Fundamentals of Refrigeration and Air Conditioning
Why the fundamentals carry the paper
This domain tests whether you understand why heat moves and how a vapour-compression machine moves it. Expect units and definitions (tonne of refrigeration, sensible versus latent heat, superheat, subcooling), cycle analysis (COP, Carnot COP), and basic psychrometry (DBT, WBT, dew point, RH). If you can read a pressure–enthalpy diagram, most later questions become arithmetic.
Core rules
- 1 TR = 3.517 kW = 12 000 BTU/h = 210 kJ/min; it is a rate of heat removal, not a mass.
- Sensible heat changes temperature; latent heat changes state at constant temperature. Total heat = sensible + latent.
- Refrigeration effect occurs in the evaporator; heat rejected in the condenser = refrigerating effect + compressor work.
- COP = refrigerating effect ÷ work input. Carnot COP = T_evap ÷ (T_cond − T_evap), both temperatures in kelvin.
- Superheat is measured at the evaporator outlet; subcooling at the condenser outlet.
- Comfort cooling sits near 24 ± 2 °C DBT and 40–60 % RH.
Worked example
A 5 TR plant runs at COP 3.0. Refrigerating effect = 5 × 3.517 = 17.6 kW. Work = 17.6 ÷ 3 = 5.9 kW, so the compressor draws about 5.9 kW and the condenser rejects 17.6 + 5.9 = 23.5 kW. The condenser is always larger than the evaporator load - which is exactly why condenser airflow matters so much in service.
Traps
Putting °C into the Carnot formula (the answer comes out absurd), calling subcooling 'superheat', treating relative humidity as the moisture content of the air, and forgetting that COP is dimensionless while TR is a rate in kW.