T E C H N I C A L R E P O R T S S E R I E S No. 1 2 2

Air Filters for Use

at Nuclear Facilities

$ I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y , V I E N N A , 1 9 7 0

AIR FILTERS FOR USE

AT NUCLEAR FACILITIES

The following States are Members of the International Atomic Energy Agency:

AFGHANISTAN GREECE NORWAY ALBANIA GUATEMALA PAKISTAN ALGERIA HAITI PANAMA ARGENTINA HOLY SEE PARAGUAY AUSTRALIA HUNGARY PERU AUSTRIA ICELAND PHILIPPINES BELGIUM INDIA 'POLAND BOLIVIA INDONESIA PORTUGAL BRAZIL IRAN ROMANIA BULGARIA IRAQ SAUDI ARABIA BURMA IRELAND SENEGAL BYELORUSSIAN SOVIET ISRAEL SIERRA LEONE

SOCIALIST REPUBLIC ITALY SINGAPORE CAMBODIA IVORY COAST SOUTH AFRICA CAMEROON JAMAICA SPAIN CANADA JAPAN SUDAN CEYLON JORDAN . SWEDEN CHILE KENYA SWITZERLAND CHINA , KOREA. REPUBLIC OF .. ' SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND CONGO, DEMOCRATIC LEBANON TUNISIA

REPUBLIC OF LIBERIA TURKEY COSTA RICA LIBYAN ARAB REPUBUC UGANDA CUBA LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST CYPRUS LUXEMBOURG REPUBUC CZECHOSLOVAK SOCIALIST MADAGASCAR UNION OF SOVIET SOCIALIST

REPUBLIC MALAYSIA REPUBLICS DENMARK MALI UNITED ARAB REPUBLIC DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT ECUADOR MONACO BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA GABON NIGER VIET-NAM GERMANY, FEDERAL REPUBLIC OF NIGERIA YUGOSLAVIA GHANA ZAMBIA

The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is " to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world".

© I A E A . 1970

Permission to reproduce or translate the information contained in this publication may be obtained by writing to the InternationalAtomic Energy Agency, Karntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria.

Printed by the IAEA in Austria December 1970

TECHNICAL REPORTS SERIES No. 122

AIR FILTERS FOR USE

AT NUCLEAR FACILITIES

A Guidebook p r e p a r e d by

P . LINDER

Akt iebo lage t A t o m e n e r g i , Studsvik , Nykoping, Sweden

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1970

AIR F I L T E R S FOR USE AT NUCLEAR FACILITIES IAEA, VIENNA, 1970

S T l / D O C / l O / 1 2 2

FOREWORD

One of the m a i n p r o b l e m s in the planning, des ign , c o n s t r u c t i o n and o p e r a t i o n of any n u c l e a r f ac i l i t y , be it l a r g e o r s m a l l , i s tha t of r e m o v i n g p a r t i c u l a t e m a t t e r f r o m ven t i l a t ion exhaus t a i r b e f o r e it i s d i s c h a r g e d to the s u r r o u n d i n g a t m o s p h e r e .

T h i s Guidebook was p r e p a r e d ch ie f ly f o r t he benef i t of M e m b e r S ta t e s that a r e developing, o r a r e about to develop , t h e i r own n u c l e a r p r o g r a m s and of t h o s e that a r e p lanning to u s e n u c l e a r p o w e r in the f u t u r e . It d i f f e r s f r o m o the r publ i shed l i t e r a t u r e on f i l t e r s in the i n t e r n a t i o n a l r a t h e r than na t iona l c h a r a c t e r of t he i n f o r m a t i o n it con t a in s . Al though e m p h a s i s i s h e r e p laced on f i l t e r s , i n f o r m a t i o n on ven t i l a t ion s y s t e m s i s a l s o inc luded .

M r . Edward W. W i e d e r h o l d of the IAEA, ac t ing a s P r o j e c t O f f i c e r , was r e s p o n s i b l e f o r o rgan i z ing and d i r e c t i n g "the work of p r e p a r a t i o n . The Guidebook p r e s e n t s in e a s i l y r e a d a b l e f o r m the t h e o r y of f i b r o u s f i l t r a t i o n , and the t e s t i ng , o p e r a t i o n and m a i n t e n a n c e of a i r f i l t e r i n s t a l l a t i o n s at n u c l e a r f a c i l i t i e s .

CONTENTS

1. . I N T R O D U C T I O N . . . . . . . . . • 1

1 .1 . S c o p e . . . ; 1

1. 2.: N e c e s s i t y of a i r c lean ing at n u c l e a r f a c i l i t i e s 1

1. 3. N a t u r e of d i f f e r e n t types of a i r b o r n e rad ioap t ive , . i m p u r i t i e s . . 4

1. 4. P r i n c i p l e s of r e m o v i n g i m p u r i t i e s f r o m a gas s t r e a m . . . 7

2. THEORY OF FIBROUS FILTRATION. . . . . . . . ... . . . . 9

3. TYPES OF F I L T E R 12

4. DESIGN OF F I L T E R UNITS • . . 16

5. F I L T E R P R O P E R T I E S 20

5. 1. A i r - f l o w capac i ty and g e o m e t r i c a l s i ze 20

5. 2. P r e s s u r e d rop 20

5 . 3 . P e n e t r a t i o n 21

5. 4. Dus t -ho ld ing capac i ty 24 5. 5. R e s i s t a n c e to heat , f i r e , humidi ty and c h e m i c a l a t tack . . 27

5. 6. Mechan ica l r e s i s t a n c e 28

6. TESTING METHODS 29

6 . 1 . G e n e r a l t e s t s 29

6. 2. Re ten t ion e f f i c i ency t e s t s 33

6. 3. Leakage t e s t s 37

6 . 4 . Dust loading t e s t s 38

6. 5. I n - p l a c e t e s t i n g t echn iques 38

7. F I L T E R INSTALLATION AND CHANGING TECHNIQUES 42

7. 1. Vent i la t ion s y s t e m , g e n e r a l layout 42

7. 2. F i l t e r s y s t e m s , p r e f i l t e r s and f ina l f i l t e r s 48

7. 3. F i l t e r mount ing and changing a r r a n g e m e n t s 50

7 . 4 . L a b o r a t o r i e s 58

7. 5. R e a c t o r bui ldings 62

7. 6. I n s t r u m e n t a t i o n 63

8. F I L T E R SELECTION AND SPECIFICATION 65

9. F I L T E R HANDLING AND INSPECTION 69

9 . 1 . Handl ing '. 69

9 . 2 . Inspec t ion 69

9. 3. Inspec t ion on de l i ve ry 70

9 . 4 . S to rage 70

9. 5. Changing 71

9. 6. T e s t i n g and inspec t ion 72

9 . 7 . D i s p o s a l 73

10. ECONOMIC ASPECTS 73

R E F E R E N C E S 75

1. INTRODUCTION

1 . 1 . Scope

The vent i la t ion s y s t e m of a n u c l e a r f ac i l i ty p lays a v i ta l ro l e in e n s u r i n g that the a i r in work ing a r e a s and the env i ronmen t r e m a i n s f r e e f r o m rad ioac t ive con tamina t ion . An e a r l i e r IAEA publ icat ion, T e c h n i q u e s f o r Cont ro l l ing Air Pol lu t ion f r o m the Opera t ion of N u c l e a r F a c i l i t i e s , Safety S e r i e s No. 17 [ 1] , d e a l s with the des ign and o p e r a t i o n of ven t i l a -t ion s y s t e m s at n u c l e a r f a c i l i t i e s . T h e s e s y s t e m s a r e u sua l ly provided with a i r - c l e a n i n g d e v i c e s which r e m o v e the con taminan t s f r o m the a i r .

T h i s publ ica t ion i s intended a s a guide to those who a r e conce rned with the des ign of a i r - f i l t e r i n g s y s t e m s and with the t es t ing , ope ra t i on and m a i n t e n a n c e of a i r - f i l t e r i n s t a l l a t i o n s at n u c l e a r f a c i l i t i e s . E m p h a s i s i s ma in ly placed on so - ca l l ed h i g h - e f f i c i e n c y pa r t i cu l a t e a i r f i l t e r s (HEPA f i l t e r s ) and on providing g e n e r a l i n f o r m a t i o n on t h e m . B e s i d e s d e s c r i b i n g the u sua l f i l t e r t ypes , t h e i r d i m e n s i o n s and c o n s t r u c t i o n m a t e r i a l s , the guidebook a t t e m p t s to expla in t h e i r p r o p e r t i e s and behav-i o u r unde r d i f f e r en t o p e r a t i n g condi t ions . It a l so gives advice on t e s t i n g and handl ing the f i l t e r s so that e f fec t ive and sa fe p e r f o r m a n c e i s e n s u r e d .

The guidebook should s e r v e a s an in t roduc t ion to the use of high-e f f i c i e n c y pa r t i cu l a t e a i r f i l t e r s in c o u n t r i e s w h e r e work with r ad ioac t i ve m a t e r i a l s ha s only r e c e n t l y c o m m e n c e d . The l i s t of r e f e r e n c e s at the end of the book i n d i c a t e s s o u r c e s of m o r e advanced i n f o r m a t i o n f o r those who a l r e a d y have c o m p r e h e n s i v e e x p e r i e n c e in t h i s f i e ld . It i s a s s u m e d h e r e that the f i l t e r s a r e obtained f r o m a m a n u f a c t u r e r , and the guidebook thus con ta ins no i n f o r m a t i o n on the des ign and deve lopment of the f i l t e r i t s e l f , no r does it dea l with the c lean ing of the in take a i r to a plant, with g a s so rp t ion o r p ro t ec t i ve r e s p i r a t o r y equ ipmen t .

1. 2. N e c e s s i t y of a i r c l ean ing at n u c l e a r f a c i l i t i e s

All work involving the handl ing of r a d i o a c t i v e m a t e r i a l may c a u s e the a i r to b e c o m e c o n t a m i n a t e d . Dur ing the f a b r i c a t i o n of fue l e l e m e n t s f o r n u c l e a r r e a c t o r s u r a n i u m dust i s f o r m e d , o r in a r e a c t o r plant i t se l f the f i s s i o n p roduc t s and the induced ac t iv i ty in the cool ing m e d i u m m a y a p p e a r ou ts ide the c losed s y s t e m and con tamina te the a i r . In a r e p r o c e s s -ing plant l a r g e a m o u n t s of f i s s i o n p roduc t s a r e handled , and in i so tope produc t ion p lants and i so tope l a b o r a t o r i e s m a n y k inds of nuc l ides a p p e a r . T h e p e r s o n n e l in such p lan ts can be p ro tec ted by a well des igned ven t i l a -t ion s y s t e m and by us ing p ro tec t ive d e v i c e s such a s f u m e - h o o d s and g l o v e - b o x e s o r r e s p i r a t o r y equ ipmen t . Outs ide the bui lding, however , an unaccep tab le l eve l of r ad ioac t i ve con tamina t ion could o c c u r , even if t he a i r w e r e di luted when r e l e a s e d to the a t m o s p h e r e . Not only i s t h e r e a h a z a r d f r o m a high concen t r a t i on in the a i r , but a l so f r o m poss ib l e depos i t ion on the ground and uptake by p l an t s . V a r i o u s pa ths thus ex i s t by which r ad ioac t iv i t y can r e a c h m a n , f o r e x a m p l e through foods such a s mi lk o r v e g e t a b l e s .

The d e g r e e of r e m o v a l n e c e s s a r y might have to be d e t e r m i n e d on t h e b a s i s of m a x i m u m c r e d i b l e r e l e a s e u n d e r acc ident condi t ions r a t h e r t han on that of the n o r m a l ope ra t i ng l eve l of con tamina t ion .

1

Methods for Particle Size

Analysis

«

-Impingers--Ultramicrosocope-

- Electron Microscope -

-Ultracentrifuge-

K X-Ray Diffraction-

-Centr i fuge-

Sieves -Microscope

U* K 1—Elutriation -

-Turbidimetry-- Sedimentation -

-Adsorption — I-Permeability—

-Nuclei Counter4 -Light Scattering-

-Scanners

-Electrical Conductivity-

- Sieving-Furnishes average particle diameter but no size distribution.

+ + S i z e distribution may be obtained by special calibration.

-Visible to Eye-

Machine Tools (Micrometers, Calipers, etc.)-

Types of Gas Cleaning

Equipment '

Ultrasonics (very limited industrial application)

k --Settl ing Chambers

Centrifugal Separators--Liquid Scubbersl >

-Cloth Collectors--Packed Beds—

-High Efficiency Air Filters— I

Thermal Precipitation

i- Common Air Filters-—Impingement Separators-

fused only tor sampling)

- Electrical Precipitators-~ T

1 0 -

-Mechanical Separators-

Terminal Gravitational

Settling* for spheres,! sp. gr. 2.0 J

Reynolds Number 10" 1 Iff7

_J 10" 10 10"', 10"

Settling Velocity, cm/sec. 10"

Reynolds Number |ur , 5io"14 io"13

I I I Settling Velocity,

.cm/sec. 10- 10 , 2 3 5 | 235 I ill I 111

1 0 " I

10"' 10"1 2 3 5 . 2 3 5

10, 10", 10". t i r i

io"! io", io":

10"° 10 , io" i ? i i

10" 1 0 " I-

10°, 10' I . -1

10' I

1 0 ' 10 iff

10 10

10J

I

10 10

Particle Diffusion -. Coefficient'

cm 2/sec.

In Air at 25-C..

latm. _ In Water

at 25-C.

10"!

10"' 10"?" 10" 10" 10"° „ 2 | 5 3 2 | 5 3 2 , 5 3 2 , 5 3 2 , 65 1 ' I - i I I I I I i I ' I I I L I I I I L1_L

1 0 " ".6 5 4 3" 2 1 0 " I I I

1 0 " 10"' 1 0 " • 1 0 " 10"

10- 10" 1 0 "

•StokesCunmngham factor included in values given for air but not included for water

2 3 4 5 6 S i 2• 3 4 56 '2 3 4 56 S | 2 3 4 5 6 8 2 3 4 56 2 3 4 56. 0.0001 0.001

(Imp) .0.01 . 0.1 1

Particle Diameter, microns (p)

10 100 1,000 (lmm.)

10,000 (lcm.)

PREWRED BY C E LAPPLE

"FIG.l ." Charac te r i s t i c ' s of p a r t i c l e s and p a r t i c l e d i spe r so ids . (Cour t e sy C . E . Lapp ie , S tanford Research I n s t i t u t e . )

Not only the h a z a r d to hea l th h a s to be cons ide red when dec id ing the e f f i c i ency needed f o r an a i r - c l e a n i n g in s t a l l a t i on . S o m e t i m e s t e c h n i c a l r e a s o n s will domina t e , such a s the n e c e s s i t y f o r low background c o n t a m i n a -t ion in the plant , f o r example to p ro tec t m a t e r i a l such a s pho tographic f i l m that i s s ens i t i ve to r ad i a t i on o r con tamina t ion . B e f o r e a big ven t i l a -t ion and a i r - c l e a n i n g plant i s c o n s t r u c t e d , a l l m e a n s of m i n i m i z i n g the f o r m a t i o n of a e r o s o l s in the bui lding should have been s tudied , such a s wet handl ing and reduc t ion of the amount of r ad ioac t ive m a t e r i a l involved in the p r o c e s s e s . If the f o r m a t i o n of d u s t s can s t i l l not be avoided , the h a z a r d s f r o m a e r o s o l s can be r educed by enc los ing the p r o c e s s in hoods , f u m e cupboa rds o r g love -boxes . The s m a l l e r vo lume of a i r involved r e d u c e s the s i z e of the f i l t e r ins t a l l a t ion in the s a m e p ropor t ion , even if a h i g h e r dust concen t ra t ion l e a d s to m o r e f r equen t changing of the f i l t e r s .

1 . 3 . N a t u r e of d i f f e r en t t ypes of a i r b o r n e r ad ioac t i ve i m p u r i t i e s

Def in i t ions of the d i f f e r en t t e r m s used to d e s c r i b e a i r b o r n e r ad ioac t ive po l lu tan t s a r e given below.

A e r o s o l s a r e v a r i o u s s y s t e m s of p a r t i c l e s d i s p e r s e d in the a i r , such a s dus t , f u m e , smoke and m i s t .

Dus t s a r e solid p a r t i c l e s f o r m e d by the d i s i n t eg ra t i on of m a t e r i a l s by such p r o c e s s e s as c ru sh ing , mi l l ing , gr inding, mach in ing o r s iev ing and o t h e r s i z e r educ t ion o r s e g r e g a t i o n p r o c e s s e s . The s i z e of the p a r t i c l e s that e s c a p e m a y v a r y f r o m s u b - m i c r o n to s o m e t e n s of m i c r o n s 1 .

F u m e s a r e f o r m e d by such p r o c e s s e s a s sub l imat ion , combus t ion and condensa t ion and a r e u sua l ly l e s s than 1 jum in s i z e . Lead f u m e , z inc oxide, u r a n i u m oxide, b e r y l l i u m oxide and f e r r i c oxide a r e typ ica l m e t a l l u r g i c a l f u m e s .

Smokes a r e p a r t i c u l a r l y c h a r a c t e r i z e d by having an a p p r e c i a b l e op t ica l dens i ty . Commonly s m o k e s a r e s y s t e m s that a r e o rgan i c in o r i g i n such a s s m o k e f r o m coal , oil , wood and o t h e r c a r b o n a c e o u s f u e l s . Smoke p a r t i c l e s c o m p a r e in s i ze to m e t a l l u r g i c a l f u m e s .

M i s t s a r e f o r m e d by vapour condensa t ion o r a tomiza t ion of l iquids . P a r t i c l e s i z e s can v a r y g r e a t l y depending on the c i r c u m s t a n c e s of f o r m a t i o n .

Another g roup of a i r pol lu tan ts i s f o r m e d by v a p o u r s and g a s e s , which a r e m o l e c u l a r d i s p e r s i o n s and m a y include the noble g a s e s , t r i t i u m , iodine and i t s vo la t i l e compounds , e t c .

A e r o s o l s can be r emoved f r o m the a i r by m e c h a n i c a l m e a n s , such a s cyc lones o r f i b r o u s f i l t e r s . F o r the r e m o v a l of v a p o u r s and g a s e s , a n o t h e r technique i s n e c e s s a r y , even if, f o r example , e l e m e n t a r y iodine can be adso rbed on the f i b r e s in f i l t e r s to a c e r t a i n extent . T h i s guidebook, however , only d i s c u s s e s the r e m o v a l of a e r o s o l s f r o m the a i r .

A e r o s o l s can be roughly c h a r a c t e r i z e d by pa r t i c l e s ize , s i ze d i s t r i b u -tion, and concen t r a t i on . P a r t i c l e s i z e s of s o m e typica l a e r o s o l s and pu lve r i zed m a t e r i a l a r e p r e s e n t e d in F i g 1. Only v e r y f ine p a r t i c l e s can r e m a i n a i r b o r n e f o r any length of t i m e , pe rmanen t a t m o s p h e r i c p a r t i c l e s be ing s m a l l e r than 1 jum. P a r t i c l e s l a r g e r than 10 ^ m s e t t l e r e l a t i v e l y r ap id ly . If inha la t ion i s a s s u m e d to be the n o r m a l way that p a r t i c u l a t e m a t t e r e n t e r s the body, the s m a l l e r s i ze f r a c t i o n s alone a r e of i n t e r e s t , a s only p a r t i c l e s with d i a m e t e r s up to ten o r p e r h a p s

1 1 mic ron ( | i m ) = 10~5 m e t e r (m) .

4

twenty m i c r o n s can p e n e t r a t e into the r e s p i r a t o r y s y s t e m . T h e s e p a r t i c l e s a r e not v i s ib le to the naked eye, and have thus to be ana lysed with an i n s t r u m e n t . Opt ical m i c r o s c o p y can be used f o r p a r t i c l e s down to 0 . 5 p m o r , unde r f a v o u r a b l e condi t ions , down to about 0 .2 jum. By us ing an e l e c t r o n m i c r o s c o p e , the r e so lu t ion r ange can be extended by one o r two o r d e r s of magn i tude . The u l t ima te r e s o l u t i o n f o r an e l e c t r o n m i c r o -scope i s of the o r d e r of 0 . 0 0 1 /um. When a pa r t i c l e i s o b s e r v e d by th i s m e a n s , the d i a m e t e r i s u sua l ly def ined a s the a r e a d i a m e t e r , e . g . the d i a m e t e r of a c i r c l e having the s a m e a r e a a s the p ro j ec t ed a r e a of the p a r t i c l e . When o t h e r m e t h o d s a r e used f o r o b s e r v i n g the p a r t i c l e s , the d i a m e t e r i s def ined a c c o r d i n g to the c h a r a c t e r i s t i c s used f o r ana lys ing the p a r t i c l e s , f o r example the se t t l ing ve loc i ty in a c e n t r i f u g e o r the l i g h t - s c a t t e r i n g p r o p e r t i e s in an au toma t i c pa r t i c l e c o u n t e r .

When taking a s a m p l e f r o m d u s t - l a d e n gas , a s i ze g r ad ing can be m a d e by us ing, f o r example , a c a s c a d e i m p a c t o r . Once the i m p a c t o r i s c a l i b r a t e d , the m e a n d i a m e t e r of the p a r t i c l e s on e v e r y s t a g e need not be d e t e r m i n e d f o r e v e r y s a m p l e . Only the amount of dus t co l lec ted on the d i f f e r e n t s t a g e s of the i m p a c t o r m u s t be ana lysed by counting the n u m b e r of p a r t i c l e s o r , in the c a s e of r a d i o a c t i v e dus t , by m e a s u r i n g the d i s i n t e g r a -t ion r a t e of the d i f f e r en t s l i d e s .

Since d e t e r m i n i n g the s i ze s p e c t r u m of p a r t i c l e s i s t ime consuming , i n d i r e c t m e t h o d s a r e of ten used , and in many c a s e s it i s su f f i c i en t to know the to ta l amount of i m p u r i t i e s p e r unit vo lume of the a i r . Methods1

f o r s ampl ing a e r o s o l s and f o r m e a s u r i n g the p a r t i c l e s i ze a r e d e s c r i b e d in de ta i l in the l i t e r a t u r e [ 35, 36],

P a r t i c l e s i ze d i s t r i bu t i on i s the r e l a t i v e a m o u n t s of p a r t i c l e s with d i f f e r e n t d i a m e t e r s . N o r m a l l y it i s r e p r e s e n t e d by a cumula t ive weight f r a c t i o n c u r v e in which the p ropo r t i on of p a r t i c l e s on a weight b a s i s , s m a l l e r than a c e r t a i n s ize , i s p lo t t ed ' aga ins t the d i m e n s i o n of the p a r t i c l e s . S o m e t i m e s the d i s t r i bu t i on i s d e s c r i b e d acco rd ing to the n u m b e r of p a r t i c l e s . The d i f f e r en t c u r v e s can be t r a n s f o r m e d into each o t h e r if the s p e c i f i c weight of the p a r t i c l e m a t e r i a l i s known. F r o m the d i s t r i bu t ion c u r v e s the m e a n d i a m e t e r can be obta ined, but it is i m p o r t a n t to r ecogn ize the d i f f e r e n c e be tween the m e a n d i a m e t e r by weight and the m e a n d i a m e t e r by n u m b e r . The m e a n d i a m e t e r by weight i s much l a r g e r , s ince the l a r g e p a r t i c l e s con t r ibu te m o r e to the weight than do the s m a l l but m o r e f r e q u e n t p a r t i c l e s . In m o s t c a s e s the l o g a r i t h m i c n o r m a l - p r o b a b i l i t y equat ion app l i e s to the d i s t r i bu t ion [4] , If a s p e c i a l s c a l e , based on the probabi l i ty i n t e g r a l , i s used , the cumula t ive p e r c e n t a g e plotted aga ins t the l o g a r i t h m of the d i a m e t e r of the p a r t i c l e s will be a s t r a i g h t l i ne . If s o m e g rad ing device i s used when sampl ing the p a r t i c l e s , an a s s u m e d d i s t r i bu t i on c u r v e can thus be obtained even if only a few s i z e f r a c t i o n s a r e d e t e r m i n e d .

Concen t ra t ion of a i r i m p u r i t i e s i s the c h a r a c t e r i s t i c that i s m o s t e a s i l y d e t e r m i n e d . It i s usua l ly e x p r e s s e d a s m i l l i g r a m s p e r cubic m e t r e ( m g / m 3 ) which i s n u m e r i c a l l y equal to m i c r o g r a m s p e r l i t r e o r m i c r o - o u n c e s p e r cubic foot . S o m e t i m e s g r a i n s p e r cubic foot o r g r a i n s p e r 1000 f t 3 i s u sed . One g r a i n p e r 1000 f t 3 i s equa l to 2 . 3 m g / m 3 . In F i g . 2 s o m e typica l c o n c e n t r a t i o n s a r e p r e s e n t e d . T h i s f i g u r e d e m o n -s t r a t e s that h i g h - e f f i c i e n c y f i l t e r s a r e su i tab le only f o r c o n c e n t r a t i o n s l o w e r than about 1 m g / m 3 . The need to change f i l t e r s will o c c u r f r e q u e n t l y if the concen t r a t i on i s much h i g h e r . A reduc t ion of the concen t r a t i on i s obtained by us ing a p p r o p r i a t e p r e f i l t e r s .

5

05

103 102 10 1 10"' 10-2 10"3 10"* JO" 5 10"6 I0"7 10"8 ICT9 ICT10 10"" 10"12 10-'3

L o w p r e s s u r e p n e u m a t i c D u s t s t o r m - I n d u s t r i a l d i s t r i c t " P o l l e n

co n v e yi n g , a i r .

E x p l o s i v e " C l o u d b u r s t M o d e r a t e D r i z z [ e F o g ^ C i t y a i r

c o n c e n t r a t i o n r o ' n a n d m i s t S u b u r b on a n d o f a i r b o r n e d u s t p . . " "" \

r l y - o s n ru ra l a i r

e f f l u e n t

I n d u s t r i a l s t a c k e f f l u e n t s F o u n d r y w o r k r o o m

S t o n e d r y i n g B r a s s f o u n d r y a i r B | a s t f u r n o c e C o t t o n m i l l w o r k r o o m

M i n e a i r

I n d u s t r i a l d u s t , m a x i m u m a c c e p t a b l e c o n c e n t r a t i o n s

F e 2 0 3 M n S b A s H g , C d T e , B e

S i l i c a t e s

R a d i o a c t i v e . m a t e r i a l s, m a x i m u m a c c e p t a b l e c o n c e n t r a t i o n s

! 2 3 9 P u 2 2 6 R a 9 0 S r 3 2 P . 2 4 N a 1 3 1 l

- 4 x T O ' 1

C o n c e n t r a t i o n s d e a l t w i t h by h i g h - e f f i c i e n c y f i l t e r s

C o n c e n t r a t i o n s d e a l t w i t h b y C o n c e n t r a t i o n s d e a l t w i th by

c y c l o n e s a i r - c o n d i t i o n i n g f i l t e r s

C o n c e n t r a t i o n s d e o l t w i t h by

i n d u s t r i a l f i l t e r s

C o n c e n t r a t i o n s dea l w i th by

e l e c t r o s t a t i c p r e c i p i t a t o r s

FIG. 2. Dust concent ra t ions occurr ing in d i f fe ren t s i tuat ions ( g / m 3 ) . (Courtesy Butterworth and Co . L t d . , London . )

F r o m F i g . 2 it can a lso be recognized that- the m a x i m u m accep tab le concen t ra t ions of r ad ioac t ive m a t e r i a l s a r e m a n y . o r d e r s of magni tude . , s m a l l e r than t hose . fo r indus t r i a l dus t s . This explains why a spec ia l a i r - c l e a n i n g technique has been developed f o r use in f ac i l i t i e s dealing, with radioac t ive m a t e r i a l s . The h igh-e f f i c iency f i l t e r s a r e well adapted to coping with the r e q u i r e m e n t s fo r cleaning rad ioac t ive ly contaminated a i r . Such f i l t e r s mus t p o s s e s s high ef f ic iency fo r s m a l l pa r t i c l e s , even f o r pa r t i c l e s in the s u b - m i c r o n range , and they should p r i m a r i l y deal with low concent ra t ions , e . g . below the region of conventional c leaning dev ices . - . . .. . ,

The or igin of the a i r b o r n e radioac.tive impur i t i e s depends on the p r o c e s s e s involved. General ly , the rad ioac t ive nucl ides a r e fo rmed by neutron act ivat ion of va r ious subs tances or as a r e s u l t of f i s s ion : r eac t ion . Na tura l rad ioac t ive m a t e r i a l a l so occurs , such as. uranium and r a d i u m . :

In r ad iochemica l , rad io iso tope and other laboratories, the a i r is contaminated dur ing the production of a r t i f i ca l , i so topes , in the p rocess^ ing of i r r ad i a t ed m a t e r i a l s and by syn thes i s of. label led compounds i

In hot l a b o r a t p r i e s , rad ioac t ive produc ts can escape dur ing the mechan i -ca l and chemica l p r o c e s s i n g of i r r a d i a t e d f i s s ionab le m a t e r i a l s .

In u ran ium product ion plants and fue l e lement f a c t o r i e s , l a rge amounts of na tu ra l rad ioac t ive m a t e r i a l a r e handled. Even if the spec i f i c act ivi ty i s low, d u s t - f o r m i n g p r o c e s s e s will enta i l health p r o b l e m s if a r r a n g e -m e n t s are. not. made to cont ro l the d i s p e r s i o n of dus t .

In power r e a c t o r s , f i s s ion produc ts can appear in the cool ing.medium if t h e r e a r e holes , even as s m a l l as pinholes, in the cladding of the fuel . e l e m e n t s . Cor ros ion products and the cooling medium i t s e l f , can : be act ivated by neut ron cap tu re . F r o m the, o f f - g a s sys t em and by leakage f r o m the p r i m a r y sys t em both radioac t ive a e r o s o l s and g a s e s can appea r in the a i r .

In r e s e a r c h r e a c t o r s , rad ioac t ive nucl ides are. a lso produced in the , i r r a d i a t i o n f ac i l i t i e s and the re i s a lways a r i s k of damage to the enc losu re that would lead, to a contaminat ion of the fac i l i ty and the a i r . .

In r e p r o c e s s i n g plants, where the plutonium and the r ema in ing uran ium a r e r ecove red f r o m i r r a d i a t e d fuel e l emen t s , r igid cont ro l of the airborne rad ioac t iv i ty is an absolute n e c e s s i t y . Leakprqof .enclosur .es , extensive vent i la t ion ins ta l la t ions a.nd ef fec t ive a i r - c l e a n i n g dev ices a r e ind i spensab le .

1 . 4 . P r i nc ip l e s of removing i m p u r i t i e s f r o m a gas s t r e a m

In nuc lea r f ac i l i t i e s , a i r cleaning is usual ly done by f ib rous f i l t r a t ion , s ince th is i s the only method with the r equ i r ed high e f f ic iency fo r the r e m o v a l of sma l l p a r t i c l e s . If the a e r o s o l concent ra t ion is .high, some type of p rec lean ing device i s needed, and f o r th is purpose some of the . me thods desc r ibed .in th is chap te r can be used . , ,.

T h e r e a r e five p r inc ipa l methods, of removing p a r t i c l e s f r o m a gas, s t r e a m [ 1 , 4 ] : g rav i ta t iona l set t l ing; d r y ine r t i a l and cen t r i fuga l s e p a r a -tion; .washing and wet scrubbing; e l e c t r o s t a t i c prec ip i ta t ion; f i l t r a t ion .

Two s teps a r e n e c e s s a r y in r emoving par t i cu la te m a t t e r f r o m the a i r . The f i r s t c o n s i s t s in p r e p a r i n g the p a r t i c l e s fo r sepa ra t ion , o r . > . • condit ioning them, by f loccula t ing o r wetting to make deposi t ion e a s i e r by gravi ty o r cen t r i fuga l f o r c e s o r in charg ing them f o r subsequent

7

e l e c t r o s t a t i c p rec ip i t a t ion . F o r the ac tua l s e p a r a t i o n one o r s e v e r a l of the fol lowing f o r c e s a r e used : g rav i ty , i n e r t i a l o r c e n t r i f u g a l f o r c e s , d i f fus ion and e l e c t r i c a l f o r c e s . The e f fec t of s ieving and in t e r cep t ion i s a l so used to s o m e extent in f i l t e r s .

Grav i t a t i ona l se t t l ing i s i m p r a c t i c a l f o r p a r t i c l e s s m a l l e r than 50-100 (im b e c a u s e of t he i r s low se t t l ing ve loc i ty , which would lead to a l a r g e s p a c e r e q u i r e m e n t even f o r m o d e r a t e e f f i c i ency . S o m e t i m e s l a r g e p a r t i c l e s se t t l e in ven t i l a t ion duc t s and c a u s e r ad ia t ion p r o b l e m s .

Dry i n e r t i a l and c e n t r i f u g a l s e p a r a t o r s i n c r e a s e the se t t l ing ve loc i ty by i n e r t i a l f o r c e s , e i t h e r th rough de f l ec t ion of the a i r s t r e a m o r th rough a s p i r a l movemen t of the a i r in a cyc lone . The l a t t e r type i s p robab ly the m o s t widely used pa r t i c l e r e c o v e r y unit in i n d u s t r y . Owing to i t s r obus t s i m p l i c i t y and r e l a t i v e l y low r e s i s t a n c e to gas flow, the cyclone i s well su i t ed to r e m o v i n g p a r t i c l e s l a r g e r than 20 ium, and m a y even be used f o r p a r t i c l e s down to 5 /um. High dust c o n c e n t r a t i o n s can be handled and such s e p a r a t o r s can be used f o r e i t h e r hot o r cold g a s e s . Cyclones a r e o f t en used a s p r i m a r y s e p a r a t o r s .

With the washing and wet s c r u b b i n g method , the m a i n p rob lem i s to b r i n g the p a r t i c l e s into contac t with the wash ing l iquid. In a wet s c r u b b e r the liquid i s sp r ayed into the gas s t r e a m o r the gas s t r e a m i s used f o r the d i s p e r s i o n of the w a t e r . Gene ra l l y wet s c r u b b e r s a r e not v e r y e f f ec t i ve f o r s m a l l p a r t i c l e s , but they a r e v e r y u s e f u l fo r adso rp t ion of acid m i s t s and v a p o u r s . The gas i s cooled by the s p r a y wa te r , which i s s o m e t i m e s a d i sadvantage , a s the s tack l i f t and plume r i s e a r e r e d u c e d . In s p r a y t o w e r s and cyclone s c r u b b e r s only the liquid d r o p l e t s ca tch the dus t p a r t i c l e s , but in wet f i l t e r s and packed t o w e r s the contac t s u r f a c e i s i n c r e a s e d by the g l a s s o r p l a s t i c f i b r e f i l t e r s and the packing. The v e n t u r i s c r u b b e r i s the m o s t e f f i c ien t of the wet c leaning dev ices , but the p r e s s u r e d r o p i s high.

