The invention concerns a method for facilitating web threading in a paper or board machine's single-wire drying section, where the web travels supported by a wire from a first cylinder via a reversing roll to a second cylinder, the reversing roll being located at a distance of 10-30 mm, preferably 15-20 mm, from the first cylinder and, correspondingly, from the second cylinder, in which method into a pocket defined by the cylinders and the reversing roll a runnability component is fitted, with which vacuum is produced in the pocket in order to stabilize web travel.

The invention also concerns an arrangement for facilitating web threading in a paper or board machine's single-wire drying section, where a web is arranged to travel supported by a wire from a first cylinder via a reversing roll to a second cylinder, the reversing roll is located at a distance of 10-30 mm, preferably 15-20 mm, from the first cylinder and, correspondingly, from the second cylinder, and the cylinders and the reversing roll in between them define a pocket, into which a runnability component is fitted, with which vacuum can be produced in the pocket in order to stabilize web travel.

Single-wire draw is often used in the cylinder drying sections of paper machines, in which the web is carried supported by the drying wire through the entire group of cylinders in such a way that on the drying cylinders the wire presses the web against the heated cylinder surfaces, and on the reversing rolls the web will remain on the side of the outer curve. Two adjacent drying cylinders and a reversing roll fitted in between them in a separate row define in between them a pocket, which is limited by the wire. In connection with the pocket there are opening and closing gaps. An opening gap means an area, where the web and wire come off the cylinder or reversing roll and, correspondingly, a closing gap means an area, where the web and wire arrive at the reversing roll or cylinder. Airflows travelling along with the surfaces moving at a high speed bring about the result that vacuum tends to build up in the opening gap, whereas overpressure tends to build up in the closing gap.

With increasing paper machine velocities, efforts have been made to prevent run- nability problems from occurring in the area of single-wire draw by using different kinds of runnability components, such as blow boxes, suction boxes and blow- suction boxes. With the aid of such a runnability component vacuum is arranged in connection with the pocket, which affects the web and wire travel from a cylinder to a reversing roll and from the reversing roll to the next cylinder. The need for vacuum may differ in various parts of the pocket. High vacuum is needed especially at the gap opening in between the cylinder and the wire in order to detach the web from the cylinder and to attach it to the wire, whereas elsewhere in the pocket lower vacuum would usually suffice. In most cases, the vacuum brought about by runnability components is similar over the entire area to which the effect of the runnability component extends. The most popular reversing roll in use is a grooved and/or perforated roll, in connection with which vacuum is arranged in order to make sure that the web will remain attached to the wire as they travel on the surface of the reversing roll.

In a drying section marketed by the applicant under the name of Compact Sym- Run, the cylinders and reversing rolls are located very close to each other, so that only a very small space is left in between them, a space of preferably just 15-20 mm. On the inlet side, the gap opening in between the cylinder and the wire is very close to the gap closing in between the wire and the reversing roll. The bend of the wire under the effect of vacuum is very slight, as the wire travels only a short distance without support. The wrap angle of the web on the cylinder can be made larger with a shorter distance between the cylinder and the reversing roll. Thus, the web can be heated over a longer distance than before. Minor web bend will reduce wearing of the runnability component's seals and will thus reduce vacuum leaks. It is easier to direct the vacuum to a limited area with a small number of seals. However, the pocket in between the cylinders and the reversing roll is relatively narrow, and in problematic situations paper may pack in between the cylinder and the reversing roll.

In the web spreading-out stage it would be advantageous to bring about in the pocket a vacuum that is at least as high than in the running situation. For example, when the running speed is 2000 m/min, the vacuum requirement may be 1500 Pa, depending on the paper grade and on the dry-matter content of the web. Threading of the web is the easier the higher vacuum there is in the pocket waiting for the web threading.

When preparing for web threading and during the web threading, the airflow through the wire into the pocket is higher than in the running situation and it be- comes more difficult to keep up the vacuum needed in the web threading. However, vacuum is needed to make sure that the tail end will come off the cylinder, that it attaches to the wire and follows it to the reversing roll. The need for vacuum may be reduced, among other things, by increasing the draw difference, but the web quality will thereby suffer. Besides, there are always response times in adjustments. Adjustments may also impair runnability later on along the machine line. One way of solving the problem would be by increasing the suction capacity for the time of web threading, but this will easily lead to excessive blower power and to an over-dimensioned system of air ducts for a normal run.

