j. aerosol sci., - prairie swine...

J. Aerosol Sci., Vol. 23, No. 7, pp. 723-735, 1992 0021-8502/92 $5.00 + 0.00 Printed in Great Britain. ~.~ 1992 Pergamon Press Ltd

P E N E T R A T I O N A N D P R E S S U R E D R O P O F A H E P A F I L T E R D U R I N G L O A D I N G W I T H S U B M I C R O N L I Q U I D P A R T I C L E S

S. PAYET, *t D. BOULAUD, *$ G. MADELAINE* and A. RENOUX t

* Institut de Protection et de Sfiret6 Nucl6aire, D6partement de Protection de rEnvironnement et des Installations, Laboratoire de Physique et M&rologie des A6rosols, IPSN/CEA, BP 6, 92265 Fontenay-aux-Roses C6dex, France

and t Laboratoire de Physique des A6rosols et Transfert des Contaminations, Universit6 Paris XII, France

(Received 22 January 1992; and in final form 30 June 1992)

Abstract--Experiments and a model have been made of the effects of mass loading of a HEPA fiber filter during filtration of submicron liquid aerosol particles. The measurements reveal that penetration of the test medium increased during clogging by a liquid aerosol, irrespective of particle size within the chosen range (0.024).5/~m). The physicochemical properties of the test aerosol did not seem to affect this phenomenon. Application of a non-stationary filtration model by Payet [1991, Th6se, Universit6 Paris, 150 p] (based on the correlation of Liu and Rubow [1990, 5th World Filt. Congress, Nice 3, 112] showed that the increase in penetration can be explained in part by an increase in interstitial velocity and in part by a decrease in the number of fibers available for capture of particles.

INTRODUCTION

The basic parameters describing filtration are:

• pressure drop across the filter, Ap, and • penetration, P.

The behaviour of a filter over time can be divided into two phases:

• in the first phase it is assumed that changes occurring in filter structure due to deposition of particles are sufficiently negligible so that filter efficiency is unaffected. Particles making contact with a fiber are captured without modification of filtration mechanisms. During this 'stationary phase', P and Ap do not change with time;

• gradual build-up of particles at the fiber surface causes secondary effects, such as formation of aggregates with solid aerosols or bridges with liquid aerosols. This results in changes in P and Ap during this 'non-stationary phase' of filtration.

Various studies devoted to the stationary phase have given rise to several models which allow prediction of the 'instantaneous' penetration of a filter. In contrast, non-stationary filtration remains poorly understood, notably in the case of liquid where some authors (Billard et al., 1963; Mohrmann and Marchlewitz, 1974; Accomazzo et al., 1984) have reported anomalies, such as increases in filter penetration P as the pressure drop Ap across the filter rises, unlike that seen with solid aerosols. Yet the theoretical explanations of this phenomenon remain entirely qualitative.

It is important to understand liquid aerosol filtration as it is involved in several fields of study, such as filter testing with DOP (dioctyphthalate), industrial filtration, individual protection and so forth.

The aim of this study was therefore to improve understanding of the behavior of high efficiency fibrous filters, with liquid aerosols, and to try and develop a simple calculation for estimating changes in P during clogging of the filter with liquid particles. The basic model used for non-stationary filtration is the correlation of Lee and Liu (1982) and that of Liu and Rubow (1990) as improved by Payet (1991).

Author to whom correspondence should be addressed.

723

724 ;'~ ' , ~ - ~ 4~'~ ~

B A C K G R O U N D ON F I L T R A T I O N MECHANISM,3

The penetration P of the filter medium is defined by the following expression:

number of particles crossing the filter P =

number of particles incident on the filter

P is related to the overall efficiency E by the equation P = 1 - E and can be expressed as follows:

with t' = exp(-- ~/A) (1)

4~h A - zr(1-~)Df ' (2)

where t/is the single fiber efficiency, h the filter thickness, ~ the filter solidity, and Df the fiber diameter. ~ is related to the porosity e by ~ = 1 - e .

The single fiber efficiency q depends on the partial efficiencies of the different mechanisms of aerosol capture, the principal mechanisms being Brownian diffusion, interception and inertial impaction.

