AU2018200356B2

AU2018200356B2 - Blender apparatus and method

Info

Publication number: AU2018200356B2
Application number: AU2018200356A
Authority: AU
Inventors: Jorge ARRIBAU
Original assignee: NOV Condor LLC
Current assignee: NOV Condor LLC
Priority date: 2012-09-17
Filing date: 2018-01-16
Publication date: 2019-04-18
Anticipated expiration: 2032-09-17
Also published as: EP2895258A1; CA2884578C; AU2012389829B2; RU2604628C2; CA2884578A1; CN104812471A; EP2895258A4; RU2015114174A; CN104812471B; AU2012389829A1; AU2018200356A1; WO2014042655A1

Abstract

Abstract A method and apparatus for blending liquids and granular materials in which an impeller assembly (27) is mounted for rotation within a housing (20) at a lower end (13) of a particles inlet (12) and characterized in particular by circumferentially spaced impeller vanes (28) in outer concentric relation to a series of expeller vanes (29, 29') surrounding the particles inlet and which together propel the solid particles outwardly to intermix with the liquid introduced into the annulus surrounding the impeller assembly, and different selected vane configurations (52, 61) are provided with circumferential portions protruding into the path of counterflow of the slurry from the annulus in order to keep the eye or central area of the impeller assembly dry. WO 2014/042655 PCT/US2012/055731 27 37'

Description

2018200356 16 Jan 2018

BLENDER APPARATUS AND METHOD

BACKGROUND AND FIELD OF INVENTION

The disclosure of the complete specification of Australian

Patent Application No. 2012389829 as originally filed is incorporated herein by reference.

The following relates to a novel and improved method and apparatus for controlling the introduction of solids into a chamber containing a pressurized fluid, such as, for example, blenders for intermixing and pumping large volumes of liquid/sand slurries in downhole fracking operations.

Previously I have devised different blade or vane designs for a given ratio of impeller diameters. In the past, the vanes were designed to balance the point at which the solids and liquids were intermixed between the outer space surrounding the impeller vanes and the center of the impeller assembly in order to allow the introduction of dry sand through the center of the impeller. Among other considerations in determining the design of the impeller vanes is the mass flow rate or capacity of flow of the solid particles as well as their density for a given speed of rotation of the impeller vanes; and to multiply the RPMs or speed by the number of vanes which in turn will aid in establishing the spacing between the vanes as well as their depth.

Still another variable to be taken into consideration is the rate at which the sand is ejected from the center to the impeller region and which may be influenced 1

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2018200356 16 Jan 2018 both by the utilization of expeller blades and a generally conical or raised center. Further, once the diameter of the expeller and its number of vanes is established based on the desired flow rate of sand particles, the diameter of the impeller and shape of its vanes can be determined in order to achieve optimum rate of flow of the sand particles through the impeller region. Conversely, it is important to compute the rate of counterflow of liquids through the spaces between the impeller vanes toward the center of the impeller assembly.

From that, one is able to determine the optimum balance point or size and position of vanes necessary to reverse the inward flow and force the slurry to return to the outer annular space surrounding the impeller assembly.

In accordance with my U.S. Letters Patent No. 7,967,500, there is disclosed an arrangement or configuration of vanes in which the liquid would follow a path between the primary vanes toward the center of the impeller, until it reached the next vane which would cause it to reverse and flow away from the center. Nevertheless, there is a need for utilizing blocking vanes in the spaces between the primary vanes in order to keep the eye of the impeller dry and to regulate the balance point between the solids and slurry in a region radially outwardly of the eye while pumping the slurry over a wide range of mass flow rates. Further, there is a continuing need for impeller vane designs which not only

2018200356 08 Mar 2019 achieve the foregoing but minimize the energy expended and reduce wear over long-term use while further simplifying the construction and minimizing the number of parts required in preventing liquid or slurry leakage back into the eye or central area of the assembly.

SUMMARY

In view of the foregoing, there is a need for an improved method and apparatus for blending liquid and solid particles with a simplified impeller assembly which minimizes wear, expenditure of energy and replacement of parts while maintaining optimum blending conditions and preventing the counterflow of liquid or slurry back into the eye of the impeller .

It would be desirable for a method of designing an impeller which takes into consideration a number of variables including flow rates, density and size of particles for a given number and speed of rotation of the impeller vanes as well as their spacing.

