The role of variable frequency speed regulation in evaporator fan pump loads

1 Introduction

The application of variable frequency speed regulation on fans and pump loads has significant energy-saving effects and has excellent control characteristics of soft start and soft stop. Therefore, frequency converters are first widely used in the control fields of metallurgy, electricity, petrochemicals, heating and civil fans and water pumps. With the in-depth development of my country's market economy, the increase in production, energy conservation and emission reduction, and efficiency improvement of paper mills and pulp mills have been mentioned in an important position, and equipment and electrical transformation are imperative.

The main function of the evaporation fan is to recover the waste liquid steam injected into the evaporator and then send it to the heat sink to increase the pressure and temperature. It should be noted that before the evaporation fan is turned on, fresh steam needs to be replenished. When the temperature reaches above 96°C, the evaporation fan can be turned on to achieve self-circulation and close the fresh steam to save energy. The evaporated condensate can be recycled after cooling, thus saving clean water resources.

During the waste liquid evaporation process, the required air volume and pressure have different requirements at different stages of the evaporator evaporation efficiency. Among them, the damper adjustment efficiency is poor. It artificially changes the resistance curve and increases the wind resistance. The more it is adjusted, the worse the performance will be. If a frequency converter is used to change the evaporation fan speed, it can be operated at a better operating point, and the fan can operate at a higher efficiency in a larger range, the energy-saving effect will be better.

The evaporation fan pump of a paper mill is the core component of the evaporation system. Its reliable and stable operation is the fundamental guarantee for the normal operation of the entire evaporation system. Due to the special function of the evaporation fan pump in the entire system, it is a special load type, which is neither a fan nor an ordinary pump load. The evaporation fan pump has high power and is a high-energy-consuming equipment in the evaporation section. If it can be successfully transformed into a variable frequency energy-saving device, it will produce very considerable benefits.

2. Evaporation fan section and equipment introduction

Figure 1 Flow chart of evaporation section The flow chart of evaporation section of paper mill is shown in Figure 1. The motor used by evaporation fan is 10kV/3000kW three-phase squirrel cage asynchronous motor (motor parameters are shown in Table 1). According to the requirements of evaporation fan, 2559kW and 2564 rpm can meet the requirements, but there is no such model and gear in the motor selection. In order to adapt to the needs and starting characteristics of the fan, a larger motor must be selected. According to the speed requirements of evaporation fan, there is no motor with such specification speed. If a high-level motor is selected, the motor's own rotational inertia cannot meet the starting requirements. Therefore, according to the above requirements, it is necessary to choose to increase (1.7x motor rated speed) speed increaser to meet the requirements. Although it meets the process and production requirements, it is a waste of energy and increases production costs.

The evaporation fan used in the evaporation section is a turbine fan of the evaporator. The speed is required to reach 151.3T/H at 2564rpm. The compressed steam volume is changed by adjusting the damper. The damper changes regularly according to the start-up characteristics and evaporation efficiency. Before the evaporator works, fresh steam is added inside the evaporator to increase the temperature to above 96℃ and then stop. Then the damper automatically reaches 30% and then the motor is turned on. Then the waste liquid is added and the damper is adjusted to 75% for normal startup.

Modern society has entered an era of competition with high production capacity and low consumption. According to energy identification, it can be found that there is a huge waste of energy in the motors, speed increasers and evaporation fans in the evaporation section. It is urgent to choose energy-saving and consumption-reducing solutions.

3 Application Examples

Shandong Sun Paper Co., Ltd. is a world-leading multinational papermaking group and forest-pulp-paper integrated enterprise. It is China's largest private papermaking enterprise, China's largest high-end coated packaging paperboard manufacturer, and China's leading food-grade packaging paper enterprise. The group has total assets of 18.6 billion yuan and an annual pulp and paper production capacity of 3.5 million tons. Its products mainly serve China, the United States, Africa, Southeast Asia, Hong Kong and other countries and regions.

