Flame Suppression by a Physical Mechanism?

13

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 9, 2018 at 16:05:02 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Halogenated Fire Extinguishants: Flame Suppression by a Physical Mechanism? ERIC R. LARSEN Halogens Research Laboratory, The Dow Chemical Co., Midland, Mich. 48640

The mechanism by which halogenated agents act as flame suppressants has been the subject of intensive research for about twenty five years. In the early 1950's Fryburg (1) proposed that the outstanding effectiveness of these agents may be due to a mechanism that involved the chemistry of the combustion processes rather than one or another of the mechanisms which were thought to be applicable to the inert gases. A chemically based mechanism was felt to be attractive since i t might explain the fact that certain halohydrocarbons, especially those containing bromine, were from five to eight times as effective as carbon dioxide and nitrogen. Fryburg's proposal led directly to very extensive studies to elucidate the chemistry of flames into which halogenated materials were introduced. These studies, in turn, led to the development of the radical trap mechanism for flame suppression that is in vogue today. That the halogenated agents enter into the chemistry of the various combustion processes has been adequately demonstrated by studies too numerous to into here. That this chemical reactivity is involved in the primary mechanism by which the halogenated agents act as flame suppressants i s , however, open to question. Several years ago it was reported by the author that an investigation of the flammability of halohydro-carbons showed that these compounds were nonflammable in air only when they contained more than about 70 wt.% halogen (2,3). The type of halogen present did not seem to be important. In other words, when the halogen concentration was expressed on a weight basis, rather than on the commonly employed molar basis, the various halogens were essentially equivalent. This was later shown to also be true for mixtures of halogenated 376 Gann; Halogenated Fire Suppressants ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

13.

LARSEN

Flame Suppression Mechanism

377

agents and h y d r o c a r b o n based f u e l s when the h a l o g e n c o n t e n t o f the m i x t u r e o f agent and f u e l was d e t e r mined a t the " f l a m m a b i l i t y peak" (A) . The h a l o g e n c o n t e n t o f peak c o m p o s i t i o n f o r some s i x t y agents i n h e p t a n e - a i r systems were found t o obey the r e l a t i o n ship: 1.

[ [ A ]

+

3 [ p ]

. 100 = 69.8

±

3.5%

where [H] i s the weight o f h a l o g e n i n t h e m i x t u r e o f agent [A] and f u e l [F] t h a t i s p r e s e n t a t the f l a m m a b i l i t y peak when a l l c o n c e n t r a t i o n s a r e e x p r e s s e d i n mg/l. T h i s r e l a t i o n s h i p was found t o h o l d r e g a r d l e s s o f the t y p e o f h a l o g e n p r e s e n t i n the agent, and showed t h a t the halogens were e f f e c t i v e flame supp r e s s a n t s i n d i r e c t p r o p o r t i o n t o t h e i r atomic w e i g h t s , i . e . F:Cl:Br:I::1.0:1.9:4.2:6.7. These r e s u l t s a r e c l e a r l y i n c o n s i s t e n t w i t h the p o s t u l a t e d c h e m i c a l mechanisms o f flame s u p p r e s s i o n . P r i o r t o d i s c u s s i n g the p r i m a r y mechanism by which h a l o g e n a t e d compounds a c t as flame s u p p r e s s a n t s i t i s n e c e s s a r y t o re-examine the r o l e o f the i n e r t gases, s i n c e we must e s t a b l i s h the b a s e l i n e b e h a v i o r o f t h e s e agents b e f o r e we can hope t o s t u d y the v a r i a n t b e h a v i o r o f t h e h a l o g e n a t e d a g e n t s . The i n e r t gas a g e n t s , e.g. He, A r , N2, C02# CF4, e t c . , a r e g e n e r a l l y conceded t o a c t by a p h y s i c a l mechanism s i n c e t h e y do not e n t e r i n the c h e m i s t r y o f the v a r i o u s combustion p r o c e s s e s t o any s i g n i f i c a n t e x t e n t . F i g u r e 1 shows the i n f l u e n c e o f t h e i n e r t gases upon the f l a m m a b i l i t y l i m i t s o f p e n t a n e - a i r mixt u r e s ( 5 ) . The p o i n t a t which the upper and lower l i m i t c u r v e s meet i s commonly c a l l e d the f l a m m a b i l i t y peak and r e p r e s e n t s the minimum agent c o n c e n t r a t i o n needed t o p r e v e n t flame p r o p a g a t i o n i n any and a l l f u e l - o x i d a n t m i x t u r e s . While the minimum agent conc e n t r a t i o n , o r peak v a l u e s , o f N2 and CO2 a r e r e l a t i v e l y h i g h - 42 v o l % and 29 v o l %, r e s p e c t i v e l y the h a l o g e n a t e d agents g e n e r a l l y show peak v a l u e s o f l e s s than 10 v o l %. A l t h o u g h the most e f f i c i e n t h a l o genated agent (CF2Br2, 4.4 v o l % i n heptane) and n i t r o g e n d i f f e r i n peak c o n c e n t r a t i o n s by about a f a c t o r o f 10, t h e d i f f e r e n c e between CF2Br2 and C3F8 one o f the most e f f e c t i v e i n e r t gases - i s o n l y about a f a c t o r o f two. T h i s r e l a t i v e l y narrow s p r e a d i n d i f f e r e n c e s h a r d l y seems t o j u s t i f y a c o m p l e t e l y new mechanism. A c l o s e i n s p e c t i o n o f the i n e r t gas agents shows t h a t when the c o n c e n t r a t i o n s o f pentane, oxygen, and i n e r t gas a r e r e p l o t t e d on a heat c a p a c i t y b a s i s , o r

Gann; Halogenated Fire Suppressants ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

HALOGENATED FIRE SUPPRESSANTS

,1

0

I 10

I I 20 30 Added Inert, (Vol %)

I 40

I 50

Bureau of Mines Bulletin

Figure 1. Limits of flammability of various n-pentane-inert gas-air mixtures at 25°C and atmospheric pressure (5)


13.

LARSEN


Figure 2. Limits of flammobility of various n-pentane-inert gas-'air" mixtures. Heat capacity basis.

κ IF/

InertV?

• 38.7% 0 • 31.3% 02 a 21.0% 0 2

2

Figure 3. Limits of fiammability of various n-pentane—inert gas-air mixtures at 25°C and atmos pheric pressure. N from air deleted. Heat capacity basis. 8


380


f o r t h a t m a t t e r on a weight b a s i s , the d i f f e r e n c e s between t h e v a r i o u s i n e r t gases d i s a p p e a r s (6). F i g u r e 2 shows such a p l o t u s i n g t r i a n g u l a r c o o r d i n a t e s . The heat c a p a c i t y c o n t r i b u t i o n o f the i n e r t gases i s shown here as the sum o f the i n d i v i d u a l c o n t r i b u t i o n s o f the i n e r t gases i . e . N2 and C F A # p r e s e n t i n t h e system. The upper l i m i t (I) and lower l i m i t ( J ) o f pentane i n oxygen a r e shown a l o n g the base o f the diagram. The upper l i m i t (G) and lower l i m i t (H) i n a i r a r e shown a l o n g the l i n e AD. P o i n t F, the p o i n t a t which t h e l i m i t c u r v e s converge, r e p r e s e n t s t h e t h e o r e t i c a l f l a m m a b i l i t y peak, and p o i n t Ρ the ex p e r i m e n t a l l y d e t e r m i n e d f l a m m a b i l i t y peak. The d i f f e r e n c e s between t h e two i s p r o b a b l y due t o a v a r i e t y o f f a c t o r s , c h i e f o f which appear t o be t h e s t r e n g t h o f the i g n i t i o n s o u r c e and the p r a c t i c a l d i f f i c u l t y o f making up a c c u r a t e gas m i x t u r e s when t h e d i f f e r ences between the upper and lower l i m i t become v e r y small. T h a t the e x p e r i m e n t a l peak l a y s on t h e f u e l r i c h s i d e o f the s t o i c h i o m e t r i c m i x t u r e l i n e , i . e . C s t C , i s i n k e e p i n g w i t h the f i n d i n g t h a t the most r e a d i l y i g n i t a b l e mixtures are s l i g h t l y f u e l r i c h . The e x p e r i m e n t a l p o i n t s shown i n F i g u r e 2 a r e d e r i v e d from a s t u d y , c a r r i e d o u t by Moran and B e r t s c h y (1), on the e f f e c t o f C F on the flamma b i l i t y l i m i t s o f p e n t a n e - " a i r " systems. The " a i r " employed h e r e c o n t a i n e d e i t h e r 21.0%, 31.3%, o r 38.7% oxygen. The s o l i d l i n e r e p r e s e n t s the flammable l i m i t c u r v e s f o r t h e system p e n t a n e - o x y g e n - n i t r o g e n . I t i s o b v i o u s from t h i s diagram t h a t t h e system pentane-oxygen-CF s h o u l d e x h i b i t e s s e n t i a l l y the same l i m i t diagram as p e n t a n e - o x y g e n - n i t r o g e n . The r e g i o n bound by the t r i a n g l e ADC i s shown i n g r e a t e r d e t a i l i n F i g u r e 3. In t h i s f i g u r e the con t r i b u t i o n t o the heat c a p a c i t y due t o the n i t r o g e n c o n t e n t o f the a i r employed as the o x i d a n t was d e l e t e d , and t h e d a t a r e n o r m a l i z e d . I t i s o b v i o u s from t h e s e f i g u r e s t h a t a l l o f t h e i n e r t gases show e s s e n t i a l l y the same flammable l i m i t s , and t h a t t h e i r heat capac i t y c o n t r i b u t i o n s are a d d i t i v e . There a r e s e v e r a l p o i n t s i n F i g u r e 2 t h a t a r e i m p o r t a n t t o the a n a l y s i s o f f l a m m a b i l i t y peak (P) d a t a ; t h e s e a r e shown as K and K j p . K i s the c o m p o s i t i o n o f t h a t i n e r t gas-oxygen m i x t u r e which w i l l g i v e a peak (P) m i x t u r e upon the a d d i t i o n o f f u e l . S i m i l a r l y , Kjp i s the c o m p o s i t i o n o f i n e r t gas which upon a d d i t i o n o f oxygen w i l l y i e l d a peak m i x t u r e . These c o m p o s i t i o n s a r e r e p r e s e n t e d by t h e e q u a t i o n s : [I]Cp 4

4

I 0

I 0

T


13.

