Abstract:
A rotary high pressure feeder for processing cellulosic fibrous material including: a rotor rotatable about an axis and comprising at least one pocket comprising a conduit extending through the rotor in a direction perpendicular to the axis and having end openings at opposite ends of the pocket, wherein each of said end openings of each pocket functions as both an inlet and outlet depending upon an angular position of the rotor within a housing; the housing encloses said rotor and said housing includes a low pressure inlet port and a low pressure outlet port aligned vertically, and a high pressure inlet port and a high pressure outlet port aligned horizontally, wherein the ports are arranged on the housing for registry with the end openings of at least one pocket, and a screen grid seated in or adjacent to the low pressure outlet port, wherein the screen grid includes an array of screen slots and bars forming a uniform pattern of slots and bars extending without interruption across an entirety of the low pressure outlet port.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/169,378 filed on Apr. 15, 2009, the entirety of which is incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to high pressure feeders (HPF) typically used to pressurize slurries of comminuted cellulosic material, such as wood chips. The present invention particularly relates to screens that retain cellulosic material in the rotor of the HPF and allow liquid to be discharged through a low pressure outlet of the HPF. 
         [0003]    High pressure feeders are typically included in chip feed systems that deliver a high pressure slurry of wood chips to a pressurized digester vessel, such as used in Kraft pulping. HPFs are described in U.S. Pat. Nos. 4,107,843; 6,120,646; 5,236,285, 6,468,006 and 6,616,384, and in published international patent applications WO 94/21855 and WO 99/42653. 
         [0004]    HPFs typically have a rotor with four through passages that fill and empty with cellulosic material and liquid as the rotor turns in a housing. HPFs typically have a low pressure outlet through which passes liquor separated from the slurry of cellulosic material. 
         [0005]    To prevent the loss of cellulosic material, a screen grid typically covers the low pressure outlet. The screen grid passes low pressure liquor through to the low pressure outlet and blocks cellulosic material from exiting through the low pressure outlet. In conventional HPFs, the screen grid has a narrow solid section in the center that is aligned with a divider bar in the casing of the HPF. The center-section of the screen grid abuts the outlet divider bar in the middle of the low pressure outlet of the HPF housing and below the screen grid. The divider bar is a narrow support wall extending across the center of the low pressure outlet. 
         [0006]    The divider bar and narrow solid section of the screen grid are generally aligned with a center solid circumferential portion of the rotor of the HPF. This alignment minimizes the flow obstruction caused by the divider bar and narrow solid section. The alignment between the outlets in the rotor and the screen grid and the divider wall changes as the rotor is moved a relatively short distance, e.g., 5 inches (127 millimeters (mm), axially with respect to the liner and casing of the HPF. The rotor is periodically moved to compensate for wear on the surfaces of the rotor and the liner. Moving the rotor axially changes the alignment between the outlets in the rotor and the center solid portion of the screen grid and the divider wall. As the alignment changes, the center solid portion of the screen grid and the divider wall may become misaligned with the center of the rotor and thereby obstruct the flow of liquor passing through the low pressure outlet of the HPF. 
         [0007]    Conventional HPFs have dual screen grids with slots that do not extend continuously over the entire area of the screen grid exposed to the ports in the rotor and the low pressure outlet in the casing. Because the region of slots is not continuous, the screen grid has solid sections that obstruct the low pressure flow of liquor from the HPF. The rotor is obstructed specifically near the outlet divider bar and the drive end of the housing. The divider bar and narrow solid section of the screen grid obstruct the flow of liquor through the outlets in the rotor and the low pressure outlet in the casing of the HPF. The obstruction of the flow of liquor to the low pressure outlet reduces the efficiency and capacity of the HPF. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    An unobstructed flow of liquor to the low pressure outlet would allow for liquor to more easily flow through and out of the rotor. Allowing the liquor to flow through and out of the rotor without obstruction enhances the flow of wood chips through the HPF and to the high pressure outlet of the HPF. An unobstructed flow of liquor through the low pressure outlet would lower the pressure in the flow passing through the rotor while the rotor is aligned with the low pressure inlet to the HPF. An unobstructed flow of liquor through the low pressure outlet would increase the amount of chips that fill the rotor as the low pressure slurry of chips and liquor enters the HPF and the rotor. 
