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Smoke Movement

There are two main factors which determine the movement of smoke and hot 'gases from a fire in a building. These are:

a) The smoke's own mobility which is due to the fact that it usually consists of hot gases which are less dense than the surrounding air.

b) The normal air movement inside the building, which may have nothing to do with the fire but which can carry smoke around a building.

The relative magnitude of these two "smoke moving" factors will depend on particular circumstances and will certainly differ from place to place inside a building. In general it may be expected that close to the fire factor "a" will dominate and as the distance from the fire increases and the smoke gets cooler factor "b" will become more important. The movement caused by the smoke's mobility is due to pressure differentials developed:

a) by the expansion of the gases being heated by the fire;

b) by the difference in density between the hot gases above the flames and the cooler air which surrounded the fire.

The normal air movement in the building can be caused by three separate factors:

a) The pressure differential due to the air inside a building being at a different temperature from the air outside. This will cause the air inside a building to move upwards or downwards depending on whether the air inside the building is warmer or colder than the air outside.

b) Buildings are to a greater or lesser extent leaky, and wind penetrating through the leaks contributes to internal air movement.

c) Any mechanical air handling system inside the building.

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