Carbon dioxide (CO2) is a widely used and effective agent for extinguishing fires. Its ability to quickly suppress flames makes it a common choice in various settings. Understanding how CO2 works provides insight into its effectiveness. This gas operates through specific mechanisms to remove the elements that allow a fire to burn.
The Elements of Fire
Fire requires three essential components to burn: fuel, heat, and oxygen. These are commonly represented as the “fire triangle.” Fuel is any combustible material like wood, paper, or flammable liquids. Heat provides the energy to raise fuel to its ignition temperature, initiating the combustion process.
Oxygen acts as the oxidizing agent, supporting the chemical reaction that produces fire. Air contains about 21% oxygen. For most fires to sustain combustion, oxygen concentration in the surrounding atmosphere must remain at or above 16%. Removing any one of these elements extinguishes a fire.
Smothering the Flames
Carbon dioxide extinguishes fire by displacing oxygen. CO2 is significantly denser than the air we breathe. When released from an extinguisher, pressurized liquid CO2 rapidly expands into a gas, forming a cloud over the burning material.
This heavier CO2 gas pushes away lighter oxygen from the fire’s vicinity. By reducing oxygen concentration below the critical level needed for combustion, the fire becomes starved of its essential oxidizing agent. This process, known as smothering, quickly interrupts the chemical reaction, causing flames to die out.
Cooling the Heat
Carbon dioxide also contributes to fire extinguishment through a cooling effect, addressing the heat component of the fire triangle. CO2 is stored under high pressure as a liquid. When released, it undergoes a rapid change of state, expanding quickly from a liquid to a gas.
This rapid expansion causes a significant temperature drop, often reaching approximately -78°C. This phenomenon is due to the Joule-Thomson effect, where an expanding gas absorbs heat from its surroundings. The extremely cold CO2 gas and any resulting “dry ice” particles cool the burning fuel, lowering its temperature below its ignition point and inhibiting the fire.
Where CO2 Extinguishers Are Used
CO2 extinguishers are effective on specific fire types. They are preferred for Class B fires (flammable liquids like gasoline, oils, and paints) and Class C fires (electrical fires involving energized equipment). The gas smothers these fires and does not conduct electricity, making it safe for live electrical components.
A significant advantage of CO2 extinguishers is that they leave no residue behind after discharge, as the gas dissipates into the atmosphere. This non-residue feature makes them ideal for sensitive equipment like computers, electronics, and machinery. However, CO2 extinguishers are not suitable for Class A fires (ordinary combustible materials like wood or paper), as they may not cool the material sufficiently to prevent re-ignition. Using CO2 in confined spaces is hazardous because it displaces oxygen, posing an asphyxiation risk.