Refrigeration Fundamentals
Core concepts
Refrigeration moves heat from a place where it is unwanted to a place where it can be rejected. Understand the four-stage vapour-compression cycle: compression (low-pressure vapour to high-pressure, high-temperature vapour), condensation (rejecting latent heat to become liquid), expansion (throttling to low pressure and temperature), and evaporation (absorbing heat from the load). Know that the expansion valve is an isenthalpic device, while the compressor is assumed isentropic in ideal analysis.
Key quantities: sensible heat changes temperature, latent heat changes state without temperature change. Superheat is vapour temperature above saturation; subcooling is liquid temperature below saturation. The pressure-enthalpy diagram is the working tool - locate the evaporating and condensing pressures, then read refrigerating effect (enthalpy difference across the evaporator) and compressor work (enthalpy rise across compression).
Traps in questions
- Confusing COP with capacity: COP = refrigerating effect ÷ work input, a dimensionless ratio, not a kW figure.
- Forgetting that lower evaporating temperature raises the compression ratio and reduces volumetric efficiency.
- Mixing up absolute and gauge pressure when reading saturation tables.
- Assuming superheat increases capacity - useful superheat protects the compressor but excessive superheat reduces mass flow and capacity.
Rule of thumb
A rough check: raising evaporating temperature by 1 K improves COP by roughly 2–4 % for typical halocarbon systems. Use this to sanity-check answers claiming large efficiency swings.
Worked example
Evaporator enthalpy in = 250 kJ/kg, out = 400 kJ/kg, compressor work = 50 kJ/kg. Refrigerating effect = 150 kJ/kg; COP = 150/50 = 3.0.