A centrifugal separator for cleaning gas

Abstract

The present invention relates to a centrifugal separator for cleaning gas containing contaminants and a separation member (9') for the separator. The centrifugal separator (1) comprises a stationary casing (2), a gas inlet (20) and a plurality of separation members (9') comprising a central aperture (80) defining an inner periphery (91) of the separation member (9') and a rotary member (7) arranged to rotate around an axis (X) of rotation in the separation space (3) and further comprises a rotatable shaft (8) and a separation unit (16) comprising a plurality of the separation members (9') attached to the shaft. At least a portion (90) of the inner periphery (91) of the separation member (9') is arranged in contact with the shaft (8) when attached to the shaft (8), and wherein the portion (90) is provided with one or more flexible elements (100). By the flexible elements, a press fit of the separation members to the shaft is obtained.

Description

Field of the Invention

[0001] The present invention relates to the field of centrifugal separators for cleaning a gas containing liquid contaminants. In particular, the present invention relates to a separator for cleaning crankcase gases of an internal combustion engine from oil particles.Background of the Invention

[0002] It is well known that a mixture of fluids having different densities may be separated from one another using a centrifugal separator. One specific use of such a separator is in the separation of oil from gas that is vented from a crankcase forming part of an internal combustion engine. The high-pressure gas present in the combustion chambers of the internal combustion engine may leak past associated piston rings and into the crankcase of the engine. This continuous leaking of gas into the crankcase can lead to an undesirable increase of pressure within the crankcase and, as a consequence, to a need to vent gas from the casing. Such gas vented from the crankcase typically carries a quantity of engine oil (as droplets or a fine mist), which is picked up from the reservoir of oil held in the crankcase.

[0003] In order to allow vented gas to be introduced into the inlet system without also introducing unwanted oil (particularly into a turbocharging system wherein the efficiency of the compressor can be adversely affected by the presence of oil), it is necessary to clean the vented gas, i.e., to remove the oil carried by the gas, prior to the gas being introduced into the inlet system. This cleaning process may be undertaken by a centrifugal separator, which is mounted on or adjacent the crankcase and which directs cleaned gas to the inlet system and directs separated oil back to the crankcase. An example of such a separator is disclosed e.g., in US 8,657,908, in which a centrifugal separator comprising a housing defining an inner space, and a rotor assembly for imparting a rotary motion onto a mixture of substances to be separated is shown. The rotor assembly is located in said inner space and is rotatable about an axis relative to the housing. The rotor assembly comprises an inlet for receiving said mixture of substances, an outlet from which said substances are ejected from the rotor assembly during use, and a flow path for providing fluid communication between the inlet and outlet, wherein the outlet is positioned more radially outward from said axis than the inlet. The rotor assembly further comprises a rotary shaft having a longitudinal axis coincident with the axis of rotation.

[0004] In the centrifugal separator, a plurality of separation discs is mounted to the rotary shaft to form a disc stack. In the existing solutions, the fitting of the separation discs on the shaft can be either press fit (fit with friction between the parts to obtain tight fit) or play fit (fit with certain play or movement between the parts), due to the tolerances on the plastic on both parts. This may create a problem during the assembly of the disc stack, especially in case of high volumes, since a variable amount of force may be required to press the disc stack onto the shaft. Furthermore, the play fit may cause movement of the discs in respect of the shaft, causing imbalance and vibrations, thereby reducing the lifetime of the separator parts.

[0005] There is thus a need for an improved disc design to facilitate and improve the assembly and the fit of the disc stack onto the shaft.Summary of the Invention

[0006] It is an objective of the present invention to at least partly solve the problem with prior art solutions and thus overcome one or more limitations of the prior art. In particular it is an objective to provide a centrifugal separator with an improved disc design to facilitate and improve the assembly of the disc stack onto the shaft and reduce wear of the shaft during the operation of the centrifugal separator.

[0007] It has been noted that that it may be advantageous that there is always a press fit between the discs in the stack and the shaft. As mentioned above, if there is a play fit, there is a risk of movement of the discs which can cause wear on the shaft. The wear may cause imbalance in rotation, which in turn causes vibrations and reduced lifetime. Further, if there is play fit, there is a risk that at least some of the discs are not centered and therefore the disc stack is unbalanced.

[0008] According to the present invention, the problem with the fitting of the separation discs on the shaft to obtain a desirable press fit and balance is solved by providing a flexible element to a portion of the disc, which is in contact with the shaft of the separator when in use. By the flexible element is meant an element, which is able to bend easily without breaking and which is provided to the disc for example by forming a part of the disc as the flexible element or by providing an external element as the flexible element to the disc. This element is configured to be in contact with the shaft and provides a flexible press fit between the disc and the shaft. It compensates for the tolerances on both the disc and shaft, which may include e.g. plastic material. The flexible element thus ensures that there is always a certain amount of press fit, but due to the flexibility of the element, this will not become too tight. In this way, the wear on the shaft can be reduced and lifetime of the separator parts can be prolonged.

