Total restraint drying

Abstract: In a machine for manufacturing a paper web, the machine includes a dry end comprising a plurality of dryer groups I - VIII arranged one after the other along the path of the web through the dry end; each dryer group comprising: a single row of heatable drying cylinders 10; a reversal roll 11 between each two drying cylinders in the group; and endless, web support belt 12 which supports the web to travel together with the support belt through the dryer group; guide rolls 13 in each dryer group for guiding the support belt in an endless loop through the respective dryer group; the support belt 12 passing through the dryer group I - VIII in meander path so that the web on the support belt directly contacts a drying cylinder 10 in the respective dryer group, the support belt then touching a reversal roll 11 in sequence and the web then touching the next drying cylinder in sequence so that the web comes into direct contact with the drying cylinders and the support belt comes into direct contact with the reversal rolls; one of said dryer groups I - VIII being a top-felted dryer group wherein a pressure differential box 14 is located between the dryer cylinders 10 and above the respective reversal rolls 11; a second dryer group located immediately downstream from said top-felted dryer group; said second dryer group being a bottom felted dryer group wherein a pressure differential box 14 is located between the dryer cylinders 10 and below the reversal rolls 11; and an open draw transfer between said first dryer group and said second dryer group.

Description: The present invention relates to a machine for producing a fiber web, particularly a paper web, and particularly relates to a machine of the kind as defined in the preamble of claim 1. The machine has a press section followed in the web path by a dryer section. The dryer section comprises a plurality of separated dryer groups, each operable at a respective different speed. Each dryer group includes a plurality of dryers, a plurality of web path reversal rolls, one between each two dryers, and an endless loop web support belt, which is sometimes a dryer felt, passing around the dryer group in a meander path past the dryer cylinders and the reversal rolls and past guide means which guide the support belt in the endless loop. The pocket between the dryer cylinders may have a differential pressure box within it. After the web to be produced from a fiber suspension is formed and partly dewatered in a wire end or forming end or wet end of the machine, the web is dewatered mechanically as far as possible in a press. Then the web is dried in a dry end comprised of heatable drying cylinders. In the prior art single tier dryer sections, the web is restrained while travelling around the dryer cylinder by having a felt press the web against the dryer cylinder. Between dryer cylinders, a vacuum roll was located to restrain the web against the felt during passage around the vacuum roll and thereby restraining the web against cross-machine directional shrinkage. However, the web is not restrained between the place where the web leaves the dryer cylinder and the place where the web is picked up by the vacuum roll. In order to increase the runnability, it is known to keep the web as reliably as possible on the web support belt at the place where the web runs off from each individual drying cylinder to the reversing roll, and on the straight travel path from the reversing roll to the following drying cylinder. In this respect, the initial region of the dry end presents a particular problem because the paper web is still relatively wet there and it has a tendency to adhere to the wall of the drying cylinder and to detach itself temporarily from the support belt as the web leaves each dryer cylinder. In other words, a so called bubble is formed here between the web of paper and the support belt. In order to reduce the danger of the web of paper tearing, it is attempted to keep the bubble as small as possible. For this purpose, it is known to form a vacuum zone at the run-off place, shown in U.S. Pat. No. 4,359,828, FIG. 3. Another known measure to reduce the danger of the web of paper tearing consists of reducing the distance between the drying cylinder and the adjacent reversing roll as much as possible, shown in International Application WO 83/00514, FIG. 2, or U.S. Pat. No. 4,905,379, FIG. 1. A machine of the kind as defined in the preamble of claim 1 is disclosed in DE-U-92 07 656. US-A-5 299 363 discloses a dry end of a paper making machine comprising alternately arranged top-felted dryer groups. SUMMARY OF THE INVENTION The dryer section of the machine according to the present invention and as defined in the claims consists of a number of dryer groups having dryer cylinders and one reversal roll between each two dryers within a dryer group. The dryer groups are top-felted dryer groups. A differential pressure box may be placed in the top-felted dryer groups between the dryer cylinders and above the reversal rolls. Drive means for driving the drying cylinders in each of the dryer groups rotates the dryers to move the web through the dryer group. Each of the drive means for each of the dryer groups has a drive which is adapted to produce a selectively variable rotation speed for the drying cylinders in that dryer group, and a respective speed control device for the drying cylinders of each of the dryer groups for enabling adjustment to produce a difference in rotation speeds between the drying cylinders of adjacent dryer groups. According to the invention all