Air Filtration Fundamentals

. . . . . a primer on high efficiency air filtration

Air Filtration Pu.ndamentals -- Table of Contents AIR FILTRATION FUNDAMENTALS

I. .. . . - TABLE OF CONTENTS

Introduction

The HEPA Filter

Function 1. Components

Medium 2. Particle Capture Mechanisms

Separators Frame Adhesive Gasket Faceguards Assembly

3. Conditions for Design and Testing Static Pressure Drop 18 Flow Rate 19 Temperature 2 1 Additional Testing 2 1

4. Factory Efficiency Testing 2 2 DOP Aerosol Measuring Equipment 23

5. Field Testing 2 6

Carbon Adsorption Systems Components

(1) Media Nuclear Carbon Types of Adsorption

Physical Chemical Particle Size Ma) (imum Apparent Density Ignition Temperature

Media Certifications Frame, Perforated Material and Fire Protection

General Construction Adsorber Types Adsorber Label Carbon Filling 'Testing

R11 Test Detection Apparatus Discrimination Test Penetration Measurements

Air Filtration Fundamentals -- Table of Contents

Test Examples ......................................... 53 .................... Continuous Monitoring Detections 56

Calibration .................................... 57 ...................... R11 Generators 57

R11 Detector Operation .............. 59 Gas Chromatograph 60

Continuous Detection Operation 6 0

Adsorber Design and Test Conditions ............. 62 Standard Criteria For An Agentsystem ............ 63 Desorption ...................................... 64 Temperature ..................................... 65

........................................ Pressure 65

.. Poisons ......................................... 66

Testing of Carbon. Adsorbers and Systems .......................... 67 Laboratory Testing of Carbon .................... 67 Tests For Used Carbons ........... ........'....... 70

........................... Factory Testing of Individual Adsorbers 71 Field Testing ................................... 72

11 . When to Change A Filter ..................................... 78 ...................................... HEPA 78

Carbon - Nuclear Syst .................. 78 ................... Carbon -"Agent" Systems 79

Summary ..................................................... 80 References .................................................. 81 Figure Source Acknowledgments ............................... 83

ERRATA

AIR FILTRATION FUNDAMENTALS

JAMES R. EDWARDS, AUTHOR CHRISTINE GOLDEN, EDITOR KENNETH HEFFLEY, GRAPHICS EDITOR

THE ERRATA BELOW ARE NOT TO CORRECT MINOR EDITING PROBLEMS, SUCH AS THE INCONSISTENT INDENT IN THE FOREWORD OR THE LACK OF A CAPITAL "T" ON PAGE 9.

RATHER, THE ERRATA ARE TO CORRECT TECHNICAL ERRORS IN THE TEXT.

THE ERRATA ARE AS FOLLOWS:

1. PAGE 11 - EXTEND THE IMPACTION CURVE TO INTERCEPT THE PENETRATION AXIS AT ZERO.

2. PAGE 52 - THE LINEARITY DISCUSSION WAS BASED ON AN OLDER VALCO MODEL GC. EVEN SO, THE ONLY WAY TO BE SURE OF CONSISTENT FREON DETECTION PERFORMANCE OVER A RANGE OF UPSTREAM CONCENT-TION VALUES IS TO CALIBRATE THE DETECTOR WITH CALIBRATION GASSES OVER THE ENTIRE RANGE OF THOSE VALUES. THE NCS LMP-10 IS CALIBRATED AT lOPPM FOR UPSTREAM AND 10 PPB FOR DOWNSTREAM. OPERATING AT UPSTREAM VALUES WHICH ARE (SAY) +/ - 208 OF THE CALIBRATION VALUES MAY LEAD TO ERRORS.

TEST HAVE SHOWN EXCELLENT LINEARITY WITH THE NCS LMP-10, BUT TO MAKE A BLANKET STATEMENT FOR ALL LMP-10' S WOULD BE INACCURATE.

A REASONABLE ARGUMENT CAN BE MADE THAT BOTH THE UP AND DOWN VALUES WOULD BE NON-LINEAR IN THE SAME RATIO, AND SO TESTING VALIDITY IS STILL PRESENT. HOWEVER, THE READER SHOULD BE AWARE OF THE QUESTION OF LINEARITY.

3. PAGE 59 - THE READER SHOULD KNOW THAT THE VALVES HAVE APPROXIMATE RATIOS OF 1000:1,. WITH THE UPSTREAM VALVE HAVING A VOLUME OF APPROXIMENTLY 1 MICROLITER (ul) . THE DOWNSTREAM VALVE WILL WVE A VOLUME OF 1000 TIMES THAT.

4. PAGE 65 - THE FIGURE NEEDS SOME ADDITIONAL CONSIDERATION BY THE READER. THE FIGURE HAS MORE DATA ON IT THAN IS INDICATED IN THE TEXT.

5. THE 80C TEMPERATURE ON THE G W H SHOULD BE 70c TO BE CONSISTENT WITH THE TEXT.

1 AIR FILTRATION FUNDAMENTAL ERRATA PAGE 2 OF 2

6. PAGE 71 - "AEROSOL" SHOULD BE "VAPOR".

I 7. REFERENCE #8 HAS AN ERROR IN THE DATE " 182" SHOULD BE " 1982 " .

8. AT THE RECOMMENDATION OF A LIBRARIAN, THE NAME OF THE BOOK WAS CHANGED FROM :HIGH EFFICIENCY AIR FILTRATION" TO "AIR FILTRATION FUNDAMENTALS". THE REFERENCE IN THE FOREWORD TO "HIGH EFFICIENCY AIR FILTRATION" SHOULD BE "AIR FILTRATION F'UNDAMENTALS"

AIR FILTRATION FUNDAMENTALS A PRIMER ON HIGH EFFICIENCY AIR FILTRATION

By: James R. Edwards, BS, MAEd, EdS Christine Flis Golden, BSME

Edwards has over 20 years experience in the air filter industry. He started his career by working with Flanders Filters from 1967 to 1977. In 1977, he founded Charcoal Service Corporation, and continues as president.

Edwards has presented papers on the subject of air filtration at many conferences, and along with Fred Leckie of NCS (Columbus, OH) , and the Duke Power Training Center (Charlotte), founded the Duke Power In-place Test Training School.

Golden has over 12 years in the nuclear air cleaning industry, conducting in-place testing, air cleaning unit troubleshooting and design change implementation. She is currently employed as an HVAC System Engineer in an operating nuclear facility. She serves on the American Society of Mechanical Engineers (ASMJ3) Committee on Nuclear Air and Gas Treatment, its Subcommittee on Ventilation Air Cleaning Equipment, and is on the Board of Directors for the International Society of Nuclear Air Treatment Technologies. She Chairs the American Nuclear Society Committee which is presently revising ANS 59.2, Safety Criteria for Ventilation Systems Located Outside Primary Containment.

Heffley holds a BS Degree in chemistry from the University of North Carolina Chapel Hill. In addition to computer skills, he has been involved in adsorption by activated and impregnated carbon for over eight years. He has been the lead engineer at CSC in the start-up of complicated metal working machinery, carbon impregnation equipment, and many other tasks associated with high efficiency air filtration.

The author and editors can be contacted as follows:

JAMES R. EDWARDS AND KENNETH W. HEFFLEY CSC PO BOX 3 BATH, NC 27808

CHRISTINE FLIS GOLDEN INDIANA MICHIGAN POWER COMPANY DONALD C. COOK PLANT ONE COOK PLACE BRIDGMAN, M I 49106

Air Filtration Fundamentals is a comprehensive work about air filtration. It is applied to containment systems, which is a filter system used to capture and "containn an undesirable contaminant. The primary focus is on two filtering devices:

1. High Efficiency Particulate Air (HEPA) filters, and how they work

2. Carbon adsorbers, and how they work

Each of these filtering devices is explored by discussing the following topics:

1. Components 2. Design considerations 3. Assembly 4 . Factory Testing 5. Field testing

This work is not intended to replace any plant QA program or documents. All legal document, such as plant Technical Specifications, FSARs, and others take precedence over the'information herein.

High Efficiency Air Filtration is intended to be an informational handbook to supplement background knowledge of the air filtering process.

DISCLAIMER

Theinformationcontainedinthisworkwas developedbythe authors fromexperience, research, exposure t o researchby o thers , and l o r e obtained by assoc ia t ion with exper ts i n t h e f i e l d of a i r f i l t r a t i o n .

The authors be l i eve t h a t t h e information is t r u e and accurate. However, as i n any work, mistakes can be made.

Before t h e use r relies on any of t h e information here in i n any s i t u a t i o n which may have an e f f e c t on t h e heal th and s a f e t y o f t h e p u b l i c , p lantpersonnel , oranyotherhumans, any process, o r any product, t h e user should v e r i f y t h e u s e r ' s s p e c i f i c designand applicationbyhavingcompetent review by engineering profess ionals , and o thers who have f irs t-hand knowledge of t h e appl ica t ion.

.*. 111

INTRODUCTION

An a i r f i l t r a t i o n system is designed to :

(I) remove p a r t i c l e s from t h e airstream, ( 2 ) remove unwanted gases and vapors from t h e airstream, o r ( 3 ) remove both p a r t i c l e s and gases,

Par t i cu la te , thosecomponentscommonlyinthe formof dust , smoke, o r pollen, are removed by p a r t i c u l a t e f i l t e r s through the f i l t r a t i o n process. G a s e s o r vapors are removed by carbon f i l t e r s , through t h e adsorption process. These l a t t e r f i l t e r s a r e thus re fe r red t o as carbon adsorbers . The required capab i l i t y of an a i r f i l t r a t i o n system t o remove unwanted p a r t i c l e s w i l l depend on i t s spec i f i c applicat ion. An a i r conditioning system, f o r example, may requ i re t h a t only gross pa r t i cu l a t e be control led, so it might havepar t i cu la te f i l t e r s w h i c h a r e 50% ( o r e v e n l e s s ) e f f i c i e n t f o r removingpart icles 50micrometersor larger . A r ec i r cu l a t i ng odor control carbon system might require t h e i n s t a l l a t i o n of a carbon adsorber t h a t has an e f f ic iency of only 25%- An a i r handling un i t f o r an off i c e buildingmight have both types and e f f i c i enc i e s i n s t a l l e d , Each a i r f i l t r a t i ~ n ~ s y s t e m is designed t o m e e t s p e c i f i c needs.

A high eff ic iency f i l t r a t i o n system w i l l have components which are designed t o remove and contain t a r g e t part icu- l a t e , vapors, o r both. A s a ru l e , these contaminants a r e dangerous, tox ic , o r noxious, and e f f i c i e n c i e s of greater than 99.9% a r e required.

~lmosteveryaircleaningsystemwillhave componentsother thanthosewhichperformthe s p e c i f i c t a sks r e q u i r e d o f t h e system. These additionalcomponentsare des ignedtodo one th ing, t h a t is t o p ro t ec t t h e p a r t i c u l a t e and gas f i l t e r s . For example, a moisture separa tor might be i n s t a l l e d t o remove sensible w a t e r f romthe airstream. Sensible water has a very detr imental e f f e c t on both pa r t i cu l a t e and gas f i l t e r s .

Water d rop l e t s c o l l e c t i n g on p a r t i c u l a t e f i l t e r s , f o r example, w i l l increase pressure drop t o a point t h a t t h e fan is incapable of moving t h e r e q u i s i t e amount of a i r through t h e uni t . For carbon adsorbers, t h e problem water c r ea t e s is not with pressure drop. Sensible w a t e r coa t s t h e carbon granules, which prevents t h e carbon's pore s t ruc tu re from in t e r ac t i ng with t h e airborne gas o r vapor, preventing adsorption from tak ing place.

Another component designed t o p ro tec t both t h e high e f f i c iency p a r t i c u l a t e f i l t e r and t h e adsorber is t h e p r e f i l t e r , sometimes r e f e r r ed t o as a roughing f i l t e r . These f i l t e r s remove l a rge p a r t i c l e s from t h e airs tream and prevent t h e high e f f i c i ency f i l t e r s from "face loading" with t h e l a rge p a r t i d l e s . This saves t h e more expensive high e f f i c iency f i l t e r s f o r removing small p a r t i c l e s , which a r e h a r d e r t o capture. P r e f i l t e r s a l s o prevent l a rge p a r t i c l e s from agglomerating on t h e perfora ted screens of t h e adsorbers. Large p a r t i c l e s co l l ec t i ng on t h e adsorbers increase t h e pressure drop across t h e adsorber, reduce airf low, andobscure t h e carbon granules f romthe airstream. The a p p l i c a b i l i t y of t h i s funct ion depends on t h e pos i t ion of t h e adsorber i n t h e chain of components. P r e f i l t e r s and moisture separa to rs are among t h e f i r s t components t h a t an a i r s t r e a m i s exposedtowhenthey a r e i n s t a l l e d i n t h e system f o r p ro tec t ion of t h e more s e n s i t i v e components.

