Question 25 QMED04 - Boiler Technician-Watertender

The Correct Answer is A **Why option A ("CO2") is correct:** Carbon Dioxide ($\text{CO}_2$) is a gaseous extinguishing agent that primarily works by smothering the fire, displacing oxygen below the concentration needed for combustion (usually around 15-16%). While it is very effective at quickly extinguishing surface fires, it has very poor cooling capabilities. Since $\text{CO}_2$ does not significantly cool the remaining fuel—which may still be well above its flashpoint or autoignition temperature—the introduction of fresh oxygen (e.g., from opening a door or restarting ventilation) immediately after the $\text{CO}_2$ concentration dissipates can quickly lead to re-ignition or "reflash." This makes $\text{CO}_2$ systems prone to reflash if the fuel source is still hot. **Why each of the other options is incorrect:** * **B) Water stream:** A heavy water stream is highly effective at cooling the fuel by absorbing large amounts of heat as the water turns to steam (latent heat of vaporization). The primary goal and mechanism of a water stream are to reduce the temperature of the fuel below its ignition temperature, making reflash very unlikely once sufficient cooling has occurred. * **C) Foam:** Firefighting foam works by creating a stable blanket over the fuel, separating the fuel from the oxygen. In addition to smothering, the water component of the foam provides substantial cooling. Because foam provides both separation and cooling, it is very effective at preventing reflash, particularly on flammable liquid spills. * **D) Water fog:** Water fog (or spray) uses finely atomized water droplets. It is highly effective at absorbing heat and cooling the combustion area, particularly in confined spaces, due to the greatly increased surface area of the water droplets. Like a water stream, its primary action is heat absorption and cooling, making it effective at preventing reflash.