Abstract:
A pulp digester includes a vessel having an inlet, an outlet and a wall extending between the inlet and the outlet, the wall of the vessel having a curved interior surface. The digester includes a screen assembly positioned inside the vessel adjacent to the curved interior surface of the wall for removing liquid from pulp material. The screen assembly is movable relative to the curved interior surface of the vessel wall. The digester includes at least one support element permanently attached to the vessel wall for limiting movement of the screen assembly relative to the curved interior surface of the vessel wall. In certain embodiments, the at least one support, such as a ledge permanently attached to the vessel wall, supports a portion of the screen assembly. The screen assembly may have an inner face that is concave in a horizontal direction and convex in a vertical direction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/583,885, filed Jun. 29, 2004, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present application is generally directed to making pulp and is more specifically directed to screen assemblies for pulp digesters.  
         [0003]     Continuous digesters are used in the paper and pulp industry to remove lignin from wood chips. The digesters generally include a series of tubular reactors that are arranged in a vertical orientation. The digester is made up of a plurality of reaction and extraction stages for carrying out the pulp-making process in a specific sequence. In certain stages, chemicals are introduced into the digester for chemically treating the woodchips. These chemicals may include hydrosulfide and sodium hydroxide, commonly referred to as liquor. In other stages, the chemicals are removed from the reactor through screens provided inside the digester.  
         [0004]      FIGS. 1A and 1B  show a conventional pulp digester  20  having a vessel wall  22  with an interior surface  24  and an exterior surface  26 . The vessel  22  has a step out section  28  and a straight section  30  located below the step out section. The step out section  28  diverges outwardly for expanding the diameter of the vessel  22  between an upstream section of the digester and a downstream section of the digester.  
         [0005]     The digester  20  includes a bar screen  32  having an upper end  34  and a lower end  36 . The bar screen  32  includes a plurality of vertically extending, cylindrical bars  38  that are spaced from one another. The upper end  34  of the bar screen  32  is attached to the vessel via an upper support element  40  and the lower end  36  of the bar screen  32  is attached to the vessel via a lower support element  42 . The bar screen is permanently fixed to the vessel wall  22 , such as by welding the upper and lower ends  34 ,  36  of the bar screen  32  to the upper and lower support elements  40 ,  42 . The bar screen  32  has a diameter that remains substantially constant between the upper and lower ends  34 ,  36  thereof.  
         [0006]     The digester also includes an upper cover plate  44  having an upper end  46  and a lower end  48 . The upper end  46  of the upper cover plate  44  is secured to the interior surface  24  of the vessel at a location above the step out section  28 . The lower end  48  of the upper cover plate  44  is secured to the upper support element  40 . The digester also has a lower cover plate  50  having an upper end attached to lower support element  42 . The lower cover plate  50  extends downwardly from the bar screen  32  to a downstream section (not shown) of the digester. The digester has an outlet  52  for removing liquid from the vessel.  
         [0007]     Permanently fixing the bar screen to the wall  22  involves a significant amount of work when assembling the digester. For example, when welding is used, the manufacturing process requires hundreds of welds to attach one bar screen to the vessel wall. For a vessel having many bar screens, the number of required welds may be in the thousands. The large number of welds increases the chances that one or more of the welds will crack, which may adversely affect operation of The digester or may require the digester to be taken off-line for repairs.  
         [0008]     The production of quality pulp involves introducing and removing liquor from the digester at certain time periods. The liquor is typically removed from digesters by passing the liquor through screens in a radial direction. The radial removal of the liquor causes compression of the wood chips onto the screen. This may prevent the wood chips from continuing to move toward the bottom of the digester, which is critical for the proper treatment of the chips. Extreme radial compression may also limit the amount of liquor that can be removed from the digester. Both of these situations may adversely affect the quality of the pulp produced using the digester.  
         [0009]      FIGS. 2A and 2B  show a pulp digester having a diverging screen assembly. The digester  120  has a vessel wall  122  with an interior surface  124  and an exterior surface  126 . The vessel  122  has a step out section  128  and a straight section  130  located below the step out section. The step out section  128  diverges outwardly for expanding the diameter of the vessel  122  between an upstream stage and a downstream stage of the digester.  
