Carbon Dioxide used in extinguishers is in liquid form, and under pressure. When pressure is released by squeezing the handles, carbon dioxide is forced out, turning into gas at the same time.

Pressurised powder comes out through the hose, using a special nozzle to deliver a spray.

How does a stored pressure fire extinguisher work?

Most fire extinguishers contain either water, foam, powder or carbon dioxide. A fire can be extinguished by cooling it, for which we use water, foam or carbon dioxide. Or it can be extinguished by smothering it, for which we use foam, powder or carbon dioxide. The fire extinguisher is designed to get these contents spread over the fire as quickly and safely as possible.

water

First the extinguisher is almost filled with water, then compressed nitrogen is pumped in. It now contains water under pressure. When the safety pin in pulled out, the handles can be squeezed together, and a valve opens just inside the hose. Water under pressure is forced through the valve, through the hose, and out. (Some water extinguishers are 'jet' type where the water comes out as though from a hosepipe, some are 'spray' type, see foam for how spray is made.)

Foam

First a little concentrated foam solution is poured in, then the extinguisher is almost filled with water, then compressed nitrogen. It now contains diluted foam solution under pressure and operates in the same way as the water extinguisher. At the end of the hose is a spray nozzle, which 'swirls' round as the foam solution passes through it, so that the foam comes out as a fine spray.

Powder

Instead of water, dry powder is used. The pressurised powder comes out through the hose, and again a special nozzle is used to deliver a spray.

Carbon dioxide

A slightly different method. The carbon dioxide used in extinguishers is in liquid form, and under pressure. When pressure is released by pulling the safety pin and squeezing the handles, carbon dioxide is forced out, turning into gas at the same time. The pressure is much greater than in any of the above extinguishers, and carbon dioxide extinguishers should be treated with great care.

Cartridge operated

Not all extinguishers have 'stored pressure' ie compressed nitrogen. Some contain water, foam or powder and a cartridge containing carbon dioxide under pressure. Squeezing the handles causes a spike to press down and puncture the carbon dioxide cartridge as the valve is opened, pressurising the extinguisher at the time of use, instead of all the time.