The results of a five-year effort at correcting the operational deficiencies of a 4,000-ton central chilled-water plant are presented. The results show what primary considerations need to be taken to reduce pumping costs, maximize the use of existing tonnage, and increase the overall temperature difference of the system. The system discussed uses primary/secondary variable-speed pumping systems combined with a global direct digital control (DDC) system. Comparisons are made with other systems, and the function of key hydronic components is described. Modifications to the system reduced the number of distribution pumps required and improved the operational efficiency of the chillers by raising the water temperature differential from 6 deg F (3.3 deg C) to 15 deg F (8.3 deg C). Control problems were reduced, and increased system flexibility was observed. Past changes, as well as future installations, are presented.

KEYWORDS: designing, variable speed motors, pumps, water pumps, chilled water supply, efficiency, temperature difference, computer controls, comparing, controls, optimisation.