Which unit is used to express the Heat Release Rate (HRR) of a fire?

Which unit is used to express the Heat Release Rate (HRR) of a fire?

• A. Kilowatts (kW)
• B. Kilojoules/Gram (kJ/g)
• C. British Thermal Unit (Btu)
• D. Kilowatts per meter squared (kW/m2)

Answer: (A)

Heat release rate is the amount of heat a fire releases per unit time, i.e., the fire’s power. Since power is measured in kilowatts, expressing HRR in kilowatts makes the quantity directly describe how rapidly energy is being liberated. The other options don’t fit because they describe different things: kilojoules per gram is energy per mass (not a rate); the British thermal unit is a unit of energy (not rate); and kilowatts per meter squared is heat flux—heat transfer per area—which is a rate per area, not the total rate of heat release. Remember, if you were looking at heat release per area instead of total HRR, you’d use kW/m^2, but for total HRR the unit is kW.

Heat release rate is the amount of heat a fire releases per unit time, i.e., the fire’s power. Since power is measured in kilowatts, expressing HRR in kilowatts makes the quantity directly describe how rapidly energy is being liberated. The other options don’t fit because they describe different things: kilojoules per gram is energy per mass (not a rate); the British thermal unit is a unit of energy (not rate); and kilowatts per meter squared is heat flux—heat transfer per area—which is a rate per area, not the total rate of heat release. Remember, if you were looking at heat release per area instead of total HRR, you’d use kW/m^2, but for total HRR the unit is kW.