (Q, inFigure 1) to less than a certain value. It wasfound tha t a t higher flow rates the loss could besubstan tial and the measurement result could bein error. A simple experiment showed that themeasured efficiency of a filter decreased withincreasing flow rate throug h the electrical detec-tor. The appareht dr op in the efficiency is causedby particle loss by impaction in the in strume nt.Consequently, in performing the filter efficiencymeasurements, the flow rate of the electricaldetector was kept at 4 l/min or below, which wasfound to be satisfactory.Since the aerosol passing through the 85Krneutralizer was not completely neutralized andwas only brought to a state of charge equilib-rium given by the Boltzmann's law , som e experi-ments were perform ed to determ ine the effect ofthe residual particle charge on the measuredefficiencies. This was done in a way similar totha t used previously by L iu and Lee (1976).Theefficiency was first measured with the chargeequilibrated aerosol and then with the aerosolthat had first been passed through a precipitat-ing electric field to remove the charged particles.The same result was obtained, indicating anegligible influence of the charge particles.Since the filter efficiency measured by theprocedure described here was the total efficiencywith the filter placed in the holder, it wasnecessary to correct for the contribution of theholder to the total efficiency measured. T o m akethis correction, and also to account for thedifference n ae rosol losses in the filter line and inthe bypass line, a separate series of measure-

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K . W. LeeandB. Y. H. Liu

ments was made in which the efficiency of anempty filter holder was measured using the sameapparatus and the same procedure.In order to verify a filtration theory com-pletely, it is necessary to have data not only onthe filtration efficiency but also on the filterpressure drop. The filter pressure drop wasmeasured across pressure taps installed on theupstream and downstream sides of the filterholder.A micromanometer was used to measurefilter pressure drop to a precision of 0.001 in.H 2 0 or pressure drops of up to 2 in. H 2 0 .Forpressure drops above 2 in.H,O, a conventional

inclined manometer was used. The pressuredrop across the filter was obtained by subtract-ing the measured pressure drop across theempty filter from the measured pressure dropacross the entire holder plus filter assembly.

RESULTSResults for Dacron Filter ATable 1contains a summary of the filter dimen-sions used and the filter pressure drop charac-teristics. The dimensionlessfiber drag parameterbased on a face velocity of 5 cm/sec is plottedagainst the filter solidity in Figure 3, where theparameter is obtained from pressure dropA P bythe following equation:

whereR is the dimensionless fiber dragparameter,C,, the drag coefficient,Re the Reynolds number,AP the pressure drop across the filter mat,L the filter mat thickness,a the filter solidity,D, the filter fiber diameter,U , the face velocity, andp the air viscosity.

It was also found that the dimensionless fiberdrag parameter remains constant for a facevelocity of up to 30 cm/sec.

WE SOLIDITY , QnFIGURE 3. Dimensionless fiber drag parameter forDacron filters A ( 0 ) nd B (0)s a function ofsolidity.

The results of the filter efficiency measure-ments are shown in Figures 4--9.If we denote thetrue filter efficiency by E, the filtration efficiencydue to the filter holder by E,, and the totalFIGURE 4. Single fiber efficiency of Dacron filterA (a = 0.0086).

PARTICLE DIAMETER , p m

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Experimental Study of Aerosol Filtration 41

PARTICLE DIAMETER , pm0001

FIGURE 5. Single fiber efficiency of Dacron filter FIGURE 7. Single fiber efficiency of Dacron filterA ( a = 0.0168). A ( a = 0.0823).

II

' ' ~ l l ~ ~ ~1 1 1 l l 1

eficiency of the filter plus the holder by E,, we The measurement results reported have all beenhave corrected by means of Eq. (2).E [ = l - ( l - E ) ( l - E h ) , In order to obtain the single-fiber eficiency q,the following relation was used:

002 0 1 2PARTICLE DIAMETER , p m

from which we haveE = ( E t - E h ) / ( l - E h ) .FIGURE 6. Single fiber efficiency of Dacron filterA (a = 0.0434).

