CENTRIFUGAL SEPARATOR WITH RECIRCULATION OF SEPARATED SLUDGE

Abstract: A centrifugal separator has for returning separated sludge to the separation chamber of the rotor a recirculation circuit (27, 28, 33, 39, 41, 43, 32, 30, 29) which is completely free of flow restricting nozzles. The amount of separated sludge allowed to leave the centrifugal separator through the outlet conduit (39) without being recirculated is determined by setting valves (22, 40) in the stationary outlet conduits (21, 39) for the separated sludge and clarified liquid, respectively.

Description: This invention is concerned with centrifugal separators. More particularly it relates to a centri­ fugal separator comprising: a rotor including a separation chamber, a central inlet for a mixture of components to be separated, with a first central out­ let for separated light component and a second central outlet for separated heavy component; first stationary means forming an inlet channel for supplying the mixture to the rotor inlet; second stationary means forming an outlet channel for receiving separated light component from the first central outlet; third stationary means forming an outlet channel for receiving separated heavy component from the second central outlet; a central inlet chamber in the rotor arranged to receive the mixture entering through the central inlet; first channels in the rotor connecting the central inlet chamber with the separation chamber; second channels in the rotor connecting the radially innermost parts of the separation chamber with the first central outlet; third channels in the rotor connecting the radially outermost parts of the separation chamber with the second central outlet; and means for recirculating to the separation chamber part of the separated heavy component conducted from the separation chamber through said third channels. Such centrifugal separators are used when the content of the heavy component, referred to hereinafter as the "sludge", in a mixture varies heavily or is constantly low, whereas it is desired to obtain a separated sludge with a constant predetermined concen­ tration. In a centrifugal separator known from U.S.-PS 4,278,200, the rotor has sludge outlets in the form of radially extending so-called concentrate pipes, in which narrow nozzles are arranged. After its passage through the concentrate pipes the sludge is collected in a central chamber in the rotor and from which chamber it is discharged by means of a stationary paring member. Part of the discharged sludge is returned to the rotor, while the rest is carried off. Special sensing means may be used for automatic control of the amount of sludge returned to the rotor. When determining the required number of nozzles and the through-flow area of each nozzle in a centri­ fuge rotor of this construction, consideration must be given to several different factors. Thus, noticeable factors are, among others, the particle size and separability of the incoming sludge, the sludge content of the incoming mixture, and the desired concentration of the sludge leaving the centrifuge rotor. It often proves difficult to optimize the centrifuge rotor design such that an exactly desired concentration is obtained for the sludge leaving the rotor. For instance, it is frequently desired that a very high sludge concentration should be obtained, which often cannot be fulfilled, however, due to the risk of the outlet nozzles clogging. Part of the difficulty in optimizing the centri­ fuge rotor design resides in the fact that the necessary flow restricting property of the nozzles limits the free­ dom of choice as to the desired degree of sludge re­ circulation. Depending upon the separability of the sludge, a certain minimum flow velocity has to be maintained in the concentrate pipes extending from the radially outermost parts of the separation chamber to the central chamber. For the above reasons it will be obvious that the known centrifuge rotor has to be provided with special nozzles for each new application and, further­ more, there is a very small margin available for control of the sludge recirculation when a high sludge concentration is required. A particular problem in connection with the known centrifuge rotors, especially at a low content of sludge in the mixture supplied to the rotor, is that the same sludge particles have to be subjected to repeated recirculation and thus pass through the nozzles several times. The repeated pressure changes to which the sludge is subjected during these passages may have an adverse effect on certain sensitive sludge particles. The object of the present invention is to provide an improved centrifugal separator allowing narrow nozzles in the recirculation circuits to be avoided and enabling the flow velocity of the recirculation flow to be independent of a chosen setting for the desired concen­ tration of the sludge leaving the rotor. In accordance with the invention there is pro­ vided a centrifugal separator as initially described hereinabove, characterised in that: sealing means are so arranged at connections of said first, second and third stationary means to the central inlet and the first and second outlets of the rotor, respectively, that during operation, the supplied mixture and discharged separated components are kept out of contact with each other and with the atmosphere surrounding the rotor; pumping means is provided for supplying said mixture of components to the separation chamber and to maintain a hydraulically rigid connection, in the operation