In the paper industry, the most important department is the drying department. In general, after the wet paper is pressed, the dryness is only about 30-40%. Even if the composite press is used for paper pressing, the dryness is only 40-50%. Therefore, we need to use Yankee hood to dry the water, and increase the dryness to more than 10%.
In the paper-making process, the drying section mainly removes the moisture of the wet paper after the pressing section. At the same time, it 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 water in the wet paper entering the drying section: free water, capillary water and combined water.
At the initial stage of drying, the fibers can slide freely with each other. After removing the free water, the surface tension of water begins to pull the fibers together, but before the paper dryness is less than 40%, the fiber bonding is not obvious. Once the dryness reaches a certain critical value, the fiber shrinkage in the paper begins to produce hydrogen bond bonding. When the paper's dryness reaches more than 55%, with the decrease of moisture content, or with the increase of dryness, the paper's strengths, such as tensile strength and bursting resistance, which are based on the hydrogen bond bonding force, increase rapidly.
In the drying process of wet paper, the strength development process is basically consistent with the paper shrinkage process. When the paper dryness reaches about 55%, the paper shrinkage also occurs rapidly. When the dryness reaches 80%, the shrinkage is generally completed.
Drying not only affects the mechanical strength of paper, but also the compactness, absorptivity, air permeability, smoothness, sizing and other properties of paper. The change of these properties is closely related to the way of drying. High temperature intensified drying with rapid temperature rise is beneficial to increase the softness, porosity, absorptivity and air permeability of paper, and to reduce the compactness, transparency and mechanical strength of paper; on the contrary, low temperature drying with slow temperature rise is just the opposite. As for vacuum drying paper, it is loose, compact, transparent, sizing and mechanical strength are low.
The drying section of the paper machine is composed of several drying cylinders. The number and arrangement of the drying cylinders are determined by the type of paper produced and the speed of the paper. The dryer of the common multi cylinder paper machine used to produce general paper is arranged in two layers. The running route of the paper in the dryer makes the two sides of the paper in turn contact with the dryer to eliminate the two side difference of the paper smoothness.
In order to facilitate the paper introduction, make the paper better contact with the drying cylinder and improve the heat transfer efficiency of the airtight hood, the drying cylinder is wrapped with a dry blanket, and the wrapping angle between the dry blanket and the drying cylinder is generally 180 °. The drying cylinders are divided into arrays, each group is usually composed of 6-12 drying cylinders, each group of the upper and lower rows of drying cylinders each share a dry blanket. The running line of the dry blanket is provided with a blanket guide roller, a tension roller, a correction roller and a blanket drying cylinder, etc. At the end of the drying section, there are usually one or two cold cylinders. In addition, each drying cylinder has a scraper device and a transmission device. The medium and high speed paper machine also has a paper rope guiding device.
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