Working of a Fire Extinguisher

What is fire?

Someone striking a match into a flame next to a matchbox.

Photo: Fire is a chemical reaction that needs activation energy to start it off, provided by something like a match, the heat of the sun, or an overheating machine.

Ask most people what a fire is and they’ll tell you it’s something frightening and destructive involving flames. But to a scientist, a fire is something much more precise. A fire is actually a chemical reaction called combustion. When combustion happens, substances like woodpaper, oil, or coal (all of which are made from chemicals, even if you don’t immediately think of them that way) combine with oxygen in the air to produce water, carbon dioxide, waste gases—and an awful lot of heat. Combustion doesn’t normally happen all by itself: things don’t burst into flames without help. It usually takes some activation energy (provided by a spark or a match) to kick off the reaction. Once combustion is underway, the fire seems to continue all by itself.

The fire triangle

Simple artwork showing the fire triangle made up of heat, oxygen, and fuel

Photo: You need to to take away one or more of heat, air (oxygen), or fuel to break the triangle and put the fire.

That’s not quite true. Fire happens when three things are in the same place at the same time:

  1. Fuel (something to burn—such as wood or coal).
  2. Oxygen (usually from the air).
  3. Heat.

A fire can burn when all these things are present; it will stop when at least one of them is removed. As any fire-fighter will tell you, putting out a fire involves breaking the fire triangle—which means removing either the fuel, the heat, or the oxygen. Suppose a fire breaks out in a pan on top of your cooker, the first thing you normally do is switch off the heat. If that doesn’t work, you might soak a towel with water and place it very carefully over the pan (or, better still, use a fire blanket). The towel is designed to block off the supply of oxygen to the fire (the water stops the towel from catching fire and making things worse). Every fire-fighting technique you can think of involves removing heat, oxygen, or fuel—sometimes more than one of those things at the same time. Fire extinguishers work by removing heat, air, or both.

Types of fire extinguishers

Basic dry powder fire extinguisher with (inset) pressure gauge needle showing 14 bars.

Photo: Some extinguishers have pressure gauges on top so you can check they’re correctly pressurized and safe to operate. If the pressure is either too high or too low, the needle moves into the upper or lower red zone. On this dry-powder extinguisher, the needle is right in the middle: still safely in the green zone, pressurized to about 14 times atmospheric pressure (the normal pressure of the air around us).

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 makes a huge white jet. CO2 attacks the fire triangle in two ways: it smothers the oxygen and, when it turns from a liquid back to a gas, it “sucks” in a massive amount of heat from its surroundings (the latent heat of vaporization), which cools whatever you spray it on by removing heat.

That classifies extinguishers by what they contain. You’ll also find fire extinguishers classified by the types of fires you can use them on. This gives us five different kinds:

  • A: Green: For wood, cloth, and paper.
  • B: Red: For combustible and flammable liquids such as oil, gasoline, and paint.
  • C: Blue: For electrical equipment and tools.
  • D: Orange:: For flammable metals.
  • K: Black: For animal or vegetable oils or cooking fats.

It’s important always to use the right extinguisher for the fire. Using the wrong extinguisher can put your life in danger and make the fire worse. For example, you must never use water extinguishers on electrical fires because you could electrocute yourself and the people nearby. If you’re in the slightest doubt about tackling a fire, leave it alone and get yourself to safety.