Superheating is a technical expression for a refrigerant in gas phase at a temperature higher than the corresponding evaporation temperature related to the actual pressure.
In a refrigeration plant, superheating is a term used on the evaporator and on the discharge line from the compressor.
All refrigerants have a liquid, evaporation and gas phase like water at 1 bar: liquid below 100 °C, evaporation at 100°C, and gas (steam) above 100 °C.
The boiling temperature for a refrigerant is pressure dependent, where at a curtain pressure relates to a specific boiling temperature. For example, R717 ammonia at 2.9 bar boils at -10°C, whereas at 11.7 bar, it boils at 30 °C. Ammonia at 11.7 bar is seen as superheated at 35 °C, which is 5 °C above its boiling temperature of 30°C.
In the compressor, the refrigerant is superheated by absorbing the energy input necessary to compress the refrigerant from suction to discharge pressure.
In an evaporator, the refrigerant evaporation at a given temperature causes the refrigerant to be superheated to a temperature close to the cooled media.
Superheating in the evaporator in a refrigeration plant with direct expansion is necessary to avoid the liquid reaching the compressor. This will trigger breakdowns.