Views: 2 Author: Site Editor Publish Time: 2021-11-20 Origin: Site
The net trough structure of the cylinder paper machine is usually made of wood, and the main part of the trough ring is sometimes lined with metal (usually stainless steel), and sometimes also made of plastic. The so-called mesh groove ring refers to the annular gap between the surface of the mesh cage and the semicircular body. The ring of the mesh slot must be carefully constructed, and its shape must be symmetrical and smooth.
There are many forms of paper machine mesh slots. The longest used form is the downstream mesh trough and the countercurrent mesh trough. At present, the most widely used in our country is the movable arc-shaped plate type mesh groove, as well as the dry type mesh groove, the rotary former and the drum vacuum former, etc., in the more advanced paper machine, the suction type cylinder is used.
The structure of the paper machine's slot has a great influence on the output and quality of paper. Various net slots have certain characteristics. These characteristics may be advantages for the production of one paper grade, but may not be important for another paper grade. The mesh slot should be designed or selected according to specific production requirements. The general requirements for the design of the mesh tank structure are: the fibers of the pulp in the mesh tank are evenly dispersed without agglomeration; the pulp in the mesh tank has a uniform flow rate across the width; the pulp channels of the mesh tank are smooth and straight, and the corners should be smooth. Transition to prevent slurry hanging and sedimentation; easy to clean and repair.
There are mainly the following types of online slots for paper machines:
1. Ordinary downstream network slot
The fiber flow direction in the ring of the mesh groove is the same as the rotation direction of the mesh cage. The slurry flow enters the mesh trough through the branch spreader or the cone spreader and other slurry distribution devices. First, it is fully mixed in the flow part (head turning box) to promote the uniform dispersion of the fibers and remove the foam, and then overflows the spray plate and flows into the mesh. Inside the cage. The distance between the spray plate and the mesh surface of the mesh cage can be adjusted to control the flow rate of slurry entering the mesh tank. Lip cloth (also called skirt cloth) is a piece of rubber with a thickness of 1-2mm, which can prevent the slurry flow from directly rushing to the surface of the forming wire, and make the slurry flow change direction and flow down along the arc-shaped bottom of the mesh groove. The height of the adjustment lip cloth position is adjustable to control the position where the paper web begins to form.
The arc-shaped bottom of the mesh trough and the surface of the mesh cage form a horn-shaped pulp channel. The size and shape of the horns have a great influence on the quality of the paper. Usually, the distance between the curved bottom and the net cage is 130-140mm at the entrance of the slurry; then it is gradually reduced to 100-110mm at the bottom of the net trough; it is 70-80mm at the front of the overflow box. If the horn channel is too small, the pulp flow speed is too fast, which is easy to bring in air and form bubbles; when the horn channel is too large, the pulp speed is too slow, increasing the friction between the pulp and the wet paper web already formed on the mesh cage , Affect the evenness of the paper.
There is an overflow box at the end of the horn channel to eliminate the settlement of the fibers in the mesh groove and avoid the occurrence of excessively concentrated pulp mass. The width of the overflow box is 100-200mm and the depth is 200-300mm.
Downstream mesh tanks are characterized by a large dewatering arc length, and can use low-concentration pulp. The finished paper has better uniformity, greater tightness, and smoother. Its structure is relatively simple, it can be adjusted appropriately by spray plate and lip cloth, and it is easy to clean. It is suitable for papermaking of various cultural paper, general thin paper, base paper, paper string paper, oil-sealed paper, etc.
2. Counter-current mesh trough
The basic components of the countercurrent mesh trough are the same as the downstream mesh trough, but the direction of rotation of the slurry flow and the mesh cage is opposite. The pulp concentration is greatest where the paper web begins to take shape. The rotation of the net cage has some stirring effect on the pulp, causing some intertwining of the fibers, so the ratio of the vertical and horizontal tensile strength of the finished paper is small; the wet paper formed by it is looser, and the surface has upright fibers, which makes the paper layer and paper It is easy to combine the layers, so the counter-flow mesh groove is often used for making thick paper, pulp board or cardboard with multi-cylinder net.
The feature of the counter-current mesh tank is that the requirements for pulp are not strict, and the same structure of the mesh tank can be used for pulp with very different properties, which is easy to operate and control. The mesh trough has a simple structure, no overflow and lip cloth, and is easy to clean. The disadvantage is that the evenness of the finished paper is low and the speed cannot be too high.
Downstream mesh troughs have better uniformity than countercurrent mesh troughs, while countercurrent mesh troughs can take a higher quantitative. Therefore, in practice, the downstream mesh grooves are often used to produce the outer paper web, and the countercurrent mesh grooves are used to produce the middle layer paper web.
