What is the internal drying of the paper machine?

What is the internal drying of the paper machine?

The dryness of the wet paper out of the press section is generally only 30%~40%. In the new paper machine with compound press, the dryness is only 40%~50%. Therefore, it is necessary to use the drying cylinder to evaporate the water to make the paper dry. increased to 92%~95%.

In the process of papermaking, the dryer section mainly removes the moisture of the wet paper after the press section, and at the same time, improves the strength of the paper, enhances the smoothness of the paper and completes the sizing effect of the paper.

There are three different forms of moisture in the wet paper entering the dryer section: free water, capillary water and bound water.

In the early stage of drying, fibers can slide freely between each other. After removing free moisture, the surface tension of water begins to pull the fibers together. However, before the dryness of the paper is less than 40%, the fiber bonding is not obvious. Once the dryness reaches a certain critical value , the shrinkage of fibers in the paper begins to produce hydrogen bonding. When the dryness of the paper reaches more than 55%, with the decrease of the moisture content, or the increase of the dryness, the strength of the paper based on the hydrogen bonding force, such as tensile strength, bursting strength, etc. Rapid growth.

During the drying process of wet paper, the strength development process is basically consistent with the shrinkage process of the paper. When the dryness of the paper reaches about 55%, the shrinkage of the paper also occurs rapidly, and when the dryness reaches 80%, the shrinkage is generally completed.

Drying affects not only the mechanical strength of the paper, but also other properties such as tightness, absorbency, air permeability, smoothness, and sizing. The change of these properties is closely related to the drying method. High temperature drying with rapid heating is beneficial to increase the softness, porosity, absorbency and air permeability of the paper, and help to reduce the tightness, transparency and mechanical strength of the paper; on the contrary, the low temperature drying with slow heating has the opposite result. As for the vacuum-dried paper, it is relatively loose, less tight, low in transparency, sizing and mechanical strength.

The drying section of the paper machine consists of several drying cylinders. The number and arrangement of the drying cylinders are determined by factors such as the type of paper produced and the speed of the vehicle. The drying cylinders in the drying section of an ordinary multi-cylinder paper machine used to produce general paper are arranged in two layers.

In order to facilitate paper threading, make the paper better contact with the drying cylinder and improve the heat transfer efficiency, the drying cylinder is surrounded by a dry blanket, and the wrapping angle between the dry blanket and the drying cylinder is generally 180°. The drying cylinders are divided into groups, each group is usually composed of 6~12 drying cylinders, and the upper and lower drying cylinders of each group share a drying blanket.

There are guide rollers, tension rollers, correction rollers and drying felt cylinders on the dry blanket running line. There are usually 1~2 cold cylinders at the end of the drying section. In addition, each drying cylinder has a scraper device and a transmission device, and a medium and high-speed paper machine also has a threading rope device.