US3904714A

US3904714A - Low-speed mechanical aerator impeller

Info

Publication number: US3904714A
Authority: US
Inventors: Thomas C Rooney; Robert E Crandall
Original assignee: Rexnord Inc
Current assignee: Siemens Water Technologies Holding Corp
Priority date: 1973-09-26
Filing date: 1973-09-26
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09
Also published as: NL7412642A; JPS5059858A; GB1441502A; BE819964A; FR2257799A1; AU7349074A; IT1030060B; SE7412092L; JPS5852686B2

Abstract

A low speed impeller of large size and pumping capacity comprises a conical shroud and 3, 4 or 5 flat blades projecting therefrom. The shroud and blades are disposed and proportioned to provide a wider operating range and are particularly adapted for fabrication of flat plates which are welded directly to the shroud.

Description

United States Patent [191 Rooney et a].

[ Sept. 9, 1975 LOW-SPEED MECHANICAL AERATOR IMPELLER Inventors: Thomas C. Rooney, Waukesha;

Robert E. Crandall, Greendale, both of Wis.

Assignee: Rexnord Inc., Milwaukee, Wis.

Filed: Sept. 26, 1973 Appl. No.: 401,100

US. Cl. 261/91; 210/219; 210/220; 259/8; 259/108; 416/188 Int. Cl. BOIF 7/22 Field of Search 259/8, 23, 24, 43, 44, 259/107, 108, 122; 416/188; 261/92, 91; 210/220, 219

References Cited UNITED STATES PATENTS Durdin 210/8 2,346,366 4/1944 Durdin 259/97 2,354,653 8/1944 Allen 3,323,782 6/1967 Clough 3,470,092 9/1969 Bernard 210/220 X Primary Examiner-Harvey C. Hornsby Assistant Examiner.lames A. Niegowski [5 7] ABSTRACT A low speed impeller of large size and pumping capacity comprises a conical shroud and 3, 4 or 5 flat blades projecting therefrom. The shroud and blades are disposed and proportioned to provide a wider operating range and are particularly adapted for fabrication of flat plates which are welded directly to the shroud.

4 Claims, 6 Drawing Figures LOW-SPEED MECHANICAL AERATOR IMPELLER BACKGROUND OF THE INVENTION 1. Field of the Invention Fixed vertical axis rotating single stirrer for fixed re ceptacle. Class 259/8.

2. Description of the Prior Art Impellers in general are designed for inducing an axial flow or a radial flow, the latter including centrifugal impellers. If the discharge is partially axial and partially radial, the impeller is a mixed flow impeller.

Impellers have some number of vanes or blades projecting from the hub which is mounted on the shaft which supports and rotates the impeller.

In large impellers the efficiency of the blades is generally improved by a shroud around the hub which blocks off the center space and directs the flow smoothly past or around the hub. The hub and shroud may of course comprise the same structure.

Each blade of an impeller has a boundary edge which includes the portion where it is attached to the hub or where it meets the shroud. The boundary edge also has three other portions which are generally defined in terms of impeller rotation and the direction of the flow, viz. the leading edge, the inlet edge, and the outlet edge.

Other considerations which enter into the design of an impeller include its balance, its configuration such that it is readily fabricated, and the presence of strings and rags which may wrap themselves on the blades.

One method of aerating waste water or industrial wastes for its biological treatment is by the use of a ro tating impeller located at the surface of the water. Such impellers are generally fixed on the lower end of the vertical output shaft of a motor driven gear reducer which is supported either by a fixed structure or by floats or pontoons.

A high speed aerator is one which generally includes an axial flow impeller in a vertical draft tube with an upper member which deflects the water in a circular pattern having a considerable upward trajectory. In terms of pumping a given quantity of water per horsepower or current consumption, are of lower first cost but of less efficiency.

