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 the 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.
by Ron Walker