As the a i r l eav ing a wet s c r u b b e r i s o f ten s a t u r a t e d with w a t e r o r con ta ins s o m e m i s t if the w a t e r e l imina t ion i s not comple te , a sub-sequent high e f f i c i ency c l e a n - u p m a y b e d i f f i cu l t . The r e l a t i ve humidi ty of the a i r h a s to be d e c r e a s e d e i t h e r by hea t ing the air. o r by mixing it with d r y a i r .

E l e c t r o s t a t i c p rec ip i t a t ion i s achieved by giving the p a r t i c l e s an e l e c t r i c a l c h a r g e with ioniz ing e l e c t r o d e s and then co l lec t ing them on c h a r g e d p l a t e s . The accumula t ed dus t m a y be r emoved f r o m the co l l e c t -ing p la tes by d r y (mechan i ca l shaking) o r wet (washing) t echn iques . The advantage of e l e c t r o s t a t i c p r e c i p i t a t o r s i s that v e r y l a r g e v o l u m e s of a i r can be handled at v e r y low p r e s s u r e d r o p s and that they can be used at high t e m p e r a t u r e s . In p r a c t i c e , a g r ea t dea l of a t ten t ion i s r e q u i r e d if high e f f i c i e n c i e s a r e to be m a i n t a i n e d . Usua l ly the c o a r s e r p a r t i c l e s a r e r emoved by a s e p a r a t e c lean ing dev ice . In r ad ioac t i ve work , the pos s ib i l i t y of a g g l o m e r a t e s be ing dis lodged f r o m the p r e c i p i t a t o r m e a n s that a f i l t e r m u s t be i n s t a l l e d . T h i s h a s undoubtedly made the use of e l e c t r o s t a t i c p r e c i p i t a t o r s u n a t t r a c t i v e . N e v e r t h e l e s s , they can provide e f f ec t ive in le t a i r f i l t r a t i o n .

F i l t r a t i o n i s a c c o m p l i s h e d by a bed of f i l t e r m e d i u m cons i s t i ng of w i r e s c r e e n , m e t a l wool, g l a s s wool o r d i f f e r e n t kinds of f ine f i b r e that a r r e s t the p a r t i c l e s by d i r e c t i n t e r cep t ion , i n e r t i a l impact ion , d i f f u s i o n and e l e c t r o s t a t i c f o r c e s . The m e c h a n i s m of f i l t r a t i o n i s d e s c r i b e d in c h a p t e r 2.

8

F i l t e r s a r e u sua l ly used f o r r e m o v i n g s m a l l p a r t i c l e s in low con-cen t r a t i on , f o r e x a m p l e a t m o s p h e r i c po l lu t an t s . The f i l t e r s u sua l ly a r e th rown away when they a r e loaded with dust , but s o m e types m a d e of m o r e m e c h a n i c a l l y r e s i s t a n t m a t e r i a l s , such a s m e t a l wool, can be c leaned and r e - u s e d .

A s p e c i a l type of f i l t e r i s the cloth f i l t e r , which can be used f o r heavy dust l oad ings . T h i s f i l t e r can be c leaned and i s o f t en used f o r r e c o v e r i n g u s e f u l m a t e r i a l .

To r e m o v e v a p o u r s and g a s e s , p r i n c i p l e s o the r then those f o r pa r t i cu l a t e m a t t e r have to be app l i ed . The fol lowing b a s i c m e t h o d s can be used : ad so rp t i on and c h e m i s o r p t i o n , abso rp t ion , and r e t en t ion in s t o r a g e unti l t he i so tope has subs t an t i a l l y d e c a y e d .

2. THEORY OF FIBROUS FILTRATION

The t h e o r y of f i l t r a t i o n i s he lp fu l in p r e d i c t i n g the e f f ec t of v a r i a t i o n in o p e r a t i n g p a r a m e t e r s on f i l t e r p e r f o r m a n c e . The m a i n m e c h a n i s m of f i l t r a t i o n i s t h e r e f o r e b r i e f l y d e s c r i b e d in th i s c h a p t e r [4] ,

A f i b r o u s f i l t e r c o n s i s t s of f ine f i b r e s in the f o r m of a th in p a p e r - l i k e m a t e r i a l o r of a l o o s e r and t h i c k e r pad . The i n t e r f i b r e d i s t a n c e s a r e l a r g e in c o m p a r i s o n with the r a d i i of t he f i b r e s and the p a r t i c l e s to be r e m o v e d . A f i b r o u s a i r f i l t e r i s in no way a s i e v e in the u s u a l s e n s e of t he t e r m .

The pack ing dens i ty , def ined as the vo lume of f i b r e s divided by the to ta l vo lume of t he f i l t e r , i s n o r m a l l y l e s s t han one ten th . The f i l t r a t i o n e f f i c i e n c y i s highly dependent on f i b r e r a d i u s and, in many h i g h - e f f i c i e n c y f i l t e r s , t he g r e a t m a j o r i t y of f i b r e s a r e of a r a d i u s l e s s than 2 /urn, a l though l a r g e r f i b r e s m a y be p r e s e n t to act a s c a r r i e r s f o r t h e i r f i n e r and m o r e f r a g i l e n e i g h b o u r s .

The d i f f e r e n t ways in which a p a r t i c l e i s caught by the f i b r e a r e u s u a l l y def ined as i n t e r c e p t i o n , p a r t i c l e i n e r t i a , d i f fus ion and e l e c t r o s t a t i c f o r c e s . It i s a s s u m e d that a l l p a r t i c l e s tha t c o m e into contac t with the f i b r e a d h e r e to the f i b r e and r e m a i n t h e r e .

I n t e r cep t ion . All p a r t i c l e s tha t a r e caught by the f i b r e without any devia t ion f r o m the s t r e a m l i n e s a round the f i b r e a r e sa id to be caught by i n t e r c e p t i o n (F ig . 3). Th i s m e a n s tha t p a r t i c l e s with the r a d i u s r p wi l l be caught if the c e n t r e of the p a r t i c l e i s a s s o c i a t e d with a s t r e a m l i n e at a d i s t a n c e rp f r o m the f i b r e . T h i s m e c h a n i s m is t h e r e f o r e m o r e i m p o r t a n t f o r p a r t i c l e s of l a r g e r r a d i i and i s independent of ve loc i ty a s long a s the f low p a t t e r n r e m a i n s unchanged .

F I G . 3 . P a r t i c l e c a u g h t by i n t e r c e p t i o n .

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FIG. 6. The e f fec t s of iner t ia , diffusion and in tercept ion on the p e n e t r a t i o n - p a r t i c l e - s i z e curve .

• FIG. 7. The e f fec t s of ine r t i a , diffusion and in tercept ion on the pene t r a t i on -ve loc i ty c u r v e .

I n e r t i a . When the s t r e a m l i n e s bend on m e e t i n g an o b s t a c l e , t he p a r t i c l e s in the a i r tend to m o v e s t r a i g h t f o r w a r d , depending on the i n e r t i a of the p a r t i c l e (Fig, 4). The i n e r t i a l f o r c e s depend on the m a s s and ve loc i ty of the p a r t i c l e and the r e m o v a l e f f ec t by i n e r t i a wil l be g r e a t e r f o r h e a v i e r p a r t i c l e s and h i g h e r v e l o c i t i e s .

D i f fus ion . F o r s m a l l p a r t i c l e s the Brownian mot ion - the e f f ec t on p a r t i c l e s of the t h e r m a l mot ion of the m o l e c u l e s in the a i r - c a u s e s devia t ion f r o m the s t r e a m l i n e s (F ig . 5). The devia t ion and the pos s ib i l i t y of the p a r t i c l e be ing r e m o v e d f r o m the a i r by the f i b r e a r e g r e a t e r with d e c r e a s i n g p a r t i c l e s i z e and ve loc i ty .

The d i f f e r en t e f f e c t s of p a r t i c l e s i z e and gas ve loc i ty on the p e n e t r a t i o n of p a r t i c l e s th rough a f i l t e r a r e shown s c h e m a t i c a l l y in F i g . 6.

Since p e n e t r a t i o n = 1 - r e m o v a l e f f i c i ency , the p e n e t r a t i o n of s m a l l p a r t i c l e s i s low due to r e m o v a l of the p a r t i c l e s by d i f fus ion e f f e c t s . F o r s m a l l p a r t i c l e s p e n e t r a t i o n i n c r e a s e s with i n c r e a s i n g p a r t i c l e s i ze , s i n c e d i f f u s i o n b e c o m e s l e s s e f f ec t ive f o r p a r t i c l e r e m o v a l . As the p a r t i c l e s i z e i n c r e a s e s f u r t h e r , i n t e r c e p t i o n and i n e r t i a b e c o m e m o r e i m p o r t a n t and the p e n e t r a t i o n d e c r e a s e s . Th is m e a n s tha t t h e r e i s a m a x i m u m p e n e t r a t i o n f o r p a r t i c l e s of a c e r t a i n s i ze , which m u s t be kept in mind when t e s t i n g f i l t e r s . A t e s t a e r o s o l with an a p p r e c i a b l e n u m b e r of p a r t i c l e s in the c r i t i c a l s i z e r a n g e i s u sua l ly u s e d .

The in f luence of ve loc i ty can be s t a t ed as fo l lows : at low ve loc i t i e s enough t i m e i s ava i l ab le f o r the p a r t i c l e s to d i f fuse to t h e s u r f a c e of the f i b r e s ; at h ighe r v e l o c i t i e s the d i f fu s ion wil l be l e s s i m p o r t a n t but the i n e r t i a l f o r c e s wi l l be g r e a t e r ( see F i g . 7); the i n t e r c e p t i o n e f f ec t i s independent of the ve loc i ty .

E l e c t r o s t a t i c f o r c e s . In f i e ld u s e , f i b r o u s f i l t e r s a r e r e l a t i v e l y u n c h a r g e d and g a s - b o r n e p a r t i c l e s p a s s i n g th rough the f i l t e r u sua l ly c a r r y a r e l a t i v e l y low c h a r g e . In the l a b o r a t o r y , howeve r , t h e s e r e l a t i v e l y u n c h a r g e d f i l t e r s a r e t e s t e d with spec i a l l y p r e p a r e d t e s t a e r o s o l s which a r e u sua l ly a s s u m e d to be e l e c t r i c a l l y n e u t r a l but may ac tua l ly be highly c h a r g e d . Condi t ions encoun te r ed in many f i b r o u s f i l t e r s a r e such tha t co l lec t ion r e s u l t i n g f r o m p a r t i c l e c h a r g e can be the p r i m a r y m e c h a n i s m f o r . p a r t i c l e co l l ec t ion . Th i s i s e s p e c i a l l y t r u e f o r p a r t i c l e s in the s i ze r a n g e 0 . 1 to 1 /um d i a m e t e r , a s t h i s i s the p a r t i c l e - s i z e r a n g e f o r m a x i m u m n e u t r a l a e r o s o l p e n e t r a t i o n . P a r t i c l e s l a r g e r than 1 /jm a r e co l lec ted m a i n l y by impac t ion , while p a r t i c l e s s m a l l e r t han 0. 1 jum a r e co l lec ted p r i n c i p a l l y by Brownian d i f fus ion .

2 . 1 . P e n e t r a t i o n and p r e s s u r e d rop

The b a s i c f i l t r a t i o n equat ion f o r a m o n o d i s p e r s e p a r t i c u l a t e cloud and idea l i zed m a t i s

n = noe -?

w h e r e n = c o n c e n t r a t i o n of p a r t i c l e s p e n e t r a t i n g the f i l t e r , n0 = concen t r a t i on of p a r t i c l e s e n t e r i n g the f i l t e r , and y .= index of f i l t r a t i o n of the m a t , p r o p o r t i o n a l to the depth of the m a t .

If the p r e s s u r e d r o p i s i n c r e a s e d by i n c r e a s i n g the t h i c k n e s s of t he m a t o r by i n c r e a s i n g the n u m b e r s of l a y e r s of p a p e r , the p r e s s u r e d rop

11

i s proportional to the depth and the index of filtration thus becomes proportional to the pressure drop.

'7 = k- p

f = = e^P

log f = -kj* p

k 1 = - log f

k j = quali ty f a c t o r of the f i l t e r med ium, f = f r a c t i o n of pene t ra t ion , and p = p r e s s u r e drop at defined veloci ty f o r c lean f i l t e r .

The pene t ra t ion plot ted agains t the p r e s s u r e d rop fo r va r i ous f i l t e r med ia will t heo re t i ca l ly be a s t r a igh t l ine in l i n - l o g - d i a g r a m with the p r e s s u r e drop as a b s c i s s a and the loga r i thm of pene t ra t ion as o rd ina te ( see F i g . 8).

100

t 0.1

0.001 0 10 20 30 U0 50

P r e s s u r e d rop mm w.g.

FIG. 8 . P e n e t r a t i o n versus pressure drop for va r ious f i l t e r m e d i a .

1 2 3 4 Velocity cm/sec

FIG. 9. Pressure drop versus v e l o c i t y through fibrous f i l t e r s .

The a i r veloci ty and p a r t i c l e s i ze m u s t be kept constant f o r any c o m p a r i s o n . Genera l ly speaking, however , to obtain a h igher e f f i c iency m o r e ene rgy has to be spent on ove rcoming the h igher p r e s s u r e d rop .

2. 2. The a i r - f l ow r e s i s t a n c e

Since the f i b r e s a r e ve ry s m a l l the flow through the f i l t e r i s l a m i n a r , which m e a n s that the p r e s s u r e drop i n c r e a s e s d i rec t ly p ropor t iona l to the a i r ve loci ty (see F i g . 9).

3. T Y P E S OF F ILTER

The quali ty of f i l t e r s f o r a i r - c l e a n i n g p u r p o s e s v a r i e s within a wide r a n g e . Air -condi t ioning uni t s f o r l a r g e a i r vo lumes with low dust loads a r e f i t ted with v i scous impingement , pane l type f i l t e r s and a r e v e r y inef f ic ien t f o r s m a l l p a r t i c l e s , about 20% of 1-jum p a r t i c l e s being r e m o v e d .

12

With h igh-e f f i c iency p a r t i c u l a t e a i r f i l t e r s (HEPA f i l t e r s ) , u s e d when a high d e g r e e of a i r c l ean ing i s r e q u i r e d , 99.99% of l-yum p a r t i c l e s a r e r e m o v e d and p r a c t i c a l l y al l p a r t i c l e s g r e a t e r than 5 /um.

C o m m o n f i l t e r s can be c l a s s i f i e d in t h r e e g roups and the e f f i c i e n c y of the d i f f e r e n t g roups i s given in Tab le I [ 6 ]. The r e p o r t e d e f f i c i ency f o r f i l t e r s depends upon the t e s t me thod u s e d and the u s e r should e x a m i n e f i l t e r e f f i c i ency da t a v e r y c a r e f u l l y to m a k e s u r e tha t he u n d e r s t a n d s what they m e a n .

The Group I f i l t e r s in Tab le I a r e pane l type f i l t e r s . They a r e m a d e with a f r a m e f i l l ed with f i l t e r m e d i a of g l a s s , wool, c e l l u l o s e o r p l a s t i c f i b r e s o r s o m e m e t a l m e d i u m such as w i r e s c r e e n , m e t a l m e s h o r m e t a l wool . The f i l t e r m e d i a a r e wet with o i l to i m p r o v e p a r t i c l e r e t e n t i o n and such f i l t e r s a r e t hus o f t en ca l led v i s c o u s i m p i n g e m e n t f i l t e r s . The f i l t e r s a r e e i t h e r t h rown away a f t e r u s e o r t he f i l t e r m e d i a only a r e r e p l a c e d in a p e r m a n e n t f r a m e (F ig . 10). F i l t e r s m a d e of m e c h a n i c a l l y m o r e r e s i s t a n t m a t e r i a l can be c leaned and r e u s e d , but g e n e r a l l y only once o r tw ice . F i l t e r s with a m e t a l m e d i u m can be c l eaned and wet so tha t the o r i g i n a l condi t ion i s r e s t o r e d .

T A B L E I . CLASSIFICATION OF COMMON AIR F I L T E R S

Group Eff ic iency Fi l ter type NBS e f f i c i e n c y 3

07°)

I Low Viscous i m p i n g e m e n t , panel type 5 - 3 5 b

II Modera te Extended m e d i u m , dry type 40 - 7 5 b

III High Extended m e d i u m , dry type 80 - 98°

a Nat ional Bureau of Standards, Dil l Dust-Spot Method [ D i l l , R . S . , A Test Method for Air Fi l ters , Nat ional Bureau of Standards ( 1 9 3 8 ) ] .

b Test with synthet ic dust . c Test with a tmospher ic dus t .

T h e r e a r e a l so au toma t i c f i l t e r s (F ig . 11), e i t h e r with s e c t i o n s of m e t a l s c r e e n s tha t a r e s e l f - c l e a n e d in an oil ba th o r a r o l l of t h r o w - a w a y f i l t e r m e d i u m tha t i s r o l l e d f o r w a r d when the p r e s s u r e d r o p h a s i n c r e a s e d to a p r e s e t va lue . The Group I f i l t e r s can be u s e d as p r e f i l t e r s f o r the high qual i ty f i l t e r s in Groups II and III.

F i l t e r s in Groups II and ILL a r e m a d e of d i f f e r e n t g r a d e s of f i n e r f i b r e s , f o r m e d as m a t s . Since t h e s e f i l t e r m e d i a have a h ighe r p r e s s u r e d rop than the c o a r s e f i l t e r s in Group I, they a r e des igned with an ex tended s u r f a c e to r e d u c e the ve loc i ty t h r o u g h the m e d i a and, consequent ly , the p r e s s u r e d r o p . Thus h ighe r e f f i c i ency can be obta ined with a m o d e r a t e i n c r e a s e in a i r - f l o w r e s i s t a n c e . The ex tended s u r f a c e i s ach ieved by des ign ing the f i l t e r with deep p l e a t s o r in the f o r m of long b a g s ( see F i g . 12).

A s p e c i a l type of f i l t e r that i s not u s e d f o r a i r - c o n d i t i o n i n g p l a n t s i s t he cloth f i l t e r . A c o m m o n cloth f i l t e r c o n s i s t s of t u b u l a r bags of cot ton, wool o r syn the t ic f i b r e s , i s highly e f f i c i en t and can dea l with high dust c o n c e n t r a t i o n s . Such f i l t e r s a r e c leaned by shak ing the b a g s o r with an a i r j e t , and a r e o f t en u s e d f o r r e c o v e r i n g va luab le m a t e r i a l .

13

F I G . 1 0 . Panel f i l t e r , throwaway type and c l e a n a b l e type . (Cour tesy A m e r i c a n Ai r -F i l t e r C o . )

F I G . 1 1 . Automat ic f i l ters , se l f -winding fibrous f i l ter and s e l f - c l e a n i n g oil f i l t e r . (Courtesy De lbag -Luf t f i l t e r G m b H . )

F I G . 1 2 . Extended m e d i u m , dry type f i l t e r , r ep laceab le m e d i u m type .

(Courtesy Cambr idge Fi l ter Corpora t ion . )

3 . 1 . H E P A - f i l t e r s

When c lean ing a i r of r a d i o a c t i v e o r tox ic c o n t a m i n a n t s , even s m a l l p a r t i c l e s have to be r e m o v e d with high e f f i c i ency a s s m a l l c o n c e n t r a t i o n s in the a i r can be h a r m f u l (F ig . 2). H E P A - f i l t e r s a r e s u p e r i o r to c o m m o n a i r f i l t e r s f o r r e m o v a l of s m a l l p a r t i c l e s (Table II) [ 6 ] a s a c o n s e q u e n c e of t he v e r y f i ne f i b r e s ( l e s s than 1 jum d i a m . ) of t he f i l t e r p a p e r u s e d in t h e m . Since the f i l t e r uni t i s des igned with m a n y deep p l e a t s , the a i r ve loc i ty t h r o u g h the f i l t e r m e d i u m i s low and the p r e s s u r e d r o p i s m o d e r a t e , about 25 m m w. g . , in sp i te of the high e f f i c i e n c y of 99. 99%. By u s i n g a s l igh t ly mod i f i ed f i l t e r p a p e r in the s a m e f i l t e r des ign , a good p r e f i l t e r i s ob ta ined with an e f f i c i ency b e t t e r than tha t of Group III a i r f i l t e r s , o r about 95% f o r 0. 3-jum p a r t i c l e s , and a low p r e s s u r e d rop , about 10 m m w. g.

T A B L E H. COMPARISON OF AIR F I L T E R S BY P E R CENT REMOVAL E F F I C I E N C Y FOR P A R T I C L E S OF VARIOUS SIZES

Group Eff ic iency

Removal e f f i c i e n c y p7o) for pa r t i c l e s ize of:

Group Eff ic iency 0 . 3 /. i m 1 . 0 p m 5 . 0 p m 10 .0 | im

I Low 0 - 2 1 0 - 3 0 4 0 - 7 0 9 0 - 9 8

II Modera te 1 0 - 40 4 0 - 7 0 8 5 - 9 5 9 8 - 9 9

III High 4 5 - 85 7 5 - 9 9 9 9 - 9 9 . 9 9 9 . 9

HEPA Extreme 9 9 . 9 7 m i n 9 9 . 9 9 100 100

The f i l t e r p a p e r u s e d in H E P A f i l t e r s i s m a n u f a c t u r e d by only a few f i r m s . M i c r o g l a s s f i b r e s of su i t ab l e qual i ty in the s i z e r a n g e 0. 5 to 1 jj.m a r e not ava i l ab le in many c o u n t r i e s , but by u s ing loca l ly ava i l ab le m a t e r i a l t h i s d i f f icu l ty can be o v e r c o m e . If a s b e s t o s f i b r e s a r e ava i l ab le , 70% a s b e s t o s and 30% ce l lu lose g ives op t imum p r o p e r t i e s with r e s p e c t to f i l t r a t i o n e f f i c i e n c y and dus t -ho ld ing capac i t y . The m e d i u m i s , howeve r , weak, and a s t r o n g e r p a p e r i s obta ined if g l a s s f i b r e s a r e added. A r e a s o n a b l e c o m p o s i -t ion migh t be 50% a s b e s t o s , 30% ce l lu lo se and 20% g l a s s [7] . The c e l l u l o s e f i b r e s su i t ab le f o r f i l t e r s a r e e s p a r t o g r a s s f i b r e s , and if t h e s e a r e not ava i l ab le c e r t a i n qua l i t i e s of r a g pulp can be u s e d [7] , If g l a s s f i b r e p a p e r i s not ava i l ab le , a f i l t e r c an be m a d e of a f i b r e m a t , f o r e x a m p l e 50% cot ton and 50% a s b e s t o s , a s d e s c r i b e d on page 141 in Ref . [4] , To r e d u c e t h e need f o r i m p o r t e d m a t e r i a l , a s and f i l t e r i s a p o s s i b l e so lu t ion . The des ign of such f i l t e r s and o p e r a t i n g e x p e r i e n c e s with them a r e r e p o r t e d in R e f s [ 8 , 9 ] .

4 . DESIGN OF F I L T E R UNITS

With a l a r g e vo lume of a i r , f o r e x a m p l e 100 000 m 3 / h , a l a r g e a r e a of f i l t e r m e d i u m i s r e q u i r e d to m a k e the p r e s s u r e d rop accep t ab l e and to obta in good f i l t r a t i o n e f f i c i e n c y . The advent of f i l t e r p a p e r s m a d e it p o s s i b l e to conta in a l a r g e f i l t r a t i o n a r e a within a l i m i t e d v o l u m e . The

16

commonly u s e d f i l t e r i s des igned a s i l l u s t r a t e d in F i g . 13 [4 ] . The f i l t e r p a p e r i s fo lded b a c k and f o r t h into p l e a t s and s u r r o u n d e d by a r e c t a n g u l a r f r a m e o r c a s i n g with s e a l i n g f a c e s on one o r both s i d e s . The indiv idual p l e a t s a r e u sua l ly s e p a r a t e d by c o r r u g a t e d s p a c e r s . Seal ing of the p l ea t ed f i l t e r med ium into the f r a m e i s i m p o r t a n t f o r high e f f i c i ency f i l t e r s , and v a r i o u s c e m e n t s and a d h e s i v e s a r e u s e d a c c o r d i n g to the o p e r a t i n g condi t ions .

In s o m e c a s e s s ea l i ng i s obta ined by c o m p r e s s i n g the ends with f ine f i b r e p a d s . The f i l t e r i s p rov ided with s e a l i n g g a s k e t s on one o r two s i d e s , depending upon the me thod of f i l t e r i n s t a l l a t i on . Some t y p e s of f i l t e r have no s p a c e r s and s p a c i n g i s ach ieved by p lea t ing the f i l t e r p a p e r in a s p e c i a l way. F i l t e r s m a d e without s p a c e r s a r e not s t r o n g enough f o r app l ica t ion to con tamina ted exhaus t [6] , and t h e r e i s no i n c r e a s e in t h e i r d u s t - l o a d i n g capac i ty with a t m o s p h e r i c a e r o s o l s [ 10]. Smal l f i l t e r un i t s a r e m a d e f o r g love-boxes .

A combina t ion of su i t ab le m a t e r i a l s in the d i f f e r e n t p a r t s of the f i l t e r e n s u r e s tha t i t h a s the p r o p e r t i e s n e c e s s a r y f o r a l l o p e r a t i n g cond i t ions . The u s u a l c o n s t r u c t i o n m a t e r i a l s u s e d in H E P A - f i l t e r s a r e d e s c r i b e d be low [ 1 , 4 , 6 ] .

4 . 1 . F i l t e r m e d i u m

The m o s t commonly u s e d f i l t e r m e d i a a r e c e l l u l o s e - a s b e s t o s , g l a s s , g l a s s - a s b e s t o s , p l a s t i c f i b r e s , and c e r a m i c s . The e a r l i e s t h i g h - e f f i c i e n c y f i b r o u s f i l t e r s w e r e bui l t up by s e v e r a l l a y e r s of f i b r e p a d s . Now the u s u a l f i l t e r med ium i s p a p e r of v a r i o u s c o m p o s i t i o n s .

FIG.13 . T y p i c a l HEPA f i l te r des ign. (Courtesy Cambr idge Fil ter Corpora t ion . )

17

Cel lu lose - a s b e s t o s p a p e r . The compos i t i on of th i s p a p e r m a y v a r y with the m a n u f a c t u r e r , but n o r m a l l y i t c o n s i s t s of about 50% e s p a r t o , 20% blue a s b e s t o s and 25% cot ton with a l i t t l e h e m p . In a good p a p e r t h e r e i s e f f ec t ive d i s p e r s i o n of the f ine a s b e s t o s f i b r e s . A p a p e r i s su i t ab le f o r mak ing p lea ted f i l t e r s even if c a r e m u s t be t aken to avoid c r a c k i n g at the f o l d s . The t e m p e r a t u r e r e s i s t a n c e i s l im i t ed and the m a t e r i a l i s not su i t ab le f o r cont inuous o p e r a t i o n above 100° C, the s a f e p e r i o d at 170° C being one h o u r . The ce l l u lo se i s i n f l a m m a b l e and s p a r k s m a y se t f i r e to both the dus t in the f i l t e r and the f i l t e r i t s e l f . A s p a r k a r r e s t e r in f r o n t of the f i l t e r would obvia te t h i s r i s k . The m e d i u m i s not r e s i s t a n t to high humid i ty , but t r e a t i n g t h e p a p e r with s i l i cone m a k e s i t m o r e r e s i s t a n t . The c e l l u l o s e - a s b e s t o s f i b r e p a p e r i s the l e a s t expens ive type , and i s u s e d when no s p e c i a l r e s i s t a n c e to t e m p e r a t u r e , f i r e o r humid i ty i s r e q u i r e d . T h i s type of f i l t e r i s m o r e r e s i s t a n t to hydrogen f l u o r i d e (HF) a t t ack than , f o r e x a m p l e , g l a s s p a p e r .

G l a s s - f i b r e p a p e r s . With the deve lopment of advanced t e c h n i q u e s of mak ing g l a s s - f i b r e p a p e r , t h i s type of f i l t e r m e d i u m has b e c o m e v e r y c o m m o n . The f i b r e s have , to a l a r g e ex ten t , d i a m e t e r s of l e s s t han 1 n m and a s p e c i a l p a p e r - m a k i n g p r o c e s s p r o d u c e s a su i tab le p a p e r . The g l a s s f i b r e s a r e of ten mixed with s o m e a s b e s t o s f i b r e s . The p lea t ing m u s t be c a r r i e d out c a r e f u l l y to avoid c r a c k i n g at the fo lds - o f ten a s o u r c e of i n c r e a s e d p a r t i c u l a t e p e n e t r a t i o n . T h i s p a p e r h a s good hea t r e s i s t a n c e and can wi ths tand a t e m p e r a t u r e of 500° C, but i t s s t r e n g t h i s m a r k e d l y r e d u c e d above 200° C, p robab ly due to burnoff of the b i n d e r . It i s non-i n f l a m m a b l e , which m e a n s tha t , even if a s p a r k ign i t e s the dus t in the f i l t e r , the f i l t e r med ium i tsel f does not con t r i bu t e to the f i r e . G l a s s - f i b r e p a p e r s given w a t e r p r o o f i n g t r e a t m e n t have good e f f i c i ency even u n d e r high humid i ty condi t ions . They a l so have good r e s i s t a n c e to m o s t c h e m i c a l s and to c o r r o s i v e a t m o s p h e r e s .

G l a s s - a s b e s t o s b lends a r e m o r e r e s i s t a n t to high t e m p e r a t u r e s . T h e r m o p l a s t i c f i b r e s . F i b r e s have been m a d e of polyethylene , nylon

and p o l y s t y r e n e . The l a s t - m e n t i o n e d p l a s t i c m a t e r i a l i s u s e d in a c o m m e r -c ia l ly ava i lab le H E P A - f i l t e r . The f i b r e s have d i a m e t e r s r ang ing be tween 0. 5 and 1. 5 /urn. The f i l t e r m e d i u m can be moulded into a f r a m e of the s a m e m a t e r i a l . F o r d i s p o s a l of the u s e d f i l t e r the whole uni t can be d i s s o l v e d and a f t e r evapo ra t i on the vo lume i s r e d u c e d to a m i n i m u m . The p l a s t i c - f i b r e f i l t e r a l so o f f e r s a me thod of r e c o v e r i n g va luab le r a d i o a c t i v e c o m p o n e n t s . The med ium has good r e s i s t a n c e to c h e m i c a l a t t ack but low hea t r e s i s t a n c e . F o r p o l y s t y r e n e the t e m p e r a t u r e i s l i m i t e d to 80° C. The m a t e r i a l i s i n f l a m m a b l e and i t s igni t ion t e m p e r a t u r e i s about 120° C.

C e r a m i c s . The need f o r f i l t e r s capable of wi ths tanding t e m p e r a t u r e s above 500° C f o r p ro longed p e r i o d s h a s r e s u l t e d in the u s e of c e r a m i c m a t e r i a l , o f t en of a l u m i n o s i l i c a t e . T h i s i s ava i lab le in the f o r m of f i b r e s tha t can be u s e d a s a th in p a p e r o r a s a deep bed , but not ye t in a l o w - p r i c e d un i t . In s t r e n g t h and c h e m i c a l r e s i s t a n c e it i s a s good as g l a s s and wil l wi ths tand t e m p e r a t u r e s up to 1000° C. At t h i s high t e m p e r a t u r e mi ld s t e e l cannot be u s e d in the f r a m e and t h e r e a r e f i l t e r un i t s f a b r i c a t e d e n t i r e l y of c e r a m i c m a t e r i a l s .

4 . 2 . F r a m e ( f i l t e r cas ing)

F r a m e s a r e gene ra l l y m a d e of plywood, s t e e l , r e i n f o r c e d p l a s t i c , a lumin ium, o r c e r a m i c s .

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Plywood f r a m e s a r e of l o w - c o s t c o n s t r u c t i o n and m o s t commonly u s e d . The d u s t - l o a d e d f i l t e r i s e a s i l y d i sposed of by c o m p r e s s i o n o r by i n c i n e r a -t ion . The t h i c k n e s s of the plywood should be 3 /4 in.(20 m m ) to e n s u r e l e a k t i g h t n e s s of t he f r a m e , to give r i g id i t y and to r e s i s t the c o m p r e s s i o n f o r c e s when the f i l t e r i s c l a m p e d to the mount ing f r a m e . E x t e r i o r g rade plywood i s needed f o r good m o i s t u r e r e s i s t a n c e . To m e e t f i r e p r o t e c t i o n r e q u i r e m e n t s , f i r e - r e t a r d a n t plywood i s s p e c i f i e d . P r e s s u r e i m p r e g n a t i o n i s u s e d to e n s u r e good i m p r e g n a t i o n . When such f r a m e s a r e u s e d in t r o p i c a l a r e a s , it m a y be n e c e s s a r y to inc lude t r e a t m e n t to i n c r e a s e r e s i s t a n c e to funga l g rowth . The t e m p e r a t u r e should not exceed 100° C. W o o d - p a r t i c l e b o a r d i s a l so u s e d .

Steel f r a m e s a r e u s e d f o r h i g h - t e m p e r a t u r e app l i ca t ions and when a n o n - i n f l a m m a b l e c o n s t r u c t i o n i s needed . Such f r a m e s can wi ths tand o p e r a t i n g t e m p e r a t u r e s up to 500° C. C a d m i u m - p l a t e d c a r b o n s t e e l i s u sua l l y u s e d to avoid c o r r o s i o n . Mild s t e e l coated with epoxy pa in t i s a l so r e s i s t a n t to m o i s t u r e . F i l t e r s with s t a i n l e s s - s t e e l f r a m e s a r e much m o r e expens ive , cos t ing about twice as m u c h a s t h o s e with wooden f r a m e s .

R e i n f o r c e d p l a s t i c and c e r a m i c s a r e u s e d in combina t ion with f i l t e r m e d i a of c o r r e s p o n d i n g m a t e r i a l .

4 . 3 . S p a c e r s o r s e p a r a t o r s

The m a t e r i a l s u s e d f o r s p a c e r s o r s e p a r a t o r s a r e k r a f t p a p e r , a lumin ium, p l a s t i c , a s b e s t o s p a p e r , s t a i n l e s s s t ee l , o r c e r a m i c s .

K r a f t p a p e r s e p a r a t o r s a r e the l e a s t expens ive but they have poor r e s i s t a n c e to hea t , humid i ty and c h e m i c a l a t t ack .

Aluminium s p a c e r s a r e g e n e r a l l y m o r e u s e f u l . They can wi ths tand a t e m p e r a t u r e of at l e a s t 300°C and they have good w a t e r r e s i s t a n c e . T h e i r c h e m i c a l r e s i s t a n c e i s r e l a t i v e l y good, except f o r s t r e a m s of a high c a u s t i c conten t .

P l a s t i c h a s poor hea t and f i r e r e s i s t a n c e . A s b e s t o s p a p e r s a r e u sua l l y t r e a t e d to i n c r e a s e t h e i r m o i s t u r e

r e s i s t a n c e . S t a in l e s s s t e e l s e p a r a t o r s a r e e x t r e m e l y expens ive . S o m e t i m e s s p a c e r s a r e p rov ided with a so f t edge on one s ide o v e r which

the f i l t e r m e d i u m can be s a f e l y fo lded .

