GB2190305A - Centrifugal mixing impeller - Google Patents

Abstract

A mixing impeller including a plurality of blades (5, 11, 17) circumferentially secured around a substantially conical frame (2A-2D), which is hollow. The impeller, when rotating within a fluid takes in and impels the fluid internally through open and generally lateral exhaust outlets between the blades for discharge of the fluid. On rotation of the impeller there is provided a more efficient purging circulatory flow for destratification of fluids and solid-fuel suspension. <IMAGE>

Description

SPECIFICATION Spider mounted centrifugal mixing impeller FIELD OF THE INVENTION The present invention relates to a mixing impeller particularly suited for the destratification of fluids and solid-fluid suspension and having a rearwardly contracting preferably conical configuration for providing a faster and highly efficient purging circulatory flow within a mixing reservoir while producing a high volumetric flow output at low torque values.

The present invention also operates in such a manner as to produce lower pressure differentials between the blade face and blade back reducing cavitation and reducing noise at the mixing impeller as well as noise otherwise radiating from the mixing unit as a result of vibration caused by cavitation.

BACKGROUND OF THE INVENTION Fluid mixing is vital to most production systems in the chemical process and allied industries.

It has extreme importance in the mining, food petroleum, chemicals, pharmaceuticals, pulp and paper, and power industries, and in municipal and industrial waste treatment facilities.

In an axial-flow mixing impeller a flow is generated parallel to the mixing impeller shaft, along the impeller axis. Axial-flow impellers generally generate more fluid flow per horsepower than radial impellers.

A significant drawback resulting from the use of a conventional mixing impeller is the low efficiencies of the device when run at higher speeds to decrease mixing time. A general value for efficiency may be between 0.3% to 1.1%, while at lower speeds may increase to as high as 7.0%. This efficiency value is a reflection of how much power is consumed per unit of volume mixed. The less power consumed to mix a given volume of fluid, the more efficient the device.

Two of the most significant factors that affect the overall efficiency of a mixing device is the production of a strong, purging circulatory flow within a mixing reservoir and the pressure differential between the back and the face of the blade which on a conventional impeller have a foil-like construction rotated in a narrow plane resulting in very significant pressure differentials between the face and the back of the blade.

SUMMARY OF THE PRESENT INVENTION The present invention provides an elongated, rearwardly contracting mixing impeller comprising a generally hollow frame supporting a plurality of impeller blades secured circumferentially around the frame. The frame has an interior flow path and a substantially open forward end for intake of fluids to the flow path. The adjacent blades on the frame are opened from one another for circulatory flow from the center flow path outwardly between the blades.

BRIEF DESCRIPTION OF THE DRA WINGS Detailed features of the present invention will be described according to the preferred embodiments of the present invention in which; Figure 1 is a plan view showing a general schematic embodiment of the present invention as it would be used to mix fluids; Figure 2 is a perspective view of the mixing impeller from Fig. 1; Figures 3 through 5 are sectional views along the lines 3-3, 4-4, 5-5 respectively of Fig. 2; Figures 6 and 7 are rear and front views respectively of the mixing impeller of Fig. 2; Figure 8 is a flow diagram of the fluid inflow and outflow produced by rotation of the mixing impeller of Fig. 2.

DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION Fig. 1 shows a mixing impeller indicated at 1 mounted in a mixing reservoir beneath the surface of the fluid to be mixed. As will be clearly seen in Fig. 1, the mixing impeller has a generally conical rearwardly contracting configuration, the details of which are well shown in Fig.

2.

The overall mixing impeller comprises a supporting frame including support portions 2A, 2B, 2C and 2D. Located along the length of the mixing impeller are a plurality of blades, fixedly secured in position with respect to the supporting frame. These blades, according to the preferred embodiments shown in the drawings, are arranged in three separate stages or groupings indicated at rearward stage 4, intermediate stage 10 and forward stage 16.

The rearward stage comprises a plurality of blades 5, the intermediate stage comprises a plurality of blades 11 and the forward stage comprises a plurality of blades 17. All of these blades are mounted to the frame opened outwardly from the adjacent blade in each stage, but if closed upon one another to a 0 degree angle, would form a generally conical configuration, or at least a frustrum of a cone. It is to be noted that all of the blades in the mixing impeller extend in a generally horizontal orientation and are slightly turned along their lengths as will be described later in detail.

