NUREG/CR-r4264

LA-i10436-MS

RI

Investigation of High.-Efficiency

Particulate Air Filter Plurgging

by Combustion Aerosols;.

D. L. Fenton*.

M. V. Gunaji*

W. S. Gregory

R. A. Martin

Manuscript submitted: April 1985

Date published: May 1985

Prepared for

Division of Risk Analysis

Off ice of Nuclear Regulatory Research

US Nuclear Regulatory Commission

Washington, DC 20555

NRIC FIN No. A7029

Mechanical Engineering Department, New Mexico State University, Las Cruces, NM 88003.

ELOS Los Alamos National Laboratory Los Alamos, New Mexico 087545

INVESTIGUATION OF IHIGH-EFFICIENCY PARTICULATE AIR FILTER

PLUGGING Bf COMBUSTION AEROSOLS

by

0., L.: Fenton, M. Vý. Gunaji,

W.S. Gregory,' and R. A. Martin

ABSTRACTý

Experiments, were conducted to investigate high-efficiency par-,

tic'ulate air (HEPA) filter plugging by combustilon aerosols. These.

tests were done to obtain empirical data to improve our modeling of'

-filter plugging phenomena using'.the Los Alamos National Laboratory

fire accident analysis code FIRAC. Commiercially available 0.61-rn ,.by 0.61-rn square flilters were tested in a specially desi~gned facili-

.ty to determine how airflow resistance variles with increased filter.

loading by combustion aerosols. Two organic fuels normally found

in nuclear fuel cycle facilities,..polystyrene (PS) and polymethyl"-

methcryate(PMMA), were burned under varied conditions to generate combustion aerosols.. The test facility included a comibustor,. a

I,23-rn-long duct,.and a specially designed gtravimetric balance for

determining the aerosol mass gain of the filters.

'Tes .t results in .clude correlations of HEPA filter resistance ra-7-

tio's (actual resistance/i .nitial resistance) with aerosol mass gain.

T7he -mass gain of. plugged HEPA filters, was, found to. correlate ýwit~h

.the -airborne mas~s concentration of material in the size range greater than approximately 2.0 prn. Also, the fuel1 wi~th a smaller soot

fraction, IPMMA_, produced f~ilter 'pl~ugging at-lower accumulated aerosol -mass dep~os its on. or within the filter.

1. :INTRODUCTION

High-efficiency particulate air (HEPA) filters are used commonly in, nuclear

reactor or fuel processing plant ventilation systems to protect the outside en;-

vironment and plant workers from airborne radioactive particles. Additional applications include hospitals, where bacteria-free atmospheres are required for

operating rooms, and numerous other facilities where clean air is necessary.

Exposing a HEPA fi~lter to heat and smoke significantly alters its operatingý

characteristics. This invest igation focuses on the characteristics related to

1
filter plugging or the increase-i~n pressure drop across a filter as a result of

,the coll~ection.:of particulate material on the filtration media.' Such information is required to better understand how filters are plugged with combustiongenerated aerosols and will improve our estimates of ventilation system response

to compartment fires using an existing Los Alamos fire accident analysis computer code called FIRAC.

I I. OBJECTIVES

An experiment that provided the required HEPA filter plugging data and was

based on the combustion of two. solid fuels was desig~ned. The fuels we selected

bracketed the particle material generation rate (the mass fraction of the solid,

fuel that converts to aeros~ol, particles) that would be expected in a typical nuclear facility.2 The two fuels 'selected were polystyrene (PS):and ýpolymethylmethacryl-ate.(PMMA); their soot fractions are nominally 0.33 and..0.021, respectively. For these numbers, both the soot and low vapor pres'sure liquids-are

included in the aerosolized combustion product mass normalized by the unburned

fuel mass for overventilated conditions'.3 We had six specific questio~ns about

the problem w~e investigated.

(1) What is the correlation between accumulated combustion particulate

mass-on the HEPA filter- and fl1ow res istance across :the fIlter?

(2)1 What is the effect of soot 'fraction (as manifested by fuel type) on

'HEPA-fi~lter plugging?

(3) What is the effect of the fuel mass burning rate on HEPA filter plugýging ?

(4) How far do th~e aerosols penetrate within the filtration medium?

(5) Does the HEPA filter upstream faceguard influence fil terplugging?

(6) What is the magnitude of particle-deposition within the duct upstream

from the HEPA filter?

The experimental work was directed at answering these questions. The equipment

included a combustion chamber where. the fuels were burned, an airflow facility

composed of a fan and duct, .and a specially designed gravimetric balance with an

accuracy of 2 to 3 g for measuring the accumulated mass collected by the HEPA

filter.

2
III. FI LTRATI ONz AND PLUGGING THEOR(J

FjIltr at ion' involves the captureý of ai~rborne particles by a filter's medium.

Analysi~s of particle capture treats the medium as. al long cyl-inder:.folded :Uponitself a number of times. S hort-r~ange mechani-sms of. particl~e capture by a l1Ong

circulAr yinder include inertial impaction,inecpoaddfusn.

A. Inertial Impqaction

If an indivi~dual particlemiovi~ng with uniform velocity is introduced -in a

fluid ahead of a circular cylinder,, it will tend to follow the streamlines. 'However,. because the particle necessarily must-have!,mass, the particle motion may.'

'diverge from that of the streamline and possibly hit the circular cylinder.. The

actual deviation of particle motion from the fluid streamlines' is greater for.'

particles of larger mass and for higher-veloc~ity flIows-.

B.. Interception

With the inertial. imp action mechanism, it was assumed that'. the pa rticle has:

.mass but no sizes but in interception the assumption made is that the .particle

has no-mass but has ai finite size.. Thus, the particle approaching a- cylinde r

will not diverge from-the fluid streaml~ines. Interception of the particle will

occur when the distance between the cylinder surface and, parti~cle'' center is not,

greater than the'radius of the 'particle.

