Abstract:
A radial impeller includes a cover having a frusto-conical portion and multiple blades extending radially outwardly. Related mixing systems involve positioning the radial impeller with blades adjacent a flow opening of a tank to draw water through the flow opening when the impeller is rotated.

Description:
TECHNICAL FIELD 
     The present invention relates to mixing arrangements for use in tanks, and more particularly to mixing arrangements which utilize radial flow impellers. 
     BACKGROUND 
     Radial flow impellers have long been used in continuous flow mixing systems. Such systems have, for example, been used for copper extraction applications and may accomplish their extraction operation by rotating a radial flow impeller near the base of a mixing chamber to expedite the flow of a mineral rich solution into the mixing chamber through a draft opening in the base of the chamber. Such applications are commonly referred to as pumper-mixers or lifter turbines. Exemplary systems using radial flow impellers include those described in U.S. Pat. Nos. 3,233,876, 4,207,275, 5,501,523 and 5,511,881. Other types of mixing systems, including batch systems, utilizing radial flow impellers are also known. 
     Many prior art radial flow impellers used in such systems consist of a flat plate cap and blades of uniform height positioned generally along the radial direction of the lower surface of the plate. Because the cap is flat and the blades are a uniform height, each blade extends a uniform depth below the cap all along its length from the inner to the outer radius of the cap. While in some applications the blades may trace the radii of the plate exactly, it is also known to use curved blades. Where the blades are of uniform height, and because adjacent blades necessarily are positioned closer to one another at the inner radius of the plate than at the outer radius of the plate, there is an increase in flow area between blades as fluid is pumped from the inner to the outer edge of the radial flow impeller. As a result of this increase in flow area, the flow velocity of fluids that are being pumped decreases near the outer radius of the impeller causing losses in pumping efficiency. 
     Accordingly, it would be desirable to provide mixing systems incorporating an improved radial flow impeller assembly. 
     SUMMARY OF THE INVENTION 
     In one aspect, a continuous flow mixing system for mixing two materials entering an inlet port of a tank, the tank including an outlet positioned above the inlet, is provided. The system includes a drive shaft extending within the tank and aligned with the inlet port. An impeller assembly including a plurality of blades and a cap member is coupled to the drive shaft to be rotated by the drive shaft. The plurality of blades extend radially outwardly, each blade including a lower side, the lower sides being substantially coplanar and lying adjacent the inlet port of the tank, each blade including an upper side. The cap member includes a substantially planar portion and a frusto-conical portion extending radially outward from the substantially planar portion and terminating in a circular rim. Rotation of the impeller assembly produces a head pressure for drawing material in the inlet port and raising a fluid level in the tank to at least a level of the tank outlet. The upper side of each blade is positioned adjacent an inner surface of the cap member and the lower side of each blade is positioned below a plane defied by the circular rim. A radially inner end of each blade is spaced between a central axis of the cap member and an intersection circle defined by intersection of the substantially planar portion and the frusto-conical portion. Each blade extends radially outwardly to at least the circular rim. A radius of the intersection circle is between about thirty percent (30%) and about sixty percent (60%) of a radius of the circular rim. A radial flow area defined by the inner surface of the cap member and adjacent blades of the impeller assembly remains substantially constant from a radial point starting at the intersection circle and extending to the circular rim. 
     In another aspect, a continuous flow mixing system for mixing two materials entering an inlet port of a tank, the tank including an outlet positioned above the inlet, is provided. The mixing system includes a drive shaft extending within the tank and aligned with the inlet port. An impeller assembly including a plurality of blades and a cap member is coupled to the drive shaft to be rotated by the drive shaft. The plurality of blades extends radially outwardly, each blade including a lower side, the lower sides being substantially coplanar and lying adjacent the inlet port of the tank, each blade including an upper side. The cap member includes a substantially planar portion and a frusto-conical portion extends radially outward from the substantially planar portion and terminates in a circular rim. Rotation of the impeller assembly produces a head pressure for drawing material in the inlet port and raising a fluid level in the tank to at least a level of the tank outlet. The upper side of each blade is positioned adjacent an inner surface of the cap member and the lower side of each blade is positioned below a plane defined by the circular rim. A radial flow area defined by the inner surface of the cap member and adjacent blades of the impeller assembly remains substantially constant from a radial point starting at an intersection circle and extending radially outward therefrom. 
     In a further aspect, a mixing system for a tank includes a drive shaft extending within the tank and a stationary tube centrally disposed and submerged within the tank, the tube having an upper opening and a lower opening. An impeller assembly including a plurality of blades and a cap member is coupled to the drive shaft to be rotated by the drive shaft. The plurality of blades extend radially outwardly, each blade including an exposed side, the exposed sides being substantially coplanar and lying adjacent the lower opening of the tube, each blade including a covered side. The cap member includes a substantially planar portion and a frusto-conical portion extending radially outward from the substantially planar portion, the frusto-conical portion terminating in a circular rim. Rotation of the impeller assembly draws material in the upper opening of the tube, down through in the tube, out the lower opening of the tube, and back upward along an annular spaced defined between the tube and the tank. The covered side of each blade is positioned adjacent an inner surface of the cap member and the exposed side of each blade is positioned above a plane defied by the circular rim. A radial flow area defined by the inner surface of the cap member and adjacent blades of the impeller assembly remains substantially constant from a radial point starting at an intersection circle defined by intersection of the substantially planar portion and the frusto-conical portion and extending radially outward from the intersection circle. 
