Refrigeration Theory, Refrigerants, and System Components
The vapor-compression cycle
Every system is four components in a loop: evaporator, compressor, condenser, and metering device. Refrigerant boils in the evaporator at low pressure, absorbing heat; the compressor raises pressure and temperature; the condenser rejects heat and condenses the refrigerant; the metering device drops pressure back to evaporator conditions. The entire diagnostic logic of the trade rests on the pressure-temperature relationship: at a given pressure, a saturated refrigerant has one known temperature.
Superheat and subcooling
Superheat is the difference between the actual suction line temperature and the saturation temperature at suction pressure; it proves the evaporator is fully fed with liquid boiling away. Subcooling is the saturation temperature at the liquid line pressure minus the actual liquid line temperature; it proves the condenser has a solid liquid column at the metering device. Measure superheat at the evaporator outlet and subcooling at the condenser outlet. A common trap is trying to charge a TXV system by superheat alone, or reading temperature with an uninsulated probe in outdoor air.
Refrigerants in practice
R-410A is a near-azeotropic blend with slight glide and roughly 50 to 60 percent higher pressures than R-22; it uses POE oil, which absorbs moisture quickly, so systems must be evacuated properly and never left open. Blends must be charged as liquid to preserve composition. R-22 is phased out for new equipment, and lower-GWP refrigerants such as R-32 and R-454B are displacing R-410A. Venting refrigerant is prohibited; recovery, recycling, and reclamation are required under EPA Section 608.
Rule of thumb and worked example
One ton equals 12,000 Btu/h, and nominal airflow is about 400 cfm per ton. Sensible capacity equals 1.08 times cfm times temperature difference. At 400 cfm with a 20°F return-to-supply difference, sensible capacity is 1.08 x 400 x 20 = 8,640 Btu/h, about 0.72 ton of sensible capacity.