Abstract:
The free-flow pump comprises an impeller ( 11, 22, 33 ) with an impeller base that is constituted by a front side ( 14, 5 24 ) of a hub body ( 12, 23 ) projecting at the center of the impeller ( 11, 22, 33 ) and by a disk surface ( 18, 28 ) located deeper than the front side ( 14, 24 ) of the hub body ( 12, 23 ) and reaching to an outer circumference of the impeller with its maximum depth. The disk surface ( 18, 28 ) is provided with vanes ( 19, 29, 34 ) comprising open vane front sides ( 20, 30, 35 ) adjoining the hub body ( 12, 23 ) at their inner end and extending from there to the outer circumference of the impeller ( 11, 22, 33 ). To avoid material accretions in front of the impeller ( 11, 22, 33 ) it is suggested that at least within an inner third of its radius, the impeller base is not located deeper with respect to the inner end of the vane front sides ( 20, 30, 35 ) than at most one sixth of the height difference (H) between the inner end of the vane front sides ( 20, 30, 35 ) and the maximum depth of the disk surface ( 18, 28 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §371 National Phase conversion of PCT/EP2012/053261, filed Feb. 27, 2012, which claims benefit of European Application No. 11157262.4, filed Mar. 8, 2011, the disclosure of which is incorporated herein by reference. The PCT International Application was published in the English language. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a free-flow pump having an impeller that is spaced from an inlet in such a manner that a free passage for solids contained in the pumped liquid results between the inlet and an impeller exit, the impeller comprising an impeller base constituted by a front side of a hub body projecting at the center of the impeller and by a disk surface located deeper than the front side of the hub body and reaching to an outer circumference of the impeller with its maximum depth, the disk surface being provided with vanes comprising open vane front sides adjoining the hub body at their inner end and extending from there to the outer circumference of the impeller. 
     BACKGROUND OF THE INVENTION 
     Free-flow pumps of this kind, as they are known from EP 0 081 456 A1 to the applicant of the present invention, are often used in wastewater that is contaminated in particular with solid matter. In such pumps the distance between the impeller and the pump inlet is chosen such that a free flow space is formed between the inlet and the impeller exit, the free flow space constituting a passage for a sphere of a predetermined largest sphere diameter that can possibly be pumped so as to counteract the risk of clogging due to the solid components in the pumped liquid. 
     In practice, however, it has often been found that particularly tissue or knit materials consisting of fibers or yarns or other solids composed of two-dimensional and flexible materials tend to accumulate at the impeller front surface and obstruct the desired unimpeded passage through the vane-free space. More specifically, a short-term or even permanent accretion of such materials has been observed in the central area of the impeller. This material accretion in front of the impeller surface causes an undesirable reduction of the pumping head and of the efficiency or leads first to a reduction of the flow rate and ultimately to total clogging of the pump. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to develop a free-flow pump of the kind mentioned in the introduction so as to prevent the accretion of two-dimensional materials in front of the rotation surface of the impeller to ensure an undisturbed pumping operation. 
     This object is attained by the free-flow pump according to claim  1 . The dependent claims define preferred embodiments. 
     Thus, according to the invention, a free-flow pump is suggested where at least within an inner third of its radius, the base of the impeller is not located deeper with respect to the inner end of the vane front sides than at most one sixth of the height difference between the inner end of the vane front sides and the maximum depth of the disk surface. 
     For it was surprisingly found in the context of the present invention that by a thus caused reduction of the suction effect in the central area of the impeller and a resulting enlargement of the flow path around this central area, the aforementioned accretion of two-dimensional materials can be significantly reduced or even entirely prevented over the entire impeller front surface. 
     The construction of the impeller is preferably optimized such that a reduction of the pump efficiency can be kept as low as possible in order to ensure the clog-free operation of the free-flow pump in a large number of applications. According to the invention it has been found to be essential in this respect that the disk surface reaches to the outer circumference of the impeller with its maximum depth. In this manner the pressure buildup required for producing the useful flow and the acceleration of the vortex in the flow space can be kept quite high and thus a relatively high pumping head can be achieved during a clog-free operation of the free-flow pump. 
     In order to further reduce the accretion of two-dimensional and flexible materials in the inlet area of the vane channels it is suggested that at least within an inner half of its radius, the impeller base is preferably not located deeper with respect to the inner end of the vane front sides than at most two thirds of the height difference between the inner end of the vane front sides and the maximum depth of the disk surface. More preferred, the impeller base is not located deeper than at most one half of this height difference relative to the inner end of the vane front sides. 
