Abstract This paper studies the control system of a heat exchange station and develops and designs an automatic control system for the heat exchange station based on a PLC controller. The system adopts a PID control strategy to perform real-time data acquisition and control of the primary network, secondary network, and water supply system of the heat exchange station. After actual operation verification, the system is stable and performs well.
Keywords: centralized heating system; heat exchange station control system; PLC
Development and Implementation of Heat Exchanger Control System
Zhenduo Fu
(Beijing Hollysys Automation and Drive Co., Ltd, Beijing, 100176)
Abstract This paper discusses the heat exchanger control system, and develops a PLC based automatic control system with PID control method. The system accomplishes the realtime data acquisition and equipment control of the primary network, the secondary network and the water supply system. This system has been realized with a stable and good performance.
Key Words Central Heating Supply System; Control system of heat exchanger; PLC
1 Introduction
Central heating systems are an essential part of urban infrastructure and a hallmark of urban modernization. Developing centralized heating is of great significance for energy conservation, environmental improvement, and raising people's living standards. Due to its significant social, environmental, and economic benefits, it is both necessary and urgent for urban development and improvement.
Urban centralized heating information management, dispatching, and automated control systems play a vital role in ensuring high-quality heating, safe operation, economic efficiency, and environmental protection. By constructing such systems, all major information regarding heating projects throughout the city can be collected, enabling centralized monitoring, comprehensive analysis, coordinated dispatching, and improved efficiency. This transforms heating industry management from an extensive to an intensive model, leading to an overall improvement in the industry's control level.
Reducing heat loss is the primary focus of energy conservation in the heating industry. This article aims to provide heating control solutions that minimize heat loss through automatic control. By providing a process control function library, it ensures that the load is met while maximizing the efficiency of these devices under corresponding process conditions, thereby minimizing energy consumption.
2. Working principle of heat exchange station
District heating, also known as centralized heating, uses hot water and steam as energy carriers to supply heat to all heat users in a region through a pipe network. It typically involves one or more centralized heating devices, such as heating boilers, combined heat and power (CHP) units, geothermal springs, low-temperature heating nuclear reactors, and industrial waste heat. A district heating system consists of three parts: the heat source, the heat users, and the heating network. The heat source is responsible for preparing the heat medium, the heating network is responsible for transporting the heat medium, and the heat users are the places where heat is used. Heat users in a district heating system include heating, ventilation, hot water supply, air conditioning, and heat-using systems in production processes.
The heat load of each heat user's heating system can be divided into two main categories according to its nature: seasonal heat load and year-round heat load. Hot water heating systems can be classified into two types according to their airtightness: open and closed. In a closed system, the circulating water of the heating network serves only as a heat transfer medium, supplying heat to heat users, and is not extracted for use. In an open system, the circulating water of the heating network is partially or completely extracted from the heating network and used directly for production or hot water supply.
Because the heat load of heat users in a heating system is not constant—for example, the heating and ventilation heat load varies with outdoor weather conditions, and the hot water supply and heat used in production processes vary with usage conditions—it is necessary to regulate the operation of the heating system to ensure heating quality, meet the requirements of all heat users, and ensure the rational preparation and transmission of heat energy.
In urban centralized hot water heating systems, the heating load is the most important, and sometimes the only, heat load. Therefore, the heating load's variation with outdoor temperature is typically used as the basis for heating regulation. The purpose of heating regulation is to ensure that the heat output of users' radiators matches the variation in their heat load, thus preventing excessively high or low temperatures for users.
Depending on the location of the heating regulation, heating regulation can be divided into three types: centralized regulation, local regulation, and individual regulation. Centralized regulation is carried out at the heat source or heating network. Local regulation is carried out at the heat exchange station or the heat user inlet, and individual regulation is carried out directly at the heat dissipation equipment. Centralized heating regulation is easy to implement and convenient to operate and manage, making it the most common heating regulation method.
The heat exchange station in a centralized heating system is the connection point between the heating network and heat users. It houses the relevant equipment, pipes, valves, instruments, and control devices for connecting to users. Its function is to regulate and convert the heat medium transported by the heating network according to different connection methods based on the heating network's operating conditions and varying circumstances, distributing heat to the user systems to meet user needs. Simultaneously, it should also perform centralized metering and monitoring of the parameters and quantities of the heating medium as needed. Depending on its scale and location, heat exchange stations can be categorized as primary stations, district heat exchange stations, centralized heat exchange stations, and user heat exchange stations. The control of heat exchange stations is a key aspect of heat distribution network control.
The heat exchange station primarily facilitates heat exchange from the primary heating network to the secondary network, with the hot water from the secondary network typically ranging from 40℃ to 65℃. The heat exchange station monitoring system can acquire, measure, control, and remotely transmit signals related to the heating network's temperature, pressure, flow rate, and on/off status. It provides real-time monitoring of various parameters, including the temperature, pressure, and flow rate of the primary and secondary networks, the operating status of the circulating pumps and makeup water pumps, and the water tank level, enabling effective monitoring and control of the heating process. During the heating season, the supply and return water temperatures of the secondary network can be adjusted according to the outdoor temperature (manual or automatic switching is possible) to achieve on-demand heating and climate-compensated energy-saving control. Time-based and zone-based energy-saving control can also be implemented to achieve heat balance across the entire heating network and conserve energy.
The system architecture diagram is shown in Figure 1.
