Refrigeration
The cycle
The vapor-compression cycle has four processes: compression (1-2), condensation (2-3), expansion (3-4), and evaporation (4-1). On a pressure-enthalpy diagram, refrigeration effect is h1 − h4, compressor work is h2 − h1, and COP = (h1 − h4)/(h2 − h1). Heat-pump heating COP equals cooling COP plus one. Capacity in tons equals refrigerant mass flow times refrigeration effect, divided by 12,000 Btu/h.
Hardware
Reciprocating and scroll compressors dominate small systems, screws the midrange, and centrifugals large plants. Centrifugals surge at low flow and high head; screws unload with a slide valve and recips by cylinder unloading. Expansion is by TXV, electronic expansion valve, or capillary; the TXV controls superheat at the evaporator outlet, typically 8–12°F, while 8–15°F of condenser subcooling indicates a properly charged system.
Refrigerants
Know safety group (A1 low toxicity and low flammability; B2 higher in both), ODP, and GWP: R-410A is A1, zero ODP, high GWP; R-32 is A2L; ammonia (R-717) is B2L, efficient but toxic; CO2 runs at high pressure and suits cascade or transcritical heat-pump duty.
Traps
Using condensing temperature where pressure is required; mixing up where superheat and subcooling are measured; and forgetting that a colder evaporator raises capacity but lowers COP.
Worked example
h1 = 105 Btu/lb, h2 = 125, h4 = 45. COP = (105 − 45)/(125 − 105) = 60/20 = 3.0, a plausible result for roughly 20°F evaporating and 110°F condensing.