An impeller assembly may advantageously have blocking vane surfaces incorporated into the primary vanes and so spaced and arranged as to maintain optimum balance and deflection of slurry away from the eye of the impeller.

It would be useful to minimize energy consumption resulting from the counterflow of the liquid between the vanes by blocking the counterflow as close to its origin as possible and causing it to be redirected back into the annular space surrounding the impeller assembly.

2018200356 08 Mar 2019

An apparatus for blending lipids and solid particles according to the present invention comprises:

a housing having an upper funnel shaped particles inlet and a lower liquid inlet, a central drive shaft, and an outlet in an outer wall of said housing;

an impeller assembly mounted for rotation beneath the lower end of said particles inlet including a base plate and a plurality of circumferentially spaced impeller vanes extending upwardly from said base plate and positioned in surrounding relation to said shaft;

a plurality of circumferentially spaced expeller vanes extending upwardly from said base plate and being aligned with said impeller vanes and positioned in surrounding relation to said shaft;

a central impeller area between said shaft and said expeller vanes and beneath said lower end of said particles inlet;

a plurality of air relief vents disposed in surrounding relation to said shaft and in communication with said central impeller area.

In one aspect of the invention the apparatus further comprises :

a tubular member located centrally of said central impeller area and wherein said tubular member is provided with at least a portion of said air relief vents.

In another aspect of the invention each of said expeller vanes has an upper rounded end curving in a direction away from the direction of rotation of said expeller vanes.

2018200356 08 Mar 2019

In another aspect of the invention ach of said expeller vanes has an upper rounded end curving in a direction away from the direction of rotation of said expeller vanes; and said air relief vents are disposed about said shaft adjacent the lower end of said particles inlet.

In another aspect of the invention said housing includes a pair of funnel shaped portions, with one of the pair partially disposed within the other, and wherein the two funnel shaped portions are disposed apart from one another so as to create an air relief vent therebetween.

Further aspects and embodiments will become apparent by reference to the following drawings when taken together with the detailed description and it is intended that the embodiments disclosed herein are to be considered illustrative rather than limiting.

BRIEF DESCRIPTION OF DRAWINGS

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FIG. 1 is an elevational view, partially in section of a hydraulically driven mixing system with a low profile blender assembly;

FIG. 2 is another elevational view, partially in section of a hydraulically driven mixing pump;

FIG. 3 is an elevational view, partially in section of another form of mechanically driven mixing pump;

FIG. 4 is a perspective view of a first embodiment of a blender with a combined impeller/expeller assembly;

FIG. 5 is a cross-sectional view of the impeller assembly of FIG. 4 taken about lines 5-5 of FIG. 7;

FIG. 6 is a perspective view of a top cover plate over a blender assembly as illustrated in FIG. 4;

FIG. 7 is an elevational view of the impeller/expeller assembly shown in FIGS. 4 to 5;

FIG. 8 is an elevational view of a second embodiment of impeller/expeller assembly;

FIG. 9 is a cross-sectional view taken about lines

9-9 of FIG. 7;

FIG. 10 is a perspective view of a cover plate over a blender assembly illustrated in FIGs. 8 and 9;

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FIG. 11 is a perspective of a third embodiment of impeller assembly;

FIG. 12 is a perspective view of another form of a blender with the impeller assembly of FIG. 11; and

FIG. 13 is a plan view of FIG. 11.

DETAILED DESCRIPTION OF FIRST EMBODIMENT

Referring in detail to the drawings, apparatus 10 takes the form of a hydraulically driven mixer shown in FIG. 1 and which may be mounted on a truck, not shown, but shown and described in detail in my U.S. Letters Patent No. 7,967,500.

As illustrated in FIGs. 1 and 2 of that patent, a booster pump communicates with an intake port, such as, intake port 24 illustrated in FIG. 1 herein. As shown in more detail in

FIGs. 1 and 2 of my hereinbefore referred to patent, in oil and gas operations, such as, fracturing or cementing wells, the pump 10 is mounted on a truck bed along with an engine with a drive mechanism to impart rotation via a speed reducer mechanism to a central drive shaft. The solid granular matter, such as, sand is delivered from a storage area by means of an auger to the upper end of a hopper and advanced by gravity into the impeller area. The sand is mixed with a liquid which is introduced through the port 24, and the resultant slurry is discharged via an outlet port 26 through a

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2018200356 16 Jan 2018 delivery tube under sufficient pressure to be delivered to a well head. The booster pump regulates the pressure in the annulus of the impeller assembly housing and can be closely controlled to maintain a constant pressure level from the outlet of the pump to the inlet port 24 as well as to increase the pressure as desired.