In the range of particle sizes studied here (smaller than 0.5 pm), Lee and Liu (1982) consider that only the mechanisms of Brownian diffusion and interception are involved in aerosol collection. These authors make the assumptions that, for a given particle size, the contribution of one of these mechanisms is predominant, and that the influence of inertial impaction is negligible. They express q as follows:

~1 =: qd + qr, (3)

where r/d and r/r are the single fiber efficiencies of diffusion and interception, respectively. In the case of Kuwabara flow, Lee and Liu (1982) give the following expressions for ~/d

and r/f:

(" 1--0~ 1/3 r/d = 1.6 \ - K u - } Pc-2/3 (4)

( 1 - ~ ) R 2 ~f=0.6 k ~ - u I + g ' (5)

where Pe is the Peclet number, R the interception parameter and Ku the Kuwabara factor. Pe and R are defined as follows:

Pe = UoDf/D (6)

R = Dp/Df, (7)

where Dp is the particle diameter, U0 the frontal velocity of filtration, and D the diffusion coefficient of the particle.

The semi-empirical correlation of Lee and Liu (1982) applies to a continuum flow regime, characterized by a negligible Knudsen number, Knf, relative to the fiber (Kne=22/Df). When the diameter of the fiber Df is of the same magnitude as the mean free path (2) of the carrier gas molecules, the so-called effect of slip flow must be taken into account in equations (4) and (5). This effect is due to the discontinuity of fluid around the fiber.

Using the work of Pich (1966) as a basis, Liu and Rubow (1990) corrected the initial model to take this restriction into account. They proposed the following expression for single fiber efficiency:

q = l . 6 k - - ~ - u j pe-2/3Cd+0.6 - ~ - -i-~Cr (8)

Non-stationary filtration 725

with

1 + 0.388 Knf ((1 - ~t) Pe'~l/3 Cd K u J (9)

1.999 Knr C, = 1 + , (10)

R

where Cd and C, represent, respectively the slip flow correction factors for Brownian diffusion and interception.

Payet (1991) noted that the value q in equation (8) may exceed unity at low Peclet numbers. She therefore modified the correlation of Liu and Rubow (1990) by including a further correction factor to bring the value of t/< 1 for very fine particles. This correction results in a modification of the efficiency of diffusion, which is then expressed as follows:

~ld=l.6[(~u)lpe-e/3cdc'd (11)

with 1

(12) 1 + 1.6 \ Ku / Pe- 2/3 Cd

Finally, t/is given by the following equation:

r/= 1"6 \--Ku-u J Pe"- 2/a +0'6 I + R ' " (13)

Pe" and R' being, respectively, the Peclet number and the interception parameter, modified as

Pc" = Pc' C~- 3/2 (14)

Pc' = PeCk- 3/2 (15)

R ,2 R 2 I + R , = I + R Cr. (16)

MATERIALS AND METHODS

Measurement of stationary penetration The measurement of penetration is performed using the experimental set-up shown in

Fig. 1. This set-up comprises two parts:

• the first designed to produce monodisperse aerosols, • the second for measurement of the particle concentration upstream and downstream from

the test filter.

To produce monodisperse aerosols in the particle size range 0.02~.5 #m, we use an electrostatic classifier, the principle of which has been established by Liu and Pui (1974). The method of electrostatic classification is based on the existence of an unambiguous relation between the electrical mobility of a charged particle and its size. By passing charged aerosol particles through an electrical mobility analyser, it is possible to extract a monodisperse fraction. This aerosol generator comprises a Collison type atomizer, a drier and a neutralizer consisting of a krypton (SSKr) source and a differential mobility analyser (DMA).

To generate a monodisperse aerosol with this system, a polydisperse aerosol is first produced using the Collison type atomizer. The droplets are then dried and the residual aerosol obtained is passed through a neutralizer where it is exposed to a aSKr source to bring the electric charge of the particles to Boltzmann equilibrium. For a submicron-sized aerosol at Boltzmann equilibrium, the particles are principally neutral or carry one or two

726 S et ~i

r

Kr 8 5 r - ~ h i ~ 4<

[ ' ....... I N l l A,#" supply sysfem ~_~A tom,zer e pump

/ ' - .... "x\ r

- E l e c f r o s f a t i c c lass ihec c

Fig. 1. Exper imenta l set-up.

positive or negative charges. This aerosol is then electrostatically classified by passage through the DMA.