top-felted dryer groups have special drive means as defined in the characterizing part of claim 1. In a further embodiment of the invention, at least in the terminal end region of the dry end of the dryer section, an open unsupported paper path or open draw is present between two dryer groups. In other words, a place of separation is developed as an open place of separation or open draw. Preferably, at least in the second half of the dry end, places of separation between adjacent dryer groups are developed as open places of separation. This not only favors the removal of longitudinal stresses, it takes into account that slight rotary oscillations can occur from time to time in the drive elements. These oscillations cause a danger of producing a sudden, abrupt increase in the longitudinal stress in the web of paper which may cause a tear of the web. However, this danger is avoided with the invention by providing open separation places, since a sudden increase in the longitudinal stress within a free path, and especially a relatively long free path of travel of the web of paper, is less dangerous than at a closed place of separation. Further an open draw transfer between dryer groups allows for easy removal of broke between those dryer groups. At the open draw the broke is allowed to fall directly into the basement of the paper machine. The invention can be used in connection with various different types of dry ends. However, all of them share the feature that they have exclusively top-felted single tier dryer groups in the main dryer section. In a single tier drying group, all of the drying cylinders dry the same side of the web. In accordance with the invention, the dry end design has all top-felted single tier dryer groups wherein the drying cylinders of all of the dryer groups are arranged above the respective reversal rolls between adjacent drying cylinders. The transfers between top-felted dryer groups in accordance with the invention can be developed as open or closed separation places for the reasons explained above. Whether the separation spaces should be developed open depends on the type of paper being dried or on the moisture content still present in the web at the place of separation, and furthermore on the magnitude of the speed difference to be adjusted. In many cases, it is entirely possible to keep a separation place closed despite a required difference in speed between two dryer groups. One can imagine that following the place of run off of the preceding web support belt from the last cylinder of the preceding dryer group, up to contact with the succeeding web support belt of the following dryer group, the web detaches itself slightly from the last drying cylinder since a thin layer of vapor forms between the last drying cylinder and the web. Furthermore, the web initially only has loose contact with the support belt of the following dryer group. This contact only becomes more secure at the place where the support belt and the supported web reach the first reversal roll of the following dryer group. It is possible that the speed of the support belt of the following dryer group may differ by a small amount from the speed of the support belt of the preceding dryer group. This means that the web moves at different speeds in the preceding and following dryer groups. However, because small and sudden changes in speed must be expected from time to time, the resulting danger of a tear can be reduced if the "simple" separation places are also developed as open separation places. In certain cases, it may be advantageous to operate a separation place at times open and at times closed. For this purpose, one of the rolls over which the support belt of the following dryer group travels can be movably supported. In the preferred type of construction of the dry end to which the invention can be applied, the drying cylinders of all of the dryer groups are arranged above the respective reversal rolls between adjacent drying cylinders. It depends on the individual dry end and the nature of the paper web to be produced whether it is better to operate the separation places open or closed. As a rule, however, it will be advantageous to provide open separation places between the dryer groups at least in the final end region of the dry end where the residual moisture content is already very slight. Stated more precisely, at least the last separation place, or the last two or three separation places, are developed as open separation places. On the other hand, in the upstream region of the dry end, it is usually more advantageous to develop the simple separation places as closed separation places. Again it is advisable, at least in connection with some of the separation places, to provide for the possibility of changing from an open separation place to a closed separation place, or vice versa. At a separation place which is developed as an open separation place according to the invention, the web of paper travels across the separation place over a free travel path from the last drying cylinder of the preceding dryer group to the support belt or dryer of the following dryer group. The advantages of this measure are identical or similar to those in the case of the dry end constructions described further above. Better handling of paper web shrinkage during progressive drying can be done by driving the following dryer group with a slightly lower speed than the preceding dryer group. If both dryer groups were driven continuously at the same speed, then longitudinal stress would be built up in the web of paper due to its shrinkage upon drying. In