Cooling Equipment, Refrigeration Cycle, and Refrigerant Handling (EPA Section 608)
The cycle
Compressor raises refrigerant pressure and temperature; the condenser rejects heat and subcools the liquid; the metering device - TXV, EEV, or fixed piston - drops pressure into the evaporator, where refrigerant boils and absorbs heat, leaving superheated vapor for the compressor. Measure, do not guess: superheat is suction line temperature minus saturated suction temperature; subcooling is saturated liquid temperature minus liquid line temperature.
Charging and diagnosis
Fixed-orifice systems are charged by superheat to the manufacturer target, generally 8 to 12 degrees at the evaporator with indoor conditions above 65 degrees wet bulb. TXV systems are charged by subcooling, typically 8 to 12 degrees. Airflow first: low airflow drops suction pressure and makes charge readings misleading.
EPA Section 608
Technicians must be certified (Type I, II, III, or Universal). Venting refrigerant is prohibited; recover into a DOT cylinder, never fill above 80 percent by weight, and evacuate to the level the rule specifies before opening a system. Appliances with 50 pounds or more of charge carry leak-repair duties - repair within 30 days, verify, and keep records three years. R-410A is a 50/50 blend of R-32 and R-125, is not a drop-in for R-22, uses POE oil, and runs far higher pressures. A2L refrigerants such as R-454B add charge limits and ignition-source rules.
Worked example
R-410A system: suction 118 psig (40 degrees saturated) with a 52 degree suction line gives 12 degrees superheat, on target for a fixed orifice. Liquid line 320 psig (100 degrees) with an 88 degree liquid line gives 12 degrees subcooling.
Traps
Charging by gauge pressure alone; adding refrigerant to fix an airflow problem; venting during repair; brazing without nitrogen; mixing refrigerants in a recovery cylinder.