The geometric standard deviation of the distribution of the aerosol produced is less than 1.2 and the precision of the value of the median diameter is estimated as 6%.

The monodisperse aerosol is again brought to Boltzmann equilibrium and is then conveyed to the test filter. The concentration (number of particles cm-3) for each size is recorded by a continuous flow condensation nucleus counter (CNC Model No. 3020, TSI Inc, St Paul, MN, U.S.A.) before and after passage of the aerosol through the filter medium. Filter penetration (ratio of the number of particles that pass through the filter to the number of incident particles) is then deduced.

Clogging of filter.['or non-stationary experiments

The concentration of the aerosol emerging from the electrical mobility analyser is not sufficiently high to allow rapid clogging of the filter and so the filter was directly clogged by the residual polydisperse aerosol which leaves the drier. The clogging velocity was equal to the frontal velocity of filtration chosen for measurement of penetration of the filter medium. Throughout the experiments, the pressure drop Ap was monitored using a linear water manometer. When the desired value of Ap was reached, the filter penetration was measured. If the filter was clogged with polydisperse aerosol, the variation in efficiency was determined using a monodisperse aerosol. The quantity of aerosol collected during the experiment was evaluated by weighing the clogged filter.

Filter characteristics

The test medium was a Pallflex filter of Teflon-coated glass fibers from the Pall Company (Pall France, St Germain, France) with the following characteristics:

• thickness h = 0.24 mm, • density m = 4 x 10 - 2 gm -2, • solidity ~t=0.08, • mean fiber diameter O f = 1 ~tm.

Df is obtained from the empirical relation of Davies (1973):

De=8 1 °h~3/2 u°] l /2 , !17

where v is the kinematic viscosii~ ~c i}tfid.


This calculation naturally only gives an apparent fiber diameter, insofar as a filter generally comprises a mixture of fibers of different diameters. The mean fiber diameter Dt calculated from equation (17) is about 1/~m, a value' confirmed by examination in an electron microscope.

Test aerosols

The aerosols used to clog the Pallflex filter were:

• organic liquids: dioctyphthalate (DOP), diethylsebacate (DES), tributyl-phosphate (TBP), • reference mineral oils C, F, 350, standardized using reference viscometers.

Table 1 summarizes the principal physicochemical properties of these oils.

E X P E R I M E N T A L R E S U L T S

Reproducibility

Experimental reproducibility was verified by measuring, under the same experimental conditions, the penetration of 10 Pallflex filters by monodisperse particles of DOP. The 95% confidence interval for particle diameters of 0.1~).3 #m is given in Table 2.

Experimental uncertainty defined as

maximum v a l u e - m i n i m u m value

mean value

was about 35%.

Effect of the nature of the aerosol

The Pallflex filter was clogged with DES particles at a velocity of 2 cms -1. Measurements of penetration were taken alternately with monodisperse 0.15/am particles of liquid DES and of solid sodium chloride up to 2.5 times the initial pressure drop across the filter (Api = 60 Pa).

For both natures there is an initial period of improved collection followed by its decline. This is a typical well-known phenomenon corresponding to the preclogging of the filter by liquid aerosol which increase the collection efficiency due to the clogging of "microleaks".

Table 1. Properties of test aerosols

Densi ty Superficial tension Viscosity at 20°C Oils (gcm- 3) (dyne cm- 1 ) (centipoises)

Organics TBP 0.98 DES 0.91 31 DOP 0.98 31

Minerals C 0.88 32 F 0.89 33 350 0.90 33

4 20 48

100 355

1020

Table 2. 95% Confidence interval for the penetration of 10 "Pallflex" filters for DOP particles (frontal velocity 3.65 cm s-~; Api = 100 Pa)

Particle diameter (#m) 0.1 0.3

95% confidence interval 8 x 10 - a - 1.85 x 1 0 - 2 1.1 x 10-3-3.3 x 10 -a

728 5~ PAvFr et al,

e,,,, 0

( I ; C~ aP

10 -~ 'l T

U o = 2 c m / s ; 6p i = 60 Pa , 1 , q

g

P

~-. NaCl - - * - - DES

f ,P

I / /

10 -2 l l , ,I I

1,0 1,5 2,0 2,5 Relative pressure drop (6p /6p i )

Fig. 2. Penetration of the "Pallflex" filter, clogged with DES, for aerosols of DES and sodium chloride.

