Vapor-Compression Refrigeration

Vapor-compression refrigeration is the cycle behind nearly every refrigerator, freezer, air conditioner and heat pump. A compressor raises low-pressure vapour to high pressure and temperature. A condenser rejects that heat outside, and the fluid condenses to liquid. An expansion valve drops the pressure abruptly. The cold liquid passes through an evaporator, where it boils by absorbing heat from the space being cooled, and returns to the compressor. The phase changes do the work: boiling and condensing move far more energy per kilogram than a temperature change alone.

The cycle moves heat rather than creating cold, which is why its coefficient of performance — heat moved divided by electrical work in — routinely lands between two and four. Nothing is violated; the energy is pumped, not manufactured. Run the loop the other way and the same hardware becomes a heater, which is what a heat pump is.

The exhaust path is not optional. A portable unit with no duct outside puts its condenser in the same sealed room as its evaporator. The two cancel and the compressor's electrical work is added on top, so the room net-heats. Single-hose designs partly solve this but exhaust indoor air, drawing warm outside air in through gaps, which is why rated capacity depends on which test standard is quoted — Goodhart's Law in appliance labelling.

Two common devices are not refrigeration at all. An evaporative cooler lowers air temperature by adding water vapour, trading heat for humidity, and works only in dry air. An air-entrainment fan simply moves air; it cools skin by aiding sweat evaporation and slightly warms the room. Calling either air conditioning is a Plausible Mechanism confusion — cold air at an outlet is not heat leaving the building, and the building envelope is the Ground Truth.