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86 TRG-TRC016-EN
notes
period four
Chiller-Plant Control
When the system has determined that a chiller needs to be turned on or off,
the next issue is to determine the sequence in which to turn chillers on and off.
It is assumed that the first chiller in the sequence will always be turned on
whenever cooling is required.
When the system consists of identical chillers, the choice of which chiller is
turned on or off next has little impact on system efficiency. Some design
engineers and operators prefer to equalize the run time and the number of
starts for all chillers in the system. This is typically done by rotating the
sequence of chillers on a periodic basis, often every few days or weeks. This
method generally keeps the run time equalized reasonably well, and the
operator knows exactly when to expect the rotation to occur. An alternative
approach is to total the actual run hours on each chiller, in an attempt to rotate
the chillers when a significant imbalance in the run time or the number of starts
occurs. Rotation that is based on actual run time has the disadvantage of the
operator not knowing when rotation will occur. In some installations, operating
personnel prefer to manually initiate rotation.
On the other hand, some design engineers and operators believe that
equalizing run times will result in all of the chillers needing to be overhauled or
replaced at the same time. They tend to operate the most-efficient chiller first,
followed by the next-most-efficient, and so on. With this approach, all chillers
are turned on at least once a month to ensure that they will be able to start
when required.
Chiller Rotation
equally
equally
-
-
capacity chillers
capacity chillers
Figure 94