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184.108.40.206 Multi-cylinder Dryer Section
220.127.116.11.1 Types of Multi-cylinder Dryer Section
Multi-cylinder dryer sections consist of double-tier or single-tier groups or a com¬bination of both. In double-tier dryer sections (Fig. 6.54) the paper web runs around a large number of cylinders (up to about 60 in graphic paper machines and up to 90 for board and packaging paper machines) arranged in an upper and a lower row. The paper is in contact with each cylinder at an angle of about 220 to 240°. Dryer fabrics, one for each top group and one for each bottom group of cylinders, press the paper against the cylinder surface for improved heat transfer. Even together with stabilizers the dryer fabrics can only partly support the paper web during its transfer to the next cylinder. Tail threading in conventional dryer sections is usually done by means of ropes.
The state-of-the-art dryer section for a high-speed paper machine has a single-tier 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 cylinders and suction rolls (Fig. 6.55). In the top row the paper web is pressed against the drying cylinders by the dryer fabric. In the bottom row it is held on the fabric by reduced pressure in the suction rolls. Stabilizing elements ensure a safe web run along its path between cylinder and suction roll where the web is not in contact with the drying cylinder (Fig. 6.56). For high speed paper machines tail threading with only doctors and air jets is standard (Fig. 6.57). Many paper machines have some single-tier groups for runnability reasons fol¬lowed by double-tier dryer groups. Cylinder diameters today are usually 1.8 m, in older machines they are also 1.5 or 2.2 m.
The dryer section is split into several drive groups to control web tension and to account for web stretch and shrinkage. Each group has its own dryer fabric and separate drive. The steam and condensate system is also split into groups to con¬trol the heating curve. Temperature and steam pressure in the different heating groups have to follow an ascending curve so, usually, a steam cascade system is applied, supplying the blow through steam of the last drying group to the last but one etc. The remaining steam from the first group is condensed and any air ex¬tracted from the steam-condensate system.
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 fabrics. Modern dryer fabrics provide high contact area, low caliper, high stabil¬ity and abrasion resistance. Good cleaning of the dryer fabrics ensures uniform evaporation, less sheet picking and improved effectiveness of the web run stabi¬lizers. Shutdowns for cleaning can be avoided or cleaning intervals increased by appropriate cleaning devices as mentioned in Section 6.4.
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