[Abstract]: This paper analyzes the characteristics of air conditioning secondary pump variable water volume system and its load regulation method, and discusses the problems related to the constant pressure differential control of the pump variable speed regulation system. 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. [Key words]: secondary pump system Bridge tube pressure distribution 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 unit flow unchanged, the user load increases when the opposite action. Fig.1 Primary pump systemFig.2 Bridge bypass control method commonly used 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), as shown in Figure 3. 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 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. 3, the secondary pump system load regulation The secondary pump system is a variable water system, by changing the amount of circulating water to achieve user load regulation. Common variable volume control methods are the number of units and speed regulation of two. 3.1 The number of units to adjust the number of traditional primary pump system to regulate the use of more differential pressure control, the number of secondary pump system mainly uses the flow rate control, the control accuracy of the more occasions to use load control. Differential pressure control is the use of pump parallel characteristic curve, set a return water pressure fluctuation range, when the load changes caused by pipe network flow changes, the supply and return pressure also will fluctuate, when the set limit is exceeded by the pump When less than the set lower limit pump. Flow control is based on the direction and size of the water flow within the bridge tube to control the pump and the corresponding opening and closing of the cold machine. When the user load drops, the secondary flow decreases, a flow excess, the bridge of cold water from the water flow back to the water. When the flow rate is greater than 110% of the single pump flow, turn off a cold machine and the corresponding pump; when the user load increases, there is a lack of flow, cold reverse flow of water within the bridge. When the flow rate is more than 20% of the single pump flow, turn on a pump and the corresponding cold machine. The purpose of turning on the chiller in advance is to avoid a big fluctuation in the temperature of the secondary water supply. Figure 4 fan coil cooling capacity and flow Figure 4 for the end of the common air conditioning system fan coil cooling capacity and flow diagram [7]. Due to the non-linear thermal characteristics of end equipment [8] ~ [10], when the flow demand is reduced to one pump, it does not mean that the load of the user is also reduced to one capacity of the refrigerator. Therefore, the control requirements of the higher occasions should be used to load control. The load control calculates the required cooling capacity by detecting the temperature difference and the flow rate on the primary supply pipe. When the required cooling capacity is reduced to the capacity equivalent to one chiller, stop one water pump and the corresponding chiller. Compared with the flow control, load control can effectively solve the problem of uncoordinated hydraulic and thermal conditions [1]. 3.2 Variable speed adjustment The secondary pump lift to overcome the resistance, including pipe network, coil, balance valve and control valve. In the constant speed variable flow system, when the flow decreases, the pressure drop of the pipe network, the coil and the balance valve also decreases, but the circulation pump head not only did not reduce, but also increased, the difference between the two must By the control valve (two-way valve) to pay. Therefore, the variable speed water system, the energy saving effect is not obvious. At very low loads, the control valve will lose control due to excessive pressure drop, causing excess cold water to pass through the coil. Figure 5 constant speed variable water pressure on the system control valve changes using pump variable speed adjustment can overcome the above drawbacks. When the load decreases, by changing the pump speed to reduce head and flow, you can get significant energy savings. Consider the inverter efficiency and motor cooling and other factors, variable speed adjustment should have a minimum speed limit (usually 30% of rated speed). When the load changes a large range, often using multi-pump parallel variable speed regulation to achieve energy-saving operation. Figure 6 is several different modes of operation of the pump power with the load curve. Constant water flow system pump operating conditions unchanged, the same pump power; single pump constant speed system only by two-way valve throttling regulation, pump power change is not big; multi-pump variable speed system at low load can still maintain a larger Energy saving potential. Figure 6 Comparison of pump power under different operation modes 4, Control curve of pump speed regulation According to the similar law, the pump power is proportional to the third power of its rotational speed under similar conditions. Ignoring the static head, the points on the system curve are similar operating conditions, to meet the similar law. In the variable speed water system, the pump variable speed control often use constant pressure control, the control curve and system curve do not coincide. Therefore, the pump power and speed does not meet the third law. Figure 7 is a pump variable speed constant pressure control of the relationship between the curves. Pump head by the constant pressure and variable pressure differential pressure of two parts: Constant pressure differential pressure sensor control loop, by the coil, balance valve and control valve, its value does not change with the flow; variable pressure transmission and distribution Pipe network pressure drop, proportional to the square of the pipe network traffic. From the pipe network curve upward translation of a constant pressure to control the curve. It can be seen from the figure that the smaller the constant pressure difference, the better the energy saving effect of the system. Figure 7 pump variable speed control curve It should be noted that the control curve is assumed to be the user load ratio under the conditions of an average curve. For example, when the system traffic is reduced by 50%, each user traffic requirement in the system is 50%. In practice, the user load is determined according to their own needs, and the user traffic changes rarely remain consistent. The following to Figure 8, for example, calculate the different load distribution under different control modes required pump head. In order to simplify the analysis, the calculation assumes that the user design load is equal, and the flow rate replaces the user load change. Fig. 8 Air-conditioning water system with 6 identical ends Fig. 8 Head of the pump required for different load distributions with different control modes (unit: kPa) Table 1 Constant pressure differential flow at user 6 Constant pressure differential load at user 1 Focus on near-end The load is concentrated on the remote proportional load 0 m3 / h
Fuonce-Lighting , https://www.gdfuonceled.com