Title:
MIXER APPARATUS
United States Patent 3752447
Abstract:
A housing and paddle assembly adapted for use in processing and mixing highly viscous fluids is provided. The mixer employs a paddle assembly having a shaft which rotates about a horizontal axis. The assembly has at least two pairs of blade members. The members of each pair are symmetrically positioned about the shaft and extend axially in one half of the housing. The blade members of each pair may be slotted at diagonally opposite outside ends. In operation, the paddle assembly can sweep out substantially all of the housing interior and can produce simultaneously in a fluid cyclical vertical displacement, rolling action, horizontal displacement, and, even, fold over action.
US Patent References:
Plaster-mixing device
Essick - May 1927 - 1630790
Portable concrete mixing foamating and dispensing machine
Ayers - January 1962 - 3017164
PADDLE-TYPE POLYMERIZATION REACTOR
Anderson - September 1969 - 3469948
POLYMERIZER APPARATUS
Leybourne - November 1969 - 3476523
Plaster-mixing device
Essick - May 1927 - 1630790
Portable concrete mixing foamating and dispensing machine
Ayers - January 1962 - 3017164
PADDLE-TYPE POLYMERIZATION REACTOR
Anderson - September 1969 - 3469948
POLYMERIZER APPARATUS
Leybourne - November 1969 - 3476523
Application Number:
05/172147
Publication Date:
08/14/1973
Filing Date:
08/16/1971
View Patent Images:
Export Citation:
Assignee:
Monsanto Company (St. Louis, MO)
Primary Class:
Other Classes:
366/172.200, 366/149, 422/225
International Classes:
B01F7/02; B01F15/00; B01J19/20; B01F7/00; B01J19/18; B01F7/04
Field of Search:
259/9,10,7,8,5,6,109,110,21,22,23,24,25,26,43,44,45,46,178,68,69 23/252R
Primary Examiner:
Jenkins, Robert W.
Claims:
What is claimed is
1. A mixer adapted for use in processing highly viscous fluids comprising:
2. a shaft rotatably mounted at its opposite end portions in said housing and extending generally longitudinally through said interior chamber parallelly to said axis, and
3. two sets of axially adjacent blade members, each set occupying a different 1/2 region of said interior chamber, the blade members of each set being circumferentially equally spaced from one another, each blade member of each set radially projecting from said shaft and extending axially generally continuously along said shaft, the radial width of each blade member of each set at any given point along its axial length measured from the shaft axis being such as to make the ratio of the chamber radius range from 1 to 0.5 thereat,
4. The mixer of claim 1 wherein each set of axially adjacent blade members contains one pair of blade members.
5. The mixer of claim 2 wherein in said sets of blade members, one blade member of each set is slotted in the region of one outside corner, the effective total slot cross-sectional surface area in each such blade member ranging from about 3 to 50 per cent of the effective total surface area of each individual blade member.
6. The mixer of claim 3 wherein the effective total slot cross-sectional surface area in each blade ranges from about 4 to 20 per cent of the total effective surface area thereof, and wherein the slot in each respective blade of such a pair of blades is substantially equally sized and similarly located at respective diagonally opposite outside ends thereof.
7. The mixer of claim 2 wherein each of blade member of each set is similarly radially and convexly curved with respect to the direction of shaft rotation.
8. The mixer of claim 1 wherein each blade member of each set is substantially flat and planar.
9. The mixer of claim 1 wherein said chamber has dimensions such that the ratio of the length of said chamber along said axis to the maximum diameter of said chamber ranges from about 0.5 to 3.5.
10. The mixer of claim 1 wherein said shaft is substantially coaxial with said axis.
11. The mixer of claim 1 further equipped with drive means for revolvably driving said shaft including a power head and power transfer means.
1. A mixer adapted for use in processing highly viscous fluids comprising:
2. a shaft rotatably mounted at its opposite end portions in said housing and extending generally longitudinally through said interior chamber parallelly to said axis, and
3. two sets of axially adjacent blade members, each set occupying a different 1/2 region of said interior chamber, the blade members of each set being circumferentially equally spaced from one another, each blade member of each set radially projecting from said shaft and extending axially generally continuously along said shaft, the radial width of each blade member of each set at any given point along its axial length measured from the shaft axis being such as to make the ratio of the chamber radius range from 1 to 0.5 thereat,
4. The mixer of claim 1 wherein each set of axially adjacent blade members contains one pair of blade members.
5. The mixer of claim 2 wherein in said sets of blade members, one blade member of each set is slotted in the region of one outside corner, the effective total slot cross-sectional surface area in each such blade member ranging from about 3 to 50 per cent of the effective total surface area of each individual blade member.
6. The mixer of claim 3 wherein the effective total slot cross-sectional surface area in each blade ranges from about 4 to 20 per cent of the total effective surface area thereof, and wherein the slot in each respective blade of such a pair of blades is substantially equally sized and similarly located at respective diagonally opposite outside ends thereof.
7. The mixer of claim 2 wherein each of blade member of each set is similarly radially and convexly curved with respect to the direction of shaft rotation.
8. The mixer of claim 1 wherein each blade member of each set is substantially flat and planar.
9. The mixer of claim 1 wherein said chamber has dimensions such that the ratio of the length of said chamber along said axis to the maximum diameter of said chamber ranges from about 0.5 to 3.5.
10. The mixer of claim 1 wherein said shaft is substantially coaxial with said axis.
11. The mixer of claim 1 further equipped with drive means for revolvably driving said shaft including a power head and power transfer means.
Description:
BACKGROUND
In processing fluids, especially viscous fluids having viscosities generally greater than, say, about 5,000 centipoises, it is often necessary to blend therewith additives. In mixing operations involving such viscous fluids, many problems are encountered, and specialized mixing apparatus is desirable and commonly necessary. Complex mechanical and fluid forces are involved. The art has long sought new and improved mixing means adapted for use with highly viscous fluids, the end result generally desired being to achieve in such viscous fluids, or to maintain such viscous fluids in, a substantially uniform or homogeneous condition in the most efficient manner possible. One problem with conventional mixing means has been the large amount of power required to drive an agitator placed in a highly viscous fluid. Another problem therewith has been the difficulty of mixing a liquid of low viscosity (for example, one having a viscosity of less than about 10 centipoises) with one of high viscosity (for example, one having a viscosity of greater than about 10,000 centipoises). Still another problem has been the form and configuration of the agitator and of the mixer housing relative to the agitator.
One class of mixers recognized in the prior art characteristically has an agitator revolving generally (though not necessarily exactly) about a horizontal axis within a vessel or housing which is usually cylindrical. Such class of mixers is suitable (depending upon individual situations) for batch or continuous operation. There has now been discovered a new and improved mixer of this class wherein the mixing of highly viscous fluids (for example, of a low viscosity liquid into a high viscosity liquid) can proceed with unexpectedly low power and in a rapid and highly efficient manner. This mixer is especially well adapted for use conditions where the housing is only partially filled with a highly viscous fluid. The mixer is useful in a wide variety of end use applications involving mixing or agitation of fluids and produces a type of mixing action heretofore unknown.