4 . 4 . Adhes ive o r c e m e n t

The a d h e s i v e s g e n e r a l l y u s e d a r e r u b b e r b a s e , epoxide r e s i n , p o l y e s t e r r e s i n , p l a s t i c compounds , s i l i c a t e , o r r e f r a c t o r y c e m e n t .

Rubber b a s e d adhes ive i s m o s t commonly u s e d f o r n o r m a l t e m p e r a t u r e . If the f i l t e r i s in tended f o r cont inuous o p e r a t i o n at h ighe r t e m p e r a t u r e , the l i m i t i n g s e r v i c e t e m p e r a t u r e should be given by the m a n u f a c t u r e r .

P l a s t i c a d h e s i v e s have l i m i t e d t e m p e r a t u r e r e s i s t a n c e . S i l ica te o r r e f r a c t o r y c e m e n t s , which a r e h i g h - t e m p e r a t u r e a d h e s i v e s ,

should only be u s e d when n e c e s s a r y , b e c a u s e t e m p e r a t u r e r e s i s t a n c e i s not a lways combined with good s e a l i n g p r o p e r t i e s .

G l a s s f i b r e s in the f o r m of a pad a r e s o m e t i m e s u s e d i n s t ead of adhes ive as a s e a l be tween the f i l t e r pack and the f r a m e . Such a des ign i s v e r y s e n s i t i v e to m e c h a n i c a l d a m a g e , f o r e x a m p l e dur ing t r a n s p o r t a t i o n .

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4 . 5 . G a s k e t s

The: m a t e r i a l s u s e d f o r g a s k e t s a r e r u b b e r , n e o p r e n e , g l a s s - f i b r e s , o r m i n e r a l - f i b r e s .

Rubbe r g a s k e t s a r e not su i t ab le f o r t e m p e r a t u r e s above 70° C. Neop rene can s tand a s l igh t ly h i g h e r t e m p e r a t u r e . C losed ce l l n e o p r e n e

g a s k e t s a r e su i t ab l e . R e c o m m e n d e d gaske t s i z e s a r e 6 m m th ick and 19 m m wide . Gaske t s should have cut s u r f a c e s on both f a c e s b e c a u s e the ' n a t u r a l sk in ' p roduced by mold ing t ends to b r i d g e d i scon t inu i t i e s o r d e f e c t s in the s ea l i ng s u r f a c e s .

M i n e r a l f i b r e s and g l a s s f i b r e s a r e u s e d f o r h i g h - t e m p e r a t u r e f i l t e r s .

5. F I L T E R P R O P E R T I E S [4]

5 . 1 . A i r - f l o w capac i ty and g e o m e t r i c a l s i z e

Without any f o r m a l s t a n d a r d i z a t i o n , the s i z e s of the H E P A f i l t e r s a r e t r a d i t i o n a l l y of i n t e rna t i ona l l y accep ted s i z e s . As the f i l t e r s o r ig ina l l y w e r e developed in the United Sta tes of A m e r i c a , t h e i r d i m e n s i o n s a r e in i n c h e s . The d i f f e r e n t s i z e s a r e r e l a t e d to a n o r m a l a i r f low (Tab le 111). With the deve lopment of new f i l t e r p a p e r s , the a i r capac i ty h a s i n c r e a s e d , but o r i g i n a l l y the f i l t e r s had t h e i r r a t e d capac i t y at a p r e s s u r e d rop of 25 m m w. g.

S m a l l e r f i l t e r s a r e m a d e f o r u se in g l o v e - b o x e s .

T A B L E III. F I L T E R DIMENSIONS R E L A T E D TO NORMAL AIR FLOW

Normal air f low Dimensions (mm)

m 3 / h f t3 / m i n he ight width depth a

45 25 203 (8 i n . ) 203 (8 in . ) 78 (3 1 /16 i n . )

85 50 203 (8 in . ) 203 (8 i n . ) 150 (5 7 / 8 in . )

215 125 305 (12 i n . ) 305 (12 i n . ) 150 (5 7 / 8 in . )

850 500 610 (24 in . ) 610 (24 i n . ) 150 (5 7 / 8 in . )

1700 1000 610 (24 i n . ) 610 (24 i n . ) 292 (11 1 / 2 i n . )

a T h e depths exc lude the thickness of the gaskets . T h e gaskets are normal ly 6 m m ( 1 / 4 in . ) th ick and are appl ied to one or both faces .

5 . 2 . P r e s s u r e d rop

As ment ioned in c h a p t e r 2, t h e r e i s a l i n e a r r e l a t i o n s h i p be tween a i r f low and p r e s s u r e d r o p . A s t a n d a r d f i l t e r i s u sua l ly des igned f o r a p r e s s u r e d r o p of 25 m m w. g. at r a t e d a i r f low. At 50% of r a t e d a i r f low the p r e s s u r e d rop i s 12. 5 m m w. g. and at 150% a i r f low the p r e s s u r e d r o p i s 3 7 . 5 m m w. g. When des ign ing the ven t i l a t ion s y s t e m , a l l p r e s s u r e

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d r o p s in g r id s , duc ts and f i l t e r s have to be c o n s i d e r e d . One m u s t al low f o r i n c r e a s e in p r e s s u r e d r o p b e c a u s e of the dust co l lec ted in the f i l t e r . Des igns of ten take into account a d e g r e e of dus t accumula t ion c o r r e s p o n d i n g to two to fou r t i m e s the in i t i a l p r e s s u r e d rop . Th i s n o r m a l l y m e a n s a f i n a l p r e s s u r e d r o p of 50 to 100 m m w. g.

5 . 3 . P e n e t r a t i o n

(a) Col lec t ion e f f i c i e n c y and p e n e t r a t i o n

The co l lec t ion e f f i c i e n c y i s def ined as

co l l ec ted dust (weight) ^ dus t e n t e r i n g the f i l t e r (weight)

As the e f f i c i e n c y i s n o r m a l l y v e r y high, one of ten t a lk s of the f r a c t i o n of p e n e t r a t i o n

_ dus t p a s s i n g the f i l t e r (weight) dus t e n t e r i n g the f i l t e r (weight)

F r o m th i s i t i s obvious that

f = 1 - n

P e n e t r a t i o n i s a r e l e v a n t e x p r e s s i o n f o r the abi l i ty of the f i l t e r to r e m o v e the p a r t i c u l a t e f r o m the exhaus t a i r at p lan t s w h e r e r a d i o a c t i v e m a t e r i a l s a r e u s e d . The s t a n d a r d type of H E P A f i l t e r made by d i f f e r e n t m a n u f a c -t u r e r s n o r m a l l y h a s an e f f i c i e n c y of 99.97% o r a p e n e t r a t i o n of 0 .03% f o r a dus t with a d i a m e t e r of 0. 3 - 0 . 5 jam, d e t e r m i n e d with a s t a n d a r d i z e d method d e s c r i b e d in c h a p t e r 6.

FIG. 14 . Penet ra t ion versus pressure drop for various f i l ter m e d i a .

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(b) P e n e t r a t i o n and p r e s s u r e d r o p

As shown in chap t e r 2, the p r e s s u r e d r o p d e c r e a s e s with i n c r e a s i n g p e n e t r a t i o n . F i l t e r s a r e ava i lab le with a p e n e t r a t i o n of 5% and a p r e s s u r e d r o p of 10 m m w. g. down to a p e n e t r a t i o n of 0. 0001% and a p r e s s u r e d rop of only 28 m m w. g. High e f f i c i ency f i l t e r s have a h ighe r qual i ty f a c t o r , k i = log f / p , than do low e f f i c i e n c y f i l t e r s , p robab ly b e c a u s e the high e f f i c i e n c y f i l t e r ha s f i n e r f i b r e s .

When c o m p a r i n g f i l t e r s the p e n e t r a t i o n should a lways be r e g a r d e d in r e l a t i o n to the p r e s s u r e d r o p at equa l v e l o c i t i e s . In F i g . 14 p e n e t r a t i o n da t a f r o m ca ta logues a r e plot ted aga ins t the p r e s s u r e d r o p .

(c) P e n e t r a t i o n and ve loc i ty

As has a l r e a d y been ment ioned in c h a p t e r 2, the m a i n m e c h a n i s m caus ing the p a r t i c l e s to be caught i n the f i l t e r i s in f luenced by the ve loc i ty of the a i r in d i f f e r e n t ways . At low speed d i f fus ion i s e f f ec t ive and pene t r a t i on i s low. At h i g h e r speed d i f fus ion e f f e c t s d e c r e a s e and the i n e r t i a l f o r c e s a r e m o r e p r e d o m i n a n t . P e a k p e n e t r a t i o n o c c u r s at a m e d i u m speed w h e r e the d i f fus ion e f fec t i s low and the i n e r t i a l f o r c e s a r e s t i l l s m a l l (F ig . 15). Th i s peak a p p e a r s at a ve loc i ty of about 10-40 c m / s . The s m a l l e r the p a r t i c l e , the g r e a t e r i s the ve loc i ty at which peak p e n e t r a t i o n o c c u r s .

v, c m / s

FIG. 15. Varia t ion of pene t ra t ion with veloci ty through a paper f i l t e r . (Courtesy Butterworth and Co . L t d . , London.)

As the speed th rough the f i l t e r p a p e r in a s t a n d a r d H E P A f i l t e r i s about 2 c m / s , it i s obvious that the p e n e t r a t i o n wil l i n c r e a s e as long as the a i r f low th rough the f i l t e r i s i n c r e a s e d within p r a c t i c a l l i m i t s (F ig . 16). Th i s i s t r u e as long as the f i l t e r h a s no l eakage th rough p inholes o r c r a c k s in the fo lds o r in the s e a l i n g be tween the f i l t e r p a p e r and the f r a m e .

When t h e r e i s a pinhole in the f i l t e r , p a r t of the a i r wi l l p a s s th rough tha t hole p r a c t i c a l l y u n f i l t e r e d . However , the f i l t e r will m e e t the s p e c i f i -ca t ions f o r a h i g h - e f f i c i e n c y f i l t e r because the f i l t e r p a p e r i s n o r m a l l y so good that the spec i f i ed p e n e t r a t i o n a l lows f o r s o m e p inho les . Although th i s i s the c a s e at r a t ed capac i t y i t i s not a lways so at r educed capac i t y . As can be s een in F ig . 16, the p e n e t r a t i o n i n c r e a s e s as the a i r f low d e c r e a s e s [ 1 1 ] . Th i s phenomenon can be expla ined by the d i f f e r e n c e be tween the r e s i s t a n c e of the f i l t e r p a p e r i tse l f and that of the p inhole .

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The r e s i s t a n c e of the f i l t e r p a p e r i s a l i n e a r funct ion of the a i r ve loc i ty , but t u rbu len t a i r f low th rough the pinhole g ives a r e s i s t a n c e tha t v a r i e s with the s q u a r e of the ve loc i ty . At r e d u c e d capac i ty the r e l a t i v e amount of a i r p a s s i n g th rough the pinhole will be l a r g e r , which gives a h ighe r p e n e t r a t i o n as ind ica ted in F ig . 16.

F I G . 1 6 . V a r i a t i o n of p e n e t r a t i o n wi th v e l o c i t y for an unbroken and a p i n h o l e d f i l t e r .

a £L

Q2-

0.1 : r- 1 1 1 1 0 0.1 0.2 0.3 0.4 0.5

Par t ic le s i z e }jm

FIG. 17 . Variation of p e n e t r a t i o n with p a r t i c l e s i z e .

(d) P e n e t r a t i o n and p a r t i c l e s i ze •'

Smal l p a r t i c l e s a r e co l l ec ted by d i f fus ion e f f e c t s and l a r g e p a r t i c l e s by i n t e r c e p t i o n and i n e r t i a l f o r c e s . Th i s m e a n s that m a x i m u m p e n e t r a t i o n o c c u r s w i th -pa r t i c l e s of m e d i u m s i z e . F i g u r e 17 shows that t h e r e i s m a x i m u m p e n e t r a t i o n with 0. 25-/am p a r t i c l e s . The m a x i m u m m a y v a r y with the type of f i l t e r , i t s pack ing dens i ty and f i b r e d i a m e t e r , but in m o s t c a s e s i t p robab ly l i e s between d i a m e t e r s of 0. 1 and 0. 3 /am. In gene ra l , the peak wil l sh i f t to s m a l l e r s i z e s as a i r ve loc i ty i n c r e a s e s and the e f fec t of d i f fus ion d e c r e a s e s .

23

(e) P e n e t r a t i o n and dust load

The e f f i c i ency of a f i l t e r g e n e r a l l y i n c r e a s e s as the f i l t e r i s loaded with dus t . The r e a s o n ' f o r t h i s i s tha t the i n t e r s t i t i a l d i s t a n c e s b e c o m e s m a l l e r and the s u r f a c e a r e a of the f i l t e r (now f i b r e s and depos i ted a e r o s o l ) b e c o m e s g r e a t e r . In the d i a g r a m shown in F ig . 18 the plot of the l o g a r i t h m of pene t r a t i on v e r s u s the vo lume of depos i t b e c o m e s a l m o s t a s t r a i g h t l ine [ 4 ] .

FIG. 18. Loading-pene t ra t ion charac ter i s t ics of a glass paper f i l t e r . (Courtesy Butterworth and Co . L td . , London.)

T h e s e t e s t s w e r e u sua l l y p e r f o r m e d with h e t e r o d i s p e r s e c louds of sol id p a r t i c l e s . Th i s g e n e r a l e x p e r i e n c e h a s been c o n f i r m e d with a t e s t r e p o r t e d in R e f . [ l 2 ] . With the s a m e a r r a n g e m e n t a t e s t was a l so p e r -f o r m e d with an oi ly smoke , g e n e r a t e d by a s m o k e - p r o d u c e r ca l l ed ' s m o k i n g r o p e ' , and p e n e t r a t i o n i n c r e a s e d by t h r e e o r f o u r t i m e s . The f i l t e r s t r u c t u r e i s thus a s s u m e d to be d a m a g e d by a phys i ca l a n d / o r c h e m i c a l ac t ion p roduced by t a r r y o r o i ly a e r o s o l s .

5 . 4 . Dus t -ho ld ing capac i t y

Dus t -ho ld ing capac i t y i s def ined as the amount of dus t co l l ec ted in the f i l t e r f o r a defined i n c r e a s e in p r e s s u r e d rop o v e r the f i l t e r . T h e s e da ta a r e v e r y i m p o r t a n t f o r eva lua t ing the economy of the f i l t e r . In r a d i o -ac t ive p lan t s the r a d i a t i o n f r o m the f i l t e r s o m e t i m e s d e t e r m i n e s the i n t e r v a l f o r changing the f i l t e r , but in m o s t c a s e s the i n c r e a s e in p r e s s u r e d r o p n e c e s s i t a t e s the change of f i l t e r s when the des ign va lue i s r e a c h e d .

24

500 1000 Dust load g r a m s

1500

5.5 pm

2000

FIG. 19. Pressure drop increase for various dusts accumula t ed in f i l t e rs .

It i s v e r y d i f f icul t to obta in c o m p a r a b l e da ta f r o m the f i l t e r m a n u f a c t u r e r s . Most va lues a r e obtained f r o m a c c e l e r a t e d t e s t s , s o m e t i m e s with a t e s t dus t that i s not r e l e v a n t to the condi t ions u n d e r which the f i l t e r wil l o p e r a t e ( see F i g . 19). In th i s f i g u r e the c u r v e s r e f e r to the fo l lowing t e s t condi t ions :

C u r v e T e s t dus t Size Concen t r a t i on T i m e Ref . (/am) ( m g / m 3 )

A. N a t u r a l s e t t l ed dust 5 . 5 200 6 h [13]

B. P r e f i l t e r e d dust 2 . 0 40 15 h [13]

C. NaCl f u m e 0 . 5 [13]

1. C o t t r e l l p r e c i p i t a t e

without l i n t e r s 35 23 h [14] 2. A t m o s p h e r i c a e r o s o l < 1 0 . 0 5 1 y r [10]

3. C a r b o n b lack [15]

The t e s t s w e r e p e r f o r m e d with g l a s s - f i b r e f i l t e r s with e f f i c i e n c i e s of m o r e than 99.97% and a r a t e d capac i ty of 1700 m 3 / h . By c o m p a r i n g the c u r v e s A, B and C we see that a f i l t e r i s m o r e r ap id ly clogged by s m a l l p a r t i c l e s . C u r v e s 1 and 3 a r e b a s e d on da ta given by m a n u f a c t u r e r s .

It i s obvious that the c u r v e s shown in F ig . 19 r e f l e c t the d i f f e r e n c e between the dust p r o p e r t i e s r a t h e r than a d i f f e r e n c e in the behav iour of the f i l t e r un i t s t e s t e d . To ind ica te any d i f f e r e n c e be tween f i l t e r s a c o m p a r a b l e t e s t i s p r e f e r r e d . In Ref . [ 10] a t e s t with f o u r f i l t e r s f r o m v a r i o u s m a n u -f a c t u r e r s i s r e p o r t e d . They w e r e al l exposed to a t m o s p h e r i c a e r o s o l s

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with a m e a n concen t ra t ion of about 0. 05 m g / m 3 . T h r e e of the f i l t e r s showed the s a m e c h a r a c t e r i s t i c s . They w e r e g l a s s pape r f i l t e r s with a r a t e d capac i ty of 1700 m 3 / h . A f t e r about one y e a r the p r e s s u r e d rop had in -c r e a s e d to 50 m m w. g. and the load was then 700 g (curve 2, F i g . 19). One f i l t e r a ccumula t ed only 450 g, but i t had a s l ight ly d i f f e r e n t des ign . F i l t e r s of the t r a d i t i o n a l des ign al l s e e m to have about the s a m e load-, p r e s s u r e d rop c h a r a c t e r i s t i c . T h e r e does not s e e m to be any s i m p l e c o r r e l a t i o n between dus t -ho ld ing capac i ty and f i l t e r a r e a within the s a m e f i l t e r unit s i z e s .

T A B L E IV. CHARACTERISTICS O F SOME F I L T E R - M A T E R I A L S

Tempera tu r e , Fire Water Mater ia l s continuous res is tance resis tance

C C )

Fil ter m e d i a

Plastic 60 Poor Good Cel lu lose -asbestos 100 Poor Poor Glass f ibre , standard 500 Good Fair Glass f ibre, water re tardant 500 Good Good Ceramics 1000 Good Good

Separators

Plastic 60 Poor Good Kraft paper 100 Poor Poor Alumin ium 300 Good Good Asbestos 400 Good Poor Water re tardant asbestos 500 Good Good Ceramics 1000 Good Good

Frames

Reinforced plas t ic 60 Poor Good Wood, board, plywood 100 Poor Fair Fire re tardant wood 100 Fair Fair S tee l 500 Good Good Ceramics 1000 Good Good

Adhesives

Rubber base 70 Poor Good Plastic base 250 Fair Good S i l i ca t e fu rnace c e m e n t 500 Good Good Ceramics 1000 Good Good

Gaskets

Rubber 70 Poor Good Neoprene 100 Poor Good Minera l f ibre 500 Good Good Ceramics 1000 Good Good

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5 . 5 . R e s i s t a n c e to hea t , f i r e , humid i ty and c h e m i c a l a t t ack

The r e q u i r e m e n t s tha t the f i l t e r ha s to m e e t depend upon the con-di t ions unde r which the f i l t e r will o p e r a t e . It i s of ten the e x t r a o r d i n a r y s i tua t ions r a t h e r than n o r m a l condi t ions tha t d e t e r m i n e the d e m a n d s on the f i l t e r . By combin ing su i tab le c o n s t r u c t i o n m a t e r i a l s f o r f i l t e r med ia , s e p a r a t o r s , f r a m e , adhes ive and gaske t , the p r o p e r t i e s of the comple te f i l t e r can be d e t e r m i n e d . The c h a r a c t e r i s t i c s of s o m e m a t e r i a l s a r e l i s t ed in Tab le IV [ 14 ] .

With r e g a r d to c h e m i c a l r e s i s t a n c e , the v a r i e t y of c h e m i c a l cons t i -tuen t s and c o n c e n t r a t i o n s and o the r condi t ions such as t e m p e r a t u r e and humid i ty a lways m a k e a choice of m a t e r i a l d i f f icu l t . In m a n y c a s e s , the only way i s to t e s t the d i f f e r e n t m a t e r i a l s . Some b a s i c c h a r a c t e r i s t i c s of v a r i o u s m a t e r i a l s a r e ment ioned below.

G las s f i l t e r m e d i u m is the bes t g e n e r a l m e d i u m but d e t e r i o r a t e s through the act ion of hydrogen f luo r ide (HF) and caus t i c s t r e a m con ten t s . F o r high hydrogen f l uo r ide content , ce l lu lose a s b e s t o s p a p e r i s p r e f e r a b l e .

A s b e s t o s s e p a r a t o r s have a high d e g r e e of c h e m i c a l s t ab i l i t y . Alumin ium i s g e n e r a l l y u s e f u l but i s su scep t i b l e to high caus t i c s t r e a m conten t . P l a s t i c can be u s e d in m o s t a t m o s p h e r e s , but i s poor in s t r e a m s with high vapour content .

Wooden f r a m e s cannot wi ths tand high c o n c e n t r a t i o n s of c h e m i c a l s . C a d m i u m - p l a t e d s t e e l i s r e l a t i v e l y good, but r e i n f o r c e d p l a s t i c should be chosen fo r r e s i s t a n c e to s t r o n g c h e m i c a l a t tack , except when high o rgan ic vapour content i s expec t ed . The adhes ive has to be chosen in c o n f o r m i t y with the o t h e r m a t e r i a l s of the f i l t e r . F o r high o rgan i c vapour content , s i l i ca t e adhes ive should be used ins t ead of r u b b e r - o r p l a s t i c - b a s e d adhes ive . When high c h e m i c a l s t ab i l i t y i s r e q u i r e d in combina t ion with high hea t r e s i s t a n c e , a m e c h a n i c a l s e a l can be u s e d .

Rubber and n e o p r e n e g a s k e t s should be avoided when high o rgan ic vapour content i s expec ted . M i n e r a l f i b r e s have a high d e g r e e of c h e m i c a l s t ab i l i ty .

C e r a m i c f i l t e r s , u sed f o r high t e m p e r a t u r e condi t ions , a r e r e s i s t a n t to m o s t c h e m i c a l s except hydrogen f l u o r i d e .

F o r both m a n u f a c t u r e r and u s e r it i s advan tageous to s t a n d a r d i z e combina t ions of m a t e r i a l s . If only a few types of f i l t e r a r e used , the n u m b e r of f i l t e r s to be s t o r e d can be r educed and the p u r c h a s i n g rou t ine s i m p l i f i e d . F o r m o s t p u r p o s e s the f i l t e r m u s t only o p e r a t e at r o o m t e m p e r a t u r e and c h e m i c a l cons t i t uen t s wil l appea r only at v e r y low concen -t r a t i o n s in the s t r e a m . F o r s a f e t y r e a s o n s a c e r t a i n d e g r e e of f i r e r e s i s t a n c e h a s to be s p e c i f i e d . It i s not e c o n o m i c a l l y f e a s i b l e to have comple t e ly f i r ep roo f f i l t e r s when the f i r e r i s k in the plant i s m o d e r a t e . A f i r ep roo f f i l t e r i s m o r e expens ive and the d i s p o s a l c o s t s a r e a lso h i g h e r . What i s u sua l l y r e q u i r e d i s that the f i l t e r should not suppo r t f i r e o r that it should be f l a m e - r e s i s t a n t . In Tab le V s p e c i f i c a t i o n s f o r a f l a m e - r e s i s t a n t f i l t e r a r e s u g g e s t e d . Such a f i l t e r can wi ths tand the d e g r e e of humid i ty that o c c u r s in a n o r m a l plant (about 100%).

When a f i l t e r o p e r a t e s at e leva ted t e m p e r a t u r e s o r when t h e r e i s a high f i r e r i s k , a h i g h - t e m p e r a t u r e f i l t e r i s r e q u i r e d .

Specia l a t tent ion should be paid to the high f i r e r i s k tha t e x i s t s du r ing the handl ing of p y r o p h o r i c m e t a l s o r i n f l a m m a b l e so lven t s , o r when s e r i o u s r ad io log i ca l c o n s e q u e n c e s could o c c u r in the event of a f i l t e r f i r e . In

27

T A B L E V. SPECIFICATIONS FOR FLAME-RESISTANT, HIGH-T E M P E R A T U R E AND CHEMICAL-RESISTANT F I L T E R S

Type Fil ter

m e d i u m Separators F rame Adhesive Gasket Max. t emp . a

F l a m e -resistant

Glass paper

Alumin ium Fire • re tardant wood

S e l f -ext inguishing (rubber base)

Rubber, n e o -prene

100"C

High-t empera tu re

Glass paper

Alumin ium Stee l S i l i ca t e c e m e n t or m e c h a n i c a l seal ing

Minera l f ibre

400"C

C h e m i c a l -resistant

Glass paper

Plastic Reinforced plas t ic

Epoxide resin

Neo-prene

75"C

For continuous operat ion the tempera tures should be lower.

such c a s e s a h i g h - t e m p e r a t u r e f i l t e r i s p r e f e r a b l e even if the n o r m a l o p e r a t i n g t e m p e r a t u r e i s low. The h i g h - t e m p e r a t u r e f i l t e r d e s c r i b e d in Tab le V has both good f i r e r e s i s t a n c e and good humid i ty r e s i s t a n c e . If a m e c h a n i c a l s e a l i s used a f i l t e r e f f i c i e n c y t e s t should be p e r f o r m e d both at d e l i v e r y and i n - s i t u .

The h i g h - t e m p e r a t u r e f i l t e r h a s f a i r l y good c h e m i c a l r e s i s t a n c e . If, however , the c h e m i c a l a t t ack i s s t r o n g the s t e e l f r a m e wil l not be r e s i s t a n t enough. Where the exhaus t duc ts , f o r the s a m e r e a s o n , a r e made of p l a s t i c , t h e r e i s no need f o r a h i g h - t e m p e r a t u r e f i l t e r . A c h e m i c a l - r e s i s t a n t f i l t e r of p l a s t i c can be used i n s t ead . An example of such a f i l t e r i s given in Tab le V. A p la s t i c f i l t e r cannot be used when t h e r e i s a high o rgan i c vapour content in the s t r e a m ,

5 . 6 . M e c h a n i c a l r e s i s t a n c e

J u s t as a h i g h - e f f i c i e n c y p a r t i c u l a t e f i l t e r m u s t be c o n s t r u c t e d of high qual i ty f i l t e r m e d i a with a t tent ion paid to p reven t ion of l eakage , i t i s a lso of g r e a t i m p o r t a n c e f o r the f i l t e r to have enough m e c h a n i c a l s t r e n g t h to wi ths tand t r a n s p o r t a t i o n , rough handl ing and a v a r i e t y of o p e r a t i n g condi t ions .

At o p e r a t i n g condi t ions the p r e s s u r e d r o p ove r the f i l t e r m a y r i s e to 50-100 m m w. g. If the f i l t e r i s clogged by dust the m a x i m u m p r e s s u r e d rop that the f i l t e r can be exposed to i s the to ta l s t a t i c p r e s s u r e that can be developed by the fan in the s y s t e m . In a g e n e r a l vent i la t ing s y s t e m the to ta l head of the fan i s u sua l ly 100 to 200 m m w. g. The abi l i ty to wi ths tand p r e s s u r e d rop depends on the depth of the f i l t e r and i t s f ace s i z e s . A good f i l t e r with a f a c e s i ze of 610 X 610 m m m a y wi ths tand a p r e s s u r e d rop of 250 m m w. g. f o r a depth of 150 m m and of 500-600 m m w. g. f o r a depth of 292 m m . The 2 9 2 - m m - d e e p f i l t e r i s usua l ly spec i f i ed to wi th-s tand a p r e s s u r e d rop of 300 m m w. g. Such condi t ions as pu l sa t ing a i r f low r e s u l t i n g f r o m a i r t u rbu l ence , t e m p o r a r y va r i a t i on in fan p r e s s u r e o r v ib ra t ion of the f i l t e r mount ing f r a m e w o r k m a y cause s o m e s t r a i n to

28

a f i l t e r i n s t a l l a t i on . If they c a u s e s o m e damage to the f i l t e r , the un -f avou rab l e condi t ions should be e l i m i n a t e d by choos ing a p r o p e r f an f o r the o p e r a t i n g c h a r a c t e r i s t i c s of the plant and by good s t r e a m l i n e des ign of the duct u p s t r e a m of the f i l t e r s . V ib ra t ing duc ts should be adequa te ly s u p p o r t e d .

Most damage to f i l t e r s p robab ly o c c u r s du r ing t r a n s p o r t a t i o n and handl ing . F i l t e r un i t s should r e m a i n in t h e i r sh ipping c a r t o n s unt i l they a r e about to be i n s t a l l ed in a plant , thus avoiding t e a r i n g and p i e r c i n g of the f i l t e r m e d i a .

6. TESTING METHODS

6 . 1 . G e n e r a l t e s t s

V a r i o u s t e s t s can be p e r f o r m e d to d e m o n s t r a t e the high qual i ty n o r m a l l y r e q u i r e d of h i g h - e f f i c i e n c y f i l t e r s . They can be c a r r i e d out on the componen t s of the f i l t e r b e f o r e a s s e m b l i n g , on the comple t e f i l t e r b e f o r e ins ta l l a t ion and on the f i l t e r in s i tu . Some t e s t s can be c o n s i d e r e d a s p a r t of the p roduc t ion c o n t r o l and the ope ra t i on inspec t ion . O t h e r s a r e m o r e o r l e s s p a r t of e x p e r i m e n t a l w o r k f o r deve lopment of e f f i c ien t f i l t e r s and to widen the r a n g e of appl ica t ion of t h e s e f i l t e r s . U s e r s of f i l t e r s a r e s e l d o m in a pos i t ion to p e r f o r m e l a b o r a t e t e s t s . It i s u sua l ly m o r e p r a c t i c a l f o r one o r two s p e c i a l i z e d l a b o r a t o r i e s to be r e s p o n s i b l e fo r m o r e advanced t e s t s that r e q u i r e ex t ens ive f a c i l i t i e s and i n s t r u m e n t a t i o n . Any individual u s e r , however , should be able to p e r f o r m s o m e s i m p l e t e s t s to p r o v e that h i s a i r - c l e a n i n g plant c o m p l i e s with the spec i f i ed r e q u i r e m e n t s . Such t e s t s a r e deal t with h e r e in deta i l , but f o r p u r p o s e s of o r i en ta t ion o the r t e s t s a r e a l so d e s c r i b e d . The m a n u f a c t u r e r s can p rov ide i n f o r m a t i o n on t e s t s that a r e p e r f o r m e d dur ing p roduc t ion of the f i l t e r .

(a) Component t e s t s

(i) F i l t e r m e d i u m

Retent ion e f f i c i ency i s d i s c u s s e d u n d e r a s e p a r a t e heading . Flow r e s i s t a n c e . The flow r e s i s t a n c e of the f i l t e r m e d i u m is

s o m e t i m e s spec i f i ed a s having a m a x i m u m value of 40 m m w. g. at 5 . 2 c m / s [ 4 , 1 6 ] . A n o r m a l value is 30 m m w. g. at the s a m e ve loc i ty th rough the f i l t e r m e d i u m .

T e n s i l e s t r e n g t h . Tens i l e s t r e n g t h can be d e t e r m i n e d in the m a c h i n e d i r e c t i o n of the p a p e r and a c r o s s tha t d i r ec t ion . F i l t e r p a p e r h a s a l o w e r t e n s i l e s t r e n g t h a c r o s s the m a c h i n e d i r ec t i on [ 1 7 ] . A m i n i m u m t e n s i l e s t r e n g t h of 360 g / c m is r e q u i r e d in s o m e s t a n d a r d s [16 ] but m a y b e i n c r e a s e d to 450 g/cm [ 1 7 ] ,

T h i c k n e s s of the p a p e r i s m e a s u r e d by a m i c r o m e t e r gauge b e -tween p l a t e s at s e v e r a l p l a c e s ove r the s a m p l e and should not be l e s s than 0. 4 m m acco rd ing to a s t a n d a r d spec i f i ca t ion [ 1 8 ] . A f i l t e r m a n u f a c t u r e r n o r m a l l y a s k s for a t h i c k n e s s of 0. 5 - 0. 75 m m [ 4 ] .

Weight can be e x p r e s s e d in g r a m s p e r unit a r e a . A n o r m a l va lue i s 80 g / m 2 f o r a 0. 4 - m m - t h i c k p a p e r . T h e r e i s no c o r r e l a t i o n be tween the b a s i c weight and p h y s i c a l c h a r a c t e r i s t i c s and f i l t r a t i on p e r f o r m a n c e . It i s t h e r e f o r e concluded that a b a s i c weight r e q u i r e -m e n t i s not an e s s e n t i a l p a r t of the f i l t e r m e d i a spec i f i ca t i on [ 1 7 ] .

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Combus t ib le content . F o r a f i r e - r e s i s t a n t f i l t e r the p e r c e n t a g e of combus t ib l e compounds p r e s e n t in the f i l t e r could be l imi t ed to a c e r t a i n va lue , f o r e x a m p l e to 5% [ 1 8 ] .

W a t e r r e s i s t a n c e . Th i s is t e s t e d a c c o r d i n g to Ref. [ 16 ]. Heat r e s i s t a n c e . The s t r e n g t h of the p a p e r a f t e r heat ing c o m p a r e d

with the s t r eng th b e f o r e hea t ing ind ica t e s i t s t h e r m a l p r o p e r t i e s .

(ii) F r a m e

When the f r a m e is m a d e of f i r e - r e t a r d a n t wood, a f l ame s p r e a d t e s t m a y be appl ied .

(iii) Gaske t

The h a r d n e s s of g a s k e t s can be d e t e r m i n e d .

(iv) Adhes ives

The se l f - ex t ingu i sh ing p r o p e r t y o r f i r e r e s i s t a n c e of the adhes ive can be v e r i f i e d .

(b) F i l t e r unit t e s t s

The t e s t on the a s s e m b l e d f i l t e r unit is of m o s t i n t e r e s t to the u s e r of the f i l t e r , a s it g ives the ope ra t i ng c h a r a c t e r i s t i c s . .

Re ten t ion e f f i c i ency . D i f f e r en t m e t h o d s a r e d e s c r i b e d u n d e r a s e p a r a t e heading . F o r m o s t h i g h - e f f i c i e n c y f i l t e r s an e f f i c i ency of 99. 97% is spec i f i ed . Th i s va lue a l so has to be obtained a f t e r the f i l t e r h a s been sub jec t ed to o the r t e s t s .

F low r e s i s t a n c e . The m a x i m u m in i t i a l flow r e s i s t a n c e of a f i l t e r i s spec i f i ed fo r the r a t e d a i r f low.

