Abstract : This article introduces the variable-frequency speed control method for the water-refilling pump and circulating pump in the heat-exchange station of the urban centralized heat supply system, and illustrates the control method and calculates the energy saving rate with examples. Key Words : Water-refilling pump; Circulating pump; Variable-frequency speed control; Energy saving. I. Introduction To save energy, reduce urban pollution, and make full use of the waste heat from the steam turbines of thermal power plants, centralized heating is provided to northern cities in winter. The heating method involves hot water from the power plant reaching the heat exchange station in the city. The primary supply hot water temperature is over 90 degrees Celsius. After passing through the heat exchanger, the temperature of the primary return hot water drops to over 60 degrees Celsius before flowing back to the power plant. Hot water delivered to urban residents' homes flows through heat exchangers at each user's home, where heat exchange occurs, before flowing back to the heat exchange station. The secondary return water temperature entering the heat exchanger at the station is over 50 degrees Celsius, while the secondary supply water temperature is over 60 degrees Celsius. There are many such heat exchange stations in Baoji, Shaanxi Province. The equipment at these stations is relatively simple, consisting of several heat exchangers, a circulating pump group composed of several pumps, and a makeup water pump. One heat exchange station has four heat exchangers, a circulating pump group composed of four 37kW pipeline pumps, and a 3.7kW makeup water pump. The flow rate of the circulating and makeup water pumps is controlled by manually opening and closing valves, which increases the damping of the pipeline and wastes electricity. II. Variable Frequency Speed ​​Control of Heat Exchange Stations 1. Variable Frequency Speed ​​Control of Makeup Water Pump To further improve energy efficiency, Baoji Heating Company in Shaanxi Province implemented automation upgrades to the heat exchange stations from 2003 to 2004. Variable frequency speed control was used for the circulating and makeup water pumps, and the entire urban heating system was monitored by computer, achieving unmanned operation of the heat exchange stations. Hot water is circulated in the heating system via a circulating pump. Leaks in pipes and valves can cause a drop in circulating water pressure. If water is not replenished in time, the heating system will malfunction. The variable frequency pump (VFD) method for replenishing water is relatively simple. The hot water pressure in the system is 0.4 MPa. A pressure transmitter is installed on the return water main. Pressure changes in the pipeline are converted into a 4-20 mA signal by the pressure transmitter and fed back to the input of the VFD's PI controller. The VFD's setpoint is set to 4 kg. When the heating system pressure drops below 4 kg, the VFD's output frequency increases to begin replenishing water; when the pressure reaches 4 kg, the feedback signal is approximately equal to the setpoint signal, and the VFD's output frequency decreases to stop replenishing water. This example uses a Senlan BT12S 3.7kW frequency converter and a Sennas DG130W-BZ-A 1MPa pressure transmitter. The variable frequency speed control water supply system is shown in Figure 1: [align=center] Figure 1 Schematic diagram of variable frequency speed control water supply system[/align] 2. Variable frequency speed control of the circulating pump The control of the circulating pump is more complex than that of the water supply pump. The ultimate goal of the heating system is to maintain a stable indoor temperature for heat users. However, since heat users do not have room temperature regulators, and it is impossible to form a closed-loop control for the room temperature of numerous heat users, the most effective way to achieve economical operation and ensure heating quality is to control the secondary water supply temperature of the heat exchange station. Under steady-state conditions, the system's heat supply, radiator heat dissipation, and user heat consumption are equal. The steady-state secondary water supply temperature can be obtained as follows: By modifying equation (1) and considering that the indoor temperature, the ratio of the actual flow rate of the secondary pipe network to the design flow rate, and the return water temperature are approximately constant, then in the equation: a, b, and c are relevant meteorological parameters of the area where the pipe network is located. Equation (2) is the calculation method for the given value of the secondary water supply temperature. The value determined by equation (2) can track changes in outdoor temperature, so that the indoor temperature of the heat user is not affected by changes, and stable heating is achieved. Since the indoor heating system of the heat user adopts the single-pipe heating method of supplying from the top and returning from the bottom, it can be seen from the heating theory that the best adjustment method for single-pipe heating should be the comprehensive adjustment of temperature and flow rate. As can be seen from equation (1), with the change of outdoor temperature, not only should the secondary water supply temperature be adjusted in time, but the flow rate G of the circulating water should also be adjusted accordingly to avoid the phenomenon of "vertical imbalance" where the upper room temperature is seriously too high and the lower room temperature is seriously too low. The temperature of the secondary water supply is related to the temperature and flow rate of the primary water supply, the flow rate