We have two York YS screws that are in a primary secondary loop system with a hydraulic decoupler. We have also been trying make this system as efficient as possible. Our biggest problem is that either the primary loop over pumps the secondary loop and return water to the chillers is too cold (low delta t. Chillers won't load) or the secondary over pumps the primary and makes the supply water warmer than the discharge leaving the chillers.
We can remedy this by varying the flow through the chillers. I was thinking of a Vfd on the evap pump. But our service guys say that you can't do that with screws. The engineering manual shows a minimum flow and a max flow through the evap, so i was thinking it would be ok as long as I stayed in between those flow numbers. JCI won't tell me why you can't vary flow they just say you can't. Can someone enlighten me?