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Experimental inert gas generator
NATIONAL RESEARCH COUNCIL
CANADA
DIVISION O F BUILDING RESEARCH
AN E X P E R I M E N T A L INERT GAS GENERATOR
b y
J.H. McGuire
I n t e r n a l R e p o r t NO.
294
oft h e
Division
ofBuilding R e s e a r c h
OTTAWA
May
1 9 6 4
PREFACE
T h e many difficulties a s s o c i a t e d with the fighting of deep- s e a t e d building f i r e s by conventional methods h a s led t o a consideration of p o s s i b l e a l t e r n a t i v e a p p r o a c h e s . One of t h e s e , the u s e of i n e r t g a s on a m a s s i v e s c a l e h a s r e c e i v e d attention a t the B r i t i s h Joint F i r e R e s e a r c h Organization i n the U. K . , and i s of much i n t e r e s t a l s o t o the F i r e Section of the Division. Some d i s c u s s i o n of the p o s s i b i l i t i e s and r e q u i r e m e n t s a r e now p r e s e n t e d , together with a r e v i e w of t h e w o r k that h a s been c a r r i e d out t o c o n v e r t a c o m m e r c i a l l y available g a s b u r n e r into a s m a l l - s c a l e a p p a r a t u s suitable for f u r t h e r exploration of t h e method.
The a u t h o r , a p h y s i c i s t , is a r e s e a r c h officer with t h e F i r e Section concerned with the physical c h a r a c t e r i s t i c s of f i r e s in buildings.
Ottawa May
1 9 6 4
N. B. Hutcheon A s s i s t a n t D i r e c t o r
AN EXPERIMENTAL INERT GAS GENERATOR
J. H.
McGuireBuilding f i r e s can b e r e a d i l y extinguished by conventional techniques, but only when i t i s possible t o b r i n g h o s e j e t s o r fog
nozzles t o b e a r on a substantial proportion of t h e heated solid s u r f a c e s giving r i s e t o the volatiles actually involved in t h e combustion p r o c e s s . Unfortunately, t h i s l a t t e r condition can not always b e m e t and on t h e s e occasions t h e best that f i r e f i g h t e r s can do i s t o confine t h e f i r e t o one building o r compartment and t o e n s u r e that it does not s p r e a d to
adjacent property.
In G r e a t B r i t a i n and North A m e r i c a i n p a r t i c u l a r , two alternative fire-fighting techniques a r e being considered and each shows s o m e p r o m i s e for c e r t a i n c l a s s e s of fire-fighting p r o b l e m s . One approach, which will not be c o n s i d e r e d in any detail h e r e , i s the u s e of high expansion foam which was originally developed i n G r e a t B r i t a i n t o combat f i r e s in mine roadways. M a s s i v e s c a l e high e x - pansion foam g e n e r a t o r s a r e now being m a r k e t e d in t h e U. S. A. and i t i s hoped that the flow p r o p e r t i e s of the foam will allow t h e extinguish- ment of f i r e s in r e l a t i v e l y i n a c c e s s i b l e p a r t s of a building,
The approach t h a t i s the main concern of t h i s r e p o r t i s t h e m a s s i v e u s e of i n e r t gas which was f i r s t conceived in a p r a c t i c a l
manner by the B r i t i s h Joint F i r e R e s e a r c h Organization within t h e l a s t decade. In o r d e r t o a s s e s s t h e application of t h i s technique a number of f a c t o r s r e q u i r e consideration and t h e s e will .now be d i s c u s s e d individually.
P e r m i s s i b l e Oxveen Content of Inert Gas
When the oxygen concentration i n an a t m o s p h e r e i s p r o - gr e s sively reduced, t h e upper and lower flammability l i m i t s of combustible g a s e s and vapours c o m e c l o s e r together and finally
m e e t (1, 2). In m o s t c a s e s t h i s meeting point i s a t an oxygen concen- t r a t i o n (of the a t m o s p h e r e p r i o r t o t h e addition of the flammable g a s ) of m o r e than 12 p e r cent. T h e r e a r e s t a r t l i n g exceptions, however, such a s hydrogen and c a r b o n monoxide, f o r which the values a r e approximately 5 and 7 p e r cent r e s p e c t i v e l y , but it i s not likely that t h e s e g a s e s will b e g r e a t l y involved i n building f i r e s . The n a t u r e of t h e other g a s e s constituting the a t m o s p h e r e can a l s o modify t h e flammability l i m i t s , but a s t h e effect will a l m o s t invariably favour the u s e of i n e r t g a s a s an extinguishing agent it i s justifiable t o ignore it. In g e n e r a l , t h e r e f o r e , if t h e oxygen concentration of t h e a t m o s p h e r e in a burning building i s reduced below about 12 p e r cent,
A f i r e fighter might a s k that an i n e r t gas should not m e r e l y s u p p r e s s flaming combustion but should a l s o s u p p r e s s smouldering so that t h e r e i s no likelihood of the f i r e rekindling when the oxygen con- tent of the a t m o s p h e r e r e t u r n s to i t s n o r m a l level. Work r e p o r t e d by the B r i t i s h Joint F i r e R e s e a r c h Organization (3) indicates that
smouldering of c o r k dust and mixed hardwood sawdust will not continue in a t m o s p h e r e s with oxygen concentrations of about
9
and 1 2 1 p e r cent respectively at ambient t e m p e r a t u r e . At higher a t m o s p h e r i c t e m p e r - a t u r e s smouldering o r s o m e other exothermic p r o c e s s can take place a t lower oxygen concentrations. With a l l the techniques to be discussed i n t h i s r e p o r t , however, the atmospheric t e m p e r a t u r e in a building will be reduced t o below 3 0 0 ° F a f t e r injection h a s been continued for some t i m e . J . R . J u t r a s (4) finds that at a t e m p e r a t u r e of 450'F smouldering i s not readily sustained in wood sawdust at oxygen concentrations of l e s s than9.
