US 3329409 A
Description (OCR text may contain errors)
y 1967 J 5. RALEIGH DRAFT TUBE IMPELLER 2 Sheets-Sheet 1 F1 led Feb. 4, 1966 5 R. Wh a Z% J M/ Y m/% 1% ILL" ,w g?
July 4, 1967 J. 5. RALEIGH 3,329,409
DRAFT TUBE I MPELLER Filed Feb. 4, 1966 2 Sheets-Sheet 2 United States Patent Filed Feb. 4, 1966, Ser. No. 525,240 9 Claims. (Cl. 259-97 This invention relates to impellers such as are commonly mounted within draft tubes to effect accelerated mixing of flowable materials, to effect flow of material around heating or cooling elements, or to effect transportation of material from one stage to another in industrial processes.
Draft tubes are flow-directing devices used to develop a desired flow patern; ordinarily, they are hollow, openended, metal cylinders. When used to effect mixing, a draft tube is placed in a vat or other receptacle containing the material to be mixed, and a shaft upon which impeller blades are mounted is supported for rotation within the draft tube.
Conventionally, impellers are of two kinds. They comprise either a plurality of separate stirring blades or a continuous helical blade and so arranged that the ends of the stirring blades or the free edge of the helical blade substantially touch the interior of the draft tube. However, neither of these conventional blades, when used alone, is entirely satisfactory. The stirring blades provide a vigorous local mixing action but they are unable efliciently to propel viscous materials through the draft tube. A continuous helical blade, on the other hand, will merely displace viscous material vw'th very little mixing action unless high speeds are used, but the higher speeds conductive to effective mixing by such blades are not usually practical, especially when the material is highly viscous When enormous power is required to turn the continuous helical blade rapidly.
A principal object of this invention is to provide an improved impeller of helical type which possesses the desirable functional characteristics of helical and mixing blades as it rotates in a draft tube; which effectively cuts through and mixes viscous material as well as displacing it; which will displace highly viscous material without a large expenditure of power; and which is not only inexpensively made but also extremely durable and highly efiicient.
As herein illustrated, the invention comprises, in combination with a draft tube and a shaft mounted for rotation therein, a composite impeller supported by said shaft, said impeller comprising a blade having one or more smooth circular, truly helical turns for propelling fluid through the draft tube, and a mixing blade whose outer free edge is a composite of substantially straight edges which substantially form chords of the draft tube and provide corners at the intersections of said straight edges which are equally spaced about the interior of the draft tube. The several straight edges of the mixing blade are of substantially equal length and are of such pitch that four successive edges wind approximately one turn about the shaft axis. The corners nearly touch the interior of the tube and successive corners are equally spaced about the axis of the shaft.
Desirably, the mixing blade is made up of square plates, initially flat, containing central holes to receive the shaft. Each plate is split on a diagonal from one corner to the center hole. The plates are mounted in axially spaced relation on the shaft with the edges of each plate (at opposite sides of the split) joined respectively, to the opposite edges of the axially adjacent plates. The plates are fixed to the shaft and the edges of successive plates are joined to each other by welding or the like. Preferably the peripheral edges of the plates are beveled or sharpened to provide for a cutting or comminuting action.
The invention will now be described in greater detail with reference to the accompanying drawings wherein:
FIG. 1 is a vertical section through a container within which is supported a draft tube and showing the impeller in elevation;
FIG. 2-is a transverse horizontal section on line 2-2 of FIG. 1 showing the plates of the mixing blade in elevation;
FIG. 3 is a perspective view showing the impeller removed from the draft tube and drawn to larger scale;
FIG. 4 is a plan view, to small scale, of one of the plates employed in preparing the mixing blade of the impeller;
FIG. 5 is a perspective view illustrative of the plate of FIG. 4 showing the manner in which the plate of FIG. 4 is bent to provide a helical surface in mounting it on the rotatable shaft; and
FIG. 6 is a small scale fragmentary section, in the same plane as FIG. 1, illustrating one blade of a suggested modification.
Draft tubes with impellers therein are used for many purposes in industrial processes. The drawings show certain embodiments of the invention by way of example.
