NRC Publications Archive
Archives des publications du CNRC
Inert Gas Fire Extinguishment: Excessive High-Level Vents
McGuire, J. H.
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. /
La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version
acceptée du manuscrit ou la version de l’éditeur.
Publisher’s version / Version de l'éditeur:
Building Research Note, 50, p. 7, 1965-11-01
NRC Publications Record / Notice d'Archives des publications de CNRC:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?lang=en
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?lang=fr
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=fr
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Contact us / Contactez nous: [email protected].
INERT GAS FIRE EXTINGUISHMENT:
EXCESSIVE HIGH -LEVEL VENTS
SUMMARY
A previous publication specified critical a r e a s of high-level
vent which, i f exceeded, would not allow reduction of the oxygen
concentration in buildings t o zero. In fact, many building f i r e s
will be satisfactorily extinguished if the oxygen concentration i s
only reduced to about 9 p e r cent to 15 p e r cent. En this note it
is shown that vent a r e a s may exceed the critical values by a
factor of 2 or even 4 under favourable circumstances.
A recent publication (1) discussed the large-scale use of
inert gas for the extinguishment of building fires. It was shown
that the limitation of this technique will usually be concerned with
l o s s of the injected inert gas from high-level openings. An expression
was given ( s e e Appendix I) for the maximum permissible a r e a of
high-level openings that will ensure that all a i r i s excluded from the
building.
Where high-level openings exceed the critical value, a i r
will enter the compartment through low-level openings (which a r e
assumed to be prevalent) and will mix with the inert gas to give a
virtually homogeneous mixture throughout. The object of this note
i s t o extend the previous analysis t o cover this condition.
The analysis i s made in Appendix I, and the results a r e
presented in Figure 1. Two types of gas and temperature condition
a r e considered. In addition to the "N. R. C. gas" (i.e. one consisting
of products of combustion and a l a r g e proportion (>60 per cent of
water), a hypothetical inert gas with a molecular weight equal to the
It constitutes a limiting c a s e ,
m e a n value for a i r is considered.
giving t h e c u r v e on t h e e x t r e m e left of F i g u r e 1. C u r v e s for a l l
other g a s e s and t e m p e r a t u r e conditions would l i e to t h e right of
t h i s curve.
T h e f i r s t of t h e two t e m p e r a t u r e conditions c o n s i d e r e d is
constant t e m p e r a t u r e which m o r e a p p r o p r i a t e l y r e p r e s e n t s t h e
i n i t i a l s t a g e s of injection into a building heavily involved in f i r e and
a l s o t h e u s e of a g a s including a substantial proportion of w a t e r vapour
a t a t e m p e r a t u r e n e a r 100"C.
In t h e l a t t e r c a s e , h e a t t r a n s f e r t o
diluent a i r r e s u l t s not s o much f r o m cooling of t h e i n e r t g a s as f r o m
condensation of a s m a l l p r o p o r t i o n of the water vapour.
T h e second
t e m p e r a t u r e condition a s s u m e s that t h e t e m p e r a t u r e of the g a s i n the
f i r e e n c l o s u r e is t h e a r i t h m e t i c m e a n of the i n e r t g a s and diluent
a i r temperatures.
T h i s assumption l i b e r a l i z e s vent r e q u i r e m e n t s .
In fact, t h e e x t r e m e right -hand c u r v e of F i g u r e 1 is not valid f o r i t
d i s r e g a r d s t h e heat contribution given by t h e condensation of w a t e r
vapour
.
A s both the e x t r e m e c u r v e s of F i g u r e 1 a r e only of somewhat
a c a d e m i c i n t e r e s t , and as g r e a t a c c u r a c y is not c a l l e d for, it is
r e a s o n a b l e t o m a k e a generalization f r o m the figure.
T h e oxygen
content of a building need v i r t u a l l y n e v e r be lowered below 9 p e r
c e n t even t o s u p p r e s s smouldering combustion, and a t m o s p h e r e s
with 12 t o 15 p e r cent 0 will usually inhibit f l a m i n g combustion. It
c a n t h e r e f o r e b e conclu ed that high-level vent a r e a s m a y b e allowed
to exceed t h e c r i t i c a l value by a f a c t o r of between 2 and 4, depending
on whether smouldering combustion is involved.
f
A s c r i t i c a l high -level vent a r e a is l i n e a r l y r e l a t e d t o volume
r a t e of injection of i n e r t gas, t h e above s t a t e m e n t might b e m o r e
T h e capacity of a n i n e r t g a s
a p p r o p r i a t e l y o r i e n t e d a s follows.
g e n e r a t o r need only b e 1/2 t o 1/4 that specified by t h e c r i t e r i o n
concerning high -level vent.