SUMMARY
The present invention is directed to a mixer which is especially well adapted for use in processing highly viscous fluids, but which is also suitable for mixing fluid or fluidizable materials generally. The mixer incorporates a housing which encloses an interior chamber with side and end walls. The chamber walls are generally radially symmetrical (e.g., cross-sectionally circular) with respect to a longitudinal axis extending therethrough. The housing is preferably adapted to be oriented during mixer operation so that the longitudinal axis extends generally horizontally. The housing preferably has an input port above the level of the axis and has an output port preferably below the level of the axis, although a housing with only a single port may be used, if desired, as when an auxiliary fluid pump is used and the mixer is not used in a continuous operation. The housing can be formed of any convenient construction material though steel is presently preferred.
The mixer employs a paddle assembly which has a rotatably mounted shaft which extends generally longitudinally through said housing and generally parallel to said axis. From the shaft extend tow independnet but axially adjacent pairs of blade members. The blade members of each pair are symmetrically spaced from one another and radially project from the shaft preferably to near engagement with interior wall surfaces of the interior chamber of the housing. Thus, if there are only two blade members in one pair, then these blade members are diametrically opposed to one another. Each blade member is generally continuous along its axial length and radial breadth. Each member of each pair may optionally be slotted in the region of their respective diagonally opposite outside ends (or corners), with the effective total slot cross-sectional surface area in each blade ranging from about 3 to 50 percent of the total effective surface area of such blade (preferably ranging from about 4 to 20 percent thereof), though somewhat larger and smaller surface areas may be used without departing from the spirit and scope of the present invention, as those skilled in the art will appreciate. Preferably, the slots in each respective blade of such a pair of blades are substantially equally sized and similarly located at respective diagonally opposite outside ends. The diameter of each such pair of blade members in a paddle assembly in a mixer at any given location along the axis of the shaft except in slot locations is typically not less than about 90 percent of the diameter of the interior chamber of the mixer taken at about the same location. Each such pair axially occupies about one half the interior region or chamber of the housing.
The shaft of the paddle assembly is journaled by appropriate journal means at its opposite end regions in fixed relationship to the housing to adapt the shaft for rotational movements. When the shaft rotates, the blade members of each such pair thus sweep out about one half the interior chamber of the housing. Preferably, the shaft axis is coaxial with the housing's longitudinal axis. The paddle assembly can be formed of any convenient material of construction, though metals such as settl are presently preferred. Sealing means to prevent fluid leakage between the paddle assembly shaft and the housing are provided in an operating mixer, as those skilled in the art will readily appreciate.
The paddle assembly is in a mixer preferably adapted to rotate with substantially no contact between blade tips and chamber walls. For operation, a mixer is equipped with drive means for revolvably driving the paddle assembly shaft, including a power head and power transfer means. A paddle assembly when rotating during mixer operation with fluid in the chamber of the mixer housing is preferably, though not necessarily, adapted to produce a simultaneous combination of cyclical vertical displacement, rolling action, horizontal displacement, and, even, fold over action. Preferably, the clearance between blade tips and housing wall ranges from about 0.01 to 1.5 inches, depending on mixer size, though preferably, in a mixer of this invention, the ratio of the clearance between the blade tips and the adjacent chamber walls to the chamber diameter measured at about the same location at substantially all locations in the chamber along the longitudinal chamber axis except opposite slot locations ranges from about 0.01 to 0.0001. Preferably, in a mixer, the paddle assembly is operated so as to produce during operation uniform rotational movements of the paddle assembly shaft. Individual blade members may be radially and/or axially curved.
The invention is further directed to the paddle assembly itself which is employed in a mixer of this invention.
DRAWINGS
Turning to the attached drawings, there are seen various illustrations intended to provide a better understanding of the present invention, as follows:
FIG.1 is a side elevational view of one embodiment of a mixer of the present invention;
FIG. 2 is a vertical, longitudinal sectional view taken through the mixing chamber of the embodiment shown in FIG. 1;
FIG. 3 is a diagrammatic representation of the fluid mixing mechanics in an operating mixer of FIGS. 1 and 2 as seen in a vertical, longitudinal section;
FIGS. 4, 5 and 6 are diagrammatic representations of the fluid mixing mechanics in one embodiment of an operating mixer of the present invention as seen in cross-section;
FIG. 7 is an isometric view of a paddle assembly similar to that of FIGS. 1-6 but radially curved as well as slotted;
FIG. 8 is an isometric view of a paddle assembly which is radially and axially curved;
FIG. 9 is a view of a paddle assembly similar to FIG. 8 but with a different radial and axial curvature;
FIG. 10 is an isometric view of a paddle assembly having six flattened blades;
FIG. 11 is an isometric view of a paddle assembly having six flattened blades but different positioned in comparison to the blade assembly of FIG. 10.
FIGURE DESCRIPTION
Referring to the drawings more particularly, there is seen in FIGS. 1 and 2 an embodiment of a mixer of this invention which is herein designated in its entirety by the numeral 20. Mixer 20 has a housing designated herein in its entirety by the numeral 21 formed of steel or the like which encloses an interior chamber 22 (see FIG. 2). Housing 21 is formed by a central cylindrical portion 23 to which are secured at opposite ends thereof heads 24 and 25, respectively. In the embodiment depicted, head 24 is secured to one end of cylindrical portion 23 by welding along flange 27, while head 25 is secured to the opposite end of cylindrical portion 23 by a series of bolts 28 with mating nuts 29 extending through adjoining flanges 30 and 31 on cylindrical portion 23 and head 25, respectively.
Housing 21 has formed therein an input port 33 located in the top mid-region of cylindrical portion 23. An appropriately flanged conduit 35 connects port 33 to a feed dome 34, conduit 35 and dome 34 being secured together by bolts 36 which extend through the flange of feed dome 34 into threaded rolls in the flange of head 35. Through dome 34 extend feed pipes 37 and 38. Pipe 37 terminates within dome 34 in a spray head 40 so located as to be adapted to spray material into a wide area of chamber 22 according to a preselected pattern, while pipe 38 terminates within dome 34 in a conventional orifice (not detailed) which delivers material into chamber 22 as a stream.
Housing 21 also has formed therein an output port 41. An appropriately flanged conduit 42 connects port 41 to outlet pipe 43, pipe 43 and conduit 42 being similarly secured together by bolts 44. Pump means (not shown) may be provided to deliver material to, or to take material from, chamber 22, via feed pipe 37 and/or 38, or via pipe 43, respectively. Additional input and output ports on a mixer 20 may be employed, of course, as desired.
