Smoke problem in building fires

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Smoke problem in building fires

NATIONAL R E S E A R C H COUNCIL

CANADA

DIVISION O F BUILDING RESEARCH

THE SMOKE P R O B L E M IN BUILDING F E E S

b y J. H. M c G u i r e

ANALYZED

I n t e r n a l R e p o r t No. 287 of t h e D i v i s i o n of Building R e s e a r c h OTTAWA M a r c h

1 9 6 4

PREFACE

Smoke i s one of the s e r i o u s a s p e c t s of building f i r e s . It often h a m p e r 6 the e s c a p e or r e s c u e of occupants a s well a s the subsequent efforts t o fight t h e f i r e . Consequently, it becomes an important topic f o r study and r e s e a r c h and a p r e l i m i n a r y study of it i s now reported.

The author i s a r e s e a r c h officer with the F i r e Section of the Division having i n t e r e s t s in the d e v e l o ~ m e n t and s p r e a d of f i r e .

Ottawa March

1964

N.

B.

Hutcheon, Assistant Director.

T H E SMOKE P R O B L E M IN BUILDING

FIRES

J. H. M c G u i r e S u m m a r y T h e s i g n i f i c a n c e of t h e n e u t r a l p r e s s u r e p l a n e i n a f i r e a r e a i s outlined a n d s o m e i l l u s t r a t i v e e x p e r i m e n t s a r e d e s c r i b e d .

A

t e c h n i q u e i s s u g g e s t e d w h e r e b y t h e s m o k e g e n e r a t e d i n a t y p i c a l building f i r e could b e confined t o t h e i m m e d i a t e f i r e a r e a . It h a s b e e n a p p r e c i a t e d f o r m a n y y e a r s t h a t t h e smoke:* g e n e r

-

a t e d b y f i r e s i n b u i l d i n g s f r e q u e n t l y c r e a t e s p r o b l e m s both f o r o c c u p a n t s a t t e m p t i n g t o e s c a p e and f o r f i r e f i g h t e r s a t t e m p t i n g t o c o m b a t t h e f i r e s . T h e planned m o v e m e n t of o c c u p a n t s t o w a r d a n exit, e v e n i n f a m i l i a r a r e a s , c a n r a p i d l y b e c o m e i m p o s s i b l e a s s m o k e l e v e l s build up, b e c a u s e of t h e r e d u c t i o n i n v i s i b i l i t y a n d i r r i t a t i o n of t h e e y e a n d b r e a t h i n g p a s s a g e s . T h e e n s u i n g d e l a y i n e s c a p i n g m a y b e f a t a l , d e a t h r e s u l t i n g , f o r e x a m p l e , f r o m t h e t o x i c c o m p o n e n t s of t h e p r o d u c t s of c o m b u s t i o n . T h e f i r e f i g h t e r c a n e l i m i n a t e t h e t o x i c g a s a n d i r r i t a t i o n p r o b l e m s b y w e a r i n g b r e a t h i n g a p p a r a t u s but h i s e f f i c i e n c y i s s o m e t i m e s i m p a i r e d by h i s i n a b i l i t y t o l o c a t e t h e o r i g i n of t h e f i r e . T h i s p r o b l e m , h o w e v e r , i s f a r l e s s i m p o r t a n t t h a n t h e h a z a r d s t h a t s m o k e c r e a t e s f o r t h e o c c u p a n t s of buildings. In o r d e r t o a s s e s s t h e m a g n i t u d e of t h i s l a t t e r p r o b l e m t h e r e l e v a n t e x p e r i m e n t a l w o r k a t t h e N a t i o n a l R e s e a r c h Council w i l l be r e v i e w e d . T y p i c a l S m o k e L e v e l s : E x p e r i m e n t a l W o r k D u r i n g t h e w i n t e r of 1957 -58, t h e D i v i s i o n of Building R e s e a r c h of t h e N a t i o n a l R e s e a r c h Council, a s p a r t of a n e x p e r i m e n t a l i n v e s t i g a t i o n known a s t h e St. L a w r e n c e B u r n s

(I),

b u r n e d s i x h o u s e s t o gain knowledge about t h e d e v e l o p m e n t of f i r e .

T h r e e of t h e h o u s e s h a d h i g h l y f l a m m a b l e i n t e r i o r l i n i n g s s u c h a s unfinished f i b r e b o a r d and p r e s s e d p a p e r ; t h e o t h e r t h r e e had m u c h l e s s f l a m m a b l e linings s u c h a s p l a s t e r b o a r d o r p l a s t e r , which w e r e c o l l e c t i v e l y d e s c r i b e d a s n o n - c o m b u s t i b l e linings. In e a c h h o u s e t h e f i r e w a s s t a r t e d in wooden c r i b s , l o c a t e d i n t h e l i v i n g r o o m , but t h e r e w a s no o t h e r

T h r o u g h o u t t h i s r e p o r t t h e t e r m " s m o k e " i s a p p l i e d t o a l l g a s e o u s p r o d - u c t s of c o m b u s t i o n w h e t h e r v i s i b l e o r not.

e x t r a n e o u s f u e l ( s u c h as f u r n i t u r e ) i n a n y p a r t of t h e h o u s e . One down- s t a i r s window o r door w a s left p a r t l y open.

