Multi-cylinder dryer section

The state-of-the-art dryer section for a high-speed paper machine has a singletier configuration with steam heated cylinders in the top row and suction rolls in the bottom row. The paper web is always supported by the dryer fabric and runs jointly around the drying cyHnders and suction roUs (Fig. 6.55). In the top row the paper web is pressed against the drying cyHnders by the dryer fabric. In the bottom 

In the field of dryer fabrics excessive air entrainment at high machine speeds had to be reduced without losing drying capacity by decreased air permeability of 

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. 

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% dryness 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. 

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 cyhnders to the web as weU as the vapor exhaust conditions close to the web have to be uniform. Additional CD moisture control tools such as sectioned cylinders, blow boxes or moisturizers are also used. 

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Continuous paper machines have undergone extensive mechanical developments since the 1950s, although the principles employed have changed Utde. Cylinder machines stUl are operated and involve multiples of five to seven cylinders they are used to produce heavy multi-ply boards. Fourdriniers are common in the industry and can be used to produce virtually any grade of paper or paperboard. They vary from 1 to 10 meters in width and, including the press and dryer sections, may be more than 200 m long. 

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