[0009] As a first aspect of the invention, there is provided a centrifugal separator for cleaning gas containing contaminants. The centrifugal separator comprises a stationary casing, enclosing a separation space through which a gas flow is permitted and a gas inlet extending through the stationary casing and permitting the supply of the gas to be cleaned to the stationary casing. Further, the centrifugal separator comprises a separation member for cleaning of the gas and comprising a central aperture defining an inner periphery of the separation member and a rotating member arranged to rotate around an axis of rotation in the separation space and comprising a rotatable shaft and a separation unit comprising a plurality of the separation members attached to the shaft. The rotating member is rotated by a drive member. The separator further comprises a gas outlet arranged through the stationary casing and configured to permit the discharge of cleaned gas out from the stationary casing, and a drainage outlet arranged in the stationary casing and configured to permit discharge of liquid contaminants that have been separated from the gas out from the stationary casing.

[0010] According to the invention at least a portion of the inner periphery of the separation member is arranged to be in contact with the shaft when attached to the shaft. The portion is provided with one or more flexible elements.

[0011] The flexible elements provide a press fit towards shaft, while they assure that the fit will not be too tight.

[0012] The flexible element may be formed by at least one slit or recess projecting radially outwards from the portion at the inner periphery of the separation member. This way, a simple and robust design is provided.

[0013] According to a variant, the flexible element is formed between a pair of adjacent slits or recesses. Thereby a simple design is achieved.

[0014] The flexible element may have a longer radial extension into the central aperture than other parts of the inner periphery. In this way it can be ensured that the flexible element will come into contact with the shaft and that the contact is kept also during use of the separator, when the rotating member rotates. Thereby the movement of the separation member in respect of the shaft can be avoided and thus wear of the shaft can be further prevented.

[0015] The flexible element may have a tapering thickness radially towards the central aperture. In this way, the flexibility is further improved.

[0016] The separation member may comprise at least three flexible elements evenly distributed in the portion around the inner periphery of the separation member. In this way balance can be improved in addition to improved press fit around the inner periphery of the disc.

[0017] The shaft may comprise a number of evenly distributed axially and radially outwards in respect of the shaft protruding splines. The splines prevent rotation of the separation members in respect of the shaft. Consequently, the separation member comprises a corresponding number of at least partially mating spline recesses to receive the splines. The flexible elements may be placed in between the spline recesses in the portion around the inner periphery of the separation member. Thus, the design ensures press fit even when the shaft comprises splines.

[0018] The spline recesses may comprise a tip portion and a root portion. The radial extension of the flexible element may be smaller than the radial extension of the tip portion and larger than the radial extension of the root portion. The radial extension in this context is the extension outwards from the inner periphery of the separation member. In this way, the flexible element can provide for good flexibility, while the mechanical properties of the disc are not negatively affected.

[0019] The separation member may be a separation disc comprising a flat portion, which extends perpendicularly to the axis of rotation, and a conical portion, which extends in the axial direction and outwardly from the direction of the axis of rotation, and wherein the flat portion comprises the central aperture. The shape of the disc is thus frustoconical.

[0020] According to a second aspect the invention relates to a separator member for a centrifugal separator as defined above. The separator member comprises a central aperture defining an inner periphery and is configured to be attached to the shaft of the centrifugal separator via the central aperture. At least a portion of the inner periphery is provided with one or more flexible elements.

[0021] In analogy with the centrifugal separator described above, the same advantages are obtained by the disc, i.e. that the flexible elements can provide a press fit towards shaft when assembled thereto, while they assure that the fit will not be too tight.

[0022] The flexible element may be formed by at least one slit or recess projecting radially outwards from the portion at the inner periphery of the separation member. This way, a simple and robust design which does not complicate the manufacture of the separation member essentially is provided.

[0023] The flexible element may have a longer radial extension into the central aperture than other parts of the inner periphery. This way, contact with the shaft during assembly and rotation of the centrifugal separator can be ensured, and thereby a press fit obtained both during the assembly and the use.

[0024] The flexible element may have a tapering thickness towards the central aperture. The thickness may taper radially towards the central aperture. In this way the flexibility can be improved.

[0025] The separation member may comprise at least three flexible elements evenly distributed around the inner periphery of the separation member. In this way, the separation member is well-balanced during the use.

[0026] The separation member may be a frustoconical disc comprising a circular flat portion comprising the central aperture, and a conical portion extending axially and radially outwards of the flat portion, and wherein the flat portion comprises a plurality of through holes, which are located radially outside of the flexible elements. The flexible element is especially suitable for this disc type.

[0027] As used herein, the term "axially" denotes a direction which is parallel to the rotational axis (X). Accordingly, relative terms such as "above", "upper", "top", "below", "lower", and "bottom" refer to relative positions along the rotational axis (X). Correspondingly, the term "radially" denotes a direction extending radially from the rotational axis (X). A "radially inner position" thus refers to a position closer to the rotational axis (X) compared to "a radially outer position". A radial plane is a plane having its normal parallel to the axis of rotation (X). An axial plane is a plane having its normal perpendicular to the axis of rotation (X).

[0028] The contaminants in the gas may comprise liquid contaminants, such as oil, and soot. Thus, in embodiments, the contaminants comprise oil.

