The Professor: Compressor Overheating

April 5, 2010
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My column in the March 8 issue of The NEWS dealt mainly with reciprocating compressors experiencing low head pressure and high suction pressure from bad valves. The article pointed out reasons why a compressor’s valves may become inefficient from being warped or having carbon and/or sludge deposits on them, preventing them from sealing.

Listed below are those reasons:

• Compressor overheating from low suction or high head pressures (high compression ratios);

• Slugging of refrigerant and/or oil;

• Moisture and heat causing sludging problems;

• Refrigerant migration problems;

• Refrigerant flooding problems;

• Acids and/or sludge in the system deteriorating parts;

• TXV set wrong - too much superheat causing compressor overheating;

• Undercharge of refrigerant causing compressor overheating from high compressor superheat; and

• Lack of oil sealing the valves.

Compressor overheating is still today’s most serious and frequently occurring field problem a service technician will face. In fact, the first bullet point above mentions high compression ratio. For this column, I want to take a more in-depth look at high compression ratios that cause compressor overheating along with high compressor discharge temperatures.

Figure 1.

HIGH COMPRESSION RATIOS

High compression ratios are caused from:

• High head pressures;

• Low suction pressures; and

• A combination of both.

High compression ratios will cause high compressor discharge temperatures. The compressor’s discharge temperature can tell the service technician what is going on inside a refrigeration or air conditioning system. The compressor’s discharge temperature is a reflection of the hottest part of a refrigeration system, and there are limits as to how hot a discharge temperature should be.

The compressor’s discharge temperature can be measured by placing an insulated thermistor or thermocouple on the discharge line about 3-4 inches from the compressor (Figure 1). 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 about 350°. If this occurs, serious overheating problems will happen. And, remember, compressor overheating problems are today’s most serious compressor field problems. Service technicians must always monitor compressor discharge temperatures and keep them under 225°.

CAUSES

Some main reasons for high compressor discharge temperatures are:

• High condensing pressures;

• Low suction pressures;

• High compressor superheats; and

• High compression ratios.

Below are some of the causes:

Causes for High Condensing Pressures:

• Dirty condenser coils;

• Burned-out condenser fans;

• Broken fan belts;

• Undersized condenser coils;

• Overcharge of refrigerant;

• Noncondensables in the system;

• High ambient temperature; and

• 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.

Causes for Low Suction Pressure:

• 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; and

• 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.

Causes for High Compression Ratios:

• Low suction pressures;

• High head pressures; and

• A 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.

Causes for High Compressor Superheats:

High compressor superheats can be caused from the evaporator being starved of refrigerant or a suction line seeing too much heat. Listed below are causes for high compressor superheats:

• A restricted liquid line;

• Undercharge;

• Plugged filter drier;

• Kinked liquid line;

• TXV or capillary tube underfeeding;

• Suction line too long; and

• Uninsulated suction line.

In conclusion, compressor discharge temperatures reflect all of the latent heat absorbed in the evaporator, the evaporator superheat, all of the suction line superheat, 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. A high compressor discharge temperature usually means an overheated compressor.

Publication date: 04/05/2010

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