Thread Content
【Abstract】 An intelligent control system for heat exchange stations, composed of components such as programmables, frequency converters, pressure transmitters, and temperature transmitters, mainly introduces the structure of the heat exchange station system and the design of the automatic control system. This control system has been tested in multiple power plants, and users are quite satisfied with it. The system has a high degree of automation, enables remote monitoring, and is safe and reliable, making it easy to repair and maintain. 1 Introduction: With economic development, there is an increasing demand for environmental protection and energy conservation across the country. Utilizing advanced scientific and technological methods to allocate heat efficiently and ensure that existing thermal energy is fully utilized in order to provide better heating services to more users is the primary task of heating companies. Introducing microcomputer monitoring and automated control into heating systems enables a transition from manual to automatic regulation of these systems, which is necessary to meet the heating demands in the new circumstances. 2 Overview of the heat exchange station: The heat exchange station primarily uses heat exchanger equipment to create a closed system in which the high-temperature water generated by the thermoelectric plant is mixed with the low-temperature return water from the user’s secondary network, thereby allowing for heat exchange. Circulation pumps are then used to overcome the resistance present in the secondary network, and the hot water is finally delivered to the heating devices inside the buildings. 3 Control of the heat exchange station system 3.1 Control objectives and main controlled devices The purpose of controlling the heat exchange station system is to adjust the operating mode of the system under various conditions by controlling the circulation pumps, make-up water pumps, heat exchangers, temperature control valves, and regulating valves in the system pipelines, thereby providing a stable water supply temperature at the end points in the most economical manner. At the same time, it enhances the automation level of the system, improves its management efficiency, and reduces the workload associated with management. http://www.tx7878.cn/d/file/jishu/2016-10-10/cbbb05cbec02ed99e36a0ec144425761.gif As shown in Figure 1, the control cabinet collects data on the pressure and temperature in the pipes, and then controls the circulation pump, make-up water pump, temperature control valves, as well as the water level in the tank. 3.2 Design of the electrical control system (1) Based on the equipment’s operation process and the electrical components used for control, arrange the interfaces of the PLC and prepare an I/O address allocation table. (2) The touch screen communicates via the PLCCOM1 port, while the printed circuit board communicates via the PLCCOM2 port. (3) Programming. ①Control process: After the PLC is powered on and started up, the program is initialized first; control parameters such as the supply and return water pressure and temperature on the secondary side, the return water pressure and temperature on the primary side, the outlet water temperature of the heat exchanger, and the over-temperature threshold are set via the touch screen. After reaching the automatic position, activate control. The program performs periodic scanning and PID control; it samples values such as pressure and temperature. After internal calculations, these values are compared with the set points, and PID adjustments are made based on the results. The calculated values generate digital and analog signals that control the start/stop of the water pump as well as the frequency adjustment of the inverter. ②Programmable programming: Automatic control systems are composed of main electrical components such as PLCs, frequency converters, pressure transmitters, temperature transmitters, and level transmitters. The make-up water pump is started and stopped based on the return water pressure ； The circulation pump operates at variable frequency, with the frequency being adjusted based on the pressure difference between the supply and return water streams ； The condensation pump operates based on the water level in the condensation tank. Control principle of the make-up water pump: The current signal is transmitted to the PLC via a return water pressure transmitter; after processing by the PLC, the feedback value is compared with the set value. When the pressure reaches the set lower limit, the PLC sends a signal to automatically start the make-up water pump ； When the pressure reaches the set upper limit, the PLC sends a signal to automatically stop the make-up water pump ； At the same time, when the water level in the water replenishment tank falls below the protective value, the water replenishment pump stops operating automatically. Control principle of the circulation pump: Current signals are sent to the PLC via a return water pressure transmitter and a supply water pressure transmitter respectively. Through calculations performed by the PLC, the feedback value representing the pressure difference between supply and return water is compared with the set pressure difference. When this value is lower than the set value, the PLC outputs a voltage signal to the inverter, causing the frequency of the circulation pump to increase linearly and thus its speed to rise ； When it exceeds the set value, the PLC also outputs a voltage signal to the frequency converter, causing the frequency of the circulation pump to decrease linearly and its speed to drop ； When approaching the set value, the circulation pump runs at a constant speed. Control principle of the condensation pump: The magnetic flap level gauge in the condensation water tank transmits electrical signals to the PLC. Through calculations performed by the PLC, the feedback value is compared with the set value; when the water level in the tank reaches the set upper limit, the PLC sends a signal to automatically start the condensation pump ； When the water level reaches the set lower limit, the PLC sends a signal to automatically stop the condensation pump. Example: The program shown in the figure below is used for collecting temperature and pressure data. First, the analog data collected by AIW0 (Channel 1) is transmitted to VW140, where temperature or pressure values are calculated through computations ； The program shown in the diagram can also detect whether the transmitter is damaged; if it is damaged, a message will be issued indicating that the transmitter needs to be repaired. This detection function can also be manually disabled. http://www.tx7878.cn/d/file/jishu/2016-10-10/2979644d2946b480074960d93e74e536.gif ③ Touchscreen programming: This device uses Siemens 6AV6648-0AE11-3AX0 series touchscreens, which enable real-time monitoring of the operating status and parameters of the system and equipment; they can also report any faults that occur along with solutions, as well as allow for the operation of the on-site equipment. 4 Conclusion The control system for the heat exchange station utilizes PLC control technology, frequency conversion tuning technology, and electronic signal technology to achieve automatic control of pressure and temperature in the heat exchange station. This not only reduces equipment costs but also enhances the level of automation, ensuring reliable and stable operation of the equipment. It can also communicate over Ethernet, allowing the control room to receive various signals; parameters can be set via a touch screen, and start/stop operations can be performed with a single button, enabling remote monitoring.