WO1998045047A1

WO1998045047A1 - A centrifugal separator having sludge screw conveyors

Info

Publication number: WO1998045047A1
Application number: PCT/SE1998/000550
Authority: WO
Inventors: Ingvar HÄLLGREN; Hans Martin; Berndt NYSTRÖM
Original assignee: Alfa Laval Ab
Priority date: 1997-04-04
Filing date: 1998-03-26
Publication date: 1998-10-15
Also published as: SE9701225D0; AU6860398A

Abstract

In a centrifugal separator, which has a centrifugal rotor (2), the separation chamber (20) of which has a larger axial extension than the radial extension thereof, several sludge conveyor screws (29) are distributed around the rotational axis (4) of the centrifugal rotor and extend substantially parallel therewith adjacent to the surrounding wall (17) of the rotor. A centrifugal separator of this kind is used to separate from a liquid, which has a predetermined density, particles suspended in the liquid and having a larger density than the liquid. According to the invention the density of at least part of each one of said sludge conveyor screws is chosen such that it is in the area of 50-150 %, preferably 90-110 %, of said predetermined density of the liquid. Further, a sludge conveyor screw has different density per unit of length of its axial extension in the separation chamber (20).

Description

A centrifugal separator having sludge screw conveyors

The present invention relates to centrifugal separators for freeing a liquid from solid particles suspended therein and having a density larger than that of the liquid. Particularly concerned is a centrifugal separator of the kind described for instance in US-A-3,685,721 and comprising

a rotor that has a centre axis and comprises two axially spaced end walls and a surrounding wall, which is arranged axially between the end walls and surrounds together with these a separation chamber, the axial extension of which is substantially larger than its radial extension, the rotor further having at least one inlet for introducing said liquid into the separation chamber, at least one liquid outlet for discharging liquid having been freed from particles and at least one sludge outlet for discharging solids having been separated from the liquid,

a driving means for rotation of the rotor around the centre axis,

- at least two sludge conveyor screws extending substantially axially in the separation chamber close to the surrounding wall of the rotor and rotatable relative thereto for axial transportation of separated particles along the surrounding wall to said sludge outlet and

- a transmission means for rotation of the sludge conveyor screws relative to the surrounding wall of the rotor during rotation of the rotor. In operation of a centrifugal separator of the kind here concerned said sludge conveyor screws rotate at a large speed around the centre axis of the rotor. This creates an undesired load on the bearings, which support the sludge conveyor screws in the rotor and which are intended to facilitate the rotation of the sludge conveyor screws around their own centre axes. An undesired load on said bearings is also created by forces which strive at subjecting the sludge conveyor screws to bending between said bearings as a consequence of the rotation of the sludge conveyor screws around the centre axis of the rotor.

It is desirable, therefore, with an unloading of said bearings from forces which emanate from the rotation of the sludge conveyor screws around the centre axis of the rotor, and one object of the present invention is to make possible such an unloading.

According to the invention this object may be achieved when a liquid having a predetermined density is to be freed from particles having a larger density than the liquid by means of a centrifugal separator of the above described kind in that the density of each one of said sludge conveyor screws, at least such parts of the sludge conveyor screws which are situated far from the sludge outlet and therefore during operation of the rotor will be substantially completely surrounded by said liquid, is chosen such that it is in the area of 50-150 % of said predetermined density of the liquid. Preferably, the density of said parts of the sludge conveyor screw is chosen such that it is in the area of 60-110 %, and if possible in the area of 75-95 %, of the density of the liquid.

In this way it can be achieved that the sludge conveyor screws during the rotation of the rotor will be kept substantially suspended in the liquid, in which they are present in the separation chamber, and that, thereby, the above-mentioned bearings are loaded as little as possible. Furthermore, the bending of the sludge conveyor screws will thereby be minimized.

Suitably the density of each one of the sludge conveyor screws is chosen such that in such first parts of the sludge conveyor screw, which are situated in the vicinity of the said sludge outlet of the rotor, it is larger than in such second parts of the sludge conveyor screw, which are situated farther from the sludge outlets. Preferably, the density is chosen such that in said first parts it is in the area of 100-125 % and in said second parts in the area of 75-100 % of said predetermined density of said liquid.