~theraccessorieswhichmightbeincludedinhighefficiency . a i r f i l t r a t i o n system design are f i r e detec t ion, instrumen- t a t i o n , hea te r s , dampers, adsorbent t e s t c an i s t e r s , and sampling connections. This t e x t w i l l dea l exclusively with a i r f i l t e r sy s t ems tha t contain High E f f i c i e n c y p a r t i c u l a t e A i r (HEPA) filtersandadsorberswithactivated, impregnated carbon as ' t h e adsorbing. medium.

This t e x t usual ly r e f e r s t o a Nuclear A i r Treatment System (NATS) , u s e d t o controlradioactivematerialinacommercial l i g h t water reactor . This is due t o t h e f a c t t h a t those involved i n t h e publ ica t ion have experience i n t h e f i e l d and NATS contains some of t h e most s t r i ngen t requirements f o r a i r f i l t r a t i o n of any process t o be found. The f i l t r a t i o n technology discussed here in can be t r ans f e r r ed t o any o ther applicationusingthesetypesof f i l t e r s . O t h e r u s e s i n c ~ u d e

t h e con t ro l of pathogens, control of dangerous o r t ox i c gases, vapors, of p a r t i c u l a t e (such as m i l i t a r y chemical agen ts ) , recombinant RNA and DNA mater ia l , and con t ro l of noxious mater ia l (such as particlesthataffectthethrough- put of e l ec t ron i c micro-chip manufacturing, o r t h e through- put of a pharmaceutical p l a n t ) .

This t e x t w i l l f i r s t descr ibe t h e components of a HEPA f i l t e r , and its capture mechanisms. The equipment used t o test HEPA f i l t e r s w i l l then be presented. Final ly , t h e various tests required f o r HEPA f i l t e r s a r e included. Since t h e nuclear indust ry has t h e most rigidHEPA spec i f i ca t i ons a n d t e s t requirements, t h e d i s c u s s i o n w i l l b e f o r t h e n u c l e a r appl ica t ion. Other operat ions may have less s t r i ngen t requirements. The reader. can determine t h e appropriate l e v e l of f i l t r a t i o n required f o r t h e i r appl ica t ion. Adsorbers w i l l be addressed i n t h e same order as HEPA f i l t e r s .

I t is not poss ib le t o address t h e subject of f i l t r a t i o n thoroughly without some mention of t h e f i l t e r system which houses t h e components. The incorporat ion of HEPA f i l ters and adsorbers i n t o a f i l t e r system, and t h e successful t e s t i n g of them, w i l l conclude t h e discussion. Even though HEPAs and adsorbers a r e t h e main' t op i c , o the r f a c t o r s w i l l have necessary r o l e s i n t h e development of t h e HEPA and adsorber cha rac t e r i s t i c s . Reference t o o the r f i l t e r system components w i l l be included i n t h i s document. These may include moisture separa tors , a i r heaters , p r e f i l t e r s , fans, and performance monitoring systems. These components w i l l not be discussed i n d e t a i l , however.

READER HELPS

The author recommends t h a t t h e reader have copies of t h e publ ica t ions denoted' i n t h e REFERENCES sec t ion a t t h e i r d isposal . The references are not a requirement f o r use of t h i s text, but r e f e r r a l t o them where noted w i l l lend much understanding t o t h e reading of t h i s work.

NOTE: The United S t a t e s Nuclear Regulatory Commission (NRC) w i l l be referenced herein. The authors are not represen- tativesoftheNRCandhavenoNRCconnections. NRCpositions are being r e l a t e d a s they a r e understood by t h e authors.

THE HEPA FILTER

Since t h e nuclear indust ry is believed t o have t h e most r igidHEPAspecif icat ions and requirements, t h e discussion w i l l be fornucleargradeHEPAfilters. Other app l ica t ions may have less s t r i n g e n t requirements. The reader is l e f t t o determine t h e appropr ia te l e v e l of f i l t r a t i o n f o r t h e i r applicat ion.

I n t h e paragraph above, "most r ig id" and "less s t r ingen t" a r e terms applied t o t h e s t r u c t u r a l requirements of t h e HEPA f i l t e r , not t h e e f f i c i enc i e s . Some c lean room appl ica t ions requ i re HEPA f i l te rs with e f f i c i e n c i e s t h a t exceed 99.999%, f a r higher than t h e 99.97% required f o r a NATS . Function

The function of a HEPA f i l t e r is t o remove s m a l l p a r t i c l e s f romtheai rs t ream. Smallparticlesaredefinedtobethose with a diameter range of 5 pm t o 0.001 pn. This s i z e range is se lec ted f o r two primary reasons. F i r s t , roughing f i l t e r s a r e used t o remove p a r t i c l e s l a r g e r than 5 pm. Secondly, p a r t i c l e s less than 0.001 pn are i n a t r a n s i t i o n s tage , and more near ly obey t h e gas l a w s than t h e laws of p a r t i c u l a t e physics.

A study of t h e cha r t (Figure 1. ) on t h e next page is very i n s t ruc t i ve .

HEPA Components

A HEPA f i l t e r typ ica l ly used i n a United S ta tes commercial nuclear industry has a f i l t e r i n g medium, separators, a frame, adhesive ( t o glue themediumtothe frame), agaske t , face guards, and a labe l as the necessary components. The arrangement of some of these componentsis showninFigures -

2A and 2B.

F i q u r e 2 A B.LmmFibrRck u zz and lntspal Frame

Filter Medium Figure 2B

The f i l t e r i n g medium is a matrix of boron s i l i c a t e g l a s s f ibers . These f i b e r s have a range of diameters, from sub- micrometer t o about 5 p.

The matrix is formedbythe standard paper-makingprocess. That is, a s lu r ry of f i b e r mix is s luicedonto a fourdr inier screen, and a vacuum p u l l s a s much water from the s lu r ry a s possible. A waterproofing material , c a l l e d a "binder", is applied. The remainder of the water i n the media and binder is removed by drying the continuously moving sheet of paper i n an oven. After drying, t h e media, now ca l led "paper", is wound around a spool t o make a r o l l of paper about 26 inches wide and about 24 inches i n diameter. The r o l l weighs about 50 The "basis weight" of t h i s paper is 50 pounds per 3000 square f ee t . The paper is about 18 m i l s thick.

Each media manufacturer has t h e i r own formulae f o r paper making. The formulae w i l l consider two primary fac tors ,

(1) The desired eff ic iency of t h e paper, and ( 2 ) The desired pressure drop of the paper.

Paper can be made more e f f i c i e n t by increas ing t h e number of s m a l l f i b e r s , and reducing the number of l a rge ones. Combining t h e f i b e r s i n t h i s manner c rea tes a higher pressure drop f o r a given eff iciency. The paper can be made more porousandless eff ic ient ,withalowerpressuredrop, by adding more la rge f i b e r s , and fewer s m a l l ones. Since smaller f i b e r s a r e more expensive t o manufacture, paper makers try t o minimize t h e i r use. However, using few s m a l l diameter f i b e r s requires a th icker matrix of l a rge r f i b e r s f o r a given e f f ic iency . Each manufacturer derives a paper formula a f t e r considering t h e manufacturing cos t , benef i t o f t h e formula, and t h e intended applicat ion o f t h e f i l t e r .

Mil i tary Standard M i l F 51079 is t h e reference specif ica- t i o n f o r t h e manufacture of ' f i l t e r paper f o r nuclear use. (Note: The U.S. Army plans t o "abandon" M i l F 51079 a t t h e end of 1995. The requirements i n M i l F 51079 w i l l be incorporated i n t o an appendix of Section FC of ANSI/ASME AG-1. )

Figure 3 is a photomicrograph of t h e f i l t e r media under lOOOx magnification. The meniscus between t h e f i b e r s is t h e waterproof binder used t o make t h e media water r e s i s t an t . Notethattherandomlypositionedfiberscreate i n t e r s t i c e s which are very la rge when compared t o a sub- micrometer p a r t i c l e . The means used t o remove these small p a r t i c l e s w i l l be discussed next.

Figure

PARTICLE CAPTURE MECHANISMS

A HEPA f i l t e r captures p a r t i c l e s by several mechanisms. These include s t ra in ing ( a l so ca l led s ieving) , intercep- t ion , i n e r t i a (a l so ca l led impaction), and diffusion. Figure 4 depicts these capture mechanisms.

To present a c lear , graphic representat ion of these capture mechanisms, a s t r i c t mathematical in te rpre ta t ion of the capture mechanisms must be discarded f o r a general in terpreta t ion.

FILTRATION MECHANISM

Figure 4

Sieving, a l s o cal led s t ra in ing , is the process whereby a pa r t i c l e is removed from the airstream by becoming lodged among several f i l t e r media f ibe r s . Large p a r t i c l e s a r e easilycapturedbysieving,whichisacollectionphenomena

of minor e f fec t iveness . S t r a in ing is an undesirable capturemechanism, sinceaparticlecapturedinthismanner has e f f e c t i v e l y screened a por t ion of t h e f i l t e r from f u r t h e r p a r t i c l e capture.

Sieving occurs more f requent ly with p a r t i c l e s l a r g e r t h a n 5 p.m. These l a r g e r p a r t i c l e s block access i n t o t h e f i l t e r media, andpreventsmallerparticles from penetra t ing i n t o t h e depths of t h e f i l t e r media. The screened por t ion of t h e HEPA f i l t e r can no longer capture p a r t i c l e s i n t h e most de s i r ab l e s i z e range. Since add i t iona l p a r t i c l e s cannot pene t ra te i n t o t h e depths of t h e media, new incoming p a r t i c l e s agglomerate on t h e s ieved l a r g e r p a r t i c l e s , and face load t h e HEPA. Face loading blocks o f f a port ion of t h e f i l t e r media from f u r t h e r air flow, and t h i s blockage i n c r e a s e s t h e pressure drop ac ross t h e f i l t e r , r e d u c i n g i t s l i f e span.

Interceptionoccurswhenaparticle, whichotherwisemight avoid a f i b e r , has s u f f i c i e n t s i z e (i .e. , r a d i a l s i z e ) t h a t , merely because of its s i z e , it comes i n contact with a f i b e r and is captured. I t is a t r a n s i t i o n a l s t age between impaction and d i f fu s ion and does not play a major r o l e i n p a r t i c l e capture

Impaction occurs when t h e p a r t i c l e has s u f f i c i e n t s i z e ( thus , momentum) t h a t it cannot continue i n t h e path of t h e gasmoleculeswhichcarryit. Thegas stream, a n d p a r t i c l e , might e a s i l y go around many f i l t e r f i b e r s , but eventual ly i ts path is blocked by a f i l t e r f i b e r . t h a t is, t h e gas stream can go around t h e f i l t e r f i b e r , but because of t h e p a r t i c l e ' s momentum, it cannot avoidbeing driven onto t h e f i b e r . Diffusion occurs when a p a r t i c l e less than 0.3 pm behaves similar t o a gas molecule. That is, t h e p a r t i c l e is suhjected t o Brownian motion, and has a random path i n t h e media. This random, t he re fo re l eng thypa th c o n t r i b u t e s t o capture by t h i s means. P a r t i c l e s want t o achieve equil ibrium with o the r p a r t i c l e s of a s i m i l a r s i z e , thus a p a r t i c l e d i f f u s e s i n t o volumes with a lower p a r t i c l e population i n o rder t o reach t h a t equilibrium. One of t h e volumes t h a t have a low p a r t i c l e population is occupied by

a submicrometer f i l t e r f iber . Thus, t h e small p a r t i c l e is a t t rac ted t o the f i l t e r f i b e r and is captured.

Pa r t i c l e s i n the s i z e range of 5 pm t o 0.001 pm discre te ly a t tach themselves t o a media f iber . Because of the great number of f i b e r s i n t he media of a 24 inch x 24 inch x 11.5 inch HEPA f i l t e r , there is a large number of s i t e s t h a t a re available f o r p a r t i c l e capture. Eachfibercanaccommodate a large number of par t ic les . Thus, a HEPA f i l t e r has a huge capacity f o r these s m a l l pa r t ic les . Figure 5 shows t h i s i n a conceptual way.

Figure 5

Small p a r t i c l e s which come i n t o contact with any media f i b e r a r e held onto the f i b e r s by V a n der Waalsl force, which i s an e l e c t r o s t a t i c force. Once a p a r t i c l e is captured, Van der Waalsg force w i l l keep t h e p a r t i c l e permanently attached t o the f ibe r . (Note: some migration of par t icu la te through the f i l t e r is possible, but t h i s migration is negligible.)

The a b i l i t y of a f i l t e r t o capture p a r t i c l e s is expressed i n terms of "efficiency" (.E) and "penetration" (P) . These terms a re of ten used interchangeably, but a r e not synonymous. Percent eff ic iency is defined t o be

Even though the terms a re used interchangeably, they do have d is t inc tappl ica t ions . E f f i c i ency i s atermusedwhen the penetration of a HEPA f i l te r is obtained under laboratory conditions. W e say the f i l t e r has "99.97%

Efficiency". Penetra t ion is a term used t o express t h e t r a v e l of contaminants through a f i l t e r when it is s u b j e c t e d t o f i e l d test conditions. These condit ions are not poss ible t o con t ro l as c lose ly as t h e conditions i n t h e laboratory. Also, a f i e l d test determines t h e amount of contaminants which a c t u a l l y penetra te t h e f i l t e r system. W e say a f i l t e r system has "0.05% Penetration".