-C. D~iffusion

Particles that are, submi-cron in size rarely are collected by inertialim

paction,:or interception-because.,they are small1 and can follow the'.streaml~ines or

move across them in a zigzag way. This zigzag movement is caused by continued.

and irregular bombardment of the molecules of the,,fluid and is called Brownian

motion. The capture of aerosols by diffusion is more likely when the free

stream velocity is l ow. and the: particles, are very small, (d <' 1 uin).

D. Combination of Filtration Mechanisms

Real particles have a definite mass and, size; he nce, the inertiail impaction and-interception mech~anisms do not. act independently,. It is ýmore likely

that two-or mor e. mechani sms act in combination, With one dominating over th e

other.. For example, with- particles in the micron size range .and larger, iner-,

tial impaction and interception will dominate, whereas -diffusion ,is -likely to

.dominate f or_. s ubm icron -sized particles.
E. 'Effects of High Temperature on-Particle Collection Mechanisms

The effect of high temperature on the particle 'coll~ectio mehanisms was

studied by Thring and Strauss. 5With an increase in temperature_, the efficiency of collection by interception and i~ne~rtial imp~action. decrea~ses-but th at

by diffusion increases. This increase in filtration efficiency by'diffusion is

a result of the increased level of Brownian motion at increased temperatures.

F. Filter Resistance

The resistance of HEPA filters to flow is defined as6

where AP is pressure drop across the filter in centimeters water gauge and Q is.

the flow rate in cubic meters per hour. The relative resistance is defined to

be WIW 0, where W0 is the clean filter resistance, AP /Q.

Based on the resistance of cylinders placed parallel. to the flow, an

expression for the pressure drop is6

- 2~nQ (2)

Ar f

where q is the coefficient of fluid~dynamic vi scosity (Ne'wton-second per square

meter), a is the packing density of fibers, Q is the flow rate (cubic meters per

second),, A is the face area .of the filter (square meters ), -is give'n'by

'(ins + 2s -2 _2-l b is the filter thickness (meters), and rf is the-mean

fiber radiu~s (meters). The factor a is incorporated because the fibers are perpendicular to the airflow. Usually, the value, of 0 ranges from-O.5 to 1.5.

In calculating the fl~ow resistance of HEPA filters, the,-parameters used are,

AP, pressure drop;

Q, flow rate;-

A, area;

b, thickness of the filter;

4
nCoefficient ýof dyai v iscos it of the fluid,

p.$ densi~ty" of, fluid,

~,microscopic mean free path of the fuid molecules,: and.

rf mean-fiber radius'.

Davies did a dimensional. .analysis coupled with experiments5' and gave the

following expressi-on for the press'ure drop-..

.1.5 3 - 6riQbo b .1 +56a 3

Ar2f

where the quantiti~es Ili Q, b, a, A,, and rf are, defined the same as for Eq.ý (2) .

.Davies stated thaIt, E-q., (3)' is v'alid.,for packing densities (s) l ess than .0. 02.

For filters with-higher packing densities, he gave the following expressi~on for

pressure drop'..

_: 17.5r)Q81 5b-1 ( -528s 4

Ar2

fe

where n, Qi b, o,,and A:are same as for Eq. (2),1 but rfeis the effective

fiber radius.

G.Filter''Plugging,

Filter pluggi~ng involves-the deposition of solid particles on f-ib ers, which

-changes the filter's-operational characteristics. This deposition causes an in-

.crease in the flow resistance of the filter. There may be -consider~able changes,

in the filtration characteristics' while the deposition of solid particles is

building up-~and the filter' is becoming plugged. The deposited particles may not

distribute evenly over the surface of the fibers but-may act as very fine fibers

themselves by constructing- chain structures, which results in an increase of

filtration efficiency. 6

The change in flow resistance across the filter because of plugging depends

.7 . .

on the following factors.

5
()-Particle Phase. Aerosols, of liqu'id. droplets ýwet the filter and may

.draw fibers together by capillary action, increase pore size, and

decrease both filter resistance and efficiency. Thus, liquid aerosols will cause a smaller change in flow resistance than so~lid particle aerosols. The penetration of liquid aerosols has been found to

increase with time. 6

(2) Particle Size. Smaller particles usually cause a higher flow resistance than larger particles for a given total particulate mass.8

(3) Filter Structure. Depending on the filter structure, the particles

are deposited either on the face of the filter or inside it. The penetration of the particles into the filter depends on the air velocity

and filter structure.

(4) Mass of Collected Particles.. This is related to the unit area of the

-filter. The pressure drop across the filter increases with increasing particle loading on the filter. It may be considered as the sum

of fil-1terý-pressure-drop and the pressure drop of a layer consisting

of--the deposited particles.7

1. Dendrite Model*. C. Tien et al. 9 put forward a theory for the formation and growth of particle dendrites .on a collector placed in an aerosol stream.

The theory is based on (1) the finite size and (2) the randomness of the location of'particles -(aerosols) in the fluid stream. Tien made the assumption that

the particle trajectory depends only on the particle inertia and the drag force.

Because of the fin~it~eness of the particle size, shadowing and chain deposition effects become important. When a particle is deposited on a fiber surface, protrusion above the surface occurs, providing greater surface area than

before.. There is greater chance for the capture of subsequent parti~cles,,because

of the increased surface area. When these subsequent particles are captured by

particles that were deposited earlier, a chainlike particle dendrite is formed.

This process proceeds with the dendrite having a greater surface than before and

consequently greater possibility of particle capture.

In this model of filter plugging, there isý a region on the fiber surface

called a shadow where no particles can deposit. thus, the capture of particles

by the interception mechanism occurs, which implies the assumption that particle

deposits do not disturb the airflow streamlines.

