Physics of a Fire

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Fire is a chemical reaction whcih needs three things to be present so it can happen: Oxygen, Fuel, and Heat.

If one of these is not present, the fire cannot start. If one of these is taken from a fire it will go out.

But how does this all work?


We all breathe Oxygen (O²) everyday - in fact, without it we would suffocate. But did you know that fire breathes Oxygen too? And, like us, without Oxygen a fire will also suffocate.

When Oxygen in the air combines with flammable vapours given off by Fuels - heat is produced and then ignition can occur.

Without enough Oxygen, ignition cannot happen. In the opposite way, if there is too much Oxygen then the vapours won't be concentrated enough to ignite. The ratio of vapour to Oxygen is known as the 'explosive' or 'flammable' limit and is different for each gas or vapour.


Combustion occurs when flammable vapours mix with air (Oxygen) and are ignited by a spark or flame.

Solids give off flammable vapours by being heated. Certain solids such as paper or flour appear to ignite almost instantly. This is because they give off vapours and reach a flammable temperature almost immediately. In fact, fine dusts dispersed in the air can explode because they give off vapours and ignite so quickly it appear to happen instantly.

Other solids like timber take longer to ignite because they are more dense and so don't give off flammable vapours so easily.


So, in our fire triangle we've got Oxygen and Heat, but we also need something that will burn - this is our Fuel.

Fuels can take almost any form:

Solids like wood, fabric, rubber and plastic.

Liquids such as petrol, oil, cooking oil or even nail varnish remover.

Gases like propane, butane and 'natural' gas.

If a fire broke out in your home today, would you automatically know which fire extinguisher to use? What would happen if you used a Class A fire extinguisher on a fire in the electrical service panel in your basement? Answer: you'd possibly be electrocuted if the extinguishing agent is a liquid!

A long time ago, the fire protection industry recognized the need to classify extinguishers according to the many kinds of burning materials encountered in a fire. For example, Class A, water-type fire extinguishers cannot be used on the electrical fires because the extinguisher operator could be seriously injured by the conduction of electricity by the stream of water from their extinguisher.
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