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Military firefighter using a carbon dioxide extinguisher to put out a fire

Fire extinguishers

by Chris Woodford. Last updated: August 24, 2020.

Fire is one of humankind's oldest discoveries; it's also one of our biggest threats. A fire can destroy in a matter of minutes a home or business that has taken decades to establish. That's why methods of putting out fires are so important. Many buildings are equipped with fire extinguishers, but why are there so many different kinds? What do they do to a fire? And how exactly do they work?

Photo: The characteristic blast of "frozen snow" from a carbon dioxide (CO2) extinguisher. Note how the firefighter wears protective gloves and avoids touching the black end of the extinguisher horn, which gets very cold. Photo by Christopher O'Grady courtesy of US Navy.

Contents

  1. Fire is dangerous!
  2. What is fire?
  3. Breaking the fire triangle
  4. Types of fire extinguishers
  5. How do fire extinguishers work?
  6. Find out more

Fire is dangerous!

A boy being taught how to use a fire extinguisher on a blazing fire

Photo: Fire fighters, like this one from the US Navy, are always happy to show you how to use fire extinguishers the correct way. Photo by Rachel McMarr courtesy of US Navy.

Before we start, here's something to note: fire is extremely dangerous. Never ever play with fires or anything that can trigger a fire. You could put your life in danger and risk the lives of other people.

Never play with fire-fighting equipment like fire extinguishers and hoses. They're designed to save lives in emergencies, not to set off like toys. If you set off fire alarms as a prank, people are more likely to ignore them when they go off for real, and someone might die as a result. What if that person was a friend of yours or someone in your family?

If you want to try out a fire extinguisher, go visit your local fire department or arrange for them to come to your school. They'll often be happy to oblige. Many fire departments have open days when you can look around, peek inside a fire engine, and even slide down the fire station poles! Keep an eye out in the local media for events like this.

What is fire?

Military firefighter using an extinguisher to put out a fire

Photo: Putting out a fire caused, by a mortar attack, with a carbon dioxide (CO2) extinguisher. The white cloud coming from the horn is formed when liquid carbon dioxide stored under pressure in the extinguisher turns back to a freezing cold gas. Photo by Sgt. Jeremiah Johnson courtesy of US Army.

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 wood, paper, 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 that make air pollution—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, the heat of the sun, or an overheating machine) to kick off the reaction. Once combustion is underway, the fire seems to continue all by itself.

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Breaking the fire triangle

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.

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

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

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.

Although professional firefighters have other, more comprehensive ways of understanding fires, the fire triangle is still probably the simplest and most useful rule of thumb for the rest of us.

Types of fire extinguishers

The four main types of fire extinguishers (water, dry powder, foam, and CO2) and how they break the fire triangle.

Artwork: The four main types of extinguishers and how they break the fire triangle. 1) Water extinguishers work mainly by cooling (removing heat), but a dense spray of water droplets also helps to cut off oxygen. 2) Dry powder extinguishers soak up heat, melt on the fuel, and cut off oxygen, but they also neutralize the fuel (cutting it off from the fire). 3) Foam extinguishers typically float a thin layer of foam over something like a burning liquid, so cutting off oxygen. They also cool a fire. 4) CO2 extinguishers mostly work by cooling a fire, but they also help to cut off its oxygen.

There are four main types of extinguishers and they work in slightly different ways:

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:

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. Once you've done that, call the fire department.

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 (the propellant). Typically, this is nitrogen or carbon dioxide.

  1. 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.
  2. Inside the sturdy steel case, there's a canister containing high-pressure gas (orange with blue hashing).
  3. Most of the extinguisher is filled with water (blue).
  4. A tube runs right up the inside of the tube to a nozzle outside (gray).
  5. The nozzle often ends in a piece of bendy plastic so you can easily direct it toward the base of a fire.
  6. To operate the extinguisher, you pull the ring and press the handle.
  7. Pressing the handle opens a valve (shown here as a green arrow) that releases the pressurized gas from the canister.
  8. The gas immediately expands and fills the inside of the extinguisher, pushing the water downward
  9. As the water is pushed down, it rises up the tube
  10. 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:

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.

Artwork: A typical carbon dioxide fire extinguisher. From US Patent 3,901,322: Fire Extinguisher Discharge Horn by Jack Winston, Brooks Equipment Co., Inc., August 26, 1975, courtesy of US Patent and Trademark Office.

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).

Foam extinguishers

These are similar to water extinguishers but, instead of containing just water and a propellant, they also have a concentrated foaming solution inside them. Liquid water is almost impossible to compress, so an ordinary water extinguisher can't produce more water for fighting a fire than the volume of the extinguisher itself, which is usually no more than about 6–9 liters. A foam extinguisher, on the other hand, works a bit more like a carbon dioxide extinguisher when the nozzle is open: the water and foaming solution swirl together in the nozzle, producing a much bigger volume of foam than the volume of the can itself. Foam extinguishers are often called AFFFs (aqueous film-forming foam), which is simply a technical way of describing how they tackle a fire: they use water (aqueous) to make a foam that sits like a film over burning fuel, cutting off its air supply.

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On this site

Hand grenade fire extinguisher patented by John Harden in 1884.

Artwork: Fire extinguishers didn't always come in bright red metal cans. Here's a common design from the late 19th century: the fire hand grenade. It consisted of a sturdy glass sphere (A) that you tossed into a fire. Inside, there was a pressurized fire extinguishing gas (B) or solution, and solid particles of something like quartz sand (C), which helped the glass break on impact. If the glass didn't smash straight away, energy from the fire soon conducted through the wire wrapped around the outside (D and E), concentrating the heat and making the glass crack open. Artwork from US Patent 297,075: Hand-grenade fire extinguisher by John J. Harden, courtesy of US Patent and Trademark Office.

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For older readers

For younger readers

Articles

Books

Patents

For deeper technical detail, here are a few examples of fire extinguisher patents. One thing to note is that the design of a fire extinguisher's nozzle is as important as (often more important than) the rest of the mechanism, because how the extinguishing chemicals arrive at the fire is critical. That's why a lot of patents focus on the specifics of the nozzle.

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Text copyright © Chris Woodford 2007, 2020. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007/2020) Fire extinguishers. Retrieved from https://www.explainthatstuff.com/fireextinguisher.html. [Accessed (Insert date here)]

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