Dewatering process of vacuum suction box of Fourdrinier paper machine

On the vacuum water tank of the paper machine, the wet paper web has been basically shaped, and its main purpose is to dewater.

For the vacuum suction box at low speed and low vacuum, the dehydration process of the wet paper web on a vacuum suction box can be roughly divided into three stages. Initially, the moisture content of the wet paper web is very high, and the moisture is filtered and discharged under the pressure difference caused by the vacuum, which is often called the free dehydration stage. Subsequently, the wet paper web is compressed under the action of the pressure difference, and compression dehydration occurs. Finally, the air begins to penetrate the paper web, bringing part of the moisture between the fibers into the suction box with the airflow, forming the so-called aerodynamic dehydration.

The point where the air penetrates the slurry layer is the "trunk", also known as the "waterline" position. A part of the fiber in the pulp before the "waterline" is suspended in the remaining water. The surface of the suspension above the wet paper web on the net is a flat liquid surface, which reflects light and forms a bright mirror surface. When the pulp is further dehydrated on the vacuum suction box, all the free water in the suspension is removed, and the fibers are exposed on the surface of the slurry, scatter the light, and the mirror disappears immediately. The boundary between these two light and dark surfaces is the so-called "waterline". The position of the waterline is generally between the second and third vacuum suction tanks or later, the pulp concentration is generally about 7%, and the exact pulp concentration varies with the quantity and variety. Observing the position and shape of the waterline can intuitively understand the quality and dryness of the paper web formed on the screen. The normal waterline should be straight or roughly straight in shape. If there is a partially protruding tongue on the waterline, it means that there may be uneven flow or concentration of the online slurry;

If there is a flapping tongue on the waterline, it generally proves that the slurry flow in the headbox is unstable, and there is a phenomenon of skipping in the sprayed slurry flow.

In the free dehydration and compression dehydration stages of the vacuum box of the paper machine, the dehydration amount is related to the dehydration time (or the width of the dehydration box) and the square root of the vacuum degree; when the width of the suction box is doubled, the vacuum degree can be increased by 4 times The same effect, but increasing the degree of vacuum is extremely unfavorable to the abrasion and power consumption of the forming wire. In the aerodynamic dewatering stage, the dewatering efficiency is very low. Inappropriately increasing the width of the dewatering box under the same vacuum degree will increase the abrasion and power consumption of the forming wire but cannot significantly improve the dryness of the wet paper. Therefore, a reasonable method is to gradually increase the vacuum degree of dehydration, so that the dehydration process is mainly at the stage of free dehydration and compression dehydration.

In fact, the time for free dehydration and compression dehydration on a vacuum suction box is very short. Theoretically, the effective width of the vacuum suction box is very small. However, considering the structure and convenient maintenance, the vacuum box cannot be made too narrow or Variations in width.

In addition, the amount of water discharged in the aerodynamic dehydration stage is not large, but it can blow off the moisture under the net and in the mesh with air flow, which is beneficial to the dehydration of the next vacuum suction tank, which is the actual dehydration process of the vacuum suction tank. Necessary, for this reason, the vacuum suction boxes currently used on paper machines are often used in a small number of structural forms with large widths.

On a high-speed paper machine, the time that the wet paper web stays on the vacuum suction box is very short, and the dehydration process that occurs on it is more similar to the dehydration process of squeezing, and the dehydration is completed under a very short pressure pulse.

The conventional vacuum box system uses 5-6 vacuum boxes with a width of 15-40cm and the vacuum degree is gradually increased. The wet-end vacuum box operates at a relatively low vacuum (6.7-10kPa). The vacuum degree of the rear vacuum box is gradually increased to 20-26.7kPa. To control the vacuum degree of the vacuum suction box, it should also be combined with the "waterline" position. Generally, the "waterline" is required to appear in the middle of all vacuum boxes, or one vacuum suction box in the middle. An increasingly common practice is to use only 4 vacuum boxes and operate at a higher vacuum degree, that is, the initial vacuum degree is 10-13.3kPa. Gradually increase to 26.7-40kPa, the dehydration volume is equal or increased, and the traction load decreases.