Table 3. Variation of the relative penetration of the "Pallflex" filters, clogged with DOP, in function of the relative pressure drop (Ap/ApO for three particle diameters (frontal velocity Uo=3.65cms-~:

Api = 100 Pa)

Ap~ 1 1.5 2.0 2.5 3

0 . ~ 1 1.9 2.8 4.0 4.5 0.12 1 1.9 2.3 3.0 3.7 0.30 1 1.75 2.5 3.4

The curves in Fig. 2 show that the penetration was essentially the same whatever the test aerosol used. Consequently, for a filter clogged with liquid aerosol, the penetration of solid or liquid aerosols increases with a rise in pressure drop above 1.25 Api.

Effect of viscosity

The curves in Fig. 3 show the experimental change in spectral penetration of three Pallflex filters as a function of the increase in pressure drop when the filters are clogged with liquid particles of DOP, DES or TBP.

These curves reveal increased penetration P of the filter medium during clogging, i.e. as the pressure drop increases, in contrast to what occurs with solid aerosols. The increase in P was particularly great around the penetration maximum, which seemed to shift towards the finer particles during clogging: 0.15 #m for a clean filter and 0.12 #m for the same filter clogged at 2 Api. This suggests that the velocity increases within the filter as the pressure drop rises. Note that the variation in P as a function of Ap/Api was essentially the same for all the aerosols used to clog the filter.

The results concerning relative penetration (ratio of the penetration of the clean filter to that of the clogged filter) for three particle diameters are given in Table 3.

The same experiments were performed with mineral oils. Figure 4 presents the relative variations in maximum penetration of the Pallflex filter as a function of the increase in

Non-stat ionary filtration 729

r - o

.4- . r o

oJ r - ( lJ

CL

10 -1

10 -z

10-3

__--

[95 % Confidence interval

DOP

AP/APi=I 1.5 . . . . . 2

A P i = 1 0 0 P a Uo= 3 . 6 5 c m l s -1

/// \:

DES

E

10 -6 i I I I II[ i I I l l l l I I l l i l l l l l I I l l l l l l I I l l l l i l l

10 -2 10 -1 10 -z 10 -~ 10 -2 10 -1 1

I I I I 1 1 1 1 1

Part ic le diameter, pm

Fig. 3. Variations of the penetration of the "Pallflex" filter with particle diameters, as a function of the pressure drop for three liquid aerosols (DOP, DES and TBP).

Table 4. Relative penetration of the "Pallflex" filters clogged with DES, as a function of the relative pressure drop (Ap/Apl), for different particle diameters (frontal velocity Uo = 2 crn s - ~ ; Api = 60 Pa)

p/Api 1 2 3 4 5 8 9 10 11

0.12 1 2.5 3.9 4.8 10 17 18 17.7 20.8 0.15 1 2.2 4 4.7 8.6 13 14.6 15.5 18 0.20 1 2.0 4.4 5.1 4.0 8.8 l0 9.7 14.3 0.26 1 2.1 2.9 4.3 3.8 7.3 10.8 8.7 0.34 1 2.0 3.2 2.0 3.3 10 10.9 7.1

initial pressure drop, when the filter is clogged with oils C, F or 350. The results obtained with DOP, DES and TBP are also indicated in this figure.

It can be seen that these variations in maximum penetration are very similar, the precision of the measurement being about 35%. Approximately the same data scatter was seen for measurements at 1.5 and 3 Api.

As these various liquids have different viscosities (from 4 to 355 centipoise), it can be deduced that viscosity does not play a significant role in the increase in penetration of a filter clogged by a liquid aerosol.

Test with substantial clogging

Changes in filter penetration were monitored up to an increase in pressure drop of 11 Api, using DES particles at a face velocity of 2 cm s- 1. The measurements of relative penetration for various particle diameters are given in Table 4.