     To maintain a quite high pump efficiency, the height difference of the disk surface within a middle third of the radius of the impeller is preferably larger than half, more preferred larger than two thirds, of the height difference between the inner end of the vane front sides and the maximum depth of the disk surface. 
     An effective flow through the impeller can be achieved in that the disk surface comprises a surface portion continuously declining towards the outer circumference. Preferably, this surface portion extends over at least one third, more preferred over at least half, of the impeller radius. Most preferred, the continuously declining surface portion extends over at least two thirds of the impeller radius. With such an impeller geometry, a pump efficiency that is sufficient for many applications and the prevention of an undesirable accretion of two-dimensional materials in front of the impeller surface can be advantageously combined. In an advantageous embodiment of the invention, the continuously declining surface portion reaches to the outer circumference of the impeller. 
     Alternatively, the disk surface may comprise an essentially flat surface portion that extends at most over the outer two thirds, preferably at most over the outer half of the impeller radius. In this case, the flat disk surface may e.g. directly adjoin to the front side of the hub body along an abrupt rise in height. Thus, for example, the disk surface may exhibit a substantially stepped decline within a middle third of its radius. 
     Another advantageous embodiment of the impeller according to the invention may comprise that the disk surface adjoins the front side of the hub body continuously along a curved surface portion. The curvature may contribute to the prevention of an accretion of two-dimensional materials in the impeller inlet area. In particular, a convex curvature may be employed. It may further be useful in this respect that the open vane front sides may adjoin the hub body in the area of the front side thereof. Furthermore it can be advantageous in this respect that the front side of the hub body has a substantially flat configuration. However, a steeper shape of the surfaces on the front side may also be contemplated. 
     To achieve optimum HQ characteristics, which characterize the functional dependence between the pumping head and the flow rate, a curved shape of the vane front sides towards the outer circumference of the impeller may be advantageous. 
     According to another advantageous embodiment of the invention, the height of at least two vanes increases towards the outer circumference of the impeller. This may contribute to an increase in pump efficiency as in this manner an increased force is applied to the pumped liquid exiting the impeller in the radial direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail hereinafter by means of preferred embodiments with reference to the drawings which illustrate further properties and advantages of the invention. The figures, the description, and the claims comprise numerous features in combination that one skilled in the art may also contemplate separately and use in further appropriate combinations. The drawings show: 
         FIG. 1 : a meridian section through a free-flow pump according to a first embodiment; 
         FIG. 2 : a front view of the impeller according to II of the free-flow pump shown in  FIG. 1 ; 
         FIG. 3 : a cross-section of the impeller according to III of the free-flow pump shown in  FIG. 1 ; 
         FIG. 4 : a meridian section through a free-flow pump according to a second embodiment; 
         FIG. 5 : a front view of the impeller according to V of the free-flow pump shown in  FIG. 4 ; 
         FIG. 6 : a cross-section of the impeller according to VI of the free-flow pump shown in  FIG. 4 ; 
         FIG. 7 : a meridian section through a free-flow pump according to a third embodiment; 
         FIG. 8 : a front view of the impeller according to VIII of the free-flow pump shown in  FIG. 7 ; and 
         FIG. 9 : a cross-section of the impeller according to IX of the free-flow pump shown in  FIG. 7 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A free-flow pump  1  shown in  FIG. 1  comprises a pump enclosure  2  having a frontal inlet opening  3  and a laterally arranged outlet opening  4 . Pump enclosure  2  encloses an impeller chamber  6 . 
     In impeller chamber  6 , an impeller  11  is arranged at such a distance from inlet opening  3  that a free passage  7  for solids contained in the pumped liquid results towards outlet opening  4 . Impeller  11  has a hub body  12  in which a shaft  8  is fastened. Shaft  8  extends along longitudinal axis  5  into the rearward part of pump enclosure  2  where it is connected to a drive not represented in the figure. 
     Hub body  12  includes a front plate  25  whose free surface  24  forms the central portion of the front side  14  of hub body  12 . The surface  24  of front plate  25  has a substantially flat shape. Front plate  25  has a central bore for receiving a screw  9  and a gently rounded edge that is followed in the radially outward direction by a flat frontal surface portion  13  of hub body  12 . Thus, front side  14  of hub body  12  has a substantially flat overall shape and extends over a little more than a third of the total radius of impeller  11 . 
