Abstract: This paper analyzes the characteristics and load regulation methods of air conditioning secondary pump variable water volume system, and discusses the related problems of system constant pressure differential control in water pump variable speed regulation. The conclusion shows that the secondary pump system through the bridge set to achieve the isolation of hydraulic conditions, with good hydraulic stability; pump speed control with remote fixed pressure, the pump head demand and load distribution. Keywords: secondary pump system Bridge tube fixed pressure control load distribution 1. Introduction In recent years, with the extensive use of central air conditioning, China's building energy consumption increased rapidly. According to statistics, the average energy consumption of buildings during the ten years from 1990 to 2000 increased by 5.8% annually, much higher than the 2.4% growth rate of energy production in the same period. In the air conditioning energy consumption, the system transport energy consumption accounts for about 1/3 [1]. Therefore, the variable flow technology in energy-saving design of air-conditioning system has been paid more and more attention. For air-conditioning water system, the proportion of transmission energy consumption in the total energy consumption increases with the system size. Variable water volume system (VWV) can reduce the system energy consumption by changing the flow of cold water in the transmission network to meet user load requirements. 2, the design of the secondary pump system As mentioned above, the user load changes can be achieved by changing the system cold water flow. However, in order to ensure stable hydromechanical conditions, the range of flow allowed by chillers is very small. There are usually two ways to solve this contradiction. Figure 1 is more commonly used in domestic design pressure bypass control method. When the load is reduced, the user valve is closed, the manifold differential pressure increases, the electric control valve opens large, some of the cold water by-pass short circuit to maintain the unit flow unchanged, the user load increases when the opposite action. Fig. 1 Primary pump system Fig. 2 is the common bridge bypass control method in foreign design [2] [3]. By setting the bridge to separate the entire system into two relatively independent hydraulic conditions of the circuit: cold water production and cold water delivery. Each district has a circulating pump is responsible for providing the district cycle power. When the load of the refrigerator is equal to the user load, the flow in the bridge is zero. When the load of the user is reduced, the flow in the bridge flows from the water supply to the backwater. Figure 2 secondary pump system For large-scale regional cooling system, often used three pump system (PST: Primary-Secondary-Tertiary Pumping System), shown in Figure 3 [4]. From the system form point of view, three pump system is only expanded bridge application, still belong to the secondary pump system category. Figure 3 Three-pump system The three-pump system divides the cold water into three separate circuits: Production, Transmission and Distribution. From the circulating pump set point of view, three pump system is a distributed pressure pump system [5]. A pump is responsible for cold water, the second pump is responsible for cold water delivery, the third pump is responsible for cold water distribution. The hydraulic conditions between the circuits are relatively independent, and the coupling among users is small, and the most unfavorable users exist, and the hydraulic stability of the system is better [6]. Users of the three-pump system can configure the corresponding circulating pumps according to their respective needs and adjust the pump speed to match the load requirements. The arrangement of the bridge pipes effectively avoids the interference among the users in adjusting the working conditions. Under ideal conditions, the sum of the head of the primary pump and the secondary pump is equal to the pump head of the primary pump system. Therefore, the pump energy consumption of the three pump system will not be higher than a pump system.
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