A scroll compressor has a fixed spiral and a second spiral that "orbits" inside it without rotating; as the pockets between them shrink towards the centre, the refrigerant is compressed. There are no valves, pistons or crankshaft; the part count is about a third of a reciprocating compressor’s. The result: low vibration, low noise, little wear and high efficiency.
In commercial refrigeration, hermetic scroll compressors in the 2–40 kW range (Copeland, Danfoss, Bitzer Orbit) are the standard choice for supermarkets, cold rooms and housed condensing units. They are especially efficient at medium and high evaporating temperatures (−10 °C and above); vapour-injection models reach low temperatures as well. Digital capacity-controlled and inverter versions save energy at part load.
Caution: a scroll compressor turns in one direction only; if the phase sequence of a three-phase model is connected in reverse it does not compress and heats up quickly. It tolerates liquid refrigerant reaching the suction somewhat better than a reciprocating compressor, but superheat control is still essential. A large share of Sonkar housed condensing units use scroll compressors.