Claims:Claims
1. Centrifuge basket (2) for separating suspensions in a discontinuous centrifuge (1) into a liquid phase and a solid phase, the centrifuge basket (2) being adapted to be mounted in a discontinuous 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 elongate openings
characterized in that
the elongate openings have a shape with a rectangular middle portion and two elliptical end portions.
2. Centrifuge basket (2) according to claim 1, wherein the centrifuge basket casing (4) is a cylindrical centrifuge basket casing.
3. Centrifuge basket (2) according to claim 1 or 2, wherein the two elliptical end portions are aligned with each other on opposite sides of the rectangular middle portion.
4. Centrifuge basket (2) according to claim 3, wherein 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.
5. Centrifuge basket (2) according to claim 4, wherein for each half of an ellipse, the length (C) of the half of the major axis is in a range of 5.0 mm to 12.0 mm whereas the length (B) of the minor axis is in a range of 4.0 mm to 10.0 mm, in particular the length (C) of the half of the major axis being in a range of 6.25 mm to 10.0 mm and the length (B) of the minor axis being in a range of 5.0 mm to 8.0 mm.
6. Centrifuge basket (2) according to claim 4 or 5, wherein for each half of an ellipse, the length (B) of the minor axis is in a range of 30 % to 60 % of the length of the major axis, in particular in a range of 35 % to 45 % of the length of the major axis, most particularly 40 % of the length of the major axis.
7. Centrifuge basket (2) according to any of claims 1 to 6, wherein the maximum overall extension of the elongate opening is in the range of 300 % to 400 % of the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening, in particular in the range of 325 % to 340 % of the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening.
8. Centrifuge basket (2) according to any of claims 1 to 7, wherein the contribution of the extension of the rectangular middle portion to the maximum overall extension of the elongate opening is in the range of 20 % to 30 % of the maximum overall extension of the elongate opening, in particular in the range of 24.0 % to 29.0 % of the maximum overall extension of the elongate opening.
9 Centrifuge basket (2) according to any of claims 1 to 8, wherein the elongate 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).
10. Centrifuge basket (2) according to claim 9, wherein the elongate openings of adjacent rows are arranged in a staggered pattern.
11. Centrifuge basket (2) according to claim 10, wherein each elongate opening of a row is arranged at a circumferential position that is in the middle of the distance between elongate openings in the adjacent row.
12. Centrifuge basket (2) according to any of claims 9 to 11, wherein adjacent elongate openings within each row are separated from each other by a distance of at least 40 mm and at maximum 75 mm.
13. Centrifuge basket (2) according to any of claims 9 to 12, wherein adjacent rows are separated from each other by a distance of at least 50 mm and at maximum 60 mm.
14. Discontinuous centrifuge (1) with a centrifuge basket (2) according to any of claims 1 to 13.

, Description:Centrifuge basket for a discontinuous centrifuge

The invention is made in the field of centrifuge technology and relates to a centrifuge basket for separating suspensions in a discontinuous centrifuge into a liquid phase and a solid phase, the centrifuge basket being adapted to be mounted in a discontinuous 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 charactristics) 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 discontinuous 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 discontinuous 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 discontinuous 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 discontinuous centrifuge into a liquid phase and a solid phase, the centrifuge basket being adapted to be mounted in a discontinuous 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 elongate openings, wherein the elongate openings have a shape with a rectangular middle portion and two elliptical end portions.
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 discontinuous centrifuge is a centrifuge specifically adapted for discontinuous operation. Therefore, a discontinuous centrifuge does not necessarily require means for continuously removing the separated solid phase during operation. In some cases, a discontinuous centrifuge does also not provide means for continuously adding new suspension during operation. Consequently, the general design of discontinuous centrifuges differs considerably from the design of continuous centrifuges.
