CENTRIFUGAL SEPARATOR HAVING ENERGY TRANSFORMATION

Abstract: A centrifugal separator having a device for the transformation of kinetic energy of a liquid rotating in a chamber (12) to pressure energy comprising a discharge element (17) for the discharge of liquid out of the chamber. The discharge element (17) has a surface (20) arranged to be so located in the rotating liquid body that liquid flows in a predetermined direction along and in contact with the surface (20). The discharge element (17) forms an outlet channel (21) having an inlet opening (22) located in said surface (20), and limited downstreams by a cross edge (25) from which the outlet channel (21) extends a bit essentially in said predetermined direction. An increased outlet pressure is achieved by the fact that at least two passages (27, 28) are arranged in the discharge element (17) connecting a part each of the outlet channel located at axial ends of the cross edge (25), respectively, to the chamber (12) in such a manner that liquid flows through the passages (27, 28).

Description: Centrifugal separator having energy transformation The present invention concerns a centrifugal separator having a device for the transformation of kinetic energy of a liquid rotating in a chamber around a rotational axis to pressure energy. More precisely determined the device comprises a prefer¬ ably stationary discharge element for the discharge of liquid out of the chamber, which discharge element has a surface surrounding the rotational axis arranged to be so located in the rotating liquid body that liquid flows in a predetermined direction along and in contact iwith the surface. The discharge element forms an outlet channel having an inlet opening located in said surface, which downstreams is limited by a cross edge extending axially from which the outlet channel extends a bit essentially in said predetermined direction. In a centrifugal separator, which is provided with an energy transformation device of said kind, parts of the rotor of the centrifugal separator form an outlet chamber, in which liquid rotates. The outlet chamber is arranged to receive a separated liquid continuously from the separation chamber of the centri¬ fugal rotor. This liquid forms a rotating liquid body in the outlet chamber. Centrally in the outlet chamber there is arran¬ ged a discharge element through which liquid is discharged out of the outlet chamber and further out of the centrifugal rotor. A centrifugal separator of this kind is shown in WO 88/7893 for instance. In many cases it is important that the energy transformation device can transform as much as possible of the energy stored in the rotating liquid to pressure energy. How high pressure you then can achieve as a maximum is determined by the equation of Bernoullis for the pressure along a flow line of the liquid. p stat + p dyn = konst The static pressure P stat at the inlet opening is composed by the pressure from the part of the rotating liquid body, which is located radially inside the inlet opening, and the pressure which acts on this part of the liquid body. The dynamic pressure P< yn is in each point along a flow line determined by the equation P dyn - 12 ? W 2 in which P is the density of the liquid and W is the flow rate of the liquid in the point looked upon. Outside the inlet opening the liquid has a total pressure, which is the sum of the static and dynamic pressure there. However, in the device in a centrifugal separator known by WO 88/7893 only a minor part of the dynamic pressure can be recovered in the form of a liquid pressure in the outlet. Therefore, another device has been suggested for separators for the recovery of the kinetic energy of the rotating liquid, which is to be discharged out of the chamber of the centrifugal rotor. This device comprises a discharge device, which has a radial extension and an inlet opening in its radial outer portion facing the flow direction, of the liquid. By facing the inlet opening in this way a greater part of the dynamic pressure of the rotating liquid outside the discharge device can be recovered in the form of a liquid pressure. However, a discharge device designed in this manner has a great slowing down effect on the liquid in the chamber. Furthermore, is has a heavy agitating effect on the liquid, which results in partly a great risk for the admixture of air in the discharged liquid, partly a possibly damaging mechanical influence of the liquid. The object of the present invention is to accomplish a centrifugal separator having a device of the kind initially described for the transformation of kinetic energy of a rotating liquid to pressure energy, which device is able to recover a greater part of the static and the dynamic pressure in the rotating liquid than previously known such devices without involving an increasing risk for the admixture of air in the liquid. The object is furthermore that the device shall be able to do this without resulting in a too great retarding effect and too heavy stresses on the liquid, and without increasing the risk for oscillating movements of the rotating system. This is achieved according to the present invention by providing a centrifugal separator with a device of the said kind which has at least two passages arranged in the outlet device connecting a part of the outlet channel each located at axial ends of the cross edge respectively to the chamber in a way such that liquid flows through the passages. By designing the device in this manner a far greater pressure can be achieved in the outlet of the discharge element than by hitherto known devices. Hereby, a pump arranged in the outlet conduit can possibly be avoided, or the radial dimensions of the outlet element can be reduced, whereby a wanted liquid pressure in the outlet can be achieved with less energy losses. This is possible without creating great stresses on the separated liquid and without resulting in an increased risk of admixture of air or unstable operation conditions. The improved recovery of the dynamic pressure in the rotating liquid can be explained by the fact that the passages result in that a greater part than previously of the liquid flowing along the surface of the discharge element is conducted into outlet channel and towards the cross edge. A part of the liquid flowing through the inlet opening is conducted passing the cross edge and further through the outlet channel towards an outlet, an other part flows out again out of the outlet channel through the inlet opening. Along many of the flow lines, which during operation thus extend into the inlet opening, the flow rate decreases considerably nearby the cross edge, whereby a great part of the dynamic pressure in the rotating liquid is