T a b l e I l i s t s t h e m o s t i m p o r t a n t r e s u l t s t h a t a r e r e l e v a n t t o t h i s r e p o r t . Of t h e five c r i t e r i a t h a t a r e involved i n T a b l e I, four a r e

c o n c e r n e d with conditions u n d e r which a h u m a n being m i g h t not s u r v i v e . T h e fifth c r i t e r i o n , v i s i b i l i t y , i s r e l a t e d t o likelihood of e s c a p e , F r o m t h e point of view of t h i s r e p o r t t h e i m p o r t a n t o b s e r v a t i o n t h a t c a n b e m a d e f r o m T a b l e I i s t h a t t h e m i g r a t i o n of s m o k e u n d e r t h e c o n d i t i o n s c o n s i d e r e d c o n s t i t u t e s a m o s t s e r i o u s h a z a r d even t o t h e o c c u p a n t s of a b e d r o o m , t h e door of which i s c l o s e d . T h e r e s u l t s i n T a b l e I a r e r e l a t e d t o s m o k e conditions i n t h e u p p e r s t o r e y of a dwelling when a f i r e i s i n p r o g r e s s on t h e l o w e r s t o r e y . Another s e t of c i r c u m s t a n c e s of i n t e r e s t t o f i r e a n d building o f f i c i a l s i s t h e d e n s i t y of s m o k e t o b e e x p e c t e d i n t h e c o r r i d o r of a building when a f i r e d e v e l o p s in a r o o m opening f r o m t h e c o r r i d o r , t h e door t o t h e r o o m b e i n g shut. Two t e s t s i l l u s t r a t i n g t h e s e conditions w e r e t h e r e f o r e c a r r i e d out i n a c o r r i d o r - r o o m t e s t a s s e m b l y which w a s c o n s t r u c t e d f o r a n e x p e r i m e n t a l p r o g r a m t o i n v e s t i g a t e t h e s p r e a d of f i r e in s u c h c o n s t r u c t i o n s T h e c o r r i d o r w a s

60

ft long, 8 ft wide, and 8 ft high and t h e r o o m w a s 12 f t long by 8 ft wide. T h e w a l l s of t h e s t r u c t u r e w e r e of c o n c r e t e b l o c k s , and t h e c e i l i n g lining m a t e r i a l w a s a s b e s t o s s h e e t i n g on which l a y s e v e r a l i n c h e s of exfoliated v e r m i c u l i t e insulation. Between t h e c o r r i d o r and t h e r o o m a 1/4-in. a s b e s t o s s h e e t door w a s i n s t a l l e d a n d a 1

-

t o 2-in. c r a c k w a s l e f t between t h e top of t h e door and t h e bottom of t h e lintel. During t h e f i r s t t e s t a n opening of about 2 s q ft w a s left a t a low l e v e l b e t w e e n t h e r o o m and t h e e x t e r i o r . D u r i n g t h e second t e s t t h e a r e a of t h i s opening w a s r e d u c e d t o about 1 s q ft and a n a d d i t i o n a l opening of about 1 . 8 s q ft w a s l e f t a t

a

high level.

In e a c h c a s e the f u e l c o n s i s t e d of five 4- by 8-ft s h e e t s of u n t r e a t e d f i b r e b o a r d s t a c k e d p a r a l l e l t o e a c h o t h e r and s e p a r a t e d b y about

1 ft. T h e f i r e w a s s t a r t e d with about half a pint of gasoline.

T h e r e s u l t s of t h e two t e s t s w e r e t h a t t h e c o r r i d o r b e c a m e smoke-logged i n about

6

and 12 m i n , r e s p e c t i v e l y . A s m o k e m e t e r w a s u s e d t o d e t e r m i n e t h e o p t i c a l t r a n s m i s s i v i t y of t h e s m o k e and w a s l o c a t e d a t t h e c e n t r e of t h e c o r r i d o r . F u r t h e r d e s c r i p t i o n of t h e a p p a r a t u s a n d a n y p r e c i s e definition of t h e t e r m "smoke-logged" i s h a r d l y n e c e s s a r y , h o w e v e r , f o r t h e t r a n s m i s s i o n o v e r t h e 1. 73 -ft path l e n g t h of t h e s m o k e m e t e r d e c r e a s e d f r o m 8 0 p e r c e n t t o l e s s t h a n 8 p e r c e n t between t h e 5th and 7th m i n in t h e f i r s t t e s t and between t h e 1 l t h and 13th mi11 in t h e s e c o n d t e s t . In addition, o b s e r v a t i o n s i n d i c a t e d t h a t v e r y l i t t l e t i m e e l a p s e d

A r e a s o n a b l y adequate s u m m a r y of the r e s u l t s would be t h a t untenable conditions developed i n t h e c o r r i d o r soon a f t e r t h e c o m m e n c e

-

m e n t of t h e f i r e i n t h e r o o m .

E f f e c t of F u e l

P r a c t i c a l e x p e r i e n c e s u g g e s t s t h a t s m o k e l e v e l s c o m p a r a b l e t o t h o s e quoted above w i l l p r o b a b l y p r e v a i l i n m a n y buildings r e g a r d l e s s

of t h e n a t u r e of the fuel involved, F o r t h i s r e a s o n , t h e r e f o r e , t h e p r i n c i p a l r e c o m m e n d a t i o n given i n t h i s r e p o r t i s not c o n c e r n e d with

p o s s i b l e r e s t r i c t i o n s on t h e m a t e r i a l s t o be i n c o r p o r a t e d in buildings e i t h e r a s c o n t e n t s o r a s p a r t of t h e c o n s t r u c t i o n . N e v e r t h e l e s s , t h e p r o p e r t i e s of t h e m a t e r i a l s l i k e l y t o c o n s t i t u t e fuel c a n have a s u b s t a n t i a l influence on t h e s m o k e conditions t o b e expected. P r o b a b l y t h e m o s t i m p o r t a n t r e l e v a n t p r o p e r t y i s t h e f l a m e s p r e a d index, a s t h i s w i l l i n f l u e n c e t h e likelihood of t h e development of a f i r e . T h i s question, h o w e v e r , c o m e s within the g e n e r a l field of f i r e p r e v e n t i o n and w i l l not be d i s c u s s e d in t h i s r e p o r t , t h e s u b j e c t m a t t e r of which i s confined t o m e a s u r e s s p e c i f i - c a l l y a i m e d a t combatting s m o k e .