By "density" of the whole or part of a sludge conveyor screw is meant in this connection the whole or a part, respectively, of the weight of the sludge conveyor screw divided by the displacing volume. In other words, if the sludge conveyor screw or parts thereof contains a cavity, which will not be filled by liquid, when the centrifugal separator is in operation, the volume of the material, from which the conveyor screw is made, as well as the volume of said cavity shall be included in said displacing volume.

The present invention also concerns a centrifugal separator having a feature that makes possible that the bearings of the sludge conveyor screws can be unloaded to an optimum during operation of the rotor.

According to the invention, in a centrifugal separator of the initially defined kind, each of the sludge conveyor screws has a different density per unit of length of its axial extension in the separation chamber. Hereby, said density per unity of length of different parts of the sludge conveyor screw may be chosen with regard to whether these parts during the operation of the rotor are expected to be situated in a more or less thick layer of separated solid particles or in an area of the separation chamber that is substantially free from separated solid particles. As was initially indicated, said particles have a larger density than the liquid. Depending upon the amount of separated particles in different parts of the separation chamber the sludge conveyor screw, thus, may be formed so that the best possible balance is achieved between the forces by which the respective parts of the sludge conveyor screw are pressed radially outwardly as a consequence of the rotation of the rotor and the hydraulic forces, which act on these parts in the opposite direction, i.e. towards the centre of the rotor, as a consequence of centrifugally generated pressure in the separated particles and the liquid, respectively, in the separation chamber.

Each of the sludge conveyor screws preferably has a larger density per unit of length in an area situated adjacent to said sludge outlet than in an area situated at a larger distance from the same sludge outlet. For instance, each one of the sludge conveyor screws may be produced from a relatively light material, e.g. plastic, and be formed with at least one cavity that may contain a relatively heavy material, e.g. metal. One or more cavities of this kind may, if desired, contain two or more bodies of different materials having different densities.

In a possible embodiment of the invention every sludge conveyor screw has a first cavity which contains a relatively heavy material and which is situated in a first area in the vicinity of said sludge outlet, and a second cavity without any such relatively heavy material, which cavity is situated in a second area remote from said sludge outlet, a part of the sludge conveyor screw situated between said first and second areas being without a cavity. In another embodiment of the invention the sludge conveyor screw may have substantially the same outer dimensions along the whole of its length, part of the sludge conveyor screw having a cavity making this part of the sludge conveyor screw lighter per unit of length than other parts thereof. For instance, the sludge conveyor screw may have a central bore having a continuously or stepwise varying diameter along the whole or part of the length of the sludge conveyor screw.

The invention is described in the following with reference to the accompanying drawing, in which

Figure 1 shows in a longitudinal section a centrifugal separator in which the invention may be used.

Figure 2 shows a section along the line II-II in figure 1.

Figure 3 shows an enlarged part of figure 1.

Figure 4 shows a cross-section through the centrifugal rotor that is shown in figure 1 , taken between two adjacent separation discs.

Figure 5 shows a separation disc of the kind shown in figure 4.

Figure 6 shows (a) a sludge conveyor screw having a central cavity, and (b) the same conveyor screw provided with bodies of different material in the central cavity.

Figure 1 shows a centrifugal separator for freeing a liquid from solid particles suspended therein and having a density larger than that of the liquid. The centrifugal separator comprises a frame 1 , a rotor 2 that is rotatably supported by and within the frame 1 and a motor 3 that is also supported by the frame but on its outside and that is adapted for rotation of the rotor 2 about a vertical rotational axis 4 coinciding with the centre axis of the rotor. For this purpose the motor 3 supports on its drive shaft a belt pulley 5 and the rotor supports at its lower part a belt pulley 6. A drive belt 7 surrounds both the belt pulleys 5 and 6.

The motor 3 supports on its drive shaft a further belt pulley 8, which together with an annular transmission member 9 arranged coaxially with the rotor 2 and rotatable relative thereto is surrounded by a further drive belt 10.

