Currently, building water supply systems have gradually abandoned traditional technologies such as water towers, elevated water tanks, and pressure tanks, and instead adopted computer control combined with variable frequency drives (VFDs) for stepless speed regulation of water pump motors and constant pressure water supply. This technology has made great progress in stabilizing water pressure, reducing equipment size, and saving energy. However, due to the use of expensive and technically complex VFDs, the performance-price ratio of the water supply system has been reduced. To solve this problem, the most effective method is to reduce the selected capacity of the VFD without significantly reducing the performance of the water supply system. Market research shows that the larger the capacity of the VFD, the greater the impact on the project cost. Therefore, when designing a large-capacity water supply system, reducing the capacity of the VFD is the most effective technical approach to improve the performance-price ratio of the project. 1 Binary Variable Flow Pump Combination Pressure Stabilization Water Supply Method Reference [1] proposes an automatic pressure stabilization water supply method that does not use a VFD (or pressure tank), namely the binary variable flow pump combination pressure stabilization water supply method, which is introduced below: Figure 1 is a schematic diagram of a system constructed using the binary variable flow pump combination pressure stabilization water supply method. The system consists of four water pumps, M0, M1, M2, and M3, operating in parallel to supply water. Each pump has the same rated head, but their rated flow rates vary by a factor of two. For example, if the rated flow rate of M0 is q, then the rated flow rates of the other three pumps, M1, M2, and M3, are 2q, 4q, and 8q, respectively. The number 1 represents pump operation, and 0 represents pump shutdown. Therefore, the operating state of each of the four pumps is represented by a single binary number a0, a1, a2, and a3. Their combined operating state is represented by a four-bit binary number a3a2a1a0. As shown in Table 1, the four-bit binary number has 16 possible variations. These variations represent not only the pump combination at that time but also the outlet flow rate Qt that the pump combination can supply to the water system at that time (the flow loss caused by the parallel operation of the pumps is approximately ignored when calculating the outlet flow rate for each operating condition). The larger this number, the larger the outlet flow rate; the smaller this number, the smaller the outlet flow rate. This led to a method for adjusting the system's outlet flow rate based on user usage to ensure stable water pressure. The working principle is as follows: the electric contact pressure gauge sets an upper limit pressure H2 and a lower limit pressure H1, with H1 and H2 forming a stable pressure range. If the actual water pressure H is low (H < H1), the programmable controller switches the pump combination's operating state according to the increasing rule of binary numbers a3a2a1a0 shown in Table 1, increasing the system's outlet flow rate and raising the water pressure until H ≥ H1. If the water pressure H is high (H > H2), the programmable controller switches the pump combination's operating state according to the decreasing rule of a3a2a1a0, reducing the system flow rate and lowering the water pressure until H < H2. Thus, during normal operation, H1 < H < H2, and the actual water pressure H of the water supply system is stabilized within the range specified by H2 and H1, achieving the purpose of stable water pressure. This technology allows for stable water supply without a variable frequency drive (VFD) when pressure stabilization accuracy is low and water load fluctuations are infrequent. However, when pressure stabilization accuracy is high, without a VFD, a larger number of pumps must be used (though this is significantly less than the traditional parallel pump combination method), which is detrimental to optimizing engineering design and facilitating construction. Furthermore, when user load changes significantly, not using a VFD will cause frequent switching of pump combinations, greatly reducing the dynamic pressure stabilization accuracy of the system, and the constant starting and stopping of the motors exacerbates energy consumption. A more ideal solution is to combine VFD constant pressure water supply technology with binary variable flow pump combination pressure stabilization water supply technology. 2. Pump Combination Optimization VFD Constant Pressure Water Supply Scheme As shown in Figure 2, the system has three pumps (the pumps shown by the dashed line are not included): P0, P1, and P2. The rated flow rates of P0 and P1 are q, while the rated flow rate of P2 is larger at 2q. All three pumps have the same rated head. In addition, only P0 uses a frequency converter to continuously control its speed, while P1 and P2 are directly controlled by a power frequency switch. Compared with a typical variable frequency constant pressure water supply system, this pump system typically uses two identical pumps of the same size, one controlled by a frequency converter and the other by a power frequency switch, thus using an extra small pump. However, since the flow rate of the pump controlled by the frequency converter is reduced by half, the capacity of the frequency converter used is also roughly halved. This achieves the replacement of a large-capacity frequency converter with a small-capacity one, reducing the overall system's performance-price ratio. The working principle of the water supply system in Figure 2 is described below. For the pumps P1 and P2, which are controlled by switches, the number 1 represents the pump working and the number 0 represents the pump stopping. Therefore, the combined working state of P1 and P2 is represented by a two-bit binary number a2a1 (as shown in Table 2). P0 uses a frequency converter to continuously adjust the motor speed. Combining this with the combined working state of P1 and P2, the flow rate of the entire water supply system can continuously vary in the range of 0≤Qt≤4q (when calculating Qt, the flow loss caused by the parallel operation of the pumps is approximately ignored). The difference between Table 2 and Table 1 is that, due to the addition of the variable frequency speed-regulating water pump P0, the water supply flow can be continuously adjusted across the entire flow range of 0 ≤ Qt ≤ 4q. Therefore, theoretically, high-precision constant pressure control can be achieved, rather than the pressure stabilization control within a certain range described in Table 1. Furthermore, compared to traditional constant pressure variable frequency speed-regulating water supply systems, the capacity of the frequency converter can be reduced by half. Therefore, this scheme combines the advantages of both the binary variable flow pump combination scheme and the typical variable frequency speed-regulating constant pressure water supply scheme. [align=center]Table 2 Optimized Variable Frequency Drive Design for Two Water Pumps[/align] As shown in Figure 2, if another water pump P3 with a capacity of 4q (shown as a dashed line) is added, based on the same working principle, the outlet flow rate of the water supply system can be continuously adjusted within the range of 0 < Qt < 8q, while the water pressure remains basically constant (see Table 3). Table 3 Optimized Variable Frequency Drive Design for Three Water Pumps From the above two examples, it can be concluded that if the design flow rate of the water supply system is Q, the capacity of the variable frequency water pump can be designed as q = Q/2n (n = 1, 2, 3...). At the same time, n water pumps controlled by power frequency switches are equipped. These n water pumps have the same rated head and are consistent with the head of the variable frequency water pump, but the rated flow design value changes by a factor of two, that is, from small to large: q, 2q, 4q...2n-1q. The pump combination optimization variable frequency speed control water supply system, consisting of (n+1) pumps (1 variable frequency speed control and n power frequency switch control), achieves high-quality constant pressure water supply across the entire flow range, while reducing the design capacity of the frequency converter to q=Q/2n, thus lowering the engineering budget price of the frequency converter and improving the performance-price ratio of the entire water supply system. 3 Conclusion This paper proposes a pump combination optimization variable frequency speed control water supply scheme. It retains the advantages of the variable frequency speed control scheme: high-precision constant pressure water supply across the entire flow range, while significantly reducing the selected capacity of the frequency converter by utilizing pump combination technology, thereby improving the performance-price ratio of the system. [b]References:[/b] ［1］Jiang Zhijian, Wang Xizhong. Binary variable flow pump combination pressure stabilization water supply method［J］. Journal of Harbin University of Architecture, 1998, 5. Editor: He Shiping