EP0598099A1

EP0598099A1 - Centrifugal separator.

Info

Publication number: EP0598099A1
Application number: EP93913701A
Authority: EP
Inventors: Leonard Borgstroem; Patrik Brehmer; Claes-Goeran Carlsson; Peter Franzen; Claes Inge; Torgny Lagerstedt; Hans Moberg
Original assignee: Alfa Laval Separation AB
Current assignee: Alfa Laval Separation AB
Priority date: 1992-06-16
Filing date: 1993-05-19
Publication date: 1994-05-25
Anticipated expiration: 2013-05-19
Also published as: SE9201838D0; DE69312643T2; DE69312643D1; SE470348B; EP0598099B1; ES2106350T3; SE9201838L; WO1993025314A1; JPH06509749A

Abstract

Séparateur centrifuge permettant d'enlever, à partir d'un liquide, une substance d'une densité inférieure à celle du liquide, ce séparateur comprenant un rotor formant une chambre de séparation (6) dans laquelle est disposée une pile de disques de séparation coniques (15). Le liquide, et la substance qui y est dispersée, s'écoulent radialement vers l'extérieur dans des espaces interstitiels ménagés entre les disques. Afin d'accroître la capacité de séparation du séparateur à un débit d'écoulement élevé, les disques de séparation (15) présentent une zone radiale interne (29a) qui n'oppose pas d'obstacle à un écoulement principal dans la direction circonférentielle, et une zone radiale externe (28a) qui oppose à l'écoulement de liquide, dans la direction circonférentielle des obstacles (27a) allongés, s'étendant à travers la zone externe (28a) et répartis de manière égale autour de l'axe de rotation, en un nombre tel que l'écoulement principal dans cette zone (28a) est dirigé radialement vers l'extérieur entre les obstacles (27a). Ces derniers (27a) présentent une longueur et une orientation telles, par rapport à l'écoulement principal, que l'écoulement du liquide dans la couche la plus proche de la couche de substance influence cette dernière de telle manière que la substance séparée est accumulée sur l'obstacle (27a) se trouvant en avant et s'écoule radialement vers l'intérieur le long de cet obstacle.Centrifugal separator for removing, from a liquid, a substance with a density lower than that of the liquid, this separator comprising a rotor forming a separation chamber (6) in which is arranged a stack of conical separation discs (15). The liquid, and the substance dispersed therein, flow radially outward through interstitial spaces between the discs. In order to increase the separation capacity of the separator at a high flow rate, the separation discs (15) have an internal radial zone (29a) which does not oppose any obstacle to a main flow in the circumferential direction, and an outer radial zone (28a) which opposes the flow of liquid, in the circumferential direction, with elongated obstacles (27a), extending through the outer zone (28a) and distributed equally around the axis of rotation, in such a number that the main flow in this zone (28a) is directed radially outwards between the obstacles (27a). The latter (27a) have a length and an orientation such, with respect to the main flow, that the flow of the liquid in the layer closest to the layer of substance influences the latter in such a way that the separated substance is accumulated on the obstacle (27a) lying ahead and flows radially inwards along this obstacle.

Description

Centrifugal separator

The present invention concerns a centrifugal separator to clean a liquid from a substance dispersed therein, which has lower density than the liquid, comprising a rotor rotatable around a rotational axis. Inside itself the rotor forms an inlet chamber, a separation chamber, which is connected to the inlet chamber, and an outlet chamber connected to the separation chamber for a li- quid, which during operation is cleaned from the substance. In the separation chamber a stack of several frusto-conical separation discs is arranged coaxially with the rotational axis. The separation discs are provided with distance elements, which keep these discs at a distance from each other in a way such that they in pairs form interspaces. The centrifugal separator also comprises means arranged to conduct liquid and substance dispersed therein during operation from the inlet chamber to a central part of the interspaces in a way such that liquid flows radially outwards in the interspaces.

Centrifugal separators of this kind are known since a long time ago. The liquid mixture to be centrifugally treated is normally conducted into the interspaces via supply holes centrally located in the separation discs. The distance elements generally consist of spot-like elements or of distance elements extending radially between the separation discs. If the distance elements consist of spot-like elements, these have no essential influence on the flow in the interspaces. Thereby, the geostrophic flow, which is created upon a so called geostrophic balance in the interspaces essentially will be directed in the circumferential direction. Hereby, the radially outwards directed flow of liquid will take place in thin so-called Ekman-layers along the upper- and underside of the separation discs.