transformed into static pressure, which becomes effective in the outlet channel and in an outlet connected thereto. In order to give the best possible effect the passage preferably is not arranged in the cross edge itself but extends from a part of the outlet element, which together with the cross edge surrounds the outlet channel. Suitably the device comprises an even number of passages which are located symmetrically relative to a middle line through the inlet opening extending in the predetermined direction. In a preferred embodiment the passages extend essentially in the predetermined direction. The greatest outlet pressure is then achieved if the passages extend from the cross edge seen in the predetermined direction. In another preferred embodiment the inlet opening seen in the predetermined direction is limited by two side edges, each one of which extends towards and is turned into a limiting surface of one of the said passages. To advantage the side edges can diverge seen in the predetermined direction, having such a direction relative to the flow direction of the rotating liquid body that liquid crossing a side edge flows towards the inlet opening. At least one of said side edges can posses a curved shape, the radius of the curvature of the side edge preferably varies along the side edge in a way such that in the flow direction from being convex towards the inlet opening it turns to be concave towards the same. In the following the invention will be described more closely with reference to the accompanying drawings, in which Fig. 1 schematically shows an axial section through a part of a centrifugal separator, which is provided with a device according to the invention, and each one of Fig. 2 and Fig. 3 schematically shows a three dimensional view of an embodiment of an outlet element in a device according to the invention. The centrifugal separator shown in figure 1 comprises a rotor, which has a lower part 1 and an upper part 2, which are joined together axially by means of a locking ring 3. Inside the centrifugal separator shown as an example there is arranged an axially movable valve slide 4. This valve slide 4 delimits together with the upper part 2 a separation chamber 5 and is arranged to open and close an annular gap towards the outlet openings 6 for a component, which during operation is separated out of a mixture supplied to the rotor and is collected at the periphery of the separation chamber 5• The valve slide 4 delimits together with the lower part 1 a closing chamber 7, which is provided with an inlet 8 and a throttled outlet 9 for closing liquid. Inside the separation chamber 5 there is arranged a disk stack 10 consisting of a number of conical separation discs between a distributor 11 and the upper part 2. The upper part 2 forms at its upper end, shown in the figure, a chamber 12, to which in this case a specific lighter liquid component of the mixture can flow from the separation chamber 5 via an inlet 13. The liquid present in the chamber 12 during operation of the rotor forms a rotating liquid body having a radially inwards facing free liquid surface 14. Centrally through the chamber 12 a stationary inlet tube 15 extends, which opens in the interior of the distributor 11. Around the inlet tube 15 there is arranged a stationary outlet tube 16 for the specific lighter liquid component in the chamber 12. In the hamber a discharge element 17 is arranged around the inlet tube 15 and connected to the outlet tube 16. The discharge element is stationary, but in an alternative outlet arrangement a similar discharge element can be arranged to rotate with a rotational speed which is lower than the rotational speed of the rotor. The discharge element 17 extends radially outwards and has outside the radial level of the free liquid surface 14 of the rotational liquid body a part, which has at least one inlet opening 18. This, inlet opening 18 is connected to the interior of the outlet tube 16 via an outlet channel 19 formed in the discharge element 17. In figures 2 and 3 there is shown two examples of how a discharge element in a centrifugal separator can be designed according to the present invention. The discharge element shown in figure 2 has a circular cylindrical surface 20, which during operation is located in the rotating liquid body in the chamber 12 and along which the liquid flows in a predetermined direction. Inside the discharge element an outlet channel 21 extends, which has an inlet opening 22 in said surface and in its opposite end is connected to the interior of an outlet tube (not shown). In this example the inlet opening 22 seen in the flow direction of the liquid is limited by two straight side edges 23 and 24. Downstreams and upstreams the inlet opening 22 is limited by cross edges 25 and 26 respectively. At the connections between the cross edge 25 located downstreams of the inlet opening and the two side edges 23 and 24 two passages 27 and 28 open into the outlet channel 21. These are symmetrically located on each side of a middle line to the inlet opening 22 extending in the flow direction of the liquid and connecting the outer channel to the surroundings of the discharge element. From its connection to the outlet channel each one of the passages 27 and 28 extends essentially in the flow direction of the liquid. The passages 27 and 28 in this example are straight and has rectangular cross sections. The shown cross sections are open towards the surroundings of the discharge element along the surface 20. However, the passages also can be designed with closed cross sections. In the shown example the outlet channel 21 is limited i.a. by two limiting surfaces 29 and 30, which at the surface 20 are connected to one of the cross sections 25 and 26 each. In figure 3 there is shown another embodiment of the discharge element in a device according to the invention. The discharge element in Fig. 3 differs from the one shown in figure 2 in that the side edges 31 and 32 diverge in the flow direction of the liquid and has a curved shape. The radius of the curvature of the side edges is then varied along the side edges seen in the flow direction of the liquid in a way such that the side edges from being convex towards the inlet opening 33 turn to be concave towards the same. In the same manner as in figure 2 the two straight passages 34 and 35 open into the outlet channel 36. In the shown embodiments the inlet openings are designed in a circular cylindrical surface and directed radially and the passages open in a surface facing axially towards the chamber 12. However, the invention is also applicable on discharge elements, the inlet openings of which are formed in surfaces which are directed in other directions, for instance axially.

