How Does a Fire Extinguisher Work?
Most fire extinguishers smother a fire by shutting off the supply of air with certain chemicals. The carbon dioxide extinguisher commonly used in homes, for example, consists of a steel cylinder of carbon dioxide gas under high pressure.
When it is released, the gas produces a dry, snow like solid, and this can cool the burning material below its kindling point—the temperature below which it will not burn. More important, the heavy gas itself does not burn. It spreads over the burning material, cutting off the free oxygen in the air, thus smothering the fire.
Inside, a fire extinguisher is quite like an aerosol can, often with two different substances inside. One of them is a solid, liquid, or gas substance for fighting the fire. The other one is called a propellant and is a pressurized chemical that makes the fire-fighting substance come out when you press the extinguisher handle.
There are three main types of extinguisher and they work in slightly different ways:
Water extinguishers, which are the most common, are essentially tanks full of water with compressed (tightly squeezed) air as the propellant to make them come out. Water extinguishers work by removing heat from the fire.
Dry chemical extinguishers are tanks of foam or dry powder with compressed nitrogen as the propellant. They work by smothering the fire: when you put a layer of powder or foam on the fire, you cut the fuel off from the oxygen around it, and the fire goes out.
Carbon dioxide (CO2) extinguishers contain a mixture of liquid and gaseous carbon dioxide, a nonflammable gas. CO2 is normally a gas at room temperature and pressure. It has to be stored under high pressure to make it a liquid.
When you release the pressure, the gas expands enormously and cools to make a huge white jet. CO2 attacks the fire triangle in two ways: it smothers the oxygen and, because it’s so cold, it also removes heat.