[0029] Consequently, the centrifugal separator may be configured for separating liquid contaminants, such as oil, from gas. The gas may be crankcase gas of a combustion engine. However, the centrifugal separator may also be suitable for cleaning gases from other sources, for instance the environment of machine tools which frequently contains large amounts of liquid contaminants in the form of oil droplets or oil mist.

[0030] The stationary casing of the centrifugal separator may comprise a surrounding side wall, and an upper end wall and a lower end wall, which enclose the separation space. The stationary casing may have a cylindrical shape with circular cross-section having a radius R from the axis (X) of rotation to the surrounding side wall. This radius R may be constant at least with respect to a major part of the circumference of the surrounding side wall. The stationary casing may also be slightly conical. The surrounding side wall may be divided into a lower surrounding sidewall that may be cylindrical in shape and may be joined to an upper surrounding sidewall e.g., by welding or by fastening members such as screws. This facilitates mounting of the rotating member inside the stationary casing. The lower surrounding annular sidewall may extend down to the lower end wall of the stationary casing. The lower end wall may thus be the lower wall extending in the radial direction, such as in the radial plane or with a small angle relative the radial plane. In some embodiments, also the lower end wall is part of the lower surrounding annular sidewall.

[0031] The gas inlet of the centrifugal separator may be arranged through the upper end wall or through the surrounding side wall close to the upper end wall, thus at the top of the separator, such that gas entering through the gas inlet is directed to the separation space. The downstream portion of the gas inlet may be centred around the axis of rotation (X). The gas inlet may further comprise an upstream portion in the form of an inlet conduit. This conduit may extend radially or axially from the centrifugal separator, or in any other direction therebetween. During operation, gas to be cleaned may be directed centrally through the separation unit from the gas inlet and through the separation unit radially outwards.

[0032] The rotating member is arranged for rotation during operation by means of the drive member. The rotating member comprises a separation unit arranged in the separation space. The separation unit of the rotating member may be a surface-enlarging insert, such as a stack of separation discs (a disc stack) or one or several filters, which promotes separation of contaminants from the gas.

[0033] Accordingly, the separation unit may comprise a plurality of separation members. The separation members of the rotating member are surface-enlarging inserts that promote separation of contaminants from the gas. The plurality of separation members may be arranged as a stack of separation discs, such as a stack of frustoconical separation discs. Alternatively, the separation unit may comprise a rotatable filter or a rotatable filter stack.

[0034] A frustoconical disc may have a planar portion extending in a plane that is perpendicular to the axis of rotation, and a frustoconical portion that may extend upwards or downwards. The planar portion may be closer to the rotational axis than the frustoconical portion. As an alternative, the discs of the stack may be radial discs, in which substantially the whole disc extends in a plane that is perpendicular to the axis of rotation. Such discs may have an outer radius and an inner radius, thus forming a central opening in the disc.

[0035] The rotating member may be journaled within the stationary casing via at least one bearing, such as via an upper and lower bearing arranged axially above and below the separation unit, respectively. In embodiments of the first aspect, the rotating member comprises an axial shaft that is supported by the at least one bearing. The axial shaft may thus be centred at the axis of rotation (X). The separation unit may be arranged around such axial shaft.

[0036] The centrifugal separator may be configured to convey gas to be cleaned, such as crankcase gases, from the gas inlet into a central portion of the rotating member. In this manner the crankcase gases may be "pumped" from the central portion of the rotating member into the interspaces between the separation discs in the stack of separation discs by the rotation of the rotating member. Thus, the centrifugal separator may work according to the concurrent flow principle, in which the gas flows in the disc stack from a radial inner part to a radial outer part, which is opposite to a separator operating according to the counter-current flow principle, in which the gas is conducted into the separation space at the periphery and conveyed towards a central part of the separation space.

[0037] The drive member may for example comprise a turbine wheel, rotated by means of an oil jet from the lubrication oil system of the combustion engine or a free jet wheel comprising a blow-back disc. However, the drive member may also be independent of the combustion engine and comprise an electrical motor, a hydraulic motor, a pneumatic motor or a mechanical drive.

[0038] The outlet for cleaned gas (gas outlet) may be in the form of an opening or a gas outlet tube extending through a wall of the stationary casing, such as through a lower portion of the surrounding side wall of the stationary casing. The gas outlet tube may be a generally cylindrical tube or have another cross-section, such as an oval cross-section.

