METHOD AND ARRANGEMENT FOR HANDLING PAPER OR CARDBOARD WEBS

Abstract: A method and apparatus for producing paper or board, according to which the paper or board web is transferred continuously supported through a drying section of a paper or cardboard machine and both sides of the web are treated with a treating agent on a long nip applicator included to the drying section of the machine. The web is transferred from the last press nip of the press section to the first dryer of the dryer section with a continuous loop-like support member, for example by a transfer belt, transfer felt or one of the press felts of the press section. The web is advantageously dried in the drying section with a blow unit on a porous drying felt by blowing hot air on the web with impingement dryers and the drying felt may be cooled.

Description: Method and arrangement for handling paper or cardboard webs The present invention relates to handling of paper or cardboard webs formed m a paper or cardboard machine, especially to leading the web during the subsequent handling process after the last nip of the press section of the machine . Background of the invention In a paper or board making machine the web is formed on by feeding a slurry of fibrous material from a head box onto a wire. Water is removeα from the weo on the wire and different kind of pressing stages. At the Deginmng of the formation of the web its strength is extremely low since the web contains only small amount of fibers compared to the amount of water. In the press section the solids content of the web increases rapidly, but the web is still quite wet and its strength low. After the press section the web is dried by different dryers using heat. The dried web is then normally treated on different processed m order to improve its properties according to desired use of the web. The most common finishing processes are calandenng, sizing and coating. On the press section of the machine the web is supported by wire or felt, but on subsequent drying stage and during finishing the web is guided by rolls or m modern machines also by belts, felts or wires. When the web is transferred forwards m the manufacturing process, it has to be supported on different sides especially wnen the web is coateα. The path of the web comprises usually several open draws wherein the web travels unsupported between two supporting members through open air. Since the web has to be tensed m order to keep it on the supporting members and to compensate the changes m web length and width during processing, a speed difference has to be set over the open draws on the supporting and driving members guiding the web. These open draws are necessary for following reasons. First, the shrinking, elongation and widening of the web has to be compensated as stateα above. The largest dimensional changes of the web are caused by moisture content variations during processing and if the variations are big, open draws are necessary since the dimensional changes can not be compensated when the web is supported on a roll, belt or similar dimensionally rigid member. Second, it is difficult to remove a wet web from a support member and transfer it to a following member without special measures, since the web usually tenαs to follow the surface on which it is already attached to. It is often easier to remove the web from one support member and transfer it over an open draw to the next support member. Third, some of the apparatuses used for paper and cardboard making require that the web is lead along a defined path through the process whereby it may be difficult to lead the web supportedly to the apparatus and from it. In some apparatuses like roll calanders or coaters the web can not be supported continuously without changing the characteristic of the process. In calanders the performance of the nip changes if, for example, a support belt is lead through the roll nips. In coating the wet coated side of the web can not be touched, whereby the web has to be supported on the dry side of the web, and there has to be a open draw between successive support members. The length of the open draw is determined by diameters of the rolls guiding the support member. Even though the open draws are often necessary, they cause several problems. After the web is formed m the press section, it is still very wet and breaks easily. When such a web is taken over an open draw, the speed differences of the web transferring elements are supposed to keep the web on a predetermined tension so that no slacking or wrinkling of the web occurs. However, the speeds of the wires, rolls, belts and other elements transferring the web may vary slightly whereby the tensions over the open draws changes. The tensile strength of tne web may also vary for example because of variations m the moisture content. Further, there may be defects m the web. These and other factor may lead to a situation where the tension of the web exceeds the strength of the web, first usually on a limited area of the web, whereby the web creaks. The web breaks are the mam reason for production stops . One of the factors that influence heavily on the quality of the product and production costs n paper and board manufacture is drying of the web. As stated above, a web is first formed by a head box, wire and press section into a reasonably solid web and the web is then dried to a desired moisture content, which is about 2 - 4% of the weight of the web. If the web is calandered, it has to be wetted thereafter to a higher moisture content depending on the calander type and if the web is coated or sized, the web absorbs water from the coating or sizing mixture. The water added to the web on calanαenng evaporates normally during the calandermg process, but the water absorbed m the web during coating or sizing has to be dried. The drying process requires a lot of energy and the dryers are quite costly and especially on fast machines very long drying sections have to be used, whereby