Extinguishers.

For extinguishing the fire it is necessary to stop burning of fire substance. To halt burning means to stop the exothermal reaction that takes place in the thin light layer of burning area called as reaction zone.


 

In this zone where is heat release, by which combustion products are heated to certain energy state. Simultaneously there is heat transfer with heat release and heating of combustion products. Increase of temperature is stopped, when rate of heat release becomes equal to rate of heat transfer. Therefore, the combustion temperature of substances and materials is not constant and changes depending on correlation of heat release and transfer ratios in the reaction zone.

At burning of the airfuel poured out, when an oxygen concentration is constant and chemical composition of fire substance is fixed, rate of heat release depends on diffusion rate of reacting agents in the reaction zone, and also on heat of burning and completeness of combustion of burning substance.

It is possible to decrease the temperature of substance being burn, and thereby to stop the combustion process both by the increase of rate of heat removal, and by reduction of speed of heat transfer. Decreasing of temperature of burning reaction zone is achieved by a few methods. To stop burning means to eliminate one or more conditions of burning.

It is possible to increase rate of heat removal from the reaction zone (in other words to cool) by the collision of reaction zone with lesser-heated noncombustible substance (water, aqueous solution of foaming agent, solid carbon dioxide) or to increase its specific surface (by use of powder-like compositions, flame arresters).

To increase heat-releasing surface at fires extinguishing of the airfuel poured out, powder-like extinguishants can be used, that are supplied in the burning zone under pressure in the form of streams.

 

Cooling of substances (materials) being burn, and quantity of burning zone, dilution of reactive matters, and also isolation of reacting agents from the burning zone (reaction) belongs to the physical methods of inhibition of burning reaction. The termination of burning by cooling is achieved by reduction of decay rate, evaporation of combustible components and their input into burning zone. At dilution of reacting agents there is a concentration pulldown in the reaction zone, reduction of reaction rate and thus decline of rate of heat release, and thus of their temperature combustion temperature. The termination of burning by the isolation of reacting agents from the burning (reaction) zone takes place due to lowering the concentration in it of one of their reacting components of the system.

The chemical method of termination of burning reaction is based on reducing of concentration of active sites in the reaction zone.

It takes place because of inputting chemically unstable substances to this zone which under the pressure of thermal energy emitted by flame decompose on radicals able to react with active sites & to neutralize them.

There are 3 used methods of termination of burning at extinguishing the fires on broken-down aircraft, where an aviation fuel is basic fuel substance: dilution, cooling and isolation of materials being burn.

All extinguishants, getting in burning zone, operate complex that is cool, dilute hot streams from the fire substance or oxidant, and inhibit the combustion process.

However each of them has only one basic, especially expressed, prevailing above other effect. Therefore the extinguishants correspond to every technique of extinguishing:


 

— for dilution of oxidant or combustible vapors and gases (carbon dioxide, nitrogen, water steam, atomized water, and also aqueous solutions of foaming agents);

— for isolation burning zone from the combustible substance or oxidant (air-mechanical foams of a different compressibility, chemical foam, powders, sheet materials and etc.);

— for chemical termination of burning reaction (ethyl bromide; compositions 3.5 and 4 I.p.; SGB; Freon & etc.).

 








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