Refrigeration Principles, Thermodynamics, and System Components
This domain tests the refrigeration cycle, pressure–temperature relationships, and how each component changes refrigerant state. Expect questions on superheat, subcooling, metering devices, compressors, and diagnosing abnormal readings.
The cycle and its measurements
The compressor raises pressure and temperature; the condenser rejects sensible and latent heat; the metering device drops pressure; the evaporator absorbs heat. Saturation temperature is the temperature at which liquid and vapor coexist at a given pressure. Superheat is suction-line temperature minus evaporator saturation temperature. Subcooling is condenser saturation temperature minus liquid-line temperature.
Worked example
An R-410A system has a suction pressure of 118 psig. From published PT data, saturation temperature is about 40°F. The suction line measures 52°F. Superheat = 52 − 40 = 12°F. On a fixed-orifice system with a 10°F target, 12°F suggests slightly low charge or a load issue; confirm before adding refrigerant. On a TXV system, charge is set by subcooling, not superheat.
What costs marks
- Using gauge pressure where absolute pressure is required; gauge plus atmospheric equals absolute.
- Treating zeotropic blends as single-temperature fluids; use bubble and dew points.
- Measuring superheat at the compressor instead of the evaporator outlet.
- Assuming low suction pressure always means low charge; restrictions and airflow also cause it.
- Ignoring that a restricted liquid line raises superheat and lowers subcooling.