EXCHANGEABLE SEPARATION INSERT

Abstract

An exchangeable separation insert for a centrifugal separator includes a rotor casing enclosing a separation space in which a stack of separation discs is arranged, and first and second stationary portions. A feed inlet supplies a fluid mixture to the separation space. The insert includes a light phase outlet and a heavy phase outlet. The feed inlet is arranged at a first axial end of the rotor casing. One of the light phase outlet and heavy phase outlet is arranged at a second axial end. A first rotatable seal seals and connects the feed inlet and a second rotatable seal seals and connects one of the light phase outlet and heavy phase outlet.

Claims

1. An exchangeable separation insert for a centrifugal separator comprising: a rotor casing enclosing a separation space in which a stack of separation discs is arranged, the rotor casing being arranged to rotate around an axis of rotation; a first stationary portion and a second stationary portion, said rotor casing being axially arranged between said first and said second stationary portions; a feed inlet for supply of a fluid mixture to be separated to said separation space; a light phase outlet for discharge of a separated phase of a first density, and a heavy phase outlet for discharge of a separated phase of a second density higher than said first density, wherein said feed inlet is arranged at a first axial end of said rotor casing (2), and wherein one of said light phase outlet and heavy phase outlet is arranged at a second axial end, opposite the first axial end, of the rotor casing; a first rotatable seal for sealing and connecting said feed inlet to a stationary inlet conduit in said first stationary portion; and a second rotatable seal for sealing and connecting one of said light phase outlet and heavy phase outlet to a stationary outlet conduit in said second stationary portion.

2. The exchangeable separation insert according to claim 1, wherein said light phase outlet is arranged at the first axial end and the heavy phase outlet is arranged at the second axial end, and wherein said second rotatable seal is for sealing and connecting said heavy phase outlet to a stationary outlet conduit in said second stationary portion.

3. The exchangeable separation insert according to claim 2, wherein said first rotatable seal also is arranged for sealing and connecting said light phase outlet to a stationary outlet conduit in said first stationary portion.

4. The exchangeable separation insert according to claim 1, wherein the rotor casing is free of any further outlets for separated phases.

5. The exchangeable separation insert according to claim 1, wherein the heavy phase collection space of the separation space extends from a first axial position to a second axial position, and wherein the inner diameter of the separation space continuously increases from said first to said second axial position.

6. The exchangeable separation insert according to claim 1, further comprising at least one outlet conduit arranged for transporting a separated heavy phase from a radially outer position of the separation space to the heavy phase outlet.

7. The exchangeable separation insert according to claim 6, wherein said at least one outlet conduit is arranged at the axially upper portion of the separation space.

8. The exchangeable separation insert according to claim 1, wherein the first stationary portion is arranged at an axial distance that is less than 20 cm from the heavy phase collection space of said separation space.

9. The exchangeable separation insert according to claim 1, wherein the stationary inlet conduit is arranged at the rotational axis.

10. The exchangeable separation insert according to claim 1, wherein the stationary outlet conduit for the separated heavy phase is arranged at the rotational axis.

11. The exchangeable separation insert according to claim 1, wherein the rotor casing is arranged to be solely externally supported by external bearings.

12. The exchangeable separation insert according to claim 1, wherein the outer surface of the rotor casing comprises a first and second frustoconical portion defining the separation space therein, wherein the first frustoconical portion has an opening angle that is larger than the opening angle of the second frustoconical portion, and wherein the imaginary apex of the first and second frustoconical portions both point in the same axial direction along the rotational axis.

13. The exchangeable separation insert according to claim 12, wherein the opening angle of the second conical portion is such that the outer surface of second frustoconical portion forms an angle α relative the rotational axis that is less than 10 degrees.

14. The exchangeable separation insert according to claim 1, wherein the stack of separation discs comprises frustoconical separation discs.

15. The exchangeable separation insert according to claim 14, wherein said frustoconical separation discs are arranged with the imaginary apex pointing towards said first stationary portion.

16. The exchangeable separation insert according to claim 15, wherein the first stationary portion is arranged at an axial distance that is less than 20 cm from the heavy phase collection space of said separation space.

17. The exchangeable separation insert according to claim 14, wherein said imaginary apex of the axially lowermost separation disc that is closest to the first end of the insert is arranged less than 10 cm from the first stationary portion.

