Refrigeration and Air Conditioning
Domain overview
This section covers the Refrigeration and Air Conditioning domain of the craft apprenticeship curriculum. The assessment checks whether you can explain the vapour-compression cycle, interpret pressure–temperature readings, apply safe refrigerant handling, and relate air-conditioning performance to airflow and psychrometrics.
Core rules
- The cycle has four steady-flow processes: compression, condensation, expansion and evaporation. The compressor raises refrigerant pressure and saturation temperature; the condenser rejects heat; the expansion device drops pressure; the evaporator absorbs heat.
- Superheat = suction-line temperature − evaporating saturation temperature. It protects the compressor from liquid return and is controlled by the TXV/EEV.
- Subcooling = condensing saturation temperature − liquid-line temperature. It shows the liquid seal and condenser margin; low subcooling can indicate undercharge or flash gas.
- Use pressure–temperature charts with the correct gauge/absolute basis. Test with dry nitrogen, never oxygen; evacuate and hold a vacuum to the manufacturer’s limit; charge by weight where possible.
- Air-conditioning performance depends on sensible and latent load: dry-bulb change shows sensible cooling, while wet-bulb or dew-point change shows moisture removal. Check airflow, filter pressure drop and condensate drainage before adjusting charge.
- Isolate and prove dead before electrical work; check overloads, capacitors and contactor operation.
Worked example
A system using R404A reads suction pressure equivalent to 2°C saturation and a suction-line temperature of 8°C. Superheat = 8 − 2 = 6 K. The liquid-line temperature is 38°C and condensing saturation is 45°C, so subcooling = 45 − 38 = 7 K. Reasoning: both values compare a measured line temperature with the saturation temperature at the same pressure-not with ambient or room temperature. A 6 K superheat is a reading to compare with the manufacturer’s target, not a universal pass mark.
Traps
Confusing superheat with subcooling; using the liquid line for superheat; taking readings before the system stabilises; ignoring gauge versus absolute pressure; failing to insulate the temperature probe; using air or oxygen for pressure testing; and overlooking airflow, filter or condensate-drain faults in air-conditioning diagnosis. Do not add refrigerant to correct a low superheat caused by a restricted TXV or poor airflow.