Refrigeration Principles and Refrigerant Handling
The Cycle, Superheat, and Subcooling
The vapor-compression cycle has four processes: compression, condensation, expansion, and evaporation. Every refrigerant has a saturation pressure-temperature relationship, and that relationship drives diagnostics. Superheat is measured suction line temperature minus the saturation temperature at suction pressure; subcooling is the saturation temperature at liquid-line pressure minus the measured liquid-line temperature. Fixed-orifice systems are charged to a target superheat that changes with indoor wet bulb and outdoor dry bulb, while TXV systems are charged to subcooling, commonly 8 to 12 degrees Fahrenheit. Low superheat with a flooded coil risks liquid slugging; high superheat suggests undercharge, restriction, or low load.
R-410A operates at roughly 50 to 70 percent higher pressures than R-22 and uses POE oil, which absorbs moisture readily, so system openings must be minimized and evacuation must be thorough. Newer A2L refrigerants such as R-32 and R-454B are mildly flammable and require attention to ignition sources, ventilation, and charge limits. Under EPA Section 608, technicians must be certified, intentional venting is prohibited, recovery cylinders may be filled to no more than 80 percent by weight, and leak repair and recordkeeping requirements apply to larger charges. Use nitrogen, never oxygen or compressed air, for pressure testing, and always use a scale.
Worked example: R-410A suction pressure of 118 psig corresponds to about 40 degrees Fahrenheit saturation. A measured suction line temperature of 52 degrees gives 12 degrees of superheat.