ROTATABLE SUPPLY AND DISCHARGE UNIT OF A CENTRIFUGAL SEPARATOR

Abstract: A centrifugal separator has a flexible member (28) for the transport of liquid to and/or from a centri­ fuge rotor (15), which rotates around the rotor in the same direction but with half the speed. The flexible member (28) is provided with a braided casing to give it a large stiffness against torsion, good tension strength and an outside surface with good wear resistance. The casing comprises plaited threads (32) of e.g. metal or plastics, some of which extend helically with a certain pitch around the flexible member and others of which extend helically in the opposite direction. The plaiting of the threads (32) is such that the casing per se is axially contractable and extensible with simultaneous radial expansion and contraction, respectively. When in use the casing is axially extended so that it is radially contracted into frictional engagement with the flexible member (28).

Description: The present invention relates to centrifugal separators, and more particularly refers to a centri­ fugal separator comprising a rotor with a separation chamber, and a flexible member forming at least one channel for fluid transport to and/or from the separa­ tion chamber during the rotation of the rotor, the flexible member extending away from the rotor at the rotor axis and subsequently passing around the rotor periphery to a position on the rotor axis at the other side of the rotor. A centrifugal separator of this kind is known e.g. from US-A-3,586,413 (reissue No. 29 738), US-A-4,109,855 and US-A-4,114,802. As is particularly evident from US-A-3,586,413 it is possible to maintain fixed connections without movable sealing means between the flexible member and the rotor as well as between the flexible member and the non-rotatable member, but the flexible member in one way or another has to be brought into a rotational movement around the centrifuge rotor in the same direction as and at half the rotational speed of the rotor. This means tht the portions of the flexible member which are not entirely straight are subjected to bending, the direction of which is gradually changed around the longitudinal axis of the flexible member during the operation of the centrifugal separator. It is a problem to form the flexible member in a way such that it will be durable over a long time. Depending upon how it is connected with the centrifuge rotor and possibly is supported by supporting means separate from the centrifuge rotor, it is subjected to large forces of different kinds during the operation of the centrifuge rotor. If the flexible member is allowed to rotate freely without any support radially outside the centrifuge rotor, e.g. as is shown in US-A-4,109,855, large forces will appear in its longitudinal direction. Then, without reduction of its flexibility it has to be made very strong in its longitudinal direction. However, even if it is possible to make the flexible member very strong in its longitudinal direction, separate guiding means normally have to be arranged to prevent too strong bending of the flexible member at the places of its connection, when it rotates. Such guiding means by abutting against the flexible member will cause shearing forces in it, since the flexible member during operation has to perform a rotational movement relative to each such guiding means. Consequently, in this case the flexible member also has to be formed relatively stiff against torsion. A relatively complicated alternative to making the flexible member stiff against torsion is shown in US-­ 4,114,802. According to this alternative the flexible member is guided close to its connection with the centri­ fuge rotor by a guiding member which itself is rotatable relative to the rotor around an axis perpendicular to the rotational axis of the rotor and, thereby, does not create substantial shearing forces in the flexible member. Apart from the above mentioned requirements on strength and stiffness against torsion of the flexible member it should be considered that, if the flexible member during its rotation lacks a separate support radi­ ally outside the rotor, there is the disadvantage that the flexible member will take the form of a bow between its places of connection, which puts certain limits on the shape of the rotor. For all of the above mentioned reasons it proves suitable in practice, particularly if the rotor is rela­ tively large and the flexible member should transport relatively large liquid flows, to provide a separate supporting member for the flexible member during its rotation around the rotor. Such a separate supporting member, which has to rotate together with the flexible member, is preferably formed as a stiff tube, in which the flexible member may turn around its own longitudinal axis (see e.g. US-A-3,586,413). Even though a support member of this kind will to some extent