The rotor 2, which is rotatably suspended in the frame by means of a lower bearing 11 and an upper bearing 12, comprises a lower end wall 13 and an upper end wall 14. The lower end wall 13 is formed in one piece with a shaft 15 that extends all the way from the area of the lower bearing 11 past the lower end wall 13 up to the upper end wall 14 and through a central opening therein. By means of a locking member 16 the upper end wall 14 is fixed relative to the shaft 15 at least in a way such that it cannot move in a direction away from the lower end wall 13. By means not shown the end wall 14 is also fixed relative to the shaft 15 in its circumferential direction. The rotor further comprises a circular-cylindrical surrounding wall 17, which extends axially between the end walls 13 and 14, coaxially surrounding the shaft 15 spaced therefrom. The surrounding wall 17 shall not transfer any substantial axial force and, therefore, is not firmly connected with the end walls 13 and 14. Instead, the surrounding wall 17 abuts at its ends through annular gaskets 18 and 19 radially against the respective end walls 13 and 14. However, the surrounding wall 17 is formed such that it can take up very large forces in its circumferential direction and, therefore, is reinforced by carbon or glass fibres extending substantially in said circumferential direction.

The end walls 13, 14 and the surrounding wall 17 surrounds a separation chamber 20 within the rotor. The separation chamber 20 surrounds the shaft 15 and has an axial extension substantially larger than its radial extension.

Within the separation chamber 20 coaxially with the rotor a stack of frusto- conical separation discs 21 is arranged between the end walls 13, 14. By means of spacing members the separation discs are maintained at some axial distance from each other.

The upper end wall 14 on its outside is connected with an inlet member 22 forming a central, vertical inlet channel 23. This inlet channel 23 communicates at its upper end with an inlet 24 for liquid to be treated within the rotor, and it branches off at its lower end in several branch channels 25. The branch channels 25 which are formed partly in the inlet member 22 and partly in the upper end wall 14 open into the upper part of the separation chamber 20 radially about half-way between the central rotor shaft 15 and the surrounding wall 17.

The lower end wall 13 has a number of branch channels 26 intended for liquid having been treated in the rotor. The branch channels 26 start from the lower part of the separation chamber 20 and extend to a common outlet channel 27 which in its turn extends further centrally in the rotor shaft 15 to and out through the lower end thereof. For solid particles having been separated from said liquid in the separation chamber 20 the rotor has several outlet channels 28 extending from the uppermost part of the separation chamber axially through the upper end wall 14 at the radially outermost portion thereof. For transportation of particles separated in the separation chamber 20 to the outlet channels 28 the rotor comprises a conveyor screw device including several conveyor screws 29. These extend axially through the separation chamber 20 close to the surrounding wall 17 and are evenly distributed around the rotor shaft 15. Each conveyor screw 29 is journalled at its ends in the respective end walls 13, 14 and is rotatable around its centre axis relative to the rotor walls during the rotation of the rotor around its rotational axis 4.

For the rotation of the conveyor screws 29 relative to the rotor the rotor shaft 15 supports immediately below the lower end wall 13 the above said annular member 9. This member 9 surrounds the rotor shaft 15 and is adapted by means of the motor 3 through the driving belt 10 to be rotated around the rotational axis 4 of the rotor at a speed different than that of the rotor. The annular member 9 has on its outside axially above the driving belt 10 a gear ring 30 engaging several bearing members 31 evenly distributed around the rotor shaft 15. Each bearing member 31 is connected with a conveyor screw 29 and forms part of a slide bearing through which the conveyor screw 29 is journalled in the lower end wall 13. The bearing member 31 and its co-operation with the gear ring 30, the end wall 13 and the conveyor screw 29 is described more in detail below with reference to figure 3.

