Claims:Claims
1. Centrifuge basket (2) for separating suspensions in a continuous centrifuge (1) into a liquid phase and a solid phase, the centrifuge basket (2) being adapted to be mounted in a continuous centrifuge (1) for rotating about an axis of rotation and having a centrifuge basket casing (4),
wherein the centrifuge basket casing (4) is arranged at the outer perimeter of the centrifuge basket (2),
wherein the centrifuge basket casing (4) has a plurality of openings adapted for discharging the liquid phase, and
wherein all or a part of the openings are rectangularly extended openings,
characterized in that
the rectangularly extended openings have a shape with a rectangular middle portion and two elliptical end portions arranged on opposite sides of the rectangular middle portion.
2. Centrifuge basket (2) according to claim 1, wherein the centrifuge basket (2) is a conical centrifuge basket.
3. Centrifuge basket (2) according to claim 1 or 2, wherein the centrifuge basket casing (4) has a lower section in which rectangularly extended openings are provided having a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings, wherein in transverse direction, the maximum extension of the rectangular middle portion is smaller than the maximum extension of the elliptical end portions.
4. Centrifuge basket (2) according to claim 3, wherein in the lower section of the centrifuge basket casing (4), the maximum extension of the rectangularly extended openings in longitudinal direction is in a range of 85 mm to 200 mm and the maximum extension of the rectangularly extended openings in transverse direction is in a range of 3.0 mm to 10.0 mm, in particular of 5.0 mm to 8.0 mm.
5. Centrifuge basket (2) according to claim 3 or 4, wherein the maximum extension of the rectangular middle portion in transverse direction divided by the maximum extension of the elliptical end portion in transverse direction is at least 0.3 and at maximum 0.9, in particular at least 0.45 and at maximum 0.75.
6. Centrifuge basket (2) according to any of claims 3 to 5, wherein each elliptical end portion of the rectangularly extended openings in the lower section of the centrifuge basket casing (4) is a segment of an ellipse with a minor axis and a major axis.
7. Centrifuge basket (2) according to claim 6, wherein for each of the rectangularly extended openings in the lower section of the centrifuge basket casing (4), the major axes of the ellipses of the elliptical end portions are aligned in longitudinal direction of the rectangularly extended opening so that the minor axes of the ellipses define the maximum extension of the rectangularly extended opening in transverse direction.
8. Centrifuge basket (2) according to any of claims 6 or 7, wherein for each elliptical end portion of the rectangularly extended openings in the lower section of the centrifuge basket casing (4), the length (C) of a half of the major axis is in a range of 5.0 mm to 12.0 mm, in particular in a range of 6.25 mm to 10.0 mm.
9. Centrifuge basket (2) according to any of claims 1 to 8, wherein the centrifuge basket casing (4) has an upper section in which rectangularly extended openings are provided having a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings wherein in transverse direction, the maximum extension of the rectangular middle portion is at least substantially equal to the maximum extension of the elliptical end portions.
10. Centrifuge basket (2) according to claim 9, wherein in the upper section of the centrifuge basket casing (4), the maximum extension of the rectangularly extended openings in longitudinal direction is in a range of 14 mm to 30 mm, in particular of 17.5 mm to 28.0 mm, and the maximum extension of the rectangularly extended openings in transverse direction is in a range of 4.0 mm to 10.0 mm, in particular of 5.0 mm to 8.0 mm.
11. Centrifuge basket (2) according to claim 9 or 10, wherein in the upper section of the centrifuge basket casing (4), each elliptical end portion has the shape of one half of an ellipse with a minor axis and a major axis, wherein each half of an ellipse includes the minor axis and one half of the major axis with the major axes of the ellipses of the elliptical end portions being aligned in longitudinal direction of the rectangularly extended opening.
12. Centrifuge basket (2) according to any of claims 9 to 11, wherein in the upper section of the centrifuge basket casing (4), the contribution of the extension of the rectangular middle portion to the maximum overall extension of the rectangularly extended opening is in the range of 20 % to 30 % of the maximum overall extension of the rectangularly extended opening, in particular in the range of 24.0 % to 29.0 % of the maximum overall extension of the rectangularly extended opening.
13 Centrifuge basket (2) according to any of claims 1 to 12, wherein the rectangularly extended openings are oriented in the centrifuge basket casing (4) in rows with each row being located in a plane at least substantially perpendicular to the axis of rotation of the centrifuge basket (2).
14. Centrifuge basket (2) according to claim 13, wherein the rectangularly extended openings of adjacent rows are arranged in a staggered pattern.
15. Centrifuge basket (2) according to claim 14, wherein each rectangularly extended opening of a row is arranged at a circumferential position that is in the middle of the distance between rectangularly extended openings in the adjacent row.
16. Continuous centrifuge (1) with a centrifuge basket (2) according to any of claims 1 to 15.
, Description:Centrifuge basket for a continuous centrifuge

The invention is made in the field of centrifuge technology and relates to a centrifuge basket for separating suspensions in a continuous centrifuge into a liquid phase and a solid phase, the centrifuge basket being adapted to be mounted in a continuous centrifuge for rotating about an axis of rotation and having a centrifuge basket casing, wherein the centrifuge basket casing is arranged at the outer perimeter of the centrifuge basket, and wherein the centrifuge basket casing has a plurality of openings adapted for discharging the liquid phase. In addition, the invention relates to a centrifuge with such a centrifuge basket.
For separating a suspension into a solid phase and a liquid phase, different techniques are known in the art. Most commonly, in industrial applications, centrifuges are used for performing this task. In a centrifuge, a container having at least one compartment is rotated at a high rotational speed about a rotational axis, thus exerting high centrifugal forces to the content of the compartment in an outward direction perpendicular to the axis of rotation. For suspensions, the rotation (spinning) of the container results in a coerced sedimentation of the solid phase as well as of the liquid phase. Resulting from the centrifugal acceleration, components of the suspension that have a higher density move faster outwards in radial direction than components having a lower density.
Most industrial centrifuges are specifically optimized for high throughput rates thus being designed as filtration centrifuges (screen centrifuges). In filtration centrifuges, the rotating container is a centrifuge basket with only one large compartment that is delimited by a perforated centrifuge basket casing (centrifuge basket wall) forming the outer wall (peripheral wall) of the centrifuge basket. Within the compartment, the perforation in the centrifuge basket casing is covered by a mesh or screen. The mesh width of this mesh or screen (i.e., the distance between adjacent wires, strands, or fibres) is chosen corresponding to the diameter of the particles in the solid phase that shall be retained within the centrifuge basket. While the actually separation is effected by the screen, the openings of the perforation (drainage holes) in the centrifuge basket casing are provided for draining off the liquid passing the screen meshes: When rotating the centrifuge basket, both phases are displaced in radial direction towards the screen. The liquid phase of the suspension moves (together with particles having diameters smaller than the width of the screen mesh openings) through the meshes of the screen towards the centrifuge basket casing. Once the liquid and the smaller particles reach the centrifuge basket casing of the rotating centrifuge basket, they can leave the container though the openings of the perforation in the centrifuge basket casing. Different from this, the particles having larger diameters are retained by the screen within the centrifuge basket and can be removed in order to complete the separation process.
Industrial centrifuges may work either in a continuous operation or in a discontinuous operation. In a continuously operated centrifuge (continuous centrifuge), the suspension to be separated is fed in continuously, so the separated solid phase needs to be removed likewise continuously during operation. Different from this, in a discontinuously operated centrifuge (discontinuous centrifuge, batch centrifuge, batch-type centrifuge), the separated solid phase is removed at the end of the operation period (wherein the suspension to be separated may be either fed into the compartment over a longer period or charged in the beginning of the operation period). Due to the different operating conditions for both types, it is evident that requirements and design of continuous centrifuges are considerably different from requirements and design of discontinuous centrifuges. For example, the centrifuge basket of a discontinuous centrifuge has typically the shape of a cylindrical drum in order to provide in axial direction largely homogeneous separation conditions while the centrifuge basket of a continuous centrifuge is typically designed in the shape of a truncated cone in order to allow the solid phase to move in axial direction during operation for being discharged at the upper edge of the centrifuge basket.
For centrifuges, one important field of application is the production of sugar. Typically, in this process, biologic material such as sugar cane or sugar beets is shredded and milled in order to obtain a sugar-rich juice. After removal of impurities (e.g., by precipitation after addition of lime), the cleared juice is subjected to evaporation of water (for example, in falling-film evaporators), thus obtaining a concentrated syrup.
