Abstract:
A method for drying a fibrous web, particularly a paper, cardboard or tissue web whereby the moving fibrous web is treated with got air in the area of a pre-definable drying zone the fibrous web is treated, at least in some areas inside the drying zone with steam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of PCT application No. PCT/EP2007/064290, entitled “METHOD AND APPARATUS FOR DRYING A FIBROUS WEB”, filed Dec. 20, 2007, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for drying a fibrous web, especially a paper, cardboard or tissue web, whereby the moving fibrous web is treated with hot air in the area of a pre-definable drying zone. It further relates to a corresponding drying apparatus as well as to a machine for the production of a tissue web with such a drying apparatus. 
     2. Description of the Related Art 
     A method which serves to produce a voluminous tissue web and in which a so-called belt press in conjunction with a hot air hood or, alternatively a steam hood is utilized to dewater a fibrous web to a certain dry content is already known from WO 2005/075737 A1. With tissue machines it is important to reduce the energy consumption especially during the drying process in order to achieve a pre-determinable dry content. There is also a requirement to increase the dry content at reduced energy consumption. 
     What is needed in the art is an improved method, as well as an improved apparatus for the drying process for the production of a tissue web, which is optimized, especially in consideration of the energy requirement for dewatering the tissue web. 
     SUMMARY OF THE INVENTION 
     Regarding the method of the present invention, the fibrous web is treated, at least in some areas, with steam inside the drying zone. Accordingly, hot air and steam are used in combination together for drying the fibrous web, preferably a tissue web. The fibrous web is advantageously treated with steam within the first half of the total drying zone length, when viewed in the direction of web travel. In this arrangement the fibrous web is treated with steam, at least at the beginning of the drying zone, when viewed in the direction of web travel. 
     Viewed in the direction of web travel, the fibrous web can initially be treated with steam and subsequently with hot air. According to an alternative practical arrangement it is also possible to treat the fibrous web, when viewed in direction of web travel, initially with hot air, subsequently with steam and then again with hot air. 
     In certain instances it is advantageous if the fibrous web, viewed in the direction of web travel, is treated at least essentially over the entire length of the drying zone with steam. 
     According to an alternative practical arrangement of the inventive method it is also possible to treat the fibrous web with steam, at least essentially only within the first half of the total length of the drying zone when viewed in direction of web travel. In this case the fibrous web is treated with steam, preferably at least essentially over only the first half of the total length of the drying zone, viewed in the direction of web travel. 
     According to an additional advantageous arrangement the fibrous web is treated with steam, at least essentially only within the first third of the total length of the drying zone, and moreover preferably at least essentially over this first third, viewed in the direction of web travel. 
     In certain cases it is also advantageous if the fibrous web is treated with steam, at least essentially only within the first quarter of the total length of the drying zone, and moreover hereby preferably at least essentially over this first quarter, viewed in the direction of web travel. 
     According to an additional alternative arrangement of the inventive method the fibrous web is treated with steam only at the beginning of the drying zone, viewed in the direction of web travel. 
     In another embodiment of the present invention the fibrous web is treated with hot air over the pre-determinable drying zone. At least in this instance the drying zone can be defined, at least essentially through the area in which the fibrous web is treated with hot air. In this case the fibrous web may be treated with steam, particularly inside and/or prior to this drying zone. 
     The fibrous web is advantageously treated at least in some areas simultaneously with hot air, as well as with steam, viewed in the direction of web travel. Under simultaneous treatment it is to be understood that a respective area of the fibrous web is treated with hot air, as well as also with steam. 
     According to a useful practical arrangement of the present invention the fibrous web can be carried through the drying zone together with a permeable fabric, especially a structured fabric or a TAD-fabric (TAD=Through Air Drying). In this case, hot air or steam, which has not condensed in the web, flow initially through the fibrous web, and subsequently through the permeable fabric. The inventive combined hot air and steam treatment can therefore also be used, in a TAD drying process. 
     Another embodiment of the present invention distinguishes itself in that the fibrous web, together with at least one permeable fabric, especially a structured fabric is guided through the drying zone, whereby hot air or steam flow initially through the permeable fabric and subsequently through the fibrous web. 
