Application of frequency converter in paper mill

      Using frequency converters can improve the running performance of paper machines and further increase the economic benefits.

      Analysis of Application of Frequency Converter in Paper Machine Attachment

      The auxiliary facilities of the paper machine include the following systems: slurry supply system, white water system, vacuum system, compressed air system, chemical preparation and delivery system, water supply system, steam system, etc. In order for a paper machine to operate continuously and evenly, its capacity for auxiliary facilities should generally exceed 15% to 30% of the maximum capacity of a paper machine.

      The application of this site has not attracted enough attention. This is also the most effective means and method to increase the share of frequency converters.

     1 、Application of frequency converter for slurry supply system

      The slurry supply system must meet the following conditions:

     (1) The slurry delivered to the paper machine must be stable, and the error cannot exceed ± 5% ;

     (2) The ratio and concentration of the slurry should be stable and uniform;

     (3) stock a certain amount of slurry, so that the pulp supply capacity can be adjusted to adapt to changes in the speed and variety of paper machines;

      (4) Purify the slurry;

      (5) Disposal of broke in all parts of the paper machine.

     Under normal circumstances, the slurry supply system consists of a slurry pump, a slurry pump, a pressure filter, and a slag remover for purifying facilities. In order to achieve the above five objectives, the most important point is that the slurry pump and the slush pump The full-speed operation is transformed into an adjustable-speed variable-speed operation, which ultimately satisfies pulp supply automation.

      Taking the slush pump as an example to illustrate the speed control process of the inverter: The frequency conversion control should adopt the speed control mode of the double closed loop system, the outer ring is the speed closed loop, the inner loop is the current or torque closed loop. The set value of the slush pump speed is obtained on the one hand by the change of the pulp speed and the speed ratio, and on the other hand is the pressure controller from the headbox. The former is the main theme and the latter is fine-tuning. The pulp speed and speed ratio of the paper machine are basically constant, so once the speed of the paper machine changes, the rotational speed of the slushing pump also follows changes; in order to improve the accuracy of the speed regulator and reflect the actual process of the head box Normally, a change of ±5% in the pressure PID control output value of the headbox is also required as an additional speed setting value of the fan pump. The actual value of the speed is taken from the actual speed of the drive motor and can be obtained by a rotating speed measuring motor or a photoelectric rotary encoder. The set value of the current is taken from the output signal of the speed loop, and the actual value of the current is taken from the measured value of the current transformer of the output terminal of the AC frequency converter of each transmission point. Therefore, for frequency conversion speed regulation of the slush pump, it is necessary to perform PID control, and it is necessary to properly select the speed feedback mode and various parameters of the PID . Understanding this point is very important for choosing the type of inverter.

 

       2. Application of frequency converter in compressed air system

 

      Compressed air is commonly used in pneumatic pressurizing and lifting devices for nets and press sections of paper machines, correction devices for net blankets, air-cushion headboxes, threading equipment, coating air knives, and various pneumatic instruments and adjustment devices.

      In the compressed air system, the main equipments include air compressors, gas tanks, pressure reducing valves, air filters, steam separators, and safety valves. The pressure on paper machines is usually around 5 to 6 bar . In most mills, a through 2 or more compressors in parallel, and then through the tank to maintain the pressure constant.

      Because the compressor power is large and the control pressure is generally regulated by loading or unloading, the motor is always at full speed. Practice shows that this control method consumes a lot of energy and wastes heavily. Therefore, the current trend has been to use a frequency converter control, multiple direct-frequency operation to control the compressor group, and to form a pressure closed-loop system.

 

      3 , the frequency changer application of chemical preparation and transmission system

 

      Due to the large amount of chemicals used in deinking, pulping, coating, sizing, etc., the amount used is proportional to the speed of the multi-drive of the paper machine, so it must be adopted in chemical delivery systems ( such as pumps ). Stepless AC speed control system, its preferred frequency converter. Mainly based on the small power of the chemical pump, generally between 0.4kW and 5.5kW , and the performance-cost ratio of the frequency converter is already optimal, and it is still widely used electromagnetic control years ago. And stepless speed regulation gear pumps have already faced elimination. According to the latest market statistics, the medium-brand lowest-power inverter has fallen below 1,000 yuan /kW .

