In a multiple compressor system with change in air demand, the compressors may rapidly cycle trying to find the right capacity match. This happens when:
- the Base load and Variable load (trim) compressors are not correctly sized to meet the changing air demand. Whenever the plant flow falls between the capacity of the undersized variable capacity compressor and the oversized base compressor, the Central or Master Controller finds it difficult to pick up the right capacity compressor.
- when the Central or Master Controller is set to control the pressure in a very narrow band (or receiver size is not big enough).
- there is a delay in picking up the next compressor.
Following chart shows undesirable & too frequent load / unload of the base compressor:
It is advisable to put the set point of VSD compressor control under the control of the Master Controller, and then the other air compressors will work in concert. And to adjust the system, one merely changes the single “target” pressure control point.