Capacity may be controlled on the basis of suction pressure, discharge pressure, flow or a combination of these variables. Capacity can be varied by use of unloaders, clearance pockets, and/or by speed variation when the driver is a variable-speed machine.
Whenever possible, constant-speed operation is recommended in order to avoid possible excitation of torsional or acoustical resonances. This is of particular importance on complex compressors with three or more stages, or when the unit(s) are to be located on an offshore platform. (The acoustical simulation becomes increasingly difficult with variable-speed machines, as does torsional vibration analysis.) The complex steel structure of an offshore platform has numerous vibratory natural frequencies which can respond to mechanical or acoustical excitations generated by the compressor. The risk of encountering such resonances are greater with variable speed compressors.