18. The exchangeable separation insert according to claim 1, wherein the exchangeable separation insert forms a pre-assembled insert configured to be handled as one unit.

19. A method for separating at least two components of a fluid mixture which are of different densities comprising the steps of: a) providing a centrifugal separator comprising the exchangeable separation insert according to claim 1; b) supplying said fluid mixture to the feed inlet to said separation space; c) discharging a separated light phase from said separation space via the light phase outlet; and d) discharging a separated heavy phase from said separation space via the heavy phase outlet.

20. The method as claimed in claim 19, wherein the fluid mixture is a cell culture mixture, such as a mammalian cell culture mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] 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.

[0080] FIG. 1 is a schematic outer side view of an exchangeable separation insert according to the present disclosure.

[0081] FIG. 2 is a schematic section of a centrifugal separator comprising an exchangeable insert according to the present disclosure.

[0082] FIG. 3 is a schematic section view of an exchangeable separation insert according to the present disclosure.

DETAILED DESCRIPTION

[0083] FIG. 1 shows an outer side view of an exchangeable separation insert 1 according to the present disclosure. The insert 1 comprises a rotor casing 2 arranged between a first, lower stationary portion 3 and a second, upper stationary portion 4, as seen in the axial direction defined by rotational axis (X). The insert 1 comprises the first stationary portion 3 which is arranged at the lower axial end 5 of the insert 1. The insert 1 comprises the second stationary portion 4 which is arranged at the upper axial end 6 of the insert 1.

[0084] The feed inlet is in this example arranged at the axial lower end 5, and the feed is supplied via a stationary inlet conduit 7 arranged in the first stationary portion 3. The stationary inlet conduit 7 is arranged at the rotational axis (X). The first stationary portion 3 further comprises a stationary outlet conduit 9 for the separated liquid phase of lower density, also called the separated liquid light phase.

[0085] There is further a stationary outlet conduit 8 arranged in the upper stationary portion 4 for discharge of the separated phase of higher density, also called the liquid heavy phase. Thus, in this embodiment, the feed is supplied via the lower axial end 5, the separated light phase is discharged via the lower axial end 5, whereas the separated heavy phase is discharged via the upper axial end 6.

[0086] The outer surface of the rotor casing 2 comprises a first 10 and second 11 frustoconical portion. The first frustoconical portion 10 is arranged axially below the second frustoconical portion 11. The outer surface is arranged such that the imaginary apex of the first 10 and second 11 frustoconical portions both point in the same axial direction along the rotational axis (X), which in this case is axially down towards the lower axial end 5 of the insert 1.

[0087] Furthermore, the first frustoconical portion 10 has an opening angle that is larger than the opening angle of the second frustoconical portion 11. The opening angle of the first frustoconical portion may be substantially the same as the opening angle of a stack of separation discs contained within the separation space 17 of the rotor casing 2. The opening angle of the second frustoconical portion 11 may be smaller than the opening angle of a stack of separation discs contained within the separation space of the rotor casing 2. As an example, the opening angle of the second frustoconical portion 11 may be such that the outer surface forms an angle α with rotational axis that is less than 10 degrees, such as less than 5 degrees. The rotor casing 2 having the two frustoconical portions 10 and 11 with imaginary apexes pointing downwards allows for the insert 1 to be inserted into a rotatable member 30 from above. Thus, the shape of the outer surface increases the compatibility with an external rotatable member 30, which may engage the whole, or part of the outer surface of the rotor casing 2, such as engage the first 10 and second 11 frustoconical portions.

[0088] There is a lower rotatable seal arranged within lower seal housing 12 which separates the rotor casing 2 from the first stationary portion 3 and an upper rotatable seal arranged within upper seal housing 13 which separates the rotor casing 2 from the second stationary portion 4. The axial position of the sealing interface within the lower seal housing 12 is denoted 15c, and the axial position of the sealing interface within the upper seal housing 13 is denoted 16c. Thus, the sealing interfaces formed between such stationary part 15a, 16a and rotatable part 15b, 16b of the first 15 and second 16 rotatable seals also form the interfaces or border between the rotor casing 2 and the first 15 and second 16 stationary portions of the insert 1.

