The present invention concerns a group of drying cylinders in a multiple cylinder dryer for a web, in particular for paper, having a drying felt or wire which is common for all drying cylinders.
The cylinders in the dryer which dry the web are frequently placed in two rows one above the other, and the web travels along a zigzag path alternately under the lower cylinders and over the upper cylinders. In the longitudinal direction of the machine the cylinders are usually grouped to form a few drive groups, the peripheral velocities of each cylinder being constant within one such group. It is common practice to provide in each drive group both for the upper cylinders and for the lower cylinders a felt or wire loop of their own, which loops urge the web against the surface of the cylinders. It is thus understood that the web has to travel from one cylinder row to the other completely without support. It may be specified as a characteristic feature of the arrangement just described that as a rule the drive group comprises two felt or wire loops, outside which the drying cylinders are located and which, both separately, urge the web in the direction of travel of the web against every second cylinder, that is, the lower felt or wire urges it against the lower cylinders and the upper felt or wire urges it against the upper cylinders.
Particularly in the course of the recent continuous increase of the output of multiple cylinder dryers, that is of their breadth and running speed, certain drawbacks of the construction just described have become ever more appreciable.
1. The web passing over from one cylinder to another, which has no mechanical support, is susceptible to suffer breaks particularly in the wet condition. This susceptibility increases with increasing speed, in which connection the tendency of the web margins to flutter under effect of various disturbing factors quite often increases. It has been found that this tendency is often in machines provided with drying wires if the permeability of the wires is too high and the placement of the rollers is unfavourable, and the air currents produced by the wires in the interstices between cylinders are strong, so as to increase the fluttering of the edges.
2. In a multiple cylinder dryer a so-called "drying pocket" is formed adjacent to each cylinder, which is a space open only at its ends and confined by the bare cylinder surface, by the drying felt or wire passing around the adjacent roller and by the web arriving at and departing from the cylinder. It is a rather difficult task to ventilate this space, and as a result of deficient ventilation the drying process is often non-uniform and the ultimate product has an objectionable moisture content profile. In order to rectify this situation, auxiliary equipment for ventilating the pockets of the dryer is frequently mounted within the pockets themselves in machines having drying felts, or on their outside when drying wires are employed. Such equipment is invariably impractical to a certain extent and even tends to cause additional breaks or other kinds of trouble, nor is its use successful in all instances, because it is difficult to correct major moisture content errors.
3. In the event of a web break, waste material tends to remain in the space between the drying felt loops, where the drying cylinders are located, and its removal is not always easy, certainly not in the case of machines having a great width. The fact that this space in particular has to be watchable during operation and that, especially in a machine with felts, air flows are present at the ends of the drying pockets, forbids the practice of constructing both longitudinal sides of the dryer as closed partitions. This implies the necessity of using machine frame structures assembled of beams and columns. These are permeable to air flows, for which reason it is furthermore common practice to enclose the entire multiple cylinder dryer in a hood--either an open or an enclosed hood.
It is known in prior art to support the web between cylinders in the manner disclosed by the U.S. Pat. No. 3,503,139. Instead of providing for the upper as well as the lower drying cylinders of a given drive group each their own drying felt or wire loop, outside which the drying cylinders are positioned, as in the conventional design solution, which was considered in the foregoing, any given set (group) of drying cylinders in the multiple cylinder dryer according to said patent has only one single felt loop, with one of the two cylinder rows within this loop and the other cylinder row remaining outside this loop. The web will then travel from one cylinder row to the other under guidance by the felt. It is a drawback, however, that on every second cylinder the web lies on top of the felt, not between the felt and the cylinder. As a consequence, in association with these cylinders the felt constitutes an insulating layer between the cylinder and the web, which impedes the heat transfer from the cylinder to the web. In an attempt to compensate for this, it is suggested that in the respective cylinders a higher steam pressure should be maintained. This may possibly damage the drying felt lapping these drying cylinders, yet without being able to increase the specific evaporation (evaporation/cylinder surface area) to be on a level with that of the drying cylinders whose surface is immediately lapped by the web.
