Man's physical universe

xanabras

COMBUSTION 649

Huge thermite incendiary bombs were used in World War II very

effectively.

Fire Extinguishers Employ Completely Oxidized Carbon Compounds.

Water extinguishes fires by lowering the temperature of the burning

object below its kindling point and by shutting off the oxygen supply.

The steam formed may be a factor also. The

various types of more efficient fire extinguishers

are based on the principle of smothering the

fire by preventing an access of air (oxygen),

just as a blanket wrapped around a person

whose clothes are on fire will smother the fiame.

An excellent example of this smothering action is

the use of foam to extinguish fires in liquid fuels.

The majority of fire extinguishers use carbon

dioxide for one reason or another. One type of

fire extinguisher, called the soda-acid type,

consists of a fairly large cylinder containing a

solution of sodium bicarbonate and a bottle of

sulfuric acid. The acid pours into the baking

soda solution when the cylinder is inverted,

thus generating carbon dioxide. The carbon

dioxide liberated in the above reaction produces

a pressure that causes relatively large amounts

of the carbon dioxide to dissolve in the solution

and forces the resulting solution out through a

nozzle. In this case, the water serves to extinguish

the fire, but the carbon dioxide has

little if any value except as an expellant. In guisher. (Courtesy of

the

the foam

American-La

type of extinguisher, a licorice extract

France and Foamite Industries,

Inc.)

is added to the carbonate solution to produce a

foam, and alum (aluminum sulfate, Al2(S04)3)

replaces the sulfuric acid of the soda-acid type. The alum hydrolyzes

to produce sulfuric acid in this case. The foam issues from the nozzle,

each bubble being filled with carbon dioxide.

Some very efficient fire extinguishers use liquid carbon dioxide

which produces carbon dioxide snow as it is sprayed on a fire and serves

to smother (not cool) the fire by diluting the inflammable mixture of

air and fuel.

In another type of fire extinguisher finely divided particles

of magnesium carbonate, MgCOs, or sodium bicarbonate, NaHCOs,

are forced onto the fire with the pressure produced by liquid carbon

dioxide or compressed nitrogen.

Fig. 283. Section of

a soda-acid fire extin-

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