[0089] There are further a seal fluid inlet 15d and a seal fluid outlet 15e for supplying and withdrawing a seal fluid, such as a cooling liquid, to the first rotatable seal 15 and in analogy, a seal fluid inlet 16d and a seal fluid outlet 16e for supplying and withdrawing a seal fluid, such as a cooling liquid, to the second rotatable seal 16.

[0090] Shown in FIG. 1 is also the axial positions of the separation space 17 enclosed within the rotor casing 2. In this embodiment, the separation space is substantially positioned within the second frustoconical portion 11 of the rotor casing 2. The heavy phase collection space (17c) of the separation space 17 extends from a first, lower, axial position 17a to a second, upper, axial position 17b. The inner peripheral surface of the separation space 17 may form an angle with the rotational axis (X) that is substantially the same as angle α, i.e. the angle between the outer surface of the second frustoconical portion 11 and the rotational axis (X). The inner diameter of the separation space 17 may thus increase continuously from the first axial position 17a to the second axial position 17b. Angle α may be less than 10 degrees, such as less than 5 degrees.

[0091] The exchangeable separation insert 1 has a compact form that increases the maneuverability and handling of the insert 1 by an operator. As an example, the axial distance between the separation space 17 and the first stationary portion 3 at the lower axial end 5 of the insert may be less than 20 cm, such as less than 15 cm. This distance is denoted d1 in FIG. 1 and is in this embodiment the distance from the lowest axial position 17a of the heavy phase collection space (17c) of the separation space 17 to the sealing interface 15c of the first rotatable seal 15. As a further example, if the separation space 17 comprises a stack of frustoconical separation discs, the frustoconical separation disc that is axially lowest in the stack and closest to the first stationary portion 3, may be arranged with the imaginary apex 18 positioned at an axial distance d2 from the first stationary portion 3 that is less than 10 cm, such as less than 5 cm. Distance d2 is in this embodiment the distance from the imaginary apex 18 of the axially lowermost separation disc to the sealing interface of the first rotatable seal 15.

[0092] FIG. 2 shows a schematic drawing of the exchangeable separation insert 1 being inserted within centrifugal separator 100, which comprises a stationary frame 30 and a rotatable member 31 that is supported by the frame by means of supporting means in the form of an upper and lower ball bearing 33a, 33b. There is also a drive unit 34, which in this case is arranged for rotating the rotatable member 31 around the axis of rotation 31 via drive belt 32. However, other driving means are possible, such as an electrical direct drive.

[0093] The exchangeable separation insert 1 is inserted and secured within rotatable member 31. The rotatable member 31 thus comprises a through hole with an inner surface for engaging with the outer surface of the rotor casing 2. That is, the rotor casing 2 of the insert 1 is secured within the rotatable member 31. The first and second stationary portions 3, 4 extend out of the rotatable member 31 and are secured in the centrifugal separator 100 such that they remain stationary during use of the centrifugal separator 100.

[0094] After mounting of the insert 1, the upper and lower ball bearings 33a, 33b are both positioned axially below the separation space 17 within the rotor casing 2 such that the cylindrical portion 14 of the outer surface of the rotor casing 2 is positioned axially at the bearing planes. The cylindrical portion 14 thus facilitates mounting of the insert within at least one large ball bearing. The upper and lower ball bearings 33a, 33b may have an inner diameter of at least 80 mm, such as at least 120 mm.

[0095] Further, as seen in FIG. 2, the insert 1 is positioned within rotatable member 31 such that the imaginary apex 18 of the lowermost separation disc is positioned axially at or below at least one bearing plane of the upper and lower ball bearings 33a, 33b.

[0096] Moreover, the separation insert is mounted within the separator 1 such that the axial lower part 5 of the insert 1 is positioned axially below the supporting means, i.e. the upper and lower bearings 33a, 33b. The rotor casing 2 is in this example arranged to be solely externally supported by the rotatable member 31. The separation insert 1 is further mounted within the separator 100 to allow easy access to the inlet, outlets and rotatable seals from the outside of the insert 1.

[0097] FIG. 3 shows a schematic illustration of cross-section of an embodiment of exchangeable separation insert 1 of the present disclosure. The insert 1 comprises a rotor casing 2 arranged to rotate around rotational axis (X), a first, lower stationary portion 3, and a second, upper stationary portion 4. The rotor casing 1 is arranged between the first stationary portion 3 and the second stationary portion 4. The first stationary portion 3 is thus arranged at the lower axial end 5 of the insert, whereas the second stationary portion 4 is arranged at the upper axial end 6 of the insert 1.