         [0009]    There is a long felt need to improve the efficiency and capacity of HPFs. Obstructions to the low pressure outlet flow in a HPF tend to reduce the capacity and efficiency of a HPF. Increasing the capacity of HPFs allows for greater amounts of wood chips to flow through and be pressurized by the HPF. Reducing the obstructions to the low pressure outlet tends to increase the efficiency of the HPF by increasing the capacity of the HPF without requiring an increase in power applied to drive the HPF. 
         [0010]    The slots in the screen grid have been increased to form a large single continuous slotted area in the screen grid which corresponds to the entire area of the low pressure outlet and to the areas of the ports in the rotor. The portion of casing for the drive coupling of the rotor may be decreased in size, e.g. by one inch (25 mm), to allow for an expansion of the open area of the low pressure outlet in the casing and allow for a larger slotted surface area of the screen grid. The center bar in the low pressure outlet of the liner may be removed. The expanded outlet of the casing and the screen grid slotted surface allow for an increase flow of low pressure liquid through the outlets in the pocketed rotor and out of the HPF. The fluid flow through the slots contributes to the increased efficiency of the screen grid. 
         [0011]    In addition, the outlet divider bar in the low pressure outlet may be shortened to avoid abutting against the screen grid. A gap formed between the inner end of the divider bar and the screen grid allows the liquid to flow through and out of the center region of the screen grid. 
         [0012]    Further, by increasing the slot opening width to between 8 millimeters (mm) to 25 mm in the screen grid and increasing the area of the slots region in the screen grid, the capacity and efficiency of the flow of liquid through the screen grid is increased. 
         [0013]    A high pressure transfer device is disclosed comprising: a pocketed rotor containing a plurality of through flow pockets, said rotor rotatable about an axis of rotation and said pockets having opposite end openings which function as both inlets and outlets depending upon an angular position of the rotor within the device, and said pockets are provided in at least first and second sets; a housing enclosing said rotor, said housing having a low pressure inlet port aligned with a low pressure outlet port, and a high pressure inlet port aligned with a high pressure outlet port, wherein said ports are arranged for registry with the inlets to and outlets from said through going pockets and said rotor being mounted in said housing for rotation with respect to said ports about said given axis of rotation, and a screen grid seated in or adjacent to the low pressure outlet port wherein the screen grid has an array of screen slots and bars wherein a regular pattern of the slots and bars extends across an entirety of the low pressure outlet port. 
         [0014]    A rotary high pressure feeder for processing cellulosic fibrous material comprising: a rotor rotatable about an axis and comprising at least one pocket comprising a conduit extending through the rotor in a direction perpendicular to the axis and having end openings at opposite ends of the pocket, wherein each of said end openings of each pocket functions as both an inlet and outlet depending upon an angular position of the rotor within a housing; the housing encloses said rotor and said housing includes a low pressure inlet port adapted to receive a low pressure slurry of the cellulosic fibrous material and a liquid, and a low pressure outlet port aligned vertically with the low pressure inlet port and adapted to discharge low pressure liquid extracted from the low pressure slurry passing through the low pressure inlet, and a high pressure inlet port adapted to receive a liquid flow under high pressure, and a high pressure outlet port aligned horizontally with the high pressure inlet port and adapted to discharge under high pressure the cellulosic fibrous material which passed through the low pressure inlet, wherein the ports are arranged on the housing for registry with the end openings of at least one pocket, and a screen grid seated in or adjacent to the low pressure outlet port, wherein the screen grid includes an array of screen slots and bars to allow passage of the liquid and block passage of the cellulosic fibrous material, wherein the array forms a uniform pattern of slots and bars extending without interruption across an entirety of the low pressure outlet port. 
         [0015]    A rotary high pressure feeder for processing cellulosic fibrous material comprising: a rotor rotatable about an axis and comprising at least one pocket comprising a conduit extending through the rotor in a direction perpendicular to the axis and having end openings at opposite ends of the pocket, wherein each of said end openings of each pocket functions as both an inlet and outlet depending upon an angular position of the rotor within a housing; the housing encloses said rotor and said housing includes a low pressure inlet port adapted to receive a low pressure slurry of the cellulosic fibrous material and a liquid, and a low pressure outlet port aligned vertically with the low pressure inlet port and adapted to discharge low pressure liquid extracted from the low pressure slurry passing through the low pressure inlet, and a high pressure inlet port adapted to receive a liquid flow under high pressure, and a high pressure outlet port aligned horizontally with the high pressure inlet port and adapted to discharge under high pressure the cellulosic fibrous material which passed through the low pressure inlet, wherein the ports are arranged on the housing for registry with the end openings of at least one pocket; a screen grid seated in or adjacent to the low pressure outlet port, wherein the screen grid includes an array of screen slots and bars to allow passage of the liquid and block passage of the cellulosic fibrous material, and a gap between the screen grid and an edge of a divider wall in the housing and traversing the low pressure outlet port, wherein liquid flowing through the slots passes through the gap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIGS. 1 and 2  are schematic views of a conventional high pressure feeder (HPF) in a feed system for a digester vessel. 