the extreme case, together with other disturbing factors, this might cause a tear in the web of paper. However, if necessary, a slight positive draw can also be run between two dryer groups across an open draw in order to create tension in the web so the web does not flutter across the open draw. Since the web is constantly shrinking as it progresses downstream, a transfer without any draw still is done under tension between the dryer groups. Driving the two dryer groups in question with a slightly different speed can be attempted even if the place of separation between them is closed. However, at the place where the paper web contacts the last drying cylinder of the preceding dryer group and the support belt of the following dryer group, there is a danger that the surface of the web will be damaged due to the difference in support belt speeds. There is the further factor that in the respective drives for each of the dryer groups of the dry end, as already mentioned, oscillations in drying cylinder rotation sometimes occur. These are more likely to cause a tear of the paper web at a closed separation place than at an open separation place. In a further embodiment, the use of vacuum rolls, reversal rolls with pressure differential boxes adjacent, and reversal rolls having a perforated cylinder or grooved roll are varied by dryer group depending on the moisture content of the paper and the type of paper being made. In one embodiment, in the early dryer groups perforated cylinders with pressure differential boxes are used. This is because the greatest water content is present in the web. This high water content in the web causes a bubble in the web when it leaves a heated surface of the dryer cylinder. In order to assist the web in removing the bubble, a draw is created between the dryer cylinder and the perforated cylinder. This draw allows pressure differential forces created by the pressure differential boxes and other forces acting on the web to pull the bubble out of the web. In the prior art, a short draw of 2-4" between the dryer cylinder and vacuum roll, does not provide a large enough area and enough time for the dryer group to work the bubble out before the paper is held to a vacuum roll. This causes wrinkling or creping of the paper. In later groups of the dryer section, guide rolls having perforations, but no means for creating a vacuum are provided. This is because at the end of the dryer section, the paper has a low moisture content, it needs a chance to relax and relieve longitudinal stresses. Therefore only perforated guide rolls or rolls having circumferential grooves are necessary between dryer cylinders within a dryer group. Other objects, features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B diagrammatically show a dry end of a paper manufacturing machine in which a mixture of top and bottom felted dryer groups are arranged. Such a machine is not claimed. FIG. 2A and 2B diagrammatically show the dry end of a paper manufacturing machine in which, according to the present invention, all the dryer groups are top-felted. FIG. 3 shows a transfer between a top and bottom felted dryer group as shown in FIG. 1 on a larger scale. Such transfers are not part of the invention. FIG. 4 shows drive arrangements in a single tier dryer group according to the present invention. FIG. 5 shows drive arrangements in single tier dryer groups according to the present invention. In a machine for manufacturing a paper web according to the prior art, as shown in Fig. 1 in many of the dryer groups throughout the dryer section, and all the dryer groups in the initial, or upstream or wetter region of the dryer section, only the lower side of the web comes into contact with the drying cylinders 10. In other words, in most of the dryer groups, the drying cylinders 10 all lie above the neighboring reversing rolls 11 with which the dryer cylinders 10 alternate along the web path. Only the second to the last dryer group, VI, for instance, has a reverse arrangement in which the drying cylinders 10 lie below the reversing rolls 11 so that the top side of the paper web comes into direct contact with the drying cylinders 10 of that group. The reversing rolls 11 as shown in Figure 1 are perforated cylinders. Between the dryer cylinders 10 and above the reversal rolls 11 in dryer groups I, II, III, IV, V, VII and VIII is located a pressure differential box 14. Between the dryer cylinders 10 and below reversal rolls 11 in dryer group VI is located a pressure differential box 14. This is preferably a Duostabilizer or vacuum box, however, other pressure differential boxes, such as blow boxes may also be used. Accordingly, within the initial region of the dry end, for instance between each two of the first five dryer groups, there are only so called "simple" places of separation between the adjacent dryer groups. This means that the web support belt 22 of the next following succeeding dryer group contacts the last drying cylinder 10 of the preceding dryer group at a place where the web of paper is no longer covered by the web support belt 12 of the preceding dryer group. Such a known development of the place of separation is advantageous in two respects. The threading of the web of paper, for instance upon the starting of the paper machine operation or after a tear of the paper web, takes place completely automatically, without rope guidance being necessary, as is required in older arrangements. The web of paper travels just as reliably during the normal operation of the dry end from each preceding dryer group to the following dryer group. In an