Thereafter, the concentrated syrup is subjected to further evaporation of water in vacuum pans in typically three stages - the first stage, the second stage, and the third stage. In the first stage, the concentrated syrup is boiled further in a vacuum pan of the first stage. During this first stage, sugar crystals obtained in the subsequent second stage are used as seed crystals for promoting crystal growth. In the first stage, growth of sugar crystals takes place in the vacuum pan of the first stage resulting in formation of a dense mixture of syrup and sugar crystals (the “massecuite” of the first stage). In order to separate the fully-grown sugar crystals of the first stage from the syrup, the massecuite of the first stage is subjected to discontinuous batch-type centrifugation. After this batch centrifugation step, the sugar crystals retained in the centrifuge are sized and packed. Once the sugar crystals are removed from the centrifuge, the centrifuge can be used in the next batch operation.
The liquid phase obtained in this batch centrifugation step (the “molasses” of the first stage) is subjected to further evaporation in a vacuum pan of the second stage. During this second stage, sugar crystals obtained in the subsequent third stage are used as seed crystals for promoting crystal growth. In the second stage, growth of sugar crystals takes place in the vacuum pan of the second stage resulting in formation of a further dense mixture of syrup and sugar crystals (the “massecuite” of the second stage). In order to separate the sugar crystals of the second stage from the syrup, the massecuite of the second stage is subjected to continuous centrifugation. After this continuous centrifugation step, the sugar crystals retained in the centrifuge are used in the first stage as seed crystals, as outlined above.
The liquid phase obtained in this continuous centrifugation step (the “molasses” of the second stage) is subjected to further boiling and evaporation in a vacuum pan of the third stage. In this process, a further dense mixture of syrup and sugar crystals is formed (the “massecuite” of the third stage). In order to separate the sugar crystals of the third stage from the syrup, the massecuite of the third stage is subjected to continuous centrifugation. After this continuous centrifugation step, the sugar crystals retained in the centrifuge are used in the second stage as seed crystals, as outlined above. The liquid phase separated in the continuous centrifugation of the third stage is the final molasses, which is typically used as a by-product.
Accordingly, for removing supernatant concentrated syrup from precipitated sugar crystals, both types of centrifuges are employed, continuous centrifuges as well as discontinuous centrifuges. When comparing the starting materials, it is evident that for each centrifugation step, the mass fractions as well as the crystallite sizes of the sugar crystals are different (and so are the densities and viscosities). Accordingly, in each of the three centrifugation steps - the continuous centrifugation steps in the second stage and in the third stage as well as the final discontinuous centrifugation step in the first stage - individual operating conditions are required that are different from the operating conditions in the respective other two steps. Therefore, the centrifuges used in each step have different designs. In particular, this applies for the continuous centrifuge and the discontinuous centrifuge as the respective designs are fundamentally dissimilar. However, this also applies for the different continuous centrifuges as the mass fractions and hence the densities of the suspensions to be separated are considerably different (leading to completely different separation characteristics) as well all the forces arising in the centrifugation.
It is evident that the overall drainage area is defined by the total sum of the cross-sections of all openings through which the supernatant solution is released from the centrifuge basket. In order to facilitate and, thus, accelerate the discharging of the liquid phase, it is required to increase the overall drainage area resulting from the cross-sections of the openings. However, when increasing the drainage area simply by providing more openings and/or larger openings, also the stress for the centrifuge basket casing increases as the solid areas between the openings is decreased. Even though in theory, such additional stress could be counteracted by increasing the thickness of the wall, this would likewise result in an increase in inertia, thus being disadvantageous for centrifuges operated at high rotational speed. Due to this additional stress, conventional centrifuges with increased drainage areas can be operated only within limited operating conditions (e.g., at lower rotational speed).
Most of the centrifuge baskets have centrifuge basket casings with circular openings since such openings can be formed in the peripheral wall by very simple process steps (e.g., by drilling). On the downside, for such circular openings, the stress arising from increasing its number or diameter is considerably high. In order to reduce the stress, it was suggested to employ elliptical (elliptic) openings in order to provide for a uniform stress distribution within the basket casing. However, even such elliptical openings do not result in an optimal design since the stress arising at the edges of such openings is still high.
Therefore, it was an object of the invention to provide a centrifuge basket for a continuous centrifuge that has an increased drainage area while keeping the stress arising at the openings and the centrifuge basket casing lower than the stress arising in centrifuges known in the prior art. Furthermore, it was an object to provide a continuous centrifuge that allows for quick drainage of the suspension to be separated, thereby increasing the capacity of the centrifuge and reducing the content of the liquid phase in the separated solid phase discharged from the centrifuge.
This objective is achieved by a centrifuge basket and a continuous centrifuge with the features of the independent claims. Useful further implementations are evident from the sub-claims, from the following description as well as from the drawings.
In particular, the invention comprises a centrifuge basket for separating suspensions in a continuous centrifuge into a liquid phase and a solid phase, the centrifuge basket being adapted to be mounted in a continuous centrifuge for rotating about an axis of rotation and having a centrifuge basket casing, wherein the centrifuge basket casing is arranged at the outer perimeter of the centrifuge basket, wherein the centrifuge basket casing has a plurality of openings adapted for discharging the liquid phase, and wherein all or a part of the openings are rectangularly extended openings, wherein the rectangularly extended openings have a shape with a rectangular middle portion and two elliptical end portions arranged on opposite sides of the rectangular middle portion.
In general, a centrifuge is a device with at least one rotating compartment that uses centrifugal forces to separate substances of different densities or consistencies (e.g., Stokes radii), to remove moisture from a solid phase, or to simulate gravitational effects. In particular, centrifuges are commonly employed for separating suspensions. A suspension (slurry) is a heterogeneous mixture in which a solid phase (particulate phase) formed by particles of solid, semisolid, or liquid materials are more or less evenly dispersed in a liquid phase. The solid phase of a suspension may consist of a homogeneous material or of mixtures or aggregates of substances, each comprising different materials. Likewise, the liquid phase of the suspension may be a homogeneous liquid material or a mixture of different liquid materials (including solutions). A continuous centrifuge is a centrifuge specifically adapted for continuous operation. Therefore, a continuous centrifuge has means for continuously adding new suspension during operation as well as means for continuously removing the separated solid phase during operation. Consequently, the general design of continuous centrifuges differs considerably from the design of discontinuous centrifuges.
For separating a suspension in a continuous centrifuge, the suspension is continuously filled into a centrifuge basket of the continuous centrifuge during operation. Typically, the suspension is filled into the centrifuge basket in a continuous manner during operation at a feeding point which is located in the lower part of the centrifuge basket, namely close to the bottom of the centrifuge basket. However, for specific setups, it is also possible to rely on a feeding point that is located at a higher level. Rotating the centrifuge basket about its central axis (axis of rotation, rotational axis) is effected by means of a drive system which includes a drive unit and a drive shaft connected to the centrifuge basket. Typically, the centrifuge is constructed for a rotation in horizontal direction about a vertical axis of rotation (here, the term “vertical” means that the deviation from exact vertical orientation is less than 3 ° while the term “horizontal” means that the deviation from exact horizontal orientation is likewise less than 3 °); however, there are also centrifuges having a different design such as centrifuges for a rotation about a horizontal axis of rotation.
The centrifuge basket as one of the main components of the centrifuge is a container for receiving the suspension to be separated. The centrifuge basket is rotatably mounted inside the centrifuge in order to allow the centrifuge basket to be rotated at high rotational speed for effecting the separation between the solid phase and the liquid phase during operation of the centrifuge. Accordingly, the centrifuge basket is largely designed as a symmetric body of rotation in order to avoid unbalanced mass during rotation about its central axis (axis of rotation, rotational axis). The centrifuge basket comprises a centrifuge basket casing that constitutes the outer sidewall of the centrifuge basket. The centrifuge basket casing is arranged at the outer perimeter of the centrifuge basket. The centrifuge basket casing has a plurality of openings connecting the inner compartment of the centrifuge basket with the exterior of the centrifuge basket. Through these openings, the liquid phase leaves the centrifuge basket during operation while the solid phase remains within the basket.
In particular, the centrifuge basket may be a conical centrifuge basket. By this design, the centrifuge basket has a shape derived from a truncated cone (conical frustum; circular frustum) having two circular bases and a lateral surface (circumferential surface) wherein the two circular bases are arranged in parallel to each other but differ in size. In this design, the smaller circular base constitutes the lower side of the centrifuge basket while the larger circular base constitutes the upper side of the centrifuge basket. For a conical centrifuge basket, the centrifuge basket casing corresponds to the lateral surface of the truncated cone. Accordingly, the extension of the centrifuge basket casing is slanted with respect to axial direction by a half of the apex angle of the cone from which the truncated cone is derived. The term “conical centrifuge basket” also includes centrifuge baskets with other rotationally symmetric shapes that are derived from the shape of a cone such as a truncated half of a hyperboloid of one sheet or even more complex shapes with bent generatrix lines. In general, other designs are also possible for centrifuge baskets of continuous centrifuges as long as they are rotationally symmetric with a radius that monotonically increases in axial direction from the bottom (allowing for a continuous discharge of the solid phase).