As a setting for the first embodiment, there is illustrated in FIG. 1 an apparatus 10 having a generally funnel-shaped hopper 12 converging downwardly and terminating in a lower end 13 mounted by circumferentially spaced struts in closely spaced relation to and above the inner wall 16 of a suspension mount for an impeller assembly 27 in the housing 20. The housing 20 is supported on a base mount 22 and includes the intake port 24 and outlet port 26 which are in open communication with an annulus in the housing 20 surrounding impeller assembly 27.

A drive shaft 30 is mounted centrally of the hopper with the lower end journaled in a hub 32 at the center of the base plate 34 of the impeller assembly 27, and its upper end 36 is mounted in bearings 38 beneath a drive motor 11. In the first embodiment, the sand and other dry chemicals mixed with the sand are advanced by gravity into the central blender area and driven outwardly in a manner to be described to form a slurry with liquids, mainly comprising water, which are

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2018200356 16 Jan 2018 introduced through the Intake pert kt and into the? .aahuiu/s. surrounding the impeller assembly 27.

Flos, 4~n illustrate in/ itnre detail the /first embodiment of a. fclendtt anit< 17 which .Is comprised of the hate plate 34. and which supports/ enter,, upwardly extending impeller vanes 2 If and Inner c/oncent ric expe l ter vane s 22, 29/¹ mounted on the Pass plate .34 and ih. shirounding relatiun to the lower open, end of the hopper 12». Pa shown ih FIG. $_f a cover piste 35.' is provided with a plurality /of circumferentially spaded ribs If extending radially along the upper surface of the cover plate 35 from an inner circular rip 38. Each of fhe ribs 31 is of uniform.· thickness toward. the enter circular .edge of the: cover' plate 35 and. coGpe/ratns In preventing the radially inward flow of slurry toward the central s/rpsp of the blender surrounding the shaft 32. In the slf:srnat./iye._f the cover· plate .35 and cage 38 may be of. the type shown In FIG«^:. 3 and 10 hereinafter described.

the impeller vanes /18-are circumferentially spaced, /arguahe generally 3~slded vanes/ extending 'Upwardly from the base plate .3/4/ between the outer edges of the expeller vansw 29. and outer dlrcuiau /edge of the base plate 31. Each of; the impeller vanes IB has opposite sides 39 .and 4.Θ Converging outwardly from an end surface 42 to terminate at at apex. 44 at or hear an outer, uircamferehtlai edge of the base plate 34.

In. tprn₇ the end surface. 42· extends substantially in a radial

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2018200356 16 Jan 2018 direction from an inner radial edge 42'. One of the sides 39 is of generally convex configuration and the opposite side 40 is of generally concave configuration and taper or converge outwardly toward one another with the convex surface 39 terminating in a curved surface portion 39' which substantially conforms to the curvature of the outer peripheral edge of the base plate 34. In this way, the wider end of each vane 28 toward the center is closest to the leading end of the next adjacent vane 28 and tends to restrict the inward radial counterflow designated at arrow A of the slurry and deflect it back into the annular space between the impeller vanes 28 and outer housing wall 20.

In addition, FIGs. 4 and 6 illustrate in more detail the expeller vane assembly in which a series of expeller vanes are made up of a combination of alternating longer, curved radial vanes 29 extending from the shaft 30 and substantially shorter but taller vanes 29' extending radially inwardly from the outer edge of the base plate 34. Each vane 29, 29' undergoes an arcuate curvature from the central area in a radially outward direction so that its convex side is the leading surface as the vanes undergo rotation in a clockwise direction. Further, each vane 29, 29' has its outer edge aligned with one of the inner radial edges of the impeller vanes 28 so that the solid particles are directed uniformally in an outward radial direction between the impeller vanes 28.

