Abstract Based on the load balance of the aluminum electrolysis cell flue gas purification system, the induced draft volume of the three high-power exhaust fan systems is calculated to have a margin of approximately (15%+17%+17%). The original exhaust system adjusted the air volume by adjusting the dampers, resulting in significant energy loss. After technical transformation, the air volume was adjusted using frequency converters, resulting in significant energy savings. Keywords High-voltage frequency converter; exhaust fan; energy-saving application calculation 0 Introduction The exhaust fans of the electrolysis purification system of Qinghai Datong Company are responsible for the flue gas purification of 260 electrolysis cells. The treatment of electrolysis flue gas adopts three sets of dry purification systems (located in the east, central, and west of the 260 electrolysis cells respectively), with each system treating the electrolysis flue gas of 100, 80, and 80 cells respectively. Currently, the eastern purification system is a newly built flue gas purification system with four 500kW asynchronous motors for the exhaust fans. The older systems (central and western) use synchronous motors with a power of 800kW and a rated voltage of 6kW. These motors use asynchronous full-voltage direct start, resulting in high starting current, significant impact on the power grid, and reduced motor lifespan. The exhaust fans are manually started and stopped, a relatively outdated technology. Airflow adjustment in the control system is achieved by adjusting dampers, which not only reduces fan efficiency but also wastes considerable energy on the dampers. When the purification system needs maintenance, the exhaust fan inlet valves must be closed, yet the fans continue operating normally, wasting a significant amount of electricity. Since frequency converters can achieve soft start and soft stop for high-power motors, avoiding voltage surges during startup, reducing motor failure rates, extending lifespan, and lowering grid capacity requirements and reactive power losses, we have decided to upgrade all three systems using frequency converters. 1. Electrolytic Cell and Exhaust Fan Technical Parameters 1.1 Electrolytic Cell Main Technical Parameters Electrolytic Cell Type: 180 kA prebaked anode electrolytic cell; Number of Electrolytic Cells: 260 units; Single Cell Exhaust Volume: 8800 m³/h; Flue Gas Temperature: 120℃; Electrolytic Cell Gas Collection Efficiency: 98%; Local Atmospheric Pressure: 77327 Pa 1.2 Eastern Exhaust Fan System Technical Parameters 1.2.1 Motor Parameters Model: Y450-6; Rated Power: 500 kW; Rated Voltage: 6.3 kV; Rated Frequency: 50 Hz; Rated Current: 59.9 A; Rated Speed: 960 r/min; Quantity: 4 units; Operation Mode: 2 units operating in parallel simultaneously. 1.2.2 Fan Parameters Model: Y4-73NO.22F; Rated Flow Rate: 302400 m³/h; Rated Speed: 960 r/min; Shaft Power: 500 kW; Total Pressure: 4.316 kPa 1.3 Technical parameters of the exhaust fan system in the central and western regions of Taiwan 1.3.1 Motor parameters Model TD118/448 Rated power 800 kW Rated voltage 6.3 kV Rated frequency 50 Hz Rated current 90.5A Rated power factor 0.9～1.0 Rated speed 750 r/min 1.3.2 Fan parameters Model Y4-73-11NO28D Rated flow 500,000 m3/h Rated speed 730 r/min Shaft power 800 kW Total pressure 3.842 kPa 2 Calculation of the load of the purification system Aluminum Corporation of China Qinghai Branch has 260 180 kA prebaked anode electrolytic cells, with an annual output of 120,000 tons of primary aluminum. The treatment of electrolytic flue gas adopts a dry purification system. 2.1 Calculation of the load of the eastern system 2.1.1 Total air volume of the system The total air volume of the system QE (m3/h) is calculated according to formula (1). 2.1.2 Exhaust Fan Air Volume 2.2 Calculation of Western (Central) System Load 2.2.1 Total Air Volume of the System The total air volume of the system, QW (m3/h), is calculated according to formula (3). 2.2.2 Exhaust Fan Air Volume 3 Modification Scheme 3.1 Control Scheme The exhaust fan motors of the three systems are equipped with high-voltage frequency converters, which control the operation of the exhaust fan. During the start-up process of the exhaust fan, the frequency of the frequency converter smoothly increases from low to high, and the speed of the motor gradually increases from zero to the rated speed. The start-up process is stable and has little impact on the power grid. When the exhaust fan is running normally, the parameters of the high-voltage frequency converter can be set according to the process requirements to achieve the control of the motor speed, thereby achieving the purpose of adjusting the exhaust fan air volume. At the same time, a constant pressure intelligent closed-loop control system for the fan is established. The control system consists of a controller (industrial computer, programmable controller), high-voltage frequency converter, pressure sensor, etc. When the exhaust fan is running normally, this control system adjusts the speed of the exhaust fan motor by regulating the frequency converter through the programmable controller based on the pressure value detected by the pressure sensor in the flue gas purification system, thereby adjusting the exhaust fan's air volume. 