           [0017]      FIG. 3  is a perspective view of a conventional high pressure feeder. 
           [0018]      FIG. 4  is a perspective view of the rotor, liner and screen grid of the conventional high pressure feeder shown in  FIGS. 1 to 3 . 
           [0019]      FIGS. 5A and 5B  are cross-sectional views of a lower half of a conventional high pressure feeder. 
           [0020]      FIGS. 6A and 6B  are cross-sectional view of a lower half of high pressure feeder having an expanded low pressure outlet, an elongated screen grid, and a shortened divider wall. 
           [0021]      FIG. 7  is a perspective view of a liner and screen grid shown in  FIGS. 6A and 6B . 
           [0022]      FIG. 8  is a graph of exposed slotted screen area as a function of the axial position of the rotor in the HPF casing for a conventional screen grid and the elongated screen grid. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIGS. 1 and 2  schematically illustrate the operation of a high pressure transfer device, which is also referred to as a high pressure feeder (HPF)  10 . As is conventional, the HPF  10  is connected to a chip chute  11 , which is supplied with steamed wood chips (or other cellulosic material) from a conventional steaming vessel  9 , the chips are slurried with liquid (liquor) from a liquid supply conduit  12 . The chute  11  is connected to a first port (low pressure inlet)  13  of a housing (casing) of the HPF  10 . The housing also has a second port (high pressure inlet)  15 , a third port (low pressure outlet)  16 , and a fourth port (high pressure outlet)  17 . These ports  13 ,  15 ,  16  and  17  are disposed around the casing  14  at intervals of approximately 90 degrees. A pocketed rotor  19  rotates (see direction of rotation  18 ) within the housing  14 . 
         [0024]    The rotor  19  typically has four pockets  21  that each form a flow passage extending through the rotor in a direction perpendicular to the axis of rotation of the rotor. The rotational axis is perpendicular to the plane of  FIGS. 1 and 2 . The rotor rotates continuously to alternatively align the pockets with ports  1  and  3  (which are in a vertical alignment) and then with ports  2  and  4  (which are in horizontal alignment).  FIG. 1  shows a pocket  21  of the rotor aligned vertically with the first and third ports, which form a low pressure passage to allow wood chips and liquor from the chip tube  11  to enter and fill the pocket  21  through the low pressure inlet (first port)  13  and for liquor to simultaneously flow out the low pressure outlet (third port)  16 . 
         [0025]      FIG. 2  shows the pocket  21  aligned horizontally with the high pressure inlet (second port)  15  and a high pressure outlet (fourth port)  17 . During each quarter turn of the rotor, the pockets  21  are alternatively aligned with the low pressure inlet and outlet of the housing and then with the high pressure inlet and outlet. When aligned with the low pressure inlet and outlet  13 ,  16 , a chip slurry fills the pocket  21  and liquid from the slurry drains through the outlet port  16 . As the pocket turns with the rotor, the chips are trapped in the pocket until the pocket is aligned with the high pressure inlet and outlet  15 ,  17 , at which point high pressure liquid flows through the inlet  15  and flushes the chips and liquid to a high pressure conduit  22  connected to the high pressure outlet  17 . 
