Overview
The compressor is the heart of the vapour-compression refrigeration cycle. It raises the pressure and temperature of low-pressure refrigerant vapour from the evaporator so that heat can be rejected in the condenser. F-Gas candidates must understand how different compressor types operate, how to diagnose faults, and how compressor selection affects system efficiency — all topics examined under EU Implementing Regulation 2015/2067.
Compressor Types
- Reciprocating (piston) — uses pistons driven by a crankshaft. Common in commercial refrigeration. Available as open, semi-hermetic, or hermetic designs.
- Scroll — two interleaving scrolls compress vapour with smooth, continuous motion. Quiet, efficient, and widely used in air conditioning and heat pumps.
- Screw (rotary screw) — two helical rotors mesh to compress gas. Used in larger commercial and industrial systems. Offers good part-load efficiency via slide-valve control.
- Hermetic vs. semi-hermetic — hermetic compressors have a welded shell (motor and compressor sealed together; not field-serviceable). Semi-hermetic compressors have a bolted casing, allowing valve and motor repair on-site.
Oil Management
Compressors rely on lubricating oil to reduce friction, seal clearances, and remove heat. The oil circulates with the refrigerant and must return reliably to the compressor crankcase. Oil type must be compatible with the refrigerant — for example, POE (polyolester) oil is used with HFC and HFO refrigerants, while mineral oil is used with HCFC and CFC systems.
Poor oil return leads to compressor damage. Signs include low oil level in the sight glass, excessive noise, and elevated discharge temperatures.
Common Faults and Diagnostics
- Liquid slugging — liquid refrigerant enters the compressor, which cannot compress a liquid. Caused by a flooded evaporator or rapid defrost. Damages valves and pistons.
- High discharge temperature — may indicate low charge, high superheat, faulty valves, or poor condenser performance.
- Short cycling — frequent on/off operation caused by pressure-switch trips, often due to airflow restrictions or incorrect settings.
- Electrical faults — check motor windings for continuity and insulation resistance. A burnout contaminates the entire refrigerant circuit with acid.
Capacity Control
Modern systems vary compressor output to match load:
- Inverter (variable-speed) drives adjust motor speed for precise capacity modulation.
- Cylinder unloading in reciprocating compressors deactivates individual cylinders.
- Hot-gas bypass diverts discharge gas back to the suction side (less efficient, used as a last resort).
Exam Tip: Know the symptoms of liquid slugging versus high superheat. Be able to identify compressor type from a description or diagram. Exam questions often link compressor faults to incorrect charge levels or expansion valve problems.
