Cooling Systems and Refrigeration Cycle
This domain tests the vapor-compression cycle, refrigerant behavior and field verification of charge using superheat and subcooling rather than pressure alone.
Core rules
- Four components: compressor, condenser, metering device, evaporator; four processes: compression, condensing, expansion, evaporation.
- Saturated pressure and temperature are paired. Superheat = suction line temperature minus evaporator saturation temperature. Subcooling = condenser saturation temperature minus liquid line temperature.
- At design conditions with a TXV, expect roughly 8 to 12F superheat and 8 to 12F subcooling unless the manufacturer states otherwise.
- Compression ratio = (discharge psia) / (suction psia). High ratios raise discharge temperature and reduce capacity.
- Refrigerant safety class matters: R-410A is A1 with negligible glide; R-32 and R-454B are A2L, mildly flammable, with different charge limits and handling.
Worked example
An R-410A system shows 118 psig suction, 58F suction line temperature, 340 psig liquid pressure and 95F liquid line temperature. Saturation at 118 psig is about 40F, so superheat = 18F, above the target. Saturation at 340 psig is about 105F, so subcooling = 10F, normal. High superheat with normal subcooling points to an undercharge or a restriction at the metering device, not an overcharge. Compression ratio = (340 + 14.7) / (118 + 14.7) = 2.67.
Traps that cost marks
- Charging by pressure or by feel instead of superheat or subcooling.
- Reading charge before the system reaches steady state, or with a dirty condenser or evaporator coil.
- Ignoring indoor wet-bulb when evaluating capacity.
- Mixing up superheat (evaporator) and subcooling (condenser).
- Treating A2L refrigerants exactly like A1 for charge limits and ignition-risk procedures.