Refrigeration and Air Conditioning
Refrigeration and Air Conditioning
This domain tests the vapor-compression cycle, refrigerant states, metering devices, superheat and subcooling, airflow, and basic psychrometrics. Fourth Class questions focus on identifying components and diagnosing abnormal operation.
Core rules
- The compressor raises refrigerant pressure and temperature; the condenser rejects heat and condenses liquid.
- The metering device drops pressure and feeds the evaporator; the evaporator absorbs heat and boils refrigerant.
- Superheat is actual suction-line temperature minus evaporator saturation temperature.
- Subcooling is condenser saturation temperature minus liquid-line temperature.
- Low airflow causes low suction pressure, icing, and poor comfort; refrigerant charge alone may not be the fault.
Worked example
A suction line measures 45°F and the evaporator saturation temperature is 35°F. Superheat = 45 - 35 = 10°F. Reason: superheat confirms vapor leaving the evaporator and protects the compressor from liquid. Too low suggests flooding; too high suggests starvation or a restricted metering device.
Traps
Do not charge refrigerant by sight glass alone; do not confuse saturation temperature with line temperature; do not diagnose low suction pressure as low charge without checking airflow and metering; and do not ignore non-condensables, which raise head pressure.