6
W.Bergma~n. et. al. 1 asue ha th 6pressureý dosruligfomte

fil1ter. f ibers and: the, particle 'dendrites- aire i~ndepe'ndent and.- when added gve ,the. pressure drop 'across the filter.' They deri ved, th olwnxres~sionfor'

a filter consisting of fibers and particle dendr~ites.:

rmf

'Equation (5) 'also can be' used for .a filter having ,two discrete

The .pressure d rop, AP0,P is giv en by

fiber sizes.

Ap nV I Ab 0 2 J

SRf f (6)

The parameters involved.inEqs. (5) and (6) ,are

~,coefficient of flu~id dynamic viscosity;

ýV, f luid velocity;-

b, filter thickness;

R.fiber radius;

.r, particle dendrite radius;-

volume fraction of the trapped particles; and

afiber volume fraction.

Bergman et al. further simpl~ified Eq; .(5), for low .particl~e loading-by neglec.- ting the nonlinear parti.-Consequently, Eq. .(5) reduces to

A.P =AP 0(+fP (7)

Equat~ion:(7) predicts the increased pressure drop becau Se- of particle load-ing will be in~versely. prop-ortional to the particle size..

.7
2.Increasing Fiber Model. ega e t al-.a1aso Assumed that the

part-icles~ form a deposit that. inpreases the :fiber'~s ýsize' without changing Its

ýshape. On the basis of deposýit'geomety tegave. the, following expression.

for the increase in fiber rad'ius (Rf)

( l~ci /2

Rf Rl+- I I (8

f /8

where Ris the original. fiber ýraidiu's "and andaf are. theý same as before.

Us ing

Al' AP(t)()

for pressure drop, Eq. (8) can be ',rearranged. to. y-ield

/ 12a\ /

0p AP~ (: + (10)

where AP' is, defined by Eq (7.-Comparing ,Eqs.ý' (5): and (10.) indicates that

pressure drop for the increasing f~iber ..model is les~s ýthan the pressure drop-for

the dendrite model with the same particle volum~e deposited on-the filter.

F~or fil'ters Undergoing: reali stic. pluggi ng con"di t ions, :Bergman et al.' 0

state that the mechanisms of the increasing,,-fiber: and'particle' dendrite models

might act in combination. At very low particle loadings on the filter, the

increasing fiber model gives a major contribution, but: after a small amount of

loading, the dendrite model prevail~s. They concluded that the dendri~te and

i ncreasin Ig fiber models are limiting cases of a more general filter plugging

model.

8
3. Phenomenological Model. Previous filter plugging tests with dry stear-,

ic acid aerosol'1 '12 strongly Idiscredit the two models described. earlier; evidence of discrepancies also is given in Bergman's work.10 Rather than trying

to construct any realistic physical mechanism for the cause of plugging, we pro`-ý

pose a phenomenological approach; namely, we assume the following relation.

where W is the resistance coefficient defined in Eq. (1) and Wois the value

of W for a clean filter. F is a monotonically increasing function of M p which

is the total mass of particulate accumulated on the filter, and is the relative

resistance. To satisfy the clean filter requirement, we must have

F (M p O )1 (12)

W4e believe the following expression should be adequate for all practical purposes.

F-i1 + a4 + OM2 (3

p p(3

where a and o are two coefficients determined by experimentation. Their evaluation forms the basis of the filter plugging test program.

IV. MODIFIED HEPA FILTER LOADING FACILITf

The existing HEPA filter loading facility was modified to accommodate the*

requirements associated with combustion aerosol generation and filter plugging

experiments. The original filter loading facility is described in Ref. 11 and

Ref. 12. Specifically, the modifications included the addition of the solid fuel

combustor, the installation of a turbulent mixing grid, the construction and

9
installation of -a metal duct, and- the addition of approximate~ly 7.4. m. of

straight duct upstream f rom -the test filter location.

Size .5 HEPA f ilters,- which have a nominal-airflow capacity of 1690:,mIh

'face di~mensions of 0.61 by 0.61 m, and a depth of 0.292 m, were tested in this13.

investigation. The unplugged filters had almass of approximately 16*kg.i

.The Mine Safety Appliance Corporation manufactured the HEPA filters tested.

A. Air-Handling Equipment and HEPA Filter Test Section

The air-handling equipment and HEPA filter test section are described in

detail elsewhere. 11"12 We will discuss only the features used for combustion

.aerosol experiments. For example, 2.4 m of stainless-steel duct. was installed

immediately downstream-from theHEPA. filter -prefilter to facilitate additi on of

the. test fuel combustor. A turbulence mixing grid was added at the ,downstream

edge of this steel duct to disperse the combustion products over the duct airflow.: The grid was composed of 2.2-cma-o.d. copper tubing spaced appr oximately

5.08 cm apart center to center.:

Two pressure measurements .were required. The first was the static pressure drop across the HEPA filter.. Four static pressure taps were' located at the

center of each duct wall 0.45 m upstream or downstream from th e "filter face; a

Dwyer vertical manometer capable of a 1-mm-w.g. resolution was used to measure

the static pressure drop. The second pressure measured was the centerline velocity pressure; a Dwyer inclined manometer capable of a 0.005-.cm-w.g. resolution

was used. The centerline velocity was measured with a pitot-static probe in the

circular cross-section duct downstream from the HEPA filter test section. Horizontal and vertical traverses across the circular duct yielded velocity profiles

that were used to calculate the volume~tric flow rate of the ai~r. A centerline

coefficient that-relates the volumetric airflow rate to the centerline velocity

then was determined. Repeating these, calculations for the volumetric flows expected in a typical HEPA. filter pl~ugging test relates the centerline velocity

.to the volumet~ric airflow rate. Figure 1 shows the centerline coefficient as a

f-unction of volumetric flow where the coeffic~ient Y is .given by the equation

Y =0.832.0 + 0.000-04618(Q) - .Q.65 x 10- Q2 where, Q is the vollumetric flow

rate.. The centerline: coefficient was used to determine all ,the volumetric,

ai rflows.I

10
* .895.

