To achieve maximum effect, the quantity of heat energy absorbed should be as great as possible. The properties of a good cooling agent are therefore:
- High specific heat capacity (thermal capacity)
- High latent heat of vaporisation
- High heat of decomposition
Water is a good cooling agent because of its high thermal capacity and latent heat of vaporisation. This, combined with the fact it is available in large quantities, makes it by far the most widely useful fire extinguishing agent.
The role of decomposition is insignificant in the case of water but very relevant with certain substances, such as carbon dioxide, that absorb heat in this way.
Smothering
If the oxygen supply to the burning material can be sufficiently reduced, burning will cease. The general procedure is to prevent fresh air from reaching the seat of the fire, allowing the combustion to reduce the oxygen content in the confined atmosphere until it extinguishes itself, for example by:
- Snuffing out candles
- Smothering a pan with a fire blanket
- Wrapping a person in a fire blanket
- Applying a blanket of foam over the burning surface, thus separating the fuel from the air
Smothering can also be achieved by removing the oxygen in the atmosphere, thus extinguishing the fire, for example, by:
- Introducing carbon dioxide (CO2) to the immediate vicinity of the fire
- Introducing an inert gas to the immediate vicinity of the fire, such as through systems installed to protect computer server rooms
Starvation
In some cases, a fire can be extinguished simply by removing the fuel source. This may be accomplished in a number of ways, such as stopping the flow of liquid or gaseous fuel, removing solid fuel in the path of the fire or allowing the fire to burn until all of the fuel is consumed.
Fires can be starved of fuel by removing potential fuel from the vicinity of the fire, for example:
- Back burning forestry fires
- Draining fuel from burning oil tanks
- Removing cargo from a ship's hold
- Creating firebreaks in peat, heathland and forest fires
- Removing vehicles in the proximity of the fire
- Creating firebreaks in thatch roofs
- Removing tyres not affected by the fire from a tyre dump
Interrupting the combustion process
Dry powder, Bromochlorodifluoromethane (BCF) and other halon extinguishers work by releasing atoms that interrupt the chemical chain reaction. They also create an inert gas barrier.
