INLET DEVICE IN A CENTRIFUGAL SEPARATOR

Abstract: A centrifugal separator has a rotor with an inlet device comprising a stack of annular discs (10), and an inlet member (8) serves to supply a liquid mixture to be separated into a receiving chamber (12) formed within the disc stack. The liquid mixture flows in thin layers through passages formed between the discs and by friction between the discs and the mixture is accelerated to the rotational speed of the rotor. The inlet member (8) is arranged for liquid to enter the receiving chamber (12) in a way such that a continuous liquid phase is maintained between mixture present in the supply member (8) and mixture already supplied and present within the receiving chamber (12).

Description: This invention relates to centrifugal separators. A very old problem encountered with continuous centrifugal separation of two or more components from a liquid mixture is that of accelerating the mixture to the rotational speed it is to have in the separa­ tion chamber of the centrifuge rotor in such a way that it does not cause difficulties for the subsequent separation. The problem, more closely defined, is to prevent the mixture under acceleration being subjected to too large shearing forces, for instance by turbu­ lence, or being subjected to splitting since that results in one or more of the mixture components being disrupted to an undesired degree. Many different solutions of this problem have been suggested since the invention of centrifugal separators of the kind here in question. Thus it has been suggested for instance that the mixture should be given a certain rotational movement while in a stationary supply device and before it is transferred to the rotor. There have also been proposed members of various designs placed within the rotor and intended to pro­ vide gradual acceleration of the incoming mixture on its way to the rotor separation chamber. None of the solutions proposed so far has elimin­ ated the problem, however, and it still remains to a substantial extent. A solution to the problem suggested in 1940, but which does not seem to have enjoyed any wide practical appliance, is described in the US-A-­ 2,302,381. Shown therein is a cetrifugal separator comprising a rotor forming a separation chamber, and supply means for supplying mixture to be separated and having an opening located centrally within the rotor, the rotor having an inlet arrangement including several annular discs coaxial with the rotor and forming a central receiving chamber for mixture supp­ lied through the supply means, and passages connecting the central receiving chamber with the rotor separat­ ing chamber being formed between the discs. More particularly a stationary supply pipe extends from below into a rotor having a vertical axis of rotation. The end of the supply pipe is below the central receiving chamber and has an axially directed opening which is strongly throttled. Upon supply of mixture through the supply pipe there is formed by the throttled opening a jet which passes axially through the whole of the receiving chamber and impinges against a conical deflection member rotating with the rotor. The jet is deflected by the deflect­ ion member radially towards the annular discs in order to flow through the passages therebetween. According to the US patent specification 2,302, 381 this inlet arrangement is said to give the result that the mixture supplied will be rapidly accelerated to the speed of the rotor without being subjected to violent shocks. The annular discs are said to bring the mixture rapidly by friction to rotate with the same speed as the rotor without the mixture having to strike against any radially extending wings with surfaces moving perpendicular to the flow direction of the mixture. As already mentioned above, not even this proposed inlet arrangement has found any wide practical applic­ ation in spite of the alleged advantageous effect of the annular discs. The aim of the present invention is to provide an inlet device which comprises acceleration discs of the same general form as the inlet arrangement accord­ ing to the US-A-2,302,381, but which is substantially improved as regards the gentleness of the treatment of a mixture supplied to the centrifuge rotor. A centrifugal separator according to the invention is characterised in that - the receiving chamber at an area along the axial length thereof communicates with a channel for conducting gas away therefrom, - the opening of the supply member is so posi­ tioned that the radially inwardly open ends of several of said passages are axially between said opening and said area of the receiving chamber, - means are arranged to maintain at the opening of the supply member a body of liquid extending through at least some of said passages during opera­ tion of the rotor, and - the supply member is so arranged that said opening is located within said liquid body during operation of the rotor, and liquid mixture supplied through the supply member forms a liquid phase con­ tinuous with said liquid body. This invention is based on the realisation that annular discs arranged in a centrifuge rotor in the manner shown in the US-A-2,302,381 has a gentle effect on a mixture accelerated between the discs to the speed of the rotor. However, the invention is also based on the recognition that with the inlet arrange­ ment of the US patent specification supply of liquid to the central receiving chamber within the annular discs cannot be performed in a way which is also gentle on the mixture. Both the strong throttling of