Sun Paper currently has four evaporation fans, which are used in parallel. Depending on the amount of waste liquid, three fans are usually enough to meet the requirements. One fan is used as a backup and is turned on when the evaporation efficiency of one fan is low to replace the evaporator with low evaporation efficiency for cleaning.

After repeated discussions, the persons in charge of on-site production, equipment, electricity and instrumentation decided that the scheme of energy saving and optimized control would be feasible by adopting the method of variable frequency speed regulation to control the air volume and the start and stop of the motor.

The leaders of Shandong Sun Paper Heli Company reported this suggestion to leaders at all levels. After careful comparison and review, they decided to use the Fengguang brand JD-BP38-3000F (3000kW/10kV, 4 sets) high-voltage frequency conversion system produced by Shandong New Fengguang Electronic Technology Development Co., Ltd. to transform the 4 evaporation motors of the evaporation fan motors based on the cost of the transformation. After the high-voltage frequency converter was put into operation, the equipment has been running well so far. After the inspection by Shandong Sun Paper Technology Center, all performance indicators met the design requirements, the evaporation fan achieved effective operation, and the transformation achieved the expected purpose.

The four evaporation fan equipment used by Shandong Sun Paper have the same on-site parameters, and the parameters of one of the equipment are as follows.

Table 1 shows the on-site motor parameters, and Table 2 shows the fan parameters.

3.1 Advantages of frequency conversion speed regulation for evaporator fan

With the application of power electronics technology, microelectronics technology, information technology and modern control theory in speed control systems, and due to the power shortage in recent years, variable frequency speed control technology has become a development direction of modern power transmission. The excellent speed control performance makes the energy saving effect of frequency converters in pulp and paper production more and more significant. Therefore, the evaporator fan is changed to frequency converter control, combining traditional motor speed control technology, modern power electronics technology and computer control technology. The evaporator fan motor speed is adjusted according to the changes in evaporator efficiency, so that the fan can run effectively at an economical speed, thereby achieving the effect of saving electricity. The characteristics of variable frequency speed control energy-saving control device: (1) high speed control efficiency; (2) large speed control range, the speed can be adjusted at any time according to the production process; (3) high speed control accuracy; (4) small starting current, and easy to achieve closed-loop control. Since the original ordinary AC asynchronous motor can be used, it is particularly suitable for the technical transformation of the original old equipment. It not only maintains the advantages of the original fan and motor without replacement, but also achieves a significant effect of saving electricity.

3.2 Shandong Xinfengguang Electronics JD-BP38-3000F High Voltage Inverter

The block diagram of the new Fengguang Electronics JD-BP38-3000F high-voltage inverter system used by Shandong Sun Paper is shown in Figure 2.

Figure 2 High-voltage inverter system block diagram

(1) Main performance indicators of JD-BP38-3000F high voltage inverter

Inverter power: 3000kW

Input frequency: 50Hz

Input voltage: 10kV±20%

Output voltage: three-phase sine wave voltage 0-10kV

Rated current: 216A

Output frequency: 0-60Hz

Frequency resolution: 0.01Hz

Acceleration time: can be set according to process requirements (0-3600s optional)

Deceleration time: can be set according to process requirements (0-3600s optional)

Fault diagnosis and detection: automatic detection, automatic positioning

Grid-side power factor: 0.95 (at high speed)

Overload protection: 120% for 1 minute, 150% for immediate protection

Protection level: IP31

Ambient humidity: 90%, non-condensing

(2) Performance characteristics

Full Chinese WINDOWS operating interface, color LCD touch screen;

High input power factor, less current harmonics, no need for power factor compensation/harmonic suppression devices;

Output step sine PWM waveform, no output filter device is required, no damage to cables and motor insulation, few motor harmonics, reduce mechanical vibration of bearings and blades, and the output line can be as long as 1000 meters;

Modular design of power circuit, easy maintenance;

The high-voltage main circuit and the controller are connected by optical fiber, which is safe and reliable;

Complete fault monitoring circuit and accurate fault alarm protection;

Built-in PLC makes it easy to change the control logic relationship to adapt to changing site needs;