5 p e r cent.The above considerations f o r m the b a s i s for the i n e r t gas approach t o the extinguishment of building f i r e s , t o be discussed in this report.
Required R a t e of Generation
The r a t e of generation of the gas m u s t be sufficient t o m e e t two r e q u i r e m e n t s . F i r s t l y , the equilibrium conditions that will be
established m u s t be satisfactory, and secondly the t i m e taken t o attain equilibrium conditions m u s t be reasonably low since it will correspond, approximately, t o the t i m e a t which a f i r e m a n would d e s c r i b e the f i r e a s "under control.
The existence of the fir s t requirement allows simplification of the calculation of the t i m e to attain equilibrium which will r e s u l t f r o m one of two mechanisms. The injection p r o c e s s might c o r r e s p o n d to the filling of a bucket with water (or p r e c i s e l y the i n v e r s e ) in which c a s e equilibrium will be established when one volume of the g a s i s injected into the enclosure. At the other e x t r e m e , if p e r f e c t mixing o c c u r r e d , the behaviour would be a s shown in Table I which h a s been derived f r o m Equation 3 of Appendix A. In t h i s c a s e equilibrium can be considered to be virtually established when t h r e e volumes of gas have been injected. F o r the purpose of this r e p o r t an estimation of this t i m e t o a g r e a t e r a c c u r a c y i s not called for.
In general, f a i l u r e t o establish s a t i s f a c t o r y equilibrium conditions, in the f i r e compartment will r e s u l t f r o m density differences between the g a s e s within the building and the a i r outside. T o derive the most stringent injection r e q u i r e m e n t s it i s w i s e t o consider the phenomenon t o be exactly analogous t o the filling of a leaky bucket with water (or ~ r e c i s e l y the i n v e r s e i f the density of the injected g a s e s i s
lower t h a n t h a t of a i r ) . T h e r e l e v a n t v a r i a b l e s a r e r e l a t e d by Equation 5 (Appendix B ) and a s t h e d e n s i t y of t h e i n j e c t e d g a s i s involved t h e e x p r e s s i o n m u s t b e e v a l u a t e d s e p a r a t e l y for e a c h g a s c o n s i d e r e d .
R e q u i r e d D e l i v e r y P r e s s u r e
As a m a t t e r of convenience g a s e s a r e often s t o r e d a t high p r e s s u r e a n d t h i s f a c t , t o g e t h e r with t h e development i n G r e a t B r i t a i n of a m a s s i v e i n e r t g a s g e n e r a t o r b a s e d on a n a i r c r a f t g a s t u r b i n e engine, h a s l e d s o m e l a y c i r c l e s t o a s s u m e t h a t a p p r e c i a b l e p r e s s u r e s a r e r e q u i r e d f o r r a p i d d e l i v e r y . T h i s a s s u m p t i o n i s f a r f r o m the t r u t h a n d i n fact p r e v a i l i n g p r e s s u r e s w i l l u s u a l l y b e of t h e o r d e r of i n c h e s of w a t e r . T h e l a r g e s t i n e r t g a s g e n e r a t o r i n e x i s t e n c e ( t h e B r i t i s h "jet e n g i n e t t g e n e r a t o r ) h a s a n output of about 40, 000 c f m and s u c h a n output could b e conveniently conducted t h r o u g h a 3 -ft d i a m e t e r duct a t a v e l o c i t y of 90 f t / s e c (60 mph). T h e v e l o c i t y h e a d under t h e s e conditions, depending on t h e n a t u r e a n d t e m p e r a t u r e of t h e g a s e s , would c o r r e s p o n d t o between 1 and 2 i n , of w a t e r and t h e p r e s s u r e d r o p o v e r a 100 -ft length of duct would be l e s s than 10 in. of w a t e r .
S o u r c e s of I n e r t G a s
T h e i d e a l c h o i c e of i n e r t g a s would b e one t h a t , a t 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 , had t h e s a m e d e n s i t y a s a i r a t a m b i e n t t e m p e r a t u r e . In t h i s c a s e t h e r e would be n o g a s e o u s l o s s e s of t h e t y p e d i s c u s s e d i n a p r e v i o u s p a r a g r a p h b y analogy with t h e filling of
a l e a k y bucket. C a r b o n dioxide a p p e a r s t o b e a v e r y s u i t a b l e c h o i c e f r o m t h i s point of view for i t s m o l e c u l a r weight i s 44 c o m p a r e d t o a value of 29 which i s a p p r o x i m a t e l y t h e m e a n m o l e c u l a r weight of a i r . At a t e m p e r a t u r e of 1 4 0 ° C GO2 w i l l t h u s h a v e t h e s a m e d e n s i t y a s a i r a t 0 ° C .