Housing 21 further has formed therein a vent port 46 in the top mid-region of cylindrical portion 23. An appropriately flanged conduit 47 connects port 46 to pipe 48, conduit 47 and pipe 48 being similarly secured together by bolts 49. During a mixing operation, port 46 may serve as a safety valve permitting escape of pressurized gases from chamber 22 in the event of excessive pressure build-up in housing 21, as through rupture of a rupture disc 50. To isolate the interior of chamber 22 from the atmosphere and prevent during operation of mixer 20 leakage of fluid therefrom, appropriate seals 51 (for head 25 and cylindrical portion 23), seal 52 (for conduit 35 and dome 34), seal 53 (for conduit 42 and pipe 43), and seal 54 (for conduit 47 and pipe 48) are provided. Vent 46 is also useful when mixer 20 is to be employed as a reactor wherein mixing of viscous fluids takes place, and wherein a reflux condenser (not shown) is connected with vent 46.
Housing 21 in mixer 20 is effectively formed by two walls, an inner wall 56 and an outer wall 57 with a space 58 thus being defined therebetween. This space 58 between walls 56 and 57 is conveniently maintained by such means as flanges 30 and 31, conduits 34, 43 and 47, flange 27, and the like, with appropriate welds (not shown). Space 58 provides a cooling, or heating jacket for delivering heat to, or removing heat from chanber 22, as desired or necessary during operation of mixer 20 by circulating a fluid coolant, such as water, or a heated fluid, such as hot oil, hot water, steam, or the like in space 58, such a cooled or heated fluid (not shown) being fed to space 58 through input ports 59 and 60, and being removed from space 58 through output ports 61, 62, 63 and 63A. Any conventional means for insulating housing 21 may be used for a mixer 20, if insulation is desired, as those skilled in the art will readily appreciate. Whether or not a mixer 20 needs insulation depends, of course, on the particular end use to which such is intended to be put in accordance with individual wishes.
Positioned and contained within chamber 22 of housing 21 is a paddle assembly designated herein in its entirety by the numeral 66. Paddle assembly 66 serves as an agitator in mixer 20 and revolves on a shaft 67. Shaft 67 in mixer 20 is generally coaxial with the longitudinal, horizontally extending axis 69 of housing 21, and extends through respective housing 21 heads 24 and 25 into conventional bearing assemblies 70 and 71, respectively. Any convenient bearing means may obviously be employed. Bearing assembly 70 is supported by and secured to, as by welding, bearing support spars 72 which, in turn, are similarly secured at their respective bases to head 24, while bearing assembly 71 is supported by and secured to, as by welding, bearing support spars 73 which, in turn, are similarly secured at their respective bases to head 25. In order to make shaft 67 be in sealing engagement with housing 21, and thereby prevent fluid leakage from housing 21 around shaft 67 during operation of mixer 20, a pair of conventional packing glands 74 and 75 are provided, one each in, respectively, head 24 and head 25, circumferentially about shaft 67. Any convenient sealing means between shaft and housing may obviously be employed. Pressure upon packing 76 and 77 in respective glands 74 and 75 is adjustable and is maintained at a predetermined value by means of tensioning nuts 78 on bolts 79 and nuts 80 on bolts 81, respectively. Thus, shaft 67 is mounted for sealed, rotational movements within housing 21.
Two pairs of independent, but axially adjacent blade members, are each secured as by weling to shaft 67. In FIG. 2, the members of one pair are designated as 83 and 84. These blades are diammetrically opposed to one another and radially project from shaft 67 to near engagement with interior wall surfaces of housing 21. Blades 83 and 84 are generally vertical (e.g. in the plane of the paper of the drawing). The members of the other pair of blade members are in a plane perpendicular to blades 83 and 84 and only one of the blades designated as 83A currently shows in FIG. 2 A. Each blade member of each pair is generally continuous along its axial length and radial breadth. Each pair of blade members occupies about one-half of the interior region of housing 21. When shaft 67 rotates, the blade members of each such pair sweep out one-half of the interior region of housing 21. Each of the blade members in FIG. 2 extends straight along shaft 67 without spiral.
A mixer 20 is adapted to achieve and maintain substantial homogeneity and uniformity in a liquid agitated by paddle assembly 66. Preferably, a mixer 20 has a chamber 22 whose dimensions such that the ratio of the length of axis 69 in chamber 22 to the maximum diameter of chamber 22 range about 0.5 to 3.5, and more preferably, from about 1.5 to 2.5
To rotatably drive the shaft 67, an electric motor 88 is provided which interconnects with shaft 67 through a transmission 89 and a drive shaft 90. Transmission 89 is equipped with a safety clutch 91 to prevent overloads. Clutch 91 can be considered to interconnect drive shaft 90 with shaft 67. Any convenient means may be used to rotatably drive a paddle assembly-electrical, magnetic, mechanical, or the like.
Conveniently, mixer 20 has a base 93 wherein a pedestal 94 supports the drive assembly (motor 88, shaft 90, transmission 89 and clutch 91) while leg assemblies 95 and 96 together support housing 21, paddle assembly 66 and their associated elements.
FIG. 3-6 illustrate bulk mixing principles for a highly viscous fluid 97 in a mixer 98 of this invention, the level of such fluid 97 in mixer 98 being such as to only partially fill mixer 98. In the vertical transverse views of FIGS. 3, 4 and 5, the broad surfaces of a paddle assembly 99 assure a good shear field (as in regions 101, 102 and 103). As paddle assembly 99 rotates, gravitational forces cooperate to produce effective fluid randomization (as in region 106) via fluid extension (as in regions 104 and 105) and foldover action (as in region 107). If radially curved, the curvature may be convex or concave with respect to the direction of paddle assembly 99 rotation.
Concurrently with the type of mixing activity illustrated in FIGS. 3-6, fluid 97 is being mixed through axial recirculation in mixer 98 by means of the slot openings 108 and 109 in paddle assembly 99 (see FIG. 3). Fluid 97 flows through slot openings 108 and 109 as paddle assembly 99 rotates on its axis in mixer 98 as diagrammed. The openings 108 and 109 give rise to equal and opposite mass gradients and paddle nip pressures which function to produce remarkably rapid axial recirculation.
Preferably, though not necessarily, a mixer 98 is not completely filled with fluid 98 in a mixing operation. A generally constant large exposed surface area on fluid 97 in mixer 98, and a generally continuously fresh surface regeneration rate, provide a maximum initial distribution of a material 110 being fed into mixer 98 for mixing with fluid 97, especially when such an input material 110 is fed into mixer 98 in a spray form, as from a spray head 111. In FIG. 6, the solid line configuration illustrates approximate fluid 97 surface position with the paddle assembly 99 in the vertical position shown and the spirally looped arrow indicates generally the pattern of axial fluid flow. The dotted line and dotted arrow illustrate approximate fluid 97 surface position and flow pattern when the paddle assembly 99 has subsequently revolved through 180°.