FIGURE 8. Single fiber efficiency of Dacron filterA (a = 0.151).

-PARTICLE DIAMETER , prn PARTICLE DIAMETER , prn

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42 K . W. Lee andB. Y . H. Liu

FIGURE 9. Single fiber efficiency of Dacron filterA (a = 0.299).

The resulting single-fiber efficiency is plotted as afunction of particle size for various filter solidityvalues, as seen in Figures 4-9. Deta iled efficiencyvalues in tabular form have been reportedelsewhere (Lee, 1977). It is seen tha t in all casesthe curves are similar in shape: For smallparticles, the efficiency first decreases with in-creasing particle size, showing the predomin-ance of the diffusion mechanism. After reachinga minimum, the efficiency then increases withincreasing particle size, show ing th e increasinglyimportant role of the interception and impac-tion m echanism s. These results will be discussedfurther when comparison with the theories ismade.Results for Dacron Filter BThe pressure dro p characteristics from D acronfilter B are shown in Table 1 and Figure 3. Theefficiency results covering the solidity range of0.0964.420 are shown in Figures 1G13. Forthese filters, the face velocity ranged from 1 to300 cm/sec.COMPARISON WITH EXISTING THEORIESA nu mb er of filtraton theories are available thatincorporate various filtration mechanisms andvarious flow fields. Most of these theories are

D1 = 12.9 p ma = 009630.1 1 2

PARTICLE DIAMETER , p mFIGURE 10. Single fiber efficiency of Dacron filterB ( a = 0.0963).based on the assumption that the filter is madeof uniform circular cylinders and the particlesare monodisperse. Since the present experi-me ntal da ta can m eet this assump tion, a series ofcomparisons was made between the data andFIGURE 11 . Single fiber efficiency of Dacron filterB (a = 0.175).

DACRON FILTER BDf = 12.9 p ma = 0.175

0.0010.02 01 1 2PARTICLE DIAMETER , pin

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Expe rimental Study of Aerosol Filtration 43

a3

0.1

GZwuLLWuWm 0.01LLW-1azV)

DACRON FILTER Bto o 100

0 30 + 300

0 . 0 O l O.02 0.1 1 2PARTICLE DIAMETER , p mFIGURE 12. Single fiber efficiency of D acron filterB ( a = 0.271).the existing theories. The theories comparedinclude those of Spieln~anand Goren (1968),Dawson (1969), Harrop and Stenhouse (1969),Stechkina et al. (1969), and Yeh and Liu (1974).These theories take into account the interferenceFIGURE 13. Single fiber efficiency of Dacron filterB ( a = 0.420).

DACRON FILTER B- A 10 O 100 Df= 12.9 prn -O 30 + 300 a = 0420

0.001 1 1 1 1 1 1 1 I 1 1 1 1 1 1 10.02 0.1 1 2PARTICLE DIAMETER , pm

effects of neighboring fibers. Additional resultsof the comparisons that are not reported herecan be found in Lee (1977).Figure 14 is a comparison of the experimentalresults with the theory of Spielman and Goren(1968). The measurement results for filter soli-dities of 0.0086, 0.0434, and 0.1513 are used inthis comparison. First, the pressure drop of thefilter with various solidities was compared withthe theoretical predictions. The comparisonshowed that Dacron filters can be well rep-resented by the result for the Spielman-Gorentwo-dimensional random fiber model. The

stream function parameters as defined in thetheory for the relevant filter solidities were foundto be 2.479, 1.523, and 0.812, respectively. Usingthese values, the theoretical efficiencies werecalculated. The comparison shows that theagreement between the theory and the experi-ment is very good for < '13RPe''3> 1. However,FIGURE 14. Correlation of filtration data using thetheory of Spielman and Goren (1968).

