Refrigeration and Air Conditioning Fundamentals
Core concepts
You must be fluent in the vapour-compression cycle: evaporation absorbs heat at low pressure, compression raises pressure and temperature, condensation rejects heat, and expansion meters liquid to the evaporator. Understand superheat and subcooling, saturation, latent versus sensible heat, and the pressure-temperature relationship that underpins every diagnostic reading. Know the refrigerant families and their trade-offs: CFC and HCFC phase-out, HFC synthetic greenhouse gases, natural refrigerants such as ammonia, carbon dioxide and hydrocarbons, plus safety class, glide, oil miscibility, and critical point behaviour for CO2 systems. Psychrometrics matters for air conditioning: dry-bulb, wet-bulb, dew point, relative humidity, enthalpy, and the sensible heat ratio. Traps include confusing gauge with absolute pressure, misreading a PT chart for a zeotropic blend without accounting for glide and dew versus bubble points, and assuming a low suction pressure always means low charge.
Traps
Questions often describe symptoms that fit several causes; the correct answer is the one consistent with all readings. Distinguish saturation temperature from actual pipe temperature before judging superheat.
Worked example
R-410A system reads 8 bar gauge suction with 12 K superheat and 4 K subcooling. Convert to absolute (about 9 bar), find the dew-point saturation temperature on the PT chart, then compare with measured suction line temperature. If superheat is in range, the evaporator is being fed correctly; look elsewhere for capacity complaints.
Rule of thumb
Always convert gauge to absolute before using a PT chart, and use dew point for superheat and bubble point for subcooling with blends.