As shown, for example, in FIGS. 3-6, the paddle assembly 99 sweeps out substantially the entire mixer 98 interior volume with each revolution (except for slots 108 and 109), thereby eliminating the possibility of low-turnover stagnation regions in a mixer 98, and assuring a good cross-sectional shear field throughout fluid 97. It will be appreciated by those familiar with fluid mechanics that in, for example, a mixer 98 of this invention, which is filled to about 10 to 90 percent by volume with a fluid or liquid, during revolutions of a paddle assembly 99 therein, three distinct types of mixing (agitation or flow patterns) may be discerned occurring simultaneously. One type may be termed cyclical vertical displacement; another, rolling action; and the third, horizontal displacement.
The cyclical vertical displacement occurs typically at a cycle rate in the range from about 1/2 to 60 times per minute. First, liquid in the mixer is subjected to a vertical lifting force (exerted by a paddle blade in a paddle assembly) which force is greater than that exerted downwardly by gravity, yet is at least sufficient to move vertically a portion of the total volume of liquid in the mixer from a gravitationally lower region to a gravitationally higher region in the mixer. Secondly, such so displaced liquid is subject to a gravitational falling force by effective removal of such lifting force therefrom (as the paddle blade continues to rotate). The total gravitational falling force applied to the liquid is at least sufficient to return substantially all of such so displaced liquid to the gravitationally lower region before a vertical displacement cycle is repeated on such so displaced liquid.
The rolling action occurs in a generally peripherally located and generally horizontally extending region which extends generally circumferentially about the entire internal periphery of the mixer. This region is continuously moving in a direction which is generally normal to the horizontal. This rolling action is produced by a similarly so moving band of pressure (created by the paddle blades in a paddle assembly) which is located adjacent to but following behind said region. The region of rolling action may be considered to be dicontinuous in slot regions depending upon the slot design in any given mixer. The paddle blades create a zone of pressure which exerts a force on the liquid in this region of rolling action at least sufficient to cause movement of a portion of such liquid in such region along (or in) a roughly cross-sectionally circular path. This path moves (or extends) normally away from the adjacent internal periphery of the mixer housing adjacent to such band of pressure (e.g., adjacent to a paddle blade) towards the interior of the mixer a distance which is generally less than the internal diameter across the mixer housing at a given peripheral position, then back towards such internal periphery forwardly of said band of pressure, and then towards the band of pressure adjacent such internal periphery. Between the blade tips and the mixer housing wall at and in the zone of pressure, there generally exists a shear rate of at least about 5 sec. - 1 , the exact value of this shear rate depending on this clearance and the blade tip speed in any given mixer. This shear rate can be as great as 10,000 sec. -1 , or even greater if desired, but it is presently preferred that such rate sec. lower than 10,000 sec. -1 to avoid application of excessive force to a given liquid which could deteriorate same.
The horizontal displacement occurs in a longitudinal, circulatory manner in a mixer at a cycle rate such that the actual volume of the liquid moved form one end region of the mixer to the opposite end region thereof within one minute is equivalent to from about one-tenth to 30 times the total volume of the liquid in the mixer. Such equivalent volume, and the horizontal circulation rate for such liquid so moved, respectively, are each approximately proportionately to said cyclical vertical displacement cycle rate in any given instance. Such horizontal displacement is produced by the slots in the blades of the paddle assembly.
The cyclical vertical displacement, the rolling action, and the horizontal displacement take place while containuously maintaining substantially the total volume of the liquid in the mixer under laminar flow conditions. Preferably, the cyclical vertical displacement in combination with the rolling action produces fold-over action in the liquid in the mixer.
It is possible by using different paddle assemblies within the scope of this invention, as those skilled in the art will appreciate, to produce mixing action in a mixer of this invention which does not have well defined therein the afore-described cyclical vertical displacement, rolling action, and horizontal displacement and such paddle assemblies are, of course, nevertheless very useful for achieving mixing of fluids, especially viscous fluids, as taught herein. However, it is greatly preferred to employ paddle assemblies within the scope of this invention which produce mixing involving the three different kinds of mixing action just indicated.
FIGS. 2 through 6 illustrate a type of preferred paddle blade construction wherein the radially outermost, axially extending edge portions of each paddle blade are equipped with an adjustable, radially extensible or retractable knife member 113. Such adjustability is achieved by bolting knife 113 to each adjoining paddle by nut and bolt assemblies 114 which fit through slots 115 in each knife 113 and extend through mating holes (not shown) in each paddle. By means of a knife member 113, the distance between the tip of each paddle blade and the interior wall surface of a mixer may be conveniently adjusted, and in addition, the total effective land area in a given paddle assembly may be adjusted as desired. For present purposes, the land area of an individual paddle blade may be considered to refer to that part of a paddle blade edge which lies on the radially outermost peripheral outside longitudinally axially extending edge portion thereof and is thus approximately adjacent to mixer interior side wall surfaces.
It is preferred that a paddle blade not actually engage or scrape the interior wall surfaces of a mixer 98 during rotation of a paddle assembly 99 therein, and that there be a relatively small finite clearance between blade tips and interior wall surfaces of a mixer, as indicated above, which clearance is generally too small to show in, for example, FIGS. 2 through 6.
As shown in FIG. 3, as well as in FIG. 2, mixers of this invention are conveniently equipped with spray heads for feeding liquid material into a mixer. If desired, the spray head can be mounted in an exit or vent port and such a configuration is advantageous where the input liquid material is used to condense materials in a vapor phase above a fluid being mixed in a mixer as when one is using a mixer of this invention in combination with a reflux condenser to conduct a polymerization reaction.
Shown in FIGS. 7 through 11 are various forms of embodiments of paddle assemblies of the present invention adapted for use in mixers of the present invention. A two pair configuration wherein each pair of blades comprises a pair of diammetrically opposed blade members is a generally preferred class of paddle assemblies (see FIGS. 7, 8, and 9). If desired, the blades of a paddle assembly can be slotted as, for example, shown in FIG. 7. Each slot 85 and 86 can range in size from about 3 to 50 per cent of the total effective surface area of its associated blade 85A or 85B. Preferably, the slot size is from about 4 to 20 per cent of the total effective surface area thereof. The exact cross-sectional size and location of a slot 85 or 86 in a blade can vary widely. Thus, a slot 85 or 86 may be opened (not joined to or circumferentially by) on one or two sides by a blade 85A or 85B, for example, in general, a slot 85 or 86 does not extend longitudinally beyond the mid line of its blade transversely. A slot 85 or 86 may extend radially from the shaft outwardly to the edge of the blade. A pair of slots is preferably mathematically symmetrical as respects size and location in blades 85A and 85B, one to the other. Slots, of course, may be used whether or not an individual blade member is radially and/or axially curved. The effect of the slot in mixing depends, of course, somewhat on the relative size of the slot in a paddle assembly and on the viscosity of the fluid being mixed. In general, if the slot is small in size and the fluid is highly viscous, the slot is substantially invisible or ineffective as respects the total blade profile as a blade assembly revolves in a mixer. In FIGS. 8 and 9, two different types of axial curvature are suggested. FIG. 8 uses a screw pump principle whereas FIG. 9 has the axial curvature of each pair of blades adjusted so that the type of material transmission and material concentration in one region of a pair of paddle blades as opposed to another region thereof tends to beovercome, and the material tends to remain relatively uniformly distributed throughout the axial length of a mixer.