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The expeller vanes 39 and 40 have similar configurations, each having an upright generally rectangular support block 42 and an upper right-angled blade portion 44 in order to channel the outward passage of the solid particles into the spaces between the impeller vanes 39, and their slight curvatures will enable smooth transition of the solid particles in an outward radial direction. Also, the upper blade portions 44 are of increasing width toward their outer peripheries and disposed at right angles to the upright portions 42. In operation, the shorter vanes 29’ will contact the sand along the outer region of the expeller and tend to drive the sand sideways and outwardly without contacting the longer vanes; and the longer vanes 29 will contact sand along the inner region of the expeller and force the sand in a circumferential and radially outward direction with little or no contact with the shorter vanes. Again, the shorter vanes 29' are of greater height than the longer vanes 29 and cover substantially the same area as the longer but lower profile vanes and in this way equalize the amount of sand engaged by each set of vanes 29 and 29' respectively, in order to avoid imbalance.

The first embodiment herein described lends itself particularly well to use in low profile impeller assemblies of the type illustrated in FIG. 1 and known in the trade as an open inlet blender of the type shown and described in my U.S. Letters Patent Nos. 4,239,396 and 4,460,276 in which the

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2018200356 16 Jan 2018 impeller assembly is capable of developing an angular velocity which will prevent reverse flow of intermixed materials through the impeller into the solids inlet. In units of this type, it is essential that not only are balanced pressure conditions maintained throughout the system while achieving continuous high volume mixing of the materials, but to avoid pressure build-up of air in the solids inlet and blockage of the sand and other granular material. This is achieved in part by utilization of an expeller arrangement in surrounding relation to an enlarged center shaft and by permitting the escape of air at a point directly adjacent to the solids inlet. In FIG. 1, for example, air is permitted to escape at the juncture of the lower end of the funnel by air relief passages or vents 17 between the wall 16 and lower edge of the funnel 12 and which is in communication with the circular opening leading into the central impeller area above the housing 20 surrounding the impeller assembly 28.

FIGs. 2 and 3 illustrate other applications of the blender of FIGs. 4 to 6 to mixing pumps, FIG. 2 being a hydraulically driven mixing pump 10' with a hydraulic motor designated at 11 at the upper end of a drive shaft 30' and once again provided with relief vents or openings 17' between the funnel 12' and upper end of the central opening leading into the central impeller area within the housing 20'. In FIG. 2 the blender or impeller assembly 27’ is modified by the

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2018200356 16 Jan 2018 addition of lower impeller blades 28' to deliver water under pressure into the annulus or housing 20' surrounding the impeller assembly 27'.

A similar application of the impeller assembly 27 of

FIGs. 4 to 6 is illustrated in FIG. 3 of a mechanically driven mixing pump 10 in which gearing M is located beneath the blender for a drive shaft 30 extending upwardly into the blender assembly 27 with lower impeller blades 28 and affixed by a lower conical end nut 80. A perforated tube 82 extends upwardly through a funnel-shaped solids inlet 12.

The solids inlet 12 is of two piece construction to permit the escape of air from the solids materials and through spaced openings in the perforated tube 82 to prevent packing and jamming of the sand and pressure build-up of air at the inlet area.

DETAILED DESCRIPTION OF SECOND EMBODIMENT

There is illustrated in FIGs. 8-10 a second embodiment in which like or similar parts to those of FIGs. 46 are correspondingly enumerated. Thus, the expeller vanes 29, 29' correspond to those of FIGs. 4-6 and are mounted within a modified impeller assembly 27' in which a series of impeller vanes 52 are arranged in equally spaced circumferential relation to one another in the same manner as

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2018200356 16 Jan 2018 the vanes 28 in FIGs. 4-6. However, each of the impeller vanes 52 is curved along its entire length from its inner radial edge 54, which is in abutting relation to one of the expeller vanes 29, 29', to its outer radial edge 56 at the outer circumferential edge of the base plate 34. Each vane 52 is of uniform width or thickness along its length and of a height corresponding to the height of the shorter expeller vanes 29'; however at its inner radial end, each vane 52 includes a V-shaped lateral extension or deflector 54 which juts into the path of counterflow designated by arrow A' of any slurry attempting to return to the center or eye of the blender 2. FIGs. 8 to 10 illustrate a modified form of cover plate 35' having a raised surface 36' with U-shaped grooves

36' at uniformly spaced intervals around the cover plate with the open ends of the grooves extending radially outwardly.

