Mechanical, Refrigeration, and Airflow Fundamentals
This domain tests the sealed system and the mechanical and airflow conditions that make it work. Questions often give pressures, temperatures, frost patterns, or static readings and ask for the likely cause.
The refrigeration cycle
- Compressor raises pressure and temperature; condenser rejects heat; metering device drops pressure; evaporator absorbs heat. The evaporator is cold because refrigerant boils, not because it is "filled."
- Saturation temperature is the boiling point at a given pressure. Superheat = suction line temperature minus evaporator saturation temperature. Subcooling = condenser saturation temperature minus liquid line temperature.
- Metering: capillary tube, thermostatic expansion valve, or automatic orifice. A restricted capillary shows low suction and a starved evaporator.
- Refrigerants: R-134a, R-600a (flammable isobutane), and R-290 (propane) are not interchangeable. Use the nameplate charge and charge by weight.
- Components: filter drier, accumulator, heat exchanger, and access valves. A restricted drier causes a temperature drop across it.
Airflow and mechanical
- Airflow is part of the refrigeration system. A dirty condenser raises head pressure; a dirty evaporator or weak fan lowers load and can freeze the coil.
- Static pressure, duct size, filter pressure drop, and fan speed determine delivery. Dryer vents must be short, smooth, and lint-free.
- Belts, pulleys, bearings, mounts, and alignment: check tension, wear, and vibration. Lubricate only where specified.
Worked example
A refrigerator freezer is cold but the fresh-food section is warm. You find heavy frost on the evaporator and no frost pattern variation. Suction pressure is 10 psig on R-134a, about −7 °C saturation; suction line temperature is 1 °C, so superheat is about 8 K, which is not grossly abnormal. The frost and warm compartment point to a defrost or airflow problem, not a charge problem. You test the defrost heater, termination thermostat, and timer/board, and check the evaporator fan and damper before touching the sealed system.
Traps
- Charging by suction pressure alone.
- Adding refrigerant to a system with a leak.
- Diagnosing a sealed system before confirming airflow and defrost.
- Mixing refrigerants or using a drop-in substitute not approved by the manufacturer.