Superheat
Also known as: superheated vapour, suction superheat
Short definitionHow many kelvin the vapour leaving the evaporator is warmer than the evaporating temperature at that pressure. Normal is 5–10 K, and it is the measurement that says most about the health of the system.
While the refrigerant evaporates in the evaporator, its temperature stays constant. Once all the liquid has turned to vapour, it keeps warming along the pipe; these extra degrees are the superheat. If the suction pressure corresponds to a saturation temperature of −10 °C and −3 °C is measured on the pipe, the superheat is 7 K. Measuring it takes a gauge on the suction side and a temperature probe that sits well on the pipe. For zeotropic blends the dew column of the table is read.
It is the first value checked at commissioning. Superheat close to zero shows that the evaporator is flooded and liquid may reach the compressor. A value above 15 K says the evaporator is starved: the charge is short, the expansion valve is throttled or the filter is blocked; capacity drops and the compressor runs hot. A thermostatic valve is set to 5–8 K, and an electronic valve can hold 3–5 K.