Waterside economizer integrated with more than one chiller

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Do you have to model one integrated WSE for the chiller loop, or can you model one non-integrated WSE for each chiller? If the second option is ok, place one HX just before each chiller. Both HXs will receive the same condenser water from the condenser loop, so On/Off sequences should be the same.

If not, I'm afraid you may be stuck using EMS to override the availability of the second chiller. Trying to alter the Plant Equipment Operation Scheme will affect the operation of the entire plant, including the chiller pump and WSE. This won't work for your application since you want the WSE to operate while the chillers are off.

The following approach DOES NOT WORK.

I faced the same issue and did some experimenting. Hopefully it will save someone else some time.

Here's what I tried: Add the HX before the splitters on the CHW and CW loops. Instead of selecting the actual chiller inlet and outlet, select the CHW splitter inlet and CW splitter inlet. The thought being that E+ might stop the flow at these nodes when the HX is off. The simulation crashes because of the incorrect node selection. I confirmed that this is the cause by successfully running the simulation using "CoolingDifferentialOnOff" as the control type.

The Chilled Water Loop:

The Condenser Water Loop:

Julien, I'm sure you've solved the problem by now, but the best results I've been able to get come with using a combination of Operation Schemes, EMS to actuate the schemes via their control scheme schedules and ensuring that the HX control type is set to OperationSchemeModulated or OperationSchemeOnOff. Each chiller and HX should be on its own branch. It doesn't seem to matter where the HX is in the branch order.

A subsequent issue with this strategy is that the chiller operation stops responding to a sequential loop load distribution scheme (I haven't tested optimum), and just runs the current plant equipment list uniformly. To get back to sequential, I manually set up the applicable load ranges within PlantEquipmentOperation:CoolingLoad. So in your case you would end up with three separate plant equipment lists.

I'm not sure if this is possible in OS, as I set this up by directly creating the IDF. Sorry!