Servicing and maintaining refrigeration equipment often requires technicians to measure the pressures and temperatures, and calculate the condition of the refrigerant at several different locations throughout a system. One aspect that is sometimes overlooked is the superheat value of the refrigerant at the inlet of the compressor. This is commonly referred to as the system’s total superheat.

Most compressor manufacturers recommend that this measurement be taken approximately 6 inches from the suction service valve. This can be an important measurement for reciprocating compressors; it ensures that the returning refrigerant will not cause damage to the compressor.

Reciprocating compressor manufacturers normally state the accepted range of superheat values for refrigerant returning to their compressors. Superheat that is too low or too high may cause damage to a compressor.

A refrigerant with a 0°F superheat value (saturated refrigerant) at this location indicates that some amount of liquid refrigerant is returning to the compressor. In this condition, the amount of liquid refrigerant returning is unknown, but any amount is too much. Liquid refrigerant returning to a compressor can damage a compressor in one of two ways:

1. Liquid refrigerant can mix with the compressor’s lubricating oil. As the liquid refrigerant comes into contact with the bearing surfaces, it will wash away the oil film, causing bearing wear. Depending on the percentage of liquid refrigerant to oil, the wear could be mild to severe.

2. If a sufficient amount of refrigerant returns to the compressor, it may be possible for liquid to enter the compressor cylinder(s) and cause further damage to the compressor as it attempts to compress a liquid.

TOO LOW, TOO HIGH

Too low of a superheat is also a problem. Even though liquid refrigerant may not currently be returning to the compressor, it may be possible that during low load conditions the superheat value of the refrigerant will drop to 0° - then some amount of liquid refrigerant will return to the compressor. One compressor manufacturer recommends that the superheat value returning to its compressor be no less than 20°.

Too high of a superheat can cause the heat of compression to increase, causing the temperature at the discharge valves to increase. If the temperature increases beyond its safe operating temperature, it will cause damage to the compressor.

For a reciprocating compressor to operate properly, the temperature of the discharge valves should never rise above 300°. At temperatures between 300° and 320° inside the compressor, the oil will start to lose its ability to lubricate, causing premature wear of the compressor’s cylinder and piston rings. At temperatures above 350° the oil will start to break down, causing accelerated bearing wear.

When you are servicing or maintaining refrigeration systems, check the superheat of the refrigerant returning to the compressor and compare it to the specifications provided by the compressor manufacturer. This is one way you can make sure the compressor is working within safe operating conditions.

Publication date: 08/06/2007