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Fire Safety for Facilities Management Personnel – How fire works – Part 56

July 27, 2020 1:46 pm

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 55, LWF began to look at the components of fire and what fire actually is. In part 56, we will look at the combustion of solid materials.

Flame itself is actually a gas-phase reaction between fuel vapour and oxygen, when solids and liquids are burning. The fuel is converted into flammable vapour.

When a liquid fuel is burning, the flame is fed by evaporation from the surface of the liquid.

However, with a solid fuel, the large molecules aren’t able to evaporate and so have to undergo chemical degradation, known as pyrolysis, and that produces smaller molecules which can evaporate from the surface.

If we consider a piece of wood as the fuel in a fire, the chemical components of the wood – mainly cellulose – break down and release vapours which are a complex mix of flammable compounds. It is these vapours which ignite and burn, forming a flame.

The combustion of the vapours combined with oxygen produces heat and much of that heat is transferred back to the surface of the wood and the cycle begins again with more wood breaking down.

While the majority of fires ignite due to an external ignition source, occasionally, some materials can self-heat and result in a spontaneous combustion. If linseed oil-soaked rags were left in a warm cupboard, for instance, they would have a high chance of self-ignition.

It should also be noted that the burn rate of solid fuel is affected by how thin or thick the fuel is. We know instinctively that a piece of paper would ignite more easily and burn more fiercely than a piece of wood or a book. The reason is that less heat is required to raise the temperature to a point at which fuel vapours are produced. This is known as the ‘heat sink effect’ and therefore the heat sink effect of paper is less than that of a book.

In part 57, LWF will begin to look at the combustion of flammable liquids in more detail. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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