Two remarks can be made on the basis of these data:

• the increase in penetration of the filter was essentially the same for all selected particle sizes (0.12-0.34 #m) up to four times the initial pressure drop, Api. Hence, at 2 Api the

730 :'~ i > ' + ' + E T e t a/.

r~

O - °

O

?

0J o .

t o

t ~

4 -

2 -

i

, P

~ ! ~ - ~ - ] viscosity (cP}

F - ] - i T--! 3s5 V,-, ,oo

1 nno i /.8

U~, ~- 3.6;(:m/S ;/~oi = 100Pa

0

0

_ _ , 1 I I 2 3 4

Relative pressure drop, Ap/Api

Fig. 4. Relative maximum penetration of the "Pallflex" filter as a function of the relative pressure drop for different oils.

penetration was increased by a factor of 2 or 2.5, whereas the increase was 4-fold at 3 Api (except for particles larger than 0.25 #m, for which the relative penetration was lower).

• the relative penetration of the filter decreased with increasing particle size above 5 Apr A potential explanation of this effect is a rise in interstitial velocity which affects the process of aerosol capture by diffusion, ~/a, and impaction, rh. Indeed, the decrease in ~/d for the finer particles (0.12, 0.15, 0.20 #m) may explain the increase in relative penetration. The increasing contribution of ~/i for the largest particles (0,26, 0.34 #m) may slow the rise in relative penetration of the filter between 5 and 9 Ap~, and decrease it above 10 Ap~.

DISCUSSION

Background

Electron microscope observations by Liew and Conder (1985) suggest that the distribution of liquid in a filter medium essentially depends on filter solidity, ~t.

Hence, when ct is less than 0.04, the interstices between the fibers are too large to be completely filled by liquid, which therefore forms large drops (of about one hundred microns) at the intersection of two or more fibers. When ct is greater than 0.04, liquid wetting


of the fibers assumes several forms:

(i) a film uniformly coating the fibers, (ii) droplets forming liquid bridges between several fibers.

The consequences of this are:

(i) an increase in th6 apparent fiber diameter, (ii) a decrease in the number of "efficient" fibers in the filter.

As observed by Liew and Conder (1985), this last point is due to the fact that since the distance between fibers and between points of intersection is small the liquid forms irregular pools or patches spanning several fibers. In this condition a significant proportion of the smaller interstices between the fibers is filled with collected liquid and not available for gas flow. The fibers bounding theses filled interstices are not available for aerosol collection.

Furthermore, addition of liquid to the filter would decrease porosity thus resulting in an increase in interstitial velocity.

Modeling

Payet (1991) has shown that the potential increase in fiber diameter has no real effect on the single fiber efficiency. The increase in P is not due to a change in Df but rather to:

(i) the decrease in the number of "efficient" fibers in the filter, and (ii) the increase in interstitial velocity.

Another possible explanation for the decrease in collection efficiency may be the increased re-entrainment effect. With the increase in Ap, there should be a corresponding increase in the interstitial velocity which, in turn, may cause an increase in re-entrainment. This explanation was very simple to reject because when the flow through the clogged filter was free of particles, the concentration downstream the filter was zero too, and there is no evidence of the re-entrainment effect for our experimental conditions.

Furthermore, when the ratio Ap/Api reached a value of 10 the interstitial velocity increased only a factor of 2 but remains too weak (4 cm s- 1) to induce a re-entrainment effect.

So, only the two first explanations ((i) and (ii)) were retained to develop our model. To estimate the fraction of fibers still available in the clogged filter, it is assumed that all wetted fibers cease to participate in the capture of incident aerosols, due to the formation of bridges between fibers, resulting in a drop in the number of efficient fibers and hence a reduction in the effective filtration surface area.

This reduction can be calculated as follows. If F< denotes the volume of liquid retained in the filter SF the initial filtration surface area, ( the average liquid film thickness in the filter, the wetted fiber surface, S~ is given by:

s'~ = V</l.

The fraction of fiber surface rendered ineffective is

s'F/s~ = v</(s~t).

From observations using an optical microscope we can consider that the liquid is spread through all the filter by capillarity and the liquid film thickness can be taken as the same as the filter thickness, h.

S'~/SF = V</Srh.