Vibra t ion t e s t . To d e m o n s t r a t e the m e c h a n i c a l s t r e n g t h of the f i l t e r it can be v i b r a t e d with, f o r example , 300 p u l s e s / m i n at an ampl i tude of about 10 m m [ 4 ].

R e s i s t a n c e to p r e s s u r e . By i n c r e a s i n g the a i r flow th rough the f i l t e r o r by clogging it with dus t , the p r e s s u r e d rop ove r the f i l t e r can be in-c r e a s e d to 250 - 300 m m w. g. (10-12 in. ). T h e s e va lues a r e of ten spec i f i ed fo r H E P A f i l t e r s .

Heat t e s t . The e f f i c i ency of the f i l t e r i s f i r s t d e t e r m i n e d and then the f i l t e r is p laced in an oven, the t e m p e r a t u r e i s slowly r a i s e d to the r e q u i r e d d e g r e e and ma in t a ined fo r s e v e r a l h o u r s . Af t e r cool ing down the f i l t e r i s inspec ted and t e s t e d f o r r e t en t i on e f f i c i ency .

The t e s t s d e s c r i b e d above a r e the t ypes of t e s t tha t a r e r e g u l a r l y p e r f o r m e d to e n s u r e that the high qual i ty of the f i l t e r i s cont inuously ma in t a ined . F o r s p e c i a l app l i ca t ions a t e s t m a y be c a r r i e d out to d e t e r m i n e w h e t h e r a h i g h - e f f i c i e n c y f i l t e r can o p e r a t e at e x t r e m e condi t ions o r s tand acc iden t condi t ions without comple t e b r e a k - d o w n .

(c) H o t - a i r and f i r e - r e s i s t a n c e t e s t

S e r i o u s f i r e s in combus t ib l e f i l t e r s have led to the deve lopment of f i l t e r s m o r e r e s i s t a n t to high t e m p e r a t u r e s , and t e s t s have been p e r f o r m e d on such f i l t e r s as r e p o r t e d in Ref . [ 1 9 ] . The t e s t equ ipment c o n s i s t e d of

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a l a r g e capac i ty , n a t u r a l - g a s b u r n e r and s ec t i ons of duct ing with b a f f l e s to give good mix ing u p s t r e a m f r o m the f i l t e r . T h e r m o c o u p l e s w e r e i n s e r t e d into the f i l t e r at d i f f e r e n t pos i t i ons .

A s t a n d a r d qua l i f ica t ion t e s t h a s been worked out by U n d e r w r i t e r s ' L a b o r a t o r i e s a s spec i f i ed in UL-586 [ 2 0 ] . It inc ludes a h o t - a i r t e s t at a t e m p e r a t u r e of 370°C f o r f ive m i n u t e s with the f i l t e r o p e r a t e d at r a t e d capac i ty . Although the s t a n d a r d p e r m i t s a r educ t i on of 3% in f i l t r a t i o n e f f i c i ency , the r e s u l t s u sua l ly show a r educ t ion of l e s s than 0. 3%. The s t a n d a r d a l so inc ludes a spot f l a m e t e s t in which the f l a m e f r o m a Bunsen b u r n e r i s d i r e c t e d aga ins t the u p s t r e a m face of the f i l t e r f o r f ive m i n u t e s while the f i l t e r i s o p e r a t e d at r a t e d capac i ty . T h i s i s a combus t ib i l i t y t e s t and wil l b r e a c h the f i l t e r but i s s u c c e s s f u l if no sus t a ined burn ing t a k e s p l ace on the d o w n s t r e a m face of the f i l t e r .

(d) W a t e r r e s i s t a n c e t e s t

Some f i l t r a t i o n s y s t e m s a r e in tended f o r u s e dur ing acc iden t condi t ions with w a t e r - m o d e r a t e d and - c o o l e d r e a c t o r s . A pipe r u p t u r e would involve a r e l e a s e of s t e a m and hot w a t e r and the f i l t e r would be exposed to s t e a m and w a t e r - a i r m i x t u r e s that m igh t . be s a t u r a t e d o r even s u p e r - s a t u r a t e d . -In the l a t t e r c a s e t h e r e a r e d r o p l e t s in the a i r . If w a t e r d rop l e t s a r e expec ted in the a i r , a d rople t s e p a r a t o r should be p laced u p s t r e a m f r o m the f i l t e r . To p rove the abi l i ty of the f i l t e r to wi ths tand the p r e s u m e d acc ident condi t ions , the f i l t e r with i t s m o i s t u r e s e p a r a t o r s can be t e s t e d a s d e s c r i b e d in Ref . [ 21 ]. If the w a t e r content in the a i r i s c o n s i d e r a b l e , f i l t e r s t e s t e d without s e p a r a t o r s b e c o m e plugged with w a t e r .

An e l a b o r a t e method f o r t e s t i ng w a t e r - r e p e l l e n t m e d i a of high wet s t r e n g t h h a s been developed at Savannah R i v e r L a b o r a t o r y [ 2 2 ] . Th i s method i s m o r e su i t ab le f o r r e s e a r c h and deve lopment w o r k than fo r rou t ine t e s t s . Sonic ve loc i ty m e a s u r e m e n t s a r e used to d e t e r m i n e the s t r e n g t h of the p a p e r , which i s exposed to a s t e a m - a i r m i x t u r e .

To check the e f f e c t i v e n e s s of the w a t e r r e p e l l e n c y t r e a t m e n t of the f i l t e r m e d i a , a s i m p l e t e s t can be p e r f o r m e d on a bench s c a l e [ 2 3 ] . Sma l l s a m p l e s of f i l t e r m e d i a a r e exposed to wet s t e a m flow and the p r e s s u r e d rop i s m e a s u r e d . S a m p l e s that a r e not t r e a t e d f o r w a t e r r e p e l l e n c y a r e p lugged v e r y r ap id ly . W a t e r - r e p e l l e n t m e d i a s t i l l show low p r e s s u r e d r o p s a f t e r f ive m i n u t e s of e x p o s u r e . With a l o n g - t e r m e x p o s u r e to d r y s t e a m flow, the leach ing e f f e c t s on the w a t e r p r o o f i n g can be s tudied . With a d ry s t e a m flow one s a m p l e r u p t u r e d a f t e r 54 h o u r s , 'but a f t e r 60 h o u r s a s a m p l e f r o m ano the r m a n u f a c t u r e r s t i l l , had a p r e s s u r e d rop of only 175 m m (7 in. ) w.'g.

Another s i m p l e t e s t i s to i m m e r s e the f i l t e r in w a t e r . Th i s t e s t can be spec i f i ed as fo l lows: the comple t e f i l t e r sha l l be i m m e r s e d in w a t e r at 20°C ( f i l t e r p e r p e n d i c u l a r ) f o r a pe r iod of not l e s s than f ive m i n u t e s . The f i l t e r should then be d r a ined f o r a p e r i o d of not m o r e than f ive m i n u t e s . A f t e r d ra in ing , the f i l t e r i s to be i m m e d i a t e l y t r a n s f e r r e d to a t e s t s tand capab le of r a t e d a i r flow to d e t e r m i n e p r e s s u r e d rop a c r o s s the f i l t e r a f t e r only t en m i n u t e s of dry ing with r o o m a i r at r a t e d capac i ty . The p r e s s u r e d rop should not be m o r e than 50 m m (2 in. ) w. g. Af t e r the f i l t e r h a s b e e n ' c o m p l e t e l y d r i e d in r o o m a i r , the o r i g i n a l p r e s s u r e d rop (25 m m w. g . ) should have been r e s t o r e d and the m a x i m u m p e n e t r a t i o n should be 0. 075% (or ig ina l p e n e t r a t i o n 0. 03%) [ 1 4 ] ,

31

FIG. 20 . DOP test r ig .

(e) Aerodynamic shock t e s t

By rup tu r ing a d iaphragm, a HEPA f i l t e r was exposed to an in s t an -taneous p r e s s u r e d i f f e rence a c r o s s the f i l t e r in a t e s t d e s c r i b e d in Ref . , [4 ] . The f i l t e r r e m a i n e d intact up to a p r e s s u r e d i f f e rence of about 0. 6 k g / c m 2 , when a n u m b e r of s m a l l ho les appea red in the f i l t e r med ium.

A shock-wave t e s t was p e r f o r m e d with a conica l shock tube [24 ] . The b la s t wave obtained in such a tube s i m u l a t e s a l a r g e explosive cha rge . F o r such a wave the pe r iod of o v e r p r e s s u r e a f t e r the peak has p a s s e d is r e l a t i ve ly long (50 mi l l i seconds) and th i s g rea t ly i n c r e a s e s the damaging e f f ec t s . The peak p r e s s u r e , e x p r e s s e d as the s ide -on (hydrostat ic) p r e s s u r e that a 610 m m X 610 m m X 292 m m (24 in. X 24 in. X 12 in . ) f i l t e r could r e s i s t , was 0.22 k g / c m 2 (3. 2 l b / i n 2 ) . F o r o the r s i z e s it was found that the s m a l l e r the individual unit and the th i cke r the unit of a given s ize , the b e t t e r was i t s abil i ty to withstand the shock wave. The dust loading had a r e l a t ive ly s m a l l e f fec t on the shock r e s i s t a n c e , but the pro tec t ion s c r e e n of h a r d w a r e w i re cloth on the f i l t e r face contr ibuted essen t ia l ly to the r e s i s t a n c e .

6. 2. Retention e f f ic iency t e s t s

An in te rna t iona l a g r e e m e n t has not yet been r eached on a s tandard re ten t ion e f f i c i ency test. The d i f fe ren t t e s t s cannot be s a t i s f a c t o r i l y c o m -p a r e d , as both s ize and composi t ion of the p a r t i c l e s and the m a n n e r in which the t e s t i s conducted inf luence the r e s u l t s .

(a) Dioctyl phthalate t e s t (DOP) (see F ig . 20)

This t e s t was or ig ina l ly developed in the United Sta tes of A m e r i c a and i s now p e r f o r m e d in acco rdance with an ins t ruc t ion manua l [25 ] . T h e r m a l -type a e r o s o l g e n e r a t o r s a r e used to p roduce the a e r o s o l by a vapo r i za t i on -condensat ion m e c h a n i s m and the p a r t i c l e s i ze i s cont ro l led by the d i f f e rence in the t e m p e r a t u r e s of the vapour s t r e a m and the quenching (condensing) a i r s t r e a m . The p a r t i c l e s i ze of the a e r o s o l is a s c e r t a i n e d by pas s ing s a m p l e s through a p a r t i c l e - s i z e m e t e r and observ ing the degree of p o l a r -ization of a light b e a m . The g e n e r a t o r g ives a m o n o d i s p e r s e a e r o s o l of dioctyl phthala te with a p a r t i c l e d i a m e t e r of 0. 3 f im. The smoke m i x t u r e i s in t roduced into the f i l t e r at a concent ra t ion of 100 m g / m 3

(40 g r a i n s / 1 0 0 0 f t 3 ) of a i r . Samples of the a i r a r e drawn b e f o r e and a f t e r f i l t r a t ion and the e f f ic iency i s de te rmined by compar ing the concent ra t ion with a f o rwa rd l i g h t - s c a t t e r i n g m e t e r and a pho toe lec t r i c ce l l device known a s a p e n e t r o m e t e r which i s sens i t ive to concent ra t ion d i f f e r e n c e s of 0. 001%. The tes t , i s p e r f o r m e d at r a t e d a i r flow and at 20% of r a t ed a i r flow to indicate pinhole e f f e c t s (see chap te r 5).

The method i s u s e f u l when t h e r e a r e many f i l t e r s to be tes ted , s ince reading i s p r ac t i ca l l y ins tan taneous and the p a r t i c l e s i ze can be continuously checked. F o r l e s s f r equen t use the m e t h o d ' i s l e s s sui table , as the . appa ra tu s i s f a i r l y expensive and needs c a r e f u l opera t ion and main tenance . The DOP a e r o s o l cannot be used at e levated t e m p e r a t u r e s .

DOP a e r o s o l can a lso be genera ted by pneumat ic a t o m i z e r s , p a r t i c u l a r l y fo r in -p lace t es t ing p u r p o s e s .

33

(b) Methylene blue t e s t

The methylene blue tes t , p e r f o r m e d accord ing to B r i t i s h Standard 2831 [26] , i s c a r r i e d out by a tomizing, f r o m a Col l i son- type a t o m i z e r , a 1 % solution of methylene blue dye in d is t i l led wa t e r . The m o i s t u r e evapo ra t e s , leaving a dry methylene blue cloud which v a r i e s in s i ze f r o m 0. 01 m to approximate ly 1. 5 /um with a m a s s median s ize of approximate ly 0. 5 - 0 , 6 ( i m . The ef f ic iency of the f i l t e r is de te rmined by taking s e p a r a t e s a m p l e s of the a i r downs t r eam and u p s t r e a m of the f i l t e r . The s a m p l e s a r e p a s s e d through s e p a r a t e f i l t e r p a p e r s . The u p s t r e a m and downs t r eam s a m p l e vo lumes a r e adjus ted and sampl ing is r epea t ed unti l the s t a ins on both of the sampl ing f i l t e r p a p e r s a r e of equal densi ty , and the methylene blue pene t ra t ion is then ca lcu la ted f r o m the r a t i o of the known vo lumes sampled u p s t r e a m and downs t r eam f r o m the f i l t e r .

An advantage of the methylene blue t es t i s that it needs only v e r y s imple appa ra tus and may be used over a wide r ange of pene t r a t i ons (0. 01 to 50%). On the o the r hand it is a r e l a t ive ly slow t e s t .

(c) Sodium f l ame t e s t (see F ig . 21)

In the sod ium-ch lo r ide t e s t , p e r f o r m e d accord ing to Br i t i sh Standard 3928 [27] , a 2% aqueous solut ion of sodium chlor ide i s a tomized . The m o i s t u r e evapo ra t e s , leaving a dry cloud of solid cubic sa l t p a r t i c l e s , which range in s i ze f r o m 0. 01 to 1. 7 /urn, with a m a s s med ian s ize of about 0. 6 (im. A sample of the a i r which is p a s s e d through the f i l t e r i s d i r ec ted to a m e t e r i n g device , which m a k e s use of the c h a r a c t e r i s t i c yellowing of a hydrogen f l ame when sodium chlor ide p a r t i c l e s p a s s through it . The b r i g h t n e s s of th i s yellow light is d i r ec t ly p ropor t iona l to the m a s s of sal t p r e s e n t . The light p roduced i s a r r a n g e d to fa l l on a pho to - sens i t ive ce l l and the c u r r e n t f r o m it is m e a s u r e d on a sens i t ive m e t e r , giving a d i rec t indication of the amount of sa l t p r e s e n t .

ORIFICE PLATE

SAMPLING P O I N T

M I X I N G BAFFLE ,100 P.S.I. AIR SUPPLY

HEATER U N I T

AIR F L O W FROM FAN

S O D I U M FLAME RIG H Y D R O G E N SUPPLY

FIG.21 . Sodium f l a m e test r ig . (Courtesy Vokes L imi ted , Guildford, Surrey. )

34

The f l a m e p h o t o m e t e r i s c a l i b r a t e d by taking a s a m p l e of t e s t c loud in f r o n t of the f i l t e r and diluting it with c l e a n a i r in v a r i e d quan t i t i e s .

The t e s t me thod i s a l m o s t i n s t an t aneous in r e s p o n s e , i s u sed f o r p e n e t r a t i o n s f r o m 0. 001% on and i s r e p l a c i n g the me thy l ene blue t e s t f o r f i l t e r s of l e s s than 0 .1% p e n e t r a t i o n .

The sod ium f l a m e and me thy lene b lue t e s t s give about the s a m e p e n e t r a t i o n va lues ; at p e n e t r a t i o n s of about 0 .1% the r e s u l t s a r e a l m o s t iden t ica l . The t e s t cloud has l i t t l e e f fec t on the f i l t e r and a f u r t h e r advantage of the sod ium f l a m e t e s t i s tha t it can be u s e d fo r t e s t i n g at e l eva ted t e m p e r a t u r e s up to about 300°C fo r sod ium c h l o r i d e and p robab ly up to 500°C if sod ium iodide i s u s e d [ 4 ] .

(d) Uran ine t e s t

In F r a n c e a t e s t a e r o s o l of u r a n i n e h a s been u s e d [28] for eva lua t ion of H E P A f i l t e r s . The method i s b a s e d upon the u s e of f l u o r i m e t r y , which a f f o r d s p a r t i c u l a r l y good sens i t i v i t y . The bes t a e r o s o l i s p roduced by a tomiz ing a w a t e r solut ion of u r a n i n e ( f o r m u l a C 2 0 H 1 0 O 5 N a 2 ) . A f t e r the e l imina t ion of l a r g e d r o p l e t s by i n e r t i a l s e p a r a t i o n , the r e m a i n i n g l iquid a e r o s o l i s e v a p o r a t e d in a hea ted tube . A f ine sol id a e r o s o l i s obta ined,

ATOMIZER SEPARATOR EVAPORATOR

© ® © g A T O M I Z E R

1 Uranine solution

2 Atomizer head

3 A i r inlet

4 Solut ion suction pipe

5 BaFries

6 Aerosol outlet

7 Manometer

8 Va l ve

S E P A R A T O R

9 Separator

10 Inlet 'or i f ice

11 Separator pipe

12 Reservoir for removed l iquid

13 Filter

14 Out let nozzle

15 Final aerosol outlet

F I G . 2 2 . U r a n i n e ae roso l g e n e r a t o r . (Cour tesy J . P r ade l and J . Brion, CEA, F o n t e n a y - a u x - R o s e s . )

35

which i s i n j ec t ed u p s t r e a m f r o m the f i l t e r tha t i s be ing t e s t e d . S a m p l e s a r e co l l ec ted u p s t r e a m and d o w n s t r e a m f r o m the f i l t e r on f la t f i l t e r s , f r o m which the u ran ine i s e x t r a c t e d by wash ing . The concen t ra t ion of u r an ine in the solut ion i s d e t e r m i n e d by m e a s u r i n g the f l u o r e s c e n c e .

The a e r o s o l g e n e r a t o r c o n s i s t s of t h r e e p a r t s : an a t o m i z e r , a s e p a r -a t o r and an e v a p o r a t o r (see F ig . 22). The a t o m i z e r is t he Co l l i son - type , which i s a l so used fo r the me thy lene b lue t e s t , and it i s fed with c o m p r e s s e d a i r . The s e p a r a t o r , i n sp i r ed by the ' c e n t r i p e t e r 1 of Hounam, c o n s i s t s of two s t a g e s , and the l a r g e r d r o p l e t s a r e e l i m i n a t e d by i n e r t i a l s e p a r a t i o n . The e v a p o r a t o r i s a s ingle tube hea ted by a so lenoid .

The a t o m i z e r i s n o r m a l l y f i l led with a l iquid solut ion of 1% u r a n i n e and the feeding a i r p r e s s u r e i s 2 b a r s . About 95% of the a e r o s o l m a s s i s r e t a i n e d in the s e p a r a t o r . The output of the d ry sol id a e r o s o l i s 20 m g / h which c o r r e s p o n d s to a concen t r a t i on of 10 m g / m 3 at the exi t of the g e n e r a t o r . The med ian d i a m e t e r by n u m b e r is 0 . 1 5 n m and the m e d i a n d i a m e t e r by m a s s is 0. 3 (jra,

Sampl ing i s done with two d i f f e r en t f i l t e r s in s e r i e s and with an a i r f low of 2. 5 to 3 m 3 / h . As the f l u o r e s c e n c e depends on the pH va lue , an a m m o n i a washing solut ion i s u s e d . By choos ing a p p r o p r i a t e op t i ca l f i l t e r s and a su i t ab le wavelength of the l ight s o u r c e , a s ens i t i v i ty of 10 - 8 m g / c m 3

i s obta ined. The method h a s been c o m p a r e d with the DOP method , and f o r p lane f i l t e r s a m p l e s at a ve loc i ty of 5 c m / s to 14 c m / s it was found tha t the u r a n i n e method gave much l o w e r v a l u e s than the DOP method . Th is m e a n s that the u r a n i n e a e r o s o l i s m u c h m o r e p e n e t r a t i n g than the DOP a e r o s o l and that the method i s m u c h m o r e s e n s i t i v e f o r t e s t i n g f i l t e r s with high e f f i c i ency . When t e s t i ng f i l t e r un i t s at a ve loc i ty of 0. 5 to 1 c m / s the r e s u l t s obtained with the two m e t h o d s w e r e about the s a m e . The r e a s o n was p robab ly the p r e s e n c e of p inholes and the inf luence of the ve loc i ty t h rough the f i l t e r .

(e) O the r m e t h o d s

' At the S t aub fo r schungs ins t i t u t (Dust R e s e a r c h Inst i tute) in Bonn, F e d e r a l Republ ic of G e r m a n y , t e s t s a r e p e r f o r m e d with t h r e e t y p e s of a e r o s o l [ 2 9 ] . The e n t i r e f i l t e r e l emen t i s t e s t e d with oil fog, in which a l l d r o p s have a d i a m e t e r below 1 j jm and m o r e than 80% of the d r o p s have a d i a m e t e r be tween 0. 3 and 0. 5 Aim. Two q u a l i t y - c o n t r o l c h e c k s a r e m a d e on the f i l t e r m e d i u m , one with r ad ioac t i ve ly tagged n a t u r a l a t m o s p h e r i c a e r o s o l s , in which the d i a m e t e r of the p a r t i c l e s r a n g e s f r o m 0. 3 jum down to s e v e r a l m i l l i m i c r o n s and the m e a n d i a m e t e r i s about 0. 07 /Jm, and the o the r with f r e s h l y ground q u a r t z dus t s m a l l e r than 10 ium in d i a m e t e r , in which the d i a m e t e r of m o r e than 80% of the p a r t i c l e s m u s t be be tween 0. 5 and 2 (im and the m e a n d i a m e t e r i s about 1. 3 (um.

The a r g u m e n t fo r us ing v a r i o u s a e r o s o l s to t e s t the f i l t e r m e d i u m is tha t the e f f i c i ency of p a r t i c l e r e m o v a l i s dependent on the d i a m e t e r of the p a r t i c l e s . Only the oi l fog t e s t i s u s e d on c o m p l e t e f i l t e r un i t s , but a s no d i r e c t r ead ing is pos s ib l e it i s r a t h e r t i m e - c o n s u m i n g f o r quant i ta t ive t e s t i ng .

A method in which oil d r o p s with d i a m e t e r s f r o m 0. 2 to 0. 3 |um a r e used i s employed fo r inspec t ing f i l t e r s in the USSR, a s men t ioned in Ref . [ 4 ] .

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An advantage of the m e t h o d s tha t give an in s t an taneous indica t ion of the p e n e t r a t i o n , such as the DOP and sod ium f l ame t e s t s , i s tha t a s e a r c h p r o b e can be u s e d fo r faul t - f inding p u r p o s e s . The o the r m e t h o d s have c o r r e s p o n d i n g d i s a d v a n t a g e s .

6. 3. Leakage t e s t s

To d e t e r m i n e the t i g h t n e s s of the gaske t and the f r a m e , a s i m p l e p r e s s u r e l eakage t e s t can be p e r f o r m e d . The f i l t e r is connec ted to a box and the open f ace i s c o v e r e d with a s t e e l p la te . With a fan (vacuum c l e a n e r ) an u n d e r p r e s s u r e i s , m a i n t a i n e d in the box and the l eakage r a t e i s d e t e r m i n e d with a flow m e t e r . To f ind the l eakage po in t s , the box i s put at o v e r p r e s s u r e and a soap solut ion i s penc i l l ed along the g a s k e t s and the c o r n e r of the f r a m e . Any damage that could c a u s e l eakage i s e a s i l y de tec ted (Fig. 23) [ 6 ] .

To d i s c o v e r c r a c k s o r r u p t u r e s in the f i l t e r i n g m e d i u m of a f i l t e r uni t , a s i m p l e smoke l e akage t e s t can be used (Fig. 24). A f ine a e r o s o l of, f o r example , p a r a f f i n oil is g e n e r a t e d by a pneumat i c a t o m i z e r . A g e n e r a t o r p roduc ing p a r t i c l e s in s i z e s f r o m 0. 5 to 5 /um has p roved s a t i s f a c t o r y . The a e r o s o l i s evenly d i s t r i bu t ed into a mix ing c h a m b e r , on top of which the f i l t e r being checked i s p l aced . With a fan a slow a i r m o v e m e n t th rough the f i l t e r i s e f f ec t ed , about 1 c m / s f ace ve loc i ty .

F I G . 2 3 . Pressure l eakage test .

/"o~\ r o T \ r~o\ — L i g h t s

/l\ /l\

" "

B l a c k v e l v e t

H E P A - f i l t e r

Mix ing c h a m b e r

rr™— v y A e r o s o l g e n e r a t o r

F I G . 2 4 . Smoke l eakage tes t .

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In a s t rong l ight p inholes can be de tec ted a s s m a l l smoke s t r i n g s aga ins t the b l ack s c r e e n . The method i s s e n s i t i v e . Even f i l t e r s which m e e t the r e q u i r e m e n t of p e n e t r a t i o n below 0. 03% can have s o m e s m a l l indica t ion of l eakage . Th i s t e s t i s , of c o u r s e , only qua l i t a t ive but can be u s e d to de tec t obvious damage of f i l t e r s a f t e r t r a n s p o r t o r c a r e l e s s handl ing. The pos i t ion of a l eak i s a l s o c l e a r l y ind ica ted .

6. 4. Dus t - load ing t e s t s

One of the m o s t i m p o r t a n t i t e m s of i n f o r m a t i o n about a f i l t e r , b e s i d e s i t s e f f i c i ency , i s the expec ted l i fe of the f i l t e r o r i t s dus t -ho ld ing capac i ty . As d e s c r i b e d in sec t ion 5. 5, the p r e s s u r e d r o p - l o a d c h a r a c t e r i s t i c of a f i l t e r i s v e r y much dependent on the p r o p e r t i e s of the dust co l l ec ted in the f i l t e r . With s m a l l - s i z e d p a r t i c l e s of dus t the f i l t e r i s m o r e rap id ly c logged. As the type of dust tha t wi l l a r i s e in a p lant i s not a lways known, the l i f e - t i m e of a f i l t e r cannot be p r e d i c t e d even if the f i l t e r dus t -ho ld ing capac i t y i s known. To e s t i m a t e the capac i ty of v a r i o u s f i l t e r s , a c o m -p a r i s o n t e s t i s r e c o m m e n d e d . It can be m a d e in the plant if the f i l t e r be ing c o m p a r e d i s exposed to the s a m e dust concen t r a t ion and a i r f low. The condi t ions a r e m o r e e a s i l y c o n t r o l l e d if the t e s t i s run on a s e p a r a t e t e s t r i g , with, fo r e x a m p l e , a t m o s p h e r i c dus t [ 1 0 ] . A c c e l e r a t e d t e s t s can be p e r f o r m e d with a r t i f i c i a l dust [ 1 3 ] . E a c h t e s t should be p e r f o r m e d with the comple t e f i l t e r unit r a t h e r than with a s a m p l e of the f i l t e r m e d i u m , a s the des ign of the folded f i l t e r pack in f luences the capac i ty of the f i l t e r to a c c u m u l a t e dus t .

6. 5. In -p lace t e s t i ng t e chn iques

When h i g h - e f f i c i e n c y f i l t e r s a r e u sed , s t r i ngen t r e q u i r e m e n t s a r e of ten spec i f i ed f o r the r e m o v a l e f f i c i ency . Even if e v e r y f i l t e r i s t e s t e d b e f o r e leav ing the m a n u f a c t u r e r , d a m a g e dur ing handling, t r a n s p o r t a t i o n o r s t o r a g e m a y j e o p a r d i z e the good funct ioning of the f i l t e r . I n c o r r e c t ins t a l l a t ion caus ing l eakage p a s t the f i l t e r o r the f i l t e r mount ing f r a m e m a y lead to a high p e n e t r a t i o n of r a d i o a c t i v e dus t . Tes t i ng the f i l t e r b e f o r e it i s ins ta l l ed and p r e f e r a b l y a l s o with the f i l t e r i n - p l a c e a f t e r i t h a s been ins t a l l ed i s r e c o m m e n d a b l e .

Some p r o b l e m s have to be c o n s i d e r e d when t e s t i ng f i l t e r s in s i tu . As a f i l t e r s y s t e m usua l l y dea l s with a l a r g e vo lume of a i r and con ta ins many f i l t e r un i t s , it i s d i f f icu l t to d i s t r i b u t e an a r t i f i c i a l a e r o s o l evenly o v e r the whole vo lume. The a e r o s o l g e n e r a t o r m u s t have a high capac i ty and good mixing of the a i r m u s t be a t ta ined b e f o r e a s a m p l e is t aken .

S a m p l e s of the f i l t e r e d and u n f i l t e r e d a e r o s o l m u s t be r e p r e s e n t a t i v e of the a e r o s o l in the s y s t e m duct . It i s s o m e t i m e s n e c e s s a r y to induce mixing a r t i f i c i a l l y by m e a n s of b a f f l e s , e t c . A so-ca l led S t a i r m a r d d i sc i s o f t en r e c o m m e n d e d . Th i s i s an annu la r p la te with a f ace a r e a equal to half the c r o s s - s e c t i o n a l a r e a of the duct . To r e d u c e p r e s s u r e d rop , the d i sc i s u sua l l y pivoted so that it can be t u rned p a r a l l e l to the ax i s of the duct when not in u s e . If a r e p r e s e n t a t i v e s a m p l e cannot be wi thdrawn d o w n s t r e a m f r o m the f i l t e r , the concen t r a t i on can be d e t e r m i n e d by mu l t i p l e s ampl ing and ave rag ing t echn iques . To check the mix ing up -s t r e a m f r o m the f i l t e r , mul t ip le sampl ing m a y be n e c e s s a r y . As a l l f i l t e r s have to be exposed to the s a m e concen t r a t i on , the mix ing h a s to be i m p r o v e d if s a m p l e s ind ica te an unaccep tab l e v a r i a t i o n .

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S o m e t i m e s the duc ts and channe l s connec ted with the f i l t e r bank a r e not e a s i l y a c c e s s i b l e . Shielding o r o t h e r f a c t o r s m a y l im i t a c c e s s to su i t ab le a e r o s o l in j ec t ion o r s a m p l i n g l oca t i ons . P e r m a n e n t l y i n s t a l l e d sampl ing and in jec t ion l i ne s , loca ted ins ide the duc t s , m a y f a c i l i t a t e the t e s t i n g p r o c e d u r e . In -p lace t e s t i ng of f u t u r e i n s t a l l a t i ons can be s i m p l i f i e d by c a r e f u l planning in the des ign s t a g e .

The t e s t a p p a r a t u s should be r ig id and e a s i l y t r a n s p o r t a b l e . It would be a big advantage if it w e r e capab le of being u s e d to loca te the l e akage po in t s , a s any s y s t e m not m e e t i n g the r e q u i r e m e n t s m u s t be c o r r e c t e d . The m e t e r i n g device can be a s i m p l e one if it c o v e r s at l e a s t f o u r d e c a d e s , a s no di lut ion a r r a n g e m e n t f o r the s ampl ing flow i s then needed .

The m e t h o d s can be c h a r a c t e r i z e d by the t e s t a e r o s o l and the m e t e r i n g device u sed . The fol lowing m e t h o d s a r e d e s c r i b e d :

A e r o s o l M e t e r i n g device

DOP, p o l y d i s p e r s e pneuma t i c g e n e r a t o r P h o t o m e t r i c l i g h t - s c a t t e r i n g p n e u m a t i c - t h e r m a l g e n e r a t o r m e t e r

Uran ine , pneuma t i ca l l y Sampl ing , f l u o r i m e t r i c g e n e r a t e d m e a s u r i n g

N a t u r a l a e r o s o l Po l l ak coun te r

Coal g a s f l a m e

A t m o s p h e r i c r ad ioac t i v i t y Sampling, r a d i o a c t i v e m e a s u r i n g

The t echn iques appl ied in any given c a s e wi l l depend on the i n s t r u m e n t s ava i lab le at the p lant , the n u m b e r of s y s t e m s to be t e s t e d and the s i ze of the f i l t e r b a n k s . F o r a big e s t a b l i s h m e n t with m a n y f i l t e r i n s t a l l a t i ons a comple t e se t of t e s t i n g equ ipment , even if expens ive , m a y be jus t i f i ed . A s m a l l ins t a l l a t ion can r e q u e s t an ins t i tu t ion p o s s e s s i n g the n e c e s s a r y equ ipment to p rov ide the t e s t i n g s e r v i c e , o r a s i m p l e device i n c o r p o r a t i n g the ava i lab le i n s t r u m e n t s m a y be u sed .

(a) DOP p o l y d i s p e r s e a e r o s o l

In the DOP t e s t a me thod involving m o n o d i s p e r s e a e r o s o l , g e n e r a t e d by v a p o r i z a t i o n and subsequen t condensa t ion , i s g e n e r a l l y u s e d . Diocty l ph tha la te can a l so be u s e d in a pneuma t i c g e n e r a t o r o r in a p n e u m a t i c -t h e r m a l g e n e r a t o r , both of which a r e e a s i l y t r a n s p o r t e d . The a e r o s o l wi l l be p o l y d i s p e r s e , but a s the i n - p l a c e t e s t i s ma in ly in tended f o r l eakage indica t ion the p a r t i c l e - s i z e d i s t r i bu t ion is not c r i t i c a l [ 3 1 ] .

With a pneuma t i c g e n e r a t o r , the s o - c a l l e d p o r t a b l e , a i r - o p e r a t e d DOP a e r o s o l g e n e r a t o r d e s c r i b e d in Ref . [ 3 2 ] , the to ta l p e n e t r a t i o n obta ined with the two types of a e r o s o l c o r r e l a t e v e r y wel l [ 3 3 ] . In Ref . [34] it i s sugges t ed that a i r - g e n e r a t e d DOP g ives a l i t t l e l e s s p e n e t r a t i o n than i s obta ined with 0. 3-/jm m o n o d i s p e r s e DOP a e r o s o l . The p o l y d i s p e r s e a e r o s o l m e e t s the r e q u i r e m e n t s p r o p o s e d in a s t a n d a r d by the United S ta tes of A m e r i c a S t a n d a r d s Ins t i tu te : "The a e r o s o l s h a l l have an a v e r a g e p a r t i c l e d i a m e t e r of the o r d e r of 0. 5 m i c r o n (ium). The a e r o s o l sha l l have 95%

39

of the p a r t i c l e s l e s s than 1. 0 m i c r o n d i a m e t e r " . P h o t o m e t r i c l igh t -s c a t t e r i n g equipment i s u s e d f o r m e a s u r i n g the concen t ra t ion of the a e r o s o l .

