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The present invention relates to an arrangement in connection with the press and dryer section of a web-forming machine, which arrangement includes
Even in modern web-forming machines, locations remain, in which the web is carried over an open gap. Such a location occurs, for example, between the web-forming machine's press section and dryer section. In one known problem location, the web is guided, from the surface of a press roll belonging to the press section, to the dryer wire belonging to the dryer section. The dryer wire is arranged in a loop supported by rolls, one of which is arranged near to the press roll. In practice, the web is guided onto the dryer wire at this point. In addition, a closing throat is formed between the web and the dryer wire on the surface of the roll. The web travels on to the dryer section, supported on the dryer wire.
At the other end of the endless loop of the first dryer wire, the web is transferred to the following dryer wire. In its return cycle, the dryer wire is empty and finally again travels past the roll arranged near to the press roll. Particularly during the return cycle, the dryer wire transports an air layer with it, which finally reaches the closing throat formed by the web and the dryer wire. Some of the air exits upwards from the throat and through the dryer wire into the vacuum roll. However, the amount of air is so great that a considerable excess pressure arises in the throat and causes bagginess in the web, especially in the edges, which are looser than the rest of the web. In the cross-direction of the web-forming machine, the bagginess is indeterminate, which hinders the travel of the web. The excess pressure also causes cross-direction currents, which cause web flutter. The bagginess and the indeterminate movement of the web stretch the web and reduce its quality. Attempts have been made to avoid this problem by rotating the dryer section at a higher speed than the press section, in order to tension the web. However, in modern high-speed web-forming machines, it is difficult to increase the speed differential. In addition, at high speeds, even a small relative difference in speed will create a large absolute change, which leads to varying forces in the machine direction. The motion of the web then becomes jerky and the bagging problem appears at least occasionally. The possibility of web breaks in the web-forming machine also increases, thus reducing the total output.
U.S. Pat. No. 545,593 discloses a device, which includes a deflector fitted before the throat and a vacuum box set inside the loop. Wear occurs in both the seals of the vacuum box and in the dryer wire. In addition, the deflector is far from the throat.
The invention is intended to create a new type of arrangement, by means of which flutter and bagging in the web can be avoided. The characteristic features of this invention are stated in the accompanying Claims. The use of the arrangement according to the invention prevents air layer from travelling into the closing throat. Thus, the pressure is essentially the same on both sides of the web, thus stabilizing the travel of the web. The arrangement can be assembled from various components, so that the problem can be avoided at each position. In addition, the number and dimensioning of the components provide a simple way to regulate the amount of air being removed.
In the following, the invention is examined in detail with reference to the accompanying drawings showing some embodiments of the invention, in which
FIG. 1 shows a schematic drawing of a side view of the arrangement according to the invention,
FIG. 2a shows a schematic drawing of a side view of a second embodiment of the arrangement according to the invention,
FIG. 2b shows a schematic drawing of a side view of a third embodiment of the arrangement according to the invention.
FIG. 1 shows an arrangement according to the invention, in connection with the press section and the dryer section of a web-forming machine. In this case, the arrangement includes first means 10 in the press section, for transporting the web 11 being made on the web-forming machine. In FIG. 1, only parts of the press section and the dryer section are shown. In the press section, the web 11 is transported on a press felt 12, which is led through a press nip formed by two press rolls 13 and 14. The press felt 12 is an endless loop and is supported from the structure of the web-forming machine, on turnover rolls 15. Correspondingly, there are second means 16 in the dryer section, for transporting the web 11 being made on the web-forming machine onwards. In this case, the second means 16 include a dryer wire 17 and a roll 18 arranged to support it. A vacuum is also arranged inside the roll 18, which is used to hold the web 11 on the dryer wire 17. In the arrangement, there is an open gap 19 between the first and second means 10 and 16, across which the web is arranged to be transferred. FIG. 1 shows the web 11, which travels from the surface of the press roll 14 to the dryer wire 17, in the open gap 19. From the press roll 14, the web 11 is thus guided to the area of the vacuum roll 18, in which the web 11 and the dryer wire 17 form a closing throat 20.
