The compressor is the heart of the refrigeration system: it draws in the refrigerant, compresses it and discharges it to the condenser. Compression is what allows the refrigerant to give the heat it took from the room to the warmer outside air. 80–90% of the system’s electricity consumption is in the compressor; efficiency, noise and service life depend largely on the compressor choice.
By construction there are hermetic (sealed, non-repairable, small capacities), semi-hermetic (bolted body, serviceable, medium to large capacities) and open-type (separate motor, ammonia systems) compressors. By compression mechanism there are reciprocating (piston), scroll, screw and rotary types; each has its own capacity range and the temperature range it suits.
Three things are checked in selection: required capacity, evaporating temperature (high, medium or low temperature application according to room temperature) and refrigerant. The same compressor gives half the capacity in a −25 °C room that it gives in a 0 °C room; that is why a low-temperature series, and sometimes a two-stage or economiser compressor, is chosen for frozen stores.
For a compressor to run healthily, no liquid must reach the suction (superheat), oil must return, the high and low pressure limits must be protected by pressure switches and it must not short-cycle. Sonkar condensing units use compressors from brands such as Bitzer, Copeland, Frascold and Tecumseh; the catalog gives compressor type and capacity for every unit.