Fire is a chemical reaction between a fuel and oxygen involving heat. Fire is graphically represented as a tetrahedron to show that removing one side of the tetrahedron causes it to collapse in the same way that removing one element of fire causes it to be extinguished.
The Tetrahedron of Fire
An example of how fuel can be removed from a fire would be closing a valve on a ruptured pipe, removing oxygen would be putting a lid on a burning saucepan and heat would be throwing a bucket of water on a fire. The chemical reaction of fire is interfered with by converting flammable vapour to non-flammable vapour.
A solid or liquid substance itself does not burn, it is vapour from the substance that burns. For a solid or liquid to burn, it must be above the temperature required for it to give off sufficient flammable vapours to be ignited (its flash point), and the vapour must be ignited.
The heat from the fire maintains the substance's temperature above its fire point so that it continues to produce vapour to fuel the fire, this is known as the chain reaction of fire.
A flammable gas or vapour is already in a burnable state and therefore requires no heat to start the combustion process, only an ignition source is needed.
A substance can be ignited without an ignition source if heated sufficiently. The temperature at which this occurs is the autoignition temperature.
There are four ways that fire can spread:
Convection involves the movement of air, so an example of a fire spreading by convection would be a spark being carried from a fire causing another fire.
Conduction is the transmission of energy by a substance. An example of a fire spreading by conduction would be heat travelling through a steel beam from a fire on one side of a wall starting a fire on the other side of the wall.
Radiation is the diffusion of a form of energy. Heat radiates from a bar heater and clothes catching fire in front of a bar heater would be an example of fire spreading by radiation.
Direct burning is the contact of the flame from a fire onto a flammable material, and is actually a combination of the other means of fire spread.
To enable types of fires to be identified, fire is categorised into four classes and a sub-class:
Class Material Examples A Carbonaceous matter Wood, cloth B Flammable liquids Petrol, oil C Flammable gases LPG, acetylene D Reactive metals Sodium, magnesium [E] Electricity involved F Animal Fat/ Vegetable Oil Cooking Oil
Electrical fires involve a material burning with electricity involved, this is why they are referred to as a sub-class. If the electricity is isolated from the fire, the fire will usually be an A class fire.
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