     Front side  14  of hub body  12  abruptly connects to an outer wall  15  of hub body  12  and forms a step therewith. This surface portion  15  adjoining the front side  14  of hub body  12  extends substantially in parallel with respect to the longitudinal axis  5  of pump enclosure  2  over half of the impeller depth and is then followed by a concavely curved portion  16 . 
     The concavely curved surface portion  16  of hub body  12  extends approximately over the middle third of the radius r of impeller  11  and then reaches its maximum depth relative to front side  14  of hub body  12 . At this point, the concavely curved portion  16  is followed by a flat surface portion  17  that extends substantially perpendicularly to the longitudinal axis  5  of pump enclosure  2 . This flat portion  17  extends over the entire outer third of the radius of impeller  11  and reaches to its outer circumference. 
     The disk surface  18  formed by surface portions  15 - 17  is provided with vanes  19 . Vanes  19  each extend from their inner ends  42  adjoining portion  15  of hub body  12 , which is substantially parallel to longitudinal axis  5  to the outer circumference of impeller  11 . Vanes  19  have a substantially constant height characteristics. The height H of vanes  19  is equal to the height difference Hn between the flat surface portion  17  and the abrupt junction between front side  14  and external wall  15  of hub body  12 , or slightly smaller. 
       FIG. 2  shows a top view of front side  14  of hub body  12  and of the surrounding disk surface  18  constituting the impeller base of impeller  11 . Twelve vanes  19  are arranged around disk surface  18  at regular intervals. The open vane front sides  20  of vanes  19  adjoin the junction between front side  14  of hub body  12  and disk surface  18 . From there, vane front sides  20  extend to the outer circumference of impeller  11  in a curved shape while their thickness remains constant. The direction of curvature of vanes  19  is opposed to the direction of rotation R of impeller  1 . 
       FIG. 3  shows a cross-sectional view of impeller  11  according to section III in  FIG. 1 . This corresponds to a section through impeller  11  along half of the height difference H between the inner end of vane front sides  20  and the maximum depth of disk surface  18 , measured by its distance from the surface portion of the inner ends of vane front sides  20  which is closest to the inlet side. As follows from  FIG. 3 , in this depth range of impeller  11 , disk surface  18  lies at the same height as surface portion  15  of hub body  12  that is located in the middle third of the radius of the impeller  11 . 
     The free-flow pump  1  described above allows pumping liquids that are e.g. contaminated with cloths or rags without clogging impeller chamber  6 . The tendency of two-dimensional materials to deposit on the front side of impeller  11  can be effectively counteracted by the described geometry of impeller  11 . 
     In  FIG. 4  a free-flow pump  21  according to a second embodiment is illustrated. Components that are designed identically with regard to free-flow pump  1  shown in  FIG. 1  are designated by the same reference numerals. The essential difference of free-flow pump  21  as compared to the previously described free-flow pump  1  consists in a different geometry of its impeller  22 . On one hand, this impeller geometry also allows avoiding clogging of impeller chamber  6  by two-dimensional materials, and on the other hand, the losses in efficiency of free-flow pump  21  can be kept sufficiently small for many applications. In particular, the following constructive measures are provided: 
     Impeller  22  has a hub body  23  whose front side  24  extends over approximately one third of the radius r of impeller  22 . Front side  24  of hub body  23  is substantially constituted by the free surface of front plate  25  that forms a continuous junction with a surrounding convex curvature  26  on the external wall of hub body  23 . The free surface of front plate  25  consists of the flat middle surface portion comprising the central bore for receiving screw  9  and of the gently rounded outer taper to which the convex curvature  26  on the external wall of hub body  23  adjoins. The flat middle surface portion extends over more than two thirds of the radius of front plate  25 . 
     The disk surface  28  around front side  24  of hub body  23  extends over the outer two thirds of the radius of impeller  22 . Disk surface  28  consists of the convexely curved surface portion  26  and of an adjoining concavely curved surface portion  27  both of which extend along the external wall of hub body  23 . The convexely curved surface portion  26  here only corresponds to about a seventh of the radius of disk surface  28 . 