S .885

.87

7C.

-~.865

.855 I

400 500, '600 700 800 900 1000' 1100 1200 1300

:Volumetric Flw Rae f/mn

Fig 19.

Centerl-ine coefficient as .af fun'cti on of volumetric -flow.

Air temperature measurements wer e made at five positions, and the inliet

and outlet dry-and wet bulb temperatures were measured with a psychrometer.

-Copper-constantan (type T7) therm~ocouples were used to measure the..temperature

of the ai~rflow at the branch-section And-upstream and downstream of the test

HEPA filter. A thermocouple digital temperature indicator was used to monitor 0. the thermocouples, which were accurate to +0.5 'C at'the 95% confidence leviel.

The measurements made at the filter test section included particulate massý.

on the filter,' Pressure drop across the filter., particu .late mass concentration,

and: particle size distribution.

B:. rest Fuel Combustion

The'test fuels' (PS and PMMA) were burned in a combustor desi~gned at

Battelle Pacific Northwest Laboratory. Figure 2 is an assembly drawing of the:.

combustor. The burner wa.s 'positioned on the underside of the stainless-steel

duct immediately downstream from the HEPA prefilter~and secured by a-flanged

steel, pipe section using eight O.,79-cm-diam bolts.

11
CONWTROL VLV

Fig. 2.

Assembly drawing of the, combust-or.

A major feature of the combustor is a cup holder where the fuel was contained and heated-. The chamber walIls provided the capability of performing combustion in a positive pressure atmosphere (the duct interior). Fuel preheat

temperatures were nominally 500 0C and were achieved by a 400,- W el~ectrical'Lring

heater controlled by a Ch~romalox on-off temperature controlle'r. 'The flame was

diffusion'-controlled, and the burning rate was regulated by .adjusting the airflow rate to the -combustor by means of a valve and rotameter.. A-gate valve at

the top of the burner (under the'.stainless-steel duct) was maintained-in the,

full-open position after the fire was..started.

12
C. -Combustion 'Product,. Characteri zat ion'

lCharacterization of the' combustion product icueonly,-the,. parti~culate.I

constituents,., the combustion products were not monitored: continuously; samples.

were takenh from t-ime to t ime 'and w~ere,:considered adequate..

Particulate mass concentration: (milligrams per~cubic meter) was ýdeter~mined

with 'Anderson Mark :III stainless-steel, in-stack, inertial, impactors incorpora-,

ting straight nozzles.: These impactors also-measured aerodynamic particl'e diameter. (bsdo ntdniyshrs hough seven stages of particle colleto

and .'a back-up fle. Preimp~actors for u-se,.in conjunction with the impactor's

were* determi ned''to 'be unnecessar~y for:this. application., Real-time particle sizing equipment also,.wa-s used (a Royco Model 225 Aerosol ýParticle Counter and a

TSI Model 3030. Electrical' Aerosol "Analyzier). Theý real-time equipment .,was used

Iinteremitt~ently to -.determine the particle sizeý character'istics :and mas's concen-.

trat~ion variationh with the burn time. The optical counter-sized particles si-,

multaneou~sly in all the. ranges and thus was suitable fo~r monitoring~ changing

concentrations. However, the Electrical Aerosol Analyzer sized particles 'seq:uentiAlly over a cycle tIme of about;3 mmn and thus was not able to characterize the aerosoll accurately. 'In contrast, 'the ilhert..ial impactors were. operated

in such a manner ý(nozzle diameter and sampling time) to- sample over the entir~e

fuel burn per~iod-'-up to.20.mn., For~this reason, the impactor size data were

chosen forl application to 'the HEPA "filter plugging ,stud~ies.,

V. EXPERIMENTAL PROCEDURES

The fuels tested required es sentially the same -test procedures,* which are

as follows.

.(I) Each filter was examined for holes by observing it agai nst, a. lightsource. 'The filter frame was checked for squarenessi, which was

necessary to prevent leaks at the flexible duct HEPA filter flange.

(2) "the test fuel cups were weighed when empty and then with the fuel

sample so that the mass was. known for determining the apparent burn.-

ing rate. The' cups were placed on a hot plate with the temperature

automatically kept at 500Q' (9320F).

(3). An aluminum platel(7.8 by 7.8 by 0.ý2 cm) was-'attached to, the bottom

of the HEPA filter being tested to provide a smooth .l~oad cel I contact

po i:nt.

I 3
(4.) The HEPA fi~lter -was placed on, the pneu~matic-cylind-ers, raised., and.

,attached to the gravimetric. balance beam assembly with four screws.

(5)ý T:he force transducer assembly was posi~tioned under the filter and

bolted to the support structure. The direct current power supply was

:.adjusted to +15 Vdc, maintained at this level for all the filter

tests, and never turned off. This was done, to achieve steady-state

operation of the power supply. The pneumatic cylinders then were

lowered.

(6) The HEPA filter was nu~lled,-or balanced by adjusting. the counterweights

.on the gravimetric, balance assembly. The filter-was considered nulled

if the output of the transducer wa's in the range of 20 to 40 mV.

(7), Calibrati'ng the force transducer involve d placing known weights on the

nulled f-ilter and recording the output. The output, was recorded from

two to four times and',then averaged for each known weight. The cali-.

bration equation was determined: at the conclusion, of the HEPA filter

plugging test.

