Carbon dioxide (\(\text{CO}_2\)) fire extinguishers are common safety devices in commercial and industrial settings. They rely on the unique physical and chemical characteristics of \(\text{CO}_2\) to rapidly suppress flames. The effectiveness of \(\text{CO}_2\) is rooted in its ability to interrupt the fire process while being safe for use where other extinguishing agents might cause damage.
The Mechanism of Fire Suppression
Combustion requires fuel, heat, and an oxidizing agent, typically oxygen, often called the fire triangle. \(\text{CO}_2\) primarily extinguishes a fire by physically smothering it, removing the oxygen component. When discharged, the gas rapidly displaces the surrounding air near the combustion zone. Fire requires an oxygen concentration of around 21% to burn, and \(\text{CO}_2\) reduces this level below 15%, effectively starving the fire.
A secondary effect of using \(\text{CO}_2\) is the rapid cooling of the burning material. \(\text{CO}_2\) is stored as a pressurized liquid, and its release causes an endothermic phase change, meaning it absorbs heat from the surroundings. The gas exits the extinguisher at an extremely low temperature, often around \(-78^\circ\text{C}\). This thermal shock helps lower the fuel’s temperature, inhibiting combustion and reducing the chance of re-ignition.
Physical Properties Supporting Extinguishment
\(\text{CO}_2\) gas is approximately 50% denser than ambient air, with a relative density of about 1.52 (air = 1). This density ensures the gas settles and forms a heavy layer over the fire. This layer prevents fresh oxygen from reaching the burning surface. The blanket of \(\text{CO}_2\) is maintained for a short period, allowing the fire to be fully contained and suppressed.
The high-pressure storage state enables the gas to be deployed effectively over a wide area. \(\text{CO}_2\) is contained in the cylinder as a liquid under high pressure, typically ranging from 50 to 70 bar. Upon release, the liquid rapidly converts to a large volume of gas, resulting in a high expansion ratio. This rapid volume increase ensures that oxygen around the fire is displaced quickly, maximizing the smothering effect.
Selection for Specific Fire Classes
\(\text{CO}_2\) extinguishers are primarily used for Class B (flammable liquids) and Class C (energized electrical equipment) fires. \(\text{CO}_2\) is a non-conductive gas, meaning it does not transmit electricity, which makes it suitable for fighting electrical fires. Applying water to energized equipment risks electrocution and damage, but \(\text{CO}_2\) can be used safely in these situations.
For fires involving flammable liquids like oils or solvents, \(\text{CO}_2\) is preferred because its gas application avoids spreading the fire, which can happen with water-based agents. \(\text{CO}_2\) is also residue-free, dissipating completely into the atmosphere. This is beneficial in environments with sensitive assets, such as data centers or laboratories. Unlike dry chemical powders or foam, the lack of corrosive or damaging residue minimizes equipment downtime and reduces post-fire recovery costs.