According to conventional construction, the diameter of a mixing impeller is readily determined as the outer circumference around the individual blades on the mixing impeller. According to the present invention, the diameter of the conical mixing impeller is determined as the effective or maximum diameter at the front end of the mixing impeller.

The value of Q (volumetric outflow rate) produced by the conical mixing impeller as shown in the drawings is increased by increasing the ratio L/D with L being the length of the mixing impeller and D being the effective diameter of the mixing impeller as shown in Fig. 2.

With the conical configuration, L increases at a faster rate than D along the length of the mixing impeller. This provides a high value of blade surface area relative to the effective diameter enabling large volumes of fluid to be moved at relatively low rotational speeds.

This feature, according to the present invention, substantially increases the amount of fluid expelled by the device and in turn uses much less energy to do so. Torque values on the drive shaft of the device and the mixing impeller iteself are substantially reduced as a result of this type of conical configuration, producing a much lower Power Number value given by the equation: 1 .523*1013P Np= N3D5p where P= impeller horsepower N= impeller speed, rpm D= impeller diameter, in.

p= fluid viscosity, cP Due to the lower pressure differential between the blade face and the blade back while the device is rotating, the advent of cavitation is less likely to occur if at all. This would occur in conventional mixing impellers at higher rotational speeds. In terms of equations this can be seen as: Ap=p1 -p2 where p1 pressure on blade face p2=pressure on blade back The reactionary thrust that is exerted on a fluid by an impeller is shown in the following general and accepted equation.

T=Ap*A where T=thrust A=area of rotating body As a result of these parameters and the unique configuration of the mixing impeller, a strong axial inflow is generated as well as a strong purging circulatory outflow necessary in an efficient mixing impeller.

Returning to the drawings, the mixing impeller itself includes a shaft S, generally central of the impeller. In the arrangement shown in the drawings, rearward support portion 2A is centrally apertured to allow fitting of the shaft which extends completely through the impeller to the forward support portion 2D, including a further centrally located securing portion for stabilizing of the mixing impeller as it is rotated by the shaft.

As the mixing impeller is rotated, fluid is taken in by the substantially open forward end as shown in Fig. 7, the individual blades scoop the fluid from the inside of the impeller and force the fluid out and upwardly of the openings between the circumferentially attached blades. The fluid is replaced internally from the outside of the mixing impeller through the open forward end due to the subsequent low pressure field generated internally as a result of the rotation of the impeller, producing a constant flow of fluid. The inflow has an axial as well as radial velocity due to forced vortex production from the rotation of the device. The flow is highly controlled due to its being controlled over an extended period and a substantial distance as determined by the overall length of the mixing impeller. This results in an efficient transfer of energy.

In the preferred embodiment the device is situated close to the bottom of the mixing reservoir, fluid taken from the bottom of the reservoir is replaced by fluid at the top of the reservoir by means of the forced circulation caused by rotation of the device. This circulatory circuit of fluid has substantial energy necessary for the effective mixing of solid-fluid suspension or the destratification of fluids. The energy within said circulatory circuit will break any interface that exists between the fluids to be mixed at a substantially faster rate than conventional systems used in the mixing industry.

Another feature of the present invention, when run at high rotational speeds, if such speed is required, is that all of the blade surface area is peripheral to the accelerated flow, thereby eliminating any central cavitation within the fluid flow generated by the mixing impeller. Such cavitation may develop in an in-line mixing operation.

Such stated benefits make the device ideal for use as a mixer in mining, food petroleum, chemical, pharmaceutical, pulp and paper, and power generating industries as well as in municipal and industrial waste treatment facilities.

In order to enhance stable operation of the mixing impeller, it is important that the flow pattern along the length of the blade, as well as from stage to stage, have a constant velocity.

This velocity is determined by two factors, namely the tip velocity of each blade at any one point and the inlet/outlet area between the blades at any one diameter. The inlet/outlet area determines the mass or amount of fluid taken in and subsequently expelled from the device.

In accordance with the conical construction, the tip velocity along the length of each blade, as well as the tip velocity for blades from one stage to the next, decreases from front to back of the mixing impeller due to its rearwardly decreasing diameter. Therefore, since tip velocity is decreasing, the blade angle increases along the length of each blade and between blades of different stages from front to rear of the mixing impeller.

