Fire Extinguisher Types
A fire extinguisher is a portable device containing a liquid or powder that is discharged on a fire to extinguish it in its early stages. The size and weight of the extinguisher permit it to be easily carried or wheeled to the scene of a fire. Basically, an extinguisher consists of a container, an extinguishing agent, a pressure-producing device or agent, and a discharge orifice or hose and nozzle. Modern extinguishers also incorporate an operating control valve.
Types of Extinguishers
The water-filled leather buckets that householders kept in former times were the first portable fire extinguishers. Another early type of extinguisher was the "squirt," a pumplike device consisting of a cylinder and plunger to discharge water. There were also "fire annihilators" that were simply large containers of water designed to burst when thrown on a burning object, some with internal charges of gunpowder that scattered the water when they exploded.
The oldest modern portable extinguisher is the soda-acid extinguisher, invented in about 1837. It consists of a metal cylinder, usually of 2.5-gallon (9.25-liter) capacity, filled with a solution of bicarbonate of soda and water and containing a stoppered bottle holding acid. When the extinguisher is inverted, the acid flows from the bottle at a predetermined rate and is mixed with the water solution to form a gas; this creates pressure to expel the' liquid through a small handheld hose. While the soda-acid extinguisher is gradually being supplanted by other types, it is still the most commonly found type of extinguisher.
Newer types of extinguishers and extinguishing agents have been developed for specific kinds of fires. A major development was the introduction in about 1960 of a multipurpose dry-chemical extinguisher, which for the first time provided a device effective on all types of fires except those involving combustible metals.
Uses of Extinguishers
As a means of indicating what extinguishers and extinguishing agents are suitable, fires have been classified according to the principal material burning.
- Class A fires involve ordinary combustibles, such as paper, wood, and cloth. They are extinguished by the heat-absorbing effects of water or water-based liquids, or by coating with certain kinds of combustion-retarding dry chemicals. Extinguishers for this purpose include pump tanks and stored-pressure devices, containing water or water with an antifreeze chemical. The pump tank is operated manually; stored-pressure devices are operated by the force of air stored under pressure. The soda-acid extinguisher and the multipurpose dry-chemical extinguisher, which uses an agent with a monammonium phosphate base, are both effective on Class A fires.
- Class B fires involve flammable and combustible liquids, greases, and similar materials. They are most readily extinguished by excluding air, inhibiting the release of combustible vapors, or interrupting the combustion chain reaction. Extinguishers for this purpose include the "regular" dry-chemical types, employing agents with a sodium bicarbonate, potassium bicarbonate, or potassium chloride base, and the multipurpose dry-chemical type, using monammonium phosphate, discharged by an expellant gas. Other extinguishers for Class B fires include the carbon dioxide type, in which the carbon dioxide is stored under pressure as a liquid and discharged as a gas; the foam type, which uses aluminum sulfate mixed with a sodium bicarbonate water-based solution to generate pressure; and the bromotrifluoromethane type, which operates on the same principles as the carbon dioxide type.
- Class C fires involve energized electrical equipment, and they must be extinguished with a nonconductive agent to avoid shocking the user; this rules out the use of water. Dry-chemical, carbon dioxide, and bromotrifluoromethane extinguishers are useful. In fighting an electrical fire the first step is to cut the power, if possible; in this way the Class C fire is converted into a Class A or B fire.
- Class D fires involve combustible metals, such as magnesium, sodium, potassium, titanium, and zirconium. These fires require a smothering and heat-absorbing extinguishing agent that does not react with the burning metals. Agents of this type include liquids, such as trimethoxyboroxine, and powders, such as screened graphitized coke, which are applied by pressurized extinguishers or handscoops.