Principles Of Heat Transmission & Fire Spread

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Principles Of Heat Transmission and Fire Spread

Once a fire has started it can spread by four different methods:

  • Direct burning,
  • Convection,
  • Conduction, and
  • Radiation.

In a real fire situation, all four methods may apply.

Direct Burning

The simplest method of fire spread, where a flame front moves along or through the burning material. For
example, if the corner of a piece of paper catches fire, the flame front will spread across the paper.

Convection

The principle that hot air rises and cold air sinks. Hot gases generated by the fire rise straight up from the fire:

Inside a building these hot gases will hit the ceiling and then spread out to form a layer underneath
the ceiling. When these hot gases touch any combustible material (such as a wooden curtain pole) they
may heat that material up sufficiently so that it bursts into flame.

Outdoors these convection currents will contain burning embers that are carried on the currents until
the air cools and the embers are dropped to the ground. This is a common way for forest fires to travel
and jump over obstacles (such as roads).

Conduction

The principle that heat can be transmitted through solid materials. Some metals, in particular, conduct heat very efficiently (e.g. copper). Any pipes, wires, ducts or services running from room to room can act as
conduits for heat and spread the fire.

Radiation

Heat energy can be radiated through air in the form of infrared heat waves, which travel in straight lines (just like light) and can pass through transparent surfaces (such as glass). Radiant heat generated by a fire shines onto nearby surfaces and is absorbed. If the material heats up sufficiently it can burst into flames.

Classification of Fires:

Class A – solid materials, usually organic, such as paper, wood, coal and textiles.

Class B – flammable liquids, such as petrol, oil and solvents.

Class C – gases, such as methane, propane and acetylene.

Class D – metals, such as aluminium or magnesium.

Class F – high-temperature fats and oils, such as cooking fat.

Note that there is no Class E fire. This classification was avoided because of potential confusion between Class E and electricity. Electricity is not a fuel (though it can be an ignition source).

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