A capacity control system intends to adjust the cooling capacity of the compressor to the thermal load of the installation, at each moment of operation. Ideally, the power absorbed would be proportional to the partial cooling power at each moment. Some systems approach more others less than would be desirable.
The speed variation system is the closest to that ideal situation; but it is still not economically viable in most cases. The most common system in industrial compressors is the opening of suction valves by some external force (oil pressure is more common).
The limitations indicated must be followed by the manufacturer for operation at partial loads, according to the operating regime. This capacity control system also allows the start-up under vacuum, which is another desirable feature.
