fire extinguisher is an absolute necessity in any home or
office. While there's a good chance that the extinguisher will
sit on the wall for years, collecting dust, it could end up
saving your property and even your life.
In this edition of HowStuffWorks,
we'll see exactly what fire extinguishers do and how they do
it. We'll also find out what causes fire in the
first place, learn the correct way to use an extinguisher and
see what sort of fire suppressant works best on different
types of fires.
What is Fire? Fire is the
result of a chemical combustion reaction, typically a
reaction between oxygen in the atmosphere and some sort
of fuel (wood or gasoline,
for example). Of course, wood and gasoline don't spontaneously
catch on fire just because they're surrounded by oxygen. For
the combustion reaction to take place, the fuel has to be
heated to its ignition temperature.
Here's the sequence of events in a typical wood fire:
Something heats the wood to very high temperatures. This
could be any number of things -- focused light,
friction, something else that is already burning.
When the wood reaches about 500 degrees Fahrenheit (260
degrees Celsius), the heat decomposes some of the
cellulose material that makes up the wood.
Decomposed material is released as volatile gases,
typically a compound of hydrogen, carbon and oxygen.
When the gas is hot enough, the compound molecules break
apart, and the atoms
recombine with the oxygen to form water, carbon dioxide and
The gases, which rise through the air, make up the
flame. Carbon atoms rising in the flame emit light as
they heat up. (Check out How Light
Bulbs Work to find out why heated objects emit light.)
The heat of the flame keeps the fuel at the ignition
temperature, so it continues to burn as long as there is
fuel and oxygen.
As you can see, there are three
essential elements involved in this process:
Oxygen (or similar gas)
Fire extinguishers are designed to
remove at least one of these elements so that a fire will die
out. There are several different ways of doing this, as we'll
see in the next section.
Inside an Extinguisher In the last section,
we saw that there are three essential elements involved in
producing fire -- heat, oxygen and fuel.
To put a fire out, you need to effectively remove one of these
The best way to remove heat is to dump water on
the fire. This cools the fuel to below the ignition point,
interrupting the combustion cycle.
To remove oxygen, you can smother the fire
so it is not exposed to air. One way to smother a small fire
is to cover it with a heavy blanket. Another way is to dump
nonflammable material, such as sand or baking
soda on top of it.
Removing the fuel is the most difficult approach
for most fires. In a house fire, for example, the house
itself is potential fuel. The fuel will only be removed once
the fire has burned all of it up.
Fire extinguishers are sturdy metal cylinders filled with
water or a smothering material. When you depress a lever at
the top of the cylinder, the material is expelled by high
pressure, similar to the way material is forced out of an aerosol
can. The diagram below shows a typical design.
In this extinguisher, a plastic siphon tube leads from the
bottom of the fire-suppressant reservoir to the top of the
extinguisher. A spring-mounted valve blocks the passageway
from the siphon to the nozzle. At the top of the cylinder,
there is a smaller cylinder filled with a compressed gas --
liquid carbon dioxide, for example. A release valve
keeps the compressed gas from escaping.
Most dry-chemical fire extinguishers have a
built-in pressure gauge. If the gauge indicator is
pointing to "recharge," the pressure in the extinguisher
may be too low to expel the contents. The National
Fire Protection Association recommends having dry
extinguishers inspected every six years, even if the
gauge indicates correct
To use the extinguisher, you pull out the safety pin and
depress the operating lever. The lever pushes on an actuating
rod, which presses the spring-mounted valve down to open up
the passage to the nozzle. The bottom of the actuating rod has
a sharp point, which pierces the gas cylinder release valve.
The metal safety pin prevents the operating
lever from closing
The operating lever pushes down on an
actuating rod (the blue piece).
The compressed gas escapes, applying downward pressure on
the fire-suppressant material. This drives the material up the
siphon and out the nozzle with considerable force. The proper
way to use the extinguisher is to aim it directly at the
fuel, rather than the flames themselves, and move the
stream with a sweeping motion.
In the next section, we'll look at the major types of
Types of Extinguishers Water is the most
familiar extinguishing material, and it is one of the most
effective. But it can be dangerous in the wrong situation. A
water extinguisher can put out things like burning
wood, paper or cardboard, but it does not work well on
electrical fires or fires involving inflammable
liquids. In an electrical fire, the water may conduct the
current, which can electrocute you. Water will only spread out
an inflammable liquid, which will most likely make the fire
are rated by the type of fire they can put out.
Class A extinguishers can put out fires from
"ordinary combustibles" such as wood, plastic or paper.
Class B can put out burning liquids such as gasoline
or grease. Class C can put out electrical fires.
Extinguishers marked A, B and C can put out all types.
Class D extinguishers, designed to put out
burning metal, are rare. Check the letters and pictures
on your extinguisher to find out what it can handle.
extinguisher material is pure carbon dioxide. In a
carbon dioxide extinguisher, the carbon dioxide is kept in
pressurized liquid form in the cylinder. When the container is
opened, the carbon dioxide expands to form a gas in the
atmosphere. Carbon dioxide gas is heavier than oxygen, so it
displaces the oxygen surrounding the burning fuel. This sort
of fire extinguisher is common in restaurants because it won't
contaminate the cooking equipment or food.
The most popular extinguisher material is dry
chemical foam or powder, typically made of sodium
bicarbonate (normal baking soda), potassium bicarbonate
(nearly identical to baking soda), or monoammonium phosphate.
soda starts to decompose at only 158 degrees Fahrenheit
(70 degrees Celsius), and when it decomposes, it releases
carbon dioxide. The carbon dioxide, along with the
insulation of the foam, works to smother the fire.
Most fire extinguishers contain a fairly small amount of
fire-suppressant material -- you can use it all up in a matter
of seconds. For this reason, extinguishers are only effective
on relatively small, contained fires. To put out a larger
fire, you need much bigger equipment -- a fire
engine, for example -- and the professionals who know how
to use it. But for the dangerous flames that can pop up in
your house, a fire extinguisher is an invaluable lifesaver.
To learn more about fire extinguishers and other
fire-fighting equipment, check out the links on the next page.