         [0010]     The digester  120  includes a diverging bar screen  132  having an upper end  134  and a lower end  136 . The bar screen  132  includes a plurality of vertically extending, cylindrical bars  138  that are spaced from one another. The upper end  134  of the bar screen  132  is attached to the vessel via an upper support element  140  and the lower end  136  of the bar screen  132  is attached to the vessel via a lower support element  142 . The bar screen continuously diverges between upper and lower ends thereof for reducing the compression loading of the wood chips on the screen. The digester also includes an upper cover plate  144  having an upper end  146  and a lower end  148 . The upper end  146  of the upper cover plate  144  is secured to the interior surface  124  of the vessel at a location above the step out section  128 . The lower end  148  of the upper cover plate  144  is secured to the upper support element  140 . The digester also has a lower cover plate  150  having an upper end attached to lower support element  142 . The lower cover plate  150  extends downwardly from the bar screen  132  to another downstream stage (not shown) of the digester. The digester has an outlet  152  for removing liquid from the vessel.  
         [0011]     In spite of the above advances, there is a need to provide continuous digesters that more efficiently introduce and remove liquor from the digester vessel at various stages of the pulp making process. There also remains a need for simplified methods for building digesters. Specifically, there remains a need for simpler methods for assembling screen assemblies inside digesters that minimize the number of welds needed for securing the screen inside the digester. Further, there remains a need for screen assemblies that are less subject to breakage during digester operations.  
       SUMMARY OF THE INVENTION  
       [0012]     In certain preferred embodiments of the present invention, a pulp digester includes a vessel having an inlet, an outlet and a wall extending between the inlet and the outlet, the wall of the vessel having a curved interior surface. The digester preferably includes a screen assembly positioned inside the vessel adjacent to the curved interior surface for removing liquid from pulp material. The screen assembly is desirably movable relative to the curved interior surface of the vessel wall. The digester also preferably includes at least one support element permanently attached to the vessel wall for limiting movement of the screen assembly relative to the curved interior surface of the vessel wall.  
         [0013]     The at least one support element preferably supports a portion of the screen assembly. In certain preferred embodiments, the at least one support element supports a lower portion of the screen assembly. The at least one support element may include a ledge immovably attached to the vessel wall. The ledge may be welded to the vessel wall.  
         [0014]     The at least one support element may also include at least one cover plate overlying the interior surface of the wall and being permanently attached to the wall, with the at least one cover plate being in contact with the screen assembly for limiting movement of the screen assembly over the curved interior surface of the vessel wall. In certain preferred embodiments, the at least one cover plate desirably includes an upper cover plate in contact with an upper end of the screen assembly and a lower cover plate in contact with a lower end of the screen assembly, the cover plates being permanently attached to the wall.  
         [0015]     The at least one cover plate is preferably in contact with the screen assembly for limiting movement of the screen assembly relative to the curved interior surface of the vessel wall. The upper cover plate may diverge in a flow direction of the pulp material through the vessel. The digester may also include a pair of lateral cover plates in contact with respective sides of the screen assembly, the lateral cover plates extending between the upper and lower ends of the screen assembly and being permanently attached to the wall.  
         [0016]     Although the present invention is not limited by any particular theory of operation, it is believed that providing screen assemblies that are movable relative to the vessel wall will solve a number of problems associated with digesters. First, the movable screen assembly of the present invention can better accommodate pressure changes within the vessel because the screen assembly can move in response to high-pressure areas within the vessel. Thus, the movable screen of the present invention enables pressure to be more evenly distributed throughout the vessel. Second, the ability of the screen assemblies of the present invention to move tends to minimize the formation of cracks in either the screen assembly or the welds used to position the screen assembly within the vessel. This feature overcomes problems found in prior art digesters that use thousands of welds to hold screens in place. These welds tend to crack under pressure or during long-term use, which results in maintenance problems or downtime for the digester. Third, the movable screen assemblies of the present invention can be used in a wide variety of digesters having different inner wall surfaces. As is well known to those skilled in the art, no two digesters are the same. This often makes it difficult to weld prior art screens to the vessel wall. However, this problem of matching the contour of the screen to the contour of the vessel wall is solved with the present invention because the screen floats and/or is movable relative to the vessel wall, thereby minimizing the need for providing exacting tolerances between the contour of the screen assembly and the contour of the vessel wall. The present invention also enables digesters to be assembled in less time and at lower cost. This is due, in part, to the fact that fewer welds are required for positioning the screens within a vessel. In many instances, the number of fewer welds may be in the hundreds or thousands.  