For separating a suspension in a discontinuous centrifuge, the suspension is filled into a centrifuge basket of the discontinuous centrifuge. Typically, the suspension is fed into the basket in the lower speed zone of the operation cycle. However, for specific setups, it is also possible to fill the suspension into the centrifuge basket in a continuous manner during operation. Thereafter, the operation in started by rotating the centrifuge basket about its central axis (axis of rotation, rotational axis) 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 monitor case 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 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. Typically, for a discontinuous centrifuge, it may be useful if in the centrifuge basket, the centrifuge basket casing is a cylindrical centrifuge basket casing. In this case, the centrifuge basket has a shape derived from a circular cylinder having a circular base and a lateral surface (circumferential surface) so that the centrifuge basket comprises a centrifuge basket casing constituting the outer sidewall of the centrifuge basket and that the centrifuge basket casing is arranged at the outer perimeter of the centrifuge basket. For a centrifuge basket derived from the shape of the cylinder, the centrifuge basket casing corresponds to the lateral surface of the cylinder.
According to the invention, all or a part of the openings are elongate openings each having a shape with a rectangular middle portion and two elliptical end portions. The rectangular middle portion forms the central part of the elongate 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 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). In general, a rectangle is a quadrilateral with four right angles in which the opposing edges have the same length (also comprising a square as a limiting case).
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 elongate 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 (midpoint, central point) 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.
Accordingly, in the rectangular middle portion of the elongate 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 elongate 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. Resulting from the rectangular middle portion, the elongate 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 elongate 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 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).
For the elongate opening, the direction through the opening (as defined by the thickness of the centrifuge basket casing) corresponds to the depth direction. For a cylindrical centrifuge basket, the depth direction is in radial direction. 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 cylindrical centrifuge basket, the longitudinal direction is in circumferential direction (i.e., in parallel to the direction of rotation). Likewise, the direction perpendicular to the depth direction as well as to the longitudinal direction corresponds to the transverse direction. For a cylindrical centrifuge basket, the transverse direction is in parallel to the axial direction. Accordingly, the extension in depth direction is the depth extension while the extension in longitudinal direction is the longitudinal extension (with the maximum extension in longitudinal direction being the maximum longitudinal extension) and the extension in transverse direction is the transverse extension (with the maximum extension in transverse direction being the maximum transverse extension). According to the present invention, the elongate opening has a maximum transverse extension that corresponds at least substantially to the maximum extension of the rectangular middle portion in transverse direction, namely the distance between the two parallel straight edges in the rectangular middle portion as well as 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 even being zero.
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.
In addition, in the centrifuge basket, the two elliptical end portions may be aligned with each other on opposite sides of the rectangular middle portion. By this design, the stress arising at the edges of the elongate openings during operation of the centrifuge is significantly homogeneously distributed over the edges of the elongate openings. Moreover, 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 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 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 openings during operation of the centrifuge.
In particular, in the centrifuge basket, 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 includes the minor axis and one half of the major axis. In other words, each of the aligned end portions may have the shape of one half of an ellipse (thus being a semi-ellipse) wherein each ellipse has a major axis and a minor axis. 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, one half of an ellipse that includes the minor axis and one half of the major axis corresponds to an ellipse that is divided on its minor axis into two halves, each half including one half of the major axis as well as the (whole) minor axis. By this measure, the symmetry of the resulting opening is further improved, thus additionally minimizing the stress arising at the edges of the opening.
It is noted that even if the shape of both end portions of each elongate opening is identical, this does not necessarily result in the shapes of the different elongate openings being identical. Of course, all elongate openings in a centrifuge basket casing of the centrifuge basket may be designed to be identical. Moreover, the shape of the openings may vary on the centrifuge basket casing. For example, the size of the openings may vary over the height of the centrifuge basket (i.e., 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 elongate openings, it is also possible that the centrifuge basket casing has different types of openings. For example, the shape of the openings may vary over the height of the centrifuge basket (i.e., in parallel to the direction of the axis of rotation of the centrifuge basket) so that elongate 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 linearly stretched ellipses 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 if all openings are designed as linearly stretched ellipses.