[0098] The feed inlet 20 is in this example arranged at the axial lower end 5, and the feed is supplied via a stationary inlet conduit 7 arranged in the first stationary portion 3. The stationary inlet conduit 7 may comprise a tubing, such as a plastic tubing. The stationary inlet conduit 7 is arranged at the rotational axis (X) so that the material to be separated is supplied at the rotational centre. The feed inlet 20 is for receiving the fluid mixture to be separated.

[0099] The feed inlet 20 is in this embodiment arranged at the apex of an inlet cone 10a, which on the outside of the insert 1 also forms the first frustoconical outer surface 10. There is further a distributor 24 arranged in the feed inlet for distributing the fluid mixture from the inlet 24 to the separation space 17.

[0100] The separation space 17 comprises a radially outer heavy phase collection space 17c that extends axially from a first, lower axial position 17a to a second, upper axial position 17b. The separation space further comprises a radially inner space formed by the interspaces between the separation discs of the stack 19.

[0101] The distributor 24 has in this embodiment a conical outer surface with the apex at the rotational axis (X) and pointing toward the lower end 5 of the insert 1. The outer surface of the distributor 24 has the same conical angle as the inlet cone 10a. There is further a plurality of distributing channels 24a extending along the outer surface for guiding the fluid mixture to be separated continuously axially upwards from an axially lower position at the inlet to an axially upper position in the separation space 17. This axially upper position is substantially the same as the first, lower axial position 17a of the heavy phase collection space 17c of the separation space 17. The distribution channels 24a may for example have a straight shape or a curved shape, and thus extend between the outer surface of the distributor 24 and the inlet cone 24a. The distribution channels 24 may be diverging from an axially lower position to an axially upper position. Furthermore, the distribution channels 24 may be in the form of tubes extending from an axially lower position to an axially upper position.

[0102] However, the distribution channels 24a may also be arranged to supply the liquid or fluid to be separated to the separation space at a radial position that is within the stack of separation discs, e.g. by axial distribution openings in the distributor and/or the stack of separation discs. Such openings may form axial distribution channels within the stack.

[0103] There is further a stack 19 of frustoconical separation discs arranged coaxially in the separation space 17. The separation discs in the stack 19 are arranged with the imaginary apex pointing to the axially lower end 5 of the separation insert, i.e. towards the inlet 20. The imaginary apex 18 of the lowermost separation disc in the stack 19 may be arranged at a distance that is less than 10 cm from the first stationary portion 3 in the axial lower end 5 of the insert 1. The stack 19 may comprise at least 20 separation discs, such as at least 40 separation discs, such as at least 50 separation discs, such as at least 100 separation discs, such as at least 150 separation discs. For clarity reasons, only a few discs are shown in FIG. 1. In this example, the stack 19 of separation discs is arranged on top of the distributor 24, and the conical outer surface of the distributor 24 may thus have the same angle relative the rotational axis (X) as the conical portion of the frustoconical separation discs. The conical shape of the distributor 24 has a diameter that is about the same or larger than the outer diameter of the separation discs in the stack 19. Thus, the distribution channels 24a may thus be arranged to guide the fluid mixture to be separated to an axial position 17a in the separation space 17 that is at a radial position P.sub.1 that is outside the radial position of the outer circumference of the frustoconical separation discs in the stack 19.

[0104] The heavy phase collection space 17c of the separation space 17 has in this embodiment an inner diameter that continuously increases from the first, lower axial position 17a to the second, upper axial position 17b. There is further an outlet conduit 23 for transporting a separated heavy phase from the separation space 17. This conduit 23 extends from a radially outer position of the separation space 17 to the heavy phase outlet 22. In this example, the conduit is in the form of a single pipe extending from a central position radially out into the separation space 17. However, there may be at least two such outlet conduits 23, such as at least three, such as at least five, outlet conduits 23. The outlet conduit 23 has thus a conduit inlet 23a arranged at the radially outer position and a conduit outlet 23b at a radially inner position, and the outlet conduit 23 is arranged with an upward tilt from the conduit inlet 23a to the conduit outlet 23b. As an example, the outlet conduit may be tilted with an upward tilt of at least 2 degrees, such as at least five degrees, such as at least ten degrees, relative the radial plane.

