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Fire extinguishers can be classified according to the type of extinguishing agent that can be found inside. Knowing their characteristics may help the user to choose the best fire extinguisher for his needs.

Dry chemical fire extinguishers
They are available in different sizes, from 1 Kg portable fire extinguishers to 250 Kg wheeled fire extinguishers, but the most common in workplaces are the 6, 12, 50, 100 and 250 Kg ones. If they are heavier than 12 Kg, they come with a trolley; the trolley for the 250 Kg fire extinguisher can be trained by a vehicle. Commonly, this kind of fire extinguisher uses ABC dry chemical as extinguishing agent (if their dielectricity is guaranteed, they can also be used on E-class fires, but their use in this case is not advisable, since their residues could damage the electrical apparatus). Before choosing one of these fire extinguishers, it should be remembered that ammonium phosphate is aggressive towards copper and its alloys.
The chemicals are in a container inside the tank and they are pressurized with a gas that will cause their expulsion. Portable dry chemical fire extinguishers can be stored pressure or with an inner tank. In this case, there is also an external 300 g pressurized liquid CO2 tank. The tank containing the powder is made of steel sheet and tested at 40/50 bar with a safety valve.
Stored pressure fire extinguishers are made of a tank, that contains the extinguishing agent, and a valve assembly with dip tube and hose. The extinguishing agent inside of it is pressurized with nitrogen at 12-14 bar. It is possible to control its pressure thanks to a pressure gauge near the valve. The container with the extinguishing agent is usually the same containing the gas, so the pressure is kept constant. It is a kind of fire extinguisher that can be found in residential buildings.

Carbon dioxide fire extinguishers
CO2 fire extinguishers are extremely common. Their size may vary, they go from 2 to 60 Kg of load. Carbon dioxide inside the fire extinguisher is liquid and pressurized. On the top of the tank there is a valve with dip tube that will allow the carbon dioxide flow to exit through a horn. Only 75% of a CO2 fire extinguisher tank is filled, 67% in very hot climates or wherever they could be exposed to a strong irradiation.
Since liquid carbon dioxide can reach extremely low temperatures, this type of fire extinguisher should not be directed  on people, as it could seriously burn them. At the same time, the user should be careful because he could burn his own hands while using it. It should be reminded that high concentrations of carbon dioxide in an enclosed space do not allow humans to breathe.

Foam fire extinguishers
Foam fire extinguishers are particularly effective on class-B fires. Please remember to direct the flow towards the inner side of the container with the flammable liquid, and not towards the liquid itself, when you use it, so to avoid spurts and the consequent spreading of the fire, and so to make the foam cover the entire surface of the liquid, suffocating it and preventing a reignition. Unlike carbon dioxide fire extinguishers, these allow humans to breathe.
If there is no antifreeze inside foam fire extinguishers, they are not suitable for class-E fires or whenever the temperature is below -5°C, because of the high concentration of water in the foam.