I f t h e terminology w a s cons i s ten t , t h e term "penetration" would be used, and not "efficiency". Whether t e s t i n g i n t h e laboratory o r i n t h e i n s t a l l e d appl ica t ion, it is always t h e pene t ra t ion through a f i l t e r which is ac tua l ly being measured.

I n t h e e f f e c t i v e s i z e range f o r HEPA f i l t e r s , l a r g e r p a r t i c l e s are captured by severa l methods already discussed. These methods are grouped together i n t o a s ing l e capture mechanism which s h a l l be defined a s 110 ( in te rcep t ion , impaction, o t h e r ) . Refer t o t h e e f f ic iency curve shown i n Figure 6. Notice t h a t as t h e p a r t i c l e s i z e increases , t h e e f f i c i ency a t t r i b u t e d t o I10 increases. Obviously, t h e r e i s a p a r t i c l e i n t h e l a rge r s i z e range which w i l l not penetra te t h e f i l t e r media. That is, Penetrat ion = 0, as shown.

Zero Percent Penetration 100 Percent Eficiency 0.00 .A 100.00 %

- - - - - - - 99.97% Percent

Penetration

smaller particles- 0 3 pm - larger particles

Particle Size in pm Figure 6

S m a l l e r p a r t i c l e s are captured by diffusion. A s t h e p a r t i c l e s i z e decreases, t h e eff ic iency a t t r i b u t e d t o di f fus ion increases. When t h e p a r t i c l e approaches molecular s i z e ( i .e . , l e s s than 0.001 pm). t h e eff ic iency curve f o r d i f fus ion begins t o t u r n down toward higher penetration.

These capture mechanisms are working simultaneously. Therefore, t he penetrat ion curves a r e addi t ive . The r e su l t an t curve is marked a s such.

There are two important po in t s i n t h i s curve:

1. The most penetra t ing - p a r t i c l e s i z e i n t h i s i l l u s t r a t i o n is 0.3 pm.

2. The penetrat ion f o r t h i s p a r t i c l e s i ze , i n t h i s i l l u s t r a t i o n , i s 0.03%.

The de f in i t i on of a HEPA f i l t e r is derived from these two important points.

BYDEFINITION,AHEPAFILTERISONEWHICHEXHIBITSAMAXIMUM PENETRATION OF 0.03% ON 0.3 pm PARTICLES.

The reader should avoid u s i n g t h e phrase "99.97% e f f i c i e n t on 0.3 pm p a r t i c l e s and larger", as it shows a misunder- standing of t h e nature of t h e performance of a HEPA f i l t e r . The r e su l t an t curve from t h e graph shows t h a t as t h e p a r t i c l e s i z e increases from 0.3 pm, t h e eff ic iency increases. A s t h e p a r t i c l e decreases from 0.3 pm, t h e eff ic iency a l so increases. I t is appropriate, therefore , t o state t h a t t h e f i l t e r exh ib i t s "0.03% maximum penetrat ion on 0.3 pm par t ic les" , s ince t h e penetrat ion decreases as t h e p a r t i c l e s i z e increases o r decreases.

The HEPA f i l t e r de f in i t i on w a s derived from work done i n t h e 1950's a t t h e US Navy Laboratory, Edgewood Arsenal, and by pr iva te contractors . Recent t e s t i n g with l a s e r p a r t i c l e s i zeana lyze r shas shownthatthemostpenetrating p a r t i c l e is 0.12 pn. This s h i f t i n t h e curve is important

f o r c lean room work, where p a r t i c l e s i z e is a c r i t i c a l considerat ion. However, t h e United S t a t e s ~ u c l e a r Regu- l a t o r y Commission (USNRC, o r NRC) has not required a change i n its c r i t e r i a from0.3 w t o 0.12 pm. Its pos i t ion is t h a t t h e impact of t h e recent d iscover ies t o t h e Health and Safety of t h e Public is not s i gn i f i can t . The N R C t s cha r t e r is t o p r o t e c t t h e hea l th and sa f e ty of t h e public . For t h i s d iscuss ion, 0.3 pm w i l l be considered t h e most penet ra t ing p a r t i c l e s i ze .

A summary of p a r t i c l e capture theory and p a r t i c l e behavior can be i l l u s t r a t e d by t h e following example.

Consider a s i x foo t t a l l person, fac ing a f o r e s t of a quar te r of a mile th ick . .There i s a moderate amount of undergrowth i n t h e f o r e s t . This person is representing a 0.3 pm p a r t i c l e i n its path through a f i l t e r system. The quar te r m i l e t h i c k woods, with i ts moderate undergrowth, is t h e f i l t e r media wi th both s m a l l and l a rge f i b e r s .

The person i n t h i s example begins t o go through t h e woods, t ak ing c a r e t o avoid touching a tree o r any brush, and continuing a t a uniform veloci ty . The l ike l ihood is high t h a t t h e person, i n s p i t e of e f f o r t s t o avoid them, w i l l come i n con tac t with a t r e e , bush, o r l i m b . When t h a t happens, t h e person is "captured". The same happens t o a small p a r t i c l e by 110.

A very s m a l l person would be more l i k e l y t o avoid a tree, but lacking cons i s t en t d i r ec t i on and cons tan tve loc i ty , is more l i k e l y t o wander i n t o a low-growing bush. This is analogous t o a p a r t i c l e smaller than 0.3 pm. A very l a rge personmay not be very a g i l e , and wouldblunder i n t o a t r e e , b u s h o r l i m b q u i t e r e a d i l y . This is analogous t o a p a r t i c l e l a r g e r than 0.3 p.m.

Suppose t h a t t h e r e is a v i s i b l e , narrow path through t h i s fo r e s t . A t t h e i n i t i a l ve loc i ty , t h e person tries t o reach t h e path. With many obs tac les i n t h e way, t h e person is captured. I f t h e person slows down t o 20% of t h e i n i t i a l ve loc i ty , however, t h e r e is a much b e t t e r chance of avoiding t h e trees, bushes and l i m b s , and t h e path can be reached. This is analogous t o t h e pinhole leak previously

described. (Note: Pinhole leaks w i l l be discussed l a t e r . The reader may wish t o r e f e r t o t h e sect ion e n t i t l e d "FLOW RATE", pages 1 9 through 21. )

~f 10,000 various s ized people began t h e t r i p through t h e fo re s t , t h r ee would make it through without touching anything. This represents t h e 0.03% penetrat ion allowed i n a HEPA f i l t e r .

Separators a r e corrugated spacers made of aluminum, k r a f t paper, o r o ther material . They a r e about 4i inch high and keepthe f i l . t e rmedia separated, s o t h a t t h e e n t i r e surface area of t h e media w i l l be exposed t o airflow. The sepa ra to r sa l so s e r v e a s f l u t e s , whichchannel t h e incoming air i n t o t h e depths of t h e HEPA f i l t e r . They a l s o provide some s t r u c t u r a l r i g i d i t y t o t h e f i l t e r .

Some nuclear grade HEPA f i l t e r s have been developed w i t h . t h e separatorsmoldedintothemediaduringthe fourdr inier

process.

The assembly of t h e f i l t e r media and t h e separators is ca l led a "pack", o r "slug".

FRAME

The f i l t e r frame is a metal, wood, p l a s t i c , o r o ther impenetrable material enclosure t h a t surrounds and contains t h e assembled f i l t e r pack. The nuclear industry has standardized on a corrosion r e s i s t a n t sheet metal frame . ADHESIVE

The adhesive is t h e glue which binds and seals t h e f i l t e r pack t o t h e frame.

GASKET

The gasket is t h e in t e r f ace between t h e HEPA f i l t e r frame (as defined above) and t h e framing systemwhich supports

t h e HEPA f i l t e r i n t h e NATS f i l t e r housing. The terms "filter f rame" and "framing system" a r e not synonymous. The gasket is normally composed of c lose-cel l sponge neoprene mater ia l and is secured t o t h e f i l t e r frame with adhesive.

FACE GUARDS

A face guard is normally kW hardware c l o t h ( m e t a l mesh) i n s t a l l e d over t h e e n t i r e face of t h e f i l t e r . I t is at tached t o t h e +" recess between f i l t e r pack and t h e frame. Its purpose is t o p ro t ec t t h e f r a g i l e f i l t e r media from damage. Another bene f i t of t h e face guards is t h e added r i g i d i t y which they provide t o t h e f i l t e r assembly.

FILTER ASSEMBLY

HEPA f i l t e r components are assembled i n t o a f i l t e r i n t h e following manner:

A continuous sheet of f i l t e r media i s woven i n t o a pack by placing separa to rs f i r s t on one s i d e of t h e sheet of media, and then t h e o ther . This process is con t inuedunt i l t h e pack is about 23 inches high. The pack is 26 inches wide, due t o t h e width of t h e media. The th ickness is approximately 11 inches. The f i l t e r frame is composed of four "boards", a term used t o descr ibe t h e frame components. There are two s i d e boards, as w e l l as t h e top and bottom boards. The boards are % inches th ick . Wooden frames a r e usual ly made of %I1 plywood. Metal frames a r e usual ly made of 1 4 gage material, with a %I1 double-turned f lange as t h e ou t e r perimeter. .The f i l t e r pack is trimmed by a double blade band s a w t o a dimension t h a t , when assembled with t h e 3" boards, makes a f i l t e r with one 24" dimension. Theo therd imens ion is at tainedbyremovingone o r more separa tors (and media) u n t i l t h e dimension, with t h e boards, is a l s o 24". The depth of t h e f i l t e r is 11 4". The depth of t h e f i l t e r pack, however, is lltl. This means t h a t t h e pack is recessed i n t o t h e frame by on both t h e f r o n t and back of t h e f i l t e r . This HEPA f i l t e r is re fe r red t o as "24 x 24 x 11 5". Whenever a d i f f e r e n t s i z e f i l t e r is thus described, t h e f i r s t dimension always r e f e r s t o t h e height of t h e f i l t e r .

The trimmed pack is glued i n t o t h e frame. More cor rec t ly , t h e frame i s assembled around t h e pack, gluing t h e i n t e r f ace between t h e pack and t h e frame.

A l aye r of adhesive is applied t o t h e frame j o i n t s t o prevent a i r f low bypass which could occur through t h e jo in ts . Wood frames a r e na i l ed together a t t h e j o i n t s and metal frames a r e assembled with r i v e t s .

Astudyoftheassemblypicture (Figure 2A) revea l s a n a r e a of spec ia l concern, i n terms of s ea l i ng t h e pack t o t h e frame. This involves t h e "cut edge" of t h e pack,

The e n t i r e roughcut edgemustbe sealed, o r t h e f i l t e r w i l l leak. That is, contaminants i n t h e a i r w i l l by-pass t h e f i l t e r media. To s e a l t h e 'cut edge, it is "potted" with glue so t h a t , by c a p i l l a r y ac t ion, t h e glue creeps i n t o t h e f i l t e r pack f o r approximately + inch, c rea t ing a sea l .

Other adhesivetechniques havebeenused tomake th i s s e a l , including two p a r t urethane which "grow" i n t o t h e c u t edge as they cure, and s o l i d urethane, which form a s o l i d glue sheet t h a t covers t h e c u t edge.

The "f lap edge" of t h e frame is secured by a s i n g l e bead of glue. This s i ng l e bead extends 'from one s i d e board t o t h e o ther , on both t h e t o p and bottom. The s ing l e bead of glue s e a l s t h e pack t o t h e t o p and bottom boards.

Gaskets used on HEPA f i l t e r s a r e 3 inches wide, and % inch th ick . The gaskets a r e i n s t a l l e d by gluing s t r i p s of t h e material t o t h e t inch face of t h e frame boards. Care is taken t o ensure t h a t t h e gasket is an i n t e g r a l e n t i t y , s ince any gaps i n t h e gasket seal w i l l allow un f i l t e r ed a i r t o bypass t h e f i l t e r media. The gasket j o i n t s can be b u t t jo in t s , doveta i led jo in t s , o r any other configurat ion whichwi l lmee t the c o n t r a c t t e m s . The gasket is i n s t a l l e d onone face , o r b o t h f a c e s o f t h e f i l t e r , a t t h e i n s t r u c t i o n of t h e con t rac t f o r f i l t e r purchase,

The face guards, are i n s t a l l e d by gluing them t o t h e frame boards, j u s t i n s ide t h e i n t e r i o r o f t h e frame, i n t h e % i n c h recess previously described.

The label , with a l l per t inent test data, is attached t o the frame.