Fundamen ta l l y , the e f f i c i ency t e s t shown s c h e m a t i c a l l y in F ig . 25 [32] c o n s i s t s of d i s cha rg ing a p o l y d i s p e r s e a e r o s o l of DOP into the a i r in take , m e a s u r i n g with the p h o t o m e t e r the concen t r a t ion of the a e r o s o l in s a m p l e s r e m o v e d f r o m the duct b e f o r e and a f t e r f i l t r a t i on , and ca lcu la t ing f i l t r a t i o n e f f i c i ency f r o m the two concen t r a t i on va lues . The a e r o s o l i s i n t roduced into the s y s t e m f a r enough in f r o n t of the f i l t e r s to e n s u r e that it i s thoroughly mixed with the a i r s t r e a m b e f o r e it r e a c h e s t h e m . By m a n i -folding a su f f i c ien t n u m b e r of a i r - o p e r a t e d a e r o s o l g e n e r a t o r s , even l a r g e s y s t e m s can be t e s t e d . The t e s t i ng equipment and t e s t i ng p e r f o r m a n c e a r e d e s c r i b e d in de ta i l in Ref. [ 3 2 ] .

F I G . 2 5 . Schemat i c of i n - p l a c e DOP test of HEPA fi l ter sys tem. (Courtesy ORNL, Oak Ridge, T e n n . )

P n e u m a t i c - t h e r m a l a e r o s o l g e n e r a t o r s a r e d e s c r i b e d in Ref . [35 ] and F ig . 26 shows a g e n e r a l s c h e m a t i c d i a g r a m of the m a j o r componen t s .

F r o m the oil r e s e r v o i r - m i x i n g c h a m b e r , which .holds a p p r o x i m a t e l y 200 c m 3 of l iquid, a f e e d e r nozz le c a r r i e s a m i x t u r e of the C0 2 and l iquid to the hea t ing uni t . H e r e it i s r a i s e d to a high t e m p e r a t u r e b e f o r e being e j e c t e d f r o m the g e n e r a t o r n o z z l e s as an a e r o s o l . The d i s p e r s i o n r a t e i s 5 - 1 5 g / m i n with an ope ra t i ng p r e s s u r e of be tween 0. 7 and 2. 8 k g / c m 2

(10 and 40 l b / i n 2 ) gauge p r e s s u r e . The t h e r m o s t a t i s ad jus t ed to a t e m p e r a t u r e of 300CC. T e s t s on H E P A f i l t e r pene t r a t i on , p e r f o r m e d with a p n e u m a t i c g e n e r a t o r and two t y p e s of p n e u m a t i c - t h e r m a l g e n e r a t o r , showed v e r y good a g r e e m e n t [ 3 5 ] . The d i f f e r e n t g e n e r a t o r s p roduced a e r o s o l s that gave , f o r about, the s a m e c o n c e n t r a t i o n s , the s a m e r ead ing on a f o r w a r d l i g h t - s c a t t e r i n g m e t e r . The l i g h t - s c a t t e r i n g m e a n d i a m e t e r f o r the g e n e r a t o r s w a s in the r ange 0. 75 - 0. 95 pim u n d e r n o r m a l g e n e r a t o r ope ra t i ng condi t ions .

40

FIG.26 . Major components of p n e u m a t i c - t h e r m a l aerosol genera tor . (Courtesy USAEC.)

The p r i n c i p a l advan tages of the p n e u m a t i c - t h e r m a l g e n e r a t o r a r e c o m p a c t n e s s and ,e l imina t ion of the need f o r c o m p r e s s e d a i r when t e s t i n g h igh -vo lume a i r - f i l t r a t i o n s y s t e m s . The p n e u m a t i c - t h e r m a l g e n e r a t o r s a r e capable of p roduc ing enough a e r o s o l to t e s t s y s t e m s with an a i r flow a s high as 25 000 m 3 / h . With r e d u c e d sens i t i v i ty down to about 0. 01%, depending upon the m e t e r i n g i n s t r u m e n t , an a i r flow of about 50 000 m 3 / h o r m o r e could be t e s t e d .

The t h e r m a l g e n e r a t o r s a r e c o n s i d e r a b l y m o r e c o m p l e x than the a i r - o p e r a t e d g e n e r a t o r s and m o r e m e c h a n i c a l and e l e c t r i c a l m a i n t e n a n c e i s n e c e s s a r y .

Concen t r a t ion m e a s u r e m e n t can be p e r f o r m e d with p h o t o m e t r i c l i gh t -s c a t t e r i n g m e t e r s . Both f o r w a r d l i g h t - s c a t t e r i n g p h o t o m e t e r s and r i g h t - a n g l e s c a t t e r i n g p h o t o m e t e r s a r e c o m m e r c i a l l y ava i l ab le . When us ing the pneuma t i c t h e r m a l g e n e r a t o r f o r s m a l l p l an t s , only a f r a c t i o n of the a e r o s o l can be u sed , s ince an e f f ec t of ' ove r load ing 1 the f o r w a r d l i gh t - s ca t t e r i ng m e t e r was o b s e r v e d when the DOP concen t r a t i on was o v e r 200 p g / l i t r e [ 3 5 ] . The i n s t r u m e n t r e s p o n s e was not l i n e a r at h ighe r c o n c e n t r a t i o n s .

(b) Uran ine a e r o s o l

As d e s c r i b e d in sec t ion 6. 2, the t e s t a e r o s o l i s p roduced by an a i r -o p e r a t e d g e n e r a t o r that p r o d u c e s an a e r o s o l f r o m a 1% u r a n i n e ( f o r m u l a C 2 0 H 1 0 O 5 N a 2 ) w a t e r solut ion. The m a s s m e d i a n d i a m e t e r i s 0 . 3 (im. S a m p l e s a r e co l l ec ted on f i l t e r s u p s t r e a m and d o w n s t r e a m f r o m the t e s t e d f i l t e r and the u r a n i n e i s e x t r a c t e d by wash ing . The c o n c e n t r a t i o n s of u r a n i n e in the so lu t ions a r e d e t e r m i n e d by m e a s u r i n g the f l u o r e s c e n c e .

When the background con tamina t ion f r o m the m e m b r a n e f i l t e r u s e d a s the second sampl ing f i l t e r i s t aken into account , t he m i n i m u m m e a s u r a b l e concen t r a t ion in the wash ing solut ion i s about 5 X 10"11 g / c m 3 . With a wash ing vo lume of 10 c m 3 , a s ampl ing a i r f low of 2. 5 m 3 / h and a s ampl ing

t i m e of one hour , the m i n i m u m m e a s u r a b l e concen t r a t i on in a i r i s 2 X 10"10 g / c m 3 . To t e s t a f i l t e r p lant with an e f f i c i ency of 99. 9%, the concen t r a t i on b e f o r e the f i l t e r should be at l e a s t 2 X 10"7 g / m 3 . An a e r o s o l g e n e r a t o r with a capac i ty of 20 m g / h i s capab le of p rov id ing the r e q u i r e d concen t r a t i on to a f low of 100 000 m 3 / h [ 28 ].

(c) A t m o s p h e r i c a e r o s o l — P o l l a k coun te r

The P o l l a k coun te r can be u s e d f o r m e a s u r e m e n t of the concen t r a t i on of nuc le i u p s t r e a m and d o w n s t r e a m f r o m a f i l t e r [ 3 6 ] . Aitken condensa t ion nuc l e i (10"1 to 10"3 /um in d i a m e t e r ) f o r m the c o r e of the w a t e r d r o p l e t s cons t i tu t ing n a t u r a l o r a r t i f i c i a l l y p r o d u c e d c louds and fogs . The Po l l ak condensa t ion nuc l e i coun te r m e a s u r e s , by m e a n s of a photocel l , the ex t inc t ion of a l ight b e a m p a s s i n g th rough the fog f o r m e d by ad iaba t ic expans ion of a i r in a tube s a t u r a t e d with w a t e r v a p o u r . The method h a s been c o m p a r e d with the sod ium f l a m e t e s t and an a g r e e m e n t within a f a c t o r of 2 h a s g e n e r a l l y b e e n obta ined. The r a n g e of nuc le i c o n c e n t r a t i o n s which can be m e a s u r e d with conf idence by the c o u n t e r c o v e r s four o r d e r s of magn i tude , so it i s p o s s i b l e to m e a s u r e s y s t e m s of 0. 01 % p e n e t r a t i o n . The n a t u r a l l y o c c u r r i n g condensa t ion nuc le i concen t r a t i on can, if n e c e s s a r y , be i n c r e a s e d by burn ing a coa l g a s f l a m e . The opera t ion of the coun te r i s not cont inuous (one r e a d i n g e v e r y 90 s) and i s not so convenient f o r loca t ing l e a k s in faul ty s y s t e m s [ 3 7 ] .

(d) A t m o s p h e r i c r ad ioac t iv i t y

At f a c i l i t i e s w h e r e the expense of s p e c i a l i n s t r u m e n t a t i o n i s not j u s t i f i ed b e c a u s e of the s m a l l n u m b e r of f i l t e r s y s t e m s o r w h e r e , f o r o the r r e a s o n s , e l a b o r a t e i n - p l a c e t e s t i n g i s not j u s t i f i ed , the radon daugh te r me thod may be u s e f u l . The method i s l imi t ed to d e t e r m i n i n g p e n e t r a t i o n of a few t e n t h s of a p e r c e n t o r g r e a t e r . The equ ipment r e q u i r e d c o n s i s t s of d e v i c e s that a r e n o r m a l l y ava i l ab le at a r a d i o a c t i v e l a b o r a t o r y . N a t u r a l r a d i o a c t i v e a e r o s o l i s co l l ec ted f o r a p e r i o d of 30 m i n u t e s o r m o r e in sampl ing f i l t e r p a p e r s u p s t r e a m and d o w n s t r e a m f r o m the t e s t e d f i l t e r , a s d e s c r i b e d in Ref. [ 3 8 ] . The f i l t e r p a p e r s a r e ana lysed in a lpha o r b e t a c o u n t e r s and the p e n e t r a t i o n ca l cu l a t ed f r o m the ac t iv i ty v a l u e s obta ined. Th is me thod cannot be used if the n a t u r a l r adon daugh te r s a r e p r e v e n t e d f r o m e n t e r i n g the ven t i l a t ion s y s t e m by an e f f i c i en t p r e f i l t e r o r m a s k e d by the p r e s e n c e of o the r r a d i o a c t i v e s u b s t a n c e s . In the l a t t e r c a s e , the o the r r ad ioac t i v i t y p r e s e n t migh t we l l cons t i tu te the t e s t a e r o s o l in p lace of the r adon d a u g h t e r s .

7. F I L T E R INSTALLATION AND CHANGING TECHNIQUES

7 . 1 . Vent i la t ion s y s t e m , g e n e r a l layout [1 ,39]

A wel l des igned ven t i l a t ion plant is the p r inc ipa l m e a n s of p reven t ing the s p r e a d of r a d i o a c t i v i t y both in s ide and outs ide the bui lding.

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7 . 1 . 1 . Concep ts

(1) The ven t i l a t ion plant sha l l supply .clean a i r to work ing a r e a s . (2) The plant sha l l be des igned on a o n c e - t h r o u g h b a s i s . (3) The plant sha l l ma in t a in d i r e c t i o n a l f lows f r o m a r e a s of l e a s t con-

t amina t ion to tha t of g r e a t e s t con tamina t ion . (4) D r a f t - f r e e condi t ions sha l l be ma in ta ined in the p e r s o n n e l w o r k a r e a s

and con t ro l led a i r ve loc i t i e s and lower a i r p r e s s u r e in hoods , boxes , c e l l s , e t c .

(5) The exhaus t a i r sha l l be c leaned b e f o r e it is d i s c h a r g e d to the a t m o s -p h e r e .

(6) Acc iden t condi t ions m u s t be c o n s i d e r e d when des ign ing the plant and evalua t ing the d e g r e e of a i r c l ean ing n e c e s s a r y fo r an a r e a .

(7) The cos t of the plant should be r e d u c e d by con ta inmen t p r o c e s s e s and by s e g r e g a t i n g exhaus t s y s t e m s .

(8) Depos i t ion of a i r b o r n e dus t in the duc t s should be avo ided .

T h e s e concep t s a r e d i s c u s s e d be low.

(1) The ven t i l a t ion plant sha l l supply a i r to work ing a r e a s .

The f i r s t p r o b l e m i s to a s s e s s the v o l u m e of a i r r e q u i r e d for the p lant . The quan t i ty of a i r wi l l depend upon the v o l u m e s of the work ing s p a c e s and the f r e q u e n c y of a i r change needed f o r the a r e a s involved . , Spread of r a d i o a c t i v e p roduc t s f r o m the p r o c e s s e s d e t e r m i n e s the need for a i r change . A s it i s u sua l l y v e r y d i f f i cu l t to p r ed i c t the d e g r e e of a i r con tamina t ion , a i r change f i g u r e s a c q u i r e d by e x p e r i e n c e a r e u sua l ly app l ied .

F o r th is p u r p o s e the a r e a s can be s e p a r a t e d into zones of va ry ing d e g r e e s of h a z a r d with r e s p e c t to r a d i o a c t i v i t y . F o u r zones can usua l ly be r e c o g n i z e d . In a d r a f t fo r ISO r e c o m m e n d a t i o n [40] , the zones a r e n u m b e r e d f r o m 1 to 4 , the h a z a r d s be ing g r e a t e r f o r the higher, n u m b e r s . In Re f . [6] the zones a r e n u m b e r e d in the r e v e r s e o r d e r , but the zones a r e in both c a s e s def ined a p p r o x i m a t e l y a s i s shown in Tab le VI.

Zone 1 inc ludes o f f i c e s , con t ro l r o o m s and shop a r e a s . The ven t i l a t ion r a t e i s decided by convent iona l s t a n d a r d s .

Zone 2 inc ludes m o s t of the opera t ing and g e n e r a l work ing a r e a s such a s l a b o r a t o r i e s . In R e f . [40] the fol lowing f i g u r e s f o r a i r changes a r e g iven for gu idance : 2 -3 v o l u m e s per h o u r . R e f e r e n c e [1] men t ions tha t a wide r a n g e of r a d i o a c t i v e l a b o r a t o r i e s have ven t i l a t ion r a t e s be tween 6 and 10 changes pe r h o u r .

Zone 3 c o v e r s s e r v i c e and m a i n t e n a n c e a r e a s fo r p r o c e s s equ ipment , hot c e l l s and g love-boxes . In the ISO r e c o m m e n d a t i o n , a ven t i l a t ion r a t e of 5 -10 v o l u m e s pe r hour i s g iven fo r gu idance . In the m o s t a c t i ve a r e a s 20 changes pe r hour a r e not uncommon [1] .

Zone 4 c o m p r i s e s hot c e l l s , g love -boxes and the p r o c e s s equipment i t s e l f . As th i s zone i s not n o r m a l l y occupied , the vent i la t ion r a t e is dec ided by the p r o c e s s i t se l f ( see sec t ion 7 .4 ) .

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TABLE VI. ZONES OF VARYING DEGREES OF HAZARD

Zone Design dose Clothing Access

1 < 3 / 1 0 MPD No spec i f i c c lo th ing

No l imi t

2 < 1 MPD Protec t ive c lo th ing

Permanent ly for occupa t iona l workers only

3 < 1 MPD Ful l -body pro tec t ion ; respirators if needed

Limit imposed by durat ion of occupancy or e f f i c i ency of pro tec t ive equ ipment

4 < 1 MPD Ful l -body pro tec t ion ; respirators

Entry forbidden unless decon t amina t ed or rad ia t ion source removed

MPD = Annual m a x i m u m permiss ible dose.

In high r o o m s the a i r quant i ty r e q u i r e d can be based on the f i r s t t h r e e m e t r e s of the height of the r o o m , to c o n s e r v e a i r r e q u i r e m e n t s .

When planning a r e s e a r c h l a b o r a t o r y , the f u t u r e u s e r s of ten cannot te l l exac t ly what type of w o r k wi l l be p e r f o r m e d in the d i f f e r e n t a r e a s . An i n h e r e n t c h a r a c t e r i s t i c of r e s e a r c h w o r k i s that i t s a i m s and d i r e c -t ions change a s new r e s u l t s open f u r t h e r p o s s i b i l i t i e s . It i s t h e r e f o r e of ten m o r e r a t i o n a l to des ign l a b o r a t o r i e s for d i f f e r e n t types of w o r k and for c e r t a i n l eve l s of r a d i o a c t i v i t y . The exhaus t a i r f r o m the n o r m a l , s t a n d a r d l a b o r a t o r y does not need c lean ing ; the med ium- l eve l r a d i o a c t i v e l a b o r a t o r y needs an i n c r e a s e d n u m b e r of a i r changes and exhaus t c lean ing for the hood exhaus t ; the high s t a n d a r d l a b o r a t o r y , intended fo r w o r k with high l e v e l s of r a d i o a c t i v i t y , i s equipped with ven t i l a t ion connec t ions f o r g love-boxes and lead c e l l s . Al l i n s t a l l a t i ons in the l a b o r a t o r i e s should be of a qua l i ty in a c c o r d a n c e with the r a d i o a c t i v i t y l eve l of the a r e a s . The w o r k p e r f o r m e d in the d i f f e r e n t l a b o r a t o r i e s has then to be adapted to the capac i t y of the i n s t a l l a t i ons to p reven t undue r i s k to p e r s o n n e l and s u r r o u n d i n g s .

The a i r quant i ty may s o m e t i m e s be dec ided by the n u m b e r and the s i z e of the ins ta l l ed hoods in a r o o m . In a p r o c e s s p lant , fo r e x a m p l e a r e a c t o r bui lding, the r e m o v a l of heat f r o m a r o o m may d e t e r m i n e the ven t i l a t ion r e q u i r e m e n t . By ins t a l l ing cool ing uni ts with a i r c i r c u l a t i o n the f r e s h a i r quant i ty needed fo r the r o o m can be r e d u c e d .

T h e a i r changes needed in r a d i o a c t i v e a r e a s for r e a s o n s of c o m f o r t a r e , a s a ru le , cove red by what i s needed f o r o ther r e a s o n s .

The i m p o r t a n c e of c lean ing the supply a i r i s d i s c u s s e d in sec t ion 2.

(2) The plant sha l l be des igned on a o n c e - t h r o u g h b a s i s

In r a d i o a c t i v e f a c i l i t i e s the exhaus t a i r i s p robab ly neve r r e c i r c u l a t e d in the plant fo r the sake of economy with r e g a r d to heat ing or cool ing

44

capac i t y . F o r s a f e t y r e a s o n s , the a i r i s r e l e a s e d a f t e r p a s s i n g th rough the plant , even if e f f ec t ive exhaus t a i r c l ean ing i s p rov ided .

(3) The plant sha l l ma in t a in d i r e c t i o n a l f lows f r o m a r e a s of l e a s t con tamina t ion to that of g r e a t e s t con tamina t ion

The o f f i c e s and c o r r i d o r s a r e the a r e a s of l e a s t con tamina t ion , the l a b o r a t o r y the a r e a of i n t e r m e d i a t e con tamina t ion and the l a b o r a t o r y hood that of g r e a t e s t con t amina t ion . A c c o r d i n g to th i s concep t , supply a:ir should en t e r the of f ice a t a p r e s s u r e s l igh t ly above a t m o s p h e r i c , flow th rough the c o r r i d o r , which i s kept a t a t m o s p h e r i c p r e s s u r e , p a s s on to the l a b o r a t o r y , which i s kept a t a p r e s s u r e below a t m o s p h e r i c , and f ina l ly be exhaus ted f r o m the hood. P a r t of the a i r has to be suppl ied d i r e c t l y to the l a b o r a t o r y and p a r t of the a i r p a s s e s f r o m the c o r r i d o r which w a r r a n t s a def ined a i r flow d i r e c t i o n and s t a b i l i z e s the p r e s s u r e d i f f e r e n c e be tween c o r r i d o r and l a b o r a t o r y . To ma in t a in a high p r e s s u r e d i f f e r e n c e of the o r d e r of 5 m m w . g . , double d o o r s o r a i r l ocks a r e needed . It m u s t be b o r n e in mind tha t the d i r e c t i o n a l a i r flow r e q u i r e d i s ach ieved by main ta in ing lower a i r p r e s s u r e in the m o r e con tamina ted a r e a s t o w a r d s which the a i r i s intended to f low.

The d r o p of p r e s s u r e in each zone m a y be s e v e r a l m m w . g . fo r zone 2, 5 -10 m m w . g . fo r zone 3, 15-45 m m w . g . f o r zone 4 [40] .

(4) D r a f t - f r e e condi t ions sha l l be ma in ta ined in the p e r s o n n e l w o r k a r e a s and con t ro l l ed a i r ve loc i t i e s and lower a i r p r e s s u r e in hoods , b o x e s , c e l l s , e t c .

The l a r g e a i r vo lume of ten needed in r a d i o a c t i v e f a c i l i t i e s i m p l i e s that the a i r - s u p p l y d e v i c e s m u s t be des igned such tha t d r a f t in work ing zones i s avo ided . A p e r f o r a t e d double ce i l ing i s of ten needed if the height of the r o o m is too low fo r o ther types of a i r i n j ec t ion s y s t e m s . The v e n t i -la t ion of hoods , b o x e s , and c e l l s i s dea l t wi th in ano the r s ec t ion of th i s cha p t e r .

(5) T h e exhaus t a i r sha l l be c leaned b e f o r e i t i s d i s c h a r g e d to the a t m o s p h e r e

Th i s concept wi l l be d i s c u s s e d in the fol lowing s e c t i o n s .

(6) Acc iden t condi t ions m u s t be c o n s i d e r e d when des ign ing the plant and eva lua t ing the d e g r e e of a i r c lean ing n e c e s s a r y fo r an a r e a

The a b n o r m a l condi t ions or acc iden t condi t ions fo r which the s y s t e m has to be des igned c a n b e s o d i v e r s e tha t a s p e c i a l e m e r g e n c y s y s t e m has to be i n s t a l l e d . The c lean ing r e q u i r e m e n t s and phys ica l condi t ions such

Jas t e m p e r a t u r e and humid i ty m a y ca l l f o r a s e p a r a t e s y s t e m to t ake over the ven t i l a t ion in c a s e of e m e r g e n c y .

T h e s y s t e m m u s t be des igned to p r e v e n t the s p r e a d of f i r e . In m o s t c a s e s the amount of c o m b u s t i b l e m a t e r i a l i s s m a l l in c o m p a r i s o n with the a i r v o l u m e s in a bui ld ing. In a bui lding w h e r e h igh- level r a d i o a c t i v e m a t e r i a l s a r e handled , n o n - c o m b u s t i b l e m a t e r i a l s should a lways be used w h e r e p o s s i b l e . If the whole ven t i l a t ion plant i s shu t down in the event of f i r e ,

45

the f i r e wi l l not be choked by lack of oxygen, but the smoke wil l m a k e f ight ing the f i r e d i f f icu l t and the s p r e a d of r ad ioac t i v i t y wi l l c a u s e m o r e t r oub l e than the f i r e i t s e l f . The exhaus t s y s t e m can t h e r e f o r e be kept work ing and only the supply s y s t e m shut off , a t l e a s t a s long a s the f i r e . i s not c a t a s t r o p h i c .

(7) The cos t of the plant should be r educed by con ta inment p r o c e s s e s and by s e g r e g a t i n g exhaus t s y s t e m s

Al l f e a s i b l e m e a s u r e s should be t aken to r e d u c e the quant i ty of a i r needed , a s the cos t of the vent i la t ing and f i l t e r i ng plant wi l l be p r o -por t iona l t o the a i r vo lume handled .

F i r s t of a l l , the p r o c e s s , a s a whole , should be s u r v e y e d . The m a t e r i a l involved can s o m e t i m e s be r e p l a c e d by a l e s s h a r m f u l one, or the quant i ty r e d u c e d by applying ano the r r e s e a r c h me thod . In a p r o d u c -tion plant , the type and amount of m a t e r i a l cannot be changed, but the s p r e a d of dus t could be r e d u c e d by changing the p r o c e s s , fo r example by tu rn ing f r o m d r y to wet handl ing.

Al l m e a n s of conta ining the p r o c e s s a r e well wor th t ry ing out . A v a r i e t y of types of e n c l o s u r e (hoods, g love-boxes and ce l l s ) p r e v e n t s h a z a r d o u s p roduc t s f r o m being s p r e a d in the r o o m a i r and r e d u c e s the a i r vo lume con tamina ted with r a d i o a c t i v e dus t . By s e g r e g a t i n g the exhaus t s y s t e m into d i f f e r e n t l i n e s , the d e g r e e of c lean ing can be adapted to the qua l i ty of the a i r in e v e r y l ine . In F i g . 27 the d i f f e r e n t c a t e g o r i e s of exhaus t l ine a r e ind ica ted . In w o r k - r o o m s w h e r e t h e r e i s no need for r e s p i r a t o r s or b r ea th ing a p p a r a t u s , the a i r does not n o r m a l l y have to be f i l t e r e d b e f o r e r e l e a s e into the a t m o s p h e r e . The exhaust m u s t be f i l t e r e d , howeve r , f r o m a w o r k - r o o m with a high dust c o n c e n t r a t i o n .

FIG.27 . Flow sheet of ex t r ac t vent i la t ion air s t r e a m . (Courtesy Butterworth and Co . L t d . , London.)

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The d e t e r m i n i n g f a c t o r m a y be the potent ia l r i s k of sp read , of ac t iv i ty in a b n o r m a l or e m e r g e n c y cond i t ions . Spec ia l dus t e n c l o s u r e s a r e suppl ied with loca l p r e f i l t e r s to r e d u c e the dus t concen t r a t i on to a l eve l su i t ab l e for t he h i g h - e f f i c i e n c y f i l t e r s . D u s t - c o l l e c t i o n nozz le s m a y need a h igh -capac i ty dus t c o l l e c t o r , such a s a cyc lone or s o m e other s p e c i a l cleaning, dev ice . B o o s t e r f ans can be ins t a l l ed in the l i nes f r o m v a r i o u s e n c l o s u r e s that r e q u i r e high p r e s s u r e d i f f e r e n t i a l s . T h e s e g r e g a t i o n m a k e s i t pos s ib l e to des ign duc t s and f i l t e r s lead ing f r o m the s p e c i a l e n c l o s u r e s of c o r r o s i o n - r e s i s t a n t m a t e r i a l to the point w h e r e the a i r i s di luted in a l a r g e v o l u m e of a i r t o make the c o n c e n t r a t i o n of c o r r o s i v e i m p u r i t i e s h a r m l e s s . '

The to ta l a i r quant i ty wi l l a l s o depend on the extent to which the a i r can p a s s s e v e r a l a r e a s in s e r i e s . The of f ice ven t i l a t ion a i r can p a s s the c o r r i d o r or be used fo r supplying a i r t o the l a b o r a t o r y w h e r e the amount of a i r needed m a y be d e t e r m i n e d by the n u m b e r of f u m e cupboa rds i n s t a l l ed .

Loca l i zed con ta inmen t can be of two k inds : an e s s e n t i a l l y to ta l en -c l o s u r e , fo r e x a m p l e a glove-box, [or a s e m i - e n c l o s u r e , such a s a f u m e -hood or c u p b o a r d .

Since a i r quan t i t i e s a r e r e d u c e d in p ropo r t i on to the r e d u c t i o n in open a r e a th rough which the a i r f lows , a m o r e e c o n o m i c a l u se of a i r i s ach ieved if the s o u r c e is enc losed .

(8) Depos i t ion of a i r b o r n e dus t in the duc ts should be avoided

Th i s i s p a r t i c u l a r l y u n d e s i r a b l e with c o m b u s t i b l e m a t e r i a l s and with g a m m a - a c t i v e m a t e r i a l s . A p r e f i l t e r n e a r the exhaus t point i s used for that r e a s o n . The des ign of the duct ing m a y a l s o be in f luenced by th is r e -q u i r e m e n t . A high ve loc i ty , no s h a r p c o r n e r s and the poss ib i l i t y of c lean ing the duc ts would be f a v o u r a b l e in such c a s e s .

7 . 1 . 2 . Individual or c e n t r a l i z e d ven t i l a t ion s y s t e m

When making the flow s h e e t , the d e g r e e to which the exhaus t a i r s y s t e m should c e n t r a l i z e d m u s t be dec ided .

E i t h e r t he individual r o o m s , hoods or bui ldings have an exhaus t fan and a s t a c k , or l a r g e , pa i r ed fans a r e ins ta l l ed a t a c e n t r a l loca t ion and the r o o m s , hoods or bui ld ings a r e connec ted to t h e s e f ans th rough l a r g e exhaus t d u c t s . Both s y s t e m s have, advan tages and d i s a d v a n t a g e s .

The advan t ages of indiv idual exhaus t a r e : (1) E a s i e r des ign ; (2) B e t t e r des ign and c o n s t r u c t i o n s chedu l e s due to r e a d y ava i l ab i l i t y of s m a l l f ans ; (3) Reduced c o s t s r e s u l t i n g f r o m : - (a) s m a l l e r duc twork , (b) lower f r i c t i o n l o s s e s .

The d i s advan t ages of individual exhaus t a r e : (1) L e s s o v e r a l l s y s t e m r e l i a b i l i t y b e c a u s e of d i f f i cu l ty in p rov id ing s t a n d - b y exhaus t equ ipment ; (2) P r a c t i c a l i m p o s s i b i l i t y of provid ing e m e r g e n c y power b e c a u s e of the s i z e of the s y s t e m , the n u m b e r of uni ts and the e l e c t r i c a l load; (3) Higher c o s t s r e s u l t i n g f r o m : (a) l a r g e r s p a c e r e q u i r e m e n t s needed to house f a n s , (b) ex tens ive i n s t a l l a t i on c o s t s f o r power f e e d s , m o t o r s , s t a r t e r s , e t c . , (c) n u m b e r of d i s c h a r g e s t a c k s r e q u i r e d , (d) i n spec t ion and m a i n t e n a n c e of many s m a l l un i t s , (e) m a n y exhaus t m o n i -to r ing s y s t e m s r e q u i r e d .

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The a r g u m e n t s fo r and aga in s t indiv idual exhaus t s y s t e m s a r e r e v e r s e d with c e n t r a l i z e d exhaus t s y s t e m s . F o r a h i g h - l e v e l r a d i o a c t i v e p lant , the a r g u m e n t s a r e in favour of c e n t r a l i z a t i o n . When high r e l i a b i l i t y i s needed , a c e n t r a l i z e d s y s t e m i s m o r e su i t ab l e , and a high s t ack and an advanced mon i to r ing s y s t e m , n e c e s s a r y fo r advanced r a d i o a c t i v e p l an t s , a l s o favour a c e n t r a l i z e d s y s t e m . In a big plant a combina t ion of the two s y s t e m s i s the b e s t so lu t ion . F o r non- rad ioac t ive and low-ac t iv i t y a r e a s individual exhaus t s y s t e m s can be u sed , but fo r a r e a s with high con tamina t ion l e v e l s a c e n t r a l i z e d s y s t e m should be i n s t a l l ed . The p r e s s u r e ba l ance between the d i f f e r e n t zones i s e a s i e r to ma in ta in in a c e n t r a l i z e d s y s t e m b e c a u s e a cons tan t p r e s s u r e can be kept in the m a i n exhaus t duc t . The a i r ba l ance i s not j eopa rd i zed by d i s t u r b a n c e s of i n -d iv idua l f ans or clogging of individual f i l t e r s .

When a r educ t i on of ope ra t i ng c o s t s is to be achieved by r e d u c i n g a i r flow dur ing non-work ing h o u r s , a c e n t r a l i z e d s y s t e m is m o r e conven ien t , s i n c e r e d u c e d a i r flow can be obtained by speed r egu l a t i on or by v a r y i n g the n u m b e r of f ans in ope ra t ion .

7 . 2 . F i l t e r s y s t e m s , p r e f i l t e r s and f ina l f i l t e r s

If the m a i n p r i n c i p l e s d e s c r i b e d in sec t ion 7 . 1 a r e app l ied , a b a s i c flow shee t can be obtained fo r the ven t i l a t ion s y s t e m . The r e q u i r e m e n t s for the d i f f e r e n t f i l t e r s a r e then eva lua ted in m o r e de t a i l .

P h y s i c a l condi t ions such a s humidi ty and t e m p e r a t u r e which might a p p e a r in a f i l t e r s y s t e m m u s t a l s o be c o n s i d e r e d in r e l a t i o n to the p e r -f o r m a n c e of the f i l t e r . F u r t h e r , such phys ica l condi t ions have a l s o t o be c o n s i d e r e d in r e l a t i o n to a b n o r m a l or e m e r g e n c y condi t ions .

The ope ra t i ng t e m p e r a t u r e of the f i l t e r does not in f luence the cho ice of f i l t e r a s long a s i t i s below 60°C, which i s u sua l ly the c a s e in v e n t i l a -t ion s y s t e m s fo r bu i ld ings . F o r h igher t e m p e r a t u r e s p las t i c f i l t e r s a r e not su i t ab l e and the f i l t e r m a t e r i a l should be chosen on the b a s i s of the data in sec t ion 5 . 5 . The v a r i a t i o n in r e m o v a l e f f i c i ency a s a func t ion of the v i s c o s i t y of the gas i s neg l ig ib le . T e s t s with g l a s s pape r showed v e r y l i t t l e change in e f f i c i ency , even a t t e m p e r a t u r e s c l o s e to that w h e r e the g l a s s would f a i l .

If the humidi ty of the a i r i s over 80%, ce l l u lo se pape r i s not su i t ab l e . T r e a t e d g l a s s - p a p e r m e d i u m can wi ths tand 100% humidi ty , but if the humid i ty a p p r o a c h e s 100%, the a i r or gas wi l l v e r y l ikely conta in s o m e liquid w a t e r in the f o r m of m i s t or d r o p l e t s . An accumula t ion of w a t e r m a y then occur in the f i l t e r . If a f i l t e r i s p laced a f t e r a s c r u b b e r , the a i r should be hea ted b e f o r e it e n t e r s the f i l t e r , so that a l l s m a l l d r o p l e t s that might p a s s the d r o p e l i m i n a t o r of the s c r u b b e r would be v a p o r i z e d .

" If ac id v a p o u r s a r e l ike ly to be e n c o u n t e r e d , fo r e x a m p l e f r o m f u m e -cupboa rd e x t r a c t s , then both the f i l t e r and a s s o c i a t e d duc twork m u s t be ac id r e s i s t a n t .

