Refrigeration Principles and System Components
The vapor-compression cycle
Every question in this domain starts from four components and four states. The compressor raises pressure and temperature and discharges hot high-pressure vapor to the condenser, where heat is rejected and the refrigerant condenses to a high-pressure liquid. The metering device (TXV, EEV, or fixed-bore piston) drops pressure, and the refrigerant boils in the evaporator, absorbing heat, then returns as superheated vapor to the compressor.
What to measure and where
Saturation temperature follows pressure alone: read the gauge, convert with a PT chart for that exact refrigerant, and compare with a line temperature. Superheat = suction line temperature − evaporating saturation temperature, measured at the evaporator outlet. Subcooling = condensing saturation temperature − liquid line temperature, measured at the condenser outlet. A TXV controls superheat (roughly 8–12°F) and subcooling is set by charge; a fixed orifice does the reverse.
Traps
Using the wrong refrigerant's PT chart, taking superheat at the condensing unit on a long line set without accounting for line loss, treating zeotropic blends (R-407C, ~10°F glide) as single-point fluids - use dew point for superheat and bubble point for subcooling - and forgetting that R-410A runs 50–70% higher pressure than R-22 and requires POE oil, which absorbs moisture fast. Charging a blended refrigerant as vapor pulls the fractionation problem into the system; charge liquid.
Worked example
R-410A system: suction pressure 118 psig ≈ 40°F saturation. Suction line temperature at the evaporator outlet reads 52°F. Superheat = 52 − 40 = 12°F, acceptable for a TXV. If the same reading were 2°F, suspect an overfeeding valve or an overcharge, not a low charge.