For a fire to occur, there must be available oxygen, a supply of fuel, and enough heat to kindle the fuel. Therefore, the three basic ways of extinguishing fire are to smother it, to cut off the fuel supply, or to cool it below the flammability temperature. Fires are classified into four types: those in solids, e.g., wood, paper, and cloth; those in flammable liquids, e.g., gasoline, alcohol, oils, lacquers, and paints; those in electrical apparatus; and those in flammable metals such as magnesium. These are called, respectively, class A, B, C, and D fires.Characteristics of Extinguishing Substances
Certain dry materials that melt and coat the burning material, thus excluding air, are useful against all classes of fire. In certain cases inert gases such as argon or nitrogen are used to fight fires in materials that would react dangerously with water or with other extinguishing agents; sodium and water, for example, is a dangerous combination.
Water, although supplanted somewhat by other materials, is still the most common substance used for quenching class A fires, which are the most common types of fire; water both cools and helps smother the fuel. Buckets of water are the simplest equipment for fighting small fires in solids. More effective are fire extinguishers capable of directing a stream of water. Wetting agents called detergents make water more penetrating, especially for such objects as cotton bales and mattresses.
Class B fires cannot be fought with water unless it is sprayed in a fine mist, for flammable liquids will usually float on water and spread. Foam is most often used to suffocate class B fires, particularly oil fires.
Since both water and foam conduct electricity, neither can be used against class C fires unless a fog nozzle, which produces tiny droplets that burst into a smothering blanket of steam, is employed. Halogen compounds and carbon dioxide are effective agents in fighting class C fires and are also used against flammable liquids and small fires in solids. Halogen compounds such as carbon tetrachloride turn into a vapor that settles over a fire, smothering it. Unfortunately, most halogen vapors are both toxic and corrosive; but for enclosed spaces where water damage would be as disastrous as fire damage, it is the agent of choice. In any case, nearly all professional firefighters today are equipped with oxygen tanks. Dry-chemical extinguishing agents, such as fine sodium bicarbonate, can be used on class B and C fires but are especially effective against class B fires.
Buildings are protected against fire most effectively by protective sprinkler systems. In most sprinkler systems, water circulates through overhead pipes whose outlets are normally closed; at high temperatures the outlets open, spraying water on the fire. Most large buildings also provide water for fire fighting through a standpipe system with hose connections on each floor. Forest and brush fires are fought by making a firebreak and by covering the fire with extinguishing substances. A narrow strip is cut and cleared in front of the fire down to mineral soil. Embers flying into the strip are put out, while water and other fire-extinguishing substances are spread from land-based vehicles or are dropped on the fire from the air. Oil-field fires demand multiple approaches: water streams, fogs, foams, and explosives may all be used simultaneously to quench a fire and prevent its reignition.