LARSEN

381

Flame Suppression Mechanism [IlCPj

3

*

K

IF

[I]C

P l

+ [Fuel]Cp

F

*

1

0

0

where [ I ] C p j , [02]CPQ and [FuelJCpp r e p r e s e n t the i n d i v i d u a l heat capacity c o n t r i b u t i o n s of the i n e r t g a s e s , o x y g e n , and f u e l , r e s p e c t i v e l y . The r a t i o o f the heat c a p a c i t y c o n t r i b u t i o n s of a l l of the i n e r t g a s e s , Σ [ I ] C p j , a t t h e p e a k (P) t o t h e t o t a l h e a t c a p a c i t y o f t h e s y s t e m c a n be r e p r e s e n t e d b y t h e equation: 2

w h e r e Σ [ C ] C p i s t h e sum o f t h e i n d i v i d u a l h e a t c a p a c i t y c o n t r i b u t i o n s o f a l l components o f the system (3,6) . T a b l e I shows t h e f l a m m a b i l i t y peak c o m p o s i t i o n s f o r v a r i o u s methane-oxygen-agent and p e n t a n e - o x y g e n agent systems. Those systems marked w i t h an a s t e r i s k c o n t a i n o n l y a s i n g l e i n e r t gas w h i l e t h e o t h e r s y s t e m s c o n t a i n N2 a l o n g w i t h t h e i n d i c a t e d a g e n t . This d a t a shows q u i t e c l e a r l y t h e a d d i t i v i t y o f t h e h e a t c a p a c i t y c o n t r i b u t i o n s o f the i n e r t gases t o the peak c o m p o s i t i o n , and c o n f i r m s the f a c t t h a t a l l o f the i n e r t g a s e s behave as f l a m e s u p p r e s s a n t s by v i r t u e of t h e i r c o n t r i b u t i n g to the t o t a l heat c a p a c i t y of the system w i t h o u t c o n t r i b u t i n g to the heat producing capacity of the system. T h e d a t a f o r A r a n d He a l s o suggests that n e i t h e r d i l u t i o n of the f u e l or oxygen, nor the thermal c o n d u c t i v i t y of the agent are p a r t i c u l a r l y important to the flame suppression process. T h e mean v a l u e o f K (86.9 ± 1.2) f o r t h e i n e r t g a s e s i s p a r t i c u l a r l y germane t o t h e mechanism by w h i c h t h e h a l o g e n a t e d a g e n t s a c t as f l a m e s u p p r e s s a n t s . R e c e n t l y , C r e i t z on the b a s i s o f an e x t e n s i v e s t u d y of t h e f l a m e i n h i b i t i n g a b i l i t y o f CF3Br d e t e r m i n e d t h e c o n c e n t r a t i o n of CFoBr r e q u i r e d t o prevent flame p r o p a g a t i o n i n v a r i o u s f u e l - o x y g e n m i x t u r e s (8). The " l i m i t i n g c o n c e n t r a t i o n of oxygen-agent atmosphere" d e t e r m i n e d b y C r e i t z f o r t h e f u e l s CH4, C2H5, C3H8F a n d C4H10 a r e s h o w n i n T a b l e I I a l o n g w i t h t h e e q u i v a l e n t v a l u e s f o r v a r i o u s other oxygen-agent atmo spheres . When CF3Br i s c o m p a r e d t o t h e i n e r t g a s e s , i . e . A r , H e , a n d N2, o n a v o l u m e % b a s i s i t w o u l d appear t h a t the d a t a might s u p p o r t a c h e m i c a l mech anism of flame suppression. Such s u p p o r t i s l e s s c l e a r w h e n CF3Br i s c o m p a r e d w i t h CF4. When t h e v a r i o u s agents are compared on a heat c a p a c i t y b a s i s i t would appear t h a t such support i s e n t i r e l y absent. c

I

0



382

Table I F l a m m a b i l i t y Peak C o m p o s i t i o n s

Fuel CH

C

H

4

5 12

Agent

Fuel Vol%

Peak Agent Vol%

(7,11,12)

KCp

K

IO

K

T F

6.3

81.5

80.3

86.9

91.3

7.5

22.3

77.8

84.7

89.9

He

5.8

36.3

78.8

85.5

91.0

He*

5.0

85.0

79.0

85.8

92.6

Ar

5.0

48.4

88.0

92.2

Ar*

4.0

88.2

81.7 83.2

88.8

93.9

N

2.1

86.2

80.8

88.0

90.9 88.7

w

2 co

2

2 co

2

2.8

28.0

77.5

CF

4

2.5

26.0

79.7

86.1 86.5

3.0

15.7

79.7

87.1

91.0 90.4

2.8

10.6

80.6

87.1

91.5

S F

C

6 F

3 8

Mean V a l u e s • S o l e Agent

KCp = 80 .2 ± 1.7 K K

I 0

TTTI

= 86 .9 ± 1.2 = 91.0 ± 1.5

E x p e r i m e n t a l C o n d i t i o n s : Fuel-C^-No- Agent M i x t u r e s w i t h Flame P r o p a g a t i o n V e r t i c a l l y t h r o u g h Tube


13.

LARSEN


383

Table I I L i m i t i n g C o n c e n t r a t i o n s o f Oxygen-Agent

Fuel

Agent

o

2

Vol%

Atmospheres

Agent Vol%

02 C %

Agent Cp%

P

CH

4

Ar

8.2

91.8

11.2

88.8

CH

4

He

10.5

85.9

14.2

85.8

CH

4

N

2

13.0

87.0

13.0

87.0

2

11.9

88.1

12.0

88.0

12.0

88.0

12.0

88.0

25.0

75.0

13.7

86.3

34.0

66.0

17.9

82.1 83.4

C

3 8

H

N

C

5 12

H

N

C

H

5 12 CH * 4

2 CF

4

CF Br 3

CF Br

32.0

68.0

16.6

H

CF Br

33.3

66.7

17.5

82.5

H

CF Br

33.8

65.3

18.0

82.0

W

3

C

3 8*

C

4 10*

3

2

• D i f f u s i o n Flame - E . C. C r e i t z , F i r e Tech., 8_ 131 (1972) (8)



384

The v a l u e which appears i n t h e l a s t column (Agent Cp) o f T a b l e I I i s the v a l u e f o r K discussed previously. The d i f f e r e n c e between t h e v a l u e o f K J Q f o r the i n e r t gases ( 8 6 . 9 ± 1 . 2 ) and f o r CF-Br ( 8 2 . 5 ± 0 . 6 ) i s s m a l l enough t o be a t t r i b u t a b l e t o the f a c t t h a t the i n e r t gas d a t a i s d e r i v e d from premixed combustion tube d e t e r m i n a t i o n s which the d a t a f o r CF3Br i s d e r i v e d from a s t u d y on the agent c o n c e n t r a t i o n s r e q u i r e d t o extinguish small d i f f u s i o n flames. T a b l e I I I shows t h e f l a m m a b i l i t y peak c o m p o s i t i o n s f o r a number o f h a l o g e n a t e d agents t h a t were s t u d i e d by the Purdue Research F o u n d a t i o n (9). These agents a r e r e p r e s e n t a t i v e o f a t o t a l o f 4 6 agents s t u d i e d w i t h r e s p e c t t o t h e i r a b i l i t y t o s u p p r e s s flame p r o p a g a t i o n i n h e p t a n e - a i r m i x t u r e s . T h i s s t u d y by Purdue has been f r e q u e n t l y employed t o s u p p o r t the premise t h a t the h a l o g e n a t e d agent a c t as flame s u p p r e s s a n t s by a c h e m i c a l , r a t h e r t h a n p h y s i c a l , mechanism (1) . The v a r i e t y o f agents shown a r e r e p r e s e n t a t i v e o f the d i f f e r e n t h a l o g e n s , s i n g l e l y and i n c o m b i n a t i o n . The v a l i d i t y o f t h i s premise i s q u e s t i o n a b l e when the agents a r e compared on a heat c a p a c i t y b a s i s r a t h e r than on t h e u s u a l volume b a s i s . When compared on a h e a t c a p a c i t y b a s i s no one t y p e o f h a l o g e n s t a n d s o u t as c l e a r l y s u p e r i o r t o any o t h e r . I t i s a x i o m a t i c o f the r a d i c a l t r a p mechanism t h a t bromine c o n t a i n i n g a g e n t s , which e n t e r i n t o the combustion c h e m i s t r y , s h o u l d be s u p e r i o r t o the f l u o r i n e c o n t a i n i n g compounds and CO2 which behave as i n e r t g a s e s . The mean v a l u e o f K f o r t h e s e systems i s 8 4 . 0 ± 2 . 9 which l a y s midway between the v a l u e s o f K J O found f o r the i n e r t gas systems ( 8 6 . 9 ± 1 . 2 , T a b l e I) and f o r C F B r ( 8 2 . 5 ± 0 . 6 , T a b l e I I ) d e t e r mined from C r e i t z ' s d a t a . The v a l u e o f Kjp found f o r t h e s e h a l o g e n a t e d agents compares c l o s e l y w i t h t h a t found f o r the i n e r t g a s e s ; 8 7 . 9 ± 2 . 5 and 9 1 . 0 ± 1 . 5 , respectively. In a more r e c e n t study by B a j p a i and Wagner ( 1 0 ) t h e e f f e c t o f t h e agents CF3Br and C F 2 C l B r upon the f l a m m a b i l i t y l i m i t s o f s e v e r a l h y d r o c a r b o n f u e l s were determined i n a m o d i f i e d combustion t u b e . The f l a m m a b i l i t y peak v a l u e s d e r i v e d from t h i s s t u d y a r e shown i n T a b l e IV, a l o n g w i t h the c o r r e s p o n d i n g v a l u e s o f KCp, KJO# and K . T h i s study tends t o bear o u t the o v e r a l l f i n d i n g s c i t e d above, and a g a i n f a i l s t o s u p p o r t any c l e a r s u p e r i o r i t y o f the h a l o g e n a t e d agents o v e r the i n e r t g a s e s . The v a l u e o f Kjp h e r e c o r r e s p o n d s even more c l o s e l y w i t h t h a t found f o r t h e i n e r t gases t h a n does t h e Purdue study (9). The v a l u e o f K j which i n t h i s c a s e i s the sum o f the h e a t I