the supply pipe opening and the impact of the jet formed thereby against the conical deflection member will cause a heavy turbulence and splitting of the components in the mixture. This undesired effect is of such severity that this known inlet arrangement, seen as a whole, is not any more advantageous than other known inlet arrangements. The prerequisites for a substantially improved separation result are destroyed by the turbulent supply of mixture to the central receiving chamber. With an inlet arrangement according to the inven­ tion the supply member for mixture is, during opera­ tion of the rotor, kept partly immersed in liquid already supplied to the rotor. This is a prerequisite for entering mixture not to split up when it enters the rotor. It has proved that relative motion between mixture already supplied and the supply member itself will not create any substantial shearing forces in the supplied mixture. By the invention the contact of the supplied mixture with air or other gases in the rotor centre is reduced to a minimum. Primarily, the invention is intended to be used in cases where the supply member is stationary, i.e. non-rotatable. Nevertheless, the invention is also applicable to separators with a supply member which for one reason or another is rotatable. As in the known inlet arrangement according to US-A-2,302,381 the annular discs of the inlet device according to the invention are preferably entirely planar. However, even non-planar, for instance frusto-­ conical discs may be employed. If the discs are frusto-conical, the passages therebetween may be used also for pre-separation of the component mixture being accelerated therein. The invention may be used irrespective of the orientation of the centrifuge rotor axis and irrespec­ tive of the direction in which mixture is supplied into the rotor. In the first place, however, the invention is intended for a centrifuge rotor having a vertical rotational axis and a supply member extending downwardly from above into the rotor. In a preferred embodiment of the invention the upper part of the central receiving chamber then communicates with the channel for leading away gas, the supply member extending through and having its opening situ­ ated below this upper part of the receiving chamber. Preferably, the supply member extends entirely through the receiving chamber, so that its opening is situated below this chamber. Thereby the opening of the supply member may be kept immersed in liquid even if a supply flow of liquid to the rotor is very small. At a relatively small supply flow of mixture through the supply member liquid flows through only those passages closest to the supply member opening, the remainder of the passages being only partly filled with mixture with the infilled portions thereof closest to the receiving chamber and containing gas forming part of the receiving chamber communicating with the gas venting channel. At a relatively large supply flow of mixture substantially more of the passages and a larger part of the receiving chamber will be filled by liquid and, thus, the pumping effect of the discs is correspondingly greater. A corresponding change of the pumping effect of the inlet device is obtained with variations in the counter pressure met by the flow of mixture after it has passed through the inlet device. During normal operation of the centrifuge rotor there is thus preferably maintained a free liquid surface within the receiving chamber radially inside the annular acceleration discs. To assist a clear understanding of the invention some embodiments are described in more detail below with reference to the accompanying drawings, in which: Figure 1 shows schematically, in axial cross-­ section, a centrifuge rotor according to the inven­ tion; Figure 2 is a similar view of a second embodiment; and Figure 3 is a simplified representation, in axial section, of a further embodiment. In the centrifuge rotor of Figure 1 a rotor body 1 is supported at the upper end of a vertical drive shaft 2. Within the rotor body there is formed a sep­ aration chamber 3 containing a conventional set of frusto-conical separation discs 4. A central member within the rotor has a tubular upper part 5 and a frusto-conical lower part 6. Bet­ ween the lower part 6 and the upper end wall of the rotor body 1 the separation discs 4 are kept in place in the separation chamber 3. (In practice the said upper end wall is formed separate from the rest of the rotor body and is kept together therewith axially by threads or the like.) Extending axially through the set of separation discs 4 are several channels 7 formed by aligned holes in the separation discs. Extending axially downwardly into the rotor body 1 is a stationary supply pipe 8 for conducting a mix­ ture of components to be separated into the rotor. The pipe 8 extends axially through the central member 5, 6 in the rotor and has an opening 9 in the lower part of the interior of the rotor body interior. Below the frusto-conical lower part of the central member 5, 6 there is arranged a pile of coaxial plane annular discs 10. These discs are supported and kept axially spaced from each other by radially and axially extending wings 11 placed substantially radially out­ side the discs 10 and distributed around the rotor axis. Otherwise there are no spacing means between the discs 10 so the passages therebetween are sub­ stantially annular. Centrally within the pile of discs 10 there is formed a receiving chamber 12 in which the opening 9 of the supply pipe 8 is situated. The upper part of the space