Scheduling and control of Chilled water meters and water economizer chillers

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A question for the group that's stumping me right now.

I'm modeling a building served by a campus chiller plant. The chiller plant
is only available during summer months, with a 7-5 weekdays-only schedule
and snap operation limiting the plant to 70degF OAT and above. This
building will require cooling at times when the plant is unavailable, so the
design team is considering using a cooling tower as a "water economizer
chiller". I'm familiar with the equest workaround for water economizers,
and have used it before.

The difficulty is in setting up a control scheme that allows the primary
chilled water loop to operate in any condition that requires cooling AND
allows the two chilled water sources to operate as intended. To summarize
the sequence requirements:

* The chilled water meter (representing the campus plant)

* Is the only chilled water source when both of these conditions are
met -

* During scheduled operation
* When the OAT is above snap temperature

* The water economizer cooling tower

* Operates when there is cooling demand outside of summer months
* Operates during scheduled operation of the campus plant when OAT is
below the chiller plant snap temperature AND there is cooling demand

I've tried playing around with equipment control and load management, and
can't seem to get a sequence that works. I'm about ready to simply let the
chilled water run without schedule restrictions and figure out what the
plant vs cooling tower operation hours are through hourly report
post-processing.

Anyone have any suggestions?

Steven Savich, LEED AP

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