Abstract:
A twin fabric hybrid forming section for paper making machine is described in which: the pitch of the fabric support elements decreases progressively in the machine direction; the level of vacuum applied to the forming fabrics through the dewatering boxes increases in the machine direction; the two forming fabrics together with the stock sandwiched between them traverse at least four separate and distinct vacuum zones within the forming section as they proceed in the machine direction; the level of vacuum applied to the last of the at least four separate and distinct vacuum zones is higher than the level of vacuum applied to the first of the separate and distinct vacuum zones; the level of vacuum applied to the at least four separate and distinct vacuum zones follows a preselected profile; and the dewatering boxes carrying the fabric support elements are arranged so that the fabric support elements are located in an alternating sequence on the machine sides of both of the forming fabrics.

Description:
BACKGROUND 
     This invention relates to a twin fabric hybrid forming section for use in a paper making machine. In a hybrid forming section the stock jet is ejected from a headbox slice onto a first forming fabric that is travelling in a horizontal plane in the machine direction over a series of dewatering boxes comprising a conventional open surface single fabric forming section. A second forming fabric is then brought into intimate contact with the exposed upper sheet surface at the beginning of the hybrid two fabric forming section. The partially formed sheet and the undrained stock is sandwiched between two forming fabrics; drainage then occurs through both forming fabrics. The second forming fabric is separated from the upper surface of the formed sheet at the end of the hybrid two fabric forming section and the sheet is conveyed to the press section on the first forming fabric. This invention is concerned with that portion of the hybrid two fabric forming section between the locus at which the first and second forming fabrics come together to sandwich the stock between them and the locus at which the first and second forming fabrics separate with the sheet continuing on the first forming fabric. Although the forming section described here includes a single second forming fabric section this invention is not so limited. It is common to have more than one hybrid two fabric forming section, and to have a second headbox delivering a second layer of stock onto the first forming fabric ahead of the second hybrid two fabric forming section. 
     In a hybrid type forming section the two forming fabrics do not follow a linear path. The fabrics together pass over a sequence of rolls and dewatering boxes which are located on alternate sides of the two fabrics and thus define the sinuous path of the two fabrics. Each dewatering box has a curved surface, which carries a group of fabric support elements, such as blades, which are in contact with the machine sides of the forming fabrics. Each dewatering box may also be connected to a source of controlled vacuum. These curved surfaces cause the moving forming fabrics to follow the desired sinuous path. The application of a controlled level of vacuum to the dewatering boxes has two effects: it promotes the removal of water from the stock between the two moving forming fabrics, and it deflects the path of the two moving forming fabrics into the gaps between the fabric support elements. This deflection of the two moving forming fabrics generates a positive pressure pulse within the stock layer sandwiched between them that creates fluid movement within the stock in the machine direction; this causes a shearing action within the stock which serves to break up fibre flocs. 
     The actual magnitude of each pressure pulse generated by the deflection angle of the moving forming fabrics at the edges of each fabric support element has a significant impact on the quality of the final sheet produced. The strength of the pressure pulse generated by each fabric support element should be chosen to match the stock conditions and properties at that fabric support element. Hence, there exists a need to be able to modify the strength and/or magnitude of the pressure pulses as more water is drained from the stock and the incipient paper web is formed. 
     Poor control of the fabric deflection within the forming section has been found to have an adverse effect on the formation process, which will in turn have a negative impact on the quality of the paper product being made. 
     The actual fabric deflection angle at the edge of each fabric support element in an operating twin fabric forming section has been found to be controlled by several factors. These include: 
     1. the geometric layout of the physical components used in the construction of the forming zone; including the element-to-element pitch for the fabric support elements, the machine direction width of the fabric support elements, and the radius of curvature of the surfaces to which the fabric support elements are attached; 
     2. the level of vacuum applied to the dewatering boxes which controls the degree to which the moving forming fabrics are deflected into the gaps between the fabric support elements; and 
     3. the amount of machine direction tension applied to each of the two moving forming fabrics. 