Figure 1 Working principle of the heat exchange station
3. Design of the heat exchange station control system
The on-site PLC control station mainly consists of two parts: an LCD display operator terminal and a control system. The color LCD screen primarily displays various monitoring images and acquired parameters, and accepts input information for parameter settings. The control system uses the cost-effective Hollysys LM series PLC, including a CPU module, I/O modules, backplane, etc. The system integrates multiple communication interfaces, making it suitable for various communication networks.
The main function of the on-site PLC control station is to monitor and collect real-time operating parameters (primary and secondary network temperatures, pressures, flow rates, etc.) of each heat exchange station, each node along the heating line, and heat users, as well as the operating status of various equipment. Based on changes in meteorological conditions and load, it automatically adjusts the circulating pumps, makeup water pumps, and regulating valves of the heat exchange stations according to pre-set control strategies to achieve fully automatic control of the heat exchange units. With the support of a communication network, the on-site monitoring station can also transmit data reflecting the heating operation to the dispatch center and simultaneously receive dispatch control commands from the dispatch center.
The connection diagram of LM is shown in Figure 3-1.
Figure 3-1 LM Connection Diagram
4. Main functions of the heat exchange station control system
ì Primary network electric regulating valve control
The heat exchange station control system features climate compensation and constant temperature water supply functions, meaning it automatically adjusts the heating capacity according to climate changes. Utilizing a programmable logic controller (PLC), it determines the heating capacity of the secondary grid side based on outdoor temperature variations and the local heat load curve. After comparing the measured heating capacity with the set value, PID closed-loop regulation is performed. The controller outputs a signal to the electric regulating valve, adjusting its opening to change the flow rate on the primary grid side, thereby regulating the heating capacity of the secondary grid side.
ì Variable frequency control of secondary network circulating pump
This system implements PID control of the supply/return water pressure difference in the secondary network, thereby ensuring normal heating at the most unfavorable point. The pressure difference setpoint can be set based on empirical parameters or empirical curves.
ì Variable frequency control of water supply pump
Automatic water replenishment is achieved by comparing the pressure signal detected by the pressure transmitter on the secondary network return water pipeline with the return water pressure set value on the controller, and then outputting a control signal. The speed of the water replenishment pump is adjusted by PID control, thereby realizing automatic replenishment of the secondary network return water.
ì Protective function
Pressure loss protection: When the return water pressure on the secondary network side falls below the ultra-low limit setting, the circulating pump will automatically stop running, the electric regulating valve will close, and the automatic water replenishment system will be activated to begin replenishing water. If the return water pressure on the secondary network side continues to decrease after the automatic water replenishment system is activated, an audible and visual alarm will be triggered.
Power failure protection: After a power outage, the electric regulating valve is automatically closed, the heat source is cut off, the controller and frequency converter are automatically reset, and all setting parameters and operating status parameters are kept at their original settings before the power outage.
Over-temperature protection: When the secondary network supply water temperature exceeds 80℃ (adjustable setting on the control panel), the primary network side electric regulating valve closes. When the primary network return water temperature exceeds 70℃ (adjustable setting on the control panel), the high-limit protection is activated, controlling the opening of the electric regulating valve based on the primary network return water temperature. When the primary network supply water temperature exceeds 120℃, the primary network side electric regulating valve immediately closes and an alarm sounds.
Overpressure protection: When the secondary network water supply pressure exceeds the set overpressure limit (adjustable on the control panel), the circulating pump stops running and the electric regulating valve on the primary network side closes.
It monitors the water tank level and has an alarm protection function. The water tank level is input via a 4-20mA signal, and the signal to stop the water supply pump is sent out by the controller. The pump stop water level can be manually set.
By establishing such an automatic control system for the heat exchange station, unmanned operation of the heat exchange station can be achieved, bringing the following functional effects:
n To have a macro-level understanding of the operating status and quality of the heating system.
n Ensure the operating parameters of the heating system. Implement fully automatic adjustment of the hydraulic and thermal conditions of the heating network, resolve the coupling effects of various heat exchange stations, eliminate horizontal imbalances in the heating network, and balance the heating effect.
With the goal of saving total heat supply, the aim is to reduce the total heat supply as much as possible while meeting the basic heating requirements of heating network users, thereby improving economic efficiency.
To better maintain and manage heating system equipment. To promptly detect and report heating system malfunctions, preventing problems before they arise.
This provides an analytical basis and foundation for how to ensure the economical and efficient operation of heating networks. By comparing and analyzing historical data of heating network operation at the end of a heating season with previous data, the main sources of energy consumption can be identified, providing a basis for future energy-saving renovations.
5. Conclusion
This automated centralized monitoring system has the following advantages: ① The system automatically controls temperature and pressure, ensuring continuous and stable operation; ② It provides a complete human-machine interface, allowing users to monitor and operate the system via a touchscreen; ③ It uses control algorithms to control the heating time and temperature curves, achieving energy savings.
The PLC control system acts as the control station, realizing automatic control of heat exchange stations or heat exchange units. It achieves excellent human-machine interaction, allowing operators to easily access real-time data and set parameters or perform manual control on the touchscreen. Simultaneously, the PLC control system provides automatic fault protection. Maintenance personnel can directly configure and easily troubleshoot equipment malfunctions on the touchscreen, achieving both intuitive display and cost savings. This design has been implemented by Beijing Hollysys Automation Drive Technology Co., Ltd. in multiple heating network projects, yielding excellent practical application results.
References
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[4] Hollysys PLC - A PLC for Solutions [J], Domestic and International Mechatronics Technology
