Patent Application
20050000670
The present invention relates to an arrangement for controlling the web in a press section of a paper of board machine, which press section includes
In the press section of a web-formation machine, such as a paper or board machine, a great deal of water is removed from the web. The water is removed using a press nip formed from two rolls. Usually the web is led to the press nip between two fabrics. After the press nip, the web is guided onwards in the web-forming machine, supported by a fabric.
Before the press section or the press nip, the edges of the web are evened, usually by water cutting. In addition, the edge parts of the fabric become dirty more rapidly than the rest of the fabric. The previous processing of the web also make the edges different to the rest of the web while the edge parts also stretch. Thus, the edges of the web tend to adhere to the first fabric, even though the rest of the web is guided to another fabric after the press nip using the vacuum effect of a suction roll. Usually the edges of the web detach from the fabric after a delay, which further stretches the edges and causes flutter in the web. The flutter is also increased by the low tension in the web and the airflow caused by the fabrics. In addition, centrifugal force in the area of the suction roll further increases the stretching of the edges. The stretched and even creased edges of the web must be cut off later, which reduces production efficiency. In the worst case, the irregular edges of the web may even cause the web to break during transfer, or during processing in the paper or board machine. The irregular and stretched edges are a major problem particularly when coating the web.
Attempts have been made to eliminate the problem described by increasing the vacuum in the suction roll and altering the tension of the fabrics. Different kinds of fabric have also been tried. Despite these attempts, the problem still persists, or at least it appears soon after changing a new fabric. In practice, increasing the level of vacuum in the suction roll has little effect, as the vacuum effect only extends to the very surface of the fabric. Thus it is nearly impossible to use a suction roll to pull an edge that has already separated back onto the surface of the fabric. The problem is exacerbated when the velocity of the web is increased.
European patent application number 1101864 discloses an arrangement in the press section of a paper machine. The core invention relates to a felt guide roll transfer in transferring the web from the press felt to the next press felt after a suction roll. In the second embodiment of the invention, a suction box is disclosed between the press nip and the suction roll, which is intended to prevent the re-wetting of the press felts, and thus the web, after the press nip.
The suction box presented is long. In that case, the production of a vacuum in the full-width suction box consumes much energy. Due to friction, the large suction box also acts as a brake on the press felt and wears it rapidly. The suction box has a low controlling effect on the web, as there is an obvious discontinuity point in the vacuum effect in the machine direction, due to the mutual positioning of the suction box and the suction roll. It is also difficult to set a large suction box in the correct position while its support requires large structures in an otherwise cramped position.
The invention provides a new type of arrangement for controlling the web in a press section of a paper or board machine, which can be easily fitted to the press section and by means of which the control of the web is better and easier than previously.
More particularly the invention includes an arrangement for controlling the web in a press section of a paper of board machine, which press section includes
In a specific embodiment, the suction apparatus may include two or four suction cases, which are arranged in different edge areas of the web. The suction case may include side seals extending, as a continuation of the cover, for a distance from the point of contact of the fabric and the suction roll, in order to create an essentially continuous suction effect between the suction case and the suction roll. In addition to the suction case, the side seals may also be shaped to conform to the shape of the suction roll.
The suction case may be delimited not only by end pieces, but also by a bottom piece, which is arranged at a distance from the cover, in order to create a suction case that is open on the side facing the throat. There may be a lubricant connection in connection with the first edge of the cover in the direction of travel of the fabric, in order to lead lubricant between the cover and the fabric. The suction case may include a transverse bearer fitted to the end piece, for adjusting the position of the suction case relative to the fabric and the angle of the suction case relative to the suction roll. The vacuum connection, forming part of the suction case for creating a vacuum, may be arranged to act as the bearer.
The length of the essentially continuous suction-effect area from the point of contact between the fabric and the suction roll is about 100-300 mm, preferably 150-250 mm, in the direction of travel of the web, the proportion of the cover of which length is at most half. The width of the suction-effect area created using the suction case may be 50-500 mm, preferably 150-250 mm. The arrangement may include a concave plate-like guide arranged before the suction case, and which is arranged to touch the fabric with its edge. At the first edge of the suction case in the direction of rotation of the suction roll, there may be a doctor blade for removing air and water from the surface of the suction roll. The cover of the suction case fitted in the opening throat may be convex.
The use of the arrangement according to the invention brings the web, and particularly its edges securely under control. Despite the effective operation, the vacuum capacity required is small and there is little effect on the movement and wear of the fabric. The suction case belonging to the arrangement can be easily applied in different kinds of press sections. In addition, the suction case according to the invention requires little installation space and its adjustment is versatile but easy.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.
a shows a schematic diagram of a second arrangement according to the invention in a press section,
b shows a variation of the arrangement of
The web is made in the web-forming section (not shown) preceding the press section. When the web is led to the press section, it contains a great deal of water, which is removed in the press nip. The web, which has very little tensile strength, is transported with the aid of fabrics 14 and 15. In the press section, the fabrics are usually press felts, which absorb water and at least partly allow air to pass through them. Usually, there are two fabrics at a press nip, which are arranged to travel through the press nip. During the actual pressing event, the web lies between the fabrics. In
The press section also includes a suction roll 18, which is arranged after the press nip 10 and with is also termed a felt suction roll. The suction roll in question is used to transfer the web to the desired fabric. When travelling between the press nip and the suction roll, the web is between the fabrics. When the fabrics again separate, the said suction roll is used to transfer the web to the desired fabric in a controlled manner. In other words, the web is forced by the suction roll to travel on the surface of a specific fabric. The drawbacks of the prior art relate precisely to the situation in which the web detaches, in which the edges of the web attempt to follow the wrong fabric. In FIG 1, the web 13 has been transferred, after the press nip 10, to travel on the surface of the lower fabric 15, from which the web 13 is transferred, using a second transfer suction roll 19, to the next fabric 20, for example, to the felt of the next press, or to the drying wire of the dryer section. The fabrics shown are endless loops, of which only part is shown in this connection.
