Abstract:
A the operation of pulp screening apparatus may be improved by employing a multi element foil having a leading foil section and a trailing foil section spaced from and trailing leading section so that adjacent surfaces of the sections one formed by a portion of a pressure side of the leading section and the other by the leading end of the trailing foil section define opposed walls of a passage for fluid directing fluid flow from the pressure side of the leading foil section to a cambered low pressure side of the trailing section. The angle of attack (α) of the complete multi element foil is set to be significantly less than the angle of attack (θ) of the trailing foil section to increase the negative pressure pulse generated by the trailing section and thereby improve operation of the screening device.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to an improved screening apparatus more particularly the present invention relates an improved pulp (as used in the paper industry) employing a hydrofoil to pump pulp through the screen and to clean the screen.  
         BACKGROUND OF THE PRESENT INVENTION  
         [0002]    The use of rotors with foils for cleaning pulp screens by generating pressure pulses as the foil is moved past the screen is a well-known and common technique that has been practiced in the industry for many years. The pressure pulse, specifically the negative pulse, clears the apertures by causing a flow reversal that backflushes the fibres in the apertures. This cleaning technique is reasonably effective, but the maximum negative pressure pulses that conventional foils or rotors can generate effectively are limited. Some specific examples of those found in the art are described below.  
           [0003]    PCT application—PCT/SE89/00568 WO 90/05807 published May 31 1990 inventor Lundberg et al. discloses a typical screening apparatus and teaches the use of wing elements on the rotor (as opposed to foils) constructed so that the leading end of the wing in the direction of rotation is spaced closer to the screen than the trailing end and the wing has a dimension measured in the direction of movement (circumferential direction) that is at least twice the radial dimension of the screen to generate a suction force to draw liquid that has already passed through the screed to the outlet side back through the screen to the inlet side to dilute the pulp on the inlet side and to clean the pores of the screen.  
           [0004]    PCT application no PCT/FI/00151—WO 93/22494 published Nov. 11, 1993 to Alajaaski et al. describes a special pulse generator that tends to locally confine the pulse to thereby improve the cleaning operation of the pulse generator which in turn increases screening efficiency  
           [0005]    PCT application PCT/US94/04582—WO 94/25183 published Nov. 10 1994 inventor Egan et al. describes the use of a special adjustable hydrofoil having a moveable section projecting out from it&#39;s cambered surface. The position of this moveable section is adjusted to obtain the optimum spacing between the screen and rotor to thereby improve the operation of the screening device.  
           [0006]    EP 0950 754 A1 published Oct. 20 1998 by Alkawa describes a stirring device in the form of a foil that applies fluid pressure against the screen adjacent to the leading end of the foil and a negative pressure for cleaning the screen adjacent to the trailing end of the foil.  
           [0007]    U.S. Pat. No. 5,799.798 issued Sep. 1, 1998 to Chen teaches the use of conventional stirrers or foil and uses specially designed screen bars to improve the operation of the screening system.  
           [0008]    Japanese patent 93-243392 shows the use of angular bars on the low-pressure side of the screen to improve the operation of the screening device.  
           [0009]    In the aircraft industry higher angle of attacks are achieved without separation of the air passing along the foil from the camber surface of the foil by employing cambered airfoils with multi-element configurations. This results in being able to attain higher lift forces by using multi-element airfoils which in effect delay the onset of flow separation from the foil (stall) and allow higher angles of attack and increased camber. The stall condition is delayed by allowing air from the high-pressure side of the wing or foil to pass into the boundary layer of the low-pressure side of the wing. This injection of air re-energizes the boundary layer enabling the flow to remain attached to the foil. Multi-element airfoils are commonly used in aerodynamic applications.  
         BRIEF DESCRIPTION OF THE PRESENT INVENTION  
         [0010]    It is an object of the present invention to provide an improved foil for improving the effectiveness of the screening process.  