Component specif icat ions , assembly ins t ruct ions , and t e s t ins t ruc t ions f o r a nuclear grade HEPA f i l t e r a r e found i n Mili tary Specification M i l F 51068. The U. S. Army plans t o "abandon" M i l F 51068 a t the end of 1995. The requirements i n M i l F 51068 w i l l be incorporated i n t o Section FC of ANSI/ASME AG-1.

CONDITIONS FOR DESIGN AND TESTING

~ i l ~ 51068 describesl imitingconditions f o r t h e opera t ion o f ' a HEPA f i l t e r . These a r e discussed a s follows:

Static pressure drop (dp) across the filter

S t a t i c pressure drop (dp) is t h e measurement of t h e force requ i red tomovethe a i r throughthe f i l t e r . Aclean f i l t e r has a dp of less than 1.0 inch water gage. A f i l t e r manufacturer w i l l warrant a nuclear grade HEPA f i l t e r t o operate properly a t 10.0 inches water gage.

The design dp is a funct ion of economics, ergonomics, and engineering considerat ions. Economic considerat ions involve opera t ing t h e u n i t with a d i r t y f i l t e r versus t h e cos t of changing t h e f i l t e r . I f access t o t h e f i l t e r is l imi ted due t o high r ad i a t i on , f o r example, then t h e cos t o f t h e ene rgy to r u n t h e f i l t e r t o a h i g h d p isminimal when compared t o ALARA consequences t o maintenance personnel.

Another economic example is i n t h e case of production opera t ionswhichre lyonHEPAfi l t ra t ion . I f shuttingdown a n o p e r a t i o n t o change filters meanslos tproduct ion, then it may bemore economical t o operate t h e f i l t e r s a t a h i g h e r dp, keeping t h e opera t ion functioning.

An example of an ergonomic considerat ion is t h a t a higher dp w i l l generate a higher l e v e l of noise. I f t h e f i l t e r system is near ( o r i n ) t h e con t ro l room of a nuclear p l an t (oranyotherareawherequietsurroundingsaredesirable), t h e noise could be an unnecessary annoyance. This would argue fo rchanging the f i l t e r a t a l o w d p . Lowerdprequires less fan ,horsepower t o . move a i r through t h e f i l t e r , r e s u l t i n g i n q u i e t e r operat ion. Another example is i n c lean rooms, which rou t ine ly use HEPA f i l ters as p a r t of t h e room boundary. Noise from high ve loc i ty a i r t r a v e l l i n g through t h e f i l t e r s , o r from t h e blower (which must be l a r g e r t o overcome t h e high s t a t i c p ressure ) , may have an undesirable e f f e c t .

An example of an engineering considerat ion is t h a t dp is r e l a t ed t o f an horsepower (hp) as a cubic function. That

is, it takes 8 t i m e s t h e hp f o r a given f an t o move t h e same amount of a i r through a f i l t e r with a 2.0 inch w a t e r gage dp a s through one with a 1.0 inch w a t e r gage dp.

For p r a c t i c a l considerat ions, an unconventional ven t i l a - t i o n fan with high hp, l a rge s i z e , and w i d e opera t ing range s t a b i l i t y would have t o be chosen f o r a NATS i f t h e HEPA f i l t e r were t o opera te from t h e lower t o t h e upper range of its a b i l i t y .

Flow Rate

F i l t e r flow r a t e , i n cubic f e e t pe r minute (cfrn) is a function of t h e square feet of f i l t e r media i n t h e HEPA. M i l F 51068 l i m i t s t h e allowed ve loc i t y of a i r f low through the media t o a maximum of 5 l i n e a l f e e t pe r minute. Older HEPA designs contained about 200 square f e e t of media, meaningthat thevolumetr ic flow r a t e o f t h e f i l t e r w a s 1000 cfm. Even though separa to r f l u t e height has decreased (which allowed more media t o be put i n t o t h e HEPA), and t h e medium was designed f o r b e t t e r poros i ty , ' t h e design flow o f t h e HE~Aremains a t1000 cfm. Thismeansthat t h e i n i t i a l pressure drop has been reduced from 1.0 inch water gage ( I t

wg) a t 1000 cfm t o a l e s s e r value- A t y p i c a l pressure drop value a t 1000 cfm, f o r a new, modern f i l t e r , is about 0.80tt wg. Al ternat ive designs f o r 1500 cfm and 2000 cfm a r e avai lable , w i t h h i g h e r i n i t i a l d p . Onlythe 1000 cfm design is discussed here.

Although flow rate changes w i l l a f f e c t t h e f i l t r a t i o n mechanisms, a f i l t e r w i l l funct ion under var iab le flow ra tes . A s t h e flow rate increases , t h e e f f i c iency due t o impaction, and in te rcep t ion , w i l l increase . The e f f i - ciency due t o d i f fus ion .a lone , however, w i l l decrease.

The converse is a l s o t r u e - A s t h e ve loc i t y through t h e media decreases, t h e e f f i c i ency due t o impaction (and t o a lesser extent , i n t e r cep t ion ) w i l l decrease, bu t t h e ef f ic iency due t o d i f fu s ion w i l l increase. These statements are self-evident considering momentum as a funct ion of veloci ty .

Even though f i l t e r s w i l l funct ion a t va r i ab l e flow rates,

t h e nuclear indust ry requi res t h e veloci ty through t h e media not exceed 5 f e e t per minute, i n accordance with M i l F 51068, which is referenced by U.S. NRC Reg Guide 1.52.

F i l t e r s a r e leak t e s t e d a t 1000 cfm using a t e s t aerosol. After t h i s 1000 cfm t e s t , t h e t e s t airf low (with i t s entrained aerosol) is reduced t o 200 cfm. This reduction of airf low through t h e f i l t e r w i l l de tec t a "pinhole" leak, I f a pinhole leak e x i s t s i n t h e f i l t e r , t h e penetrat ion through t h e pinhole w i l l be disproport ionately high a t t h e lower f lowra te . Disregardingrigorousproof, theapparent penetrat ion (%P) through t h e f i l t e r m e d i a is approximately equal t o t h e roo t flow, o r

P = K ( f low)El/2, where K is approximately 1. I f t h e flow is l o o % , t h e apparent penetra t ion is 1.

When t h e flow is reduced t o 200 cfm, which is 20% of t h e HEPA f i l t e r standard test flow, t h e apparent penetrat ion (from t h e preceding formula) is approximately 0.45. The 0.45 obtained from t h e 20% flow r a t e is disproport ionately l a r g e r t h a n t h e 1 . 0 obtainedfromthelOO% f lowra te . Refer t o Figure 7. f o r a representa t ion of t h i s phenomenon.

Figure 7

I n c a s e A, a i r i s a p p r o a c h i n g t h e m e d i a a t 5 f ee tpe rminu te (maximum). Fewparticlesmovingatthatvelocity can alter t h e i r path t o reach t h e pinhole, even though t h e dp i n t h e v i c i n i t y of t h e pinhole is less than through t h e media. In t h e second case, however, a i r is approaching t h e media a t 1 foo t per minute (maximum). A t t h i s slow veloci ty , t h e lower pressure drop i n t h e v i c i n i t y of t he pinhole allows

particles to alter their path, and many more of them can flow through the hole. Hence, the higher apparent penetration.

If a HEPA filter exhibits a penetration of 0.01% MORE at 20% flow than at 100% flow, the filter is deemed to have a pinhole leak which would impair its function. Typical acceptable values are as follows:

%P at 100% of total flow = 0.010% %P at 20% of total flow = 0.012%

Unacceptable values at the same flow rates are:

%P at 100% of total flow =.0.010% %P at 20% of total flow = 0.022%.

Temperature

. A nuclear grade HEPA filter is designed to operate continuously at 250°F. The filter manufacturer can also make the filter from components which have a tolerance for higher temperatures.

Additional Testing

MIL F 51068 defines other tests which the filter must pass before inclusion on a government Qualified Products List (QPL). These include over-pressure tests, environmental tests, tests for resistanceto radiationexposure, andmany others. At present, only Edgewood arsenal and Rocky Flats can perform these qualification tests. Only filters designs that have passed these qualification tests can be used in a nuclear power plant. ANSI/ASME N509, in its sectiononHEPAfilters, usesthephrase"otherwisepassing these tests", which means that the HEPA manufacturer has the alternative to perform some parts of the qualification testing. The test results of filter designs claimedto have passed the qualification tests by means other than a recognized national laboratory should be audited carefully to assure compliance with the qualification requirements.

FACTORY EFFICIENCY TESTING OF HEPA FILTERS

Assumingthat0.3 pmisthemostpenetratingparticle s i ze , it is d e s i r a b l e t o t e s t a H E P A f i l t e r w i t h 0.3 p p a r t i c l e s . This is done during t h e factory penetrat ion test.

D i Octyl Phthalate (DOP) is the agent used as a challenge aerosol ,

NOTE: DOPisbeingreplacedastheprimarytestagent. DOP's replacement has not y e t been standardized, but there a r e several equivalent products available.

The aerosol is generated i n t h e following manner ( r e f e r t o Figure 8 ) :

THERMAL DOP PENETROMETER W E T E R TO

MEASURE INLET HE4TER

. . . . - -. . DILUnONUR

SAMPLING

Figure 8

HEPA f i l t e r e d ambient a i r is pushed i n t o a posi t ive pressure plenum upstream of the f i l t e r by a blower. The airstream is then divided i n t o th ree ducts.

The top duct is equipped with tempering devices, so t h a t t h e ambient air canbechangedtohave spec i f ic temperature and r e l a t i v e humidity cha rac te r i s t i c s .

The center duct conveys un-tempered ambient a i r .

T h e , t h i r d duct c a r r i e s a f r ac t ion of t h e air past open containers f i l l e d with heated l i qu id DOP. This airstream e n t r a i n s t h e DOPvaporandtransportsthe vapor to aplenum where t h e a i r ex i t i ng a l l th ree of the ducts merge. The center duct a i r mass, t h e top duct tempered air , and t h e bottom duct DOP vapor laden a i r is mixed i n t h i s plenum.

A "fogm forms under these conditions. The resultant fog is a DOP aerosol. of predominantly 0.3 p. Because most of the particulate in the fog is 0.3 pm, the fog is termed a MONO-DISPERSED aerosol.

This air/aerosol mixture is pushed through the filter to be tested. A penetration instrument measures the concen- tration (C) of the DOP mass upstream (u) of the filter, and downstream (d) of the filter. A penetration value (%P) is calculated from the universal penetration equation:

Efficiency (%E) can then be calculated from the penetration:

DOP AEROSOL MEASURING EQUIPMENT

The DOP aerosol measuring equipment makes use of the Mie theory of light, which simple states that if light is scattered, it is scattered in a forward direction proportional to its concentration.

To measure aerosol concentrations, a device is used as shown in Figure 9.

PERcENl

-4 -- -- --ng -- VACUUM

PUMP

FLOW CHART 1'1 TDA-2D TEST

PmEE RLTW

Figure 9

The DOP ae roso l de t ec to r funct ions as follows:

A l i g h t source genera tes l i g h t rays. These rays, which a r e generated i n a forward d i r ec t i on , are made p a r a l l e l by a lens. A second l e n s focuses t h e l i g h t rays t o a point . The second l e n s has its cen t e r blacked ou t by lamp black, by a p iece of black fe l t , o r by some o t h e r means t o block l i g h t from passing. The r e s u l t is a cone of darkness surrounded by a l i g h t e d sheathe. The cone of darkness is focused t o t h e same poin t as t h e l i gh t ed sheathe, and s imi l a r ly diverges. The t h i r d l e n s focuses only t h e dark cone onto t h e photo m u l t i p l i e r tube. This p a r t of t h e device is c a l l e d a scattering chamber. The s c a t t e r i n g chamber has t h e same general shape as t h e l i g h t and dark cones i n s i d e of it.

During opera t ion of t h e de tec to r , an a i r sample is drawn i n t o t h e s c a t t e r i n g chamber. The sample passes th roughthe pointwhere t h e d a r k cone, su r roundedbya l igh ted sheathe, converge. P a r t i c l e s i n t h e a i r sample s c a t t e r t h e l i g h t

, i n t o t h e divergent dark cone. Since t h e dark cone is focused on t h e photo m u l t i p l i e r tube, t h e tube then senses s ca t t e r ed l i g h t . The l i g h t it senses is proport ional t o t h e concentrat ion of p a r t i c l e s i n t h e aerosol . Under t e s t condit ions, opera t ion of t h e aerosol detec tor is a s follows:

1. A i r is continuously drawn through a s m a l l i n t e r n a l HEPA f i l t e r , which c leans it f o r use i n t h e de tec to r . I t then passes through t h e s c a t t e r i n g chamber a t a r a t e of 1 cubic foot pe r minute (cfm).

2. Using t h i s c lean air, any s t r a y l i g h t s igna l s , o r e l e c t r o n i c signals, are damped out by e l e c t r o n i c means. The de t ec to r is now reading zero penet ra t ion .

