To calculate superheat for a system using a low-GWP refrigerant, which method is correct?

• A. Read suction line temperature and compare to the saturated vapor temperature at the system pressure; subtract to obtain superheat per manufacturer spec.
• B. Read suction line temperature and subtract it from ambient temperature.
• C. Read liquid line temperature and subtract it from the suction line temperature.
• D. Compare the suction line temperature to the saturated liquid temperature.

Answer: (A)

Superheat is the amount by which the suction gas temperature exceeds the saturated vapor temperature at the evaporator (suction) pressure. To calculate it, read the suction line temperature, determine the saturated vapor temperature for the system’s suction pressure, and subtract that saturation temperature from the measured suction temperature. The result shows how much the refrigerant vapor has heated above its phase-change point, which is the essential measure used to assess proper evaporator operation and compressor protection. The other methods don’t reflect the refrigerant’s pressure-enthalpy state: ambient temperature has no relation to the refrigerant’s pressure, subtracting suction from liquid line temperature mixes phases and isn’t a correct superheat measure, and comparing suction temperature to the saturated liquid temperature would relate to subcooling rather than superheat.