IndianJournalof Fibre& TextileResearchVol.22, September1997,pp. 163-168

Some critical appraisal of particle size distribution on woven filter fabric

A Mukhopadhyay,K N Chatterjee& A M SinghTheTechnologicalInstituteof TextileandSciences,Bhiwani127021, India

Received 19 February 1996; accepted 9 April 1996

Various ways of characterising the captured particle by filter fabric of different constructions arepresented for plain, twill and satin cloth. With increase in filtration time, the volume and surface meandiameter decrease gradually for normal and below 53p dust. With the increase in picks/em for plain,twill and satin, the volume mean diameter and surface mean diameter show a reduction. Twill weaveshows maximum volume and surface mean diameter followed by satin and plain of same cover atdifferent durations of filtration. 90% volume percentile [D(V,0.9)] and 50% volume percentile[D(V,0.5)] decrease, whereas 10 % volume percentile [D(V,O.l)] increases with the increase in picks/ernfor all types of fabric. Plain fabric of same cover shows maximum value of [D(V,0.9)], [D(V,0.5)] and[D(V,O.l)]. During filtration of dust of particle size <53p, decrease in [D(V,0.9)] and [D(V,0.5)] is lessas compared to that in case of normal cement dust. The captured particle characteristics are influencedby unstable cake formation as confirmed by Malvern particle size analyser.

Keywords: Fabric construction, Filtration time, Surface mean diameter, Volume mean diameter,Volume percentile, Woven filter fabric

1 IntroductionThe efficiency of a filter is a measure of its

ability to collect and retain particles whileallowing the fluid carrying the particles to passthrough. The three most important mechanismsinvolved in capturing and retaining airborneparticles are inertial impaction, direct interceptionand brownian diffusion 1-4. Other mechanisms forcapturing and retaining airborne particles aresieving or straining, gravitational settling, andelectrostatic capture", Changing particle size has adramatic effect on the efficiency of an aerosolfilter. The diffusion mechanism predominates forsmall particle sizes, interception for intermediateparticle sizes and the inertial for large particlesizes. Particle penetration through a fabric firstincreases with increasing particle size, as thediffusion mechanism becomes less effective, andthen decreases because the interception andimpaction mechanisms become more effective asthe particle size progressively increases'. Fabricconstruction also exerts strong influence onparticle capturing mechanism. However, very littleknowledge exists on the relationship betweenwoven fabric design parameters and their capturedparticle characteristics

This paper aims at developing an understandingof different parameters characterising dustparticles captured by filter fabric of differentdesign parameters.

2 Materials and Methods

2.1 Materials

Nine fabrics with varymg constructionparameters (Table 1) were prepared on a 36 in.wide Cimmco automatic 100m.

2.2 Methods

Cement dust of particle size below l18j.J and ofparticle size below 53p were used for studying thefiltration characteristics.

The filter fabric testing apparatus used is shownin Fig. 1. It consists of dust feeder, filter fabricholder, orifice meter, manometers across the fabricand orifice meter, thimble, suction pump andcontrol valves. The dust feeder feeds the dust at therate of 0.5 g/min.

The dust was fed for 1 min, 3 min and 5 mindurations. For measuring the filtration efficiency,both fabric and thimble weight were taken (afterkeeping in desiccator) at fixed time intervals.

164 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1997

Table 1- Details offabric samples

[Warp (polyester-viscose, 65:35) count, 118 tex; Weft (jute) count, 138 tex]

Sample Weave Ends/em Picks/em Fabric Fabric Fractional fabriccode weight thickness cover (calculated)

g/m? mm

FI Plain 12 6 270 1.05 0.68F2 Plain 12 9 287 1.08 0.77F3 Plain 12 12 353 1.16 0.87F4 212 Twill 16 6 358 1.58 0.78F5 2/2 Twill 16 9 372 1.62 0.84F6 2/2 Twill 16 12 410 1.67 0.90F7 5 end Satin 19 6 378 2.03 0.86F8 5 end Satin 19 9 422 2.13 0.90F9 5 end Satin 19 12 468 2.15 0.94

r;=~~~=~~~~~~~~~OMPRESSOR

MANOMETER

THIMBLEHOLDER

VALVE

~==============~~~==~~r1 PUMP

ORIFICE METER

Fig. I-Experimental set-up for dust characterization

Filtration efficiency (%)

Wt. of dust collected by fabric x 100Total wt. of dust collected by both filtersDuring filtration, 120 Llmin air flow rate was

maintained. Area of test specimen was 28.3 em",The particles retained by fabric filter were

analysed by Malvern Particle SIze Analyser (Series2600). The particle size was measured by the laserdiffraction technique which generates importantparameters as

Volume mean diameter [D(4,3)]Surface mean diameter [D(3,2)]90% Volume percentile [D(V,0.9)]10% Volume percentile [D(V,O.I)]50% Volume percentile [D(V,0.5)]Span defined as (D90-DIO)/D50

For the measurement of particles retained byfabric, 15 specimens were taken from each fabric

Table 2- Particle size characteristicsCharacteristic Particle size

<118,u <53,u

D (4,3) 57.87 31.98D (3,2) 39.59 26.30D (V,0.9) 114.23 48.31D(V,O.I) 25.64 22.32D (V,0.5) 53.37 32.63(D90-DIO)/050 1.81 0.91

and the average of the results is presented. Thecharacteristics of cement dust particles under testare given in Table 2. Propanol-2 was used asdispersant for the cement dust.