3. The photo m u l t i p l i e r tube is then exposed t o an i n t e r n a l , c a l i b r a t ed , secondary l i g h t source. The d e t e c t o r is i n s t r u c t e d t o def ine t h i s l i g h t source as 100% penet ra t ion .

4. DoP aerosol is then injected i n t o the t e s t volume. Concentration is measuredby movingthe detector s sampling valve so t h a t t he upstream sample is drawn through the sca t t e r ing chamber. The control panel of a typ ica l detector is shown i n Figure 10.

F i g u r e 10

Theconcentrationof i n j e c t e d D O ~ i s a d j u s t e d s o t h a t t he detection device reads a s close t o w l O O % M a s prac t ica l . That is, the photo mul t ip l ie r tube is sensing a l l o f t h e l ightwhich it can detect . The DOP aerosol density, under these conditions, is 100 micrograms per l i ter . Small adjustments can be made on the detector t o ensure t h a t the detector is reading a 100% concentration. The l i g h t from t h e ca l ibra t ion source is equal t o t h e l i g h t density t h a t t he tube is sensing (from the l i g h t scat tered by t h e upstreamDOPparticles). It shouldbenotedtha t a successful penetration t e s t can be run with almost any upstream concentration of aerosol. The concentration need not be 100 pg/l.

5 . In the next s tep, the sampling valve is moved t o the downstream posit ion. A t t h i s posit ion, t h e downstream concentration is being measured. An electronic at tenuator is moved t o successively more sens i t ive posit ions, untilameasurementof the downstream concentration can be made.

Knowing the upstream and downstream DOP concen- tration values, the penetration through the filter can be calculated from the standard penetration equation.

These tests and the associated test equipment are fully described inMilitary Standard 282. The Standard contains complete specifications for the equipment, including drawings and operating instructions, as well as test acceptance criteria.

FIELD TESTING

The objective of field testing is different than that of factory testing. The objective of field testing is to assure that the components (which have passed all factory tests) have not been damaged since leaving the factory, have been installed properly, and that the system into which it has been placed has the necessary design and manufacture integrity.

A field test can detect flaws in the filters, incorrectly installed filters, and inadequacies in filter design. The amount of downstream penetration of an upstream challenge aerosol will determine if flaws exist in the system which will prevent it from performing its design function.

Although the objectives of the field test are different than those of the manufacturer's test, the method is similar. In fact, the DOP detection equipment is essentially the same. The DOP generating equipment is different; however, apd the generated aerosol has different size characteristics. Instead of a homogeneous aerosol of 0.3 , the DOP generator will produce a POLY-DISPERSED aerosol with a size range as follows:

99+% of the particles will be less than 3.0 pm 50+% of the particles will be less than 0.7 pm lo+% of the particles will be less than 0.4 p.

This aerosol can be produced by severa l methods. The most common method is shown i n Figure 11.

. .. . AND .... -

VALVE OOP HEATER

RESERVOlR B L C C K

Figure 11

An i n e r t gas p ressur izes a rese rvo i r of l i q u i d DOP. The l i q u i d DOP is metered through a valve which can ad jus t t h e output of t h e generator. The metered DOP is then forced through an o r i f i c e by t h e i n e r t gas pressure, which conver t s the DOP l i q u i d i n t o a spray. The spray continues i n t o a heated chamber, and t h e heat llcracksv t h e spray . i n t o an aerosol with t h e s i z e d i s t r i b u t i o n previously noted.

Another method of generat ing t h e r e q u i s i t e aerosol is by using a Laskin nozzle. This method was developed by Echols and Young a t t h e US Navy Laboratory. The method uses compressed a i r t o 'generate t h e aerosol . I n the diagrams below (Figures 12A and 12B), t h e Laskin nozzle is submerged i n t o l i q u i d DOP. A i r pressure is appl ied through t h e pipe. A i r e x i t s t h e pipe from t h e four holes d r i l l e d i n t o t h e pipe. These four holes are d i r e c t l y above four s imi l a r holes d r i l l e d i n t o t h e c o l l a r , as n o t e d i n thediagram. LiquidDOPispul ledup fromthe r e se rvo i r through t h e holes i n t h e c o l l a r by Bernoulli I s Pr inciple . The a i r e x i t i n g t h e pipe shears off t h e DOP l i q u i d i n t o an aerosol with t h e proper s i z e cha rac t e r i s t i c s .

Cold DOP Aerosol Generator

Figure 1 2 A Ll

Figure 12B

A survey of t h e referenced l i t e r a t u r e w i l l show severa l more methodsof aerosol generation, but these two a r e themost common.

CARElON ADSORPTION SYSTEMS

INTRODUCTION

~dsorption is the process by which activated carbon can control many gas and vapor phase airborne contaminants. The adsorption process will be discussed in detail in a later section.

Compound is the term most commonly used to describe an airborne contaminant. Unlessthetextreferstoaspecific gas, or a specific vapor, the term compound will be used to apply to gaseous airborne contamination.

~t is impractical to describe the adsorption characteris- tics of every airborne compound. This discussion will focus on two of the most stringent applications. They are :

(1) Radioactive Iodides, which could become air- borne nuclear contaminants, and

( 2 ) Chemical Agent Contaminants.

These two are chosen because they could easily impact the health and safety of the public. The text will focus on the nuclear application, but the agent application requires essentially the same high standards of design and manufacture. Many other uses of adsorption material are not discussed, such as odor control and liquid filtration. However, the reader can apply the data herein to a specific application of this type.

Activated carbon is an excellent adsorber. It has been called the "universal adsorbent". This discussion will focus on activated carbon only. Activated carbon has been used for many years to remove unwanted airborne material. Doughboys in World War I, for example, used gas masks that contained nut-shell activated carbon to protect them from poisonous gasses.

With the advent of commercial nuclear power, the NRC was forced to address the health effects of radioactive materials potentially released from nuclear power plants. They concluded that radioactive methyl iodide (MEI, which

is was the most v o l a t i l e of the gases o r vapors t h a t would threaten human health. M E 1 is a body seeker, t akenupby the thyro id gland. I t e m i t s ion iz ingradia t ion which a l t e r s body c e l l s t ruc ture , making cancer of the thyroid more l ike ly . I f the most v o l a t i l e ME1 is controlled, therefore, t he l e s s v o l a t i l e species of organic iodide w i l l a l so be controlled, minimizing the t h r e a t t o the thyroid.

There are , of course, o ther gas and vapor considerations a t a nuclear powerplant. Noble off-gas in aBoilingWater Reactor (BWR) is one example. Precautionary f i l t e r s , such a s those used t o supply a i r t o the plant control room, a r e another example. T h i s t e x t w i l l d i s c u s s M E ~ removalonly, i n a nuclear exhaust f i l t e r application.

The subject of adsorption w i l l be developed i n a manner s imi la r t o t h a t of HEPA f i l t e r s . F i r s t , an adsorber's components and t h e i r func t ionswi l l be described. Second, the assembly of the components i n t o an adsorber un i t w i l l be discussed. Third, factory t e s t i n g o f t h e adsorber un i t w i l l be developed. The f i n a l section w i l l cover t e s t i n g of the i n s t a l l e d adsorbers i n an a i r f i l t r a t i o n system. A s o ther f ac to r s impact the carbon adsorber, these f ac to r s w i l l be discussed.

CARBON ADSORBER COMPO#E~S

A carbon adsorber has the following components:

( 1 ) F i l t e r media ( 2 ) Perforated re ta in ing metal ( 3 ) Frame ( 4 ) Interface between the perforated material and the frame ( 5 ) Gasket ( 6 ) Label

These components a r e described below.

(1) FILTER MEDIA:

The f i l t e r media i n a carbon 'adsorber is activated carbon. The process of adsorption is as follows:

Gas molecules, obeying basic laws of chemistry, readi ly dif fuse i n t o a given volume and a r e e l ec t ros t a t i ca l ly a t t rac ted t o any surface within t h a t volume. Once a gas molecule comes near enough t o a surface t h a t t he molecule and the surface can share e lectrons , the molecule is adsorbed. The sharingofelectronsbetweenthesurface and the compound is ca l led an interface. A material with a large surface and an abundance of e lectrons is desirable f o r adsorbing. Activated carbon has these two character- . i s t ics . I t has a very la rge surface area per un i t volume, and it has an abundance of electrons.

Activated carbon can be produced from any carbonaceous material. To produce act ivated carbon, the material is heatedtoahightemperatureintheabsenceofoxygen, using a device cal led a r e t o r t . Under these conditions, many v o l a t i l e compounds i n the material are driven from the carbon substrate. The amount of v o l a t i l e material driven out of the carbon substra te increases as a function of time spent i n the r e t o r t .

Figure 13 is an a r t i s t ' s concept of a port ion of a carbon granule a f t e r act ivat ion. Note t h e d i f fe ren t ia t ion between a micropore and a macropore. A micropore is a port (cavi ty) with an in t e rna l radius of l e s s than 100 A . The surface pore s i z e i n t h i s i l l u s t r a t i o n is about 0.02 p.

CONCEPT OF MOLECULAR SCREENING IN MICROPORES

Figure 13

It is i n t e r e s t i n g t o note t h a t once t h e v o l a t i l e mater ia l begins t o leave t h e carbon subs t r a t e du r ing the carbonizing process, it is routed i n t o t h e furnace of t h e r e t o r t , f o r use a s f u e l t o d r ive o ther v o l a t i l e substances from t h e carbon. Once t h e ac t i va t i on process is begun, it can sus t a in i t s e l f . This type of process is c a l l e d an exothermic react ion.

Figure 14 is a diagram of a t y p i c a l v e r t i c a l s i n g l e t h r o a t r e t o r t . Other types of r e t o r t s are used. A l l of them involve high temperatures, beginning t h e carbonizing cycle a t about 500°C. A t t h e e n d o f t h e ac t i va t i onp roces s , a t t h e bottom of t h e r e t o r t , t h e temperature can reach as much as 1 0 0 0 ° C . The e n t i r e process t akes p lace i n t h e absence of oxygen.

RAW MATERIAL IN

GAS PATH

Figure 1 4

Inthef igureshown, there i sanindicat ionof theact ivated carbon being exposed t o steam. Water (H20), i n t h e form of steam, r e a c t s with t h e hot carbon t o enhance t h e micropore s t r u c t u r e of t h e ac t iva ted carbon. This process was-invented i n Germany around t h e t u r n of t h e century.

NUCLEAR CARBON

Activated carbon f o r nuclear app l ica t ions is pr imar i ly made u s i n g c o c o n u t s h e l l a s t h e r a w carbonaceousmaterial . Coconut s h e l l carbon is very hard, and can be used e f f ec t i ve ly i n an a i r f i l t e r system s ince it does not f r ac tu re ea s i l y . Coconut s h e l l carbon can be ac t iva ted t o a high state. She l l carbon resists coking, which make t h e granule "puff up".

A mental image of ac t i va t ed carbon's s t ruc tu re is a s follows:

Imagine d r iv ing along a road c u t through a mountain. The mountain's c u t revea l s many l aye r s of rock, in terspersed with l aye r s of s o i l . Trees and g ra s s grow i n t h e s o i l layers . Now imagine t h e same p i c tu re , with everything removed except t h e rock s t r a t a . Assuming t h a t t h e rocks remain i n t h e same pos i t i ons as before t h e removal of t h e non-rock mater ia l , a l l t h a t is l e f t is a multitude of l aye r s of rock. These l a y e r s make caverns, ledges, and o ther penet ra t ions i n t o t h e mountain. From a s i n g l e surface, r i s i n g v e r t i c a l l y f romthe s i d e o f the . road , t he re is now a mult i tude of su r faces , beginning a t t h e road and penet ra t ing i n t o t h e mountain.

This example is a k i n t o a c t i v a t i n g a g r a n u l e o f carbon. The surface a r ea has been g r e a t l y increased. I n f a c t , t h e surface area of a s i n g l e gram of 60% ac t iva ted carbon can be measured t o be more than 1100 square meters! The openings c r e a t e d b y t h e a c t i v a t i o n process a r e about t h r e e ni trogen atoms i n dimension.

A reference w a s made i n , t h e preceding paragraph t o "60% ac t iva ted carbon". Whatexactlyismeantby"60% ac t iva ted carbon"? The American Society f o r Test ing and Materials (ASTM) has devised a series of physical p roper t i es tests t o which ac t i va t ed carbon can be subjected. These tests a r e referenced i n ASME AG-1, which r e f l e c t s work done by t h e Committee on Nuclear A i r and G a s Treatment (CONAGT) under t h e ASME auspices. They are t h e ba t t e ry of tests whichmust be s a t i s f a c t o r i l y completedto qua l i fy a carbon f o r use i n t h e nuclear industry. One of these tests is

"activation per t h e carbon te t rachlor ide (CC14) t es t " , CC14, o r carbon t e t , may be recognizable t o many, a s it is the odor associated with dry cleaning establishments. The carbontet tes t i sperformedina sophisticatedmanner, but our discussion can simplify it i n t o a simple " s m e l l " t e s t by u t i l i z i n g the following method:

NO!J!E: I f one does t h i s t e s t f o r any reason, take care t o sn i f f t h e beaker i n a laboratory-safe manner. Be sure t o read t h e carbon t e t Material SafetyDataSheet (MSDS), s inceca rbon te t i snow considered a carcinogen. Butane is i n the process of replacing carbon t e t .

