Paper dryers

Very expensive Rapid drying of large objects suited to this method Applications for final stages of paper dryers Successful on foam rubber. Not fully developed on other materials... 

The value of the coefficient of heat transfer from steam to sheet is determined by the conditions prevailing on the inside and on the surface of the dryers. Low coefficients may be caused by (1) poor removal of air or other noncondensables from the steam in the cylinders, (2) poor removal of condensate, (3) accumulation of oil or rust on the interior of the drums, and (4) accumulation of a fiber lint on the outer surface of the drums. In a test reported by Lewis et al. [Pulp Pap. Mag. Can., 22 (February 1927)] on a sulfite-paper dryer, in which the actual sheet temperatures were measured, a value of 187 W/(m °C) [33 Btu/(h-fr-°F)] was obtained for the coefficient of heat flow between the steam and the paper sheet. 

FIGURE 18.29 Normalized Planck distributions of gas (1300 K) and electric (2500 K) paper dryers, along with the spectral absorptivity of a typical wet paper product [156],... 

FIGURE 18.33 Radiation paper dryers using (a) gas firing and (ft) electric sources [156],... 

More advanced models of the drying process are discussed by Kirk [88]. More recent reviews include those written by Nederveen et al. [94] and Wilhelmsson et al. [95]. The latter authors identify 20 models that have been proposed to simulate multicylinder paper dryers. In some of these models, the physical transfer processes are simulated in detail, while others adopt a less detailed viewpoint and rely on more empirical coefficients to fit the data. 

The number of dryers and the auxiliary equipment used in a dryer section depend on the particular grade to be produced and the speed of the machine. Up to 70 cylinders may be used in a Kraft dryer section 50-55 would be adequate for a typical newsprint or fine paper dryer section. On the other hand, although the breaker stack and size press are necessary for Kraft bag and the hot press, size press, and intercalender dryers for linerboard, none of these is used for corrugating medium [4],... 

FIGURE 39.10 Dryer section pocket (A, paper dryer B, felt roll C, felt D, paper). 

FIGURE 39.19 The position of a floater dryer in a sack paper dryer unit. (Courtesy of Flakt Ind. AB, Tiruverkadu, Chennai.)... 

Although the conventional steam-heated cylinder dryers are still predominant in the paper industry, their share in pulp drying has diminished rapidly after the application of air floater dryers. The dryers are 1.2- to 1.5-m diameter case iron cylinders and the steam systems and air systems are similar to the paper dryer system described earlier. 

The air floater dryers are also similar to those described in the paper-drying section. Hot air is impinged to the web from blow boxes above and below, and the web floats supported by the airflow. The low sheet tension ensnies a greater ability to tolerate disturbances or sheet defects, and the quality of pulp is less affected. Pulp dryers also consist of stacked drying decks, as shown in Figure 39.16. Pulp dryers are larger than paper dryers as pulp is air dried from the press section to the cutter. 

Direct control and online measurement of the solids moisture content would enable significant improvements in dryer control by providing an immediate measure of the moisture content at the dryer exit and by automatic compensation for factors that distmb the control action. In certain dryers, these are already implanented industrially (e.g., in paper dryers). 

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