A fire extinguisher is a pressurized safety device designed to control or extinguish small fires in emergency situations. The contents of the canister vary significantly, and there is no single gas used across all types. The specific composition is determined by the class of fire it is designed to fight, such as those involving ordinary combustibles, flammable liquids, or electrical equipment. The term “gas” in the context of fire extinguishers can refer to either the propellant that forces the contents out or the agent that actually suppresses the fire.
The Role of the Propellant Gas
Most fire extinguishers rely on a non-flammable, inert gas to create the pressure necessary to expel the extinguishing agent. This propellant gas is stored under high pressure within the canister, sometimes in a separate cartridge or mixed with the agent itself. Nitrogen is the most common gas used for this purpose in dry chemical extinguishers.
Other extinguishers, such as those containing water or foam, typically use compressed air as the propellant. These propellant gases are chemically inert, meaning they do not participate in the fire suppression process. Their sole function is forcing the liquid or powder agent through the nozzle and onto the flames.
Carbon Dioxide: The Self-Propelling Agent
Carbon Dioxide (\(\text{CO}_2\)) is unique because the gas itself is the extinguishing agent, making it self-propelling. The \(\text{CO}_2\) is stored as a liquid under high pressure inside the metal cylinder. When the handle is squeezed, the pressure is released, and the liquid \(\text{CO}_2\) rapidly expands into a dense gas as it exits the nozzle.
This heavy, non-flammable gas works by displacing the oxygen surrounding the fire, effectively smothering it. \(\text{CO}_2\) extinguishers are primarily used for Class B fires involving flammable liquids and Class C fires involving energized electrical equipment. A significant advantage of \(\text{CO}_2\) is that it leaves no residue, making it the preferred choice for sensitive electronics and machinery.
Non-Gaseous Agents and Their Composition
The simplest agent is water, which is used only on Class A fires involving ordinary combustibles like wood or paper. Foam agents, such as Aqueous Film-Forming Foam (AFFF), are used on both Class A and Class B fires.
Dry chemical extinguishers contain fine powders that are effective against a wider range of fires. Multi-purpose, or ABC, dry chemical is typically a mixture of monoammonium phosphate and ammonium sulfate. Another common agent is sodium bicarbonate, which is used for BC-rated fires involving flammable liquids and electrical equipment. These powders are specially treated to prevent clumping and ensure proper flow when discharged.
The Science of Fire Suppression
All fire suppression methods operate by removing at least one element of the fire triangle: heat, fuel, or oxygen. Water-based agents work primarily by cooling the burning material, lowering its temperature below the ignition point. Foam and \(\text{CO}_2\) agents suppress fire by smothering the flames, which creates a barrier that prevents oxygen from reaching the fuel.
Dry chemical agents, particularly the multipurpose powders, employ a different mechanism by interrupting the fire’s chemical chain reaction. As the powder is heated, it releases substances that interfere with the combustion process at a molecular level, quickly halting the fire.