     As used herein, then following terms are to be taken to have the following meanings: 
     (i) the term machine direction, or MD, refers to a direction generally parallel to the direction of movement of the forming fabrics away from a headbox slice; 
     (ii) the term “pitch” refers to the centre to centre spacing of successive fabric support elements in the machine direction; and 
     (iii) the terms “fabric support element” and “fabric support elements” refer: 
     either to moving surfaces such as rolls over which a forming fabric moves in rolling contact, 
     or to static surfaces such as blades, foils or the like over which a forming fabric moves in sliding contact. 
     In the initial stages of sheet formation, when the level of vacuum applied to the machine side of the forming fabric, and consequently to the incipient paper web, is low, the predominant factors controlling forming fabric deflection are the geometry of the forming section and the tension applied to both of the forming fabrics. Further, although the tension applied to the two forming fabrics is usually the same, two different tension levels can be used. The two tensions are set, within the overall pattern of adjustments, to obtain the desired level of pressure pulses within the stock sandwiched between the two moving forming fabrics. 
     From the point at which the stock is first sandwiched between the two moving forming fabrics until the point at which the two forming fabrics separate, the consistency of the stock is continually increasing as water is drained from the incipient paper web. At the same time as the stock consistency increases, there is also a corresponding decrease in individual fiber mobility within the stock. These changes require a stronger pressure pulse to provide beneficial fiber movement which will improve the sheet properties in the incipient paper web. However, the incipient paper web eventually reaches a consistency at which no further beneficial fiber movement can occur. From that point onwards until the two moving forming fabrics separate the pressure pulse strength must be controlled by careful selection of the required vacuum level so that drainage continues, and by careful selection of the radius, fabric support element pitch and fabric support element width so that the pressure pulse strength is controlled to a level which will not act to impair formation of the incipient paper web. 
     During the initial sheet forming period where beneficial fiber movement can still occur, the need for a larger pressure pulse may increase at a faster rate than can be achieved by control of the vacuum level applied to the forming fabrics alone. This is because the vacuum level must be limited to a value which does not cause excessive drainage which will both reduce fiber mobility and set the sheet properties before the desired formation benefits can be achieved. It is therefore essential to obtain a larger pressure pulse by causing a higher deflection of the forming fabrics at the edges of the fabric support elements by utilizing a wider pitch between them and/or by utilizing a higher radius of curvature in the structure to which the fabric contacting fabric support elements are attached, and/or by utilizing opposed fabric support elements, such as blades, located to increase fabric deflection into the gaps between the fabric support elements. 
     SUMMARY 
     It is thus apparent that there is a matrix of variables which must be considered in order to optimise the quality of the sheet product. The present invention is based on the realization that the following factors must to be taken into account in the creation of an improved twin fabric hybrid type forming section for paper making machine: 
     (a) the pitch of the fabric support elements should decrease progressively in the machine direction; 
     (b) the level of vacuum applied to the forming fabrics through the dewatering boxes should increase in the machine direction; 
     (c) the two forming fabrics together with the stock sandwiched between them should traverse at least four separate and distinct vacuum zones within the forming section as they proceed in the machine direction; 
     (d) the level of vacuum applied to the last of the at least four separate and distinct vacuum zones must be higher than the level of vacuum applied to the first of the separate and distinct vacuum zones; 
     (e) the level of vacuum applied to the at least four separate and distinct vacuum zones must follow a preselected profile; and 
     (f) the dewatering boxes carrying the fabric support elements should be arranged so that the fabric support elements are located in an alternating sequence on the machine sides of both of the forming fabrics. 