the drying of the web after formation and during finishing forms a notable part of the manufacture costs and the cost of the machine. One possibility to decrease the investment costs of the drying and dryers on a paper or board manufacturing machine is to combine coating and sizing with the drying of the web on an on-line manufacturing line so that the treating agent is spread on the web m the drying section of the manufacturing line. The problems of this solution are the mechanical stresses imposed on a wet web on application of the treating agent and increased moisture of the wet web that requires efficient drying process. Summary of the invention According to the present invention the web is transferred continuously supported through a drying section of a paper or cardboard machine and both sides of the web are treated with a treating agent on a long nip applicator included to the drying section of the machine. According to the other aspects of the present invention, the web is transferred from the last press nip of the press section to the first dryer of the dryer section with a continuous loop-like support member, for example by a transfer belt, transfer felt or one of the press felts of the press section. The web is advantageously dried m the αrying section with a blow unit on a porous drying felt by blowing hot air on the web with impingement dryers and the drying felt may be cooled. Other objects and features of the invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. Brief description of the drawings Fig. 1 shows diagrammatically one embodiment of the invention. Fig. 2 shows diagrammatically another embodiment of the invention. Fig. 3 shows diagrammatically another embodiment of the invention. Fig. 4 shows diagrammatically another embodiment of the invention. Detailed description of the presently preferred embodiments In the following text a treating agent is a substance that is used for treatment of paper or cardboard web and contains water that makes the agent spreadable on the surface of the web. An extended nip is a nip having a greater length than a normal nip between two rolls, normally minimum length of an extended nip is considered to be 50 mm. In the arrangement shown m figure 1 the web 12 enters to the last pressing nip of the press section of a paper or cardboard machine on a felt 1. The press nip is formed between a press shoe 3 and a backing roll 4. A transfer belt 2 is arranged to run through the nip 3,4 guided by guide rolls 13. On the last pressing nip water is removed from the web and the web 12 is transferred on the surface of the transfer belt 2. Downstream from the transfer belt 12 is arranged a transfer felt 5 that is pressed on transfer belt 2 with a suction roll 6. The felt 5 is air- permeable and the negative pressure produced by the suction roll attaches the web 12 onto the felt 5. The transfer felt 6 transfers the web to a drying felt 8, that is pressed on the transfer felt 5 by a second suction roll and the web 12 is transferred on the drying felt 8. The drying felt 8 is arranged to run linearly over rolls 11 and blow boxes 10 that support the drying felt 8. The blow boxes 10 may be of suction/blow design wherein the air blown towards the felt 8 is sucked back on the box with the moisture removed from the web 12. The rolls may be vacuum rolls or coated rolls, for example. On the web side of the drying felt is an impingement dryer unit 9 that blows hot air towards the web in order to dry the web. The drying process is described more closely below. Downstream from the first dryer unit 9 is the first unit for applying treating agent on the surface of the web 12. The applicator unit comprises a transfer fabric 14, that is pressed against the drying felt 8 by a suction roll 15. The transfer fabric 14 picks the web 12 from the drying felt and transfers it to the nip of the applicator unit. The application nip is formed by a shoe press 16 and a backing roll 17 forming an extended nip and a coating transfer belt 18 is arranged to travel through the nip. The coating transfer belt 18 is smooth belt made of metal, for example steel, or polymer or composite material. The coating transfer belt 18 is arranged to travel guided by guide rolls 13 and m this position the coating transfer belt 18 is below the transfer fabric 14. The treating agent is spread on the coating transfer belt 18 by an applicator unit 19. The applicator unit may be a short-dwell applicator, spray applicator unit, a film transfer unit or any other applicator unit capable of spreading an uniform coating layer on the smooth surface of the belt 18. On the path of the transfer fabric is arranged cleaning means for removing moisture, contaminants and material adhered onto the fabric from said fabric. An applicator unit comprising an extended nip and a transfer belt provides essential benefits when the treatment of the web is performed on the drying section of a paper or board machine. First, the treating agent includes always water and on the extended application nip the web is pressed between a permeable fabric 14 and impermeable belt in a long nip. The long nip removes water efficiently from the web onto the fabric 14. Second, the treating agent is pressed in the nip to the pores of the web, which is particularly beneficial in application of sizing agent and makes it possible to use large amounts of size, which provides a strong web. The long web also smoothes the surface of the web whereby the treated web has good smoothness and other properties affecting the printability of the web. The smoothing effect of the extended nip is further intensified by the high water content of the web that makes the web easily deformable. Further, a characteristic feature of a shoe press or extended nip is that it provides good surface properties but because of a low pressure in the nip, the nip does not decrease the volume of the web whereby the bulk of the web is maintained. This is very important because better bulk