Abstract:
A papermaker&#39;s fabric, usable with a dryer section on a paper machine, has a first layer and a second layer of cross-machine-direction (CD) yarns. Interwoven with the CD yarns in a duplex weave is a system of MD yarns. The MD yarns are provided in groups of at least two adjacent MD yarns. Each group has one first MD yarn and one or more second MD yarn. The first MD yarn interweaves between the first and second layers of CD yarns, each time binding with only one CD yarn. Each second MD yarn also interweaves between first and second layers of CD yarns, each time binding with only one CD yarn in the first layer, but floating over at least two CD yarns in the second layer. The first MD yarn is between the one or more second MD yarns in its group and those of an adjacent group, and defines a continuous air channel on the surface of the fabric.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker&#39;s or dryer fabric for use on the dryer section of a paper machine, such as on a single-run dryer section. 
     During the papermaking process, a fibrous web is formed by depositing a fibrous slurry on a forming fabric in the forming section of a paper machine. A large amount of water drains from the slurry through the forming fabric, leaving the fibrous web on the surface thereof. 
     The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom. This water is accepted by the press fabric or fabrics and, ideally, does not return to the web. 
     The web, by now a sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders which are heated from within by steam. The sheet is directed in a serpentine path sequentially around each in the series of drums by one or more dryer fabrics, which hold it closely against the surfaces of the drums. The heated drums reduce the water content of the sheet to a desirable level through evaporation. 
     In a dryer section, the dryer cylinders may be arranged in a top and a bottom row or tier. Those in the bottom tier are staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it passes alternately between the top and bottom tiers as it passes first around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section. 
     As shown in  FIG. 5 , in dryer sections, the top and bottom tiers of dryer cylinders may each be clothed with a separate dryer fabric  99 . In such a situation, paper sheet  98  being dried passes unsupported across the space, or “pocket”, between each dryer cylinder and the next dryer cylinder on the other tier. 
     In a single tier dryer section, a single row of cylinders along with a number of turning cylinders or rolls may be used. The turning rolls may be solid or vented. 
     In order to increase production rates and to minimize disturbance to the sheet, single-run dryer sections are used to transport the sheet being dried at high speeds. In a single-run dryer section, such as that shown in  FIG. 6 , a paper sheet  198  is transported by use of a single dryer fabric  199  which follows a serpentine path sequentially about dryer cylinders  200  in the top and bottom tiers. 
     It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks. “Drop off” can also reduce machine efficiency if it leads to sheet breaks. 
     Many paper mills have addressed this problem by machining grooves into the dryer cylinders or rolls or by adding a vacuum source to those dryer rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric, although both are expensive. 
     The present invention provides a solution to this problem in the form of a dryer fabric having void volume on the surface which does not come into contact with the paper web, that is, on the backside surface. The void volume gives the air carried into the compression wedge somewhere to go other than through the fabric. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is a dryer fabric, although it may find application in any of the forming, press and dryer sections of a paper machine. 
     The papermaker&#39;s fabric includes a first layer and a second layer of cross-machine-direction (CD) yarns. Interwoven with the CD yarns is a system of machinedirection (MD) yarns. 
     The MD yarns are provided in groups of at least two adjacent MD yarns. Each group has a first MD yarn and at least one second MD yarn. 
     The first MD yarn in each group is interwoven with the CD yarns of the first and second layers in a duplex weave, binding with only one CD yarn of the first layer and with only one CD yarn of the second layer when so interweaving. 
     The second MD yarn or yarns in each group is also interwoven with the CD yarns of the first and second layers in a duplex weave. When a group includes more than one second MD yarn, they weave with the CD yarns side by side as one yarn. The second MD yarn or yarns bind with only one CD yarn of the first layer when interweaving therewith, but float over at least two consecutive CD yarns of the second layer when interweaving therewith. 
     The first MD yarn in each group is between the one or more second MD yarns in the same group and a second MD yarn of an adjacent group. As such, the first MD yarns form continuous air channels between second MD yarns separated by them. 
     The fabric is disposed on the dryer section in endless form, such that the continuous air channels reside on the inner, or back-side, surface thereof. The continuous air channels provide void volume for air carried into the compression wedge formed between the fabric and a dryer cylinder when the fabric is used on a dryer section such as a single-run dryer section. 