The radial flow resistance in these interspaces becomes high, which results in that the flow is equally distributed between the different interspaces. Besides, the minor flow resistance in the circumferential direction means that the flow in each interspace becomes equally distributed in the circumferential direction.

However, the fact that the radially outwards directed flow of liquid is distributed in thin Ekman-layers means that the flow velocity in these layers becomes high. This means that the layer of substance, which during operation has been separated in an interspace and been accumulated on a radially outwards directed side of a separation disc, which generally is its upper side, is exposed to a heavy shearing force, which strives to bring the substance radially outwards. If this shearing force exceeds the centrifugal force, which strives to bring the layer of substance radially inwards, there is a risk of having the substance entrained in the flow of the liquid, and be brought with the liquid out of the centrifugal separator. This delimits the possibility to get the liquid clean from the substance.

Centrifugal separators of this kind are for instance used to clean water, which is polluted by oil. Until now you have only been able at low flows through the centrifugal separators to achieve such a good separation result in these centrifugal separators that it has been possible to let out the cleaned water directly into the sea. The object of the present invention is to provide a centrifugal separator of the kind initially described, which has a satisfactory capability of separation also at a higher flow capacity than the one of hitherto known centrifugal separators upon separation of a liquid and a substance dispersed in the liquid, which has a lower density than the liquid. The substance might consist of solid particles or fractions of another liquid having a lower density than the first mentioned liquid.

This object is achieved according to the invention by the fact that you in a centrifugal separator of this kind design the separation discs with a radial inner zone, which essentially has no obstacles for a primary flow, a so called geostrophic flow, in circumferential direction so that the radial flow of liquid and of dispersed substance during operation in this zone takes place in very thin layers, so-called Ekman-layers, along the conical surfaces of the discs and by the fact that you design the separation discs with a radial outer zone, which connects onto the radial inner zone at its radial outer portion and has such a large number of elongated, equally around the rotational axis distributed, and radially through the radial outer zone extending obstacles for a liquid flow in the circumferential direction that primary flow in this radial outer zone essentially takes place radially outwards between two adjacent flow obstacles, these flow obstacles having such a length and such a direction in relation to the rotational direction that a secondary flow caused by the primary flow in the outer zone in a layer, so-called Ekman-layer, on a radial outwards directed surface of a separation disc influences the layer of substance separated onto this side with a shearing force in a direction towards a flow obstacle ahead in the rotational direction, in a way such that separated substance is accumulated at and flows radially outwards along the flow obstacle ahead.

By designing a centrifugal separator in this way the flow through each interspace becomes equally distributed in the circumferential direction and a resistance for flow radially outwards through the interspaces is obtained high enough to obtain an equal distribution of the flow over the different interspaces in the stack. At the same time the radially outwards directed flow of liquid is changed from taking place in thin Ekman-layers in the radial inner zone to take place in an essentially thicker layer of primary flow in the radial outer zone. The primary flow in circumferential direction ceases almost completely when the liquid flow through the interspaces exceeds a certain value. Hereby, the flow velocity of the liquid in the radially outer zone becomes lower, which in turn means that the layer of substance, which has been accumulated on the radially outwards directed side of the separation disc in the outer zone, is influenced by a lower radially outwards directed shearing force from the liquid flow. It is especially important in this, the radially outer zone, to keep these shearing forces low, since the layers of substance in this zone are thin and thereby the centrifugal force acting radially inwards on the layer substance is small.

In a preferred embodiment there is suggested for manufacture economical reasons that the flow obstacles in the radial outer zone are straight and that they are preferably directed essentially quite radially. Suitably, the flow obstacles are at least as long as the greatest distance in the circumferential direction between two adjacent flow obstacles.

In a special embodiment there is suggested that the flow obstacles are curved forward in the rotational direction seen radially outwards. Hereby, the primary flow is directed forwardly in the rotational direction. Since the shearing force, with which the primary flow influences the layer of substance, is directed 45° to the right seen in the direction of primary flow, the shearing force is directed in a direction such that it counteracts the centrifugal force less, whereby the separation result is improved.

