Abstract: This paper briefly introduces the composition and function of a water treatment DCS system, and discusses the principle and software design for temperature monitoring of circulating water based on the DCS. The system operates well and has significant energy-saving effects. Keywords: DCS; frequency converter; main/slave control 1. Overview Industrial circulating water plays a very important role in the steel rolling industry. Its water temperature directly affects the safety and service life of equipment such as heating furnaces and rolling mills. In order to monitor circulating water in a timely and accurate manner, the water treatment system of small and medium-sized workshops in Laiwu Steel Rolling Mill adopts the SUPCON JX-300X DCS control system from Zhejiang University Control Technology Co., Ltd. The DCS control system integrates computer, instrumentation and electrical control technology, realizing functions such as automatic data acquisition, processing, process screen display, parameter over-limit alarm, equipment fault alarm and report printing. On this basis, a closed-loop temperature control for circulating water temperature has been developed. 2. System Introduction The SUPCON JX-300X DCS adopts high-speed industrial Ethernet Scnet as its process control network. The system consists of engineering stations, operator stations, control stations, process control units, etc., and is flexibly configured. Based on the scale of the water treatment system, the SUPCON DCS system is designed with one control station and one operator station (which can also function as an engineering station by setting a safety level), forming a two-tier structure. The control station is the field level, directly exchanging data with field equipment to complete real-time monitoring of the entire industrial process. The operator station (which also functions as an engineering station) is the supervisory control level, capable of configuring the system and managing the field-level detection, control, and protection functions. The SUPCON JX-300X DCS system's SCKey configuration software is a comprehensive configuration software platform that supports various control schemes for the system. Through the SCKey configuration software, the entire system's control station configuration (system I/O configuration, control scheme configuration, etc.) and operator station configuration (screen settings, system flowchart configuration, system report configuration, etc.) can be completed. The system structure is shown in Figure 1. 3. Temperature Monitoring Scheme 3.1 Scheme Design Industrial water circulating from the production hot line flows into suction wells #1 and #2 for heat dissipation. It is then cooled by fans #1 and #2 at the top of the suction well towers. Finally, a shared outlet pipe at the bottom of suction wells #1 and #2 supplies water to the next process. The water temperature must be controlled below 30ºC. This system installs a Cu50 resistance temperature detector (RTD) at the suction well outlet pipe to monitor the water temperature. The temperature signal T1 is sent to the DCS processor. After calculation based on the control law, a control signal is given to control the frequency converter, which changes the input voltage frequency of the motor, thereby changing the motor speed and driving the fans for cooling, thus forming a closed-loop temperature control system. Two SANKEN (300KW) general-purpose frequency converters from Japan are selected. The control principle is shown in Figure 2. Based on the heat exchange characteristics of the circulating water at the bottom of the suction wells, the motors of the two fans in suction wells #1 and #2 are controlled as main/auxiliary motors. The main and auxiliary motors drive the fans according to different speed curves. Motors #1 and #2 can be selected as either primary or secondary motors, greatly simplifying operation for operators. 3.2 Software Implementation To facilitate operation, a circulating water temperature monitoring screen is created at the operator station. This screen displays the circulating water temperature, inverter speed feedback, alarms, etc., online, and allows for start/stop control and primary/secondary selection control of motors #1 and #2. Temperature, speed, and other control parameters in the primary/secondary motor control loops can be modified online by switching safety levels. Addressing the significant lag in circulating water temperature regulation, a segmented control method is adopted for motors #1 and #2. The control curves for the primary/secondary motors are shown in Figure 3: When the circulating water temperature is below 20ºC, the output voltage frequency of the inverters for both primary and secondary fan motors is 20Hz. As the circulating water temperature increases, the output voltage frequency of the inverter in the primary control loop increases accordingly. When the circulating water temperature exceeds 24ºC, the primary motor operates at full operating frequency. When the circulating water temperature rises to 23ºC, the auxiliary motor's input voltage frequency is increased by the auxiliary control circuit, thus increasing the fan speed. When the circulating water temperature exceeds 28ºC, the auxiliary motor operates at full frequency, working with the main motor to rapidly cool the circulating water. 4. Application Results Since the circulating water temperature monitoring system was put into operation, it has been running stably and effectively. In the summer when ambient temperatures are high, it can effectively control the circulating water temperature below 30ºC , meeting the requirements for hot-line production. In the spring, autumn, and winter seasons when ambient temperatures are low, the fan motor operates according to the control curve, reducing energy consumption by 35% compared to the fan's full frequency operation before the control function was added. 5. Conclusion Utilizing the flexible configuration of the DCS system, production monitoring of the water treatment process is carried out at a lower cost, creating a highly efficient working environment for on-site operators. Based on the DCS system, temperature monitoring using existing equipment has achieved the expected energy-saving and consumption-reducing effects.