During t h e e a r l y s t a g e s of combustion, s o m e m a t e r i a l s

g e n e r a t e s m o k e a t a m u c h g r e a t e r r a t e t h a n o t h e r s and t e s t s e x i s t w h e r e b y t h i s p r o p e r t y of a m a t e r i a l m a y be q u a n t i t a t i v e l y a s s e s s e d i n t e r m s of a

"smoke index. " Although t h e g e n e r a t i o n of smoke, when a m a t e r i a l b u r n s ,

is highly dependent on the c o m p l e t e n e s s of combustion, s o m e d e g r e e of i n c o m p l e t e c o m b u s t i o n i s a l w a y s t o b e found i n building f i r e s , a t one s t a g e o r a n o t h e r , and h e n c e t h e s m o k e i n d i c e s of t h e m a t e r i a l s involved c a n be expected t o i n f l u e n c e t h e t o t a l volume of s m o k e g e n e r a t e d . F o r t h i s r e a s o n , s o m e r e s t r i c t i o n on t h e u s e of m a t e r i a l s , by r e g u l a t i o n of p e r m i s s i b l e

s m o k e i n d i c e s , is a u s e f u l s t e p i n combatting s m o k e p r o b l e m s .

Unfortunately, t h e p r e s e n t l e v e l of knowledge on t h e s u b j e c t d o e s not allow t h e f o r m u l a t i o n of t r u l y sound r e c o m m e n d a t i o n s . One i n t e r e s t i n g s u g g e s t i o n i s t h a t t h e s m o k e index of a n y m a t e r i a l t o be used in a c o m p a r t m e n t should not be a n y h i g h e r t h a n t h e h i g h e s t i n d e x of m a t e r i a l s whose u s e i s unavoidable. In m a n y c a s e s t h e i n d e x t h a t w i l l r e s u l t f r o m s u c h a c o n s i d e r a t i o n will be t h a t of s o m e s p e c i e s of wood. T h e o r e t i c a l A n a l y s i s of P r o b l e m S m o k e m o v e m e n t f r o m a burning a r e a t o a n a d j a c e n t one r e s u l t s m a i n l y f r o m t w o p h e n o m e n a : ( 1 ) the t h e r m a l expansion of the g a s e s in t h e f i r e a r e a , a s t h e i r t e m p e r a t u r e r i s e s , and ( 2 ) the continuous d i s c h a r g e of g a s e s , a t a high l e v e l , f r o m t h e f i r e a r e a and t h e i r r e p l a c e m e n t by

a i r e n t e r i n g a t a low level. T h e s e c o n d phenolmenon will be d i s c u s s e d

f i r s t , p a r t l y b e c a u s e i t is t h e m o r e i m p o r t a n t and p a r t l y b e c a u s e the m e a n s t h a t will be s u g g e s t e d f o r combatting i t w i l l l a r g e l y solve the p r o b l e m

T h e m e c h a n i s m r e s p o n s i b l e f o r t h e continuous m o v e m e n t of g a s e s i n t o and out of a f i r e a r e a i s i l l u s t r a t e d i n F i g u r e 1 which a l s o

i l l u s t r a t e s t h e conditions t h a t w i l l g o v e r n t h e flow of g a s t o a d j a c e n t a r e a s . A d e t a i l e d a n a l y s i s of t h e m e c h a n i s m i s given i n Appendix A. T h e s a l i e n t f e a t u r e i s t h a t a t low l e v e l s t h e p r e s s u r e within t h e f i r e a r e a w i l l be l o w e r t h a n t h a t o u t s i d e , s o t h a t a i r w i l l flow in t h r o u g h a n y l o w - l e v e l openings, while a t high l e v e l s t h e p r e s s u r e w i l l be g r e a t e r , and t h e r e f o r e g a s e s w i l l flow out of h i g h - l e v e l openings. In t h e i d e a l i z e d c a s e a n e u t r a l p r e s s u r e p l a n e will e x i s t a t which t h e p r e s s u r e within t h e f i r e a r e a e q u a l s t h a t o u t -

s i d e .

Whether s m o k e w i l l flow f r o m a f i r e a r e a t o an a d j a c e n t a r e a , s a y , a c o r r i d o r , depends on t h e r e l a t i v e h e i g h t s of t h e connecting openings and t h e n e u t r a l p r e s s u r e p l a n e . If, a s shown i n F i g u r e l b , t h e c o n n e c t i n g openings a r e below t h e l e v e l of t h e n e u t r a l p l a n e , t h e n t h e s e openings w i l l c o n s t i t u t e i n l e t s t o t h e f i r e a r e a and t h e " c o r r i d o r " or o t h e r a d j a c e n t a r e a will r e m a i n s m o k e f r e e . One o t h e r f e a t u r e h a s , i n f a c t , b e e n i n t r o d u c e d in F i g u r e l b , v i z . , t h e s m a l l opening t o t h e e x t e r i o r i l l u s t r a t e d a t t h e b o t t o m r i g h t - h a n d c o r n e r of t h e d i a g r a m . T h i s opening i s n e c e s s a r y t o e n s u r e t h a t t h e p r e s s u r e s a t a l l l e v e l s i n t h e s m o k e - f r e e a r e a a r e t h e s a m e a s t h o s e out

-

s i d e , s o t h a t t h e l e v e l of t h e one n e u t r a l p r e s s u r e p l a n e , a s d r a w n , w i l l g o v e r n t h e g a s flow f r o m t h e f i r e a r e a t o both t h e e x t e r i o r and t h e s m o k e

-

f r e e a r e a . F a i l u r e t o include t h i s s m a l l opening would give m o r e c o m p l i - c a t e d conditions t h a t would c e r t a i n l y l e a d t o a flow of s m o k e i n t o t h e

" c o r r i d o r . I ' A n a l y s i s would show t h e e x i s t e n c e of two n e u t r a l p l a n e s , one

r e l a t e d t o t h e flow t o t h e e x t e r i o r and t h e o t h e r t o t h e flow t o t h e " c o r r i d o r . I '

T h u s , t o m a i n t a i n a n a r e a s m o k e f r e e , t h e e s s e n t i a l r e q u i r e m e n t i s t h a t openings c o m m u n i c a t i n g with t h e f i r e a r e a should be a t a l o w e r l e v e l t h a n t h e n e u t r a l p r e s s u r e plane. In g e n e r a l t h e l e v e l of t h e s e openings w i l l be s e t b y c o n s i d e r a t i o n s o t h e r t h a n f i r e and it i s t h e r e f o r e d e s i r a b l e t h a t t h e l e v e l of t h e n e u t r a l p l a n e should b e a c o n t r o l l a b l e v a r i a b l e .

