Mixer, especially for foodstuffs

Abstract: A mixer (1) comprises a vessel (2) having a base portion (3) and an upper portion (4) whose interiors have an axis (S) rotational symmetry, and arms (7) each carrying a plurality of blades (6) which are rotatable about an axis (R) so that edges (9) of the blades (6) sweep across the interior surface of at least the vessel base potion (3), characterised in that said blades (6) are loosely mounted on said arms (7) so that the blades (6) are freely movable in relation to the arms with sufficient freedom of movement about each of three orthogonal axes that their sweeping edges (9) can maintain contact with said interior surface as they sweep across it, and in that the blades (6) are spaced apart along each arm (7) and staggered as between one arm and the next thereby to allow sweeping of substantially the full extent of the interior surface of the base portion (3) of the mixing vessel (2).

Description: This invention relates to mixers and has particular but not exclusive reference to mixers for use in the food industry and in the pharmaceutical and chemical industries. Mixers used in the food industry typically have a capacity of between 600 and 1,500 kg. Such mixers are typically used in production of syrups and sauces and they may incorporate a heat exchange jacket should it be required to chill or cook the syrup or sauce. Such mixers usually include a mixing vessel which has a hemispherical base and a cylindrical upper portion. The food to be processed is contained within the hemispherical base portion of the vessel and is stirred by blades. It is also known in the food processing industry to make use of vessels having a conical base portion or torispherical (hemi-ellipsoidal) base portion and to use vessels which are plain cylindrical and in which the contents of the vessel are also stirred by blades. Such mixers may also be used in the pharmaceutical industry and the chemical industry generally. Hitherto, in one commercially available form of mixer, the mixer blades have been mounted to their arms by hairpin clips threaded between pairs of spaced flanges located on the arm arid a blade carrier respectively. This allows the blade carrier to rotate about the pivot axis defined by the hairpin clip, which, in the usual arrangement, is substantially parallel to a plane which is tangent to the surface of the mixing vessel at the instantaneous location of that blade carrier, and is also parallel to the leading edge of the blade. A similar effect is given by the blade mountings illustrated in US Patent 3752057. The commercially known blade carrier comprises a backing plate whose side margins are bent up and over to form lips which define grooves into which a blade proper may be slid and where it is held in position by a hook. Such a conventional blade may also pivot about that hook, that is, about an axis which may be considered as being effectively normal to the (moving) tangent plane referred to. There is however very little freedom of movement allowed in the third orthogonal direction namely that which allows pivotal movement of the leading edge of the blade about an axis which corresponds with the direction of advance of that leading edge. Accordingly, while the leading edge of such a blade may be moved towards or away from the interior surface of the mixing vessel of a known mixer, and while the angle such leading edge makes with its direction of advance may also be varied, there is no possibility of self-adjustment of the leading edge of the blade in cases where only one leading corner of the blade makes contact with the mixing vessel. A similar problem arises when the blade mounting construction of US 4818116 is adopted. It will be appreciated that it is desirable that each blade should sweep clear its sweeping area of the base of the mixing vessel. In order to solve this problem, DE 2815946A proposes mounting its blades on posts provided on stirrer arms. The posts, like the blades themselves, extend normally to the local surface of the mixer vessel (that is radially of the spindle) and the blades are spring-biassed radially outwardly. Such an arrangement allows a clean sweeping of the area swept by each blade, so that material being mixed is substantially prevented from sticking to the interior surface of the base of the mixing vessel, where it could burn if the vessel was heated, andlor give rise to hygiene problems. However, the blade mounting proposed in DE 2815946A is rather complicated, and the springs required may need to be formed from special material so that they will not contaminate food products and so that they themselves can resist corrosion, for example by sauces containing vinegar. EP 761296 proposes a mixer for mixing substances such as ointments and creams which is not intended for use in the food industry. Blades are mounted upon studs projecting from arms rotatable about an axis to sweep the blades across the interior