     In the drying zone the fibrous web can be covered advantageously by at least one additional permeable fabric, especially a press belt, whereby in this case hot air or steam flow initially through the additional permeable fabric or press belt, subsequently through the first permeable fabric or structured fabric and finally through the fibrous web. Moreover, in the use of a press belt a type of belt press is created through which, in addition to the mechanical pressure, the inventive combined hot air and steam drying process is applied. 
     A dewatering fabric, especially a felt can additionally be run through the drying zone together with the fibrous web, whereby hot air or steam, as far as this has not condensed on the web, as previously mentioned, initially flow through the additional permeable fabric or press belt, subsequently through the first permeable fabric or structured fabric and the fibrous web and finally through the additional dewatering fabric. 
     It is also conceivable to subject the fibrous web in the drying zone, at least in some areas, to impingement drying. In this scenario the inventive combined hot air and steam application is used within the scope of such an impingement drying. The fibrous web may be subjected, at least in some areas, also to through-air drying. 
     An objective of the invention is not inventively by an apparatus for drying a fibrous web, especially a paper, cardboard or tissue web, including a drying zone where the moving fibrous web is treated with hot air and whereby this apparatus is characterized in that the fibrous web can be treated with steam, in at least some areas inside the drying zone. 
     For the treatment of the fibrous web with hot air, at least one hot air hood is provided. In this arrangement the drying zone can be defined particularly through the dimensions of the hot air hood. A steam treatment of the fibrous web is advantageously contemplated inside and/or especially before the drying zone. 
     At least one steam blow device, especially a steam blow pipe or steam blow box, is advantageously provided for the treatment of the fibrous web with steam. 
     The steam blow device extends advantageously at least essentially over the entire width of the hot air hood as measured across the direction of web travel. It is also especially advantageous if the steam blow device is located, at least partially, inside the hot air hood. According to one embodiment of the present invention the steam blow device may also be located directly before the hot air hood, viewed in the direction of web travel. 
     The steam blow device can moreover be arranged, designed and/or controlled so that the fibrous web, viewed in the direction of web travel, is treated simultaneously with hot air as well as with steam over only a part of the total length of the drying zone or over the entire drying zone. 
     If the steam blow device includes a steam blow pipe, then the diameter of the orifice of this steam blow pipe is in a range of approximately 5 to approximately 1 mm, and preferably in a range of approximately 4 to approximately 2.5 mm. The diameter preferably has an upper limit, since a certain speed is necessary for the steam jet. 
     If the fibrous web is covered by at least one permeable fabric, for example a permeable press belt in the area of the drying zone, then the distance between the steam blow device and the outer permeable fabric, for example a press belt, covering the fibrous web is &lt;30 mm, especially &lt;20 mm, particularly &lt;15 mm and preferably ≦10 mm. 
     If the steam blow device includes a steam blow pipe its orifices can be advantageously located from each other at a distance of &lt;20 mm, particularly &lt;10 mm and preferably &lt;7.5 mm. 
     If the steam blow device includes at least one steam blow box, the moisture profile of the fibrous web can advantageously be adjusted and/or regulated through it. 
     If the steam blow device includes at least one steam blow pipe, the dry content of the fibrous web can be influenced or adjusted and/or regulated at least essentially through this steam blow pipe. 
     In principle the steam blow device may include only at least one steam blow box or only at least one steam blow pipe, or at least one steam blow box as well as at least one steam blow pipe. 
     If the fibrous web is covered by at least one permeable fabric in the area of the drying zone, a device such as a doctor blade or similar devices are advantageously provided in order to remove the boundary layer of air that is carried along by the outer permeable fabric which covers the fibrous web before the fabric enters the drying area. 
     The hot air for the hot air hood in the drying zone can be taken, at least partially, from the hood allocated to a drying cylinder, especially a Yankee-Cylinder. Energy recovery of this type is possible since the temperature of the exhaust air of such a hood allocated to a Yankee-Cylinder is very much higher than the temperature that is necessary for the hot air to supply the hot air hood in the drying zone. The temperature of the hot air taken from the hood of a drying cylinder, specifically a Yankee-Cylinder can, for example, be approximately 300° C. 
     The hot air hood in the dryer zone is supplied, at least partially, with hot air whose temperature is in a range of &lt;250° C., especially &lt;200° C. and preferably in a range of approximately 150° C. to approximately 200° C. 
     The temperature of the hot air for the supply of the hot air hood can be accordingly adjustable and/or controllable for optimization of the operating point with regard to the energy consumption. As a rule, a higher temperature does not result in a more efficient drying. 
