FireArrest

Fires – How They Start and How They Spread

How Fires Start

Fire is a chemical process that requires three components to be present:

  • Heat – enough to make the fuel burn
  • Oxygen
  • Fuel – anything that will burn whether solid, liquid or gas

The process is called oxidation which is the process of combining oxygen with other substances. A fire starts because the heat/energy created as part of the oxidation process is created faster than can be released, quickly causing combustion and the resulting flames. Whilst oxidation is a process that happens everyday such as when metal rusts, the difference is the speed of which it occurs. Fire is a rapid oxidation process that spreads quickly and will keep burning until one or more of the three necessary components is removed.

How Fires Spread

Once a fire has started it grows through the transfer of the heat energy from the flames. This can happen in three different ways:

  • Conduction – Fire spreads through direct contact between materials. Materials like metal that are good conductors absorb the heat from the fire and pass this throughout the molecules of the material. This can cause any combustible materials that come into contact with the item to ignite.
  • Convection – The flow of fluid or gas from hot areas to cooler areas. The fire heats the air around it which rises, taking the smoke along with it, but can become trapped by the ceiling if in an enclosed space. As a result the hot air is forced to travel horizontally and eventually downwards, spreading the heat from the fire across a wider area and making it very dangerous very quickly.
  • Radiation – Heat is transferred via electromagnetic waves in the air. This heat is then absorbed by combustible objects in its path causing them to heat up and possibly ignite without even touching the fire. Although some materials such as concrete do not allow radiation to pass through them, the radiated heat from a burning building can cause surrounding structures to catch fire.

 

There are 4 different stages to the life a fire which depending on the circumstances of each individual fire may vary in in the time it takes to get from one to the other but will still follow the same pattern:

  • Ignition/incipient – When the oxidation process begins and a small fire starts that is controllable with a fire extinguisher.
  • Growth – Additional fuel ignites due to the heat being generated by the fire causing the fire to grow.
  • Fully Developed – Any combustible materials and fuel sources available have caught fire and temperatures have reached their highest.
  • Decay (Burnout) – As the available fuel has already been consumed by the fire, temperatures decrease and the fire begins to get less intense.

Image:www.journeytofirefighter.com

When considering fires outdoors there are further elements to consider such as the wind which may affect the speed of the fire, direction it travels in, intensity of the fire and more. In regard to direction a fire will burn faster uphill as the flames have easier access to more unburnt fuel which is likely to have been pre heated by the radiation from the fire making it even more flammable.

Classification of Fires

Depending on the type of fuel that is burning fires can be separated into 5 different classifications which makes it easier to choose the most appropriate method to fight the fire.

  • Class A – Fires involving ordinary combustible materials. These are things such as wood and textiles and are the most common type of fire, occurring when materials become heated to ignition temperature.
  • Class B – Fires involving flammable liquids. Petrol, alcohol and paint are all examples of flammable liquids. They spread quickly, burn easily, have an ignition temperature less than 100°C and produce thick toxic smoke.
  • Class C – Fires involving flammable gases. Gases such as butane and propane have the potential to explode through a single spark and can be one of the most dangerous types of fire to fight.
  • Class D – Fires involving burning metals. Although a lot of heat is required to ignite most metals, they are good conductors so transfer heat quickly to their surroundings. Some metals can also burn when in contact with air and water, meaning that if the wrong fire fighting method is used explosive reactions could happen as a result.
  • Class F – Fires involving cooking oil. Due to the high temperatures involved in cooking oil and fats these fires can pose a difficult challenge. Common in both homes and professional kitchens water should be avoided as it can cause the flames to rapidly spread out, making the fire worse.
  • Electrical Fires – As electricity does not actually burn but is the element that sets surrounding material alight (a source of ignition rather than a fuel), an electrical fire can fall into any of the above classifications. For this reason it does not have its own classification but does have it’s own fire safety requirements.