Fire extinguishers are an essential part of fire safety in any home or commercial, public and industrial building. The chemicals they contain are tailored to fight different types of fires. They are usually kept in pressured, hand-held cylinders and can be discharged at the fire.
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Water extinguishes a fire by cooling the burning objects. It vaporises when it hits the burning object and in the process, extracts heat to form the steam. A high pressure water jet from a cylinder can be directed at the source of a fire. The pressure in a portable water fire extinguisher is provided by a cartridge containing carbon dioxide. Water should not be used to extinguish electrical fires. It can cause shorting, create sparks, cause the ignition of more material and spread the fire.
Foams are a stable mass of air bubbles that cover the fire, starve it of oxygen and smother the blaze. They are used to extinguish fires in paints, oil spills, and aviation, marine and motor accidents. The bubbles are formed by aerating a solution of foam concentrate in water such as aqueous film forming foam (AFFF). This consists of fluorochemical and hydrocarbon surfactants. A surfactant alters the surface properties of water so that it can spread like a thin film over the burning material. The film itself carries the foam that smothers the fire.
Pressurised carbon dioxide can be kept in liquid form in a cylinder. When a valve on the cylinder is opened to release the gas, the carbon dioxide expands rapidly into a cloud of very cold gas. It is denser than air so falls on the burning object like a blanket and smothers the fire. However, the pressurised gas can disperse small particles of burning matter and spread the fire.
Halon is a compound of carbon, fluorine, chlorine and bromine with the chemical name of bromochlorodifluoromethane, or BCF. It is easily liquefied under pressure and kept in a cylinder, vaporising on release to smother the fire. Its pressure is lower than that for a carbon dioxide jet so it won’t disperse the fire. This type of extinguisher is suitable for all types of fires. However, halon has some of the highest ozone depleting capacity of any commonly used chemical. It was banned in 1993 under the 1987 Montreal Protocol but has been reserved for use on aircraft and some military applications.
Ammonium phosphate is the main extinguishing agent in a dry powder fire extinguisher that can be used in homes and workplaces for many types of fires such as wood, plastic, oils and electrical equipment. Its disadvantage is that breaks up chemically in the presence of atmospheric water vapour into phosphoric acid. This substance can corrode ferrous metals and alloys and attack plastics and rubber. The dry powder causes breathing difficulties and skin irritation in humans. This extinguisher cannot be used in food preparation areas, laboratories and emergency fire escape areas.
Wet chemical extinguishers are suitable for putting out cooking fires in domestic and commercial kitchens. They are usually a solution of potassium acetate with some potassium citrate and potassium bicarbonate. The chemicals emerge as foam and smother the fire with a greater cooling effect than AFFF foams. The chemicals also saponify burning animal and vegetable fats. This is a chemical reaction that forms a soap solution and prevents splashing of burning fats.
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