      In the preparation of chemicals, a large number of grinding equipments are used, such as ball mill, colloid mill, sand mill, and high-shear dispersion stirrer. Their biggest feature is high power, high energy consumption, and bad use environment. At present, some manufacturers have achieved good results by using frequency converters in grinding equipment.

       Taking a sand mill as an example, its working principle is that the paint to be milled is fed into a cylinder by a feed pump, and is driven by a high-speed rotating disperse disk and is dispersed and mixed into a solvent by being strongly impinged by the grinding medium and grinding. Made of qualified paint, then filtered out through the top screen. The main motor of this equipment is 200kW . Before the inverter is used, it is usually in the early stage of start-up. It is repeated many times (3 times or more ) in a jog mode to make the paint and grinding media mixed evenly; for different coatings, sometimes different The speed of the process, but in fact can only run at full speed; can not control the amount of feed to ensure that the main motor is not overloaded; energy consumption is very serious. The use of 200kW inverters solves the above problems well. It is easy to set the jogging speed and slow running time to ensure that the paint and the medium are mixed to achieve the most uniform; it can be steplessly adjusted on-line, and different grinding speeds can be used for different varieties. The amount of feed as long as the actual operating current from the motor can control the amount of feed, and there is overload pre-alarm function and no trip function; energy saving rate, generally up to 20% ; reduce the loss of the gear box, to avoid work The frequency starts the impact on the gear box; since the current is gentle at startup, the impact on the power grid is avoided and the safe operation of the power grid is improved. At present, papermaking enterprises in Shandong, Heilongjiang, and Hainan have already applied in small batches.

 

       4. Application of frequency converter in ventilation system of drying section

 

      In the drying section, all vapors evaporated from the sheet are absorbed by the air and must be continuously removed from the paper mill by forced ventilation. Whether the drying section is well ventilated or not directly affects the evaporation rate of the moisture in the paper sheet and the economy of the entire drying process . The ventilation is good and the steam saturation in the air can be reduced, thereby reducing the consumption of dryer steam and improving the drying performance. Dry speed.

      The amount of air necessary to eliminate the evaporation of water from the drying section is related to the temperature and humidity of the incoming and outgoing air, and also related to the ventilation system, weather conditions, and seasons used. In general, forced air circulation is used in modern paper machines for high efficiency. Dry air heated to about 80 degrees is fed into the lower layer of the drying section by an air intake blower, so that hot air is adsorbed between dryers to form an upward airflow. Then, the hot and humid air collected in the hood is drawn out of the room (the last recovered waste heat ) through an exhaust fan. In high-speed paper machines, due to the increase in the number of dryers, they are usually divided into several stages of blowers and exhaust fans. After using the drive, according to the amount of ventilation air expert formula, always adjust the amount of intake air (blower speed) and the displacement (speed exhaust fan), without the use of conventional throttle control, to further reduce energy consumption, reduce Fan noise, improve mechanical life.

 

       5 , the frequency system application of the water system

 

      The paper machine is a large consumer of water, including white water systems, sewage systems, sealed water systems, sprinkler systems, clean water systems, etc. It is usually necessary to use pipe network constant pressure water supply, but the traditional control pressure is through the bypass and The adjustment valve is used for the inverter and is rarely used. However, due to the general lack of domestic water resources in China, the application of frequency converters will save 10% of water and 30% of energy, which will inevitably reduce the daily operating costs of paper mills.

     Inverter is usually used in the water system, there are two modes: fixed frequency conversion, frequency conversion cycle.

 

      (1) Frequency fixing method

      Frequency inverter output fixedly controls one pump while the rest of the pumps are directly powered by the power frequency grid. Their start and stop signals are controlled by PLC logic;

      (2) Frequency conversion cycle

      The frequency converter drives the pumps in turn according to a certain sequence. The frequency converter can automatically determine the number of operating pumps ( within the set range ) according to the pressure closed-loop control requirements. Only one pump is driven by the inverter at the same time. When the pump driven by the inverter runs to the set upper limit frequency and needs to increase the pump, the inverter will switch the pump to the power frequency operation and drive the other pump to run at the same time.

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