Fire is a chemical reaction known as combustion which occurs when fuel and oxygen are brought together with sufficient heat to cause ignition.
A simple representation of this reaction is the “fire triangle” which shows the components required for fire to occur.
A critical temperature must be reached for ignition to take place, but once a fire has started it will generally maintain its own heat supply.
Heat can be applied in any number of ways. It may be deliberate (arson is the major cause of fire in industry and commerce), or it may be accidental, for example: heaters being placed too close to furnishings or paper; power points being overloaded, or plugs incorrectly fused, causing wiring to overheat.
Blocked machinery vents on a production line or factory floor, or air inlets on a desktop personal computer covered by memos or cartons, can cause machinery to overheat, becoming a fire risk.
Fuel is best understood as a combustible substance, liquid, solid or gaseous.
Solids, such as a block of wood, may be quite difficult to ignite, but reduce the block to shavings and the shavings will catch fire readily.
A piece of paper can start a major fire. Add heat to paper and it produces vapours which ignite, in turn producing more heat. Oxygen feeds the flames and the combustion cycle continues.
While solid fuels may need an outside source of heat to raise them to a sufficient temperature to give off flammable vapours, there are many flammable liquids which produce such vapours even at temperatures below freezing. Petrol, for example, does so down to 43ºC below zero.
These vapours alone are extremely dangerous. A car petrol tank for example, is far more dangerous when it is empty because it is full of flammable vapours.
Gases, since they are already vapours, ignite instantly. Turn on a gas supply, add heat, and flames appear. Some gases are lighter than air, and others are heavier, so if gas can be smelt it means quite a significant amount is already present.
Extreme care must always be exercised when dealing with flammable gases. LPG is very efficient but highly dangerous if handled incorrectly.
Oxygen is normally present in the air around us in a sufficient quantity to support fire (approximately 21%).