The published WO application 2007085698 discloses a drying section, in which the web is carried from a cylinder to a reversing roll as a short transfer, and during the transfer the web is subjected to a vacuum effect produced by a runnability component, whose impact area is limited to be directed to the web transfer by sealing of the runnability component to the cylinder surface and, correspondingly, to the reversing roll's surface. The publication does not describe any separate solution to the problems of web threading. The published WO application 2007085699 discloses another drying section, in which the web is taken from a cylinder to a reversing roll and further from the reversing roll to the next cylinder as a short transfer. By a vacuum component located in a pocket space a pressure difference is produced to attach the web to the wire in such a way that the web's shrinkage in the cross-machine direction can be controlled. To keep up vacuum in the pocket space there are seal elements arranged in connection with the vacuum component, with which the pocket space is sealed against the surfaces of the cylinders. In addition, one seal element can be fitted in between the vacuum component and the reversing roll to seal the gap closing to the reversing roll.

The printed US patent specification 6574884 discloses a blow box, which is equipped with at least two blow nozzles. The first nozzle is fitted at the gap open- ing from the cylinder to blow air away from the gap in between the wire and the blowing device, and the second nozzle is fitted in the wire travel direction at a distance from said opening gap. The air jets discharging from the nozzles will maintain a vacuum in between the blowing device and the wire. At a short distance from said opening gap a throttling element is fitted, which extends towards the wire and which divides the vacuum space formed in between the first and second nozzles into a first area with boosted vacuum and limited to the opening gap and into a second lower-vacuum area. The blow box of the presented kind requires a large space, for which reason it cannot be accommodated into the narrowest pocket spaces. The blow box functions in the same manner both during a normal run and during web threading.

The published WO application 9848109 discloses a blow box, which is formed by two parts, in between which there is a passageway, which can be opened and closed either by moving one part of the box or through an actuator. At the revers- ing roll end of the passageway there is a blow opening, which can be opened and closed through an actuator, so that the blow opening is open during the web threading situation. This gives the possibility in a web threading situation of increasing the air discharge from the closing gap on the inlet side. The device requires a relatively large space and it cannot necessarily be accommodated into the narrowest pockets. The publication presents only arrangements for adjusting the distance of seals and for increasing the vacuum effect, but it does not tell how to direct the suction by limiting the vacuum area. The blow seal at the same time functions as a vacuum producer, whereby it is located far away from the closing gap. The passageways required for blowing on the box bottom further limit introduction of blowing air into the pocket. Thus, the device is only suitable for large pockets and it is only able to produce modest vacuums.

The invention aims at facilitating web threading in the drying section of a paper or board machine. It especially aims at improving the pocket vacuum before web threading and during web threading, whereby web threading may comprise either tail threading and spreading out to form a full- width web, or threading of a full- width web.

The method according to the invention is characterized by the features presented in the characterizing part of claim 1. Similarly, the arrangement according to the invention is characterized by the features presented in the characterizing part of claim 5.

According to the invention, during the web threading the vacuum generated by the runnability component is only directed in the web travel direction to the closing gap on the inlet side, whereas during a production run the same vacuum is directed in the web travel direction to a larger pocket area. When the vacuum is directed to the gap closing on the reversing roll, it will through the wire also affect the gap opening from the first cylinder.

The vacuum in the gap closing on the inlet side can be improved by preventing the air boundary layer travelling along with the reversing roll from being admitted to the gap closing on the reversing roll, for example, with the aid of at least one guide plate, which for the time of web threading is turned to be in front of the airflow in the space between the runnability component and the reversing roll. By using a similar sealing list in connection with both suction openings it is possible in between the suction openings to limit a suction-free area, which will reduce the suction volume and make it easier to direct the suction. In the suction-free area there is no suction in the direction of the runnability component, but there may be suction in the reversing roll's direction. At the same time the sealing on the pocket's outlet side can be opened, so that air is allowed to flow out of the pocket along with the wire transfer which is directed upwards. In addition, a similar sealing may be used in connection with the cylinders, whereby in between the runnability component's upper seal and the seal on the edge of the suction opening there remains an area reducing the suction volume, which area is brought about with the aid of either fixed or opening seals. Hereby it is possible when desired to arrange in the limited space also some suction, the magnitude of which can be adjusted.