3. Movable curved plate mesh slot
In the head turning box part of the movable arc-shaped plate type mesh tank, a bubble grid (or called an exhaust grid) is arranged near the arc-shaped plate to discharge the air in the slurry. The pulp first enters the directional pulp channel of the pulp flow from the tip box. The directional pulp channel is composed of a fixed arc groove bottom at the bottom of the mesh groove, a lip cloth and the lower arc of the movable arc plate, which is used to control the direction of the pulp on the net and stabilize the pulp flow. Subsequently, the pulp enters the fixed-speed pulp channel of the mesh trough, that is, the pulp channel composed of the movable arc-shaped plate and the mesh cage. The curved surface of the movable arc plate is a circular arc made with the center of the cage moving down 20-40mm, and the radius of the cage larger than the radius of 40-60mm. There is a movable pivot in the middle of the movable arc plate, which can be pushed by screws for horizontal translation to adjust the distance between the movable arc plate and the surface of the net cage, so as to control the flow rate of the pulp and meet the requirements of vehicle speed and paper web basis weight. . At the same time, the movable arc-shaped plate can also use the screw on the upper edge of the mesh groove to rotate around the movable pivot, thereby changing the shape of the pulp channel, that is, changing the size of the entrance and outflow end of the pulp channel to adapt to the nature of the pulp and the overflow rate. size. The overflow groove of the pulp is arranged at the upper end of the movable arc plate.
The movable arc-shaped mesh trough is characterized by greater adjustability and is suitable for variable production varieties; the pulp sizing head is larger, and correspondingly has a larger dewatering capacity; but the white water concentration is large, and the fiber loss is more. The side of the paper against the net is rough. The movable arc-shaped mesh slot can be used in the production of printing paper, letterpress paper, writing paper, general thin paper, toilet paper, packaging paper and other products. It is a widely used mesh slot structure type at present.
4. Suction type rotary screen and pressure type mesh trough
(1) Aspiration type rotary screen
The various mesh slots mentioned above can generally only adapt to lower vehicle speeds. When the vehicle speed is increased, the phenomenon of pulp shuffling and slipping is prone to occur, and it is difficult to ensure the quality of the finished paper. The use of aspirated rotary screen is beneficial to increase the speed of the rotary screen.
Close the two side boxes (that is, the ear boxes) of the downstream mesh trough, block both ends of the mesh cage (white water in the mesh is drained by the water legs), and seal the part of the mesh cage that exposes the slurry surface with a rubber cloth to make the mesh cage The inside and outside of the cage are basically isolated; a low vacuum of 0.8~1kPa is created in the cage with an exhaust fan.
The use of this kind of mesh groove enhances the water filtering capacity of the mesh cage, increases the adhesion of the pulp on the mesh surface, and reduces the influence of centrifugal force, thereby creating favorable conditions for increasing the speed of the circular mesh. In addition, the suction type mesh groove can improve the uniformity of the paper, reduce the moisture when the wet paper web enters the flat roller, and reduce the phenomenon of water and pulp flutter from the flat roller. The transparent spots often found on the paper made by the rotary screen are mainly caused when the wet paper web leaves the pulp surface. The air-exhausting mesh groove can eliminate this paper defect.
(2) Pressure type mesh trough
The mechanism is to cover the forming part of the mesh trough on the basis of the common downstream mesh trough, and use a blower to send in 400-500Pa air of up to 1kPa for pressurization. The main purpose of this former is to improve the forming of the paper web to improve the evenness of the paper, at the same time to increase the speed of the car, increase the output, and also to eliminate the phenomenon of pulp rejection caused by high speed.
There are two types of pressurized rotary screen: BF type and BO type.
The BF type is a mesh trough similar to the movable arc-shaped plate in our country. It uses multiple pipes to feed the slurry, then enter the diffusion mixing chamber, and then go online through a homogenizing roller. The forming part is covered and closed by air and pressurized. It is used for papermaking tissue paper, the design speed is 300m/min, and the papermaking basis weight is less than 30g/m2. When the vehicle speed exceeds 200m/min, the inside of the cage needs to be exhausted.
The BO type is a downstream mesh tank, which also uses multiple pipes to feed the slurry, then enter the diffusion mixing chamber, and then enter the arc-shaped channel through the homogenization roller to go online. The forming part is covered and closed by air and pressurized. Used for papermaking of multi-layer paperboard, the speed of high-grade white paperboard is 122m/min, and the general paperboard can reach 160m/min. The papermaking weight of each net is more than 30g/m2. The bottom net is 8-12 mesh stainless steel net when producing paperboard. The surface mesh is 55 mesh copper mesh or stainless steel mesh. The sizing concentration is about 0.2%.
The characteristic of the pressurized cylinder is large dewatering capacity; the interlayer of the paper produced is well combined and is not easy to peel off; because the forming part has a small positive pressure, the paper web is formed with a filtering property. Compared with the vacuum cylinder, the paper There is less loss of fine fibers on the web surface, and the two-sidedness of the paper is small; the evenness of the finished paper is better than that of the ordinary cylinder; the lateral basis weight difference of the paper web is better than that of the paper copied by the ordinary cylinder; the speed of the paper can be increased.
5. Dry-type net troughs and restricted net troughs
(1) Dry net trough
A simple sealing device is installed in the mesh groove ring to confine the slurry suspension to a shorter area of the mesh groove ring. Due to the reduced length of papermaking, the degree of uncontrolled turbulence in the forming area is reduced; at the same time, white water can enter the net cage smoothly to avoid accumulation in the net cage. The width of the whole machine is more uniform in dewatering, which improves the quantitative distribution of the banner accordingly. The scouring effect of the countercurrent and downstream mesh grooves is greatly reduced, while the overall forming capacity is not reduced.
(2) Restrictive mesh slot (half mesh slot)
It is actually a dry mesh tank with "empty" semi-dewatering.
The above is the type of the network slot of the cylinder paper machine.