Volute for a centrifugal pump
The present invention relates to a volute for a rotary hydraulic pump of the centrifugal type, comprising a hollow body having an axial intake duct and a tangential delivery duct, for the entry and exit of a liquid flow respectively, and associated electric heating means able to heat the liquid flow which flows during operation between the intake duct and the delivery duct.
In rotary hydraulic pumps of the centrifugal type, the problem of cavitation, i.e. the formation of air bubbles mixed with the liquid which flows from the intake duct to the delivery duct, is known. This phenomenon may cause the wear and corrosion of the impeller and alteration of the liquid flow, resulting in the formation of turbulent currents which adversely affect the delivery of the pump.
One object of the present invention is to propose an improved volute which improves the efficiency of the pump with which it is associated, resulting in a greater delivery together with a reduction in the risks of cavitation for the abovementioned pump.
These and other objects are achieved according to the invention by means of a volute for a centrifugal pump in which the heating means are shaped and arranged so that they are able to impart during operation a spinning movement to said liquid flow about the axis of the intake duct and the pump impeller, in the same direction as the rotation imparted to the liquid flow by the impeller.
Owing to these characteristics, the heating members arranged inside the volute allow the formation of a plurality of layers of liquid, in particular water, inside the hollow body, having a different temperature and density, and provide the movement of the liquid from the intake duct towards the
delivery duct with a spinning (or helical) component.
In this condition, the vortical and turbulent flows of the liquid inside the volute, which contribute towards the phenomenon of cavitation, are reduced and at the same time the delivery of the pump is improved.
Further characteristic features and advantages of the invention will emerge from the detailed description which follows, provided purely by way of a non-limiting example with reference to the accompanying drawings in which:
Figure 1 is an exploded view of a first embodiment of the volute according to the invention;
Figure 2 is a cross-sectional view of the intake duct of the volute shown in Figure 1, along the line II-II;
Figure 2a is a variation of embodiment of the intake duct shown in Figure 2;
Figure 3 shows a three-dimensional view of a thick-film heating device of another embodiment of the volute;
Figure 4 is a schematic illustration of a particular spatial arrangement of thick-film heating devices inside the volute;
Figure 5 is a schematic illustration of a further arrangement similar to that of Figure 4;
Figure 6 is an exploded view of an embodiment of the volute incorporating thick-film heating devices inside the intake duct;
Figure 7 is an exploded view, partly cross-sectioned, similar to Figure 6, of a volute which has thick-film heating devices inside its hollow body;
Figure 8 is a transversely sectioned view of a further embodiment of a volute according to the invention; and
Figure 9 is a cross-sectional view along the line IX-IX of the volute according to Figure 8.
With reference in particular to Figure 1, 10 denotes
generally a rotary hydraulic pump of the centrifugal type, in particular for electric household appliances and the like, comprising a volute 12, an impeller 14, a bottom closing element 16 and an electric motor (not shown) .
The impeller 14 is provided with vanes 24 and is intended to be connected to the shaft of the electric motor (details not shown) so as to be actuated in a direction of rotation indicated by A. The impeller 14 is housed inside the volute 12 which is joined by means of snap-engagement to the closing element 16 so as to ensure a liquid-tight seal.
The volute 12, which is for example made of plastic or aluminium or alloys thereof, comprises a hollow body 18 having an axial intake duct 20 and a tangential delivery duct 22.
During operation, the pump 10 draws a liquid flow from the intake duct 20 into the hollow body 18 as a result of rotation of the impeller 14 actuated by the electric motor. The liquid arriving from the intake duct 20 is pushed by the effect of the centrifugal force towards the periphery of the hollow body 18 and is directed towards the delivery duct 22.
The volute 12 comprises furthermore heating members, intended to heat the liquid flowing from the intake duct 20 to the delivery duct 22. Different embodiments are envisaged for these heating members and will be illustrated below, said embodiments having in common the fact that these heating members are shaped so as to impart, during operation of the pump 10, a spinning movement to the liquid flow about the axis B-B of the intake duct 20 and the impeller 14, in the same direction A as the rotation imparted to the liquid flow by the impeller 14 of the pump.
As will be clarified in the remainder of the description, in some embodiments the heating members may be inserted inside
the intake duct 20 and/or inside the hollow body 18.
In the embodiment according to Figure 1, the heating members comprise at least one spiral-shaped resistive element 26 fixed inside the intake duct 20. This resistive element has conveniently an impermeable sheath 30, in particular a metal casing, which ensures insulation thereof with respect to the pumped liquid (Figure 2) . Alternatively, the sheath 30 may be made of thermoplastic material or may consist of a layer of resin which surrounds the resistive element 26.
In Figures 1 and 2, by way of example, the cross-section of the sheath 30 has been shown with a circular form, but it may equally well be square or triangular or have any prismatic cross-section.
In variations of embodiment not shown, the spiral-shaped element 26 may be fixed, for example welded or glued, to a support base-piece made of metallic or thermoplastic material, in turn fixed to the duct 20.