[0039] The drainage outlet may be in an annular collection groove or may be formed as several through holes at the lower end wall of the stationary casing, i.e. at the bottom of the separator. The drainage outlet may be arranged such that contaminants, such as oil, are drained through a bearing arranged for journaling the rotating member. The drainage outlet may be formed so that oil can flow through the lower bearing, or radially outside the lower bearing, down into the turbine housing.Brief description of the Drawings

[0040] The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise. Fig. 1 shows a schematic drawing of the cross-section of an embodiment of a centrifugal separator for cleaning gas. Fig. 2 shows schematically in a perspective side view an example of a rotating shaft with splines and a top disc mounted to the shaft. Fig. 3 shows schematically in a perspective bottom view an embodiment of a frustoconical separation disc suitable for mounting to the shaft shown in Fig. 1. Fig. 4a shows schematically in a top view the separation disc of Fig. 2 with flexible elements according an embodiment of the present disclosure. Fig. 4b shows an enlarged view the flexible elements of Fig. 4a. Fig. 4c shows in enlarged view a further example of flexible elements. Fig. 5a shows a central portion of a separation disc with flexible elements and a shaft to which the disc is attached. Fig. 5b shows an enlarged view the flexible element of Fig. 5a. Fig. 5c shows in an enlarged side view a flexible element with tapering shape towards a shaft. Fig. 6 shows schematically a rotating member with balancing balls in the top disc and the lower end plate. Detailed Description

[0041] The centrifugal separator according to the present disclosure will be further illustrated by the following description with reference to the accompanying drawings.

[0042] Fig. 1 shows schematically a cross-section of a centrifugal separator 1 according to the present disclosure. The centrifugal separator 1 comprises a stationary casing 2, which is configured to be mounted to a combustion engine (not shown), especially an internal combustion engine, such as a diesel engine, at a suitable position, such as on top or at the side of the internal combustion engine or to the engine block of the internal combustion engine.

[0043] The centrifugal separator 1 as disclosed herein may also be suitable for cleaning gases from other sources than internal combustion engines, for instance the environment of machine tools which frequently contains large amounts of liquid contaminants in the form of oil droplets or oil mist.

[0044] The stationary casing 2 encloses a separation space 3 through which a gas flow is permitted. The stationary casing 2 comprises, or is formed by, a surrounding side wall 4, an upper end wall 5 and a lower end wall 6.

[0045] The centrifugal separator 1 comprises a rotating member 7, which is arranged to rotate around a longitudinal axis (X) of rotation. It should be noted that the stationary casing 2 is stationary in relation to the rotating member 7, and preferably in relation to the internal combustion engine to which it may be mounted.

[0046] The stationary casing 2 has a radius from the axis (X) of rotation to the surrounding side wall 4 that is constant at least with respect to a major part of the circumference of the surrounding side wall 4. The surrounding side wall 4 thus has a circular, or substantially, circular cross-section in a radial plane.

[0047] The rotating member 7 comprises a rotatable shaft, i.e., spindle 8 and a separation unit 16 attached to the spindle 8. The separation unit 16 comprises a plurality of separation members 9', which in the illustrated example are separation discs 9' arranged in a stack 9. The separation discs 9' of the stack 9 are provided between a top disc 10 and a lower end plate 11. The spindle 8, and thus the rotating member 7, is rotatably supported in the stationary casing 2 by means of an upper bearing 12 and a lower bearing 13, the bearings being arranged one on each axial side of the stack 9 of separation discs 9', only one of which is provided with a reference sign. However, the bearings 12, 13 could for example both be arranged axially below or above the stack 9 of separation discs 9'.

[0048] The separation discs 9' of the disc stack 9 are frustoconical and extend outwardly from the direction of the axis of rotation and upwardly from the spindle 8. The separation discs thus comprise a flat portion 9a, which extends perpendicularly to the axis of rotation (X), and a conical portion 9b, that extends outwardly and upwardly, in this case towards a gas inlet 20, from the flat portion 9a. As an alternative, the separation discs could also extend outwardly and downwardly, and / or even radially.

[0049] The separation discs of the stack 9 are provided at a distance from each other by means of distance members (not disclosed) in order to form interspaces 14 between adjacent separation discs 9, i.e., an interspace 14 between each pair of adjacent separation discs 9. The axial thickness of each interspace 14 may e.g., be in the order of 0.5-2 mm, such as 1-2 mm.

[0050] The separation discs 9' of the stack 9 may comprise or be made of plastic or metal. The number of separation discs in the stack 9 is normally higher than indicated in Fig. 1 and may be for instance 50 to 100 separation discs 9 depending on the size of the centrifugal separator 1.

[0051] In the illustrated example, the centrifugal separator 1 comprises an oil nozzle 24 arranged for being connected to an engine oil circuit of an internal combustion engine. During running of the internal combustion engine, oil is pumped through the oil nozzle 24 onto a turbine wheel 22, which is arranged in turbine housing 26. Since turbine wheel 22 is connected to the spindle 8, the rotating member 7, and thus the stack 9 of separation discs 9', also rotate upon rotation of wheel 22. As an alternative, the centrifugal separator 1 may comprise an electric motor arranged to rotate the spindle 8 and rotating member 7. As a further alternative, the centrifugal separator 3 may comprise a turbine wheel connected to the spindle 8, where the turbine wheel is arranged to be driven by exhaust gases from the internal combustion engine to rotate the spindle 8 and the rotating member 7. The rotating member 7 may also be arranged for being rotated by a mechanical drive unit. Thus, the centrifugal separator may comprise a mechanical drive unit for rotating the rotating member 7.