At its upper end each conveyor screw 29 is journalled in the upper end wall 14 by means of a pin 32. Radially outside the uppermost end portion of each conveyor screw 29 and axially in the area of the uppermost separation disc 21 there is delimited in the separation chamber 20 by the upper end wall 14 a space or a pocket 33 which extends radially outwardly from said end portion of the conveyor screw to a level radially outside the inside of the surrounding wall 17. At the radially outermost part of each such pocket one of the afore-mentioned outlet channels 28 is situated. Figure 2 shows a section through the upper end wall 14, taken along the line II-II in figure 1. Between adjacent pockets 33 parts of the end wall 14 form filler pieces which prevent communication between the sludge pockets in the circumferential direction of the rotor.

For closing and intermittent uncovering of the outlet channels 28 the rotor is provided with an axially movable slide 34. By means of springs 35 arranged between the inlet member 22 and the slide 34 the slide 34 is kept pressed against the outside of the upper end wall 14, axial protuberances of the slide 34 abutting sealingly against the end wall around the openings of the respective outlet channels 28.

Between the slide 34 and the end wall 14 there is delimited radially inside the outlet channels 28 a so called opening chamber 36, which via channels through the inlet member 22 and the rotor shaft 15 communicates with the interior of a narrow tube 37 extending axially upwardly through and out of the inlet member 22 to an upper pressure source of air (not shown). Through supply of pressurised air to said opening chamber 36 the slide 34 during rotation of the rotor may be caused to move axially upwardly against the action of springs 35, so that the outlet channels 28 are uncovered.

Figure 3 shows in an enlarged scale part of figure 1. It can thus be seen from figure 3 that the bearing member 31 is supported by a transmission shaft in the form of a short tap 38 extending within a bore 39 in the end wall 13 and connected with the conveyor screw 29. The bearing member 31 with its tap 38 as well as the conveyor screw 29 may be made of plastic. A sealing device 40 is arranged in the bore 39 and is adapted to seal between the tap 38 and the end wall 13.

The bearing member 31 has a tubular surrounding part 41 , which on its outside is provided with cogs 42 and on its inside has a slide bearing surface 43. The cogs 42 engage the gear ring 30 of the annular member 9, and the slide bearing surface 43 co-operates with a corresponding slide bearing surface 44 formed on an annular protuberance 45 on the outside of the end wall 13. The protuberance 45 which may have a surface layer of a ceramic material provided with said slide bearing surface 44 surrounds the opening of the bore 39 in the end wall 13, and the two co-operating slide bearing surfaces 43, 44 thus have a substantially larger circumference than the bore 39.

On the outside of the rotor end wall 13 there is mounted an annular further wall 46. This confines between itself and the outside of the end wall 13 an annular transmission chamber 47, which is closed radially outwardly but open radially inwardly towards the rotor shaft 15. The chamber 47 during operation of the rotor may be filled with liquid, e.g. water, trough a supply pipe 48 and is intended always to be filled during operation of the rotor. A radially inner edge 49 of the further wall 46 may serve as an overflow outlet for liquid being supplied to the chamber 47.

As can be seen from figure 3, a substantial part of the bearing member 31 will be present during operation of the rotor in liquid present in the chamber 47. This liquid has two purposes; firstly, it shall operate as a lubricator between the slide bearing surfaces 43 and 44, when the conveyor screw 29 rotates relative to the rotor, and secondly it shall create an hydraulic force acting on the bearing member 31 in a direction towards the rotational axis 4 of the rotor. The hydraulic force counteracts the centrifugal force to which the bearing member is subjected during its rotation around the rotational axis 4 of the rotor and, thereby, acts unloading on the slide bearing formed by the bearing member 31 and the protuberance 45 on the rotor end wall 13.

Figure 4 shows a cross section through the rotor 2 in figure 1. The section is taken between two adjacent conical separation discs 21. Figure 5 shows a single separation disc 21 of the kind also shown in figure 4.

From figure 4 it can be seen further that the centre shaft 15 of the rotor has axial grooves 50 forming axial flow paths radially inside the separation discs 21 for liquid which has been freed from solid particles in the separation chamber 20. The separation discs 21 are supported radially in all directions by the shaft 15. Figure 4 also shows that the separation discs 21 are provided with several conventional spacing members 51 which are evenly distributed around the shaft 15 and keeps the separation discs at a desired distance from each other.

