Polyolester (POE) lubricant will start to decompose at approximately 400°F, where mineral oil will start decomposition at 350°F. Decomposition of oil simply means that the oil molecules will start to combine into larger molecules. The oil first appears darker than usual and then starts to form sludge before finally turning to a powder. This process is often referred to as polymerization. Sludge and powders from decomposed oil will plug oil galleys and screens within a refrigeration system, and ultimately, there will be loss of lubrication and bearing failure.

TXVs, control valves, and the piping itself can become clogged with sludge over time. It is for these reasons a service technician should always take the compressor’s discharge temperature when servicing and troubleshooting. The compressor’s discharge temperature is a reflection of what is going on inside the compressor, and because of this, 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 for a service technician to measure. The next closest place is the discharge line coming out of the compressor.

Measurements and Limits

The compressor’s discharge line temperature can tell the service technician what is going on inside a refrigeration or air conditioning system. It is a reflection of the hottest part of a refrigeration system, so there are limits as to how hot the discharge temperature should be. The compressor’s discharge line temperature can be measured by placing an insulated thermistor or thermocouple on the discharge line about 3 to 4 inches from the compressor. (See Figure 1 - top.)

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, and in this case, that would be the condensing pressure for the high side of the system. 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.

The limit to any compressor discharge line temperature is 225°F. If the discharge temperature gets higher than 225°F, the system may start to fail from worn rings, acid formations, and oil breakdown. Remember, if the discharge temperature is 225°F, the actual discharge valve’s back will be about 75°F hotter, which brings the actual compressor’s discharge valve to 300°F.

It is a known fact that some mineral oils may start to break down (decompose) at 350°F, and if this occurs, serious overheating problems will happen. Since compressor overheating problems are today’s most serious compressor field issue, service technicians must always monitor compressor discharge temperatures and keep them under 225°F.

The four main reasons for high compressor discharge temperatures are as follows:

• High condensing pressures;
• Low suction pressures;
• High compression ratios; and
• High compressor superheats.

System Pressures

Three of the main causes for high discharge temperatures are high head pressures, low suction pressure, and a combination of both and high compression ratios. Listed below are some causes for high head pressures:

• Dirty condenser coil;
• Air in the system;
• Too small of a condenser coil;
• Condenser fan gone bad;
• Overcharge of refrigerant;
• High ambient temperature;
• Recirculated air over condenser;
• Broken fan motor belt; and
• Non-condensables in the system.

Listed below are causes for low suction pressures:

• Iced or frosted evaporator coil;
• Dirty evaporator coil;
• Evaporator fan motor gone bad;
• End of the cycle;
• Kinked suction line;
• Defrosts set wrong;
• Defrost clock bad;
• Defrost heater bad;
• Pressure drop in the suction line;
• Dirty liquid line filter drier;
• Kinked liquid line;
• Dirty suction line filter or drier;
• Undercharge;
• TXV set wrong (starving);
• Low evaporator heat loads; and
• Plugged compressor inlet screen.

Compressor overheating is also caused by high compression ratios, which are a combination of high condensing (head) pressures and low evaporating (suction) pressure. Compression ratio is simply the high-side absolute pressure divided by the low-side absolute pressure. If a system is running a head pressure of 235 psig and a suction pressure of 10 psig, the compression ratio would be:

(235 + 15) psia ÷ (10 + 15) psia, which equals 250 psia ÷ 25 psia, which equals 10 to 1 or 10:1

A compression ratio of 10:1 simply means that the high-side pressure is 10 times greater than the low-side pressure. The higher the compression ratio, the higher the heat of compression and the hotter the discharge temperature will be. The discharge temperature is the temperature measured coming out of the compressor on the discharge line. Once the discharge temperature reaches 250°F, the compressor is in danger of overheating the valve plate and valves. At 275°F, failure will occur in a matter of a short time.

Compressor Superheat

Another cause for high compressor discharge temperatures and compressor overheating is superheat that is too high coming back to the compressor. The higher the compressor superheat, the higher the compressor’s discharge temperature. Take the temperature of the suction line entering the compressor and the suction pressure at that point and convert it to a saturation temperature. The difference between the two will be the compressor superheat. Compressor superheat is often referred to as total superheat because it consists of suction line and evaporator superheat.

Refrigerant-cooled compressors rely on their return gas to be cool enough to cool their motor windings and cylinder walls. Compressors should have enough compressor superheat to ensure that when the TXV hunts, there will not be liquid coming back to the compressor. However, some compressor manufacturers require that the compressor return gas temperature should not be hotter than 65°F or the gas will not be dense enough for motor winding and cylinder cooling. Always consult with the compressor manufacturer for maximum return gas temperatures.

High compressor superheats can be caused from the evaporator and/or compressor 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 then must start to be rejected in the discharge line and condenser. It is of utmost importance for the service technician to measure this temperature when servicing and troubleshooting a refrigeration or air conditioning system.