Abstract : This paper introduces the application of frequency converters in boiler heating systems, focusing on the system modification process and the selection of optimal parameters. It also summarizes some maintenance experience in engineering applications through practical operation. This is applicable to practical engineering projects. Keywords : Frequency converter; Closed-loop control; PI controller 1 Introduction With the widespread application of frequency converters in various industries, they are increasingly being used in boiler heating systems. For example, frequency converters are widely used to directly control the motor speeds of boiler induced draft fans, forced draft fans, water supply pumps, circulating hot water pumps, and grates to regulate related flow rates. This significantly reduces the mechanical wear of boiler operators adjusting dampers, valves, and motors, reducing equipment maintenance and extending service life, resulting in significant overall benefits. However, many application units often only use open-loop control, relying on boiler operators' experience to manually control the frequency set by the frequency converter, making it difficult to ensure the boiler operates in optimal condition. This paper proposes a single closed-loop control system composed of a PI controller and a related transmitter (i.e., a domestic Type III electric instrument). Our institute has successfully upgraded the boiler heating system currently using frequency converters to control fans and pumps, achieving significant benefits and providing a feasible solution for selecting optimal parameters. On-site commissioning and operation results show that this upgrade is simple, reliable, energy-saving, and offers substantial overall benefits. 2. Closed-Loop Control System for Boiler Induced Draft Fan To improve the reliability of the boiler system, reduce energy consumption, and optimize coal combustion in the furnace, a micro differential pressure transmitter is installed in the furnace. This transmitter monitors the negative pressure during combustion in real time and transmits the signal to a PI controller. After comparison with the set value, the difference is sent to the frequency converter controlling the induced draft fan. The output frequency is then synchronously adjusted to achieve optimal combustion. In the 60t boiler heating system used on-site, the induced draft fan motor capacity is 110KW. In the upgrade, an IPF-110K Sanken frequency converter was selected; the controller is also compact. The MCAII-U type Fuji PI controller is easy to install and can provide DC24V power internally. The sensor uses the high-precision, adjustable-range (0-250mmH2O range) and adjustable-damping-time STD110 series differential pressure transmitter manufactured by Honeywell, USA. Since this project involves the renovation of an existing boiler, the measurement point of the differential pressure transmitter probe is selected at the original pre-reserved probe port on the boiler side wall to measure the negative pressure inside the furnace. Therefore, the negative pressure value reflected in the furnace will vary depending on the measurement location. This point can also monitor the combustion effect in the furnace in real time. When the combustion effect in the furnace is optimal, i.e., when the induced draft is optimal, the negative pressure value at this point in the furnace will remain at a constant value, indicating optimal combustion. At this time, by continuously measuring 10 sets of furnace negative pressure values, it can be found that the actual measured value basically fluctuates around a constant value. As shown in Table 1, the magnitude of this fluctuation is related to the damping time of the differential pressure transmitter. Therefore, the damping time was adjusted to 32s. After determining the optimal value of the furnace negative pressure, this value was transmitted to the PI controller, and the setpoint of the PI controller was calculated. The DC24V power supply of the transmitter was provided by the controller. The PI controller compared the actual measured value with the setpoint, obtained the deviation of the controlled variable, and generated an output signal (DCO-10V) according to a certain regulation law. This signal was sent to the frequency converter to control the air volume of the induced draft fan. To avoid overshooting of the output signal and causing severe system oscillations, during on-site commissioning, the proportional controller (P value) of the controller was generally set to about 1, and the integral controller (I value) was set to more than 15s. This ensured a stable and reliable system response with minimal fluctuations. The DCO-10V signal transmitted by the PI controller controlled the output frequency of the frequency converter through the input unit of the frequency converter, thereby controlling the speed of the induced draft fan motor and thus controlling the air volume of the fan blades. This ensured the oxygen consumption for combustion in the boiler furnace. This also ensures that the negative pressure in the boiler furnace remains constant during optimal combustion. The damper that originally controlled the induced draft volume must now be fully open. During on-site commissioning, due to the large rotational inertia of the induced draft fan, the fan motor was in a generator state during startup and shutdown, causing the inverter controlling it to trip and shut down. Therefore, the soft-start time (acceleration time) and soft-stop time (deceleration time) of the inverter controlling the motor need to be extended to over 90 seconds to prevent the inverter from shutting down due to overvoltage protection. 3. Closed-Loop Control System of Boiler Makeup Water Pump To maintain the water level in the boiler drum and promptly replenish water lost due to runoff, leakage, and evaporation, a pressure transmitter is installed in the return water channel of the boiler circulating hot water to monitor the water pressure entering the boiler drum in real time, thus maintaining a constant water level in the drum. The signal is transmitted to the PI controller, and after comparison with the set value, the difference is sent as an analog quantity to the frequency converter controlling the make-up water pump motor to adjust the amount of water supplied by the pump, ensuring that the boiler drum reaches the optimal water level. In the 60t boiler heating system used on-site, the make-up water pump motor capacity is 22KW. During the retrofit, an IPF-22K Sanken frequency converter was selected. The controller used is an MCAII-U type Fuji PI controller, and the sensor is a domestically produced DDZIII type electric combination instrument, with a DBY-1300 pressure transmitter manufactured by Dalian Instrument Factory, with a range of 0-2.5MPa. The most common fault in the make-up water pump closed-loop system retrofit is: due to long-term disrepair of the one-way check valve at the make-up water pump outlet, leakage occurs, causing the motor to reverse, resulting in overvoltage or overcurrent protection failure and shutdown of the frequency converter. This fault generally occurs during motor deceleration. This issue is most likely to occur when the inverter's output frequency is relatively low, therefore regular maintenance and inspection are necessary, and special attention should be paid. 4. Conclusion In summary, through the closed-loop transformation of the existing boiler heating system, we have improved the boiler's combustion effect and heating efficiency, and achieved automatic real-time control of boiler operation, which has significant practical implications for boiler system transformation. If inverter control is also used for the motors controlling the blower, circulating hot water pump, grate, etc., it can lay the material foundation for centralized computer management and control of the entire boiler heating system, allowing advanced energy-saving technologies to be naturally integrated into the boiler heating system. From the actual operation results, its energy-saving effect is significantly improved compared to open-loop operation, and the equipment investment cost can generally be recovered within one and a half years. The benefits are considerable and worthy of practical promotion. [b][align=center]For more details, please click: The Closed-Loop Control Role of Inverters in Boiler Heating Systems[/align][/b]