To stop a combustion reaction, one of the 3 elements of the fire-triangle has to be removed:
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Suppression of the combustible : closing the valve fueling the combustion, sufficient distance between fire and flame, exhaust for hot smoke (containing unburnt elements), …
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Suppression of the oxidizer (choking) : use of a carbon dioxide fire-extinguisher, a blanket, or spraying a sufficient amount of water on a solid combustible
(water vapor removes fresh air) …
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Suppression of activation energy (cooling down) : spray water in a mix of air + combustible particles), absorbing the heat (Davy Miners), exhaust to remove hot smoke, …
Role of water in fire-fighting
Water can have 2 different roles:
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In the case of a solid combustible, the limitation factor is the oxidizer (air) intake, as the fire self-produces sufficient heat ;
water chokes the fire by vapor production, which removes fresh air and thus oxygen.
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In the case of a mixed atmosphere, the combustible and the oxidizer cannot be separated, the only possible action therefore consists of cooling down to prevent
flames from spreading (water vapor will also dilute the atmosphere).
There are also cases for which the ignition factor is not the source of activation energy. For example, a smoke explosion is a very violent combustion of unburnt gases contained
in smoke created by a sudden oxygen input (oxidizer input). The interval in which an air/gas mix can burn is restricted to the explosive limits of the air.
This interval can vary from a few percentage points (kerosene) to several dozen percents (acetylene).
Role of water additives
The role of water in extinguishing a fire can be summarized as follows:
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Cooling down the fire by absorption of heat energy
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Removing Oxygen from the fire
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Diluting the atmosphere by adding vapor inside the flammable air mixture
The limits to the use of water are directly linked to the physio-chemical characteristics of water itself.
Water cannot be used on certain types of fires :
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Hydrocarbon fires (B class) as it will only spread the fire due to the difference of density
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Metal fires (D class) as these fires produce huge amounts of energy (up to 7.550 calories/kg for Aluminum) water can create violent chemical
reactions with burning metal (by oxidization)
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Fat fires (F class) as vapor will carry and spread burning oil.
Water (H²O) contains Oxygen (O²), and considering the fact that a water molecule will be modified at +/- 1.500°C (mainly due to the excessive speed of the atoms,
which will lead to connection failures), and add highly flammable Hydrogen (H) + Oxygen (O²) to the fire.
Science has well studied and understood the above mentioned reactions, it has thus been possible to create specific water-additives which allow:
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Better heat absorption with a higher density than water
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Bringing free radical catchers to the fire
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Bringing foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release
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Bringing specific reactives which react with and change the nature of the burning material
Water-additives are generally designed to be effective on several categories of fires (class A + class B or even class A + class B + class F),
meaning a better global performance and polyvalence of the fire-extinguisher.
1 - Fire
2 - Combustive agent
3 - Chemistry combustion