     According to another embodiment of the present invention at least one suction equipped device, especially a suction box and/or suction roll, is located in the area of the drying zone, on the side of the fibrous web or the additional dewatering fabric facing away from the hot air hood. Moreover, the suction equipped device may include a suction roll with a suction box that defines a suction zone. 
     As already mentioned, a belt press is created by an additional permeable fabric in the form of a press belt that is under tension. To this end the press belt is subjected to a high tension in the range of approximately 40 to approximately 60 kN/m, in order to exert a pressing pressure in the range of approximately 0.5 to approximately 1.5 bar in a press zone. It is also especially advantageous if the length of the press zone, viewed in the direction of web travel, which is formed by the permeable press belt  80 , is defined by the area of the wrap over which the press belt wraps around the suction roll. 
     The length of the press zone, viewed in direction of web travel, which is formed by the permeable press belt, can correspond also to the length of the suction zone or respectively the suction box of the suction roll. 
     The drying zone viewed in direction of web travel can be shorter than the press zone. In certain instances it is however also advantageous if the drying zone, viewed in direction of web travel, is the same length as, or longer than the press zone. 
     The throughput volume (1/min.) of steam is preferably less than the throughput volume (1/min.) of hot air. Moreover, at atmospheric pressure the throughput volume of steam can advantageously be less than 0.5 times, especially less than 0.3 times and preferably less than 0.2 times the throughput volume of hot air. 
     The steam causes an increase in the temperature of the fibrous web in order to reduce the viscosity of the water in the fibrous web. To that end the steam in the fibrous web, especially the tissue web must condense so that the appropriate temperature increase can be achieved. This temperature increase may, for example, be adjusted through an appropriate selection of the correct temperature level for the hot air. Preferably the temperature of the hot air treating the fibrous web is adjustable, especially for the purpose of influencing the condensation of the steam in the fibrous web. 
     If the temperature is too low the steam condenses immediately prior to entering the fibrous web. This is due to the fact that the steam is cooled by the housing of the hot air hood and by the incoming colder fabrics. This could occur especially when using a so-called belt press, since the steam in this case must penetrate two outer fabrics, the outer permeable fabric, in particular the press belt and possibly a permeable structured fabric, before it enters the fibrous web. 
     If the fibrous web is covered by a permeable press belt in the drying zone, then this arrangement advantageously has a permeability of &gt;100 cfm, especially &gt;300 cfm, particularly &gt;500 cfm and preferably &gt;700 cfm. 
     If the fibrous web is carried through the drying zone together with a permeable structured fabric, then this arrangement preferably has a permeability of &gt;100 cfm, especially 300 cfm, particularly 500 cfm and preferably &gt;700 cfm. 
     It is also especially advantageous if the fibrous web is covered in the drying zone by a permeable press belt which consists at least essentially of a synthetic material, especially polyamide, polyethylene, polyurethane, etc. 
     According to another embodiment of the present invention the fibrous web can also be covered in the drying zone by a permeable press belt which is formed be a metal fabric. Preferably at least one fabric, which runs through the drying zone together with the fibrous web, is pre-heated before the drying zone, viewed in the direction of web travel. This is especially advantageous in the case where a press belt consisting of metal is used. 
     For pre-heating a steam heating device, an IR heating device and/or a hot water heating device may used. A hot water heating device is advantageous for an inner fabric, such as an additional dewatering fabric that runs through the drying zone together with the fibrous web. 
     As already mentioned the boundary layer of air that is carried along on the surface of the outer fabric can advantageously be removed by a doctor blade which is located before the hot air hood and which extends across the width of the hot air hood. This also causes an accordingly higher temperature since the cooling of the steam is avoided prior to entering the fibrous web. Therefore, a lower hot air temperature can be selected. 