The runnability component can in a manner known as such be a suction box or a blow box or a blow-suction box. During a production run, the vacuum produced by the runnability component can be directed into the entire pocket through two or more suction openings affecting in the web travel direction/machine direction and/or no more than one blow nozzle. During web threading, some of the runnability component's suction openings are closed in order to direct the vacuum together with the flow restrictions only to a part of the pocket area. Besides, in order to control the spreading out of the tail end, suction openings may also be closed in the cross-machine direction either together with restrictions of the cross- machine flow or without these.

The seals and guide plates arranged in connection with the runnability component may be mechanical seals or blow seals. The reversing roll may be located eccentrically in relation to the cylinders, if it is desirable to minimize the wire's unsupported length as it arrives at the reversing roll. In addition, the runnability component may be located eccentrically in relation to the pocket, and in this way space can be obtained, for example, for the flow leaving the pocket on the outlet side.

The reversing roll used can preferably be a suction roll that is marketed by the applicant under the product name of VacRoll and which has a grooved surface and through-holes in the bottom of the grooves. The roll in question is a large- diameter roll, which is connected to a vacuum source and the vacuum effect of which extends over the entire roll periphery.

Owing to the invention the times needed for web threading are shortened, because the machine running parameters, such as draw differences or the discharge ratio, need not be changed for web threading. Energy is also saved, when the time spent on breaks is shortened. In short transfers the wire bending is slight, whereby seal wearing and the resulting leaks are reduced. It is easy to direct the vacuum to a limited area with a small number of seals.

The invention will be explained in greater detail in the following by referring to the figures in the appended drawings, but there is no intention in any way to restrict the invention narrowly to the details shown in the figures.

Figure 1 is a schematic side view of a pocket in a drying section and of airflows occurring in the pocket when running without a web.

Figure 2 illustrates the operation of a runnability component according to the invention in a web threading situation.

Figure 3 illustrates the operation of the runnability component according to the invention during a production run. Figure 4 shows a solution corresponding with Figure 3, in which there are more seals limiting the suction area.

Figure 1 shows a part of a drying cylinder group applying single-wire draw in a paper or board machine, where cylinders 10, 11 are fitted in an upper row and reversing rolls 12 in a lower row. The drying fabric, that is, the wire 13 moves down from the first cylinder 10 to the reversing roll 12, turns around the reversing roll 12 and moves upward further on to the second cylinder 11, in accordance with normal single-wire draw. At the cylinders 10, 11, the wire 13 presses the web (not shown) against the heated cylinder surface and at the reversing roll 12 the web travels on the outer surface of the wire 13. In between the first cylinder 10, the reversing roll 12 and the second cylinder 11 is outlined a pocket 14, which is partly limited by the wire 13. Into this pocket 14 is fitted a runnability component 15, which extends in the cross direction of the machine over the whole width of the web and which is sealed with the aid of seals 16, 17 against the wire 13 travelling on the surface of cylinders 10 and 11. The seals 16, 17 may be, for example, mechanical seals or blow seals. With the aid of the runnability component 15 a vacuum is generated in the pocket 14, which during a production run will stabilize the web's travel and will keep it attached to the wire 13. In this case the runnability component 15 is a suction box, but in its place there might just as well be a blow box or a blow-suction box. The suction box 15 comprises two suction openings 21 and 22 extending in the cross direction of the machine over the whole width of the web, of which suction openings the first one is directed towards the closing gap 23 on the inlet side and the second one is directed towards the opening gap 24 on the outlet side.

The cylinders 10, 11 and the reversing roll 12 are located compactly close to each other, whereby the distances the wire 13 travels without support from the first cylinder 10 to the reversing roll 12 and from the reversing roll 12 to the second cylinder 11 are very short. The gap d remaining in between the first cylinder 10 and the reversing roll 12 is in an order of 10-30 mm, preferably only 15-20 mm, and the distance between the reversing roll 12 and the second cylinder 11 is in the same order. As the distance the wire travels without support from the cylinder to the reversing roll is minimized, many advantages are achieved, one of which is reduced bending of the wire. In a web threading situation, where there is no web sealing the wire, leaking of vacuum is less with a short distance between the cylinder and the reversing roll. Thus it is possible in web threading to achieve a higher pocket vacuum and more reliable web threading. The compact location of cylinders and rolls also reduces the drying section's need for space, especially in the height direction. The reversing roll's higher location allows a direct drive for the concerned roll, whereby gearing or articulated shaft can be left out.