3 Results and DiscussionThe behaviour of air permeability and filtration

efficiency with pressure drop for different fabricsamples is shown in Table 3. Most of the abovecharacteristics can be explained from the basicfiltration theory.

3.1 Effect of Filtration TimeIt is observed from Table 4 that with the

increase in filtration time, both volume meandiameter and surface mean diameter decrease fornormal dust and dust of particle size below 53f.1.This is due to the fact that during filtration ofparticles, gradually smaller particles are capturedand at the same time a fraction of larger particlespasses through the fabric pores (ascertained fromthe. Malvern Instrument chart). The passingthrough of the larger particles may be due to theinstable cake formation. As the filtration proceeds,

MUKHOPADHYAY et al.: PARTICLE SIZE DISTRIBUTION ON WOVEN FILTER FABRIC 165

Table 3-Effect of fabric constructional parameters on filtration characteristics

Sample Air Filtration efficiency (%)/ Pressure drop' (cm-WG)code permeability Normal dust Dust of particle size below 53,u

(cc/crrr/s) I min 3 min 5min 1 min 3min 5minat

10mm-WGpressure head

FI 142.9 85.77 87.32 88.12 82.72 83.95 84.94(0.8) ( \.8) (2.2) (0.6) (1.1) (\,7)

F2 80.3 86.70 88.52 89.61 84.36 85.62 86.16(\,2) (2.0) (2.4) (\.0) (1.3) (\.9)

F3 28.3 89.86 91.57 93.56 85.89 86.29 86.53(\,4) (2.2) (2.6) (\,2) (\.6) (2.0)

F4 123.6 89.92 90.87 91.26 84.67 85.32 85.92(1.4) (2.0) (2.4) (1.3) (\.8) (2.2)

F5 72.3 90.89 91.78 92.73 85.38 85.89 86.16(\.6) (2.2) (2.5) (\.5) (2.0) (2.4)

F6 20.4 9\,46 92.61 93.71 85.89 86.12 86.47(\.8) (2.3) (2.6) (1.8) (2.2) (2.5)

F7 \02.8 90.17 92.04 93.36 84.21 85.63 86.32(\.5) (\.9) (2.4) (\.5) (\.9) (2.3)

F8 67.4 91.68 92.87 94.02 84.99 86.07 86.78(\.8) (2.2) (2.5) (\,7) (2.0) (2.4)

F9 16.3 92.46 93.98 94.59 86.63 87.38 88.01(2.0) (2.4) (2.7) (\.8) (2.3) (2.5)

a Corresponding pressure drop values are given in parentheses below filtration efficiency values

Table 4--Effect of fabric construction on volume mean diameter and surface area mean diameter

Sample Volume mean diam. (Surface mean diam.)code Normal dust Dust of particle size below 53,u

1 min 3 min 5 min I min 3 min 5 min

FI 43.72 39.84 28.20 37.92 36.01 27.08(23.19) (22.97) (19.83) (31.30) (28.74) (22.19)

F2 40.16 30.95 26.08 36.02 32.61 26.87(23.05) (2\.61) (18.53) (30.55) (26.74) (22.08)

F3 35.69 22.71 18.60 35.48 31.82 22.92(22.42) (16.35) (13.87) (28.93) (21.39) (18.96)

F4 42.85 36.61 24.32 42.73 38.38 35.33(26.37) (24.32) (22.89) (36.95) (28.85) (27.37)

Contd.

166 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1997

Table 4--Effect of fabric construction on volume mean diameter and surface area mean diameter -Contd

Sample Volume mean diam. (Surface mean diam.)code Normal dust Dust ofpartic1e size below S3J.l

I min 3 min Smin I min 3 min S min

F5 31.80 28.85 23.86 38.11 28.33 27.89(2S.49) (23.30) (22.IS) (28.80) (26.03) (18.11)

F6 30.20 28.33 21.21 35.60 22.11 18.12(24.33) (17.05) (16.01) (22.7S) (17.40) (16.98)

F7 36.72 26.60 22.17 37.04 30.69 28.05(24.19) (23.46) (20.13) (26.25) (25.13) (2S.09)

F8 28.63 26.91 21.56 30.72 28.64 24.86(23.18) (18.11) (16.82) (24.36) (21.59) (20.13)

F9 18.85 17.93 15.98 30.09 27.82 23.92(16.23) ( IS.77) (14.69) (23.85) ( 18.05) (16.07)

pressure builds up across the fabric which forcesthe larger particles to pass through the fabrics.Again, the difference between volume and surfacemean diameter gradually reduces with the increaseIn duration of filtration for both types of dustparticles. This can be explained as the smallerparticles increase over the filter fabric, the specificsurface area of particles increases, resulting inlowering of the difference between volume andsurface mean diameter and after 5 min offiltration, the values come closer.

