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Application Considerations
SYS-APM001-EN Chiller System Design and Control 21
than at full load. Variable frequency drives for unloading tower fans and
chilled-water pumps may provide benefits, depending on the costs, system
operating hours, system type, and outdoor air conditions. (See “System
Controls” on page 87.)
Mid-Sized Chilled-Water Systems
(3-5 Chillers)
Figure 18. Mid-sized chilled-water system schematic
In addition to the design decisions faced by the small chilled-water system
designer, the following objectives may be encountered by the mid-sized
system designer.
Managing control complexity
As chilled-water systems get larger (Figure 18), control system design and
execution become more critical and more complex. There are simply more
combinations of equipment and operating scenarios. On the other hand,
systems this size generally have more highly-skilled operators who can
understand proper operation and maintenance. To help operators
understand expected system operation, chiller plant controls are usually
more customized and sophisticated.
Preferential vs. equalized loading and run-time
With more chillers, sequencing options might include preferentially loading
the most efficient chiller or equalizing the run time of chillers. The decision
hinges on how different these chillers are and the preferred maintenance
routine. For example, a chiller plant with one quite old—though still reliable—
chiller may periodically enable that chiller to ensure it continues to function,
Loads
Distribution Pumps