[0052] The rotating member 7 defines a central space 15. The central space 15 is in this example formed by a through hole 150 in each of the separation discs 9. In the embodiments of Fig. 1, the central space 15 is formed by a plurality of through holes 150 (only two of which are depicted by a reference sign), each extending through the top disc 10 and through each of the separation discs 9', but not through the lower end plate 11. The through holes 150 are arranged in at least partially in the flat portions 9a of the separation discs 9'. The through holes 150 may extend to the conical portion of the separation discs, but the peripheral portion of the discs is solid.

[0053] The gas inlet 20 extends through the stationary casing 2, and more precisely through upper end wall 5, and is arranged for supplying gas to be cleaned to the separation space 3. The gas inlet 20 is formed by the axially extending inlet conduit 18, which forms an upstream portion, and by through channels 21 that form a downstream portion of the inlet 20.

[0054] The through channels 21 are in fluid connection with the central space 15 and are arranged radially outside the upper bearing 12. Thus, the gas inlet 20 communicates with the central space 15 so that the gas to be cleaned is conveyed from the inlet 20 via the central space 15 to the interspaces 14 of the stack of separation discs 9. The gas inlet 20 is configured to communicate with the crankcase of the combustion engine, or any other source, via the inlet conduit 18 permitting the supply of crankcase gas from the crankcase to the gas inlet 20 and further to the central space 15 and the interspaces 14 as explained above.

[0055] The centrifugal separator 1 comprises a drainage outlet 29, which is arranged in the lower portion of the stationary casing 2 and configured to permit discharge of liquid contaminants separated from the gas. The drainage outlet 29 is in this embodiment in the form of through holes 150 arranged in the lower end wall 6 so that separated liquid contaminants flow through the lower bearing 13 as they are drained from the separation space 3 to the turbine housing 26. The separated oil, and other particles and / or substances, is led to an oil outlet 25 of the centrifugal separator 1, which together with oil from the oil nozzle 24 used to drive the wheel 22, may be led back to the engine oil circuit of an internal combustion engine. The drainage outlet 29 in this embodiment is placed centrally on the lower end wall 6. In alternative embodiments, the drainage outlet 29 may be placed elsewhere on the lower end wall 6.

[0056] The gas outlet tube 28 of the centrifugal separator 1 is arranged through the stationary casing 2 and is configured to permit discharge of cleaned gas.

[0057] During operation of the centrifugal separator as shown in Fig. 1, the rotating member 7 is kept in rotation by the oil nozzle 24 supplying oil against the wheel 22. As an example, the rotational speed may be in the range of 7.500-12.000 rpm.

[0058] Contaminated gas, e.g., crankcase gas from the crankcase of an internal combustion engine, is supplied to the gas inlet 20 via conduit 18. This gas is conducted further into the central space 15 and from there into and through the interspaces 14 between the separation discs of the stack 9. Because of the rotation of the rotating member 7 the gas is brought to rotate, whereby it is pumped further on radially outwardly through gaps or interspaces 14.

[0059] During the rotation of the gas in the interspaces 14, solid or liquid particles such as oil suspended in the gas are separated therefrom. The particles settle on the insides of the conical portions 9b of the separation discs and slide or run after that radially outwardly thereon. When the particles and / or liquid drops have reached out to the radial outer edges of the separation discs of the stack 9, they are thrown away from the rotating member 7 and hit the inner surface 40 of the surrounding side wall 4. Separated oil particles may form a film on the inner surface of the stationary casing 2. From there, oil may be pulled by gravity downwardly to bottom end wall 6 and then leave the separation space 3 through the drainage outlet 29. For this purpose, the inner wall of the bottom end wall 6 may be tilted radially inwards, so that oil leaving the surrounding inner wall of the stationary casing 2 may be pulled by gravity towards drainage outlet 29. The path of the contaminants in the gas is schematically illustrated by arrows "D" in Fig. 1.

[0060] Cleaned gas freed from particles and exiting from the stack of separation discs 9 leaves the stationary casing 2 through the gas outlet tube 28. The path of the gas through the centrifugal separator 1 is schematically shown by arrows "C" in Fig. 1.

[0061] The centrifugal separator of Fig. 1 may also comprise a stationary insert 50, which is arranged in a lower portion of the stationary casing 2, and more precisely axially below the stack 9 of separation discs. The stationary insert 50 may be fastened to the surrounding side wall 4 of the stationary casing, and or / to lower end wall 6 via supporting legs (not shown). The stationary insert 50 comprises an outer annular wall member 51 and a central frustoconical cup member 53 with conically extending sidewalls. The lower part of the end plate 11 is arranged within the central cup member 53. The annular wall member 51 extends axially downwards from the outer edge of the cup member 53.

[0062] The stationary insert 50 forms a physical barrier between cleaned gas and separated contaminants prior to the cleaned gas and separated contaminants exit the stationary casing 2. For this purpose, the stationary insert 50 is arranged in the stationary housing 2 so as to form an annular vertical channel 52 for the separated contaminants between the inner surface 40 of the stationary casing 2 and the annular wall member 51 and a flow path for clean gas within the central cup member 53 towards the gas outlet 28. Thus, the annular vertical channel 52 may form an annular slit and, during use of the separator 1, may collect separated oil droplets that run downwards on the inner surface 40 of the surrounding side wall 4 under the action of gravity. Also, the action of a downwards spiralling gas flow may force the oil on the inner surface 40 downwards. The annular vertical channel 52 thus shields the separated liquid particles flowing down on inner wall 40 from the rotating gas, so as to decrease the risk of separated oil being pulled from the inner wall 40 on its way down to drainage outlet 29. The stationary insert 50 may be a moulded unit, preferably of a polymer or a plastic material.