Referring to Figs. 2 through 5, it will be seen that blades 5 in third stage 4 have first and second ends 7 and 9 respectively; blades 11 in second stage 10 have first and second ends 13 and 15 respectively; and blades 17 in first stage 16 have first and second ends 19 and 21 respectively. Each of the blades have a reduced blade angle between its first and second ends.

In Fig. 3, it will be seen that blade 5 has a reduced angle from its first end 7 to its second end 9; in Fig. 4 it will be seen that blade 11 has a reduced blade angle from its first end 13 to its second end 15; and in Fig. 5 it will be seen that blade 17 has a reduced blade angle from its first end 19 to its second end 21.

Furthermore, it will be clearly seen in Figs. 3 through 6 that the inlet between blades is reduced from the third rearward through to the first forward stage. In Fig. 3 the gap or inlet between blades 5 is increased relative to the gap or inlet between blades 11, shown in Fig. 4, which in turn is greater than the gap or inlet between blades 17, shown in Fig. 5.

From the above, it will be clearly seen that the blade angles are greater at the smaller diameter end of the mixing impeller than they are at the larger diameter end of the mixing impeller with the difference in the blade angles being determined according to the diameter of the mixing impeller.

As the impeller shown in the drawings rotates within the fluid contained in a mixing reservoir, the blades are subject to different fluid densities at different fluid depths or pressures, depending on the axial plane that the mixing impeller is rotated upon. In order to maintain blade stability, each of the blades may be cambered to compensate for any appreciable density differences within the fluid.

The number of blades, five of which are shown in the drawings, in each stage is a function of the load and output requirements of the mixing device, as well as the structural and space limitations in a mixing reservoir. Similarly, the number of stages affect the flow characteristics and the structural requirements of the mixing impeller.

As a result of its efficient operation, the mixing impeller of the present invention produces substantially less friction and thermal energy than found in conventional mixing impellers. This in turn results in less wear on the impeller which can therefore be made from less expensive materials, than has been possible in the past.

Furthermore, due to the lower pressure differentials that exist between the blade face and back, the mixing impeller of the present invention will produce a corresponding reduction in generated noise and vibration that can be transmitted to equipment attached to the said mixing impeller.

Although various preferred embodiments of the present invention have been described herein in detail, it will be appreciated by those skilled in the art, that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (9)

1. An elongated, rearwardly contracting mixing impeller comprising a frame supporting a plurality of impeller blades, said frame being generally hollow with an interior flow path therethrough and a substantially open forward end for intake of fluid to said flow path, said impeller blades being secured circumferentially spaced from one another around said frame outwardly of said interior flow path and being opened from adjacent blades for circulatory flow from said center flow path outwardly between said blades.

2. A mixing impeller as claimed in Claim 1, having a generally conical configuration.

3. A mixing impeller as claimed in Claim 1, including at least one stage in which said blades in said one stage are correspondingly opened on said frame to provide a substantially uniform axial fluid intake internally of said mixing impeller and subsequent strong purging circulatory flow in a mixing reservoir.

4. A mixing impeller as claimed in Claim 3, wherein said blades decrease in blade angle from back to front of said one stage.

5. A mixing impeller as claimed in Claim 1, wherein said mixing impeller comprises a plurality of stages, the blades within an individual stage being correspondingly opened, and the blades within each stage and from stage to stage increasing in blade angle from front to back of said mixing impeller, allowing for efficient circulatory motion within a mixing reservoir.

6. A mixing impeller as claimed in Claim 5, wherein said mixing impeller extends from a rearward smaller diameter to a forward larger diameter and wherein each blade has tip velocity upon rotation of said mixing impeller which varies from the smaller to the larger diameter by a factor proportional to the diameter of the mixing impeller resulting in a strong, uniform flow rate lengthwise along said mixing impeller.

7. A mixing impeller as claimed in Claim 5, comprising at least three stages.

8. A mixing impeller as claimed in Claim 5, wherein each stage comprises at least 5 blades.

9. An elongated, rearwardly contracting mixing impeller substantially as herein before described with reference to the accompanying drawings.