The approach temperature for the evaporator is the difference between the refrigerant boiling temperature and which temperature?

The approach temperature for the evaporator is the difference between the refrigerant boiling temperature and which temperature?

• A. The difference in the suction and head pressure converted to temperature
• B. The difference in the refrigerant boiling temperature and the suction-line temperature
• C. The difference in the refrigerant boiling temperature and the inlet water temperature
• D. The difference in the refrigerant boiling temperature and the leaving-water temperature

Answer: (D)

In an evaporator, the key heat-transfer driver is how close the water can come to the refrigerant’s boiling temperature at the heat-transfer boundary. The approach temperature is the difference between the refrigerant’s boiling (evaporation) temperature and the leaving-water temperature. This leaves-water value reflects the hottest point of the water as it exits the exchanger, which effectively sets how near the two streams come to each other at the interface and thus how much driving force remains for heat transfer. The other measures—like converting suction and head pressures to a temperature, or using the inlet water temperature—do not represent the actual temperature difference at the heat-transfer boundary where the refrigerant is boiling and transferring heat to the water.

In an evaporator, the key heat-transfer driver is how close the water can come to the refrigerant’s boiling temperature at the heat-transfer boundary. The approach temperature is the difference between the refrigerant’s boiling (evaporation) temperature and the leaving-water temperature. This leaves-water value reflects the hottest point of the water as it exits the exchanger, which effectively sets how near the two streams come to each other at the interface and thus how much driving force remains for heat transfer.

The other measures—like converting suction and head pressures to a temperature, or using the inlet water temperature—do not represent the actual temperature difference at the heat-transfer boundary where the refrigerant is boiling and transferring heat to the water.