a oooal o o ~ 0 4 3 4{iH:i:523

re o 008

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44 K. W.LeeandB. Y. H. Liu

the theory tends to predict higher efficiencyvalues for small values of < - ' I3RPe 13.The theories of Harrop (1969) and Harropand Stenhouse (1969)are for filtration by impac-tion and interception and use the Happel flowfield (Happel 1959).Figure 15 is a comparison ofthese theories with the experimental data. Inorder to provide experimental data in theimpaction and interception regimes, the resultsfor Dacron filter B with a solidity of 0.0963 andvelocities of 100 and 300 cm/sec are used. Thepressure drop predicted by the Happel flow wascompared with the measured pressure drop toarrive at an empirical factor to account for theinhomogeneity in fiber distribution. An in-homogeneity factor of 1.72 was found.Figure 15 shows that the theory overestimatesthe efficiency at high Stokes numbers. At lowStokes numbers it is not clear whether thetheory would still overestimate the efficiency,FIGURE 15. Comparison of data with the theoriesof Harrop (1969) ( and Dawson (1969) (---)in inertial impaction regime.

/-0 001 I I I I l l l l l 1 1 1 1 1 1 1 1

0.1 I 10S tokes Number, Stk

since at small Stokes numbers the experimentaldata may be influenced by diffusion effects. Itshould be noted that while the theory alwayspredicts a higher efficiency for a larger particlesize at the same Stokes number, the experi-mental data show that smaller particles can havea higher efficiency at the same Stokes numberunder some instances. For example, the ef-ficiency for the 0.7 pm particle with Stokesnumber of 0.925 is higher than that for the 1.3pm particle with Stokes number of 0.971. Theefficiency for the 0.5 pm particle with Stokesnumber 0.509 is also higher than that for the 1.0pm particle with Stokes number of 0.594. Apossible explanation for this would be as fol-lows. As may be noted in Figure 15, theReynolds number involved in the filtration for0.7 and 0.5 pm size particles in the example is2.82, while that for 1.3 and 1pm particles is 0.94.The filtration efficiency at such a high Reynoldsnumber should increase as an effect of increasingthe fluid inertia, and it is possible that this issufficiently significant that even small particlesat a comparable Stokes number will have ahigher efficiency than large particles. The theoryof Harrop does not account for the effect of anincreasing Reynolds number. Therefore, it isdifficult to determine the quantitative Reynoldsnumber dependence of efficiency.The theories of Stechkina et al. (1969)andYehand Liu (1974) involve calculations of the fil-tration efficiency for all three mechanisms. Acomparison of these theories with the measure-ments obtained in the present study is shown inFigure 16 for Dacron filter A and a solidity of0.151. By comparing the measured pressuredrop and the predicted pressure drop, an in-homogeneity factor of 1.52 and an effective fiberlength factor of 2.62 were determined. Thecomparisons show that the overall agreementbetween the theories and the experiment is verygood. However, the theory of Stechkina et al.(1969) shows some discrepancies at low filtrationvelocities. The inertial impaction efficiency pre-dicted by the theory is also found to be too high.Regarding the impaction efficiency, Stechkina etal. (1970) later stated that the assumptions theyemployed in the theory for impaction were not

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ExperimentalStudy of Aerosol Filtration

FIGURE 16. Comparison of data with thetheories of Stechkina et al. (1969) (- - )and of Yeh and Liu (1974) (-) forDacron filter A ( a = 0.151).

adequate. On the other hand, the agreementbetween the experimental data and the theory ofYeh and Liu (1974) is good at low filtrationvelocities. As velocity increases, the theorybegins to underestimate the efficiency some-what, and the discrepancy becomes larger as thevelocity is increased. It is also seen that theparticle size for greatest penetration predictedby the theory is in good agreement with thedata.

