What are the different types of feeder slots for circular mesh paper machines?
The grid structure of the circular screen paper machine is usually made of wood, and the main part of the mesh ring is sometimes lined with metal (often stainless steel), and sometimes plastic. The so-called groove ring refers to the ring gap between the mesh cage surface and the semicircle. The grooved rings must be carefully constructed and their shape must be uniform and smooth.
There are many forms of mesh slots. The forms that follow Z long are downstream and counterflow slots. At present, China's more widely used is the movable arc plate mesh groove, as well as dry mesh groove, rotary former and drum vacuum former, etc., and the extraction type round screen is used on more advanced paper machines.
The structural form of the mesh groove has a great influence on the yield and quality of the paper. All kinds of mesh grooves have certain characteristics. These properties may be advantageous for the production of one grade, but may not be important for another. Mesh grooves should be designed or selected according to specific production requirements. The general requirements for the design of the mesh structure are: the fibers of the pulp in the mesh are evenly dispersed and do not agglomerate; The pulp in the mesh groove has a uniform flow rate on the width of the sheet; Each slurry channel of the mesh groove is smooth and straight, and the corners should be smooth and transition to prevent the phenomenon of hanging slurry and grouting; Easy to clean and service.
1. Ordinary downstream type mesh groove
Fig. 4-32(a) shows a normal downstream grid structure. The fiber flow direction in the groove ring is the same as the rotation direction of the mesh cage. The slurry flow enters the mesh groove through the pulp distribution device such as a branch pipe distributor or conical spreader, and is first fully mixed in the flow part (pulp turning box) to promote uniform dispersion of fibers and remove foam, and then overflow the spray plate into the mesh cage groove. The distance between the spray plate and the mesh surface of the mesh cage can be adjusted to control the flow rate into the groove slurry. Lip cloth (also known as skirt cloth) is a piece of rubber 1-2mm thick, which can prevent the slurry flow from rushing directly to the surface of the forming mesh, and change the direction of the slurry flow and flow down the arc-shaped bottom of the mesh groove. Adjust the position of the lip cloth to control where the web begins to form.
The arc-shaped bottom of the mesh groove and the surface of the mesh cage form a horn-shaped pulp channel. The size and shape of the horn road has a great influence on the quality of the paper. Generally, the distance between the arc bottom and the mesh cage is 130-140mm at the entrance of the slurry; Then gradually shrink to 100-110mm at the bottom of the Z of the mesh slot; To the front of the overflow box, it is 70-80mm. If the horn channel is too small, the pulp flow speed is too fast, and it is easy to bring in air and form bubbles; When the horn road is too large, the pulp speed is too slow, which increases the friction between the slurry and the wet paper web that has been formed on the mesh cage, and affects the uniformity of the paper.
An overflow box is provided at the end of the horn road to eliminate the settlement of fibers in the mesh groove and avoid overly concentrated pulps. The width of the overflow box is 100-200mm and the depth is 200-300mm.
The downstream mesh groove is characterized by a large dewatering arc length, which can use pulp with lower concentration, and the evenness of the paper is better, the tightness is larger and smoother. Its structure is relatively simple, and it can be moderately adjusted by spray plate and lip cloth, which is easy to clean. It is suitable for copying all kinds of cultural paper, general tissue paper, base paper, paper rope paper, oil seal paper, etc.
2. Counterflow mesh slot
The basic components of the counterflow mesh groove are the same as the downstream mesh groove, but the slurry flow rotates in the opposite direction to the mesh cage. This is shown in Figure 4-32(b). Where the paper web begins to form, the slurry concentration Z is large. The rotation of the mesh cage has some stirring effect on the pulp, so that the fibers have some entanglement, so the ratio of the longitudinal and transverse tensile strength of the paper is small; The wet paper formed is looser, and there are vertical fibers on the surface, so that the paper layer and the paper layer are easy to combine, so the counterflow mesh groove is often used for multi-circle mesh copying thick paper, pulp board or cardboard.
The characteristics of counterflow mesh groove are not strict requirements for pulp, and the same structure of mesh groove can be used for pulp with different properties, which is easy to operate and control. The mesh groove structure is simple, there is no overflow and lip cloth, and it is easy to clean. The disadvantage is that the uniformity of the paper is low, and the speed cannot be too high.
Downstream grooves have better uniformity than counter-flow grooves, while counterflow grooves allow higher quantities. Therefore, in practice, it is commonly used to produce the outer paper web with the downstream mesh groove, and the middle layer paper web with the counterflow mesh groove.
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