Improved Control Strategy of Variable Speed Pumps in Complex Chilled Water Systems Involving Plate Heat Exchangers

Abstract

The low delta-T syndrome is one of the major faults that affect the operation and energy performance of the chilled water systems in real applications, particularly for the complex chilled water systems. Low delta-T syndrome refers to the situation where the measured mean temperature difference of the overall terminal air-handling units is much lower than the expected value. This paper presents an online robust control strategy for practical applications to avoid the low delta-T syndrome for chilled water systems including complex systems. In this robust control strategy, a temperature set-point reset scheme is developed aiming at providing the reliable temperature set-point for enhancing the operation reliability of chilled water pumps. In addition, a flow-limiting control scheme is employed to perform the function of actively eliminating the deficit flow in the bypass line by a feedback mechanism. This robust pump speed control strategy has been implemented and evaluated on a real com-plex chilled water system in a high-rise building. The site test results show that the temperature set-point given by the proposed strategy is reliable and the system temperature difference is significantly raised by eliminating the deficit flow problem. When compared to the conventional control strategies, 77.8% of the total chilled water pump energy was saved in the test period. The actual total pump energy saving was 140,776 kWh (39.4%) in the test of a year after implementing the robust control strategy in the studied system.