     Disk surface  28  is provided with vanes  29  comprising open vane front sides  30 . Vane front sides  30  adjoin the front side  24  of hub body  23  in the area of its convexely curved junction  26  with disk surface  28 . From there, vanes  29  extend to the outer circumference of impeller  22 . Vanes  29  exhibit a constant height characteristics, their height H substantially corresponding to the height difference between the concavely curved surface portion  27  at the outer circumference of impeller  22  and the convexely curved junction  26  with disk surface  28 . 
     The maximum depth of disk surface  28  is equal to its maximum height difference H from the surface portion of the inner ends  43  of vane front sides  30  which is closest to the inlet side. Thus, disk surface  28  only reaches its maximum depth along its outer circumference where the concavely curved surface portion  27  reaches the outer circumference of impeller  22 . 
     Accordingly, the impeller base of impeller  22 , constituted as a whole by front side  24  of hub body  23  and by the surrounding disk surface  28 , in its inner radial third only consists of the front side  24  of hub body  23 . Therefore, the height variation of the impeller base in this area substantially corresponds to the height characteristic of front plate  25 , which in its outer edge area only exhibits a small height variation as compared to the height difference H. 
       FIG. 5  shows a top view of front side  24  of hub body  23  and of the surrounding disk surface  28  forming the impeller base. Twelve vanes  29  are arranged in regular intervals around disk surface  28 . Starting from the junction between the front side  24  of hub body  23  and disk surface  28 , the vanes  29  extend to the outer circumference of impeller  22 . The vane front sides  30  of vanes  29  exhibit a curved shape. 
       FIG. 6  shows a cross-sectional view of impeller  22  according to section VI in  FIG. 4 . This corresponds to a section through impeller  22  along half of the height difference H between the inner end of vane front sides  20  and the maximum depth of disk surface  28  relative to the inner end of vane front sides  20 . As follows from  FIG. 6 , in this depth range, disk surface  28  lies in the middle of the radius of impeller  22  within the concavely curved surface portion  27  of the latter. 
     In  FIG. 7  a free-flow pump  32  according to a third embodiment is illustrated. Components that are designed identically with regard to free-flow pump  1 ,  21  shown in  FIG. 1  and  FIG. 4  are designated by the same reference numerals. Free-flow pump  21  substantially corresponds to the previously described free-flow pump  21  with the difference that the vane geometry of impeller  22  is modified in order to improve the pump efficiency. 
     In addition to vanes  29  of constant height, impeller  33  of free-flow pump  32  further comprises vanes  34  of variable height. At their inner ends, the open vane front sides  35  of vanes  34  of variable height also adjoin to front side  24  of hub body  23  in the area of its convexely curved junction  26  with disk surface  28 . From there, vanes  34  extend to the outer circumference of impeller  33  while their height continuously increases. The maximum height increase  36  of vanes  34  is in the outer third of the radius of impeller  33 . From there towards the outer circumference of impeller  33 , the height increase of vanes  34  declines until their height remains substantially constant over the outer tenth of the radius of impeller  33 . 
     Accordingly, the height of vanes  34  remains substantially constant over the inner radial half of the impeller base. Then, in the outer radial half of the impeller base, a rapid height increase follows where the height of vanes  34  increases about a fourth of the maximum depth of disk surface  28  relative to front side  24  of hub body  25 . In this manner, an increase in pumping head and pump efficiency is achieved without having to accept disadvantageous clogging properties due to two-dimensional materials contained in the pumped liquid. 
       FIG. 8  shows a top view of impeller  33 . Around disk surface  28 , three vanes  34  of variable height are arranged at regular intervals and in between them three vanes  29  of constant height. The free vane front sides  35  of vanes  34  of variable height have substantially the same shape properties as vane front sides  30  of vanes  29  of constant height, particularly with regard to their relative distance to neighboring vanes  29  and their curved shape. 
     The arrangement of vanes  29  of constant height therebetween serves the purpose of temporarily ensuring the opening of free passage  7  for the passage of larger solids in the pumped liquid during an impeller rotation. 
       FIG. 9  shows a cross-sectional view of impeller  33  according to section IX in  FIG. 7 . This corresponds to a section through impeller  33  along half of the height difference H between the inner end of vane front sides  30 ,  35  and the maximum depth of disk surface  28 . As follows from a comparison of  FIG. 6  to  FIG. 9 , this section is identical to the equivalent cross-section VI through impeller  22  of free-flow pump  21  shown in  FIG. 4 . 
     From the foregoing description, numerous modifications of the free-flow pump according to the invention are apparent to one skilled in the art without leaving the scope of protection of the invention that is solely defined by the claims.