Fire is a chemical reaction that needs three elements in sufficient quantities to take place and continue. These three elements are called the fire triangle given below,
- Ignition source/heat
Scientists developed the concept of a fire triangle to help in understanding the cause of fires and how they can be prevented and extinguished.
Classes of Fire
There are six different classes of fire.
- Class A – fires involving solid materials such as wood, paper, or textiles.
- Class B – fires involving flammable liquids such as petrol, diesel, or oils.
- Class C – fires involving gases.
- Class D – fires involving metals.
- Class E – fires involving live electrical apparatus. (Technically ‘Class E’ doesn’t exist, however, this is used for convenience here)
- Class F – fires involving cooking oils such as in deep-fat fryers.
Principles of Fire Extinction
- Cooling: cooling removes heat from the fire triangle. Water is most effective for this purpose as it absorbs heat generated by fire. But water is not suitable for oil, fat and electrical fire as water evaporates in the case of oil fire and carries grease particles and in the case of electricity, it is a good conductor.
- Starvation: starvation means the removal of fuel from the fire triangle.
- Smothering: smothering means the removal of oxygen from the fire triangle.
Water extinguishers are one of the most cost-effective ways to fight Class A fires.
There are four different types of water extinguishers:
- Water jet extinguishers work by spraying a jet of water at the burning materials, cooling them, and preventing re-ignition.
- Water spray extinguishers use a very fine spray of water droplets, each droplet is surrounded by air that is non-conductive.
- Water extinguishers with additives or foaming chemicals are more effective because the water loses its natural surface tension meaning that it can soak into the burning materials more effectively. Adding chemicals to the water means that a smaller extinguisher can produce the same fire rating as a larger, water-only, fire extinguisher.
- Water mist extinguishers apply water in the form of mist, or fog, the droplets are much smaller than those from the water spray extinguisher. The smaller the droplet, the larger its surface area in relation to its size, and the quicker the droplet evaporates which absorbs the heat energy faster.
All water extinguishers have a red label.
Foam fire extinguishers can be used on Class A and B fires. They are most suited to extinguishing liquid fires such as petrol or diesel and are more versatile than water extinguishers because they can also be used on solids such as wood and paper. The foam extinguishes the liquid fires by sealing the surface of the liquid, preventing flammable vapor from reaching the air, and starving the fire of fuel. Foam extinguishers are not suitable for use on free-flowing liquid fires.
Foam extinguishers have a cream label.
Powder extinguishers are good multi-purpose fire extinguishers because they can be used on Class A, B, and C fires. They can also be used on fires involving electrical equipment however, they do not cool the fire so it can re-ignite. Powder extinguishers can create a loss of visibility and may create breathing problems while used during emergencies. They are not generally recommended for use in a confined space unless no alternative is available.
Powder extinguishers have a blue label.
Carbon Dioxide Extinguishers (CO2)
Carbon dioxide extinguishers are ideal for places with a lot of electrical equipment such as server rooms because they are safe to use on fires involving electrical apparatus. They do not leave any residue, unlike a foam extinguishers. They can also be used on Class B fires, those involving flammable liquids such as petrol. These extinguishers work by smothering the fire and breaking the fire triangle by cutting off the supply of air.
Carbon Dioxide Extinguishers have a black label.
Wet Chemical Extinguishers
Wet chemical extinguishers are suitable for use on Class F fires involving cooking oils and fats, such as sunflower oil, and butter. They are extremely effective when used properly. The wet chemical rapidly knocks the flames out, cools the burning oil, and chemically reacts to form a soap-like solution, sealing the surface and preventing re-ignition
Wet chemical extinguishers have a yellow label.
The following table will illustrate the use of fire extinguishers for each type of fire.
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