Appendix A shows t h a t t h e l e v e l of t h e n e u t r a l p l a n e i s given by t h e e x p r e s s i o n

( T h e s y m b o l s i n t h i s e x p r e s s i o n a r e defined by F i g u r e 2. ) The two t e m p e r

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a t u r e s involved c a n h a r d l y b e d e s c r i b e d a s c o n t r o l l a b l e v a r i a b l e s and i t i s p o s s i b l e t h a t t h e r e i s a n i r r e d u c i b l e m i n i m u m f o r t h e value of A l l t h e i n l e t a r e a . T h e o u t l e t a r e a A2, h o w e v e r , i s c o n t r o l l a b l e and i n c r e a s i n g i t r a i s e s t h e n e u t r a l p l a n e , which i s i n v a r i a b l y t h e a c t i o n t h a t i s r e q u i r e d , i f any. At t h e beginning of t h i s s e c t i o n i t w a s s t a t e d t h a t s m o k e m i g r a t i o n a l s o r e s u l t s f r o m t h e t h e r m a l e x p a n s i o n of t h e g a s e s in t h e f i r e a r e a , a s t h e i r t e m p e r a t u r e r i s e s . T h e m a x i m u m t e m p e r a t u r e t o be e x p e c t e d in a

f i r e , on a n a b s o l u t e s c a l e , will g e n e r a l l y not e x c e e d five t i m e s t h e

a m b i e n t t e m p e r a t u r e s o t h a t t h e volume of t h e g a s e s t o be d i s p l a c e d w i l l e q u a l about four t i m e s t h e o r i g i n a l volume of t h e f i r e region. T h e d i s - p l a c e d g a s e s will cool s o t h a t t h e i r volume will a p p r o x i m a t e t o t h e v o l u m e of t h e f i r e r e g i o n . T h e a c t i o n r e c o m m e n d e d t o c o m b a t t h e " s t e a d y - s t a t e "

m i g r a t i o n , h o w e v e r , w i l l l a r g e l y s o l v e t h e t r a n s i e n t p r o b l e m f o r i t involves t h e p r o v i s i o n of high-level openings t o t h e e x t e r i o r . Neglecting t h e effect of winds, t h e s m o k e flow t o o t h e r p a r t s of t h e building w i l l be r e d u c e d a t l e a s t by t h e r a t i o of t h e a r e a of t h e c o m m u n i c a t i n g openings t o t h e t o t a l openings ( t h e s u m of t h e c o m m u n i c a t i n g openings and t h o s e t o t h e e x t e r i o r ) .

In f a c t , the two phenomena w i l l be o c c u r r i n g c o n c u r r e n t l y and a d e t a i l e d a n a l y s i s of t h e p r o b l e m would show t h a t t h e n e t effect of t h e t r a n s i e n t condition i s a l o w e r i n g of t h e n e u t r a l p l a n e . A s l i g h t l y m o r e

l i b e r a l p r o v i s i o n of high-level openings w i l l t h e r e f o r e c o m p l e t e l y e l i m i n a t e t h e flow of s m o k e t o a d j a c e n t a r e a s , A helpful f e a t u r e i s t h a t t h e t r a n s i e n t phenomenon w i l l o c c u r p r i o r t o t h e development of m a x i m u m t e m p e r a t u r e s and t h i s i s t h e v e r y t i m e when t h e l e v e l of t h e n e u t r a l p l a n e a s c a l c u l a t e d i n Appendix A w i l l b e high. T h e additional effect of t e m p e r a t u r e r i s e m i g h t only be, t h e r e f o r e , t o d r o p t h e l e v e l of t h e n e u t r a l p l a n e t o t h e final value i t w i l l a s s u m e i n t h e s t e a d y s t a t e when t h e t e m p e r a t u r e i s a t a m a x i m u m .

Appendix B d i s c u s s e s t h e c o m p o s i t e p r o b l e m and s h o w s t h a t t h e s i z e of opening t o t h e e x t e r i o r , needed t o e l i m i n a t e g a s flow t o a d j a c e n t a r e a s a s a r e s u l t of t h e r m a l expansion, will u s u a l l y be of t h e o r d e r 2 s q ft p e r 1 0 , 0 0 0 cu ft volume of t h e f i r e c o m p a r t m e n t .

T h e U s e of F a n s

T h e t h e o r e t i c a l a n a l y s i s of t h e p r o b l e m h a s c l a r i f i e d t h e m e c h a n i s m s involved i n s m o k e m o v e m e n t and h a s s u g g e s t e d a m e a n s of combatting it. A f u r t h e r technique i s a l s o w o r t h i n v e s t i g a t i n g : t h e u s e of f a n s t o p r e s s u r i z e t h e a r e a which i s t o be m a i n t a i n e d s m o k e f r e e , T h i s s u b j e c t i s e x a m i n e d in Ap2endix

G

w h e r e it i s shown t h a t , if no o t h e r m e a s u r e s w e r e t a k e n , i t might be n e c e s s a r y t o e s t a b l i s h p r e s s u r e s of up t o 0 . _{1 in. of w a t e r . T o a c h i e v e t h i s would u s u a l l y involve high c a p a c i t y}

f a n s a s l e a k a g e t o o t h e r a d j a c e n t a r e a s would c o n s t i t u t e a s u b s t a n t i a l

p r o b l e m . T h u s , a s i s s t a t e d in Appendix C, t h e throughput beneath a t y p i c a l door might be a s high a s 200 cu ft/min.

E x p e r i m e n t a l C o n f i r m a t i o n of R e c o m m e n d a t i o n

The r e c o m m e n d a t i o n , a s f o r m u l a t e d s o f a r , h a s been b a s e d on t h e a s s u m p t i o n t h a t t h e g a s e s within t h e f i r e a r e a a r e s t a g n a n t , T h e o v e r -

a l l upward v e l o c i t y of t h e g a s e s m a y c e r t a i n l y be neglected i n c o m p a r i s o n with t h e i n l e t and outlet v e l o c i t i e s , but t h e v e l o c i t i e s a s s o c i a t e d with

check on t h e validity of t h e r e s u l t given in Appendix A i s t h e r e f o r e d e s i r

-

able.