around the discs 10, which is divided into separate compartments by the wings 11, communicates directly with the separation chamber 3 in axial alignment with the channels 7 through the set of separating discs 4. The radially inner free edge 13 of the upper end wall of the rotor body serves as an overflow outlet from the separation chamber 3 during operation of the rotor. The annular channel 14 defined between the suplly pipe 8 and the member 5, 6 communicates the upper part of the central receiving chamber 12 with the atmosphere surrounding the rotor body. While the rotor (including all the parts shown in Figure 1 except the supply pipe 8) is rotating, a liquid mixture of components to be separated is supplied through the pipe 8. In the receiving chamber 12 and in the uppermost passages between the discs 10 there is formed a free liquid surface of a coherent liquid body extending from the interior of the pipe 8, into the receiving chamber and on through the passages between the lowermost discs 10. During the operation of the rotor the pipe 8 is thus partly sub­ merged in liquid present in the rotor. The mixture entering the receiving chamber 12 flows in very thin layers through a larger or smaller number of passages between the discs 10. In these passages the mixture is brought substantially to the same rotational speed as the rotor by the friction between the discs and the mixture. When the mixture reaches the wings 11, it has substanti­ ally the same speed as they have and it is directed thereby upwardly into the separation chamber 3. The space around the discs 10 communicates with the separation chamber 3 in the area of the uppermost discs 10, whereas the opening 9 of the inlet pipe 8 is situated in the area of the lowermost discs 10. This arrangement ensures a continuous throughflow of the whole space around the discs 10, even if incom­ ing mixture does not flow through all of the disc interspaces. In the separation chamber a relatively heavy component of the mixture is separated from a rela­ tively light component. It is presumed for continu­ ous operation of the rotor that the relatively light separated component is in liquid form, so that it can flow radially inwards through the passages bet­ ween the separation discs 4. The relatively heavy component may be in a liquid form or be solids. The separated heavy com­ ponent collects in the radially outermost part of the separation chamber. The inner free edge 13 of the upper end wall of the rotor forms an overflow outlet from the separa­ tion chamber 3 for the separated light liquid com­ ponent. Thereby the edge 13 also constitutes one of the means necessary to maintain, for a given supply flow of liquid into the rotor, the above men­ tioned free liquid surface in the receiving chamber 12, such that the supply pipe 8 will remain partly immersed in liquid. In Figure 1 there are shown (with full lines) both the free liquid surface formed in the separation chamber 3 during operation, and the free liquid surface formed in the receiving chamber 12 for a certain supply flow of mixture. If the supply mixture through the pipe 8 increases, the free liquid surface in the partly liquid filled passages between the discs 10 will move radially inwards. Simultaneously the liquid level rises along the outside of the pipe 8 in the central part of the receiving chamber 12, to a position shown in Figure 1 with a dotted line. As can be seen, a larger total surface of the discs 10 then will have contact with the mixture supplied, and the pumping effect of the discs on the supplied mixture will increase. Thus, the pumping effect of the inlet device increases with an increasing flow of supplied mixture. Correspondingly, the pumping effect of the discs decreases with a decreasing supply of mixture, since then the free liquid surface will move radially out­ wards and downwards. As can be seen from Figure 1, the inner diameter of the discs 10 decreases axially upwards. This means that every additional disc, which as a consequence of an increased supply flow of liquid takes part in the pumping thereof, has a somewhat larger pumping effect than the underlying adjacent disc. A similar result is achieved because, as also to be seen from Figure 1, the discs 10 have outer diameters which increase in the direction axially upwards. Air or other gases separated from the supplied mixture in the receiving chamber 12 leave upwardly through an annular channel 14. In Figure 2 there is shown an alternative embodi­ ment of the invention. The parts thereof having counterparts in the embodiment according to Figure 1 have been given the same reference numerals as in Figure 1. Wings corresponding to the wings 11 in Figure 1 have not been shown in Figure 2, however, for the sake of clarity. In Figure 2 the tubular part of the member 5, 6 arranged centrally within the rotor is provided at its upper end with an internal annular flange 15. The acceleration discs 10 in this case are arranged axially between this flange 15 and the frusto-conical lower part of the central member 5, 6. The space radially outside of the discs 10 communicates at its lower end with the rotor separation chamber 3 through channels 16 formed between radial wings (not shown) evenly distributed around the rotor axis. The opening 9 of the supply pipe 8 in Figure 2 is situated at a distance axially below the discs 10. Between the opening 9 and the lowermost disc 10 the pipe 8 supports an external annular flange 17. The flange 17 having the form of a lens with an elliptical axial cross section is releasably mounted on the