The aim of the present invention is to eliminate in multiple cylinder web dryers the above-drawbacks, which have been encountered in them. The invention is characterized in that, with the aid of guiding rollers or equivalent, the drying felt or wire together with the web has been guided to travel from one drying cylinder to the next drying cylinder in the drying cylinder group in such manner that the drying cylinders will lie outside the drying felt or wire loop, in order that the web to be dried might always be compelled to travel between the surface of the drying cylinder and the drying felt or wire.
The drying cylinder group may comprise two substantially parallel rows of drying cylinders, the web being directed from the last cylinder in the row which is first in the direction of travel of the web to the second drying cylinder row. The guiding rollers belonging to the drying cylinder group may be placed between each two adjacent drying cylinders in the rows. It is advantageous if the guiding rollers are so disposed between the drying cylinders in the rows that the planes passing through their axes lie outside the planes passing through the axes of the drying cylinders, and most appropriately so that the planes passing through the axes of the guiding rollers lie outside the tangential planes of the drying cylinders. It is advantageous if the open sides of the felt or wire loop have been closed with walls in order to produce an enclosed space within the felt or wire loop and if in said enclosed space a vacuum has been arranged to prevail in order that the web, while travelling upon the drying felt or wire, will be attached to it by suction. Said guiding rollers are most appropriately foraminous so that the vacuum within the drying felt or wire loop causes the web to be attached by suction to the drying felt or wire passing over the guiding rollers. In said enclosed space a vacuum of such height may be arranged to act that the guiding rollers may operate as traverse flow dryers. It is then advantageous if the guiding rollers have been provided with a blow hood containing a heat source. The drying cylinder group may also comprise a drying cylinder intended for the drying of the drying felt and which may be located inside or outside the felt loop.
The invention is described in closer detail in the following with reference to the embodiment examples presented in the attached drawings.
In the drawing,
FIG. 1 presents as a longitudinal sectional view a cylinder group of a multiple cylinder dryer according to the invention.
FIG. 2 shows, in longitudinal section, a cylinder group of a multiple cylinder dryer according to another embodiment of the invention.
FIG. 3 shows, as a longitudinal section, a cylinder group of a multiple cylinder dryer according to a third embodiment of the invention.
FIG. 4 shows the section along line IV--IV in FIG. 3. FIGS. 5, 6, 7, 8 and 9 present examples of the longitudinal sections of multiple cylinder dryers according to the present invention.
FIG. 1 represents the invention in its simplest form. The reference numerals 1, 2, 3, 4, 5 and 6 indicate drying cylinders disposed in two rows. The drawing has been so executed that 1, 2 and 3 are lower cylinders and 4, 5 and 6 are upper cylinders, but in actual fact the positioning of the rows with regard to gravity may be arbitrary and the rows might therefore equally be vertical. It should be noted that the drying cylinders need not even necessarily lie in rows. The numerals 7 refer to drying felt (or wire) guiding rollers belonging to the group, 8 indicates the drying felt (or wire), 9 its tensioning means, 10 a conveyor which conveys the web 11 from the wet end from the press section, or from the preceding group, to the group under consideration. It is observed that the web 11 is transferred from the conveyor 10 onto the drying felt (or wire) 8 with fully closed conduction without even a single moment at which it would not receive support from one or the other conveyor. It is a characteristic feature of the arrangement that the group comprises only one drying felt (or wire) 8 and that the drying cylinders 1, 2, 3, 4, 5 and 6 of the group all are located outside the felt (or wire) loop. It is thus understood that the web 11 is supported by the drying felt (or wire) throughout its travel within the group and that at all drying cylinders it is interposed between the drying felt (or wire) and the cylinder surface, whereby an efficient drying action is ensured at all cylinders. Within one group the web is in contact with all cylinders with one and the same side. If it is desired, for instance in view of the gloss of the web, to change the side which is in contact with the cylinder, the dryer may be constructed to have a pluralitiy of groups in succession, the side of the web changing at transition from one group to another. Examples of various possible arrangements are seen in FIGS. 5-8. It may be noted that in the exemplary case of FIG. 1 the number of cylinders may naturally be another than six; it may in principle by any number from two upwards.