I

0

0

3

I F

0


13.

LARSEN

385

Flame Suppression Mechanism Table Flammability

III

Peak

Compositions

Experimental Conditions: n-Heptane-Air Mixtures with Flame P r o p a g a t i o n V e r t i c a l l y t h r o u g h Tube, Spark Ignition Purdue Res. Found, and Dept. o f Chem.: Sum. R e p ' t S e p t . 1 , 1947 t o A u g . 3 1 , 1 9 4 8 , P u r d u e U n i v . (Contract W - 4 4 - 0 0 9 e n g - 5 0 7 w i t h A r m y E n g . R&D L a b s ) (9)

Agent

Agent Vol%

Peak

Heptane Vol%

KCp

CF Br 2

2

4.2

4.4

67.2

81.2

79.5

CH Br

2

5.2

2.1

73.5

80.9

89.0

C H I

5.6

3.2

70.5

81.4

84.7

CH Br

6.1

2.1

73.4

80.7

88.8

CH I

6.1

2.1

73.1

80.6

88.8

CF Br

6.1

3.0

71.8

81.8

85.4

C H Br

6.2

2.3

73.5

81.6

88.4

11.2

3.2

74.6

84.6

86.4

12.3

3.3

74.0

84.4

85.4

13.4

3.0

77.8

86.6

88.6

14.9

3.9

73.8

85.2

84.7

17.5

3.6

74.6

85.9

85.2

17.8

2.2

77.3

84.8

89.7

18.1

2.3

85.3

90.5

89.7

2

2

5

3

3

3

2

5

cci

4

CC1F C

3

2 6 F

2 2 CHC1 C F

C 1

3

CHF

3

c-C F 4

8

CF

4

26.0

1.8

82.1

87.7

92.7

co

2

26.0

1.8

78.8

85.5

91.1

Mean

Values

KCp = 7 5 . 1 ± 4 . 4 IO K „ IF K

T

=

8 4 . 0 ± 2 .9

= 8 7 . 9 ± 2 .5



386

Table Flammability

Fuel CH

C

Agent

4

n-C H 4

1 Q

AgenP t eak F u e l Vol% Vol%

(10)

KCp

KTF

6.8

9.2

74

82

88

CF ClBr 2

4.3

9.9

72

81

87

CF Br

8.0

4.3

75

83

89

CF ClBr 2

7.0

5.2

73

82

87

CF Br

7.2

3.0

75

82

90

CF ClBr 2

6.3

2.8

75

82

90

CF Br

7.2

4.0

73

82

87

CF ClBr

6.3

2.9

75

82

90

3

H

Peak C o m p o s i t i o n s

CF Br 3

3 8

IV

3

3

2

Mean V a l u e s

KCp = 7 4 . 0

± 1.2

K

IO

=

82.0

± 0.5

K

IF

=

88.5

± 1.4


13.

LARSEN


387

c a p a c i t y c o n t r i b u t i o n s o f the agent and t h e n i t r o g e n from the a i r employed as the o x i d a n t a g r e e s e x a c t l y w i t h t h e v a l u e f o r CF3Br d e r i v e d from C r e i t z ' s d a t a ( 8 ) . On t h e b a s i s o f t h e d a t a d e v e l o p e d h e r e and from the p r e v i o u s f i n d i n g t h a t t h e h a l o g e n s a r e e f f e c t i v e as flame s u p p r e s s a n t s i n d i r e c t p r o p o r t i o n t o t h e i r atomic weight i t i s f e l t t h a t the h a l o g e n a t e d agents probably p l a y a d u e l r o l e i n the s u p p r e s s i o n of flames. T h e i r primary r o l e i s s i m i l a r t o t h a t of the i n e r t gases, t h a t i s , t h e y a c t as h e a t s i n k s w i t h o u t cont r i b u t i n g s i g n i f i c a n t l y t o the h e a t p r o d u c i n g p o t e n t i a l o f the system. In t h i s r e s p e c t i t i s a n t i c i p a t e d t h a t they a c t i n t h e r e g i o n ahead o f the moving flame f r o n t r a t h e r than w i t h i n the flame p e r s e . In a d d i t i o n t o t h i s p r i m a r y mechanism t h e h a l o genated a g e n t s , w i t h a few e x c e p t i o n s , a l s o appear t o a c t as f u e l s once they a r e h e a t e d t o a h i g h enough temperature. T h i s a b i l i t y t o a c t as a f u e l i s c o n s i s t e n t w i t h the r e l a t i o n s h i p shown i n e q u a t i o n 1 and i t i s p r o b a b l y i n t h i s r o l e t h a t t h e s e agents p a r t i c i p a t e i n t h e c h e m i s t r y o f f l a m e s . In t h i s r o l e t h e h a l o genated agents would be flame s u p p r e s s a n t s o n l y w i t h i n the c o n t e x t t h a t the a d d i t i o n o f e x c e s s f u e l t o a f l a m mable m i x t u r e s produces a non-flammable m i x t u r e . In o r d e r t o make any k i n d o f a s t r o n g c a s e f o r a more complex mechanism o f flame i n h i b i t i o n t h a n i s found f o r the i n e r t gases i t would appear t o be nece s s a r y t o show t h a t t h e agent produces a d e f i n i t e e f f e c t upon t h e o v e r a l l flammable e n v e l o p e . T h i s w i l l be d i f f i c u l t t o a c c o m p l i s h u s i n g a i r as the o x i d a n t as has been done w i t h t h e g r e a t b u l k o f t h e work c a r r i e d o u t t o d a t e , and i t i s s t r o n g l y recommended t h a t f u t u r e s t u d i e s on t h e mechanism o f flame i n h i b i t i o n be c a r r i e d o u t u s i n g a i r t h a t i s s i g n i f i c a n t l y e n r i c h e d w i t h oxygen, eg. >50% O2. Experimental

Data

The d a t a r e l i e d upon i n t h i s paper were d e v e l o p e d by a number o f d i f f e r e n t groups, i n c l u d i n g Coward and coworkers (11,12), Z a b e t a k i s ( 5 ) , Moran and B e r t s c h y (7) 9 C r e i t z (S), B a j p a i and Wagner (10) and o t h e r s ( 9 ) . The " f l a m m a b i l i t y peak" v a l u e s d e r i v e d from t h e s e r e f e r e n c e s are g i v e n i n the v a r i o u s t a b l e s . The r e a d e r i s r e f e r r e d t o t h e o r i g i n a l a r t i c l e s f o r the complete data. Only t h o s e s t u d i e s were e v a l u a t e d i n which a number o f i n e r t gases on h a l o g e n a t e d agents were e v a l u a t e d f o r t h e i r a b i l i t y t o p r e v e n t flame p r o p a g a t i o n by d e t e r m i n i n g the e f f e c t o f the added i n e r t



388

gases upon t h e l i m i t s o f f l a m m a b i l i t y o f v a r i o u s f u e l oxidant mixtures. I n most c a s e s t h e l i m i t d a t a was o b t a i n e d by upward flame p r o p a g a t i o n i n an a p p a r a t u s s i m i l a r t o t h a t d e s c r i b e d by t h e Bureau o f Mines (12), the one e x c e p t i o n b e i n g C r e i t z * s study (£) which em ployed a d i f f u s i o n flame. I n t h e combustion tube s t u d i e s t h e tubes employed were s u f f i c i e n t l y l a r g e so t h a t i n c r e a s i n g t h e d i a m e t e r o f t h e tube would have l i t t l e e f f e c t upon t h e v a l u e s o b t a i n e d . F o r t h e method employed t o c o n v e r t t h e c o m p o s i t i o n o f t h e systems from t h e normal volume p e r c e n t b a s i s t o t h e heat c a p a c i t y b a s i s employed here t h e r e a d e r i s referred t o reference ( 6 ) .

Literature Cited 1. 2. 3. 4. 4 5. 6. 7. 8. 9.

10. 11. 12.

