How far a vapour's temperature is above its saturation temperature at the same pressure.
Superheat is the temperature of refrigerant vapour minus the saturation temperature that matches its pressure. If the suction line reads 12 °C and the suction pressure corresponds to 4 °C saturation, the vapour has 8 K (8 degrees Celsius) of superheat.
It tells you whether the evaporator has finished boiling off the liquid it was fed. High superheat means the coil is starved: not enough refrigerant for the heat load. Low superheat means liquid is getting close to the end of the coil and could reach the compressor. Evaporator superheat is measured near the coil outlet (where a TXV bulb sits); total superheat is measured near the compressor.
On fixed-orifice systems superheat is the usual charging check, read against the manufacturer's chart. On TXV systems the valve holds superheat roughly steady, so the charge is checked with subcooling instead.
Exam work areas
E · Commissioning and start-up F · Maintenance, service and troubleshooting