Claims: Claims 1. Centrifugal separator having a device for the transformation of kinetic energy of a liquid rotating in a chamber around a rotational axis to pressure energy, comprising a preferably stationary discharge element (17) for the discharge of liquid out of the chamber (12), which discharge element (17) has a surface (20) surrounding the rotational axis arranged to be so located in the rotating liquid body that the liquid flows in a predetermined direction along and in contact with the surface (20), the discharge element (17) forming an outlet channel (19, 21, 36) with an inlet opening (18, 22, 33) located in said surface and limited downstreams by an axially extending cross edge (25), from which the outlet channel (19, 21, 36) extends a bit essentially in said predetermined direction, c h a ¬ r a c t e r i z e d i n that at least two passages (27, 28, 34, 35) are arranged in the discharge element (17) connecting one part each of the outlet channel (19j 21 36) located at ends of the cross edge (25), respectively, to the chamber (12) in a way such that liquid flows through the passages (27j 28, 34, 35). 2. A device according to claim 1, c h a r a c t e r i z e d i n that the passages (27, 28, 34, 35) extend from a part of the discharge element (17), which together with the cross edge (25) surround the outlet channel (19, 21, 36). 3. A device according to claim 1 or 2, c h a r a c t e ¬ r i z e d i n that the passages (27, 28, 34, 35) extend essentially in the predetermined direction. 4. A device according to claim 3, c h a r a c t e r i z e d in that the passages (27, 28, 34, 35) seen in the predetermined direction extend from the cross edge (25). 5. A device according to claim 3, c h a r a c t e r i z e d i n that the passages (27, 28, 34, 35) seen in the predetermined direction extend towards the cross edge (32). 6. A device according to any of the previous claims, c h a ¬ r a c t e r i z e d i n that it comprises an even number of passages (27, 28, 34, 35), which are symmetrically located rela¬ tive to a middle line through the inlet opening (18, 22, 33). 7. A device according to claim 6, c h a r a c t e r i z e d i n that the inlet opening (18, 22, 33) is delimited seen in the predetermined direction by two side edges (23, 24, 31, 32), each one of which extends towards and turns into a delimiting surface of the passages (27, 28, 34, 35). 8. A device according to claim 7, c h a r a c t e r i z e d i n that said side edges (31, 32) diverge in the predetermined direction and have such a direction relative to the flow direc- tion of the liquid in the rotating liquid body that liquid cros¬ sing a side edge (31, 32) flows towards the inlet opening (33). 9. A device according to claim 7, c h a r a c t e r i z e d i n that at least one of said edges (31, 32) has a curved shape, the radius of the curvature of the side edge (31, 32) is varying along the side edge (31, 32) in a way such that it seen in the predetermined direction from being convex towards the inlet opening turns to be concave towards the same. 10. A device according to any of the previous claims, c h a ¬ r a c t e r i z e d i n that the passages (27j 28, 34, 35) are straight and have rectangular cross sections. 11. A device accordng to any of the previous claims, c h a - r a c t e r i z e d i n that the passages (27, 28, 34, 35) have cross sections which are open towards said chambers (12). 12. A device according to any of the previous claims, c h a ¬ r a c t e r i z e d i n that said surface (20) surrounding the rotational axis is facing radially. 13. A device according to claim 12, c h a r a c t e r i ¬ z e d i that the passages (27, 28, 34, 35) open towards the chamber (12) in an axially facing surface of the discharge element (17). 14. A device according to any of the previous claims, c h a ¬ r a c t e r i z e d i n that said chamber (12) is formed in a rotating casing (2).

Inventors: BORGSTROEM LEONARD, CARLSSON CLAES-GOERAN, FRANZEN PETER, INGE CLAES, LAGERSTEDT TORGNY, MOBERG HANS, NAABO OLLE, BORGSTROEM, LEONARD, CARLSSON, CLAES-GOERAN, FRANZEN, PETER, INGE, CLAES, LAGERSTEDT, TORGNY, MOBERG, HANS, NAABO, OLLE
Applicants: ALFA LAVAL SEPARATION AB, ALFA-LAVAL SEPARATION AB
Application year: 1990
Main classification: B04B11
IPC codes: B04B1, B04B11