CA1072538A - Bottom supported magnetic stirring element - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The specification describes a stirring element for use with magnetic stirrers comprising a central disc portion adapted for rotation about a vertical axis, vane members upstanding and depending from the central disc portion, the central disc portion and the vane members being integrally molded from a non magnetic material and a permanent magnet disposed horizontally in the disc portion.

Description

- The present invention relates to stirring apparatus which utilize a rragnetic stirring element in connection with magnetic stirring apparatus, and more particularly to such a stirring element haviny a vaned or finned structure.
Magnetic stirring apparatus is well-known in the art. Briefly, such apparatus includes a ro~atably driven drive magnet. A vessel containing a liquid tc) be mixed is placed over this drive magnet and a magnetic stirring element dropped into the vessel. The drive magnet and stirring element are magnetically coupled through the container walls so that when the drive magnet is rotated the magnetic stirring element in the vessel is rotatably driven to stir and mix the liquid contents of the vessel.
A magnetic stirring element is simply a bar or disc magnet encased in an inert material such as plastic. ~ lements of this general type are shown for exa~ple, in U.S~ Patent Nos.

2,951,689 granted on September 6, 1960 to Howard L. Asp et al;
2,518,758 granted on August 15, 1950 to George B. Cook; and

3,749,369 granted on July 31, 1973 to Kurt Landsberger. In addition, magnetic stirring elements having some sort of vaned structure are also known in the art and one such element is illustrated in U.S. Patent No. 3,0~8,716 granted on May 7, 1963 to Frank Dudley Scott. The present invention is parti-cularly concerned with a new and improved magnetic stirring element having a vaned structure.
There are disadvantages to the vaned magnetic stirring elements of the prior art. For example, the typical construction is to have vanes or fins placed only on the top surface of the element above the magnetic portion. This construction tends to make vaned stirring elements of the prior art unstable hecause of the relatively large moment arm created by the magnet-to-vane distance. Also, the vaned element of the prior axt must settle on the bottom of the vessel vane side up so ~ 3~
,~ ~
`that some care must be used in puttiny the stirring el~ment into the vessel. Also, with the vane posltion as shown in - the prior art, the vanes are located at some distance above ~
the bottoTn of the vessel so that there is little or no mixing along the bottom which makes it difficult to dissolve powder or other solids which might settle to the bottom of the vessel.
In the present invention, a magnetic stirring element is provided with a central disc portion having vanes both upstanding and depending from the disc for better and more efficient mixing. The magnet is intermediate the upper and lower vanes so that there is a balancing of the magnet-to-vane moment arms thereby making the magnetic stirring element of the present invention relatively stable over a wide range of rotational speeds. Moreover, the present invention contemplates a variation in the shape and placement of the vanes to adapt the stirring elements to any specific mixing function.
; In this latter respect, different applications may call for differe~t stirring and/or mixing functions. For example, in some biomedical applications such as cell growth in a liquid culture, very gentle stirring is required with little or no agitation or mixing. In certain chemical reactions requiring the absorption of air or other gases, it is desirable to create a deep vortex in the liquid. In other reactions or mixing applications where aeration is undesirable, agitation without vortex formation is needed. Rather than discussing the fluid mechanics involved in agitation or mixing of a liquid, it would be suf~icient for purposes of the present invention merely to say that the rotation of the stirring element may produce agitation with or without the formation of a vortex and the formation of a vortex is not necessarily a measure of the amount of agitation or mixing which is being accomplished.

The present invention may be characterized in one aspect thereof by the provision of a stirring element used with , ~,, . , ,,.~j :~O~S~3 ~agnetic stirrers, the element havin~ at least three vanes both upstanding and depending from a central disc, and a permanent magnet disposed at least partly in said disc wherein said vanes and disc are integrally molded of non-magnetic material about said permanent magnet.
Broadly defined the present invention is a stirring element for use with magnetic stirrers comprising: a central disc portion adapted for rotation about a vertical axis; vane me~bers upstanding and depending from the central disc portion;
the central disc portion and vane memhers being integrally molded from a non-magnetic material; and a permanent magnet disposed horizontally in the disc portion.
These and other features and advantages of the present ~ invention will become more apparent upon the consideration of .. the following detailed description thereof when taken in `~ connection with the accompanying drawings depicting the sameO
:,~ Figure 1 is a perspective view showing one embodiment ~; of the magnetic stirring element of the present invention;
Figure 2 is a top view of the embodiment shown in Figure 1 partly broken away and in section, the bottom view being identical;
Figure 3 is a side elevation view of the embodiment shown in Figure 2;
Figure 4 is a schematic view of the magnetic mixing apparatus employing the stirring element of the present invention;
Figures 5-7 are views similar to Figure 2 only showing stirrers having different vane configurations;
Figure 8 is a view similar to Figure 3 only showing another embodiment of the invention;
Figure 9 is a fragmentary plan view showing a different configuration for the ends of the vanes;