unload the flexible member from some tension forces, its supporting function will, however, cause the flexible member to be subjected to large friction forces when it is forced during its rotation relative to the rotor to turn relative to the supporting member. Large requirements on stiffness against torsion are therefore put on the flexible member and on wear resistance of its outer surface. The present invention has for its aim to improve the durability of the flexible member in a centrifugal separator of the form described, and thereby to enable practical use of such centrifugal separators in industry where the centrifuge rotors have to be relatively large and have a capacity for relatively large liquid flows. According to the invention there is provided a centrifugal seaparator as initially described, character­ ised in that the flexible member has a torsion stiff but flexible casing comprising at least one first thread extending with a certain pitch helically around the member and at least one second thread extending helically around the member with a pitch directed opposite to that of the first thread, said threads being plaited together such that the casing per se is axially contract­ able and extensible with simultaneous radial expansion and contraction, respectively, and and the casing is in frictional engagement with the flexible member due to radial contraction of the casing caused by axial extension thereof. In a preferred embodiment of the invention the casing comprises several first threads, which are arranged in groups extending helically around the flex­ ible member like the threads of a multiple-threaded screw, and several second threads also arranged in groups, said groups of first threads being plaited together with the groups of second threads. By means of a braided casing of this form the flexible member may be given a large tensile strength as well as a large stiffness against torsion. Further­ more, the casing is given an outside which by said plait­ ing of the threads is both resistant to wear and offers a small friction against a possible support for the flex­ ible member radially outside the rotor. The threads in the casing can preferably be made of metal, for instance stainless steel. It has proved that a casing of threads like this will be sufficiently stiff against torsion to enable driving of the rotor only by means of the flexible member. In such an arrangement the casing is preferably connected firmly both with the rotor and with a non-rotatable member, and is connected to a driving device such that it can be brought into rotation around the rotor with a speed half of the desired speed of the rotor. Alternatively, the threads may be made of plastics, whereby the casing can have a very small weight and provide an outside surface offering a very small friction against a surrounding supporting member, for instance of metal, against which it has to abut under movement relative thereto during the operation of the rotor. Even a casing consisting of plastic threads is very stiff against torsion. In a particular embodiment of a centrifugal separator according to the invention the rotor comprises both a rotor body and a separate member which forms said separation chamber and is movable relative to the rotor body. In this case the flexible member is connected with said separate member, the torsion resistant casing of the flexible member also being connected with the separate member and thus being movable together therewith relative to the rotor body. The separate member may be formed by an end portion of the flexible member, arranged in a cavity of the rotor body. To assist a full understanding of the invention some embodiments are described below with reference to the accompanying drawing, in which:- Figure 1 shows a centrifugal separator according to the invention; Figure 2 shows the flexible member of the separator and with two channels for liquid transport to and from, respectively, a separation chamber in a centrifuge rotor; Figure 3 shows a sectional view of an end portion of the flexible member in Figure 2; Figure 4 shows a cross sectional view along the line IV-IVin Figure 3; and Figures 5 and 6 show alternative surface structures for a casing around a flexible member according to Figure 2. In Figure 1 there is shown a centrifugal separator comprising a stationary housing 1, a motor 2 mounted there­ in, and a bearing housing 3. The bearing housing 3 supports two bearings 4 and 5, in which there is journalled a vertical spindle 6. The spindle is rotatable around a rotational axis C. On the upper end of the spindle there is mounted an upwardly open cylindrical container 7. The motor 2 is arranged to rotate the spindle 6 through two gear wheels 8, 9 and a gear belt 10. Mounted coaxially within the cylindrical container 7 is a sleeve 11 mounted on the bottom of the container. On its inside the sleeve 