[0063] As an alternative, the annular wall member 51 could also be left out, so that the stationary insert 50 is mainly defined by the conical cup member which could be sufficient to direct the gas flow in connection to the gas outlet.

[0064] Reference is now made to Fig. 2 illustrating a rotatable shaft 8 with a top disc 10 of a separation unit 16 indicated in Fig. 1. The top disc 10 has a frustoconical shape. The top disc 10 is fastened to the rotatable shaft 8 at an upper portion of the shaft 8. The top disc 10 comprises a flat portion 10a with through holes 150, and a conical portion 10b extending axially upwards and outwards in respect of the shaft 8, as also described in connection with Fig. 1. The outer shape and size of the top disc 10 may essentially correspond to the outer shape and size of separation discs 9', an example of which is shown in Fig. 3. The shaft 8 in Fig. 2 comprises a number of splines or strips 82 which extend axially along a length of the rotary shaft 8. The splines may extend a length corresponding to the axial extension of the disc stack 9 comprising a plurality of separation discs 9'. The top disc 10 is located outside but connected to the splines 82. The splines are used to prevent rotation of the discs in the disc stack in respect of the shaft 8 during the use of the centrifugal separator, i.e. when the rotary member 7 with the disc stack rotates (see Fig. 1). However, the splines could in other variants be omitted. The shape of the splines may vary, but in the cross-sectional view, each of the splines may include a tip portion 83 and one or more root portions 84, wherein only one of each is indicated with a reference number. The tip portion 83 may have a larger radial extension outwards from the shaft than other portions of the shaft 8. The tip portion 83 provides a free end to the spline. The root portions 84 are located radially inward of the tip portion 83. The root portions 84 may have a greater circumferential dimension than the tip portion 83. Generally, the disc 9' may be made of or comprise plastic material, glass fibres and / or metal. The material may be a composite material. The thickness of the disc may be for example about 0.5 mm but is not limited thereto and can be altered depending on the size and type of the separator.

[0065] Further with reference to Fig. 3, the separation disc 9' comprises the central aperture 80, which defines an inner periphery 91 of the disc 9'. The shape of the central aperture 80 and the consequently the shape of the inner periphery 91 of the disc 9' may at least partly correspond to the outer contour or shape of the shaft 8, which may comprise the splines 82 to prevent the movement of the discs in respect of the shaft during rotation of the rotating member 7 and the shaft 8. That is, the shape of the inner periphery 91 may correspond to the shape of the shaft with the splines 82 in a cross-section taken perpendicular to the axis through the rotary shaft 8. The inner periphery 91 comprises portions 90 that are arranged to be in contact with the shaft 8. During the use, these portions provide a contact surface of the disc 9' with the shaft 8.

[0066] As mentioned above, the fitting of the discs on the shaft can be either press fit or fit with certain play or movement, herein referred to as a play fit, due to the tolerances of the parts. During the assembly of the discs in high volumes a problem can be created due to the tolerances: sometimes it takes a lot of force to press the disc stack onto the shaft and sometimes no force at all is needed. A further problem is that the discs may not be equally centered towards the shaft, which leads to unbalanced disc stack. Therefore, it is advantageous that there is always a press fit between the discs in the stack and the shaft. The play fit increases a risk of movement which can cause wear on the shaft. The imbalance may cause vibrations and reduced lifetime. For example, a gap between the disc and the shaft may be from about 0.05 mm to 0.15 mm. If the gap is 0.05 mm and the disc stack weighs 500g, there may be an imbalance of 25g / mm, which is close to an allowed upper limit. The problem of play fit and / or imbalance can be solved by the flexible element as described below with reference to Fig. 4a, 4b and 5a-5c.

[0067] Reference is made to Fig. 4a-4c. In Fig. 4a, a view from above of an embodiment of a separation disc 9' is schematically shown. The disc 9' is has a circular cross-sectional shape. The disc 9' may be frustoconical and comprises a central aperture 80 and a plurality of through holes 150 for the passage of the gas to be cleaned. As mentioned above, the present invention aims to solve the problems above by providing one or more flexible elements 100 to respective one or more portions 90 at the inner periphery 91 of the disc. The portion 90 is arranged to be in contact with the shaft 8 when the discs 9' are assembled to the shaft 8. The flexible element 100 may be provided to the disc 9' in different ways.

[0068] Fig. 4b is an enlarged view of a centre portion of the disc 9' shown in Fig. 4a and shows a variant of the flexible element 100. The flexible element 100 can be formed by at least one slit or recess projecting radially outwards from the inner periphery 91 of the disc 9'. In the example shown in Fig. 4a, the flexible element 100 is formed between a pair of adjacent slits 101, 102.

