Staged Chiller Control Methodlogy

[69FIREBIRD]

Hi can anyone recommend the best way to program a 4 stage chiller (actually 2x 4 stage chillers in parallel). should I keep it simple and look at differentials from a setpoint coupled with "interstage" time delays and dead bands to bring on each stage or is their a better / more energy efficient / accurate way? - PID with stages set a % of output? I’m just trying not to reinvent the wheel - if someone has a tried and tested way to stage control this type of equipment I'd much appreciate some insight. Cheers.

[PdL]

Depends on how the compressors are controlled. Do they have capacity control? Speed control by VFD?

[lostcontrol]

One would think that if it was a brand new install, that they would be new units, and have their own staging/un-staging control, with perhaps only a master control that brings the chillers in/out, based on plant demand. Do the chillers have their own control systems, that do their own safeties etc, or is the staging to be determined by an external source? Edited January 18, 2010 by lostcontrol

[69FIREBIRD]

Hi Chillers are around 20-30 years old original control has been removed some time ago and current control is via PLC. Their are 4 stages controlled via solenoid valves and compressor start up is via some sort or partial winding method. their is no money for a replacing the chillers at present but we are going to start from scratch re control as the existing setup has been modified so many times, their are no drawings, and the PLC control is over complicated an buggie. So I think best option is keep it simple and look at differentials from a setpoint coupled with "interstage" time delays and dead bands to bring on each stage. But I'm interested before I jump into it to get some feedback on other ways or more efficient or accurate ways to control. Thanks