exceptional case and despite the favorable manner of construction described above, if a tear should take place in the web of paper, then the reject paper or broke moves readily downward from ALL drying cylinders 10 of the dryer groups into the basement provided below the dry end. In Figure 1, the dry end following the press section comprises eight dryer groups I to VIII. Each dryer group has its own respective web support belt 12 and has a plurality of drying cylinders 10 which alternate with a respective reversal roll 11 and a pressure differential box 14 between adjacent drying cylinders 10 within each dryer group. As shown in Figure 1, these reversal rolls 11 are perforated cylinders. Furthermore, there are guide means comprising customary additional guide rolls 13 for guiding, tensioning and regulating each endless loop support belt 12. Horizontal rows of drying cylinders 10 are shown. However, vertical or inclined rows of cylinders can also be provided. Each of the dryer groups I to VIII has its own respective drive. In the dryer groups I, II, III, IV, V, VII and VIII, the drying cylinders 10 are above the adjacent, alternate in the web path, reversal rolls 11 so that within these dryer groups, only the bottom side of the paper web comes into direct contact with the drying cylinders 10. The top side of the web rides on the outside of the support belt 12 around the rolls 11. In the dryer group VI, the drying cylinders 10 lie below the reversal rolls 11 so that the opposite top side of the web come into contact with the drying cylinders 10. Between dryer groups V and VI and VI and VII, all separation places between two adjacent dryer groups are developed as turn over or web side reversal separation places. At all of these turn over separation places, the web of paper travels over a free path of travel or open draw from the web support belt 12 of the preceding dryer group to the web support belt 22 of the following dryer group. Here, all separation places are developed as open separation places. In an embodiment according to the present invention shown in FIG. 2, the drying cylinders 10 are arranged above the reversal rolls 11 in all of the dryer groups I-VIII. Accordingly, only the bottom side of the web comes into contact with the drying cylinders 10 within the entire dry end. The separation places present within the dry end are therefore developed as so called "simple" separation places. This means, for instance, that at the separation place 100, the support belt 22 of the following dryer group II contacts the last drying cylinder 10 of the first dryer group I. That support belt 22 wraps around that cylinder 10 to a greater or less extent. This contact takes place at the point where the web of paper is no longer covered by the first support belt 12. This, therefore, is a "closed simple" separation place. For the above indicated reasons, an "open simple" separation place can also be provided. Here, a guide roll 23 and the first reversal roll 11 for the support belt 22 of the second dryer group II would be arranged behind the last drying cylinder 10 of the first dryer group I so that the support belt 22 passes at a slight distance away from the drying cylinder 10. Finally, it is possible to operate a simple separation place optionally either open or closed by displacing a guide roll 23. The guide roll 23 would be supported to be moveable. Figure 3 shows an enlarged view of figure 1 at the transfers between the top-felted single tier dryer groups and the bottom felted single tier dryer group. There is an open draw transfer between the dryer group V and dryer group VI. The web 60 is totally unsupported between the guide roll 61 and the dryer cylinder 62. A similar open draw transfer occurs between dryer group VI and dryer group VII. Figures 4 and 5 illustrate according to the present invention a drive concept used with the dryer sections of Figure 2. As shown in Figures 4 and 5, there are no gear boxes. According to the present invention the dryer groups are driven by driving the reversing rolls 101. Because of the small wrap angle, the last dryer cylinder 102 of each dryer group is also driven. The first dryer cylinder 103 of each dryer group may also be driven because of the small wrap angle and because of the vapor layer created between the sheet and the dryer surface. Figures 4 and 5 show the driven rolls and dryers of different size dryer groups. The drives for the reversing rolls 101 of a dryer group are equal. The drives of the driven dryers 102 or 103 of a dryer group are equal. The drives of the dryers 102 or 103 are smaller than the drives of the reversing rolls 101. The drives of each dryer group are independently controlled. The drives of each dryer group, in turn, move the respective endless support belt 104. The web is moved by the driven support belts 104 and the speed of the drive units determines the speeds of the drying cylinders 102 and 103, of the support belts 104 and therefore of the web. The separation places between dryer groups make it possible for a certain difference in speed to be adjusted, at least on some of the separation places between the adjacent drive units. The speed of the dryer group refers to the speed of the web moving through the dryer group. A positive difference in speed can be provided, for example, between the first two dryer groups I and II as shown in Figure 2 at the separation place 100. In other words, the web in the second dryer group II travels slightly faster than the web in the first dryer group I. Two adjacent dryer groups, for instance, groups II and III, can, if necessary, also