Refrigeration / Compressors / Refrigerants & Reclaim

Ice Breaker: What Frosted Suction Lines Don't Tell You

June 6, 2011

What does frost on a suction line indicate about a refrigeration system? Does it indicate a properly working system? Does it indicate a shortage or overcharge of refrigerant? Does it indicate another system problem?

Actually, frost on the suction line only indicates that at the location where the frost is present, the suction line piping is below the dew point temperature of the surrounding ambient air and at or below 32°F. That’s it. It is neither an indication of a properly operating system nor a system defect. Although it is normal for many refrigeration systems to have frost on the suction line, it is not a very accurate indication of a properly operating system.

The temperature of the suction line can be an indication of how the system is operating. However, this measurement must also be taken with the pressure of the refrigerant and the refrigerant’s superheat value calculated. It is the refrigerant’s superheat value that is a useful measurement when analyzing the operation of a system. While troubleshooting a system, technicians normally measure the refrigerant superheat value at two locations along the suction line.

The first location is at the outlet of the evaporator, typically referred to as the “evaporator’s superheat.” Measuring the evaporator’s superheat value is an important part of analyzing a system’s performance. If a lower than normal value is measured, too much refrigerant is entering the evaporator for the present heat load. Many technicians refer to this as a “flooded evaporator.” If a higher than normal value is measured, too little refrigerant is entering the evaporator for the present heat load. Technicians generally refer to this as a “starved evaporator.”

What is the correct superheat value of the refrigerant leaving an evaporator? It is based on the system’s application and the manufacturer’s design. Always refer to the manufacturer’s specifications for the specific design superheat value. However, as a rule of thumb, many medium temperature systems will generally have a superheat of 8 to 10 degrees and many low temperature applications of 4 to 6 degrees.

The second typical location to measure the refrigerant’s superheat is at the inlet of the compressor, commonly referred to as the system’s “total superheat.” Most compressor manufacturers recommend this measurement be taken approximately 6 inches from the suction service valve. This can be an important measurement for compressors as it ensures that the returning refrigerant will not cause damage to the compressor. A saturated refrigerant (0° superheat) or excessively high superheat value may cause damage to a compressor.

A saturated refrigerant at this location indicates that some amount of liquid refrigerant is re- turning to the compressor. The amount of liquid refrigerant returning is unknown; however, any amount is too much. Liquid refrigerant returning to a compressor can damage it in one of two ways:

• First, it can mix with the compressor’s lubricating oil. As the liquid refrigerant comes in contact with the bearing surfaces it will wash away the oil film and cause wearing of the bearings. Depending on the percentage of liquid refrigerant to oil, the bearing wear could be mild to severe.

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

While troubleshooting refrigeration systems, technicians should measure both the temperature and pressure and calculate the refrigerant’s superheat value to help determine the true cause of a system problem. Just looking at the suction line temperature or pressure only is not an accurate means of determining the true cause of a system problem.

Publication date: 06/06/2011

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