           [0011]    Broadly the present invention relates to a pulp screening apparatus comprising a substantially cylindrical screen having a cylindrical axis, a foil, means for mounting said foil for rotation on said cylindrical axis, said foil having a leading foil section and a trailing foil section, said leading foil section leading in a direction of movement of said foil as it is rotated around said cylindrical axis and said trailing section spaced from and trailing said leading section in said direction of movement to provide a space separating said a trailing end of said leading foil section and a leading end of said trailing foil section and defining a passage for fluid, each of said foil sections having a high pressure side facing away from said screen and a cambered low pressure side facing and positioned adjacent to said screen, said trailing end of said leading foil section having a portion adjacent to which said leading end of said trailing foil section is positioned so that a surface of said portion of said pressure face on said leading foil section and an adjacent surface of said leading end of said trailing foil section define opposite walls of said passage, said high pressure side of said leading foil section, said opposite walls of said passage and said cambered low pressure side of said trailing foil section being relatively positioned so that fluid passing across said high pressure side of said leading foil section passes through said passage and along said cambered low pressure side of said trailing section, said foil being set at a first angle of attack (α) and said trailing foil section being set a second angle of attack (θ).  
           [0012]    Preferably said first (α) and second (θ) angles of attack are different.  
           [0013]    Preferably said second angle of attack (θ) is larger than said first angle of attack (α).  
           [0014]    Preferably said first angle of attack (α) will be in the range of 0 to 30°, more preferably 5 to 15°, and said second angle of attack (θ) will be in the range of 0 to 60°, more preferably 5 to 15°.  
           [0015]    Preferably, said passage has a substantially uniform width measured parallel to said axis said width tapering from its mouth at the intersection of high pressure surface of said leading foil section with said cavity and minimum width (w) between opposite surfaces.  
           [0016]    Preferably said minimum dimension (w) measured will be in the range of 0.1 to 5 centimeters (cm.), more preferably in the range of 0.5 to 2 centimeters.  
           [0017]    Preferably said leading foil section has a first length (x+y) measured along its cambered surface and said trailing foil section has a second length (z) measured along its cambered surface and the ratio of said first length to said second length will be in the range of 1 to 2 and 1 to 0.1, more preferably 1 to 1 and 1 to 0.25.  
           [0018]    Preferably said portion comprise a nesting cavity formed in said leading foil and said leading end of said trailing foil is received in said nesting cavity. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0019]    Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which;  
         [0020]    [0020]FIG. 1 is a schematic axial view of a pulp screening apparatus incorporation the present invention  
         [0021]    [0021]FIG. 2 is a schematic cross section showing a multi element foil (MEF) of the present invention.  
         [0022]    [0022]FIG. 3 is a section similar to FIG. 2 but showing a leading foil section with an aerodynamically shaped cavity into which the leading end of the trailing foil section is received. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    [0023]FIG. 1 shows a typical right cylindrical pulp screen  10  having a cylindrical axis represented by the point  12 . A rotor is represented in Figure by a plurality of foils  14  that are mounted for rotation around the axis  12  as schematically represented by the arrow  16 .  
         [0024]    As in conventional operations the pulp to be cleaned in the illustrated arrangement is introduce in side of the screen  10  and the cleaned pulp that passes through the screen  12  as indicated by the arrow  20  is collected in the surrounding chamber  18  and from there directed to the next step in the operation. While the arrows  20  indicate the preferred direction of flow it is known to operate screens with the flow in the opposite direction so that the chamber  16  is the inlet chamber and the screened pulp is collect inside the screen  10 . The present invention can be adapted to either type of operation i.e. pulp flow toward or away from the axis  12 , however the disclosed embodiment show flow away from the axis  12 . One skilled in the art can easily convert to flow in the opposite direction.  
         [0025]    The present invention replaces the conventional foils or rotor elements normally employed in such screen rotors with multi-element foils (MEF)  14  of the type that will be disclosed in greater detail here in below. The function of each the foil  14  is to operate in the conventional manner to facilitate the screening operation. As above described one of the principal operations of the foil is to generate a negative pressure pulse at the trailing end of the foil to pull material back through and clean the screen.  
         [0026]    Foils also may be shaped to generate a positive pressure adjacent to the leading end of the foil to drive material through the screen. The foil  14  in the illustrated embodiments is configured to generate a pressure pulse adjacent to the leading end of the foil  14 .  
         [0027]    The use of the MEF  14  of the present invention permits improving the operation of the screen by increasing the magnitude of the pressure pulses, particularly the negative pressure pulse generated at the trailing end of the foil  14 .  
