Fiber web drying station has controlled parameters for the drying blanket tension and the temp of the drying cylinders for a trouble-free web passage through the station at high speeds

摘要

To dry a fiber web, of paper or cardboard or tissue, at least one drying cylinder (2) is used with an air-permeable cylinder mantle in contact with the web, and a compressed air supply is connected to the interior of the cylinder. The drying cylinder (2) is heated at the start of the web drying station to achieve a min. dry content on or over a relatively high min. temp. The min. temp. level reduces or prevents sticking of the web at each drying cylinder to an acceptable rate. The tension of the drying blanket (3) is sufficiently high on one side to give a trouble-free control of the web tension and, on the other side, sufficiently low to give the required drawing action on the web to give a trouble-free pick-up of the web (1) from the drying cylinder (2). The selection of the parameters depends on the conditions and especially related to the type, quality and/or characteristics of the paper product. They are also determined according to the paper quality, the running performance, prodn. speed and/or prodn. costs. The relationship between the intensities of the settings is established between three groups of process stages according to the conditions, and especially the second and third. The cylinder mantle is perforated over the whole contact range with the web (1), or the contact zone is covered by the smoothest possible air-permeable cylinder belt, as a woven strip or the drying blanket (3). The compressed air flow is heated in the drying cylinder (2). The cylinder mantle of the drying cylinder (2) is heated to a temp. of -200 deg C and pref. 200-400 deg C. The mantle heating is by infra red radiation, gas burners and/or induction heating. The pressure of the compressed air is set sufficiently high to form a compressed air cushion between the cylinder mantle and the web (1), with a thickness of max. 2 mm and pref. 0.5-1.0 mm. One or more drying cylinders (2) are deployed at the start of the web drying station. The min. temp. decreases as a tendency or steadily along the direction (4) of web travel through the drying station, and the min. temp. is raised on an increase in the web speed. The tension of the drying blanket (3) is selected according to the permeability of the web (1) and the temp. of the drying cylinders (2). The blanket (3) tension and/or the drying cylinder (2) temp. is set according to the web (1) speed, the disposable web pull, and the target drying level. An Independent claim is included for a web drying station where the fiber web (1) is in contact with drying cylinders (2), together with a drying blanket (3) under a controlled tension. Preferred Features: The assembly has a control system to set the drying blanket (3) tension and/or the temp. of the drying cylinders (2).