pipe 8. The lowermost portion of the pipe 8 is externally slightly conical - as is the hole or inner surface of the annular flange 17. Upon removal of the pipe 8 from the rotor the flange 17 will become detached and remain in the rotor, the flange then being supported on a central bowl-like surface 18 in the rotor. After reinsertion of the pipe 8 and supply of liquid therethrough, the liquid will flow through the central hole in the flange and pass under the flange and radially outwards between the flange and the concave surface 18, the flange thereby being pressed axially upwards to the position in which it is shown in Figure 2. The convex underneath side of the flange 17 guarantees that no gas or air collects below the flange. After the incoming component mixture has passed through the space between the flange 17 and the sur­ face 18 it turns axially upwards, and flows around the edge of the flange 17 and into the receiving chamber 12. Depending upon the magnitude of the incoming flow, the mixture will pass through a larger or smaller number of the passages between the discs 10. The mixture then flows axially downwards and through the channels 16 to enter the separation chamber 3. In the remaining passages between the discs 10 a free liquid surface is formed, as illu­ strated in Figure 2. As the discs 10 in Figure 1, the discs 10 in Figure 2 have an outer diameter which increases upwardly through the stack of discs. The reason why the incoming mixture flows axi­ ally upwards towards the receiving chamber 12, in­ stead of joining the axially downwards directed flow towards the channels 16, is that the latter flow is rotating at substantially the same speed as the rotor, whereas the incoming mixture below the flange 17 does not have any substantial rotational speed. The object of arranging a flange 17 on the supply pipe 8 is primarily to enable a very small supply flow of mixture through the pipe 8 while maintaining a continuous liquid phase between mixture present within the pipe and mixture present outside the pipe within the rotor. A secondary object of the flange 17 is to prevent incoming mixture being split up by splashing up into the receiving chamber 12. The discs 10 in Figure 2, instead of being supported by means of wings similar to the wings 11 in Figure 1, may be suspended from the flange 15. Thus, a number of rods may be connected with the flange 15 and extend axially downwards through the pile of discs 10. Rods of this kind, which prefer­ ably extend through the radially outermost parts of the discs, may support between the discs spacing members for keeping the discs at a desired distance from each other. In Figure 3 there is shown schematically a pile of annular discs 10 surrounding a stationary supply pipe 8. The pipe 8 at its lower end is provided with circular members 19 and 20 forming together with wings or the like (not shown) radially directed chan­ nels 21 forming a continuation of the channel through the pipe 8. Thus, in this case, the stationary supply pipe 8, 19, 20 has radially directed openings. If desired, the channels 21 may be replaced by a single substantially annular channel. As can be seen from Figure 3 the distances between the discs 10 gradually decrease in a direction from the supply member opening and upwards. This means that the lower part of the disc stack has a smaller pumping effect than the upper part of the disc stack, which is desirable so that a continuous liquid phase may be maintained from the interior of the supply member 8, 19, 20 to the separation chamber 3 even with a very small flow of mixture through the supply member. The variation of the disc interspace width has the same effect as the variation of the inner dia­ meter and outer diameter of the discs 10 shown in Figure 1. The member 19 in Figure 3 has substantially the same function as the flange 17 in Figure 2. The above mentioned pumping effect of the discs 10 is obtained mainly as a consequence of so-called Ekman layers, formed closest to the surfaces of the discs 10. The thickness of these Ekman layers depends, among other things, on the viscosity of the liquid in question. Typical Ekman layer thick­ nesses for liquids treated in centrifugal separators of this kind are between 30µ and 350µ. The small­ est distance which should be present between adjacent discs for the obtainment of the desired gentle acceleration of liquid between the discs is twice the relevant Ekman layer thickness. However, solids present in a liquid supplied to the centrifugal separator will often set a different limit for the space between adjacent discs. This limit is frequently substantially above twice the relevant Ekman layer thickness. In practice the space between adjacent discs would seldom be smaller than 300µ. It is assumed that a common distance between the discs will be between 0.3 mm and 5.0 mm. The pumping effect of the discs 10 may be ampli­ fied where desired by means of for instance radial ribs bridging the whole or a part of the distance between adjacent discs. In the embodiments of the invention according to Figure 1 and Figure 2, the channel 14 communicates with the atmosphere surrounding the rotor. This is not always necessary. The reason for the channel 14 is primarily to enable at least a certain displace­ ment of air or other gases out of the central receiv­ ing chamber 12, so that a substantial number of acceleration discs 10 are not rendered ineffective as a consequence of gases being trapped in the receiving chamber and, thus, preventing inflow of mixture into the passages between those discs.

