John Tomczyk

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.

Causes for high compressor discharge temperatures can be high condensing temperature, low evaporator temperatures, high compression ratios, and high compressor superheat. We will look at each in detail.

High Condensing Temp

Many causes can be associated with high compressor discharge temperatures. A high condensing temperature is one of them. When the condensing temperature is high, the compressor must compress the refrigerant from the low-side (evaporating) pressure to an elevated high-side (condensing) pressure. This added work of compression done by the compressor will make the heat of compression higher. Thus, the compressor’s discharge temperature will be higher.

Remember, condensing temperatures occur when the refrigerant is changing from a vapor to a liquid in the condenser. There is a pressure/temperature relationship with the condensing temperature because of the phase change. A gauge reading on the high side of the system is all that is needed to find the condensing temperature. Convert this pressure to a temperature using a pressure/temperature chart, and this will be the condensing temperature.

However, there are many causes for high condensing temperatures, which will also be causes for high discharge temperatures and high discharge-valve temperatures. Here are some of the causes:

• Dirty condenser;

• High ambient temperature;

• Noncondensable (air) in the system;

• Condenser fan out;

• Restricted airflow over condenser;

• Refrigerant overcharge;

• Wrong refrigerant;

• High heat load on evaporator; and

• Recirculated air over the condenser.

Low Evaporator Pressures and Temperatures

Low evaporator pressures will also cause a high compressor discharge temperature, causing high discharge valve temperatures. When evaporator pressures are low, the compressor must compress refrigerant from this lower evaporator pressure to the condensing temperature. This added work of compression done by the compressor will make the heat of compression be higher. Thus, the compressor’s discharge temperature will be higher.

Remember, evaporator temperatures are defined when the refrigerant is changing from a liquid to a vapor in the evaporator. There is a pressure/temperature relationship with the evaporating temperature because of the phase change. A gauge reading on the low side of the refrigeration system is all that is needed to find the evaporating temperature. Convert this pressure to a temperature using a pressure/temperature chart. This will be the evaporating temperature. However, there are many causes for low evaporating pressures and temperatures, which will also be causes for high compressor discharge temperatures, since low evaporating pressures cause high compressor discharge temperatures. Listed below are causes for low evaporator pressures:

• Dirty evaporator coil;

• Iced-up evaporator coil;

• Evaporator fan motor out;

• Shortage of airflow over the evaporator;

• Frosted evaporator coil from high humidity;

• Frosted evaporator coil from a bad defrost heater or other defrost component malfunction;

• Low heat load on the evaporator coil;

• Defrost intervals set too far apart on the time clock;

• Undercharge of refrigerant;

• End of the running cycle;

• Partially plugged filter drier;

• Compressor inlet screen partially plugged;

• Restricted liquid line;

• Wrong refrigerant; and

• Metering device starving.

High Compression Ratios

High compression ratios are a result of high condensing pressures or low evaporator pressures, or both. So, any time there are high condensing pressures or low evaporator pressures, or both, there will be high compression ratios. And, anytime there is a high compression ratio either from high condensing or low evaporator pressures, or both, there will be more work added to the compression stroke of the compressor. This will cause the heat of compression to increase, and the compressor will have a higher compressor discharge temperature and discharge valve temperature.

Compression ratio is defined as the absolute discharge pressure divided by the absolute suction pressure. Discharge pressure and condensing pressure are one and the same and will be used interchangeably throughout this article. The same holds true for suction pressure and evaporating pressure.

Compression ratio = Absolute discharge pressure ÷ Absolute suction pressure

A compression ratio of 8:1 simply means the discharge pressure is eight times the magnitude of the suction pressure.

Again, a compression ratio of 12.3:1 simply indicates to the technician that the absolute, or true, discharge pressure is 12.3 times as great as the absolute suction pressure.

The volumetric efficiency is expressed as a percentage from 0-100 percent. Volumetric efficiency is defined as the ratio of the actual volume of the refrigerant gas pumped by the compressor to the volume displaced by the compressor pistons. A high volumetric efficiency means that more of the piston’s cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases. The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder each down-stroke of the piston, and thus more refrigerant will be circulated with each revolution of the crankshaft. The system will now have better capacity and a higher efficiency. So, the lower the discharge pressure, the less re-expansion of discharge gases to suction pressure. Also, the higher the suction pressure, the less re-expansion of discharge gases because of the discharge gases experiencing less re-expansion to the higher suction pressure and the suction valve will open sooner.

Volumetric Efficiency

The compressor’s volumetric efficiency depends mainly on system pressures. In fact, the farther apart the discharge pressure’s magnitude is from the suction pressure’s magnitude, the lower the volumetric efficiency because of more re-expansion of discharge gases to the suction pressure before the suction valve opens. Compression ratio is the ratio that measures how many times greater the discharge pressure is than the suction pressure, in other words, their relative magnitudes. Remember, a compression ratio of 10:1 indicates that the discharge pressure is 10 times as great as the suction pressure, and a certain amount of re-expansion of the vapors will occur in the cylinder before new suction gases will enter. This is why lower compression ratios will cause higher volumetric efficiencies and lower discharge temperatures. So, keep those compression ratios as low as possible by keeping condensing pressures low and evaporator pressures high, or both.

High Compressor Superheats

Compressor superheat is all of the superheat the compressor sees. It consists of evaporator superheat and suction line superheat. High compressor superheats cause the suction gases entering the compressor to be hotter and more expanded (less dense). The compressor will now compress these hotter suction gases and add even more superheat from the heat of compression. This will cause the compressor’s discharge temperature, thus valve temperature, to be hotter. Here are some causes for high compressor superheats:

• Metering device starving the evaporator;

• Restricted liquid line;

• Partially plugged filter drier;

• Undercharge of refrigerant;

• Long suction line;

• Uninsulated suction line; and

• Suction line ran in a hot environment (roof or suspended ceiling).

Publication date: 5/5/2014 

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