3.2 System Configuration 3.2.1 Frequency Converter Control Method Selection The eastern flue gas purification system has a total of 5 exhaust fans (1 spare), equipped with 2 frequency converters (1 spare). During normal system operation, 3 fans operate at mains frequency, and 1 fan operates at variable speed. Any fan in this system can achieve soft start and switch from frequency converter to mains frequency operation. The central and western flue gas purification systems consist of 1 set, each with 3 exhaust fans (1 spare), equipped with 2 frequency converters (1 spare). During normal system operation, 1 fan operates at mains frequency, and 1 fan operates at variable speed. Any fan in this system can achieve soft start and switch from frequency converter to mains frequency operation. 3.2.2 Control Method and Monitoring: The start-stop design for the central and western system fans is based on dual-location control, with control from both the field and the control room. Control cabinets, frequency converters, and PLCs are installed on-site, requiring a total of four control cabinets. Two industrial control computers are installed in the main control room of the cleanroom, using configuration software and a Profibus-DP communication network to monitor the real-time operating status of each fan, displaying the data on both the field and the control room. The monitoring function detects the fan outlet air pressure, bearing temperature, fan bearing vibration, and motor current. The control system can adjust the operating status of the exhaust fan according to the actual needs of the production process, and can remotely monitor all operating parameters of the exhaust fan. When an abnormality occurs, the control system can immediately stop the exhaust fan and issue an alarm. The control system can achieve dual-location control (remote control and field control). The start-stop design for the eastern system fans is based on dual-location control, with control from both the field and the control room. Control cabinets, frequency converters, and PLCs are installed on-site, requiring a total of four control cabinets. One industrial control computer is installed in the main control room of the eastern purification system. It uses configuration software and Profibus-DP communication network to monitor the working status of each fan in real time, and displays the data in both the field and the control room. Other functions are the same as those in the central and western systems. 3.3 Technical requirements for high-voltage frequency converters Due to the expansion of the electrolytic cell (the cell current is increased from 160kA to 180kA), the purification task undertaken by the exhaust fan has also increased. Therefore, the high-voltage frequency converter matched with the exhaust fan is required to have high reliability, be able to adapt to harsh operating environments, have complete digital control functions, standard digital communication interfaces, and meet the speed regulation requirements of the exhaust fan during operation. 4 Economic benefit analysis 4.1 Investment estimate 1) High-voltage frequency converters The frequency converters selected in the eastern system cost approximately RMB 1.2 million per unit, totaling RMB 2.4 million; the frequency converters selected in the western and central systems cost approximately RMB 1.25 million per unit, totaling RMB 5 million. 2) Cables and air conditioning: approximately 1.2 million yuan; 3) Control and sampling system: approximately 800,000 yuan; 4) Equipment installation and other: approximately 400,000 yuan. Total investment: approximately 9.8 million yuan. 4.2 Profit Estimation 4.2.1 Eastern Fans 4.2.2 Central and Western Fans 5 Conclusion Based on the calculation results, the technical transformation of our factory's exhaust fans using high-voltage frequency converters not only has significant energy-saving effects, but also enables soft starting of the fans, greatly reducing the impact of starting current on the exhaust fan system and the power grid. The indirect economic benefits are also considerable. About the Author : Mao Yahong (1976-), female, electrical engineer, graduated from Kunming University of Science and Technology in 1999, engaged in the technical management of electrolytic electrical equipment. References : [1] Peng Hongcai. Principles of Electric Motors and Drives [M]. Beijing: Machinery Industry Press, 1994. [2] Tong Chunhou. Speed ​​Regulation Principles of Frequency Converters [M]. Beijing: Metallurgical Industry Press, 1984.