About paper machine pressing section
December 10, 2020
Basic research on wet pressing has shown that the most important requirement in press design is to provide the shortest path for dewatering from the nip, and the shortest distance is equal to the thickness of the felt (usually called the "vertical" direction).
Therefore, the main water flow should be perpendicular to the felt, and the lateral flow should be minimal. A press that mainly flows vertically is called a vertical press.
The original press section used flat rolls. Because of this type of press, water can only be discharged from the nip side by lateral flow, which greatly restricts the flow of water.
The vacuum press developed as early as the 19th century was the first cross-flow press developed. The perforations of the perforated roll provide a convenient channel for dehydration.
Through the fixed vacuum box set in the roller, the water in the holes is stored and discharged on the outlet side of the nip, and it is thrown away by centrifugal force when the vehicle speed is about 300m/min.
At low vehicle speeds, most of the water is drawn into the vacuum chamber and discharged through the vacuum system. Some water may flow directly into the holes, but most also have to move laterally.
This perforated hollow shell structure limits the pressure that can be applied. Stress fatigue damage of rollers has been a persistent problem for many years.
A special alloy has been developed to increase its corrosion resistance and reduce the damage of the vacuum roll. This material is most needed on wide-format high-speed paper machines.
In 1963, the horizontal press design of the grooved roll was introduced.
The grooves on the surface of the roller can easily absorb the water discharged from the paper. Spiral cut and milled grooves, generally depth 2.54mm, width 0.5mm, center distance 3.2mm.
Regarding the geometric dimensions of the grooves, with the improvement of the roll coating material, there are respective optimized designs for different vehicle speeds and different linear pressure requirements.
The water in the roller grooves is thrown out by its own centrifugal force at high speeds, and water sprayers and scrapers can be used to keep it clean at low speeds.
In addition to the above-mentioned pressing forms, there are blind hole presses, lined presses, blanket presses, and new wide-nip presses. After continuous improvement, wide-nip presses have produced the most advanced shoe press today.