Role of Water in Fire-fighting
Water can have two different roles:
1. In the case of a solid combustible, the solid fuel produce pyrolyzing products under the influence of heat, commonly radiation. This process is halted by the application of water, since water is more easily evaporated than the fuel is pyrolyzed. Thereby energy is removed from the fuel surface and it is cooled and the pyrolyze is stopped, removing the fuel supply to the flames. In fire fighting, this is referred to as surface cooling
2. In the gas phase, i.e. in the flames or in the smoke, the combustible can not be separated from the oxidizer, the only possible action consists of cooling down. In this case, water droplets are evaporated in the gas phase, thereby lowering the temperature and adding water vapour making the gas mixture non combustible. This requires droplets of a size less than about 0.2 mm. In fire fighting, this is referred to as gas cooling or smoke cooling. There also exist cases where the ignition factor is not the activation energy. For example, smoke explosion is a very violent combustion of unburned gases contained in the smoke created by a sudden fresh air input (oxidizer input). The interval in which an air/gas mix can burn is limited by the explosive limits of the air. This interval can be very small (kerosene) or large (acetylene).
Read more about this topic: Fire Triangle
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