7 . 2 . 1 . Inle t a i r f i l t r a t i o n

A t m o s p h e r i c a i r has a c o n c e n t r a t i o n of dus t that v a r i e s f r o m 0 . 0 5 m g / m 3

in r u r a l a i r t o 1 m g / m 3 in c i ty a i r , and in i n d u s t r i a l d i s t r i c t s the dus t con -tent of a t m o s p h e r i c a i r i s even h i g h e r . If the in le t a i r i s not f i l t e r e d , the

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exhaus t f i l t e r i s exposed to a l l the a t m o s p h e r i c d u s t . T e s t s [10] have shown tha t , even with the m i n i m u m a t m o s p h e r i c dus t c o n c e n t r a t i o n of 0 . 0 5 m g / m 3 , the p r e s s u r e d r o p of a HEPA f i l t e r i n c r e a s e s to twice i t s o r ig ina l va lue a f t e r one y e a r of cont inuous o p e r a t i o n . With high c o n c e n t r a t i o n v a l u e s , a h i g h - e f f i c i e n c y f i l t e r would be plugged a f t e r a v e r y s h o r t t i m e . With e f f ic ien t exhaus t f i l t e r s , high qua l i ty inle t f i l t r a t i o n i s w o r t h ins t a l l ing and i s l e s s c o s t l y than the HEPA f i l t e r . M o r e o v e r , the exhaus t f i l t e r s a r e con tamina ted by r a d i o a c t i v i t y which m a k e s handl ing and d i s p o s a l m o r e expens ive . It i s a l s o d e s i r a b l e to keep the dus t c o n c e n t r a t i o n in the work ing a r e a s to a m i n i m u m to r e d u c e s u r f a c e con tamina t ion . A c c o r d i n g l y , in a plant w h e r e highly r a d i o a c t i v e m a t e r i a l i s handled , the in le t a i r f i l t r a t i o n r e q u i r e m e n t i s h ighe r than i s n o r m a l l y r e q u i r e d ' m e r e l y fo r r e a s o n s of c o m f o r t in o r d i n a r y ven t i l a t ion p l an t s . A f i l t e r of e i the r Group II or G r o u p III, a s d e s c r i b e d in c h a p t e r 3, should be u s e d . The a i r in take should be s o pos i t ioned that e x p o s u r e to dus t i s m i n i m i z e d and t h e r e i s no p ick-up of exhaus t f r o m o ther s y s t e m s . Lower heal th phys i c s c o s t s a r e u sua l l y a s s o c i a t e d with a lower l eve l of dus t in the intake a i r and t h e r e i s thus an e c o n o m i c a l advan tage to f i l t r a t i o n .

7 . 2 . 2 . P r e f i l t e r s

In bui ld ings w h e r e r a d i o a c t i v e m a t e r i a l s a r e handled , the dus t p roduc t ion in the p lant is kept low by h igh -qua l i t y f loor and bench c o v e r i n g s and by g e n e r a l c l e a n l i n e s s . The p r o c e s s i t se l f wi l l , h o w e v e r , g e n e r a t e dus t and a s the HEPA f i l t e r s should not w o r k with dus t c o n c e n t r a t i o n s h ighe r than 1 m g / m 3 , a p r e c l e a n i n g of the a i r i s o f ten n e c e s s a r y . The to ta l c o s t of f i l t e r i n g the a i r should be c o n s i d e r e d , f i r s t f r o m the point of view of the added c o n s t r u c t i o n cos t and the added c o s t of changing the p r e f i l t e r s and aga in f r o m tha t of having no p r e f i l t e r s and thus be ing fo rced to change the HEPA f i l t e r s m o r e of ten . Depending upon the concen t r a t i on of dus t , e s p e c i a l l y of l a r g e r p a r t i c l e s , and o ther condi t ions , the to ta l c o s t of f i l t e r i n g a g iven vo lume of a i r wi th p r e f i l t e r i n g can be e i the r lower or h i g h e r . The economy to be ach ieved with p r e -f i l t e r s is i l l u s t r a t e d in F i g . 28. A cyc lone i s su i t ab le when t h e r e i s high

F i l ter l i t e , m o n t h s

FIG.28 . Filter l i fe with opt imum pref i l ter . (Courtesy Butterworth and Co . L td . , London.)

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dust p roduc t ion f r o m a m a c h i n e . In o ther c a s e s a p r e f i l t e r a t the exhaus t point i s u s e f u l . Such a f i l t e r a l s o p r o t e c t s the duc ts f r o m dust depos i t ion that would be highly u n d e s i r a b l e if the dus t we,re py rophor i c or g a m m a -r a d i o a c t i v e .

7 . 2 . 3 . F i n a l f i l t e r s

To avoid l eakage of dus t f r o m the duc t s and f i l t e r hous ings , the f i l t e r s should c o m e b e f o r e the f a n s . By th is m e a n s the f i l t e r housing can be kept a t nega t ive p r e s s u r e and the fan i s kept f r e e of d u s t , which f a c i l i t a t e s m a i n t e n a n c e w o r k . The f ina l f i l t e r s should be ins t a l l ed in a s e p a r a t e r o o m , p r e f e r a b l y of f i r e p r o o f des ign . A s t h e s e f i l t e r s r e p r e s e n t the l a s t l ine of p r o t e c t i o n for the s u r r o u n d i n g s , i t i s i m p o r t a n t that they be p ro t ec t ed f r o m d e s t r u c t i o n by a f i r e in the bui ld ing. Dur ing f i l t e r changing s o m e dus t migh t be s p r e a d , but it would not in f luence the a c c e s s i b i l i t y of t he work ing a r e a s if a s p e c i a l f i l t e r r o o m w e r e p rov ided .

7 . 2 . 4 . F i r e h a z a r d s in a f i l t e r i n s t a l l a t i on [1]

F i l t e r s of n o n - c o m b u s t i b l e m a t e r i a l s should be used when the r i s k of f i r e i s g r e a t , f o r e x a m p l e when handling py rophor i c m a t e r i a l or when the c o n s e q u e n c e s of a f i l t e r d a m a g e d by f i r e migh t be s e r i o u s . F o r many app l i ca t ions f l a m e - r e s i s t a n t f i l t e r s a r e j u s t i f i a b l e , s ince the dus t a c -cumula t ed in the f i l t e r a l w a y s involves a f i r e h a z a r d . With a p r e f i l t e r a t a c o n s i d e r a b l e d i s t ance u p s t r e a m f r o m the f ina l f i l t e r , the l a s t b a r r i e r wi l l poss ib ly r e m a i n in t ac t . With an a i r - s t r e a m content of unneu t r a l i z ed h y d r o f l u o r i c ac id , the c e l l u l o s e f i l t e r m e d i u m has to be u s e d . The f i l t e r s can be p ro t ec t ed by m e a n s of s p a r k a r r e s t o r s f r o m s p a r k s that might r e a c h t h e m .

7 . 3 . F i l t e r mount ing and changing a r r a n g e m e n t s

It i s v e r y i m p o r t a n t fo r the in s t a l l a t ion a r r a n g e m e n t s for h i g h - e f f i c i e n c y f i l t e r s to be c a r e f u l l y des igned and c o n s t r u c t e d . The p e r f o r m a n c e of a HEPA f i l t e r can e a s i l y be spoi led by fau l ty in s t a l l a t ion and handl ing. When des ign ing the f i l t e r i n s t a l l a t i on , i t i s e s s e n t i a l to e l im ina t e any poss ib i l i t y of un f i l t e r ed a i r l eak ing pas t the uni t . As the e f f i c i ency of a HEPA f i l t e r i s u sua l l y 99. 97% and a s t a n d a r d uni t has a c apac i t y of 1700 m 3 / h , a l eakage of 0.03% or 0 . 5 m 3 / h would i n c r e a s e the p e n e t r a t i o n to twice i t s nomina l va lue . At a p r e s s u r e d r o p of 25 m m w . g . a c r o s s the f i l te r , a hole with a d i a m e t e r of about 3 m m would al low l eakage of that o r d e r . Th i s p r o b l e m should a lways be c o n s i d e r e d with r e s p e c t t o the type of f i l t e r i n s t a l l a t i on being u s e d .

The mount ing a r r a n g e m e n t s a r e in f luenced by the n u m b e r of f i l t e r s i n s t a l l e d , the changing t echn ique , the type and concen t r a t i on of r a d i o a c t i v e c o n t a m i n a n t s , the ava i l ab le s p a c e , the duct ing, e t c . As the f i l t e r s a r e to be handled by p e r s o n n e l d r e s s e d in bulky p r o t e c t i v e c lothing and equipped with r e s p i r a t o r s or m a s k s , t h e r e should be enough s p a c e for m a i n t e n a n c e w o r k and the t r a n s p o r t a t i o n of f i l t e r s .

Open-bank ins t a l l a t ion can be used a s shown in F i g . 29. P r e f a b r i c a t e d hous ings su i t ab le fo r changing f i l t e r s with p l a s t i c bags a r e c o m m e r c i a l l y

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/

F I G . 2 9 . Open bank instal la t ion of f i l te rs . (Courtesy ORNL, Oak Ridge, T e n n . )

ava i l ab l e (F ig . 30). The f i l t e r can be of the s e l f - c o n t a i n e d type shown in F i g . 31, which can be connected to duc t s with s l e e v e s . Th is type of f i l t e r i s a l s o ca l led an enc lo sed f i l t e r . The u sua l open - f aced f i l t e r can a l s o , be connected to a duct with f l a n g e s , a s s e e n in F i g . 32.

7 . 3 . 1. G e n e r a l des ign c o n s i d e r a t i o n s

Many e x a m p l e s of f i l t e r i n s t a l l a t i ons and de ta i led de s igns a r e given in Re f . [6] and s o m e g e n e r a l r e m a r k s a r e p r e s e n t e d h e r e .

To obtain the n e c e s s a r y s ea l i ng be tween the f i l t e r and the mount ing f r a m e , the f i l t e r s ea l i ng s u r f a c e should be r i g i d , f l a t , smoo th , c l ean and f r e e of weld s p a t t e r . T o l e r a n c e s should be provided to al low fo r p o s s i b l e swel l ing of a wood f i l t e r f r a m e f r o m e x p o s u r e to m o i s t a i r and to f a c i l i t a t e e a s y r e p l a c e m e n t of the f i l t e r uni t . F o r a bank in s t a l l a t i on , the de f l ec t ion of the f r a m e du r ing ope ra t ion a t m a x i m u m dus t loading of the f i l t e r m u s t be c o n s i d e r e d . F o r a f i l t e r bank with a p r e s s u r e d r o p of 250 m m w . g . , the a i r p r e s s u r e e x e r t s a f o r c e of 250 k g / m 2 . A p r a c t i c a l r u l e i s that the b e a m s should de f l ec t no m o r e than 0.1% of the length unde r a loading equiva lent to 1 .5 t i m e s the m a x i m u m d i r t y - f i l t e r p r e s -s u r e d r o p a c r o s s the bank . F i l t e r mount ing f r a m e s should be shop f a b r i c a t e d , a s it i s n e a r l y i m p o s s i b l e to avoid m i s a l i g n m e n t , wa rp ing and d i s t o r t i o n in f ie ld f a b r i c a t i o n .

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FIG.30 . C o m m e r c i a l l y ava i l ab le housings. (Courtesy Sa in t -Gobain Service N u c l £ a i r e . )

ULTRA AIRE SPACE FILTER

FIG. 32. O p e n - f a c e d f i l ter f ixed be tween duct f langes . (Courtesy Mine Safety Appl iances C o m p a n y , Pit tsburgh, P a . )

The f i l t e r uni t m u s t be c l amped to the mount ing f r a m e with enough p r e s s u r e to enable the gaske t to ma in t a in a r e l i a b l e s e a l under condi t ions of v ib ra t ion , t h e r m a l expans ion , f r a m e f l e x u r e , shock , o v e r p r e s s u r e , and widely v a r y i n g t e m p e r a t u r e and humid i ty that can be expec ted in s e r v i c e . E x p e r i e n c e has shown that a s i m p l e b o l t - a n d - n u t s y s t e m g ives the m o s t s a t i s f a c t o r y s e r v i c e under t h e s e cond i t ions . The c l amp ing f o r c e r e q u i r e d depends on the h a r d n e s s of the g a s k e t , the t o l e r a n c e s of the g a s k e t and the condi t ion of the s ea l i ng s u r f a c e s . If the t h i c k n e s s of the gaske t i s ma in ta ined to a t o l e r a n c e of ± 0 . 2 5 m m and the mount ing s u r f a c e i s f la t to a t o l e r a n c e of + 0 . 2 5 m m , a c o m p r e s s i o n of only 20% is needed . However , t h e s e t o l e r a n c e s a r e too r e s t r i c t i v e fo r e c o n o m i c a l c o n s t r u c -tion and e x p e r i e n c e has shown tha t a g a s k e t c o m p r e s s i o n of a t l e a s t 80% i s u sua l ly n e c e s s a r y to e f f ec t a r e l i a b l e s e a l over long p e r i o d s . T h i s d e g r e e of c o m p r e s s i o n r e q u i r e s a loading of a p p r o x i m a t e l y 1 .4 k g / c m 2 of the ga ske t a r e a , which gives a to ta l c l amping load of 650 kg fo r a 610X610 m m f i l t e r [6] . When no p r e s s u r e - d i s t r i b u t i o n f r a m e is p rov ided , eight p r e s s u r e points a r e r e c o m m e n d e d per 610 X 610 m m f i l t e r uni t . With p r e s s u r e -d i s t r i bu t ion f r a m e s , four p r e s s u r e points a r e s u f f i c i e n t . Individual c l amping of each f i l t e r i s r e c o m m e n d e d .

The o r i en ta t ion of the f i l t e r should be d e t e r m i n e d not only on the b a s i s of an ea sy connec t ion to duc ts and f ans but a l s o by the r e q u i r e m e n t s fo r f i l t e r p e r f o r m a n c e and m a i n t e n a n c e . The v e r t i c a l pos i t ion of the f i l t e r (hor izon ta l a i r flow) i s p r e f e r r e d to the ho r i zon ta l pos i t ion ( v e r t i c a l a i r flow) [30] . F u r t h e r , the f i l t e r should be placed with the p l ea t s and s e p a -r a t o r s v e r t i c a l . Th i s e l i m i n a t e s sagging of the p l ea t s f r o m the weight of the f i l t e r med ia and the a c c u m u l a t e d d u s t . The f i l t e r i s a l s o ea sy to handle in the v e r t i c a l pos i t ion . When changing f i l t e r s , m a t e r i a l co l l ec ted in the f i l t e r wil l not fa l l out and the f i l t e r can s l ide on a f r a m e when moved into

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or out of a f i l t e r hous ing . A co l lec t ion of condensa t e in the f i l t e r , which could poss ib ly happen in a ho r i zon t a l bank, i s avoided .

No c l e a r - c u t r u l e ex i s t s fo r mount ing f i l t e r s on e i the r the u p s t r e a m s ide of the mount ing f r a m e or the d o w n s t r e a m s ide (see F i g . 33). Both me thods of ins t a l l a t ion a r e used s u c c e s s f u l l y [6] . The u p s t r e a m mount ing i s the m o s t usua l one, p robab ly b e c a u s e it s e e m s m o s t n a t u r a l to m a k e u s e of the a e r o d y n a m i c f o r c e s to p r e s s the f i l t e r aga in s t the sea l ing s u r f a c e . F o r HEPA f i l t e r s , howeve r , the c l amping f o r c e s a r e unusua l ly high, 650 kg c o m p a r e d with the a e r o d y n a m i c load of 37 kg fo r a 610 X 610 m m f i l t e r a t 100 m m w . g . p r e s s u r e d r o p . The advan tages of u p s t r e a m mount ing a r e :

(1) F i l t e r s a r e handled in the con tamina ted s ide of the s y s t e m dur ing a f i l t e r change . No con tamina ted m a t e r i a l s a r e b rought into the c l ean s ide of the s y s t e m .

FIG. 33 . Ver t i ca l mount ing (hor izonta l air f low).

(2) The a i r flow tends to load the f i l t e r gaske t du r ing ope ra t ion .

The d i s advan tages of u p s t r e a m mount ing a r e :

(1) The f i l t e r c l amping d e v i c e s a r e loca ted in the d i r t y s ide of the s y s t e m w h e r e they a r e m o s t exposed to c o r r o s i o n and d i r t . (2) P e r s o n n e l have to work in a highly con tamina ted zone dur ing a f i l t e r change and con tamina t ion can be t r a c k e d or c a r r i e d out by w o r k m e n , un-l e s s a housing des igned fo r f i l t e r change with p l a s t i c bags is u s e d . (3) The ou te r s u r f a c e s of the f i l t e r c a s e a r e c o n t a m i n a t e d .

F o r d o w n s t r e a m mount ing the oppos i te advan tages and d i s advan t ages app ly . Leak probing m a y b e m o r e s e n s i t i v e , s i n c e , if t h e r e a r e gaske t or c a s ing l e a k s , the t e s t a e r o s o l p e n e t r a t e s the leaking points and can be de tec ted b e f o r e i t m i x e s with the a i r pa s s ing the f i l t e r .

When f i l t e r banks m u s t be h o r i z o n t a l ( v e r t i c a l a i r .flow), upflow i s p r e -f e r r e d to downflow b e c a u s e sagging of the f i l t e r c o r e i s o f f se t to s o m e extent by a i r p r e s s u r e and b e c a u s e t h e r e i s l e s s chance of c r o s s con tamina t ion f r o m the d i r t y s ide to the c l ean s ide of the s y s t e m . With downflow, con-t amina t ed dust d is lodged dur ing a f i l t e r change wi l l fa l l into the c l ean s ide of the s y s t e m . An advan tage of d o w n s t r e a m flow i s , however , tha t the dus t i s co l lec ted within the unit and not d i s lodged dur ing s e r v i c e [4] .

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&

FIG.34 . Horizontal moun t ing (ver t ica l air f low).

When f i l t e r s a r e in s t a l l ed in h o r i z o n t a l banks , they should be mounted on the upper s ide of the mount ing f r a m e ( see F i g . 34). R e p l a c e m e n t of f i l t e r s is e a s i e r with the f i l t e r s on the f l oo r of a f i l t e r r o o m than if they have to be changed f r o m below [6] . On the o ther hand, when a housing i s used that p e r m i t s changing f r o m the s i de , the f i l t e r can be mounted on the lower s ide ( u p s t r e a m s ide) to al low the f i l t e r to s l ide on the gaske t f r e e f a c e .

A s the weight of the f i l t e r (15-20 kg) and the a e r o d y n a m i c f o r c e s (36 kg for a 610 X 610 m m s i z e a t a p r e s s u r e d r o p of 100 m m w. g . ) a r e s m a l l c o m p a r e d with the c l amping f o r c e s (650 kg), e f f ec t ive c o m p r e s s i o n of the gaske t can a lways be obtained with a p r o p e r c l amping dev ice .

When c o m p a r i n g the a e r o d y n a m i c f o r c e s (36 kg), the weight of the c o r e ( - 10 kg) and the m i n i m u m m e c h a n i c a l s t r e n g t h of the f i l t e r c o r e (300 m m w . g . fo r a 610 X 610 m m f i l t e r s i z e g ives 110 kg), it i s obvious that the s a f e t y m a r g i n i s l a r g e r if the net load on the f i l t e r c o r e i s 26 kg, a s in the upflow c a s e , i n s t ead of 46 kg, a s in the downflow c a s e . The p e r c e n t a g e loads a r e 24% and 42%, r e s p e c t i v e l y , of the p e r m i s s i b l e load fo r a d u s t - l o a d e d f i l t e r . A s the s t r e n g t h of the f i l t e r c o r e m a y d e t e r i o r a t e , an i n c r e a s e in the m a r g i n i s u s e f u l . On the o ther hand, the m e c h a n i c a l s t r e n g t h of a f i l t e r 292 m m deep i s u sua l ly m o r e than the spec i f i ed 300 m m w . g . A downflow mount ing i s consequen t ly not ac tua l ly r e c o m m e n d e d but p e r m i s s i b l e . The s a m e a s p e c t s a s apply to upflow in s t a l l a t i on can be appl ied to the o r i en t a t i on of the f i l t e r on the upper and lower s i d e s .

7 . 3 . 2 . Bank s y s t e m

T h e mount ing f r a m e s a r e u sua l ly provided by the f i l t e r m a n u f a c t u r e r , a l though in c e r t a i n i n s t a n c e s they m a y be m a n u f a c t u r e d independen t ly . Not only m u s t the s ea l i ng s u r f a c e s be tween f i l t e r and f r a m e be of good qua l i ty , but the mount ing f r a m e m u s t a l s o be wel l f ixed and sea l ed to the wa l l of the f i l t e r c h a m b e r . With open i n s t a l l a t i ons the f i l t e r s m u s t be changed by p e r s o n n e l d r e s s e d in whole -body p r o t e c t i v e su i t s suppl ied with f r e s h a i r . T h u s , f a c i l i t i e s for ' f r o g m a n 1 s e r v i c e m u s t be a r r a n g e d and the n e c e s s a r y a i r locks provided to allow p e r s o n s to en te r the f i l t e r c h a m b e r .

7 . 3 . 3 . Hous ings

C o m m e r c i a l l y ava i l ab l e hous ings should be p roven by p ro to type t e s t b e f o r e being spec i f i ed on a job. They m u s t be r ig id enough to allow a

55

c o m p r e s s i o n load of a t l e a s t 1 .5 k g / c m 2 on the f i l t e r s ea l i ng g a s k e t . T h e housing m u s t be v i r t u a l l y l eak t igh t to the s u r r o u n d i n g s and be ab le to w i th s t and , without d a m a g e , the nega t ive p r e s s u r e that can occur in the s y s t e m . No l eakage should take p lace f r o m the d i r t y s ide to the c l ean s i d e of the f i l t e r hous ing . With a d e s i g n such a s tha t shown in F i g . 35, with a double f l ange sea l ing , no l eakage can occur f r o m the c l ean s ide to the d i r t y s i de , e i t he r through the s ea l i ng be tween the f i l t e r and the mount ing f r a m e or th rough a leak in the hous ing i t s e l f .

1. Ab so l u te fi lter

2 . Pressure e q u a l i z i n g

open i n g w i th f i lter

3 . C l a m p i n g screws for

f i lter units

4 . Soft p l a s t i c connec t i on

5 . Dampe r , l ocked in pos i t i on

6 . Connec t i on s for p res sure-

drop a n d o i r - f l o w measurement

7 . D r a i n a g e v a l v e

F I G . 3 5 . Housing with double f lange sea l ing .

Hous ings can be provided with a d a p t o r s to enable c losed changing of the f i l t e r to be c a r r i e d out with the p l a s t i c bag technique d e s c r i b e d in de ta i l in s ec t i on 9 . 3 . Enough s p a c e m u s t be l e f t in f r o n t of the f i l t e r hous ing fo r a c c e s s dur ing f i l t e r changing , a t l e a s t 1 m fo r f i l t e r s i z e 610 X 610 m m . The a i r th rough the hous ing unit m u s t be. shut off dur ing changing. To keep dus t f r o m s p r e a d i n g , the hous ing should be kept at a s l igh t negat ive p r e s s u r e of a few m m w . g . to f ac i l i t a t e handling the p l a s t i c bag . Work with the p l a s t i c bag i s d i f f i cu l t when t h e r e i s a high p r e s s u r e d i f f e r e n c e , b e c a u s e the p las t i c bag can be sucked in and wil l then s t i ck to the s u r f a c e of the f i l t e r . When t h e r e a r e s e v e r a l housing uni ts work ing in p a r a l l e l , the fan can be runn ing dur ing f i l t e r changing . D a m p e r s m u s t then be ins t a l l ed to i so l a t e each housing unit du r ing th i s ope ra t i on . A s the d a m p e r s a lways have s o m e l e a k a g e , the u n d e r p r e s s u r e wi l l be too high if no con t ro l l ed a i r in le t to the hous ing is a r r a n g e d . F o r s a f e t y r e a s o n s a f i l t e r i s connec ted to th is a i r in le t du r ing the changing p r o c e d u r e to avoid the s p r e a d of r a d i o -ac t iv i ty in the event of o v e r p r e s s u r e o c c u r r i n g in the hous ing .

7 . 3 . 4 . Se l f - con ta ined f i l t e r s

Se l f -con ta ined f i l t e r s a r e ava i l ab l e f r o m many m a n u f a c t u r e r s in a l l s t a n d a r d HEPA f i l t e r s i z e s . The f i l t e r m e d i u m is buil t into a f r a m e or housing of plywood or m e t a l and the hous ing i s provided with n ipples fo r a t t ach ing the f i l t e r a s an i n t e g r a l p a r t of the duc t . T h e comple t e uni t i s t e s t ed by the m a n u f a c t u r e r and t h e r e i s no poss ib i l i t y of d i r t y a i r bypas s ing

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the f i l t e r . As the f i l t e r m e d i u m is we l l p r o t e c t e d , i t wi l l not be d a m a g e d dur ing the i n s t a l l a t i on of the f i l t e r . The r i s k of s p r e a d i n g co l l ec ted m a -t e r i a l dur ing handl ing of t he d i r t y f i l t e r i s a l s o neg l ig ib le . Since the f i l t e r should be kept a t nega t ive p r e s s u r e r e l a t i v e to the s u r r o u n d i n g a r e a , a c r a c k in the f r a m e r e s u l t s in h a r m l e s s l eakage of c l e a n a i r in to the exhaus t s y s t e m (F ig . 36).

Clean air

F I G . 3 6 . Se l f - con ta ined f i l t e r , harmless l e a k a g e .

If the whole f i l t e r i s placed in the con tamina ted a r e a any l eakage down-s t r e a m f r o m the f i l t e r m e d i u m is not a c c e p t a b l e . It i s i m p o r t a n t fo r the m a n u f a c t u r e r 1 s t e s t to inc lude the outs ide l eakage t e s t fo r f i l t e r s in s t a l l ed in the d i r t y a r e a .

S i m i l a r to the s e l f - c o n t a i n e d f i l t e r is the doub le - f l ange connect ion shown in F i g . 32. One p r o b l e m is the d i f f i cu l ty of in s t a l l ing and m a i n -ta in ing the f l anges p a r a l l e l to e n s u r e u n i f o r m gaske t c o m p r e s s i o n and good sea l i ng p r e s s u r e .

7 . 3 . 5 . Choice of f i l t e r i n s t a l l a t i on p r i n c i p l e s

F o r a s m a l l a i r - c l e a n i n g plant with only one or a few f i l t e r s , c o m -m e r c i a l l y ava i l ab l e hous ings fo r the p l a s t i c bag changing technique could be u s e d . If hous ings with doub le - f l ange s y s t e m s a r e chosen , t h e r e is l ess , need fo r i n - p l a c e t e s t i ng a f t e r changing. F o r s m a l l s ing le p lants i t i s an advan tage not to have to buy equ ipment for ' f r o g m a n ' w o r k and in -p lace f i l t e r t e s t i n g , s ince it would be used v e r y i n f r e q u e n t l y . In addi t ion , s e l f - c o n t a i n e d f i l t e r s or doub le - f l ange i n s t a l l a t i ons can be used fo r s y s t e m s intended to dea l with m a t e r i a l s of m o d e r a t e r ad io tox i c i t y .

M e d i u m - s i z e d plants with 10 or 20 f i l t e r s could be equipped with hous ings or bank i n s t a l l a t i o n s . The i n v e s t m e n t cos t and m a i n t e n a n c e c o s t s have to be c o m p a r e d and t h e s a f e t y p r o b l e m s c o n s i d e r e d . The bank s y s t e m in - , vo lves lower i n v e s t m e n t c o s t s but the c o s t s of c o n s t r u c t i o n of f i l t e r c h a m -b e r s should be inc luded .

With hous ing s y s t e m s the c o m p l e t e in s t a l l a t ion can be des igned and d e l i v e r e d by the c o n t r a c t o r for the ven t i l a t ion plant and the a r r a n g e m e n t in the bui lding can be v e r y s i m p l e .

F o r l a r g e p l an t s , open i n s t a l l a t i on with f i l t e r banks i s c h e a p e r . In e s t a b l i s h m e n t s such a s r e s e a r c h s t a t ions and r e p r o c e s s i n g f a c t o r i e s , t h e r e a r e many ven t i l a t ion p lants of d i f f e r e n t c a p a c i t i e s and spec i a l l y t r a i n e d people a r e ava i l ab le to ma in t a in and t e s t the f i l t e r p l a n t s . In such c a s e s , bank s y s t e m s a r e m o r e f r e q u e n t l y used than they would be f o r s ing le ven t i -la t ion p l a n t s .

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7 . 4 . L a b o r a t o r i e s

T h e r e a r e m a n y types of l a b o r a t o r y f o r r a d i o a c t i v e w o r k . Some types a r e planned only fo r s p e c i a l w o r k and o t h e r s a r e in tended for g e n e r a l a p -p l i ca t i ons . F o r s p e c i a l t ypes of l a b o r a t o r y , the a i r - c l e a n i n g r e q u i r e m e n t can be b a s e d upon the w o r k to be p e r f o r m e d . In a g e n e r a l l a b o r a t o r y th i s i s not p o s s i b l e , and it i s b e s t to des ign the ven t i l a t ion plant and the c lean ing d e v i c e s c o n s i s t e n t with the r e s t of t he plant . F o r a h igh -ac t i v i t y l a b o r a t o r y , the exhaus t a i r wi l l be c l eaned , of ten with HEPA f i l t e r s . S e v e r a l types of exhaus t s y s t e m could be ins t a l l ed in such a l a b o r a t o r y . A g e n e r a l exhaus t s y s t e m p rov ides the n e c e s s a r y a i r changes in a l l r o o m s . If the potent ia l acc iden t h a z a r d s in the plant i m p l y unaccep tab le c o n s e q u e n c e s , s a f e t y f i l t e r s can be i n s t a l l e d . Spec ia l equ ipment exhaus t s y s t e m s a r e a l s o of ten r e q u i r e d to con ta in the r a d i o a c t i v e w o r k p e r f o r m e d in the l a b o r a t o r y . Such equ ipment inc ludes f u m e c u p b o a r d s , hoods , g love-boxes , lead c e l l s , e t c .

7 . 4 . 1 . F u m e cupboard

F u m e c u p b o a r d s fo r w o r k with t r a c e l e v e l s of r a d i o a c t i v i t y could be connected to a g e n e r a l exhaus t s y s t e m . When w o r k tha t g e n e r a t e s dus t i s p e r f o r m e d in a cupboa rd , c lean ing of the exhaus t a i r i s r e q u i r e d . A c e n t r a l bank of HEPA f i l t e r s can s e r v e many f u m e c u p b o a r d s . E v e r y f u m e

F I G . 3 7 . Fi l ter for f u m e cupboards . (Courtesy AB A t o m e n e r g i , Studsvik . )

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cupboard i s p rov ided with loca l p r e f i l t e r s which can be buil t in to the cup -b o a r d o r p laced in a s e p a r a t e f i l t e r hous ing . If they a r e mounted in the c u p b o a r d , changing the f i l t e r i s d i f f i cu l t w h e r e bulky l a b o r a t o r y a p p a r a t u s r e d u c e s the a c c e s s i b i l i t y of the f i l t e r .

A s e p a r a t e hous ing i s a m o r e f l ex ib le des ign that a l lows a f i l t e r to be ins t a l l ed in any type of f u m e cupboa rd , provided tha t enough s p a c e i s a v a i l a b l e . The f i l t e r s can be changed without d i s t u r b i n g the work in the cupboard ( see F i g . 37).

7 . 4 . 2 . Open boxes (F ig . 38)

To r e d u c e the a i r v o l u m e s needed to m a i n t a i n a high ve loc i ty through the a c c e s s a r e a in a f u m e cupboa rd , d i f f e r e n t kinds of boxes with s m a l l openings can be u s e d . Th i s i s e s p e c i a l l y u s e f u l fo r r ou t i ne o p e r a t i o n s , when the equ ipment used can be m o r e or l e s s p e r m a n e n t l y in s t a l l ed in the box, fo r e x a m p l e in a p roduc t ion l ine fo r i so topes or fue l p e l l e t s . The openings should be l a r g e enough to p e r m i t e a s y a c c e s s to the box (<j> 15 t o 20 cm) and an a i r ve loc i ty of 0. 5 to 1 m / s should be ma in ta ined th rough t h e m , th i s a i r flow g e n e r a l l y provid ing enough a i r changes in the box . S o m e t i m e s the openings a r e c o v e r e d with r u b b e r s t r i p s fo r f u r t h e r r e d u c t i o n of the a i r

F I G . 3 8 . Open box. (Courtesy AB Atomene rg i , Studsvik.)

59

f low, but th is i s not n e c e s s a r y and m a y give r i s e to a s p r e a d of r a d i o a c t i v i t y a s the s t r i p s e a s i l y b e c o m e c o n t a m i n a t e d . T h e f i l t e r r e q u i r e m e n t s a r e the s a m e a s those fo r f u m e c u p b o a r d s , but the c o m p a r a t i v e l y s m a l l a m o u n t of a i r r e d u c e s the s i z e and n u m b e r of f i l t e r s needed .

7 . 4 . 3 . Glove-boxes

When m a t e r i a l of high r a d i o t o x i c i t y , such a s p lu tonium, i s being handled, comple t e con ta inment of the work ing s p a c e i s d e s i r a b l e . To en -s u r e that any l eakage wi l l be f r o m outs ide into the box, the box i s kept a t a s l ight negat ive p r e s s u r e of 20 to 50 m m w . g . , depending upon the con -t a m i n a t i o n r i s k of the w o r k in the box. As the s ens i t i v i t y when work ing with g loves i s inf luenced by nega t ive p r e s s u r e over 25 m m w . g . , the p r e s s u r e d i f f e r e n t i a l i s r e d u c e d when s a f e t y c o n s i d e r a t i o n s p e r m i t . The a p p r e c i a b l e vo lume of the g loves m u s t be t aken into accoun t , howeve r , s i n c e r a p i d m o v e m e n t with t hem e a s i l y c a u s e s o v e r p r e s s u r e in the box if a nega t ive p r e s s u r e of 20 to 30 m m w . g . i s not m a i n t a i n e d .

Since the g loves a r e f i t ted in to the a c c e s s po r t of the t ight box, no a i r flow i s needed to ma in t a in an inward flow through any openings . Dus t , f u m e s and hea t developed in the box have , howeve r , to be r e m o v e d , and th i s can be ach ieved e i t h e r by a s i n g l e - p a s s ven t i l a t ion s y s t e m or by a r e c i r c u l a t i n g s y s t e m .

(a) S i n g l e - p a s s ven t i l a t ion s y s t e m

Vent i la t ion a i r flow r e q u i r e m e n t s can v a r y within wide l i m i t s , but , f o r l e s s h a z a r d o u s w o r k in a glove-box, 10 changes per hour wi l l s u f f i c e . In des igning the exhaus t s y s t e m , h o w e v e r , s a f e t y in the event of a m a j o r b r e a k , such a s a glove f a i l u r e , m u s t be c o n s i d e r e d . The u sua l glove f a i l u r e i s only a m i n o r t e a r , but in the bas i c des ign a r e a s o n a b l e ve loc i ty over the glove por t i s a s s u m e d . F o r a d i a m e t e r of 15 c m (6 i n . ) and a ve loc i ty of 0. 5 to 1 m / s , the r e q u i r e d a i r flow is 40 to 80 m 3 / h . A s the w o r k p e r f o r m e d in glove-boxes i s a lways h a z a r d o u s , the exhaus t f i l t r a t i o n m u s t be e f f i c i en t . Glove-box f i l t e r s with a p e n e t r a t i o n of l e s s than 10"4 a r e a v a i l a b l e .