If C 0 2 w e r e t o b e u s e d on a building of d i m e n s i o n s 50 b y 100 b y 40 ft high, t h e quantity r e q u i r e d , in t h e f i r s t i n s t a n c e , would b e s o m e t h i n g of t h e o r d e r of t h e v o l u m e of t h e building, 200, 000 c u ft. I t s weight would be about 10 t o n s and, if t r a n s p o r t e d i n t h e 4 ft by
9
in. d i a m e t e r s i z e of c y l i n d e r , t h e c y l i n d e r s would account f o r a n o t h e r 10 t o n s o r s o . T h e c o s t of t h e GO2 would p r o b a b l y b e of t h e o r d e r of $2,000 s o t h a t t h e d e m e r i t s of t h e t e c h n i q u e a r e p r o b a b l y m a i n l y a s s o c i a t e d with c o s t and d e l i v e r y . N i t r o g e n is a n o t h e r g a s t h a t i s w o r t h c o n s i d e r i n g f o r f i r e - fighting a p p l i c a t i o n s . In t h i s c a s e t h e m o l e c u l a r weight a p p r o x i m a t e s t o t h e m e a n m o l e c u l a r weight of a i r and h e n c e n i t r o g e n could b e m o r e r e a d i l y r e t a i n e d in a building t h a n could CO a s the building cooleddown. A s cooling might involve s o m e s u b s t a n t i a l p e r i o d of t i m e t h e composite u s e of C 0 2 and nitrogen would b e t h e o r e t i c a l l y d e s i r a b l e . In the f i r s t i n s t a n c e C 0 2 would b e i n j e c t e d and a s t h e building cooled down n i t r o g e n would be used. T h e p r i n c i p a l d e m e r i t of n i t r o g e n would be that i t s cost i s m a n y t i m e s g r e a t e r than t h a t of GO2.
Without going into f u r t h e r d e t a i l on t h e application of
s t o r e d c o m m e r c i a l g a s e s it will be a p p r e c i a t e d that c o s t i s an i m p o r t a n t factor and a l t e r n a t i v e g e n e r a t i n g techniques a r e worth investigating with a view t o producing an a p p r o p r i a t e gas at a lower c o s t .
Generation of I n e r t G a s by Combustion
F o r m a n y y e a r s equipment h a s e x i s t e d in which the oxygen content of the output h a s been s u b s t a n t i a l l y r e d u c e d by burning any convenient fuel and i t h a s p r o v e n v e r y effective in combatting f i r e s in s h i p s ' holds
( 6 ) .
The output f r o m t h e combustion c h a m b e r of s u c h a g e n e r a t o r i s , of c o u r s e , not i m m e d i a t e l y suitable for u s e b e c a u s e of i t s high
t e m p e r a t u r e and i t i s c u s t o m a r y t o cool i t with w a t e r . T h i s can be achieved e i t h e r by a h e a t e x c h a n g e r , allowing the w a t e r ( o r s t e a m ) t o r u n t o w a s t e or a l t e r n a t i v e l y by vapourizing w a t e r in the output. The l a t t e r technique h a s the advantage t h a t i t i n i t i a l l y a u g m e n t s the g a s
supply, but i t a l s o h a s the disadvantage that t h e m e a n density of t h e out- put will be r e d u c e d thus giving r i s e t o g r e a t e r l o s s e s f r o m high l e v e l openings a s d i s c u s s e d e a r l i e r .
The output of the combustion c h a m b e r will be l a r g e l y nitrogen which h a s a m o l e c u l a r weight of 28, a p p r o x i m a t i n g c l o s e l y t o the m e a n m o l e c u l a r weight of a i r , about 29. Water vapour on t h e other hand h a s a m o l e c u l a r weight of 18. T h e volume of water vapour
g e n e r a t e d in lowering t h e t e m p e r a t u r e t o about 1 0 0 ° C will in g e n e r a l be of t h e s a m e o r d e r a s the output s o t h a t t h e m e a n m o l e c u l a r weight will drop t o about 23. T h e f a i l u r e t o r e d u c e t h e t e m p e r a t u r e t o n e a r ambient will a l s o influence d e n s i t y t o an even g r e a t e r extent.
F o r t h e s e and other r e a s o n s i t h a s been c u s t o m a r y t o cool the combustion c h a m b e r exhaust g a s e s with h e a t exchanges and t o p r e s e n t an output which c o n s i s t s m a i n l y of n i t r o g e n together with s o m e m i s c e l l a n e o u s p r o d u c t s of combustion s u c h a s C 0 2 and w a t e r .
Until r e c e n t l y t h e g r e a t e s t output f r o m a g e n e r a t o r of t h i s type h a s been of t h e o r d e r of 600 c u f t p e r m i n and by both t h e c r i t e r i a
d i s c u s s e d in t h e s e c t i o n "Required R a t e of Generation" t h i s i s quite low. T h u s i f a c o m p a r t m e n t in which a f i r e i s o c c u r r i n g h a s a volume
of 1 0 0 , 0 0 0 cu f t , o v e r 3 h o u r s w i l l e l a p s e b e f o r e one volume of g a s h a s b e e n injected. If t h e height of t h e c o m p a r t m e n t i s 20 ft and t h e m e a n g a s t e m p e r a t u r e is 100" C , t h e m a x i m u m p e r m i s s i b l e high l e v e l opening ( f r o m E q u a t i o n 5, Appendix B ) w i l l b e l e s s t h a n 1 s q ft.
