Balancing and control of variable flow chilled water distribution system

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Balancing and control of variable flow chilled water distribution system

 

Author: Wong, Kwok-keung
Title: Balancing and control of variable flow chilled water distribution system
Year: 1996
Subject: Water -- Distribution
Refrigeration and refrigerating machinery -- Automatic control
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xiii, 77 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1246559
URI: http://theses.lib.polyu.edu.hk/handle/200/3342
Abstract: For constant pump-speed chilled water distribution systems, water balancing is a desirable part of the commissioning work. The conventional proportional balancing method, which involves a trial and error procedure, is rather clumsy and time consuming to carry out manually. In this work, a computer program has been developed to faciliate efficient determination of the required pressure drops across the balancing valves, and hence the minimum number of turns required for the balancing valves, at individual branches of a piping system. With it, the balancing work can be accomplished with less time and effort, and the pressure drop across the balancing valves, and hence pump power consumption can be maintained at the minimum possible. The prediction accuracy of the program has been verified by comparison with data obtained from laboratory experiment. The deviations between the predicted and the measured flow rate was found to be satisfactory (within +-2%). For variable pump-speed designs, many engineers consider that water balancing should not be carried out at all. This is because, for a pre-balanced system, the air-handling equipment close to the pumps may suffer from insufficient chilled water supply when the air-handling equipments at the far end of the piping system are under very light loads or are shutdown (Avery 1990). When such condition arises, the total flow rate and hence the pressure drop along the main supply and return pipes will drop leading to an increase in the differential pressure across the two main pipes. In response, the control system will lower the pump speed so as to maintain a constant differential pressure across the sensing points at the supply and return main pipes. The pressure difference across other branches will be reduced at the same time. Because of the pressure drops incurred by the balancing valves, the reduced supply pressure for those branches close to the pump may become insufficient to sustain the required chilled water flow rates through the air-handling equipment even when the control valves at these equipments are fully open. This has been proven to be true by computer simulation studies on possible scenarios that will lead to the aforesaid conditions (e.g. non-office hour and off-season period for the Peninsula Hotel). The computer simulation program used in the study was a pipe network analysis program developed on the basis of the Hardy Cross Method. Design flow rate and pressure drop data of system components in the reference building, the Peninsula Hotel, were input to the program in the analysis. It has been found from this study that for a variable flow chilled water distribution system with variable pumping speed control, water balancing should not be carried out. Care however should be taken in selecting control valves (CIBSE 1985, Hansen 1985, Lam 1989) which depends on both the extent of designed water flow rate variation and the rangeability of control valves. A minimum valve authority of 0.3 (based on the recommendation from manufacturers) is found to be acceptable since the control valve stroke is then limited to 0.7<s<1 if the variation in water flow rate is 75% of design condition.

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