REFERENCES
1.
McGuire, J. H. L a r g e s c a l e u s e of i n e r t g a s to extinguish building
f i r e s . T h e Engineering Journal, Engineering Institute of Canada,
Vol. 48, No. 3, M a r c h 1965, p. 29-33.
APPENDIX
-
I
EXCESSIVE HIGH -LEVEL V E N T S
It has been shown that the relation between
high-level l o s s e s f r o m a building and high-level vent a r e a i s :
A
. . . (1)
=
T
0
The symbols used in this Appendix a r e defined in Appendix 11.
The upper limit of a r e a which will still ensure that a l l a i r
excluded fro m the building i s :
T o proceed further, expressions must be derived for v,
m and T1. In comparing A and Ac it will be assumed that the s a m e
gas generator i s involved s o that the r a t e of injection of i n e r t gas i s
the same.
Since v and v a r e quantities normalized to the s a m e
G
temperature, i t ther efor,e follows that
Use of the factor n i s v e r y convenient since i t follows f r o m i t s
definition that the oxygen concentration of the atmosphere in the f i r e
enclosure will be 21n p e r cent. It i s a knowledge of this quantity in
t e r m s of A/A which i s required.
C
In t e r m s of
A,
Two conditions of temperature will be of interest, one being
where dilution of the atmosphere in the f i r e enclosure by a i r has little
influence on temperature or T t o 'F. Substituting t h i s and expressions
( 4 ) and ( 5 ) in expression ( 3 ) gives:
If it is assumed that the diluent a i r cools the atmosphere
in the f i r e enclosure, then in evaluating T * it is convenient to assume
that different gases, at constant pressure, have the same molar heat.
The gases of interest in the present context will range from diatomic
with 5 degrees of freedom t o polyatsmic, with vibrating atoms, with
8 degrees of freedom, which will have a molar heat (at constant
p r e s s u r e ) some 40 p e r cent higher. The most valuable justification
for the assumption, however, follows f r s m ' t h e end result of the
numerical calculations, when it will. be seen that (a ) the assumption
of diluent cooling does not have a great effect on values of A/A and
and in o r i e r to
(b ) this latter effect permits greater ratios of A/A
cr
ensure f i r e extinguishment it i s best assumed not to occur.
As surning invariant molar heat,
The assumlption of diluent cooling implies that the gas t e m perature i s little affected by heat transfer from the walls of the
enclosure. This assumption i s implicit in expression (7 ) and will
be taken t o apply to expression ( 2 ) so that in ( 2 ) and (3) T =
T~
Using this information and expresaiotxs ( 4 ) and ( 5 ) in
expression (3 ) gives
'
APPENDIX I1
LIST O F SYMBOLS
T
0
= Absolute ambient temperature of external atmosphere.
r
Absolute inlet temperature of inert gas.
T
r
Absolute temperature within f i r e enclosure (critical c a s e ) .
T'
r
Absolute temperature within f i r e enclosure when dilution with
a i r i s occurring.
z
Mean molecular weight of air.
T~
m
0
Molecular weight of inert gas.
mG
=
m
= Mean molecular weight of gas in f i r e enclosure.
n
x
A
C
Proportion (by volume) of a i r in f i r e enclosure :(OCn< I ) .
= Critical a r e a of high-level vent.
A
= Area of high-level vent.
k
= Constant, itself dependent on height of building.
r
VG
v
Volume r a t e of loss of inert gas, normalized to a temperature
T
0'
= ,Total volume r a t e of loss ,of atmosphere from f i r e enclosure,
normalized t o a temperature T
0'
-" N R C
Gas"
--- H y p o t h e t i c a l
i n e r t g a s with
s a m e m o l e c u l a r weight a s a i r
A/Ac
( R A T I O OF H I G H LEVEL VENT AREA TO THE C R I T I C A L VALUE)
FIGURE
1
OXYGEN C O N C E N T R A T I O N v s VENT A R E A
flR3474