W h e r e t h e c o m p a r t m e n t c o n c e r n e d i s t h e hold of a s h i p , battening down c a n m e e t t h e second r e q u i r e m e n t and t h e l o n g - t i m e
s c a l e i s t o l e r a b l e on t h e g r o u n d s t h a t no o t h e r f i r e - f i g h t i n g t e c h n i q u e i s l i k e l y t o b e s u c c e s s f u l .
F o r w i d e r a p p l i c a t i o n m u c h g r e a t e r outputs a r e d e s i r a b l e , but t h i s h a s been difficult t o a c h i e v e owing t o a l a c k of r e a d i l y t r a n s - p o r t a b l e high output c o m b u s t i o n d e v i c e s . If, a s in a p r e v i o u s e x a m p l e , we c o n s i d e r a building 50 by 100 b y 40 f t high, t h e d i s c u s s i o n given e a r l i e r in t h i s r e p o r t w i l l give t h e conclusion t h a t t h e i n j e c t i o n r a t e should b e of t h e o r d e r of 40, 000 c u ft /min. D e l i v e r y will t h u s b e a t t h e r a t e of 1 volume e v e r y 5 m i n and, n e g l e c t i n g l o s s e s , t h e oxygen c o n c e n t r a t i o n g e n e r a l l y should b e w e l l down Within 1 5 m i n which would a p p e a r t o b e a n a p p r o p r i a t e o r d e r of t i m e . Subject t o s o m e v a r i a t i o n , depending on t h e t e m p e r a t u r e and t h e n a t u r e of t h e output g a s , E q u a t i o n 5 (Appendix B ) i n d i c a t e s that t h e m a x i m u m p e r m i s s i b l e high l e v e l
opening in a 40 -ft high building w i l l b e of t h e o r d e r of 25 s q ft. It would p r o b a b l y b e unwise t o design a m a c h i n e intended f o r wide application which r e q u i r e d high l e v e l openings i n buildings t o b e s u b s t a n t i a l l y l o w e r than t h i s value.
T h e o r d e r of h e a t g e n e r a t i o n of a m a c h i n e with a n output of 40, 000 cu ft/min i s 60 m i l l i o n ~ t u / h r and u n t i l v e r y r e c e n t l y i t had
been a s s u m e d t h a t a s i m p l e c o m b u s t i o n d e v i c e b u r n i n g a t t h i s r a t e would h a v e s u b s t a n t i a l p h y s i c a l d i m e n s i o n s . F o r e x a m p l e , m a n y e n g i n e e r s adopt t h e r o u g h r u l e t h a t t h e output f r o m a c o m b u s t i o n c h a m b e r w i l l not u s u a l l y e x c e e d 50, 000 ~ t u / h r / c u ft. On t h i s b a s i s a n a p p r o p r i a t e
c o m b u s t i o n c h a m b e r , t o give a n output of 60 m i l l i o n ~ t u / h r , would h a v e a volume of 1000 cu f t , o r , s a y , i t would b e 10 ft3 neglecting w a l l t h i c k - n e s s . It i s with t h i s b a c k g r o u n d of i n f o r m a t i o n t h a t i t i s a p p r o p r i a t e t o c o n s i d e r t h e development i n G r e a t B r i t a i n of a n i n e r t g a s g e n e r a t o r b a s e d on an a i r c r a f t g a s t u r b i n e engine. B r i t i s h " J e t Engine" I n e r t G a s G e n e r a t o r Within t h e l a s t d e c a d e D. J. R a s b a s h , of t h e J o i n t F i r e R e s e a r c h O r g a n i z a t i o n in G r e a t B r i t a i n , h a s r e c o g n i z e d t h e p o t e n t i - a l i t i e s of a m a s s i v e i n e r t g a s g e n e r a t o r in f i g h t i n g f i r e s i n c e r t a i n c l a s s e s of buildings. F a c e d with t h e p r o b l e m of achieving combustion a t a high r a t e p e r unit volume h e t u r n e d t o t h e only d e v i c e t h e n e x i s t i n g
which c a m e n e a r t o m e e t i n g h i s r e q u i r e m e n t s , viz. t h e a i r c r a f t g a s t u r b i n e .
T h e output of a g a s t u r b i n e i s not i m m e d i a t e l y s u i t a b l e f o r u s e a s an extinguishing agent, m a i n l y b e c a u s e i t s t e m p e r a t u r e i s e x c e s s i v e and t o a l e s s e r extent b e c a u s e i t h a s a s u b s t a n t i a l oxygen content. T h e National Gas T u r b i n e E s t a b l i s h m e n t looked into t h e s e p r o b l e m s . Oxygen content w a s r e d u c e d by t h e u s e of an a f t e r - b u r n e r and cooling was achieved by d i r e c t injection and vapourization of w a t e r . In t h e prototype m o d e l de1ivere.d t o t h e F i r e R e s e a r c h Station t h e
composition of t h e output g a s ( b y v o l u m e ) , a t a d e l i v e r y r a t e of
45, 000 c f m , w a s oxygen 7 p e r c e n t , n i t r o g e n 46 p e r c e n t , C 0 2 3 p e r cent and w a t e r vapour 4 4 p e r cent.