The separation discs 21 have several through-holes 52, each being placed between two adjacent spacing members 51 , the holes being axially aligned with corresponding holes in the other separation discs 21. The holes 52 form axial so called distribution channels 53 (fig 1 ) through the stack of separation discs axially aligned with the openings of the previously mentioned branch channels 25 in the upper rotor end wall 14. A filler piece 54 extends inside the surrounding wall 17 around the stack of separation discs 21 and the conveyor screws 29. This filler piece has recesses for the conveyor screws 29 and extends in the areas between the conveyor screws radially inwardly forming axially extending ridges which have contact with the separation discs 21. The separation discs which are relatively thin and may be made of plastic receive during operation of the rotor, therefore, radial support from the said ridges of the filler piece 54. Between the conveyor screws the filler piece is so formed that solid particles which during operation of the rotor are separated from the liquid and move radially outwardly between the separation discs will slide on the hills 55 of said ridges in a direction towards the conveyor screws and in between their threads.

Figure 5 shows that each separation disc has both recesses 56 for the conveyor screws 29 and recesses 57 for the filler piece 54.

The filler piece 54 may be formed in one piece, suitably from plastic or some other relatively light material. Alternatively, it may be composed of several annular elements having the cross-sectional form shown by the filler piece 54 in figure 4, or from several straight axially extending elements, which are evenly distributed around the rotational axis of the rotor. For covering of the interspaces which may remain between annular or straight elements of this kind a lining 58 of plastic or other material, as shown in figure 6, may be arranged on the inside of the filler piece 54.

Figure 6a shows a conveyor screw 29 having a circular-cylindrical core 58 and a sludge conveyor thread 59 formed in one piece therewith on its outside. The sludge conveyor thread 59 has the same pitch and the same radial extension along the whole axial extension of the conveyor screw 29, but of course this is not necessary.

Within the core 58 a cylindrical cavity 60 extends axially and centrally from the one (upper) end of the core to but not through the bearing member 31 at the other (lower) end of the core. In the upper end of the cavity there is inserted the previously mentioned pin 32, by means of which the conveyor screw 29 is journalled in the upper end wall 13 of the centrifugal rotor (see figure 1 ). The bearing member 31 and the core 58 preferably are formed in one piece of the same material, e.g. plastic.

Figure 6b shows the same conveyor screw 29 as figure 6a but equipped with three bodies 61-63 placed after each other in the cavity 60. The bodies 61-63 have different densities, differing from the density of the material in the core 58 and the bearing member 31. The largest density has the body 61 , which will be situated closest to the sludge outlet 28 of the centrifugal rotor (see figure 1 ) and which, thereby, to a great part will be situated immersed in separated sludge during operation of the centrifugal rotor. The lowest density has the body 63 which during operation of the rotor will be surrounded substantially completely by liquid having been freed from solid particles.

As can be seen from figure 6b the bodies 61-63 may have different axial extensions. Thus, the density of the conveyor screw per unit of length of its axial extension may be adapted with regard to desire. This means that different parts of the conveyor screw may be given a density which is the same or in the same order of magnitude as the density of the medium in which the different parts will be situated during operation of the centrifugal rotor. Thereby, the conveyor screw may be allowed to float or be suspended in said medium or media, so that the bearings of the conveyor screw will be loaded as little as possible as a consequence of the rotation of the centrifugal rotor, and the conveyor screw subjected to forces as small as possible, striving at bending the conveyor screw.

In a certain embodiment of the invention each sludge conveyor screw may have to be somewhat lighter than the separated liquid in its parts situated closest to the lower end wall 13 in the rotor (see figure 1 ) and somewhat heavier than the separated liquid in its parts situated closest to the upper end wall 14. The reason why the sludge conveyor screw may need to be somewhat lighter than the liquid in its parts closest to the lower rotor end wall 13 is that in the area of this rotor end wall it supports parts of the sealing means 40 (see figure 3) and that it may be difficult to construct these parts of the sealing means in a way such that they become as light as adjacent parts of the sludge conveyor screw. The sludge conveyor screw, thus, may have to have a particularly large floating ability in this relevant area in order to compensate for the weight of said parts of the sealing means.

