period four
Chiller-Plant Control
notes
Chiller Rotation
Figure 94
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
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