According to the invention, all or a part of the openings are rectangularly extended openings each having a shape with a rectangular middle portion and two elliptical end portions arranged on opposite sides of the rectangular middle portion. The rectangular middle portion forms the central part of the rectangularly extended opening.
A rectangular middle portion is a portion that has substantially the shape of a rectangle and that is located in the middle between two other portions - in the present case, between the two elliptical end portions. In general, a rectangle is a quadrilateral with four right angles in which the opposing edges have the same length, thus also comprising a square as a limiting case. (In this context, the term “substantially” means that the shape is not necessarily an exact rectangle but may also show minor deviations from an exact rectangle - such as differences in the angles differing by less than 5 ° or differences in the length of the edges differing by less than 5 % - as long as its shape results in the overall impression of a rectangle).
An elliptical end portion is a portion that has substantially the shape of a part (segment) of an ellipse that is located at the end of an object - in the present case, at the end of the rectangularly extended opening. In general, an ellipse is a plane curve that surrounds two focal points in a way that for all points on this curve, the sum of the two distances to the focal points is a constant. In an ellipse, the largest diameter through the centre (central point, midpoint) constitutes the major axis joining the two focal points (foci) of the ellipse. Likewise, the smallest diameter of the ellipse through the centre constitutes the minor axis being perpendicular to the major axis (both axes meet in the centre of the ellipse between the two focal points). The two points in which the major axis intersects the ellipse are the vertex points (vertices) while the two points in which the minor axis intersects the ellipse are the co-vertex points (co-vertices). Accordingly, the major axis is the straight line from one vertex of the ellipse via the centre of the ellipse to the other vertex of the ellipse while the minor axis is the straight line from one co-vertex of the ellipse via the centre of the ellipse to the other co-vertex of the ellipse. In the context of the present invention, an ellipse is to be distinguished from a circle so the term “ellipse” does not comprise circles. Accordingly, for an ellipse, the length of the major axis is different from the length of the minor axis - in particular, the length of the major axis is bigger than the length of the minor axis by at least 10 % of the length of the minor axis.
In the rectangularly extended opening, the two elliptical end portions are arranged on opposite sides of the rectangular middle portion. Accordingly, in the rectangular middle portion of the rectangularly extended opening, the edges delimiting the opening are two straight edges opposing each other and being oriented in parallel to each other while in the elliptical end portions of the rectangularly extended opening, the edges delimiting the opening have the shape of truncated ellipses. The length of the parallel edges of the rectangular middle portion defines the amount by which the area of the opening is increased as compared to an opening having an exact elliptical shape. Therefore, the parallel edges of the rectangular middle portion have a length of at least 1 mm, preferentially of at least 2 mm, most preferentially of at least 3 mm. In other words, the rectangularly extended opening is an opening that may be described as a rectangle in which two opposing edges define the edges of the opening in its central part wherein each of the two other opposing edges of the rectangle are replaced by a part (segment) of an ellipse, thus constituting one single (interconnected) opening. By this specific design, the stress is considerably reduced for the centrifuge basket so it is possible to employ a centrifuge basket casing having a lower wall thickness and a larger drainage area.
For the rectangularly extended opening, the direction through the opening (as defined by the thickness of the centrifuge basket casing) corresponds to the depth direction. For a centrifuge basket having the shape of a truncated cone, the depth direction is perpendicular to the lateral surface of the cone (thus, the depth direction is tilted with respect to radial direction by a half of the apex angle of the cone from which the truncated cone is derived). Moreover, the direction perpendicular to the depth direction and in parallel to the direction of the two parallel opposing edges of the rectangular middle portion corresponds to the longitudinal direction. For a centrifuge basket having the shape of a truncated cone, the longitudinal direction is in circumferential direction of the centrifuge basket casing (i.e., in parallel to the direction of rotation of the centrifuge basket). Likewise, the direction perpendicular to the depth direction as well as to the longitudinal direction corresponds to the transverse direction. For a centrifuge basket having the shape of a truncated cone, the transverse direction corresponds to the direction of the generatrix lines of the cone from which the truncated cone is derived (thus, the transverse direction corresponds to a projection of the axial direction onto the lateral surface of the truncated cone). Accordingly, the extension in depth direction of the rectangularly extended opening is the depth extension while the extension in longitudinal direction of the rectangularly extended opening is the longitudinal extension (with the maximum extension in longitudinal direction being the maximum longitudinal extension) and the extension in transverse direction of the rectangularly extended opening is the transverse extension (with the maximum extension in transverse direction being the maximum transverse extension).
It is noted that in general, the shapes of the different rectangularly extended openings may be identical or different. Of course, all rectangularly extended openings in a centrifuge basket casing of the centrifuge basket may be designed to be identical. However, the shape of the openings on the centrifuge basket casing may also vary.
For example, the size of the openings may vary over the height of the centrifuge basket (i.e., vary in parallel to the direction of the axis of rotation of the centrifuge basket). However, since the present invention also may include a centrifuge basket in which only a part of the openings is designed as rectangularly extended openings, it is also possible that the centrifuge basket casing has different types (shapes) of openings. For example, the shape of the openings may vary over the height of the centrifuge basket so that rectangularly extended openings are only present in a specific region of the centrifuge basket casing. All in all, the functionality of the centrifuge basket may be considerably improved even if only a part of the openings are designed as rectangularly extended openings while the remaining openings have a different design - for example being circular openings or (exact) elliptical openings. For most applications, indeed, the best results are achieved when all openings are designed as rectangularly extended openings.
Moreover, in the centrifuge basket, the centrifuge basket casing may have a lower section in which rectangularly extended openings are provided having a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings, wherein in transverse direction, the maximum extension of the rectangular middle portion is smaller than the maximum extension of the elliptical end portions. In other words, in the lower section, a rectangularly extended opening may have a shape similar to the shape of a historic barbell used in weightlifting (but with elliptical ends instead of globes). Such shape may be described as a rectangle with two end sections wherein each end of the rectangle transitions into an ellipse with the transverse extensions of the ellipses being larger than the transverse extension of the rectangle (thus forming a slit with elliptical widenings in each end). With respect to the contour of the rectangularly extended opening, the transition from the rectangular middle portion to the elliptical end portion may be a sharp bend (edged) or a slight bend (curved). The term “lower” in “lower section” refers to the axial direction of the centrifuge basket, implicating that also an “upper section” exist. When the centrifuge basket casing has openings in its lower section that are formed as slits with elliptical widenings in each end, the upper section may also exhibit openings (for example, barbell-shaped or linearly stretched ellipses or having a different contour). Alternatively, the upper section may exhibit no opening at all. Usually, in conventional centrifuge baskets, the upper section of the centrifuge basket casing had no openings at all resulting in a low drainage area. With conventional circular openings or slits, it was not possible to extend the drainage area of the centrifuge basket in the continuous centrifuge to the upper section without modifying the centrifuge basket casing, for example by using a centrifuge basket casing with a larger thickness. This limitation also applies for barbell-shaped openings that have the contour of a slit with circular widenings in each end. By using rectangularly extended openings having the contour of slits with elliptical widenings in each end section of the centrifuge basket casing, it was possible to extend the perforated area of the centrifuge basket casing also to its upper section without further modification of the centrifuge basket casing, thus maximising the overall drainage area of the centrifuge basket.
Moreover, it may be useful if in the centrifuge basket, in the lower section of the centrifuge basket casing, the maximum extension of the rectangularly extended openings in longitudinal direction is in a range of 85 mm to 200 mm and the maximum extension of the rectangularly extended openings in transverse direction is in a range of 3.0 mm to 10.0 mm, in particular of 5.0 mm to 8.0 mm. Likewise, it is useful if in the centrifuge basket, the maximum extension of the rectangular middle portion in transverse direction divided by the maximum extension of the elliptical end portion in transverse direction is at least 0.4 and at maximum 0.9, in particular at least 0.45 and at maximum 0.75. Such dimensions of the rectangularly extended openings result in a particularly useful design for providing large area openings that allow for a fast discharge of the liquid phase from the centrifuge basket, at the same time keeping the stresses low that arise during operation of the centrifuge.