(8) The pneumatic cylinders were raised, forcing the HEPA filter against

the stoos. This ,was the filter's position for operation and particulate loading. The flex ibl~educts then were clamped around the filter.

(9) The test fuel cup ring heater. kwithin the combustor) was switched on,

*and the Chromalox on-off c'ýontroller was adjusted to:...500 0C. Approximately 40 min were required for the cup to achieve the set temperature. During this time interval, the fan prefilter was, changed, all

electronic instruments were rechecked, and the pressure manometers.

.were adjusted for. correct zeros.

(1.0) The airflow facility 'fan, was turned on.

(11):The barometric pressure was recorded and corrected for temperaturean

loc-al elevation.

(L2) The airflow adjusting door located at the fan box was adjusted to

yield the desired velocity pressure (Volumetric airflow) measured in

the circular cross-section duct downstream from the test HEPA filter.

ý(13) remperatures at the inlet, at the branch 'downstream from combustor),

upstream from test HEPA fi~lter, downstream from HEPA filter, and at

the exit were recorded-.' The static pressure drop across the Filter

also was recorded.

Ji4
.,(14). The, airflow facility f an.' was turned :off to.;"st'art'-the fi re. Thefuel.

test cup was placed. in :the combustor cup hol der,. and the top gate

ýval ve was, opened slig~htly N o air -was supplied to the comutr

which all !owed. Vapors .to accumulate, i~n the volumeý above the::cup.

After 5-10 min, the igniter coil was energ iz ed el1ect ri1call y,. (Fuel

vapor ignition occurred most consistent~ly in this manner'.) After

combustion was initiated, the top gate valve was moved to full open,

and the combustion airflow was :adjusted to the required flow,

(0'.00047-or 0.0019 m /s) on, the rotameter by means of a Valve.

(15) The. airflow fac il1ity fan was sw itc~hed on , and the thermocouple temn-

.ýper~ature ýmeasureme~nts were recorded after. -10 min had elapsed.

(16) -The data acquisition process involved the following items.

*The time at which the flame first:.appeared was recorded.

*The time at which. the'flame di sappeared -was recorded.

e. After flame disappearance, the top gate valve was closed, and the,

inlet and exit dry and wet b~ul~b temperatures, HEPA filte .r static

pressure drop, and velocity pressure in the circular cross-section

-duct were recorded.

9 The airflow facility fanlwas turned 1off, the test HEPA filter was

released from the flexible duct, And the pneumatic cylinders were

lowered. The force transducer zero output was recorded. The HEPA

filter was lowered' carefully onto the force transducer so that no

horizontal 'forces were present, and. the output was recorded. .The

HEPA filter weight gain measureement was repeated four times.

.(17) The HEPA filter was -returned to the test ,position by raising the

pneumatic cylinders and reinstalling the fl~ex.ible duct.

(18) The next fuel test cup wa s' placed inside the c .ombu ,stor and t Ihe v apors

were ignited as before.

(19) The above procedure was repeated until..the volumetricarlwrt

was reduced to one-half of the initial flow ratej, 845 m.1min, that

is, fully plugged conditions. This condition was assumed as fully

pl-ugged, and the test was concluded,.,

ýThe difference between procedures used for PS and PMMA combustion involves

the ignition of the fuel and the number of cups burned between data recordings.

'For,.PS,, the ignition technique-used the heating coil as described above. How-

-ev'er,* a propane torch was used in place of the coil for ignition for the PMMA,

15
f uel. ý'One :cup ofPS fu~e~l-:was bu~rned betweeen data recordings, whereas Ifrom -three.

to fi ve cups of PMMA fue'l were: burned' to, obtain an prpriate asgi on the

HEPA f ilIter.`

VI. EXPERIMENTAL RESULTS-.-

Table I is a summary of the conditions associated With the combustion of

the test fuels. The sto'ichiometric airflow rate was bas~ed'oin the.given fuel

chemistry and the apparent fuel mass burning rate observed during,:preliminary

-tests.

The experimental results and the analyses: are presented for -the PS and PMMA

HEPA filter .plugging tests., Separate section's are used for the discussions be-

,cause of the-difference in the soot fraction with combustion of the two fuels.

Physical :characterization of the particulate mater-ialT is presented first; then

the HEPA filter plugg~in-g data are discussed.- Experimental results on smoke -

transport, and depo~sition.[see qujestion (6) in Sec., II], are reported elsewhere

(R e-f.14).

A. PS Combus~ti on` and HEPA Filter Plugging

- Cascade. impactors were used to.m~easure both total. particulate mass concen-, -~ 14 trati on and particle size on-the basis- of aerodynamic diameter. -Figure:3

shows the PS particle size measuremet drectly upstream from the HEPA fite

and gives the particuilate mass concentrations at the same location. Note that

:.for both the high and l ow mass burning rates, the particle size distribution is

TABLE I

TE ST FUEL COM4BUSTION CONDITIONS

Stoich-1

- ometric Ratio of Actual Chemical ,Combustion.- Ai rflIow to Stoichiometric Fuel Formula. Efficiency Rate' (tn3

/h) AirflowRates Commnents

P-S- CH 06 7.4.- 0.23 Low Burn Rhte, (granular) Underventilated.- -

0.93 High BurnRate, - Underventilated

- PMMA .CH1.60.4 O ._9 .4.0 0.85-, Low Burn Rate,

Underventil1ated

2.1ý Hgh Burn Rate, Overventi 1ated.ý

16
1oo.ct

HIGH BURN RATE

0 .pp = 0.13 k/m*

30.0- p, = 0.17 g/m3

LOW BURN RATE

E~ PP ~0.093 = g/M3 m

:3. 10.0 .: p = 0.07 g/M 3 *oa

Ix on OA

<3.0-e

1.0- -W

* 0.3 -

0.1 **I .

