Abstract:
An apparatus for dewatering a fiber web in a paper machine includes a heated drying surface, a permeable fabric carrying the fiber web, and a condensing region adjacent a portion of the permeable fabric on a side opposite the fiber web. The fiber web is interposed between and contacts each of the heated drying surface and the permeable fabric. The condensation region has: a) a temperature which is less than a temperature of the permeable fabric; and/or b) a pressure which is greater than an ambient pressure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a method and to an apparatus for manufacturing a fiber web, in particular a web of tissue or hygiene material, provided with a three-dimensional surface structure. It further relates to a method and an apparatus for drying a fiber web, in particular a web of tissue or hygiene material.  
         [0003]     2. Description of the Related Art  
         [0004]     The imprinting of a three-dimensional structure into the surface of a paper web, in particular of a tissue web, in particular of hand tissue, is known (see, for example, WO 99/47749, WO 01/18307). It is further known that a very good paper quality can be achieved by so-called through-air drying (TAD). However, it is disadvantageous that the use of TAD dryers is very complex and correspondingly expensive.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides an improved method and an improved apparatus of the kind initially mentioned with which in particular a high quality of the end product can be achieved in an economic and correspondingly favorably priced manner even without the use of a larger TAD drying apparatus. In this connection, a corresponding quality should be reached in particular with respect to the water retention capability, the water absorption rate, the bulk, etc.  
         [0006]     The present invention provides an additional drying device positioned upstream from a drying cylinder. The additional drying device includes a drying surface which evaporates moisture in the fiber web, and a condensation region with an associated higher pressure and/or colder temperature.  
         [0007]     The invention comprises, in one form thereof, an apparatus for dewatering a fiber web in a paper machine, including a heated drying surface, a permeable fabric carrying the fiber web, and a condensing region adjacent a portion of the permeable fabric on a side opposite the fiber web. The fiber web is interposed between and contacts each of the heated drying surface and the permeable fabric. The condensation region has: a) a temperature which is less than a temperature of the permeable fabric; and/or b) a pressure which is greater than an ambient pressure.  
         [0008]     The invention comprises, in another form thereof, a method of dewatering a fiber web, including the steps of: carrying the fiber web with a permeable fabric to a heated drying surface; contacting the fiber web on one side with the heated drying surface and on an other side with the permeable fabric; evaporating moisture in the fiber web using heat from the heated drying surface; and condensing the evaporated moisture in a condensing fabric on a side of the permeable fabric opposite the fiber web, the condensing fabric having: a) a temperature which is less than a temperature of the permeable fabric, and/or b) a pressure which is greater than an ambient pressure.  
         [0009]     An advantage of the present invention is that the moisture content of the fiber web is reduced by evaporation using the heated drying surface of the additional drying device.  
         [0010]     Another advantage is that the evaporated moisture is condensed in a region away from the permeable fabric using a higher pressure and/or lower temperature for subsequent removal.  
         [0011]     Yet another advantage is that the fiber web is drawn into the coarse permeable fabric prior to pressing so that the physical properties of the fiber web are not substantially adversely affected by the pressure in the “knuckle areas”.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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 embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a schematic illustration of an embodiment of a paper machine of the present invention; and  
         [0014]      FIG. 2  is a schematic illustration of another embodiment of a paper machine of the present invention. 
     
    
       [0015]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Referring now to the drawings, and more particularly to  FIG. 1 , there is shown an embodiment of a paper machine  10  of the present invention, which generally includes a double wire forming section  12 , a drying device  14 , and a press assembly  16  including a drying cylinder  18 .  
         [0017]     In the embodiment shown, double wire forming section  12  is a crescent former type forming section including a forming roll  20  carrying a permeable fabric  22  (instead of a conventional felt) and an outer forming wire  24  around a portion of the periphery thereof. Forming roll  20  may include an optional suction zone  26 . Suction device  28  is positioned at the downstream side of the extended nip formed between permeable fabric  22  and outer forming wire  24  and separates fiber web  30  from outer forming wire  24 . Fiber web  30  may be of any suitable type, such as a paper web or more particularly a tissue web. Wire forming section  12  receives fiber suspension from head box  32 , which forms fiber web  30 .  
         [0018]     Permeable fabric  22  is a coarse permeable fabric, such as a structured permeable fabric forming a structured three-dimensional fiber web. In one embodiment, permeable fabric  22  is configured as a through air drying fabric allowing air or another gaseous medium to flow therethrough.  
