Entraining device for a centrifugal separator The present invention concerns an entraining device for a centrifugal separator having a rotor rotatable around a rotational axis, the rotor forming an inlet chamber, in which an inlet tube opens for the supply during operation of a mixture of components to be separated, the inlet chamber having an outlet opening at a certain axiell position in the inlet chamber. The rotor also forms a separation chamber, which communicates with the outlet opening of the inlet chamber via at least one flow channel but otherwise is departed from the inlet chamber by means of a partition wall, which surrounds the rotational axis and has an axiell extension and the inside of which delimits the inlet chamber radially outwardly. Furthermore, the rotor forms at least one outlet for a component separated during operation. The entraining device is arranged in the inlet chamber fixedly connected to a part rotating with the rotor and extends axially along substantially all the axial length of the inlet chamber and comprises at least one liquid conducting element having two axial delimiting surfaces, the one of which being turned axially towards and the other of which being turned axially away from the outlet opening of the inlet chamber, and which extend radially and circumferentially in the inlet chamber and are at least partly located in a part of the inlet chamber, which during operation is filled up with liquid, the liquid conducting element having a radial inner edge, over which liquid can flow during operation when the liquid level in the inlet chamber is located radial inside this edge and at least one flow passage being arranged in the inlet chamber nearby the radial inside of the partition wall,

In each one of US-A-4, 701, 158, US-A-4, 721, 505 and WO-A-95/12082 there is disclosed a centrifugal separator, which has an entraining device of this kind in the inlet chamber in the form of a number of discs, which surround the rotational axis and extend radially and in circumferential direction and between themselves forms interspaces, through which the supplied liquid flows radially outwardly.

Thanks to the fact that the entraining devices in these centrifugal separators have large contact surfaces, which during operation entrain the supplied liquid mixture into the rotation of the rotor, extend radially and in circumferential direction the entrainment takes place gently along large surfaces in these centrifugal separators. The higher the flow of the liquid mixture supplied to the centrifugal separator is the more discs attend automatically to the increased need of entrainment by the fact that liquid overflows the radial inner edges of the liquid conducting elements and radially outwardly in more interspaces.

However, above all when the flow of the supplied liquid mixture is low liquid do not flow radially outwardly in all interspaces, which means that there is no axial flow along a portion of the insides of the partition walls, which delimits the inlet chambers radially outwardly towards the separa- tion chambers in the hereby previously known centrifugal separators. In many cases this means that sludge particles are deposited on the insides of the partition walls. Due to the fact that there is no space in these centri- fugal separators to design the insides of the partition walls with such a large angle with the rotational axis that these sludge particles can slip by the centrifugal force along the insides of the partition walls in direction towards the outlet openings of the inlet chamber the sludge particles will accumulate on these portions of the insides of the partition walls. Is this allowed to continue the inlet chambers eventually will be clogged where

these portions of the partition walls are located and then the centrifugal separation has to be interrupted for cleaning of the centrifugal separator.

In DE-C-30 41 210 and WO-A-97/17139 proposals are disclosed for cleaning the interior of the centrifugal separator. However, in the two proposals the centrifugal separation has to be interrupted and valable time of production will be lost. In many cases you can not get the centrifugal separator clean enough by the proposed methods but the centrifugal separator has to be disassembled, cleaned and re-assembled, which is a very labour- and time consuming operation.

The object of the present invention is to accomplish an entraining device for a centrifugal separator, which entrains the supplied liquid mixture, and which makes it possible to operate the centrifugal separator during long periods of time without the inlet chamber being clogged.

According to the present invention this is accomplished thanks to the fact that the one of the delimiting surfaces turned away from the outlet opening of the inlet chamber comprises a surface portion, which during operation at least partly is located in a part of the inlet chamber that is filled up with liquid, and which in circumferential direction extends axially in such a way that the surface portion seen in the rotational direction extends towards the outlet opening of the inlet chamber. In other words the delimiting surface, which is turned away from the outlet opening of the intet chamber, has a normalcy, which has a component in the rotational direction.

In one embodiment of the invention the axial extension in the circumfe- rential direction is more 0, 5 mm per revolution but less than 100 mm per revolution.

In another embodiment of the invention the surface portion consist of the entire delimiting surface, which is turned axially away from the outlet ope- ning of the inlet chamber and that the axial extension of it in the circumfe- rential direction is the same all along this delimiting surface.

In a further embodiment of the invention the two delimiting surfaces are substantially planar. Suitably, the two delimiting surfaces are parallel.

In a preferred embodiment of the invention the delimiting surfaces extend in the circumferential direction in a helically shaped path at least one revolution around the rotational axis.

Preferably, the flow passage is annular surrounding the rotational axis.

In a special embodiment of the invention the inlet chamber has an axial end, in which the inlet tube opens and in which the outlet opening is located.

In the following the invention is described more closely with reference to the attached drawing, in which the figure shows one embodiment of an entraining device according to the invention in a centrifugal separator.