     Thus in a first broad embodiment this invention seeks to provide a two fabric hybrid type forming section for a paper making machine having a first forming fabric and at least one second forming fabric, such that: 
     (i) each of the forming fabrics has a paper side and a machine side; 
     (ii) the forming fabrics move together in close proximity with each other in the machine direction with a layer of stock sandwiched in between; 
     (iii) the forming fabrics are supported by a series of rolls and/or a series of static fabric contacting fabric support elements over which the machine sides of each of the forming fabrics pass in sliding contact, the fabric support elements being supported on a sequence of dewatering boxes, the dewatering boxes each having a curved fabric support element supporting surface; and 
     (iv) the dewatering boxes provide separate drainage zones at least some of which are connected to a source of vacuum to provide separate vacuum zones, 
     wherein: 
     (a) the forming zone comprises that portion of the forming section between the locus at which the forming fabrics come together to sandwich the stock between them and the locus at which the two forming fabrics separate with the stock continuing on one of them; 
     (b) the dewatering boxes provide at least four separate and distinct vacuum zones within the forming section; 
     (c) either: the radii of curvature of the curved surfaces located over those dewatering boxes which are connected to a source of vacuum supporting the fabric supporting elements decreases progressively in the machine direction, 
     or: the radii of curvature of the curved surfaces located over those dewatering boxes which are connected to a source of vacuum supporting the fabric support elements decreases on successive support surfaces in the machine direction; 
     (d) either: the pitch of the fabric support elements within each vacuum zone is constant, and the pitch of the fabric support elements on successive vacuum zones decreases in the machine direction; 
     or: the pitch of successive fabric support elements within each vacuum zone decreases in the machine direction. 
     (e) the dewatering boxes supporting the fabric support elements are constructed and arranged to locate the fabric support elements in contact with the machine sides of the first forming fabric and the second forming fabric in an alternating sequence in the machine direction; 
     (f) on all of the dewatering boxes: 
     either: all of the fabric support elements are the same width in the machine direction; 
     or: all of the fabric support elements are not the same width in the machine direction. 
     Preferably, the fabric support element pitch within each vacuum zone is constant, and the fabric support element pitch within successive vacuum zones decreases in the machine direction. Alternatively, the fabric support element pitch within each vacuum zone is not constant, and the fabric support element pitch within each successive vacuum zone decreases in the machine direction. 
     Preferably, the radii of curvature of the curved surfaces supporting the fabric support elements on successive vacuum zones decreases in the machine direction. Alternatively, the radii of curvature of the curved surfaces supporting the fabric support elements on successive vacuum zones decrease progressively in the machine direction. 
     Preferably, each dewatering box provides at least one vacuum zone. More preferably, at least one dewatering box provides at least two vacuum zones. Most preferably all of the dewatering boxes provide more than one vacuum zone. 
     Preferably, the ratio of the width of the fabric support elements to the width of the gap between them varies from about 1:10 down to about 1:0.5. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the attached figures in which: 
         FIG. 1  shows schematically a two fabric hybrid type forming section according to first embodiment of the invention; 
         FIG. 2  shows schematically in more detail the hybrid forming zone of  FIG. 1 ; 
         FIG. 3  shows schematically an alternative construction to  FIG. 2 ; and 
         FIG. 4  shows schematically a further alternative construction to that shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIG. 1 , a two fabric hybrid type forming section  1  is shown. The forming section  1  is arranged substantially horizontally; the arrow A indicates the horizontal direction. 
     In the forming section of this invention, the formation zone  60  where the sheet is formed on the first forming fabric  2  extends from the breast roll  50  to the couch roll  57 . A layer of stock  7  is ejected from the headbox slice  8  onto the first forming fabric  2 . Within this zone  60  the two fabric hybrid forming section extends from the locus where the first forming fabric  2  carrying the layer of stock  7  contacts the second forming fabric  4  at lead-in box  53  sandwiching the stock  7  between them, to the locus of the turning roll  9  and transfer box  55  where the first and second forming fabrics separate. The sheet continues towards the press section on the first forming fabric  2 . The two forming fabrics move together through the hybrid forming section  1  so that the sheet moves in the machine direction as indicated by arrow A. 
     Although the hybrid forming section  1  shown in  FIG. 1  includes a single so-called “top wire” forming unit  61 , located on the first forming fabric  2 , other arrangements are possible. For example more than one own headbox delivering additional stock onto the first forming fabric  2 . Each additional unit  61  can also be provided with its own headbox delivering additional stock onto the first forming fabric  2 . 
