Refrigeration Systems: Cycle, Components, and Refrigerant Safety
What this domain tests
Refrigeration items cover the vapour-compression cycle, the function of each component, refrigerant properties and safe handling, and the operating faults that show up as pressure or temperature symptoms. Expect questions on superheat, subcooling, metering devices, and refrigerant classification.
Core rules and concepts
- Vapour-compression cycle: compression (low-pressure vapour to high-pressure vapour), condensation (rejecting heat to become liquid), expansion (pressure drop through a metering device), and evaporation (absorbing heat).
- Superheat is the temperature of vapour above saturation at the evaporator outlet; it ensures no liquid reaches the compressor. Subcooling is liquid below saturation at the condenser outlet; it ensures solid liquid at the metering device.
- Metering devices: thermostatic expansion valves maintain superheat; capillary tubes and fixed orifices are fixed-restriction devices; electronic expansion valves respond to a controller.
- Refrigerants are classified by safety group (toxicity and flammability). Handling requires recovery, never venting, and awareness that refrigerant is heavier than air in a confined space.
- Fault symptoms: low charge shows low suction pressure and high superheat; a restricted liquid line shows high superheat with low subcooling; an overcharged system shows high subcooling and high discharge pressure.
Worked example
A system has a suction pressure corresponding to 5 C saturation, and the suction line measures 12 C. What is the superheat, and what does it indicate?
Reasoning: superheat = 12 - 5 = 7 K. A typical target is 5-8 K for a TXV system. If superheat were 20 K, the valve would be starving the evaporator or the charge would be low, reducing capacity. If superheat were 0 K, liquid could reach the compressor and cause slugging and oil dilution. Adjust the TXV only after confirming the bulb is secure and insulated.
Traps that cost marks
- Measuring superheat with the gauge at the wrong point; it must be at the evaporator outlet.
- Confusing superheat (evaporator) with subcooling (condenser).
- Adding refrigerant to fix high superheat without checking for a restriction or a faulty TXV.
- Ignoring that refrigerant is heavier than air and can displace oxygen in pits and basements.