     The current invention also relates to a machine for the production of a tissue web which is characterized in that it includes an inventive drying apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic depiction of a conventional drying apparatus which operates with steam only, as well as of the corresponding dry content increase and the corresponding temperature progression; 
         FIG. 2  is a schematic depiction of a conventional drying apparatus which operates only with hot air, as well as of the corresponding dry content increase and the corresponding temperature progression; 
         FIG. 3  is a schematic depiction of an embodiment of a machine for the production of a tissue web, including a drying apparatus of the present invention; and 
         FIG. 4  is a simplified schematic depiction of a drying apparatus, as well as of the corresponding dry content increase and the corresponding temperature progression of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a schematic depiction of a conventional drying apparatus which operates with steam only and includes one suction roll  12  with a suction zone  10 , and one steam blow box  14  in the initial area, opposite suction zone  10 . The tissue web  16  is carried over suction roll  12  between an inside dewatering fabric  18  or felt and a structured fabric  20 , together with an outside press belt  22  which, in this example is metal. Fabrics  18  through  20  respectively are permeable. Press belt  22  is carried over guide rolls  24  and in the area of suction zone  10 , presses fabrics  18  through  22 , as well as tissue web  16  against suction roll  12 . 
     The temperature T increases in the area of steam blow box  14 . Subsequently tissue web  16  cools off drastically inside suction zone  10 , with the taken in ambient air. As seen in  FIG. 1 , a dry content increase of approximately 0.2% occurs, however only in the area of steam blow box  14 . 
     Now, additionally referring to  FIG. 2  there is shown a schematic depiction of a conventional drying apparatus which operates with hot air only. This drying apparatus includes a suction roll  12  with a suction zone  10  and a hot air hood  26  opposite suction zone  10 , which extents across its entire width when viewed in the direction of web travel L. Tissue web  16  is again carried over suction zone  10  of suction roll  12  between a permeable dewatering fabric  18  or felt and a permeable structured fabric  20 , together with an outside permeable metal press belt  22 . With this drying apparatus in which tissue web  16  is dried by hot air flowing through it the dry content increase D amounts to approximately 1.5%. The temperature T increases only insignificantly in the area of the suction zone  10  and hot air hood  26 . 
     Now, additionally referring to  FIG. 3  there is shown a schematic depiction of an embodiment of the present invention in the form of machine  28  for the production of a fibrous web, in this case, for example, a tissue web, with an apparatus  30 . Drying apparatus  30  includes a suction roll  32  with a suction zone  34 , which is defined by an integrated suction box, and a hot air hood  36 , which is allocated to suction roll  32 . 
     Fibrous web  38  is carried over suction roll  32  together with a permeable structured fabric  40 , whereby fibrous web  38  is located between permeable structured fabric  40  and suction roll  32 . A permeable press belt  80 , which is under high pressure, is wrapped around suction roll  32  on the outside in the area of suction zone  34 , thereby creating a belt press  80 . Press belt  80  which is merely indicated in  FIG. 1  is more clearly recognizable in  FIG. 4 . The hot air flows from hot air hood  36  successively through permeable press belt  80 , permeable structured fabric  40  and fibrous web  38  into suction zone  34  of suction roll  32 . 
     In addition, dewatering fabric  42 , for example felt which is located between suction roll  32  and permeable structured fabric  40  and through which the hot air flows into suction zone  34  of suction roll  32 , can be guided around suction roll  32 . In the present example therefore the hot air flows successively through permeable press belt  80 , permeable structured fabric  40 , fibrous web  38  and dewatering fabric  42 . 
     Moving fibrous web  38  is treated with hot air by a drying zone, whereby this drying zone can be defined by a hot air hood  36 . Moreover, this drying zone can extend, at least essentially over suction zone  34  of suction roll  32 , or for example also beyond it, viewed in the direction of web travel L. 
     According to the present invention fibrous web  38  is now treated with hot air in the area of this drying zone, and at least in some areas with steam. 
     To this end fibrous web  38  may be treated with steam at least at the beginning of the drying zone, viewed in direction of web travel L. In the present example according to  FIG. 3  and viewed in direction of web travel L, fibrous web  38  is treated only at the beginning of this drying section with steam. Viewed in direction of web travel it is initially treated with steam and subsequently with hot air. 
     At least one steam blow pipe or steam blow device  44 , such as a steam blow pipe or steam blow box is provided for treatment of fibrous web  38  with steam. In the present example this steam blow device  44  includes a steam blow pipe, located preferably at the beginning of the drying zone. 
     The steam blow device  44  can extend across the entire width of hot air hood  36 , measured across the direction of web travel L. Advantageously it is located at least partially inside hot air hood  36 . 
     As can be seen in the example depicted in  FIG. 4 , steam blow device  44  may also include, at least one steam blow box  44 . In this case too steam blow box  44  is located again at the beginning of the drying zone which is defined by hot air hood  36  and is located inside hot air hood  36 . Therefore, in this arrangement too, fibrous web  38  is initially treated with steam and subsequently with hot air. 
