CENTRIFUGAL SEPARATOR ARRANGED FOR DISCHARGE AF A SEPARATED PRODUCT WITH A PREDETERMINED CONCENTRATION

Abstract: To control the sludge concentration of a separated component, separated sludge is recirculated back to the separation chamber of a centrifugal separator at a controlled rate by letting the sludge pass through a channel formed such that the magnitude of the recirculated flow will be dependent on the sludge viscosity. If the viscosity of the separated sludge increases, the amount of recirculated sludge is reduced, and vice versa. The separator of the invention provides for such sludge recirculation with­ in the centrifuge rotor itself. Arranged centrally in the rotor, in addition to a reception chamber (10) for separated sludge, is a recirculation chamber (14) to which sludge is transferred from the reception chamber (10). An overflow outlet (15) maintains a free liquid surface in the recirculation chamber (14) at a predetermined radial level. Recirculation channels (16), formed so that the flow therethrough will be controlled automatically according to the viscosity of the separated sludge, conducts sludge from the re­ circulation chamber (14) back to the radially outer­ most parts of the separation chamber (5).

Description: The present invention relates to centrifugal separators. In particular the invention concerns a centrifugal separator having a rotor enclosing a separation chamber provided with an inlet for a mixture of components to be separated, a first out­ let for a separated component having a reduced sludge content and a second outlet for a separated, sludge enriched component, the second outlet having a flow restriction and means being arrnged for recirculating some separated sludge enriched component having passed therethrough for the recirculated component to flow again through the restriction, and the recirculation means including one or more recirculation passages so arranged that the recirculation flow of sludge enriched component decreased with increasing viscosity of the component and increases with decreasing viscosity of the component. A centrifugal separator as mentioned above has been proposed in the U.S. patent specification 4,162,760, and this separator has a rotor with outlet nozzles around its periphery for discharging the separated sludge enriched component. Located outside the rotor is a reception vessel with an overflow outlet and a bottom outlet, the latter being in communication with a passage for recirculating of part of the separated sludge enriched component back to the centrifuge rotor. The re­ circulation passage is so formed that it lets through a flow that increases with decreasing viscosity and decreases with increasing viscosity of the separated sludge enriched component. This arrangement is intended to provide a constant concentration of the separated sludge enriched component leaving the centrifugal separator through the overflow outlet of the reception vessel. In the known separator the reception vessel has a relatively large volume which is unavoidable because it extends around the whole of the rotor. This means that the concentration control cannot be made as accurate as is desirable, since it takes substantial time for separated sludge enriched component to flow from the outlet nozzles of the separation chamber, to the viscosity sensitive recirculation passage at the bottom of the reception vessel. Furthermore, the known construction requires a large space and is expensive. According to the present invention there is pro­ vided a centrifugal separator as set forth hereinabove, characterised by: means in the rotor defining a reception chamber for the separated sludge enriched component, the reception chamber communicating with the separation chamber through said second outlet, means for conducting separated sludge enriched component from the reception chamber to maintain a flow of sludge enriched component from the separation chamber to the reception chamber, means in the rotor defining a recirculation chamber, said recirculation passage or passages comm­ unicating with the recirculation chamber, means arranged to transfer sludge enriched com­ ponent from the reception chamber to the recirculation chamber, and means arranged to maintain a predetermined liquid surface level in the recirculation chamber, said level being close enough to the rotor axis for sludge enriched component to flow from the recirculation chamber through the recirculation passage or passages. With such a centrifugal separator it is possible to control the concentration of the separated sludge enriched component substantially more accurately than can be achieved by means of an arrangement according to U.S. 4,162,760. Furthermore, this improvement can be obtained by means of equipment which is less complex, less expensive and less space requiring than the corres­ ponding equipment according to U.S. 4,162,760. In a separator according to the invention the entire concentration control equipment may be arranged within the rotor. Furthermore, both the reception chamber and the recirculation chamber may have a very small total volume and may communicate directly with the separation chamber. A change of the concentration of the flow leaving the separation chamber through said second outlet will, as a result, immediately influence the viscosity sensitive flow in the recirculation passage or passages. As a consequence the concentration control will be very accurate. Obtained automatically due to the centrifugal force is a pressure difference across said recirculation passage or passages substantially larger than can be accomplished in the arrangement according to U.S. 4,162, 760. This allows the recirculation passage or passages, to be more readily formed so that laminar flow is obtained with certainty within them. The means for maintaining the liquid surface at the desired level in the recirculation chamber may comprise paring members or the like. By means of such members the liquid level