FIG. 9 illustrates a six-bladed paddle assembly wherein each pair of blades comprises a set of three symmetrically spaced circumferentially blades. FIG. 10, on the other hand, illustrates a paddle assembly having the same total number of individual blade members, but wherein the distribution of blades is varied so that one pair of blades comprises a set of four individual blades, while the other thereof comprises a pair of blades.
In general, it is preferred to have blade members of a set comprising one pair of blades in a paddle assembly be asymmetrically, circumferentially, equally spaced from one another, although such need not necessarily be the case as a requirement. Preferably, one pair of blades in an assembly wherein the individual blade members of each pair are circumferentially symmetrically positioned with respect to each other on the associated shaft is so related to the blade members of the other pair that each blade member in one set radially extgnds midway between a pair of blade members in the adjoining pair of blade members as shown in all figures, for example, 7 through 11.
In operating a mixer of this invention, upon a highly viscous fluid, it is preferred to employ rotational speeds for the shaft of the paddle assembly therein at laminar flow in such viscous fluid. Commonly, speeds of from about 3 to 30 RPM are employed, although higher and lower RPMs can be used without departing from the spirit and scope of this invention.
The mixer of the present invention is characterized by a relatively uniform consumption of power during rotational movements of a paddle assembly (torque can be used as a measure of power consumption). The type of mixing action achieved here is thus distinctly different from the type of mixing action achieved, for example, when using a mixer of the type described in copending application of George A. Latinen, Ser. No. 172,059, filed on even date herewith. Observe, however, that the tyoe of mixer here described and the type of mixer described in that invention both utilize the type of simultaneously achievable three different types of mixing action - cyclical vertical displacement, rolling action, and axial recirculation. The effect of each individual type of mixing action being emphasized or minimized by particular specifics of agitator assembly (e.g. paddle assembly) design.
It will be appreciated that, while the embodiments of the present invention as shown and described herein are necessarily limited to a few forms of the present invention, many variations and modifications thereof are feasible and practical without departing from the spirit and scope of the present invention disclosed and claimed herein.
In processing fluids, especially viscous fluids having viscosities generally greater than, say, about 5,000 centipoises, it is often necessary to blend therewith additives. In mixing operations involving such viscous fluids, many problems are encountered, and specialized mixing apparatus is desirable and commonly necessary. Complex mechanical and fluid forces are involved. The art has long sought new and improved mixing means adapted for use with highly viscous fluids, the end result generally desired being to achieve in such viscous fluids, or to maintain such viscous fluids in, a substantially uniform or homogeneous condition in the most efficient manner possible. One problem with conventional mixing means has been the large amount of power required to drive an agitator placed in a highly viscous fluid. Another problem therewith has been the difficulty of mixing a liquid of low viscosity (for example, one having a viscosity of less than about 10 centipoises) with one of high viscosity (for example, one having a viscosity of greater than about 10,000 centipoises). Still another problem has been the form and configuration of the agitator and of the mixer housing relative to the agitator.
One class of mixers recognized in the prior art characteristically has an agitator revolving generally (though not necessarily exactly) about a horizontal axis within a vessel or housing which is usually cylindrical. Such class of mixers is suitable (depending upon individual situations) for batch or continuous operation. There has now been discovered a new and improved mixer of this class wherein the mixing of highly viscous fluids (for example, of a low viscosity liquid into a high viscosity liquid) can proceed with unexpectedly low power and in a rapid and highly efficient manner. This mixer is especially well adapted for use conditions where the housing is only partially filled with a highly viscous fluid. The mixer is useful in a wide variety of end use applications involving mixing or agitation of fluids and produces a type of mixing action heretofore unknown.
SUMMARY
The present invention is directed to a mixer which is especially well adapted for use in processing highly viscous fluids, but which is also suitable for mixing fluid or fluidizable materials generally. The mixer incorporates a housing which encloses an interior chamber with side and end walls. The chamber walls are generally radially symmetrical (e.g., cross-sectionally circular) with respect to a longitudinal axis extending therethrough. The housing is preferably adapted to be oriented during mixer operation so that the longitudinal axis extends generally horizontally. The housing preferably has an input port above the level of the axis and has an output port preferably below the level of the axis, although a housing with only a single port may be used, if desired, as when an auxiliary fluid pump is used and the mixer is not used in a continuous operation. The housing can be formed of any convenient construction material though steel is presently preferred.
The mixer employs a paddle assembly which has a rotatably mounted shaft which extends generally longitudinally through said housing and generally parallel to said axis. From the shaft extend tow independnet but axially adjacent pairs of blade members. The blade members of each pair are symmetrically spaced from one another and radially project from the shaft preferably to near engagement with interior wall surfaces of the interior chamber of the housing. Thus, if there are only two blade members in one pair, then these blade members are diametrically opposed to one another. Each blade member is generally continuous along its axial length and radial breadth. Each member of each pair may optionally be slotted in the region of their respective diagonally opposite outside ends (or corners), with the effective total slot cross-sectional surface area in each blade ranging from about 3 to 50 percent of the total effective surface area of such blade (preferably ranging from about 4 to 20 percent thereof), though somewhat larger and smaller surface areas may be used without departing from the spirit and scope of the present invention, as those skilled in the art will appreciate. Preferably, the slots in each respective blade of such a pair of blades are substantially equally sized and similarly located at respective diagonally opposite outside ends. The diameter of each such pair of blade members in a paddle assembly in a mixer at any given location along the axis of the shaft except in slot locations is typically not less than about 90 percent of the diameter of the interior chamber of the mixer taken at about the same location. Each such pair axially occupies about one half the interior region or chamber of the housing.
The shaft of the paddle assembly is journaled by appropriate journal means at its opposite end regions in fixed relationship to the housing to adapt the shaft for rotational movements. When the shaft rotates, the blade members of each such pair thus sweep out about one half the interior chamber of the housing. Preferably, the shaft axis is coaxial with the housing's longitudinal axis. The paddle assembly can be formed of any convenient material of construction, though metals such as settl are presently preferred. Sealing means to prevent fluid leakage between the paddle assembly shaft and the housing are provided in an operating mixer, as those skilled in the art will readily appreciate.