of the rotor, from said central inlet through the separation chamber to the first and second central outlets; means adjustable during operation of the rotor is provided for selecting the relation between the flows of separated light and heavy components through said outlet channels, respectively; and said recirculating means forms a closed return path for said recirculated part of the separated heavy component whereby said recirculated part is kept out of contact with the atmosphere surrounding the rotor. With a separator embodying the invention a desired concentration of the sludge leaving the rotor may readily be set, and changed if desired, by merely adjusting the relation between the flows of separated sludge and separated liquid freed from sludge, respectively, dis­ charged through the said outlet channels. Furthermore, the recirculation of sludge may be set as desired according to need and may be allowed to increase or decrease during the operation of the rotor without having any influence on the set concentration of the sludge that is discharged. Throttles in the recirculation circuit are not needed and should not be present. Within the scope of the invention the recirculated sludge may be returned to the separation chamber through different ways, for instance together with the mixture of components supplied by said pump means. In a preferred embodiment of the invention a further pumping means is connected to either the recir­ culation channel or the outlet channel for returning part of the sludge flowing through the outlet channel. Hereby, the recirculated sludge may be supplied downstream of a stationary pump means connected to the rotor inlet. Said recirculating means preferably comprises a stationary part forming a recirculation channel and connected to said outlet channel for separated heavy component, and a part rotatable with the rotor and form­ ing in the rotor recirculation channels separate from the central inlet chamber and the channels connected to it. By this arrangement distribution of the recirculated sludge in the newly supplied mixture and, thus, having to be separated again is avoided. Sealing means can be arranged to ensure such a connection of the stationary part to the rotatable part of the recirculating means that the recirculated sludge is kept out of contact with the atmosphere surrounding the rotor as well as with the supplied mixture and separated light component. The said sludge recirculation channels in the rotor preferably open at the radially outermost parts of the separation chamber at positions situated between the above mentioned third channels through which sludge is flowing towards the rotor centre. This makes it possible, during the operation of the rotor, to maintain a constant flow of sludge along the outermost parts of the separation chamber, so that sludge cushions cannot be formed in these parts. Furthermore, cleaning of the separation chamber after finished operation is facili­ tated, since cleaning liquid may be forced through the said channels via the radially outermost parts of the separation chamber. The invention will be further described with reference to the accompanying drawing which shows a preferred embodiment thereof. In the drawing there is shown a rotor comprising two parts 1 and 2, which are axially held together by means of a locking ring 3. The rotor is supported by a vertical drive shaft 4. Within the rotor there is formed a separation chamber 5, in which a stack of conical separation discs 6 rest on the lower part of a distributor 7. The distributor has radial distribution wings 8 on its underside and through the wings 8 rests on a conical partition 9 positioned centrally in the rotor. Within the distributor 7 there is formed a central inlet chamber 10 which communicates through the passages between the wings 8 with the separation chamber 5 in the region of the radially outer edge of the lowermost separation disc. The distributor 7 supports a central pipe 11 extending axially out of the rotor and forming an inlet channel 12 in communication with the inlet chamber 10. The pipe 11 is surrounded by a partly tubular member 13, which is supported by the rotor part 1 and is attached thereto by a locking ring 14. Formed between the pipe 11 and the member 13 is an annular channel 15 which communicates directly with the central parts of the separation chamber 5 and constitutes an outlet channel therefrom. A stationary member 16 is arranged around the pipe 11 and the member 13, and is provided with an inlet 17 for a liquid mixture to be supplied to the rotor and an outlet 18 for liquid having been separated in the rotor. The inlet 17 communicates with the inlet channel 12, whereas the outlet 18 communicates with the annular outlet channel 15. To the inlet 17 there is connected an inlet conduit 19 including a pump 20, and to the out­ let 18 there is connected an outlet conduit 21 having an adjustable valve 22. Located between the stationary member 16 and the inlet tube 11 is a first mechanical seal 23, and located between the stationary member 16 and the partly tubular member 13 is a second mechanical seal 24, 25. The first mechanical seal 23 is of a single kind with one stationary sealing ring and one rotatable sealing ring, which sealing rings abut axially against each other. The second mechanical seal 24, 25 is of a so-called double kind with two pairs of such sealing rings. In a manner known per se the double mechanical seal should be charged with so-called sealing liquid in the space between