Refrigeration Cycle, Components, and Refrigerants
Four processes, two saturated points
The vapor-compression cycle has four steps: compression (low-pressure vapor to high-pressure vapor), condensation (rejecting heat at saturated condensing temperature), expansion through a metering device, and evaporation (absorbing heat at saturated evaporating temperature). The compressor raises pressure, not temperature directly - temperature follows the refrigerant's pressure-temperature relationship.
Two measurements diagnose a charge. Superheat = suction line temperature minus the saturated temperature at suction pressure; it proves the evaporator is fully fed. Subcooling = saturated temperature at liquid pressure minus liquid line temperature; it proves the condenser is fully condensed. Fixed-orifice and piston systems are charged by superheat; TXV and EEV systems by subcooling. Typical targets run roughly 8–12°F superheat (fixed orifice, at the evaporator outlet) and 8–12°F subcooling (TXV), but always use the manufacturer's charging chart.
Traps: reading superheat at the compressor instead of the evaporator outlet (pickup from a hot motor skews it); charging by pressure alone on a blend; treating a restricted filter-drier or a dirty condenser as low charge; ignoring non-condensables; forgetting that zeotropic blends have temperature glide, so bubble and dew points differ.
Refrigerants are classified by safety group (A1 lower flammability, A2L lower flammability but flammable). A2L systems demand leak detection, ventilation, and charge limits from the applicable safety standard, and all refrigerant handling follows recovery/recycling rules.
Worked example: R-410A at 118 psig suction has a saturation temperature near 40°F. If the suction line measures 52°F, superheat = 52 − 40 = 12°F.