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Compressor Superheat:
Compressor superheat is a critical value that must be checked. Check the compressor
superheat as follows:
1. Determine the suction pressure at the suction service valve of the compressor.
2. Determine the saturation temperature at the observed suction pressure using
refrigeration pressure temperature tables.
3. Measure the suction line temperature 6 -10 inches away from the compressor.
4. Subtract the saturation temperature (step 2) from the measured temperature
(step 3). The difference is the superheat of suction gas.
A low suction superheat can cause liquid to return to the compressor. This will cause
dilution of the oil and eventual failure of the bearings, rings and valves. A high suction
superheat will cause excessive discharge temperatures, which cause a breakdown of
the oil. This causes piston ring wear, and piston and cylinder wall damage. System
capacity decreases as the suction superheat increases. For maximum system capacity,
keep the suction superheat as low as practical. Copeland requires a minimum
compressor superheat of 20°F; however, to improve compressor life, 25°F to 40°F is
preferred. Adjust the expansion valve at the evaporator when adjustments to the
suction superheat are necessary. Refer to “Evaporator Superheat” on the next 2 pages
for more information.
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