Low speed aerators are generally of two types and are driven by motors or engines of up to 175 hp. One type churns large quantities of water at the surface for aeration. The other type is designed to draw as much water as desired from as deep a level below the unit as is necessary to effect mixing and scouring of the bottom of the basin in which the unit is operating. The water is then discharged horizontally to create a large flow across the surface of the water toward the wall of the tank. This up-draft type in a fixed structure might include a large draft tube having an inlet near the floor of the tank. These units generally include an axial flow impeller and the discharge may have a vertical component similar to that of the high speed aerator. The use of an open-style impeller without a draft-tube is preferred because of its more simple structure, and many open-style impellers are presently claimed to be of the up-draft type but their effectiveness at depths below feet is relatively modest. Additionally, slight change of their position relative to the water surface greatly affects their operation. Reference here is made to pumping capacity and submergence range and these two factors are somewhat differently important to both the design of a floating mechanical aerator and to the design of a sewage or waste treatment tank, pond or lagoon in which the units are mounted on piers or bridges.

In most sewage or waste treatment tanks the flow through the tank and the elevation of the water level are controlled by effluent weirs which operate according to the formula K Q/L= V 8gH% where L is the weir length and H is the water level above a straight, sharp crested weir.

The formula will show that at higher and lower flow rates (Q) the water level will be higher and lower respectively, for a given weir length.

In a typical sewage treatment tank having fixed surface aerators this relationship may be utilized so that, for example, during the day and at a high flow rate, the impeller will be more supmerged and operate at maxi mum capacity and during the night when the flow rate is greatly diminished, the impeller will operate at reduced capacity because the lowered water level causes the impeller to be less submerged. This self-regulation can be utilized to great advantage in respect of the biological processes of the organisms which utilize the oxygen which is being supplied to them. Additionally, of course, a saving in power costs is realized in the reduced power requirements at low flows, having reference here to pumping efficiency as well as capacity.

However, the known prior art impellers exhibit a wide range of pumping capacity over a narrow range of submergence. This means that in terms of overall tank design, the tank must be provided with impossibly long weirs in order to hold the water level within the narrow range allowed by the impeller. Alternatively, mechanically operated weirs have been provided to maintain a constant level.

Such a device is of course the type of mechanical complication which the selection of maintenance-free low speed aerators would be intended to avoid, and ad ditionally. it can be seen that such a device which does in fact maintain a constant liquid level entirely eliminates the several advantages of the self-regulating feature which have been described.

On the other hand, many industrial waste treatment systems operate with relatively constant conditions over long periods of time and the low speed mechanical aerators may more readily be mounted on floats which support the units at a given elevation relative to the water levelj The displacement of each of these floats must not only be adequate to support each aerator but additionally must hold the unit steady against the downward reaction of the pumping effort. The pumping action is always subject to some pulsing and the entire unit is susceptible to bobbing in the water. Imperceptible and unpredictable interactions of some number of units in the same tank, pond or lagoon will inexplicably cause one or some to rise and fall. When several fall in unison the combined excess current draw may overload the electrical supply and actuate the protective circuit breaker. This is most likely to occur when one or some of the units are started up or shut down.

With reference to the impeller characteristics here, an impeller which imposes a much larger torque load on the motor when the impeller is only slightly deeper in the water, is likely to exhibit some such instability.

The object of the present invention, thus, is to provide a more stable impeller having the desired pumping capacity and submergence range which is required for and characteristic of a typical biological treatment tank utilizing surface aeration.

SUMMARY OF THE INVENTION The impeller of the present invention has a 45 conical shroud and three, four or five flat blades which ex tend downwardly therefrom so that their inlet edges are substantially below the apex of the shroud. The angle between the leading edge of each blade and the impeller axis is in the order of 30. In general terms, the overall height of the impeller is in the order of two-thirds of its diameter. A prime characteristic of the impeller is its wide band submergence limits for a typical perfor mance range.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a representative biological waste treatment tank with a bridge mounted mechanical aerator.

FIG. 2 is a central vertical cross section of the tank shown in FIG. 1.

FIG. 3 is a plan view of the blades of the impeller. That is, the view is that of the blades looking down wardly. The drive shaft is shown in section and the outline of the shroud is shown in broken lines.

FIG. 4 is a side elevation of the impeller of FIG. 3.

FIG. 5 is a vertical elevation of one-half of the impeller taken on line 55 of FIG. 3.