In a special embodiment the radial inner zone is designed annular and surrounding the rotational axis. However, it is quite possible within the scope of the present invention to design a separation disc with a radial inner zone, which does not surround the rotational axis. A small number of the flow obstacles arranged in the outer zone might for instance extend radially inwards to a central part of the separation disc and delimit the radial inner zone in the circumferential direction. Hereby, the radial inner zone will only include a sector of the separation disc. However, this sector should have an angle at the centre, which at least is 45°, preferably at least 60°, to have a sufficient primary flow, a geostrophic flow, in circumferential direction created in this zone.

In still another embodiment there is suggested that the separation discs are designed with centrally located supply holes in order to have the supplied liquid mixture not to entrain and re-admix already separated and centrally in the separation chamber accumulated substance.

In the following the invention will be described more closely with reference to the attached drawings.

On these

figure 1 schematically shows an axial section through a rotor in a centrifugal separator according to the invention,

figure 2 shows a separation disc in a centrifugal separator according to figure 1 seen from above, and

figure 3 shows another embodiment of a separation disc in a centrifugal separator according to figure 1 seen from above.

The rotor shown in figure 1 comprises an upper part 1 and a lower part 2, which parts are kept together by a locking ring 3. The rotor is supported by a driving shaft 4, which is connected to the lower part 2. Inside the rotor there is a valve slide 5 arranged axially movable in the lower part 2. The valve slide 5 forms together with the upper part 1 a separation chamber 6 and is arranged to open and close an annular gap at the largest periphery of the separation chamber 6 between the separation chamber 6 and the outlet openings 7 to let out intermittently a component, which during operation has been separated out of a liquid mixture supplied to the rotor and accumulated at the periphery of the separation chamber 6. The valve slide 5 delimits together with the lower part 2 a closing chamber 8, which is provided with an inlet 9 and a throttled outlet 10 for a closing liquid.

Centrally in the rotor a distributor 11 is arranged, which surrounds a stationary inlet tube 12 and inside itself forms an inlet chamber 13. The inlet chamber 13 is connected to the separation chamber 6 via relatively centrally located holes 14 in the conical lower part of the distributor 11. Inside the separation chamber 6 a stack of a number of frusto-conical separation discs 15 is arranged coaxially with the rotational axis. The stack is supported by and guided by the distributor 11. At least a part of the separation discs 15 are identical.

In its in the figure shown upper end the upper part forms a central outlet chamber 16 for the discharge of a liquid during operation being cleaned from the substance and a central outlet chamber 17 for the discharge of a substance separated during operation. The first mentioned outlet chamber 16 communicates with the separation chamber 6 via an outlet channel 18 formed in the upper part 1 and an overflow outlet 19. The channel 18 formed in the upper part 1 opens in a radially outer portion of the separation chamber 6. The last mentioned outlet chamber 17 communicates via an overflow outlet 20 with a central part of the separation chamber 6.

In the two outlet chambers 16 and 17 a stationary discharge device 21, 22, respectively, is arranged in a known manner to discharge liquid and substance, respectively, through internal outlet channels 23, 24, respectively, towards an outlet 25, 26, respectively. Figure 2 shows a separation disc 15a seen from above. An arrow A shows the rotational direction during operation of the rotor and thereby the rotational direction during operation of the separation disc.

On its upper side the frusto-conical separation disc 15a has several straight elongated flow obstacles 27a for the liquid flow in the circumferential direction, which are equally distributed around the centre of the separation disc and extend quite radially through a radially outer zone 28a of the separation disc 15a. The flow obstacles 27a constitute at the same time distance elements, which keep the separation discs at a distance from each other in the stack in a way such that they in pairs form an interspace. Radially inside the outer zone 28a the separation disc is designed with a radially inner zone 29a, which has no obstacles for the liquid flow in the circumferential direction and from the inner radius of the separation disc extends radially outwards towards the radial inner portion of the radial outer zone 28a. In a radial inner part of the radial inner zone 29a a number of supply holes 30a are arranged equally distributed around the centre of the separation disc 15a for the supply of liquid to be treated.

Figure 3 shows another embodiment of a separation disc 15b seen from above. As in figure 2 an arrow A shows the rotational direction during operation of the rotor and the separation disc.