One hundred (100) grams of an act ivated carbon a r e put i n t o a beaker. Ten ( 10 ) grams of CC14 a r e added and the beaker is sniffed.

No smell should come from the beaker a t t h i s point , s ince the carbon has adsorbed a l l of t h e CC14 vapors. After adding 10 more grams of carbon t e t , the beaker is sniffed again and the process'repeated. Assume t h a t a f t e r adding a t o t a l of 60 grams of carbon t e t , no s m e l l i s detected. I f , a f t e r adding one addit ional gram of CC14, t he CC14 odor can be detected, t he carbon has a 60% ac t iv i ty . That is, it can hold 60% of its weight i n CC14 before becoming "saturated". Sa tura tedmeans tha t the e x i t concentration equals t h e i n l e t concentration. Similarly, a 20% ac t ive carbon w i l l hold 20% CC14 by weight before saturat ion, and so on. The laboratory t e s t i s found i n ASTM D 3467.

Carbon activated t o t h e 60% ac t iva t ion value must remain i n the r e t o r t longer, and is heated t o a higher temperature than 40% ac t ive carbon, Therefore, 60% act ivated carbon is more expensive t o manufacture than 40% ac t ive carbon.

Nuclear grade act ivated carbon must have 60% min imi ac t iv i ty . A 60% ac t ive carbon w i l l have an approximate 50% retent ion of MEI, The NRC requires t h a t new activated carbon have a 97% re ten t ion of MEI. In order t o resolve t h i s apparent contradict ion between what is possible, and what is required by the NRC, it is necessary t o understand the adsorption mechanism.

Adsorption is the process whereby a gas molecule i s a t t r ac t ed t o a surface. Sawdust, chicken feathers, egg she l l s , o r any other material has surface. But, a s has already been discussed, act ivated carbon has a very large surface per un i t volume. Avolume of activatedcarboncan, therefore, hold an almost uncountable number of gas molecules.

TYPES OF ADSORPTION

Physical Adsorption

Gas molecules a r e a t t r a c t e d t o t h e surface of the carbon. These inner surfaces a r e cor rec t ly termed the micropore s t r u c t u r e of the carbon. The phenomenon of diffusion is what a t t r a c t s the gas molecules t o these inner pores. Once they d i f fuse i n t o these inner pores, they come i n close contact with the carbon surface. This close contact is ca l l ed the in te r face . A t t h e in te r face , theybegin to share electrons. The molecule is thus physically adsorbed.

Chemical Adsorption

Sometimes a gas molecule, a compound, o r a vapor has an a f f i n i t y t o combine with another gas molecule, compound, o r v a p o r t o formanew compound. Thisphenomenonisnothing more than obeying the laws of valance chemistry. For example, sulphur and hydrogen spontaneously combine t o form H 2 S . Oxygen and hydrogen form H 2 0 . I f a compound is pre-adsorbed onto t h e carbon, and i f t h i s compound has a high a f f i n i t y f o r t h e compound t h a t is targeted f o r removal, then the t a r g e t compound can be removed by combining. with the pre-adsorbed compound.

This type of removal is ca l led chemical adsorption, o r chemisorption. If a compound which has an a f f i n i t y f o r the targeted compound is pre-adsorbed on ac t ive carbon, t h i s compound pre-adsorption is termed impregnation.

physical adsorption and chemisorption a c t simultaneously. To achieve higher re ten t ion of a targeted compound than physical adsorption alone can achieve, it is necessary t o

impregnate (pre-adsorb) t h e a c t i v e carbonwithachemical t h a t has an a f f i n i t y f o r t h e t a r g e t compound. For example, carbon impregnatedwith s u l p h u r w i l l control mercury vapor. The pre- adsorbed sulphur w i l l r e a c t with t h e mercury vapors i n t h e airstream, and combine t o form mercuric su l f ide . Many such combinations e x i s t .

Carbon impregnated with T r i Ethylene D i Amine (TEDA) w i l l combine with ME1 t o form a new compound. The TEDA is f ixed on t h e carbon, and t h e r e s u l t a n t compound adheres t o t h e carbon a lso . The reac t ion of one compound with a second compound t o form a new, t h i r d compound is ca l l ed complexing. The reac t ion f o r t h e formation of t h i s s p e c i f i c compound is as follows:

I n add i t ion t o TEDA, another compound which can help con t ro l ME1 is KI3. This is a stable iodine i n combination with potassium. It is i n t h e formof K I + 12 ,where the I ~ i s l o o s e l y bound t o t h e K I .

When an ME1 compound comes i n t o c lo se proximity t o t h e KI3, t h e radioact ive iodine on t h e ME1 and t h e stable iodine on t h e K13 spontaneously, i so top i ca l l y interchange, A t t h i s po in t , t h e airborne, v o l a t i l e ME1 has a stable iodine molecule, and theimpregnatedK13hasaradioactiveiodinemolecule a s a p a r t of it. This is exact ly t h e des i red end r e s u l t . The reac t ion is as follows:

The reac t ion does not s t op a t t h e f i r s t react ion. I n t h e next ins tance , t h e radioact ive iodine, previously interchanged, may undergo another interchange. That is, t h e a i rborne stable iodine i n t h e ME1 may interchange with a radioact ive iodine previously adsorbed on t h e carbon, making t h e a i rborne ME1 radioact ive again! This cycle, with a given radioact ive iodine, may take place mi l l ions of t i m e s . Each interchange requ i res a d i s c r e t e amount of time. I f enough time is allowed fortheinterchanges,theiodinewillspontaneouslydecayinto stable xenon. Since t h e ha l f - l i f e of 1311 is about 8.5 days, t h e delay time required is ,about 64 days.

The ha l f - l i fe of 1311 is about 8.5 days. After eight half- l ives, methyl iodine w i l l decay t o a leve l which is below background. Therefore, if there is enough carbon i n an a i r cleaning u n i t t o delay the radioactive iodine f o r asufficientperiodoftime, t h e radioactiveiodinedecays. This con t ro l s the amount of radioactivematerialreleased.

Activated carbon has an eff ic iency of about 50% f o r MEI. Impregnated carbon w i l l have an eff ic iency of a t l e a s t 97% f o r ME1 . To summarize t h i s f i r s t sect ion on adsorption:

1. Gas molecules a r e a t t r a c t e d t o any surface. I f the surface has an excess of electrons, t he molecule shares some of those e lectrons a t the interface.

2. Carbonaceous material is act ivated t o increase its surface area.

3. Activated carbon has both excess electrons and a large surface area.

4. There a r e two types of adsorption which a re of in t e res t :

a ) Physical adsorption b ) Chemical adsorption

5. Chemicals can be pre-adsorbed (impregnated) onto activated carbon t o enhance its a b i l i t y t o control a t a rge t compound.

6. When K13 is t h e impregnated compound, ME1 is controlled by i so topic interchange. In the case of TEDA impregnate, M E 1 i s controlled by complexing t h e ME1 i n t o the TEDA molecular s t ructure .

Carbon Particle S i z e

Before explaining t h e methods of adsorpt ion i n t h e previous sec t ion , ASTM tests f o r nuclear carbon w e r e being discussed. To r e tu rn t o t h i s sub jec t , another of t h e required ASTM tests w i l l be introduced. This test is t h e s i z e range test f o r carbon granules. This ASTM test is referenced i n AG-1.

Carbon s i z e is designated by s i eve s i ze . Nuclear grade carbon i s 8 x16 USmesh, a n d t h e al lowablerange for8x16USmeshcarbon is a s follows:

Retained on a #6 screen 0.1% maximum Retained on a 88 screen 5.0% maximum Through a 88, on #12 60.0% maximum Through a #12, on #16 40.0% minimum Through a #16 5.0% maximum Through a #18 1.0% maximum

Itisimportanttonotethatgranuledistributionina s i z e range can vary s ign i f i can t ly . This va r i a t i on w i l l have a not iceable e f f e c t on pressure drop on otherwise i d e n t i c a l adsorbers, I f . t h e granules tend toward t h e smaller s i z e s i n a range, t h e dp f o r ag iven adso rbe rwi l l beh ighe r . I f thegranu les tend toward t h e l a r g e r s i z e s i n t h e range, t h e dp across an adsorber w i l l be lower.

The s i z e d i s t r i b u t i o n of carbon granules w i l l a l s o have an a f f e c t on contaminant removal ef f ic iency. Under design opera t ing condit ions, however, s i z e d i s t r i b u t i o n variances a r e not s i g n i f i c a n t -

There is no reason r e l a t e d t o adsorption f o r mandating t h a t nuclear carbon must be 8 x 1 6 mesh- This s i z e is t h e nuclear s tandard because m e t a l pe r fo ra to rs , i n t h e 19601s, could not cons i s ten t ly pe r fo ra t e 24 gage s t a i n l e s s steel with less than 0.045 inch diameter holes. Perforated s t a i n l e s s s t e e l is used t o contain carbon i n an adsorber bed. A 0.045 inch hole w i l l r e t a i n a 16 mesh carbon. So, i n i t i a l spec i f i ca t i ons invoked' only what w a s poss ib le t o ob ta in a t t h e time. Early research i n t h e r e t en t ion of ME1 showed t h a t a 6 x 16 mesh carbon performs as s a t i s f a c t o r i l y a s an 8 x 1 6 mesh. I n f a c t , some of t h e e a r l y spec i f i ca t i ons required a 6 x 16 mesh carbon. But t h e 8 mesh, as an upper l i m i t i n s i z e , is more conservative and eventual ly became t h e standard. The ASTM reference f o r p a r t i c l e s i z e is ASTM 2862.

MAXIMUM APPARENT DEWSITP (MAD) Per ASTM D2854:

Another important physical property of carbon is its density. A carbon bed packed t o its Maximum Apparent Density (MAD) w i l l not s e t t l e , Se t t l i ng is undesirable, s ince it can c rea te v o i d s i n t h e carbonbed, producing leak paths,

The ASTM D2854 procedure f o r apparent density t e s t i n g has the following elements:

1. A sample of carbon t o be t e s t e d is placed i n a shallow reservoir with an open end. The reservoir is t i l t e d downward very s l i g h t l y toward t h e open end,

2. The reservoir is vibrated.

3. Carbon slowly migrates toward the open end, where it t r i c k l e s out and f a l l s i n t o a graduated cylinder. Each i r regular ly shaped granule must have time t o reach its natural r e s t ing place i n t h e cylinder. I f the carbon granules a re not al lowedto reach t h e i r natural res t ing place, some granules may "bridge" between two other granules, and the density w i l l not be a t its maximum.

4. A specif ied amount of carbon is thus t r i ck led i n t o the cylinder.

5. The sampleisweighed, andthemaximumapparent density is calculated.

Maximum apparent density is an important physical charac te r i s t ic and the concept w i l l be used during the discussion on f i l l i n g and t e s t ing .

IGNITION TEMPERATURE (Per ASTM D 3466)

Ignit ion is the temperature a t which activated, impreg- nated carbon w i l l combust spontaneously. The ASTM D3466 requirement is t h a t t he minimum temperature be 330°C. Activated carbon routinely m e e t s t h a t requirement.

New ac t iva ted carbon does not r ead i ly burn, Sustained i g n i t i o n almost always requ i res a secondary heat source. If a mass of new carbon is ign i t ed by a heat source, such as a gas torch, t h e i g n i t i o n w i l l be sustained a s long a s t h e heat source is present . When t h e heat source is removed, t h e carbon w i l l usual ly self-ext inguish.

Used ac t iva ted carbon w i l l burn read i ly , due t o flammable mater ia l which has been adsorbed on it. Upon ign i t i on , used carbonwillcontinuetoburndespitemosteffortsto extinguish it. Water sprays w i l l no t r ead i ly ext inguish burning carbon. The "burn" is not a flame, but r a t h e r glowing embers. The f u e l f o r t h e burn is i n t h e carbon's pore s t ruc tu re , and sens ib le water does not r ead i ly pene t ra te i n t o t h e pore s t ruc tu re .

Carbon f i r e s i n NATS are control led by a very few means. The most common is t o i s o l a t e t h e NATS by c los ing t h e i n l e t and o u t l e t dampers, and once t h e carbon uses t h e ava i lab le oxygen, t h e f i r e d i e s ou t , There have been very few carbon f i r e s i n NATS from which t o gain experience i n extinguishing them.

MEDIA CERTIFICATIONS

According t o US NRC Reg Guide 1.52, a batch of carbon is a m a s s of carbon with a volume'not t o exceed 350 f t 3 . This equates t o a weight of no more than about 10,000 pounds.