provides better stiffness on same volumetric weight, and on the other hand, same stiffness can be obtained by using less fiber material which decreases the material consumption and cost of the product. Downstream of the applicator unit is a second drying unit 20 that is similar to the first drying unit 7 - 11. The coating transfer belt 18 transfers the web 12 to the second drying felt that in this position is arranged above the path of the web. Downstream of the second drying unit is a second applicator unit 21 which also is in inverted position to the first applicator unit 14 - 20. After the second applicator unit is a third drying unit whereafter the web is transferred to a roller of further processing. In the embodiment of the invention shown in figure 1 the drying method of the drying unit includes heating and drying of the web by means of impingement drying on the surface of a material, which is permeable, has a sufficient porosity volume. In the drying and coating method the paper or cardboard web is transferred as closed transfer from the last press nip on a surface of a transfer belt and press felt and with help of the transfer suction roll to a transfer felt and further with another transfer roll into the first impingement drying unit. If the web is transferred from the last press nip on the surface of the upper fabric, it can be transferred from the surface of this upper fabric with the transfer suction roll directly into the first upper impingement drying unit. It is not necessary to have the impingement blow of the first impingement drying unit from upwards. The web can be transferred into the first lower impingement drying phase directly from the lower fabric of the press nip with help of the transfer suction roll. The impingement drying from underneath generates hereby a greater content of dry matter for the lower surface than for the upper surface and the runnability of the possibly following cylinder dryer section in regarding the adhesion on the cylinders is better after the impingement drying. The drying wire is preferably quite smooth and has a good absorbency. The drying wire is most advantageously cooled up before the impingement drying area with a cooling unit and possibly also during the impingement drying by cold air blow boxes in order to form a temperature gradient on the web. The water and the fibers at the surface on the impingement dried side get warmer when the impingement drying with hot air or with superheated steam is done against the drying wire which most preferably is cooled, and when the temperature is high enough (over 65 °C) evaporation of water occurs. One part of the water evaporated from the web is transferred into the cooler drying wire and condenses m it. The water condensed m the dryer fabric is removed after the impingement drying unit by centrifugal drying, suction unit and/or blowing unit, which can act also as a cooling device for dryer fabric. The dryer fabric is supported along its almost horizontal run f.ex. with rolls, which can be non-smooth rolls or Vac-rolls, and also with support of blow suction boxes. Coating is carried out after the first or the few first impingement drying units. Characteristics for the coating is the closed web transfer into the coating phase from the drying unit with help of the transfer fabric, dosing of the coating agent (e.g. size or pigment coating mix) at the coating unit on the surface of the belt transferring the coating agent and pressing the coating to web surface most advantageously with extended nip press. The web is transferred after the press nip as closed transfer on the surface, of the belt into the next handling phase (drying or coating). The coating unit can be e.g. a blade coater, rod coater, roll coater, spraying device or a combination thereof. Alternatively the both fabrics going through the same coating unit can be belts and the coating is metered on the surface of both of them and/or the intermediate calendering is realized by the double-belted unit. There can be an intermediate drying phase after the first coating phase of the other web side and after that the coating of the other side with the same method. There can be impingement drying unit or units after the later coating phase and after that an ordinary cylinder drying unit or the final drying of the web can alternatively be realized with the mentioned impingement drying method. Paper or cardboard web is transferred as a closed transfer from the last press nip on the surface of the transfer belt or the press felt and with help of the transfer suction roll onto the transfer fabric and further with help of the other transfer suction roll onto drying fabric of the first impingement drying unit. Also the web transfer without the transfer fabric into the impingement drying is possible (see fig. 2) . The drying method of the drying unit includes the heating of the web and the drying of the web means of impingement drying on the surface of material, which is permeable, has a sufficient porosity volume, is preferably quite smooth-surfaced and has a good absorbency. Also two nested fabric loops, fine and rough wires, can be used. They can be combined into a single fabric. The drying fabric is most advantageously cooled before the impingement drying with cooling units and possibly also during the impingement drying with cold air blow boxes. Also an embodiment without cooling is possible. The water and the fibers at the surface on the impingement dried side first get warm when the impingement drying with hot air or with superheated steam is done against the drying wire which most preferably is cooled and when the temperature is high enough (over 65 °C) water evaporation from the web begins to occur effectively. One part of the water evaporated from the web transfers into the cooler drying wire and condenses in it. This drying method ensures an efficient and a process that saves both energy and machine length. The water condensated in the dryer fabric