The arrangement also includes a suction apparatus in connection with the suction roll 18. In the suction apparatus, there is a cover equipped with suction openings, and it is arranged in connection with the fabrics. According to the invention, the suction apparatus 21 includes at least one suction case 24 or 24′, which is arranged in the closing throat 25, and/or in the opening throat 25′ formed by the fabric 15 and the suction roll 18. According to
The problems are eliminated especially by arranging one or two suction cases at both edges of the fabric. Thus, the suction apparatus preferably includes two or four suction cases, which are arranged in the various edges areas of the web. In narrow and otherwise small suction cases, a significant vacuum can be used, without greatly affecting the travel of the fabrics and without significant increases in operating costs. If necessary, suction cases are also arranged in the opening throat, in which case the suction apparatus includes four suction cases.
The suction case 24 is hollow and is delimited by a bottom piece 30, in addition to the end pieces 29. According to the invention, at the front edge of the suction case 24, the bottom piece 30 is arranged at a distance from the cover 23 (
The end and bottom pieces of the suction case are preferably triangular in shape. In addition, they have a curved shape, allowing the suction case to be placed deep in the throat. In addition to the suction case, the side seals are also shaped to conform to the suction roll. This allows the side seals to even be in contact with the surface of the suction roll. The side seals are, however, intended to be adjusted to a distance of 0.5-3 mm from the surface of the suction roll, which will avoid wear in the side seals, but achieve a sufficient seal. One side of the bottom piece is also concave, so that water and air coming with the fabric will be guided smoothly down by the suction case. In order to ensure the operation of the suction case 24, the arrangement also includes a concave plate-like guide 31 arranged before the suction case 24 (
Various guides are important in order to avoid dirtying. However, the ceramic cover continuously rubs against the fabric, with the vacuum further increasing the friction. In order to avoid damage, a lubricant connection 33 is arranged in association with the first edge of the cover 23 in the direction of travel of the fabric. Lubricant is led between the cover 23 and the fabric from the lubricant connection 33, thus reducing friction. The lubricant is preferably water. In this case, the lubricant connection 33 terminates in a gap 34 opening out to the cover 23.
The suction case according to the invention is small and light. The suction case used in tests weighed only about 40 kg, making it easy to support. According to the invention, the suction case 24, or 24′ includes a transverse bearer 35 fitted to the end piece 29. Thus, both the position of the suction case relative to the fabric and the angle of the suction case relative to the suction roll can be adjusted using the same bearer. Particularly when using a cylindrical bearer 35, the suction case 24 can be rotated steplessly (
In the disclosed and other known press sections, the angle of cover of the fabric is small before the detaching of the fabric. In other words, in the prior art the area of the vacuum effect is extremely short, despite the extensive vacuum zone of the suction roll. According to the invention, the length of the continuous vacuum-effect area from the point of contact of the fabric and the suction roll is about 100-300 mm, preferably 150-250 mm, in the direction of travel of the web. This is many times more than in the prior art. The proportion of the cover of the said length is at most half, so that a significant part of the vacuum area is also formed with the aid of the space delimited by the side seals.
The suction case has also be made intentionally narrow, allowing the stretching and fraying problems of the edges of the web to be resolved precisely. In practice, fraying of the edges nearly always causes a web break. According to the invention, the width of the vacuum-effect area created by the suction case is 50-500 mm, preferably 150-250 mm. Thus, even a large vacuum can be used, as the surface area in which a vacuum is to be created is small compared, for example, to the suction roll, a full-web-width suction box. In practice, in tests the vacuum used inside the vacuum case is about 5-50 kPa, preferably about 15-30 kPa.
In the suction roll 18 shown, some of the holes 41 are so-called countersunk. Grooves 42 connecting the holes 41 can also be used, so that the effect of the vacuum can be increased and its area of influence extended. The operation of the suction case according to the invention can be further ensured by aligning the nozzle 43 into the throat opening out after the suction roll 18, in order to create a jet of air (
The configuration of the suction apparatus is selected as required. A small and light suction case according to the invention can be easily and quickly installed in a desired position, even as a retrofit. The number of the suction cases is mainly affected by the dimensioning of the suction roll and the cover angle of the fabric on the suction roll. The paper grade being manufactured, for example, the solids in the web, and the running speed also affect the behaviour of the edges. Particularly in situations, in which the vacuum effect is small and the cover angle is small and the running speed is high, a suction case is preferably used on both sides of the suction roll. On the other hand, in more favourable conditions, a suction case is used on one side or other of the suction roll. In
The end and bottom pieces of the suction case are preferably made form a sheet-metal material. The cover, on the other hand, is usually ceramic. The seals used are of wear and heat-resistant plastic. This avoids unnecessary wear of the fabric and provides a good seal.
The use of the arrangement according to the invention solves an annoying problem in the press section. In addition, the purchase and operating costs of the suction case according to the invention are low. Further, the suction case is highly suitable for use in different kinds of press sections. The upper and lower fabrics referred to in the descriptions of the embodiments can change places in different kinds of press section. The essential feature is the web travelling between two fabrics, the edges of which web are made to adhere to the surface of the desired fabric using the suction case according to the invention.
Although the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20035118 | Jul 2003 | FI | national |