Extinguishment t e s t s with t h e a p p a r a t u s h a v e p r o v e n quite s u c c e s s f u l ( 7 , 8 ,
9 )
and a p p e a r t o c o n f i r m t h e concepts given e a r l i e r in t h i s r e p o r t under t h e headings I 1 P e r m i s s i b l e Oxygen Content of I n e r t Gas" and "Required R a t e of Generation.In g e n e r a l t h e equipment will not s u p p r e s s s m o u l d e r i n g and it i s intended t h a t f i r e m e n should p e r f o r m t h i s function s o m e t i m e
subsequent t o t h e elimination of flaming combustion. T h e g e n e r a t o r i s quite v e r s a t i l e and t h e composition of t h e g a s can be a d j u s t e d t o m a k e it t r a n s l u c e n t , a n e c e s s a r y r e q u i r e m e n t i f f i r e m e n a r e t o o p e r a t e in the a t m o s p h e r e .
Canadian Conditions
A l t e r n a t i v e s t o conventional f i r e -fighting techniques a r e a s n e c e s s a r y in Canada a s in m o s t other p a r t s of t h e w o r l d , but i t is not a u t o m a t i c that t h e B r i t i s h g a s t u r b i n e i n e r t g a s . g e n e r a t o r would p r o v e a s valuable a tool in C a n a d a a s it i s l i k e l y t o do in t h e U. K. So f a r a s t h e U. K. i s concerned i t i s not c o n s i d e r e d e s s e n t i a l t h a t t h e "jet engine1' g e n e r a t o r should s u p p r e s s s m o u l d e r i n g a s i t is thought t h a t t h i s
a c t i v i t y c a n be left t o f i r e m e n a t a l a t e r s t a g e of t h e p r o c e e d i n g s . With Canadian c o n s t r u c t i o n s , however, one of the m o s t i m p o r t a n t f e a t u r e s of f i r e fighting i s the s u p p r e s s i o n of f i r e in t h e concealed s p a c e s which a r e t o be found i n a l m o s t a l l buildings, whether old o r new. While t h i s f e a t u r e m a k e s t h e m a s s i v e u s e of i n e r t g a s even m o r e d e s i r a b l e t h a n i n t h e U. K. it a l s o s u g g e s t s that the oxygen content of t h e gas should be sufficiently low a s t o be c a p a b l e of s u p p r e s s i n g smouldering. T h e oxygen content in t h e output of t h e B r i t i s h device, which can be r e d u c e d t o 7 p e r cent a f t e r t h e injection of w a t e r and
12 p e r cent b e f o r e , would s e e m t o be b o r d e r l i n e and lower v a l u e s a r e d e s i r a b l e .
Achieving a high r a t e of combustion in a s m a l l volume i s now considered (1 1) to be l a r g e l y a physical r a t h e r than a chemical problem and conventional combustion c h a m b e r s a r e generally f a r l a r g e r than the limit set by physical considerations. O r d e r s of
magnitude a r e involved and it should not be n e c e s s a r y t o r e s o r t to gas turbines to achieve, in a conveniently sized chamber, combustion a t the r a t e s discussed e a r l i e r in this r e p o r t . In fact it should be possible to design a combustion chamber. operating a t substantially atmospheric p r e s s u r e having an output comparable to that of an a i r c r a f t gas turbine and a volume of a s i m i l a r o r d e r .
At about the time that the National R e s e a r c h Council was considering this problem, it came to light that such a combustion chamber, was, in fact, being developed by a U . S . company (Black, Sivalls and Bryson of Oklahoma City),. and
a
complete a s s e m b l y . w a s purchased. At the t i m e , development of a v e r y high output model had not been completed and the model purchased had an output of6.
5million ~ t u / h r . If a machine with an output of 40, 000 cfm of inert gas i s a r b i t r a r i l y described a s " a full-scale i n e r t gas g e n e r a t o r , 'I a generator based on a
6. 5
million ~ t u / h r would constitute a one-tenth s c a l e machine.This choice of s c a l e h a s proven to be v e r y appropriate for two r e a s o n s . F i r s t l y the device was sufficiently s m a l l t o facilitAte experimental development of the water injection techniques. Secondly this scale of i n e r t gas generator needs to be developed in i t s own right, for application in the fighting of f i r e s in the holds of ships and in
inaccessible attic s p a c e s in s m a l l buildings. T h e burner fuel i s gaseous propane supplied a t a p r e s s u r e of about
, 9
p s i and a t a r a t ecorresponding t o a little l e s s than 1 gal/min in liquid phase. Combustion a i r i s supplied a t a r a t e of approximately 1000 cfm, a t a p r e s s u r e not exceeding 20 in. of w a t e r , by a 550 volt, 3 phase, 10 hp motor. The output t e m p e r a t u r e i s said to be in the region of 3000°F.
T o c r e a t e a chamber into which water could be injected, stainless s t e e l cylinders of various diameters w e r e attached t o the
9
-in. diameter burner outlet. Unstable combustion resulted although con- ditions improved a s the diameter of the chamber was i n c r e a s e d . It was thought undesirable, from considerations of s i z e , to u s e a diameter l a r g e r than 2 ft and with t h i s diameter of cylinder, other approaches t o the problem were investigated. A twofold solution was a r r i v e d at. In the burner p r o p e r , a flame stabilizer baffle was installed in the region where the fuel and the combustion a i r f i r s t begin t o mix, and in the water injection chamber a horizontal vane was installed a s i l l u s t r a t e d i n F i g u r e 1 to cut down the component of rotational velocity in the gas flows in that region.Water injection w a s f i r s t a t t e m p t e d a x i a l l y , with h y d r a u l i c atomizing n o z z l e s , but p r o v e d u n s u c c e s s f u l . Heat t r a n s f e r t o t h e n o z z l e s w a s t o o g r e a t and s t e a m i n s t e a d of w a t e r i s s u e d f r o m the
n o z z l e s . C i r c u m f e r e n t i a l injection, h o w e v e r , a s i l l u s t r a t e d in F i g u r e 1 gave r i s e t o no p r o b l e m s .
