fig1 shows a centrifugal separator 1 according to a first embodiment of the invention . the centrifugal separator 1 comprises a rotor 2 situated uppermost on a vertical rotor shaft 3 . the rotor shaft 3 is journalled by an upper bearing ( a so - called top bearing 4 ) and a lower bearing ( a so - called bottom bearing 5 ) in a frame 6 . the rotor shaft 3 is thus arranged to support the rotor 2 for rotation about a vertical axis of rotation r in the frame 6 . a motor m is adapted to driving the rotor 2 about the axis of rotation r . the rotor 2 has a rotor wall 7 a , 7 b which surrounds an inner space with a separating chamber 8 in which the main centrifugal separation takes place . the inner space also comprises other spaces within the rotor , e . g . inlet passages for the mixture which is to be separated in the separation space 8 , and at least one outlet chamber for a separated component . the rotor wall is divided into a lower part 7 a and an upper part 7 b which are held together by a locking ring 9 . the lower part 7 a includes a planar wall portion 7 e positioned on a bottom portion of the rotor 2 . a compressible unit 10 comprising a stack of truncated conical separating discs 11 is situated in the inner space , centrally within the rotor . a more detailed description of the compressible unit 10 appears in specification wo 2008 / 111889 a1 . the stack of separating discs 11 is situated in the separating chamber 8 and arranged to form between mutually adjacent separating discs thin intermediate spaces in which the main separation takes place during operation of the centrifugal separator . fig1 shows schematically a small number of separating discs 11 with relatively large intermediate spaces between them . in practice , however , a large number of separating discs are stacked on one another , and the surfaces of the separating discs are provided with spacing elements to maintain the thin intermediate spaces between mutually adjacent separating discs . the rotor 2 comprises a hub 12 situated on the outside of the lower part 7 a of the rotor wall and outside the rotor &# 39 ; s inner space . the hub 12 is tubular and extends axially outwards / downwards from the lower part 7 a , and the tubular hub is configured to radially surround a portion of the rotor shaft 3 . both the tubular hub 12 and the rotor shaft 3 are cylindrical with circular cross - sections . a releasable clamping device 13 is arranged to connect the hub 12 to said portion of the rotor shaft 3 in a frictional fastening . the clamping device 13 is fitted in ( e . g ., slid axially into ) an annular space which is arranged radially between the hub 12 and the rotor shaft 3 . the clamping device 13 has a flange 17 f that has an outer sleeve 14 and an inner sleeve 15 extending axially therefrom . the outer sleeve 14 is expandable radially outwards and configured for frictional engagement with the hub 12 , and the inner sleeve 15 is expandable radially inwards and configured for frictional engagement with the rotor shaft 3 . an annular gap 16 extends axially between the outer sleeve 14 and the inner sleeve 15 , and a pressurizing screw 17 is provided for pressurization of a pressure medium in the gap 16 in order to expand the outer sleeve 14 and the inner sleeve 15 to a state in which they lock the hub 12 and the rotor shaft 3 . the pressurizing screw 17 may be slackened to reduce the pressure on the pressure medium so that the expandable sleeves 14 , 15 revert to their original shape and the fastening is thus undone so that the clamping device 13 the rotor shaft 3 and the rotor 2 are released from one another without axially separating the rotor shaft 3 from the rotor 2 , as shown in fig1 . as shown in fig3 , the hub 12 extends axially downward from the planar wall portion 7 e a distance d 1 sufficient to receive the inner sleeve 15 and the outer sleeve 14 . the hub 12 terminates at an unobstructed axial end 12 f that engages a portion 17 e of the flange 17 f of the clamping device 17 . the hub 12 has an unobstructed radially outward surfaces 12 x configured to provide access to the clamping device 17 whereby the clamping device 12 is operable from the outside of the rotor 2 in order to releasably connect the hub 12 to the rotor shaft 3 . the centrifugal separator in fig1 has an inlet comprising a duct 18 which extends axially through the rotor shaft 3 , and the rotor 2 has a connection 19 for the duct 18 . the connection 19 is disposed within the hub 12 and arranged to communicate with the separating chamber 8 via a number of liquid passages in the form of distribution ducts 20 which extend straight outwards radially from the connection 19 and lead into the separating chamber 8 . the distribution ducts 20 are disposed at equal spacing in the circumferential direction within the rotor 2 . the distribution ducts 20 form an angle of 90 ° with the axis of rotation r . consequently , the connection 