1. 第十八次课 Drying

2. Introduction The papermachine drying section and its operating principle has remained virtually unchanged since its initial development. For most paper and board grades, contact drying with steam-heated cylinders is still the most common and effective method. Other methods have been developed over the years and the following are the most common drying methods used today in the paper and board machine industry: Multi-cylinder drying for Printing papers, base papers & Boards Yankee, or MG cylinder drying for Soft tissues, Boards & Machine glazed papers.

3. Infrared drying for surface sizing and coating. Web heating used for Moisture profile control Impingement drying (Hoods located on the paper machine dryer section and placed against the web supported on a drying cylinder, roll or fabric)used to increase drying capacity for coated boards/papers, also unrestrained drying of sack kraft paper Through air drying (TAD)used for Soft tissues, Fibre and filter fabrics

4. The dryer section of a paper or board machine is required to achieve the following: Produce flat paper with an even moisture profile across the machine Consistent moisture content in the machine direction Maximum evaporation of water for the range of paper and board grades produced.

5. Good runnability for overall machine efficiency. Sheet breaks occurring in the dryers are often the major causes of lost production time and this becomes more significant as the running speed of the machine increases. Dryer system design should minimize energy consumption and recover surplus heat from other sections of the process.

7. The main objective of the Drying Section of the paper machine is to remove any residual moisture in the pressed paper web, usually by means of evaporation. The drying section commonly consists of steam heated drying cylinders around which the paper web is passed. The paper web is held in close contact with the drying cylinders by means of tensioned woven dryer fabrics. This assists in heat transfer from the cylinder to the paper and hence increases the rate at which moisture evaporates from the web. Evaporation mainly occurs in the open draws between cylinders, but also occurs through the fabric whilst the sheet is in contact with the cylinder.

8. Early drying cylinders are used to warm up the paper gently. Later cylinders are heated to much higher temperatures and are responsible for the majority of the evaporation. Drying cylinders can be arranged either as a single tier, a two-tier or a combination of both. The MG (Yankee) dryer, the introduction of Infra-red dryers and through air drying (TAD - primarily for tissue machines) are examples of different/modifications to the dryer part. The drying part of the paper machine is costly in terms of both the capital cost required (i.e. number of drying cylinders required) and in terms of operating cost for the production of steam to heat the cylinders. Improving the evaporation rate from the sheet is a cost effective goal of drying.

10. Steam and Condensate Systems are used for multi cylinder drying applications. Contact drying with a multi-cylinder dryer is the most common drying method in paper and board manufacture. Cylinder drying has several benefits. Steam as the heat source suits the energy infrastructure of the paper and board industry.

12. The photos show an example of a dryer fabric on the paper machine, a close-up of two dryer fabrics, one with a fine yarn (bottom) and one with a coarser yarn (top) finally a pocket ventilation unit between the two drying cylinders. Fabric selection at each dryer position uses a balance of fabric properties to yield maximum life at optimum production rates while maintaining runnability, without adversely influencing the quality of paper being manufactured. Maintaining paper quality during the drying process requires undisturbed web to fabric contact and non-marking surface and seam structures.

13. Depending upon the machine conditions, the fabric life should be about 6 - 18 months. Choice of fabric material has a strong impact on life. Improved cleaning techniques including the use of chemicals, has prolonged the life of fabrics. The most common material used in fabric manufacture is heat stabilised polyester (PET). Other synthetic materials are also used dependant on the conditions found within the dryer section. Materials behave differently at different humidity and temperatures.

14. Drying 难点

15. New words

16. Homework( 中译英 ) The wet web from the press section containing 55%-60% moisture (40-45% dryness) is passed over a series of rotating steam-heated cylinders where water is evaporated and carried away by ventilation air. Uaually three to five dryer sections are grouped for independent steam pressure control.