Moreover, it may be useful if in the centrifuge basket, for each half of an ellipse, the length of the half of the major axis is in a range of 5.0 mm to 12.0 mm whereas the length of the minor axis is in a range of 4.0 mm to 10.0 mm, in particular the length of the half of the major axis being in a range of 6.25 mm to 10.0 mm and the length of the minor axis being in a range of 5.0 mm to 8.0 mm. Likewise, in the centrifuge basket, for each half of an ellipse, the length of the minor axis may be in a range of 30 % to 60 % of the length of the major axis, in particular in a range of 35 % to 45 % of the length of the major axis, most particularly 40 % of the length of the major axis. Such dimensions of the end portions of the elongate 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, the maximum overall extension of the elongate opening may be in the range of 300 % to 400 % of the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening, in particular in the range of 325 % to 340 % of the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening. For elongate openings with aligned elliptical end portions, the maximum overall extension of the elongate opening is the distance between the two vertices (corresponding to the maximum longitudinal extension of the elongate opening) while the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening corresponds to the distance between the two parallel edges in the rectangular middle portion as well as to the length of the minor axes of the elliptical end portions (corresponding to the maximum transverse extension of the elongate opening). Furthermore, in the centrifuge basket, the contribution of the extension of the rectangular middle portion to the maximum overall extension of the elongate opening may be in the range of 20 % to 30 % of the maximum overall extension of the elongate opening, in particular in the range of 24.0 % to 29.0 % of the maximum overall extension of the elongate 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 elongate 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 elongate 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 elongate 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 elongate openings are arranged annularly on the centrifuge basket casing with each row of elongate openings forming a circle on the centrifuge basket casing. In this context, the term “at least substantially perpendicular“ means that the rows 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 10 ° with respect to such exact perpendicular orientation. By such design, the elongate openings are oriented in a way that the shape of the elongate opening is suited best to withstand the stresses occurring at the edges of the elongate openings during rotation of the centrifuge basket.
For a row arrangement, it is useful if in the centrifuge basket, the elongate openings of adjacent rows are arranged in a staggered pattern. The term “staggered” means that the elongate openings are arranged in a pattern so that a line connecting the centre of an elongate opening in a first row with the centre of an elongate opening in a second row adjacent to the first row is not running in transverse direction (i.e., parallel to the axis of rotation) 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 elongate openings of adjacent rows are aligned in transverse direction. In particular, in the centrifuge basket, each elongate opening of a row may be arranged at a circumferential position that is in the middle of the distance between elongate openings in the adjacent row. The arrangement of an elongate opening at a circumferential position refers to the position of the elongate opening in the circumferential plane on the centrifuge basket casing. Such a design results in a diamond-shaped distribution of the elongate openings on the centrifuge basket casings. By the equidistant distribution of adjacent elongate openings in adjacent rows resulting from this arrangement, a centrifuge basket having a uniform stress resistance without predetermined breaking points is realised, at the same time allowing for a fast drainage of the liquid phase from the centrifuge basket.
Additionally, in the centrifuge basket, adjacent elongate openings within each row may be separated from each other by a distance of at least 40 mm and at maximum 75 mm. The term “separated by a distance of” does not refer to the distance between the centres of adjacent elongate openings but refers to the extension of solid portions of the centrifuge basket between two adjacent elongate openings. Similarly, in the centrifuge basket, adjacent rows may be separated from each other by a distance of at least 50 mm and at maximum 60 mm.
Finally, the invention includes a discontinuous centrifuge with a centrifuge basket having the above features.
Of course, even though not explicitly mentioned, a centrifuge basket as well as a discontinuous 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 in its upper part, a side view of a part of a centrifuge basket casing of a conventional centrifuge basket in a discontinuous centrifuge, and in its lower part, a detail view of an opening in such conventional centrifuge basket;
Fig. 2 in its upper part, a side view of a of a part of a centrifuge basket casing of a centrifuge basket in a discontinuous centrifuge according to the present invention, and in its lower part, a detail view of an opening in such inventive centrifuge basket;
Fig. 3 a sectional view of a part of a discontinuous centrifuge having a centrifuge basket according to the present invention at a process stage in which the centrifuge basket is rotated while the suspension to be separated is fed into the centrifuge; and
Fig. 4 a sectional view of a part of the discontinuous centrifuge shown in Fig. 3 in which the centrifuge basket is stopped while the separated solid phase is discharged from the centrifuge.
