Analyses of operations of control valves in air-conditioning systems comprising multiple chillers and chilled water pumps

摘要

In an earlier paper, the guidelines given in the literature on the selection of chilled water flow control valves for air-handling equipment were reviewed and the proper way of selecting this type of control valves was presented. Complementary to that paper, this paper attempts to answer some questions related to the selection and operation of control valves in air-conditioning systems comprising multiple chillers and chilled water pumps. Explanations are given on the limits imposed on the cooling outputs of air-handling units when they are equipped with three-way control valves and are fed with chilled water from a central plant comprising multiple chillers and pumps while some of the chillers and pumps have been shutdown under part-load conditions. The Interactions between the control valves and the chilled water pumps, as they are cycled on and off, in a single-loop and a two-loop pumping system are analysed. The conclusions Include that the differential pressure bypass control valve in a single-loop pumping system should be sized for a flow handling capacity significantly greater than the flow rate of one chiller. For minimising the disturbances to the temperature control function of air-conditioning systems, control valves with linear Inherent characteristics should be used for differential pressure bypass control. Furthermore, for a two-loop pumping system, both the chiller on and off settings for a conventional de-coupler bypass control system should be based on surplus flow rates, and the on/off control over the secondary-loop pumps should avoid requiring each pump to handle a flow rate significantly greater than its rated flow rate.