[0069] As explained in connection with Fig. 2, the shaft 8 may comprise a number of axially and radially outwards in respect of the shaft protruding splines 82. The splines 82 are distributed evenly around the periphery of the shaft 8. As shown in Fig. 4a and in Fig. 4b, the separation disc 9' therefore comprises a corresponding number of mating spline recesses 92 to receive the splines 82 which aim to prevent the rotation of the disc in respect of the shaft 8. As shown in Fig. 4b, the flexible elements 100 are placed in between the spline recesses 92 around the inner periphery 91 of the separation disc 9' in the portions 90 of the inner periphery.

[0070] The splines 82 and thus the mating spline recesses 92 may comprise a tip portion 93 and a root portion 94, which may correspond to a tip portion 83 and root portion 84 of the splines. However, the splines and the recesses do not need to have completely mating shapes, as long as the splines can be fitted to the recesses. The root portion 94 in the disc 9' may comprise an additional intermediate root portion 94' as shown in Fig. 4b and 4c, but these root portions do not have to exactly mate with the root portion 84 of the spline 82. The root portions 94, 94' in the disc 9' can be connected by rounded transition sections.

[0071] In the shown examples in Fig. 4a-4c, the flexible element 100 is formed by two adjacent slits 101, 102. In Fig. 4c it is shown that the respective radial extension r4, r5 of the slits 101, 102 is smaller than the radial extension r1 of the tip portion 93 and larger than the respective radial extension r3 of the root portion 94 and the respective radial extension r4 of the root portion 94' of the disc 9'. Generally, the dimensions of the discs and the dimensions of the inner periphery 91, portions 90, the spline recesses 92 and the flexible elements 100 are adapted to the shaft 8 of the centrifugal separator.

[0072] To further ensure that a press fit for the disc is obtained, the flexible element 100 may have a longer radial extension towards the centre of the separation disc 9' than the other portions of the inner periphery 91 and thus form an end portion 100'. Since the end portion 100' has a longer extension towards the centre it will be in contact with the periphery of the shaft 8 when the disc 9' is attached to the shaft. The end portion 100' may further improve the press fit towards the shaft and improve the balance of the disc stack 9. The end portion 100' is schematically shown in Fig. 4c, in an enlarged detailed view of a central section of a separation disc 9' according to an embodiment. According to a variant shown in Fig. 4c, one of the slits transitions to the root portion of an adjacent spline recess 92, but in the variant shown in Fig. 4b, both of the slits 101, 102 extend from the portion 90, which is configured to be in contact with the shaft 8.

[0073] Reference is made to Fig. 5a-5c, which show enlarged central views of further embodiments of the invention. In Fig. 5a, a shaft 8 comprising three splines 82 evenly distributed around the periphery of the shaft is shown. The spline 82 comprises a tip portion 83 having a radial extension R1' and a root portion 84 having a radial extension R2', which is smaller than R1'. The tip and root portions 83, 84 of the spline 82 are fitted to corresponding spline recesses 92 of the disc 9' and have a tip portion 93 and a root portion 94'. The root portion 94 of the recess does not receive a corresponding root portion of the shaft in the shown embodiment.

[0074] In a similar way as in Fig. 4a-4c, the flexible element 100 is provided by two adjacent, radially outwards from the inner periphery 91 extending slits 101, 102, as shown in more detail in Fig 5b. The flexible element 100 may have a longer radial extension into the central aperture 80 than other parts of the inner periphery 91 and thereby an end portion 100' is provided to the flexible element 100. Fig. 5b also shows that the flexible element 100 is provided at a portion 90 of the inner periphery, which is in contact with the shaft 8, when the disc 9' is assembled to the shaft. In the shown embodiment of Fig. 5a and 5b, the shaft 8 comprises a protruding portion 85, which is arranged between two adjacent splines 82. The protruding portion 85 is arranged to be in contact with the end portion 100' of the flexible element 100. In this way the flexible element can press towards the protruding portion 85 and thereby the balancing of the disc stack can be further improved.

[0075] Fig. 5c shows a cross-sectional view in an axial direction of the disc 9' attached to the shaft 8. It can be seen that the flexible element has a radially tapering thickness towards the central aperture 80, whereby the thickness d1 decreases in radial direction to a thickness d2 at the end portion 100' of the flexible element and the thickness d2 is smaller than the thickness d1. The tapering shape is arranged on the top side of the disc. Thus, the flexibility of the flexible element 100 can be further improved.

[0076] In the shown examples of Fig. 4a-4c and 5a-5c, the separation disc 9' comprises at least three flexible elements 100 arranged around the inner periphery 91 of the disc 9'. To ensure that the discs are well balanced during the rotation, the flexible elements 100 are equally distributed around the inner periphery 91. The number of the flexible elements can vary and can be for example between 3 to 6.