F o r d r y handl ing of p lu tonium an o v e r a l l p e n e t r a t i o n of l e s s than 10"6

i s r e q u i r e d . F i l t e r s should p r e f e r a b l y be of a n o n - c o m b u s t i b l e type , but t he n e c e s s a r y c o r r o s i o n r e s i s t a n c e should a l s o be c o n s i d e r e d . C o m m e r c i a l f i l t e r s a r e ava i l ab l e which a r e s m a l l enough to be pa s sed in and out th rough a r e g u l a r t r a n s f e r p o r t . E v e n if the r e q u i r e d e f f i c i ency can be obtained with the glove-box f i l t e r , a b a c k i n g - u p f i l t e r should be ins ta l l ed in the exhaus t s y s t e m , such a f i l t e r be ing capab le of s e r v i n g m a n y g love-boxes .

A i r fo r the vent i la t ion of the box i s taken f r o m the r o o m and p a s s e d in to ' t he box th rough a f i l t e r e d in le t . The in le t f i l t e r should have such d i m e n s i o n s that the p r e s s u r e d r o p of the d i r t y f i l t e r can be o v e r c o m e by the p r e s s u r e d i f f e r e n c e be tween the r o o m and the i n t e r i o r of the box. The in le t f i l t e r p ro longs the l i f e of the exhaus t f i l t e r and p r e v e n t s s p r e a d of con tamina t ion in the event of the glove-box becoming acc iden t ly p r e s s u r i z e d . T h e e f f i c i e n c y needed for the f i l t e r i s m o d e r a t e — 90 to 99% of the r o o m dus t should be r e m o v e d . The in le t and out le t of the box can be c l o s e d , and a s the box i t se l f i s v e r y t ight i t c an be d i sconnec ted and moved to a s e r v i c e

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a r e a . The m a x i m u m accep tab le leak r a t e is usual ly l e s s than 0.5% of the box volume per hour at a p r e s s u r e d i f f e r e n c e of 100 m m w . g . The box can a l s o be purged with inert gas ins tead of r o o m a i r , in which ca se the flow r a t e is low and the box v e r y t ight .

(b) Rec i r cu l a t i ng s y s t e m s

' In such s y s t e m s the gas i s r e c i r c u l a t e d through a g a s - p u r i f y i n g and - c i r cu la t ing plant that mus t be designed to r e m o v e c o r r o s i v e , explos ive and other u n d e s i r a b l e g a s e s and vapours f r o m the box a t m o s p h e r e and to cont ro l the p r e s s u r e in the s y s t e m . T h e s e s y s t e m s a r e su i tab le for p r o -c e s s e s with d r y a i r or i ne r t gas a t m o s p h e r e s .

The boxes mus t be v e r y t ight . The spec i f ied m a x i m u m leak r a t e is usual ly 0. 05% of the box volume per hour at a negat ive p r e s s u r e of 100 m m w . g . The box might be connected to an exhaust s y s t e m to be used in c a s e of e m e r g e n c y . The s a m e r e q u i r e m e n t s may be spec i f ied as for the s ing le -pass s y s t e m , i . e . that it should p reven t sp read of contaminat ion in ca se of a glove f a i l u r e .

7 . 4 . 4 . Cel ls

When l a r g e quant i t ies of g a m m a - e m i t t i n g m a t e r i a l a r e handled shielding space mus t be provided . If d u s t - s p r e a d i n g opera t ions a r e involved, t h e r e i s a contaminat ion r i s k and the g e n e r a l r e q u i r e m e n t s for hot ce l l vent i la t ion and f i l t e r ing a r e , i n p r inc ip le , the s a m e as those for glove-boxes . Smal l ce l l s a r e often shie lded with lead blocks but l a r g e ce l l s a r e built up with thick conc re t e wal l s and have a c c e s s d o o r s , through which f r o g m a n can enter the ce l l for main tenance w o r k . In a ce l l 10 000 Ci can be a c c o m m o d a t e d . The high ac t iv i ty leve l and the s i ze of the ce l l s n e c e s s i t a t e r e q u i r e m e n t s in addit ion to those given for glove-boxes .

A cont ro l led s i n g l e - p a s s s y s t e m is usual ly used for hot c e l l s . A i r f r o m the ma in tenance zone i s supplied to the ce l l via an inlet f i l t e r , at a control led flow r a t e which depends on the work c a r r i e d out in the ce l l . The pe rmanen t c h a r a c t e r of a c o n c r e t e ce l l building m a k e s it n e c e s s a r y to des ign the vent i la t ion s y s t e m for any kind of work that can poss ib ly be p e r f o r m e d in the ce l l . The hot ce l l s a r e of ten designed f o r a vent i la t ion r a t e of 60 changes per hou r . If poss ib le , the supply and exhaust po r t s should be located so that a i r f lows f r o m the top of the ce l l to the bot tom. The a i r flow the reby con t r ibu tes to the se t t l ing of dust p a r t i c l e s in the ce l l . When opening the shie lded plug door to the ce l l , a veloci ty of 0. 5 to 1 m / s through the opening is d e s i r a b l e to avoid the s p r e a d of r ad ioac t i ve dust into the main tenance a r e a . The a i r flow for th is purpose wil l probably be the cont ro l l ing f ac to r for the s i ze of the exhaust f i l t e r . A p r i m a r y HEPA f i l t e r of non-combus t ib le type should p r e f e r a b l y be placed in the ce l l so that it can be changed and the used f i l t e r enclosed in p ro tec t ive cover ing inside the ce l l by a- f r o g m a n during main tenance work . In c a s e the r ad ia t ion level f r o m the f i l t e r is too high, the f i l t e r m u s t be r e m o v e d by the manipu la to r in the ce l l . A f inal HEPA f i l t e r is r e q u i r e d to r e d u c e the dust c o n c e n t r a -tion of the exhaust a i r and a s a s a f e ty m e a s u r e . As the l a r g e ce l l s a r e often used for handling i r r a d i a t e d fuel e l e m e n t s , some impregna ted c h a r c o a l beds m a y be needed to r e m o v e the f i s s i o n product iodine, which may cause heal th h a z a r d s if r e l e a s e d to the s u r r o u n d i n g s .

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7 . 5 . R e a c t o r bui ldings

The c lean ing r e q u i r e m e n t s for a i r and g a s e s leaving a r e a c t o r plant d i f f e r a c c o r d i n g to the s y s t e m c o n c e r n e d . T h e r e a r e ven t i l a t ion s y s t e m s fo r d i f f e r e n t a r e a s of the r e a c t o r bu i ld ings , which usua l ly a r e divided into zones of va ry ing a c c e s s i b i l i t y : u n r e s t r i c t e d a d m i s s i o n , r e s t r i c t e d a d m i s s i o n and no a d m i s s i o n . An o f f - g a s s y s t e m is provided to co l l ec t g a s e s and v a -p o u r s f r o m the p r o c e s s s y s t e m s . F o r acc iden t condi t ions a s p e c i a l e m e r -gency ven t i l a t ion s y s t e m is a r r a n g e d .

F o r the g e n e r a l ven t i l a t ion of a r e a s which a r e a c c e s s i b l e du r ing o p e r a -t ion , the con tamina t ion leve l m u s t be kept low. Since p e r s o n s a r e p r e s e n t in t h e s e a r e a s dur ing work ing h o u r s , the concen t r a t i on of r a d i o a c t i v e c o n t a -mina t ion m u s t be kept we l l below the m a x i m u m p e r m i s s i b l e v a l u e s . When the a i r i s r e l e a s e d th rough the s t a c k , it is di luted by many o r d e r s of m a g n i -tude and the concen t r a t i on in the s u r r o u n d i n g s wil l be r e d u c e d by a t l e a s t a f a c t o r of 10, which is the r e l a t i o n s h i p be tween occupa t iona l dose and the dose accep ted for m e m b e r s of the populat ion l iving a round a r e a c t o r p lan t . Clean ing of the a i r may not be r e q u i r e d .

F a c i l i t i e s which a c c o m m o d a t e the p r i m a r y coolant s y s t e m s m a y have a high concen t r a t i on of ac t iv i ty in the a i r f r o m leakage of the coolant m e d i u m , which con ta ins ac t iva ted c o r r o s i o n p roduc t s a s wel l a s f i s s i o n p roduc t s f r o m punc tured fue l e l e m e n t s . A high ac t iv i ty concen t r a t i on in the a i r would not in f luence the ope ra t ion , a s the a r e a s a r e not a c c e s s i b l e b e c a u s e of the high r a d i a t i o n leve l fo r the p r i m a r y coo lan t . The r e l e a s e of f i s s i o n p r o d u c t s f r o m fue l e l e m e n t s tha t have c r a c k s or p inholes in the canning depends v e r y much on the ope ra t ing t e m p e r a t u r e of the f u e l . At t e m p e r a t u r e s above 1600° and 1700°C, the r e l e a s e of iodine i s a c c e l e r a t e d . F o r a r e a c t o r with a high fue l t e m p e r a t u r e , the c lean ing dev ice should c o n s i s t of a p a r t i c u l a t e f i l t e r , fo l lowed by an iodine f i l t e r such a s an i m p r e g n a t e d c h a r c o a l bed .

Al l r e a c t o r s have s o m e kind of o f f - g a s s y s t e m to co l l ec t g a s e s f r o m the r e a c t o r s y s t e m f o r t r e a t m e n t b e f o r e r e l e a s e through the s t a c k . A p a r t f r o m a e r o s o l s , the o f f - g a s m a y conta in noble g a s e s and iodine and a h igh-e f f i c i ency p a r t i c u l a t e f i l t e r i s thus a lways inc luded in the c lean ing s y s t e m . A s the iodine i s of ten the m o s t h a r m f u l of the f i s s i o n p roduc t s that may l eak f r o m the fue l , an iodine a d s o r p t i o n f i l t e r i s needed , but should be posi t ioned a f t e r the a e r o s o l f i l t e r , a s s o m e of the iodine is a d s o r b e d on the a e r o s o l s and a d e s o r p t i o n m a y o c c u r . The noble g a s e s a r e only d e -layed a s h o r t whi le in a c h a r c o a l bed a t n o r m a l t e m p e r a t u r e , and to obtain an a p p r e c i a b l e de lay the bed m u s t be cooled to a low t e m p e r a t u r e . A de l ay tank i s of ten used to obtain the d e s i r e d decay . Since the decay p roduc t s wi l l f o r m r a d i o a c t i v e a e r o s o l s , the decay tanks should be followed by an a e r o s o l f i l t e r .

When any vo lume n e a r the c o r e , w h e r e t h e r e i s an a p p r e c i a b l e neu t ron r a d i a t i o n f ie ld , i s f i l led with a i r , i t b e c o m e s r a d i o a c t i v e b y ' n e u t r o n a c t i v a -t ion of a r g o n and any dus t p a r t i c l e s exposed to neu t ron f lux . The p roduc t ion of a r g o n i s wel l def ined by the neu t ron f lux, and , even a t s m a l l ven t i l a t ion r a t e s , m o s t of t he a r g o n produced wil l be r e l e a s e d to the s u r r o u n d i n g s . A f i l t e r i s needed to r e m o v e r a d i o a c t i v e p a r t i c l e s , and , e s p e c i a l l y when the a i r i s u sed fo r cooling p u r p o s e s , the in le t a i r should be c leaned to m i n i m i z e induced r a d i o a c t i v e con t amina t ion . An exhaus t f i l t e r m u s t a l s o be i n s t a l l e d .

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C o m m e r c i a l p o w e r r e a c t o r s u s u a l l y h a v e a c o n t a i n m e n t d e s i g n e d to c o n f i n e t h e p r i m a r y s y s t e m o r p a r t of i t and a l s o have r a p i d - c l o s i n g v a l v e s to i s o l a t e the p ipes p a s s i n g t h r o u g h the c o n t a i n m e n t . E m e r g e n c y v e n t i l a -t ion s y s t e m s a r e u s e d t o t ake c a r e of t he l e a k a g e f r o m the c o n t a i n m e n t . C i r c u l a t i o n s y s t e m s c a n be p l a c e d i n s i d e the c o n t a i n m e n t f o r coo l ing and c l e a n i n g p u r p o s e s .

T e m p e r a t u r e c o n d i t i o n s m u s t be c o n s i d e r e d , and f o r w a t e r - c o o l e d r e a c t o r s the h u m i d i t y of t h e - a i r i s h igh . S ince iod ine i s t he m o s t h a z a r d o u s f i s s i o n p r o d u c t , the c l e a n i n g d e v i c e m a y c o n s i s t of a p a r t i c u l a t e f i l t e r and a n iod ine a d s o r b e r . T o p r e v e n t t he p a r t i c u l a t e f i l t e r f r o m b e c o m i n g w e t f r o m a i r m i x e d wi th w a t e r d r o p l e t s and to m a i n t a i n the e f f i c i e n c y of the c h a r c o a l bed a t h igh h u m i d i t y , a m o i s t u r e s e p a r a t o r a n d , if p o s s i b l e , h e a t i n g c o i l s should be p l a c e d u p s t r e a m f r o m the f i l t e r s . T h e nob le g a s e s a r e not r e d u c e d wi th s u c h c l e a n i n g a r r a n g e m e n t s , bu t , s i n c e t h e y a r e c o n t a i n e d , t he s m a l l a m o u n t t h a t l e a k s out c a n be r e l e a s e d t h r o u g h the s t a c k o r , when t h e r e q u i r e m e n t s a r e m o r e s t r i n g e n t , t he l e a k a g e can be p u m p e d b a c k i n to t h e c o n t a i n m e n t . F o r s m a l l r e s e a r c h r e a c t o r s the f i s s i o n - p r o d u c t i n v e n -t o r y i s m o d e r a t e and t h e r e a c t o r bu i ld ing c a n b e v e n t e d by an e m e r g e n c y v e n t i l a t i o n s y s t e m , a s d e s c r i b e d a b o v e , wh ich i s c o n n e c t e d t o a s t a c k . T y p i c a l c l e a n i n g s y s t e m a r r a n g e m e n t s a r e shown in F i g . 39 [49] .

Indian Point I

r r a f o

Oconee 1 and 2 • Q - M K r t

d af c a f

Millstone Point

d rr af c ' af

Browns Ferry 1 and 2

F I G . 3 9 . Typ ica l C o m p o n e n t Arrangements . (Courtesy ORNL, Oak Ridge, T e n n . )

7 . 6 . I n s t r u m e n t a t i o n

It i s v e r y i m p o r t a n t f o r t h e v e n t i l a t i o n p l a n t to be p r o p e r l y i n s p e c t e d d u r i n g s t a r t - u p . T h e a i r flow in e v e r y w o r k i n g a r e a shou ld be a d j u s t e d to the p r e d e t e r m i n e d v a l u e s and t h e p r e s s u r e d i f f e r e n c e s b e t w e e n d i f f e r e n t z o n e s and r o o m s c h e c k e d . T h e p r e s s u r e l e v e l s shou ld be m a i n t a i n e d t h r o u g h the p r e s s u r e d r o p of a i r s t r e a m s r a t h e r than by r e g u l a t i n g d e v i c e s .

&

63

Once the plant i s ba lanced nothing should be changed , except fo r the p r e s s u r e d r o p in the f i l t e r s when they b e c o m e loaded with dus t . M a n o m e t e r s should be f i t ted a c r o s s the f i l t e r s to ind ica te the change in p r e s s u r e d r o p and the need f o r f i l t e r r e n e w a l .

F o r v a r i o u s r e a s o n s it m a y be n e c e s s a r y to know the a i r t e m p e r a t u r e and humid i ty , fo r e x a m p l e to avoid p o s s i b l e d a m a g e to f i l t e r m a t e r i a l .

In IAEA Safe ty S e r i e s No. 17 [1] in c h a p t e r 6, " C o n t r o l and Moni tor ing of Vent i la t ion and A i r - C l e a n i n g S y s t e m s " , f u r t h e r i n f o r m a t i o n can be found on i n s t r u m e n t s fo r mon i to r ing and dus t s a m p l i n g and Appendix IV of th is publ ica t ion con ta ins e x a m p l e s of a i r - m o n i t o r i n g s e r v i c e s .

When a p a r t i c u l a r i n s t r u m e n t r o o m is used fo r a m o r e e l a b o r a t e m o n i -to r ing s y s t e m , including, p e r h a p s , iodine and noble gas mon i to r i ng , a t r a n s m i s s i o n l ine of c o n s i d e r a b l e length wi l l s o m e t i m e s be needed ( F i g . 4 0 ) .

F I G . 4 0 . Sampl ing sys tem.

In such c a s e s , t he l o s s of p a r t i c l e s in the t r a n s m i s s i o n l ine th rough depo-s i t ion m u s t be c o n s i d e r e d . The depos i t ion of p a r t i c l e s in n o z z l e s , mani fo ld and t r a n s m i s s i o n l ine tubes and bends depends on the v o l u m e t r i c flow and the Reyno lds n u m b e r (Re). Depos i t ion i n c r e a s e s s h a r p l y a s the Re i s i n c r e a s e d in the r e g i o n of t u rbu len t pipe flow at a cons t an t vo lume flow b e c a u s e of t u r b u l e n t i m p i n g e m e n t of p a r t i c l e s on the tube wa l l . At a su f f i c i en t ly low Re (< 2100), the gas s t r e a m is l a m i n a r , but s e d i m e n t a t i o n of l a r g e p a r t i c l e s in ho r i zon t a l p a r t s of the s a m p l i n g l ine b e c o m e s i m p o r t a n t b e c a u s e of the i n c r e a s e d r e s i d e n c e t i m e . Inves t iga t ions have shown that fo r l a r g e p a r t i c l e s the t r a n s m i s s i o n in a s a m p l i n g l ine cons i s t i ng of equal p a r t s of ho r i zon t a l and v e r t i c a l p i e c e s of tubing has a m a x i m u m with in the Re r a n g e be tween 3000 and 10 000 [41] .

If the v o l u m e t r i c flow is i n c r e a s e d a t cons t an t R e , l o s s e s wi l l b e c o m e s m a l l e r . Advantage can be t aken of th i s e f fec t by des ign ing the s a m p l i n g s y s t e m a s shown in F i g . 40. In a c i r c u i t wi th a low p r e s s u r e d r o p , a r e l a t i v e l y l a r g e s a m p l e flow is ma in ta ined with a s m a l l b lower ( s y s t e m I). F r o m that s y s t e m a s m a l l s a m p l e flow is led to the mon i to r ing d e v i c e s , such a s co l l ec t ing f i l t e r s or r a d i a t i o n d e t e c t o r s .

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8. F I L T E R SELECTION AND SPECIFICATION

When s e l e c t i n g f i l t e r s f o r a p lant , t he r e q u i r e d l e v e l of r e m o v a l e f f i c i ency should f i r s t of a l l be d e t e r m i n e d to p rov ide a b a s i s f o r choos ing a su i t ab le f i l t e r . The f low of a i r to be t r e a t e d should be ca l cu l a t ed , as an aid in dec id ing how l a r g e the c l ean ing dev ices need to be . An e s t i m a t i o n of the s i z e d i s t r i bu t i on of t he dus t and the dus t c o n c e n t r a t i o n wi l l i nd ica t e whe the r a H E P A f i l t e r i s su i t ab le and whe the r a p r e c l e a n i n g device i s r e q u i r e d . The p h y s i c a l condi t ions u n d e r which the f i l t e r wi l l o p e r a t e should be spec i f i ed , f o r example the work ing t e m p e r a t u r e and humid i ty o r any m e c h a n i c a l s t r e s s f r o m v i b r a t i o n o r a e r o d y n a m i c f o r c e s . Any pos s ib i l i t y of c h e m i c a l a t t ack m u s t be i nves t i ga t ed . The f i l t e r p e r f o r m a n c e r e q u i r e m e n t s u n d e r acc iden t condi t ions , such as f i r e , should a l s o be ana lysed .

The e f f i c i ency needed in a f i l t e r p lant i s ca l cu la t ed f r o m the an t ic ipa ted amount of r a d i o a c t i v i t y in the exhaus t a i r and the accep t ab l e c o n c e n t r a t i o n s i n the s u r r o u n d i n g s . It i s o f t en v e r y d i f f icu l t to e s t i m a t e the amount of a i r b o r n e r ad ioac t i v i t y , but an u p p e r l i m i t can be obta ined by a s s u m i n g tha t a f r a c t i o n of the m a t e r i a l dea l t with in the plant migh t be a i r b o r n e . Dur ing wet handl ing , t he l o s s of m a t e r i a l should be wel l be low 1%, and even dur ing the handl ing of d ry powder , i t would be v e r y unl ike ly f o r m o r e than 1% to b e c o m e a i r b o r n e . In t h i s way a rough e s t i m a t i o n of the p o s s i b l e quan t i t i e s of r a d i o a c t i v i t y in the exhaus t a i r can be m a d e .

The accep t ab l e r e l e a s e f r o m a s t a c k depends on the d i lu t ion r a t e , which i s a func t ion of the a t m o s p h e r i c t u r b u l e n c e , and on the m a x i m u m p e r m i s s i b l e c o n c e n t r a t i o n s (MPC) of t he r a d i o n u c l i d e s c o n c e r n e d . T h e r e a r e many m e t h o d s of ca lcu la t ing the di lut ion in the a t m o s p h e r e . R e f e r e n c e [ 1] d e s c r i b e s t h e s e m e t h o d s and p r o v i d e s many u s e f u l r e f e r e n c e s . The i n s t a n t a n e o u s d i f fus ion p a t t e r n in the wind di rect ion- f o r c e r t a i n s t ab i l i t y p a r a m e t e r s i s u s u a l l y given in the l i t e r a t u r e . Among o the r condi t ions , wind d i r e c t i o n changes with t i m e on both a r a p i d and a s low s c a l e . Annual a v e r a g e s of c o n c e n t r a t i o n wi l l t hus be f a r below the i n s t an t aneous c o n c e n t r a -t ions ; the m a x i m u m ground l e v e l c o n c e n t r a t i o n f o r d i f f e r e n t s t ab i l i t y condi t ions i s a f a c t o r of 20 to 50 h i g h e r t han the annual m e a n c o n c e n t r a t i o n in the m o s t exposed d i r e c t i o n f r o m a 50 -m s t a c k [1] .

Since a di lut ion r a t e of m a n y o r d e r s of magn i tude i s ob ta ined , an in i t i a l d i lu t ion in the s t a ck does not have much e f f e c t . The r e s u l t i n g c o n c e n t r a t i o n m a y be ca l cu la t ed with the f o r m u l a given be low:

q " q j + q 2

w h e r e Q = r e s u l t i n g di lut ion r a t e (m 3 / s ) , . Q 2 = a t m o s p h e r i c di lut ion r a t e ( m 3 / s ) , and Q 2 = i n i t i a l d i lu t ion r a t e ( m 3 / s ) .

An i n c r e a s e in the amount of a i r exhaus ted th rough the s t a c k wi l l i n d i r e c t l y c a u s e b e t t e r di lut ion by i n c r e a s i n g the e f f ec t i ve s t a c k height , s i n c e h ighe r ve loc i ty in the s t a ck gives a h ighe r p l u m e r i s e [ 1].

On the o t h e r hand, s u r r o u n d i n g s t r u c t u r e s and the t opography m a y in f luence the r e s u l t i n g c o n c e n t r a t i o n at ground l e v e l . As a r u l e of t humb, i t can be accep ted tha t , with a s t a c k two and a half t i m e s a s high a s any s u r r o u n d i n g bui ld ing, t he p lume r e m a i n s u n a f f e c t e d . In o the r c a s e s the in f luence of the bui ld ing height can be c o n s i d e r e d a s d e s c r i b e d in Ref . [42] .

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The height of the a i r i n t akes of s u r r o u n d i n g bui ld ings m u s t a l so be t a k e n into accoun t . As a m i n i m u m , the r e l e a s e tube should d i s c h a r g e 5 m above the gable of the roof of a bui ld ing; o r , if t h e r e a r e s e v e r a l ne ighbour ing bui ld ings within a r a d i u s of 50 m, the tube should be no l e s s than 5 rri h ighe r than the gable of the roof of the h ighes t bui ld ing .

Max imum p e r m i s s i b l e c o n c e n t r a t i o n s of r ad ionuc l i de s f o r the occupa t iona l e x p o s u r e of ind iv idua ls and f o r the e n v i r o n m e n t a l e x p o s u r e of m e m b e r s of the g e n e r a l publ ic can be obta ined f r o m pub l i ca t ions such a s the ICRP r e c o m m e n d a t i o n s [43] , t he A g e n c y ' s B a s i c Safety S t anda rds f o r Rad ia t ion P r o t e c t i o n [44] and na t iona l r e c o m m e n d a t i o n s .

S o m e t i m e s the cumula t i ve h a z a r d s f r o m depos i t ion m a y be m o r e s ign i f i can t than the inha la t ion h a z a r d s . In a l l c a s e s the ca lcu la ted e x p o s u r e should be wel l below the m a x i m u m p e r m i s s i b l e one.

While a c c u r a t e ca lcu la t ion of t he dose r e c e i v e d by m e m b e r s of t he g e n e r a l publ ic m a y r e q u i r e ex t ens ive knowledge of m i c r o m e t e o r o l o g y and biology, the des ign of s m a l l exhaus t s y s t e m s does not n e c e s s i t a t e e l a b o r a t e and soph i s t i c a t ed c a l c u l a t i o n s . E f f ec t i ve c l ean ing with a good s a f e t y m a r g i n i s j u s t i f i ed in such cases . '

Unce r t a in ty in p r e d i c t i n g the a e r o s o l p roduc t ion in a plant can be o v e r c o m e by p rov id ing s p a c e f o r mount ing f r a m e s f o r H E P A f i l t e r s of s t a n d a r d s i z e . T h e r e a r e f i l t e r s c o m m e r c i a l l y ava i lab le with p e n e t r a t i o n in the r a n g e f r o m 5% down to 0. 005%. By-a i r s a m p l i n g du r ing the in i t i a l o p e r a t i n g p e r i o d , the1 concen t r a t i on in the exhaus t a i r can be d e t e r m i n e d and a s a f e and e c o n o m i c a l f i l t e r qual i ty chosen .

The a e r o s o l s p r e s e n t u n d e r n o r m a l o p e r a t i n g condi t ions m a y not a lways d e t e r m i n e the n e c e s s a r y d e g r e e of r e m o v a l . The r e q u i r e m e n t s m a y p r o v e m o r e r e s t r i c t i v e f o r acc iden t condi t ions , when the c l ean ing s y s t e m m a y have to cope with g r e a t l y i n c r e a s e d a e r o s o l concen t r a t i on . With acc iden t condi t ions , a m o m e n t a r y dose equivalent to q u a r t e r l y o r annual cont inuous dose m a y be accep ted , but a h ighe r concen t r a t i on u p s t r e a m f r o m the f i l t e r and a l o w e r d i lu t ion r a t e at i n s t an t aneous r e l e a s e may ca l l f o r h ighe r r e m o v a l e f f i c i ency .

The a i r f low to be c leaned should be kept a s low as p o s s i b l e by a p p r o p r i a t e work ing m e t h o d s , con ta inment of t he dust s o u r c e and • s e g r e g a t i o n of the exhaus t s t r e a m s . In a c e n t r a l i z e d exhaus t s y s t e m , the capac i ty of the f ina l f i l t e r can be l e s s than the sum of a l l the i n s t a l l e d connec t ions . F o r e x a m p l e , in a l a b o r a t o r y a d i v e r s i t y f a c t o r m a y be a s s u m e d . All the i n s t a l l e d equ ipment m a y not be in u s e at the s a m e t i m e .

When the d e s i g n e r h a s an idea of the e f f i c i ency and capac i ty r e q u i r e d , a su i t ab le c lean ing device has to be chosen . If t he p a r t i c l e s i z e s a r e in the s u b - m i c r o n r a n g e and the e f f i c i ency r e q u i r e d i s m o r e than 99%, a H E P A f i l t e r i s l ike ly to be u s e d . F o r p a r t i c l e s l a r g e r than 5 f tm, d i f f e r e n t s e p a r a t o r s a r e ava i l ab le a s d e s c r i b e d in Ref . [ l ] . T h e s e s e p a r a t o r s m a y a l so be needed f o r p r e c l e a n i n g if the dus t concen t r a t i on i s too high f o r HEPA f i l t e r s . H igh -e f f i c i ency f i l t e r s a r e only su i t ab le f o r dus t c o n c e n t r a t i o n s l o w e r than about 1 m g / m 3 . Th i s i s b e c a u s e the dus t capac i ty of ava i l ab le f i l t e r s i s l i m i t e d at present ," and if ca l l ed upon to t r e a t h ighe r concen t r a t i ons they would need changing too f r e q u e n t l y f o r e c o n o m i c a l o p e r a t i o n (about once e v e r y month at 1 m g / m 3 ) .

When a H E P A type of f i l t e r h a s b e e n s e l e c t e d , the m a t e r i a l s of c o n s t r u c t i o n should be -chosen a c c o r d i n g to the p h y s i c a l cond i t ions ' t o b e

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withstood by the f i l t e r both du r ing n o r m a l ope ra t i on and with acc iden t s i t u a t i o n s . F i l t e r s in exhaus t s y s t e m s f r o m l a b o r a t o r i e s and o t h e r work ing f a c i l i t i e s wi l l o p e r a t e at r o o m t e m p e r a t u r e . If a h i g h e r t e m p e r a t u r e i s expec ted , the o p e r a t i n g t e m p e r a t u r e can usua l ly be ca l cu l a t ed and the f i l t e r m a t e r i a l chosen on the b a s i s of the da ta g iven in c h a p t e r 5. Most f i l t e r s wi l l wi ths tand a r e l a t i v e humid i ty of 80%. If h ighe r humid i ty i s expec ted , m a t e r i a l s with p o o r w a t e r r e s i s t a n c e should be avoided and the f i l t e r should n e v e r be al lowed to get we t . A d rop l e t s e p a r a t o r o r d e m i s t e r ' should be p l aced u p s t r e a m f r o m the f i l t e r and the a i r should be hea ted o r di luted b e f o r e i t i s f i l t e r e d . M a t e r i a l s with f a i r w a t e r r e s i s t a n c e wi l l t hus be s a t i s f a c t o r y in m o s t c a s e s i

Ant ic ipa t ing the d e g r e e of c h e m i c a l a t t ack tha t a f i l t e r wi l l be exposed to i s o f ten d i f f i cu l t . If a c e n t r a l i z e d exhaus t s y s t e m i s u s e d in a l a b o r a t o r y , t he f i n a l H E P A f i l t e r i s not exposed to high vapou r c o n c e n t r a t i o n s , s i n c e the a i r i s d i lu ted b e f o r e r e a c h i n g the f i l t e r . F o r s p e c i a l p r o c e s s ven t i l a t ion , t he f i l t e r c o n s t r u c t i o n m a t e r i a l should be chosen in a c c o r d a n c e with the p r o c e s s plant i t se l f and o the r componen t s of the ven t i l a t ion p lan t . It m a y s o m e t i m e s be wor th whi le to t r y an inexpens ive s t a n d a r d qual i ty f i l t e r , and if t h i s p r o v e s to be u n s a t i s f a c t o r y a m o r e expens ive f i l t e r can be chosen .

Mechan ica l s t r e s s e s f r o m v i b r a t i o n and a e r o d y n a m i c f o r c e s should be avoided. If t h i s i s not p o s s i b l e , the s p e c i a l r e q u i r e m e n t s f o r the f i l t e r should be d i s c u s s e d with the f i l t e r m a n u f a c t u r e r .

As d e s c r i b e d in c h a p t e r 5, a f l a m e - r e s i s t a n t f i l t e r can s o m e t i m e s be used. , When a high f i r e r i s k i s an t i c ipa ted o r t he c o n s e q u e n c e s of a f i r e would be s e r i o u s , a f i r e p r o o f o r n o n - c o m b u s t i b l e f i l t e r i s n e c e s s a r y . The h i g h - t e m p e r a t u r e f i l t e r d e s c r i b e d in Tab le V i s e f f ec t i ve up to 400° C and al l m a t e r i a l s a r e n o n - c o m b u s t i b l e .

8 . 1 . F i l t e r s p e c i f i c a t i o n s

When o r d e r i n g a f i l t e r t he b u y e r m u s t s p e c i f y exac t ly what he wants , to be s u r e tha t the d e l i v e r e d p roduc t s a t i s f i e s h i s r e q u i r e m e n t s . Gene ra l l y , the only r e q u i r e m e n t s tha t should be s p e c i f i e d a r e t h o s e tha t can be v e r i f i e d and t e s t e d by e i t h e r the m a n u f a c t u r e r o r t he b u y e r , p r e f e r a b l y by the l a t t e r . Consequent ly , the c o n s t r u c t i o n m a t e r i a l should be. spec i f i ed r a t h e r than the r e s i s t a n c e to t e m p e r a t u r e , f i r e , humid i ty , and c h e m i c a l a t t a c k . F o r the p e r f o r m a n c e da ta s p e c i f i e d , t he t e s t me thod should a l s o be given.

When c o r r e s p o n d i n g with f i l t e r m a n u f a c t u r e r s , t he da ta l i s t e d below should be inc luded f o r each type of f i l t e r . The s p e c i f i c a t i o n s f o r a s t a n d a r d s i z e H E P A f i l t e r a r e given f o r t he sake of e x a m p l e .

N o m e n c l a t u r e High e f f i c i ency p a r t i c u l a t e a i r f i l t e r , f l a m e - r e s i s t a n t .

N u m b e r of f i l t e r s r e q u i r e d -

E f f i c i e n c y 99. 97% f o r a m a s s m e d i u m p a r t i c l e s i z e of 0. 3 fim at an a i r f low down to 20% of r a t e d capac i t y .

Ra ted a i r f low capac i ty 1700 m^/h.

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R e s i s t a n c e to a i r f low

F i l t e r d i m e n s i o n s and t o l e r a n c e

Heat r e s i s t a n c e

F l a m e r e s i s t a n c e

In i t i a l r e s i s t a n c e sha l l not exceed 25 m m w a t e r gauge s t a t i c p r e s s u r e d r o p at r a t e d a i r f low at 20° C and 760 m m of m e r c u r y .

Height 610 ± 2 m m (24 in . ± 1 / 1 6 i n . ) Width 610 ± 2 m m (24 in. ± 1 / 1 6 i n . ) Depth 292 ± 1 m m (11 1 /2 in . ± 1 / 3 2 i n . ) The depth exc ludes the gaske t . The longes t d iagonal should not exceed 867 m m (34 1 /8 i n . ) .

T h e f i l t e r should be able to o p e r a t e cont inuous ly at 70° C.

The e n t i r e f i l t e r o r any component t he reo f sha l l not suppor t a f l a m e of i t s own a c c o r d .

Humidi ty r e s i s t a n c e

M e c h a n i c a l s t r e n g t h

M a t e r i a l

F r a m e

Gaske t

T e s t i n g

The f i l t e r sha l l wi ths tand 100% r e l a t i v e humid i ty .

When the f i l t e r i s c logged with dus t , no f r a c t u r e o r i n c r e a s e d p e n e t r a t i o n s h a l l o c c u r b e f o r e a p r e s s u r e d r o p of 300 m m (12 i n . ) w. g.

F i l t e r m e d i u m : g l a s s f i b r e s . F r a m e : S e p a r a t o r s : A d h e s i v e s :

Gaske t :

f i r e r e t a r d a n t wood, a lumin ium, r u b b e r - b a s e d , s e l f -ex t inguish ing , r u b b e r o r n e o p r e n e .

