Discharge temperature and condensing temperature are often confused. Condensing temperature is the saturation equivalent of the discharge pressure in the chart; for example 45 °C. Discharge temperature, by contrast, is the actual temperature of the gas leaving the compressor as measured with a thermometer, and because it carries the heat of compression and the motor heat it is far above this. In the same system the discharge pipe may be at 80–90 °C. In the condenser the gas first gives up this superheat and then begins to condense. The two describe different things and are measured separately.
The measurement is made 10–15 cm from the compressor outlet, with an insulated probe in good contact with the discharge pipe. The value read on the outside of the pipe is 15–25 K lower than the actual temperature at the valve plate. This is why the practical limit is taken as 105–110 °C on the pipe; when the temperature inside exceeds 130 °C the oil thins, then carbonises, carbon builds up on the valves and acid forms. Most compressors have a discharge temperature thermostat that stops them at this limit.
The factors that raise the discharge temperature: a high compression ratio (the combination of low suction pressure and high discharge pressure), high suction superheat, low charge, a dirty condenser, non-condensable gases, leaking valves. The refrigerant is also decisive: R448A and R449A discharge 10–15 K hotter than R404A, and R32 10–20 K hotter than R410A. Low-temperature applications, that is, evaporating temperatures of −30 °C and below, are the zone of greatest risk. A long, uninsulated suction line also produces the same result by increasing the superheat.
The measures are chosen according to the cause: cleaning the condenser, correcting the charge and the superheat, insulating the suction line. At the design level a compressor head fan, liquid injection, and compression with an economiser or in two stages are used. A discharge temperature lower than normal is also a warning: it shows that liquid is returning to the compressor. The discharge temperature is expected to be at least 20–30 K above the condensing temperature. For comparison you can take the saturation values from our refrigerant pressure–temperature chart. If the measured values are written on the maintenance form, a rise over time is easily seen.