In the centrifuge basket, each elliptical end portion of the rectangularly extended openings in the lower section of the centrifuge basket casing may be a segment of an ellipse with a minor axis and a major axis. A segment of an ellipse is formed by an elliptical arc and by a chord. For a rectangularly extended opening having the shape of a rectangle with elliptically widened ends, the elliptical end portions are linked to the rectangular middle portion so that the chord of each segment of the ellipse is identical with one of the two opposing edges of the rectangular middle portion that run in transverse direction. Accordingly, it is evident that the length of the chord is smaller than the minor axis of the ellipse. By this design, the stress arising at the edges of the rectangularly extended opening during operation of the centrifuge is reduced as it is homogeneously distributed over a larger edge zone. The transition area between the rectangular middle portion and the elliptical end portions may form a sharp bend or may be rounded.
In the centrifuge basket, the chord of the segment of the ellipse may be in parallel to the minor axis. In particular, in the centrifuge basket, for each of the rectangularly extended openings in the lower section of the centrifuge basket casing, the major axes of the ellipses of the elliptical end portions may be aligned in longitudinal direction of the rectangularly extended opening so that the minor axes of the ellipses define the maximum extension of the rectangularly extended opening in transverse direction. A much better alignment can be realized when the two elliptical end portions have the same spatial extensions or - even better - if for each rectangularly extended opening, both elliptical end portions have identical shapes. By this alignment, the opening is mirror symmetric having the centre line (centre axis, middle axis, middle line) of the rectangular middle portion in transverse direction as the mirror plane. Even better results are achieved if the rectangularly extended opening is doubly mirror symmetric so that both centre lines of the rectangular middle portion serve as mirror planes, the centre line in transverse direction as well as the centre line in longitudinal direction. Such design can be achieved when for both elliptical end portions of a rectangularly extended opening, the chord of the segment of the ellipse is in parallel to the minor axis wherein both elliptical end portions have the same shape. By this design, the two elliptical end portions are aligned in a way that both ellipses whose parts form the outer edges of the elliptical end portions are oriented with respect to each other in a way that the major axes of both elliptical end portions are aligned with each other and also with the centre line in longitudinal direction of the rectangular middle portion. Resulting from this design, a highly symmetric opening is realized thus minimizing the local stress arising at the edges of the rectangularly extended openings during operation of the centrifuge.
Moreover, in the centrifuge basket, for each elliptical end portion of the rectangularly extended openings in the lower section of the centrifuge basket casing, the length of a half of the major axis may be in a range of 5.0 mm to 12.0 mm, in particular in a range of 6.25 mm to 10.0 mm. Such dimensions of the end portions of the rectangularly extended openings result in a fast drainage of the liquid phase, at the same time keeping the load low that arises on the centrifuge basket casing during operation of the centrifuge.
Furthermore, it may be useful if in the centrifuge basket, the centrifuge basket casing has an upper section in which rectangularly extended openings are provided having a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings wherein in transverse direction, the maximum extension of the rectangular middle portion is at least substantially equal to the maximum extension of the elliptical end portions. The maximum extension of the rectangular middle portion in transverse direction corresponds to the distance between the two parallel straight edges in the rectangular middle portion. This distance is least substantially equal to the maximum extension of both elliptical end portions in transverse direction. “At least substantially” means that for the different portions, the difference between the maximum extensions is less than 5 % of the larger extension, in particular being close to zero or being zero.
According to this design, resulting from the rectangular middle portion of the rectangularly extended opening, the rectangularly extended opening has the shape of a linearly stretched ellipse with a central part in which the opening is delimited by two straight parallel edges and with two side parts in which the edges are formed by truncated ellipses. In other words, the rectangularly extended opening is an elongate elliptical opening that may be described as a rectangle in which two opposing edges define the edges of the opening in its central part wherein each of the two other opposing edges of the rectangle are replaced by a part (segment) of an ellipse, thus constituting one single (interconnected) opening. The term “linearly stretched ellipse” implicates that the resulting contour of this rectangularly extended opening has two straight-line segments running in parallel to each other, thus being considerably different from an ordinary “stretched ellipse” where no straight-line segments run in parallel to each other (still having its two co-vertices). The term “upper” in “upper section” refers to the axial direction of the centrifuge basket, implicating that also a “lower section” exist. When the centrifuge basket casing has openings in its upper section that are formed as linearly stretched ellipses, the lower section may also exhibit openings (for example, barbell-shaped or linearly stretched ellipses or having a different contour). Alternatively, the lower section may exhibit no opening at all (so that only the upper section of the centrifuge basket casing can serve as drainage area). In conventional centrifuge baskets, it was not possible to introduce openings into the upper section of the centrifuge basket casing in the continuous centrifuge without further modification of the centrifuge basket casing. Thus, it is most useful if the centrifuge basket casing has rectangularly extended openings with the contour of a slit with elliptical widenings in its lower section while it has rectangularly extended openings with the contour of linearly stretched ellipses in its upper section in order to provide the largest drainage area without major modification of the centrifuge basket casing.
In the centrifuge basket, in the upper section of the centrifuge basket casing, each elliptical end portion may have the shape of one half of an ellipse with a minor axis and a major axis, wherein each half of an ellipse may include the minor axis and one half of the major axis with the major axes of the ellipses of the elliptical end portions being aligned in longitudinal direction of the rectangularly extended opening. In general, one half of the major axis corresponds to the major semi-axis (semi-major axis), namely to a line from the centre of the ellipse to one of the vertices of the ellipse. Likewise, the minor semi-axis (semi-minor axis) corresponds to one half of the minor axis, namely to a line from the centre of the ellipse to one of the co-vertices of the ellipse. Thus, when dividing an ellipse on its minor axis into two halves, each half including one half of the major axis as well as the (whole) minor axis, a half of an ellipse is obtained that includes the minor axis and one half of the major axis. In other words, each of the aligned end portions may be a semi-ellipse formed by an elliptical arc and by a chord wherein the chord does not only run in parallel to the minor axis but is substantial equal to the minor axis. In this design, the major axes of the ellipses of the elliptical end portions are aligned in longitudinal direction of the rectangularly extended opening. Basically, the two elliptical end portions are aligned with each other on opposite sides of the rectangular middle portion, so the stress arising at the edges of the rectangularly extended openings during operation of the centrifuge is homogeneously distributed over the edges of the elongate elliptical openings. A particularly good alignment can be realized when the two elliptical end portions have the same spatial extensions or - even better - if for each elongate elliptical opening, both elliptical end portions have identical shapes. By this design, the opening is mirror symmetric having the centre line of the rectangular middle portion in transverse direction as the mirror plane. Even better results are achieved if the elongate elliptical opening is doubly mirror symmetric so that both centre lines of the rectangular middle portion serve as mirror planes, the centre line in transverse direction as well as the centre line in longitudinal direction. By this alignment, the two elliptical end portions are aligned in a way that both ellipses whose parts form the outer edges of the elliptical end portions are oriented with respect to each other in a way that the major axes of both elliptical end portions are aligned with each other and also with the centre line in longitudinal direction of the rectangular middle portion. Resulting from this design, a highly symmetric opening is realized thus minimizing the local stress arising at the edges of the elongate elliptical openings during operation of the centrifuge.
Moreover, it may be useful if in the centrifuge basket, in the upper section of the centrifuge basket casing, the maximum extension of the rectangularly extended openings in longitudinal direction may be in a range of 14 mm to 30 mm, in particular of 17.5 mm to 27.0 mm, and the maximum extension of the rectangularly extended openings in transverse direction may be in a range of 4.0 mm to 10.0 mm, in particular of 5.0 mm to 8.0 mm. Such dimensions of the elongate elliptical opening result in a particularly useful design for providing large area openings that allow for a fast discharge of the liquid phase from the centrifuge basket, at the same time keeping the stresses low that arise during operation of the centrifuge.
Similarly, in the centrifuge basket, in the upper section of the centrifuge basket casing, the contribution of the extension of the rectangular middle portion to the maximum overall extension of the rectangularly extended opening is in the range of 20 % to 30 % of the maximum overall extension of the rectangularly extended opening, in particular in the range of 24.0 % to 29.0 % of the maximum overall extension of the rectangularly extended opening. Also by such design, the openings have an optimum shape in order to provide a large overall drainage area at the same time providing sufficiently good stability of the centrifuge basket casing even for lower wall thicknesses.