Before the open gap, in the direction of travel of the dryer wire 19, the arrangement according to the invention further includes deflector means 21, which are arranged on the side of the loop formed from the dryer wire 17 opposite to the roll 18 (FIGS. 1, 2a, and 2b). In addition, the deflector means are arranged close to the dryer wire, in order to restrict the air layer travelling with the dryer wire, and thus to prevent an excess pressure forming in the said closing throat. The deflector means are thus used to remove, or other wise deflect a significant part of the air layer travelling with the dryer wire. Particularly on the outer surface of the loop, the air layer increases significantly in the aforementioned position and causes problems in the open gap.
The amount of air transported by the dryer wire is affecting by several factors, of which the most important are the speed of the dryer wire and the open formation length. The air near the surface of the dryer wire travels at the speed of the dryer wire and the boundary layer tends to increase continually. To create a sufficient preventive effect, the deflector means are arranged at 1-12 mm from the dryer wire. However, it is preferable to locate the deflector means as close to the dryer wire as possible, so that an increasing part of the boundary layer and particularly its rapidly travelling portion can be removed. The location of the deflector means relative to the closing throat also affects their operation. Generally, it is preferable to install the deflector means as close to the throat as possible, so that the formation of a new boundary layer of air will be prevented. In other words, the formation length will remain advantageously short. The distance of the deflector means according to the invention from the point of contact between the roll and dryer wire is less than 3000 mm, preferably 50-1500 mm. The boundary layer of air restricted by the deflector means will then no longer be able to increase again before the throat.
The air layer can be restricted in various ways. Generally, according to the invention, the deflector means are arranged to be passive or active. Active deflector means are formed of elements arranged to move or rotate, or of airflow devices. In FIG. 1, the deflector means 21 is the aforementioned press felt 12, which travels in the opposite direction to the dryer wire 17. The press felt travels at a speed nearly as great as, but in the opposite direction to that of the dryer wire, making the removal of the air layer extremely effective. FIG. 2b too shows an active deflector means 21, which in this case is an auxiliary roll 22 arranged to rotate. The auxiliary roll is preferably rotated against the direction of travel of the dryer wire. The effect of active deflector means has, in practice, a specific reach. Nevertheless, the aim is to locate the deflector means as close to the dryer wire as possible. According to the invention, the distance of the active deflector means from the dryer wire is 3-12 mm. The effect of the deflector means will then extend as far as the dryer wire, thus removing a substantial part of the air layer.
Correspondingly, passive deflector means are formed of air baffles or doctors. As passive deflector means lack the reach effect, they must be set closer to the dryer wire than active deflector means. According to the invention, the distance of passive deflector means from the dryer wire is 1-6 mm. Doctors, for example, can even be set to touch the dryer wire. However, deflector means that touch the wire require maintenance and may wear the dryer wire, even if they are manufactured to conform to the shape of the surface they touch. FIG. 2a shows an air baffle 23, which is used to guide a significant part of the air layer away from the surface of the dryer wire. Particularly passive air baffles can be easily installed with several one after the other. This will remove the air layer in stages. The air baffle can also be combined with a vacuum, by means of which air can be removed from the position. Active and passive deflector means can also be combined, or fitted after each other, thus preventing the greatest possible part of the boundary air layer from travelling into the closing throat.
The dryer wire also transports a certain amount of air on its inner surface. The air in question travels into the closing throat formed by the dryer wire and the roll and from there through the dryer wire to the outside of the loop. As all excess air causes problems in the open gap, that the arrangement further includes runnability components arranged inside the loop formed from the dryer wire. The runnability components are preferably fitted in connection with the roll and are arranged to restrict the air layer travelling with the inner surface of the dryer wire. This will prevent both an excess pressure forming in the closing throat formed by the dyer wire and the roll, and air permeating through the dryer wire. In FIG. 2a, the runnability component 24 is a blow box 25, which is used to create a blast 29 that restricts the boundary air layer on the inside of the loop. At the lower corner of the blow box, there is also a second blast 29, which is used to create a vacuum inside the loop, which pulls the web being transported on the under surface of the dryer wire onto the dryer wire. Beside pulling the web, both blasts also remove air that may have moved inside the loop. The same components as outside the loop can be used inside the loop, though there is little installation space. On the other hand, passive air baffles require only a small installation space.
