Yankee hood has the ability to dry paper. In order to improve the drying ability and drying rate of the paper machine, the paper machine basically needs to be equipped with oxygen high temperature Yankee hood. It mainly uses section steel, pipe fittings as the valuation, and uses aluminum shell as the shell to prevent the temperature from being emitted into the air.
In the production of tissue machine, it is necessary to equip with good drying equipment to improve the quality of paper web.
The quality of tissue can be improved through the following aspects:
1) adding short fiber, coating and chemical issue paper in raw pulp has good softness;
2) in the press part of the paper machine, the loose thickness of tissue paper can be improved with lower pressure load and lower inlet dryness;
3) tissue paper is less dried on Yankee dryer, more dried on Yankee hood, and the softness and loose thickness of paper is further improved.
What are the reasons for the improvement of tissue sheets?
In the production of high-speed and high-temperature Yankee hood, high-temperature air is sprayed on the surface of the paper for drying, which is the problem of moisture transferring heat and quality in a very complex form. When the wet web is introduced into the pressing area, the drying operation occurs at the moment when the web contacts the cylinder block, and then the web is pasted on the cylinder block surface, and the heat conduction between the two dominates the drying process of the web.
When the moisture in the web becomes steam, it flows from the closed space of the wet web to its surface, migrates in the form of liquid, fills all the voids, and then emits to the external environment.
When the web is dry, the evaporation rate of its moisture is in a constant stage, and its surface is always wet. At this time, only in the place where the fiber is covered / exposed, the air in other places cannot enter into the web structure. Under the combined action of capillary force of fiber and surface tension of moisture itself, the moisture in the paper web is led to the surface of cylinder block, and the tiny space on the surface of cylinder block is filled with evaporated moisture. At this time, if only relying on the heat energy released by the steam inside the cylinder block to dry the paper web without the hot air from the hood, the moisture on the open surface of the paper web is still However, due to the capillary force and moisture stress of the fiber, the pulp fiber is pulled back to the closed space near the surface of the cylinder block, resulting in the close and low accumulation of pulp fiber on the surface of the cylinder block, resulting in a dry web.
In the drying process of paper web, if Yankee drying cylinder is used and new high temperature Yankee hood is used, the paper web is operated together. The heat energy of wet paper web is transferred by conduction and convection in the cylinder body of drying cylinder and high temperature Yankee hood respectively, that is to say, the drying rate is accelerated, the fiber structure of paper web is improved and good effect is achieved. The rapid evaporation of water on the open surface of the web makes the water vapor in the semi closed space move to the open surface. High temperature air spray on the open surface of the web can easily occupy the space nearby and keep it open. The moisture in the paper web in the closed space changes into water vapor due to the heating of the cylinder block surface, which is easy to move outward and reach the open surface. At this time, the moisture / water vapor existing in the closed space of the web moves towards two heat sources at the same time during the drying process, so that the web structure has good opening performance.
Increase the air temperature of the dryer hood and reduce the steam pressure inside the dryer hood. When the Yankee hood is increased by 10 ℃, the steam pressure of the dryer will be reduced by 0.5 PA after drying. Now many large-scale paper machines use Yankee hood basically, because it requires high drying rate, and the web also has a good open structure.
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