Superheating Suction Vapor
When all of the liquid refrigerant in the evaporator circuit has boiled off only vapor remains in the circuits. At this point the remaining vapor is still very cold. As the cold vapor continues to travel through the evaporative coil, it continues to pick up heat from the warm indoor air passing across the coil. Because there is no liquid left to boil, the refrigerant vapor temperature begins to rise above the saturation temperature. Once the refrigerant vapor temperature rises above saturation temperature, it now becomes Superheated. The heat added to the refrigerant vapor after all of the liquid has boiled off is called Suction Vapor Superheat.In this example, we have 44F saturated refrigerant entering the evaporative circuit. We know this temperature because, using our gauges, we have converted our suction pressure of 130 PSIG (R-410A) or ---PSIG (R-22) . A digital clamp on temperature probe is then placed at the suction line service gauge port, it reads 54F. The temperature of the suction line indicating that we have heated up the refrigerant vapor by 10 degrees, therefore the superheat level is 10F.
Since the changing of state (which consumes massive amounts of heat energy) has ceased, the cooling process of the now cool vapor drops off dramatically. The cool vapor’s temperature rises very rapidly at this point. Although superheating of the refrigerant vapor at the evaporator coil only provides minimal cooling, it is an important part of the refrigeration cycle. Superheating the refrigerant vapor ensures that no liquid is present on the compressor. As a compressor cannot compress a liquid, the superheating of the refrigerant vapor protect the compressor from damage.
Steps to Measuring Superheat
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Attach your low side (vapor side) refrigerant gage on the vapor line service port at the condenser coil. You can identify the vapor line because it is the larger of the two refrigerant lines.
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Place a clamp on digital temperature probe near the inlet to the condenser coil. Somewhere near the service port.
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Read and record the pressure and corresponding temperature from your low side gauge. In this example, 130 PSIG for R-410A and 74.5 PSIG for R-22, both of these temperatures correspond to 44F.
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Read and record the temperature from you digital temperature probe. In this example, 54F.
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Subtracting the digital temperature from the temperature record from your gauges shows that the refrigerant leaving the coil is 10F warmer than the saturation temperature.
54F – 44F = 10F. This example shows 10 degrees of superheating.