Cooling water compositions and carbon dioxide.
The termination of burn by cooling consists in the decline of temperature of burning zone below than flash point. Cooling rate of hard materials and liquids being burn by extinguishants depends on a contact surface, temperature difference and heat-transfer coefficient.
Water, having a large heat capacity and heat of vaporization (2.26 MJ/kg), has significant cooling capability. Its thermal stability far exceeds thermal one of other noncombustible substances, used as extinguishants, for example, carbon tetrachloride, ethyl bromide, methylene bromide, tetrafluordibromomethane & etc.
Extinguishing the fire with a water of most hard materials (wood, plastics, fabrics and etc.) is safely, as their combustion temperature does not exceed 1300 oC, and decomposition of water and aqueous comes at temperatures, that exceed 1700 0C.
Water, getting on the surface of materials being burn cools it and partially gets to the pores of material, moistening him. At the same time there is direct dependence between the size of the moistened surface and degree of its cooling
. Different foam formers are used as humectants: ПО-1, ПО-1Д, ПО-3А, ПО-6к, “Сампо”, Поток-2 & etc.
The usage of aqueous solutions of foam formers instead of water advances efficiency of extinguishing in 1.2-1.3 times that result in reduction of discharge of composition supply or at the same discharge allows to extinguish the large areas of fire.
Water or aqueous solutions of foam formers are supplied on their surface by continuous or sawed streams for cooling of substances, materials and equipment being burn. When it is possible approach closely to burning zone, streams are most effective at extinguishing the fires in the middle of case, and for extinguishing free (mirror) liquid and effluent streams of combustible liquids and those, that are inflammable. Thin-sawed drops of water extinguishant considerably reduce the temperature of surface layer of combustible substance and sharply decrease evaporation rate.
Except for it, there is subsidence of drops of water under the layer of aviation fuel in connection with the difference of water density and aviation fuel one, as a result there is the additional cooling of volume of the last mentioned.
Water and aqueous solutions of foam formers are electro conductive, therefore before extinguishing the electrical supply networks and installations being energized must be de-energized at extinguishing.
It is impossible also to supply water and its solutions on the substances, which at interaction with them create burning gases with significant heat release (calcium carbide CaC2, calcium oxide CaO). The fact that the water and aqueous solutions freeze at the negative temperatures of ambient air is the following disadvantage of water, and this, in turn, causes certain difficulties at extinguishing the fires.
At fire extinguishing with the method of cooling, except for water and aqueous solution, hard carbon dioxide is used, that is supplied by means of socket-and-snow-creator of carbon-dioxide fire-fighting equipment.
Hard (snow) carbon dioxide presents by itself a fine-crystalline mass with a density 1.54 t/m3, which at heating to the temperature 79 oC goes into gas, omitting the liquid state (it is sublimated), that allows extinguish by it the matters and materials, that deteriorate at the supply on them the water or aqueous solution.
On application of CO2 cooling effect is created mainly due to the considerable temperature difference of burning zone and extinguisher. Most quickly hard carbon dioxide cools gas volumes and substances being burn, as they have the large collision surface with it.
On an aircraft hard carbon dioxide can be used for extinguishing the fires of power plants, baggage compartments, service compartments and channels, passenger cabins under absence of people there, that have not respiratory protective devices, and also different electrical installations, that are energized.
It is not allowed to extinguish the fires by hard carbon dioxide in passenger cabins at presence of passengers and crewmen there, power plants and organs of landing at burning of magnesium alloys.
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