Do You Know The Dehydration Process of Vacuum Suction Box?

Dehydration Process of Vacuum Suction Box for Foudrinier Paper Machine:

On the vacuum suction box, the wet paper web has been basically formed, and its main purpose is dehydration.

For vacuum suction box with low vehicle speed and low vacuum degree, the dehydration process of wet paper on a vacuum suction box can be roughly divided into three stages. At first, the moisture content of wet paper web was very high, and the moisture was filtered and discharged under the pressure difference caused by vacuum, commonly known as the free dehydration stage. Subsequently, the wet paper web is compressed under pressure differential, resulting in compression dehydration. Finally, air begins to penetrate the paper web, carrying a portion of the moisture between the fibers into the suction box with the airflow, forming what is known as aerodynamic dehydration.

The point where air penetrates the pulp layer is the position of the "main line", also known as the "water line". A portion of the fibers in the pulp before the "water line" are suspended in the remaining water, and the surface of the suspension above the wet paper web on the forming fabric is a flat liquid surface, reflecting light and forming a bright mirror. 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 pulp, scattering light and causing the mirror surface to disappear. The boundary between these two light and dark surfaces is called the "waterline". The position of the waterline is usually between the second and third vacuum suction box or later, and its pulp concentration is generally about 7%, with the exact pulp concentration varying depending on the quantity and variety. Observing the position and shape of the waterline can provide a preliminary understanding of the quality and dryness of the formed paper on the forming fabric. A normal waterline should have a straight or roughly straight shape. If there are locally protruding tongue like shapes on the waterline, it indicates that there may be uneven flow or concentration of the pulp flow on the forming fabric;

If there are tongue shaped protrusions on the waterline, it generally indicates that the pulp flow in the headbox is unstable and there is a phenomenon of protrusion in the sprayed pulp flow.

In the free dehydration and compression dehydration stages of the vacuum box, the amount of dehydration is related to the dehydration time (or dehydration box width) and the square root of the vacuum degree; When the width of the suction box is doubled, the same effect of increasing the vacuum degree by 4 times can be achieved, but increasing the vacuum degree is extremely detrimental to the wear and power consumption of the forming fabric. In the aerodynamic dehydration stage, the dehydration efficiency is very low. Improper increase in the width of the dehydration box under the same vacuum degree will increase the wear and power consumption of the forming fabric, 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 in the stages 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 a vacuum suction box is very small. However, from a structural perspective and convenient maintenance, the vacuum box cannot be made too narrow or uneven in width.

In addition, although the amount of water discharged during the aerodynamic dehydration stage is not large, it can use air flow to blow away the water under and in the mesh, which is beneficial for the dehydration of the next vacuum suction box. This is necessary in the actual dehydration process of the vacuum suction box. Therefore, the vacuum suction box currently used on paper machines often adopts a structure with a small quantity but a large width.

On high-speed paper machines, the wet paper web stays on the vacuum suction box for a very short period of time, and the dehydration process that occurs on it is more similar to the dehydration process of pressing, which is completed under very short pressure pulses.

The conventional vacuum box system uses 5-6 vacuum boxes with a width of 15-40cm that gradually increase in vacuum degree. The wet end vacuum box operates at a relatively low vacuum degree (6.7-10kPa). The vacuum degree of the vacuum box gradually increases to 20-26.7kPa. To control the vacuum degree of the vacuum suction box, the position of the "water line" should also be considered. Generally, it is required that the "water line" appears in the middle of all vacuum box, or in the middle, one vacuum suction box ahead. An increasingly common practice is to use only four vacuum chambers and operate them at a higher vacuum level, with an initial vacuum level of 10-13.3kPa. Gradually increasing to 26.7-40kPa with equal or increased water removal capacity, the traction load decreases.