         [0019]     A wet suction box  34  is positioned adjacent to and in fluid communication with a portion of permeable fabric  22 . Wet suction box  34  is positioned between double wire forming section  12  and drying device  14  relative to the fiber web travel direction  36 .  
         [0020]     Drying device  14  includes a heated drying surface  38 , which is in the form of a heated drying cylinder in the embodiment shown. Drying device  14  is internally referred to as a “Boost Dryer” within the assignee of the present invention. Drying cylinder  38  has a surface temperature of between approximately 100° C. to 250° C.; and more particularly has a surface temperature of between approximately 120° C. to 180° C.; This surface temperature range has been found to be effective to evaporate a portion of the moisture within fiber web  30  carried by drying cylinder  38 .  
         [0021]     A condensation fabric  40  is positioned adjacent to permeable fabric  22  on a side opposite fiber web  30 . Condensation fabric  40  is a permeable fabric allowing the evaporated moisture to accumulate and condense therein, as will be described in more detail hereinafter. An impermeable membrane  42  is positioned adjacent to condensation fabric  40  on a side opposite permeable fabric  22 . Impermeable membrane  42  allows pressure to be applied to condensation fabric  40 , permeable fabric  42  and fiber web  30 , and also is thermally conductive.  
         [0022]     Drying device  14  also includes a pressurized hood  44  which surrounds and is substantially sealed with a portion of drying cylinder  38 . Pressurized hood  44  is in communication with a plurality of outlets  46 , one of which is visible in  FIG. 1 , which in turn are fluidly coupled with a header  48 . Header  48  is coupled on the discharge side thereof with a pump  50 , which in turn pressurizes the circulating fluid medium (e.g., pressurized steam, water, air or other gas) and discharges the pressurized fluid medium to one or more inlets  52  in communication with pressurized hood  44 . Pressurizing the fluid medium using pump  50  not only increases the pressure of the fluid medium to a pressure greater than ambient pressure, but also reduces the temperature of the fluid medium. In one embodiment, the fluid medium circulated within pressurized hood  44  is pressurized to a pressure of between approximately 0.5×10 5  N/m 2  and 1×10 6 N/m 2 .  
         [0023]     Water which is condensed within the pores of condensation fabric  40  is removed from condensation fabric  40  by conditioners/suction device  54  positioned on opposite sides of condensation fabric  40  in the return loop after exiting from the discharge side of drying cylinder  38 .  
         [0024]     Fiber web  30  exits drying device  14  and is carried from drying device  14  on the bottom side of permeable fabric  22 . Optional cooling device  55  is positioned adjacent permeable fabric  22  on a side opposite from fiber web  30 .  
         [0025]     Positioned downstream from cooling device  55  is turning roll  56  located adjacent permeable fabric  22  on a side opposite from fiber web  30 . Turning roll  56  is positioned between drying device  14  and press assembly  16  relative to fiber web travel direction  36 . Turning roll  56  is in the form of a suction turning roll with a suction zone  58  in the embodiment shown in  FIG. 1 .  
         [0026]     An additional suction box (not shown) can be positioned upstream from turning roll  56 , between turning roll  56  and drying device  14 .  
         [0027]     Press assembly  16  includes press member  60  defining a press nip with drying cylinder  18 . In the embodiment shown, press member  60  is in the form of a suction press roll, but may also be configured differently, such as a shoe press roll, etc. Fiber web  30  and permeable fabric  22  travel through the press nip between press member  60  and drying cylinder  18 . Fiber web  30  is immediately adjacent to drying cylinder  18 , and permeable fabric  22  is immediately adjacent to press member  60 . Drying cylinder  18  and press member  16  are each positioned downstream from drying device  14  relative to web travel direction  36 .  
         [0028]     Drying cylinder  18  preferably is configured as a Yankee drying cylinder having a large diameter and corresponding large circumference. Drying cylinder  18  thus provides the dual functionality of both acting as a drying cylinder and being used as part of press assembly  16 . A creping doctor  62  is positioned on the downstream side of drying cylinder  18 .  
         [0029]     A hot air drying hood  64  is positioned adjacent to and partially surrounds drying cylinder  18 . Hot air drying hood  64  thus is in fluid communication with permeable fabric  22  and fiber web  30 . Hot air drying hood  64  further assists in drying of fiber web  30  as it is carried about the periphery of drying cylinder  18 .  