1.0 10. 50 90 99 99.9 CUMULATIVE PERCENT LESS THAN STATED SIZE

* Fig. 3.

Polystyrene combustion particulate mass- size distribution.

nearly',the' same for part~icles less than 2.0 1um.. However, for particle diameters

greater than 2.0 u~m there is. ainfcntly'greater relative number of particles at the higher burning-rate compared with the lower burning rate.

The particulate mass concentration data, in conjunction with the mass burning rate data, imply an important feat ure associated with PS combustion. This

,feature, is that the total particulate mass concentration measured near the HEPA

filter is proportional to the mass burning rate. This is established by ratioing the- average high burning rate to the average lowburning rate in Table IL

-and obtlaini'ng the value 1.7. Calculating the corresponding ratio for-average

total, particulate mass concentrations using the data in Fig. 3 also0 gives 1. 7.

.The correspondence betwe en the burning rate 'and particulate mass conce .ntrati~on

implies a constant soot fraction for underventilated conditions.

A summary 'of the results for both fuels, is given in Table II., These datag

are the result of burning many cups of fuel in each case, and thus the mass

burning rates are average values. The HEPA filters tested with PS had protec-

.tive metal face screens (1- by 1-cm mesh size), with the exception of filters

number 6 and 7. The HEPA filters tested with PM?4A all had the metal screens

removed.

17
TABL.E I I

SU?4MAIt OF COMBUSTION PRODUCT. PLUG33IWG OF HEPA .FILTERS.

Clean Static Apparent Mass Total ParticulateMass

Test pressure Drop Burning Rate Collected at Plugginga

Fuel No. (an u.g.) (glmin) (g)

PS 1 2.1 19.4.(high) 527

PS 2 2.1 21.3 (high) 432

PS 3 2.1 15.4 (low) 1256

PS 4 2.0 21.7 (high) 405

PS 5 2.1 17.5 (low) 1207

PS 6 2.1 22.9 (high) 432

PS 7 2.1 21.3 (high) 391

PWIA 8 2.2 .11.6 (high) 233

Pt4A 9 2.2 12.4 (low) 257

PF44A 10 2.2 . 15.6 (high) .186

PMI'A 11 2.2 13.4 (low) 309.

dine HEVA fllter resistance ratio has a value of..12.0 for each filter.

The detail~s of the plugging process for each filter are shown in Fig. 4

The actual resistance is normalized by the initiall clean filter resistance

(Wo = Initial ,R in Fig. 4).. The da~ta show the-influence of the PS mass burnin g rate on the quantity of the 'particulate material collected thiat is necessary for plugging. -For a g .iven combustion condi-tion'(fuel type and mass burn'

rate), the following functional form is seen to correl~ate the plugging data

WIW =(1 am +am2) '(14) 0 p

where W/-W is the filter airflow resistance ratio, M4 is the accumulated par- .0 p

ticulate. mass. on the HEPA filter (grams), and the variables a and a are ldete~rmined to give the best fit to,,the data. From Fig. 4, the..importance of the pr~o-m

tective screen on HEPA filter, plugging,,can be seen to be negligible. Table III

shows the values for a and'B for the combustion conditions tested.' The resis-.

tance ratio is observed to be a strong function of the-linear term in (4 -the a

values are relatively small.. The filter plugging coefficients given lin Table III

,can be used in the input data deck of. the Los Alamos fire accident analysis corn-.

puter code FIRAC. i This code is designed,;to model and~predict fire-induced

'flows and thermal and material transport within complex nuclear fuel.,cycle-

.facilities.

18
Fig. 4.

Variation of resistance ratio with accumulated

mas Isfor polystyrene combustion.

TABLE III

COMPILATION OF HEPA FILTER RESISTANCE RATIO

FUNCTIONa COEFFICIENTS

Fuel

PS

PS

PWMMA

PMMA

Combustion:Condition.

High, 02

Low 0 2

Underventi lated

Overventilated

0.022248

0.0057105

'0.0476796

0.0641064

0

0.64249 x 10-

0.17108 x 10-5

-0.308264

-0.57276 x 10-4

GJhe HEPA filter resistance ratio has a value of 12.0 for each filter.

19
Particle size data, ca~n be. used .t~o exp-lain the vrainin, -PS combustion

patclt asnecessary to 'plug the HEPA filters.. From Table II, the mean

.total mass collected- by the filters when nearly plugge~d (resistance-ratio =12),

for the low and high burning rates Are 1232 and 437 gs,respecti~vely. Table IV

presents particle size data for both fuels. "These data were obtained using the

cascade impactors with sampling during the combustion'of individual cups 1of

fuel. The quantities in Table IV include the particulate mass concentration' for.

particles larger than 2.0 um (p k), the particulate mass concentration for parp

ticles smaller than 2.0 um (p, ), the total..fuel mass burned (Mf),.and the mass

PSf .burning rate (mf). Combining these'into the ratio op2ZMff yields a parameter

that modifies the concentration according to the combustion,,rate. The ratio of

.thi .s parameter for those particles greater than 2.0 um gives the value 2.7 when

the average value of thi~s parameter for high mass burning, ,ra~te is divided by the

low mass burning rate average value. When the corr'espondi ng r~atio for accumulated particulate mass gains for the fil~ter-at the flow-,resistance ratio, value

of 12.0 is computed using the data of Table, .11, the number is 2.8. Numbers this

close for-these two ratios indicate that the particles greater than 2.0 um in

diameter dominate the plugging of'the filter., Partic~les-less than 2.0 pim appar-,

enty entrtethe filter medium, contributing to the mass of collected parti

culate material but contributing little to increase the static pressure drop

TABLE IV

COMBUSTION PARTICULATE MASS CHARACTERIZATION.