if required may be moved radially during the operation of the rotor. This can be performed for instance by moving the paring member radially within the centrifuge rotor, or by actuating an adjustable throttle valve in the liquid channel of the paring member to let a larger or smaller flow out through the paring member. However, if there is no need for any variation of the liquid level in the recirculation chamber, the means for determining the liquid level therein is pre­ ferably constituted by an overflow outlet. This over­ flow outlet may either lead directly to a stationary collection vessel outside the centrifuge rotor or lead to an outlet chamber within the centrifuge rotor, from which it can be conducted away by means of a paring member or the like. According to a particular embodiment of the invention the overflow outlet, instead, leads to the aforementioned reception chamber, one and the same member being arranged to conduct separated sludge en­ riched component from the reception chamber and to transfer part of it to the recirculation chamber, the rest of it passing out of the centrifuge rotor. A paring member or the like is preferably used to conduct separated sludge enriched component from the reception chamber. Thereby, if desirable, the liquid level in the reception chamber may be moved radially during the operation of the rotor in the same manner as described above in connection with the liquid level in the recirculation chamber. Some embodiments of the invention are described in detail below with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment in axial cross-section; Figure 2 shows part of Figure 1 on an enlarged scale; Figure 3 shows a second embodiment of the invention in axial section; and Figure 4 shows a modification for simplifying cleaning of a centrifuge rotor designed according to Figure 3. In Figure 1 there is shown a centrifuge rotor composed by two parts 1 and 2, which are held together axially by means of a locking ring 3. The rotor is supported by a vertical drive shaft 4 connected with the rotor part 2. Within the rotor there is formed a separation chamber 5 in which a pile of conical separation discs 6 rest on the lower part of a so-called distributor 7, which in turn rests through radially extending wings 8 upon a partly conical partition 9 supported by the rotor part 2. Formed between the rotor part 2 and the partition 9 is a central chamber 10 which communicates through several radially extending pipes 11 connected to the partition 9 with the radially outermost parts of the separation chamber 5. Each pipe 11 has a throttle 12 at its radially innermost end. A further partition 13 with a smaller radial extension than the partition 9 is connected with the latter such that a radially inwardly open annular chamber 14 is formed between the partitions 9 and 13. The lower partition 9 has a central opening, and the annular edge of the partition 9 formed thereby constitutes an overflow outlet 15 from the chamber 14 to the chamber 10. The partition 13 also has a central opening, the diameter of which is smaller than that of the opening through the partition 9, however. The chamber 14 communicates through pipes 16 connected to the partition 13 with the radially outer­ most parts of the separation chamber 5. The pipes 11 and 16 are evenly distributed around the rotor axis, so that each pipe 11 is situated between two adjacent pipes 16. The pipes 11 have a substantially larger internal diameter than the pipes 16, and the throttles 12 of the pipes 11 (see Figure 2) are entirely responsible for determining the flow through the pipes 11. Each throttle 12 has a very small extension in the through-flow direction, so that changes expeceted during operation in the viscosity of a separated sludge enriched component flowing through the pipes 11 should not influence the through-flow to a substantial degree. In contrast thereto, each pipe 16 along the whole of its length has a through-flow area which is so small in relation to its length that flow of separated sludge enriched component through the pipes 16 is to a substan­ tial degree influenced by the viscosity of the component. Thus, an increasing viscosity will result in a decreased flow through the pipes 16 if conditions are otherwise unchanged. Extending axially into the centrifuge rotor is a stationary member having one central channel 17 and two annular channels 18 and 19, respectively, surrounding coaxially the central channel. The central channel 17 constitutes an outlet channel and communicates through an opening 20 with the interior of a paring tube 21 extending into the chamber 10. In line with the annular chamber 14 there is a small opening 22 in the stationary member, which provides for a small flow to pass from the channel 17 into the chamber 14. In the channel 17 outside the rotor there is a constant pressure valve 23 shown schematically in Figure 1. A similar valve (not shown) may be arranged in the outlet channel 19 for the separated liquid. The channel 18 constitutes an inlet channel and communicates through openings 24 with a central inlet chamber 25 in the rotor. The channel 19 constitutes an outlet channel and communicates with the interior of a paring disc 26. The central inlet chamber 25 communicates with the separation chamber 5 through the spaces between the radial wings 8 and through holes 27 in the lower part of the distributor 7. The embodiment according to Figure 1 is intended to operate in the following manner for the separation of sludge, for instance yeast, from a liquid. The mixture of sludge and liquid is introduced through the channel 18 into the rotor inlet chamber 25, from where it flows between the wings 8 and through the holes 27 to the separation chamber 5. Here the sludge is separated and collects at the radially outermost parts of the separation chamber, in the so-called sludge space, while the clarified liquid flows towards the rotor centre and is continuously discharged from the rotor through the