Refrigeration Principles and System Components
Heat, pressure, and saturation
Refrigeration removes heat from a space and rejects it outdoors; it never "adds cold." Candidates must be fluent with the pressure–temperature (P–T) chart: for any refrigerant, saturation temperature corresponds exactly to pressure, so gauges are the fastest way to know what the refrigerant is doing. Know the four components and their jobs - compressor (raises pressure and temperature, moves mass flow), condenser (rejects sensible and latent heat, producing subcooled liquid), metering device (drops pressure, creating a flash-gas mixture), and evaporator (absorbs heat, producing superheated vapor).
Superheat and subcooling
Total superheat = suction-line temperature minus evaporator saturation temperature. Subcooling = condenser saturation temperature minus liquid-line temperature. These two measurements describe the whole charge. Mixing them up is the most common exam trap: superheat is measured at the evaporator outlet (or service valve), subcooling at the condenser outlet. Target superheat rises as indoor wet-bulb rises and outdoor temperature falls; fixed-orifice systems are charged by superheat, TXV systems by subcooling.
Components and traps
Expect to know reciprocating, scroll, and rotary compressors; accumulator vs. receiver (accumulator on the suction side of a heat pump, receiver on the liquid line); filter driers, sight glasses, crankcase heaters, oil separators, and suction-line heat exchangers. Trap: a compressor does not "make cold" and does not change state - it raises pressure and therefore saturation temperature. Another: zeotropic blends such as R-410A have glide, so read bubble/dew columns and charge as liquid.
Rule of thumb
R-410A at 118 psig saturates near 40°F. If the suction line reads 52°F, superheat is 12°F - reasonable for a fixed orifice at a moderate indoor load.