PnQ Pn~ -

Combustion >Mm) (< ~m). rn..m

Fuel Condit Iion (g/m) (g/mJ) (_) _______) NjmfM

PS High, O:2 0.0051 0.074 186.6 20.1 0.47 06

PS High 02 0.059 .0.050. 223.8 22.2 0.59 0.50

PS Low 0 2 0.012 0..,08 1, 207.1ý 11.7 0.2114

OS Low- 02 . 0.0094 0:.052: 223.3- 13.3 0.19 0Q.87

PMM~A UnderventilAted 0.10022 0.014 600.5 ý12.0 .0.11 0.70

PMMA Underventilated '0..0027 0.013 600.0 11.5 04.1 ý0.68

PhylA Overventilated 0.0018 0.0031 1003.5 12.3 0.15- 0.25

P.MMA' Overventilated 0..001.3 0.0035 1000.0 11. 7 0.11 0.30ý

- 20.
across the filter.- ýRemoval, of the ýHEPA filters.._from the, test, apparatus suppports'

thist sp ecu lation because the upstream face had a thick "mat", of fragile particu-ý

-late mat eri a I.that tended to fall~ awy f rom th6 filter even when disturbe'd .by'

the most: c arefu U handling.

B . -PIMMA Co~mbustion. and HEPA FilterPlugging

Figure 5 give's athe pa~rticle a~erodyn~amic di~ameter size distributions-, de-

.termined by cascade6 impactors directly upstream from the test HEPA filter.. With

:the combustion of the PMflA fuel, the low burning rate particle size distributions are consis~tent, but the high burning-rate distributions are apparently

different. However,,the burning* rate does.. influence the particle si ze di stri bu-

.tion.,

Also indicated in Fig. 5:areý the particulate mass concentrations ýassoci-

-ated, with each condition. In contrast to-the.PS combustion characteristics, the

.average fuel mass burn rate (Table II) is not correlated with the airborne average particulate mass concentration. (See. Fig. 5.) The mas s burn rate-ratio of

te-t~wo conditions. is only 0.9, and the mass concentration ratio is greater

100.0

High Burn Rate

*pi, = 0.049 /M 3

*PP= 0,0448g/m3

30.0 Low Bumn Rate (Linderventilated)

Ap,=0.015.k/m 3

*p, 0.016 g/m 3

10.0

3.0*A

* 1 .0 on *1S

1.0 10 50:.9 99 99.9

Cumulative Per Cent Less Than Stated Size

F ig. 5.

ýPolymethylethaicrylate combustion particulate mass-.size distributi-on..

21
th an 3'. This i's true because telwbrrat crespod to underven'tilIated

combustion c onditifons and the ,high burn rate' corresponds to overventilated conditions-the soot fractions are diffe~rent.:

four HEPA filters-were plugged with the PMMA comb~ustion products,;

Tables II--IV and Fig. 6 show the results. All four of the HEPA filters had the

protective metal screen removed., and the last filter (number 11) incorporated

asbestos separators within the ;filtration media in place, of'the alumnum, separa-.

tors used in all the previous filters. Figure 6-shows that HEPA "filter number

11 displays essentially normal behavior regarding plugging, indicating that the

separator-material does not'discernibly influence the plugging characteristics

fr PfA combustion products. Equation (14). agai ucsflycreae h

filter resistance ratio in terms of M for the two PMMA combustion conditions.

p

'Table IV suggests',that the parameter p ~Mf/mf (which was successful with,

the PS plugging data) is not successful-for the PMMA tests. The corresponding

parameter (p M /Mf ratio with the under~venti~lated values di.vided by the overPSI f f

ventilated values'yields'the number 2.5. The average accumulated masses on the

filter causing plugging at a resistance ratio of 12 are 210 and 283 for the

0 Filter, .11

CO2

P 4-

'0.0 320

F ig. 6.

Variation'of resistance ratio with

_polymethymethacrylate c~ombu~sti~on.

accumulated mass

22
overvent ilIAtdd to underventilatedý mass burning rates The ratio-of h vr

ventilated to,'underventilated HEPA filter ac'cumulated;-masses is 0.74., Consequently, 'this par~ame .ter is not appropriate for. the PMMA tests.-Rather, the corresponding particul~ate mass conc Ientration for particles greaterthan 2.0 um wnen

divid~ed yields a value of 1.04. This implies that again the larger particles

are. important -for a HEPA filter'~s plugging characteristics. Addi~tionally, going

back to the PS data and using the same ratio of ppgives a value 5.0, which,

is approximately double the HEPA filter mass gain ratio and thus isAnot appro-I

pri ate.

IV CONCLUSIONS.

The test resul~ts quantitatively establish HEPA filter plugging characterist ics for PS and PMMA combu~stion. aerosols.. Thelse tests were performed to support

devdlopment of the Lo's Alamos'.FIRAC fi~re accident analysis computer code. Tentative qualitative, conclusions ;can be made regarding the plugging characteristics involving particulate mass concentration and accumulated particulate mass

on th'el filter.

* Normalized HEPA filter flow resistance can be correlated with the.

accumulated mass gain on the filter for a given combustion aerosol

(PS and PMMA). When the combustion conditions change (fuel And/or

..oxygen availability), the HEPA filter plugging characteristics change

and are explained with the particu'late mass concentration for particles greater than 2.0 um.

* The lower the fuel soot fraction, the smaller the accumulated mass

gain on the HEPA filter required for plugging. -This results.,from the

physical characteristics of the particulate material-mass concentration and particle s.i ze.

R.ýEFERENCES

1. -P. A. F. White and S. E. Smith, Hi .h Efficiency Air Filtration (Butter- worths, London, 1964).

2.M. K. W. Chan, "Testing and Calibration Results of the Filex Smoke Gener- atos,"Pacific Northwest Laboratory draft report (March 30, 1982).