         [0026]    Connected to the second port  15  (high pressure inlet) is a high pressure pump  20  or other source of high pressure liquid (liquor). As illustrated in  FIG. 2 , the pump  20  provides liquid under high pressure to the second port  15  which, when the pocket  21  is horizontal, flushes the wood chips or other cellulosic fibrous material within the pocket  21  out under high pressure through the fourth port  17  (high pressure outlet) into the circulation, high pressure conduit  22  associated with a continuous digester vessel  23 . The high pressure conduit feeds the slurried pressurized chips under above-atmospheric pressure to a top inlet  24  of the digester vessel  23 . At the top inlet  24 , a conventional solids and liquid separator is provided which returns some of the liquid via conduit  25  that may direct some or all of the liquid to the inlet to the pump  20 . The liquid in conduits  22 ,  25  typically is white liquor and may include steam condensate and sometimes black liquor, and which may be supplemented with make up liquor conduit  26 . For non-kraft situations, the liquid in lines  22 ,  25  could be water, solvent pulping liquid, etc. 
         [0027]    Connected to the third port  16  (low pressure outlet), and providing a suction thereto, is a low pressure discharge conduit  27  connected to a low pressure pump  28 , the pump  28  in turn being connected to the conduit  12  to supply slurrying liquid to the chip chute  11 . 
         [0028]    A conventional screen grid  29  is mounted within the housing  14  at the third port  13 . As seen in  FIG. 1 , the screen grid  29  allows liquid under a low pressure, e.g., atmospheric pressure (or slightly above atmospheric pressure due to the chip tube) to pass into the conduit under the influence of the suction of pump  28  and gravity. The screen grid prevents chips or other cellulosic fibrous material from passing through the third port  13 . The screen grid ensures that chips or other cellulosic fibrous material remain in and fills the pocket  21  of the rotor  19 , while the pocket is vertically aligned with the first and third ports. 
         [0029]      FIG. 3  is a perspective view of a conventional high pressure feeder (HPF)  10 . The ports  13 ,  15 ,  16  and  17  are provided in the casing  14  of the HPF. Each port is generally rectangular in shape, but may have a shape adapted to accommodate the flow into or out of the rotor and to a conduit connected to the inlet. The elongated rectangular ports (see  13 ,  17  in  FIG. 3 ) are aligned with the four pockets of the rotor  19 . A divider wall  30  is centered in each of the ports  13 ,  15 ,  16  and  17  and is preferably aligned between the pockets of the rotor to avoid obstructing flow through the pockets. The divider walls  30  is a structural support for ports of the casing. An inner end of the divider wall  30  is adjacent a housing liner  32  or an inner wall of the casing  14 . 
         [0030]    The rotor  19  is attached to shafts  34  coaxial to the rotational axis of the rotor. The shafts are supported by bearings in the casing. A motor is coupled to one of the shafts to rotationally drive the rotor. The shafts allow for limited axial adjustment of the rotor within the casing. 
         [0031]    An adjustment device (represented by hand turn wheel  33 ) moves the shafts and rotor axially with respect to the casing. Typically, the rotor is moved axially in the casing to adjust the clearance between the rotor and the liner or inner wall of the casing. The rotor and liner/inner wall of the casing may each have a slight taper along their length to their otherwise cylindrical shape. Because of the taper in the rotor and liner/inner wall, moving the rotor axially changes the clearance between the rotor and liner/inner wall. The clearance may be adjusted to compensate for wear of the rotor, liner and inner wall. 
         [0032]      FIG. 4  is an exploded view of the rotor  19 , housing liner  32  and a conventional screen grid  29 . The rotor is tapered from a first end  36  to the second end  38 . The rotor is seated in and rotates with respect to the liner. The liner  32  is tapered in a corresponding manner to the rotor. The rotor  19  includes one or more (e.g. four) diametrically through-going pockets  21 . 
         [0033]    The pockets  21  each form a flow passage extending through the rotor in a direction transverse to the rotational axis. Each pocket has ports  40 ,  42  that alternatively allow flow into and out of the pocket. Each pocket  21  has a pair of ports  40  or  42  that are aligned along a flow passage extending perpendicular to the rotor axis. The ports  40  in the left-hand side of the rotor are provided for the pair of pockets  21  in the left hand side of the rotor. The ports  42  are provided in the rotor for the pair of pockets  21  in the right-hand side of the rotor. 
         [0034]    As the rotor turns in the liner, the ports  40 ,  42  of the pockets move into and out of alignment with the openings  46  in the liner. Each of the openings  46  in the liner correspond to and is aligned with one of the ports  13 ,  15 ,  16  and  17  in the HPF. A center circumferential section  63  of the liner is conventionally solid. This center section  63  of the liner is aligned with the mid-section of the screen grid  60  and is generally aligned with the center circumferential section  62  of the rotor. 