paring disc 26 and the outlet channel 19. Sludge collected in the sludge space, mixed with a small amount of liquid, flows radially inward through the so-called concentrate pipes 11 to the reception chamber 10, from which the sludge is pared off by the paring tube 21 and passes to the outlet channel 17 and out of the rotor. Some sludge passes from the outlet channel 17 through the hole 22 and enters the chamber 14. From there part of it flows further through the recirculation pipes 16 to the peripheral parts of the separation chamber 5, i.e. to the so-called sludge space, whereas excess sludge flows over the overflow outlet 15 back to the reception chamber 10. During operation the constant pressure valve 23 is automatically controlled such that the free liquid surface in the reception chamber 10 is maintained by the paring tube 21 at a predetermined radial level. In a corresponding manner the free liquid surface of the clarified liquid is maintained in the rotor at a radial level closer to the rotor axis. Hereby the said trans­ portation of sludge from the sludge space through the concentrate pipes 11 to the reception chamber 10 is accomplished. Because the valve 23 maintains a constant pressure in the outlet channel 17, independently of the flow through the channel 17, a constant flow of sludge is obtained through the hole 22 to the recirculation chamber 14. It is assumed here that the extension of the hole 22 in the flow direction is so short that the flow there­ through is substantially independent of any changes occurring in the sludge viscosity. However, depending upon the sludge viscosity (concentration) more or less of the sludge entering the chamber 14 will flow back to the sludge space through the pipes 16, or flow across the overflow outlet 15 back to the chamber 10, respectively. If the viscosity increases, a smaller part of the sludge will flow back through the pipes 16, while the flow across the over­ flow outlet 15 increases. Thus, if the viscosity and as a consequence the flow across the overflow outlet 15 increases, the out­ flow of sludge through the paring tube 21 and the outlet channel 17 also increases. In Figure 2 there is shown on an enlarged scale the connection of the concentrate pipe 11 to the recep­ tion chamber 10. It illustrates the flow determining throttle 12 more clearly than Figure 1. In Figure 3 there is shown an alternative embodi­ ment of the invention, according to which the reception chamber and the recirculation chamber are arranged at the top instead of at the bottom of the centrifuge rotor. Details in Figure 3 having counterparts in Figure 1 have been given the same reference numerals in Figure 3 with the addition of the letter "a". An additional member in this embodiment is con­ stituted by a conical so-called top disc 28 having a larger radial extension than the separation discs 6a. The concentrate channels 11a as well as the recirculation channels 16a are formed between the top disc 28 and the upper rotor part 1a, for instance by radial grooves in the upper side of the top disc 28. Another additional member is constituted by an upper annular end wall 29 which is attached to the rotor part 1 by means of a locking ring 30. The end wall 29 forms with the partition 9a the reception chamber 10a. Further additional members are constituted by two annular partitions 31 and 32. The partition 31 forms together with the partition 9a the recirculation chamber 14a. The partition 32 forms an annular overflow outlet 33 from the separation chamber 5 to a paring chamber 34 surrounding the paring disc 26a for clarified liquid. The embodiment according to Figure 3 is intended to operate in the following manner. A sludge containing liquid mixture is supplied to the rotor through the inlet channel 18a and flows through the reception chamber 25a and the holes 27a into the separation chamber 5a. Clarified liquid leaves the separation chamber 5a via the overflow outlet 33, the paring chamber 34, the paring disc 26a and the outlet channel 19a. Separated sludge flows from the sludge space through the concentrate channels 11a and the throttles 12a into the reception chamber 10a, from where it is pared off by means of the paring disc 21a. Part of the sludge leaves the rotor through the outlet channel17a, while the rest of it is conducted through the opening 22a to the recirculation chamber 14a. Some of the sludge flows from there back to the sludge space through the recirculation channels 16a, while the rest flows across the overflow outlet 15a directly back to the reception chamber 10a. With a constant pressure valve arranged in the outlet channel 17a (similar to the valve 23 in Figure 1) the operation is otherwise as has been earlier des­ cribed in connection with Figure 1. For reasons of clarity no liquid levels have been shown in Figure 3. It will be understood, however, the liquid level in the separation chamber 5a is determined by the position of the overflow outlet 33 and that in the recirculation chamber 14a by the position of the overflow outlet 15a. The latter is situated at a smaller radius than the former. Furthermore, it is intended that the liquid level in the reception chamber 10a be maintained radially outside the liquid level in the separation chamber 5a by means of the above mentioned constant pressure valve (not shown) in the outlet channel 17a. In Figure 4 there is shown a modification to part of the embodiment of Figure 3, the same reference numerals being used for corresponding details. One single member has been added in Figure 4, which is an annular slide 35. The slide can be turned around its own and the rotor axis. The slide 35 has a tubular part arranged radially between the annular walls defining the outlet channels 17a and 19a, respectively. At its lower end the tubular part of the slide 35 supports an external flange defining an annular groove 36 which opens upwards. Part of the member forming the outlet channel 17a extends down into this groove. Below the groove 