7.4.1 Convection
Due to its properties hot air rises; this can be seen graphically
when smoke from a bonfi re rises and disperses
within the atmosphere or a fi re is started within a grate
and rises up through the chimney.
The convection process begins when combust ible
materials are subject to excessive levels of heat and
they give off a vapour which in turn ignites. When these
vapours are heated they expand and become less dense
than air. As they rise they leave an area of low pressure
which is replaced instantly by cooler unheated air. This
fresh air is then mixed with the vapour and heated,
assisting in the development of greater temperatures. The
process is cyclic, continuing to support the fi re process.
Convection is the most common cause of fi re
spread within buildings and structures. During a fi re
hot gases and vapours (smoke and heated air) will rise
vertically through stairwells, lift shafts and service risers
to the highest level available. They then form a layer at
that height, from which they spread out horizontally until
checked.