In general, in the centrifuge basket, the rectangularly extended openings may be oriented in the centrifuge basket casing in rows with each row being located in a plane at least substantially perpendicular to the axis of rotation of the centrifuge basket. Accordingly, the rows of the rectangularly extended openings are oriented in parallel to the direction of the rotation of the centrifuge basket casing (i.e., in a circumferential plane on the centrifuge basket casing) so that the longitudinal directions of the rectangularly extended openings run in circumferential direction of the centrifuge basket casing. For centrifuges in which the centrifuge basket rotates about a vertical axis of rotation, the plane substantially perpendicular to the axis of rotation of the centrifuge basket is a horizontal plane, so the rows are oriented at least substantially horizontally on the centrifuge basket casing. Accordingly, the rectangularly extended openings are arranged annularly on the centrifuge basket casing with each row of rectangularly extended openings forming a circle on the centrifuge basket casing. In this context, the term “at least substantially perpendicular“ means that each row is located in a plane that is either exact perpendicular to the axis of rotation to the centrifuge basket (i.e., exact perpendicular orientation) or may have an angular deviation of less than 3 ° with respect to such exact perpendicular orientation. By such design, the rectangularly extended openings are oriented in a way that the specific shape of the rectangularly extended openings is suited best to withstand the stresses occurring at the edges of the rectangularly extended openings during rotation of the centrifuge basket.
For a row arrangement, it is useful if in the centrifuge basket, the rectangularly extended openings of adjacent rows are arranged in a staggered pattern. The term “staggered” means that the rectangularly extended openings are arranged in a pattern so that a line connecting the centre of an rectangularly extended opening in a first row with the centre of an rectangularly extended opening in a second row adjacent to the first row is not running in parallel to the transverse direction but is rather skew with respect to the transverse direction. Such an arrangement results in a better drainage efficiency as well as in an improved stability of the centrifuge basket casing as compared to an arrangement wherein rectangularly extended openings of adjacent rows are aligned in transverse direction. In particular, in the centrifuge basket, each rectangularly extended opening of a row may be arranged at a circumferential position that is in the middle of the distance between rectangularly extended openings in the adjacent row. The arrangement of a rectangularly extended opening at a circumferential position refers to the position of the rectangularly extended opening in the circumferential plane on the centrifuge basket casing. Such a design results in a diamond-shaped distribution of the rectangularly extended openings on the centrifuge basket casings. By the equidistant distribution of adjacent rectangularly extended openings in adjacent rows resulting from this arrangement, a centrifuge basket having a uniform stress resistance without predetermined specific breaking points is realised, at the same time allowing for a fast drainage of the liquid phase from the centrifuge basket.
Additionally, in the lower section of the centrifuge basket casing, adjacent rectangularly extended openings within each row may be separated from each other by a distance of at least 50 mm and at maximum 160 mm. Likewise, in the upper section of the centrifuge basket casing, adjacent rectangularly extended openings within each row may be separated from each other by a distance of at least 10 mm and at maximum 30 mm. Preferentially, the rectangularly extended openings in the lower section of the centrifuge basket casing as well as in the upper section of the centrifuge basket casing meet the aforementioned limitations. The term “separated by a distance of” does not refer to the distance between the centres of adjacent rectangularly extended openings but refers to the (minimum) extension of solid portions of the centrifuge basket between two adjacent rectangularly extended openings. Similarly, in the centrifuge basket, adjacent rows may be separated from each other by a distance of at least 40 mm and at maximum 60 mm, in particular 50 mm. However, it is possible to realise other solid distances.
Finally, the invention includes a continuous centrifuge with a centrifuge basket having the above features.
Of course, even though not explicitly mentioned, a centrifuge basket as well as a continuous centrifuge according to the present invention may have further components such as an inner screen mesh and a supporting screen, such further components being typically used in centrifuge baskets, thus being commonly known to person skilled in the art.
Subsequently, the invention is illustrated with reference to the drawings. The drawings show schematically
Fig. 1: a sectional view of a part of a centrifuge basket of a continuous centrifuge;
Fig. 2: a side view of a part of a centrifuge basket casing of a centrifuge basket of a continuous centrifuge with rectangularly extended openings;
Fig. 3: a detail view of a rectangularly extended opening in the lower section of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge;
Fig. 4: a detail view of a rectangularly extended opening in the upper section of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge;
Fig. 5: a side view of a part of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge with the arrangement of rectangularly extended openings;
Fig. 6: a sectional view of a part of a continuous centrifuge having a centrifuge basket according to the present invention.
Fig. 1 schematically shows a sectional view of a part of a centrifuge basket 2 in a continuous centrifuge. The centrifuge basket 2 is a conical centrifuge basket that has a centrifuge basket casing 4 in the shape of the surface area of a truncated circular cone. The truncated cone is arranged upside-down so that the larger circular base of this cone is located on top while the smaller circular base (which is located next to the apex/vertex of the cone from which the truncated cone is derived) is located on the bottom. At the bottom of its central portion, the centrifuge basket 2 has a receptacle for accommodating a drive shaft. At the top of its central portion, the centrifuge basket 2 has a circular aperture for accommodating a tubular inlet for feeding the suspension to be separated. In radial direction, the lower part of this receptacle is shielded from the rest of the compartment of the centrifuge basket 2 by an accelerating cone 8. The accelerating cone 8 is an open bottom dome in the shape of a truncated cone. The accelerating cone 8 is designed to deflect the incoming stream of the suspension to be separated in radial direction and to accelerate the suspension to be separated in rotational direction at the inner face of the accelerating cone 8. The bottom part of the accelerating cone 8 is an opening forming an annular gap. Below the accelerating cone 8, an annular collecting tray is arranged that has inclined (conical) outer walls. At its upper part, the collecting tray merges with the centrifuge basket casing 4 that forms the lateral surface of the truncated cone. Accordingly, when the suspension to be separated leaves the accelerating cone 8 through the annular gap, it is spun onto the inclined wall of the collecting tray and is moved upwards by centrifugal forces until it reaches the centrifuge basket casing 4.
The centrifuge basket casing 4 has rectangularly extended openings (not shown in Fig. 1). In detail, in its lower section I, the centrifuge basket casing 4 has rectangularly extended openings which have a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings, wherein in transverse direction, the maximum extension of the rectangular middle portion is smaller than the maximum extension of the elliptical end portions. Thus, in lower section I, the rectangularly extended openings are slits with elliptical widenings in each end (i.e., slits terminated by ellipses). Different from this, in its upper section II, the centrifuge basket casing 4 has rectangularly extended openings which have a longitudinal direction in which the extension of the rectangularly extended openings is maximum and a transverse direction perpendicular to the longitudinal direction of the rectangularly extended openings wherein in transverse direction, the maximum extension of the rectangular middle portion is at least substantially equal to the maximum extension of the elliptical end portions. Thus, in upper section II, the rectangularly extended openings are linearly stretched ellipses. However, it is also possible that in the centrifuge basket casing 4, the rectangularly extended openings are of the same type in the lower section I and in the upper section II. Likewise, the lower section I or the upper section II may have other openings than rectangularly extended openings or even no openings at all (as long as rectangularly extended openings are provided in the other section of the centrifuge basket casing 4).
At the inner face of the centrifuge basket 2, the openings in the centrifuge basket casing 4 are covered by a screen 5 arranged on the surface of the centrifuge basket casing 4. The screen 5 has a mesh size suited for retaining the solid phase from the suspension to be separated within the centrifuge basket 2 while the liquid phase may leave the centrifuge basket 2 via the rectangularly extended openings in the centrifuge basket casing 4. Within the compartment of the rotating centrifuge basket 2, the solid phase retained by screen 5 is still moved upwards along the inclined inner face of centrifuge basket casing 4 until the solid phase reaches the upper edge of the centrifuge basket 2. From the upper edge of the centrifuge basket 2, the solid phase is spun in radial direction in order to the centrifuge basket 2. The centrifuge basket 2 may be used in a continuous centrifuge for the production of sugar.
In Fig. 2, a side view of a part of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge with rectangularly extended openings is shown schematically (without perspective foreshortening). The centrifuge basket casing is the centrifuge basket casing shown in Fig. 1. In detail, Fig. 2 shows eight rows of rectangularly extended openings wherein all eight rows are oriented in the same direction, namely in planes perpendicular to the axis of rotation of the centrifuge basket (i.e., in horizontal direction being in direction of the rotation of the conical centrifuge basket).
