Air Conditioning and Refrigeration Principles
What this domain covers
The Air Conditioning and Refrigeration Principles domain tests the vapor-compression cycle, pressure–temperature relationships, charging, efficiency ratings, and the psychrometrics that explain sensible and latent capacity. Expect questions that make you read a PT chart and interpret superheat or subcool.
The cycle
- Four components: compressor, condenser, metering device, evaporator.
- The compressor raises pressure and temperature; the condenser rejects heat and subcools liquid; the metering device drops pressure; the evaporator absorbs heat and superheats vapor.
- Saturation temperature is read from a PT chart at the measured pressure. Superheat = suction line temperature − evaporator saturation temperature. Subcool = condenser saturation temperature − liquid line temperature.
- Fixed-orifice systems are charged by superheat; TXV systems by subcool, after airflow is verified.
- Blends have glide; use the dew point for superheat and the bubble point for subcool where the chart requires it.
Efficiency and capacity
- One ton = 12,000 Btu/h. EER is a full-load rating; SEER2 uses a different test procedure and cannot be compared directly to SEER.
- Sensible heat ratio (SHR) = sensible capacity ÷ total capacity. In humid Hawaii, an oversized unit with a high SHR can leave the space cold and clammy because it short-cycles before removing moisture.
- Total capacity depends on indoor wet-bulb, outdoor dry-bulb, and airflow; low airflow lowers capacity and can freeze the coil.
Worked example
An R-410A system reads 120 psig suction. From the PT chart, saturation is about 41°F. The suction line temperature is 53°F, so superheat = 53 − 41 = 12°F. For a fixed-orifice system with a target of 8–12°F, this is at the top of range; before adding charge, verify airflow and that the line temperature is measured at the same point as the pressure.
Traps that cost marks
- Charging to a pressure target instead of superheat or subcool.
- Using an R-22 PT chart on R-410A.
- Measuring suction-line temperature at the wrong location.
- Adding refrigerant to fix a dirty coil or restricted filter.
- Treating SEER and SEER2 as interchangeable.