Fryburg, G . , NACA Tech. Note 2102, p. 27 (1950). Larsen, E. R., in Handbook of Experimental Phar macology, New Series, Volume XXX, (M. B. Chenoweth, ed.), Springer-Verlag, New York, 1972, p. 28. Larsen, E. R., Abs. 166th National ACS Meeting, Chicago, I l l i n o i s , Aug. 26-31 (1973), Paper No. INDE 054. Larsen, E. R., JFF/Fire Retardant Chemistry, 1 (1974). Zabetakis, M. G . , "Flammability Characteristics of Combustible Gases and Vapors," Bull. 672, Bur. Mines, 1965. Larsen, E. R., JFF/Fire Retardant Chemistry, 2 5 (1975). Moran, Jr., Η. Ε., and Bertschy, A. W., "Flam mability Limits for Mixtures of Hydrocarbon Fuels, A i r , and Halogen Compounds", NRL Report 4121, (1953). Creitz, E. C., Fire Tech., 8 131 (1972). Purdue Research Foundation and Department of Chemistry: Fire Extinguishing Agents, Summary Report Sept. 1, 1947 to Aug. 31, 1948, Purdue University (Contract W-44-009 eng. 507 with Army Eng. Res. and Dev. Labs.). Bajpai, S. N. and Wagner, J. P . , I&EC Product R&D, 14 54 (1975). Coward, H. F . and Hartwell, F . J., J. Chem. Soc., 1926, 1522. Coward, H. F . and Jones, G. W. : "Limits of Flam mability of Gases and Vapors", Bull. 503, Bur. Mines, 1952.


13.

LARSEN


389

DISCUSSION D. CHAMBERLAIN: D i d you i n c l u d e bromine bromide i n y o u r c o r r e l a t i o n ?

and

hydrogen

E. R. LARSEN: Data on t h e e f f e c t o f HBr was not a v a i l a b l e f o r t h e f u e l s which were d i s c u s s e d i n t h i s paper. Data t h a t i s a v a i l a b l e f o r t h e system propanea i r - h y d r o g e n bromide i n d i c a t e s t h a t HBr i s n o t s i g n i f i c a n t l y d i f f e r e n t i n i t s behavior. R. ALTMAN: Don't you need t o use the H - # 98 instead of C (298) t o demonstrate t h e v e r a c i t y o f your argument, and i s n ' t t h e Τ t o use the flame temperature o r some f r a c t i o n t h e r e o f i n s t e a d o f 298K? 2

T

d

a

t

a

E. R. LARSEN: As I p o i n t e d o u t i n r e f e r e n c e 6, t h e method used here which employs C (298) a l l o w s a good a p p r o x i m a t i o n o f the f l a m m a b i l i t p b e h a v i o r o f t h e s e systems. In a more r i g o r o u s t r e a t m e n t t h e s p e c i f i c heat o f each component s h o u l d be i n t e g r a t e d o v e r t h e temperature range between t h e temperature o f the c o l d gas and the l i m i t flame t e m p e r a t u r e . T h i s i s more i m p o r t a n t w i t h r i c h m i x t u r e s than w i t h l e a n m i x t u r e s s i n c e , w h i l e t h e l i g h t gases show a s l i g h t l i n e a r i n c r e a s e i n C w i t h i n c r e a s i n g temperature, t h e h e a v i e r more complex f u e l s and a g e n t s , e.g., Cj-H, , C 0 , CH^, CF^, e t c . show a l a r g e n o n - l i n e a r i n c r e a s e i n C o v e r t h e range o f 300-1000°K. This factor i s n o t p a r t i c u l a r l y i m p o r t a n t i n systems c o n t a i n i n g l a r g e amounts o f l i g h t components such as N and 0 , i . e . , s t u d i e s employing a i r as the o x i d a n t , and, c o n s e q u e n t l y , was f o r the most p a r t i g n o r e d i n e v a l u a t i n g the "peak" d a t a . I n some c a s e s , e s p e c i a l l y t h o s e i n v o l v i n g e x t i n c t i o n , o r flame v e l o c i t y r e d u c t i o n , i n pre-mixed gas b u r n e r s t h e use o f C (700°K) seems t o g i v e b e t t e r r e s u l t s . There a r e ? however, n o t enough examples o f such systems where a l a r g e v a r i e t y o f agents were employed t o determine whether t h i s approach o f f e r s any d i s t i n c t advantage. 2

2

2

2

A. D. LEVINE: Your c o r r e l a t i o n s based on h e a t c a p a c i t y a r e v a l i d i n a p r a c t i c a l sense f o r p r e d i c t i n g flammability l i m i t s . W i l l t h i s c o r r e l a t i o n work when agents a r e added t o a h o t flame where c h e m i c a l k i n e t i c s


390 may


be

important?

E. R. LARSEN: The c o r r e l a t i o n appears t o h o l d i n t h o s e c a s e s where t h e agent i s i n j e c t e d i n t o e i t h e r the f u e l s i d e o r t h e a i r s i d e o f a d i f f u s i o n f l a m e , or f o r t h a t m a t t e r i n t o a b u r n e r which i s f u e l e d w i t h a premixed f u e l - a i r m i x t u r e . Some adjustment o b v i o u s l y has t o be made s i n c e the b u r n e r i t s e l f may cont r i b u t e t o the heat l o s s e s i n c u r r e d by t h e system. A. S. GORDON : I f we c o n s i d e r h e a t c a p a c i t y on a molar b a s i s , t h e n CF. and CF-Br s h o u l d have t h e same h e a t c a p a c i t y / m o l e t o a good f i r s t a p p r o x i m a t i o n . Now i f we add the same quencher/fue1 r a t i o t o a g i v e n f u e l / o x y g e n mix, t h e two systems s h o u l d have t h e same h e a t r e l e a s e p o t e n t i a l and t o t a l h e a t c a p a c i t y . Yet a t a C F - B r / f u e l r a t i o where t h e flame i s e x t i n g u i s h e d , f u l l r e p l a c e m e n t o f CF~Br by CF. does n o t e x t i n g u i s h the f l a m e . E. R. LARSEN: I agree t h a t CF. and CF-Br have e s s e n t i a l l y the same h e a t c a p a c i t y . I do n o t , however, have d a t a t h a t e i t h e r s u p p o r t s o r r e f u t e s your second statement and I am n o t s u r e t h a t a s e t o f c i r c u m s t a n c e s such as you p o s t u l a t e i s i n f a c t p o s s i b l e . CF. i s e s s e n t i a l l y a t r u e i n e r t gas which c o n t r i b u t e s n o t h i n g t o t h e energy p r o d u c i n g a b i l i t y o f t h e system and c a u s e s m i n i m a l , i f any, change i n t h e f u e l - o x y g e n stoichiometry. CF-Br, on t h e o t h e r hand, does e n t e r the combustion p r o c e s s and cause a marked change i n the f u e l - o x y g e n s t o i c h i o m e t r y . CF~Br, i n f a c t , becomes p a r t o f the f u e l c o n t e n t o f t h e system. W i t h i n t h i s c o n t e x t CF-Br would a l s o a c t as a flame s u p p r e s s a n t i n t h e same manner t h a t t h e a d d i t i o n o f e x c e s s f u e l t o a flammable m i x t u r e can r e s u l t i n the p r o d u c t i o n o f a non-flammable m i x t u r e . Cons e q u e n t l y , i t would n o t s u r p r i s e me t o f i n d t h a t f u l l r e p l a c e m e n t o f CF~Br by CF. d i d n o t e x t i n g u i s h t h e flame. F. A. WILLIAMS : In f i g u r e 17 o f my paper i s shown the dependence o f measured flame temperature a t e x t i n c t i o n on t h e n i t r o g e n - CF-Br r a t i o f o r a heptane d i f f u s i o n flame b u r n i n g i n a m i x t u r e o f oxygen, n i t r o g e n and CF-Br. The temperature a t e x t i n c t i o n i n CF-Br i s more tnan 500°C above t h a t i n n i t r o g e n . T h i s t e l l s me t h a t thhe c o o l i n g caused by CF^Br a t e x t i n c t i o n i s c o n s i d e r a b l y l e s s than t h a t caused by nitrogen. The c h e m i c a l i n h i b i t i o n , i . e . , s l o w i n g o f t h e o v e r a l l r e a c t i o n , by t h e s u p p r e s s a n t a t any


13.

LARSEN


391

g i v e n flame temperature, t h e r e f o r e must be i m p o r t a n t . In a paper t o be p u b l i s h e d i n t h e F i f t e e n t h I n t e r n a t i o n a l Symposium on Combustion, Kent and I a m p l i f y this point further. Our c o n c l u s i o n i s t h a t your c o r r e l a t i o n can work w e l l because o f two competing effects that cancel. E. R. LARSEN: That the temperature a t e x t i n c t i o n i n CF~Br i s more than 500°C above t h a t i n n i t r o g e n comes as no g r e a t s u r p r i s e t o me, i n f a c t i t i s what I would have e x p e c t e d i f my t h e o r y i s c o r r e c t . As I have p o i n t e d o u t N s e r v e s s t r i c t l y as a h e a t s i n k , but CF^Br can p l a y a d u e l r o l e , i . e . , heat s i n k and fuel. In o r d e r t o e v a l u a t e you comments i t i s n e c e s s a r y t o know the c o n c e n t r a t i o n s o f a l l components o f b o t h systems. However, i n view o f the f a c t t h a t t h e i g n i t i o n energy o f a C F ~ B r - 0 - h e p t a n e m i x t u r e near the f l a m m a b i l i t y peak i s s e v e r a l o r d e r s o f magnitude h i g h e r than i s t h e i g n i t i o n energy o f a c o r r e s p o n d i n g N ~ 0 - h e p t a n e m i x t u r e I would assume t h a t a much h o t t e r flame must o c c u r i n o r d e r t o o b t a i n a s e l f - p r o p a g a t i n g flame. 2