The cover plate is mounted against the undersurface of the top wall of the housing 20 and spaced above the impeller assembly 27'. The assembly 27' is a unitary part of and extends downwardly from the cover plate 35'.

DETAILED DESCRIPTION OF THIRD EMBODIMENT

Another embodiment is illustrated in FIGs. 11 to 13 and in which a modified form of impeller assembly is illustrated in place of the impeller assembly 27 in the embodiment shown in FIGs. 4-6. Once again, a circular base

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2018200356 16 Jan 2018 plate 34 has a central opening 62 which is mounted for rotation on a central drive shaft as in the other embodiments.

A central expeller vane assembly is made up of generally triangular blades 64 of uniform thickness and diverging upwardly and outwardly from the center 62 to an outer vertical edge 65 in closely spaced facing relation to an inner surface of each of the impeller vanes 61 to be hereinafter described.

Upper inclined edge 63 of each expeller blade 64 is curved laterally in the direction of rotation of the vanes 61. In turn, each of the impeller vanes 61 has an arcuate blade 66 curving radially and outwardly from an elbow-shaped portion made up of an inner radial end 68 and a short, radially extending return portion 70. The blade elements 66, 68 and 70 are of uniform thickness and the major blade element 66 curves in an outward radial direction from its inner radial edge to an outer radial edge 72 which is flush with the outer circular edge of base plate 34. Accordingly, one side surface 73 is concave and the opposite side 74 is convex, and each return portion 70 extends radially outwardly in a direction toward an outer edge 72 of each next blade 62 in succession so as to define a limited space or gap between the adjacent vanes. In a manner to be described, the spacing between adjacent vanes is regulated to limit the counterflow of slurry toward the center of the impeller/expeller assembly.

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Although not shown, it will be evident that either one of the cover plates and the expeller assemblies of the three embodiments are interchangeable. For the purpose of illustration but not limitation, the assembly 27 of FIGs. 11 to 13 are shown as part of the blender assembly in FIG. 3 but the base plate 34 serves as a divider plate for a lower impeller assembly designated at 28. In a similar manner, the first and second embodiments are interchangeable and may be mounted as illustrated in FIGs. 1 and 2 with or without a lower impeller arrangement.

In the design of the impeller vanes, a number of factors must be taken into consideration as noted earlier and including but not limited to the velocity of the liquid toward the center of the impeller after each vane passes by a given point on the impeller. Referring to FIG. 5, for example, the arrow A represents the direction of return flow of slurry entering the space between vanes 28. In this respect, the widened end of each impeller vane will act as a deflector and can be moved outwardly to meet the fluid path as close to its origin as possible to the outer periphery of the impeller assembly. In other words, the sooner the fluid is blocked and redirected back toward the annulus the less energy will be consumed.

FIG. 9 represents an alternative approach by the utilization of the ledges or blocking vanes 54 opposite to the

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2018200356 16 Jan 2018 point of entry of the liquid from the annulus into the space between the vanes 52. This approach reduces the overall size of each vane but does require greater energy in that the deflector is located closer to the center of the impeller assembly before it is deflected back toward the annulus. Here the liquid or fluid path is represented by the arrow A'.

FIG. 13 illustrates still another approach in which the blocking vane is mounted more toward the bottom of the vane with its return end 70 being positioned in the path of slurry to prevent it from invading the center of the impeller, but requires greater energy consumption by virtue of the greater spacing between the outer end or edge 72 of each impeller and the inner end 68 of each next successive impeller. Thus, the fluid path is represented by the arrow A which is much longer and, while the fluid is blocked from reaching the center of the impeller, must be pumped back into the annulus thereby reducing the efficiency of the system. In this regard, the amount of pressure generated by the mixing pump in relation to the mass rate of flow of the sand or other granular material must be taken into consideration in determining the most efficient impeller assembly to utilize.