         [0017]     In certain preferred embodiments of the present invention, the screen assemblies may be used to replace existing screens in a digester. In certain embodiments, the existing screen assemblies in a digester may be cut out and/or removed from the digester and replaced with one or more screen assemblies of the present invention. The newly installed one or more screen assemblies of the present invention may be held in place by cover plates that overlie the one or more edges of the screen assembly. The screen assembly of the present invention is preferably moveable relative to the vessel wall of the digester. As a result, the screen assembly is able to respond to high pressure areas within the digester. In certain preferred embodiments, the screen assemblies of the present invention are assembled outside of the digester. The screen assemblies are then secured inside the digesters in a manner so that the screen assemblies are moveable within a range or area relative to the vessel wall. Due to the moveable nature of the screen assemblies of the present invention, exact tolerances between the outer face of the screen assembly and the inner face of the vessel wall are not required.  
         [0018]     In certain preferred embodiments, the screen assembly has an inner face that is concave in a horizontal direction. The screen assembly may also have an inner face that is convex in a vertical direction. In other preferred embodiments, the screen assembly may diverge in the flow direction of the pulp material.  
         [0019]     The screen assembly desirably includes a plurality of bars extending in a generally vertical direction between the upper and lower ends of the screen assembly, whereby the bars are spaced from one another so as to define gaps between the spaced bars. In certain preferred embodiments, the bars are curved in the vertical direction. The size of the gaps between the spaced bars may remain constant between the upper and lower ends of the screen assembly. In other preferred embodiments, the size of the gaps between the spaced bars may change between the upper and lower ends of the screen assembly. In still other preferred embodiments, the vessel preferably has a longitudinal axis extending between upper and lower ends thereof and each of the bars of the screen assembly is curved relative to the longitudinal axis. The bars may be of any geometric shape in cross-section, such as cylindrical, circular, oval, square or rectangular. The exterior surface of the bars may be curved in one area and flat in another area.  
         [0020]     In other preferred embodiments, the screen assembly may include one or more metal plates having one or more openings therethrough. The screen assembly may include a metal plate having a plurality of openings extending therethrough. The size and spacing of the openings may be constant or may change over the area of the plate. The openings may also be elongated in one or more directions such as slots. In further preferred embodiments, the screen assembly may include a plurality of metal plates that are assembled together with gaps or spaces between the metal plates. The size and shape of the gaps may be constant or may change.  
         [0021]     In certain preferred embodiments, the screen assembly includes a top support arch, a bottom support arch spaced from the top support arch, and at least one intermediate support arch positioned between the top and bottom support arches. The screen assembly may also include at least one frame member attached to the support arches for maintaining the support arches in a fixed, spaced orientation relative to one another, and a plurality of spaced bars extending between the top and bottom support arches, the spaced bars having upper sections permanently attached to the top support arch, lower sections permanently attached to the bottom support arch and intermediate sections in contact with, but not permanently attached to, the at least one intermediate support arch.  
         [0022]     The support arches may have inwardly extending fingers for engaging the bars. The fingers of the intermediate arch may be longer than the fingers of the top and bottom support arches for providing a curve to the bars between the top and bottom support arches. The screen assembly may include a plurality of intermediate support arches between the top and bottom support arches. The fingers of the intermediate arches near the center of the screen assembly are preferably longer than the fingers of the intermediate arches near the top and bottom support arches.  