the two pairs of sealing rings. To simplify the drawing no arrangement for supplying sealing liquid has been shown. Positioned between the conical partition 9 and the lower rotor part 2 is a short cylindrical sleeve 26 which supports several pipes 27 evenly distributed around the rotor axis and extending radially from the sleeve 26 to the radially outermost parts of the separa­ tion chamber 5. Through bores in the sleeve 26 the interior ducts of the pipes 27 communicate with channels 28 which extend radially towards the rotor centre. Further pipes 29 extending radially inwardly from the radially outermost parts of the separation chamber are supported by the conical partition 9. Through bores in the partition 9 the interior ducts of the pipes 29 communicate with channels 30 which extend radially towards the rotor centre. The vertical drive shaft 4 has a central bore through which extends a pipe 31. This pipe forms a central channel 32 and has on its outside several axial grooves forming longitudinal channels 33 between the pipe 31 and the surrounding drive shaft 4. At the rotor centre the upper end of the central channel 32 in the pipe 31 communicates with the radial channels 30, and the longitudinal channels 33 communi­ cate with the radial channels 28. To simplify the drawing no arrangement for driving and journalling of the drive shaft 4 has been shown. At its lowermost end the drive shaft is surrounded by a stationary member 34. Between this member and the drive shaft 4 and the pipe 31, respectively, there are two mechanical seals 35 and 36. The first mechanical seal 35 comprises one stationary sealing ring, which is supported by the member 34, and one rotatable sealing ring supported by the drive shaft 4. The other mech­ anical seal 36 comprises one stationary sealing ring, which is supported by the member 34, and one rotatable sealing ring supported by the pipe 31. If desired, the seal 35 may be of the double kind, like the seal 24, 25. The stationary member 34 has one inlet 37 comm­ unicating with the channel 32 in the pipe 31, and one outlet 38 communicating with the channels 33 between the pipe 31 and the drive shaft 4. The outlet 38 is connected to an outlet conduit 39, which includes an adjustable valve 40. Connected to the outlet conduit 39 between the outlet 38 and the valve 40 is a branch con­ duit 41 which is connected to the inlet of a pump 42. The outlet of the pump 42 is connected through a con­ duit 43 to the inlet 37 of the member 34. A liquid mixture comprising sludge is supplied to the rotor by means of the pump 20. The mixture enters through the channel 12 into the central receiving chamber 10 of the rotor and flows between the distribution wings 8 to the separation chamber 5. In the separation chamber 5 the sludge is separated from the mixture and collects in the radially outermost parts thereof. Clarified liquid flows radially inwardly and leaves the separation chamber through the channel 15, the outlet 18 and the conduit 21. The separated sludge and a small amount of remain­ ing liquid is pressed by the overpressure of the pump 20 radially inward through the pipes 27 and then through the channels 28 and 33 to the outlet 38. At the outlet conduit 39, part of the sludge leaves through the valve 40, while the rest of the sludge by means of the pump 42 is returned to the separation chamber 5 through the conduits 41 and 43, the channels 32 and 30 and the pipes 29. Depending upon the content of sludge in the supplied mixture and the desired concentration of the separated sludge which is to leave through the outlet conduit 39, the valves 22 and 40 are set in a way such that a certain desired relation is obtained between the flows through these valves. The valves 22 and 40 thus form adjustable throttles in the conduits 21 and 39, respectively, and are connected in a hydraulically rigid manner with each other and with the inlet pump 20 through the separation chamber 5. The pump 42 which may have a controllable speed is adjusted with respect to its capacity such that a desired degree of sludge recirculation is obtained. The amount of sludge recirculated is determined above all by the flow velocity in the pipes 27. This velocity has to be sufficiently large so that the sludge particles which move radially inwardly in the pipes 27 are not prevented from doing so by the centrifugal forces and instead be separated out of the small amount of convey­ ing liquid in which they are suspended. If the content of sludge in the supplied mixture is expected to vary during operation, means are preferably provided for changing automatically the setting of at least one of the valves 22 and 40. Such means may com­ prise sensing members of various kinds arranged for controlling of the setting of at least one of the valves 22, 40. The sensing members may be present in one of the conduits 19, 21 and 39. In the inlet conduit 19 a change of the sludge content of the supplied mixture may be directly sensed. In the outlet conduit 21 a sensor can sense if the content of remaining sludge in the clari­ fied liquid increases or decreases. In the outlet con­ duit 39 a sensor can sense if the concentration of the separated sludge increases or decreases. In all of these cases a sensed change would cause an adjustment of the setting of one or both of the valves 22 and 40. In this way the concentration of the separated sludge may be maintained substantially constant and the amount of sludge having accumulated in the centrifuge rotor may be maintained substantially unchanged.