It is therefore to be understood that while preferred methods and apparatus have been herein set forth and described, various modifications and changes may be made to the construction and arrangement of parts, and their

2018200356 08 Mar 2019 interchangeability without departing from the spirit and scope of the embodiments described herein and as defined by the appended claims.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word comprise, and variations such as comprises and comprising, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

2018200356 08 Mar 2019

Claims

The claims defining the invention are as follows:

1. Apparatus for blending liquids and solid particles comprising in combination:

a housing having an upper funnel shaped particles inlet and a lower liquid inlet, a central drive shaft, and an outlet in an outer wall of said housing;

an impeller assembly mounted for rotation beneath the lower end of said particles inlet including a base plate and a plurality of circumferentially spaced impeller vanes extending upwardly from said base plate and positioned in surrounding relation to said shaft;

a plurality of circumferentially spaced expeller vanes extending upwardly from said base plate and being aligned with said impeller vanes and positioned in surrounding relation to said shaft;

a central impeller area between said shaft and said expeller vanes and beneath said lower end of said particles inlet;

a plurality of air relief vents disposed in surrounding relation to said shaft and in communication with said central impeller area; and a tubular member located centrally of said central impeller area and wherein said tubular member is provided with at least a portion of said air relief vents.
2. Apparatus for blending liquids and solid particles comprising in combination:

a housing having an upper funnel shaped particles inlet and a lower liquid inlet, a central drive shaft, and an outlet in an outer wall of said housing;

an impeller assembly mounted for rotation beneath the lower end of said particles inlet including a base plate and a

2018200356 08 Mar 2019 plurality of circumferentially spaced impeller vanes extending upwardly from said base plate and positioned in surrounding relation to said shaft;

a plurality of circumferentially spaced expeller vanes extending upwardly from said base plate and being aligned with said impeller vanes and positioned in surrounding relation to said shaft;

a central impeller area between said shaft and said expeller vanes and beneath said lower end of said particles inlet;

a plurality of air relief vents disposed in surrounding relation to said shaft and in communication with said central impeller area; and wherein each of said expeller vanes has an upper rounded end curving in a direction away from the direction of rotation of said expeller vanes.
3. Apparatus according to Claim 2 wherein each of said expeller vanes has an upper end extending laterally in a direction away from the direction of rotation of said impeller vanes .
4. Apparatus according to Claim 2 wherein said air relief vents are positioned at vertical intervals above said impeller assembly.
5. Apparatus for blending liquids and solid particles comprising in combination:

a housing having an upper funnel shaped particles inlet and a lower liquid inlet, a central drive shaft, and an outlet in an outer wall of said housing;

2018200356 08 Mar 2019 an impeller assembly mounted for rotation beneath the lower end of said particles inlet including a base plate and a plurality of circumferentially spaced impeller vanes extending upwardly from said base plate and positioned in surrounding relation to said shaft;

a plurality of circumferentially spaced expeller vanes extending upwardly from said base plate and being aligned with said impeller vanes and positioned in surrounding relation to said shaft;

a central impeller area between said shaft and said expeller vanes and beneath said lower end of said particles inlet;

a plurality of air relief vents disposed in surrounding relation to said shaft and in communication with said central impeller area;

wherein each of said expeller vanes has an upper rounded end curving in a direction away from the direction of rotation of said expeller vanes; and wherein said air relief vents are disposed about said shaft adjacent the lower end of said particles inlet.
6. Apparatus wherein said particles inlet is in surrounding relation to said drive shaft.
7. Apparatus for blending liquids and solid particles comprising in combination:

a housing having an upper funnel shaped particles inlet and a lower liquid inlet, a central drive shaft, and an outlet in an outer wall of said housing;

an impeller assembly mounted for rotation beneath the lower end of said particles inlet including a base plate and a plurality of circumferentially spaced impeller vanes extending

2018200356 08 Mar 2019 upwardly from said base plate and positioned in surrounding relation to said shaft;

a plurality of circumferentially spaced expeller vanes extending upwardly from said base plate and being aligned with said impeller vanes and positioned in surrounding relation to said shaft;

a central impeller area between said shaft and said expeller vanes and beneath said lower end of said particles inlet;

a plurality of air relief vents disposed in surrounding relation to said shaft and in communication with said central impeller area; and wherein said housing includes a pair of funnel shaped portions, with one of the pair partially disposed within the other, and wherein the two funnel shaped portions are disposed apart from one another so as to create an air relief vent therebetween .
8. Apparatus according to Claim 7 further comprising a tubular member located centrally of said central impeller area and extending through said two funnel shaped portions, said tubular member provided with at least a portion of said air relief vents.

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