The following t e s t w a s c a r r i e d out in t h e c o r r i d o r - r o o m a s s e m b l y d e s c r i b e d e a r l i e r in t h i s r e p o r t . At a low l e v e l in one wall of t h e r o o m t h e r e w a s an opening t o t h e e x t e r i o r of a r e a 70 s q in. con-

stituting an i n l e t ; 8 4 in. above t h i s t h e r e w a s an outlet t o the e x t e r i o r of a r e a 256 s q in. T h e p r i n c i p a l opening communicating with the c o r r i d o r w a s a 1 -in. c r a c k a t t h e top of t h e door, which w a s 24 in. below t h e m e a n height of the outlet. T h e s e conditions c o r r e s p o n d qualitatively t o t h o s e of the second of the two c o r r i d o r t e s t s p r e v i o u s l y d e s c r i b e d . In t h e e a r l i e r t e s t , however, t h e r a t i o of t h e outlet t o inlet a r e a s was such t h a t t h e

n e u t r a l plane w a s a p p r e c i a b l y lower than t h e top of t h e door linking t h e r o o m and the c o r r i d o r . The fuel was the s a m e a s for t h e t e s t s previously

d e s c r i b e d : five s h e e t s of 4 - by 8-ft f i b r e b o a r d stacked with the s h o r t e r dimension v e r t i c a l , t h e f i r e being initiated with about half a pint of gasoline. T o o b s e r v e the e f f e c t i v e n e s s of f a n s , two f a n s with a t o t a l r a t e d c a p a c i t y of 500 cu ft p e r min w e r e i n s t a l l e d in t h e c o r r i d o r .

At frequent i n t e r v a l s during the t e s t an o b s e r v e r stood behind the a s b e s t o s door in t h e c o r r i d o r and noted t h e movement of s m o k e f r o m the f i r e a r e a into t h e c o r r i d o r . The l e v e l of the n e u t r a l plane could be

d e t e r m i n e d t o an a c c u r a c y of about

*

4 in. by watching t h e behaviour of t h e s m o k e at a s m a l l v e r t i c a l c r a c k adjacent t o the door.

During the whole c o u r s e of t h e t e s t , with t h e p o s s i b l e exception of t h e f i r s t 3 m i n u t e s , t h e n e u t r a l plane r a n g e d between points 1 o r 2 in. below and

6

in. above the top of the door. T h i s v a r i a t i o n w a s not obviously

r e l a t e d t o t i m e .

P e r i o d i c a l l y t h e f a n s w e r e switched on f o r s h o r t p e r i o d s , but it w a s found t h a t t h e y had l i t t l e influence on t h e l e v e l of the n e u t r a l plane. A m a n o m e t e r i n s t a l l e d between t h e c o r r i d o r and t h e e x t e r i o r indicated t h a t t h e p r e s s u r e differential c r e a t e d by t h e f a n s w a s l e s s than 0. 01 in. of w a t e r . T h e r e w e r e n u m e r o u s c r a c k s in t h e walls and e n d s of t h e c o r r i d o r and the m a r k e d effect t h e s e can have on p r e s s u r e differential i s shown in Appendix

C . T h u s , a fan c a p a c i t y of up t o 200 cfm should be provided for t h e s m a l l gap t o be found beneath a s i n g l e . d o o r . Unless v e r y high-capacity fans o r r e l a t i v e l y well s e a l e d e n c l o s u r e s a r e involved t h e p r e s s u r i z i n g of a r e a s i s not l i k e l y t o be p r a c t i c a l .

T h e f a n s p e r f o r m e d another v e r y useful function, h o w e v e r , and t h a t w a s t h e r a p i d c l e a r i n g of any s m o k e that w a s allowed t o e n t e r the

c o r r i d o r . The throughput of the fans constituted a volume change e v e r y

8 m i n s o that t h i s r e s u l t i s not s u r p r i s i n g .

T h e l i m i t e d m i g r a t i o n of s m o k e t o the c o r r i d o r r e s u l t e d a l m o s t e n t i r e l y f r o m t h e s t e a d y - s t a t e phenomenon t h a t w a s d e s c r i b e d in t h e p r e v i o u s

s e c t i o n s i n c e t h e r e l a t i o n s h i p b e t w e e n t h e o u t l e t a r e a and v o l u m e of t h e r o o m m a d e t h e t r a n s i e n t , t h e r m a l e x p a n s i o n p h e n o m e n o n i n s i g n i f i c a n t . It is t h e r e f o r e i n t e r e s t i n g t o c o m p a r e t h e l e v e l of t h e n e u t r a l p l a n e with t h e c o n d i t i o n s given b y t h e e x p r e s s i o n d e r i v e d i n Appendix B :

( T h e s y m b o l s u s e d i n t h i s e x p r e s s i o n a r e defined by F i g u r e 2,)

T h e only v a r i a b l e i n t h e above e x p r e s s i o n which w a s not m e a s u r e d d u r i n g t h e t e s t w a s

T ,

and s u b s t i t u t i n g t h e v a l u e s of t h e o t h e r p a r a m e t e r s g i v e s T

=

1190°C. T h i s v a l u e i s p r o b a b l y h i g h e r t h a n t h e m e a n g a s t e m p e r a t u r e i n t h e r o o m , but i t i s of t h e r i g h t o r d e r , i n d i c a t i n g t h a t t h e s i m p l e t h e o r y depending on t h e a s s u m p t i o n of s t a g n a n t g a s e s within t h e e n c l o s u r e i s a r e a s o n a b l y c l o s e a p p r o x i m a t i o n t o t h e t r u t h . Any m o r e a c c u r a t e d e t e r m i n a t i o n of t h e a p p r o p r i a t e t e m p e r a t u r e s t o u s e i n t h e e x p r e s s i o n , for t h e v a r i o u s c o n d i t i o n s l i k e l y t o b e e n c o u n t e r e d i n p r a c t i c e , would involve a s u b s t a n t i a l p r o g r a m of w o r k which could not b e c o n t e m p l a t e d a t t h e t i m e of w r i t i n g of t h i s r e p o r t . It i s a l s o doubtful w h e t h e r m o r e

a c c u r a t e i n f o r m a t i o n would find m u c h p r a c t i c a l application.

