Abstract: This paper introduces an energy-saving control system for boiler blowers and induced draft fans based on ABB frequency converters. The energy-saving principle and control technology of the control system are discussed, the control method is given, energy-saving analysis is conducted, and practical application proves that the control system has good control effect and significant energy-saving effect.
Keywords: ABB frequency converter; energy saving; induced draft fan; blower
1 Introduction
In recent years, with the rapid development of electronic technology, variable frequency speed control technology has been widely promoted and applied. Simultaneously, variable frequency technology has evolved from U/F ratio control and voltage space vector control to sensorless direct torque control (DTC), and power switching devices have progressed from GTO and GTR to IGBT and IGCT. The advantages of frequency converters have been fully demonstrated in various fields. Variable frequency speed control technology has also been gradually introduced into the energy-saving control of industrial and civil boilers. Utilizing this technology for stepless speed control of the blowers and induced draft fans in boiler auxiliary equipment has yielded significant energy-saving effects.
2. Control Process
Typically, when selecting a boiler's matching fan, short-term overload capacity must be considered, and an appropriate margin must be added to determine the model. When selecting a boiler, the boiler capacity must be determined based on the maximum process load and an appropriate margin. Since the selection of these two stages is limited by product specification grading, the final fan capacity is often too large. Furthermore, the adjustment of the boiler's forced draft and induced draft fans is accomplished by regulating dampers, resulting in significant energy waste in the fan system when airflow changes. To change this situation, variable frequency speed control technology is the best approach. The process diagram of the boiler forced draft and induced draft fan energy-saving control system is shown in Figure 1.
Figure 1. Process diagram of the energy-saving control system for the furnace blower and induced draft fan.(1) Measure the steam outlet temperature and transmit the measured value to the frequency converter. Together with the steam temperature setpoint, they form a differential PID control system to adjust the blower to change the air supply volume, thereby changing the furnace temperature, which is to adjust the steam temperature.
(2) Measure the negative pressure of the furnace and transmit the measured value to the frequency converter. It forms a differential PID control system with the furnace negative pressure set value to adjust the induced draft fan to change the induced draft volume and keep the furnace in a slightly negative pressure state. Its control process is simple, practical and energy-saving.
3. Control Method
(1) The original induced draft fan operates by reduced-pressure start-up and power frequency operation, and its air volume is adjusted by adjusting the damper. The disadvantages of this control method are large energy waste, poor real-time adjustment, high noise, and high labor intensity for workers. Therefore, it is modified by frequency conversion. The block diagram of the closed-loop control principle of the induced draft fan is shown in Figure 2. The pressure closed-loop circuit composed of differential transmitter, frequency converter, controller and induced draft fan is used to automatically control the speed of the induced draft fan, so that the furnace maintains a stable micro negative pressure. This not only improves the control accuracy, but also saves energy, and makes the control of the induced draft fan more reasonable.
(2) The original operating mode of the blower is reduced-pressure start-up and power frequency operation. The air volume is also adjusted by adjusting the damper. The disadvantages of this control method are large waste of electricity and fuel, poor real-time adjustment, high noise, and high labor intensity for workers. The block diagram of the blower frequency conversion closed-loop control principle is shown in Figure 3. The temperature closed-loop loop composed of temperature transmitter, frequency converter, controller and blower automatically controls the speed of blower, so that the temperature of boiler steam outlet is kept at a stable value. This not only improves the control accuracy, but also saves energy (electricity and fuel), making the blower control more reasonable.
Figure 3. Block diagram of closed-loop control principle of blower4. Energy Saving Analysis
Taking the 20-ton steam boiler of Harbin Jiancheng Group as an example, we will carry out frequency conversion speed regulation control modification on its blower and induced draft fan.
The blower is 45kW and the induced draft fan is 90kW. The investment in the frequency converters is 42,000 yuan and 80,700 yuan respectively. The total investment is 122,700 yuan. If we calculate based on the speed after speed adjustment being 70% of the rated speed: then the theoretical power saving rate of the blower = 1 - (70%)³ = 65.7%. Assuming 300 working days per year, 16 hours of operation per day, and a price of 0.6 yuan/kW·h: Power saving for the blower = 45 × 300 × 16 × 65.7% - 45 × 300 × 16 × 1.5% = 138,672 (kW·h) Power saving for the induced draft fan = 90 × 300 × 16 × 65.7% - 90 × 300 × 16 × 1.5% = 277,344 (kW·h) The 1.5% in the formula takes into account the efficiency of the frequency converter.
The electricity cost and energy savings are shown in the attached table. (Attached Table: Electricity Cost and Energy Savings After Frequency Conversion Retrofit)
The payback period for a one-time investment is approximately 0.48 years, with particularly significant benefits. The above calculations do not yet take into account the investment savings from the electric damper actuator, the economic benefits of frequency conversion enabling soft start and stop of the blower and induced draft fan, and the reduced operating current extending the lifespan of the blower and motor. If these factors are considered, the overall benefits are even more significant, and the payback period is even shorter.
5. Conclusion
This paper discusses the energy-saving principle and control method of an ABB frequency converter-based boiler blower and induced draft fan energy-saving control system. Through the commissioning of this system, the previous black smoke problem in the boiler system has been completely eliminated, and the operation of the boiler blower and induced draft fan system is rationally controlled. The system fully leverages the advantages of stepless speed regulation and energy saving of the frequency converter, resulting in a significant energy-saving effect while meeting environmental protection requirements. Therefore, this system is both energy-saving and beneficial to the country and its people.