         [0035]    The conventional screen grid  29  is typically two screen grid sections  31  seated adjacent each other in the low pressure outlet of the casing. Each screen grid section includes an integral metal body, e.g., cast metal, having a frame  50  and a plurality of bars  52 . The frame is defined by opposite sides  54  arranged generally parallel to the rotor axis, and opposite sides  56  arranged perpendicular to the rotor axis. The bars  52  may be concave to conform to the outer surface of the rotor and have a lower region along mid-line  58 . 
         [0036]    Slots are formed between the bars and are parallel to the bars. The width of the slots is typically about 8 millimeters (mm). The slots are sufficiently narrow to block the passage of wood chips and allow the passage of liquor out of the pocket  21 . 
         [0037]    A center portion  60  of the screen grid  29  is solid and has an outer surface that provides an abutment to the divider wall  30  in the outlet port  30 . The screen grid extends substantially the length of the rotor and the center portion  60  is generally aligned with a circumferential region  62  of the rotor between the ports  40 ,  42  in the left-hand and right-hand sides of the rotor. 
         [0038]    The thickness (TH) of the screen grid may taper from one end  56  to the other end  56 . The taper conforms to the taper of the liner and rotor. 
         [0039]    The screen grid  31  has a recess  100  that seats on prongs  102  extending inwardly of an inside wall  75  of the low pressure outlet of the HPF casing  14 . Fastener bolts  104  secure the screen grid  31  to the prongs  102  and the casing. 
         [0040]      FIGS. 5A and 5B  are cross-sectional views of a lower half of a conventional high pressure feeder  10 .  FIG. 5A  shows the shaft  34  and rotor  19  extended to the left in the housing of the feeder.  FIG. 5B  shows the shaft  34  and rotor  19  extended to the right in the housing of the feeder. 
         [0041]    The screen grid  29  shown in  FIGS. 5A and 5B  is a conventional grid, such as shown in  FIG. 4 . The grid is seated in the casing  14  of the HPF over the low pressure outlet (third port)  16 . The center section  60  of the screen grid abuts the inner end of the divider wall  30 . The centers section my have a slot or groove in the outer surface to receive the inner end of the divider wall. 
         [0042]    The center section  60  and inner end of the divider wall  30  obstruct a portion of the low pressure outlet  16  of the casing and the ports  40 ,  42  in the rotor. The amount of obstruction to flow through the low pressure outlet  16  varies depending on the axial position of the rotor in the casing. The center section  60  and divider wall  30  are generally aligned with the circumferential center section  62  of the rotor, especially when the rotor is backed to the left side of the casing as shown in  FIG. 5A . The center section  60  and divider wall  30  are less aligned with the circumferential center  62  of the rotor when the rotor is advanced axially towards the right side of the casing, as shown in  FIG. 5B . 
         [0043]      FIGS. 6A and 6B  show cross-sectional views of a lower half of a high pressure feeder (HPF)  70 .  FIG. 6A  shows the shaft  34  and rotor  19  extended to the left in the housing of the HPF.  FIG. 6B  shows the shaft  34  and rotor  19  extended to the right in the housing of the HPF. 
         [0044]    The HPF  70  is similar in many respects to the HPF  10 . The differences between the two HPFs  10 ,  70  include that the HPF  70  has a screen grid  72  with wide slots; the slotted region  108  of the screen grid  72  is enlarged and is continuous along the length of the screen grid to conform to the entire area of the enlarged low pressure outlet  16 . Further, the wall  74  of the low pressure outlet  16  has been shifted axially outward to expand the open area of the low pressure outlet. The end  76  of the divider wall  78  is set back from the screen grid  72  by a gap (G). In addition, the center circumferential sections  62 ,  63  of the rotor and liner, respectively, may be narrowed to allow the ports  40 ,  42  of the rotor and outlets  46  in the liner to be expanded. The center bar in the low pressure outlet of the liner has been eliminated to open the flow area through the liner. 
         [0045]    The divider wall  78  in the low pressure outlet does not abut the screen grid  72 . A gap (G), e.g., one to two inches (25 mm to 50 mm), between the end  76  of the divider wall and the outer surface of the screen grid  72  allows liquid flowing out of the screen grid to enter the outlet  16  and thereafter pass over the end  76  of the divider wall. In view of the gap (G), the slotted region of the screen grid extends through the mid-section of the screen grid. The screen grid  72  need not have a solid-center section (or a solid region at the junction of two screen grid sections  31 ). 