FIG. 6 shows a blade of the impeller with additional marginal portions which may be provided in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A representative square tank 8 is shown in FIGS. 1 and 2 and includes the baffled inlet pipe 9 and the outlet weir 10 which controls the elevation of the water level within the tank. One of the four side walls of the tank includes the weir trough 11 which extends the length of weir 10 and includes the effluent pipe 12. The mechanical aerator 13 is mounted on the bridge 14 at the center of the tank and includes the upper drive motor 15, the gear reducer 16 having an output shaft 17 extending downwardly therefrom and below bridge 14 and the impeller 21 which is mounted on the lower end of shaft 17 for rotation therewith at the surface of the tank contents.

Impeller 21 comprises the shroud 22, the several blades 23 and a hub, not shown in detail, by which the impeller is removably secured to shaft 17. The hub and shroud 22 may be of any suitable construction. A feature of the invention resides in the fact that blades 23 are of flat plates which may be welded at their inner boundaries 24 directly to shroud 22.

Shroud 22 is in the form of a right circular 45 cone and as such includes a circular upper end which is bounded by the upper edge 22a. The height of the shroud from its lower apex 22b to its upper end is thus equal to the radius of the upper edge 22a. For practical reasons, the shroud may be truncated as shown such that it has a relatively small circular lower end 22c.

The inner boundary 24 of each blade has been referred to. As shown in FIGS. 3-5, the remaining outer boundary includes the leading edge 25, the inlet edge portions 26a and 26b, and the outlet edge 27. Each blade 23 forms an angle in the order of 30 with the downward projection A of the axis of shaft 17 and the blades are equally spaced around the axis. The intersection of the plane of each blade 23 and the axis of the impeller is located at the upper end of shroud 22 such that the plane referred to may be further defined as a plane which includes a radius R of shroud '22 at its upper end and defines therewith an angle of 60.

Proceeding with such reference points, the leading edge 25 of blade 23 extends from shroud 22 to a lower elevation approximately 033R below apex 22b and the outlet edge 27 from edge 22a of shroud 22 to a point approximately at the elevation of apex 22b and radius R. The inlet edge includes two portions. Portion 26a extends horizontally at said lower elevation from leading edge 25 and inlet edge portion 26b extends there from to the lower end of outlet edge 27 which has been defined.

The basic configuration of blades 23 has been shown and described for the purpose of defining the invention and it should be understood that the actual outline of the blades may be curved or rounded by others with the th) ught that some gain in efficiency may result. Also, FIG. 6 shows by way of example the additional areas which may be provided in accordance with the invention. This may include the triangular portion 31 such that the leading edge 25a has a slight negative rake respecting its rotation.

Where a given impeller design has been set up for manufacture and an impeller of some greater capacity is required, the trailing section 32 may be added such that the outlet edge 27a extends to a radius of 1.2R as shown, but not appreciably greater than l.5R. Impeller 21 could also be provided with four similar blades and possibly with five blades. However, the blades extend 90 or more around the shroud 22 and the overlapping of the blades is thought to make six or more blades impracticable.

As has been mentioned, the usual tank, pond or lagoon is of such size that a considerable number of aerators are required at equally spaced locations. The tank 8 may be considered typical of a biological treatment tank as a single unit. The tank as shown is feet X 80 feet X 20 feet and the water level to the weir 10 is 18 feet. Such a tank would accommodate a normal flow (Q) for a typical waste in the order of 7,500 gallons per minute. Minimum and maximum flows might be 2,500 gpm. and 22,000 gpm. respectively.

For such flows the following tabulation is derived using the established formula given in the description of the prior art.

It will be seen from the above, that impeller 21 would require a weir length of only 29 feet because of its submergence range. On the other hand, to provide the same tank with the prior art impeller described, a weir length of somewhere between 80 feet and 120 feet may be required. While a weir is an especially inexpensive device, the necessity of providing a weir of such additional length for tank 8 would require extending the weir trough 11 around the tank as shown in FIG. 2 by the broken line 11a and would be a considerable complication.

The unexpected advantages of the present invention in relation to weir length should of course not be considered in every case to be more important than others which have also been mentioned or described at less length. The advantages such as in manufacturing simplicity are selfevident; the increased pumping effcicncy of the impeller which is claimed, is presently indicated by applicants efforts in the development of the present invention. The stability of an impeller is gener ally only based on observation or deduction after a power shut down has occurred.