         [0023]     In other preferred embodiments of the present invention, a pulp digester includes a vessel having an inlet, an outlet and a wall extending between the inlet and the outlet, whereby the wall of the vessel has a curved interior surface. The digester may include a screen assembly for removing liquid from the pulp material, whereby the screen assembly has an inner face that is concave in a horizontal direction and convex in a vertical direction. In this embodiment, the screen assembly may be movable over the curved interior surface of the vessel wall, The digester further comprising at least one support element permanently attached to the vessel wall for limiting movement of the screen assembly over the curved interior surface of the vessel wall.  
         [0024]     In other preferred embodiments of the present invention, a screen assembly for a pulp digester includes a top support arch, a bottom support arch spaced from the top support arch and at least one intermediate support arch positioned between the top and bottom support arches. The screen assembly desirably includes at least one frame member attached to the top support arch, the bottom support arch and the at least one intermediate support arch for maintaining the support arches is a fixed orientation relative to one another, and a plurality of spaced bars extending between the top and bottom support arches, the spaced bars having upper sections permanently attached to the top support arch, lower sections permanently attached to the bottom support arch and intermediate sections in contact with, but not permanently attached to, the at least one intermediate support arch. The bars are desirably curved between the upper and lower sections thereof.  
         [0025]     In certain preferred embodiments, the screen assembly has an inner face that is concave in a horizontal direction. The inner face may be convex in a vertical direction.  
         [0026]     Each of the support arches desirably has a convex outer face and a concave inner face having fingers that engage the respective bars. The fingers of the at least one intermediate support arch are preferably longer than the fingers of the top and bottom support arches for facilitating curving of the bars when the bars are in contact with the fingers. The inner ends of the fingers desirably have seating surfaces adapted to receive the bars. The bars have preferably have cylindrical exterior surfaces and the seating surfaces of said fingers are preferably concave for receiving the cylindrical exterior surfaces of the bars.  
         [0027]     The support arches preferably extend in directions that are parallel to one another. At least one intermediate support arch preferably includes a plurality of intermediate support arches between the top support arch and the bottom support arch. The fingers of the intermediate support arches are progressively longer for the arches closer to the center of the screen assembly.  
         [0028]     In another preferred embodiment of the present invention, a screen assembly for removing liquid from a pulp digester includes a top support arch having inwardly extending fingers, a bottom support arch spaced from the top support arch and having inwardly extending fingers, and at least one intermediate support arch positioned between the top and bottom support arches and having inwardly extending fingers. The screen assembly desirably includes at least one frame member attached to the top support arch, the bottom support arch and the at least one intermediate support arch for maintaining the support arches is a fixed orientation relative to one another, and a plurality of spaced bars extending between the top and bottom support arches, whereby each bar is in contact with one of the fingers of each support arch. The fingers of the at least one intermediate arch are preferably longer than the fingers of the top and bottom support arches for bending the bars between the top and bottom support arches.  
         [0029]     The spaced bars may have upper sections permanently attached to the fingers of the top support arch, lower sections permanently attached to fingers of the bottom support arch and intermediate sections in contact with, but not permanently attached to, the fingers of the at least one intermediate support arch.  
         [0030]     In other preferred embodiments of the present invention, a method of installing a screen assembly in a pulp digester includes providing a vessel having an inlet, an outlet and a wall extending between the inlet and the outlet, whereby the wall of the vessel has a curved interior surface. The method includes attaching a support element to the curved interior surface of the vessel wall, and installing a screen assembly inside the vessel adjacent the curved interior surface so that the screen assembly is movable relative to the curved interior surface, whereby the support element engages a portion of the screen assembly for limiting downward movement of the screen assembly through the vessel. The attaching step may include permanently securing the support element to the curved interior surface, such as by welding.  
         [0031]     In another preferred embodiment of the present invention, a method of installing a screen assembly in a pulp digester includes providing a vessel having an inlet, an outlet and a wall extending between the inlet and the outlet, whereby the wall of the vessel has a curved interior surface, and installing a screen assembly inside the vessel adjacent the curved interior surface, the screen assembly having a horizontal cross-section that is concave and a vertical cross-section that is convex. The method may include attaching a support element to the curved interior surface of the vessel wall so that the screen assembly is movable relative to the curved interior surface, whereby the support element engages a portion of the screen assembly for limiting downward movement of the screen assembly through the vessel.  