[0105] The outlet conduit 23 is arranged at an axially upper position in the separation space 17, such that the outlet conduit inlet 23a is arranged for transporting separated heavy phase from the axially uppermost position 17b of the separation space 17. The outlet conduit 23 further extends radially out into the separation space 17 so that outlet conduit inlet 23a is arranged for transporting separated heavy phase from the periphery of the separation space 17, i.e. from the radially outermost position in the separation space at the inner surface of the separation space 17.

[0106] The conduit outlet 23b of the stationary outlet conduit 23 ends at the heavy phase outlet 22, which is connected to a stationary outlet conduit 8 arranged in the second, upper stationary portion 4. Separated heavy phase is thus discharged via the top, i.e. at the upper axial end 6, of the separation insert 1.

[0107] Furthermore, separated liquid light phase, which has passed radially inwards in the separation space 17 through the stack of separation discs 19, is collected in the liquid light phase outlet 21 arranged at the axially lower end of the rotor casing 2. The liquid light phase outlet 21 is connected to a stationary outlet conduit 9 arranged in the first, lower stationary portion 3 of the insert 1. Thus, separated liquid light phase is discharged via the first, lower, axial end 5 of the exchangeable separation insert 1.

[0108] The stationary outlet conduit 9 arranged in the first stationary portion 3 and the stationary heavy phase conduit 8 arranged in the second stationary portion 4 may comprise tubing, such as plastic tubing.

[0109] There is further a lower rotatable seal 15, which separates the rotor casing 2 from the first stationary portion 3, arranged within lower seal housing 12 and an upper rotatable seal, which separates the rotor casing from the second stationary portion 4, arranged within upper seal housing 13. The first 15 and second 16 rotatable seals are hermetic seals, thus forming mechanically hermetically sealed inlet and outlets.

[0110] The lower rotatable seal 15 may be attached directly to the inlet cone 10a without any additional inlet pipe, i.e. the inlet may be formed at the apex of the inlet cone directly axially above the lower rotatable seal 15. Such an arrangement enables a firm attachment of the lower mechanical seal at a large diameter to minimize axial run-out.

[0111] The lower rotatable seal 15 seals and connects both the inlet 20 to the stationary inlet conduit 7 and seals and connects the liquid light phase outlet 21 to the stationary liquid light phase conduit 9. The lower rotatable 15 seal thus forms a concentric double mechanical seal, which allows for easy assembly with few parts. The lower rotatable seal 15 comprises a stationary part 15a arranged in the first stationary portion 3 of the insert 1 and a rotatable part 15b arranged in the axially lower portion of the rotor casing 2. The rotatable part 15b is in this embodiment a rotatable sealing ring arranged in the rotor casing 2 and the stationary part 15a is a stationary sealing ring arranged in the first stationary portion 3 of the insert 1. There are further means (not shown), such as at least one spring, for bringing the rotatable sealing ring and the stationary sealing ring into engagement with each other, thereby forming at least one sealing interface 15c between the rings. The formed sealing interface extends substantially in parallel with the radial plane with respect to the axis of rotation (X). This sealing interface 15c thus forms the border or interface between the rotor casing 2 and the first stationary portion 3 of the insert 1. There are further connections 15d and 15e arranged in the first stationary portion 3 for supplying a liquid, such as a cooling liquid, buffer liquid or barrier liquid, to the lower rotatable seal 15. This liquid may be supplied to the interface 15c between the sealing rings.

[0112] In analogy, the upper rotatable seal 16 seals and connects the heavy phase outlet 22 to the stationary outlet conduit 8. The upper mechanical seal may also be a concentric double mechanical seal. The upper rotatable seal 16 comprises a stationary part 16a arranged in the second stationary portion 4 of the insert 1 and a rotatable part 16b arranged in the axially upper portion of the rotor casing 2. The rotatable part 16b is in this embodiment a rotatable sealing ring arranged in the rotor casing 2 and the stationary part 16a is a stationary sealing ring arranged in the second stationary portion 4 of the insert 1. There are further means (not shown), such as at least one spring, for bringing the rotatable sealing ring and the stationary sealing ring into engagement with each other, thereby forming at least one sealing interface 16c between the rings. The formed sealing interface 16c extends substantially in parallel with the radial plane with respect to the axis of rotation (X). This sealing interface 16c thus forms the border or interface between the rotor casing 2 and the second stationary portion 4 of the insert 1. There are further connections 16d and 16e arranged in the second stationary portion 4 for supplying a liquid, such as a cooling liquid, buffer liquid or barrier liquid, to the upper rotatable seal 16. This liquid may be supplied to the interface 16c between the sealing rings.

