What Is a Fire Extinguisher?

A fire extinguisher is a portable device used to put out small fires by directing a stream of water or another liquid onto the flames. The water cools the flames, deprives them of oxygen and interferes with the chemical reactions that sustain them. Extinguishers are available for use in commercial buildings, retail stores, offices, schools and public buildings.

There are many types of fire extinguishers; they are often rated with a letter and a number (for example, 1-A:10-B:C). The numbers indicate the size of a room that an extinguisher can handle and the class of a fire for which it is suitable. Each type of fire is handled by a different extinguishing agent; for example, a metal fire will be put out with salt, whereas a paper or wood fire will be extinguished with foam.

Fire extinguishers come in a variety of sizes and colors. They may be mounted on a wall in an easily accessible place, or kept in vehicles like cars, boats and airplanes. Most states require that some kind of hand-held fire extinguisher be installed or carried in motor vehicles, boats and airplanes.

Hand-held fire extinguishers are typically supplied as a kit with mounting brackets and instructions for use. They are usually positioned in an area where there is high traffic and are easy to reach. They are also found in industrial areas and at sporting events.

There are two types of hand-held fire extinguishers, a stored pressure type and a cartridge-operated type. Stored-pressure units use a pressurized container of carbon dioxide to hold the extinguishing agent. They are most common in the United States. Cartridge-operated units have a metal or plastic cylinder with an internal operating mechanism that is powered by a rechargeable compressed gas source. They are most common in the rest of the world.

Dry chemicals and multipurpose extinguishers use an ammonium phosphate base, which can be used on classes A and B fires. They are not as effective on a deep-seated class A fire because of their surface coating, which does not penetrate the burning material.

Class D extinguishers contain a solution of sodium chloride salt, which melts to form an oxygen-excluding crust over the fire. This type of fire extinguisher can be used on most alkali metals such as sodium and potassium, but should not be used on lithium fires because it reacts with them to form LiCl and NaCl.

The oldest and most toxic of the vaporizing liquids, carbon tetrachloride was once commonly used in aircraft. It is now replaced by methyl bromide, which is less toxic and works by slowing or stopping the release of vapors from the liquid. It is less effective than carbon dioxide and is typically only used in very specialized applications.