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6.6.3.1.2 Multi Cylinder Drying Section : Web Handling
In order to reduce the forces acting on the web at high machine speeds a single-tier dryer section is applied where the paper web is continuously supported by a fabric. In the critical areas where the web has to be released from the drying cylinder surface stabilizers support the safe web run. Stabilizers are also used in double-tier dryer sections. In the most sensitive areas of low strength or low stretch potential the size and speed of the dryer groups have to be adjusted according to the strength, stretch potential and web shrinkage. This enables fine-tuning of sheet quality and improves machine runnability.
6.6.3.1.3 Dryer Hood
The whole dryer section is enclosed in a drying hood with doors which can be opened e. g. for inspection. It allows controlled flow of the hot and dry make-up air as well as of the vapor laden exhaust air. The pressure inside the hood should be balanced in such a way that a minimum of air is blown from the hood into the machine hall or sucked from the machine hall into the hood. For effective pocket ventilation the hot air enters via blow boxes or blow rolls and flows to both sides of the machine where it is sucked off. To prevent condensation the hood walls are insulated and make up air is supplied along both hood sides from underneath. These measures allow a low amount of make-up air, a high air dew point of the exhaust air and effective heat recovery.
6.6.3.1.4 Paper during Drying
The web strength increases with increasing dryness due to build up of hydrogen bonds between the fibers. The increase in strength from about 50 % to 95 % dry¬ness is about a factor of 10. Stretch before rupture decreases with drying and also depends on the web structure and how far the web was allowed to shrink.
The paper web shrinks during drying. The extent of shrinkage depends on the type of stock, degree of beating, the fiber orientation, and on forces that restrain the shrinkage. The shrinkage in the machine direction can be controlled by stretching the web. CD shrinkage is nonuniform and is higher at the sides than towards the center of the web. The single-tier dryer configuration changes the CD shrinkage profile compared with a conventional double-tier one (Fig. 6.58). The profile is now flatter over a large part of the sheet between the drive and tender side with low shrinkage, accompanied by less stretch potential and higher dimensional stability in this area. On the other hand there are distinctly steeper slopes in the shrinkage curve with high shrinkage numbers at the edges.
6.6 Dryer Section
Fig. 6.58 CD shrinkage profile for single-tier and double-tier dryer sections.
Paper curl is an undesirable effect when paper undergoes heating or moisten¬ing, e. g. in copy machines or in printing. Curl is due to nonsymmetrical residual stresses in the z-direction of the web which date back to nonsymmetrical drying of its top and bottom sides. The amount of curl is influenced by the degree of non-symmetrical drying. The impact increases towards the end of the drying process. The direction of curl (MD, CD or diagonal curl may occur) is defined by the paper and fiber structure. Curl can be overcome by varying the operation of the top and bottom drying cylinders if there is a conventional double-tier after-dryer section. With a pure single-tier dryer section curl can be controlled by additional tools such as by moistening (water, steam) or by additional drying (air impingement drying).
A uniform CD moisture profile of the web at the end of the dryer section is an important quality requirement so the heat transfer of the cylinders to the web as well as the vapor exhaust conditions close to the web have to be uniform. Addi¬tional CD moisture control tools such as sectioned cylinders, blow boxes or mois¬turizers are also used.
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