Refrigeration and Air Conditioning
Domain 8: Refrigeration and Air Conditioning
This domain tests the vapor-compression cycle, refrigerant properties, psychrometrics, airflow, and safe refrigerant handling.
Core rules
- The basic cycle: compressor raises pressure and temperature; condenser rejects heat and subcools liquid; expansion device drops pressure; evaporator absorbs heat and superheats vapor.
- Superheat is measured at the evaporator outlet above saturation; subcooling is measured at the condenser outlet below saturation. Charging by superheat or subcooling is more reliable than pressure alone.
- One ton of refrigeration = 12,000 Btu/hr. COP = refrigeration effect / work input.
- Psychrometrics links dry-bulb, wet-bulb, humidity ratio, and dew point. Latent load removes moisture; sensible load changes dry-bulb temperature.
- Refrigerant safety groups, oxygen displacement, and leak detection govern confined-space entry.
Worked example
A unit provides 12,000 Btu/hr of cooling while the compressor draws 3,000 Btu/hr of work. COP = 12,000 / 3,000 = 4.0. That means 4 Btu of cooling per Btu of compressor work. If the same unit were rated 2 tons, cooling would be 24,000 Btu/hr; at COP 4, compressor work would be 6,000 Btu/hr.
Traps
Confusing superheat with subcooling; charging a blend by pressure without temperature glide; ignoring airflow when low suction pressure suggests low charge; and treating wet-bulb as dry-bulb.