2

2

2

W. D. WEATHERFORD, JR.: Southwest Research I n s t i t u t e has o b t a i n e d d a t a which c o u l d be c o n s i d e r e d t o s u p p o r t a p h y s i c a l i n h i b i t i o n mechanism under c e r t a i n conditions. I w i l l mention i t a t t h i s time t o f u r t h e r i l l u s t r a t e the c o m p l e x i t i e s o f h a l o g e n compound flame i n h i b i t i o n mechanisms. We have c o n f i r m e d t h a t 5 l i q u i d volume p e r c e n t h a l o n 1011 (The h a l o n 1011 c o m p o s i t i o n was 85% (wt) 1011, 10% 1020, and 5% 1002.) i n d i e s e l engine i s s a t i s f i e d w i t h such a f u e l . In f a c t , t h e t o t a l i g n i t i o n d e l a y i n a s i n g l e c y l i n d e r r e s e a r c h d i e s e l engine i s not i n f l u e n c e d by t h e p r e s e n c e o f t h i s c o n c e n t r a t i o n o f h a l o n 1011 i n the f u e l . I n a d d i t i o n , minimum a u t o i g n i t i o n temp e r a t u r e measurements y i e l d AIT v a l u e s o f about 480°F f o r h a l o n 1011 c o o n c e n t r a t i o n s i n d i e s e l f u e l from 0 t o 30 l i q u i d volume p e r c e n t , whereas the v a l u e i s about 920°F f o r t h e f u e l - f r e e h a l o n 1011. The o b s e r v e d m i s t f l a m m a b i l i t y , b o t h a t ambient c o n d i t i o n s and i n a d i e s e l e n g i n e , p r o b a b l y stems from t h e vapor f l a m m a b i l i t y c h a r a c t e r i s t i c s shown i n the f o l l o w i n g i l l u s t r a t i o n . These d a t a were measured by Southwest Research I n s t i t u t e Army F u e l s and L u b r i c a n t s L a b o r a t o r y (Wimer, W. W., e t a l , " I g n i t i o n and F l a m m a b i l i t y P r o p e r t i e s o f F i r e - S a f e F u e l s , " NTIS, AD 784281, 1974). The r e s u l t s c l e a r l y i n d i c a t e t h a t 5 l i q u i d volume p e r c e n t t e c h grade h a l o n 1011 can be flammable under c e r t a i n c o n d i t i o n s


392


12

r

MOLE % BROMOCHLOROMETHANE *

Flammability characteristics of diesel fuel bromochloromethane * mixtures in air at 150 dfc 10°C and 1 atm


13.

Fhme Suppression Mechanism

LARSEN

and nonflammable under

393

others.

J . DEHN: The a u t h o r g i v e s as h i s most s u c c e s s f u l a n a l y s i s o f Purdue U n i v e r s i t y r e s u l t s the approximate constancy o f the r a t i o (heat c a p a c i t y of i n e r t s ) / ( t o t a l heat capacity) (Egn. 4 ) . For n-heptane+air systems he f i n d s t h a t when t h e n i t r o g e n + h a l o n i n e r t s c o n t r i b u t e 75% t o t h e h e a t c a p a c i t y o f t h e m i x t u r e t h e n no f l a m e c a n p r o p a g a t e . I submit that f o r the c a s e s he has t r e a t e d t h i s r a t i o i s b a s i c a l l y (heat capacity N )/(heat capacity a i r ) with observed v a r i a t i o n s i n t r o d u c e d by t h e o t h e r components o f t h e system and e x p e r i m e n t a l e r r o r s i n the d a t a . For example, the s p e c i f i c heats (cal/mole-°K) o f n i t r o g e n and oxygen (and so o f a i r ) a r e e q u a l t o 7 . 0 , w h i l e those o f heptane and CF~Br a r e 39.0 and 16.5 r e s p e c t i v e l y a t the i n i t i a l temperature o f t h e gas mixture. F o r p e a k v a l u e s o f 6 . 1 % C F ~ B r , 3% n - h e p t a n e a n d t h e r e s t a i r ( a u t h o r ' s T a b l e I I I ; we o b t a i n 2

.061(16.5) + .73(7.0) .03(39) + .91(7.0) + .061(16.5)

. X

X

Ω

U

η

=

U

1 Δ

*

A c t u a l l y t h e P u r d u e r e p o r t (pp 65 a n d 146) g i v e s 2 . 5 % f u e l i n s t e a d o f 3% w h i c h r a i s e s t h e a b o v e s l i g h t l y t o 73%. I f we n e g l e c t t h e h a l o n a n d t h e f u e l we o b t a i n * ' .91(7.0) 7

3

(

7

Q

)

χ 100 = 80%,

t h e a p p r o x i m a t i o n we a r e u s i n g f o r t h e p e r c e n t a g e o f nitrogen i n a i r . S i m i l a r calculations f o r the other c o m p o u n d s c i t e d r e v e a l t h e same b a s i c r e a s o n f o r t h e c l u s t e r a r o u n d 75%. I f o n l y weight f r a c t i o n s are used w i t h o u t heat capacities 0.3(100) If

.061(149) + .73(28) + .73(28) _ .18(32)

we n e g l e c t

'Mill)

+ .061(149)

X

1

Q

Q

±

υ

υ

=

?

? ,

1 /

e

%

"

t h e h a l o n a n d f u e l we h a v e

+ .18(32)

x

1

0

0

»

7

8

%

'

e

t

C

'

The a u t h o r o b s e r v e s a n o t h e r a p p r o x i m a t e l y c o n s t a n t weight r a t i o (agent)/(agent+fuel) and c l a i m s t h a t somehow t h i s d e m o n s t r a t e s a p u r e l y p h y s i c a l m e c h a n i s m . Why t h i s s h o u l d b e s o i s n o t c l e a r . A t any r a t e , I submit that t h i s r a t i o i s b a s i c a l l y u n i t y (halon


394


w e i g h t d i v i d e d by i t s e l f ) w i t h d e v i a t i o n s i n t r o d u c e d m a i n l y by t h e weight o f the f u e l and w i t h o t h e r v a r i a t i o n s due t o t h e f u e l p e r c e n t a g e found a t a peak and e x p e r i m e n t a l u n c e r t a i n t i e s . F o r example, a l l r a t i o s i n v o l v i n g n-pentane c e n t e r around 0.89 w h i l e t h o s e i n v o l v i n g t h e h e a v i e r η-heptane c e n t e r around t h e v a l u e 0.83. The a u t h o r a l s o c l a i m s t h a t h a l o n s o r hydrocarbon+halon m i x t u r e s become non-flammable i n a i r when h a l o g e n atoms a c c o u n t f o r more t h a n about 70% o f the fuel+halon weight. The examples shown i n t h e a u t h o r ' s r e f e r e n c e 4 do n o t c o n v i n c e one t h a t t h i s f i g u r e i s a p r a c t i c a l guide f o r avoiding f i r e s s i n c e v a l u e s as low as 63% and as h i g h as 80% appear t h e r e . Moreover, C 0 and N which c o n t a i n no h a l o g e n atoms f i t w e i g h t f r a c t i o n schemes elsewhere i n the paper why s h o u l d n ' t they a l s o f i t here? As f o r pure h a l o n s i n a i r b e i n g non-flammable when h a l o g e n atoms make up a t l e a s t 70% o f t h e i r w e i g h t , we might r e f e r t o t h e Z a b e t a k i s r e p o r t c i t e d by t h e a u t h o r (Bureau o f Mines B u l l 627, pp 102-5). Here d a t a i s g i v e n t o show t h a t CH~Br (84% Br by w e i g h t ) , CH^Cl^ (84% CI by weight) and C H C 1 (81% CI by weight) can be flammable i n a i r a t one atmosphere p r e s s u r e o r below, g i v e n a p r o p e r i g n i t i o n s o u r c e and c o n t a i n e r s i z e . S i m i l a r m a t t e r s a r e d i s c u s s e d i n Bur. Mines B u l l . 503, pp 101 f f . 70% by w e i g h t h a l o n i s a poor p r a c t i c a l guide. We may.ask why t h a t f r a c t i o n o f t h e t o t a l m i x t u r e h e a t c a p a c i t y c o n t r i b u t e d by t h e i n e r t s s h o u l d have any p a r t i c u l a r s i g n i f i c a n c e . I t would seem more reasonable t o look f o r a c r i t i c a l heat c a p a c i t y v a l u e of the t o t a l mixture i f heat c a p a c i t y i s indeed the dominant f a c t o r . In p u r e l y t h e r m a l t h e o r i e s o f flame p r o p a g a t i o n i n pre-mixed gases a t l e a s t two p h y s i c a l f a c t o r s a r e u s u a l l y t a k e n t o be i m p o r t a n t . These a r e the s p e c i f i c h e a t , c , and the t h e r m a l conduc t i v i t y , λ. Simple t h e o r i e s l e a d t o a flame v e l o c i t y p r o p o r t i o n a l t o some power o f the r a t i o λ/c , s i n c e a flame s h o u l d p r o p a g a t e w e l l when i t i s e a l y t o t r a n s m i t h e a t and r a i s e t h e temperature o f t h e unburned gas (at l e a s t f o r t y p i c a l h y d r o c a r b o n + a i r m i x t u r e s i n t w o - i n c h t u b e s ) . A good f i r e s u p p r e s s a n t s h o u l d d e c r e a s e λ and i n c r e a s e c , l e a d i n g t o a d e c r e a s e i n flame v e l o c i t y and e v e n t u a l e x t i n c t i o n . The T a b l e below g i v e s the average c and λ v a l u e s f o r a s t o i c h i o m e t r i c n-heptane+air fiixture and f o r Purdue peak p e r c e n t a g e m i x t u r e s w i t h v a r i o u s compounds. The N peak m i x t u r e however was t a k e n from t h e Z a b e t a k i s r e p o r t a l r e a d y c i t e d (p. 36) s i n c e t h e 2

2

2

3

2


13.