11 supports two bearings 12 and 13, in which there is journalled a sleeve-like part 14 of a rotor body 15. Spaced from the centre axis of the container 7 there is mounted on the container bottom a further sleeve 16. This supports on its inside two bearings 17 and 18, in which there is journalled a vertical shaft 19. The shaft 19 extends down through an opening in the bottom of the container 7 and supports below this bottom a gear wheel 20. The gear wheel 20 is in engagement with a gear ring 21 firmly mounted on the inside of the station­ ary housing 1. At its upper end the shaft 19 supports a gear wheel 22, which is in engagement with a gear wheel 23 mounted on and coaxial with the rotor body 15. Within the rotor body 15 there is mounted firmly relative thereto a pipe, which with a first part 24 extends along the periphery of the rotor body and with a second part 25 extends inwards towards the rotational axis C of the rotor. Within the container 7 there is mounted firmly relative thereto a further pipe 26 which extends from the central end of the pipe parts 25 downwards into the sleeve 11, radially out through an opening in the surrounding wall of the sleeve 11, further upwards with­ in the container 7 and again towards the rotational axis C at the upper part of the housing 1. A short sleeve is firmly mounted in the housing 1 coaxially with the rotor body 15 and is arranged opposite to the upper end of the pipe 26. A flexible member 28 extends from the outside of the housing 1 through the sleeve 27 and further through the pipe 26 and the pipe parts 25 and 24. Within the flexible member 28 there are confined two channels 29, 30, which partly form ways for liquid transport to and from the rotor body, and partly form a separation chamber at the end portion of the flexible members 28 situated within the rotor body 15 close to its periphery. The flexible member 28 has a smaller diameter than the sleeve 27 and the pipes 24-26, so that it can be rotated therein around its own longitudinal axis. Above the sleeve 27 there is a normally stationary, but upon need rotatable, member 31 arranged in engagement with the flexible member 28 for intermittent rotation thereof relative to the sleeve 27 and the pipes 24-26 during operation of the centrifugal separator. The centrifugal separator shown in Figure 1 operates in the following manner. Upon start of the motor 2 the spindle 6 and the container 7 are brought into rotation. Then the sleeve 16 is moved in a course around the rotational axis C, the gear wheel 20 by its engagements with the stationary gear ring 21 causing the shaft 19 to rotate relative to the sleeve 16. This rotational movement is transferred through the gear wheels 22 and 23 to the rotor body 15, which thus is caused to rotate relative to the container 7. The different gear transmissions are so calculated that the rotor body 15 with the pipe parts 24 and 25 will rotate in the same direction as and at twice the speed of the container 7 and the pipe 26 around the rotational axis C. By the arrangement for the driving of the rotor body it is possible to maintain one end of the flexible member 28 fixed relative to the housing 1 by means of the member 31, and the other end of the flexible member fixed relative to the rotor body 15, without getting the flexible member twisted during rotation of the rotor body. Independent of whether the rotor body 15 is rotating or not the flexible member 28 - when desired - may be rotated by means of the member 31 relative to the sleeve 27 and the pipes 24-26. In Figure 2 there is shown a view of the part of the flexible member 28 which is situated within the rotor body 15. Along a part of the member 28 it is shown that it has an outer braided casing comprising strands or threads 32, for instance of plastics or metal, which are interplaited. A certain number of threads thus extend helically with a certain pitch in one direction along the member 28, while an alike number of threads - interplaited with the others - extend with a correspond­ ing pitch in the other direction along the member 28. The casing of threads 32 can be considered as a separate tube in which there is inserted a flexible inner member. A tube of this kind is flexible and can be extended or shortened, its diameter then being correspondingly changed. The dimensioning of the tube is made such that, after the flexible inner member has been inserted into the tube, the latter may be extended until it gets the same length as the inner member and in this condition has an inner diameter which is as large as the outer diameter of the inner member. In this way a desired surface engagement is obtained between the threads 32 and the flexible member and, simultaneously, the tube can be used for holding the ends of the flexible member 28 relative to the housing and the rotor body 15, respectively. The threads 32 give the flexible member 28 a large