Abstract : This paper presents the energy-saving principle of frequency conversion technology, introduces its successful application in boiler control systems, and analyzes and evaluates its energy-saving effect. Keywords : frequency conversion technology; boiler control; energy saving As an important energy conversion device, the boiler plays a crucial role in industries such as power, machinery, metallurgy, chemical, textile, papermaking, and food, as well as in residential heating . Based on production load requirements, the boiler must constantly adjust its production status and change the amount of heat supplied. When selecting fans and pumps, users determine the capacity based on the maximum load specified in the process requirements, resulting in a "large horse pulling a small cart" phenomenon. Furthermore, the airflow of the boiler's induced draft fan and forced draft fan is controlled by adjusting the damper, while the speed of the motors driving the fans and pumps is not adjustable, leading to significant adjustment losses and wasted electricity. Based on this situation, this paper proposes a boiler control system that uses variable frequency speed regulation (VFD) technology to control boiler production equipment. This significantly improves operator working conditions, enhances the starting performance of the fan equipment, achieves stepless speed regulation, and saves approximately 35% of electrical energy, thereby achieving energy conservation, consumption reduction, and reduced equipment noise pollution. 1. Energy-Saving Principle of VFD Speed ​​Regulation for Fan-Type Loads Fans are devices that convert the shaft power of an electric motor into fluid. In the past, speed control methods were rarely used; instead, squirrel-cage induction motors were used for constant-speed operation. When the flow rate needed to be changed, throttle valves and baffles were adjusted. While this method is simple to control, it is inefficient, uneconomical, and has poor dynamic tracking performance. Variable frequency speed control (VFD) energy saving is relative to valve regulation. Using a VFD, the valve is fully opened, and the motor speed is changed by altering the motor power supply frequency. According to fluid mechanics, flow rate Q is directly proportional to the first power of speed n, wind pressure H is directly proportional to the square of speed n, and power P is directly proportional to the cube of speed n, i.e.: Q = Qe × (n/ne), H = He × (n/ne)², P = Pe × (n/ne)³. Where Qe is the rated flow rate of the fan, He is the rated pressure of the fan, Pe is the rated power of the fan, and ne is the rated speed of the fan. As shown in the formula above, the fan flow rate can be adjusted by regulating the rotational speed. In this case, the fan shaft output power is proportional to the cube of the rotational speed. This can be analyzed based on the fan system characteristic curve (Figure 1). Assuming the optimal efficiency operating point of the fan is point A, when the fan's air supply needs to be reduced, the traditional damper adjustment method increases system resistance to meet the requirement, shifting the fan's operating point from point A to point B. This method not only fails to save energy but also accelerates fan efficiency loss. Furthermore, inefficient operation causes higher air and structural vibrations, generating noise and damaging equipment. By adopting variable frequency speed control (VFD) technology, the asynchronous motor speed is reduced, allowing the system to re-balance, shifting the operating point from point A to point C. From point C, it can be seen that although the motor speed is reduced, it has little impact on fan efficiency. Based on the above principle, when the fan flow rate varies over a large range, using VFD to control the fan speed will achieve a very significant energy-saving effect. The relationship between fan flow rate, rotational speed, shaft power, and power supply frequency is shown in Table 1. 2. System Implementation The boiler control system is shown in Figure 2. The measurement and control components detect and control the boiler production parameters, changing the grate frequency converter based on the set boiler outlet water temperature, and simultaneously changing the forced draft frequency converter. The induced draft frequency converter is adjusted to maintain a slight negative pressure state within the furnace. [align=center]Figure 2 System Composition[/align] This boiler control system has been successfully applied in a heating project in Fushun. The entire heating system consists of one 40t/h and two 20t/h hot water boilers. The system uses Rockwell PowerFlex series products, using 15 frequency converters with a total installed capacity of 1166KW, controlling the induced draft, forced draft, grate, circulating pump, water supply pump, and slag removal motors of three boilers. The combination of the PLC and each frequency converter is shown in Figure 3. The induced draft, forced draft, and grate frequency converters have two operating modes—local control and central control. Under normal circumstances, the operator operates from the main control room, adjusting the frequency converter according to the boiler's operating parameters to achieve remote control. The working status and operating parameters of each frequency converter can be observed on the host computer. In case of emergency, the operator can operate from the frequency converter room to better understand the situation on site. When automatic control is used, the PID loop can automatically adjust and control according to the set value, keeping the system's outlet water temperature, outlet water pressure, and furnace pressure at predetermined values. 3. Energy Saving Effect Assessment Taking a heating project in Fushun as an example, the system reports show that the average shaft power output is below 50%. Conservatively calculating with a 35% energy saving rate, the energy saving of the system in one heating season (calculated as five months) is: 1166kW × 35% × 24 × 30 × 5 = 1,469,160 kWh. Calculated at an electricity price of 0.50 yuan, the electricity cost saved in one heating season is: 1,469,160 × 0.50 = 734,580 yuan. [align=center]Figure 3 Combined Application of PLC and Frequency Converter[/align] After applying frequency conversion speed regulation technology, the original operation mode of the fan has been changed, realizing remote control. This effectively regulates the boiler production process, ensuring stable system operation, maintaining efficient fan operation, and enabling soft starting of the motor with no inrush current. This significantly reduces equipment failure rate and maintenance costs. The system's application of frequency conversion speed regulation technology not only greatly saves energy but also allows fans and pumps that operate under light loads for extended periods to run at low speeds and low voltages. This results in less motor heat generation and lower temperature rise, extending service life. Frequency conversion speed regulation technology also improves the power factor, reduces grid losses, increases efficiency, and reduces fan noise, improving the production environment. Furthermore, the frequency converter has comprehensive self-detection, fault diagnosis, and protection functions, effectively preventing the escalation of accidents. 4 Conclusion The boiler control system proposed in this paper utilizes frequency conversion technology to achieve automatic control of boiler production equipment and systems. Due to its energy saving, consumption reduction, advanced technology, and convenient control, it is an important approach for product upgrading and enterprise equipment transformation. References [1] Che Yunhui. Application of variable frequency speed regulation technology in polyvinyl chloride drying system. Devices and Equipment. 2002 (6) [2] Xu Yan. Retrofit of variable frequency energy-saving system for blower. Foshan Ceramics. 2005 (9) About the author Wang Cuizhu (January 1984), male, from Chifeng, Inner Mongolia, master's student, research direction is boiler combustion optimization control. Contact information of the author Address: Wang Cuizhu (received) Class 06 (3), Shenyang University of Technology, No. 6, Nanping Middle Road, Hunnan New District, Shenyang, Liaoning Province Postcode: 110168 Tel: 15998856235 E-mail: [email protected]