     As can be seen in  FIG. 3 , a device such as a doctor blade  46  or similar devices can be provided in order to remove the boundary layer of air which is carried along by outer permeable structured fabric  40  covering fibrous web  38 , before fabric  40  enters into the drying zone. 
     In addition machine  28  includes a former with two dewatering fabrics  40  and  48  running together, whereby in the existing example the inside fabric is the permeable structured fabric  40 . The two dewatering fabrics  40  and  48  run together, thereby forming a stock infeed nip  50  and are carried over a forming element  52 , especially a forming roll  52 . 
     In the existing example permeable structured fabric  40  is in the embodiment of the inside dewatering fabric of the former, which is in contact with forming element  52 . Outside dewatering fabric  48  which is not in contact with forming element  52  is separated again from fibrous web  38  subsequent to forming element  52 . 
     The fibrous stock suspension is fed into the stock infeed nip  50  by way of a headbox  54 . 
     A suction element  56  is provided between forming element  52  and drying apparatus  30 , through which fibrous web  38  is held on permeable structured fabric  40  or, respectively is pressed against permeable structured fabric  40 . 
     After drying apparatus  30 , dewatering fabric  42  is again separated from permeable structured fabric  40 . Moreover, a pickup or separation element  58  is provided after drying apparatus  30  through which fibrous web  38  is held to permeable structured fabric  40  during the separation from dewatering fabric  42 . 
     Subsequent to this fibrous web  38 , together with permeable structured fabric  40 , is run through a press nip  64  which is formed preferably by a drying cylinder  60  in the embodiment of a Yankee-Cylinder  60  and a press element  62 , for example a press roll  62 . In the present arrangement press element  62  is for example a shoe press roll  62 . Following press nip  64  permeable structured fabric  40  is separated again from drying cylinder  60  while fibrous web  38  remains on drying cylinder  60 . A hood  66  is allocated to drying cylinder  60 . 
     A vacuum box with a hot air hood  68  or similar device can optionally be provided between suction roll  32  and drying cylinder  60 , in order to increase the sheet rigidity. 
     The hot air for hot air hood  36  which is allocated to suction roll  32  can be taken at least partially from hood  66  which is allocated to drying cylinder  60 . The hot air taken from hood  66  has a temperature in the range of approximately 300° C. which, as a rule is higher than is required for the hot air of hot air hood  36 . 
     As can be seen in  FIG. 3  the hot air taken from hood  66  which is allocated to drying cylinder  60  can be supplied to hot air hood  36  via a supply line  70  in which at least one valve  72 , especially a control valve can be located. In addition a filter  74  may also be provided, if required, in this supply line  70  for the removal specifically of short fibers, dust or similar substances. Finally, a ventilator may also be located in supply line  70 . 
     The hot air taken from hood  66  which is allocated to cylinder  60  can also be mixed with cold air that is supplied through a line  76 . Also in line  76  a valve  78 , especially a control valve, can be provided for the cold air that is to be supplied. The temperature of the air supplied to hot air hood  36  can therefore be adjusted through the mixing ratio of the hot air taken from hood  66  and the cold air. 
       FIG. 4  shows a simplified depiction of a modified design variation of the inventive drying apparatus  30 . As already mentioned, in this arrangement steam blow device  44  includes a steam blow box  44  located at least essentially inside hot air hood  36 , in place of the steam blow pipe. Viewed in direction of web travel L steam blow box  44  is located at the beginning of the drying zone which is defined here at least essentially by hot air hood  36 . 
     The present example distinguishes itself from that in  FIG. 3  moreover in that in addition to the permeable structured fabric  40  and the dewatering fabric  42  or felt a permeable press belt  80  is routed through the drying zone together with the fibrous web  38 , by way of which permeable structured fabric  40 , fibrous web  38  and permeable dewatering fabric  42  are pressed against the suction roll in the area of suction zone  34 . 
     Viewed in direction of web travel L press belt  80  is routed around a guide roll  82  before and after the drying zone respectively through which the appropriate tension for press belt  80  is produced. 
     As can be seen in  FIG. 4 , a relatively high temperature T results opposite the entire suction zone which in this arrangement, also defines the drying zone. Correspondingly, a relatively high dry content increase also occurs—in this instance approximately 3%. 
     While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.