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6.6.3.2 Tissue Dryer Section
Conventional tissue drying is a combination of contact and impingement drying. Today a combination with through drying is often used. So a tissue drying system consists either of a tissue cylinder wrapped by a hood or of a system with an additional through air dryer section ahead of the tissue cylinder. In some cases only through air dryers are used.
6.6.3.2.1 Tissue Cylinder
Tissue cylinders (Fig. 6.59), also called Yankee dryers, have a width of up to 8.2 m and a diameter of 3.6 to 5.5 m, and in special cases even up to 6.3 m. The inside of a tissue cylinder shell is usually ribbed for maximum drying rates. The drying rate is further enhanced by turbulence generators. Condensate removal is performed by two-phase flow through a sodastraw syphon system. The cylinder shell, made of special cast iron, is the most sensitive part as regards safety against catastrophic failure. It undergoes a mainly two-dimensional combined static and dynamic

 Fig. 6.59 View inside a ribbed tissue cylinder with soda straw siphons for condensate removal 
stressing. This is due to inside steam pressure, high temperature gradient across the shell thickness, high centrifugal forces and dynamic stress due to press roll loading so the admissible steam pressure has to be limited. For good creping effect a coating consisting of hemicellulose and/or synthetic agents has to be established on the cylinder shell surface. This also reduces wear; prolonged cylinder lifetime can also be achieved by covering the surface with a metal spray coating. One (or two) press roll(s) dewater the web and bring it into good contact with the cylinder for intense heat transfer.

The shell shape in CD depends on the operating condi¬tions. These must be defined in advance in order to grind the adequate crowning on both cylinder and pressure roll(s) for uniform web pressing and dewatering in CD. As operating conditions may vary in a certain range, the cylinder shape and in CD the moisture profile may also show some deviations. To end up with a uniform moisture profile and creping quality modern tissue drying includes a press roll which can better adjust to cylinder shape deviations as well as a dryer hood with a CD moisture profiling system.

6.6.3.2.2 Tissue Dryer Hood
The dryer hood spans the tissue cylinder by about 220 to 260° and consists of two halves both of which are retractable. The nozzle plate is concentric to the cylinder and includes exhaust openings. The diameter of the nozzles is about 5 to 7 mm depending on the spacing between the nozzle plate and the cylinder. The ratio of nozzle diameter to spacing has to be optimized with regard to minimum fan energy consumption and maximum drying rate. The minimum spacing is limited for runnability reasons to about 20 mm. Air temperatures of up to 700 °C and air blow velocities of up to 160 m s–1 are used. For CD moisture profile control the hood is divided in several sections across its width. The hood is insulated. Its design has to account for the large temperature differences when heated up. The air system with burners, fans and heat exchangers for heat recovery is located in a separate place outside the hood. A tissue dryer hood and its air system is shown in Fig. 6.60.

6.6.3.2.3 Through Air Dryer
This type of cylinder can be built with diameters up to 5.5 m, or in special cases up to 6.7 m, and widths of up to 9 m. It has a free outside surface area of up to 96 % in order to have a uniform air flow through the paper web. During its contact with the cylinder the paper web is supported by a revolving wire. The cylinder can run at speeds of up to 3000 m min–1. Air is usually sucked into the cylinder by reduced pressure and is supplied by a hood wrapping the cylinder by about 250°. The temperature can be up to 300 °C. The design has to withstand the heat up and cool down cycles during its operational life and always has to ensure a uniform geome¬try and drying conditions. Figure 6.61 gives an overall view of a through air drying cylinder and a closer look at the construction.