         [0032]     These and other preferred embodiments of the present invention will be described in more detail below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1A  shows a cross-sectional view of a prior art pulp digester including a vessel and a cylindrical screen mounted in the vessel.  
         [0034]      FIG. 1B  shows a fragmentary front elevation view of the prior art pulp digester of  FIG. 1A .  
         [0035]      FIG. 2A  shows a cross-sectional view of a prior art pulp digester including a vessel and a diverging screen mounted in the vessel.  
         [0036]      FIG. 2B  shows a fragmentary front elevation view of the prior art pulp digester of  FIG. 2A .  
         [0037]      FIG. 3A  shows a cross-sectional view of a pulp digester including a vessel and a screen positioned inside the vessel, in accordance with certain preferred embodiments of the present invention.  
         [0038]      FIG. 3B  shows a fragmentary front elevation view of the vessel and the screen shown in  FIG. 3A .  
         [0039]      FIG. 4  shows another cross-sectional view of the pulp digester shown in  FIG. 3A .  
         [0040]      FIG. 5  shows an expanded view of an upper section of the screen shown in  FIG. 4 .  
         [0041]      FIG. 6  shows an expanded view of an intermediate section of the screen shown in  FIG. 4 .  
         [0042]      FIG. 7  shows an expanded view of a lower section of the screen shown in  FIG. 4 .  
         [0043]      FIG. 8  shows a top plan view of a portion of the vessel and the screen shown in  FIG. 4 .  
         [0044]      FIG. 9  shows an expanded view of a lateral section of the screen shown in  FIG. 8 .  
         [0045]      FIG. 10  shows an expanded view of an intermediate section of the screen shown in  FIG. 8 .  
         [0046]      FIG. 11A  shows a bar screen having constant gaps between bars, in accordance with certain preferred embodiments of the present invention.  
         [0047]      FIG. 11B  shows a bar screen having gaps that increase in size between ends of the screen, in accordance with other preferred embodiments of the present invention.  
         [0048]      FIG. 12  shows a graph comparing relative chip compaction for a right cylinder screen assembly, a diverging screen assembly and a screen assembly in accordance with certain preferred embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0049]      FIGS. 3A and 3B  show a pulp digester  220  in accordance with certain preferred embodiments of the present invention. The pulp digester  220  desirably includes a vessel wall  222  with an interior surface  224  facing the inside of the vessel and an exterior surface  226  facing the outside of the vessel. The vessel wall  222  has a step out section  228  that diverges outwardly from an upstream stage  229  of the digester. The vessel wall  222  also has a straight section  230  located below the step out section  228 . The vessel wall  222  has a longitudinal axis or centerline C-C, and the step out section  228  diverges outwardly from the centerline C-C for expanding the interior diameter of the vessel wall  222  between the upstream stage  229  and a downstream stage.  
         [0050]     Referring to  FIGS. 3A, 3B  and  4 , the digester  220  preferably includes a bar screen  232  having an upper end  234  and a lower end  236 . The bar screen  232  desirably includes a plurality of vertically extending bars  238  that are spaced from one another. The bars  238  may be cylindrical in shape. The size of the gaps between the bars may be constant or may change over the area of the screen. The gaps between the bars  238  preferably allow fluid, such as liquor, to flow between the bars when treating pulp in the digester. The bar screen  232  preferably includes a series of support arches  260  that extend between the upper and lower ends of the screen.  
         [0051]     In certain preferred embodiments, the upper end  234  of the bar screen  232  ( FIG. 3A ) may be located anywhere in the step out section  228  of the vessel wall  222 , with the lower end  236  of the bar screen being supported by the lower support element  242 .  