In the lower section I, five rows are depicted consisting of rectangularly extended openings each having the shape of a slit with elliptically widenings at both ends. Therefore, each rectangularly extended opening in the lower section I has a longitudinal direction in which the extension of the rectangularly extended openings is maximum; this direction corresponds to horizontal direction. Similarly, each rectangularly extended opening in the lower section I has a transverse direction, which is perpendicular to the longitudinal direction of the rectangularly extended opening. For each rectangularly extended opening in the lower section I, the maximum extension of the rectangular middle portion in transverse direction is smaller than the maximum extension of the elliptical end portions in transverse direction, thus resulting in a barbell shape with elliptical ends. Moreover, each elliptical end portion of the rectangularly extended openings in the lower section I of the centrifuge basket casing is a segment of an ellipse with a minor axis and a major axis. The major axes of the ellipses of the elliptical end portions of each rectangularly extended opening in the lower section I are aligned in longitudinal direction of the rectangularly extended opening so that the minor axes of the ellipses define the maximum extension of the rectangularly extended opening in transverse direction.
In the upper section II, three rows are depicted consisting of rectangularly extended openings each having the shape of a linearly stretched ellipse. Therefore, each rectangularly extended opening in the upper section II has a longitudinal direction in which the extension of the rectangularly extended openings is maximum; this direction corresponds to horizontal direction. Similarly, each rectangularly extended opening in the upper section II has a transverse direction that is perpendicular to the longitudinal direction of the rectangularly extended opening. For each rectangularly extended opening in the upper section II, the maximum extension of the rectangular middle portion in transverse direction is at least substantially equal to the maximum extension of the elliptical end portions in transverse direction, thus resulting in a linearly stretched ellipse. Each elliptical end portion of the rectangularly extended openings in the upper section II of the centrifuge basket casing has the shape of one half of an ellipse with a minor axis and a major axis. Each half of an ellipse includes the minor axis and one half of the major axis wherein the major axes of the ellipses of the elliptical end portions are aligned in longitudinal direction of the rectangularly extended opening.
In the lowermost row of lower section I, the maximum longitudinal extension of the rectangularly extended openings is shorter than in the uppermost row of lower section I. Accordingly, the maximum longitudinal extensions of the rectangularly extended openings increase with the height position of the respective row on the centrifuge basket. Different from this, in the rows of upper section II, the maximum longitudinal extensions of the rectangularly extended openings do not change over the height position of the respective row on the centrifuge basket.
In each section - in lower section I as well as in upper section II -, the rectangularly extended openings of adjacent rows are arranged in a staggered pattern. In such a staggered pattern, the openings of adjacent lines are not aligned in a transverse direction. In Fig. 2, the staggered pattern has the shape of a diamond with the centres of the openings (i.e., the central points of the rectangular middle portions of each rectangularly extended opening) constituting the corners. Accordingly, the centre of each rectangularly extended opening in a row is arranged at a circumferential position that is located in the middle of the distance between the horizontal positions of the centre of adjacent openings in the adjacent rows. Accordingly, in transverse direction, the openings of a row are aligned with the corresponding openings in the next but one row, thus being arranged in the form of a regular grid in which only each second node has an opening. (In Fig. 2, the straight lines connecting the openings in horizontal direction indicate the rows thus merely serving as guides to the eyes.) By this staggered arrangement of the rectangularly extended openings, it is ensured that during operation of the centrifuge, when the separated liquid phase moves from the lowermost edge of the centrifuge basket casing along the inner face of the centrifuge basket casing in upward direction, the liquid phase will reach an opening through which it can leave the compartment of the centrifuge basket.
However, according to the present invention, also rectangularly extended openings with other designs may be realised, for example rectangularly extended openings in which the elliptical end portions of an opening are formed by segments of different ellipses, or in which the elliptical end portions are oriented slightly inclined with respect to each other, or in which one or both elliptical end portions are formed by segments of ellipses that are less than one half of an ellipse.
Fig. 3 schematically shows a detail of a centrifuge basket according to the present invention, namely a schematic side view (without perspective foreshortening) of a rectangularly extended opening in the lower section of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge. This rectangularly extended opening may be provided in lower section I of the centrifuge basket casing shown in Fig. 2. This opening is a rectangularly extended opening having a shape with a rectangular middle portion and two elliptical end portions. Such a rectangularly extended opening may be described as a slit with elliptical widenings at each end. In longitudinal direction, the extension of the rectangularly extended openings is maximum, thus corresponding to the direction of the longitudinal centre line of the rectangularly extended opening shown in Fig. 2 in horizontal direction. The transverse direction is perpendicular to the longitudinal direction of the rectangularly extended openings, thus corresponding to the direction of the transverse centre line of the rectangularly extended opening shown in Fig. 3 in vertical direction; in transverse direction, the maximum extension of the rectangular middle portion is smaller than the maximum extension of the elliptical end portions. Each elliptical end portion has the shape of a segment of an ellipse with a minor axis and a major axis wherein both elliptical end portions are identical. Moreover, the major axes of the ellipses of the elliptical end portions are aligned in longitudinal direction of the rectangularly extended opening so that the minor axes of the ellipses define the maximum extension of the rectangularly extended opening in transverse direction. The maximum overall extension of the rectangularly extended opening is the maximum longitudinal extension E of the rectangularly extended opening while the maximum extension of the rectangularly extended opening in direction perpendicular to the maximum overall extension of the rectangularly extended opening is the maximum transverse extension of the rectangularly extended opening, thus corresponding to the length B of the minor axes of the elliptical end portions.
For the rectangularly extended opening in the lower section of a centrifuge basket casing, the maximum extension in transverse direction is 5.0 mm and the maximum extension E in longitudinal direction depends on the row in which the rectangularly extended opening is arranged. In detail, for both elliptical end portions, the half of the major axis has a length C of 7.5 mm (corresponding to a major axis of 15.0 mm) while the length B of the minor axis is 5.0 mm. Moreover, within the rectangularly extended opening, the distance A between the centres of the two ellipses from the elliptical end portions in longitudinal direction varies from 75 mm to 175 mm, depending on the row in which the rectangularly extended opening is arranged: For rows arranged at a higher position with respect to axial direction, the circumference of the centrifuge basket is larger than for rows arranged at lower positions. Accordingly, for rectangularly extended openings in rows arranged at such higher position, the longitudinal extension of the slit may be chosen larger than for rectangularly extended openings in rows arranged at lower positions. However, in the rectangular middle portion, the maximum extension D in transverse direction is 3 mm (thus being shorter than the length B of the minor axis). Of course, it is also possible to realise rectangularly extended opening in the lower section of a centrifuge basket casing with other dimensions.
Fig. 4 schematically shows a detail of a centrifuge basket according to the present invention, namely a schematic side view (without perspective foreshortening) of a rectangularly extended opening in the upper section of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge. This rectangularly extended opening may be provided in upper section II of the centrifuge basket casing shown in Fig. 2. This opening is a rectangularly extended opening having a shape with a rectangular middle portion and two elliptical end portions. Each elliptical end portion has the shape of one half of an ellipse including the minor axis and one half of the major axis wherein both elliptical end portion are identical. Moreover, both elliptical end portions are aligned with each other on opposite sides of the rectangular middle portion in a way that the ellipses whose segments constitute the elliptical end portions are aligned with respect to its major axes (major semi-axes). The maximum overall extension of the rectangularly extended opening is the maximum longitudinal extension J of the rectangularly extended opening while the maximum extension of the rectangularly extended opening in direction perpendicular to the maximum overall extension of the rectangularly extended opening is the maximum transverse extension of the rectangularly extended opening, thus corresponding to the length G of the minor axes of the elliptical end portions as well as to the transverse extension of the rectangular middle portion.
For the rectangularly extended opening in the upper section of a centrifuge basket casing, the maximum extension in transverse direction is 5.0 mm and the maximum extension J in longitudinal direction is 20.0 mm (thus being 400 % of the maximum overall extension of the rectangularly extended opening in direction perpendicular to the maximum overall extension of the rectangularly extended opening). In detail, the rectangular middle portion is a square of 5.0 mm x 5.0 mm having a length F in longitudinal direction of 5.0 mm and a length in transverse direction of 5.0 mm as well. Moreover, for both elliptical end portions, the half of the major axis has a length H of 7.5 mm (corresponding to a major axis of 15.0 mm) while the length G of the minor axis is 5.0 mm. For both elliptical end portions, the length G of the minor axis amounts to 33 % of the length of the major axis. Furthermore, the contribution of the length F of the rectangular middle portion to the maximum overall extension J of the rectangularly extended opening is 25 % of the maximum overall extension of the rectangularly extended opening. Of course, it is also possible to realise rectangularly extended openings in the upper section of a centrifuge basket casing with different dimensions.