is removed after the impingement drying unit with centrifugal drying, by doctoring, with suction unit and/or blowing unit, which can act also as a cooling device for the dryer fabric. The dryer fabric is supported along its almost horizontal run f. ex. with rolls, which can be non-smooth rolls or Vac-rolls, and also with supports of blow suction boxes. Also impingement drying on the Vac-roll which most preferably has large diameter and is optionally cooled, is an alternative for drying in a planar position. Also the combination of the previous methods is possible. Coating phase is carried out after the first or the few first impingement drying units. Characteristics for the coating is the closed web transfer into the coating phase from the drying unit with help of the transfer fabric, dosing of the coating agent (e.g. size or pigment coating) at the coating unit on the surface of the smooth impermeable belt and pressing the coating from the belt to the web surface most preferably with extended nip press, so the nip pressure distribution is controllable to optimum within wide ranges. The dosing unit of the coater can be e.g. a blade coater, rod coater, roll coater, spraying device (the coating is sprayed onto the belt or directly onto the web possibly intensified by electric field) or combination thereof. Alternatively the both fabrics going through the same coating unit can be belts and a light coating layer is metered on the surface of them both. Instead of coating, calendering or light weight coating and calendering simultaneously can be made by double-belted unit (fig. 2) After the first, one-sided coating phase the web is transferred as supported transfer to the impingement drying phase and after that to coating of the other side with the same method. Following the second coating phase there is an impingement drying unit or units and after that may be an ordinary cylinder drying unit (CY) or units. The final drying of the web can alternatively be realized with the mentioned impingement drying method. The arrangement shown in figure 2 differs from the arrangement of figure 1 in few aspects. In this arrangement the web 12 is transferred directly on the surface of the transfer belt 2 to the first drying stage 22. The dryers are similar to the dryers of the embodiment of figure 1. After the first drying stage 22 follows treatment of the web in two successive stages. In this embodiment the coating mixture is spread on the surface of a film transfer roll 23, which operates simultaneously as a backup roll for the shoe press 16. In here the treating agent remains for a shorter time on a transfer surface than in the preceding embodiment but the nip is extended in the same way as in the embodiment above. The advantage of this embodiment is that normal film transfer rolls and applicator can be used for application of the treating agent. The treatment of the opposite side of the web follows immediately without intermediate drying, wherefore the fabric of the second applicator unit has to be replaced by a smooth belt 24 that does not adhere to the surface of the treated web. The paper web is transferred from the upper transfer belt or felt of the press section directly to the top impingement drying phase or phases which are done as in fig 1. The web is adhered from the drying fabric to a smooth belt having proper elastic properties by pressing or with sticking nip, on which belt the coating can also be metered (size/pigment coating) and from which belt the web is transferred to an extended calendering and/or coating nip. Counter roll is preferably a heatable calender roll and a light coating can be metered on its surface before the calendering and/or coating nip. After the first coating phase the web is transferred on a belt which is stickier than the first one to another "counter sided" coating phase and from that on to impingement or cylinder drying. In figure 3 is shown two last press nips of the press section of a paper or board making machine. From the last press nip the web 12 is transferred on a transfer felt 5 to a first drying felt 25 that travels around a vacuum drying cylinder 26. Around the cylinder is arranged impingement dryers 27 that blow hot air or superheated steam on the web 12 and the drying felt 25. Downstream to the vacuum dryer roll is an applicator unit 28 that is similar to the applicator units of the embodiment described in figure 1. In downstream direction follow second vacuum dryer unit 29, second applicator unit 30 and third vacuum dryer unit 31. The embodiment of the figure 4 is similar to the embodiment of the figure 1 except that the web 12 is transferred onto the transfer felt 12 of an applicator unit directly on the transfer belt 2 of the last press nip of the press section. In all embodiments of the invention the web is continuously supported by a continuos support elements arranged to run in loops. The support elements may be wires, felts, belts or other suitable bandlike elements. Advantages of the drying and coating methods, are: • paper or board web is transferred as supported transfers with low, optimal tensile differencies and strains for the web from wire section to final drying or possibly even to reeling enabling thus a great running speed and high efficiency. • Drying of the paper is made by efficient energy saving drying method, which enables on-line coating and calendering phases having supported transfers at every stage and avoiding sticking of the coating to the counter surfaces . • By these methods good, symmetrical and uncurled properties of paper or board are achieved and the properties can be optimal with wide process control value ranges of different drying, coating and/or calendering phases . Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the de.... may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same results are within the scope of the invention. Substitutions of the elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appen- ded hereto.

