Refrigeration Cycle and System Components
Four components, two diagnostic measurements
The vapor-compression cycle: the compressor raises pressure and temperature, the condenser rejects heat and subcools the liquid, the metering device drops pressure and temperature, and the evaporator absorbs heat, producing superheated vapor. Saturation temperature tracks pressure; a pressure-temperature chart converts one to the other. For zeotropic blends such as R-407C you must use bubble and dew points rather than a single saturation value.
Superheat equals suction line temperature minus evaporator saturation temperature; it proves the evaporator is properly fed. Subcooling equals condensing saturation temperature minus liquid line temperature; it proves the condenser has fully condensed the refrigerant and is the charging indicator on a TXV system. Manufacturer targets are typically in the 8 to 12 F range for each at design conditions.
Traps: measuring superheat at the compressor, which includes suction line heat gain; charging a TXV system by superheat; assuming a frosted coil means low charge when it usually means low airflow or a restriction; forgetting that fixed-orifice systems are charged by superheat while TXV systems are charged by subcooling.
Worked example: R-410A with 118 psig suction corresponds to about 40 F saturation. A suction line at 50 F gives 10 F superheat, in range. If the line reads 60 F, superheat is 20 F and you should look for low charge, a restriction, or insufficient indoor airflow.