Application of VFD (Variable-frequency Drive) in the recycled paper industry

VFD (Variable-frequency Drive) energy-saving transformation in paper mills

The use of VFD (Variable-frequency Drive) can improve the running performance of the paper machine and further improve the economic benefits.

VFD (Variable-frequency Drive) Application Analysis of Paper Machine Auxiliary Equipment

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

The application here has not attracted enough attention, which is also the most effective means and method to increase the share of VFD (Variable-frequency Drive).

1. Application of VFD (Variable-frequency Drive) in slurry supply system

The slurry supply system must meet the following conditions:

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

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

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

(4) Purify and select the slurry;

(5) Handle broken paper in various parts of the paper machine.

Normally, the slurry supply system is composed of the slurry pump of the slurry supply pipeline, the flushing pump, the pressure screen and the slag remover of the purification facility. The full-speed operation is transformed into a speed-adjustable frequency conversion operation, which finally meets the automation of pulp supply.

Take the slurry pump as an example to illustrate the speed control process of VFD (Variable-frequency Drive): The frequency conversion control should adopt the speed control method of double closed-loop system, the outer loop is the speed closed loop, and the inner loop is the current or torque closed loop. The setting value of the pulp pump speed is obtained by changing the ratio of pulp speed and wire speed on the one hand, and from the pressure controller of the headbox on the other hand. The former is the main tune, and the latter is the fine-tuning. The pulp speed and net speed ratio of the paper machine is basically constant, so once the net speed of the paper machine changes, the speed of the punching pump also changes; in order to improve the accuracy of the speed regulator and reflect the actual process of the headbox , It is usually necessary to take the ±5% change of the output value of the pressure PID control of the headbox as the additional speed setting value of the punching pump. The actual value of the speed is obtained from the actual speed sampling of the transmission motor, which can be obtained through detection devices such as a rotating speed measuring motor or a photoelectric rotary encoder. The set value of the current is obtained from the output signal of the speed loop, and the actual value of the current is obtained from the measured value of the current transformer at the output end of the AC VFD (Variable-frequency Drive) at each transmission point. Therefore, for the frequency conversion speed regulation of the slurry pump, it is necessary to carry out PID control, and it is necessary to correctly select the speed feedback mode and various parameters of PID. Understanding this is very important for choosing a VFD (Variable-frequency Drive) model.

2. Application of VFD (Variable-frequency Drive) in compressed air system

Compressed air is often used in the pneumatic pressurized lifting device of the wire section and the press section of the paper machine, the calibration device of the screen blanket, the air cushion type headbox, the paper threading equipment, the coating air knife, and various pneumatic instruments and adjustment devices.

In the compressed air system, the main equipment includes air compressors, air storage tanks, pressure reducing valves, air filters, steam-water separators and safety valves, etc. The pressure on the paper machine usually needs to be about 5 to 6 BAR. In most paper mills, more than 2 air compressors are operated in parallel, and then the pressure is kept constant through the air storage tank.

Due to the high power of the compressor and the control pressure is generally adjusted by loading or unloading, the motor is always running at full speed. Practice shows that this control method consumes a lot of energy and wastes a lot. Therefore, it has tended to adopt a VFD (Variable-frequency Drive) control and multiple direct power frequency operation to control the compressor unit and form a pressure closed-loop system.

3. Application of VFD (Variable-frequency Drive) in chemical preparation and delivery system

Since a large amount of chemicals are 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 used in the chemical delivery system (such as pumps) The first choice for stepless AC speed regulation system is VFD (Variable-frequency Drive). The main reason is that the power of the chemical pump is small, generally between 0.4kW and 5.5kW, and the performance-price ratio of VFD (Variable-frequency Drive) in this power range is already the best, and it was still widely used many years ago. Both electromagnetic speed regulation and stepless speed regulation gear pumps used are facing elimination. According to the latest market statistics, the lowest power VFD (Variable-frequency Drive) of medium brands has fallen below 1,000 yuan/kW.

A large number of grinding equipment are used in the preparation of chemicals, such as ball mills, colloid mills, sand mills, high-shear dispersing mixers, etc. Their biggest features are high power, high energy consumption, and harsh operating environments. At present, some manufacturers have adopted VFD (Variable-frequency Drive) on grinding equipment and achieved good results.

Taking the sand mill as an example, its working principle is that after the paint to be ground is fed into the barrel through the feeding pump, driven by the high-speed rotating dispersing disc, it is strongly impacted and ground by the grinding medium, and then dispersed and mixed into the solvent. Qualified paint is made, and then filtered out through the top sieve. The main motor of this equipment is 200kW. Before VFD (Variable-frequency Drive) is used, usually in the early stage of start-up, the jogging method is used for many times (more than 3 times) to mix the paint and the grinding medium evenly; for different Paint sometimes requires different process speeds, but in fact it can only run at full speed; it is impossible to control the amount of feed to ensure that the main motor is not overloaded; energy consumption is very serious. The use of 200kW VFD (Variable-frequency Drive) solves the above problems very well. It can easily set the jog speed and slow running time to ensure the most even mixing of paint and medium; it can be adjusted online steplessly, and different varieties Different grinding speeds are used; the feeding amount can be controlled only by the actual operating current of the motor, and it has overload pre-alarm function and trip-free function; the energy saving rate can generally reach more than 20%; the gear box is reduced Loss, avoiding the impact of power frequency startup on the gearbox; due to the smooth current during startup, the impact on the power grid is avoided, and the safe operation of the power grid is improved. At present, paper-making enterprises in Shandong, Heilongjiang, Hainan and other places have already applied it in small batches.

4. Application of VFD (Variable-frequency Drive) in the ventilation system of drying section

In the drying section, after all the water vapor evaporated from the paper is absorbed by the air, it must be continuously removed from the paper workshop through forced ventilation. Whether the drying section is well ventilated will directly affect the evaporation rate of moisture in the paper and the economy of the entire drying process; good ventilation can reduce the steam saturation in the air, thereby reducing the consumption of steam in the drying cylinder and improving the efficiency of the drying process. dry speed.

The air volume necessary to remove the evaporated water in the drying section is related to the temperature and humidity of the incoming and outgoing air, as well as the ventilation system used, climatic conditions and seasons. Usually, forced air circulation is used in modern paper machines for high efficiency, and the dry air heated to about 80 degrees is sent to the lower layer of the drying part by the intake blower, so that the heat vapor is absorbed between the drying cylinders to form an upward airflow, Then the hot and humid air collected in the air hood is drawn out of the room through the exhaust fan (finally recovery of waste heat). In a high-speed paper machine, due to the increase in the number of drying cylinders, it is usually divided into several sections of blower and exhaust unit. After adopting VFD (Variable-frequency Drive), it can be calculated according to the expert calculation of ventilation air volume