surface of the mixing vessel. The blades are mounted upon the studs with an element of play between them to allow a slight element of movement about two orthogonal axes. Also this allows a slight amount of linear movement with respect to the studs. The blade mounting arrangement defines crevices and recesses which could not be readily cleaned. It is an object of the present invention to provide a mixer in which the problem of proper contact between a sweeping blade and the mixing vessel is achieved with a simpler construction and which also alleviates other problems relating to hygiene and to the ease with which the mixing equipment vessel and its blades may be cleaned between charges. According to the present invention there is provided a mixer comprising a vessel having a base portion and an upper portion whose interiors have an axis (S) rotational symmetry, and arms each carrying a plurality of blades which are rotatable about an axis (R) so that edges of the blades sweep across the interior surface of at least the vessel base portion, the blades being spaced apart along each arm and staggered as between one arm and the next thereby to allow sweeping of substantially the full extent of the interior surface of the base portion of the mixing vessel and said blades being loosely mounted on said arms characterised; in that the arms are provided with blade carriers formed as curved loops upon which the blades are mounted so that the blades are freely movable in relation to the arms with sufficient freedom of movement about each of three orthogonal axes that their sweeping edges can maintain contact with said interior surface as they sweep across it. We have found that clean sweeping by the mixer blades can be achieved simply and reliably by an appropriate loose mounting which can be of very simple construction and which does not require any spring biasing of the blades. The apparatus is accordingly simpler to construct and to assemble than the apparatus proposed by DE 2815946A. Such apparatus can also have a further and very important advantage due to its simple construction; it can contain fewer or indeed no small interstices in which material being processed can lodge. We have found that by appropriate design of the blades and their mountings, it is possible to construct an apparatus which can be cleaned easily and thoroughly by using an internal spray head. Previously known mixing apparatus, on the other hand, has required dismounting of the mixing blades for a thorough cleaning and sterilisation of the mixing equipment. This is itself time consuming, and it requires the apparatus to be properly adjusted on re-assembly which can also be a complicated and time-consuming operation. Because the blades of a mixer according to the invention are carried by the arms with a sufficient degree of freedom of movement, setting up and fitting out of the mixer are very much simplified, each such blade can self-adjust to that portion of the surface of the mixing vessel against which it bears, and a thorough scraping of the interior of the vessel during mixing can more easily be achieved. Thus the mixing blades can adapt to irregularities in the vessel or in setting up, and also to any tendency for the spindle to run out of true. In order to promote this end it is preferred that the blades should have a freedom of movement of at least 2° and preferably at least 5° about each of three orthogonal axes. In preferred embodiments of the invention, contact between the base portion of the mixing vessel and the blades takes place at a leading edge of each said blade, and preferably, the blade leading edges are biassed into contact with the mixing vessel base portion by forces exerted on the blades by any contents of the mixing vessel. Advancing the blade in such a way that their leading edges make contact and they are thus inclined at an acute angle to their direction of advance promotes a lifting of the material away from the interior surface of the vessel and thus contributes towards good mixing. The blades may be, and preferably are, chamfered at their leading edges in order to promote these advantages, and also to help ensure that the leading edges of the blades are pressed into contact with the interior walls of the mixing vessel by forces exerted by the material being processed, this further promoting clean sweeping of the vessel. The foregoing addresses the specific problem of contact between any given blade and the surface of the vessel over the length of that blade, and thus over the area swept by that blade. In order to ensure that substantially the full extent of the interior surface of the base portion of the mixing vessel is swept as the blades rotate on their arms, the positions of the blades along the different arms are staggered. By staggering the blades along the respective arms it is possible to ensure that there are no areas of the base portion of the mixing vessel which are left