         [0052]     Referring to  FIGS. 3A, 4  and  5 , the uppermost end of bar screen  232  includes at least one bar  238  that is attached to a top support arch  260 A by weld  239 . Referring to  FIGS. 4 and 7 , the lowermost end  226  of bar screen  232  includes at least one bar  238  that is attached to a bottom support arch  260 K by weld  235 . Referring to  FIGS. 4 and 6 , bar screen  232  also has one or more intermediate support arches  260  between the top support arch  235  and the bottom support arch  237 . In the particular preferred embodiment shown in  FIG. 6 , the round bar  238  is welded to intermediate support arch  260  by welding  262 . This welding with an intermediate arch is generally done when the screen  232  has a substantial height. Welding and even the provision of the intermediate arch may not be necessary for screens that do not have substantial height. Thus, in other preferred embodiments, the bar  238  merely engages and/or is supported by the intermediate support arch  260  and is not attached to the intermediate support arch  260 . Significantly, none of the support arches  260 A- 260 K is attached to the vessel wall  222 . As a result, the bar screen  232  is able to move relative to the vessel wall, within an area bordered by one or more supporting elements as will be described in more detail below.  
         [0053]     Referring to  FIGS. 3A, 3B ,  4  and  5 , the digester also preferably includes an upper cover plate  244  having an upper end  246  and a lower end  248 . The upper end  246  of the upper cover plate  244  is secured to the interior surface  224  of the vessel wall  222  and the lower end  248  of the upper cover plate  244  is secured to an upper support element  240 . The upper support element  240  has an outer end that is attached to the inner wall  224  of the vessel wall  222  and an inner end that is attached to the upper cover plate  244 . Referring to  FIGS. 4 and 5 , the above-referenced attachments may be formed by attaching the outer end of the upper support element  240  to the interior surface  224  of vessel wall  222  using welding seams  241 , and attaching the inner end of the upper support element  240  to the upper cover plate  244  using welding seam  243 . The upper cover plate  244  preferably diverges outwardly from centerline C-C between the upper and lower ends of the upper cover plate. As a result, the distance between the centerline and the upper end of the upper cover plate is less than the distance between the centerline and the lower end of the upper cover plate.  
         [0054]     Referring to  FIGS. 3A, 4  and  7 , the digester  220  also preferably has a lower cover plate  250  having an upper end attached to the lower support element  242  that is attached to the vessel wall  222 . The lower cover plate  250  extends downwardly from the lower end  236  of bar screen  232  to a downstream stage (not shown) of the digester. Referring to  FIGS. 4 and 7 , the lower support element  242  has an outer end attached to vessel wall  222  by welding seam  245  and an inner end attached to lower cover plate  250  by welding seam  247 .  
         [0055]     As noted above, the support arches  260 A- 260 K of the bar screen assembly  232  are not attached to the vessel wall  222 , but are bounded by and held in position by the one or more supporting elements such as the upper and lower cover plates  244 ,  250 , the upper support element  240  or the lower support element  242 . As shown in  FIGS. 4 and 5 , the upper cover plate  244  covers or overlaps the top support arch  260 A but is not attached thereto so that the upper end  234  of the bar screen is able to float freely in back of the upper cover plate. As shown in  FIGS. 4 and 7 , the lower cover plate  250  covers or overlaps the bottom support arch  260 K but is not attached thereto so that the lower end  236  of the bar screen is able to float freely in back of the lower cover plate  250 . The lower support element  242  preferably supports the lower end of the screen assembly  232  and limits downward movement of the screen assembly relative to the vessel wall.  
         [0056]     Referring to  FIGS. 3A and 4 , the digester has at least one outlet  252  for removing fluid and/or liquid from the vessel wall  222 .  
         [0057]     Referring to  FIGS. 8-10 , digester  220  includes a first lateral support element  264  and a second lateral support element  266  that border the respective lateral sides of bar screen  232 . The first lateral support element  264  has an outer end that is attached to the interior surface  224  of vessel wall  222  by welding seams  268  and an inner end that is attached to a first lateral cover plate  270  by welding  272 . The second lateral support element  266  has an outer end that is attached to the interior surface  224  of vessel wall  222  by welding  274  and an inner end that is attached to a second lateral cover plate  276  by welding  278 .  
