Inherent characteristics of combustion-generated inert gases cooled by water injection

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la

première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Building Research Note, 1966-05-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=d5d0849c-00ff-4f8c-9855-8f937c75a4d5 https://publications-cnrc.canada.ca/fra/voir/objet/?id=d5d0849c-00ff-4f8c-9855-8f937c75a4d5

NRC Publications Archive

Archives des publications du CNRC

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.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/40000652

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Inherent characteristics of combustion-generated inert gases cooled by

water injection

I Ser

Trn

56

OF

GOMEIUSTION-GENERATED

COOLED

BY

T- . j RESEARCH COUNCIL -- I I I May

I 9 6 6

-?,#

I

.

INHERENT CHARACTERISTICS O F

COMBUSTION -GENERATED INERT GASES

COOLED

B Y

ABSTRACT

The composition of a water spray cooled,

combustion-type inert gas: is independent of the

choice of hydrocarbon fuel. The effect

on

-

perature and transparency of adding excess air

is evaluat ed,

U s e d on a massive scale, inert gas has an application in t h e f i g h t i n g of building fires, A convenient w a y of generating a suitable gas at a sufficiently high r a t e is by combustion and

subsequent cooling(l) (2). If the generator is t o b e mobile t h e

most appropriate cooling technique c u r r e n t l y available is direct

injection a n d vaporization of water.

Adwption of the vaporization cooling technique w i l l not p e r m i t seduction of t ernperatur e much below 1 00"

C ,

and obviously gives a gas with a very high water vapour content.

These features could be undesirable f o r either of the following

reasons,

(a) In some quarters it is suggested that firemen should b e able

to move around a building while it is being inert ed to carry out

various operations such as the opening or closing of doors. Not

m e r e l y would high temperatur c s b e undesirable, but a super

-

(b) The gas might be required in place of air for the genexation

of high expansion foam and it has been found that the stability of some high expansion foams falls off at high temperatur es.

It is the object of this note to report on the inherent

characteristics of the w e t combustion-typ e inert gases

relevant to t h e above p r o b l e m s and to d i s c u s s the variations that can b e effected by a reduction in the water injection rate and dilution with air,

CHOICE

OF

FUEL

The specified t y p e of i n e r t gas might be generated by

the combustion of any of a number of fuels such as gasoline, fuel o i l or propane. Simple calculations such as that given

in Appendix A indicate that the constitution of t h e i n e r t gas w i l l be virtually unaffected by the choice of fuel. Benzene, n-heptane, n - decane, and propane (all hydrocarbons) were

considered, a n d the water vapour content of the theoretical

outputs r a n g e d f r o m

66

constituent w a s atmospheric nitrogen together with any trace

elements found in a n o r m a l atmosphere,

It was assumed that a stoichiometric fuel-air mixture w a s burned. The effect of a d d i n g air w i l l be discussed later

and this w i l l cover the burning of a lean mixture, since the

sequence in which various processes a r e carried out is theor e t i c a l y unimportant.

In discussing the characteristics of various i n e r t gas-

air mixtures it w i l l be assumed t h a t the basic gas, when cooled to t h e limit by the vaporization of w a t e r , w i l l have z e r o oxygen content and a water vapour content of

68

CHARACT ERISTICS INERT GAS -AIR MIXTURES

If no air is added to the output of a generator burning

a stoichiometric mixture, then t h e composition w i l l be sub -

stantiall y as d e s c r i b e d above and visibility, in all probability,

poor, because the gas w i l l b e completely saturated with water

vapour

.

The conditions a r i s i n g when air is added have been

derived f r o m the relations given in Appendix B and are

illustrated graphically

_in

Ternperatur e does not fall off sharply with the addition

of air. The heat warming the diluent air originates m o r e f r o m the condensation

of:

adding air is m o r e that the dilueat air is warmed than that the inert gas is cooled.

Oxygen concentration has been chosen as the abscissa

of Figure 1 because, along with temperature, it is the most interesting variable influenced by the a d d i t i o n of air. The

second curve, relating t o t a l volume t o oxygen concentration, indicates that over the range covered the total volume of the

mixture is virtually the sum of the (temperature corrected) volumes of the inert gas and diluent air. The decrease in

volume resulting from water vapour condensation is negligible

despite the fact that the heat contribution is substantial.