     The present invention will now be described in more complete detail with frequent reference being made to the drawing figures, which are identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of the back-side surface of a papermaker&#39;s fabric according to an embodiment of the present invention; 
         FIG. 2  is a plan view of the paper-contacting surface of the papermaker&#39;s fabric of  FIG. 1 ; 
         FIG. 3A  is a cross-sectional view taken in the warpwise direction as indicated by line  3 — 3  in  FIG. 1 ; 
         FIG. 3B  is a cross-sectional view of a papermaker&#39;s fabric according to another embodiment of the present invention; 
         FIG. 4  is a cross-sectional view taken in the weftwise direction as indicated by line  4 — 4  in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of a dryer section; and 
         FIG. 6  is a cross-sectional view of a single-run dryer section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to these figures,  FIG. 1  is a plan view of the back-side surface  12  of the papermaker&#39;s fabric  10  of the present invention. In  FIG. 1 , the machine direction (MD) and cross-machine direction (CD) are as indicated. The spacing between the yarns of the papermaker&#39;s fabric  10  in this and other figures is exaggerated for the sake of clarity.  FIG. 1  shows two repeats of the weave pattern side by side one another. 
       FIG. 3A  is a cross-sectional view, taken as indicated by line  3 — 3  in FIG.  1 . It will be observed that fabric  10  includes two layers of CD yarns. As fabric  10  may be flat woven and subsequently joined into endless form with a seam, the CD yarns are weft, or filling, yarns in the process by which fabric  10  is produced. A first layer  14  of CD yarns includes CD yarns  21 , 23 , 25 , 27 , 29 , 31 , while a second layer  16  of CD yarns includes CD yarns  22 , 24 , 26 , 28 , 30 , 32 . As is apparent in  FIGS. 1 and 3A , the CD yarns in the two layers  14 , 16  are not in vertically stacked positions. Rather they alternate with one another in machine direction of the fabric  10 , so that both layers are visible in the view presented in FIG.  1 . In reality, CD yarns  21 , 23 , 25 , 27 , 29 , 31  of the first layer  14  may barely be visible on the back-side surface  12  of the actual fabric  10  as the spacing between the yarns is quite small. 
     Returning now to  FIG. 1 , MD yarns  41 - 52 , which are warp yarns in the process by which the fabric is woven, may be flat monofilament yarns having cross sections of substantially rectangular shape. The cross-sectional shape of MD yarns  41 - 52  is shown in  FIG. 4 , a cross-sectional view taken in the weftwise direction as indicted by line  4 — 4  in FIG.  1 . 
     MD yarns  41 - 52  are arranged in groups of three in which two MD yarns are twinned and weave as one with the CD yarns  21 - 32 . Specifically, MD yarns  42 , 43 ; MD yarns  45 , 46 ; MD yarns  48 , 49 ; and MD yarns  51 , 52  are twinned pairs, which are separated from those adjacent thereto by MD yarns  41 , 44 , 47 , 50 . These latter MD yarns  41 , 44 , 47 , 50  define continuous air channels  60  on the back-side surface  12  of the fabric  10  in a manner to be described below. 
     The twinned MD yarn pairs form long floats on the back-side surface  12  of the fabric  10 . Specifically, MD yarns  42 , 43  weave under CD yarns  21  and CD yarns  22 , over CD yarns  23 - 31 , and under CD yarns  32  in each repeat of the weave pattern, whereby MD yarns  42 , 43  float over four consecutive CD yarns  24 , 26 , 28 , 30  of the second layer  16  on the back-side surface  12  of the fabric  10 . MD yarns  48 , 49  weave in the same manner as MD yarns  42 , 43 . 
     Similarly, MD yarns  45 , 46  weave over CD yarns  21 - 25 , under CD yarns  26 - 28 , and over CD yarns  29 - 32  in each repeat of the weave pattern, whereby MD yarns  45 , 46  float over four consecutive CD yarns  30 , 32 , 22 , 24  of the second layer  16  on the back-side surface  12  of the fabric  10 . MD yarns  51 , 52  weave in the same manner as MD yarns  45 , 46 . The floats formed by MD yarns  45 , 46  and MD yarns  51 , 52  are offset in the machine direction from those formed by MD yarns  42 , 43  and MD yarns  48 , 49  by six CD yarns. 
