Fire extinguishers use two methods to expel the agent onto the fire. Stored Pressure and Cartridge Operated. Stored pressure fire extinguishers are easily the most common type. In these units, the expellant is stored in the same cylinder as the agent itself. The different agents used in the fire extinguisher may require different propellants. Dry chemical extinguishers typically use nitrogen or dry air as a means of propelling the agent onto the fire. Water and foam extinguishers typically use air.