On its upper side the frusto-conical separation disc 15b shown in figure 3 has several curved elongated flow obstacles 27b for liquid flow in the circumferential direction, which are equally distributed around the centre of the separation disc and extend radially through the radial outer zone 28b of the separation disc 15b. Seen radially outwards the flow obstacles are curved forward in the rotational direction. As the flow obstacles 27a on the separation disc 15a according to figure 2 the flow obstacles 27b on the separation disc 15b according to this embodiment also constitute distance elements. Also this embodiment has a radial inner zone 29b, which has no obstacles for the flow in the circumferential direction and extends from the inner radius of the separation disc radially outwards towards the radial inner portion of the radial outer zone 28b. A number of supply holes 30b are also in this separation disc 15b equally distributed around the centre of the separation disc 15b and a radial inner portion of the radial inner zone 29b.

Both the separation discs according to figure 2 and the separation disc according to figure 3 are provided with a further zone 31a, 31b, respectively, located radially outside the flow obstacles in the outer zone 28a, 28b, respectively. As the radial inner zone 29a, 29b, respectively, this surrounds the rotational axis and has no obstacles for liquid flow in the circumferential direction. The arrangement of such a further zone 31a, 31b, respectively, means that the distribution of flows etc. is equalized in the circumferential direction in the radial outer portion of the separation chamber 6.

A centrifugal separator, which is designed according to the invention works in the following manner:

When starting the centrifugal separator the rotor is brought to rotate and the separation chamber 6 is closed by supplying closing liquid to the closing chamber 8 through the inlet 9. Then the liquid with a substance dispersed therein to be centrifugally treated can be supplied to the separation chamber 6 via the inlet tube 12, the inlet chamber 13 and the supply hole 14 in the distributor 11. The supply liquid is distributed via the supply holes 30a or 30b out into the interspaces between the separation discs 15 where the substantial separation is taking place. During the separation the specific heavier liquid flows radially outwards and is accumulated at the radial outer portion of the separation chamber, whereas the specific lighter substance is accumulated on the radial outwards directed sides of the separation discs 15 and flows along these radially inwards.

The cleaned liquid flows out of the separation chamber 6 through the channel 18 and via the overflow outlet 19 into the outlet chamber 16. The liquid is discharged out of the outer chamber 16 through internal discharge channels 27 in a stationary discharge device 21 out towards an outlet 25.

The separated and in the central portion of the separation chamber 6 accumulated substance flows out of the separation chamber 6 via an overflow outlet 20 into the outlet chamber 17. Also, the substance is discharged out of the outlet chamber 17 through internal channels 24 in a stationary outer device 22 towards an outlet 26.

If specific heavier solid particles, sludge or the like, are accumulated during operation at the greatest radius of the separation chamber these can be discharged intermittently during operation through the opening 7 by interrupting the supply of closing liquid to the closing chamber 8 for a short period of time. During the flow of the liquid and the dispersed substance in the interspaces in the radial inner zone 29a, 29b, respectively, a so called geostrophic balance is established, at which a Coriolis-force acting on the liquid is created which is as big as a counter-directed, radially inwards directed, the force which the pressure gradient gives rise to on the liquid. When this balance is at hand the most part of the liquid flows in a primary flow, a so called geostrophic flow, perpendicular to the pressure gradient. Since there are no obstacles for the liquid flow in the circumferential direction in this zone the primary flow essentially will be directed in the circumferential direction against the rotational direction.

As a result of the primary flow in the rotating system another liquid flow, a secondary flow, will be generated in thin layers at the upper and lower sides of the separation discs, so-called Ekman-layers. In these layers liquid flows in other directions than the directions of the primary flow. The direction varies with the distance from the surface of the separation disc. Closest to such a surface the flow direction in an Ekman-layer forms an angle of 45° to the direction of the primary flow. The flow direction in the Ekman-layers in the inner zone becomes a radially outwards directed component. Thus, the radial liquid transport in this zone will take place in these thin layers. This means that the resistance for radial liquid flow through this zone is so high that the flow is distributed equally over the interspaces in the stack.

The layer of substance accumulated on the radially outwards directed upper side of the separation discs is partly influenced by the centrifugal force, which strives to bring the layer of substance in a wanted direction inwards, partly via shearing force from the liquid flow in the Ekman-layers, which has a radially outwards directed component.

The centrifugal force increases by increasing thickness of the layer of substance. However, the shearing force is often independent of the thickness of the layer.

