METHOD AND DEVICE IN CONNECTION WITH IMPINGEMENT DRYING AND/OR THROUGH-DRYING OF A PAPER WEB OR OF AN EQUIVALENT WEB-LIKE MATERIAL

Abstract: The invention concerns a method and a device in connection with impingement drying and/or through-drying of a paper web or equivalent. In the invention the paper web (W) is dried by blowing hot air or equivalent by means of an impingement drying device (10) perpendicularly to the web (W), which is passed on support of a wire (F) past the impingement drying device (10). The web (W) and the wire (F) are supported by the intermediate of blowings produced by means of vacuum blow boxes (30) across the entire width of the web. The first blowings are blown at a speed substantially higher than the speed of the drying wire (F) and the outlet direction of the blowings is substantially the same as the running direction of the drying wire (F). By means of said first blowings steam or air is ejected out of the space between the wire (F) and the wall placed in connection with the blow boxes (30). The second jets produced by means of the blow boxes are blown in the direction opposite to the running direction of the wire (F) so as to seal the area of vacuum between the wire (F) and the blow box (30). The same medium is used as the blow medium in the blow box (30) as is used in the impingement drying unit (10).

Description: Method and device in connection with impingement drying and/or through-drying of a paper web or of an equivalent web-like material The invention concerns a method in connection with impingement drying and/or through-drying of a paper web or of an equivalent web-like material, in which method the paper web or the equivalent web-like material is dried by blowing hot air and/or superheated steam and/or equivalent by means of an impingement drying device substantially perpendicularly to the paper web or the equivalent web-like material, in which method the paper web is passed on support of a wire or equival¬ ent past the impingement drying device. Also, the invention concerns a device in connection with impingement drying and/or through-drying of a paper web or of an equivalent web-like material, which device comprises an impingement drying and/or through-drying device/devices, past which the web to be dried has been fitted to pass on support of a wire or equivalent. It is known from the prior art to use various impingement-drying/through-drying units for evaporation drying of a paper web, in which units the paper web is dried so that, by means of the impingement drying devices, hot air or superheated steam is blown substantially perpendicularly to the paper at a relatively high speed. In these solutions the paper web or an equivalent web-like material often runs on support of a wire, roll, cylinder or equivalent. With respect to the prior-art solutions related to evaporation drying of a paper web and based on impingement drying, reference can be made to the US Patent 4,361,466, in which a web drying method and an equip¬ ment that employ hot air are described, and to the US Patent 5, 210, 958, in which an equipment and a method for drying of a paper web are described in which super¬ heated steam is employed. As is known from the prior art, the use of superheated steam as the drying medium is more advantageous than the use of hot air, because in such a case it is also possible to utilize the hot moisture separated from the web. When paper is dried by means of superheated steam, it is known from the prior art to use devices in which the paper web runs on support of a wire and in which, at the side of the paper web, an impingement dryer is placed which includes steam blow openings and exhaust openings for the return steam, and in which the area of the impingement dryer has been formed as a closed space by means of an enclosure. Inside the enclosure, there can also be several separate dryers. The closed space in the interior of the enclosure is, as a rule, filled with a steam whose temperature is somewhat higher than 100 °C. Compared with conventional impingement drying processes provided with an impingement drying hood, an advantage of an enclosed arrangement is that the number of potential leakage points is minimized to two, i.e. the point at which the paper web is passed into the enclosure and the point at which the paper web is passed out of the interior of the enclosure. The prior-art solutions involve, among other things, the drawback that, with such an enclosure, there should not occur any web breaks or equivalent in the interior of the enclosure, because entering into the interior of the enclosure for repair and/or maintenance operations requires a time-consuming stage of emptying of steam and a period of cooling of the equipment and, after the maintenance or cleaning, a long start-up period, during which period the air must be removed and the equipment is heated to the operation temperature, which in itself lowers the capacity