Similar to Fig. 2, in Fig. 5, a side view of a part of a centrifuge basket casing of a centrifuge basket in a continuous centrifuge is shown schematically from which the arrangement of rectangularly extended openings can be seen. While in Fig. 3 and 4, different quantities are indicated that are relevant for characterizing the different rectangularly extended openings, in Fig. 5, several quantities are depicted that are relevant for characterizing the arrangement of the rectangularly extended openings in the centrifuge basket casing: The rectangularly extended openings are arranged in rows that are spaced apart from each other by a distance N denoting the transverse distance between the centres of adjacent rectangularly extended openings in adjacent rows. The distance N also corresponds to the spacing between the uppermost row in the lower section and the lowest row in the upper section.
In the lower section of the centrifuge basket casing, rectangularly extended openings are arranged in rows wherein each opening has the shape of a slit with elliptical widenings in each end. For each rectangularly extended opening in this section, the distance A between the centres of the two ellipses in longitudinal direction is a characteristic quantity as well as the distance K between the centres of two ellipses of adjacent rectangularly extended openings in longitudinal direction (this distance K is larger than the minimum extension of solid portions of the centrifuge basket between two adjacent rectangularly extended openings by the length of the major axis of the two ellipses). For each row in the lower section, the distances should be chosen so that the circumferential length is an integer multiple of the sum of both distances (A + K) in order to allow for a homogeneous distribution of the openings in circumferential direction.
In the upper section of the centrifuge basket casing, rectangularly extended openings are arranged in rows wherein each opening has the shape of a linearly stretched ellipse. For each rectangularly extended opening in this section, the distance L between the centres of two adjacent rectangularly extended openings in longitudinal direction is a characteristic quantity. This quantity corresponds to the sum of the maximum extension J of the rectangularly extended opening in longitudinal direction and the minimum extension M of the solid portion between two adjacent rectangularly extended openings in longitudinal direction. For each row in the upper section, the distances should be chosen so that the circumferential length is an integer multiple of the distance L between the centres of two adjacent rectangularly extended openings in order to allow for a homogeneous distribution of the openings in circumferential direction.
For a conical centrifuge basket, the circumferential length of a row increases with the position of this row on the centrifuge basket. Given the above relations, it is evident that for a row at a higher position on the centrifuge basket, in longitudinal direction, the solid distances between adjacent rectangularly extended openings and/or the maximum extension of the rectangularly extended openings increases as compared to a row at a lower position on the centrifuge basket. For the sake of intelligibility, this increase of the extensions with the height position of the rows is not shown in the schematic representation given in Fig. 5 (however, this increase is adumbrated in Fig. 2).
The openings shown in Fig. 2 and Fig. 5 were cut in a flat steel sheet. Subsequently, the steel sheet was rolled in a shape of the frustum of a cone (frustoconical shape) and welded in order to form the centrifuge basket casing.
In Fig. 6, a sectional view of a part of a continuous centrifuge 1 having a centrifuge basket 2 according to the present invention is shown schematically in operation. The continuous centrifuge 1 has an external housing in which a conical centrifuge basket 2 is connected to a drive shaft 3. The centrifuge basket 2 has the shape of a truncated cone that can be rotated by the drive shaft 3 about an axis of rotation (the respective drive is not shown). The centrifuge basket 2 is arranged in the continuous centrifuge 1 to form an annular gap between the outside of the centrifuge basket casing 4 and the inner housing surrounding the centrifuge basket 2. The inside of the centrifuge basket 2 forms a compartment adapted for receiving the suspension to be separated into a solid phase and a liquid phase during centrifugation. The continuous centrifuge 1 shown in Fig. 6 is designed for being used in the production of sugar 12 from sugar cane or sugar beets. Accordingly, the suspension to be separated is a viscous mixture of sugar crystals and syrup, the massecuite 10. Consequently, the solid phase to be separated from the liquid phase is sugar 12 while the liquid phase to be separated from the solid phase is molasses 11.
In the centrifuge basket 2, the lateral surface is formed by a centrifuge basket casing 4. The centrifuge basket casing 4 is a metal sheet in the shape of the lateral surface of a truncated cone having openings formed therethrough as through-holes penetrating the metal sheet (not shown in Fig. 6). In the present case, the openings are rectangularly extended openings that have a shape with a rectangular middle portion and two elliptical end portions arranged on opposite sides of the rectangular middle portion. In detail, the rectangularly extended openings in the lower section of the centrifuge basket casing 4 have the shape of slits with elliptical widenings in each end while the rectangularly extended openings in the upper section of the centrifuge basket casing 4 have the shape of linearly stretched ellipses. The rectangularly extended openings are arranged in rows forming substantially horizontal rings on the centrifuge basket casing 4. In the lower section of the centrifuge basket casing 4 as well as in the upper section of the centrifuge basket casing 4, the rectangularly extended openings in adjacent rows of the same section are arranged in a staggered pattern with the longitudinal distances between adjacent rectangularly extended openings changing over the height of the centrifuge basket 2: In detail, for each section, the distance between adjacent rectangularly extended openings is moderately larger in upper rows than in lower rows. However, it is possible to realise other arrangements of the openings such as the distances between adjacent rectangularly extended openings being identical over the height of the centrifuge basket 2.
Within the compartment of the centrifuge basket 2, the rectangularly extended openings are covered by a screen 5 abutting on the centrifuge basket casing 4. Typically, the screen 5 is a fine wire screen or mesh made of brass or steel, nickel or the like with a mesh width in the range from 0.6 mm to 0.8 mm.
For operating the continuous centrifuge 1, the partly evaporated massecuite 10 is continuously fed into the compartment of the centrifuge basket 2 via a tubular massecuite inlet 6. The tubular massecuite inlet 6 enters the compartment through a circular aperture provided in the centre of the circular top base of the truncated cone constituting the centrifuge basket 2. The massecuite 10 is injected continuously into an accelerating cone 8. The accelerating cone 8 is an open bottom dome in the shape of a truncated cone, which is connected to of the centrifuge basket 2. Due to the rotation of the centrifuge basket 2, the incoming massecuite 10 is accelerated in rotational direction and spread over the inner face of the accelerating cone 8, thus being evenly distributed over the whole circumference of the spinning accelerating cone 8 in order to avoid imbalance. The thus spread massecuite 10 leaves the accelerating cone 8 through the lower annular opening of the cone in radial direction into an annular collecting tray located in the lower part of the centrifuge basket 2. The collecting tray has inclined walls so that upon rotation of the centrifuge basket 2, the massecuite 10 is spun onto the inclined walls. Resulting from the rotation, the massecuite 10 flows along the inclined walls in upward direction, thus reaching a screen 5 covering the inside of the centrifuge basket casing 4. The centrifuge basket casing 4 has the shape of the lateral surface of a truncated cone; this also applies for the screen 5 covering the centrifuge basket casing 4. During rotation, the massecuite 10 is pressed against the screen 5 with the liquid phase (the molasses 11) being able to go through the screen 5 and to leave the centrifuge basket 2 via the rectangularly extended openings in the centrifuge basket casing 4 while the solid phase (sugar 12 of intermediate size sugar crystals still coated with residual molasses) is retained by the screen 5. The upward movement by which the massecuite 10 / sugar 12 is transported from the lower part of the centrifuge basket 2 to its upper part is caused by centrifugal forces. At the upper edge of the centrifuge basket 2, the sugar 12 leaves the centrifuge basket 2 continuously by being centrifuged off in radial direction, thus entering an annular gap. Outside the centrifuge basket 2, the loosened sugar 12 falls down onto the circular bottom of the annular gap for being discharged by falling down into a sugar outlet 9.
After the molasses 11 traversed the rectangularly extended openings in the centrifuge basket casing 4, the molasses 11 enters an inverse-conical drainage gap between the centrifuge basket casing 4 and the housing of the centrifuge 1. In this drainage gap, the molasses 11 flows downwards until it reaches the bottom of the compartment from which it leaves the centrifuge 1 via a molasses outlet 7.
In order to find out whether or not the inventive centrifuge basket is suited for providing a larger drainage area at the same time reducing the stress arising in centrifuge basket casing, the distribution of stress was calculated for different shapes of openings in the centrifuge basket casing. In this context, three different types of rectangularly extended openings were compared with each other: According to the present invention, two types of rectangularly extended openings with elliptical end portions were analysed, namely a slit with elliptical widenings in each end (elliptically barbell-shaped) as suggested for the lower section of the centrifuge basket casing and a linearly stretched ellipse as suggested for the upper section of the centrifuge basket casing. The slit with elliptical widenings had a distance between the centres of the two ellipses in longitudinal direction from 75.0 mm to 175.0 mm, a length of the minor axis of the elliptical end portion of 6.0 mm, and a length of a half of the major axis of the elliptical end portion of 7.5 mm. The linearly stretched ellipse had a maximum extension in longitudinal direction of 20.0 mm, a length of the minor axis of the elliptical end portion of 6.0 mm, a length of a half of the major axis of the elliptical end portion of 7.5 mm, and a length of rectangular middle portion in longitudinal direction of 5.0 mm. Both rectangularly extended openings suggested by the present invention were compared with a rectangularly extended opening not covered by the present invention, namely by a slit with circular widenings in each end (circular barbell-shaped) having maximum transverse extension of the rectangular middle portion of 3.0 mm and a diameter of the circular end portions of 6.0 mm. For the three centrifuge baskets with the different types of rectangularly extended openings, the maximum stress was identified as well as the positions in which the maximum stress arises.
