Under which arrangement of evaporator and condenser temperatures would the BTU transfer be the greatest?

• A. High evaporator temperature, high condenser temperature
• B. Low evaporator temperature, low condenser temperature
• C. High evaporator temperature, low condenser temperature
• D. Low evaporator temperature, high condenser temperature

Answer: (D)

Heat transfer in a refrigeration cycle is driven by the temperature difference between the cold side (evaporator) and the hot side (condenser). The larger that difference, the stronger the driving force for heat flow, so more heat can be moved per unit time.

If the evaporator is very cold and the condenser is very hot, you create the largest temperature lift in the cycle. For the same refrigerant flow and heat exchanger size, this means more heat can be absorbed from the space in the evaporator and more heat can be rejected to the surroundings in the condenser, increasing the BTU transfer rate.

If the evaporator and condenser temperatures are close, the driving force is small and less heat is moved. If the evaporator is high while the condenser is low, the lift is reduced, and so is the heat transfer. Therefore, the arrangement with a low evaporator temperature and a high condenser temperature yields the greatest BTU transfer. Note that this also increases the compressor work required, but the question focuses on the transfer rate.