How Does a Dry Powder Fire Extinguisher Work?

Written by isaiah david
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Fire is a chemical chain reaction. Molecules of a flammable material contain stored energy. When they are heated to a certain point, they react with oxygen, releasing that energy in the form of heat. This heat energy then causes other nearby flammable molecules to react with oxygen, continuing the cycle. The fire needs the heat, oxygen and a fuel of flammable material to continue burning. Fire extinguishers work by preventing the oxygen from contacting the fuel, removing the heat or both.

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Parts of an Extinguisher

Powder fire extinguishers contain a pressurised gas such as nitrogen, and a powder such as sodium bicarbonate (baking soda), potassium bicarbonate or some other fire retardant powder. At the mouth of the fire extinguisher is a nozzle attached to a trigger. A ring attached to a pin keeps the nozzle from being pushed. The extinguisher is encased in a solid metal canister that can withstand the internal pressure from the gas.

Smothering the Fire

When the trigger is depressed, it opens a small hole in the nozzle. Because the gas inside is at a much higher pressure than the atmosphere, the gas flows out through the nozzle to equalise the pressure, carrying the powder with it. When a fluid such as the nitrogen in the extinguisher moves from a high-pressure area to a low-pressure one, it cools down rapidly, which helps to cool the fire, removing some of the heat required to continue the chain reaction. The most important effect, however, comes from the powder. It coats whatever is burning. This coating stops the oxygen from reaching the fuel. Without oxygen to react with, the fire quickly dies.

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