The present invention relates to a drying section of a machine for manufacturing a fiber web, in particular a web of paper. The drying section has features like those in U.S. Pat. No. 4,934,067. The invention particularly concerns a so called single row or single tier drying section in which the web to be dried is conducted by a first support belt continuously from first drying cylinder to first drying cylinder in a first drying group and is then transferred from the first support belt to a second support belt in a second drying group of second drying cylinders. The transfer avoids an open length of web, i.e. an open draw. The present invention particularly concerns the placement of the axes of the drying cylinders and of the cooperating reversing rollers of a series of the drying groups.
In drying sections of that type, there is a distinction between drying groups which have an upper support belt and drying groups which have a lower support belt. In the first-mentioned drying groups, the web to be dried and the support belt for the web wrap around the upper part of the circumference of the drying cylinders. In this case, the support belt travels from the last cylinder of the drying group, over guide rollers which lie above the drying cylinders, back to the first drying cylinder of this group. In the second-mentioned drying groups having a lower support belt, the wrapping and movements of the web and the support belt are inverted opposite those of the first-mentioned drying groups. The drying groups are arranged along a single row or tier with all drying cylinders parallel.
In the known drying section, the shafts or axes of the drying cylinders of those drying groups which have a lower support belt lie at a smaller vertical distance above the longitudinal support beam of the drying section than the shafts or axes of the drying cylinders of the other drying groups which have an upper support belt.
This known arrangement has the following effect on the design of the places of web transfer between two successive drying groups: The web pick-up roller of the following drying group can be arranged at a relatively small distance from the last support belt reversing roller of the preceding drying group. Nevertheless, it is possible to make the circumferential wrapping regions of the two adjacent drying cylinders which lie at the place of web transfer relatively large so as to obtain the greatest possible transfer of heat to the web.
Although the above noted advantages are obtained, however, there are disadvantages in other respects. On the one hand, the guide rollers of the lower support belts are located in a cellar that is below the longitudinal beams. This makes maintenance of these rollers, for instance, of their bearings, difficult. On the other hand, a large amount of space is lost in the cellar by the support belt guide rollers which are arranged in the cellar and by the support belts which travel over them. That space is thus not available for other units of the web manufacturing machine.
U.S. Pat. No. 3,434,224 discloses that the drying section is customarily covered by a hood which serves primarily for containing and leading away vapors. In principle, such a containment hood is comprised of two lateral longitudinal walls, which stand on the floor of the paper machine, an upper covering wall, and two lower longitudinal walls which are in the cellar. The lower walls are necessary because the guide rollers of the lower support belts are arranged in the cellar. This arrangement has the disadvantage that the volume surrounded by the hood is relatively large. Thus, a relatively large expenditure of energy is necessary to hold the entire inside volume beneath the hood at a desired high temperature.