It follows from the short distance that the gap opening in between the first cylinder 10 and the wire 13, that is, the opening gap 26 on the inlet side, touches the gap closing in between the wire 13 and the reversing roll 12, that is, the gap 23 on the inlet side. Correspondingly, the gap opening in between the reversing roll 12 and the wire 13, that is, the opening gap 24 on the outlet side, touches the gap closing in between the second cylinder 11 and the wire 13, that is, the closing gap 27 on the outlet side.

The arrows 18, 19, 20, A and B shown in Figure 1 illustrate airflows in a situation, where there is no web covering the wire 13. This corresponds with a situation before web threading is started and partly also during web threading. Arrow 18 illustrates leakage air that is drawn through wire 13 into pocket 14. Arrow 19 illus- trates an airflow, which is brought about by the opening gap 24 on the outlet side and by the closing gap 27 on the outlet side. Arrow 20 illustrates a boundary layer flow moving along with the reversing roll's 13 surface. Arrows A and B illustrate airflows that are removed from pocket 14 through the runnability component's 15 suction openings 21 and 22. Figure 2 illustrates the use of vacuum in a web threading situation or in a situation prior to web threading, where the wire 13 is travelling without a web. In the solution according to the invention, the vacuum produced by the runnability component 15 is in the web travel direction limited to be directed to an area, which mainly comprises the closing gap 23 on the inlet side and the opening gap 26 on the inlet side. This is implemented in Figure 2 by closing the runnability component's 15 second suction opening 22 and by turning the guide plate 25 located in connection with the first suction opening 21, so that it extends from the lower surface of runnability component 15 close to the surface of reversing roll 12. With the aid of the guide plate 25 the boundary layer air is prevented from flowing along with the reversing roll's 12 surface into the closing gap 23 on the inlet side. If there is a risk of overpressure being formed on the outlet side of pocket 14, it is possible to open the seal 17 on the outlet side, so that pressure is allowed to discharge through the space formed in between the runnability component 15 and the second cylinder 11. By focusing the vacuum produced with the aid of runnability component 15 for the time of web threading to the inlet side of pocket 14 it is possible to make sure that the tail end will better than before follow the wire 13 and the time needed for web threading is shortened.

Figure 3 illustrates the operation of runnability component 15 during a normal production run. An essentially similar vacuum is directed to the area of the gap 23 closing on the inlet side and at the area of the gap 24 opening on the outlet side, whereby the entire pocket 14 is under vacuum. Pocket 14 is sealed against the wire 13 travelling over the surface of cylinders 10, 11 with the aid of seals 16 and 17. The web first travels in between the first cylinder 10 and the wire 13, then on the surface of wire 13 around reversing roll 12, and finally in between wire 13 and the second cylinder 11. The reversing roll used may be any perforated and/or grooved roll, which may co-operate with an internal or external vacuum source. The reversing roll 12 used is preferably a suction roll marketed by the applicant under the product name of VacRoll, which has a grooved and perforated surface and the inside of which is connected to a vacuum source. Figure 4 shows a runnability component 15 that has more seals than in Figure 3. The seals 16 and 28 on the inlet side reduce the airflow in between the runnability component 15 and the first cylinder 10. The seals 30 and 17 on the outlet side re- duce the airflow in between the runnability component 15 and the second cylinder 11. A first guide plate 25 guides air to flow from the closing gap 23 on the inlet side into the runnability component's 15 first suction opening 21. A second guide plate 29 guides air to flow from the opening gap 24 on the outlet side into the runnability component's 15 second suction opening 22. The opening angles of each guide plate 25, 29 are preferably adjustable. Such a solution allows a more exact adjustment of the vacuum than before in various parts of the pocket.

Many different modifications of the invention are possible within the scope defined by the claims presented in the following.