[0113] Furthermore, FIG. 3 shows the exchangeable separation insert in a transport mode. In order to secure the first stationary portion 3 to the rotor casing 2 during transport, there is a lower securing means 25 in the form of a snap fit that axially secures the lower rotatable seal 15 to the cylindrical portion 14 of rotor casing 2. Upon mounting the exchangeable insert 1 in a rotating assembly, the snap fit 25 may be released such that the rotor casing 2 becomes rotatable around axis (X) at the lower rotatable seal.

[0114] Moreover, during transport, there is an upper securing means 27a,b that secures the position of the second stationary portion 4 relative the rotor casing 2. The upper securing means is in the form of an engagement member 27a arranged on the rotor casing 2 that engages with an engagement member 27b on the second stationary portion 4, thereby securing the axial position of the second stationary portion 4. Further, there is a sleeve member 26 arranged in a transport or setup position in sealing abutment with the rotor casing 2 and the second stationary portion 4. The sleeve member 26 is further resilient and may be in the form of a rubber sleeve. The sleeve member is removable from the transport or setup position for permitting the rotor casing 2 to rotate in relation to the second stationary portion 4. Thus, the sleeve member 26 seals radially against the rotor casing 2 and radially against the second stationary portion 4 in the setup or transport position. Upon mounting the exchangeable insert 1 in a rotating assembly, the sleeve member may be removed and an axial space between engagement members 27a and 27b may be created in order to allow rotation of the rotor casing 2 relative the second stationary portion 4.

[0115] The lower and upper rotatable seals 15,16 are mechanical seals, hermetically sealing the inlet and the two outlets.

[0116] During operation, the exchangeable separation insert 1, inserted into a rotatable member 31, is brought into rotation around rotational axis (X). Liquid mixture to be separated is supplied via stationary inlet conduit 7 to the inlet 20 of the insert, and is then guided by the guiding channels 24 of the distributor 24 to the separation space 17. Thus, the liquid mixture to be separated is guided solely along an axially upwards path from the inlet conduit 7 to the separation space 17. Due to a density difference the liquid mixture is separated into a liquid light phase and a liquid heavy phase. This separation is facilitated by the interspaces between the separation discs of the stack 19 fitted in the separation space 17. The separated liquid heavy phase is collected from the periphery of the separation space 17 by outlet conduit 22 and is forced out via the heavy phase outlet 22 arranged at the rotational axis (X) to the stationary heavy phase outlet conduit 8. Separated liquid light phase is forced radially inwards through the stack 19 of separation discs and led via the liquid light phase outlet 21 out to the stationary light phase conduit 9.

[0117] Consequently, in this embodiment, the feed is supplied via the lower axial end 5, the separated light phase is discharged via the lower axial end 5, whereas the separated heavy phase is discharged via the upper axial end 6.

[0118] Further, due to the arrangement of the inlet 20, distributor 24, stack 19 of separation discs and the outlet conduit 23 as disclosed above, the exchangeable separation insert 1 is de-aerated automatically, i.e. the presence of air-pockets is eliminated or decreased so that any air present within the rotor casing is forced to travel unhindered upwards and out via the heavy phase outlet. Thus, at stand-still, there are no air pockets, and if the insert 1 is filled up through the feed inlet all air may be vented out through the heavy phase outlet 22. This also facilitates filling the separation insert 1 at standstill and start rotating the rotor casing when liquid mixture to be separated or buffer fluid for the liquid mixture is present within the insert 1.

[0119] As also seen in FIG. 3, the exchangeable separation insert 1 has a compact design. As an example, the axial distance between the imaginary apex 18 of the lowermost separation disc in the stack 19 may be less than 10 cm, such as less than 5 cm, from the first stationary portion 3, i.e. less than 10 cm, such as less than 5 cm, from the sealing interface 15c of the lower rotatable seal 15.

[0120] 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.