The lower part of Fig. 1 shows a detail of a conventional centrifuge basket in a conventional discontinuous centrifuge, namely a schematic side view of an opening in a centrifuge basket casing. The opening has a circular shape with its edges forming a circle having a diameter D.
In the upper part of Fig. 1, a side view of a part of a centrifuge basket casing is schematically shown that is a component of a conventional centrifuge basket in a discontinuous centrifuge. The centrifuge basket comprises a centrifuge basket casing forming the outer sidewall of the centrifuge basket. For a centrifuge basket derived from the shape of the cylinder, the centrifuge basket casing corresponds to the lateral surface of the cylinder. In Fig. 1, the centrifuge basket casing is arranged at the outer perimeter of the centrifuge basket, which has the shape of a cylindrical drum. The centrifuge basket casing is the outer sidewall of the centrifuge basket so it is essentially constituted by a circular metal sheet forming the lateral surface (circumferential surface) of the cylindrical drum. Consequently, the centrifuge basket has a shape derived from a circular cylinder having a circular base and a lateral surface.
The openings in this centrifuge basket casing are arranged in horizontal rows in a staggered pattern. In such a staggered pattern, the openings of adjacent lines are not aligned in a vertical direction. In the upper part of Fig. 1, the staggered pattern has the shape of a diamond with the openings constituting the corners. In this diamond pattern, each opening is located at a horizontal position that is in the middle between the horizontal positions of the adjacent openings in the adjacent rows. Accordingly, in vertical 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 the upper part of Fig. 1, the straight lines connecting the openings in horizontal direction and vertical direction merely serve as guides to the eyes.) In the centrifuge basket casing shown in the upper part of Fig. 1, each opening has a diameter D of 7.0 mm while the openings are arranged in 20 rows, each row comprising 64 openings.
The lower part of Fig. 2 shows a detail of a centrifuge basket according to the present invention, namely a schematic side view of an opening in a centrifuge basket casing. This opening is an elongate 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 (but are mirrored, of course). Moreover, both elliptical end are aligned with each other on opposite sides of the rectangular middle portion in a way that the major axes (major semi-axes) of the ellipses are aligned whose segments constitute the elliptical end portions. The maximum overall extension of the elongate opening is the maximum longitudinal extension of the elongate opening while the maximum extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening is the maximum transverse extension of the elongate opening.
For the elongate opening, the maximum extension in transverse direction is 6.0 mm and the maximum extension in longitudinal direction is 21.0 mm (thus being 350 % of the maximum overall extension of the elongate opening in direction perpendicular to the maximum overall extension of the elongate opening). In detail, the rectangular middle portion is a square of 6.0 mm x 6.0 mm having a maximum longitudinal extension A of 6.0 mm and a maximum transverse extension of also 6.0 mm. Moreover, 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 6.0 mm. For both elliptical end portions, the length B of the minor axis amounts to 40 % of the length of the major axis. Furthermore, the contribution of the extension A of the rectangular middle portion to the maximum overall extension (A + 2C) of the elongate opening is 28.6 % of the maximum overall extension of the elongate opening.
In the upper part of Fig. 2, a side view of a part of a centrifuge basket casing is schematically shown that is a component of a centrifuge basket according to the present invention in a discontinuous centrifuge. Similar to the centrifuge basket shown in the upper part of Fig. 1, the centrifuge basket shown in the upper part of Fig. 2 has a shape of a cylindrical drum with a centrifuge basket casing forming the outer sidewall of the centrifuge basket and being essentially constituted by a circular metal sheet. The centrifuge basket casing of the centrifuge basket according to the present invention is arranged at the outer perimeter of the centrifuge basket. (For the sake of clearness, also the centrifuge basket casing depicted in the upper part of Fig. 2 is shown as a flat area without a shortened perspective.)
