While sizing a new walk-in cooler, a technician must account for the motor heat load of the evaporator fan. If a 1/2 HP fan motor (mechanical output) has an efficiency of 75% and operates continuously, what is the heat contribution from the fan motor to the cooler in Watts? (1 HP = 746 W)

Question: While sizing a new walk-in cooler, a technician must account for the motor heat load of the evaporator fan. If a 1/2 HP fan motor (mechanical output) has an efficiency of 75% and operates continuously, what is the heat contribution from the fan motor to the cooler in Watts? (1 HP = 746 W)

Answer options:

• 124.3 W
• 186.5 W ✅ 248.7 W
• 373.0 W

Correct answer: 248.7 W

Explanation: The total electrical power input to the motor is 0.5 HP * 746 W/HP = 373 W. With an efficiency of 75%, the mechanical output is 373 W * 0.75 = 279.75 W. The heat contributed to the space is the difference between total power input and mechanical output (dissipated as heat) plus the mechanical work itself (also converts to heat within the space). However, for refrigeration load calculation, the entire electrical input to the fan motor is considered as heat to be removed from the refrigerated space, as the mechanical work is ultimately converted to heat through air movement and friction. So, 0.5 HP * 746 W/HP = 373 W. Let me re-read the explanation carefully. The heat generated by the motor (due to inefficiency) is part of the heat load. The mechanical work done by the fan also contributes to the heat load as it agitates the air, and this energy is dissipated as heat within the refrigerated space. Therefore, the entire electrical input to the fan motor should be counted as heat gain. The electrical input is 0.5 HP * 746 W/HP = 373 W. The closest option is 373W. I assumed the 248.7 W as 'heat contributed from the motor due to inefficiency, then add the output converted to heat'. This is a common point of confusion. For refrigeration loads, the entire energy input to the fan motor is considered a heat load. Therefore, 0.5 HP × 746 W/HP = 373 W.