stiffness against torsion, so that when needed it can be rotated around its own longitudinal axis by means of the member 31 at one of its ends. In Figure 3 there is shown a longitudinal section through the end portion of the flexible member 28, intended to be situated in the rotor body 15. As can be seen, the flexible member 28 comprises the previously described outer braided casing 32 surrounding a hose 33, for instance of soft plastics, in which the two channels 29 and 30 are formed. Between the channels 29 and 30 is a partition 34 formed by the hose material. This parti­ tion is penetrated a short distance from the end of the hose, so that a connection opening 35 is formed between the channels 29 and 30. A closing member 36 is inserted into the hose 33 from its end, and a collar 37 clamps the casing against the closing member 36, which is there­ by securely fixed within the hose 33. In Figure 4 there is shown a cross section along the line IV-IV in Figure 3. In Figures 5 and 6 there are shown alternative ways of plaiting the threads 32 of a casing of the kind surrounding a flexible member according to Figures 2-4. In Figure 5 there is shown how two groups 38 and 39 of threads extending in parallel helically around the flexible member with a certain pitch cross four groups 40-43 of threads situated beside each other and extending in parallel and helically around the flexible member but with another pitch and in the opposite direction to the threads of groups 38 and 39. As can be seen from Figure 5, the thread group 38 at the shown crossing places, extends below the thread groups 40 and 41 but above the thread groups 42 and 43, whereas the thread group 39 extends above the thread groups 40 and 43 but below the thread groups 41 and 42. Acceptable properties have been obtained with a braided casing of threads plaited according to Figure 5, and which consists of twelve first thread groups extending in parallel in one direction and twelve second thread groups extending in parallel and crossing the twelve first thread groups. Each group contains twelve threads of stainless steel and with a diameter of 0.3 mm. Another casing which has been made of plastic threads plaited in the same way has also shown the desired properties, i.e. it has given a flexible member of the kind shown in Figures 2-4 a good stiffness against torsion, a good tensile strength and a wear resistant cover. In this case the casing consisted of 2 x 12 groups of threads. Each group comprised four plastic threads with a diameter of 0.9 mm. The casing of metal threads as well as the casing of plastic threads by axial expansion and compression could be given a diameter of between about 24 mm and 32 mm, respectively. In Figure 6 there is shown a modified way of plaiting threads in a casing of a kind here in question. Several other ways of plaiting threads in a casing of this kind are possible within the scope of the definition given in the subsequent claims.

Claims: 1. A centrifugal separator comprising a rotor with a separation chamber, and a flexible member (33) forming at least one channel (29, 30) for fluid transport to and/or from the separation chamber during rotation of the rotor, the flexible member extending away from the rotor at the rotor axis (C) and subsequently passing around the periphery of the rotor to a position on the rotor axis at the other side of the rotor, characterised in that the flexible member (33) has a torsion stiff but flexible casing comprising at least one first thread (32) extending with a certain pitch helically around the member and at least one second thread (32) extending helically around the member with a pitch directed opposite to that of the first thread, said threads (32) being plaited together such that the casing per se is axially contractable and extensible with simultaneous radial expansion and contraction, respectively, and that the casing is in frictional engagement with the flexible member due to radial contraction of the cas­ ing caused by axial extension thereof. 2. A centrifugal separator according to claim 1, wherein the casing is connected at its respective ends with at least one of the rotor and a member which is not rotatable with the rotor. 3. A centrifugal separator according to claim 1 or 2, wherein the flexible member (33) consists of a hose of a kind known per se, which forms two or more channels. 4. A centrifugal separator according to any of claims 1 to 3, wherein the casing comprises several first threads arranged in groups (38, 39) extending helically around the flexible member like the threads of a multi-threaded screw, and several second threads also arranged in groups (40-43), said groups of first threads being plaited with the groups of second threads.

Inventors: ENHSTROM, LARS, WESTBERG, ERIC
Applicants: ALFA-LAVAL SEPARATION AB
Application year: 1987
Main classification: B04B11
IPC codes: B04B11, B04B5