The purpose of the dryer section is to increase the dry content of the paper web, usually to 90 to 98 %, by evaporation. During drying the fibers develop hydrogen bonds which provide the natural strength of the paper. Drying is a coupled heat
6.6 Dryer Section
and mass transfer process so heat has to be transferred from a heat source to the paper and the evaporated water has to be carried off. During drying, the paper web which has been picked up from the press section has to be guided safely through¬out the dryer section to the reel where it is wound up. Some dryer sections include a size press for paper strength improvement and/or a breaker stack for pre-cal-endering. Depending on the type of paper machine different heat transfer and drying principles as well as their combinations are applied.
188.8.131.52 Contact Drying by Steam Heated Cylinders
With this most common principle in paper drying steam condenses at the inner surface of the cylinder wall, the heat is transferred through the wall to the paper web, the web is heated and water is evaporated. Air flow takes up the evaporated water. The heat transfer rate from the steam to the cylinder shell depends on the flow pattern of the condensate motion. This flow pattern is mainly dependent on the machine speed, and to a lower degree on the amount of condensate volume in the cylinder and on the cylinder diameter. At low speeds of up to about 300 to 500 m min–1 a pond of condensate is found in the cylinder. At higher speeds – above the “rimming speed” – the condensate builds up a ring.
Acceleration during “ascending” of the condensate ring is against the rotating direction, and in the rotation direction when descending. This results in a swinging condensate motion relative to the cylinder with the effect that the condensate ring velocity is lowest and thus its thickness is highest at the culmination point and not in the bottom position. For high machine speeds the condensate motion and thus heat transfer decrease.
For heat transfer enhancement and uniform drying spoiler bars induc¬ing turbulence to the condensate layer are installed (Fig. 6.52). The condensate is removed from the inner surface of the cylinder to its axis by syphons (Fig. 6.53) in the form of a two-phase flow of steam and condensate. To overcome the high centrifugal forces and to generate the two-phase flow a pressure difference of about 0.3 to 0.5 bar is necessary. The syphons either rotate with the cylinder (for higher machine speeds) or are stationary. Heat transfer through the wall depends on the thickness and conductivity of the cylinder material which is mainly cast iron (in some cases steel). Higher steam pressure increases the temperature difference and thus the drying rates.
Accumulation of air in the cylinder has to be avoided as it would reduce the condensing temperature according to the partial pressure. Good heat transfer from the cylinder to the paper web is obtained by pressing the web tightly to the cylinder e. g. by means of dryer fabrics.
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