     In the operation of the formation zone  60 , a jet of stock is ejected from the headbox slice  8  to provide a layer  7  of very aqueous stock on the open surface portion  2 A of the first forming fabric  2 . The first forming fabric  2  and the stock layer  7  move together in the machine direction shown by arrow A, over in sequence a forming board  51 , and a series of dewatering boxes and other sundry dewatering devices indicated generally as  52 . The first forming fabric  2  carrying the stock layer  7  then enters the top wire unit  61  of the hybrid forming section  1 . The second forming fabric  4  is brought into contact with the stock layer  7  at this point, so that it becomes sandwiched between the first and second forming fabrics  2  and  4  (see  FIG. 2  for more details). The first forming fabric  2  and the second forming fabric  4 , with the stock layer  7  sandwiched between them, then pass with their respective machine sides in contact with a sequence of units. These are: a lead-in dewatering box  53 , a multi-chambered dewatering box  10 , an opposed fabric support element unit  54  and a transfer box  55 . The multi-chambered dewatering box  10  is located with its fabric support elements in contact with the machine side of the second forming fabric only (see  FIGS. 2 ,  3  and  4 ). At the end of the unit  61  the second forming fabric  4  wraps around a turning roll  9  and is thereby taken out of contact with the stock layer  7 . The stock layer  7  carried by the first forming fabric  2  then passes over further dewatering boxes  56  and finally is transferred after the couch roll  57  at the end of the forming section  61  to the press section (not shown) for further processing. 
       FIG. 2  shows a more detailed schematic view of the lower part of the two fabric hybrid forming section  1  shown in  FIG. 1 . In  FIG. 2  the second forming fabric  4  partially wraps around the forming roll  3  with the result that the stock  7 , which is conveyed in the machine direction as indicated by the arrow A, becomes sandwiched between the first forming fabric  2  and the second forming fabric  4 . The two forming fabrics  2  and  4  with the stock layer  7  sandwiched between them then pass over several dewatering devices. The machine side of the first forming fabric  2  passes in sliding contact over the lead-in dewatering box  53 , an opposed fabric support element box  54  and a transfer box  55 . At the same time, the machine side of the second forming fabric  4  passes in sliding contact with the opposed fabric support elements  73  located on the multi-chambered dewatering unit  10 . Box  54  is optional, and the support elements  71  need not all be in contact with the machine side of the fabric  2 . The two forming fabrics  2  and  4  thus pass together in sequence past these four dewatering units in the sequence box  53 , unit  54 , unit  10  and box  55 . After box  55  the second forming fabric  4  wraps around the turning roll  9  and is carried away out of contact with the stock  7 . The stock  7  is carried by the first forming fabric  2  towards the press section (not shown). 
     In  FIG. 2 , dewatering box  53 , which is referred to as a lead-in box, as shown is provided with two vacuum chambers  63 ,  64 . Box  55 , which is referred to as a transfer box, which ensures the transfer of the stock  7  from the second forming fabric  4  to the first forming fabric  2 , as shown is provided with a single vacuum chamber. Either or both of these dewatering boxes  53  and  55  may be internally divided to provide two, or more, separate vacuum chambers each of which is connected to a separate controlled vacuum supply (not shown). A further embodiment is shown in  FIG. 4 , in which Box  53  comprises a single vacuum chamber and Box  55  comprises two vacuum chambers  101 ,  102 . 
     In Box  53 , forming fabric support elements  70  are mounted on the continuously curved fabric support element supporting surface  90 . Box  54  is an opposed fabric support element unit, which is a gravity drainage box. Water removed from the machine side surface of the first forming fabric  2  drops into the box  54 , and is removed therefrom. The box  54  includes fabric support elements  71 , which are mounted on the surface  91 . As this box  54  is on the outside of the convex curve of the two fabrics  2 ,  4 , formed by the box  10 , the fabric support elements  71  can be mounted on flexible, adjustable mountings such as those disclosed by McPherson in U.S. Pat. No. 6,361,657. Box  55  is provided with a plurality of fabric support elements  72  supported by the continuously curved surface  96 . 