         [0028]    The embodiment of the present invention shown in FIG. 2 is a two foil section MEF  14  having a leading foil section  22  leading in a direction of movement of the foil  14  as it is rotated around said cylindrical axis  12  as indicated by the arrows  16  and a trailing foil section  24  trailing the leading section  22  in the direction of movement  16 .  
         [0029]    The leading foil section  22  has a cambered surface  26  facing toward the screen  10  and an aerodynamic, smooth surface  28  on the side of the section  22  opposite the camber surface  26 . The cambered surface  26  trailing the leading end  29  of section  22  is contoured and oriented to approach more closely the screen  10  till the distance between the screen and the surface  26  reaches a selected minimum as indicated at  30  a distance x from the leading end  29  and y from the trailing end  32  of the section  22 . The ratio of x/y will normally be in the range of 1 to 10 preferably 1 to 05.  
         [0030]    The trailing foil section  24  is formed primarily to generate suction (low) pressure on its cambered (low pressure) surface  34 , which faces the screen  10  which aids in producing a higher magnitude (lower pressure) negative pressure pulse at the trailing end  44  of the section  24 . A flat or high-pressure (aerodynamic smooth) surface  36  forms the side of the foil section  24  remote from the screen  10 .  
         [0031]    The cambers or shapes of the surfaces  26  and  34  are each selected based on conventional design practise.  
         [0032]    The surface  28  adjacent to the trailing end  32  of foil section  22  is formed with a nesting portion  38  that is positioned between the leading end  40  of the trailing foil section  24  and the screen  10 . The portion 38  and the leading end  40  are relatively mounted on the rotor (not shown) so that there is a space or passage  42 , the opposed walls of which are formed by the adjacent surfaces of the portion  38  and the leading end  40 . This passage  42  interconnects and directs fluid flow from the flat or high pressure side  28  of the foil section  22  to the cambered or low pressure side  34  of the trailing foil section  24 .  
         [0033]    The length z of the cambered surface  34  of foil section  24  measured from the leading end  40  to the trailing end  44  is correlated with the length x+y of the surface  26 . The location of the gap or passage  42  between the two foils  22  and  24  (i.e., the relative sizes of the foils) which ends at the trailing end  32  is chosen such that the trailing end  32  is reached before the point of stall for flow along the cambered surface  26  of foil section  22  is reached. The location of stall as is well known is a complex function of foil shape, angle of attack, etc. In practice, the length z of the trailing foil is about ½ to ¼ of the length x+y of the leading foil  22 .  
         [0034]    It will be apparent that the effective axial length of the foil  14  and thus of the foil sections  22  and  24  extending axially (parallel to the axis  12 ) will be substantially the full axial length of the screen  10 . The most likely configuration would be a series of short axial length foils  14  that extend only ¼ or ⅓ the axial length of the screen. I.e. a plurality of the shorter axial length foils  14  arranged in a staggered configuration that extends the entire axial length of the screen  10 . It will be apparent that a full length foil  14  and/or a segmented short axial length foils  14  configuration could be used.  
         [0035]    Thus the passage  42  also extends substantially the full axial length of the screen  10  and maintains a substantially uniform spacing between the leading end(s)  40  and the adjacent wall of the portion(s)  38  of the surface(s)  28  of the foil section(s)  22  along substantially the full axial length of the foil  14 .  
         [0036]    As is apparent from the illustration in FIG. 2 the passage  42  tapers in the direction of flow from the mouth of the passage  42  adjacent to the leading end  40  to the minimum width position  41  where the passage  42  has a minimum width dimension w between surfaces  38  and the adjacent surface  34  of the foil section  24  trailing the leading end  40 . This minimum distance w will normally be in the range of 0.1 to 5 cm.  
         [0037]    As is known the curvature of the surface  38  is designed so that the fluid flowing along the surface  28  remains in hugging relationship with the surface  38  defining one side of the passage  42  and at or adjacent to the minimum width position  41  flow along the surface  38  transfers to the surface  34  without generating any undue turbulence and combines with and aids in the transfer of the fluid flow leaving the surface  26  so that there is a smooth transition of fluid flow from flow along the surface  26  to flow along the surface  34  as well as flow from the surface  26  (through the passage  42 ) to the surface  34 .. In effect both foil sections  22  and  24  are aerodynamic on both the leading  29  and  40  and trailing  32  and  44  edges to eliminate any flow separations at the trailing edge  32  of the first foil The trailing end  44  is aerodynamic (sharp) such that the flows along surfaces  34  and  36  merge together smoothly which reduces the drag on the foil and reduces the power required to rotate the rotor (foil  14 ). One form of the camber that was found satisfactory is mathematically calculated and is known in the art as a National Advisory Committee for Aeronautics (NACA) shape, more particularly, a NACA 8412 airfoil cut into two airfoils and reshaped.  
