Unit 10: Assignment A
Fire tetrahedron: ref 1
A fire tetrahedron is the essentially a triangle of
combustion – it represents heat, oxygen, fuel and a
chemical chain reaction which was a necessary
component of fire. All four elements must occur for the
presence of fire – if you remove any one of these
elements the fire would be extinguished e,g, oxygen.
Heat is necessary for the ignition of a fire to occur, and
multiple materials have different temperatures at which
they can ignite. Combustion reactions also produce heat as they burn, which further
increases the fuels temperature. The heat may be cooled by applying water, but this is not
possible for all types of fires – a chip pan fire cannot use water to be extinguished.
Fuel is any materials that are flammable that can cause a fire. A fire can't start without
material available to burn. These materials could include fabrics, wool, paper and oil. This
can start in places like homes or businesses. Compared to other materials, some burn more
easily. In the tetrahedron, fuel is the hardest to remove once ignited so it is imported
flammable materials are stored appropriately so they do not become a possible fire hazard.
Oxygen is needed to maintain the combustion reaction since it’s the oxygen that’s reacting
with the fuel to release the heat and carbon dioxide. There is enough oxygen in the world to
trigger a fire as we have 21% of O2 if the other two components are present too. Some fire
extinguishers and fire blankets remove the oxygen component by removing or displacing it
therefore causing suffocation and extinguishing the combustion reaction.
Methods of extinguishing a fire;
Cooling: ref 2
Cooling method is known as a class A fire, using mostly water to extinguish the fire as the
fire can be absorbed by water. Removing heat is one of the most effective ways to put out a
fire, hence cooling with water is one of the most widely used techniques. Which is why fire
fighters use a hose with high volumes of water. Because of the high heat capacity of water,
it takes more energy to vaporize the water which means less energy is being used trying to
maintain the fire by heating the fuel. So as the water absorbs all the heat generated from the
fire, the fuel is then cooled and if the flame can't heat up the fuel enough for it to vaporize,
the flame can't feed itself and therefore will dissipate.
However, there are circumstances where water as an extinguisher is not the most efficient -
for example – forest/wildfires, this may not be a feasible strategy as it Is more difficult to get
enough water to where It is needed in such a wide stretch of land. In other situations, water
can be dangerous to use as a extinguisher – for example chip pan fires (oil fire), electricity
and other flammable liquids or materials, another extinguisher is best to be used. The
reason why using water would not be beneficial for extinguishing a fire in a chip pan fire, is
because the water is much more dense than the hot oil and therefore the water will fall
underneath the oil and produce gas quickly that pushes the particles up and as a result
, pushes the oil up and the fire- this causes the fire to rise and escalate instead of being
extinguished. It is also much harder to control this kind of fire. Ref 3.
Starving:
Starving is to remove the fuel from the fire. When fire runs out of the flammable materials, it
will eventually burn itself out. For instance, an open campfire that isn't touching any other
wood or dry grass will ultimately die out since there won't be anything to burn, and if there
isn't anything to burn, the fire won't be contained. Starving a fire works well for electrical fires
where you'd turn the power source off. In cases like cooking or stove fires, it means to turn
off the gas supply. This way there's no fuel to burn, the fire cannot grow and therefore will be
easier to control and can extinguish the fire much easier. This method isn't used in bigger
fires because there's too much fuelling the fire that turning of a power source isn't very
effective and would need another method. However, for smaller fires at home, it’s an
effective method. Ref 4.
Smothering:
Smothering tackles, the last element of the triangle, which is oxygen. Smothering occurs
when oxygen is removed from the fire. An example would be using a fire blanket in the initial
stages of a fire - a small frying pan blaze for example. This reduces the oxygen level to
below 16% and therefore by having lower levels of oxygen, the fire eventually decreases. In a
less severe case, a simple candle being extinguished by the lid, or a glass is also a way of
smothering. The fire burns out all of the oxygen inside the lid/glass creating a vacuum. This
method is usually applied to solid fuel fires. However, in cases where the fire is too big and
smothering is no longer an option as you will not be able to get close to the flame, another
type of method or fire extinguisher will have to be used.
Different fire extinguishers: Ref 5.
Fire extinguishers are also a viable method of putting out a fire. There are multiple types of
fire extinguishers that can be used in different situations. The reason why the different fire
extinguishers are effective is because different types of fuels start fires, so it needs different
extinguishers to tackle a type of fire.
There are different classes of fire. It's important to know the different classes to know which
extinguisher is most effective to put out a type of fire.
Class A fire – these are combustible materials that are caused by flammable solids. For
example: wood, fabric or paper.
Class B fires – these are the flammable liquids. For example: paint, turpentine or petrol.
Class C fires – these are the flammable gases. For example: methane, butane or hydrogen.
Class D fires – these are the combustible metals. These are chemicals such as aluminium,
magnesium or potassium.
Electrical fires – these are fire caused by electrical equipment. However, once the electrical
object is removed, the class of the fire changes.
Class F fires – these are fires caused by cooking oils, usually a chip-pan fire.