         [0046]    The inventors recognized that the end of the divider wall need not abut the screen grid. Further, the inventors recognized that providing a gap (G) between the end of the divider wall and the screen grid and providing slots through the center of the screen grid would allow liquid to flow through the center of the screen grid. 
         [0047]    The drive end of the casing can be reduced in length to effectively expand the open area of the low pressure outlet  16 . In addition, the interior wall  74  of the low pressure outlet  16  may be shifted relative to the corresponding wall  75  of a conventional HPF. For example, the wall  74  of the low pressure outlet is shifted by a distance (T) which may be about an inch (25 mm). The inventors recognized that shifting the wall, e.g., changing the slope of the wall  74 , allows more of the grid  72  to pass low pressure liquid flow by aligning with the opening  21  of the rotor. 
         [0048]      FIG. 7  is a perspective view of a screen grid having bars  82  and slots  84  between the bars. The slots may each form a gap of about 10 mm to 25 mm. Liquid flows through the slots  84  and chips are retained by the bars in the pocket of the rotor. The slots and bars extend substantially the entire length of the screen grid. The width of each of the bars may be larger, e.g., 10 mm to 25 mm, than the width of the slots. A uniform array of parallel slots and bars extends uninterrupted across the portion of the screen grid corresponding to the opening in the low pressure outlet of the HPF. Because of the uniform array, there is no wide solid bar in the center of the screen grid as there is with conventional screen grids. 
         [0049]      FIG. 7  shows that the center circumferential bar at the low pressure outlet of the liner  110  may be eliminated. Similarly, a solid center section in the screen grid is unnecessary. The bars  82  may be concave to conform to the cylindrical shape of the rotor. The length (L) of the slotted section of the screen grid  72  covers substantially the entire length of the open area of the low pressure outlet  16  and the ports  42 ,  44  to the rotor. This length (L) is longer than conventional screen grids, e.g., by about an inch, because of the increased length of the opening in the liner. 
         [0050]      FIG. 8  is a graph  90  showing the percentage of open screen based on axial rotor position in a casing of a HPF. The curved line  92  represents the percentage of open screen for the conventional HPF  10  and shows the change in this percentage as the rotor is moved axially in the casing. The curved line  94  represents the percentage of open screen for the screen grid  72  in the HPF  70  and shows that the percentage of opening remains very high, e.g., effectively 100 percent, regardless of the axial rotor position. 
         [0051]    Specifically, the curved line  92  represents a convention HPF  10  having a screen grid  29  with a nominal slot size of 8 mm, a divider bar abutting a center region of the screen grid, and an elongated drive end of the casing  14 . As shown by curved line  92 , when the rotor is positioned in the casing as shown in  FIG. 5A  (which is the rotor position used typically at the beginning of the wear life of the HPF), the efficiency of the screen grid flow is just above 75% due to the number of unobstructed slots in the screen grid and the narrow width of the slots. As the rotor wears, it is advanced axially towards the center of the casing. The advancement of the rotor results in more of the screen slots being available (because of better alignment between the center section of the screen grid and the center of the rotor) so that the flow of liquid is increased through the screen grid and the efficiency increases to almost 95 percent. As the rotor further wears and is advanced axially towards the drive end of the casing, the screen slots again become obstructed and the efficiency falls to about 85 percent. 
         [0052]    The uniform and high efficiency represented by straight line  94  is due to the unobstructed slots in the screen grid regardless of the axial position of the rotor, the wider slots in the screen grid, a single and longer screen grid (due to the added length to the screen grid by increasing the length of the liner opening, and a shortened outlet divider bar that does not block a portion of the screen grid. 
         [0053]    Some of the features and advantages provided by the HPF  70  include: 
         [0054]    A. increased efficiency of liquid flow through the screen grid over the life of the rotor. 
         [0055]    B. a slot width in the screen grid of between 10 to 25 mm, which is wider than conventional slots. 
         [0056]    C. a shorten outlet divider bar which does not abut the screen grid and thereby allows a larger single screen grid with slots facing the bar being unobstructed by the bar. 
         [0057]    D. increase length of openings in the liner to expand the low pressure outlet in the casing and allow for a longer screen grid having a greater slotted area. 
         [0058]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.