be driven at the same speed, that is, the web travels at the same speed in both groups. The difference in speed between dryer groups is preferably only 1 to 3 feet per minute. The web of paper has a tendency to adhere to the wall of each drying cylinder at the runoff point from the individual drying cylinder 10 and therefore to temporarily detach itself from the respective support belt 12 of the dryer group. In order that the so called bubble produced at the run off place remain as small as possible, the drive for the first dryer group I is adjusted to a somewhat higher speed than the drive for the press section. Consequently, the web arrives at the runoff point with a certain longitudinal pre-tension. For the same reason, the drive for the second dryer group II can be driven with a somewhat greater speed than the drive for the first dryer group I. In order to make this possible, the first reversal roll 11 of the second dryer group II can be arranged at a distance from the support belt 12 of the first dryer group I. Accordingly, the paper web travels in a free travel path or open draw from the support belt 12 to the support belt 22. Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims: A machine for manufacturing a paper web, the machine including a dry end comprising: a plurality of dryer groups (I-V, VII-VIII) arranged one after the other along the path of the web through the dry end; each dryer group (I-V, VII-VIII) comprising: a single row of heatable drying cylinders (10); a reversal roll (11) between each two drying cylinders (10) in the group; an endless web support belt (12) which supports the web to travel together with the support belt (12) through the dryer group; guide rolls (13) in each dryer group for guiding the support belt (12) in an endless loop through the respective dryer group; the support belt (12) passes through the dryer group in a meander path so that the web on the support belt (12) directly contacts one of the dry-ing cylinders (10) in the respective dryer group, the support belt (12) then touches a reversal roll (11) in sequence and the web then touches the next drying cylinder (10) in sequence so that the web comes into direct contact with the drying cylinders (10) and the support belt comes into direct contact with the reversal rolls (11); the respective drying cylinders (10), reversal rolls (11) and guide rolls (13) of each of the dryer groups being so placed as to define a respective separation place between each preceding dryer group and the succeeding dryer group thereafter in the path of the web through the dry end, and including means at the separation place for transferring the web from the preceding dryer group to the succeeding dryer group; the drying group cylinders (10) and the reversal rolls (11) in all of the plurality of the dryer groups are so placed that the drying cylinders (10) are above reversal rolls (11) such that only the bottom side of the web contacts the drying cylinders; and for at least one of the dryer groups a guide roll (13) within the support belt (12) is supported for being movable in a manner in which the separation place can be operated optionally opened for providing a space where a web or a tail runs unsupported from the preceding dryer group to the following dryer group or closed where the support belts (12) of two dryer groups are in contact and a web or tail is supported in its movement across the separation place; characterized in that each of the dryer groups comprises at least one driven last drying cylinder and at least one driven last reversal roll. The machine of claim 1 wherein each separation place is provided as an open separation place. The machine of claim 1 wherein open separation places are provided between the dryer groups at least in the final end region of the dry end. The machine of claim 1 wherein at least the last separation place is provided as an open separation place. The machine of claim 1 wherein at least the last two or three separation places are provided as open separation places. The machine of claim 1 wherein at least in the second half of the dry end, the places of operation are provided as open separation places. The machine of claim 1 wherein, within the initial region of the dry end, there are only simple places of separation which means that, in the case of a closed simple place of separation, a respective web support belt of the next following succeeding dryer group contacts the last drying cylinder of the preceding dryer group at a place where the web of paper is no longer covered by the web support belt of the preceding dryer group, or, in the case of an open simple place of separation, the respective web support belt of the next following succeeding dryer group passes at a slight distance away from the last drying cylinder of the proceeding dryer groups. The machine of claim 1 wherein a pressure differential box is located between the dryer cylinders and above the respective reversal rolls for each dryer cylinder within a dryer group for at least 70% of the dryer groups as means at the separation place for transferring the web. The machine of claim 1 wherein a pressure differential box is located between the dryer cylinders and above the respective reversal rolls for each dryer cylinder within a dryer group for 100% of the dryer groups as means at the separation place for transferring the web.

Inventors: HALMSCHLAGER GUENTER DR, OECHSLE MARKUS, HALMSCHLAGER, GUENTER, DR., OECHSLE, MARKUS, Halmschlager, Günter, Dr.
Applicants: VOITH PAPER PATENT GMBH
Application year: 1998
Main classification: D21F5
IPC codes: D21F5