C o m ~ l i c a t i n e F a c t o r s

In applying t h e s i m p l e c r i t e r i a quoted i n t h i s r e p o r t t h e fir s t c o m p l i c a t i n g f e a t u r e t h a t m u s t b e r e c o g n i z e d , although i t i s v i r t u a l l y s e l f - a p p a r e n t , i s of s u c h i m p o r t a n c e t h a t i t i s w o r t h e m p h a s i z i n g . T h e a r e a s A l , A2 and A3 r e f e r t o openings a s t h e y w i l l e x i s t a t s o m e t i m e d u r i n g t h e c o u r s e of a f i r e . T h u s , i f a r e g i o n i n c l u d e s a s u b s t a n t i a l a r e a of s i m p l e glazing, a c c o u n t m u s t b e t a k e n of t h e f a c t t h a t i t w i l l p r o b a b l y f a l l out d u r i n g t h e c o u r s e of a f i r e . A c o n s e r v a t i v e c a l c u l a t i o n would a s s u m e t h a t h i g h - l e v e l g l a z i n g would r e m a i n i n t a c t w h e r e a s low - l e v e l g l a z i n g would f a i l out.

T h e d e s i g n c r i t e r i o n t h a t follows f r o m t h e argurrlents given i n t h i s r e p o r t i s t h a t s i m p l e windows i n e x t e r i o r w a l l s , a t a l e v e l h i g h e r t h a n t h e door l i n t e l , a r e a c c e p t a b l e . Windows a t a l o w e r l e v e l m u s t b e c o n s i d e r e d a s u n d e s i r a b l e openings u n l e s s s p e c i a l p r e c a u t i o n s h a v e b e e n t a k e n t o k e e p t h e m i n p l a c e . W i r e d g l a s s , f o r e x a m p l e , m i g h t r e m a i n i n t a c t f o r up t o a n h o u r . T o e n s u r e t h a t t h e a p p r o p r i a t e d o o r s c l o s e i n t h e e v e n t of a f i r e and t h a t t h e a p p r o p r i a t e h i g h - l e v e l openings a r e e s t a b l i s h e d , i n t e r

-

c o n n e c t i o n with a f i r e d e t e c t o r s y s t e m ( o r p o s s i b l y a s p r i n k l e r s y s t e m ) is p r o b a b l y e s s e n t i a l . T h u s t h e h i g h - l e v e l opening m i g h t b e n o r m a l l y s e a l e d b y a f l a p h e l d c l o s e d by a s i m p l e e l e c t r o - m a g n e t . When t h e p o w e r i s d i s - c o n n e c t e d a s p r i n g could t h e n f o r c e t h e f l a p open. D e v i c e s of t h i s n a t u r e intended t o h o l d opet-I d o o r s t h r o u g h which t h e r e i s h e a v y t r a f f i c , a r e a l r e a d y a p p e a r i n g on t h e N o r t h A m e r i c a n m a r k e t .

A

second f e a t u r e that i s well w o r t h c o n s i d e r i n g i s the effect of e x t e r i o r wind on t h e movement of s m o k e i n a building, f o r p r e s s u r e s of about 0. 3 in. of w a t e r c a n be developed by a 25-rnph wind. In s o m e l o c a t i o n s t h e r e i s a p r e v a i l i n g wind d i r e c t i o n and no action a t a l l is n e c e s s a r y w h e r e t h e high l e v e l vent a r e a s will open i n a l e e w a r d direction. E n c l o s u r e s on t h e windward s i d e of a building, h o w e v e r , should be a r r a n g e d t o have openings t o l e e w a r d i n t h e event of a f i r e . S o m e duct w o r k would t h e r e f o r e be n e c e s s a r y and, i f t a k e n t o t h e top of t h e building, it would s e r v e a n additional function by giving a c h i m n e y action and hence v e r y g r e a t l y r a i s i n g t h e l e v e l of t h e n e u t r a l plane. Much of t h e duct w o r k could be common t o s e v e r a l e n c l o s u r e s .

A

f e a t u r e t h a t h a s b e e n ignored in t h e t r e a t m e n t , s o f a r , i s t h a t m o s t openings have finite height and t h e effective height of s u c h an opening i s not e x a c t l y equal t o t h e m e a n height above the n e u t r a l plane. F o r a n opening of c o n s t a n t width, however, t h e c o r r e s p o n d e n c e i s

sufficiently c l o s e (between 0.88 and unity) a s t o m a k e t h e d i s c r e p a n c y not too i m p o r t a n t . Where multiple openings a r e c o n s i d e r e d , t h e a c c u r a t e e x p r e s s i o n for t h e effective height of _{a n u m b e r of a r e a s A1, A 2 , A3 etc.} a t h e i g h t s h i , h2, h 3 etc. above the n e u t r a l plane is:

T h i s e x p r e s s i o n i s c u m b e r s o m e , however, and i n g e n e r a l a s i m p l e a v e r a g e on a n a r e a b a s i s would be sufficiently a c c u r a t e .

Conclusions

1. T h e movement of s m o k e in building f i r e s r e s u l t s p r i n c i p a l l y f r o m a s t e a d y - s t a t e phenomenon causing g a s e s t o flow out of h i g h - l e v e l o u t l e t s , t o be r e p l a c e d b y a i r e n t e r i n g at low-level i n l e t s . A n e u t r a l

p r e s s u r e plane e x i s t s below which openings will c o n s t i t u t e i n l e t s and above which openings will c o n s t i t u t e outlets. T h e l e v e l of t h e n e u t r a l plane i s given by t h e e x p r e s s i o n :

In o r d e r t o e s t i m a t e t h e value of A2 (high-level o u t l e t ) , n e c e s s a r y t o give a sufficiently high n e u t r a l plane, it i s r e a s o n a b l e t o a s c r i b e a value of 1450°K (approx, 1180°C) t o t h e absolute t e m p e r a t u r e T. In a n a r e a t o be maintained s m o k e f r e e i t m u s t a l s o be r e m e m b e r e d t h a t a s m a l l opening communicating with t h e e x t e r i o r i s e s s e n t i a l ( F i g u r e I b).

