A new type of water supply scheme not only retains the advantages of variable frequency speed regulation and constant pressure water supply scheme, but also has the advantages of continuous adjustable outlet flow, constant water outlet pressure and good water supply quality, and also reduces the design capacity of the inverter with pump combination technology The system-wide cost performance.

At present, the building water supply system has gradually abandoned the traditional technology of water tower, high water tank, pressure tank, etc., and the use of computer-controlled variable frequency speed regulator for the pump motor stepless speed regulation, constant pressure water supply. This technology has made great strides in terms of stabilizing water pressure, reducing equipment size, and saving energy. However, the cost-effective and technically complex frequency converter has reduced the performance-to-price ratio of water supply systems.

To solve this problem, the most effective way is to reduce the selection of inverter capacity without reducing the performance of the water supply system. Market research shows that the greater the capacity of the inverter, the greater the impact on project costs. Therefore, when designing a large capacity water supply system, how to reduce the capacity of the VFD is the most effective technical way to improve the engineering cost performance.

1, a combination of binary variable flow pump water supply method

Proposed in the literature [1] not using a frequency converter (or pressure tank) automatic regulator feed water method, that is, a combination of binary variable flow pump regulators feed water method, are described as follows:

The system has a total of four pumps M0, M1, M2, M3 parallel operation, the combination of water. The rated head of each pump is the same and the rated flow rate changes twice. If the nominal flow rate of M0 is q, then the rated flow of the other three pumps M1, M2 and M3 are 2q, 4q and 8q respectively.

The number 1 indicates that the pump is working, and the number 0 means the pump is stopped. So M0, M1, M2, M3 four working state of each pump with a binary number a0, a1, a2, a3 to be expressed. The working state when they are combined together is represented by a four-digit binary number a3a2a1a0.

There are 16 variations of four binary numbers. These changes not only represent the current combination of pumps, but also represent the outlet flow rates Qt (at the time of calculating the outlet flow rate for each operating mode) Due to the pump running in parallel caused by the flow loss). That is, the greater the number, the greater the exit traffic; the smaller this number, the smaller the exit traffic. Thus found according to the amount of user volume, adjust the system of water flow to ensure that the regulator water supply, its working principle is: electric contact pressure gauge set the upper limit pressure H2, the lower limit pressure H1, by the H1 and H2 form a pressure stability interval . If the actual water pressure H is low, when H <H1, the programmable controller switches the working state of the water pump combination according to the increasing law of the binary number a3a2a1a0 in Table 1, increases the water flow of the system, and the water pressure rises until H≥H1, H pressure is high, H> H2, the programmable logic controller according to a3a2a1a0 regressive switching pump combination of working conditions, reducing system flow, water pressure down until H <H2. In such a normal operation, H1 <H <H2, the actual water pressure H of the water supply system is stabilized within the range specified by H2 and H1 to achieve the purpose of stabilizing the water pressure.

The use of this technology, the regulator accuracy is not high, the water load fluctuations are not frequent cases, not using the frequency converter can also be regulated water supply. However, when the voltage regulation accuracy is high, if no frequency converter is used, more water pumps must be provided (of course, the number of water pumps is greatly reduced than the traditional water pump combination method), which is very unfavorable to the optimization of engineering design and convenience of construction. In addition, large changes in user load, do not use the inverter will cause frequent switching pump combination, the system greatly reduced the accuracy of dynamic regulator, the motor start and stop constantly increasing energy consumption. The comprehensive evaluation of the ideal solution is to convert the constant pressure water supply technology and binary variable flow pump combination of water supply technology used in combination.

2, pump combination to optimize frequency control constant pressure water supply program

The system has a total of three pumps (dotted line drawn pump does not count) P0, P1, P2, P0 and P1 of the rated flow q, while the P2 rated flow larger 2q, three pumps rated head the same. In addition, only P0 adopts frequency converter to continuously control the speed, while P1 and P2 direct power frequency power switch control.

This pump system is equipped with a typical variable frequency constant pressure water supply system compared to the latter generally the same size with two identical pumps, a frequency control, a power frequency switching control, the use of a small pump. However, due to the inverter control the flow of the pump doubles, so the use of inverter capacity is roughly doubled. To achieve a small capacity inverter instead of large-capacity inverter, reducing the cost-performance of the entire system.

For the pumps P1 and P2 controlled by the switch, the numeral 1 indicates the pump operation, the numeral 0 indicates the pump stop working, and the combined working status of P1 and P2 is represented by a two-digit binary number a2a1 (as shown in Table 2). P0 continuously adjusts the motor speed by frequency converter and combines it with the combination of P1P2, the flow of the whole water supply system can continuously change in the range of 0≤Qt≤4q (when Qt is approximately ignored, the flow caused by the parallel connection of pumps loss). The difference between Table 2 and Table 1 lies in that the feed water flow rate can be continuously adjusted in the entire flow range of 0≤Qt≤4q because of the addition of the variable frequency speed control pump P0 so that theoretically constant pressure control with high accuracy can be realized, Not in Table 1 described within a certain range of voltage control. At the same time, compared with the traditional constant pressure frequency conversion water supply system, the design capacity of the inverter can be reduced by half. Therefore, the program combines the advantages of binary variable flow pump combination scheme and typical variable frequency constant pressure water supply scheme.

If we add a 4q pump P3 (dotted line), according to the same working principle, the outlet flow of the water supply system can be continuously adjusted within the range of 0 <Qt <8q? And the water pressure is basically constant (see Table 3) .

Through the above two cases can be concluded, if the design flow of the water supply system is Q, the capacity of the inverter pump can be designed as q = Q / 2n (n = 1,2,3 ...). At the same time with the n frequency power switch control of the pump, which n the same rated pump head and pump head with the same frequency, but the rated flow design value is twice the change, that is, from small to large: q, 2q , 4q ... 2n-1q.

By this (n +1) pump (a frequency control, n frequency switch control) consisting of a combination of water pump frequency control system to optimize water supply, that is, in the entire flow range to achieve high-quality constant pressure water supply, Again the design capacity of the inverter reduced to q = Q / 2n, reducing the inverter project budget prices and improve the cost performance of the entire water supply system.

3, conclusion

This paper presents a combination of water pump frequency control water supply program. It not only retains the advantages of frequency control scheme: the entire flow range can achieve high precision constant pressure water supply, but also use the pump combination technology greatly reduces the capacity of the inverter selection and improve the system's cost performance.

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