A fire extinguisher is a portable or wheeled container filled with materials that may be used to fight fires. Many buildings, especially those that are not fireproof or are not equipped with sprinkler systems, are required by law to have fire extinguishers available.
The most common fire extinguishers contain water, carbon dioxide, carbon tetrachloride, or dry sodium bicarbonate. These substances extinguish fires by cooling the burning materials, by depriving the fires of oxygen, or by a combination of both methods. The kind of extinguisher that is used to fight a fire depends largely on the nature and size of the fire.
Water fire extinguishers put out fires primarily by cooling the burning materials. In addition, the fire is sometimes deprived of oxygen for a short time by the blanket of steam that is formed when the water comes into contact with the fire. Certain chemicals are sometimes added to the water to make it penetrate the burning materials more thoroughly. In some extinguishers other chemicals are mixed with the water in order to form a foam that cools the burning materials and at the same time smothers the fire.
The water is pumped out of the extinguisher by hand or is forced out by a compressed gas. In the common soda-acid extinguisher the stopper in a bottle of sulfuric acid falls off when the extinguisher is turned upside down. The acid released from the bottle reacts with a water solution of sodium bicarbonate to form carbon dioxide gas. The carbon dioxide forces the water out of the extinguisher.
Water extinguishers should not be used on electrical fires because of the danger of electric shock to the operator. This is especially true of soda-acid extinguishers, since the soda and the acid increase the electrical conductivity of the water. In addition, water extinguishers should not be used on large chemical, oil, or gasoline fires, because water might spread the fire and make it worse. However, water in the form of fog may be used to fight oil fires.
Carbon dioxide extinguishers put out fires by covering them with a blanket of carbon dioxide, which deprives the fires of oxygen. Carbon dioxide extinguishers contain liquid carbon dioxide under great pressure. When released, the carbon dioxide turns into a gas and is directed at the fire through a short hose, which is attached to a long cone. Because carbon dioxide is blown away by wind, it is usually of little use in the open air. However, carbon dioxide extinguishers are one of the best means of fighting small indoor fires, particularly electrical or chemical fires.
In one dry extinguisher a powdery mixture of sodium bicarbonate and other substances is shot out by an expanding gas. The heat of the fire causes the bicarbonate to release large amounts of gaseous carbon dioxide (CO2), which blankets the fire. The most CO, is produced at the places of greatest heat, making this procedure highly effective. The other solid particles, whose grain size is carefully controlled, cool the burning material and provide it with a solid blanket. Such extinguishers are effective in fighting burning liquids and electrical fires.
Applied to fuel fires by either mixing and expanding with air in the branch pipe (aspirated) or forming a frothy blanket over the fuel (non aspirated), preventing oxygen reaching it and suffocating the fire. Unlike powder, foam can be used to extinguish fires without flashback.