We claim:

1. ln apparatus for aerating and mixing a body of water, said apparatus comprising rotational drive means having a vertical output shaft extending downwardly therefrom, an impeller fixed to the lower end of said shaft and having a lower inlet end, and means for supporting said drive means so that the upper end of the impeller is at a selected elevation above the water surface and the lower end of the impeller is at a selected level of submergcncc; the improvement wherein said impeller comprises a conical shroud of R height and be tween three and live flat blades, the larger upper end of the shroud defining the upper end of the impeller and having a radius R, each said blade being in the form of a flat plate which is set with a 30 rearward rake having reference to the upward direction of the flow toward the propeller and disposed so that its plane intersects the impeller axis at the upper end of the impeller, the perimeter of each blade consisting of l a part which adjoins the shroud, (2) a leading edge, (3) an inner inlet edge, (4) an outer inlet edge, and (5) an outlet edge, said leading edge of each blade extending from the shroud downwardly and radially outward to the inner inlet edge ofthe blade, the outlet edge of each blade being of R height and having a radius of between R and 1.5R and extending from the outer inlet edge to the upper outer edge of the shroud, the inner inlet edges of the blades being about 0.3R below the lower end of the shroud and defining the lower end of the impeller.

2. The improvement of claim 1 in such apparatus wherein the leading edge of each blade has a slight radially rearward rake.

3. The improvement of claim 1 in such apparatus wherein the lower end of the shroud is truncated.

4. The improvement of claim 1 in such apparatus wherein the supporting means is adapted to support the drive means so that the upper end of the impeller is in the order of 0.25R-0.2R above the level of the water surface.

Claims

1. IN APPARATUS FOR AERATING AND MIXING A BODY OF WATER, SAID APPARATUS COMPRISING ROTATIONAL DRIVE MEANS HAVING A VERTICAL OUTPUT SHAFT EXTENDING DOWNWARDLY THEREFROM, AN IMPELLER FIXED TO THE LOWER END OF SAID SHAFT AND HAVING A LOWER INLET END, AND MEANS FOR SUPPORTING SAID DRIVE MEANS SO THAT THE UPPER END OF THE IMPELLER IS AT A SELECTED ELEVATION ABOVE THE WATER SURFACE AND THE LOWER END OF THE IMPELLER IS AT A SELECTED LEVEL OF SUBMERGENCE, THE IMPROVEMENT WHEREIN SAID IMPELLER COMPRISES A CONICAL SHROUD OF R HEIGHT AND BETWEEN THREE AND FIVE FLAT BLADSE, THE LARGER UPPER END OF THE SHROUD DEFINING THE UPPER END OF THE IMPELLER AND HAVING A RADIUS R, EACH SAID BLADE BEING IN THE FORM OF A FLAT PLATE WHICH IS SET WITH A 30# REARWARD RAKE HAVING REFERENCE TO THE UPWARD DIRECTION OF THE FLOW TOWARD THE PROPELLER AND DISPOSED SO THAT ITS PLANE INTERSECTS THE IMPELLER AXIS AT THE UPPER END OF THE IMPELLER, THE PERIMETER OF EACH BLADE CONSISTING OF (U) A PART WHICH ADJOINS THE SHROUD, (2) A LEADING EDGE, (3) AN INNER INLET EDGE, (4) AN OUTER INLET EDGE, AND (5) AN OUTLET EDGE, SAID LEADING EDGE OF EACH BLADE EXTENDING FROM THE SHROUD DOWNWARDLY AND RADIALLY OUTWARD TO THE INNER INLET OF THE BLADE, THE OUTLET EDGE OF EACH BLADE BEING OF R HEIGHT AND HAVING A RADIUS OF BETWEEN R AND 1.5R AND EXTENDING FROM THE OUTER INLET EDGE TO THE UPPER OUTER EDGE OF THE SHROUD, THE INNER INLET EDGES OF THE BLADES BEING ABOUT 0.3R BELOW THE LOWER EDGE OF THE SHROUD AND DEFINING THE LOWER END OF THE IMPELLER.