The following advantages are now gained by the arrangement.
1. The web is supported throughout its travel.
2. There are no drying pockets. The uniformity of ventilation within the felt (or wire) loop (in the space 12, FIG. 1) is not essential because the web is covered by the drying felt (or wire) and therefore is not sensitive to variations within this space. Those regions where the web is exposed during its travel (the runs 8' and upon the guiding rollers 7, FIG. 1) are readily approachable from the outside. In order to improve the drying profile, blowing tubes 13 adjustable in the breadth direction of the web, or blowing hoods 14, may be constructed in association with some of the guiding rollers 7 or perhaps with all of them.
3. The removal of waste becomes easier because now, owing to the closed felt (or wire) loop, waste cannot easily get into the interstices of the drying cylinders. It is particularly advantageous in view of the waste to make the rows vertical, as for instance in FIG. 2, in which case there are no horizontal surfaces on which waste could accumulate.
FIG. 2 presents a variation developed from the preceding one. Equivalent parts have been indentified by the same reference numerals. In addition to the fact that the rows of drying cylinders now are vertical, it is essential that the guiding rollers 7 have now been made with a foraminous mantle. When the ends of the wire loop formed by the wire 8 are closed with partitions (not depicted in the drawing), in its interior space 12 a vacuum may be maintained with reference to the surroundings. For the purpose of maintaining the vacuum, FIG. 2 shows schematically a suction tube 15 departing from the space 12 and a blower 16, by the aid of which said vacuum is generated. This vacuum now causes the web to be drawn by suction to adhere both to the felt (or wire) 8 at the runs 8' and to the surface of the foraminous guiding rollers 7, which enables fully automatic threading of the leading web end to be accomplished and ensures the travel of the web during operation, which is necessary particularly in the case of high machine speeds, in which instance otherwise the centrifugal force on the surface of the guiding rollers 7 may be disturbing. In the figure also doctors 17 have been shown, which during the web introduction operation also ensure the travel of the web 11 together with the felt (or wire) 8.
FIG. 3 shows a still more advanced modification of the preceding embodiment. Equivalent parts have been indicated by the same reference numerals. The difference is that now the partitions on either side of the felt (or wire) loop 8 are sturdy enough and the seals have been well enough constructed to enable in the space 12 with the aid of the flower 16 a vacuum of such magnitude to be maintained that a traverse flow through the web 11 is produced. In that case at least part of the guiding rollers 7 may be replaced by traverse flow drying cylinders 18, in themselves known in prior art, with blowing hoods 19', 19". The drying air or gas flows from the hoods in the direction of arrows a through the web and the mantle of the drying cylinder, and the spent gases flow in the direction of arrows b into the space 12 and thence into the suction pipe 15. For several traverse flow dryers either individual hoods 19' or a common hood 19" may be provided. Furthermore, in FIG. 3 dotted lines indicate the positions of the hoods 19', 19" during a web break, giving access to those parts of the mchine where waste may accumulate. It may be observed that it is to be expected that the dryer now described has a favourable specific evaporation, because the specific evaportion of each drying cylinder 1, 2, 3, 4, 5 and 6 increases owing to the lowered total pressure in the space 12 and the specific evaporation of the transverse flow dryers 18 can be made a multiple of that of a common cylinder. The ventilation and heat recovery equipment for the machines described differ completely from conventional design solutions. It may be observed, among other things, that since the described dryer may be constructed with impermeable side walls and since at the evaporating surfaces of the web 11 facing towards the room there is a flow directed into the space 12, as a result of the suction effect, it follows that a drying hood proper, in particular a closed one, is unnecessary. To that space outside the drying felts or wires of a conventional dryer around which a hood has been constructed from which the exhaust suction of a dryer of this type is effected, corresponds in the dryer according to FIG. 3 the space 12 inside the drying felt or wire loop, whence the withdrawal of exhaust air can be accomplished through the pipe 15, without need of a separate hood for collecting the exhaust air.
