What role does a PLC play in an electrostatic precipitator control system?
I. Composition and Control Requirements of Low-Voltage Control System for Electrostatic Precipitators
The low-voltage control technology of electrostatic precipitators mainly includes rapping control, insulator temperature box electric heating control, ash hopper electric heating control, ash unloading control, material level control, inlet and outlet temperature display, high-voltage isolation switch position display, and long-distance communication.
1.1 Vibration Control System
Electrode rapping cleaning is a key operation of electrostatic precipitators (ESPs). Its cleaning effect depends not only on the rapping acceleration applied to the anode and cathode, but also significantly on the rapping cycle. Traditional rapping methods are tangential rapping, which can be controlled by continuous rapping or timed rapping. When the rapping force and uniformity meet requirements, the rationality of the rapping principle greatly affects the dust removal efficiency of the ESP. Excessive rapping prevents the dust collected on the anode plate from falling into the ash hopper in clumps, leading to severe secondary re-entrainment, especially in the final stage electric field, which greatly reduces dust removal efficiency. Conversely, excessively long rapping cycles result in thick dust accumulation on the anode, causing a drop in voltage between the anode and cathode, a decrease in secondary current, reduced corona discharge, and ultimately, a decrease in dust removal efficiency. Severe dust accumulation on the anode plate can even form a back corona, allowing the dust already collected on the anode plate to re-enter the airflow. Therefore, selecting a reasonable rapping cycle will help to achieve better cleaning and improve dust removal efficiency.
1.2 Ash Removal Control System
After the dust entering the electrostatic precipitator is captured by the anode and cathode, it is shaken into the ash hopper by the rapping system. This ash should be discharged in a timely manner. If too much ash accumulates, it will not only increase the load on the ash hopper, but in severe cases, it will also cause a short circuit between the anode and cathode, making the electrostatic precipitator unable to operate normally. Conversely, if the ash hopper does not store ash, air leakage will occur at the ash hopper outlet, causing secondary dust emission and reducing the dust removal efficiency.
1.3 Extreme Thermal Control System
The heating control system includes components such as the main beam electric heater, the cathode ceramic shaft electric heater, and the ash hopper electric heater. A common control strategy is to maintain the temperature of the electric heaters based on temperature signals from the temperature measuring equipment. When the temperature reaches the lower limit, the electric heater is activated; when the temperature exceeds the upper limit, heating is stopped.
II. Application Examples of Low-Voltage Control Systems for Electrostatic Precipitators
A factory in Shandong uses a low-voltage control system for its dust removal chamber, which employs a three-field dust removal method. The electrostatic precipitator has three fields with six vibrating motors, three ash hopper heating controllers, three main beam heating controllers, and three ash discharge motors. In addition, there are alarm signal output terminals, hot air motor control, main start, fault clearing, three electric field level gauge detection inputs, and temperature and control at the main beam and ash hopper, etc., totaling 20 digital outputs, 23 digital inputs, 8 analog inputs, and 6TC temperature measurement inputs.
Based on the above calculations, the low-voltage control system of the electrostatic precipitator is mainly controlled by the CPU-224A.
The CPU224A module receives fault signals from the motor drive protector, the electric heating protection controller, the temperature signal from the temperature collector, the material level signal, and the electrical signals from the ash conveying and unloading equipment. After analysis and processing, it sends control signals to the vibrating motor, heater, alarm, etc., according to the process flow to the corresponding external devices. The digital input/output module EM223 sends the signals from the CPU module controlling the ash unloading motor to the corresponding external devices. The analog input module EM231 sends relevant signals from the high-voltage silicon rectifier, the inlet and outlet flue gas temperatures from the temperature collector, and the temperature signals from the beam, ceramic shaft, ash hopper, etc., to the CPU module for analysis and processing.
III. Software Design of Low-Voltage Control System for Electrostatic Precipitators
3.1 Vibration Control Planning
Based on years of experience operating low-voltage control systems for electrostatic precipitators, the following requirements are made for optimizing the control of the local rapping motor in order to improve the dust removal efficiency of the electrostatic precipitator and effectively reduce power consumption:
Cathode and anode rapping cannot be performed simultaneously in the same electric field;
In a multi-field dust collector, the rapping of the anodes (or cathodes) of the front and rear electric fields cannot be performed simultaneously;
In an electrostatic precipitator equipped with a rapping trough plate, the final electric field anode rapping and the trough plate rapping cannot be performed simultaneously.
3.2 Heating Operation
The heater uses constant temperature range control, meaning that the start and stop of the heater are controlled by the upper and lower amplitudes of the set temperature. When the measured temperature is lower than the set temperature by a certain margin, the heater starts working. When the measured temperature is higher than the set temperature by a certain margin, the heater stops working. The heater starts heating again when the measured temperature is lower than the set temperature by a certain margin, and the cycle repeats to complete the temperature control.
3.3 Ash Removal Control Planning
The ash removal methods for electrostatic precipitators can be divided into timed automatic ash removal, automatic ash removal at both the upper and lower material levels, and timed automatic ash removal at the upper material level. This system employs a "high material level + timing" control method, combining timed high material level ash removal with periodic ash removal. High material level ash removal refers to the system activating the corresponding ash removal equipment when the low-pressure control system detects a certain upper material level signal; after the precipitator has operated for a certain period, the corresponding ash conveying interlock system is then deactivated after a delay. To prevent ash blockage in the electrostatic precipitator due to level gauge damage or false alarms, a timed periodic ash removal function is also added to ensure automatic ash removal.
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