A l o t o f ca rbon i soneo rmoreba t ches o f t h e same typeo f carbon which has been manufactured under t h e same conditions.

Each.batch of nuclear carbon has t h e following c e r t i f i c a t i o n s :

1. A physical test repor ta showing t h a t t h e carbon media m e e t t h e ASTM physica l p rope r t i e s requirements.

2. A labora tory test r epo r t showing penet ra t ion of radio- a c t i v e ME1 and rad ioac t ive elemental Iodine.

3. A copy of t h e q u a l i f i c a t i o n test r e s u l t s p@rformed on t h e carbon t o i n i t i a l l y qua l i fy it as a nuclear carbon ( t h e manufacturer 's Q u a l i t y Assurance Department normally keeps t h i s on f i l e ) .

FRAME, PERFORATED MATERIAL, AND FIRE PROTECTION

Nuclear adsorber frames a r e made of s t a i n l e s s s t e e l , usual ly 14 gauge, type 300 series s t a i n l e s s s t e e l (ss), and t h e perforated screens are 24 gage with 0 -45 inch diameter holes, 225 holes pe r square inch, 37% open. This h ighly corros ive-res is tant m e t a l is chosen s ince t h e m e t a l salts impregnated on t h e carbon, and any mater ia l previously adsorbed on t h e carbon, w i l l convert i n t o s t rong ac ids i f wetted. Even when b u i l t of s t a i n l e s s steel, i f t h e carbon becomes w e t , it is imperative t o remove t h e carbon from t h e metal conta iners as soon as possible. Af ter wetting, t h e s t a i n l e s s steel conta iners may become s o cor roded tha t f u r t h e r use i s impract ical . For example, i f even two adjacent perfora t ion margins are destroyed, carbon w i l l l eak from t h e container , and cause a i r bypass. If t h e wett ing is discovered i n time, t h e carbon can be removed from t h e bed and t h e bed screens and frames r insed with water. Af te r drying, new carbon can be loaded and t h e required tests performed t o assure t h a t t h e system is operable.

In s p i t e of every e f f o r t t o prevent it, carbon can unintent ional ly ge t w e t from any of t h e following sources:

1. Airborne sens ib le w a t e r d rop l e t s

2. Condensationof hot , high r e l a t i v e h u m i d i t y a i r o n c o o l e r carbon

3. Agglomeration of water molecules i n t o water d rop l e t s under t h e r i g h t environmental condi t ions

4. Inadvertent ac t i va t i on of t h e w a t e r deluge system, which is designed t o ext inguish a carbon fire. (This is t h e most l i k e l y source) .

5. Leaks from valves which connect t h e deluge system t o t h e NATS.

Experiments have shown t h a t a water deluge system w i l l not ext inguish a carbon f i r e . This f a c t is recognized by t h e NRC, and they do not requ i re a w a t e r source as a means t o ext inguish f i r e i n a carbon bed. Many nuclear insur ing companies do, however, r equ i re a water de.luge system f o r heat removal.

General Construction

The general construction d e t a i l s of a carbon adsorber a r e a s follows:

Aframesupportsaseriesofperforatedmetalplates, which are positioned so t h a t they a r e a constant distance apar t . For nuclear use, t h e bed depth must be a t l e a s t 211. They a r e designed so t h a t each p a i r of p l a t e s w i l l contain the carbon, with an a i r space between t h e carbon containing p la tes . Each p a i r has a perimeter of un-perforated m a t e r i a l , t o allow a s l i g h t margin of e r r o r f o r s e t t l i n g of t h e carbon media a f t e r f i l l i n g , Air is forced from one s ide of t h e carbon bed through t o e x i t t he other s ide. The contaminated a i r is cleaned by t h e adsorption mechanisms previously described.

It is considered good design t o have as many of these beds i n a f i l t e r as possible, which w i l l r e s u l t i n a minimum pressure drop.

Adsorber wpes

A t present , t he re a r e four d i s t i n c t types of adsorbers, designated as Types I, 11, 111, and IV. Figure 15 shows t h e design concepts of Types I, 11, and 111.

Type I is characterized by a s ing le serpentine bed, as shown. Type I f i l t e r s are very r a r e l y used i n t h e modern a i r cleaning industry.

STIFFENERS

URBW BED

FTRFORATED Y E I U SCREE*

MCNPERFOLlAlED MARGIN OF SCREEN Y P A ~ A T E PLEATS TO RETARD SETTLING

WONPERF ORATED END CAP . [BAFFLE) F L w G E TO SEU A G A W NEOPRENE PAD SPOnGE NEOPRENE Y U P a TO Paeveur srpurwc OF AIR

CASING

CUING STIFFENERS

PCUtE NEOPRENE FACE GASKET EXTERUAL TO

PERFORATED SCREEN \ MARGM OR BAFFLE

PERFORATED SCREEN

LOWER CARBON BED FACE PLATE ' ,,

T b R B ( k BED

Upstream HEPA filters

Flow

Downstream HEPA filters

Figure 15. D E S I G N CONCEPTS OF TYPES I , 11, AND I11 ADSORBERS

Type I1 f i l t e r s , a l s o c a l l e d "drawer type" adsorbers, a r e character ized by two carbon beds i n p a r a l l e l . Each Type I1 adsorbe rce l l i s ra t eda t333 cfm. ThreeType I1 adsorber c e l l s would then be r a t ed a t 1000 cfm, t h e same a s a s i n g l e HEPA f i l t e r . Three Type I1 adsorbers have roughly t h e same face a rea a s a s i n g l e HEPA f i l t e r . Combinations of a s i n g l e HEPAandthree Type I1 adsorbers a r r a n g e d i n s e r i e s produce a cons is ten t geometry.

A Type I11 f i l t e r is charac te r ized by p a r a l l e l beds of a given height and depth and is b u i l t i n t o t h e a i r f i l t e r systemas a n i n t e g r a l p a r t . None o f theha rdware is removed

from t h e system during maintenance, a s opposed t o t h e Type I1 systems, where t h e adsorber c e l l s a r e removed f o r r e f i l l i n g o r sampling. I n t h e Type I11 system, only the carbon is removed. This i s done by a remote means, usual ly involving a vacuum system.

A Type I V f i l t e r is a s e r i e s of V-shaped beds, arranged i n amanner s i m i l a r t o t h e Type I f i l t e r . Figure 1 6 depic t s a Type I V f i l t e r .

F i g u r e 16 - TYPE I V ADSORBER

The i n t e r f a c e between t h e perforated mater ia l and the frame, i n Type I and Type I V f i l t e r s , i s a gasket sea l ing pad, a p o t t i n g of glue, o r another acceptable sea l ing method.

The s e a l funct ion is t o assure t h a t t h e carbon i n t h e bed w i l l not l eak out of t h e bed during t h e l i f e of t h e f i l t e r , and t h a t a i r w i l l not by-pass t h e carbon. Gaskets a r e used on Types I , 11, and I V f i l t e r s a s t h e i n t e r f a c e between the frame and t h e supporting g r i d (framing system) i n t h e a i r f i l t e r system. A Type I11 f i l t e r needs no gasket, s ince a l l its, j o i n t s a r e e i t h e r welded, o r a r e sealed by a r e se rvo i r of carbon a t t h e t o p of it. Good design p rac t i ce f o r t h e Type I11 is t o have a r e se rvo i r depth of two t i m e s t hebeddep th , because it i s d i f f i c u l t t o achieveMADduring t h e media f i l l i n g process. See Figure 1 7 f o r a graphic i l l u s t r a t i o n of t h e r e se rvo i r .

Figure 17. TYPE I11 ADSORBER ILLUSTRATING THE CARBON RESERVOIR

Each of these designs has a method of f i l l i n g t h e adsorber with media and "capping" off t h e f i l l po r t s , making an a i r t i g h t s e a l . When t h e carbon becomes sa tu ra t ed , t h e o ld carbon can be removed, and new carbon i n s t a l l e d . Af te r t e s t i n g , t h e new carbon i n t h e o ld frame i s a s good as i f a l l new components were used.

Adsorber Label

Each adsorber has a l a b e l a f f ixed t o it which shows t h a t t h e f i l t e r complies wi th t h e i n s t r u c t i o n s i n t h e con t rac t , and with appl icable standards.

CARBON FILLING

Design of an a i r f i l t e r system includes determination of t h e adsorber type.

The frames and beds a r e manufactured according t o a customer approved Q u a l i t y Assurance (QA) program. The carbon media is put i n t o t h e f i l t e r s i n a previously qua l i f i ed manner. The goal is t o achieve MAD with t h e carbon. Given t h e tendency f o r bed screens t o s t r e t c h , " o i l can" ( o i l canning is t h e spontaneous r e l i e f of s t r e s s i n a p i e c e o f m e t a l ) , orbewavy, t h e precisevolume cannot be ca lcu la ted . However, anapproximat ionof the volume can be derived. With t h e known bed volume, and t h e known MAD of t h e carbon which i s going i n t o it, a weight of carbon can be determined which is used a s a guide f o r f i l l i n g t h e bed so t h a t no s e t t l i n g w i l l occur during system use.

The process of f i l l i n g an adsorber bed is a compromise i n technology. I f t h e adsorber is t r i c k l e - f i l l e d alone, t h e method w i l l not make every screen imperfection change t o maximize t h e volume. A m a x i m u m volume i s one which has a l l screen indent ions away from t h e carbon bed. I f t h e indentions a r e toward t h e bed, and they pop out , o r "o i l can" due t o some small stress a t a l a t e r time, t h e volume of t h e bed w i l l change a t t h e onset of t h e "o i l can" e f f e c t . With t h i s new, l a r g e r volume, and a f ixed amount of carbon, t h e carbon bed w i l l se t t le below t h e baf f led perimeter, causing a i r bypass.

So, i n a d d i t i o n t o t h e t r ick lemethod, which is nothingmore than allowing each carbon granule t o f ind i ts lowest r e s t i n g p lace , t h e e n t i r e adsorber is "rough handled t o make sure t h a t t h e r e a r e no screen a r e a s which could "o i l can" l a t e r . Rough handling c o n s i s t s of subject ing the adsorber t o a drop of 3 inch a t a r a t e of 200 times per minute. Rough handling ensures t h a t a l l screens a r e "popped out" t o assure maximum volume.

After f i l l i n g , rough handling, and capping off t o f i l l t h e beds t o a maximum, a cover is secured over t h e f i l l i n g por t s . Once f i l l e d , with t h e gasket i n s t a l l e d , t h e adsorber i s ready f o r f ac to ry t e s t i n g .

TESTING OF ADSORBERS

Factory t e s t i n g of f i l l e d adsorbers is s imi la r t o fac tory t e s t i n g of HEPA f i l t e r s . The ob jec t ive is t o measure t h e upstream and downstream concentra t ions , and t o use t h e formula %P = Cd/CU x 100 t o c a l c u l a t e t h e penetra t ion.

The challenge agent has h i s t o r i c a l l y been a Freon 11 (R- 11) o r Freon 12 ( R - 1 2 ) vapor. These vapors a r e used f o r t h e following reasons:

(1) The vapor of t h e s e compounds is read i ly delayed by carbon.

( 2 ) Because of t h e i r v o l a t i l i t y , they a r e not f i rmly adsorbed. I n time, a l l of t h e R-11 o r R-12 moves

through t h e carbon bed and e x i t s it, so t h e bed i s not "poisoned" by t h e R-11 .

( 3 ) R-11 and R-12 a r e , a t t h i s t i m e , r ead i ly ava i l ab le , commercially obtained compounds.

( 4 ) R-11 and R-12 vapors can be e a s i l y measured with commercial measuring equipment, such a s a Gas Chromatograph ( G C ) .

Ozone deple t ing compounds such a s R-11 and R-12 a r e being phased out of production f o r environmental p ro tec t ion reasons. A t t h e present time, seve ra l compounds a r e being t e s t e d a s replacements. While t h e r e has been some success using o the r compounds, t h e r e i s no consensus on which w i l l replace t h e ha l ides cu r ren t ly referencedby s t a n d a r d s t h a t apply t o adsorber t e s t i n g .

Whichever compound i s se l ec ted , it w i l l produce essen- t i a l l y t h e same reac t ion a s R - 1 1 i n a halocarbon de tec to r . Therefore, t h e information and references i n t h i s book regarding R - l l w i l l apply t o R-12 and s u b s t i t u t e compounds a s well .

R-11 TEST DETECTION APPARATUS There a r e two types of apparatus used i n t h e de tec t ion of - 1 . The f i r s t apparatus t o be discussed is t h e Gas Chromatograph (GC) . The GC used is of t h e e l ec t ron capture de tec to r ( E C D ) type. The GC can be equipped with an accurate valving system which can cons i s t en t ly measure R-11 concentra t ions i n t h e 10 p a r t s per mi l l ion (ppm) and 1 0 p a r t s pe r b i l l i o n (ppb) range. I t is necessary t o measure both of these ranges s ince t h e upstream R-11 vapor w i l l be i n ppm, and t h e downstream vapor, having gone through t h e carbon, w i l l be i n ppb. Refer t o Figure 18 f o r a representa t ion of a GC.

