What is Superheat – definition, function, setting and adjusting
Proper definition: The difference between the saturated temperature (pressure/temp chart) and the actual temperature of the suction gas.
Function: To protect the compressor! Other functions such as improving the efficiency of the evaporator are secondary. The two functions can become in conflict, however, protecting the compressor is the first priority of Superheat.
If Superheat is set to low: refrigerant may flood back, washing out oil. Leading to low oil pressure, bearing wiping and other damage may ensue. Refrigerant and or oil may slug back into the compressor resulting in broken valves, rods and other parts.
If Superheat is set to high: the compressor will get insufficient cooling. Oil breakdown and sludging, oil screen clogging, high ring wear and ultimate seizure.
Steps in setting Superheat:
1. With an accurate gauge, measure the pressure at the compressor suction service valve. Convert to temperature (°F) from the appropriate gas chart.
2. With an accurate thermometer measure the temperature of the suction line entering the compressor AT the compressor.
3. Subtract 1 from 2. The difference is Superheat in °F. Most manufacturers recommend setting at about 10-12 °F. It is essential to check the specific manufacturers recommendation.
4. Now, measure pressure at the evaporator (remote bulb location) and again convert to temperature to see if the evaporator is operating at design level. If it is, all is well in the circuit. If it is not, it may be necessary to adjust the thermostatic expansion valve. However, if this is done it is essential to remeasure at the compressor to be sure that Superheat setting is still within the manufacturers recommended range.
With multiple evaporators, the steps above must be repeated. Measure first at the compressor then at each successive evaporator, final measurement to be taken back at the compressor.