Refrigeration System Components and Operation
The loop, part by part
The compressor is the only pump in the system. Know the types: fixed-displacement piston or scroll units driven through an electromagnetic clutch, and clutchless variable-displacement units whose output is set by an electrically controlled valve that changes crankcase-to-suction pressure. The condenser must have airflow, and subcooling at its outlet shows liquid is stacking. The expansion device is either a fixed orifice tube in the liquid line (system uses an accumulator on the suction side) or a TXV that modulates flow from a sensing bulb or internal charge at the evaporator outlet (system uses a receiver-drier on the liquid side). Only one of those two storage devices exists on any given system.
Lubricant matters: PAG oil with R-134a, PAG or POE with R-1234yf. Oil is hygroscopic, so cap lines and never leave the system open. Pressure switches protect the system: low-pressure cutoff, high-pressure cutoff, cooling-fan control, plus a mechanical relief valve.
Traps
Putting a receiver on an orifice-tube system; clamping a TXV bulb to the wrong pipe; condemning a clutchless compressor because it never disengages; treating R-134a and R-1234yf as interchangeable (different fittings, oils, capacities); and calling a compressor bad on low pressures before verifying charge.
Rule of thumb: verify charge on a TXV system with subcooling at the condenser outlet, typically 10-15 F. On an orifice-tube system, charge by weight and confirm with a performance test, because subcooling is less dependable.