For a fire to occur, three things are necessary:
HEAT, for example a spark. This must be hot enough to cause ignition;
OXYGEN, which is in ample supply all around us,( in the air we breathe);
FUEL, which can be a solid, a liquid or a gas.
Once there is a heat source to cause ignition and a sufficient amount of fuel and oxygen present, the fire will continue to burn. As the fire burns, large amounts of heat are produced. Heat always flows from regions of high temperature to regions of low temperature. This transfer of heat causes the fire to grow and to spread to other areas.
There are three methods by which heat may be transferred: CONDUCTION, CONVECTION & RADIATION.
This type of heat transfer occurs inside materials, typically solid materials. The heat from the fire is passed from molecule to molecule along the length of the material. The fire will generally follow the heat or sometimes the heat from conduction may cause a new fire to ignite elsewhere.
Certain materials are better than others at conducting heat. Most metals conduct heat easily and quickly and they are called conductors, on the other hand, certain plastics are very poor at conducting heat and they are called insulators. In houses, walls between rooms are built from materials which are good insulators. This helps to keep the heat in the room on a winter's night and helps to slow the rate at which fires spread through houses.
This type of heat transfer occurs only in liquids and gases. The heat from the fire can heat the air, to a very hot temperature. Hot air will always rise and it will flow under the ceiling of a room, spreading the heat from the fire. This is the main way in which a fire spreads throughout a house. When a fire is burning, large amounts of hot gases and smoke are produced. These will travel through the house in hot air currents, often igniting more combustible materials, causing the fire to spread.
In this form of heat transfer, the heat does not travel through a material like conduction nor does it flow through air or liquid currents like convection. It simply travels in rays similar to sunrays, in straight lines away from the fire. The heat from the rays can be absorbed by combustible materials which causes them to heat up and perhaps ignite. The main principle of radiation is the closer the material is to the fire, the more radiated heat it will receive. Certain materials such as concrete do not allow radiation to pass through them. Therefore, materials like concrete are good construction materials to help prevent fires spreading through houses or to nearby buildings.
Radiated heat from a burning building can, in some circumstances, give rise to fire in a nearby building.
Fires fall within five broad classifications:
Class 'A' Fire involving ordinary combustible materials
Class 'B' Fire involving flammable liquids/liquid flammable solids
Class 'C' Fire involving gases
Class 'D' Fire involving burning metals
Class 'E' Fire involving electrical fires