2. During t h e e a r l y s t a g e s of a f i r e , s m o k e m o v e m e n t will a l s o r e s u l t f r o m t h e r m a l expansion of the g a s e s in t h e f i r e a r e a . In g e n e r a l a high l e v e l opening of 2 s q ft p e r 10, 000 cu ft of volume will be sufficient t o e n s u r e t h a t the s m o k e does not flow t o other p a r t s of t h e building

( a s s u m i n g t h e a p p r o p r i a t e d o o r s , etc.

,

a r e closed).

3. Variation of the l e v e l of t h e n e u t r a l plane between one r e g i o n and another by t h e u s e of f a n s will u s u a l l y r e q u i r e v e r y high c a p a c i t y f a n s sufficient t o e s t a b l i s h a velocity of about 12 f t / s e c at a l l t h e v a r i o u s c r a c k s and other openings which will a l m o s t c e r t a i n l y exist. E v e n i f t h i s

objective is not achieved, however, a volume change e v e r y few m i n u t e s , in an a r e a t o be maintained a s f r e e of s m o k e a s p o s s i b l e , will r a p i d l y d i s - p e r s e s m o k e i n a d v e r t e n t l y admitted when, s a y , a door t o the f i r e a r e a i s t e m p o r a r i l y opened.

4. If t h e t o t a l quantity of s m o k e g e n e r a t e d during a f i r e is t o be l i m i t e d it would be helpful t o r e s t r i c t the f u e l s involved on t h e b a s i s of a "smoke-index" given by s o m e suitable t e s t .

R e f e r e n c e

1. S h o r t e r , G , W . ,

J.

H. McGuire, N. B. Hutcheon and R. F. Legget. The St. Lawrence B u r n s. Q u a r t e r l y of t h e NFPA, Vol. 53, No. 4, A p r i l 1960, p. 300-316. ( r e p r i n t e d a s NRC 5730).

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GAS DENSITY

P

0

ABSOLUTE TEMP To

- -

Po

1

GAS DENSITY

pe

- - -

P o - - -

ABSOLUTE TEMP

T = T o + B

L

J

i'

AREA A,

VOLUME

V

A P P E N D I X

A

TKE LOCATION O F THE NEUTRAL PRESSURE PLANE

If

it is a s s u m e d t h a t t h e g a s e s within

a

f i r e e n c l o s u r e a r e s t a g n a n t , t h e n F i g u r e 2 i l l u s t r a t e s t h e conditions t h a t w i l l p r e v a i l , t h e i m p o r t a n t f e a t u r e being t h a t a n e u t r a l p r e s s u r e p l a n e e x i s t s , below which all openings will constitute i n l e t s and above which all openings w i l l

c o n s t i t u t e o u t l e t s .

T h e l e v e l of t h e n e u t r a l p l a n e is a function of t h e p a r a m e t e r s defined i n F i g u r e 2 and m a y be d e t e r m i n e d b y a n a p p l i c a t i o n of B e r n o u l l i ' s e x p r e s s i o n t o t h e g a s v e l o c i t i e s v l and v2 a t t h e i n l e t and outlet

r e s p e c t i v e l y . B e r n o u l l i s t a t e s t h a t t h e v e l o c i t y (v) t h r o u g h a n o r i f i c e , f o r n o n - v i s c o u s flow, i s r e l a t e d t o t h e p r e s s u r e d i f f e r e n c e e i t h e r s i d e of t h e o r i f i c e by t h e e x p r e s s i o n :

P r e s s u r e d i f f e r e n c e

=

pvL/2

w h e r e p is the density of t h e g a s e s and v t h e i r velocity.

Applying Bernoulli's e x p r e s s i o n t o t h e two openings,

and H e n c e The p a r a m e t e r s d e n s i t y and v e l o c i t y m a y b e e l i m i n a t e d f r o m t h e f a c t t h a t : and f r o m m a s s c o n s e r v a t i o n c o n s i d e r a t i o n s , t h a t : In d i s c u s s i n g p r a c t i c a l conditions r e l a t i n g t o m a s s flow a n o r i f i c e f a c t o r

would need t o be included t o c a t e r f o r f r i c t i o n a l l o s s e s . In g e n e r a l , h o w e v e r , t h e f a c t o r s r e l a t i n g t o t h e two openings as i l l u s t r a t e d i n F i g u r e 1 w i l l h a v e t h e s a m e value ( 0 .

6)

and will not t h e n a p p e a r i n t h e solution f o r t h e l e v e l of the n e u t r a l plane. Substituting t h e s e e x p r e s s i o n s in equation ( 1 ) g i v e s

Equation ( 2 ) i s of the u t m o s t i m p o r t a n c e for i t shows that t h e location of the n e u t r a l plane m a y be r e a d i l y r a i s e d b y i n c r e a s i n g t h e r a t i o of the high level openings t o the low l e v e l openings.

APPENDIX

B

COMPOSITE

P R O B I S M (VARYING TEMPERATURE)

Where t h e t e m p e r a t u r e i s v a r y i n g i n a f i r e e n c l o s u r e , t h e conditions i l l u s t r a t e d in F i g u r e 3 can r e a d i l y be e s t a b l i s h e d . By

r e s t r i c t i n g t h e t o t a l a r e a (A1

+

A2

+

Ag) it is a l s o p o s s i b l e t h e o r e t i c a l l y , and p r o b a b l y in p r a c t i c e , t o e n s u r e t h a t

all

t h e s e a r e a s c o n s t i t u t e o u t l e t s and t h a t t h e concept of a n e u t r a l plane i s invalid. T h i s situation i s t o be1 avoided, however, and will t h e r e f o r e not be d i s c u s s e d h e r e .

