16/01/2019
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 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.
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 facts.
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.
How do fire extinguishers work?
Inside, a fire extinguisher is quite like a giant 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. Next time you see a fire extinguisher, take a good look. Have you noticed that fire extinguishers are always really strong steel canisters? That because the propellant is stored inside at a high pressure. Strong canisters are needed to stop the extinguishers exploding!
Water extinguishers
Animation showing how a fire extinguisher releases water when you press the handle.
A water extinguisher is like a giant water pistol, but instead of using pressure from your finger to fire out the water, it uses pressure from a trapped gas.
A ring or pin on the handle stops the fire extinguisher from being set off by accident. It also acts as a tamper-proof seal: if the ring is broken or missing, you know the extinguisher needs to be checked.
Inside the sturdy steel case, there's a canister containing high-pressure gas (orange with blue hashing).
Most of the extinguisher is filled with water (blue).
A tube runs right up the inside of the tube to a nozzle outside (gray).
The nozzle often ends in a piece of bendy plastic so you can easily direct it toward the base of a fire.
To operate the extinguisher, you pull the ring and press the handle.
Pressing the handle opens a valve (shown here as a green arrow) that releases the pressurized gas from the canister.
The gas immediately expands and fills the inside of the extinguisher, pushing the water downward
As the water is pushed down, it rises up the tube
A jet of water emerges from the nozzle.
Carbon dioxide extinguishers
Simple illustration showing the main parts of a carbon dioxide fire extinguisher and horn.
The most noticeable difference between a water extinguisher and one that fires carbon dioxide is the large, black, cone-shaped horn, which allows the carbon dioxide gas to expand, cool, and turn into a mixture of frozen "snow" and gas. The horn has to be designed very carefully to stop two major potential problems: it has to allow the CO2 to exit at high speed, so any snow that forms doesn't block it up, and it has to mix up the gas in a fairly turbulent way to stop it firing air from the horn at the fire as well (which would effectively make the fire burn more strongly). This typical design from a patent by Brooks Equipment in the 1970s solves both problems. I've added the coloring for clarity, but followed the original numbering of the key parts:
10. Tank containing pressurized liquid carbon dioxide.
12. Valve.
14. Trigger.
16. Discharge horn made of plastic that can survive low temperatures without cracking.
18. Reinforcing bands wrap around horn at intervals.
20. Ni**le with a screw thread to which the horn attaches.
As the carbon dioxide enters the horn, it swirls around in a turbulent flow (orange arrows) forming snow (orange blobs) and gas. The swirling turbulence stops dead air zones forming in the horn, which in turn prevents air being swept down the horn toward the fire.
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