of the equipment and causes expenses. In the prior-art solutions, the paper web often runs on support of a wire, and the web is kept in contact with the wire by means of a difference in pressure across the wire, but the drying-air jets of the impingement drying are in themselves insufficient for producing this difference in pressure, because the process between the nozzle face and the paper is somewhat dynamic, in which connection the paper web can be separated from the wire. In the prior art, mechanical seals have been suggested as a solution, which seals permit the maintaining of a certain difference in pressure between the top and bottom portions of the enclosure. Mechanical seals are, however, not favourable, because they drag against the wire and, thus, wear the wire. Further, the use of mechanical seals has the consequence that said difference in pressure must be maintained over the length of the entire enclosure from the inlet opening of the paper to the outlet opening, because cross-direction additional seals for maintaining a difference in pressure make the equipment considerably more complex. This is why, at the inlet and outlet openings of the enclosure, there is a pressure above the wire and/or a vacuum below the wire, which can, however, result in leakage of steam out of the enclosure above the wire or in flow of air into the enclosure below the wire at said openings if the solution of sealing the openings for the web is not fully sealed. Further, alignment of a mechanical seal is difficult, because the seal must coincide with the edge of the paper web with adequate precision, which is very difficult to accomplish under production conditions. If leakage occurs through the wire outside the edges of the paper web, the result is a reduced difference in pressure or a steam flow from the top side to the bottom side. Besides sealing, further important factors in applications of impingement drying of a paper web or of an equivalent web-like material, from the point of view of runnability, include the keeping of the web substantially straight and the distance of the web from the blow devices. In order to control the running of the web, it is known to use a suction box placed underneath the wire, but the faces of the suction box that drag against the wire as well as the exhaust suction produced by the suction box cause problems for the runnability of the web and affect its stability on the face of the wire. With respect to the prior art related to the present invention, reference can also be made to the applicant's FI Patent 67,107 (equivalent to US Patent No. 4,551,203), in which an arrangement is described for passing a paper web from the press section into the dryer section, in which, to the side of the drying wire that supports the web, a number of air jets are blown at a speed substantially higher than the speed of the drying wire, the outlet direction of said jets being substantially the same as the running direction of the drying wire at said location, and air is ejected by means of said blowing of air out of the space placed between the drying wire and the wall placed in connection with the members that produce said air jets. In this arrange¬ ment, blow boxes have been employed, which are fitted on the run of said drying wire at the side of said wire, which extend substantially across the entire width of the web, which include one or several nozzle slots, and in which blow boxes the walls placed facing the run of the drying wire are plane and substantially parallel to the run of the drying wire. The object of the present invention is to provide a solution by whose means, in impingement drying, in particular in a system accomplished by means of superheated steam, the runnability can be secured. It is a further object of the invention to provide a solution in which the problems and drawbacks described above do not occur. In view of achieving the objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in that in the area of the impingement drying device the web and the wire are supported from the opposite side of the wire, opposite in relation to the web, by the intermediate of blowings produced by means of a vacuum blow box/boxes substantially across the entire width of the web, that the first blowings are blown at a speed substantially higher than the speed of the drying wire, that the outlet direction of the blowings is substantially the same as the running direction of the drying wire in said area, and that by means of said first blowings steam and/or air is/are ejected out of the space between the drying wire and the wall placed in connection with the blow boxes that produce said jets, and that the second jets produced by means of the blow boxes are blown in the direction opposite to the running direction of the wire so as to seal the area of vacuum between the wire and the blow box, and that in the method substan- tially the same medium is used as the blow medium in the blow box as is used in the impingement drying unit. Further, the device in accordance with the invention is characterized in that the device further comprises a vacuum blow box/boxes in view of supporting the web and the wire by means of blowings produced by means of the vacuum blow box/- boxes substantially across the entire width of the web, that the blow medium in the vacuum blow box/boxes is substantially the same medium as the blow medium in the impingement-drying/through-drying unit/units. In accordance with the invention, at the opposite side of the paper web or an equivalent web-like material and the wire, opposite in relation to the impingement dryer, vacuum blow boxes are placed, i.e. blow devices by whose means a vacuum is produced below the wire, said vacuum blow devices generating the necessary difference in pressure across the wire, in which connection the paper web remains in contact with the wire and the runnability is secured. In the present invention it has been realized, in a novel way, in view of securing the runnability, to combine an impingement drying application with vacuum blow boxes, by whose means the keeping of the web on the wire is secured and by whose means, at the same time, the desired distance of the web from the impingement drying devices and the control of said distance can be achieved, in which connection the efficiency of evaporation can also be controlled. In this way, by means of the arrangement in accordance with the invention, the desired draw of the paper web through the enclosure in the impingement drying unit is achieved. By means of the arrangement in accordance with the invention, a sealing arrange¬ ment free of contact is achieved, in which there are no faces that drag against the wire or against the paper web. The blower in the system of runnability in accordance with the invention can also be used for emptying the enclosure from steam in a running -down situation, for example, for maintenance, and for evacuating air during a start-up procedure without having to use separate devices. When the web is dried by means of superheated steam, in the vacuum blow devices steam is used as the blow medium, which steam is preferably taken from the interior of the enclosure, primarily from below the wire in order that disturbing flows should not arise in the vertical direction in the enclosure. If necessary, in the steam supply line for the blow boxes there is a heating device, for example a steam heat exchanger or a direct supply of fresh steam, so as to maintain a suitable temperature level and, thus, to eliminate any risk of condensing. In connection with the arrange¬ ment in accordance with the invention, it is also preferable to apply a system of control of runnability and steam status, wherein, by means of a blower, steam is also sucked from above the paper web and the wire and the same amount of steam is fed back after it has been heated. In order to maintain the temperature level in the enclosure, the heating device provided in the system of runnability is probably adequate, and no other heating or supply of steam into the enclosure is needed. In the following, the invention will be described in more detail with reference to the figures in the accompanying drawing, the invention being, however, not supposed to be strictly confined to the details of said illustrations. Figure 1 is a schematic illustration of an arrangement of runnability in accordance with the invention in connection with impingement drying. Figure 2 is a schematic illustration of a group of two blow boxes for use in connec¬ tion with the arrangement in accordance with the invention. In the exemplifying embodiment shown in Fig. 1, the paper web W is dried by means of an impingement dryer 10, whose surrounding area is enclosed both above the paper web W and below the wire F by means of a box 20. From the impinge¬ ment dryer 10, superheated steam is blown towards the paper web W, which runs on support of the wire F. The enclosure 20 forms a closed space 21 , which is open at the point A, at which the web W enters into said enclosed space 21, and at the point B, at which the web W departs from the closed space 21. Said points A,B of passage of the web W are provided with support and sealing members 22A,22B. In the space 21 in the interior of the enclosure 20, underneath the wire F, a number of vacuum blow devices 30 are fitted. By means of the vacuum blow boxes 30 the run of the wire F and of the web W is stabilized, and thereby the runnability of the web W past the impingement dryer 10 and through the enclosure 20 is improved. As is shown in Fig. 2, out of the blow boxes 30 placed at the wire F side, steam and/or air jets Si are blown substantially in the running direction of the wire F, and the speed of said jets S t is substantially higher than the speed of the wire F. As is well known, the moving wire F carries steam/air along with it. If the area from which the wire F takes steam/air along with it can be sealed, a vacuum is produced in said area. In the method of the present invention, the sealing is carried out by means of blowings. Since the speed of the jets Si is higher than the speed of the wire F, an ejection effect is produced, which enhances the vacuum produced by the pumping in the wire F. The sealing of the vacuum area in the other direction takes place by means of steam and/or air jets S 2 . By the effect of the area subject to a vacuum formed between 27 the blow boxes 30 and the wire F, the steam/air attempts to flow through the wire F. Since the web W is practically impenetrable by air, the web W adheres tightly onto the wire F, and no detrimental fluttering occurs. Thus, the blow boxes stabilize the run of the wire F and of the web W in the area of the impingement dryer 10. In Fig. 1, the number of the blow boxes 30 is three groups of two blow boxes 30 placed one after the other, and in the gaps between said groups guide rolls 23 are placed. Depending on the geometry of the impingement dryer 10, there can be one or several blow boxes. Said blow boxes have a plane top face 28, which is placed at a distance Δ from the drying wire F that runs facing said face 28. Said distance Δ is preferably in the range of 5...30 mm. At both ends of the plane wall 28 in the blow boxes 30 there are nozzle slots 29, by whose means the blowings S, and S 2 described above are produced. There may also be nozzles at the edges of the blow boxes 30, the access of steam/air between the box and the wire being sealed by means of blowings produced by means of said edge nozzles. As is shown in Figs. 1 and 2, the steam needed by the blow boxes 30 is taken into the ducts 18 from the space 21 in the interior of the enclosure 20, preferably from the portion below the wire F, in order that disturbing vertical flows should not arise inside the enclosure 20. From the ducts 18 the steam is passed along a duct 40 provided with a regulator 43 to the blower 41. By means of the blower 41 , along the blow line 42, the steam is passed into each blow device 30 through the ducts 17, in which ducts regulators 44 have been fitted so as to regulate the blowing in each device 30. The blow line 42 can also include a heating device 45, for example a steam heat exchanger or a device for direct supply of steam, in which case a suitable temperature level can be maintained in the interior 21 of the enclosure 20 in order to eliminate any risk of condensation. By means of the blower 41, it is also possible to suck steam from the interior 21 of the enclosure 20 from above the wire F and the web along the ducts 51,52 and to feed steam back, after it has been heated, along the ducts 53,54. The duct 53 is provided with a regulation member 55 for regulation of the quantity of the steam to be supplied. When the interior 21 of the enclosure has to be emptied of steam, for example, for servicing of the dryer unit, the enclosure 20 is emptied of steam by means of the blower 41, and similarly, during the start-up stage of the equipment, air can be removed through the blower 41 and the ducts 18,40 from the interior 21 of the enclosure 20. Above, the invention has been described with reference to an exemplifying embodi¬ ment in which steam is used as the drying medium in the impingement dryer 10. The arrangement in accordance with the invention can also be applied, with the principles described above, in arrangements in which hot air or equivalent is used as the drying medium in the impingement dryer unit 10. Above, the invention has been described with reference to one preferred embodiment of same only, the invention being, yet, not supposed to be strictly confined to the details of said embodiment. Many variations and modifications are possible within the scope of the inventive idea defined in the following patent claims.

Claims: Claims 1. A method in connection with impingement drying and/or through-drying of a paper web or of an equivalent web-like material, in which method the paper web (W) or the equivalent web-like material is dried by blowing hot air and/or super¬ heated steam and/or equivalent by means of an impingement drying device (10) substantially perpendicularly to the paper web (W) or the equivalent web-like material, in which method the paper web (W) is passed on support of a wire (F) or equivalent past the impingement drying device (10), characterized in that in the area of the impingement drying device (10) the web (W) and the wire (F) are supported from the opposite side of the wire (F), opposite in relation to the web (W), by the intermediate of blowings (S j ^) produced by means of a vacuum blow box/boxes (30) substantially across the entire width of the web, that the first blowings (S j ) are blown at a speed substantially higher than the speed of the drying wire (F), that the outlet direction of the blowings is substantially the same as the running direction of the drying wire (F) in said area, and that by means of said first blowings (S j ) steam and/or air is/are ejected out of the space (27) between the drying wire (F) and the wall (28) placed in connection with the blow boxes (30) that produce said jets, and that the second jets (S 2 ) produced by means of the blow boxes are blown in the direction opposite to the running direction of the wire (F) so as to seal the area of vacuum (27) between the wire (F) and the blow box (30), and that in the method substantially the same medium is used as the blow medium in the blow box (30) as is used in the impingement drying unit (10). 2. A method as claimed in claim 1, characterized in that, in the method, super¬ heated steam is used as the blow medium in the vacuum blow boxes (30). 3. A method as claimed in any of the claims 1 or 2, characterized in that, in the method, the paper web (W) or an equivalent web material is passed on support of a wire (F) or equivalent through a substantially closed space (21) formed by means of an enclosure (20), into which enclosure (20) the web (W) and the wire (F) are passed through an inlet point (A) and out of which enclosure (20) they are removed through an outlet point (B), and that in the method the superheated steam is taken from the interior of the closed space (21) provided with an enclosure (20), preferably from the side of the web (W) and the wire (F) at which the vacuum blow box/boxes (30) is/are placed. 4. A method as claimed in any of the claims 1 to 3, characterized in that the steam that is passed into the vacuum blow box/boxes (30) is heated. 5. A method as claimed in any of the claims 1 to 4, characterized in that, in the method, the runnability of the web (W) and of the wire (F) by means of the vacuum area (27) and the steam space (21) in the interior of the enclosure (20) are controlled by means of the blower (41), by whose means steam is sucked from the interior of the enclosure (20) and steam is fed back after it has been heated. 6. A method as claimed in any of the claims 1 to 5, characterized in that the space (21) in the interior of the enclosure (20) is emptied of steam by means of the blower (41) of the vacuum blow-box system in a run-down stage of the system. 7. A method as claimed in any of the claims 1 to 6, characterized in that air is removed out of the interior (21) of the enclosure (20) by means of the blower (41) of the vacuum blow-box system during a start-up period. 8. A device in connection with impingement drying and/or through-drying of a paper web or of an equivalent web-like material, which device comprises an im- pingement drying and/or through-drying device/devices (10), past which the web (W) to be dried has been fitted to pass on support of a wire (F) or equivalent, char¬ acterized in that the device further comprises a vacuum blow box/boxes (30) in view of supporting the web (W) and the wire (F) by means of blowings (S j ,S 2 ) produced by means of the vacuum blow box/boxes (30) substantially across the entire width of the web, that the blow medium in the vacuum blow box/boxes (30) is substantially the same medium as the blow medium in the impingement-drying/through-drying unit/units (10). 9. A device as claimed in claim 8, characterized in that the blow medium in the blow box/boxes (30) is superheated steam, and that the impingement-drying and/or through-drying device/devices (10) is/are placed in a substantially closed space (21) provided with an enclosure (20), which space is provided with an inlet point (A) and an outlet point (B). 10. A device as claimed in claim 8 or 9, characterized in that the blow box/boxes (30) in the device communicate with a blower (41) through ducts (17) provided with regulators (44), and that the blower (41) communicates with exhaust pipes (18) fitted in the interior of the enclosed space (21) through a duct (40) provided with regula¬ tors (43) so that the blowings (Sι ,S 2 ) in the vacuum blow boxes (30) are fitted to be produced by means of the blower (41). 11. A device as claimed in claim 10, characterized in that the blower (41) com- municates with exhaust pipes (51) fitted in the interior space (21) in the enclosure (20) at the side of the impingement dryer unit (10) through an exhaust duct (52) provided with a regulator (53), and that the blower communicates with the feed pipes (54) through the feed duct (53), which feed pipes extend to the side of the impinge¬ ment dryer unit (10) in the interior (21) of the enclosure (20) in order to control the steam space in the interior space (21) in the enclosure (20). 12. A device as claimed in any of the preceding claims 8 to 11 , characterized in that in connection with the blower (41) a heating device (45) has been provided in order to heat the blow medium.

Inventors: SUNDQVIST HANS, SUNDQVIST, HANS
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1997
Main classification: D21F5
IPC codes: D21F5