The centrifugal separator described above with reference to the figures 1-6 operates briefly in the following manner.

After the rotor 2 has been caused to rotate around its rotational axis 4 and the conveyor screws 29 simultaneously have been caused to rotate around their respective rotational axes relative to the rotor 2, a suspension of liquid and particles dispersed therein and having a density larger than that of the liquid is supplied through the inlet 24. The suspension is conducted through the channels 23 and 25 to the distribution channels 53 in the stack of separation discs 21. From respective holes 52 in the separation discs 21 the suspension flows out into the spaces between the separation discs 21 and is conducted between adjacent spacing members 51 to the axial channels 50 at the rotor shaft 15 (see fig 4).

On the way between the holes 52 and the channels 50 said particles are separated from the liquid and they slide along the underside of the separation discs back radially outwardly towards the conveyor screws 29. The inclined surfaces 55 on the filler piece 54 (see figure 4) makes the particles collecting exactly in the areas of the conveyor screws 29.

In the areas of the conveyor screws 29 the particles form a sludge which by the conveyor screws is transported axially within the separation chamber 20 towards the upper rotor end wall 14. Sludge of this kind is gradually collected in the interspaces between the threads of the conveyor screws, which will be filled by sludge in an increasing degree the closer the sludge outlet the interspaces are situated. Thus, it may happen that substantially no sludge at all is present in the interspaces between the threads closest to the lower rotor end wall 14, whereas the interspaces between the threads closest to the upper end wall 13 are substantially filled by sludge.

In the end wall 14 each conveyor screw 29 extends through a short cylindrical bore which opens into a pocket 33 (see figures 1 and 2). The sludge formed in the separation chamber 20 is thus transported through these bores and out into the pockets 33. From here the sludge is discharged intermittently through the outlet channels 28 in that these are uncovered by means of the slide 34 at desired time intervals. The slide can be actuated by supply of pressurised air to the opening chamber 36. When sludge is supplied through said bore to a pocket 33, displaced liquid is con- ducted away from the pocket 33 to the separation chamber 20 in the part of the bore that is situated closest to the centre axis 4 of the rotor, where a narrow slot is formed between the threads of the conveyor screw 29 and the wall of the bore. The liquid having been freed from particles is con- ducted out of the rotor below the lower end wall 13 through the channels 26 and 27.

The slide 34 alternatively may be adapted automatically to uncover the outlet channels 28, when a predetermined resistance against turning of the conveyor screws 29 is obtained, indicating that a certain amount of sludge has been collected in the separation chamber.

Since the inlet for suspension is arranged at one end and the outlet for liquid at the opposite end of the separation chamber 20, and the outlet for sludge is arranged at the inlet end of the separation chamber, good prerequisites are obtained for liquid leaving the separation chamber to be substantially free from particles.

In the embodiments of the invention having been described above and shown in the drawings the conical separation discs are arranged in a way such that they face with their apex ends upwardly. If desired, they may instead be arranged with their apex ends facing downwardly towards the outlet for cleaned liquid. Then, the end walls 13 and 14 are suitably formed in a corresponding way, the lower end wall 13 then forming an upwardly open funnel which with its apex portion forms the central outlet for cleaned liquid. If the lower end wall 13 is formed in this way a complete emptying of the separation chamber 20 is facilitated after a finished separating operation.