The above considerations give the lowest temperatures

that can be achieved by the addition of air and a r e particularly

interesting where the generation of high expansion foam i s

involved. W h e n gas alone is in use, however, and firernen

w i s h to enter the building, temperature is again an important

consideration, but it is overshadowed by the requirement of

transparency,

The relevant information determining whether or not

a g a s is transparent is given in F i g u r e 2. The boundary of the "precluded r egf on" is merely a curve of the water vapour content of a saturated atmosphere. The characteristics of the output of a s t a n d a r d generator w i l l be represented by the cross

at the point, 68 per cent water vapour,

89°C.

the output, the point w i l l _{rn er ely} move down the curve bounding

the pr e c h d e d region. The resulting oxygen content a n d total

volume will, of course, b e as shown in F i g u r e 1 and the gas w i l l at all times give poor visibility.

T o achieve transparency, water injection should be

very slightly reduced t o give a higher output temperature.

Any

of

line to the right of the 68 per cent, 8 9 ° C point can be readily achieved and the gas w i l l be transparent,

Transparency and a lower temperature can then be achieved by dilution with air at the expense of i n c r e a s e d oxygen

content, Molar heats of various gases being approximately

the same, the conditions given by dilution with air can be most

readily d e r i v e d . They w i l l b e represented by a point on the straight line joining the t w o points representing the original conditions of the t w o constituent gases. The particular location on the l i n e w i l l depend on the proportions b y which the gases

are mixed. Thus the straight l i n e shown represents the

conditions that can b e achieved by mixing a 67 per cent water vapour gas at l Z O ° C with a d r y gas at 36°C.

Where the straight line inter s e c t s t h e boundary of the

precluded r e g i o n conditions w i l l b e r epr es ented by whichever o f t h e t w o l i n e s i s t o t h e r i g h t . Onlywherethestraightline .

is to the right of the boundary w i l l the mixture b e transparent,

A s the problem is a linear one, the oxygen concentration

of the mixture (where the straight line applies) w i l l b e as shown

on the right hand scale. A similar scale r e a d i n g from 0 to 100 per cent instead

of

DISGUSSION

OF

The u s e of an effective wet t y p e i n e r t gas -air mixture as a f i r e extinguishing agent in a building w i l l probably not

provide an atmosphere that is sufficiently cool and transparent to allow a fireman t o move freely around the building. Thus

to suppress smouldering an oxygen content of less than 10 per cent is usually necessary and such a gas would have a temper

-

ature exceeding 73

" C.

per cent w o u l d still involve a temperature greater than

60°C

(140°F).

It must also b e accepted that high-expansion foam

including an appreciable proportion of inert g a s will involve

similar temperatures, Only foaming agents capable of

producing stable foams at high temperatures would, therefore, b e acceptable for this application.

REFERENCES

1. McGuire,

J.

No.

3,

March

2. Rasbash,

D.

J.

APPENDIX A

SAMPLE

FUEL

The combustion of 1 grn mole

of

represented by

The heat of combustion of 1 grn rn ole of propane is 488.5

K

allowing a small correction for t h e heating af the gases t o about 1 0 0 ° C , 477

K

the quantity vaporized w i l l be 795 g m or 44 gxn mole. T h e

output will ther ef or e be

N2

-

H20

-

APPENDIX B

EFFECT

OF

The molar heats (at constant pressure) of nitrogen,

oxygen, carbon dioxide and water vapour, in the temperature

r a n g e ambient to 100°C, have been a s s u m e d to be the same and to have a value 7 cal/rnoleJ"C.

A l l expressions have been related to the combustion of

1 mole of propane, the heat of combustion being taken as 488.5

K

caL

The products of combustion are 3 CO -t 20 N

+

2 2 2

If M moles of air a r e added, N moles of water vaporized, and the output temperature is 0°C above ambient, t h e heat

balance will be given by

w h e r e it has been assumed that 10,800 cal a r e required to vaporize 1 m o l e of water.

There a r e t w o variables in equation (1) and the second equation governing the conditions relates to the limiting water

vapour pressure, which can exist in a saturated atmosphere,

Assuming an ambient atmospheric pressure of 760 mrn of

m e r c u r y the water vapour p r e s s u r e in the output w i l l be

From vapour pressure tables a value of ternperatur e can then

b e attained. F r o m these tables a n d the t w o expressions a number of values of 8 and N w e r e d e r i v e d for various

values of M by a trial and e r r o r process, Further refinement proved unnecessary because it w a s found that adequately

accurate values could be derived from only twa stages

of

From any one set of results, values of oxygen concentration

and proportional

21M Oxygen concentration

=

-

F I G U R E 1

EFFECT O F

D I L U T I N G

I N E R T

G A S

W I T H

A I R