The blow box is fitted close to the dryer wire, in order to achieve the most effective possible deflection effect. According to the invention, the distance of the blow box 25 from the part of the dryer wire 17 on the side of the deflector means is at most 50 mm, preferably 5-20 mm. In addition, the operation and positioning of the air baffle 23 also depend on the properties of the dryer wire 17. The air baffle is used to deflect the boundary layer on the surface of the dryer wire to a desired place, to prevent the penetration of the detrimental boundary layer between the roll 18 and the blow box 25. This will also prevent the web from detaching from the under surface of the dryer wire. In the embodiment of FIG. 2a, the air baffle 23 acts essentially on top of the blow box 25. In the situation in question, the air permeability of the dryer wire is normal, i.e. about 1000-2000 m3/h/m2. At greater permeabilities, the optimal location for the air baffle 23 will be in the area between the roll 18 and the blow box 25.
FIGS. 1 and 2a show the roll 18 as being a vacuum roll that is, as such, conventional, with a vacuum zone limited by seals 26 formed inside it. The vacuum effect acting through the perforated jacket of the vacuum roll is directed through the dryer wire to the web, which is thus made to follow the dryer wire. In FIG. 2b, the vacuum zone is limited by a special vacuum box 27, which is sealed to the surface of the vacuum roll 18. Thus a greater part of the jacket of the vacuum roll than before will be subject to a vacuum effect. At the same time, the blow box 25 restricting the boundary air layer can be easily connected to the vacuum box 27.
In principle, the boundary air layer could be removed by increasing the capacity of the vacuum roll. In practice, however, this situation is impossible. The vacuum must be held at a certain level determined by the properties of the web. The openness of the jacket has already been optimized correspondingly while, on the other hand, a lack of space often makes it impossible to increase the diameter of the jacket. Making the vacuum roll larger will also increase costs. Thus, according to the invention, the air layer, or at least a significant part of it is removed before the open gap. The vacuum roll can then be adjusted to suit the control of the web while capacity will still remain for removing the rest of the excess air. The arrangement is preferably set up in such a way as to remove the air in a controlled manner and to eliminate air currents that disturb the travel of the web. The number of deflector means and their distance from the dryer wire and throat are arranged case-specifically. In practice, the amount of air to be removed using the vacuum roll depends on these factors. Thus, an effective device set close to the dryer wire and the closing throat will remove nearly all of the boundary air layer, thus preventing the formation of an excess pressure in the throat.
In FIGS. 2a and 2b, the direction and magnitude of the boundary air layer outside the loop are shown by arrows drawn with a solid line. Correspondingly, the direction and magnitude of the boundary air layer inside the loop are shown by arrows drawn with a broken line. The dryer wire is shown with a broken line and the web with a line of dots and dashes. In reality, the web lies on the dryer wire, but for greater clarity the web is shown in the figures as separate from the dryer wire.
Particularly the positioning of active deflector means close to the dryer wire demands special care. Especially the meeting of the press felt and the dryer wire should preferably take place at a large angle, so that the fabrics will travel for a short distance in opposite directions to each other. In addition, the rolls supporting the fabrics are preferably set as close to each other as possible, to minimize stretch in the fabrics. The deflector means can then be brought advantageously close to the dryer wire, to effectively remove the boundary air layer. Though they are very close to the wire, deflector means that do not touch the wire will not wear and, if they are taken into account beforehand in the design, can even be implemented without additional costs.
The arrangement according to the invention can be applied to all forms of production and to different positions. The boundary air layer can be removed in different ways and in each case a sufficient removal capacity will be achieved. The arrangement is also economical to implement and it is simple to increase its capacity as the production speed rises, for example. In addition, the arrangement will operate with known vacuum rolls, as the movement of the boundary air layer is limited before the throat, or is removed entirely. The travel of the web is thus undisturbed and production and quality problems are avoided.