Evaporating temperature
Also known as: evaporation temperature, saturated suction temperature
Short definitionThe temperature at which the refrigerant boils inside the evaporator. It is lower than room temperature by the TD; this point sets compressor capacity, efficiency and evaporator selection.
To pull heat out of the room, the refrigerant has to be colder than the room air. In a 0 °C room it evaporates between −6 and −10 °C, in a −20 °C frozen room between −28 and −32 °C; in blast freezing it goes down to −40 °C. Suction pressure sets the value. Applications are classed by it: +5 °C and above is high temperature, −5 to −15 °C medium and −25 to −40 °C low.
In unit catalogues capacity is always stated for an evaporating temperature and an ambient condition; a capacity read from the wrong row in a quote will not cool the room. When evaporation drops by 1 K, compressor capacity falls by about 3% and consumption rises by 2–3%. An undersized, iced or dirty evaporator forces the system to run at a lower evaporating temperature. The same job is done with less capacity and at a higher cost.