Introduction to Fire Safety Management

Class C
– fi res that involve gases such as LPG (i.e. 
butane, propane) or those involving natural gas.
7.4
Principles of fi re spread
Once a fi re has started and there is suffi cient fuel and 
oxygen to sustain it, there are three recognised ways in 
which it can spread within the building: convection, con-
duction and radiation. There are also the effects of direct 
burning or heat transfer to take into account.
7.4.1 Convection
Due to its properties hot air rises; this can be seen graph-
ically 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.
As the temperature of the smoke and now the 
toxic gas/vapour layer increases, heat is radiated back 
downwards and may ignite other combustible mater ials 
in the vicinity. Modern research also shows that the 
Figure 7.13
Class ‘C’ fi res
Class D
– fi res that involve metals such as sodium, 
lithium, manganese and aluminium when in the form of 
swarf or powder.
Class F
– fi res that involve cooking mediums such 
as vegetable or animal oil and fats in cooking appliances. 
Such fi res are particularly diffi cult to extinguish as they 
retain considerable heat allowing the chemical reaction to 
restart.