With t h e conditions i l l u s t r a t e d in F i g u r e 3, t h e vena c o n t r a c t a v e l o c i t i e s v l and v2 will again be governed b y Bernoulli's e x p r e s s i o n , s o t h a t p r e c i s e l y t h e s a m e e x p r e s s i o n s will r e s u l t a s in Append'ix A, viz:

Again, t h e d e n s i t y p a r a m e t e r s m a y b e e l i m i n a t e d b y t h e r e l a t i o n s h i p po

/

pe

=

T / T o . T h i s r e l a t i o n s h i p a l s o g i v e s t h e r e s u l t t h a t (P,

-

pe)

/

po

=

B / T s o t h a t equations ( 1 ) and ( 2 ) b e c o m e

and 2 h2gf3/T

=

v

2

0 2 ( 4)

T h e m a s s c o n s e r v a t i o n equation, which m u s t now be invoked, d i f f e r s f r o m t h a t given in Appendix A, f o r i t m u s t include a t e r m c o v e r i n g the expansion i n t h e f i r e volume. It i s

w h e r e n i s a n o r i f i c e f a c t o r a s s o c i a t e d with t h e vena c o n t r a c t a and will in g e n e r a l have t h e value 0.

6 .

E l i m i n a t i n g p and p

Equations ( 3 ) , ( 4 ) and

( 6 )

a r e sufficient in n u m b e r , t h e o - r e t i c a l l y , t o allow t h e elimination of v l and v2. Unfortunately a n

a t t e m p t at a solution gives a v e r y c u m b e r s o m e q u a d r a t i c equation. The s i m p l i c i t y of t h e solution for the s p e c i a l c a s e d i s c u s s e d in Appendix A s u g g e s t s , however, t h e following technique. Consider t h e outlet a r e a t o b e divided into two p a r t s (A2 and A3) a s i l l u s t r a t e d in F i g u r e 3, the a r e a A2 being a s s o c i a t e d with t h e s t e a d y - s t a t e p r o c e s s and t h e a r e a A3 with t h e t h e r m a l expansion.

Equation

( 6 )

then divides into two p a r t s , a s follows:

and

Equations ( 3 ) , ( 4 ) and ( 7 ) then give the s a m e r e s u l t

as

i n Appendix A,

i.

e .

Equations (4) and (8) give the r e s u l t

w r i t i n g n = 0.

6.

The f o r m of equations ( 9 ) and (11) i s v e r y i n t e r e s t i n g for t h e y a r e , t o a c o n s i d e r a b l e extent, c o m p l e m e n t a r y . At t h e e a r l i e s t s t a g e s of a f i r e when T

3

T o , A2 need not b e a s l a r g e a s l a t e r on when T h a s a t t a i n e d i t s m a x i m u m . On the other hand, s i n c e both a T and a

8

l / 2 a p p e a r i n t h e denominator of equation ( 1 1) A3 will need t o be l a r g e in t h e e a r l y s t a g e s . A s an equilibrium t e m p e r a t u r e is attained (and hence d ~ / d t

=

0). A3 m a y tend t o z e r o . In p r a c t i c e A3 will n e v e r be a d e q a a t e l y l a r g e i n t h e e a r l y

Figure 4

is a g r a p h of A 3 a g a i n s t

0

f o r T o = 300" K

( 2 7 "

C ) ,

V

=

10,000 c u f t , h 2

=

2 ft, and d ~ / d t

=

1 ' ~ / s e c o r lZOO°C r i s e i n

APPENDIX C

FLOWS AND PRESSURES

The possible effect which a fan might have in preventing t h e flow of s m o k e f r o m one a r e a t o another m a y be quite r e a d i l y e s t i m a t e d t h e o r e t i c a l l y .

F r o m Appendix A the p r e s s u r e difference causing s m o k e t o flow f r o m one a r e a t o another a t a high l e v e l i s

Writing % / p e = '/To and

0

= T - T

0 gives

Where a single s t o r e y c o m p a r t m e n t i s being c o n s i d e r e d t h e m a x i m u m value which 6p i s l i k e l y t o a s s u m e i s 0. 1 in. of w a t e r , which would r e s u l t w h e r e , s a y , t h e r e i s a n opening 8 ft

6

in. above t h e n e u t r a l plane and a t e m p e r a t u r e r i s e of 1200°C i s involved.

If a fan i s t o be expected t o e s t a b l i s h a c o m p a r a b l e p r e s s u r e i n an a r e a t o be maintained f r e e of s m o k e , i t will g e n e r a l l y need t o be of high c a p a c i t y b e c a u s e of t h e v a r i o u s c r a c k s and s m a l l openings t h a t w i l l be found in t h e s u r f a c e s bounding t h e a r e a .

The a i r velocity t o be expected a t c r a c k s , e t c . can again be e s t i m a t e d f r o m an application of Bernoulli's e x p r e s s i o n which gives

where

h

= water head ( f t )

W

-

Pw = density of water

and Po = density of a i r ( s a m e units a s p )

W

The f a c t o r 0. 6 h a s been introduced a s a n o r i f i c e f a c t o r , It i s probably

m o r e fittingly applied a s an a r e a c o r r e c t i o n , but applying i t t o the p a r a m e t e r velocity gives t h e s a m e end r e s u l t s o f a r a s m a s s flow i s concerned.

3

Hence v

=

( 1 . 6

x 10

H

) 0. 5 w h e r e

H

=

w a t e r h e a d (in. )

W W 7

T h u s w h e r e

H

=

0. 1 in. w a t e r , v

=

1 2 . 7 f t / s e c . W

Such a velocity i s quite s u b s t a n t i a l and t h u s a volume t h r o u g h - put of o v e r 200 c u f t / m i n would r e s u l t beneath a t y p i c a l door ( 3 f t 3 in.

wide) with a gap of one inch between i t and t h e f l o o r . Such gaps a r e common, r a t h e r than t h e exception, often being left t o allow f o r t h e laying of c a r p e t s .