         [0030]     Using a paper machine  10  as shown in  FIG. 1 , a paper web, especially a tissue web, is dried prior to drying cylinder  18  by evaporating a part of the water in fiber web  30  at a pressure which is higher than the ambient pressure. During the drying process, fiber web  30  is in contact with a heated drying surface  38  (e.g., drying cylinder). The other side of fiber web  30  is in contact with and is transported by thin, permeable fabric  22 . The water vapor passes through permeable fabric  22  and condenses in the colder condensation fabric  40 . Pressure is provided by pressurized hood  44 . Paper machine  10  is capable of producing a bulky tissue with a high water capacity (kg water/kg fiber). The structure of fiber web  30  is provided by forming or by suctioning wet fiber web  30  into the coarse fabric structure of permeable fabric  22  using suction box  34 . This formed structure of fiber web  30  is not destroyed by the high efficiency drying process using paper machine  10 . The drying rate of drying device  14  is above 400 kg/hrm 2  and preferably above 500 kg/hrm 2 .  
         [0031]     The sheet absorbency capacity as measured by the basket method, for a nominal 20 gsm web is equal to or greater than 12 grams water per gram of fiber and often exceeds 15 grams of water per gram fiber. The sheet bulk is equal to or greater than 10 cm 3 /gm and preferably greater than 13 cm 3 /gm. The sheet bulk of toilet tissue is expected to be equal to or greater than 13 cm 3 /gm before calendering.  
         [0032]     With the basket method of measuring absorbency, five (5) grams of paper are placed into a basket. The basket containing the paper is then weighted and introduced into a small vessel of water at 20° C. for 60 seconds. After 60 seconds of soak time, the basket is removed from the water and allowed to drain for 60 seconds and then weighted again. The weight difference is then divided by the paper weight to yield the grams of water held per gram of fibers being absorbed and held in the paper.  
         [0033]     More particularly, during use, head box  32  discharges a fiber suspension into the forming section between permeable fabric  22  and outer forming wire  24 . Fiber web  30  is suctioned using wet suction box  34  to control sheet moisture. Fiber web  30  is then dried in drying device  14  and is transferred to the press nip between two press members. One press member is Yankee cylinder  18  and the other press member is a press roll  60 , such as a suction press roll or preferably a shoe press roll. The press nip can be preceded by suction turning roll  56  and/or cooling device  55 . Fiber web  30  is then dried using Yankee cylinder  18 , preferably also with hot air drying hood  64 , and is creped by doctor  62 .  
         [0034]     Pressurized hood  44  is pressurized by a hot fluid, e.g., pressurized steam or water. The temperature of fluid medium in pressurized hood  44  is less than the temperature of heated drying cylinder  38 . The condensed water in condensation fabric  40  is suctioned out by suction boxes  54 . Impermeable membrane  42  fluidly separates the fluid in pressurized hood  44  from condensation fabric  40 .  
         [0035]      FIG. 2  illustrates yet another embodiment of a paper machine  70  of the present invention. Paper machine  70  generally includes a double wire forming section  72 , drying device  14 , and press nip  16  defined by drying cylinder  18  and press roll  60 . It will be readily apparent upon comparison of  FIG. 2  with  FIG. 1  that the primary difference between paper machines  10  and  70  is the configuration of double wire forming section  72  with respect to double wire forming section  12  shown in  FIG. 1 . Particularly, in the embodiment shown in  FIG. 2 , double wire forming section  72  is in the form of a twin wire former including an upper wire  74  and a lower wire  76 . Forming roll  78  includes a suction zone  80  similar to forming roll  20  shown in  FIG. 1 . Double wire forming section  72  also includes a suction device  82  positioned immediately downstream from forming roll  78 . The structure and operation of paper machine  70  otherwise is similar to that of paper machine  10  described above, and thus will not be described in further detail.  
         [0036]     It is also possible to configure the double wire forming section as a fourdrinier wire former or an inclined wire former (not shown).  
         [0037]     According to the embodiments of paper machines  10  and  70  shown in  FIGS. 1 and 2 , drying device  14  is positioned upstream from a further drying cylinder in the form of a Yankee cylinder  18 . However, it is also possible in another embodiment (not shown) to increase the diameter of drying cylinder  38  such that fiber web  30  has a longer contact path around the periphery of drying cylinder  38 . This increases the drying effectiveness of drying cylinder  38 , and allows Yankee cylinder  18  to be eliminated in this embodiment.  
         [0038]     While this invention has been described as having a preferred design, 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.