Refrigeration principles, vapour compression cycle and system components
What this domain tests
The vapour compression cycle: why refrigeration needs work input, the four processes, reading the P-h diagram, COP, superheat, subcooling, compression ratio and the meaning of one tonne of refrigeration.
Core rules
- Heat flows only from hot to cold. A refrigerator moves heat from a low-temperature source to a higher-temperature sink by expending work (Second Law).
- Four processes: compression 1-2 (non-isentropic in practice), constant-pressure heat rejection in the condenser 2-3, isenthalpic throttling 3-4, constant-pressure heat absorption in the evaporator 4-1.
- Refrigerating effect = h1 − h4; compressor work = h2 − h1; COP = (h1 − h4) / (h2 − h1). COP falls as condensing temperature rises or evaporating temperature falls.
- Subcooling increases the refrigerating effect; suction superheat protects the compressor but raises discharge temperature.
- Compression ratio = absolute discharge pressure / absolute suction pressure. A high ratio lowers volumetric efficiency and raises discharge temperature.
- Capacity Q = mass flow x refrigerating effect.
Worked example
R22 system at evaporating −5 °C and condensing 40 °C, with h1 = 401 kJ/kg, h2 = 435 kJ/kg and h3 = h4 = 249 kJ/kg, mass flow 0.05 kg/s. Refrigerating effect = 401 − 249 = 152 kJ/kg, so capacity = 0.05 x 152 = 7.6 kW (about 2.16 TR). Compressor work = 0.05 x (435 − 401) = 1.7 kW. COP = 7.6 / 1.7 = 4.5. A calculated COP above the Carnot value for the same temperatures always signals a table or unit error.
Traps
Using gauge pressures with properties tables; confusing subcooling with superheat; dividing kW by kJ/kg without checking units; treating throttling as isentropic instead of isenthalpic; ignoring that capacity depends on mass flow, not compressor size alone.