The paddle assembly is in a mixer preferably adapted to rotate with substantially no contact between blade tips and chamber walls. For operation, a mixer is equipped with drive means for revolvably driving the paddle assembly shaft, including a power head and power transfer means. A paddle assembly when rotating during mixer operation with fluid in the chamber of the mixer housing is preferably, though not necessarily, adapted to produce a simultaneous combination of cyclical vertical displacement, rolling action, horizontal displacement, and, even, fold over action. Preferably, the clearance between blade tips and housing wall ranges from about 0.01 to 1.5 inches, depending on mixer size, though preferably, in a mixer of this invention, the ratio of the clearance between the blade tips and the adjacent chamber walls to the chamber diameter measured at about the same location at substantially all locations in the chamber along the longitudinal chamber axis except opposite slot locations ranges from about 0.01 to 0.0001. Preferably, in a mixer, the paddle assembly is operated so as to produce during operation uniform rotational movements of the paddle assembly shaft. Individual blade members may be radially and/or axially curved.
The invention is further directed to the paddle assembly itself which is employed in a mixer of this invention.
DRAWINGS
Turning to the attached drawings, there are seen various illustrations intended to provide a better understanding of the present invention, as follows:
FIG.1 is a side elevational view of one embodiment of a mixer of the present invention;
FIG. 2 is a vertical, longitudinal sectional view taken through the mixing chamber of the embodiment shown in FIG. 1;
FIG. 3 is a diagrammatic representation of the fluid mixing mechanics in an operating mixer of FIGS. 1 and 2 as seen in a vertical, longitudinal section;
FIGS. 4, 5 and 6 are diagrammatic representations of the fluid mixing mechanics in one embodiment of an operating mixer of the present invention as seen in cross-section;
FIG. 7 is an isometric view of a paddle assembly similar to that of FIGS. 1-6 but radially curved as well as slotted;
FIG. 8 is an isometric view of a paddle assembly which is radially and axially curved;
FIG. 9 is a view of a paddle assembly similar to FIG. 8 but with a different radial and axial curvature;
FIG. 10 is an isometric view of a paddle assembly having six flattened blades;
FIG. 11 is an isometric view of a paddle assembly having six flattened blades but different positioned in comparison to the blade assembly of FIG. 10.
FIGURE DESCRIPTION
Referring to the drawings more particularly, there is seen in FIGS. 1 and 2 an embodiment of a mixer of this invention which is herein designated in its entirety by the numeral 20. Mixer 20 has a housing designated herein in its entirety by the numeral 21 formed of steel or the like which encloses an interior chamber 22 (see FIG. 2). Housing 21 is formed by a central cylindrical portion 23 to which are secured at opposite ends thereof heads 24 and 25, respectively. In the embodiment depicted, head 24 is secured to one end of cylindrical portion 23 by welding along flange 27, while head 25 is secured to the opposite end of cylindrical portion 23 by a series of bolts 28 with mating nuts 29 extending through adjoining flanges 30 and 31 on cylindrical portion 23 and head 25, respectively.
Housing 21 has formed therein an input port 33 located in the top mid-region of cylindrical portion 23. An appropriately flanged conduit 35 connects port 33 to a feed dome 34, conduit 35 and dome 34 being secured together by bolts 36 which extend through the flange of feed dome 34 into threaded rolls in the flange of head 35. Through dome 34 extend feed pipes 37 and 38. Pipe 37 terminates within dome 34 in a spray head 40 so located as to be adapted to spray material into a wide area of chamber 22 according to a preselected pattern, while pipe 38 terminates within dome 34 in a conventional orifice (not detailed) which delivers material into chamber 22 as a stream.
Housing 21 also has formed therein an output port 41. An appropriately flanged conduit 42 connects port 41 to outlet pipe 43, pipe 43 and conduit 42 being similarly secured together by bolts 44. Pump means (not shown) may be provided to deliver material to, or to take material from, chamber 22, via feed pipe 37 and/or 38, or via pipe 43, respectively. Additional input and output ports on a mixer 20 may be employed, of course, as desired.
Housing 21 further has formed therein a vent port 46 in the top mid-region of cylindrical portion 23. An appropriately flanged conduit 47 connects port 46 to pipe 48, conduit 47 and pipe 48 being similarly secured together by bolts 49. During a mixing operation, port 46 may serve as a safety valve permitting escape of pressurized gases from chamber 22 in the event of excessive pressure build-up in housing 21, as through rupture of a rupture disc 50. To isolate the interior of chamber 22 from the atmosphere and prevent during operation of mixer 20 leakage of fluid therefrom, appropriate seals 51 (for head 25 and cylindrical portion 23), seal 52 (for conduit 35 and dome 34), seal 53 (for conduit 42 and pipe 43), and seal 54 (for conduit 47 and pipe 48) are provided. Vent 46 is also useful when mixer 20 is to be employed as a reactor wherein mixing of viscous fluids takes place, and wherein a reflux condenser (not shown) is connected with vent 46.
Housing 21 in mixer 20 is effectively formed by two walls, an inner wall 56 and an outer wall 57 with a space 58 thus being defined therebetween. This space 58 between walls 56 and 57 is conveniently maintained by such means as flanges 30 and 31, conduits 34, 43 and 47, flange 27, and the like, with appropriate welds (not shown). Space 58 provides a cooling, or heating jacket for delivering heat to, or removing heat from chanber 22, as desired or necessary during operation of mixer 20 by circulating a fluid coolant, such as water, or a heated fluid, such as hot oil, hot water, steam, or the like in space 58, such a cooled or heated fluid (not shown) being fed to space 58 through input ports 59 and 60, and being removed from space 58 through output ports 61, 62, 63 and 63A. Any conventional means for insulating housing 21 may be used for a mixer 20, if insulation is desired, as those skilled in the art will readily appreciate. Whether or not a mixer 20 needs insulation depends, of course, on the particular end use to which such is intended to be put in accordance with individual wishes.
Positioned and contained within chamber 22 of housing 21 is a paddle assembly designated herein in its entirety by the numeral 66. Paddle assembly 66 serves as an agitator in mixer 20 and revolves on a shaft 67. Shaft 67 in mixer 20 is generally coaxial with the longitudinal, horizontally extending axis 69 of housing 21, and extends through respective housing 21 heads 24 and 25 into conventional bearing assemblies 70 and 71, respectively. Any convenient bearing means may obviously be employed. Bearing assembly 70 is supported by and secured to, as by welding, bearing support spars 72 which, in turn, are similarly secured at their respective bases to head 24, while bearing assembly 71 is supported by and secured to, as by welding, bearing support spars 73 which, in turn, are similarly secured at their respective bases to head 25. In order to make shaft 67 be in sealing engagement with housing 21, and thereby prevent fluid leakage from housing 21 around shaft 67 during operation of mixer 20, a pair of conventional packing glands 74 and 75 are provided, one each in, respectively, head 24 and head 25, circumferentially about shaft 67. Any convenient sealing means between shaft and housing may obviously be employed. Pressure upon packing 76 and 77 in respective glands 74 and 75 is adjustable and is maintained at a predetermined value by means of tensioning nuts 78 on bolts 79 and nuts 80 on bolts 81, respectively. Thus, shaft 67 is mounted for sealed, rotational movements within housing 21.
