HVACR Fundamentals and Refrigeration Theory
This domain tests whether you can reason from the saturation table instead of guessing: pressure–temperature relationships, superheat and subcooling, the four-component cycle, metering devices, and compression ratio. Expect questions that hand you gauge readings and ask what they mean.
Saturation is the anchor
Every refrigerant has a unique pressure–temperature curve. While liquid and vapor coexist in the evaporator and condenser, temperature is fixed by pressure. Outside that saturated zone temperature moves independently - and that gap is how charge is measured.
- Superheat = suction line temperature minus evaporator saturation temperature. A TXV is set near 8–12°F evaporator superheat when the coil is properly loaded.
- Subcooling = condenser saturation temperature minus liquid line temperature. It proves a liquid seal at the metering device; TXV systems are charged by subcooling.
- Compression ratio = absolute discharge pressure ÷ absolute suction pressure. Add 14.7 to gauge readings. Above roughly 10:1, discharge temperature and wear climb sharply.
The four components
Compressor raises pressure and temperature; condenser rejects heat and condenses; the metering device drops pressure and flashes part of the liquid; the evaporator absorbs heat. Mass flow is identical everywhere - the compressor moves vapor, the metering device feeds liquid.
Worked example
R-410A: suction 118 psig (≈40°F saturation), suction line 52°F, discharge 340 psig, liquid line 95°F. Superheat = 52 − 40 = 12°F. Condensing saturation at 340 psig is ≈108°F, so subcooling = 108 − 95 = 13°F. Compression ratio = (340 + 14.7) ÷ (118 + 14.7) = 354.7 ÷ 132.7 = 2.67. These readings are consistent with a correctly charged TXV system on a warm day.
Traps
Using gauge pressure in the compression-ratio formula; measuring superheat at the compressor instead of the evaporator outlet; blaming low suction pressure on charge when a dirty filter or slow blower produces the same reading; and forgetting that superheat swings with load while subcooling stays steadier.