How does a trolley-type CO₂ fire extinguisher operate, and what is the mechanism for discharging CO₂?

In a trolley-type CO₂ fire extinguisher, the CO₂ (carbon dioxide) is stored in a high-pressure cylinder in liquid form. This pressurized CO₂ is stored at approximately 50 to 60 bar (725 to 870 psi) to ensure that a sufficient amount of CO₂ can be packed into the cylinder in its liquid state. Storing the CO₂ in liquid form increases the amount of gas that can be contained within a smaller, more manageable cylinder. The cylinder is made of durable steel or similar materials to withstand the internal pressure, and it is designed to ensure that the gas remains in liquid form until it is discharged. CO₂ remains in liquid form under these high-pressure conditions due to the gas's critical temperature and pressure characteristics.

The operation of the trolley-type CO₂ fire extinguisher begins when the release mechanism is activated. This typically involves pulling a handle or pressing a discharge lever, which in turn opens the valve located on the top of the cylinder. The valve is designed to regulate the flow of CO₂ gas from the cylinder. When the operator engages the handle or lever, the valve opens and allows the pressurized CO₂ to exit the cylinder. This is controlled by a safety pin or locking mechanism that prevents accidental discharge, ensuring that the extinguisher is not used unless intentionally activated. The valve, hose, or nozzle is engineered to direct the CO₂ in a controlled manner, allowing the user to aim and discharge the gas accurately at the fire source.

Once the CO₂ is released from the cylinder, the liquid CO₂ rapidly undergoes a phase change from a liquid to a gas. This change occurs due to the drastic pressure drop when the CO₂ exits the high-pressure environment of the cylinder into the lower pressure of the surrounding atmosphere. This phase transition causes the liquid CO₂ to expand rapidly into a gas, a process known as vaporization. As the CO₂ transforms into a gas, it expands by a factor of about 450 times its liquid volume. This expansion is what enables the extinguisher to release a large volume of CO₂, covering a wide area and effectively reducing the oxygen concentration in the vicinity of the fire. The rapid transition from liquid to gas also causes the CO₂ to cool drastically, with the gas exiting the nozzle at an extremely low temperature (around -78.5°C / -109.3°F). This cooling effect contributes to the suppression of the fire by both suffocating the flames and reducing the surrounding temperatures.

As CO₂ expands from its liquid state to a gas, it absorbs heat from the environment due to the Joule-Thomson effect, which results in the gas being very cold. This cooling effect is crucial for fire suppression, as it lowers the temperature around the fire, which further helps to inhibit the combustion process. The extreme cold can also help to freeze the fire or hot surfaces, creating a barrier to further combustion. The cooling of the surrounding materials and the fire itself further reduces the chance of re-ignition, especially in situations where the fire is fueled by flammable liquids or volatile chemicals. In addition to smothering the fire by displacing oxygen, the cooling effect helps to stabilize the environment and prevent the fire from spreading.

The primary method by which CO₂ extinguishes fire is by displacing oxygen from the fire's environment. Fires require three key elements to continue burning—fuel, heat, and oxygen—collectively known as the "fire triangle." By reducing the oxygen concentration around the fire, CO₂ directly disrupts the chemical reaction that sustains the fire. CO₂ is a heavier-than-air gas, meaning it tends to settle near the base of the fire, where it can effectively cut off the oxygen supply. This suffocation process is quick and efficient, as CO₂ works by reducing oxygen levels to below what is required for combustion, typically below 15%. Once the oxygen levels drop, the fire is extinguished. CO₂ is particularly effective in suppressing fires involving electrical equipment or flammable liquids, as it does not introduce any conductive elements (like water) that could cause short-circuiting or spread the fire.