[0077] The invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the claims set out below. The invention is not limited to the orientation of the axis of rotation (X) disclosed in the figures. The term "centrifugal separator" also comprises centrifugal separators with a substantially horizontally oriented axis of rotation. In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

[0078] Another solution disclosed herein is aimed for improving the balance of the rotor, i.e. the rotating member, in the centrifugal separator explained above in connection with Fig. 1. The balance can be improved by incorporating balancing elements, such as balls or stop screws, in the top and bottom discs of the rotating member. The balancing elements add weight to the top and bottom discs and could be of any suitable type. An example of the balancing elements is illustrated in Fig. 6, in which the rotating member 7 is schematically shown. The rotating member 7 comprises a top disc 10 and a lower end plate 11, and a disc stack 9 assembled around the rotating shaft 8 in between the top disc 10 and the lower end plate 11. The top disc 10 comprises a first cavity into which a ball 71 is pushed. In a similar way, the lower end plate 7 can comprise a second cavity into which a second ball 72 is pushed. The number of the cavities and balancing elements such as balls can be adapted to the needs in centrifugal separators, and in this way a robust way of balancing can be provided.

Claims

1. A centrifugal separator (1) for cleaning gas containing contaminants, said centrifugal separator (1) comprising: a stationary casing (2), enclosing a separation space (3) through which a gas flow is permitted; a gas inlet (20) extending through the stationary casing (2) and permitting supply of the gas to be cleaned to the stationary casing (2); a separation member (9') comprising a central aperture (80) defining an inner periphery (91) of the separation member (9'); a rotating member (7) arranged to rotate around an axis (X) of rotation in the separation space (3) and comprising a rotatable shaft (8) and a separation unit (16) comprising a plurality of the separation members (9') attached to the shaft, a drive member (22) for rotating the rotating member (7); a gas outlet (28) arranged through the stationary casing (2) and configured to permit the discharge of cleaned gas out from the stationary casing (2); and a drainage outlet (29) arranged in the stationary casing (2) and configured to permit discharge of liquid contaminants that have been separated from the gas out from the stationary casing (2); wherein at least a portion (90) of the inner periphery (91) of the separation member (9') is arranged in contact with the shaft (8) when attached to the shaft (8), and wherein the portion (90) is provided with one or more flexible elements (100).

2. The centrifugal separator (1) according to claim 1, wherein the flexible element is formed by at least one slit or recess (101; 102) projecting radially outwards from the portion (90).

3. The centrifugal separator (1) according to claim 2, wherein the flexible element is formed between a pair of adjacent slits or recesses (101, 102).

4. The centrifugal separator (1) according to any one of the preceding claims, wherein the flexible element (100) has a longer radial extension into the central aperture (80) than other parts of the inner periphery (91).

5. The centrifugal separator (1) according to any one of the preceding claims, wherein the flexible element (100) has a tapering thickness radially towards the central aperture.

6. The centrifugal separator (1) according to any one of the preceding claims, wherein each separation member (9') comprises at least three flexible elements (100) evenly distributed in the portion (90) around the inner periphery (91).

7. The centrifugal separator (1) according to any one of the preceding claims, wherein the shaft (8) comprises a number of evenly distributed axially and radially outwards in respect of the shaft protruding splines (82), and wherein the separation member (9') comprises a corresponding number of at least partially mating spline recesses (92) to receive the splines, and wherein the flexible elements (100) are placed in between the spline recesses in the portion (90) around the inner periphery of the separation member.

8. The centrifugal separator (1) according to claim 7, wherein the spline recesses comprise a tip portion (93) and a root portion (94; 94'), and wherein the radial extension of the flexible element (100) is smaller than the radial extension of the tip portion (93) and larger than the radial extension of the root portion (94; 94').

9. The centrifugal separator according to any one of the preceding claims, wherein the separation member is a separation disc (9') comprising a flat portion (9a), which extends perpendicularly to the axis of rotation, and a conical portion (9b), which extends axially and outwardly from the direction of the axis of rotation, and wherein the flat portion comprises the central aperture (80).

10. A separation member for a centrifugal separator according to any one of the preceding claims 1-9, wherein the separation member comprises a central aperture defining an inner periphery (91) of the separation member and is configured to be attached to a shaft (8) of the centrifugal separator via the central aperture (80), wherein at least a portion (90) of the inner periphery (91) is provided with one or more flexible elements (100).

11. The separation member of claim 10, wherein the flexible element (100) is formed by at least one slit or recess (101; 102) projecting radially outwards from the portion (90), or between a pair of adjacent slits or recesses (101; 102).

12. The separation member according to any one of the preceding claims 10 or 11, wherein the flexible element (100) has a longer radial extension into the central aperture (80) than other parts of the inner periphery (91).

13. The separation member according to any one of the preceding claims 10 to 12, wherein the flexible element (100) has a tapering thickness towards the central aperture.

14. The separation member according to any one of the preceding claims 10 to 13, wherein the separation member comprises at least three flexible elements (100) evenly distributed around the inner periphery (91) of the separation member.

15. The separation member according to any one of the preceding claims 10-14, wherein the separation member is a frustoconical disc comprising a circular flat portion comprising the central aperture (80), and a conical portion extending axially and radially outwards of the flat portion, and wherein the flat portion comprises a plurality of through holes (150), which are located radially outside of the flexible elements.

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