In FIG. 1 of the drawings there is shown a receptacle or tank 19 for the reception of the flowable material 12 (for example, an aqueous solution or suspension) which is to be circulated, for example, to effect a thorough mixing and/or comminution of solid particles or to maintain a uniform density within the tank. The draft tube 14 employed, in conjunction with the receptacle 10, has a hollow elongate, cylindrical chamber C and is suitably supported in the receptacle 10 to provide an entrance E for rflowable material as its lower end. Within the draft tube there is rotatably mounted a shaft 16 here shown as hollow, on which there is fixed the impeller 18 forming the subject matter of the invention. The shaft 16 is supported by means of bearings 20 and 22 at the top and bottom of the receptacle and may be driven by any suitable means (not shown). The impeller blade 18 is in general helical, that is, it winds around and along the shaft axis, although portions of it may deviate widely from strict mathematical expressions for helices, but for convenience it is here referred to as helical. It should be understood however that the term helical as used herein is intended to suggest the fact that the blade structure winds about the shaft axis.
As here illustrated and in accordance with a preferred embodiment of the invention, the impeller comprises two truly helical turns H and H which are located at the lower portion of the shafts, that is to say turns, each of which is smooth and uninterrupted and whose free pcripheral edges are circular, the function of these helical turns being to induce the entrance, through the openings E, of material from the tank into the lower part of the tube and to start the material moving in an upward direction. Since only a few of these helical irnpelling turns- (for example, two) are used, the power required for turning the shaft is not excessive even though the fluid may be quite viscous.
Above the helical turns H and H the impeller blade comprises a plurality of flights or turns B B etc., whose inner margins define a true helix, their inner edges being fixed to the shaft.
As shown in FIG. 5, each flight or turn has an inner edge 23, which is fixed to the shaft 16 and each of said flights or turns has an outer free edge which is a composite of substantially straight edges 26 of equal length which intersect at corners 28. As shown in FIG. 2 the corners 28 substantially touch the interior of the draft tube 14 and 3 the pitch of the helix defined by the edges is such that successive corners 28 are equally spaced about the axis of the shaft. As shown, the lengths of the edges 26 are such that four successive edges define, in space, slightly more than a single helical turn which gives the mixing blade a serrated appearance when viewed axially (FIG. 2).
Desirably, the mixing portion of the impeller blade comprises a plurality of rectangular plates, such as the plate P (FIG. 4) each initially flat and each of which has a central circular hole 32 for the reception of the shaft 16, and each of which is split diagonally from one corner to the central hole 32 so that the edges at opposite sides, E, E (FIG. 5) of the split may be displaced, axially of the shaft so that the plates collectively form on helical surfaces, the edges E B being joined to those respectively, of the overlying and underlying plates. The plates, bent to helical curvature, are held in axially spaced relation on the shaft by welding or otherwise fastening the edges 23 to the shaft, and the edges E, E of successive plates are fastened to each other by welding or the like. The mixing blade of the impeller, as thus formed, has a continuous helical surface lengthwise of the shaft, but the free edge of each flight B B etc., as above pointed out, is constituted by a series of substantially straight edges characterized by relatively sharp corners 28 which substantially touch the interior of the tube. With this arrangement, the action of the impeller does not produce a solid column of fluid within the draft tube at the location of the mixing blade since, as above pointed out, the edges 26 of the blades form chords of the circle defined by the draft tube 14, so that there is a substantial space between each straight edge of the mixing blade and the tube 14. On the other hand, because the straight edges of successive blades are offset from each other axially of the shaft and because successive corners of each plate are at different levels, the rotation of the mixing blade has the effect of agitating the fluid material very thoroughly and, even though the material may be very viscous, the edges of the mixing blade cut through it with case so that relatively little power is required to drive the impeller at a speed such as to accomplish a thorough mixing. Moreover, by providing the blades with beveled or sharpened edges as illustrated in FIGS. 1 and 6, their cutting action is enhanced so that if, for example, the fluid medium contains fibrous material or other solid particles, the rotating blade acts to divide such fibers of particles into smaller pieces.
In FIG. 6 a modified impeller blade is illustrated, wherein, by way of example, a portion of the shaft 16 is shown in section with a portion of the upper turn 28 of the mixing blade in radial section. The draft tube 14 is also shown with an insulating jacket I which may, for example, be of a plastic foam. As illustrated, the blade member B has an interior chamber within which there is housed an electrical heating unit U of convertional type whose terminal wires extend through an opening in the wall of the tube 16 and are connected to conductor wires in a cable A located within the shaft 16 and which may be supplied with current from a suitable source in conventional manner. By this means the helical surface which contacts the fluid may be heated to a predetermined selected temperature. Similarly, a cooling fluid might be circulated through the hollow flights of the impeller. In this way the temperature of the fluid passing through the tube 14 may be heated or cooled, as desired, Without directly affecting the temperature of the liquid outside of the tube.