Claims

1. A method of separating from a liquid having a predetermined density solid particles having a larger density than the liquid by means of a centrifugal separator comprising

a rotor (2) which has a centre axis (4) and comprises two axially spaced end walls (13,14) and a surrounding wall (17), which is arranged axially between the end walls (13,14) and together there- with surround a separation chamber (20), the axial extension of which is substantially larger than the radial extension thereof, the rotor (2) further having at least one inlet (23-25) for introducing said liquid into the separation chamber (20), at least one liquid outlet (26,27) for discharging liquid having been freed from particles, and at least one sludge outlet (28) for discharge of particles having been separated from the liquid,

a driving means (3) for rotation of the rotor (2) around the centre axis (4),

at least two sludge conveyor screws (29) which extend substantially axially in the separation chamber (20) in the vicinity of the surrounding wall (17) of the rotor and are rotatable relative thereto for axial transportation of separated particles along the surrounding wall to said sludge outlet (28) and

a transmission means (9,30,31 ,38) for rotation of the sludge conveyor screws (29) relative to the surrounding wall (17) of the rotor during rotation of the rotor, c h a r a c t e r i z e d i n that the density of a sludge conveyor screw (29), at least such parts thereof which are situated far from said sludge outlet (28) and, thus, during operation of the rotor will be substantially completely surrounded by said liquid, is chosen such that it is in the area 50-150 % of said predetermined density of the liquid.

2. A method according to claim 1 , in which the density of said parts of the sludge conveyor screw (29) is chosen so that it is in the area of 60-110 % of the predetermined density of the liquid.

3. A method according to claim 1 , in which the density of said parts of the sludge conveyor screw (29) is chosen so that it is in the area of 75-95 % of said predetermined density of the liquid.

4. A method according to any one of claims 1-3, in which the density of the sludge conveyor screw (29) is chosen so that it is larger in such first parts thereof which are situated in the vicinity of said sludge outlet (28) than in such second parts of the same which are situated farther from the sludge outlets (28).

5. A method according to claim 4, in which the density of the sludge conveyor screw (29) is chosen so that in said first parts it is in the area of 100-125 % and in said second parts is in the area of 75-100 % of said predetermined density of the liquid.

6. A centrifugal separator for freeing a liquid from solid particles suspended therein and having a density larger than that of the liquid, comprising a rotor (2) which has a centre axis (4) and comprises two axially spaced end walls (13,14) and a surrounding wall (17), which is arranged axially between the end walls (13,14) and together there- with surround a separation chamber (20), the axial extension of which is substantially larger than the radial extension thereof, the rotor further having at least one inlet (23-25) for introducing said liquid in the separation chamber (20), at least one liquid outlet (26,27) for discharging liquid having been freed from particles, and at least one sludge outlet (28) for discharging particles having been separated from the liquid,

a driving means (3) for rotation of the rotor (2) around the centre axis (4),

at least two sludge conveyor screws (29) which extend axially in the separation chamber (20) in the vicinity of the surrounding wall (17) of the rotor and are rotatable relative thereto for axial transportation of the separated particles along the surrounding wall (17) to said sludge outlet (20) and

a transmission means (9,30,31 ,38) for rotation of the sludge conveyor screws (29) relative to the surrounding wall (17) of the rotor during the rotation of the rotor,

c h a r a c t e r i z e d i n

that a sludge conveyor screw (29) has different density per unit of length of its axial extension in the separation chamber (20).

7. A centrifugal separator according to claim 6, in which the conveyor screw (29) has substantially the same outer dimensions along substantially the whole of its length and part of the sludge conveyor screw (29) has a cavity making this part of the sludge conveyor screw lighter per unit of length than other parts thereof.

8. A centrifugal separator according to claim 6 or 7, in which the sludge conveyor screw (29) has a central bore, which has a varying diameter along the whole or part of the length of the conveyor screw.

9. A centrifugal separator according to claim 6, in which the sludge conveyor screw (29) is made of a relatively light material, e.g. plastic, and is formed with at least one cavity containing a relatively heavy material, e.g. metal.

10. A centrifugal separator according to claim 9, in which the sludge conveyor screw (29) has a first cavity, which contains said relatively heavy material and which is situated in a first area in the vicinity of said sludge outlet (28), and a second cavity without such a relatively heavy material, which is situated in a second area remote from said sludge outlet (28), a part of the sludge conveyor screw (29) situated between said first and second areas being without a cavity.

11. A centrifugal separator according to any one of the claims 6-10, in which the sludge conveyor screw (29) has a larger density per unit of length in an area situated adjacent said sludge outlet than in an area situated at a larger distance from the same sludge outlet.