     MD yarns  41 , 44 , 47 , 50 , which separate the twinned MD yarn pairs from one another, weave over three CD yarns and under the following three CD yarns in a repeating pattern. Specifically, MD yarns  41 , 47  weave over CD yarns  21 , 22 , 23 , under CD yarns  24 , 25 , 26 , over CD yarns  27 , 28 , 29 , and under CD yarns  30 , 31 , 32  in each repeat of the weave pattern. On the other hand, MD yarns  44 , 50  weave over CD yarn  21 , under CD yarns  22 , 23 , 24 , over CD yarns  25 , 26 , 27 , under CD yarns  28 , 29 , 30 , and over CD yarns  31 , 32 . As such, MD yarns  44 , 50  weave with the CD yarns in a manner that is offset in the machine direction from the manner in which MD yarns  41 , 47  so interweave by two CD yarns. 
     With particular reference to  FIGS. 1 and 3A , it will be noted that MD yarn  41 , and MD yarn  47  which weaves in the same manner, does not have a long float on the back-side surface  12  of fabric  10 . Instead, MD yarns  41 , 47  weave over only CD yarns  22 , 28  of the second layer  16 , and tend to pull CD yarns  22 ,  28  inwardly with respect to the back-side surface  12 , so that the knuckles formed by MD yarns  41 , 47  when weaving with CD yarns  22 , 28  are inward of the floats formed by MD yarns  42 , 43 ;  45 , 46 ;  48 , 49 ; and  50 , 51 . As a consequence, MD yarns  41 , 47  are protected from heat and abrasion on the back-side surface  12  of the fabric  10 . 
     Similarly, MD yarn  44 , and MD yarn  50  which weaves in the same manner, also does not have a long float on the back-side surface  12  of fabric  10 . Instead, MD yarns  44 , 50  weave over only CD yarns  26 , 32  of the second layer  16 , and tend to pull CD yarns  26 , 32  inwardly with respect to the back-side surface  12 , so that the knuckles formed by MD yarns  44 , 50  when weaving with CD yarns  26 , 32  are also inward of the floats formed by MD yarns  42 , 43 ;  45 , 46 ;  48 , 49 ; and  50 , 51 . As a consequence, MD yarns  44 , 50  are also protected from heat and abrasion on the back-side surface  12  of the fabric  10 . 
     Because the knuckles formed when MD yarns  41 , 47  weave over CD yarns  22 , 28 , and when MD yarns  44 , 50  weave over CD yarns  26 , 32  are inward of the long floats formed by MD yarns  42 , 43 ;  45 , 46 ;  48 , 49 ;  50 , 51 , MD yarns  41 , 44 , 47 , 50  define continuous air channels  60  between these twinned pairs. Continuous air channels  60  provide a solution to the problem of “drop-off” in dryer sections such as single-run dryer sections. Continuous air channels  60 , which are oriented in the machine direction, perform the same function as is carried out by grooved dryer rolls. That is, they provide volume for air carried into and trapped in a compression wedge, thereby reducing the tendency for air to be forced through the fabric  10  entirely, where it may cause “drop off”. The void volume provided by continuous air channels  60  is different from that in other dryer fabric structures, both woven and spiral-link, because the void volume is continuous. Most dryer fabrics have some void volume, but generally the void volume is provided in discrete discontinuous pores or openings in the fabric. In the present invention, the void volume is continuous in a predetermined direction, such as in the machine direction. 
       FIG. 2  is a plan view of the paper-contacting surface  18  of the fabric  10 , and is the reverse of FIG.  1 .  FIGS. 2 and 3A  taken together show that MD yarns  41 - 52  bind with a single CD yarn  21 , 23 , 25 , 27 , 29 , 31  of the first layer  14  each time they weave to the first layer  14 . Specifically, MD yarns  41 , 47  bind with CD yarns  25 , 31  as they weave to the first layer  14  twice in each repeat of the weave pattern. Similarly, MD yarns  44 , 50  bind with CD yarns  23 , 29  as they weave to the first layer  14  twice in each repeat of the weave pattern. On the other hand, the twinned pairs of MD yarns  42 , 43 ;  48 , 49  bind with CD yarn  21  as they weave to the first layer  14  once in each repeat of the weave pattern, while the twinned pairs of MD yarns  45 , 46 ;  51 , 52  bind with CD yarn  27  as they weave to the first layer  14  once in each repeat of the weave pattern. As a consequence, CD yarns  21 , 23 , 25 , 27 , 29 , 31  make up most of the area of the paper-contacting surface  18  of the fabric, which surface  18  may appropriately be described as a shute-runner surface. In reality, CD yarns  22 , 24 , 26 , 28 , 30 , 32  of the second layer  16  may barely be visible on the paper-contacting surface  18  of the actual fabric  10  as the spacing between the yarns is quite small. In any event, the CD yarn-dominated nature of the paper-contacting surface  18  of the fabric  10  protects MD yarns  41 - 52  from heat and abrasion. 