As above pointed out, the impeller has a continous helical surface lengthwise of the shaft, although at the mixing zone the free edge of the blade is characterized by a series of substantially straight edges and by relatively sharp corners.
The impeller as thus constructed is comparatively simple and easy to construct since by cutting, stamping or otherwise forming a plurality of flat metal plates such as the plates P (FIG. 4) which are square and have holes at their centers; then splitting the plates diagonally (FIG. 5); and then so bending them while joining their inner edges to the shaft and connecting the split edges of adjacent plates by welding or the like, the impeller may be formed without recourse to the use of forming dies or casting.
The sharp, straight edges of the blades with their sharp corners function somewhat as a saw so that as the impeller rotates it cuts through a viscous mass while, at the same time, impelling it longitudinally of the tube, and thus the improved impeller may be driven for a mixing operation with the expenditure of much less power than has heretofore been required for effective mixing at a practical speed.
It will be understood that the present disclosure is for the purpose of illustration only, and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.
1. An impeller for use in a draft tube having a centrally located rotatable shaft, means fixed to said shaft operative to impel fluid upwardly within the lower portion of the tube, and other means fixed to said shaft operative to agitate fluid Within the upper portion of said tube, characterized in that the means for impelling and the means for agitating the fluid are portions of a continuous helical surface fixed relatively to the shaft, that portion of said helical surface which is at the lower part of the tube has a continuous curved peripheral edge which at all points is concentric with the tube and close to the latter, while the peripheral edge of the surface which functions to agitate the fluid is constituted by a series of substantially straight edge portions which intersect to form corners which are spaced from the inner surface of the tube, said corners of successive turns being offset about and axially of the shaft.
2. An impeller for use in a draft tube, said impeller comprising a rotatable shaft coaxial with the tube and means fixed to said shaft defining an uninterrupted helical surface extending from en-d-to-end of the impeller, certain turns of said helical surface having curved free edges disposed closely adjacent to and concentric with the inner surface of the tube, and certain turns of said surface having peripheral edge portions which are chordal with respect to the inner surface of the tube and which intersect to define corners which are close to but spaced from the inner surface of the tube.
3. In combination with a draft tube and a shaft mounted for rotation therein, an impeller supported by said shaft, said impeller being helical and comprising a mixing portion having a peripheral edge which is a composite of substantially straight edges situated in substantially chordal relation to the interior of the draft tube, with corners, at the intersections of said straight edges, which are equally spaced about the interior of the tube.
4. The combination according to claim 3, characterized in that said straight edges are of substantially equal length, and said corners substantially touch the interior of the tube.
5. The combination according to claim 3, wherein said straight edges are of such length that four successive edges wind approximately one turn around the shaft axis.
6. The combination according to claim 5, characterized in that the mixing portion comprises a plurality of flights, each consisting of a plate which is substantially square and which has a central hole for the reception of the shaft, each plate having a diagonal slit extending from one corner to the center hole and the several plates being mounted on the tube in axially spaced relation and means securing the plates on the shaft in spaced relation with an edge of one plate, at one side of is diagonal slit, adjacent and welded to an edge of the next plate.
7. An impeller comprising a shaft and a plurality of initially square plates mounted on the shaft, each plate containing a central hole for receiving the shaft and a diagonal slit extending from a corner to the central hole to enable the edges, at opposite sides of the slit of each plate, to he joined to the edges at the opposite sides of the slits in the next adjacent plates, means securing the plates, in axially spaced relation, at their center holes to the shaft, and means joining the edges of the slits of successive plates to each other to form a continuous helical surface having a peripheral edge comprised of a series of straight edges.
8. An impeller according to claim 2, further characterized in having means for maintaining said surfaces at a substantially constant predetermined temperature.
9. Draft tube apparatus comprising an impeller accordwith an insulating jacket, and having means whereby the impel'ling and agitating surfaces may be maintained at a predetermined selected temperature thereby to treat the fluid which is acted upon by the impeller.
References Cited UNITED STATES PATENTS 134,706 1/1873 Schultze 25997 1,284,945 11/ 1918 Swan 25997 10 3,076,637 2/1963 Moziek et al 2598 3,249,341 5/ 1966 Stanford 25997 WALTER A. SCHEEL, Primary Examiner.
ing to claim 8, and wherein the draft tube is provided 15 ROBERT W. JENKINS, Examiner.