It is seen from Table 5 that with the increase infiltration time, D(V,O.9) decreases for all fabricsand for both types of dust particles. The samephenomenon can be observed for D(V,O.5),whereas D(V,O.l) values show slight increasingtrend with the increase in filtration time. Theabove phenomenon can be explained as smallerparticles increase over the filter fabrics. Again, thechange in the values of D(V,O.9) and D(V,O.5)with different picks/em is greater for particle sizebelow 118/1 as compared to particle size below53/1. It is also observed that with the increase infiltration time the span is gradually reduced in alltypes of fabrics. This is due to decrease inD(V,O.9) value and simultaneous increase inD(V,O.l) value. Although D(V,O.5) increases butthe effect of (D9Q-DIO) is more pronounced in thepresent case.

3.2 Effect of Fabric CeastruetleaIt is seen from Table 4 that with mcrease in

picks/em for plain, twill and satin, the volumemean diameter and surface mean diameterdecrease. This is expected due to increased surfacearea and reduced pore size of the fabric which canarrest smaller particles and increase pressure drop,leading to fractional loss of larger particles. Whilecomparing the samples of different weave butsame cover (e.g. F2 & F4, F3 & F7, F6 & F8),twill weave shows maximum volume and surfacemean diameter followed by satin and plain fornormal dust. However, few exceptions are therewhile filtering dust of particle size below 53/1.Table 5 shows that D(V,O.9) and D(V,O.5)decrease, while D(V,O.I) shows an increasingtrend with the increase in picks/em in all types offabric. It is interesting to note that plain fabric ofsame cover with twill and satin shows maximumvalues of D(V,O.9), D(V,O.5) and D(V,O.l) butleast values of volume and surface mean diameter.It is also envisaged that with increase in picks/emin all types of cloth, span gradually reduces, whichcan be explained as smaller particles increase overthe filter fabric.

3.3 Effect of Dust SizeIt is observed from Table 4 that in case of dust

particles below 53/1, the difference betweenvolume and surface mean diameter is graduallyreduced with increasing filtration time andpicks/em, Filtration with dust of particle sizebelow 53/1 shows higher surface mean diameter forall fabrics at all levels of filtration time, which is

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168 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 1997

..

due to feeding of greater amount of smaller dustparticles during filtration which are captured bythe fabric. However, from Table 5 it is observedthat the decrease in D(V,O.9) with increase infiltration time is greater in case of normal dust ascompared to dust of particle size below 53J.1.D(V,O.l) shows a greater value in case of 53J.1 dustparticle at all levels of filtration time.The above phenomenon is due to capturing of a

large amount of small dust particles (below 53J.1)during filtration. For the above phenomenonD(V,O.5) is also affected.It has also been seen that span value is less for

particle size below 53J.1 which is due to reducedD(V,O.9) and greater D(V,O.I).

4 Conclusions4.1 With the increase in filtration time, volume

mean diameter and surface mean diameter reducegradually for normal and below 53J.1 dust.

4.2 With the increase in picks/em for plain, twilland satin, the volume mean diameter and surfacemean diameter show a reduction. Differencebetween volume and surface mean diameter is lessin case of dust particles below 53J.1 as compared tonormal cement dust at different durations offiltration.

4.3 With the increase in picks/em for all typesof fabrics, D(V,O.9) and D(V,O.5) decrease andD(V,O.l) increases. With the increase in filtration

time, D(V,O.9) and D(V,O.5) gradually reduce incase of normal cement dust and below 53J.1 dust.4.4 In filtering normal cement dust, twill weave

shows maximum volume and surface meandiameter followed by satin and plain of same coverat different durations of filtration. However, plainweave shows maximum value of D(V,O.9),D(V,O.5) and D(V,O.I).

4.5 With the increase in picks/em for all typesof fabrics, span value is reduced. The reduction inspan value is also observed with the increase infiltration time.

AcknowledgementThe authors wish to express their gratitude to

Prof. ReD Kaushik, Director, TIT &S, forproviding all the necessary facilities for this work.They are also thankful to the management and staffmembers of Bhiwani Textile Mills, Bhiwani, forproviding facilities in preparing the fabric samples.

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2 Cadwell G H (Jr) & Whyte W, High efficiency airfiltration in Clean room design, edited by W Whyte (JohnWiley and Sons Ltd., West Sussex, UK), 1991, 181.

3 Wright M W, Cleanrooms, Feb (1989) 32.4 Dorman R G, Filtration in aerosol science, edited by C N

Davies (Academic Press, New York, USA), 1977, 195.5 Jaroszczyk & Wake J, Critical aerosol velocity in

nonwoven filtration, in Proc, TAPP! Nonwoven COIlf,1991,125.