     In another aspect, a mixing system for a tank includes a drive shaft and an impeller assembly. The drive shaft extends within the tank and the impeller assembly is coupled to the drive shaft to be rotated by the drive shaft. The impeller assembly includes a plurality of blades and a cap member. The blades extending radially outwardly, each blade including an exposed side, the exposed sides being substantially coplanar and lying adjacent flow opening of the tank, each blade including a covered side. The cap member includes a substantially planar portion and a frusto-conical portion extending radially outwardly from the substantially planar portion and terminating in a circular rim, rotation of the impeller assembly causing a flow out of the flow opening and through the impeller assembly. The covered side of each blade is positioned adjacent an inner surface of the cap member and the exposed side of each blade is spaced from a plane defined by the circular rim. 
     In yet another aspect, a mixing system includes an impeller assembly having a plurality of blades and a cap member. The blades extend radially outwardly, each blade including an exposed side, the exposed sides being substantially coplanar, each blade including a covered side. The cap member has an inner surface including a substantially planar portion and a frusto-conical portion extending radially outward from the substantially planar portion and terminating in a circular rim. The covered side of each blade is positioned adjacent the inner surface of the cap member and the exposed side of each blade is spaced away from a plane defied by the circular rim. A radially inner end of each blade is spaced between the central axis of the cap member and an intersection circle defined by intersection of the substantially planar portion and the frusto-conical portion. A radius of the intersection circle is between about thirty percent (30%) and about sixty percent (60%) of a radius of the circular rim. A radial flow area defined by the inner surface of the cap member and adjacent blades of the impeller assembly remains substantially constant from a radial point starting at the intersection circle and extending to the circular rim. 
     In yet a further aspect, a mixing system includes an impeller assembly having a plurality of blades and a cap member. The blades extend radially outwardly away from a central axis of the assembly, each blade including an exposed side, the exposed sides being substantially coplanar, each blade including a covered side. The cap member has an inner surface including a frusto-conical portion extending radially outward away from the central axis and terminating in a circular rim. The covered side of each blade is positioned adjacent the inner surface of the cap member and the exposed side of each blade is spaced away from a plane defined by the circular rim. A radially inner end of each blade is spaced from the central axis of the cap member and an outer tip of each blade extends at least to the circular rim. A covered blade height is between about sixty-six percent (66%) and about two-hundred thirty-three percent (233%) of an exposed blade height. A plurality of flow channels are defined by the frusto-conical inner surface portion of the cap member, adjacent blades of the impeller assembly and a plane defined by the exposed sides of the blades and the flow area of each flow channel remains substantially constant along its entire radial length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation of one embodiment of a mixing system using a radial flow impeller; 
     FIG. 2 is an enlarged side elevation of one embodiment of a radial flow impeller; 
     FIGS. 3A and 3B are bottom views of radial flow impeller arrangements; 
     FIG. 4 is a side elevation of another mixing system embodiment; and 
     FIG. 5 is a side elevation of another mixing system embodiment. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, a continuous flow mixing system  10  of a tank  12  is shown. A drive shaft  14  extends within the tank  12  and a radial flow impeller assembly  16  is coupled to the drive shaft for rotation by the drive shaft. The drive shaft  14  may be rotated by any technique commonly known in the art. The impeller assembly  16  is positioned above an inlet port  18  of the tank  12  through which materials may enter the tank as shown by arrows  19 . The impeller assembly  16  includes a plurality of blades  20  and a cap member  22 . The blades  20  extend radially outward and away from a central axis of rotation of the impeller assembly  16 . 
     As best seen in the enlarged impeller assembly view of FIG. 2, the cap member  22  includes a substantially planar portion  24  which is centrally located and a frusto-conical portion  26  which extends radially outward from the substantially planar portion  24  and terminates in a circular rim  27 . Each blade  20  includes an exposed side  28 , a covered side  30 , an inner end  32  and a radially outer end or tip  34 . The exposed sides  28  of the blades  20  are substantially coplanar. The covered side  30  of each blade lies adjacent an inner surface of the cap member  22  and may be fixed thereto. 
     In one arrangement, the frusto-conical portion  26  is angled (φ) such that a radial flow area defined by the inner surface of the cap member  22  and adjacent blades  20  of the impeller assembly  16  remains substantially constant from a radial point starting at an intersection circle  36 , defined by intersection of the substantially planar portion  24  and the frusto-conical portion  26 , and extending to the circular rim  27 . In this respect, the necessary angle (φ) to achieve such a uniform flow area can be calculated as follows. In describing such calculation the term “exposed blade height” is defined as the height (H BE ) of the blade  20  from its exposed side  28  to the plane defined by the circular rim  27 . The term “covered blade height” is defined as the height (H BC ) of the blade between the plane defined by the circular rim  27  and the substantially planar portion. The covered blade height will typically be substantially equal to the height (H C ) of the cap member  22 , at least where the outer contour of the cap member follows the inner contour of the cap member. However, it is understood the contour of the outer surface of the cap member  22  could vary from that of the inner surface. To calculate the angle (φ) needed to provide a substantially uniform flow area from the intersection circle  36  to the rim  27 , the flow area at the rim (FA R ) is first determined as: 
     
       
           FA   R =( H   BE )×(2  πR   2 ) 
       
     
     FA R  must then be set equal to the flow area at the intersection circle (FA I ), where: 
     
       
           FA   I =( H   BE   +H   BC )×(2  πR   1 ). 
       
