The compressor’s discharge temperature can be measured by placing an insulated thermistor or thermocouple on the discharge line about 3 to 4 inches from the compressor. The discharge temperature is a measure of the superheated refrigerant’s vapor temperature. Remember, superheated refrigerant refers to a refrigerant vapor that is at a higher temperature than its saturation temperature for a certain pressure. The certain pressure would be the condensing pressure for the high side of the system in this case.
Since the compressor’s discharge temperature is a superheated vapor temperature measurement, a pressure-temperature relationship does not exist, and a pressure gauge cannot be used for its measurement. Pressure gauges can only be used for a pressure-temperature relationship when a saturation temperature (evaporating and/or condensing) is wanted.
Since the compressor’s discharge temperature is a reflection of what is going on inside the compressor, it must be monitored very closely. The back of the compressor’s discharge valve is actually the hottest part of the system, but it is impossible to measure by a service technician. The next closest place, however, is the discharge line of the compressor.
The limit to any compressor discharge temperature is 225°F. If the discharge temperature gets higher than 225°, the system may start to fail from worn rings, acid formations, and oil breakdown. Remember, if the discharge temperature is 225°, the actual discharge valve will be about 75° hotter. This will bring the actual compressor’s discharge valve to 300°. It is a known fact that most oil may start to break down and vaporize at 350°. If this occurs, serious overheating problems will happen. And, since compressor overheating problems are today’s most serious compressor field problems, service technicians must always monitor compressor discharge temperatures and keep them under 225°.
Some main reasons for high compressor discharge temperatures are:
• High condensing pressures;
• Low suction pressures;
• High compression ratios;
• High compressor superheats.
Causes for the above four reasons follow.
HIGH CONDENSING PRESSURESHigh condensing pressures may be due to:
• Dirty condenser coils;
• Burned-out condenser fans;
• Broken fan belts;
• Undersized condenser coils;
• Overcharge of refrigerant;
• Noncondensables in the system;
• High ambient temperature;
• Recirculated air over the condenser.
A high condensing temperature causes a high condensing pressure. Now the compressor must put more work, thus generate more heat of compression, in compressing the suction pressure to the higher condensing pressures.
LOW SUCTION PRESSURELow suction pressures can be caused by:
• Undercharged systems;
• TXV or capillary tubes underfeeding;
• Low evaporator heat loads;
• End of the cycle;
• Frosted evaporator coils;
• Evaporator fan out;
• Kinked suction lines;
• Plugged suction line;
• Liquid line filters;
• Kinked liquid lines;
• Plugged compressor inlet screens.
Again, more work, thus more heat of compression, will be generated in compressing a lower suction pressure to the condensing pressure.
HIGH COMPRESSION RATIOSHigh compression ratios can be caused by:
• Low suction pressures;
• High head pressures;
• Combination of both low suction and high head pressure.
The higher the compression ratio, the higher the compressor’s discharge temperature will be. This happens because more heat of compression will be generated when compressing the gases through a greater pressure range.
HIGH COMPRESSOR SUPERHEATSHigh compressor superheats can be caused by the evaporator being starved of refrigerant. This can happen with a restricted liquid line, undercharge, plugged filter drier, kinked liquid line, or TXV or capillary tube underfeeding.
In conclusion, compressor discharge temperatures reflect all of the latent heat absorbed in the evaporator, the evaporator superheat, all of the suction line superheat, and all of the heat of compression and motor generated heat at the compressor. It is at the discharge temperature where all of this heat is accumulated and now must start to be rejected in the discharge line and condenser.