The resistive element 26 has terminal sections 28 which pass (in a fluid-tight manner) through the wall of the duct 20 and emerge outside the latter so that they may be connected to an electric power supply and control circuit (not shown) . In this way, during operation the water flowing into the intake duct 20 in the direction of the hollow body 18 is heated by the heat generated by the resistive element 26 owing to the Joule effect.
With reference in particular to Figure 2, it can be noted that the spiral-shaped resistive element 26 has an inclination α and a pitch L which are predefined so as to provide the liquid entering the hollow body 18 with an angular velocity component in the same direction as the rotational velocity A of the impeller 14. This condition, combined with heating of the liquid, allows a partial
reduction in the turbulence affecting the liquid flow from the intake duct 20, thus allowing an increase in the delivery and a consequent decrease in the risk of cavitation.
The variation of application shown in Figure 2a exhibits a spiral-shaped element 26 produced by means of co-moulding together with the intake duct 20 made of thermoplastic material. In this case, the sheath 30 may not be envisaged.
Purely by way of example of the multiplicity of variations of application which may be obtained by means of co-moulding, the internal wall of the intake duct 20 is shown with a threaded profile 32 projecting internally.
With reference to Figure 3, this shows a heating device 34 used in another embodiment of the invention. The heating device 34, which is of a type known per se, comprises a substantially plate-like support 36 with a main surface 38 and a superficial resistive element 40 deposited using the thick-film technique. The plate-like support 36 may be made, for example, of ceramic material. The thick-film resistor 40 is connected to insulated conductors 42 which allow heating thereof due to the passage of current and which, in their connection portions, are electrically insulated and waterproofed with a layer of synthetic resin 44.
A plurality of thick-film resistive heating elements of the type according to Figure 3, or the like, is conveniently arranged inside the intake duct 20 and/or inside the hollow body 18 of a volute 12, for example according to one of the arrangements which will now be described.
Figures 4 and 5 are schematic illustrations of spatial arrangements which these heating devices 34 may assume inside the intake duct 20 or inside the hollow body 18 or simultaneously inside both of them. In both the arrangements shown, the heating devices 34 are angularly spaced from each
other uniformly about the axis B-B and have preferably the same inclination relative to the direction of entry or incidence of the liquid, indicated by the arrows C.
In Figure 4 the deflecting surfaces 38 of the heating devices 34 lie in planes which are substantially parallel to radial directions transverse to the axis B-B and form an angle β with respect to the flow lines C, in particular directed towards the rear with respect to the direction of rotation A of the impeller 14.
Figure 5 is a variation of Figure 4 in which the deflecting surfaces 38 lie in planes intersecting radial directions transverse to the axis B-B. Therefore, the surfaces 38, in addition to having an inclination of angle δ towards the rear, with respect to the direction of rotation A, are also rotated through an angle γ with respect to the transverse radial directions passing through the bottom vertex of their radially outer side.
The arrangements described above are such that the deflecting surfaces 38 are able to deviate at least one portion of the liquid flow C such that the deviated flow has a velocity component in the same direction as the rotational velocity of the impeller 14. In this way, a spinning movement is imparted to the liquid coinciding with the direction of rotation A of the impeller 14, this helping achieve a reduction in the cavitation phenomena and an increase in the delivery of the pump 10.
Figure 6 shows a volute 12 comprising four heating devices 34 arranged inside the intake duct 20 and oriented as shown in Figure 4.
Figure 7 shows a volute 12 comprising four heating devices 34 arranged inside the hollow body 18. In this variation of embodiment, the deflecting surfaces 38 of the heating devices
34 have, on the side directed towards the impeller 14, a profile shaped in a manner substantially complementing the profile of the latter.
In embodiments not shown, the heating devices 34 may be arranged both inside the intake duct 20 and inside the hollow body 18.
In further embodiments not shown, the use of a spiral-shaped resistive element 26 situated inside the intake duct 20 may be combined with the use of the heating devices 34 inside the hollow body 18.
It is therefore also possible to combine an arrangement of heating elements 34 inside the intake duct 20 with the arrangement of a spiral-shaped heating element 26 inside the hollow body 18 of the volute 12.
Figures 8 and 9 show a partial variation of embodiment of a volute 12 according to the invention.
In this variation, a plurality of fixed deflector vanes 46 which are angularly spaced, extends from the internal surface of the hollow body 18 of the volute 12. These vanes 46 are preferably planar and moulded as one piece with the hollow body 18 of the volute 12; moreover, each fixed vane 46 lies in a plane which is substantially tangential to the distal end of a vane 24 of the impeller 14 when this vane 24 of the impeller 14 is situated at a minimum distance from the free end of the fixed vane 46. This condition occurs simultaneously for all the fixed vanes 46 of the volute 12 according to Figure 8. Moreover, it is also possible to adopt other arrangements of fixed vanes 46 so that, at a certain instant, the abovementioned condition occurs only for a subset of said fixed vanes 46.
The solution described above with reference to Figures 8 and
9 helps effectively reduce the risks of cavitation and may be adopted in combination with heating means of the types previously described with reference to Figures 1 to 7.
Obviously, without modifying the principle of the invention, the embodiments and the constructional details may be widely varied with respect to that described and illustrated purely by way of a non-limiting example, without thereby departing from the scope of invention as defined in the accompanying claims.