by Chris Woodford. Last updated: April 13, 2015.
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: 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 gas. Photo by Sgt. Jeremiah Johnson courtesy of US Army.
Fire is dangerous!
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
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.
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.
What is fire?
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
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—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
That's not quite true. Fire happens when three things are in the same place at the same time:
- Fuel (something to burn—such as wood or coal).
- Oxygen (usually from the air).
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
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.
Photo: You need to to take away one or more of heat, air (oxygen), or fuel to break the triangle and put the fire.
How 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!
There are three main types of extinguisher and they work in slightly
- 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.
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.
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).
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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.