unswept. While it may be possible to achieve such a full sweep of the base portion of the mixing vessel using blades which are mounted on only two arms the achievement of this end is very much simplified when the blades are carried by three arms and such embodiments of the invention are accordingly much preferred, since we believe that this feature of the invention is of particular practical significance. As has been noted the invention in either of its aspects extends to those mixers in which the base portion is conical, but it is preferred that the base portion should be hemispherical since this offers advantages in good mixing of a food preparation contained therein. It is also preferred that the axis of rotation of the arms bearing the mixing blades should be acutely inclined to the axis of symmetry of the mixing vessel. This affords great advantages in the even mixing of the contents of the vessel. The angle between the two axes is preferably greater than 10° and is suitably between 20° and 50°. In the most preferred embodiments of the invention said blades are constituted as substantially rectangular plates of synthetic resin material. These are particularly simple to form. Suitable synthetic resins include PTFE and nylon. The blades are preferably provided with slots which are a loose fit over the curved loops. We have found that a suitable design of curved loop holder on the arm and of the blades is sufficient to ensure that the blades do not come loose from the holder during use. Retention of the blades is promoted in embodiments in which each loop is arrange to sweep across the interior surface of the base portion of the mixer vessel while its distal end is spaced from the surface of that base portion by less than the thickness of the blade which it carries. Clearly the blade cannot separate from its loop while sweeping that base portion. However, in the case of a hemispherical mixer having an offset-axis blade system it will be appreciated that the blades will periodically lift out of the base portion and thus away from the wall of the vessel and in such cases it is suitable to provide some means for retaining the blade onto its loop carrier. It is preferred therefore that at least some of said blades are each provided with a retaining pin which is insertable across a said slot within a said loop to retain the blade on the carrier. This system of blades and loop carriers is of particular advantage because it is very easily cleanable. The system may be constructed with a minimum of crevices in which food particles could lodge and in fact in the most preferred embodiments, the system is so simple to clean that proper hygiene may be maintained simply by hosing the interior of the vessel down between charges. Spray heads may be provided in the vessel for spraying cleaning fluids, for example a pre-rinse with water, a caustic wash and a following rinse. In the most preferred embodiments of the invention, the mixer is connected to a CIP (Clean-In-Place) cleaning system. The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which: Figure 1 is a vertical cross sectional view of an embodiment of mixer in accordance with this invention; Figure 2 is a diagrammatic view of an arrangement of mixer blades carried on mixer arms; and Figure 3 is a detail sectional view of a mixer blade on its curved carrier loop. In Figure 1, a mixer 1 comprises a mixing vessel 2 having a base portion 3 which is hemispherical in shape and a cylindrical upper portion 4. The vessel 2 has a roof 21 in which there is a charging hatch 22. The roof 21 also supports a motor 23 which drives a shaft 8 via a gearbox 24. and The shaft 8 carries by arms 7 on which are mounted blades 6 for mixing the contents of the vessel. The blades 6 are suitably constituted as substantially rectangular plates of synthetic resin material. The blades 6 and arms 7 are rotated about the rotational axis R of the shaft 8 which in this embodiment is arranged at an angle of 25° to the axis S of rotational symmetry of the mixing vessel 2. The blades 6 are driven in rotation by the motor 23 so that their leading edges 9 sweep across the interior surface of at least the base portion 3 of the vessel 2. The blades 6 are carried by the arms 7 on carriers 10 in the form of curved loops. The positions of the blades 6 are staggered along the different arms 7 to allow sweeping of substantially the full extent of the interior surface of the base portion 3 of the mixing vessel 2, as is most clearly shown in Figure 2. In fact as shown in Figure 2, the blades are carried by three arms 7. As shown in Figures 2 and 3 the blades are held by the curved loops 10 simply by the latter's projection through slots 11 formed in the blades. As shown in Figure 3, the slots are a loose fit around the curved loop carriers 10 and it is found that this itself provides sufficient play to give the required freedom of movement to the blades 6 that their leading sweeping edges 