Variable-flow chilled water plants are designed to maintain a relatively constant delta-T, the difference between return and supply chilled water temperature. But in almost every real chiller plant, delta-T falls well short of design levels. The result is that flow and load do not track, usually requiring that additional chillers be brought on line to maintain flow requirements even though none of the chillers is fully loaded. Both pump energy and chiller energy increase accordingly. Many design and retrofit measures have been tried to resolve the problem, but they are sometimes expensive and not always successful. In this paper, the author argues that while many causes of degrading delta-T may be eliminated, in most plants it is not possible to avoid degrading delta-T under all operating conditions. Several design and operational techniques are presented both to minimize degrading delta-T and to design plants to be efficient despite degrading delta-T.

Units: I-P