Basic Fire Theory

What is fire and how does it behave?

The Fire Triangle

This is the fire triangle, it represents the 3 elements required for fire to start and sustain itself. Removing any of these components or interrupting their interaction will extinguish the fire. Lets take a more detailed look at the individual components.

Oxygen

Oxygen supports the combustion process by reacting with the fuel and releasing energy, as well as producing byproducts like gases and smoke. In general fires require a concentration of about 16% oxygen to continue to burn freely, this need is easily met by the atmosphere of the earth which is approximately 21% oxygen. If a fire is unable to get enough oxygen the reaction becomes more incomplete, increasing the amount of smoke and toxic byproducts that were unable to be burnt up by the process.

Inside buildings it is common for a fire to use up all available oxygen before it has consumed all the available fuel. This makes opening doors or windows a high-risk action as it is possible that the heat and fuel are still present, just waiting for the reintroduction of oxygen. The phenomena of introducing oxygen into a room containing highly heated smoke is known as backdraft and is extremely dangerous. Remember, smoke is largely the unburnt fuel resulting from either a lack of oxygen or the smoke cooling below its ignition temperature.

Extinguishing a fire by removing or excluding the oxygen from the fuel is referred to as 'Smothering', some methods of removing or excluding the oxygen from a fire include:

• Using a carbon dioxide fire extinguisher on the fire which displaces the air containing oxygen

• Using an ABE dry chemical powder extinguisher which coats the fuel in a fine dust, separating the oxygen and fuel

• Using a foam fire extinguisher which coats the burning fuel in a layer, separating the oxygen and fuel

• Covering the fire with a fire blanket

• Throwing sand or dirt on top of the burning material

• Closing a door or container which the fire is in, limiting its ability to obtain oxygen-rich air

Heat

Heat is required for a fire to start and to sustain the production of flammable vapours. Initially most fires require an external source of heat to start, this is called "activation energy", however once a fire is burning the reaction becomes self sustaining by producing its own heat. Because the reaction is highly exothermic it is capable of heating more material than is currently involved. This makes fires grow exponentially (when not limited by oxygen or fuel) and can turn a relatively small and easy to manage fire into a raging inferno in a very small amount of time.

Heat spreads in 3 key ways (convection, conduction and radiation) and is the primary mechanism behind fire growth and spread.

Convection is the transfer of heat by the physical movement of hot masses of air. As air is heated it expands and becomes lighter than the surrounding air, and it rises. This is why the air near the ceiling is hotter than the air near the floor. Cooler air rushes in from the sides, where it is heated and also rises.

Conduction is the transfer of heat within the material itself, for example the hot handle of a spoon you left in a pot.

Radiation refers to the emission of energy in rays or waves. Heat moves through space as energy waves. This is the type of heat you feel when sitting in front of a fireplace or around a campfire. It travels in straight lines at the speed of light and is the reason only one side of you gets warm when facing a fire.

If a fuel is sufficiently cooled it will not produce enough vapour to continue burning. Extinguishing a fire by reducing the heat is referred to as 'Cooling', some methods of reducing the heat include:

• Using water, either from an extinguisher or a hose 

Fuel

Fuels are anything capable of burning to sustain a fire, in Australia we have 6 classes of fire that are used to identify the different type of fuels that are burning and their unique factors that need to be considered when attempting to extinguish a fire. 

Fuels exist in 3 states of matter (solid, liquid and gas), which state the base fuel is in has implications for the fires growth speed, extinguishing considerations and dangers posed to nearby people.

In order to burn all fuels need to be converted into a gas or vapour, this is via a process known as pyrolysis. During pyrolysis heat breaks down the fuel until it releases flammable vapours, which are then able to be ignited.

The gaseous state of matter is therefore the easiest to ignite as it has no need to undergo pyrolysis in order to be flammable. In order to burn however it must be in a correct ratio to the oxygen in the air, different gases have different ratios required. If there is too much fuel vapour in the air it will not burn as the mixture is "too rich", if there is not enough vapour it also will not burn and this is referred to as being "too lean", these together are referred to as the upper and lower flammability limits and are unique to each gas.