The elongate openings in this centrifuge basket casing are arranged in horizontal rows in a staggered pattern so that the elongate openings of adjacent lines are not aligned in a vertical direction. In the upper part of Fig. 2, the staggered pattern has the shape of a diamond with the elongate openings constituting the corners. Also in this diamond pattern, the elongate openings are located at horizontal positions that are in the middle between the horizontal positions of the elongate openings in each of the adjacent rows. Accordingly, in vertical direction, the elongate openings of a row are aligned with the corresponding elongate openings in the next but one row thus being arranged in the form of a regular grid in which only each second node is occupied by an elongate opening. (Likewise, in the upper part of Fig. 2, the straight lines connecting the elongate openings in horizontal direction and vertical direction are merely shown to guide the eyes.) In the centrifuge basket casing shown in the upper part of Fig. 2, the elongate openings are arranged in 20 rows, each row comprising 64 openings.
The openings shown in Fig. 2 were cut in a flat steel sheet. Subsequently, the steel sheet was rolled in a cylindrical shape and welded in order to form the centrifuge basket casing.
However, according to the present invention, also openings with other designs may be realised, for example in which the elliptical end portions of an elongate 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.
In Fig. 3 and Fig. 4, sectional views of a part of a discontinuous centrifuge 1 having a centrifuge basket 2 according to the present invention are shown in two different process stages. In the process stage shown in Fig. 3, the centrifuge basket 2 is rotated while the suspension to be separated is fed into the centrifuge 1. Different from this, in Fig. 4, the centrifuge basket 2 is stopped while the separated solid phase is discharged from the centrifuge 1.
In detail, Fig. 3 and Fig. 4 show a discontinuous centrifuge 1 with an external housing in which a drum-shaped centrifuge basket 2 is connected to a drive shaft 3. The centrifuge basket 2 has a cylinder-like shape 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 discontinuous centrifuge 1 to form an annular gap between the outside of the centrifuge basket casing 4 and the inside of the housing of the centrifuge 1. 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 discontinuous centrifuge 1 shown in Fig. 3 and Fig. 4 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 dense mixture of sugar crystals and syrup, the partly evaporated 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 that is a metal sheet in the shape of a tube section having openings formed therethrough as through-holes penetrating the metal sheet (not shown in Fig. 3 and Fig. 4). In the present case, all of the openings are elongate openings of the same size each having a rectangular middle portion and two elliptical end portions. The elongate openings are arranged in rows forming substantially horizontal rings on the centrifuge basket casing 4. In adjacent rows, the elongate openings are arranged in a staggered pattern with the distances between adjacent elongate openings being identical with respect to the height of the centrifuge basket 2 (i.e., the extension in parallel to axial direction). Within the compartment of the centrifuge basket 2, the elongate 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, a linen or the like with a mesh width in the range from 0.6 mm to 0.8 mm.
For operating the discontinuous centrifuge 1, the partly evaporated massecuite 10 is fed into the compartment of the centrifuge basket 2 via a tubular massecuite inlet 6 that is introduced into the compartment to through an annular recess provided in the circular top base of the cylinder constituting the centrifuge basket 2. Typically, the massecuite 10 is fed into the rotating centrifuge basket 2 so that the massecuite 10 is evenly distributed inside the centrifuge basket 2 by centrifugal forces, thus avoiding imbalance. However, it is also possible to fill the massecuite 10 into the stopped centrifuge basket 2, thereafter starting the rotation of the centrifuge basket 2. During rotation, the massecuite 10 is pressed against the screen 5 with the liquid phase (namely, the molasses 11) being able to go through the screen 5 and to leave the centrifuge basket 2 via the elongate openings in the centrifuge basket casing 4 while the solid phase (namely, the sugar 12) is retained by the screen 5. Accordingly, the molasses 11 enters the annular gap between the centrifuge basket casing 4 and the housing of the centrifuge. In this gap, the molasses 11 flows downwards until it reaches the inclined bottom of the compartment from which it leaves the centrifuge 1 via the molasses outlet 7.