     
     If initial impeller assembly design parameters are set as H BE =to 0.4 R 2 , and R 1 =0.5 R 2 , then: 
     
       
         
           FA 
           R 
           =FA 
           I 
         
       
     
     
       
         ( H   BE )×(2  πR   2 )=( H   BE   +H   BC )×(2  πR   1 ) 
       
     
     
       
         (0.4  R   2 )×(2  πR   2 )=(0.4  R   2   +H   BC )×(2  π( 0.5  R   2 )) 
       
     
     
       
         0.8  R   2   2 =0.4  R   2   2   +R   2   H   BC   
       
     
     
       
         0.4  R   2   2   =R   2   H   BC   
       
     
     
       
         0.4  R   2   =H   BC   
       
     
     The angle (φ) follows as: 
     
       
         φ=tan −1   [H   BC /( R   2   −R   1 )] 
       
     
     
       
         φ=tan −1 [0.4  R   2 /( R   2 −0.5  R   2 )] 
       
     
     
       
         φ=tan −1 [0.4  R   2 /(0.5  R   2 )]=tan −1 [0.8] 
       
     
     
       
         φ=38.7° 
       
     
     Thus, by setting design criteria including the exposed blade height (H BE ) and the radius of the intersection circle (R 1 ), the necessary angle (φ) can be determined. In one arrangement R 1  is selected between about thirty percent (30%) and about sixty percent (60%) of the radius (R 2 ) of the circular rim  27 . In another arrangement R 1  is selected between about forty percent (40%) and about fifty percent (50%) of the radius (R 2 ) of the circular rim  27 . Where flow area is equalized, the R 1 =(30-60%)R 2  design parameter will generally result in a covered blade height (H BC ) between about two-hundred thirty-three percent (233%) and about sixty-six percent (66%) of the exposed blade height (H BE ). The R 1 =(40-50%)R 2  design parameter will generally result in a covered blade height (H BC ) between about one-hundred fifty percent (150%) and one-hundred percent (100%) of the exposed blade height (H BE ). 
     Referring to the impeller assembly bottom views of FIGS. 3A and 3B, the blades  20  may be straight in one embodiment (FIG. 3A) or may curve in a direction away from the direction of rotation  38  of the impeller assembly in another embodiment (FIG.  3 B). Five blades  20  are shown in each embodiment, but it is recognized that the number of blades could vary according to the application. Additionally, while uniformly spaced blades are shown, in some circumstances the spacing could very, particularly where pairs of blades are positioned in close proximity to each other and are spaced from other pairs of blades. 
     Referring again to FIG. 1, the exposed sides  28  of the blades  20  are positioned at the lower end of the impeller assembly  16  and adjacent the inlet port  18  of the tank  12 . In one arrangement the spacing between the exposed sides of the blades and the tank wall defining the inlet port is maintained at less than ten percent of the impeller assembly diameter, and in another arrangement at less than five percent of the impeller assembly diameter. When the impeller assembly  16  is rotated a head pressure is created which causes liquid to flow into the tank  12  through port  18  and also causes the normal liquid level  40  of the tank  12  to rise to a level  42  which causes liquid to flow out of an overflow or other outlet  44  of the tank  12 , thus resulting in a flow through the tank  12 . 
     An alternative mixing system arrangement  50  is shown in FIG. 4 where a tank  52  includes a drive shaft  54  extending therein. The impeller assembly  16  is coupled to the drive shaft  54  for rotation, but is arranged in an upside-down position in comparison to that of FIG.  1 . The exposed sides of the blades  20  are positioned adjacent a lower opening  56  of a stationary draft tube  58  which is centrally disposed and submerged within the tank  52 . Rotation of the impeller assembly  16  causes liquid to be drawn in an upper opening  60  of the draft tube  58 , down through the draft tube  58 , out the lower opening  56  and back upward along the annular space defined between the tube  58  and the tank  52 . The draft tube  58  may include an annular plate  62  extending outward from the opening  56 , preferably to a radius which is at least as great as a radius of the impeller assembly  16 . In another embodiment shown in FIG. 5, the mixing system  70  includes a calandria or stationary tube bundle  72  in the annular space between the tube  58  and the tank  52 , which is commonly used for removing heat from the system. 
     Although the invention has been described above in detail referencing the preferred embodiments thereof, it is recognized that various changes and modifications could be made without departing from the spirit and scope of the invention.