Claims: 1. A centrifugal separator comprising a rotor defining a separation chamber (3), and a supply member (8) for supplying mixture to be separated and having an opening (9) located centrally within the rotor, the rotor having an inlet arrangement including several annular discs (10) coaxial with the rotor and forming a central receiving chamber (12) for mixture supplied through the supply member (8), and passages connecting the central receiving chamber (12) with the rotor separating chamber (3) being formed between the discs, characterised in that - the receiving chamber (12) at an area along the axial length thereof communicates with a channel (14) for conducting gas away therefrom, - the opening (9) of the supply member (8) is so positioned that the radially inwardly open ends of several of said passages are axially between said opening (9) and said area of the receiving chamber (12), - means (13) are arranged to maintain at the opening (9) of the supply member a body of liquid extending through at least some of said passages dur­ ing operation of the rotor, and - the supply member (8) is so arranged that said opening (9) is located within said liquid body during operation of the rotor, and liquid mixture supplied through the supply member (8) forms a liquid phase continuous with said liquid body. 2. A centrifugal separator according to claim 1, wherein said means (13) for maintaining the said liquid body is arranged to maintain a free liquid sur­ face of said liquid body in the central receiving chamber (12). 3. A centrifugal separator according to claim 1 or 2, wherein the rotor has a vertical axis of rotation, the supply member (8) extends downwardly into the rotor, an upper part of the central receiving chamber (12) communicates with the gas channel (14), and the supply member (8) extends through and has said open­ ing (9) below said upper part of the receiving chamber (12). 4. A centrifugal separator according to any of the preceding claims, wherein the supply member (8) extends through the receiving chamber (12) and said opening (9) is located axially outside of said chamber. 5. A centrifugal separator according to any of the preceding claims, wherein the annular discs (10) have decreasing inner diameters in the direction from the opening (9) of the supply member towards the area of the receiving chamber (12) communicating with the said gas channel (14). 6. A centrifugal separator according to any of the preceding claims, wherein the annular discs (10) have increasing outer diameters in the direction from the opening (9) of the supply member towards the area of the receiving chamber (12) communicating with said gas channel (14). 7. A centrifugal separator according to any of the preceding claims, wherein the axial distance between adjacent annular discs (10) is larger near the opening (9) of the supply member than is said distance closer to the area of the receiving chamber (12) communi­ cating with said gas channel (14). 8. A centrifugal separator according to any of the preceding claims, wherein the opening (9) of the supply member is directed axially within the rotor. 9. A centrifugal separator according to any of the preceding claims, wherein the supply member (8) has an external annular flange (17) positioned axially between said opening (9) and at least some of the annular discs (10). 10. A centrifugal separator according to claim 9, wherein said flange (17) has an outer diameter greater than the inner diameter of at least some of the annular discs (10). 11. A centrifugal separator according to claim 9 or 10, wherein the opening (9) of the supply member is directed axially in the rotor, and the side of the annular flange (17) nearest the opening (9) is convex. 12. A centrifugal separator according to any of claims 9 - 11, wherein the annular flange (17) is formed by a ring detachable from the supply member (8) to enable the supply member (8) to be withdrawn from the rotor without the ring, means (18) being arranged to retain the ring in a position in the rotor so that upon reinsertion of the supply member (8) into the rotor and subsequent supply of liquid thereto, the liquid is directed through the centre hole of the ring and to act on the ring and press the ring axially on to the supply member (8). 13. A centrifugal separator according to claim 12, wherein the supply member (8) and the ring co­ operate to limit axial movement of the ring along the supply pipe (8). 14. A centrifugal separator according to any of the preceding claims, wherein the opening (9) of the supply member (8) is located at one axial end of the discs, and a space surrounding the discs (10) communicates with the separation chamber (3) at the opposite axial end of the discs. 15. A centrifugal separator according to any of the preceding claims, wherein the discs (10) are frusto-­ conical.

Inventors: BORGSTROM, LEONARD, CARLSSON, CLAES-GORAN, FRANZEN, PETER, INGE, CLAES, LAGERSTEDT, TORGNY, MOBERG, HANS, NABO, OLLE SVEN-OLOF
Applicants: ALFA-LAVAL SEPARATION AB
Application year: 1986
Main classification: B04B11
IPC codes: B04B1, B04B11