So the new solid fraction efficient for filtration, ~', can then be expressed as follows:

~'=~(1 - ( V < / S ~ h ) ) .

In this relation (1 - Ve/SFh) represents the new fraction of surface efficient for filtration. It should be recalled that ~ represents the ratio of the fiber volume Vf to the total volume

of the filter V~.

732

The volume of liquid since:

5i. ( L ' k ~ . { f?I ~,'1

V, is determined from the mass of liquid aerosol depositee, my.

~"r ...... mo/p~, , i ~'7~

where pc denotes the mass per unit volume of the liquid constituting the aerosol. To calculate the new interstitial velocity U it is necessary to estimate the new porosity o1

the filter d, from the following equation:

~:~ = 1 - .~", i 20!

where the term :t" represents the ratio of the volume of fibers plus the volume of the deposited liquid to the total filter volume, i.e.

~ 9 + vf ~ " - (21) V~

Now, the overall penetration P of a fibrous filter in stationary filtration is expressed as follows (1) and (2).

Consequently, the penetration of the clogged filter is estimated by replacing the coefficient A in equation (2) by A':

4~t'h A' = (22)

7~(1 -- ~')Df"

Pc", in the expression of q (13), is calculated from the new interstitial velocity U:

Uo Uo U - ~' - ( 1 - ct")" (23)

e -

._o

CL

0.1

0.01

0.001

Key 6p/Api Penetration

o 1

" 1.4 exp.

o 2.4

- - I 1 1.4 calculation

2.4

U o = 3 .6Scm/s ; 6pi = lOOPa

°11" . 1 % ' ~ ~ - - - " - - - ~ ""

J 1 I I 0.1 0.2 0.3

DOP particle diameter (pm)

t q

-4

I

.4

I

Fig. 5. Comparison between calculated and experimental values of the penetration as a function of particle diameters for different relative pressure drop (Ap/Api).


Figures 5-7 illustrate the comparison of the experimental results with those obtained from the mass of liquid particles collected by the Pallflex filter, for a given change in pressure drop across the filter. We restricted the size range to 0.084).4/~m, in which Payet (1991) observed that the experimental results agree well with our stationary filtration model.

From Table 5, which gives the relation between the variation of pressure drop (Ap/Api) and the mass collected in the filter for a frontal velocity of 2 cm s-~, it is possible to represent Figs 6 and 7 as the variations of penetration as a function of collected mass by unit initial surface of filtration.

Figure 5 illustrates the calculated penetration of the medium for three pressure drops: at the initial state (V~ not measurable), at 1.4Api ( V t = 2 × 1 0 - 2 c m 3 ) , and at 2.4Api (Ve=6.5 × 10 -2 cm3). Also shown are the experimental results for the filter tested with DOP particles.

Figures 6 and 7 show the changes in penetration calculated as a function of Ap/Apl, as well as the penetration derived from experiments on filter clogging with DES particles. Experimental results presented on Figs 6 and 7 have been obtained using one filter. On these two figures it is not possible to observe the improve of the collection efficiency for a Ap/Api smaller than 1.25 (see Fig. 2). We have chosen to study the increase of the penetration for values of Ap/Ap~ corresponding to those of the Table 5, i.e. for Ap/Api greater than 2.

So, in the range of Ap/Api comprised between 1 and 2 the experimental points are binded by a dash-line.

It can be seen that the predictions of the model are satisfactory. Indeed, the calculated values of P for different particle sizes (from 0.12 to 0.34 #m) are very close to the experimental values, even for large increases in pressure drop across the filter.

Very similar agreements not reported here have been observed by Payet (1991) for particles of 0.15, 0.25 and 0.3 #m diameter.

Mass of liquid collected by unit surface (mg/cm 2)

g

0 , /

qu a .

0.1

0.01

0 t~, 6.1 7.3 8.2 8.8 9.1 t t t t T v I t I l r r I l i I u ,

DES particles (0,12 lam)

U o = 2 cm/s ; ,~pi = 50 Pa P experimentaL -1"

. s J

0.001 , I , , , I , , , I , ~ , i , , , I t a J

I 2 4 6 8 10 12

Ap/Api

Fig. 6. Comparison between calculated and experimental values of the penetration as a function of the relative pressure drop or as a function of the mass of liquid collected by unit initial surface of

filtration.