         [0058]     The bar screen  232  includes a plurality of vertically extending bars  238  having gaps  280  between the bars. In certain preferred embodiments, the bars may be cylindrical and may extend in horizontal or diagonal directions. The bar screen also includes a series of support arches  260 A- 260 K that engage the vertically extending bars  238 . Each support arch  260  has a first end  282 , a second end  284  and a plurality of fingers  286  that engage the respective bars  238 . The fingers  286  define flow channels  288  therebetween that are in communication with the gaps  280  between the bars  238 .  
         [0059]     The bar screen  232  also includes a first side frame  290  that is attached to the first end  282  of the support arch  260  by welding  292  and second side frame  294  that is attached to the second end  284  of the support arch  260  by welding (not shown). The side frames  290 ,  294  may be attached to a plurality of the support arches for holding the support arches in a fixed orientation relative to one another. The inner ends of the fingers  286  desirably have grooves  296  formed therein for effectively seating the bars. In the particular embodiment shown, the grooves  296  are concave in shape.  
         [0060]     Referring to  FIG. 8 , the support arch  260  has an outer face  298  that defines an arch and a series of projections  300  extending from the outer face  298 . The inner surface  224  of the vessel wall  222  has a radius R 3  from centerline C (the position of centerline C is shown for simplicity only and is not to scale). The curvature of the support arch  260  preferably changes between the first and second ends of the arch  260 . As shown in  FIG. 8 , the projections  300 A and  300 G adjacent the ends of the support arch are in contact with the inner surface  224  of vessel wall  222 . However, the gap  302  between the projections  300  and the inner surface  224  increases between the ends of the support arch and the center of the support arch  260 . As a result, the gap  302  between projection  300 D and inner surface  224  is greater than the gap between projections  300 C and  300 E and inner surface, which is greater than the gap between projections  300 B and  300 F and inner surface  224 . The radius of the support arch  260  is about R3 at the ends of the arch and is about R4 at an interior section of arch  260 , whereby R4&lt;R3.  
         [0061]     As shown in  FIGS. 8 and 9 , the bar screen  232  is not permanently attached to the vessel wall  222 , but is able to move relative to the vessel wall  222 . As a result, the bar screen is able to move up and down between the upper and lower support elements  240 ,  242  and laterally between the lateral support elements  264 ,  266 .  
         [0062]     Referring to  FIGS. 3A and 8 , in certain preferred embodiments, the bar screen  232  is assembled by attaching the first and second side frames  290 ,  294  to the first and second ends of support arches  260 . The support arches include top support arch  260 A, bottom support arch  260 K, and intermediate support arches  260 B- 260 J that lie between the top support arch and the bottom support arch. The support arches  260 A- 260 K extend in directions that are generally parallel to one another. In the particular embodiment shown in  FIG. 3A , the support arches are parallel to one another and extend in substantially horizontal directions relative to the length of the digester.  
         [0063]     Referring to  FIGS. 3A, 8  and  10 , the length of the fingers  286  on a single support arch preferably remain the same, however, the finger length may vary between two or more arches. In other words, a first support arch may have fingers with a length x, and a second support arch may have fingers with a length x+1. Referring to  FIG. 3A , in certain preferred embodiments, the fingers of support arch  260 F are longer than the fingers of support arches  260 E and  260 G, which are longer than the fingers of support arches  260 D and  260 H, which are longer than the fingers of support arches  260 C and  260 I, which are longer than the fingers of support arches  260 B and  260 J. As a result, when the vertically extending bars  238  are assembled with the support arches, the bars have a slight curve, with the center of the curve being located outside the vessel wall  222 . Referring to  FIG. 4 , the bars  238  are curved about a center located outside the vessel wall and having a radius designated R 1 . As shown in  FIG. 3A , the upper end of the bar screen  232  defines a diameter D 1 , and the lower end of the bar screen defines a diameter D 3  that is equal to D 1 . The center of the bar screen defines a diameter D 2  that is less than D 1  and D 3 . Thus, the inner face of the bar screen  232  has a decreasing diameter between the upper end  234  of the bar screen and a mid-point located between the upper and lower ends, and then has an increasing diameter between the mid-point of the bar screen and the lower end  236  thereof.  