Claims: What is claimed is: 1. Method for producing paper or board web, comprising - leading the web formed in a press section of a paper or board machine to a drying section continuously supported by a continuous support member, - receiving the web on a continuous support member of the drying section, - leading the web supported on a continuous support member through an application stage, - applying treating agent on a surface of the web, - pressing the treating agent on the surface of the web in an extended nip . 2. Method according to the claim 1, wherein both sides of the web are treated in an extended nip. 3. Method according to the claim 2, wherein the web is dried between application of treating agent on a second side of the web. 4. Method according to the claim 2 , wherein the treating agent is spread on a transfer belt running through the extended nip. 5. Method according to the claim 2, wherein the treating agent is spread on a transfer roll that is arranged to press the web in the nip . 6. Method according to the claim 4 wherein the web is pressed against the transfer belt by a heatable calander roll . 7. Method according to the claim 6, wherein treating agent is spread on the calander roll. 8. Method according to the claim 1, wherein the web is transferred on a drying felt and dried by blowing heating agent on the web and the drying felt is cooled in order to form a temperature gradient in the web. 9. Method according to the claim 8, wherein the web is arranged to run essentially linearly during drying and dried by linear impingement dryer. 10. Method according to the claim 8, wherein the web is arranged to run over a cooled roll and the web is heated by impingement heaters arranged around the roll. 11. Arrangement for producing paper or board web, comprising - a continuous support member for leading the web formed in a press section of a paper or board machine to a drying section continuously supported by said continuous support member, - a continuous support member for receiving the web from the continuous support member of the drying section, - at least one continuous support member for leading the web supported on said continuous support member through at least one extended nip comprising a pressing element and a counter roll forming said nip, and - means for applying treating agent on at least one side of the web before the web enters into said extended nip. 12. Arrangement according to the claim 11, comprising at least two extended nips. 13. Arrangement according to the claim 12, comprising at least one drying unit arranged between said extended nips. 14. Arrangement according to the claim 12, comprising a transfer belt arranged to run through said extended nip and means for spreading treating agent on said transfer belt. 15. Arranged according to the claim 12, comprising means for spreading treating agent on a transfer roll operating as a counter roll in the extended nip. 16. Arrangement according to the claim 14 wherein the transfer roll is a heatable calander roll. 17. Arrangement according to the claim 16, comprising means for spreading treating agent on the calander roll. 18. Arrangement according to the claim 11, comprising at least one drying felt means for blowing heating agent on the web and for cooling the drying felt in order to form a temperature gradient In the web. 19. Arrangement according to the claim 18, comprising at least one linear impingement dryer and means for guiding the web to run essentially linearly during drying by the linear impingement dryer. 20. Arrangement according to the claim 18, comprising a coolable roll over which the web is arranged to run, and impingement heaters arranged around the roll

Inventors: LAAPOTTI JORMA, LAAPOTTI, JORMA
Applicants: LAAPOTTI JORMA, VALMET CORP, LAAPOTTI, JORMA, VALMET CORPORATION
Application year: 1999
Main classification: D21F5
IPC codes: D21F3, D21F5