19 within the hub 12 is arranged to change the direction of flow by 90 ° from the duct 18 to the distribution ducts 20 which are adapted to leading the flow straight outwards in a radial direction to the separating chamber 8 . the lower part 7 a of the rotor shown has a radially inner planar wall portion and a radially outer sloping and surrounding wall portion . the distribution ducts 20 are disposed along the underside of the compressible unit 10 which comprises the separating discs 11 . the distribution ducts 20 thus lead into the separating chamber 8 at the lower portion of the compressible unit 10 , and the direction of the liquid mixture is again changed by about 90 ° ( relative to the direction in the distribution ducts ) upwards in the separating chamber 8 containing the stack of separating discs 11 . an elastic sliding element 21 is provided within the rotor 2 to open and close a number of outlet apertures 22 which are disposed peripherally in the lower part 7 a of the rotor wall . the elastic sliding element 21 is also described in specification wo 96 / 41683 a1 . the sliding element 21 has a radially inner edge 23 connected to , and axially fixed relative to , the rotor 2 , and a radially outer edge portion 24 which is axially movable relative to the rotor 2 between open and closed states of the outlet aperture 22 by elastic deformation of the sliding element 21 . the elastic sliding element 21 is deformed ( operated ) by feeding a hydraulic liquid in / out to fill / empty a closing chamber disposed between the sliding element 21 and the lower part 7 a of the rotor wall . the rotor shaft 3 shown encloses a duct 25 for supply to the closing chamber of a liquid which by centrifugal force and consequent hydraulic pressure pushes the underside of the elastic sliding element 21 to a closed state of the outlet aperture 22 . the rotor shaft 3 encloses also a duct 26 for supply of liquid to open a discharge valve through which the liquid of the closing chamber is drained . emptying the closing chamber of liquid reduces the hydraulic pressure from the underside of the sliding element 21 , with the result that the sliding element is deformed elastically so that its radially outer edge portion 24 moves axially downwards and opens the outlet aperture 22 . such a procedure of opening the outlet apertures 22 is conducted in situations where the rotor 2 needs to be emptied of the sludge which accumulates over time in the radially outer portions of the separating chamber 8 . such a sliding element is particularly applicable to the present invention , since the elastic sliding element 21 needs no axial support within the rotor 2 . in the case of a conventional sliding element ( which is not elastically deformed ) the hub which extends axially inwards in the rotor is used to support an inner edge of the sliding element which is arranged to move axially upwards / downwards on the hub during closing / opening of the outlet apertures 22 . the hub 12 comprises an axial stop 27 for the rotor shaft 3 in the form of an annular abutment surface extending radially inwards and arranged to abut against the rotor shaft &# 39 ; s free end within the hub . this is a simple way to ensure correct axial positioning of the rotor shaft 3 relative to the hub 12 . the hub 12 comprises also an axial stop 28 for the clamping device 13 , in the form of a similar abutment surface extending radially and configured to abut against an end of the clamping device 13 , which end is within the hub 12 . correct axial positioning of the clamping device 13 relative to the hub 12 is thus ensured . an outlet chamber 29 for a separated liquid component is provided in the upper portion of the compressible unit 10 . the outlet chamber 29 communicates with the separating chamber 8 via outlet passages ( not shown ). the centrifugal separator 1 also comprises members ( not shown ) for discharging the liquid component out from the outlet chamber 29 and the rotor 2 . fig2 shows a centrifugal separator 1 ′ according to a second embodiment of the invention . it should be noted that constituent parts which have the same or similar functions are designated by the same reference signs in both embodiments . this centrifugal separator 1 ′ differs from the centrifugal separator according to fig1 inter alia in the configuration of the hub 12 ′. this hub 12 ′ takes the form of a recess in the lower part 7 a ′ of the rotor wall . a portion 7 e ′ of the lower part 7 a ′ is positioned on a bottom portion of the rotor 2 ′. the portion 7 e ′ of the lower part 7 a ′ has an inside surface 17 y and an outside surface 17 x spaced apart from one another by a predetermined distance d 2 . the recess is formed in a relatively thick rotor wall 7 a ′. the recess 12 ′ may therefore be made deep enough , with no need for the hub to extend axially inwards in the rotor 2 ′. the recess 12 ′ is configured to radially surround a portion