FIG. 4 shows the section through a cylinder group according to FIG. 3 along the line IV--IV. Equivalent parts have been indicated by the same reference numerals. In addition to the components shown in the longitudinal section (FIG. 3) this figure also shows the walls 20 enclosing the space 12 and which have apertures for the mantles of the drying cylinders 1, 2, 3, 4, 5 and 6 and of the traverse flow dryer cylinders 18 and covers 21 and 22 closing these apertures, in which covers the shafts 23 and 24 of the cylinders are carried. If the dryer is provided, in particular, with drying felts, in each group of consecutive drying cylinders, which according to the present invention have one common drying felt, one or several such cylinders may be incorporated for the purpose of drying the drying felt, which are not lapped by the web.
FIG. 5 shows a dryer according to the invention having two cylinder groups A and B each with its own drying felt 8. In the group A the drying cylinders are disposed in two vertical rows, which are side by side, and the number of drying cylinders is five. Group B has four drying cylinders, which lie in two rows, one above the other. The groups have their own drying felts 8. The direction of travel of the web 11 is indicated by arrows C. Each cylinder group has one drying cylinder 25,26 for the drying felt 8. Of these, the drying felt (8) drying cylinder 25 in group A lies outside the felt loop, whereas the drying cylinder 26 for the drying felt 8 in group B is within the felt loop. Although it is expedient, as in FIG. 5, to place the felt drying cylinder 25, 26 between the first and last drying cylinders of the web 11, it may in principle also be placed between another pair of cylinders. If the felt permits, it is possible alternatively for its drying to use a traverse flow drying cylinder having a foraminous mantle.
FIG. 6 also shows a multiple cylinder dryer according to the invention. This, too, has two drying cylinder groups A and B, both with their own drying felt 8 and guiding rollers 7. Said guiding rollers are so positioned that the drying cylinders lie outside the drying felt loops. The arrow C indicates the direction of travel of the web 11. Each drying cylinder group A and B has six drying cylinders, which lie in two rows, one above the other. The dried web 11 is conducted through a calender 27 to the reeling device 28.
FIG. 7 also shows a multiple cylinder dryer according to the invention. It, too, has two drying cylinder groups A and B, similarly as the multiple cylinder dryer of FIG. 6. The multiple cylinder dryer of FIG. 7 differs from that of FIG. 6 mainly only in that both drying cylinder groups A and B have six drying cylinders each, which are disposed in two vertical rows side by side, and that the guiding rollers are traverse flow drying rollers 18 provided with a hood 19', 19". The web 11 travels in the direction of the arrow C and upon having dried, goes through the calender 27 to the reeling device 28.
FIG. 8 shows a multiple cylinder dryer similar to that of FIG. 7, and equivalent parts have been indicated by the same reference numerals. The multiple cylinder dryer depicted in this figure differs from that of FIG. 7 mainly in that in the groups the drying cylinders in two rows side by side are interjacent. It follows that both groups have six traverse flow dryers 18 with hood 19', 19", serving as guiding rollers. The conduction of the wire 11 from one cylinder group to the other is also different from the conduction of the web 11 shown in FIG. 7.
FIG. 9 shows a multiple cylinder dryer according to the invention having in an oblique position, consecutively, a plurality of drying cylinder groups A, B, C, D, E, F and G. Each one of these comprises one row of drying cylinders, each such row having its own drying felt 8 and the drying cylinders lying outside the felt loop formed by it. The felt loop is guided by guiding rollers 7. The web 11 is conducted from one group to another, supported by the drying wire or felt.
The invention is by no means confined to the embodiment examples presented in the foregoing, and it can be modified in numerous ways within the scope of the invention.