An electromechanical controller for operation of multiple compressors in single pressure band to start / stop or load / unload sequentially as per pre-set schedules of time rotation, sequence rotation which is generally suitable for equal capacity positive displacement compressors.
The simplest sequencers use a “cascade algorithm”. It is the sequential starting and loading of compressors based on falling pressure, and the reverse for rising pressure. This algorithm comes from the pre-computer age. Sequencers started their life as a mechanically driven pressure switch selectors, using relays, cams and timers. They work like this: as pressure drops, the next compressor starts and loads, and then the next starts and loads if pressure drops further. As pressure rises, the reverse occurs. The last on will load and unload once the number of compressors running stabilizes. The sequencer swaps the order around to even out wear. This was coded into simple programmed logic when programmable logic controllers (PLCs) and embedded controllers were introduced to industry. The cascade algorithm is best suited for positive displacement and reciprocating compressors. Cascade sequencers have a wide operating pressure differential.
A Master Controller is superior controller than a Sequencer.
(See also ‘Cascade pressure band’, ‘Master Controller’)