The compressor’s discharge line temperature is often an overlooked temperature when troubleshooting a refrigeration or air conditioning system. However, it is a very important temperature because it is an indication of the amount of heat absorbed in the evaporator and suction line, and any heat of compression generated by the compression process.
Since the compressor’s discharge line temperature is superheated, a pressure/temperature relationship does not exist and it must be read directly on the discharge line by some sort of temperature measuring device. (Figure 1 shows a thermistor. Figure 2 shows an insulated thermistor. Make sure you insulate the thermistor so you get a true discharge temperature.) The compressor’s discharge temperature should be taken about 1-3 inches away from the compressor on the discharge line. This discharge temperature should never exceed 225?F. If the compressor’s discharge temperature is 225?, the discharge valve’s back, located on the valve plate in the compressor’s head, will read 50-75? hotter (Figure 3). With a 225? discharge line temperature, the actual discharge valve’s back can reach temperatures of 275-300?. Carbonization and oil breakdown can occur near the compressor’s valve plate at these temperatures.