       FIG. 2  also shows a multi-chambered dewatering unit  10 . As shown, unit  10  includes four distinct vacuum zones  80 ,  81 ,  82  and  83 , each of which is provided with a separate controlled vacuum supply (not shown). Located beneath each of the separate vacuum zones  80 ,  81 , and  82  is a set of fabric support elements, as at  73 . The fabric support elements  73  are supported on the curved surfaces  92 ,  93  and  94 . 
     There are several possibilities for the radii of curvature of the three surfaces  92 ,  93  and  94 . 
     (i) The three radii of curvature can be the same, so that all three surfaces  92 ,  93  and  94  together form a single constant radius curve. 
     (ii) At least one of the three radii can be different, or all three can be different. If this arrangement is adopted, then the radius of curvature of each of the surfaces  92 ,  93  and  94  must decrease in the machine direction, so that the radius of curvature of the surface  94  is always the smallest of the three. 
     It also apparent from  FIG. 2  that the pitch of the fabric support elements  73  on the multi-chambered dewatering unit  10  is not constant. The pitch decreases in the machine direction. 
     In  FIG. 2 , fabric support element  74  which is the first element of the set  73 , is located on the upstream side of zone  80  towards the headbox slice and is a so-called autoslice blade, also known as a skimmer blade. When in use, the autoslice blade  74  skims excess water from the machine side of the second forming fabric  4  as it passes in the machine direction in sliding contact with the element  74 . 
       FIG. 3  is similar to  FIG. 2 , with the exception that on box  53  the radius of curvature of the curved fabric support element supporting surface  90  is not constant. The surface  90  is broken into successive portions having radii of curvature R 1 , R 2  and R 3 . The radius of curvature for each portion decreases in the machine direction, so that R 1  is the largest radius of curvature. By decreasing the radius of curvature of the supporting surface  90  for the fabric support elements  70  located on the lead-in box  53  so as to increase sequentially the amount of wrap of the first and second forming fabrics  2 , 4  the stock  7  is subjected to increasingly stronger pressure pulses, which induce shearing actions within the stock  7 , at each edge of the fabric support elements  70  as the forming fabrics  2 , 4  pass over them in the machine direction. This feature is also shown in each of the dewatering boxes  53 ,  54 ,  10  and  55 . 
       FIG. 4  is also similar to  FIG. 2  except that the individual or discrete fabric support elements  70  of the lead-in box  53  are replaced by the continuous curved surface  100  mounted on support surface  90 , as described by Buchanan et al. in US 2003/017438. In addition, the transfer box  55  has been internally portioned to provide two separate vacuum zones  101  and  102 , each of which is provided with its own controlled vacuum supply (not shown). 
     In the drawings the fabric support elements are all shown schematically to have the same width in the machine direction. In practise, the fabric support element width may not be the same for all of the dewatering boxes. Some dewatering boxes may require a different width fabric support element just to accommodate the volume of white water which is being drained from the forming fabrics at that location. It is also possible that a different width fabric support element may be required in order to obtain the desired level of pressure pulse within the stock at a given location. Experience shows that the ratio of the machine direction width of fabric support elements to the width of the gap between them should be from about 1:10 to about 1:0.5. 
     In the drawings dewatering boxes are shown which have more than one chamber to each of which a controlled level of vacuum is applied. If the vacuum levels in adjacent chambers or dewatering boxes are not the same, it is desirable that the surface curvatures, and possibly also the corresponding fabric support element pitch, also should not be the same. Furthermore experience shows that it is desirable that the vacuum level in a sequence of dewatering boxes or chambers should increase relatively smoothly in the machine direction. Although the vacuum level can remain constant in two adjacent dewatering boxes or chambers it should not decrease in the machine direction, and furthermore spikes of radically different pressure should be avoided. In other words, all of the variables do not necessarily change smoothly in a step wise fashion; adjacent zones can have the same values for at least some of the variables.