Control in multi-compressor installations
There are some advantages in industrial installations in having a number of separate compressors coupled together and feeding into the same supply line. Apart from the flexibility of being able to take an individual compressor out of service for maintenance and repair, there are possibilities inherent in this arrangement for designing a very efficient control for part-load operation. By adopting such a control system it has become possible to design rotary sets which approach the off-load efficiency of reciprocating units. In recent years the availability of cheap electronic control and sensing techniques has brought this type of approach into every day use.
There are several possibilities for this type of control, which in increasing order of complexity are:
• Control using system pressure sensed by a pressure switch to switch on or off individual compressors in a multi-set installation. Suitable for those installations with three or more compressors; this can be expensive because each compressor needs its own control card. The compressors could be switched in cascade or in ring sequence (to balance compressor use).
• Simple stop/start suitable for small units (up to about 20 kW each) whereby a system pressure switch activates a timer which stops a compressor if the signal persists after say one minute.
• Auto-idling where the receiver pressure is vented in response to a signal from the system pressure switch after a predetermined time. After a further preset time the set is switched off completely.
• Pulse-width modulation. The principle behind this development ensures that the individual compressor can operate at either full-load or idling. The system pressure is sensed as in the previous methods and the compression chamber pressure is switched between system pressure and atmospheric in a series of pulses with varying time widths. The receiver pressure is maintained at system pressure and isolated from the compression chamber by an unloader valve. A further modification of this system is possible if the cycling time turns out to be too rapid. In this case the PWM is tempered by operating at intermediate steps of say 66% and 33% of full load.