In the radial outer zone 28a or 28b the primary flow is directed essentially in radial direction, whereby the liquid transport radially outwards is taking place in a substantially thicker layer than the Ekman-layer, which means that the flow velocity of the liquid and the dispersed substance therein not yet separated becomes lower. This means in turn that the shearing force acting on the layer of substance becomes lower in the radial outer zone 28a or 28b.

Since the layer of substance is thin in the radial outer zone it is exceptionally to advantage to keep the shearing force low in this zone.

If the flow obstacles 27a or 27b are designed at least as long as the greatest distance between two adjacent flow obstacles, the fact that the shearing force forms an angle to the primary flow, which is 45° means that the substance to a great extent will be accumulated on a rear side of a flow obstacle ahead and flow radially inwards along the same.

If you design the flow obstacles 27b curved forward in the rotational direction seen radially outwards as shown in figure 3, the shearing force does not counteract the centrifugal force to the same extent but the resulting force gives a direction more favourable for the separation result.

Claims

1. A centrifugal separator to clean a liquid from substance dispersed therein, which has a lower density than the liquid, comprising a rotor rotatable around a rotational axis, the rotor forming

an inlet chamber (13),

- a separation chamber (6), which is connected to the inlet chamber (13), and

an outlet chamber (16) connected to the separation chamber (6) for a liquid, which dur- ing operation is cleaned from the substance,

a stack of several frusto-conical separation discs (15, 15a, 15b) being arranged in the separation chamber (6) coaxial to the rotational axis, the discs (15, 15a, 15b) being provided with distance elements, which keep these discs at a distance from each other in a way such that they in pairs form interspaces, and means ( 14, 30a, 30b, ) being arranged to conduct liquid and substance dispersed therein during operation from the inlet chamber (13) to a central part of the interspaces in a way such that liquid flow radially outward in the interspaces,

c h a r a c t e r i z e d i n

that the separation discs (15, 15a, 15b) has a radial inner zone (29a, 29b), which essen¬ tially has no obstacles for a primary flow, a so called geostrophic flow, in circumferential direction so that radial flow of liquid and of dispersed substance during operation in this zone (29a, 29b) takes place in very thin lay¬ ers, so called Ekman-layers, along the conical surfaces of the discs (15, 15a, 15b) and

that the separation discs (15, 15a, 15b,) have a radial outer zone (28a, 28b), which connects onto the radial inner zone (29a, 29b) at its radial outer portion and has such a large number of elongated and radially through the radial outer zone (28a, 28b) extending obstacles (27a, 27b) for liquid flow in the circumferential direction equally distributed around the rotational axis that primary flow in this radial outer zone (28a, 28b) essentially takes place radially outwards between adjacent flow obstacles (27a, 28b),

these flow obstacles (27a, 27b) having such a length and such a direction in relation to the rotational direction that a secondary flow caused by the primary flow in the outer zone (28a, 28b) in a layer, a so called Ekman- layer, on a radially outwards directed surface of a separation disc influences a layer of substance separated on to this side with a shearing force in a direction toward a flow obstacle (27a, 27b) ahead in the rotational direction, in a way such that separated substance is accumulated at and flows radially outwards along the flow obstacle (27a, 27b) ahead.

2. A centrifugal separator according to claim 1, c h a r a c t e r i z e d i n that said flow obstacles (27a) in the radial outer zone (28a) are straight.

3. A centrifugal separator according to claim 2, c h a r a c t e r i z e d i n that said flow obstacles (27a) in the radial outer zone (28a) are directed essentially quite radially.

4. A centrifugal separator according to claim 1, 2 or 3, c h a r a c t e r i z e d i n that the flow obstacles (27a, 27b) radially are at least as long as the greatest distance in the circumferential direction between two adjacent flow obstacles (27a, 27b).

5. A centrifugal separator according to claim 1, c h a r a c t e r i z e d i n that said flow obstacles (27b) in the radial outer zone (28b) are curved forward in the rotational direction seen radially outwards.

6. A centrifugal separator according to any of the previous claims, c h a r a c t e r i z e d i n that the radial inner zone (29a, 29b) is annular and surrounds the rotational axis.

7. A centrifugal separator according to any of the previous claims, c h a r a c t e r i z e d i n that said means consist of centrally located inlet holes (30a, 30b) in the separation discs.

8. A centrifugal separator according to any of the previous claims, c h a r a c t e r i z e d i n that said flow obstacles (27a, 27b) consist of said distance elements.