Once the separation of the massecuite 10 is finished, a scraping means such as a scraper blade 8 is inserted into the compartment through the annular recess in order to loosen the sugar crystals that adhere to the screen 5. Within the centrifuge basket 2, the thus loosened sugar 12 falls down onto the circular bottom base of the cylinder constituting the centrifuge basket 2. The circular bottom base has a circular opening. During the separation of the massecuite 10, the circular opening in the bottom is blocked by a conical plug. In order to discharge the sugar 12 from the centrifuge basket 2 after separation of the massecuite 10, the conical plug is lowered. By this, the circular opening is opened so that the sugar 12 is discharged by falling down into the sugar outlet 9.
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, elongate openings according to the present invention with a rectangular middle portion and two elliptical end portions (maximum longitudinal extension: 21 mm; maximum transverse extension: 6 mm; arranged in 20 rows each having 64 elongate openings) were compared with conventional circular openings (diameter: 7 mm; arranged in 20 rows each having 64 circular openings) as well as with elliptical openings having the shape of an exact ellipse (maximum longitudinal extension: 15 mm; maximum transverse extension: 6 mm; arranged in 20 rows each having 48 elliptical openings). For the three centrifuge baskets with the different types of openings, the maximum stress was identified as well as the positions in which the maximum stress arises. From this maximum stress, the Factor of Safety (Safety Factor) was determined by dividing the yield stress by the maximum stress identified for each centrifuge basket type (corresponding to the reasonably foreseeable/intended stress during operation). The Factor of Safety was calculated for a centrifuge basket casing made of duplex stainless steel having a yield stress of 460 MPa.
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 axial direction (i.e., at the uppermost and lowermost points of the edges of the opening in axial direction were the maximum transverse extension of the opening is maximum). For a circular opening, the maximum stress was as large as about 240 MPa. For the elliptical opening, the maximum stress was about 220 MPa. For the elongate opening, the stress was as low as about 200 MPa. Further results are shown in the table.
Shape of Opening Dimensions of Opening
[mm x mm] Number of Rows Number of Openings per Row Maximum Stress
[MPa] Factor of Safety Improvement
Circular 7 x 7 20 64 238.90 1.93 -
Elliptic 15 x 6 20 48 220.10 2.09 8 %
Elongate 21 x 6 20 64 202.99 2.27 18 %

It was surprising to see that even when increasing the drainage area of the openings, it is possible to reduce the maximum stress arising in the centrifuge basket by one sixth simply by providing a middle portion with parallel edges where the transverse extension of the opening is maximum.
Centrifuge baskets with different sizes were checked for elongate openings each having a rectangular middle portion and two elliptical end portions. For an elongate opening having a maximum extension in transverse direction of 6.0 mm and a maximum extension in longitudinal direction of 21.0 mm (i.e., an elongate opening with the extensions 21.0 mm x 6.0 mm), the rectangular middle portion was a square of 6.0 mm x 6.0 mm while each of the two elliptical end portions was one half of an ellipse with the major axis of 15.0 mm (i.e., with a major semi-axis having a length of 7.5 mm) and a minor axis is 6.0 mm (i.e., with a minor semi-axis having a length of 3.0 mm). Accordingly, the area of one individual elongate opening is 106.7 mm2.
For a large drum-shaped centrifuge basket having an inner diameter of 1,585 mm, a centrifuge basket casing thickness of 16 mm (thus corresponding to a mean diameter of 1,601 mm and an outer diameter of 1.617 mm), and an internal extension in axial direction of 1,110 mm, the mean circumference was 5,030 mm and the total area of the centrifuge basket casing was 1,110 mm x 5,030 mm = 5,583,300 mm2. Such a centrifuge basket had 20 rows, each row having 64 elongate openings (21.0 mm x 6.0 mm - with the centres being separated by a distance of 78.6 mm corresponding to a separating solid portion between adjacent elongate openings in a row of 57.6 mm), thus resulting in the total number of holes being 1,280. Consequently, the elongate openings in such centrifuge basket sum up to a total area of 136,576 mm2 corresponding to a relative drainage area of 2.45 % of the area of the centrifuge basket casing.
