Refrigeration Cycle
Refrigeration Cycle
The Refrigeration Cycle domain tests the four basic components, saturation, superheat, subcooling, and charging. Expect pressure-temperature relationships and field measurement interpretation.
Core rules
- The cycle: compressor raises pressure and temperature; condenser rejects heat and subcools liquid; metering device drops pressure; evaporator absorbs heat and superheats vapor.
- Superheat = suction-line temperature minus evaporator saturation temperature. Subcooling = liquid-line temperature minus condenser saturation temperature.
- Use a pressure-temperature chart for the specific refrigerant. R-410A operates at higher pressures than R-22; never mix oils or gauges.
- Charge by weight when possible, or by the manufacturer’s target superheat (fixed orifice) or subcooling (TXV) method.
Worked example
A system using R-410A has a suction pressure corresponding to 40°F saturation and a suction-line temperature of 55°F. Superheat = 55 − 40 = 15°F. If the manufacturer’s target is 10°F, the circuit is running high superheat, suggesting undercharge, a restricted metering device, or low load-not simply “add refrigerant.”
Traps
Measuring superheat at the compressor instead of the evaporator outlet; using the wrong PT chart; charging by gauge pressure alone; and ignoring that subcooling is measured on the liquid line.