36 and in the area of the re­ circulation chamber 14a the tubular part of the slide 35 has a radial through bore 37. In the diametrically opposite position the outer wall of the outlet channel 19a has a similar through bore 38. In the groove 36 the radially outer wall of the channel 17a has a radial through bore constituting the previously mentioned passage 22a, through which part of the separated sludge enriched component can be trans­ ferred from the reception chamber 10a through the channel 17a to the recirculation chamber 14a. To enable such transfer the radially outer side wall of the groove 36 has a corresponding through bore 39. During normal operation of the centrifuge rotor the slide 35 is maintained in the position illustrated in Figure 4. The bores 22a and 39 are then aligned so that through-flow is possible from the channel 17a to the recirculation chamber 14a. Simultaneously the bore 38 is closed by the lower part of the slide 35. When the centrifuge rotor is to be cleaned, the slide 35 is turned through 180° about its axis, so that the lower bore 37 in the slide will be aligned with the bore 38, and the bore 22a is covered by a non-perforated part of the side wall of the groove 36. Thus, liquid having entered the reception chamber 10a from the radially outer parts of the separation chamber 5a is prevented from returning to the separation chamber through the recirculation chamber 14a and the channels 16a, and all such liquid is instead conducted out of the rotor through the outlet channel 17a. However, part of the liquid having left the separa­ tion chamber 5a through the overflow outlet 33, the chamber 34 and the channel 19a, is conducted out through the bores 38 and 37 to the recirculation chamber 14a, so that this chamber and the recirculation channels 16a will be rinsed. Within the scope of the invention each throttle 12 (Figure 1 and 2) or 12a (Figure 3) may be substituted by a so-called vortex nozzle of the kind described in U.S. 4,311,270. A nozzle of this kind can be formed in a way such that a liquid flow therethrough increases with increasing viscosity of the liquid, and decreases with decreasing viscosity of the liquid. By means of vortex nozzles it is thus possible to provide an even more sensitive control of the concentra­ tion of the separated heavy component than can be obtained with the conventional throttles 12, 12a.

Claims: 1. A centrifugal separator comprising a rotor enclosing a separation chamber (5) provided with an inlet (27) for a mixture of components to be separated, a first outlet (26) for one separated component having a reduced sludge content and a second outlet (11) for another separated sludge enriched component, the second outlet having a flow restriction (12) and means being arranged for recirculating some separated sludge enriched component having passed therethrough for said recirculated component to flow again through the restriction, said recirculation means comprising one or more recirculation passages (16) so arranged that the recirculation flow of the sludge enriched component decreases with increasing viscosity of said component and increased with decreasing viscosity of said component, characterised by: means (9) in the rotor defining a reception chamber (10) for the separated sludge enriched component, the reception chamber (10) communicating with the separation chamber (5) through said second outlet (11), means (21) for conducting separated sludge enriched component from the reception chamber (10) to maintain a flow of sludge enriched component from the separation chamber (5) to the reception chamber (10), means (13) in the rotor defining a recirculation chamber (14), said recirculation passage or passages (16) communicating with the recirculation chamber, means (22) arranged to transfer sludge enriched component from the reception chamber (10) to the recirculation chamber (14), and means (15) arranged to maintain a predetermined liquid surface level in the recirculation chamber (14), said level being close enough to the rotor axis for sludge enriched component to flow from the recirculation chamber (14) through the recirculation passage or passages (16). 2. A centrifugal separator according to claim 1, wherein said means to maintain a predetermined liquid surface level in the recirculation chamber (14) com­ prises an overflow outlet (15). 3. A centrifugal separator according to claim 2, wherein the reception chamber (10) is arranged to receive sludge enriched component flowing from the recirculation chamber (14) through the overflow outlet. 4. A centrifugal separator according to claim 1, 2 or 3, wherein said means (21) for conducting sludge enriched component from the reception chamber (10) also serves as said means (22) arranged to transfer sludge enriched component from the reception chamber (10) to the recirculation chamber (14), whereby part of the component removed from the reception chamber (10) is conducted into the recirculation chamber (14), and another part is conducted out of the rotor. 5. A centrifugal separator according to any one of the preceding claims, wherein said flow restriction (12) is arranged for such a through flow of separated sludge enriched component that the magnitude of said flow does not decrease with increasing viscosity of the component. 6. A centrifugal separator according to to any one of the preceding claims, wherein the reception chamber (10) and the recirculation chamber (14) are both located centrally within the rotor, and said chambers communicate separately with the radially outer parts of the separation chamber (5) through passages (11, 16) having a radial extension. 7. A centrifugal separator according to claim 6, wherein said passages (11, 16) are evenly distributed around the rotor axis, each passage (11) communicating with the reception chamber (10) being positioned between two adjacent passages (16) communicating with the re­ circulation chamber (14).

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