     As an alternative to the arrangement previously described, the CD and MD yarns could be arranged so as to form a so-called monoplane surface wherein the CD and MD yarns both form the paper-contacting surface. Such monoplane surface arrangement would not affect the air channels. 
     The fabric  10  preferably comprises only monofilament yarns. Specifically, the CD yarns may be anticontaminant polyester monofilament. Such anticontaminant may be more deformable than standard polyester and, as a result, may more easily enable the fabric to be woven so as to have a relatively low permeability (such as 100 CFM) as compared to the more non-deformable yarns. The CD yarns may have a circular cross-sectional shape with one or more different diameters. For example, CD yarns  24 , 30  may have a diameter of 0.90 mm while CD yarns  21 - 23 ,  25 - 29 , 31 , 32  may have a diameter of 0.50 mm or 0.60 mm. That is, CD yarns  24 , 30  may be of larger diameter than the other CD yarns  21 - 23 ,  25 - 29 ,  31 , 32  as suggested in  FIGS. 1 ,  2 ,  3 A, and  4 . As twinned pairs of MD yarns  42 , 43 ;  45 , 46 ;  48 , 49 ; and  51 , 52  weave over CD yarns  24 , 30  when weaving up from or down to CD yarns  21 , 27  in the first layer  14 , the larger diameter of CD yarns  24 , 30  provides additional depth to the continuous air channels  60 . Alternatively, and as shown in  FIG. 3B , all of the CD yarns (i.e. CD yarns  21 - 32 ) may each have the same diameter such as 0.80 mm. The MD yarns  41 - 52  may be flat monofilament yarns of substantially rectangular cross-sectional shape. For example, the MD yarns  41 - 52  may have substantially rectangular cross sections which measure 0.44 mm by 0.88 mm, the longer dimension lying parallel to the plane of the back-side surface as shown in FIG.  4 . 
     The fabric  10  may be woven in a 6-harness repeat, although, in an alternate embodiment, it may be woven in a 4-harness repeat using single MD yarns of greater width in place of the twinned pairs of MD yarns shown in the figures. 
     CD yarns  21 - 32  may be monofilament yarns of circular cross section of any of the synthetic polymeric resins used in the production of such yarns for paper machine clothing. Polyester and polyamide are but two examples of such materials. Other examples of such materials are polyphenylene sulfide (PPS), which is commercially available under the name RYTON®, and a modified heat-, hydrolysis- and contaminant-resistant polyester of the variety disclosed in commonly assigned U.S. Pat. No. 5,169,499, and used in dryer fabrics sold by Albany International Corp. under the trademark THERMONETICS®. The teachings of U.S. Pat. No. 5,169,499 are incorporated herein by reference. Further, such materials as poly (cyclohexanedimethylene terephthalate-isophthalate) (PCTA), polyetheretherketone (PEEK) and others could also be used. 
     Further, in addition to a circular cross-sectional shape, one or more of the CD yarns may have other cross-sectional shapes such as a rectangular cross-sectional shape or a non-round cross-sectional shape. 
     As previously indicated, MD yarns  41 - 52  may be flat monofilament yarns of substantially rectangular cross-sectional shape. Alternatively, any or all of such MD yarns may have other cross-sectional shapes such as a circular cross-sectional shape or a non-round cross-sectional shape. Additionally, MD yarns  41 - 52  may be of any of the synthetic polymeric resins used in the production of yarns for paper machine clothing. Polyester and polyamide are but two examples, along with the other materials disclosed above. 
     The fabric  10  may be used with a single run or single tier dryer section. Alternatively, the fabric  10  may be used with other types of dryer sections, such as that shown in FIG.  5 . As is to be appreciated, in such situation, fabrics  99  would be replaced with fabrics  10 . 
     Modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the scope of the present invention. For example, while fabric  10  is typically flat-woven, and must be joined into endless form for use on the dryer section of a paper machine, it is also possible to produce the fabric  10  by endless weaving, in which case the MD yarns  41 - 52  would be weft yarns during the weaving process and the CD yarns  21 - 32  would be warp yarns. The claims to follow should be construed to cover such a situation.