395


LARSEN

Purdue work d i d n o t i n c l u d e Ν

λ n-heptane+a i r +4.2% CF Br~ + 5 . 2 % CH^Br^ + 6 . 1 % CFÎBr + 7 . 6 % CH^ClBr + 17.5% CHC1-* + 9 . 3 % HBr +25.5% HCI +29.5% C 0 + 33% H 0 + 42% N 9

9

9

2

NOTE:

56.5 54.9 54.5 54.0 53.4 49.6 52.4 50.3 51.0 50.7 56.7

7.6 8.1 8.1 8.6 8.1 9.1 7.56 7.47 8.1 8.2 7.3

6.78 6.73 6.28 6.59 5.45 6.93 6.73 6.30 6.18 7.77

λ u n i t s : 10 cal/cm-sec-deg Cp u n i t s : c a l / m o l e - d e g

With t h r e e e x c e p t i o n s ( N , HBr and HCI) c i s i n c r e a s e d compared t o t h e r u e l + a i r r e f e r e n c e , w h i l e a l l e x c e p t N lower λ i n t h i s sample. The f i r s t t h i n g we note i s t h e f a c t t h a t t h e r e i s no c r i t i c a l v a l u e o f c o r λ o r o f t h e r a t i o λ/c which c a n be c o r r e l a t e d w i t h t h e phenomenon o f a peak p e r c e n t a g e . I f we s i m p l y l o o k a t averages w i t h o u t p a y i n g a t t e n t i o n t o t h e type o f d e v i a t i o n s we might d e c e i v e o u r s e l v e s i n t o t h i n k i n g t h a t we have found c r i t i c a l v a l u e s and p r o c e e d t o argue t h a t no c h e m i c a l e x p l a n a t i o n i s needed. T h i s s h o u l d be a v o i d e d . The brominated methane examples g i v e n have r e l a t i v e l y s m a l l peak percentages i n s p i t e o f the r e l a t i v e l y high λ values of t h e m i x t u r e s and t h e c o n t r i b u t i o n s which some o f t h e s e compounds might be e x p e c t e d t o make t o t h e h e a t of combustion. T h i s s u g g e s t s t h a t f o r such compounds where d i l u t i o n i s much l e s s i m p o r t a n t t h a n f o r o t h e r compounds i n t h e T a b l e f a c t o r s b e s i d e s λ and c a r e at work. HBr and HCI a r e p a r t i c u l a r l y i n t e r e s t i n g s i n c e t h e y a c t u a l l y lower t h e average heat c a p a c i t y of t h e m i x t u r e y e t a r e more e f f e c t i v e on a volume basis. Even on a weight b a s i s HCI i s more e f f e c t i v e than C 0 . The f a c t t h a t N i n c r e a s e s t h e λ/c ratio p a r t i a l l y e x p l a i n s t h e jump between peak v a l u e s o f H 0 and Ν · We p a r t i c u l a r l y note t h a t N i s h e a v i e r than H 0 y e t more i s r e q u i r e d . I f we i n s e r t a water c u r v e on F i g u r e I o f t h e paper under d i s c u s s i o n , we see t h a t a s i m p l e c o r r e l a t i o n o f e f f e c t i v e n e s s w i t h weight breaks down i m m e d i a t e l y . F i n a l l y we note t h a t CHCl^ i s n o t e s p e c i a l l y e f f e c t i v e i n s p i t e o f i t s 2

2

p

2

2

2

2

2

2


396


f a v o r a b l e λ/c r a t i o i n the m i x t u r e . A l l of t h i s l e a v e s us w i t n o u t a c o r r e l a t i o n between e f f e c t i v e n e s s i n t h i s p a r t i c u l a r experiment and t h e s e two t h e r m a l parameters. We can e i t h e r e l a b o r a t e our p h y s i c a l e x p l a n a t i o n by i n c l u d i n g o t h e r f a c t o r s o r can t u r n to a p a r t i a l chemical e x p l a n a t i o n ( i f indeed i t i s p r o f i t a b l e t o make such d i s t i n c t i o n s ) . While I d i s p u t e t h e a u t h o r ' s c l a i m o f showing t h e r e i s o n l y one p h y s i c a l mechanism a t work i n h a l o n flame s u p p r e s s i o n , I do not n e c e s s a r i l y deny h i s assertion. He may be r i g h t i n some c a s e s i n s p i t e of t h e f a c t t h a t he has not p r o v e d h i s c a s e h e r e . My p e r s o n a l p r e j u d i c e i s toward the view t h a t some c o m b i n a t i o n o f p h y s i c a l and c h e m i c a l mechanisms i s p r o b a b l y a t work i n most i n h i b i t i o n o r promotion phenomena, w i t h one o r the o t h e r dominant perhaps i n p a r t i c u l a r c a s e s , a l t h o u g h no one has y e t e s t a b l i s h e d t h e c o n d i t i o n s which must h o l d . While s t r o n g i n d i c a t i o n s o f a t l e a s t p a r t i a l c h e m i c a l mechanisms have been p r e s e n t e d a t t h i s symposium and elsewhere, t h e r e i s s t i l l a l i n g e r i n g doubt s i n c e l i t t l e attempt i s u s u a l l y made t o e v a l u a t e p o s s i b l e p h y s i c a l f a c t o r s w h i c h must a l s o be a t work. S i m i l a r l y the p r e s e n t paper and my comments make no attempt t o e v a l u a t e t h e c h e m i s t r y o f e x p l o s i o n b u r e t t e work. I t might be w o r t h w h i l e t o employ c h e m i c a l a n a l y s i s w i t h i s o t o p i c l a b e l i n g i n such pre-mixed gas f l a m e p r o p a g a t i o n experiments as has been done i n s t a t i c o r f l o w r e a c t o r systems which p r o p a g a t i o n d i s t a n c e i s not a f a c t o r . The c h e m i s t r y o f combustion i n h i b i t i o n i s a d i f f i c u l t f i e l d which has opened up o n l y i n t h e p a s t few y e a r s . K i n e t i c i s t s a r e t o be c o n g r a t u l a t e d f o r the f a c t u a l l i g h t t h e y have a l r e a d y shed on o l d theories. While the g u l f between our p r e s e n t c h e m i c a l knowledge and the g o a l o f improved f i r e s u p p r e s s i o n i s s t i l l q u i t e wide, the p r a c t i c a l importance o f t h e g o a l i s c e r t a i n l y worth the e f f o r t . E. R. LARSEN: Dr. Dehn's c r i t i q u e o f my paper i s q u i t e l e n g t h y and d e t a i l e d and r e q u i r e s a s i m i l a r i l y d e t a i l e d answer. I t s h o u l d be n o t e d t h a t Dr. Dehn's c r i t i q u e i s s t r i c t l y a p p l i c a b l e t o a paper ( r e f . 2) t h a t I gave s e v e r a l y e a r s ago. In t h a t e a r l i e r paper I had d e a l t i n l e n g t h upon the use o f mass f r a c t i o n as a means o f e x p r e s s i n g l i m i t d a t a , w i t h o n l y a c u r s o r y r e f e r e n c e t o the f a c t t h a t e x p r e s s i n g t h e c o m p o s i t i o n on a h e a t c a p a c i t y b a s i s appeared t o be superior. In the p r e s e n t paper the o p p o s i t e t a c k was t a k e n . In s p i t e o f t h i s change Dr. Dehn, a f t e r


13.

