Portable fire extinguishers vary by extinguishing agent. There are seven types of portable fire extinguishers: water, dry chemical, carbon dioxide, class B foam, halogenated agent, dry powder and wet chemical extinguishers. What extinguishing agent is appropriate depends on the nature of the fire. For example, grease fires and electrical fires become worse when extinguished with water.
Water extinguishers are propelled by compressed air. Larger containers (17-plus gallons) are propelled with a separate canister of liquid CO2. The water on the flame becomes steam, absorbing heat. The steam displaces air from the vicinity of the flame. Water extinguishers sometimes include a wetting agent to reduce its surface tension and promote absorption into upholstery.
Water extinguishers are not all-purpose. The water can spread a grease fire and conduct the electricity of an electrical fire.
Carbon dioxide extinguishers store CO2 in liquid form. It expands violently when released from the canister, leading to a sudden drop in its temperature. The CO2 then both smothers and cools the flame so the CO2 is an expelling agent and an extinguishing agent. They can be used on electrical fires, and it does not leave a residue on electrical equipment. Carbon dioxide extinguishers are rapidly exhausted and require close proximity to the flame, so CO2 is used neither for ordinary solid combustibles nor outdoors where it is easily dispersed by the wind.
Foam extinguishers are water extinguishers with a foam solution propelled by compressed air. They are superior to water extinguishers in that they create an oxygen barrier superior to that of water. Being a water solution, however, the foam still can't be used on grease fires and electrical fires.
Dry Chemical vs. Dry Powder vs. Wet Chemical
Dry chemical extinguishers are close to being all-purpose. They can be used on electrical sockets and flammable liquids. Wet chemical extinguishers are needed, however, for grease fires (not to be confused with flammable fuels). The "wet chemical" extinguishing agent is potassium acetate, carbonate or citrate. Carbon dioxide extinguishers are preferable to dry chemical extinguishers around expensive electrical equipment because of the lack of residue.
Dry powder extinguishing agents are made especially for combustible metals such as thermite and magnesium.
Dry chemical extinguishers commonly contain ammonium phosphate, sodium bicarbonate or potassium bicarbonate. Dry powder extinguishers commonly contain plastic that melts into an oxygen-excluding barrier and sodium chloride to disperse heat.
An early version (1881) of portable fire extinguishers, called soda-acid extinguishers, contained sulphuric acid and sodium bicarbonate. The acid container was broken to create carbon dioxide to propel water. Storing carbon dioxide within the extinguisher in liquid form has superseded this method.
Before soda-acid extinguishers, potassium carbonate was propelled with compressed air.