The t h i c k n e s s of the wood sha l l be 19 m m (3 /4 i n . ) .

6 m m - ( l / 4 in . ) - th ick , 19 m m - ( 3 / 4 in . ) -wide g a s k e t s sha l l be u s e d on both f a c e s of t h e f i l t e r (or on one f a c e of t h e f i l t e r , depending on the mount ing a r r a n g e m e n t ) .

E a c h f i l t e r sha l l be t e s t e d at the f a c t o r y with a s t a n d a r d i z e d me thod and s h a l l be s t enc i l l ed with the fo l lowing da ta : F i l t e r type Ra ted a i r vo lume (m 3 / h ) R e s i s t a n c e (mm w. g . ) P e n e t r a t i o n (%) S e r i a l n u m b e r .

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E a c h f i l t e r sha l l be packed in a sturdy-c a r t o n . The c a r t o n sha l l be c l e a r l y s t a m p e d to ind ica te the type of f i l t e r . A r r o w s and the w o r d s 1 T h i s s ide up ' s h a l l be m a r k e d on the c o n t a i n e r t o e n s u r e tha t the f a c e s , p l e a t s and s e p a r a t o r s a r e s t o r e d in a v e r t i c a l pos i t i on .

A f t e r t he f i l t e r s a r e d e l i v e r e d they wi l l be sub j ec t ed to an a c c e p t a n c e t e s t by the b u y e r , to v e r i f y tha t no f i l t e r h a s been d a m a g e d du r ing t r a n s p o r t . Any f i l t e r not accep ted i s r e t u r n e d to the s u p p l i e r .

9. F I L T E R HANDLING AND INSPECTION

The h a z a r d s involved in work ing with r a d i o a c t i v e m a t e r i a l s gave r i s e t o the deve lopment of h i g h - e f f i c i e n c y f i l t e r s . The in t roduc t ion of t h e s e h igh -qua l i t y f i l t e r s ca l l ed f o r a new techn ique in handl ing and ma in t a in ing the ven t i l l a t ion p l a n t s . Many f i l t e r s have func t ioned badly a s a r e s u l t of c a r e l e s s handl ing and i m p r o p e r i n s t a l l a t i on . To m a k e peop le r e s p o n s i b l e f o r f i l t e r i n s t a l l a t i on a w a r e of the i m p o r t a n c e of c a r e f u l handl ing, r u l e s have been w r i t t e n and in spec t ion and t e s t i n g m e t h o d s developed [ 3 0 , 4 5 ] , The i n s t r u c t i o n s given be low wil l h e l p to e n s u r e the e f f i c i en t func t ion ing of f i l t r a t i o n p l a n t s . A c l e a r , c o m p r e h e n s i v e s t a t e m e n t of pol icy by the r e s p o n s i b l e m a n a g e m e n t i s a l so an inva luab le and e s s e n t i a l p a r t of t he f i l t e r - c o n t r o l p r o g r a m .

9 . 1 . Handl ing

(1) Do not u s e cha ins , s l i ngs o r hooks .

(2) Ti l t onto one c o r n e r and l i f t by oppos i te c o r n e r s .

(3) Do not d rop o r j a r . If d ropped o r j a r r e d the f i l t e r m u s t be r e - e x a m i n e d and t e s t e d .

(4) Keep f i l t e r uni t in t he sh ipp ing c a r t o n with the a r r o w on the c a r t o n point ing upward un t i l r e a d y f o r i n s t a l l a t i on .

(5) If an unpackaged f i l t e r i s p l aced on the f l o o r o r o t h e r s u r f a c e , t he s u r f a c e m u s t be c l e a r e d of a l l o b j e c t s which migh t d a m a g e the f i l t e r pack . Neve r l ay f i l t e r on e i t h e r i t s f r o n t o r b a c k s u r f a c e .

(6) The v e r t i c a l pos i t ion ing of t he p l ea t ed fo lds should be m a i n t a i n e d in handl ing and s t o r a g e .

9 . 2 . Inspec t ion

(1) C a r e f u l l y r e m o v e the f i l t e r uni t f r o m the shipping c a r t o n by opening t h e top f l a p s of the c a r t o n , fold the f l a p s back , i n v e r t t he c a r t o n gently t o p l a c e the exposed end of t he f i l t e r uni t on a smoo th c l ean s u r f a c e , and t h e n

P a c k a g i n g

Accep tance t e s t i n g

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withdraw the c a r t o n . Do not a t t empt to r e m o v e the f i l t e r unit f r o m the c a r t o n by g r a s p i n g the f i l t e r f r a m e . Th i s can c a u s e d a m a g e to the so f t f i l t e r m e d i u m .

(2) C a r e f u l l y i n spec t both f a c e s of the f i l t e r uni t f o r c r a c k s in the f i l t e r m e d i u m , damage to s e p a r a t o r s , and s e p a r a t i o n of the f i l t e r pack f r o m the f r a m e .

(3) C a r e f u l l y in spec t the f i l t e r edge gaske t . It m u s t be f i r m l y c e m e n t e d to the f i l t e r f r a m e and the gaske t p i e c e s cemen ted at the m a t i n g jo in t s .

A p a r t f r o m t h e s e g e n e r a l r u l e s , s o m e s p e c i a l f i l t e r handl ing r o u t i n e s a r e dea l t wi th .

9. 3. Inspec t ion on d e l i v e r y

Dur ing t r a n s p o r t a t i o n , t he v i b r a t i o n and jol t ing m a y cause the f i l t e r m e d i u m to b r e a k at the adhes ive l ine . A f a c t o r con t r ibu t ing to th i s d a m a g e m a y be that the p l e a t s a r e in a h o r i z o n t a l pos i t ion dur ing s h i p m e n t . Occas iona l ly a f i l t e r unit i s i m p r o p e r l y p l aced in the con ta ine r by the m a n u f a c t u r e r and s o m e t i m e s the c o n t a i n e r s a r e not p laced up r igh t in r a i l r o a d c a r s o r t r u c k s a c c o r d i n g to the d i r e c t i o n s ' T h i s s ide up ' on the c a r t o n . Consequent ly , i n spec t ion on d e l i v e r y at des t ina t ion i s n e c e s s a r y .

• Inspec t ion should s t a r t when a d e l i v e r y of f i l t e r un i t s r e a c h e s the p u r c h a s e r . As the sh ipmen t i s be ing unloaded, e v e r y c a r t o n should be i n s p e c t e d f o r e x t e r n a l d a m a g e . Damaged c a r t o n s should be se t a s i d e f o r . p a r t i c u l a r l y c a r e f u l i n spec t ion .

M a t e r i a l s u s e d in c o n s t r u c t i o n of t he f i l t e r uni t m u s t comply with the p u r c h a s e o r d e r spec i f i ca t i on , if any.

The u s e r of the f i l t e r s i s , of c o u r s e , i n t e r e s t e d in knowing whe the r the e f f i c i ency of t he f i l t e r ha s been a f f ec t ed by t r a n s p o r t a t i o n . P r o v i s i o n of a t e s t i n g r i g f o r e f f i c i e n c y t e s t s i s u sua l l y not j u s t i f i ed f o r a s ing le u s e r with m o d e r a t e f i l t e r consumpt ion . S o m e t i m e s a c e n t r a l t e s t i n g s t a t ion can s e r v e s e v e r a l f i l t e r c o n s u m e r s .

A s ingle s m o k e l e a k t e s t as d e s c r i b e d in c h a p t e r 6. 3, however , m a y be u s e f u l to de tec t t r a n s p o r t a t i o n d a m a g e in a f i l t e r un i t . . Even if the t e s t i s only qua l i ta t ive , d e t e r i o r a t i o n of the f i l t e r dur ing t r a n s p o r t a t i o n tha t could i m p a i r the e f f i c i ency of the f i l t e r wi l l p robab ly be de t ec t ed .

9 . 4 . S torage

A f t e r inspec t ion , the f i l t e r uni t should be r e p a c k e d c a r e f u l l y in the c a r t o n in which it was sh ipped and r e c e i v e d . All pack ing m a t e r i a l f o r i n t e r n a l s t r eng then ing of t he c a r t o n and f o r p r o t e c t i o n of the f i l t e r uni t should be r e p l a c e d p r o p e r l y . The f i l t e r uni t should be so p l aced tha t the p l e a t s a r e in a v e r t i c a l pos i t ion , c o n f o r m i n g with the d i r e c t i o n s on the c a r t o n . The height of a s t a ck of f i l t e r un i t s in s t o r a g e should be l i m i t e d , the l a r g e f i l t e r un i t s (610 X 610 m m f a c e a r e a ) not be ing s t acked m o r e than t h r e e f i l t e r un i t s h igh. F i l t e r un i t s should not be s t o r e d w h e r e they would b e exposed to d a m p n e s s , e x c e s s i v e hea t o r cold, o r r ap id ly changing t e m p e r a t u r e s .

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9 . 5 . Changing

The r e s i s t a n c e , o r p r e s s u r e d rop , a c r o s s the f i l t e r i s u s u a l l y the d e t e r m i n i n g f a c t o r f o r r e p l a c e m e n t of t he uni t . E v e r y f i l t e r bank should be p rov ided with a p r e s s u r e d i f f e r e n c e gauge and the ind ica ted va lue should be r e c o r d e d r e g u l a r l y , f o r example e v e r y week . The a c c e p t a b l e p r e s s u r e va lue depends on the fan capac i ty ava i l ab le . N o r m a l l y the ven t i l a t ion plant i s c o n s t r u c t e d f o r a f i l t e r p r e s s u r e d rop f r o m 50 m m w. g. to 100 m m w. g. (2 in . to 4 in . w. g . ) . R e p l a c e m e n t of t he f i l t e r uni t m a y be n e c e s s a r y f o r r e a s o n s , o the r t han p r e s s u r e d r o p . A n u m b e r of d e t e r m i n i n g f a c t o r s a r e men t ioned be low.

(1) R e s i s t a n c e , o r p r e s s u r e d rop , a c r o s s the f i l t e r un i t .

(2) High l eve l of r a d i a t i o n f r o m the f i l t e r un i t . The f i l t e r m u s t be changed b e f o r e the r a d i a t i o n l eve l b e c o m e s high enough to c a u s e d i f f i cu l ty in handl ing the f i l t e r o r r e s t r i c t i o n s in a c c e s s to the v ic in i ty of the f i l t e r .

(3) E x c e s s i v e bu i ld -up of l int o r combus t ib l e p a r t i c u l a t e m a t t e r on the f i l t e r un i t .

(4) L o s s of e f f i c i ency , d e t e r m i n e d f r o m a i r - s a m p l i n g m e a s u r e m e n t s m a d e d o w n s t r e a m f r o m the f i l t e r unit. .

(5) Vis ib le d a m a g e o r r u p t u r e of the f i l t e r med ium in a un i t .

(6) Change. in p roduc t ion me thod , l a b o r a t o r y ope ra t ion , hood applicat ion^ o r f r o m d ry a i r to m o i s t a i r e x p o s u r e .

B e f o r e ins t a l l a t ion , the f i l t e r should be i n s p e c t e d and the s u r f a c e s aga ins t which the f i l t e r uni t wi l l be s e a l e d should be examined to e n s u r e tha t they a r e t r u e , c l ean , smoo th , f l a t , r i g i d and f r e e of weld s p a t t e r . Thei f i l t e r should a lways be i n s t a l l ed with the p l e a t s and s e p a r a t o r s in the v e r t i c a l pos i t ion . T h i s wi l l e l i m i n a t e sagg ing of the p l e a t s f r o m the a c c u m u l a t e d weight of the m a t e r i a l s s topped by the f i l t e r un i t . .

Many t y p e s of f i l t e r hous ing a r e des igned f o r f i l t e r changing with the p l a s t i c - b a g t echn ique . S o m e t i m e s the f i l t e r hous ing i s des igned f o r a c c e s s f r o m e i t h e r s ide of the f i l t e r , which m a k e s changing e a s y . To save , space , the hous ing i s o f ten a c c e s s i b l e only f r o m one s i d e and the changing me thod m u s t be mod i f i ed to c o n f o r m with t h i s l i m i t a t i o n .

The changing s t e p s should be in the fo l lowing s e q u e n c e :

(1) The d a m p e r s on both s i d e s of t he f i l t e r a r e c l o s e d . If the d a m p e r s a r e t ight , tha t on the u p s t r e a m s ide of the f i l t e r i s not comple t e ly c losed to al low f o r ven t i l a t ion of the hous ing t h r o u g h o t h e r f i l t e r s p a r a l l e l with the f i l t e r be ing changed . A con t ro l l ed f i l t e r e d l e akage th rough a p r e s s u r e equa l iz ing va lve i s m a i n t a i n e d to r e g u l a t e the u n d e r p r e s s u r e in the p l a s t i c bag .

(2) The c l amp ing d e v i c e s a r e loosened and the d i r ty f i l t e r i s w i thdrawn in to a p l a s t i c bag e i t h e r r e m a i n i n g f r o m the p r e v i o u s change o r appl ied to the g roove f l ange b e f o r e changing. The p l a s t i c bag i s s e a l e d by welding, cut and r e m o v e d .

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(3) The new f i l t e r i s put into a c l ean p l a s t i c bag and the s e v e r e d p a r t of t he d i r t y bag i s moved to the ou t e r g roove to allow the new bag to be f i xed onto the i n n e r g roove .

(4) The s e v e r e d p a r t of the d i r t y b a g i s r e m o v e d by man ipu la t i ng it t h rough the c l ean bag . The new f i l t e r i s now i n s e r t e d . The f i l t e r i s f ixed and the b a g i s folded and l e f t i n s ide a l id , c o v e r i n g the changing opening.

(5) The d a m p e r s a r e opened .

When an open i n s t a l l a t i on of f i l t e r s on a mount ing f r a m e i s u s e d , t he changing has to be done by p e r s o n s w e a r i n g p r o t e c t i v e s u i t s and b r e a t h i n g m a s k s with a f r e s h - a i r supply f r o m tubes o r h o s e s . The f i l t e r s have to b e enc lo sed to p r e v e n t dus t f r o m be ing s p r e a d .

9. 6. T e s t i n g and i n s p e c t i o n '

E x p e r i e n c e in l a r g e l a b o r a t o r i e s [32] shows tha t when the f i r s t i n - p l a c e t e s t of f i l t e r s i s p e r f o r m e d , m a n y s y s t e m s a r e not capable of m e e t i n g the spec i f i ed r e q u i r e m e n t s . Only 78% of the f i l t e r banks conta in ing one o r two f i l t e r s had an e f f i c i ency h ighe r than 99. 95% and only 38% of the banks conta in ing t h r e e o r m o r e f i l t e r s w e r e accep ted . The s ign i f i can t c a u s e s of l eakage w e r e :

(1) F a u l t y s ea l i ng be tween f i l t e r and f r a m e caused by (a) l o o s e ho ld-down c l a m p s , (b) damaged g a s k e t s , (c) f o r e i g n m a t e r i a l be tween gaske t and f r a m e , (d) rough o r w a r p e d s u r f a c e s on the f i l t e r mount ing f r a m e , o r (e) too few hold-down d e v i c e s o r d e v i c e s i n c o r r e c t l y pos i t ioned .

(2) Defec t ive f i l t e r m e d i u m (a) punc tu red due to c a r e l e s s handl ing at t he t i m e of i n s t a l l a t i on , o r (b) sagg ing a s a r e s u l t of i n c o r r e c t i n s t a l l a t i on , t ha t i s , with p l e a t s h o r i z o n t a l i n s t e a d of v e r t i c a l .

(3) D e f e c t s in f i l t e r f r a m e and hous ing , such a s (a) unwelded jo in ts at c o r n e r s and s i d e s of f r a m e s , (b) a b s e n c e of a f r a m e m e m b e r be tween two ad j acen t f i l t e r s , o r (c) no p r o v i s i o n s f o r c l amping f i l t e r to f r a m e .

As a r e s u l t of r e p a i r and mod i f i ca t ion of the s y s t e m , p lus i m p r o v e m e n t i n f i l t e r handl ing and i n s t a l l a t i on m e t h o d s , the f a i l u r e r a t e was r e d u c e d in s u c c e e d i n g t e s t s .

In addi t ion to the i n i t i a l i n - p l a c e t e s t , i t i s v e r y i m p o r t a n t f o r each i n s t a l l a t i on to be t e s t e d p e r i o d i c a l l y , t he f r e q u e n c y depending on the app l ica t ion of t he s y s t e m and the condi t ions u n d e r which it o p e r a t e s . T e s t s should a l so be c a r r i e d out a f t e r e v e r y f i l t e r change and wheneve r t h e r e i s d e c r e a s e d e f f i c i ency .

F i l t e r s of the H E P A type usua l ly have an e f f i c i ency of 99. 97%, but i t can be accep ted tha t s o m e addi t ional p e n e t r a t i o n wi l l occu r due to f au l ty i n s t a l l a t i o n . An e f f i c i ency of 99. 95% m a y thus be accep ted f o r i n - p l a c e t e s t i n g .

I n -p l ace t e s t i n g m a y be p e r f o r m e d with one of t he m e t h o d s d e s c r i b e d in s ec t i on 6. 5.

A c o n t r o l p r o g r a m f o r a f i l t e r p lant m a y t ake one of the f o r m s l i s t e d be low.

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(1) A few H E P A f i l t e r s (< 10)

(a) F i l t e r hous ing , s e l l - c o n t a i n e d f i l t e r s o r double f l ange connec t ion a r e u s e d . (b) Ai r s a m p l i n g d o w n s t r e a m f r o m the f i l t e r i s p e r f o r m e d r e g u l a r l y .

(2) A s m a l l p lant with s e v e r a l i n s t a l l a t i o n s

(a) F i l t e r hous ings with double f l a n g e s a r e u s e d . (b) F i l t e r s a r e checked with a s m o k e l e a k t e s t ( sec t ion 6. 3) at d e l i v e r y and b e f o r e i n s t a l l a t i o n . (c) Ai r s a m p l i n g d o w n s t r e a m f r o m the f i l t e r s i s p e r f o r m e d r e g u l a r l y . (d) Safety f i l t e r s d imens ioned f o r a c c i d e n t s a r e t e s t e d with n a t u r a l a t m o s p h e r i c r a d i o a c t i v i t y . (e) A l t e rna t ive ly , t he i n - p l a c e t e s t i n g s e r v i c e i s h i r e d f r o m an a p p r o p r i a t e i n s t i t u t ion .

(3) A l a r g e plant with s e v e r a l l a r g e i n s t a l l a t i o n s

(a) F i l t e r s a r e i n s t a l l e d in the m o s t su i t ab le way within the l i m i t a t i o n s i m p o s e d by i n s t a l l a t i on and m a i n t e n a n c e c o s t s . Bank i n s t a l l a t i on with mount ing f r a m e s m a y be u s e d f o r l a r g e a i r f l ows . (b) E f f i c i e n c y t e s t s of f i l t e r s a r e p e r f o r m e d at d e l i v e r y and b e f o r e i n s t a l l a t i on . (c) I n -p l ace t e s t i n g of e v e r y f i l t e r i n s t a l l a t i on i s r e g u l a r l y p e r f o r m e d .

The s a m e t e s t i n g equ ipment i s u s e d f o r e f f i c i e n c y t e s t s on s ing le f i l t e r s and f o r i n - p l a c e t e s t s of f i l t e r b a n k s .

The c o n t r o l p r o g r a m s s u g g e s t e d above a r e e x a m p l e s showing how the c o n t r o l r o u t i n e can be adapted to the n u m b e r of f i l t e r s i n s t a l l e d and to o the r r e l e v a n t cond i t ions .

9 . 7 . D i s p o s a l

Used r a d i o a c t i v e f i l t e r s can be c o n s i d e r e d and t r e a t e d a s so l id r a d i o a c t i v e w a s t e s . F o r vo lume r educ t i on , f i l t e r s with wooden f r a m e s could b e b u r n t . S o m e t i m e s a g love-box fac i l i ty i s ava i l ab le w h e r e the f i l t e r s can be m e c h a n i c a l l y d i s a s s e m b l e d , t he f i l t e r pad c o m p r e s s e d and the wooden f r a m e d iagonal ly d i s t o r t e d . At Brookhaven , h i g h - e f f i c i e n c y f i l t e r s a r e be ing r e p l a c e d with un i t s in which the f i l t e r m e d i a a r e i n s t a l l e d i n demoun tab le s t a i n l e s s - s t e e l f r a m e s . The f r a m e s can be s t r i p p e d off the f i l t e r m e d i a , decon t amina t ed and s e n t b a c k to the f i l t e r m a n u f a c t u r e r f o r r e u s e . The f i l t e r m e d i a a r e c r u s h e d in a power b a l e r , t h e r e b y r e d u c i n g the w a s t e vo lume by a f a c t o r of 5 o v e r t he s p a c e r e q u i r e d to pack the s t a n d a r d f i l t e r s .

10. ECONOMIC A S P E C T S

All m e a n s of r e d u c i n g the a i r - c l e a n i n g c o s t s m u s t be c o n s i d e r e d without d i s r e g a r d i n g the need f o r r e l i a b l e and s a f e func t ion ing of t he p lan t . The r i s k of a i r con tamina t ion by h a z a r d o u s m a t e r i a l s can be r e d u c e d by

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p e r f o r m i n g r e s e a r c h work on a s m a l l s c a l e and by wet handl ing . F a c i l i t i e s t o enc lo se d u s t - f o r m i n g p r o c e s s e s should be p rov ided , such as f u m e -c u p b o a r d s , hoods and g love-boxes . Mach ines can be enc losed and have only the n e c e s s a r y a c c e s s p o r t s . The a i r quant i ty needed f o r con t ro l l ing the dus t s p r e a d can t h e r e b y be r e d u c e d to a m i n i m u m . ' By s e g r e g a t i n g the d i f f e r e n t exhaus t a i r s t r e a m s , the a p p r o p r i a t e c lean ing s y s t e m can be appl ied to each c a t e g o r y of a i r .

Depending on the r e q u i r e m e n t s of the f ac i l i ty s e r v e d , the a i r - c l e a n i n g s y s t e m m a y be o p e r a t e d f u l l - t i m e o r p a r t - t i m e o r held on s tandby f o r e m e r g e n c y s e r v i c e .

If the plant i s o p e r a t i n g with only one o r two s h i f t s , i t i s p o s s i b l e t o ' shu t off t he p lan t at n igh t . Cont inuous o p e r a t i o n of t he plant i s , h o w e v e r , s o m e t i m e s r e c o m m e n d e d [ 6], Dur ing a comple t e plant shu t -down, l e akage of r a d i o a c t i v e dus t f r o m duc t s m a y o c c u r . If the c l i m a t e c a u s e s high h e a t i n g o r cool ing c o s t s , i t i s b e t t e r f b r the plant to opera te ' at r e d u c e d capac i ty du r ing off h o u r s . Reduced a i r f low can e a s i l y be obta ined with s e v e r a l f ans in p a r a l l e l o r t w o - s p e e d m o t o r s .

Ve ry few cos t da ta a r e ava i l ab le in the l i t e r a t u r e and f i g u r e s f r o m d i f f e r e n t c o u n t r i e s may not be c o m p a r a b l e . Re la t ions be tween cap i t a l c o s t s and l abou r c o s t s m a y v a r y f r o m one coun t ry to ano the r , and th i s should be kept in mind when s tudying r e p o r t s on economic a s p e c t s .

When ana lys ing the economy of an a i r - c l e a n i n g plant , both the i n s t a l l a t i o n and ope ra t i on c o s t s should be c o n s i d e r e d . An inves t iga t ion on annual c o s t s f o r f i l t e r p l an t s [ 4 6 , 4 7 ] shows tha t cap i t a l i za t ion c o s t s a r e l e s s than 20% f o r m o s t i n s t a l l a t i o n s , but f i l t e r r e p l a c e m e n t c o s t s ( m a t e r i a l and l abour ) r e p r e s e n t 65% o r m o r e of the to t a l . As the m a i n c o s t s a r e those f o r r e p l a c e m e n t , i t could be e c o n o m i c a l to o v e r s i z e a p lant and to r u n it below nomina l capac i ty . The inves t iga t ion men t ioned above r e v e a l s tha t if a p lant i s o p e r a t e d at a capac i ty l e s s than 80% of the nomina l capac i ty , the t o t a l annual cos t p e r uni t a i r f low i n c r e a s e s v e r y r a p i d l y . If t he plant i s runn ing at ove r 80% and up to 100% of n o m i n a l capac i ty , t h e r e i s v e r y l i t t l e change in to ta l annual cos t p e r uni t a i r f low. It i s obviously not e c o n o m i c a l to o v e r s i z e the f i l t e r p lant too m u c h . '

As the power c o s t s a r e only a s m a l l p e r c e n t a g e (15%) of the annual to t a l , a r educ t i on in c o s t s would p robab ly be ach ieved if t he f i l t e r s w e r e loaded to a h ighe r p r e s s u r e d rop b e f o r e changing. The fan in many i n s t a l l a t i o n s can only o v e r c o m e a p r e s s u r e d rop of 50 m m w. g. o v e r the f i l t e r s . F o r new i n s t a l l a t i o n s , i t would be adv i sab le to des ign f o r a f an capac i ty tha t a l lows a p r e s s u r e d rop of 100 m m w. g. In addi t ion, with a s l igh t r e d u c t i o n of a i r f low, m o r e p r e s s u r e b e c o m e s ava i lab le f o r t he -f i l t e r s f r o m the r e d u c t i o n of o the r r e s i s t a n c e s in the p lan t .

To i n c r e a s e the l i f e of the f i l t e r , the dus t concen t r a t i on should be r e d u c e d as much as p o s s i b l e . T h i s can be ach ieved 'by e f fec t ive ly c l ean ing the supply a i r to t he bui lding, which would a l so he lp to keep the work ing a r e a c l ean . If the p r o c e s s e s involved p r o d u c e dus t s , p r e -c l ean ing i s n e c e s s a r y if the dus t concen t r a t i on i s h i g h e r than 1 m g / m 3

and would be e c o n o m i c a l at l o w e r c o n c e n t r a t i o n s , depending upon the c logging c h a r a c t e r i s t i c s of t he dus t ( see sec t ion 5 . 4 ) . The cos t of a H E P A f i l t e r i s s even o r eight t i m e s a s high as tha t of a p r e f i l t e r [46 ] and p r e f i l t e r s m u s t be chosen so tha t the total ' cos t , inc luding "labour f o r changing, i s as low a s p o s s i b l e .

74

Since the cos t f o r the f i l t e r i s 60% of the t o t a l annual c o s t s , i t i s i m p o r t a n t t o buy a f i l t e r which, f o r i t s p r i c e , h a s a good d u s t - l o a d i n g capac i t y . In Ref . [ 10], the o p e r a t i o n c o s t s f o r d i f f e r e n t f i l t e r s a r e c o m p a r e d , showing tha t t he ca l cu la t ion cannot be b a s e d only on the a r e a of f i l t e r p a p e r enc losed in the uni t . The r e l a t i o n be tween the amount of dus t loaded into the f i l t e r and the p r e s s u r e d rop m u s t be i nves t i ga t ed f o r e v e r y f i l t e r uni t de s ign .

R E F E R E N C E S

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[2 ] GREEN, H.L. , LANE, W.R. , Particulate Clouds, Dusts, Smokes and Mists, London, Spon. (1964). [3] DRINKER, P . , HATCH, T . , Industrial Dust, McGraw-Hill, New York (1954). [4 ] WHITE, P . A . F . . SMITH, S. E., High-Efficiency Air Filtration, Butterworths, London (1964). [5 ] KEILHOLTZ, G .W. , Filters, sorbents, and air cleaning systems as engineered safeguards, Nuclear

Installations, ORNL-NSIC-13 (1966); . [6] BURCHSTED, C . A . , FULLER, A.B. , Design, Construction and Testing of High Efficiency Air Filtration , : Systems for Nuclear Application,' ORNL-NSIC-65 Oak Ridge (1970). [7] THOMAS, K . T . , Bhabha Atomic Research Centre, India, Personal communication. [8] SYKES, G . H . , HARPER, J . A . , "Design and operation of a large sand bed for air fil tration", Treatment of

Airborne Radioactive Wastes (Proc. Symp. New York, 1968), IAEA, Vienna (1968) 215. [9 ] CHEEVER, C . L . , McFEE, D.R. , SEDLET, J . , DUFFY, T . L . , "ZPPR roof sand filtration of uranium,

. Plutonium and uranine aerosols", Ninth A EC Air Cleaning Conf . , Conf. -660904, USAEC (1967) 942. [10] LINDER, P . J . , SHOGULF, H , , "The economics of some types of absolute filters", Treatment of

Airborne Radioactive Wastes (Proc. Symp. New York, 1968), IAEA, Vienna (1968) 221. [11] ADLEY, F .E . , ANDERSON, D . E . , The Effect.of Holes on the Performance Characteristics of High-'

Efficiency Filters, HW-77912, Rev. General Electric C o . , Hanford Atomic Products Operation, Richland, Wash. (1963).

[12] PRADEL, ] . , B1LLARD, F . , MADELAINE, G. , BRION, J . , "Filter efficiency and clogging", Eight A'EC . Air Cleaning Conf . , TID-7677, USAEC (1963) 419.

[13] ADLEY, F .E . , WISEHART, D . E . , Life-loading Tests on Certain Filter Media, Seventh AEC Air Cleaning Conf . , TID-7627, USAEC (1961) 116.

[14] MINE SAFETY APPLIANCES COMPANY, Technical products release number 1508 B., Bulletin No. 1505-22, Pittsburgh.

[15] VOKES LIMITED, Guildford, United Kingdom, Bulletin HJ, Dec.1965. [16] US military standard, Mil F-51079. [17] ANDERSON, W.L. , "Government-industry meeting on filter media and media testing", Tenth AEC Air

Cleaning Conf. , Conf.-680821, USAEC (1968) 79. [18] UNITED STATES ATOMIC ENERGY COMMISSION, USAEC Health and Safety information. Issue

No.212 (1965). [19] MURROW, J . L . , "Effect of fire intrusion and heat on HEPA filter units", Ninth AEC Air Cleaning Conf . ,

Conf. -660904, USAEC (1967) 96. [20] Standard UL-586, High Efficiency Air Filter Units, Underwriters'Laboratories, Chicago, 111., June 1964. [21] N1TTEBERG. L .J . , SMITH, R .K. , "Humidity test report - NPR high efficiency filter bid evaluation",

Eight AEC Air Cleaning Conf . , TID-7677, USAEC (1963) 572. [22] SCHWALBE, H . C . , "Redevelopment of the Savannah River Laboratory moisture resistance test for filter

paper", Tenth AEC Air Cleaning Conf . , Conf-680821, USAEC (1968) 86. [23] HAYS, J .B . , "Performance of high efficiency particulate filters subjected to steam-air mixtures",

Seventh AEC Air Cleaning Conf . , TID-7627, USAEC (1961) 549. [24] ANDERSON, W.L. , ANDERSON, T . , "Effect of shock overpressures on high efficiency filter units",

Ninth AEC Air Cleaning Conf . . . Conf.66094 USAEC (1967) 79. [25] Instruction manual for the installation, operation and maintenance of penetrometer, filter testing DOP,

0 107 US Army Edgewood Arsenal Instruction Manual 136-300-175A, Edgewood Arsenal, Md. [26] BRITISH STANDARDS INSTITUTION, Methods of test for air filters used in air-conditioning and

general ventilation, B.S. 2831, London (1957).

75

[27] BRITISH STANDARDS INSTITUTION, Methods of test for low-penetration air filters, B.S. 3928 London (1965).

[28] PRADEL, J . , BRION, J . , "Methode sensible de mesure de l 'eff icaci tg des filtres & haute efficacitS au moyen d'un aerosol d 'utanine", Treatment of Airborne Radioactive Wastes (Proc, Symp. New York, 1968), IAEA, Vienna (1968) 279.

[29] HASENCLEVER, D . t "The testing of high-efficiency filters for the collection of suspended particles" Intern. Symp. on Fission Product Release and Transport Under Accident Conditions, Conf. 650407 USAEC (1965) 805.

[30] GILBERT, H. , PALMER, J . H . . High Efficiency Particulate Air Filter Units, TID-7023 USAEC (1961). .[31] THOMAS, J . W . , Aerosol penetration through pinholed filters, Hlth Phys. 11 (1965) 667. [32] PARRISH, E . C . , SCHNEIDER, R .W. , "Review of inspection and testing of installed high-efficiency

particulate air filters at ORNL", Treatment of Airborne Radioactive Wastes (Proc. Symp. New York, 1968), IAEA, Vienna (1968) 243,

[33] PARRISH, E .C . , SCHNEIDER, R.W. , Tests of High-Efficiency Filters and Filter Installations at ORNL, ORNL-3442, Oak Ridge (1963).

[34] ETTINGER, H.J . et a l . , Comparison of HEPA filter efficiencies, using thermal and air jet generated D . O . P . . J. Air Poll. Control Assoc. 17 9 (1967) 607.

[35] KNUTH, R .H . , "An evaluation of two portable thermal aerosol generators for in-place filter testing", Ninth ABC Air Cleaning Conf . , Conf-660904, USAEC (1967) 763.

[36] DAVIS, R.E. , CLIFTON, J . J . , The UseofaPol lakCounter forln-SituFil terTest ing, AEEW-M 567, UKAEA (1965).

[37] DAVIS, R.E. , CLIFTON, J . J . , The use of a Pollak counter for in-situ testing of high efficiency filters, Hlth Phys. 12 (1966) 981.

[38] BRESLIN, A . J . , A method for performing in-place fil ter tests using atmospheric radioactivity, USAEC Health and Safety Information No. 210 (1965).

[39] HALE, R .J . , Laboratory Ventilation, Savannah River Laboratory. [40] INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Fundamental principles for protection

in the design and construction of installations for work on unsealed radioactive material . Draft ISO recommendation No. 1710 (1968).

[41] STROM, L. , AB Atomenergi, Studsvik, Sweden, Personal communication. [42] SLADE, D . H . , Meteorology and Atomic Energy 1968, USAEC, TID-24190 (1968). [43] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations of the

International Commission on Radiological Protection, Pergamon Press (1958). [44] INTERNATIONAL ATOMIC ENERGY AGENCY, Basic Safety Standards for Radiation Protection, Safety

Series No.9. IAEA, Vienna (1967 Edition). [45] HELD, B .J . , WEHMANN, G . , MOUSEAU, D.R. , FLETCHER, R.D. , ANDERSON, R.G. , Hazardous

Materials Filter Manual for the National Reactor Testing Station, IDO 12032, Idaho (1963). [46] FIRST, M . W . , SILVERMAN, L., FITZGERALD, J . J . , BILLINGS, C . E . , DENNIS, R. , "Economic

survey of air and gas cleaning operations within the AEC", Seventh AEC Air Cleaning Conf , , TID-7627, USAEC (1961) 406.

[47] FIRST, M . W . , SILVERMAN, L., "Cost and effectiveness of air cleaning systems", Nuclear Safety 4 1 (1962) 61.

76

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