_ ~ _ Figure 10 is a perspective view of still another emhodiment of the invention; a~d Figure 11, located on the sheet of drawings containing Figure 1, is a view showing the flow pattern produced by one embodiment of the invention.
Referring to the drawings, Figure 1 shows the magnetic stirring element of the present invention generally indicated at 10. In the embodiments shown in Figure 1 the stirring element includes an intermediate disc portion 12 with up-10 standing and depending vane members 14, 16 respectively. Both the upstanding and depending vane members are composed of two sections (a) and (b) disposed at right angles to each other, each section extending along the diameter of disc 12.
As shown in Figures 2 and 3, the stirring member ; contains a small bar magnet 18 located within the disc portion and extending along the diameter thereof in the area of one of the vane members. While any suitable material may be used for the stirring element, a plastic material is preferred as this `~ allows the bar magnet to be encapsulated within the material - 20 of the stirring element, as by molding the stirring element --around the bar magnet. The preferred plastic material is poly-tetrafluoroethylene as this plastic, while difficult to mold, is the most inert chemically and has a very low coefficient of friction.
In use, as shown schematicallv in Figure 4, the stirring , ' ~

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element 10 is placed ln a beaker or o-ther vessel 20 containing the liquid to be rnixed. The beaker is located over a conventional magnetic stirring apparatus consisting substantially of a motor 22 which rotatably drives a magnet 24. The magnetic coupling between magnet 24 and bar magnet 18 within the stirring element causes -the stirring element to rotate about a vertical axis as the drive magnet 24 is rotated by motor 22 so that the vertical surfaces of the vanes push against the liquid. Arrows 26 indicate flow lines and show that the vaned design of the stirring element of the present invention produces considerable movement of liquid close to the bottom of the vessel wherein the arrows indicate -tha-t the liquid is forced radially along the bottom of the vessel and then deflected upwardly along the sides. Since there is considerable liquid movement along the bottom of the vessel, the mixing and dissolving of powders and the like which settle to the bottom of the vessel is facilitated by the stirring element of the present invention.
Furthermore, the vaned configura-tion of the stirring element of the present invention makes available a relatively large cross-sectional area for actually mixing fluids as opposed to conventional magnetic mixing bars. In this respec-t, Figure 3 shows the relatively large vertical area provided by the upstanding and depending vane members 1~, 16 respec-tively for "pushing" against the liquid as the stirring element rotates. This is in contrast to the vertical area of a more conventional element as shown for example in ~.S. Patent No.
2,951,689. Furthermore, when rotated 90 the embodimen-t shown in Figure 3 presents the same vertical area for "pushing"
against the liquid whereas a similar rota-tion of the aforementioned prior art elemen-t results in a greatly reduced vertical area. Consequen-tly, a smaller vaned s-tirring element j31~

of -the present invention can accomplish the same amount of stirring as any larger straight cylinder7 bar or like conventional magnetic stirring element.
The present design of having vanes upstanding and depending from a cen-tral disc portion has the further advantage of loca-ting bar magnet 18 substantially at the center of gravity of the stirring element. This provides a more stable operation of the stirring element in that even wi-th a relatively large cross-sectional mixing area, -there is a relatively short momen-t arm from the magnet to the top and -the bottom of the s-tirring element. Accordingly, during operation, the stirring element of the present inven-tion is less likely to "spin out", that is, to become uncoupled from the drive magnet and stop.
The design also produces a vortex at relatively low mixing speeds.
Having upstanding and depending vanes has a further advantage in that there is no right or wrong side to the stirring element. In this respec-t the stirring elemen-t 10 can be simply dropped into the vessel and no care need be taken to ensure that one side or the other comes to rest on the bottom of the vessel.
While a preferred embodiment has been described with reference to Figures 1-3, other vane configurations may be used depending upon the par-ticular mixing application. While two in-tersecting vane members disposed at righ-t angles and extending along the diameter of the disc is preferred for most applica-tions, a -three vane struc-ture (Figure 5) can also be used. Here the vanes 28 extend radially outward from the cen-ter of the disc. The stirring element may also have more than -two intersecting vane members 30 extending across the diameter of the disc as shown in Figure 6.

