Discharge temperature
Also known as: hot gas temperature, compressor outlet temperature
Short definitionThe temperature of the compressed gas as it leaves the compressor. Being superheated, it is well above the condensing temperature; 60–110 °C is read on the pipe, and beyond the limit the oil breaks down and the compressor is damaged.
It should not be confused with the condensing temperature read from the gauge. With condensing at 45 °C, a thermometer fixed to the discharge pipe of the same circuit can show 80–90 °C, because the gas also carries the heat of compression and of the motor. The probe goes 10–15 cm beyond the compressor outlet, clamped tightly to the pipe and insulated. The valve plate inside is 15–25 K hotter than the pipe, so 105–110 °C on the pipe is taken as the upper limit.
Freezer room projects carry the most risk, because low suction with high discharge pressure raises the compression ratio. With R449A and R448A the discharge runs 10–15 K higher than in an R404A system. In design, a head cooling fan, liquid injection or an economiser is considered and the suction line is insulated. On site, look for a dirty condenser, undercharge and high superheat. If discharge is not 20–30 K above condensing, liquid may be returning to the compressor.