Refrigeration Principles and System Components
This domain tests the vapor-compression cycle, saturation and pressure-temperature relationships, superheat and subcooling, metering devices, and the major components and oils in a split system.
The cycle
- The compressor raises pressure and temperature.
- The condenser removes sensible heat, then latent heat, leaving subcooled liquid.
- The metering device drops pressure, flashing part of the liquid.
- The evaporator absorbs heat at saturated temperature and leaves superheated vapor.
- An accumulator or crankcase heater protects the compressor.
The two diagnostic numbers
- Superheat = suction line temperature minus the saturation temperature at suction pressure. Fixed-orifice systems run about 10 to 15 F; TXV systems about 5 to 10 F, per the manufacturer.
- Subcooling = saturation temperature at liquid-line pressure minus liquid-line temperature. TXV target is commonly 8 to 12 F.
Components and oils
- TXV and EEV modulate to hold superheat; a fixed piston is a matched orifice and cannot adjust.
- Filter-drier, sight glass, receiver, accumulator, reversing valve, and crankcase heater each solve one specific problem.
- Mineral oil belongs with CFC and HCFC refrigerants; POE and PVE belong with HFC and HFO blends. POE absorbs moisture fast, so keep containers sealed.
Worked example
R-410A system, suction pressure 118 psig, suction line temperature 52 F. The PT chart gives 40 F saturation, so superheat is 52 - 40 = 12 F. Liquid pressure 410 psig gives about 119 F saturation; the liquid line measures 108 F, so subcooling is 119 - 108 = 11 F. Both readings fall in range, so the charge is acceptable.
Traps that cost marks
- Taking the suction temperature at the compressor instead of at the evaporator outlet.
- Charging a TXV system to a superheat target; TXV systems are charged by subcooling.
- Assuming a blend can be treated as a single substance when it has measurable glide.
- Blaming the charge when the real fault is low airflow.