A decision a s t o the d i a m e t e r of t h e f i n a l outlet duct w a s then r e q u i r e d and 1 2 in. w a s c o n s i d e r e d a p p r o p r i a t e . A velocity of about 90 f t / s e c could t h e n b e expected giving a velocity head of about 1 in. of w a t e r . It would c e r t a i n l y not be f e a s i b l e t o a t t e m p t d e l i v e r y through a duct of l e s s than half t h i s d i a m e t e r f o r t h i s would give m o r e than four t i m e s t h e velocity and hence a velocity h e a d of s o m e 16 in. of w a t e r . T h e r a t e d d e l i v e r y p r e s s u r e of t h e combustion blower is only 20 in. of w a t e r and only a p o r t i o n of t h i s i s available t o o v e r c o m e back p r e s s u r e due t o flow r e s t r i c t i o n s .
T r a n s i t i o n and outlet p i e c e s w e r e i n s t a l l e d a s i l l u s t r a t e d i n F i g u r e 1 and once again unstable combustion o c c u r r e d . It w a s thought that t h i s r e s u l t e d f r o m a r e s o n a n c e m e c h a n i s m in which the plug of g a s e s in t h e outlet constituted t h e o s c i l l a t i n g m a s s and the g a s e s i n t h e m a i n c h a m b e r gave r i s e t o t h e f o r c e s . A s i m p l e c a l c u l a t i o n gave t h e r e s o n a n t f r e q u e n c y a s about 50 c / s and while t h i s s e e m e d higher than t h e f r e q u e n c i e s t h a t w e r e a p p a r e n t t h e r e w a s s o m e a g r e e m e n t as t o o r d e r
.
Acoustic damping w a s i n t r o d u c e d i n t h e f o r m of two s t a i n l e s s s t e e l wool baffles located as i l l u s t r a t e d i n F i g u r e 1. In o r d e r t o p r e v e n t s e r i o u s damage t o the s t a i n l e s s s t e e l wool o v e r - i n j e c t i o n of w a t e r w a s c a l l e d f o r and w a s achieved by t h e inclusion of t h r e e h o s e s a s i l l u s t r a t e d i n F i g u r e 1. Over -injection h a d t h e additional m e r i t of giving even
s m o o t h e r combustion and t h e n o i s e l e v e l f r o m t h e m a c h i n e w a s r e d u c e d to l i t t l e m o r e than t h e n o i s e f r o m t h e 10 hp c o m b u s t i o n a i r b l o w e r .
With t h e a r r a n g e m e n t a s specified both the oxygen a n d combustion g a s contents of t h e output g a s e s w e r e w e l l under 1 p e r cent. T h e r m o c o u p l e s a t t a c h e d t o the w a l l of t h e outlet p i e c e and p l a c e d i n the ( w e t ) g a s s t r e a m r e g i s t e r e d a t e m p e r a t u r e of 7 5 ° C .
T h e t h e o r e t i c a l compositions of t h e input and output of t h e g e n e r a t o r a r e evaluated i n Appendix C but have not yet been a c c u r a t e l y investigated e x p e r i m e n t a l l y , excepting a s mentioned i n t h e p r e v i o u s p a r a g r a p h . Noting t h e l e v e l s i n t h e p r o p a n e tank, b e f o r e and a f t e r running, s u g g e s t s that t h e s c a l e of input and output i s r a t h e r g r e a t e r than t h a t given in Appendix C . T h e v e r y high w a t e r i n j e c t i o n r e q u i r e - m e n t a l s o s u p p o r t s t h i s view. T h e s p r a y n o z z l e s , supplied a t
a
p r e s s u r e of 1 2 0 p s i , a r e providing 1 2 g a l / m i n and the t h r e e -in. n o z z l e s , a t a p r e s s u r e of 50 p s i , a r e providing
5
gal/min. Water r u noff f r o m t h e equipment is 2 gal/min. T h i s c o n s t i t u t e s a t h e r m a l l o s s but i s of no r e a l significance.
F i g u r e 2 i l l u s t r a t e s t h e m a c h i n e in operation.
F u r t h e r Work
At t h e t i m e of writing, an e x p e r i m e n t a l investigation of t h e p e r f o r m a n c e of t h e m a c h i n e in extinguishing f i r e s w a s under way.
It i s l i k e l y that t h e g e n e r a t o r a t i t s p r e s e n t s i z e w i l l con- s t i t u t e a useful tool f o r fighting f i r e s i n s h i p s t h o l d s and in s m a l l i n a c c e s s i b l e a r e a s i n buildings, s u c h a s t h e a t t i c s p a c e . It i s t h e r e - f o r e d e s i r a b l e that a n e a s i l y t r a n s p o r t a b l e m a c h i n e should b e developed f r o m t h e N. R. C. e x p e r i m e n t a l g e n e r a t o r .
F o r wider application a l a r g e r s c a l e of m a c h i n e should b e developed, in the f i r s t i n s t a n c e one with a n output of 40, 000 c f m ,
i. e . c o m p a r a b l e with t h e B r i t i s h g a s t u r b i n e machine. An e v e n
higher output m o d e l i s d e s i r a b l e and s o m e c o n s i d e r a t i o n should b e given t o i t s development.