         [0064]     In certain preferred embodiments, the lower ends of the bars  238  are permanently attached to the fingers of the bottom support arch  260 K and the fingers of the top support arch  260 A, and are not permanently attached to the fingers of the intermediate arches  260 B- 260 J. This may be accomplished by seating a lower end of a bar  238  in one of the fingers of the bottom support arch  260 K and welding the bar to the finger. The bar is then seated in the fingers of the intermediate arches, starting first with arch  260 J and moving onto  260 B. The upper end of the bar is then seated in the groove of one of the fingers of top support arch  260 A and welded to the finger of the top support arch. The process is repeated for all of the vertically extending bars so that the lower ends of the bars are welded to the bottom support arch and the upper ends of the bars are welded to the top support arch. Due to the changing length of the fingers, the bars will have a slight curve between top and bottom support arches  260 A,  260 K. In certain preferred embodiments, the bars follow a curve having a center located outside the vessel wall, whereby the curve has a radius R 1  of between about 55-70 meters and more preferably about 60-65 meters. Thus, the bars  238  follow a curved path having an extremely large radius.  
         [0065]      FIG. 11A  shows bar screen  232  held in position by upper cover plate  244 , lateral cover plate  270 , and lower cover plate  250 . The bar screen is not permanently affixed to the vessel wall  222  so that the screen is able to float under the cover plates. The bar screen  232  includes a series of vertically extending bars  238  having gaps  280  therebetween. The size of the gaps  280  remains constant between the upper end  234  and the lower end  236  of the screen  232 .  
         [0066]      FIG. 11B  shows a bar screen  232 ′ held in position by upper cover plate  244 ′, lateral cover plate  276 ′ and lower cover plate  250 ′. Once again, the bar screen  232 ′ is not permanently affixed to the vessel wall  222 ′, but is able to move relative to vessel wall  222 ′. The bar screen  232 ′ includes a series of bars  238 ′ having gaps  280 ′ therebetween. The size of the gaps  280 ′ between the bars  238 ′ increases between the upper end  234 ′ and the lower end  236 ′ of the screen  232 ′.  
         [0067]      FIG. 12  shows a graph comparing relative chip compaction in a pulp digester when using the standard, right cylinder screen assembly shown in  FIG. 1A , the diverging screen assembly shown in  FIG. 2A , and the screen assembly in accordance with certain preferred embodiments of the present invention shown in  FIGS. 3A-11B . The results for the standard screen are depicted using a solid line, the results for the diverging screen are depicted using a dashed line having elongated dashes, and the results for the screen assembly of the present invention are depicted using a dashed line having shorter dashes. The X-axis in the graph represents the height of the screen assembly, starting 0.1 meter above the screen (wherein the cover plate is located). The Y-axis in the graph shows the relative compaction of the chip column. The chip columns in the three different systems (i.e. standard screen, diverging screen, screen of the present application) all have the same absolute starting value at 0.2 meter above the upper end of the screen. The graph shows that compaction for the standard screen and diverging screen increases at the transition between the cover plate and the screen. In contrast, the compaction level for the present invention actually decreases at the transition because the upper cover plate diverges. The peak value of the compaction is at the location where there is no axial flow vector at all. After peak value, the compaction is decreased rapidly by counter-current wash flow.  
         [0068]     Thus, the graph of  FIG. 12  shows that the screen assembly of the present invention provides the pressure relief where it is needed: at the top area of the screen. In contrast, the diverging screen has an integrating effect along the screen height. At the beginning there is no effect because of the diverging screen. At the end, where the effect of the diverging screen is greatest, the counter-current wash-flow dramatically reduces the compaction.  
         [0069]     These and other variations and combinations of the features discussed above can be utilized without departing from the present invention. Thus, the foregoing description of the preferred embodiments should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims.