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.
As chilled-water systems get larger (Figure above), 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.
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, but use it sparingly due to its lower efficiency. Or, a chiller may have a different fuel source, used as a hedge against either high demand or high energy consumption charges for other energy sources.
Some chilled-water systems have unequally sized chillers, allowing fewer chillers to operate.