T h e t r a n s p o r t a t i o n of p r o p a n e involves p r e s s u r e v e s s e l s and the u s e of a liquid fuel s u c h a s k e r o s e n e would b e m o r e convenient. Some thought should b e given t o t h e p o s s i b i l i t y of producing a c o m
-
bustion c h a m b e r of r e a s o n a b l e d i m e n s i o n s which would a c c e p t s u c h a fuel. Doubtless, s u c h a device would b e l a r g e r than one t h a t b u r n s g a s e o u s fuel and t h e question i s-
how m u c h l a r g e r it would be. It i s conceivable that by t h e u s e of high p r e s s u r e a t o m i z i n g n o z z l e s as u b s t a n t i a l i n c r e a s e in s i z e could be avoided.
Acknowledeements
Acknowledgement i s due t o D r .
T.
D. Northwood of t h e Division of Building R e s e a r c h and t o s e v e r a l m e m b e r s of t h e Division of Mechanical E n g i n e e r i n g f o r helpful a d v i c e leading t o t h e solution of t h e combustion p r o b l e m s mentioned in t h e r e p o r t .R e f e r e n c e s
1. Coward, H. J. and G. W. J o n e s . L i m i t s of F l a m m a b i l i t y of G a s e s and Vapours. U. S . B u r e a u of Mines Bulletin 503, 1952.
2. P e n n e r , S. S. and B. P. Mullins. E x p l o s i o n s , Detonations, F l a m m a b i l i t y and Ignition. 1959, P e r g a m o n P r e s s .
F i r e R e s e a r c h 1960. Dept. of S c . and Indus. R e s . and F i r e
Offices' C t t e . , HMSO, 1961, p. 9-1 1.
Building R e s e a r c h 1962. National R e s e a r c h Council, Division of Building R e s e a r c h , Oct. 1963, p. 40.
Fluid M e t e r s , T h e i r T h e o r y and Application. 5th Edition, 1959, A. S. M. E .
Rushbr ook, F r a n k . Ship F i r e s . Institution of Fir e E n g i n e e r s Q u a r t e r l y , Vol. XXI, No. 44, Winter 1961, p. 393-412.
Rasbash, D. J. I n e r t G a s G e n e r a t o r for Control of F i r e s in L a r g e Buildings. The E n g i n e e r , Vol. 21 5, No. 5601, 31 May 1963, p. 978-984.
F i r e R e s e a r c h 1960. Dept. of Sc. and Indus. R e s . and F i r e Offices' C t t e . , HMSO, 1961, p . 29-30.
Leete, L. W. J. F i r e Fighting with a n I n e r t G a s G e n e r a t o r . F i r e International NO. 1, J u l y 1963, p. 34-47.
P e r r y , John H. (Ed. ) C h e m i c a l Engineer s t Handbook. McGraw- Hill, 1950, 3 r d E d . , p. 244.
Longwell, John P. and
M.
A. Weiss. High T e m p e r a t u r e Reaction R a t e s i n Hydrocarbon Combustion. Indus. and Eng. Chem.,
Vol.TABLE I
OXYGEN CONCENTRATIONS (PREDICTED)
1 Oxygen Concentration M
=
5% (Gas with 5% 02)(%I
20 1 0 . 5 7 . 0 5 . 7 5. 35
dv
0 1 2 3 4 m M = O (Inert g a s )(%I
2 0 7 . 4 2. 7 1 . 0 0 . 4 0 LG A S
( V o l u m e
v
c u . f t
R a t e
I n i i n )
F I G U R E 3
G A S E O U S L O S S E S
DENS I T Y
p'
( I b l c u . f t )
p'
< % P o
f
- - -
A I R
( D e n s i t y , U o I b l c u .
f t )
r1
h f t .
A R E A
A,(sq. f t )
A I R
( D e n s i t y
po
I b l c u . f t )
Appendix A
Dilution of A t m o s p h e r i c Oxygen
Suppose a g a s G be i n j e c t e d a t a volume r a t e v i n t o a volume V which o r i g i n a l l y contained a i r .
Suppose f u r t h e r that mixing i s p e r f e c t and t h a t n o a p p r e c i - able p r e s s u r e d i f f e r e n c e s a r e involved.
Let t h e f r a c t i o n a l c o n c e n t r a t i o n of the g a s G a t a n y t i m e t be g ( O C g < 1 ) .
In a t i m e 6t t h e gain of g a s G w i l l be v. 6t.