Sample (elect KO-negat Ive spec ie s )

Scperating

Integmclng h p l i f ier

Heater

Dctcctor Cell Anode

( C r i t b t e d t o i l )

Figure 18. - GAS CHROMATOGRAPH

A GC f o r R - 1 1 t e s t i n g works i n t h e following manner:

The de tec t ing u n i t c o n s i s t s of a cy l inder t h a t has a t r i t i a t e d t i t an ium f o i l on i ts inner l i n i n g . This f o i l emitsionizingradiationwaveswhichare f a s t a n d d i f f i c u l t t o capture. A c a r r i e r gas (usua l ly high p u r i t y Nitrogen, o r a N i t r o g e n r i c h m i x t u r e , s u c h a s P-5, o r P - 1 0 ) i s i o n i z e d by t h e r ad ia t ion , knocking off a negative Beta p a r t i c l e . This negative p a r t i c l e is much slower than t h e ion iz ing p a r t i c l e , and can e a s i l y be captured by an anode located i n t h e center of t h e cy l inder . The flow of negative Beta p a r t i c l e s , from t h e ionized c a r r i e r gas t o t h e anode, e s t ab l i shes an e l e c t r i c a l cur ren t which can be measured.

The maximum amount of cu r ren t generated by t h i s ion iz ing process is c a l l e d t h e s tanding cu r ren t ( S C ) . Its magnitude canbemeasuredandexpressedas a p e a k o n a cha r t recorder. The char t recording is a chromatogram.

Any mater ia l which reduces t h i s flow of e l ec t rons w i l l reduce t h e s tanding cu r ren t . Such mater ia l is c a l l e d an electro-negative spec ies , and w i l l reduce t h e magnitude of t h e current flow.

This reduction i n cu r ren t flow can be measured and displayed a s a peak on t h e chromatogram. Oxygen is mildly electro-negative, and w i l l be a major cons t i tuen t of t h e

dilution (or concentration, depending on the GC manufac- turer) is done using different sample volumes in the valves.

It is not practical to assume that every valve will have a ratio of volumes of 1: 1000. However, these volumes can be measured, and the ratio of the volumes can be determined. T h i s r a t i o i s c a l l e d a C a l i b r a t i o n F a c t o r (CF) and must be considered in the %P formula. The CF for each valve is unique, and is a part of the certification given by the GC manufacturer. The %P formula must be modified to include the CF, which then becomes:

%P = Cd/Cu x 100 x CF.

The final CF will also include other factors and will be discussed in later text, when the entire CF is developed.

Discrimination:

It would be very confusing for the GC operator to try to distinguish between all of the possible electro-negative species in the samples, so a GC requires an operation to be performed between the valve sampling actions and the ECD. The operation is to push the sample through a heated column. The column is filled with a material which retards the progress of the constituent parts of the air sample. This retardation is a function of the molecular weight of the constituents.

Oxygen, having a lower molecular weight, goes through the column quicker than R-11, which has a higher molecular weight. The ECD can distinctly measure each of the electro-negative species which pass through it.

Given the proper column design, including operating temperature, length, and packing, the GC can be designed so that R-11 exits the ECD in a consistent time interval relative to oxygen. Under normal conditions, the R-11 passes through the ECD 13 seconds after the oxygen passes through it.

I t has a l r eady been e s t a b l i s h e d t h a t t h e oxygen component r e d u c e s t h e SC amaximumamount. Therefore, t h e oxygen peak is a reliable i n d i c a t o r f o r t h e ope ra to r t o begin looking f o r t h e R-11 peak some 13 seconds later.

The term peak comes from t h e f a c t t h a t t h e SC l e v e l is u s u a l l y measured from t h e lower ba se l i ne of t h e c h a r t r e co rde r , and any r educ t i on of t h e SC is shown as an upward peak, conforming t o GC convention. The peak is a measurement of t h e reduc t ion i n SC. The u n i t of measure is i n graph paper c h a r t u n i t s ( c u ) .

F igure 19 p r e s e n t s a t r a c i n g of an ope ra t i ng GC. The t a l l peaks a r e t h e sxygen peaks a t t h e beginning of t h e downstreamcycles. Oxygenpeaks a r e n o t v i s i b l e d u r i n g t h e upstream c y c l e because t h e sample is very s m a l l , and t h e amount of oxygen is n o t s u f f i c i e n t t o cause a peak.

Figure 19

Halfway between t h e s e oxygen peaks is t h e beginning of t h e upstream cyc l e . Sometimes, t h e r e w i l l be a b l ip i n t h e t r a c i n g when t h e GC changes cyc les . This b l i p can be a number of t h i n g s , a l l unimportant. The b l i p may be from a spu r ious e l e c t r i c a l s i g n a l induced by t h e c i r c u i t r y i n t h e GC c o n t r o l l i n g t h e valves . I t may come from an i n d i c a t i o n of t h e upstream oxygen peak. Reca l l , however, t h e upstream oxygen peak w i l l be 1000 t i m e s less apparent t h a n t h e downstreamoxygen peak. A s m a l l l e ak i n t h e t ub ing of t h e GC may induce such a b l i p .

downstream sample. Oxygen w i l l c r e a t e a very high peak a t t h e beginning of t h e downstream cycle . W e do not have an i n t e r e s t i n t h e oxygen peak as a means of measuring R-11. This peak w i l l , however, be important a s a marker f o r determining cyc l e s of t h e GC.

R - 1 1 is a l s o an e lec t ro -nega t ive mate r ia l . I f it flows through t h e d e t e c t i n g cy l inde r , it too w i l l reduce t h e c u r r e n t flow a measurable amount.

An important component of t h e GC is a valve system whereby t h e sample of R-11 a i r i s i n j e c t e d i n t o t h e GC. The valve i s designed s o t h a t a vacuum pump p u l l s a continuous a i r sample from both t h e upstream (cha l lenge) and t h e downstream ( p e n e t r a n t ) t e s t l o c a t i o n s through it.

Atthebeginningoftheupstreammeasuringcycle, t h e valve r o t a t e s t o cap ture t h e upstream sample flow, and t a k e s a "grab sample" of t h e continuous sample being pu l l ed from t h e upstream sample l oca t ion . I t i n j e c t s t h i s sample i n t o t h e GC f o r measurement.

A t t h e end of t h e measuring cyc le , t h e valve r o t a t e s t o t h e downstream sample flow, and t a k e s a "grab sample" of t h e downstreamgas volume. This "grabsample"istypically1OOo t i m e s l a r g e r than t h e upstream sample so t h a t t h e GC can measure i n t h e same u n i t s .

A t y p i c a l pene t r a t i on requirement f o r a new f i l t e r i s 0.1% maximum. This is a 1:1000 r a t i o of Concentrat ion Downstream (Cd) t o t h e Concentrat ion Upstream (C,). The u n i t s f o r C U areppm. This necessarilymeansthattheunits f o r c d a r e i n p p b . S u c h a l a r g e s e n s i t i v i t y r a n g e i s ou t s ide t h e a b i l i t y of a GC t o assimilate. Because of t h e r e s t r i c t i o n s i n t h e G C t s s e n s i t i v i t y range, e i t h e r t h e upstream sample is d i l u t e d , o r t h e downstream sample is concentra ted s o t h a t each sample w i l l have approximately t h e same volume of R-11. Therefore, e i t h e r t h e volume of t h e upstream sample must be approximately 1000 t i m e s less than thevolume o f t h e downstream, o r thedownstreamsample must be 1000 t imes l a r g e r than t h e upstream sample. This al lows t h e GC t o measure both samples wi th consis tency, s i n c e it is measuring s i m i l a r amounts of - 1 1 This

So, between any two consecutive oxygen peaks w i l l be one complete downstream and upstream cycle . The Oxygen peak r e p r e s e n t s t h e beginning o f t h e downstream cycle . Halfway between t h e two oxygen peaks w i l l be t h e beginning of t h e upstream cycle .

T e s t Penetration Measurements

F a c t o r y t e s t acceptance c r i t e r i o n i s u sua l ly 0.1% maximum pene t ra t ion . On a few occas ions , it may be 0.01% pene t ra t ion , maximum. Some rare a p p l i c a t i o n s r equ i r e a 0.001% pene t r a t i on , maximum. I t is important t o consider how t h e s e pene t r a t i ons can be measured.

Under test condi t ions , t h e downstream peak he igh t , r e p r e s e n t i n g t h e downstream concentra t ion, can be read t o t h e nea re s t 0.5 c h a r t u n i t ( c u ) . An i d e a l l y c a l i b r a t e d GC w i l l have a 1:l r a t i o between t h e upstream peak ( r ep re sen t ing t h e upstream concen t r a t i on ) , and t h e downstream peak. Therefore, an i d e a l l y c a l i b r a t e d GC must have a t least a 0.5 ppm upstream concentra t ion i n o rde r t o prove a 0.1% m a x i m u m pene t ra t ion .

I f t h e c a l i b r a t i o n is no t 1:1, tes t personnel must c a l c u l a t e t h e upstream concent ra t ion r equ i r ed tomake t h e downstream 0.5 cu measurements s i g n i f i c a n t .

I f t h e c a l i b r a t i o n is 1 : 1, a 5 ppm upstream concentra t ion of R - 1 1 i s required i n o rde r t o prove a 0.01% m a x i m u m pene t ra t ion . And, i f t h e upstream concentra t ion is 50 ppm, t h e f i l t e r can be c e r t i f i e d t o have a maximum pene t ra t ion of 0.001%.

An upstream concentra t ion of 50 ppm should be t h e maximum, s i n c e t h i s concentra t ion is about t h e upper l i m i t t h a t a GC can accu ra t e ly measure. Above 50 ppm, t h e GC l o s e s l i n e a r i t y , and t h e peak he igh t s do no t accura te ly r e f l e c t t h e concent ra t ions measured.

Test Examples

Examples of chromatograph t r ac ings and penetra t ion ca lcu la t ions a r e shown i n Figures 2 0 , 2 1 , and 22.

Thepeaks a r e n o t ac tua l t e s t da ta , butweredrawnonablank t r a c i n g t o i l l u s t r a t e t h e t e s t i n g sequence and possible r e s u l t s .

The handwritten nota t ions a r e t y p i c a l of an ac tua l chromatogram. The nota t ions " U l , U 2 ..." a r e u s e d t o denote t h e d i r e c t i o n which t h e c h a r t is t r ave l ing , and t o help l o c a t e t h e p a i r s of d a t a po in t s used i n t h e ca lcu la t ions a t a l a t e r t i m e . During t e s t i n g , it i s important t o a n n o t a t e d a t a p o i n t s ando the r important in format iononthe t r a c i n g , a s it is d i f f i c u l t t o i n t e r p r e t t h e test r e s u l t s a t a l a t e r time without these notat ions .

CASE 1 (Figure 2 0 ) :

I n Case 1, t h e r e a r e no background cons t i tuents . A t t h e handwritten nota t ion "R-11 on", R - 1 1 was in j ec ted i n t o t h e a i r c leaning system. A new'peak caused by t h e R - 1 1 i n j e c t i o n appears i n t h e upstream cycle. A new peak then emergesonthedownstreamcycle. Thisnewpeakiscons tan t .

A t t h e "R-11 o f f " no ta t ion , t h e upstream peak disappears, and t h e downstream peak diminishes during t h e next cycle. I t w i l l disappear wi thin a very few cycles , usual ly a f t e r t h e f i r s t one.

This t r a c i n g is t y p i c a l of a system with new carbon and a s m a l l mechanical leak. I t is t y p i c a l of a mechanical l eak f o r t h e downstream peak. t o rap id ly disappear a f t e r t h e upstream R-11 i s turned o f f .

CASE 2 (Figure 21):

I n Case 2 , no background R-11 is presen t . The R-11 is i n j e c t e d , and t h e downstream peak is o f f -scale high. The R-11 i s tu rned o f f , and t h e downstream r a p i d l y decreases .

This t r a c i n g i s t y p i c a l of a sys temwi th a l a rgemechanica l l eak i n it. The l e a k ( s ) must be found, r e p a i r e d o r cor rec ted , and t h e system s a t i s f a c t o r i l y r e t e s t e d before p l ac ing it i n s e rv i ce .

CASE 3 (Figure 22) :

C a s e 3 shows a cons tan t background R-11 peak du r ing t h e downstream cyc le . Th is is R-11 background, which w i l l be sub t r ac t ed when t h e pene t r a t i ons are c a l c u l a t e d , and w i l l no t a f f e c t t h e r e s u l t s of t h e t e s t .

A t t h e no t a t i on "R-11 on", t h e upstream peak appears . The background peak i n c r e a s e s wi th each cyc le . The i n c r e a s e is due t o desorpt ion. Ca l cu l a t i ons are performed f o r each cycle , and t h e r e s u l t a n t p l o t of pene t r a t i on vs . t i m e is shown i n Figure 28.