9 can self-adjust to maintain contact with the interior surface of the base portion 3 of the vessel 2 as they sweep across it. As shown in Figures 1 and 2, the blades are provided with a chamfer 13 leading towards their leading, sweeping edges 9. In use this chamfer tends to urge the leading edge 9 of the blade 6 against the interior surface of the base portion 3 of the mixing vessel 2 to ensure thorough sweeping. Also due to this chamfer, though the blades 6 are initially formed with straight leading edges 9, in practice these may quite soon wear away, especially at the ends of those edges, to leave curved leading edges 9a which correspond with the curvature of the hemispherical base portion of the mixing vessel. The slope and curvature of the blade retaining loops 10 combined with the slope 11 of the slot in the blades 6 tends to cause those blades to ride up the loops 10 so that they retain the blades without any further means being required. Such retention is ensured if each said loop 10 is arranged to sweep across the interior surface of the hemispherical base portion 3 while its distal end 10a is spaced therefrom by a distance which is less than the thickness T of the blade 6 which it carries. If required, at least some of said blades 6 are each provided with a retaining pin 12 insertable across a said slot 11 within a said loop 10 to retain the blade 6 on the carrier 10. Such an arrangement is shown in Figure 3. The hemispherical base portion 3 of the mixing vessel 2 is provided with a drain opening 25 at its lowest point, and it is enclosed within a heat-exchange jacket 14 through which may be circulated steam or hot water or a coolant fluid as desired. A spray head 30 is provided as part of a cleaning system for directing jets of appropriate cleaning fluids to the interior of the mixing vessel. Such fluids may include water, cold or hot, steam, or caustic or other washing solutions.

Claims: A mixer (1) comprising a vessel (2) having a base portion (3) and an upper portion (4) whose interiors have an axis (S) rotational symmetry, and arms (7) each carrying a plurality of blades (6) which are rotatable about an axis (R) so that edges (9) of the blades (6) sweep across the interior surface of at least the vessel base portion (3), the blades (6) being spaced apart along each arm (7) and staggered as between one arm and the next thereby to allow sweeping of substantially the full extent of the interior surface of the base portion (3) of the mixing vessel (2) and said blades (6) being loosely mounted on said arms (7) characterised in that the arms (7) are provided with blade carriers (10) formed as curved loops upon which the blades (6) are mounted so that the blades (6) are freely movable in relation to the arms with sufficient freedom of movement about each of three orthogonal axes that their sweeping edges (9) can maintain contact with said interior surface as they sweep across it. A mixer according to claim 1, wherein said blades (6) have freedom of movement of at least 2° about each of three orthogonal axes. A mixer according to claim 2, wherein said blades (6) have freedom of movement of at least 5° about each of three orthogonal axes. A mixer according to any preceding claim, wherein contact between the base portion (3) of the mixing vessel (2) and the blades (6) takes place at a leading edge (9) of each said blade (6). A mixer according to any preceding claim, wherein the blade leading edges (9) are biassed into contact with the mixing vessel base portion (3) by forces exerted on the blades (6) by any contents of the mixing vessel (2). A mixer according to any preceding claim, wherein said blades (6) are chamfered (13) towards their leading edges (9). A mixer according to any preceding claim wherein said blades (6) are constituted as substantially rectangular plates of synthetic resin. A mixer according to claim 7, wherein said blades (6) are provided with slots (11) which are a loose fit over said curved loops (10). A mixer according to claim 8, wherein at least some of said blades (6) are each provided with a retaining pin (12) insertable across a said slot (11) within a said loop (10) to retain the blade (6) on the carrier (10). A mixer according to claim 8 or 9, wherein each said loop (10) is arranged to sweep across the interior surface of said base portion (3) while its distal end (10a) is spaced therefrom by less than the thickness (T) of the blade (6) which it carries. A mixer according to any preceding claim, wherein there are three blade-carrying arms (7). A mixer according to any preceding claim, wherein said base portion (3) is hemispherical and the axis of rotation (R) of said arms (7) is inclined to the axis of rotational symmetry (S) of said vessel (3).

Inventors: WHYLES GRAHAM PETER, WHYLES, GRAHAM PETER
Applicants: S & W PROCESS ENGINEERING LTD, S & W PROCESS ENGINEERING LIMITED
Application year: 2000
Main classification: B01F15
IPC codes: B01F15, B01F7