Two pairs of independent, but axially adjacent blade members, are each secured as by weling to shaft 67. In FIG. 2, the members of one pair are designated as 83 and 84. These blades are diammetrically opposed to one another and radially project from shaft 67 to near engagement with interior wall surfaces of housing 21. Blades 83 and 84 are generally vertical (e.g. in the plane of the paper of the drawing). The members of the other pair of blade members are in a plane perpendicular to blades 83 and 84 and only one of the blades designated as 83A currently shows in FIG. 2 A. Each blade member of each pair is generally continuous along its axial length and radial breadth. Each pair of blade members occupies about one-half of the interior region of housing 21. When shaft 67 rotates, the blade members of each such pair sweep out one-half of the interior region of housing 21. Each of the blade members in FIG. 2 extends straight along shaft 67 without spiral.
A mixer 20 is adapted to achieve and maintain substantial homogeneity and uniformity in a liquid agitated by paddle assembly 66. Preferably, a mixer 20 has a chamber 22 whose dimensions such that the ratio of the length of axis 69 in chamber 22 to the maximum diameter of chamber 22 range about 0.5 to 3.5, and more preferably, from about 1.5 to 2.5
To rotatably drive the shaft 67, an electric motor 88 is provided which interconnects with shaft 67 through a transmission 89 and a drive shaft 90. Transmission 89 is equipped with a safety clutch 91 to prevent overloads. Clutch 91 can be considered to interconnect drive shaft 90 with shaft 67. Any convenient means may be used to rotatably drive a paddle assembly-electrical, magnetic, mechanical, or the like.
Conveniently, mixer 20 has a base 93 wherein a pedestal 94 supports the drive assembly (motor 88, shaft 90, transmission 89 and clutch 91) while leg assemblies 95 and 96 together support housing 21, paddle assembly 66 and their associated elements.
FIG. 3-6 illustrate bulk mixing principles for a highly viscous fluid 97 in a mixer 98 of this invention, the level of such fluid 97 in mixer 98 being such as to only partially fill mixer 98. In the vertical transverse views of FIGS. 3, 4 and 5, the broad surfaces of a paddle assembly 99 assure a good shear field (as in regions 101, 102 and 103). As paddle assembly 99 rotates, gravitational forces cooperate to produce effective fluid randomization (as in region 106) via fluid extension (as in regions 104 and 105) and foldover action (as in region 107). If radially curved, the curvature may be convex or concave with respect to the direction of paddle assembly 99 rotation.
Concurrently with the type of mixing activity illustrated in FIGS. 3-6, fluid 97 is being mixed through axial recirculation in mixer 98 by means of the slot openings 108 and 109 in paddle assembly 99 (see FIG. 3). Fluid 97 flows through slot openings 108 and 109 as paddle assembly 99 rotates on its axis in mixer 98 as diagrammed. The openings 108 and 109 give rise to equal and opposite mass gradients and paddle nip pressures which function to produce remarkably rapid axial recirculation.
Preferably, though not necessarily, a mixer 98 is not completely filled with fluid 98 in a mixing operation. A generally constant large exposed surface area on fluid 97 in mixer 98, and a generally continuously fresh surface regeneration rate, provide a maximum initial distribution of a material 110 being fed into mixer 98 for mixing with fluid 97, especially when such an input material 110 is fed into mixer 98 in a spray form, as from a spray head 111. In FIG. 6, the solid line configuration illustrates approximate fluid 97 surface position with the paddle assembly 99 in the vertical position shown and the spirally looped arrow indicates generally the pattern of axial fluid flow. The dotted line and dotted arrow illustrate approximate fluid 97 surface position and flow pattern when the paddle assembly 99 has subsequently revolved through 180°.
As shown, for example, in FIGS. 3-6, the paddle assembly 99 sweeps out substantially the entire mixer 98 interior volume with each revolution (except for slots 108 and 109), thereby eliminating the possibility of low-turnover stagnation regions in a mixer 98, and assuring a good cross-sectional shear field throughout fluid 97. It will be appreciated by those familiar with fluid mechanics that in, for example, a mixer 98 of this invention, which is filled to about 10 to 90 percent by volume with a fluid or liquid, during revolutions of a paddle assembly 99 therein, three distinct types of mixing (agitation or flow patterns) may be discerned occurring simultaneously. One type may be termed cyclical vertical displacement; another, rolling action; and the third, horizontal displacement.
The cyclical vertical displacement occurs typically at a cycle rate in the range from about 1/2 to 60 times per minute. First, liquid in the mixer is subjected to a vertical lifting force (exerted by a paddle blade in a paddle assembly) which force is greater than that exerted downwardly by gravity, yet is at least sufficient to move vertically a portion of the total volume of liquid in the mixer from a gravitationally lower region to a gravitationally higher region in the mixer. Secondly, such so displaced liquid is subject to a gravitational falling force by effective removal of such lifting force therefrom (as the paddle blade continues to rotate). The total gravitational falling force applied to the liquid is at least sufficient to return substantially all of such so displaced liquid to the gravitationally lower region before a vertical displacement cycle is repeated on such so displaced liquid.
The rolling action occurs in a generally peripherally located and generally horizontally extending region which extends generally circumferentially about the entire internal periphery of the mixer. This region is continuously moving in a direction which is generally normal to the horizontal. This rolling action is produced by a similarly so moving band of pressure (created by the paddle blades in a paddle assembly) which is located adjacent to but following behind said region. The region of rolling action may be considered to be dicontinuous in slot regions depending upon the slot design in any given mixer. The paddle blades create a zone of pressure which exerts a force on the liquid in this region of rolling action at least sufficient to cause movement of a portion of such liquid in such region along (or in) a roughly cross-sectionally circular path. This path moves (or extends) normally away from the adjacent internal periphery of the mixer housing adjacent to such band of pressure (e.g., adjacent to a paddle blade) towards the interior of the mixer a distance which is generally less than the internal diameter across the mixer housing at a given peripheral position, then back towards such internal periphery forwardly of said band of pressure, and then towards the band of pressure adjacent such internal periphery. Between the blade tips and the mixer housing wall at and in the zone of pressure, there generally exists a shear rate of at least about 5 sec. - 1 , the exact value of this shear rate depending on this clearance and the blade tip speed in any given mixer. This shear rate can be as great as 10,000 sec. -1 , or even greater if desired, but it is presently preferred that such rate sec. lower than 10,000 sec. -1 to avoid application of excessive force to a given liquid which could deteriorate same.