T h e l o s s of g a s G, by t h e e f f l u x of displaced g a s e s w i l l b e gv. 6t s o that t h e net gain of g a s G will b e ( 1
-
g) v. 6t.The change i n c o n c e n t r a t i o n 6g of the g a s G will be ( 1
-
g)v. 6 t / ~ s o t h a t
Integrating t h i s equation and invoking t h e b o u n d a r y c o n - ditions g = 0 a t t = 0 and g + l a t t + w
If t h e g a s G i s taken t o have a n oxygen c o n c e n t r a t i o n m and a i r t o h a v e a n oxygen c o n c e n t r a t i o n of 0. 2 (i. e. 20 p e r c e n t ) , t h e oxygen concentration n of t h e m i x t u r e will b e
If n and m a r e e x p r e s s e d a s p e r c e n t a g e s
M
and N Equation (2) b e c o m e sT a b l e I r e l a t e s N and v t / ~ f o r v a l u e s of M of z e r o and
Appendix
B
G a s e o u s L o s s e s
P a r t 1
-
G e n e r a lConsider a g a s , l e s s d e n s e t h a n a i r , i n j e c t e d a t a r a t e v c u ft/min into a n e n c l o s u r e a t t h e top of which t h e r e i s a n opening of a r e a A2 ( F i g u r e 3). T h e e q u i l i b r i u m condition which will be
e s t a b l i s h e d will be s u c h t h a t t h e p r e s s u r e difference e i t h e r s i d e of t h e outlet gives r i s e t o a flow equal t o t h e inlet flow. A s s u m i n g t h e g a s e s within t h e e n c l o s u r e t o be effectively stagnant and s t r a t i f i e d , and t h e flow coefficient a t t h e outlet t o a p p r o x i m a t e t o 0. 6 ( 5 ) , t h e e x p r e s s i o n governing t h e flow will b e
w h e r e
g i s a c c e l e r a t i o n due t o g r a v i t y and t h e r e m a i n i n g s y m b o l s a r e defined i n F i g u r e 1.
M a s s flow continuity c o n s i d e r a t i o n s give
w h e r e v i s t h e injection r a t e in cu ft/rnin and t h e f a c t o r 60 i s n e c e s s a r y owing t o i n c o n s i s t e n c y of u n i t s . 1 -3 1
+
Hence A-
3. 47. 10 (v/hz) [ p l / ( p o-
p) ]
2 P a r t 2-
N. R. C. G e n e r a t o r T h e d e n s i t y r a t i o f o r t h e output of t h e NRC e x p e r i m e n t a l g e n e r a t o r i s m o s t e a s i l y evaluated by t a k i n g d e n s i t y a s p r o p o r t i o n a l t o m e a n m o l e c u l a r weight/absolute t e m p e r a t u r e . T h e composition of t h e g a s i s 3 C 0 2+
2ONk+
4 8 H 2 0 s o t h a t i t s m e a n m o l e c u l a r weight i s 21. 9. T h e m e a n m o e c u l a r weight of a i r i s about 29. Taking t h e t e m p e r a t u r e of t h e i n j e c t e d g a s a s 1 0 0 ° C and ambient t e m p e r a t u r e a s O°C, t h e d e n s i t y r a t i o i n Equation 5Hence -3 1 A2
=
3 . 8 6 . 1 0 v/hz w h e r e the u n i t s ofA
a r e s q ft t h o s e of v a r e cd ft/rnin and t h o s e of h a r e ft.Appendix C
Combustion and Vapourization P r o c e s s e s
If a s t o i c h i o m e t r i c m i x t u r e of a i r and propane i s b u r n e d , t h e h e a t output of 1 gm m o l e of p r o p a n e will be 488. 5K c a l (10). T h i s f i g u r e a p p l i e s t o t h e conditions giving g e n e r a t e d w a t e r in t h e vapour p h a s e . If t h e g a s e s a r e a s s u m e d t o b e a t 1 0 0 ° C a f u r t h e r c o r r e c t i o n (approx. 11K c a l ) i s a s s o c i a t e d with t h e heating of t h e i n e r t g a s e s i n t h e combustion a i r .
The h e a t available t o v a p o u r i z e w a t e r w i l l t h e r e f o r e b e 477K ~ a l / ~ m m o l e of propane. If i t is a s s u m e d that t h e h e a t r e q u i r e d t o r a i s e t h e t e m p e r a t u r e of w a t e r f r o m an a r b i t r a r y ambient t e m p e r a t u r e t o 100" C and t o vapourize i t i s 600 ~ a l / ~ m t h e n t h e quantity of w a t e r which could be vapourized i s 477,000/600 =. 795 gm
e
44 gm mole.T h e e x p r e s s i o n f o r t h e c o m p l e t e combustion and v a p o u r i z a - tion p r o c e s s , a s s u m i n g a i r t o be c o m p o s e d only of 0 and N is t h e r e -
f o r e : 2 2
C H
+
50+
20N+
44H 0 (liquid) = 3 C 0+
4H O(vapour)+
20N+
44H O(vapour)3 8 2 2 2 2 2 2 2
T h u s for e v e r y m o l e c u l e of p r o p a n e which i s b u r n e d t h e out- put w i l l contain 71 m o l e c u l e s .
6
T h e r a t e d h e a t output of t h e b u r n e r a c q u i r e d b y NRC is 6. 5 x 10 ~ t u / h r and adopting t h e higher value of t h e h e a t of combustion of p r o p a n e (21, 646 ~ t u / l b ) (10) the input and output of t h e g e n e r a t o r w i l l be constituted a s follows:5. 0 lb/min of p r o p a n e ( 0 . 8 6 Imp gal/min)
+
89 l b / m i n of a i r (1085 cu f t / rnin a t O°C)+
90 lb/min of w a t e r (9 Imp gal/min)= 15 lb/min of C 0 2 (155 c u f t / m i n a t 75OC)
+
64 lb/min of n i t r o g e n (1040 cu ft/min a t 7 5 ° C )+
98 lb/min of w a t e r vapour (2600 c u ft/min at 1 0 0 ° C )= 1195 c u ft/min of " n a t u r a l g a s e s " ( a t 75OC)
