Fire extinguishers all work in much the same fashion, holding their contents under pressure and releasing them through a nozzle or horn when the operating lever is squeezed. The fire extinguishing agent or extinguishant inside works by removing one or more of the three things a fire needs to burn; oxygen, heat and fuel.
By removing the supply of oxygen, reducing the heat or reducing the heat around the fuel, a single fire extinguisher can extinguish a small fire in a matter of seconds.
What Happens Inside a Fire Extinguisher
When you squeeze the lever on the top of a non-gaseous fire extinguisher, you both open a vale to the nozzle and also release a highly pressurized supply of gas stored at the top of the extinguisher. This gas escapes and pushes out the extinguishing agent from the cylinder through the nozzle.
In CO2 fire extinguishers, the gas is held under tremendous pressure so it liquefies. When you squeeze the lever and release the pressure, the CO2 rapidly expands into a gas again.
Water and Water-additive fire extinguishers
Water and water additive extinguishers are filled with water and pressurised with oxygen. They work by both cooling the fire and soaking the fuel so it will not burn further.
Powder Fire Extinguishers
Regular dry powder extinguishers work by smothering the fire in a layer of power, thus eliminating the source of oxygen. Specialist Class D rated fire extinguishers for metal fires work in a similar way, but use agents that do not react with the metals and are delivered at a low pressure.
Foam (AFFF) Fire Extinguishers
Foam fire extinguishers work in the same way as powder extinguishers, by smothering the fire. Additionally, the foam forms a skin that helps cools the fire. Foam can also soak into absorbent fuel to prevent re-ignition.
Carbon Dioxide Fire Extinguishers
CO2 fire extinguishers hold CO2 gas under great pressure, so it is stored as a liquid. When the pressure is lowered by squeezing the operating lever, the liquid rapidly expands into a gas, which is extremely cold.
The cold gas cools the fire and also displaces the oxygen around it, since CO2 is heavier than air.
Wet Chemical Fire Extinguishers
Wet chemical fire extinguishers are specifically for use on Class F fires involving cooking oils and fats. These extinguishers use a chemical additive to effectively turn the surface of burning oil into a non-combustible soft soap, a process called saponification.
The process of converting the oil into this soapy foam draws heat from the oil, and the resulting layer seals the oil away from the oxygen in the air.