Some gases are lighter than air and will disperse quickly, however some are heavier than air and will sink to lower points and accumulate in "clouds" or "puddles" of gas which can pose a risk of explosion or fire if an ignition source is available.

The liquid state of matter gives off flammable vapours as it is heated, as a general rule the higher the temperature the more flammable vapours are released. This can vary wildly between liquids, as an example petrol produces enough flammable vapours to ignite at around -43°C and above, whereas diesel  won't produce enough until its over approximately 52°C.

The solid state of matter also produces flammable vapours as it is heated. They generally require more heat to produce flammable vapours than liquids and are therefore usually more difficult to ignite. The size, shape and surface area of a solid determine how well the fuel will burn. The larger the surface area of the material in relation to its mass the easier it is to ignite. All things being equal, a pile of wood shavings will be easier to ignite than a solid block of wood due to its higher surface area.

Extinguishing a fire by removing the fuel source is referred to as "starving", some methods of removing the fuel include:

• Turning off the gas feed to a gas fire

• Turning off a valve to a fuel fire

• Removing unburnt material from a leaf or litter fire

How the triangle interacts

The three sides of the fire triangle interact via the chemical chain reaction that links them, while some specific extinguishing agents exist to interrupt the reaction itself without removing or excluding any of the 3 sides this is outside the scope of this document. 

The general process of the chemical chain reaction is:

There are a few important temperature points involved in this reaction, while the effect of these points is uniform the actual temperature they occur at changes in different materials.

The flash point is the lowest temperature at which the vapours produced by a material will momentarily ignite when given an ignition source. This can be found on a materials MSDS.

The fire point is the temperature at which a material will continue to burn after ignition. As a general rule of thumb fire points are around 10°C higher than flash points.

The autoignition temperature is the lowest temperature at which a material will ignite spontaneously. The heat of the fuel is high enough to provide its own activation energy.

Stages of fire

A fire inside a room or other confined area containing normal combustibles will normally progress through three phases.

The incipient phase

This is the beginning phase of a fire. During the incipient phase the heat being produced may not be enough to be self-sustaining. The oxygen content in the area has not been reduced significantly and carbon dioxide and water are the main things being produced by the fire, as well as minor amounts of other gases.

Though the flame temperature itself may be several hundred degrees the room temperature is only slightly elevated. Fires in the incipient phase, as a general rule, are safer and easier to contain and extinguish than the other phases

The free burning phase

In the free burning phase the fire is producing enough heat to be selfsustaining and will continue to grow while fuel and oxygen are available. The fire produces hot convection currents that lead to a build up of hot, dense gases at ceiling level which will gradually descent. This thermal layer descending makes the room more dangerous and difficult for people to enter and is the reason people are told to crawl when escaping from such a fire.

The hot gases in the thermal layer will force their way out through openings and spread the fire to other areas, also contributing to the build up of heat. As the contents of a room heat up they begin to undergo pyrolysis, releasing vapours. If enough vapours are produced and the temperature is high enough the whole room may experience what is called a flashover. A flashover is the near-simultaneous ignition of all the rooms contents.

If a fire is well supplied with oxygen it may continue in the free burning phase until all the available fuel is consumed. However, if the fire is oxygen limited, for example all doors and windows are closed, it may enter the next phase.

The hot smouldering phase

This stage in fire development is reached once a fire no longer has access to enough oxygen to sustain the free burning phase. In this phase the room will fill with hot dense smoke and toxic gases well over 500°C. If a door is opened (or another source of oxygen is made available to the fire) it may trigger an explosion, referred to as a backdraft. Once a backdraft has been triggered it will usually return back to the free burning phase.

It is important to be able to recognise which phase a fire is in, in order to be able to take appropriate steps.