LARSEN


397

h e a r i n g my paper, d e c i d e d t o s t a n d by h i s c r i t i q u e . Dr. Dehn's p o i n t s a r e w e l l t a k e n and were c o n s i d e r e d d u r i n g the a n a l y s i s o f the f l a m m a b i l i t y l i m i t data t h a t i s a v a i l a b l e i n the l i t e r a t u r e . U n f o r t u n a t e l y , I can n o t agree w i t h h i s r e a s o n i n g , nor w i t h h i s c o n c l u s i o n s . F l a m m a b i l i t y l i m i t , and peak, d a t a w h i c h appears i n t h e l i t e r a t u r e has been determined i n a v a r i e t y o f systems and w i t h a v a r i e t y of i g n i t i o n sources. As I p r e v i o u s l y s t r e s s e d t h e Purdue s t u d y was c a r r i e d o u t twenty, o r so, y e a r s ago and employed a spark i g n i t i o n s o u r c e o f unknown s t r e n g t h . From our e x p e r i e n c e spark s o u r c e s employed d u r i n g t h a t p e r i o d were n o t i n t e n s e enough t o i g n i t e a l l flammable mixtures. I t i s p r o b a b l e t h a t the l i m i t s o f i g n i t a b i l i t y were b e i n g measured i n the Purdue study; r a t h e r than the l i m i t s o f f l a m m a b i l i t y . T h i s would r e s u l t i n an e x p e r i m e n t a l f l a m m a b i l i t y peak t h a t would be somewhat lower than i t s h o u l d be. Such an e f f e c t i s a p p a r e n t when a comparison i s made o f t h e Purdue peak v a l u e f o r CF~Br (6.1 v o l %) and the CF~Br peak (7.2 v o l %) c i t e d i n T a b l e IV. For t h i s reason I have s h i e d away from l a y i n g undue emphasis upon t h e peak v a l u e o f any s i n g l e agent. In s p i t e o f t h i s problem the Purdue study i s i n t e r n a l l y s e l f - c o n s i s t e n t and i s l a r g e enough (46 agents) so t h a t t h e t r e n d s drawn from i t s h o u l d be v a l i d even though t h e i n d i v i s u a l peak v a l u e s may be u n r e l i a b l e . It i s partly f o r t h i s r e a s o n t h a t the d i f f e r e n c e s between t h e v a l u e s o f KC f o r the i n e r t gases and f o r t h e h a l o n s do n o t o v e r l y d i s t u r b me. The r e a s o n i n g employed by Dr. Dehn i n c r i t i z i n g my a n a l y s i s o f t h e peak d a t a i s o b v i o u s l y i n v a l i d ; b o t h when the c o m p o s i t i o n s a r e e x p r e s s e d on a mass f r a c t i o n b a s i s o r on a h e a t c a p a c i t y b a s i s . To deny the v a l i d i t y o f a d i r e c t c o n v e r s i o n o f v o l % d a t a t o t h e s e o t h e r u n i t s , a s i m p l y m a t h e m a t i c a l manipulation» i s t o deny the v a l i d i t y o f the volume % d a t a i t s e l f . Dr. Dehn submits t h a t f o r the c a s e s i n which a i r i s employed as the o x i d a n t , as i n the Purdue s t u d y , KC i s " b a s i c a l l y (heat c a p a c i t y N ) / ( h e a t c a p a c i t y a i r ? w i t h o b s e r v e d v a r i a t i o n s i n t r o d u c e d by o t h e r components o f t h e system and e x p e r i m e n t a l e r r o r s i n the d a t a . " Dr. Dehn's r a t i o (heat c a p a c i t y N ) / ( h e a t c a p a c i t y a i r ) i s l i t e r a l l y p o i n t D i n f i g u r e 2. If the "observed v a r i a t i o n i n t r o d u c e d by the o t h e r components o f t h e system and the e x p e r i m e n t a l e r r o r i n the d a t a " a r e r e a l l y t h i s l a r g e then I submit t h a t a l l o f our s t u d i e s a r e w o r t h l e s s , s i n c e the same argument can be employed when volume % d a t a i s employed. 2

2


398


Dr. Dehn's comments must a l s o be viewed as i n v a l i d s i n c e t h e y a r e a p p l i c a b l e t o a l l p o i n t s bound by t h e t r i a n g l e ADC i n f i g u r e 2, and a p p l y e q u a l l y w e l l t o the i n e r t gases and t o the h a l o n s , i n c l u d i n g p o i n t s l a y i n g near, b u t not on, s i d e AC, and c e r t a i n l y t o a l l p o i n t s l a y i n g a l o n g l i n e AD. The w e i g h t r a t i o ( a g e n t ) / ( a g e n t + f u e l ) was e x p r e s s e d i n the c u r r e n t paper on a h e a t c a p a c i t y b a s i s r a t h e r than on a mass f r a c t i o n b a s i s as was done i n t h e paper r e f e r r e d t o by Dr. Dehn. The importance o f t h i s r a t i o i s a g a i n o b v i o u s i n f i g u r e 2, s i n c e t h i s r a t i o s i m p l y r e p r e s e n t s any c o m p o s i t i o n c o n s i s t i n g s o l e l y o f agent and f u e l : t h o s e composi t i o n s l a y i n g a l o n g s i d e AC. As i s a p p a r e n t i n f i g u r e 2 any m i x t u r e o f i n e r t gas and f u e l w h i c h has a c o m p o s i t i o n l a y i n g between p o i n t s A and Κ will, upon the a d d i t i o n o f oxygen, e v e n t u a l l y y i e l d a flammable m i x t u r e , i . e . , pass t h r o u g h the flammable envelope ( I P J ) . One would a n t i c i p a t e t h a t i f t h e h a l o n s were d i s p r o p o r t i o n a t e l y more e f f e c t i v e t h a n the i n e r t gases t h e y would show v a l u e s f o r t h i s r a t i o w h i c h a r e much s m a l l e r than i s found f o r the i n e r t gases. I t might a l s o be e x p e c t e d t h a t t h i s r a t i o would be much s m a l l e r w i t h the bromine c o n t a i n i n g h a l o n s than f o r the c h l o r i n e or f l u o r i n e c o n t a i n i n g halons. That t h i s i s not the c a s e i s shown by t h e v a l u e s o f K , i . e . , the v a l u e o f t h i s r a t i o f o r peak c o m p o s i t i o n s , shown i n T a b l e I I I . The c o n s t a n c y o f t h i s r a t i o i s s t r o n g e v i d e n c e i n s u p p o r t o f an argument t h a t t h e p r i m a r y mechanism o f flame s u p p r e s s i o n i s t h e same f o r b o t h the i n e r t gases and the h a l o n s , and, c o n v e r s e l y , i s d i f f i c u l t t o r a t i o n a l i z e away w i t h a c h e m i c a l mechanism. The o b s e r v a t i o n t h a t the h a l o n s and h a l o n h y d r o c a r b o n m i x t u r e s g e n e r a l l y become non-flammable i n a i r when t h e h a l o g e n atoms a c c o u n t f o r more t h a n 70% o f the f u e l = h a l o n w e i g h t i s d e a l t w i t h i n d e t a i l i n r e f e r e n c e 4. As p o i n t e d o u t i n r e f e r e n c e 4 the f l a m m a b i l i t y peak c o m p o s i t i o n i s most i n t e r e s t i n g from a m e c h a n i s t i c s t a n d p o i n t s i n c e i t r e p r e s e n t s t h a t c o m p o s i t i o n o c c u r r i n g a t t h e p o i n t o f convergence of the upper and lower l i m i t c u r v e s . The peak com p o s i t i o n r e p r e s e n t s t h e minimum amount o f agent r e q u i r e d t o i n e r t any and a l l f u e l - o x i d a n t m i x t u r e s . T h e r e f o r e , the c o m p o s i t i o n o f t h e system a t t h i s p o i n t i s unique w i t h r e s p e c t t o the agent and f u e l i n each a g e n t - f u e l - o x i d a n t system, and c o m b i n a t i o n s o f f u e l and agents t h a t o c c u r a t the peak can be t r e a t e d as single entities. The h a l o g e n c o n t e n t o f the m i x t u r e I F


13.

LARSEN


399

i s f i x e d , and may be c a l c u l a t e d , i n much t h e same way t h a t t h e h a l o g e n c o n t a n t o f a f i n i t e compound i s fixed. The h a l o g e n c o n t e n t f o r some 60 peak c o m p o s i t i o n s were c a l c u l a t e d and t h e mean h a l o g e n c o n t e n t o f t h e m i x t u r e s was found t o be 69.8 ± 3.5 wt %, i . e . , wt

% Halogen i n _ wt Halogen χ 100 the mixture wt Agent + wt f u e l

t o determine whether o r n o t any one s p e c i e s o f h a l o g e n was d i s p r o p o r t i o n a t e l y more e f f e c t i v e than any o t h e r , as demanded by t h e r a d i c a l t r a p t h e o r y o f flame s u p p r e s s i o n , t h e peak d a t a f o r t h e 46 compounds i n c l u d e d i n t h e Purdue s t u d y were broken down by h a l o g e n t y p e , and t h e mean v a l u e s o f κ f o r each group determined. One group (9 agents) c o n t a i n e d o n l y f l u o r i n e (κ=68.0), and a f o u r t h and f i n a l group (5 agents) c o n t a i n e d i o d i n e o r i o d i n e and f l u o r i n e (κ=73). On t h e b a s i s o f t h i s a n a l y s i s , which i s somewhat more complete i n r e f . 4, no e v i d e n c e was found t o s u p p o r t t h e c o n t e n t i o n t h a t any one h a l o g e n was more e f f e c t i v e t h a n any o t h e r , and t h e r e l a t i v e e f f e c t i v e n e s s o f t h e h a l o g e n i s shown t o be d i r e c t l y p r o p o r t i o n a l t o t h e i r atomic weights, i . e . , F : C l : Br : 1 = 1 . 0 : 1.9 : 4.2 : 6.7. The f a c t t h a t one bromine atom i s 4.2 t i m e s as e f f e c t i v e as a f l u o r i n e atom on a molar b a s i s i s not s u p p o r t i v e o f t h e con t e n t i o n t h a t t h e y f u n c t i o n by d i f f e r e n t mechanisms. The f a c t t h a t t h o s e h a l o n s and h a l o n - h y d r o c a r b o n systems s t u d i e d showed a mean v a l u e o f 70 wt % h a l o g e n does not mean t h a t t h e r e a r e no e x c e p t i o n s : t h r e e b e i n g c i t e d by Dr. Dehn. I t s h o u l d be p o i n t e d o u t t h a t w h i l e CH~Br i s flammable i n a i r even though i t c o n t a i n s 84 wt % Br, t h e Purdue s t u d y shows a peak p e r c e n t a g e f o r t h e heptane-CH-Br system than c o n t a i n s 73 wt % bromine. I would be r e m i s s here i f I d i d not t u r n Dr. Dehn's q u e s t i o n about and ask how the c h e m i c a l mechanism e x p l a i n s t h e f l a m m a b i l i t y o f CH^Br i n a i r i n view o f i t s h i g h molar c o n c e n t r a t i o n o f bromine. I do agree w i t h Dr. Dehn t h a t employing 70 wt % h a l o g e n as a p r a c t i c a l g u i d e i s poor p o l i c y , e x c e p t f o r s a y i n g i f t h e compound, o r m i x t u r e o f f u e l and h a l o n , c o n t a i n s l e s s t h a n 70 wt % h a l o g e n i t s h o u l d be r i g o r o u s l y t e s t e d b e f o r e assuming t h a t i s non flammable. I n g e n e r a l , any f l a m m a b i l i t y l i m i t d a t a o b t a i n e d u s i n g an unknown s p a r k s o u r c e i s p o t e n t i a l l y a booby-trap ( r e f . 2).