23
3.. A. T~ew~arson, "1Physico-.Chemical, and Combustion./Pyrolysis Properties of,.

,Polymeric Materials," Factory Mutual! Research.Corporation' (.Norwood,, Mass Achusetts) technical 'report RC8O-T-79 (1980)

.4. W. Strauss, Industrial Gas Cleaning (Per'gamon Press, L~td.,,Oxford, 1966),

pp. 214-243..

5.M. W. Thring arnd W. Strauss, "The Effects of High Temperature on Particle

Collection Mechanisms," Trans. :Inst..of Chemical.Engineers 41, 248,(1963).

6.. C. N. Davies,,Air Filtration .(Academic Pres's, London, .1973).

.7. C. N. Davies9: Ed., AerosolScience (Academic Press, London, 1966).

8. C. A. Snyder and.R. T..Pring, "Design Consideration in Filtration of Hot

.Gases, Ind. Eng. Chem. 47, 960 (1955)..

.9. C. Tien, C. Wang,-and D. T. Barot, ".Chainlike Formation of Particle Depoýsits. in Fluid Particle Separation," Science 196, 983-986 (1966).

10. W. Bergman.,: A. H.-' Bierman, .H. Heba'rd., B. Lum, and.W. D. Kuhl',- "E lectrostatic Filtersa'Generated by:Electric Fields," Lawrence L-ivermiore National.

Laboratory .preprinit UCRL-848.78 (January 1981).

11. J. J- Dallman, "An Investigation of the Simul'ated Combustion Aerosol-Loading of HEPA Filters," Masters.Thesis, New Mexico :State University<:(1982)..

12 . .Fetn,3 3'Daian . .Smi th, R. A. Mar-tin, 'and- W. S. Gregory,

"lThe Los Alamos NationalI Laboratory/New Mexico State UniversityFilter,

Plugging TestFacili~ty-Description and Preliminary Results,".Los .Alamos.

National' Laboratory, report in 'preparation'.

13. C. A. Burchst ed, A. B.' 'Fuller, and 3. G. Kahn', "'Nucle~ar Air Cleaning H .Andbook,ý".EnergyLRe~search'and Development Administration. H andbook'ERDA-76-21

(1976).

14. R. A. Martin and D. L. Fenton, "Full-Scale Measurementsý of-Smo'ke Transport

and Deposition in Ventillation, System Ductwork," Los Alamos.National L~aborator'y report in preparation.

15. R. W. An'drae, 3. W. Bolstad, R. D. Foster, W. . S. Gregory, F. R. Krause,

R. A.,Martin, and P. Ki. Tang, "FIRAC User's Manual," Los Alamos National.

Laboratory report in preparation.

24.
DISTRIBUTION

,Nuclear Regulatory Commission, RI, Bethe,.sda, Maryland

Technical Information; Center,. Oak Ridge, Tennessee

Los Alamos National1 Laboratory, Los Alamhos, New Mexico

Copies

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NRPC FORM 23 US NUCLEAR REGULATORY COPMMiSSION I REPORT NUMBER fAuSg,.. bl, TiDC. Soo voi. No. a sIr

16.83i

BIBLIOGRAPHIC DATA SHEET NUREG/CR-4264

LA-i 043 6-MS-. 2 Lftw ble-k

3 TITLE AN~D SUBTITLE 4 RIECIPIENT'S ACCESSION NUMBER

Investigation of High-Efficiency Particulate Air Filter _______________

Plugging by" Combustion Aerosols 5 DATE REPORT COMPLETED

#AONTI jEAR

________________________________________ April 1985 6 AUTHORISI 7 DATE REPORT ISSUED

D.L. Fenton*, M.V. Gunaji*, W.S. Gregory.' and R.A. Martin MONTm YEAR

*Mechanical Engineering Department, New Mexico State May 1985

University. La,, Crmirt - NM 88M.1p 9 PROJECT ITASKIWAFOK UNIT NUMBER

B PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (Incl.. ZIV C04*i

-Los Alamos National Laboratory_______________

.Los Alamos, NM 87545 ID FIrwN N UMBER

A7029

I I SPONSORING ORGANIZATION NAME AND MAILING ADDRESS I(neI.o Zp COME) 12a TYPE OF REPORT

.Division of Risk Analysis

Office of Nuclear Regulatory Research Informal Report

U.S. Nuclear Regulatory Commission 120 PERIOD COVERED (inclusew re5J

Washi~ngton, DC. 20555

13 SUPPLEMENTARY NOTES

14 ABSTRACT f2LV wor 01 Ar=J

Experiments were conducted to investigate high-efficiency particulate air (HEPA) filter

plugging by combustion aerosols.,These tests were done to obtain empirical data to improve

our: modeling of filter plugging phenomena using the Los Alamos National Lab~oratory fire

-accident analysis code FIRAC. Commercially available 0.61-rn by 0.61-in square fil~ters were

tested in-a specially designed facility to determine how airflow resistance varies with

increased filter loading by combustion aerosols. Two organic fuels normally found in

nucearfuel cycle facilities, polystyrene (PS) and polymethylinethacrylate.(PMMA), were

burned under varied conditions to generate combustion aerosols. The test facility included

a combustor, a 23-rn-long duct, and a specially designed gravimetric balance for determining

the aerosol mass gain of the filters.

Test results include correlations of HEPA filter res~istance ratios (actual resistance/

-.initial resistance) with aerosol mass gain. The mass gain of plugg~ed HEPA filters was

found to correlate with the airbor .ne mass concentration of material in the size range

greater that approximately 2.0 jim. Also, the fuel with a smaller soot fraction, PMMA,

.produced filter plugging at lower accumulated aerosol mass deposits on or within the

*fil1 ter.

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