The section of a part of a centrifugal separator schematically shown in the figure has a rotorbody 1, which is supported by a driving shaft 2. Inside itself the rotorbody forms a separation chamber 3. Centrally in the rotorbody 1 a wall element is arranged, which forms a partition wall 4 and together with parts of the rotorbody delimits an inlet chamber 5. The inlet chamber 5 has outlet openings 6 in its in the figure shown lower axial end and communicates with the separation chamber 3 via flow channels 7,

which are formed between the partition wall 4 and the rotorbody 1. In the separation chamber 2 a stack of frusto-conical separation discs 8 is arranged, the discs dividing the separation 3 in a number of interspaces, in which the main separation takes place. Axially though the stack of separation discs 8 a number of passages 9 extend, which are formed by right above one another located holes in the discs.

From above in the figure a stationary inlet tube 10 with an internal inlet channel 11 extends axially through a central opening in the rotorbody 1 into the rotor and further through a central opening 12 in the partition wall 4 into the inlet chamber 5. The inlet channel 11 has an opening 13, which is located in the in the figure lower axial end of the inlet chamber 5. In the inlet chamber an entraining device 14 according to the present invention is arranged fixedly connected to a part rotating with the rotor, as a suggestion to the partition wall 4 or to the rotorbody 1, and extending axially in the inlet chamber 5 along substantially all its length between the opening 13 of the inlet channel 11 and the opposite axial end of the inlet chamber 5. Closest to the opening 13 of the inlet channel 11 the entrai- ning 14 device is provided with an annular disc 15, which surrounds the inlet tube 10 leaving a gap 16 between itself and the inlet tube.

The embodiment of an entraining device 14 according to the invention shown as an example in the figure has a liquid conducting element, which is delimited axially by two delimiting surfaces 17 and 18, the one 17 of which being turned towards and the other one of which being turned from the outlet opening 6 of the inlet chamber 5. The delimiting surfaces 17 and 18 extend radially and in the circumferential direction around the inlet tube 10 and the rotational axis and is located during operation at least partly a part of the inlet chamber 5 being filled with liquid. The liquid conducting element has a radial inner edge turned towards the inlet tube

and surrounding the rotational axis enabling liquid during operating to flow over the edge when the radial liquid level, which in the figure is marked with a triangle, in the inlet chamber is located radially inside this edge 19.

An annular flow passage 20 surrounding the rotational axis is arranged between the liquid conducting element and the partition wall.

The centrifugal separator schematically shown in the figure is provided with an outlet 21 in the form of an overflow outlet for a separated specific lighter component.

The two delimiting surfaces 17 and 18 in the embodiment shown in the figure has a surface portion, which during operation at lest partly is located in a liquid filled part of the inlet chamber 5, and which in the circumferential direction extends axially in such a way that the surface portion seen in the rotational direction extends in a direction towards the inlet opening of the inlet chamber. In the shown example all of the delimiting surfaces 17 and 18 extend axially on the circumferential direction in such a way that they seen in the rotational direction extends towards the outlet opening 6 of the inlet chamber 5. Furthermore, the two delimiting surfaces 17 and 18 are substantially planar and parallel and extend in a helically shaped path several revolutions around the rotational axis.

The entraining device for a centrifugal separator shown in the figure operates in the following manner : While the rotor is rotating the liquid mixture of components to be separated is supplied through the inlet channel 11 to the inlet chamber 5.

The entering liquid fills up the inlet chamber lower part radially inwardly and eventually liquid flows through the gap 16 between the inlet tube 10

and the annular disc 15 of the entraining device where it comes in contact with the delimiting surfaces 17 and 18 of the liquid conducting element rotating with the rotor. Hereby, the delimiting surfaces 17 and 18 act entraining on the liquid. The liquid, which has not jet obtained the rotational speed of the rotor is moving opposite to the rotational direction relative to the delimiting surfaces 17 and 18, which has an axial exten- sion, which seen in the rotational direction, is directed from the outlet opening 6 of the inlet chamber 5. Hereby it is guarantied that at least a portion of the liquid flowing into the inlet chamber flows through a farther portion of the inlet chamber 5 seen from the outlet openings 6 and the flow passages 20 before it flows passing out to the separation chamber where the main separation takes place.

At a certain flow of the supplied liquid mixture to the centrifugal separator shown in the figure the free liquid surface of the rotating liquid body in the inlet chamber 5 will be positioned as illustrated by the continuous line and the little triangle in the figure. If the flow of the supplied mixture increases the liquid surface gradually will be so displaced that liquid will pass through more and more interspaces between the revolutions of the liquid conducting element.

By designing a centrifugal separator in this way a supplied liquid mixture can be entrained efficiently and gently at the same time as the danger of having the inlet chamber can be reduced.

In the embodiment shown in the figure the entraining device is provided with one single helically shaped liquid entraining element. The entraining device can, of course, be provided with more helically shaped liquid entraining elements or be composed by a number of elements distributed

axially and around the rotational axis. As a suggestion these might be shaped as vanes.

In the shown example the axial extension of the delimiting surfaces is constant but can also be varying by the distance to the outlet opening.

In the shown example the invention is used in a centrifugal separator having a vertical shaft but can, of course, also be used in centrifugal separators having a horizontal driving shaft such as in decanters.