Figure 17 is a comparison of the experimentaldata with the theory of Dawson (1969). Thecomparison is made for a Dacron filter ofsolidity 0.0434 at 30 cm/sec. Dawson introduceda model of parallel flow to account for thenonuniformity of filter. By comparing the pre-dicted pressure drop and measurement, theresistivity ratio defined by the theory was foundto be 0.62 for a Dacron filter having a solidity of0.0434. The comparison shows that the theory isin good agreement with the result of the measur-ement. The theory of Dawson (1969) on diffu-sion and interception is based on the expressionof Stechkina et al. (1969), except that Dawsonused the flow field of Spielman and Goren (1968)and employed the parallel flow model to ac-count for nonhomogeneity of the filter. For this

\ Dacron F i l ter A U,,crn/sec i

P a r t ~ c l e D ia m et er , p m

regime, good agreement with experiment, com-parable to that obtained with the theories ofStechkina et al. and Yeh and Liu, was obtained.For comparison in the region of impaction plusinterception, Dacron filter B with a solidity ofFIGURE 17 . Comparison of data with the theory ofDawson (1969) for Dacron filter A ( a = 0.0434).Uo= 30 cm/sec.

PARTICLE DIAMETER , p m

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46 K. W. Lee a n d B . Y. H. iu

0.0963 was chosen, and the efficiency valuesmeasured at the face velocity of 100 and 300cm/sec were compared with the theoreticalvalues. Figure 15 shows that the experimentaldata are in good agreement with theory.Compared with the efficiencies predicted byHarrop (1969), he efficiencies given by Dawsonare generally lower for Stokes number largerthan 1. The possible effect of increasingReynolds number on filtration by impactionremains unconfirmed.

CONCLUSIONSThe real-time experimental technique describedin the present study has been found useful formeasuring the filtration efficiencies of fibrousfilters. This technique makes it possible todetermine the filter efficiency quickly and ac-curately over wide ranges of particle size andfiltration flow rates.

The measurement results show clearly thedependence of the filter efficiency on particlesize, filtration velocity and filter solidity. Theresults also show that for a given filter at a fixedvelocity there is a particle size at which the filterefficiency is a minimum and the particle penet-ration maximum. The most occursat a particle size of 1 pm for a flow velocity of 1cm/sec, decreasing to about 0.03 pm for avelocity of 300 cm/sec. The corresponding mi-nimum efficiency decreases with increasingvelocity.The comparison of the experimental resultswith the existing filtration theories shows thatamong the theories which take into account theinterference effects of neighboring fibers, those ofSpielman and Goren (1968), Dawson (1969),Stechkina et al. (1969), and Yeh and Liu (1974)are generally in good agreement with measureddata.

This research was financially supported by the U . S.Envir onm ental Protection Agency under Research Grant No.R804600-01.

REFERENCESDawson, S . V . (1969). Theory of collection of airborneparticles by fibrous filters, Ph.D. Thesis, The HarvardSchoo l of Public Health, Boston, M A.Happel, J. (1959). AIChEJ. 5:174.Harro p, J . A. ( 1969). The effect of fiber configuration on theefficiency of aerosol filtration, Thesis, LoughboroughUniversity of Technology, England.Harrop, J. A ., and Stenhouse, J. I . T. (1969). Chem . Eng.Sci. 24: 1475.Lee, K. W . (1977). Filtration of submicron aerosols byfibrous filters, Ph.D. Thesis, University of Minnesota,Minneapolis.Liu , B. Y. H .,andLee, K. W. (1975). Am. Ind. H y g . Assoc.

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University of Illinois; Rep. 11, U.S. Atomic EnergyCommission Rep. 000-1012.Yeh, H. C . (1972). A fundamental study of aerosol filtrationby fibrous filters, Ph.D. Thesis, University of Min-nesota, Minneapolis.Yeh, H. C ., and Liu, B. Y. H. ( 1974).J. AerosolSci. 5: 191 .

Received 8 Decem ber 1981; accepted 23 December1981 .

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Experimental Study of Aerosol Filtration_pap