With respect to the position in which the maximum stress arises, the results obtained for the different centrifuge baskets are consistent: For each centrifuge basket, the maximum stress arises at the apices of the openings in transverse direction (i.e., at the uppermost and lowermost points of the edges of the opening in transverse direction were the maximum transverse extension of the opening is maximum). For the slits with circular widenings, the maximum stress was as large as about 230 MPa. For the linearly stretched ellipses, the maximum stress was about 160 MPa. For the slits with elliptical widening, the stress was as low as about 140 MPa.
It was surprising that even when increasing the drainage area of the openings, it is possible to reduce the maximum stress arising in the centrifuge basket by about 30 % simply by providing elliptical widenings instead of circular widenings.
Centrifuge baskets with different sizes were checked each having five rows of twelve slits with elliptical widening (maximum transverse extension of the rectangular middle portion: 3.3 mm) in the lower section of the centrifuge basket casing, while having three rows of linearly stretched ellipses (maximum longitudinal extension: 20 mm; maximum transverse extension: 5.0 mm) in the upper section of the centrifuge basket.
For a large conical centrifuge basket, the lowermost row of the lower section of the centrifuge basket has a perimeter of about 3,130 mm and the uppermost row of the lower section of the centrifuge basket has a perimeter of about 3,750 mm while the lowermost row of the upper section of the centrifuge basket has a perimeter of about 3,910 mm and the uppermost row of the upper section of the centrifuge basket has a perimeter of about 4,220 mm. In the lower section of the centrifuge basket, the distance between the centres of the two ellipses within each rectangularly extended opening varied from about 130 mm in the lowermost row of the lower section to about 156 mm in the uppermost row of the lower section while the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 115 mm in the lowermost row of the lower section to about 141 mm in the uppermost row of the lower section. Accordingly, in the upper section of the centrifuge basket, the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 13 mm in the lowermost row of the upper section to about 15 mm in the uppermost row of the upper section, thus resulting in 118 or 120 openings per row.
For a medium-sized conical centrifuge basket, the lowermost row of the lower section of the centrifuge basket has a perimeter of about 2,670 mm and the uppermost row of the lower section of the centrifuge basket has a perimeter of about 3,300 mm while the lowermost row of the upper section of the centrifuge basket has a perimeter of about 3,460 mm and the uppermost row of the upper section of the centrifuge basket has a perimeter of about 3,770 mm. In the lower section of the centrifuge basket, the distance between the centres of the two ellipses within each rectangularly extended opening varied from about 111 mm in the lowermost row of the lower section to about 137 mm in the uppermost row of the lower section while the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 96 mm in the lowermost row of the lower section to about 122 mm in the uppermost row of the lower section. Accordingly, in the upper section of the centrifuge basket, the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 13 mm in the lowermost row of the upper section to about 16 mm in the uppermost row of the upper section, thus resulting in 104 or 106 openings per row.
Likewise, for a smaller conical centrifuge basket, the lowermost row of the lower section of the centrifuge basket has a perimeter of about 1,840 mm and the uppermost row of the lower section of the centrifuge basket has a perimeter of about 2,470 mm while the lowermost row of the upper section of the centrifuge basket has a perimeter of about 2,620 mm and the uppermost row of the upper section of the centrifuge basket has a perimeter of about 2,940 mm. In the lower section of the centrifuge basket, the distance between the centres of the two ellipses within each rectangularly extended opening varied from about 77 mm in the lowermost row of the lower section to about 103 mm in the uppermost row of the lower section while the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 62 mm in the lowermost row of the lower section to about 88 mm in the uppermost row of the lower section. Accordingly, in the upper section of the centrifuge basket, the longitudinal extension of the solid portion between adjacent rectangularly extended openings within each row varied from about 13 mm in the lowermost row of the upper section to about 17 mm in the uppermost row of the upper section, thus resulting in 78 or 80 openings per row.
Moreover, centrifuge baskets having rectangularly extended openings with different sizes were checked. In particular, for rectangularly extended openings suggested for the lower section of the centrifuge basket casing (namely, for slits with elliptical widenings in each end), the distance A between the centres of the two ellipses within each rectangularly extended opening was varied in a range of 75 mm to 175 mm. The smallest slits with elliptical widenings in each end had elliptical end portions having a length B of the minor axis of 5.0 mm or 6.0 mm and a length C of the half of the major axis of 6.25 mm or 7.5 mm as well as a rectangular middle portion having the maximum extension D in transverse direction of 3.0 mm. Larger slits with elliptical widenings in each end had elliptical end portions having a length B of the minor axis of 6.0 mm or 7.0 mm and a length C of the half of the major axis of 7.5 mm or 8.75 mm as well as a rectangular middle portion having the maximum extension D in transverse direction of 4.0 mm. Even larger slits with elliptical widenings in each end had elliptical end portions having a length B of the minor axis of 7.0 mm or 8.0 mm and a length C of the half of the major axis of 8.75 mm or 10.0 mm as well as a rectangular middle portion having the maximum extension D in transverse direction of 5.0 mm. Of course, it is possible to use rectangularly extended openings with even smaller or even larger dimensions.
Likewise, for rectangularly extended openings suggested for the upper section of the centrifuge basket casing (namely, for linearly stretched ellipses), all rectangularly extended openings had elliptical end portions having a length G of the minor axis of 5.0 mm (or 6.0 mm or 7.0 mm or 8.0 mm) and a length H of the half of the major axis of 6.25 mm (or 7.5 mm or 8.75 mm or 10.0 mm). The smallest linearly stretched ellipses had a rectangular middle portion with an extension F in longitudinal direction of the rectangularly extended opening (this extension corresponds to the distance between the centres of the two ellipses within each rectangularly extended opening) of 4.0 mm, larger linearly stretched ellipses had a rectangular middle portion with an extension F in longitudinal direction of the rectangularly extended opening of 5.0 mm, even larger linearly stretched ellipses had a rectangular middle portion with an extension F in longitudinal direction of the rectangularly extended opening of 6.0 mm, while the largest linearly stretched ellipses had a rectangular middle portion with an extension F in longitudinal direction of the rectangularly extended opening of 7.0 mm.
All the above embodiments had an increased drainage area with respect to conventional centrifuge baskets, at the same time showing an improved stress resistance even for low wall thicknesses, thus demonstrating clearly the advantageous effects of the inventive concept.

Reference signs

A distance between the centres of the two ellipses within a rectangularly extended opening in longitudinal direction (slit with elliptical widenings in each end)
B length of minor axis of elliptical end portion (slit with elliptical widenings in each end)
C length of a half of the major axis of elliptical end portion (slit with elliptical widenings in each end)
D extension of the rectangular middle portion in transverse direction (slit with elliptical widenings in each end)
E maximum extension of rectangularly extended opening in longitudinal direction (slit with elliptical widenings in each end)
F length of rectangular middle portion in longitudinal direction (linearly stretched ellipse)
G length of minor axis of elliptical end portion (linearly stretched ellipse)
H length of a half of the major axis of elliptical end portion (linearly stretched ellipse)
J maximum extension of rectangularly extended opening in longitudinal direction (linearly stretched ellipse)
K distance between the centres of two ellipses of adjacent rectangularly extended openings in longitudinal direction (slit with elliptical widenings in each end)
L distance between the centres of two adjacent rectangularly extended openings in longitudinal direction (linearly stretched ellipse)
M minimum extension of solid portion between two adjacent rectangularly extended openings in longitudinal direction (linearly stretched ellipse)
N transverse distance between the centres of two rectangularly extended openings in adjacent rows (slit with elliptical widenings in each end and linearly stretched ellipse)

1 continuous centrifuge
2 centrifuge basket
3 drive shaft
4 centrifuge basket casing
5 screen
6 massecuite inlet
7 molasses outlet
8 accelerating cone
9 sugar outlet
10 massecuite
11 molasses
12 sugar

I lower section of the centrifuge basket casing
II upper section of the centrifuge basket casing