734 :'~. P,~VET et ai.

0.1

0.01

Mass of liquid collected by unit surface (mg/cm z)

6.6 6.1 7.3 8.2 8.8 91

D E S p a r t i c l e s (0.3/~ ~ t m )

U o : 2 cm/s ; Api = 50 Pa i - . - e - P experimental _~

+ P calculation

/ -~, -4

!

J

12

0 . 0 0 1 I I ~ L I I l , B J , , i I t J i I a i I

2 4 6 8 10

/~p/Api

Fig. 7. Comparison between calculated and experimental values of the penetration as a function of the relative pressure drop or as a function of the mass of liquid collected by unit initial surface of

filtration.

Table 5. Relation between the relative pressure drop Ap/Api and the mass and the volume of liquid aerosol collected in the

filter. The frontal velocity is Vo = 2 cm s -

Ap/Api too(g) V1 (cm 3 )

2.3 0.064 0.070 4 0.088 0.107 5 0.106 0.116 7.5 0.119 0.130 9 0.128 0.140

10 0.132 0.145

too: mass of liquid aerosols in the filter. VI: volume of liquid aerosols in the filter. SF: initial surface of filtration is 14.5 cm z.

In conclusion, the model, that we have presented here, provides a good understanding of the behavior of HEPA filters clogged with liquid aerosols. From simple formulae involving only the volume of liquid contained in the filter, which decreases the filtration surface area, and the filter porosity, it is possible to estimate correctly the changes in the penetration of these filters during clogging, in the particle size range of 0.08-0.4 #m. Application of this model shows that the increase in penetration can be explained in part by an increase in interstitial velocity and in part by a decrease in the number of fibers available.

C O N C L U S I O N S

This study of the behavior of HEPA fibrous filters with liquid aerosols in non-stationary filtration allows the following conclusions to be drawn.

(1) Filter penetration P increa~s as the pressure drop across the filter rises. This effect diminishes at larger particle sizes.


(2) Once the filter is clogged with l iquid aerosol, its measured penet ra t ion is the same for

l iquid and solid aerosols. (3) The viscosity of the l iquid const i tu t ing the aerosol does no t seem to influence the

changes in penetrat ion. (4) The experiments and the model of Liu and Rubow (1990), as modified by Payet

(1991), have allowed development of a simple formula which, for the mass of l iquid particles deposited on a filter, allows evalua t ion of overall filter penet ra t ion dur ing clogging. Agreement between the model and the experimental results is fully acceptable in the particle size range studied (0.08~3.4/~m) which represents the doma in of m a x i m u m penetrat ion.

Acknowledgement~The authors wish to gratefully thank Dr Tiret from the Centre d'Etudes du Bouchet who provided, financial support for this study, and Roselyne Gougeon, who helped with some experiments.

R E F E R E N C E S

Accomazzo, M. A., Rubow, K. L. and Liu, B. Y. H. (1984) Ultrahigh efficiency membrane filters for semiconductor process gases. Solid State Technol. 141.

Billard, F., Madelaine, G. and Pradel, J. (1963) Variation de l'efficacit6 des filtres en fonction de leur colmatage par divers types d'a6rosols. In Colloque sur la Pollution Radioactive des Milieux Gazeux, Saclay, 12-15 Novembre 1963. Presses Universitaires de France, Paris, 415.

Conder, J. R. and Liew, T. PI (1989) Fine mist filtration by wet filters-II. Efficiency of fibrous filters. J. Aerosol Sci. 20, 45.

Davies, C. N. (1952) The separation of airborne dust and particles. Proc. Inst. Mech. Eng. 18, 185. Lee, K. W. and Liu, B. Y. H. (1982) Theoretical study of aerosol filtration by fibrous filters. Aerosol Sci. Technol.

1, 147. Liew, T. P. and Conder, J. R. (1985) Fine mist filtration by wet filters-I. Liquid saturation and flow resistance of

fibrous filters. J. Aerosol Sci. 16, 497. Liu, B. Y. H. and Pui, D. Y. H. (1974) A submicron aerosol standard and the primary, absolute calibration of the

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