How To Choose Suitable Dandy Roll

Selection and installation of dandy roll on foudrinier paper machines:

The dandy roll is generally a straight and round hollow mesh roller, not a through shaft type. The two ends of the roller body are fixed with wheel amplitudes.

The following figure shows the composition and appearance of commonly used shaftless dandy roll. The roller body is a spiral copper wire frame covered with copper mesh, with end rings and small rollers at both ends. The pressure of the dandy roll on the wet paper web can be adjusted by a lever hammer mechanism. On a low-speed paper machine, the dandy roll is driven by paper sheets and fabric. As paper machines become wider and faster, the diameter and stiffness of the dandy roll increase, and it is necessary to have a built-in drive device to ensure that the roll speed is consistent with the mesh speed. On some new paper machines, the speed of the dandy roll exceeds the mesh speed by up to 5%, thereby improving the uniformity and surface characteristics of the paper sheet. To wash the mesh surface, the dandy roll is equipped with a spray pipe and a jet pipe.

The selection of dandy roll has a significant impact on the quality of the product. The surface mesh is usually a flat woven mesh, requiring good permeability, and the mesh wire is slightly thinner than the linear mesh wire. Select the appropriate diameter, face width, and number of faces based on the type of paper produced, the speed of the paper machine, the width and mesh size of the forming fabric.

Generally, for paper with fast speed and large quantity, the mesh size of the mesh should be appropriately small to facilitate dehydration, and the difference between the mesh size and the forming fabric should not be too large. Writing paper and double adhesive paper generally use a mesh of about 50 mesh, while thin paper uses a mesh of about 80 mesh. If the mesh size of the mesh is too high, it will be difficult for the dandy roll to dehydrate, and the pulp will be easily lifted by the roller at the outlet of the contact gap with the forming fabric. The pulp brought onto the surface of the paper is thrown onto the surface of the dandy roll after the roller rotates, causing transparent spots on the paper and possibly forming longitudinal "pulp channels" and numerous air bubbles forming pores on the paper.

The diameter of the dandy roll is related to the weight of the paper and the speed of the vehicle. It is generally better to choose a larger diameter for vehicles with fast speeds and large quantities, in order to avoid the formation of a negative pressure zone at the outlet due to the small diameter and fast rotation speed of the dandy roll, which can generate suction force and cause difficulty in dehydration, leading to the formation of pulp water or bubbles, affecting the uniformity of the paper and causing paper diseases. Based on experience, it is advisable to use a speed of 100r/min to determine the diameter of the dandy roll.

The selection and calculation of the width of the dandy roll surface is generally based on the following empirical formula:

b=bB+(50-100)

In the formula, b represents the width of the dandy roll surface, in millimeters, and bB represents the width of the breast roll surface, mm。

The installation position of the dandy roll is very important, usually placed on the "water line". After the roll runs on the mesh surface, it will cause the water line to elongate. At the same time, the spray water on the cleaning roll surface of the dandy roll itself will increase the moisture content of the wet paper. Therefore, the dandy roll is generally placed between two sets of vacuum suction boxes, and the number of vacuum suction boxes at the back should be greater than that at the front. It is necessary to increase the vacuum degree of the vacuum press roll or take measures to strengthen dehydration to reduce the moisture content of the wet paper at the outlet of the roller.

For the new large-diameter dandy roll, the free mesh surface below should be equal to the diameter of the entire roller. The installation of the roll should have 1/3 of the free mesh surface in front of the roller and 2/3 of the free mesh surface behind the roller. The dandy roll is located after the first suction tank and sometimes after the last vacuum cutting board box. In order to achieve better results, the moisture content under the dandy roll should be 2%.

In order to provide a volume that can accommodate excess water inside the dandy roll, a new double-layer mesh dandy roll has been designed. First, wrap a layer of extremely thick mesh, and then wrap another layer of 35-40 mesh stainless steel wire mesh. The mesh is made of spiral seams and is almost entirely made of stainless steel material. The excess water stored between the dandy roll mesh gaps must be removed, usually by using internal spray water. The water collection tray is located on the upstream side of the dandy roll. There is a sealing device at the bottom of the disc to prevent water leakage. The removed water should be drained in a timely manner. There are drainage ports on both the front and back of the large width paper machine.