400


Dr. Dehn asks why the f r a c t i o n o f the t o t a l m i x t u r e heat c a p a c i t y c o n t r i b u t e d by t h e i n e r t s s h o u l d have any p a r t i c u l a r s i g n i f i c a n c e . I do n o t have a good answer f o r t h i s q u e s t i o n a t t h i s time o t h e r than i t seems t o work. I f we examine the peak d a t a f o r the system p e n t a n e - a i r - a g e n t we f i n d t h a t t h e agent c o n c e n t r a t i o n d e c r e a s e s as t h e t o t a l h e a t c a p a c i t y o f the m i x t u r e i n c r e a s e s , but the r a t i o o f the h e a t c a p a c i t y c o n t r i b u t i o n o f the agent and the t o t a l heat c a p a c i t y o f the m i x t u r e remains r e a s o n a b l y c o n s t a n t . The N i n the a i r i s p r o p e r l y p a r t o f the agent system and i t s heat c a p a c i t y c o n t r i b u t i o n i s i n c l u d e d i n Σ[I]Cp . 2

T

Agent ( v o l %) Pentane

N (42.2) C0 (28.0) CF~ (20.6) SF* (15.7 CJ* (10.6) CF " ( 4 1 . 2 ) * C F j (46.8)** 9

9

Σ[Ι]0

Ρ ι

6.00 6.29 7.25 8.12 8.62 8.97 8.67

* Oxygen e n r i c h e d " a i r " - 31.3% ** Oxygen e n r i c h e d " a i r " - 38.7%

E[C]Cp

KC

c

7.42 8.11 9.10 10.19 10.69 11.45 11.02 0 0

p

80.8 77.5 79.5 79.7 80.6 78.7 78.3

2 2

On t h e b a s i s o f the b e h a v i o r o f t h e i n e r t gases I see l i t t l e r e a s o n t o e x p e c t t h a t a c r i t i c a l v a l u e o f Cp w i l l be found, u n l e s s t h i s r a t i o i s t h a t v a l u e . I am aware o f the f a c t t h a t i n p u r e l y t h e r m a l t h e o r i e s o f flame p r o p a g a t i o n i n pre-mixed gases a t l e a s t two p h y s i c a l f a c t o r s a r e u s u a l l y t a k e n t o be important: t h e s p e c i f i c heat (Cp) and t h e t h e r m a l c o n d u c t i v i t y (λ). As f a r as I can a s c e r t a i n λ was o r i g i n a l l y proposed as b e i n g i m p o r t a n t by Coward and H a r t w e l l i n an attempt t o j u s t i f y the f a c t t h a t h e l i u m appeared t o be more e f f e c t i v e than t h e y thought i t s h o u l d be i n view o f i t s low h e a t capacity: e s p e c i a l l y when compared t o argon which has the same h e a t c a p a c i t y but a much lower λ v a l u e . They v e r y h i g h t h e r m a l c o n d u c t i v i t y o f h e l i u m then was employed t o r a t i o n a l i z e t h e e f f i c i e n c y o f h e l i u m . In a p r e v i o u s paper ( r e f . 6 ) , I n o t e d t h a t n e i t h e r He nor Ar were s i g n i f i c a n t l y d i f f e r e n t than N , H 0, o r C 0 , when compared t o a pure heat c a p a c i t y b a s i s . I f the p r e s e n t t h e r m a l t h e o r i e s r e q u i r e t h a t λ p l a y an i m p o r t a n t r o l e i n flame p r o p a g a t i o n then I s u g g e s t 2

2


2

13.


LARSEN

401

t h a t t h e s e t h e o r i e s might r e q u i r e r e t h i n k i n g . I f Dr. Dehn had extended h i s t a b l e t o i n c l u d e more i n e r t gases he would have o b s e r v e d t h a t Cp f o r "peak" c o m p o s i t i o n s can extend from a t l e a s t 5.3 t o about 13.5 λ(10~ ) can extend from about 42 t o 295, and h i s " c o n s t a n t " r a t i o λ/Cp from about 5.5 t o 55. I agree w i t h Dr. Dehn t h a t l o o k i n g a t a v e r a g e s , e s p e c i a l l y w i t h a l i m i t e d d a t a base such as employed by Dr. Dehn, can cause one t o be d e c i e v e d i n t o t h i n k i n g t h a t we have found c r i t i c a l v a l u e s and p r o c e e d t o argue t h a t no c h e m i c a l e x p l a n a t i o n i s needed. C o n v e r s e l y , one can be d e c e i v e d by o b s e r v i n g t h a t because t h e h a l o g e n a t e d agents undergo t h e r m a l c r a c k i n g and e n t e r i n t o the flame c h e m i s t r y t h i s c h e m i s t r y must be i n h i b i t o r y i n n a t u r e . Proposed mechanisms o f flame s u p p r e s s i o n i n which the h a l o g e n behaves as a r a d i c a l t r a p g i v e r i s e t o a dichotomy which i s n o t g e n e r a l l y d e a l t w i t h . On t h e one hand, the h a l o g e n s and t h e i r compounds a r e w e l l known as e f f e c t i v e flame s u p p r e s s a n t s w i t h bromine o c c u p y i n g a prominent p o s i t i o n w i t h r e s p e c t to e f f i c i e n c y . On the o t h e r hand, i t i s a l s o known t h a t bromine i s e s p e c i a l l y e f f e c t i v e as an e f f e c t i v e o x i d a t i o n , and hence combustion, promoter. I s i t not p o s s i b l e t h a t t h e f r e e r a d i c a l c h e m i s t r y t h a t i s r e p u t e d t o r e s u l t i n flame i n h i b i t i o n might be t h e c h e m i s t r y o f flame promotion, o r the i n i t i a t i o n o f the combustion a t a temperature lower than i t might o t h e r w i s e o c c u r i n a g i v e n system. I t s h o u l d be p o i n t e d o u t here t h a t t h e d a t a f o r KCp i n T a b l e I I I does have a b i a s b u i l t i n t o i t , and t h a t t h e s i m p l e use o f a v e r a g e s can be m i s l e a d i n g . As mentioned i n my o r a l p r e s e n t a t i o n , b u t o v e r l o o k e d i n the w r i t t e n manuscript, I ran a r e g r e s s i o n a n a l y s i s o f t h i s d a t a and found i t t o f i t t h e e q u a t i o n : f

EtUCpj

= 1.0

E[C]Cp

c

-

2.0

which tends t o a c c o u n t f o r t h e f a c t t h a t t h e v a l u e o f KCp tends t o i n c r e a s e as we descend t h e column. T h i s b i a s i s a s s o c i a t e d w i t h the f a c t t h a t each a g e n t - f u e l - o x i d a n t system has i t s own unique f l a m m a b i l i t y diagram. I f we l o o k a t the diagrams f o r a s e r i e s o f a g e n t s , such as was done i n f i g u r e 3, we f i n d t h a t t h e y form a f a m i l y o f diagrams h a v i n g a common base ( l i n e AB). F o r each d i f f e r e n t agent the apex C o f t h e diagram w i l l be e v a l u a t e d above the p l a n e o f t h e page, so t h a t our c o l l e c t i o n o f diagrams open somewhat l i k e the l e a v e s o f a book.


402


The diagram which o c c u p i e s t h e page i s t h a t o f N and t h e p o i n t s f o r t h e o t h e r agents a r e shown as p r o j e c t i o n s upon t h e N diagram. I submit t h a t t h e burden o f p r o o f f o r t h e s u p p o r t o f a new and n o v e l mechanism l i e s upon t h o s e who propose i t . I do not deny t h a t t h e h a l o n s e n t e r i n t o t h e flame c h e m i s t r y and i n most c a s e s appear t o a c t as f u e l s . T h e i r b e h a v i o r i n premixed gas systems, such as employed i n combustion t u b e s , i s e s s e n t i a l l y i n d i s t i n g u i s h a b l e from t h a t o f t h e i n e r t gases. I t i s f o r t h e r e a s o n s s e t down i n t h i s paper and i n p r e v i o u s papers t h a t I m a i n t a i n t h a t t h e p r i m a r y r o l e o f t h e h a l o n s i n flame s u p p r e s s i o n i s as h e a t s i n k s , and t h a t t h e y have a common mechanism w i t h the i n e r t g a s e s . As a f u r t h e r note i n p r o o f , I would l i k e t o say t h a t j u s t p r i o r t o t h i s meeting I r e q u e s t e d Dr. R. V. P e t r e l l a o f our l a b o r a t o r y t o determine whether o r not t h e c o m p o s i t i o n 48.9 v o l % CF-Br, 7.2 v o l % 5 12' ·*·8 v o l % 0 , t h e whole a t one atmosphere p r e s s u r e , was flammable. Indeed, i t p r o v e d t o be. On a weight f r a c t i o n b a s i s the r a t i o 2 #

2

C

H

a

n

d

4

2

[CF^Br] χ 100 [C H I+[o ]+[CF Br] 5

1 2

2

79.2

3

i s r e a s o n a b l y c l o s e t o t h e r a t i o found i n the Purdue s t u d y f o r t h e system H e p t a n e - A i r - C F - B r , which was c r i t i c i z e d by Dr. Dehn above, i . e . , [CF^Br]+[N ] x 100 = [C H ]+fo ]+[N^]+[CF Br] 0

7

16

2

77.1

3


Flame Suppression by a Physical Mechanism?

Recommend Documents