of the rotor shaft 3 ′, and a clamping device 13 is provided to connect the hub to said portion of the rotor shaft 3 ′. this involves the clamping device 13 being fitted in an annular space arranged radially between the hub 12 ′ and the rotor shaft 3 ′. the clamping device 13 has in this embodiment the same configuration as in fig1 . the predetermined distance d 2 is of a magnitude sufficient to receive the inner sleeve 15 and the outer sleeve 14 , such that the outside surface 17 x defines an unobstructed axial end 12 f ′ that engages a portion of the flange 17 f of the clamping device 13 and the outside surface 17 x configures ( e . g ., slopes away from the recess as illustrated by sloped surfaces 82 and 83 ) an unobstructed area 80 to provide access to the clamping device 13 whereby the clamping device 13 is operable from the outside of the rotor 2 ′ in order to releasably connect the hub 12 ′ to the rotor shaft 3 ′. this centrifugal separator 1 ′ differs also in that the rotor 2 ′ has outlet apertures in the form of nozzles 22 ′ disposed peripherally in the rotor wall 7 a ′. such nozzles 22 ′ are adapted to continuous discharge of separated solid particles ( sludge ) from the separating chamber 8 ′. such a rotor 2 ′ is used in separation of liquid mixtures with relatively high concentrations of solid particles ( sludge ), viz . from about 6 % to 25 - 30 % ( by volume ). this rotor 2 ′ thus has nozzles 22 ′ arranged to be constantly open , with consequently no need for a discharge system with closing chamber and sliding element ( such as shown in fig1 ). nor is there any need for a duct to supply a hydraulic liquid to a closing chamber , or for a duct to supply a liquid to a discharge valve in order to empty the closing chamber of its liquid . as shown in fig2 , the centrifugal separator 1 ′ however comprises a similar inlet for the liquid mixture , in the form of a duct 18 ′ extending axially through the rotor shaft 3 ′ with a similar connection 19 ′ for the duct 18 ′ within the hub 12 ′. in this embodiment example , the connection 19 ′ is likewise arranged to communicate with the separating chamber 8 ′ via a number of liquid passages in the form of distribution ducts ( not shown ) which extend outwards radially from the connection 19 ′ and lead into the separating chamber 8 ′. these distribution ducts are likewise disposed at equal spacing in the circumferential direction within the lower portion of the separating chamber 8 ′ and extend in principle straight radially outwards from the connection 19 ′. however , the lower part 7 a ′ of the rotor shown slopes somewhat downwards in a radially outward direction , and the distribution ducts do in principle follow this slope or angle relative to the axis of rotation r . as shown in fig2 , the change of direction is about 100 °. these distribution ducts lead into the lower portion of the separating chamber 8 ′, and the liquid mixture again changes direction by about 100 ° ( relative to the direction in the distribution ducts ) upwards in the separating chamber 8 ′. this separating chamber 8 ′ is likewise provided with a stack of separating discs ( not shown in fig2 ). the hub 12 ′ shown in fig2 comprises likewise an axial stop 27 ′ for the rotor shaft 3 ′ in the form of an abutment surface with a radial extent , arranged to abut against the rotor shaft &# 39 ; s free end within the hub 12 ′. the hub 12 ′ likewise comprises an axial stop 28 ′ for the clamping device 13 , in the form of an abutment surface extending radially inwards and configured to abut against an end of the clamping device 13 , which end is within the hub 12 ′. in this embodiment example , the respective axial stops 27 ′ and 28 ′ for the rotor shaft 3 ′ and for the clamping device 13 constitute a single radial surface . the invention is not limited to the embodiment examples described but may be varied and modified within the scope of the claims set out below . according to a further embodiment , the hub may for example take the form of a shaft journal which extends radially outwards from the rotor wall . the shaft journal and the rotor shaft may therefore be connected together via a fastening which in this case takes the form of a shaft coupling . the centrifugal separator may thus be so arranged that the journal is provided with a first bearing member ( e . g . a so - called top bearing for the rotor ) mounted on the shaft journal , and the rotor shaft is supported by a second bearing member ( e . g . a so - called bottom bearing for the rotor ) in a frame . this means that the whole rotor , including the shaft journal with the first bearing member mounted on it , can be fitted in the frame with said rotor shaft and the second bearing member . the shaft journal and the rotor shaft are thereafter aligned and connected together by the shaft coupling , which is thus situated between the first bearing member and the second bearing member .