Also, while it is preferred that the vanes extend from :~)7~53~3 the center to the periphery of the disc, the vanes of any configuration may extend inwardly from -the periphery of` the disc to terminate short of the center 32 of the disc as shown in Figure 7.
Where the vessel containing the liquid to be mixed has a convex bottom, a vaned stirring element as shown in Figure 8 may be used wherein the upstanding and depending vanes have there upper and lower edges 34, 36 respectively tapered inwardly from the periphery to the center of the disc.
Figure 9 shows a further embodiment wherein the leading and trailing edges 38, 40 respectively of the vanes are provided with a curved contour as opposed to the straight sided vanes shown for example in Figure 2.
A still further embodiment is shown in Figure 10. In this embodiment the upstanding and depending vanes provided on the end faces of central disc portion 40 take the form of wedges 42.
The riser or trailing face 44 of each wedge projects substantially normal fr~m the plane of the disc. The inclined surface 46 of each wedge tapers from the top edge of one riser to the base of the following riser. Further, the top edge of each riser is inclined upward from the center 48 of the stirring element to the outer end poin-ts 50 of the top edges, so that the center point 48 lies in a plane located below the plane of end points 50. With the element ro-tating clockwise as viewed in Figure 10, the inclined surfaces of the wedges at the top of the element tend to push the liquid upward in a direction normal to the plane of ro-tation much like a screw or propeller. Meanwhile, the vertical end faces of the wedges on the bot-tom of the element tend to move liquid on the bottom of the vessel radially. Thus, the stirring element shown in Figure 10 develops a primarily vertical as .

opposed to rotary agitation so as to reduce the possibility of developing a vortex. Accordingly, this embodiment finds most utility where agitation without vortex development is required. The flow pattern produced by this embodiment is illustrated in Figure 11.
Thus, it should be appreciated that the present invention accomplishes its intended objects in providing a magnetic stirring element having a relatively large cross-sectional area available for mixing as opposed to conventional cylindrical stirring bars of comparable size, which is more stable than vaned stirring elements of the prior art and which provides a vaned s-tructure which is symmetrical about a horizontal plane so that the stirring element will operate effectively regardless of which surface is up or down.
While various vane configurations have been shown by way of example only, it should be appreciated tha-t the present invention is not limited to the embodiments described and that vanes of various o-ther cross-sections or profiles finding utility in specific applications are within the scope of the invention as claimed.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A stirring element for use with magnetic stirrers comprising:
a) a central disc portion adapted for rotation about a vertical axis;
b) vane members upstanding and depending from said central disc portion;
c) said central disc portion and vane members being integrally molded from a non-magnetic material; and d) a permanent magnet disposed horizontally in said disc portion.

2. A mixing element as set forth in Claim 1 wherein said upstanding and depending vane members are each formed by an intersecting pair of members disposed at right angles and extending along the diameter of said disc.

3. A mixing element as in Claim 2 wherein said upstanding and depending vane members are disposed one above the other wherein said mixing element is generally cross-shaped in elevation view.

4. A mixing element as in Claim 1 in which said upstanding and depending vane members extend radially inward from the periphery of said disc and terminate short of the center of said disc.

5. A mixing element as in Claim 1 wherein the top edge of each vane member tapers inward from the periphery to the center of said disc.

6. A mixing element as in Claim 1 wherein said vane members are generally wedge-shaped having an upstanding face and an inclined surface extending from the top edge of said upstanding face down to the plane of said central disc portion.

7. A mixing element as in Claim 1 wherein said magnet is located substantially at the center of gravity of said element.

8. A mixing element as in Claim 1 wherein said upstanding and depending vanes are of substantially equal height.