The refrigerant in the evaporator must be colder than the room to be able to draw heat from the room air. If the room is at 0 °C the refrigerant evaporates between −6 and −10 °C; in a −20 °C frozen room between −28 and −32 °C; in blast freezing down to −40 °C. This temperature is set by the suction pressure and is referred to as the "evaporating temperature" or simply "evaporation".
Catalog operating conditions are written in terms of this value: condensing unit capacity is given for a condition such as "−10 °C evaporating, 32 °C ambient". When the evaporating temperature falls by 1 K, compressor capacity falls by about 3% and electricity consumption rises by 2–3%; a system running at −15 instead of −10 °C does the same job with 15% less capacity and at a higher cost. That is why the evaporator must not be undersized, and icing and fouling must be prevented.
Applications are classified by evaporating temperature: high temperature (+5 °C and above: air conditioning, chillers), medium temperature (−5 to −15 °C: cold rooms, cabinets), low temperature (−25 to −40 °C: frozen storage, blast freezing). Compressors and refrigerants are selected by these classes. The evaporator and condensing unit tables in our catalog show capacity at different evaporating temperatures.