of the secondary return water, and the ambient temperature. Generally, among these factors, the temperature and flow rate of the primary water supply are not regulated at the heat exchange station; only the flow rate G of the circulating water can be regulated. The control strategy of the automated secondary water supply temperature control system is as follows: if the secondary water supply temperature is low, the flow rate G of the circulating water increases; conversely, if the secondary water supply temperature is high, the flow rate G of the circulating water decreases. However, this does not consider the impact of ambient temperature changes. If the outdoor temperature changes, to keep the indoor temperature relatively constant, one control strategy is to use the temperature difference between the secondary inlet and return water to control the speed of the circulating pump inverter, setting the temperature difference between the secondary inlet and return water to 12. When the temperature difference between the secondary inlet and return water is greater than 12, the circulating pump inverter accelerates, and the flow rate G of the circulating water increases; when the temperature difference between the secondary inlet and return water is less than 12, the circulating pump inverter decelerates, and the flow rate G of the circulating water decreases. Furthermore, considering that when the flow rate G of the circulating water is small, the speed of the circulating pump is low, and the circulating water cannot be supplied to users on the highest floors. Therefore, based on temperature difference control, the target value of the temperature difference can be appropriately adjusted within a certain range according to the minimum head required by the height of the heat user. The control signal for the variable frequency drive (VFD) speed regulation is provided by the automation system. The VFD speed regulation system diagram of the circulating pump is shown in Figure 2: In the figure, BP1 is the Senlan BT12S37kW VFD, BU is the soft starter (autotransformer starter), and the system adopts a cyclic switching method. The temperature difference signal is sent to the PLC, processed by the PLC, and then sent to the VFD as the speed regulation control signal. When the system starts, motor M1 is variable frequency driven. When the frequency rises to 50Hz, if the circulating water flow does not meet the given requirements, motor M1 is switched to the mains frequency, and M2 is operated by VFD; if the circulating water flow still does not meet the given requirements, motor M2 is switched to the mains frequency, and M3 is operated by VFD...; if the circulating water flow exceeds the given requirements for some reason, then motors M1, M2, etc., are stopped. Only one circulating pump motor is operating by VFD at any given time. If only one motor, M3, is operating at variable frequency, and the circulating water flow rate does not meet the given requirements, then motor M3 switches to mains frequency, and M4 operates at variable frequency; when M4 switches to mains frequency, M1 operates at variable frequency, returning to the initial state. Motors M1-M4 always cycle between mains frequency and variable frequency. BU1 is used as a backup, and the entire system's operating information is sent to the computer by the PLC. [align=center] Figure 2 Circulating Pump Variable Frequency Speed ​​Control System Diagram[/align] III. Energy Saving of Circulating Pumps During the design of heat exchange stations, excessive consideration is given to the long-term heating capacity before and after construction, as well as various problems that may occur during long-term operation, resulting in an excessive margin. In reality, most heat exchange stations do not reach the maximum design capacity at the beginning, but rather the heating area gradually reaches the design capacity as urban construction develops; on the other hand, it is difficult to accurately calculate the heating capacity during the design process, and the maximum heating capacity of the system is usually used as the basis for selecting the circulating pump. However, the series of circulating pumps is limited, and often, if a suitable circulating pump model cannot be selected, an even higher model is chosen, further increasing the margin. In actual heating operation, valves are commonly used for flow regulation, which increases system resistance and consumes a significant amount of energy. After the circulating pump is frequency-controlled, all valves are at their maximum opening, and system resistance is minimized. By controlling the speed of the circulating pump frequency converter based on the temperature difference between the secondary inlet and outlet water and the minimum head required by the height of the heat user, the flow rate of the circulating pump can be greatly reduced. When the average flow rate is 80% of the design flow rate, the energy saving rate can be calculated according to the calculation formula in the implementation supervision guide of the mandatory national standard GB12497 "Economic Operation of Three-Phase Asynchronous Motors", which is: energy saving rate 36%. It can be seen that the energy saving benefits are considerable. IV. Conclusion China is one of the energy-poor countries, and energy conservation and consumption reduction are our national policies. There are tens of thousands of centralized heating heat exchange stations in cities across the country. If all of them were to undergo energy-saving renovations, the amount of electricity saved would be considerable. Moreover, the system operates stably and reliably, achieving unattended operation, with significant economic and social benefits.