The horizontal displacement occurs in a longitudinal, circulatory manner in a mixer at a cycle rate such that the actual volume of the liquid moved form one end region of the mixer to the opposite end region thereof within one minute is equivalent to from about one-tenth to 30 times the total volume of the liquid in the mixer. Such equivalent volume, and the horizontal circulation rate for such liquid so moved, respectively, are each approximately proportionately to said cyclical vertical displacement cycle rate in any given instance. Such horizontal displacement is produced by the slots in the blades of the paddle assembly.
The cyclical vertical displacement, the rolling action, and the horizontal displacement take place while containuously maintaining substantially the total volume of the liquid in the mixer under laminar flow conditions. Preferably, the cyclical vertical displacement in combination with the rolling action produces fold-over action in the liquid in the mixer.
It is possible by using different paddle assemblies within the scope of this invention, as those skilled in the art will appreciate, to produce mixing action in a mixer of this invention which does not have well defined therein the afore-described cyclical vertical displacement, rolling action, and horizontal displacement and such paddle assemblies are, of course, nevertheless very useful for achieving mixing of fluids, especially viscous fluids, as taught herein. However, it is greatly preferred to employ paddle assemblies within the scope of this invention which produce mixing involving the three different kinds of mixing action just indicated.
FIGS. 2 through 6 illustrate a type of preferred paddle blade construction wherein the radially outermost, axially extending edge portions of each paddle blade are equipped with an adjustable, radially extensible or retractable knife member 113. Such adjustability is achieved by bolting knife 113 to each adjoining paddle by nut and bolt assemblies 114 which fit through slots 115 in each knife 113 and extend through mating holes (not shown) in each paddle. By means of a knife member 113, the distance between the tip of each paddle blade and the interior wall surface of a mixer may be conveniently adjusted, and in addition, the total effective land area in a given paddle assembly may be adjusted as desired. For present purposes, the land area of an individual paddle blade may be considered to refer to that part of a paddle blade edge which lies on the radially outermost peripheral outside longitudinally axially extending edge portion thereof and is thus approximately adjacent to mixer interior side wall surfaces.
It is preferred that a paddle blade not actually engage or scrape the interior wall surfaces of a mixer 98 during rotation of a paddle assembly 99 therein, and that there be a relatively small finite clearance between blade tips and interior wall surfaces of a mixer, as indicated above, which clearance is generally too small to show in, for example, FIGS. 2 through 6.
As shown in FIG. 3, as well as in FIG. 2, mixers of this invention are conveniently equipped with spray heads for feeding liquid material into a mixer. If desired, the spray head can be mounted in an exit or vent port and such a configuration is advantageous where the input liquid material is used to condense materials in a vapor phase above a fluid being mixed in a mixer as when one is using a mixer of this invention in combination with a reflux condenser to conduct a polymerization reaction.
Shown in FIGS. 7 through 11 are various forms of embodiments of paddle assemblies of the present invention adapted for use in mixers of the present invention. A two pair configuration wherein each pair of blades comprises a pair of diammetrically opposed blade members is a generally preferred class of paddle assemblies (see FIGS. 7, 8, and 9). If desired, the blades of a paddle assembly can be slotted as, for example, shown in FIG. 7. Each slot 85 and 86 can range in size from about 3 to 50 per cent of the total effective surface area of its associated blade 85A or 85B. Preferably, the slot size is from about 4 to 20 per cent of the total effective surface area thereof. The exact cross-sectional size and location of a slot 85 or 86 in a blade can vary widely. Thus, a slot 85 or 86 may be opened (not joined to or circumferentially by) on one or two sides by a blade 85A or 85B, for example, in general, a slot 85 or 86 does not extend longitudinally beyond the mid line of its blade transversely. A slot 85 or 86 may extend radially from the shaft outwardly to the edge of the blade. A pair of slots is preferably mathematically symmetrical as respects size and location in blades 85A and 85B, one to the other. Slots, of course, may be used whether or not an individual blade member is radially and/or axially curved. The effect of the slot in mixing depends, of course, somewhat on the relative size of the slot in a paddle assembly and on the viscosity of the fluid being mixed. In general, if the slot is small in size and the fluid is highly viscous, the slot is substantially invisible or ineffective as respects the total blade profile as a blade assembly revolves in a mixer. In FIGS. 8 and 9, two different types of axial curvature are suggested. FIG. 8 uses a screw pump principle whereas FIG. 9 has the axial curvature of each pair of blades adjusted so that the type of material transmission and material concentration in one region of a pair of paddle blades as opposed to another region thereof tends to beovercome, and the material tends to remain relatively uniformly distributed throughout the axial length of a mixer.
FIG. 9 illustrates a six-bladed paddle assembly wherein each pair of blades comprises a set of three symmetrically spaced circumferentially blades. FIG. 10, on the other hand, illustrates a paddle assembly having the same total number of individual blade members, but wherein the distribution of blades is varied so that one pair of blades comprises a set of four individual blades, while the other thereof comprises a pair of blades.
In general, it is preferred to have blade members of a set comprising one pair of blades in a paddle assembly be asymmetrically, circumferentially, equally spaced from one another, although such need not necessarily be the case as a requirement. Preferably, one pair of blades in an assembly wherein the individual blade members of each pair are circumferentially symmetrically positioned with respect to each other on the associated shaft is so related to the blade members of the other pair that each blade member in one set radially extgnds midway between a pair of blade members in the adjoining pair of blade members as shown in all figures, for example, 7 through 11.
In operating a mixer of this invention, upon a highly viscous fluid, it is preferred to employ rotational speeds for the shaft of the paddle assembly therein at laminar flow in such viscous fluid. Commonly, speeds of from about 3 to 30 RPM are employed, although higher and lower RPMs can be used without departing from the spirit and scope of this invention.
The mixer of the present invention is characterized by a relatively uniform consumption of power during rotational movements of a paddle assembly (torque can be used as a measure of power consumption). The type of mixing action achieved here is thus distinctly different from the type of mixing action achieved, for example, when using a mixer of the type described in copending application of George A. Latinen, Ser. No. 172,059, filed on even date herewith. Observe, however, that the tyoe of mixer here described and the type of mixer described in that invention both utilize the type of simultaneously achievable three different types of mixing action - cyclical vertical displacement, rolling action, and axial recirculation. The effect of each individual type of mixing action being emphasized or minimized by particular specifics of agitator assembly (e.g. paddle assembly) design.
It will be appreciated that, while the embodiments of the present invention as shown and described herein are necessarily limited to a few forms of the present invention, many variations and modifications thereof are feasible and practical without departing from the spirit and scope of the present invention disclosed and claimed herein.