VFD'd on the house circuit will do you no good, especially since there are 3-ways at the load points.
The 3-ways make for constant flow in the loop. You could utilize the vfd as a means to set the flow rate instead of using discharge valves.
Did anyone ever set the flow rates? Through the loop, or the chiller bundles?
To utilize a VFD properly, the loop volume needs to be variable, as it would be if the control valves at the loads were 2-way.
In either case, vfd's won't help with chiller cycling. For that, timers, buffer tanks, HGBP, or an APR valve would be needed.
Personally, I'd be looking at an APR valve added to the 1st stage compressor on each chiller module.
Did you ever get this sorted? I'm not sure it was every really explained what you need the extra volume for. The idea is, the cycling is due to a difference in chiller capacity, and system load. This results in total heat being either removed, or added to the system. This heat results in a temperature change in the system, which leads to the cycling, where the temp starts swinging the other way. There is no 'fix' to remove the cycling - and a variable flow system is going to have the same problem. All you can hope to do is slow down the cycling. This is where loop volume comes in (A chilled water buffer tank). By increasing the overall system volume, the rate of temp change for a given imbalance is greatly reduced. Slowing the rate of temperature change will slow down the staging up and down rate.
That said, there are other advantages (on the energy saving front) from using a primary/secondary system, and getting rid of the 3-way valves. Currently, you are using extra electricity most the year just to move water in a circle, for no real reason, other than the system was easier to design. You can fix two problems in one by using a chilled water buffer as the decoupler between a primary and secondary system. Put a fixed speed primary pump (they don't have to be very big) for each of the chiller modules, that only turns on when that chiller is needed. Get rid of the 3-way valves, and use a VFD on the system pumps (which would now be secondary pumps).
No, this has not been fixed yet. We have someone doing evaluations and hopefully they'll come up with a solution which can reduce cycling and not require major hardware changes.
Originally Posted by CraziFuzzy
A buffer tank would probably help, but it would be quite difficult to add due to space limitations. Putting it outside the building is not an option.
I know cycling occurs because of mismatch between capacity and demand. The modular chillers add & subtract capacity in 25 or 30-ton increments. We need another small chiller in the loop that has variable capacity. Use the modular chillers for base load and the other chiller for load-following.
Originally Posted by CraziFuzzy
Without knowing the system volume, it's impossible to determine the size tank that would be required. Still, it is likely to be much cheaper than a variable chiller. Keep in mind, cycling isn't necessarily a bad thing - but excessive cycling is. A buffer is not going to remove the cycling, just stretch it out - which should be enough. Truly variable chillers in the 30-ton range are uncommon, which usually means expensive. Most likely option would be a smaller multi-scroll - something with 5 or 6 5-ton copelands in it... even that would still cycle as needed though.
Scroll modulars typically have 2x10, 12.5 or 15 hp compressors and should be able to cycle one or both on. On the multistacks with the brazed plate HX the evaps are arranged CKT1|CHW|CKT2|CHW etc and the condensers are arranged CKT1|COND W|CKT2|COND W etc. So each module should be able to get down to half its capacity. Short cycling is an issue with low effective system mass as discussed or could be controls trying too hard to accurately maintain setpoint. Have you trended CHWS against stages? Maybe the chiller plant controller is being too aggressive in staging things on and off to tightly maintain setpoint. You might be able to fix this with software.