Claims: 1. A centrifugal separator comprising: a rotor including a separation chamber (5), a central inlet (12) for a mixture of components to be separated, a first central outlet (15) for separated light component and a second central outlet (33) for separated heavy component; first stationary means forming an inlet channel (19) for supplying the mixture to the rotor inlet; second stationary means forming an outlet channel (21) for receiving separated light component from the first central outlet; third stationary means forming an outlet channel (39) for receiving separated heavy component from the second central outlet; a central inlet chamber (10) in the rotor arranged to receive the mixture entering through the central inlet; first channels in the rotor connecting the central inlet chamber (10) with the separation chamber (5); second channels in the rotor connecting the radi­ ally innermost parts of the separation chamber (5) with the first central outlet (15); third channels (27, 28) in the rotor connecting the radially outermost parts of the separation chamber (5) with the second central outlet (33); and means (41-43) for recirculating to the separation chamber part of the separated heavy component conducted from the separation chamber through said third channels, characterised in that: sealing means (23 - 25, 35) are so arranged at connections of said first, second and third stationary means to the central inlet (12) and the first and second outlets (15, 33) of the rotor, respectively, that during operation, the supplied mixture and discharged separated components are kept out of contact with each other and with the atmosphere surrounding the rotor; pumping means (20) is provided for supplying said mixture of components to the separation chamber (5) and to maintain a hydraulically rigid connection, in the opera­ tion of the rotor, from said central inlet (12) through the separation chamber (5) to the first and second central outlets (15, 33); means (22, 40) adjustable during operation of the rotor is provided for selecting the relation between the flows of separated light and heavy components through said outlet channels (21, 39), respectively; and said recirculating means (41 - 43) forms a closed return path for said recirculated part of the separated heavy component, whereby said recirculated part is kept out of contact with the atmosphere surrounding the rotor. 2. A centrifugal separator according to claim 1, wherein said recirculating means comprises a stationary part forming a recirculation channel (41, 43) connected to said outlet channel (39) for separated heavy component, and further pumping means (42) connected to either re­ circulation channel (41, 43) on the outlet channel (39) for returning part of the separated heavy component flowing through the outlet channel (39). 3. A centrifugal separator according to claim 1 or 2, wherein said recirculating means comprises a part rotat­ able with the rotor and forming in the rotor recirculation channels (29, 30, 32) separated from the central inlet chamber (10) and the channels connected thereto, the recirculation channels opening in the rotor at the radially outermost parts of the separation chamber (5) at positions located between said third channels (27, 28) in the peripheral direction of the rotor. 4. A centrifugal separator according to any one of claims 1 to 3, wherein said recirculating means has one stationary part forming a recirculation channel (41, 43) connected to said outlet channel (39) for separated heavy component, and one part rotatable with the rotor and forming in the rotor recirculation channels (29, 30, 32) separated from the central inlet chamber (10) and from the channels connected thereto, sealing means (36) is so arranged at a connection of said stationary part to said rotatable part of the recirculating means, that returned separated heavy component is kept out of contact with atmosphere surrounding the rotor as well as the mixture supplied and the separated light component. 5. A centrifugal separator according to any one of claims 1 to 4, wherein sensing means is arranged to sense a substance concentration in one of the supplied mixture and separated components, said adjustable means (22; 40) is coupled to the sensing means and is so arranged to adjust automatically, according to the sensed concentration, the relation between the flows through the outlet channels (21, 39) for separated light and heavy component, respectively, that a substantially constant concentration of the separated heavy component leaving the rotor is obtained. 6. A centrifugal separator according to any one of claims 1 to 5, wherein said recirculating means (41 - 43) for returning part of the separated heavy component to the separation chamber is arranged for setting of a given return flow of heavy component, independently of the setting of said adjusting means (22, 40).

Inventors: KROOK, GORAN, Krook, Göran
Applicants: ALFA-LAVAL SEPARATION AB
Application year: 1987
Main classification: B04B1
IPC codes: B04B1, B04B11