         [0038]    The leading foil section  22  may be mounted on the rotor (not shown) for angular adjustment relative to a radius leading to the leading end  29  of the section  22  and to be moved radially relative to the axis  12  to be positioned closer or farther from the screen  10  as indicated schematically by the arrow  50 . The foil section  24  may be mounted to permit adjustment as indicted by the set of arrows  52 . However for a given installation when the optimum positioning has been established the positioning and orientation of the sections  22  and  24  will normally be fixed.  
         [0039]    The angle of attack α of the foil  14  which includes the two foil sections  22  and  24  and the cord  54  from which the angle of attack α is determined extends from the leading end  29  of section  22  to the trailing end  44  of section  24 . I.e. the angle of attack a of the foil  14  is the angle between the direction of relative movement of the foil through the pulp as indicated by the dotted line  56 .  
         [0040]    The angle of attack θ of the trailing foil  24  is determined by the angle θ between the cord  58  joining the leading end  40  and trailing end  44  of the cambered surface  34  and line  56  and is significantly different from the angle a.  
         [0041]    The angle θ generally will be significantly larger than the angle a to up to about tripple  
         [0042]    In some special cases for example where the foil  14  is at zero angle of attack (α=0°), the angle of attack θ of the trailing section  24  may also zero (θ=0°). Thus in some cases α may equal θ (α=θ).  
         [0043]    The first angle of attack α (of the foil  14 ) will be in the range of 0° to 45° more preferably 5° to 15° and the second angle of attack θ (of the trailing section  24 ) will be in the range of 0° to 60°, more preferably 5° to 25°.  
         [0044]    In operation fluid flowing along the surface  28  of the leading foil section  22  follows the surface of the trailing portion  38  through the space  42  between the surface  38  and the leading end  40  of the foil section  24  and then leaves the surface  38  at about the point  41  and follows the cambered surface  34  of the trailing foil section  24 . This flow along the surface  34  stabilizes the flow from the surface  26  as it passes onto and over the surface  34  so that the angle of attack θ of trailing foil section  24  may be increased significantly beyond what could normally be achieve with a conventional single element foil. To insure that the flow of fluid flows smoothly from the surface  28  into the space or passage  42  the geometry of the portion  38  of surface  28  must be aerodynamic to avoid flow separation and to reduce drag that causes undue power consumption. The surface  38  may also be designed to conform to the shape of the leading surface at the leading end  40  of foil  24  such that the both foil sections  22  and  24  together act as a single aerodynamic foil as will be described below with reference to FIG. 3.  
         [0045]    In FIG. 2 the shape of the surfaces  26  and  28  foil section  22  adjacent to its trailing end  32  and the shape of tehsurface  34  adjacent to the leading end of the airfoil section  24  are aerodynamic which enables the flow to readily pass between the two foils. In FIG. 3, the passage  42 A is more tortuous as the portion  38  is converted to a cavity shaped aerodynamic configuration which makes it more difficult for fluid to follow the surface portion  38 A and pass through the passage  42 A but has the advantage that the entire airfoil  14  is more aerodynamic and has a small drag.  
         [0046]    As indicated, the portion  38 A as illustrated in FIG. 3 and extending from or forming the trailing end of the pressure surface  28  of the leading foil  22  has been change from what is shown in FIG. 2 so that the cavity defined by the portion  38 A is adapted to receive the leading end of the trailing foil  24  with the leading end  40  and adjacent portion of the surface  34  forming one wall of the passage  42  and the surface  38 A forming the opposed surface of the passage  42 A in the same manner as the surfaces  38  and  34  form opposed walls of the passage  42  in the FIG. 2 embodiment.  
         [0047]    The invention has been described with the foil  14  composed of two foil sections  22  and  24 , but it is believed that more sections in series could be used if desired in the same manner as such multiple section foils are used in the aircraft industry.  
         [0048]    Having described the invention, modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.