Similarly, for a further large drum-shaped centrifuge basket having an inner diameter of 1,550 mm and an internal extension in axial direction of 1,160 mm, the mean circumference was 4,920 mm and the total area of the centrifuge basket casing was 5,707,200 mm2. Such a centrifuge basket had 21 rows, each row having 64 elongate openings (21.0 mm x 6.0 mm - with the centres being separated by a distance of 76.9 mm corresponding to a separating solid portion between adjacent elongate openings in a row of 55.9 mm), thus resulting in the total number of holes being 1,344. Consequently, the elongate openings in such centrifuge basket sum up to a total area of 143,405 mm2 corresponding to a relative drainage area of 2.51 % of the area of the centrifuge basket casing.
Likewise, for a medium-sized drum-shaped centrifuge basket having an inner diameter of 1,350 mm and an internal extension in axial direction of 1,100 mm, the mean circumference was 4,291 mm and the total area of the centrifuge basket casing was 4,720,100 mm2. Such a centrifuge basket had 19 rows, each row having 56 elongate openings (21.0 mm x 6.0 mm - with the centres being separated by a distance of 76.6 mm corresponding to a separating solid portion between adjacent elongate openings in a row of 55.6 mm), thus resulting in the total number of holes being 1,064. Consequently, the elongate openings in such centrifuge basket sum up to a total area of 113,529 mm2 corresponding to a relative drainage area of 2.41 % of the area of the centrifuge basket casing.
Likewise, for a smaller drum-shaped centrifuge basket having an inner diameter of 1,260 mm and an internal extension in axial direction of 1,050 mm, the mean circumference was 4,009 mm and the total area of the centrifuge basket casing was 4,209,450 mm2. Such a centrifuge basket had 19 rows, each row having 48 elongate openings (21.0 mm x 6.0 mm - with the centres being separated by a distance of 83.3 mm corresponding to a separating solid portion between adjacent elongate openings in a row of 62.3 mm), thus resulting in the total number of holes being 912. Consequently, the elongate openings in such centrifuge basket sum up to a total area of 97,310 mm2 corresponding to a relative drainage area of 2.31 % of the area of the centrifuge basket casing.
Moreover, centrifuge baskets having elongate openings with different sizes were checked. In particular, the smallest elongate openings had a rectangular middle portion with an extension A in longitudinal direction of the elongate opening of 4.0 mm and elliptical end portions having a length B of the minor axis of 5.0 mm and a length C of the half of the major axis of 6.25 mm. Larger elongate openings had a rectangular middle portion with an extension A in longitudinal direction of the elongate opening of 5.0 mm and elliptical end portions having a length B of the minor axis of 6.0 mm and a length C of the half of the major axis of 7.50 mm. Even larger elongate openings had a rectangular middle portion with an extension A in longitudinal direction of the elongate opening of 6.0 mm and elliptical end portions having a length B of the minor axis of 7.0 mm and a length C of the half of the major axis of 8.75 mm. The largest elongate openings had a rectangular middle portion with an extension A in longitudinal direction of the elongate opening of 7.0 mm and elliptical end portions having a length B of the minor axis of 8.0 mm and a length C of the half of the major axis of 10.00 mm. Of course, it is possible to use elongate openings with even smaller or even larger dimensions.
All the above embodiments had an increased drainage area with respect to centrifuge baskets with circular openings, 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 length of rectangular middle portion in longitudinal direction (of elongate opening)
B length of minor axis of elliptical end portion (of elongate opening)
C length of a half of the major axis of elliptical end portion (of elongate opening)
D diameter (of circular opening)

1 discontinuous centrifuge
2 centrifuge basket
3 drive shaft
4 centrifuge basket casing
5 screen
6 massecuite inlet
7 molasses outlet
8 scraper blade
9 sugar outlet
10 massecuite
11 molasses
12 sugar