A centrifugal separator for cleaning gas

Abstract

The present invention provides a centrifugal separator (1) for cleaning gas containing contaminants. The centrifugal separator (1) comprises a stationary casing (2), enclosing a separation space (3) through which a gas flow is permitted; a gas inlet chamber (20) for receiving the supply of the gas to be cleaned and in fluid communication with said separation space (3); a rotating member (7) comprising a stack (9) of frustoconical separation discs (50) that is arranged to rotate around an axis (X) of rotation in said separation space (3); a gas outlet (28) arranged through the stationary casing (2) and configured to permit discharge of cleaned gas out from the stationary casing (2); a drainage outlet (29) arranged in the stationary casing (2) and configured to permit discharge of liquid contaminants that have been separated from the gas out from the stationary casing and a drive member (22) for rotating the rotating member (7). The separation discs (5) of said stack (9) form an angle (α) with said axis (X) of rotation that is below 42° and the axial height (H) of said stack (9) is above 90 mm.

Description

[0001] A CENTRIFUGAL SEPARATOR FOR CLEANING GAS

[0002] Field of the Invention

[0003] The present invention relates to the field of centrifugal separators for cleaning gas. In particular, the present invention relates to a separator for cleaning crankcase gases of a combustion engine from oil.

[0004] Background of the Invention

[0005] It is well known that a mixture of fluids having different densities may be separated from one another through use of a centrifugal separator.

[0006] One specific use of such a separator is in the separation of oil droplets from gas vented from a crankcase of an internal combustion engine. High-pressure gas found in the combustion chambers of an internal combustion engine may leak past the associated piston rings and into the crankcase of the engine. This continuous leaking of gas into the crankcase can lead to an undesirable increase of pressure within the crankcase and, as a consequence, to a need to vent gas from the casing. Such gas vented from the crankcase typically carries a quantity of engine oil (as droplets or a fine mist), which is picked up from the reservoir of oil held in the crankcase.

[0007] In order to allow vented gas to be introduced into the inlet system without also introducing unwanted oil (particularly into a turbocharging system wherein the efficiency of the compressor can be adversely affected by the presence of oil), it is necessary to remove as much oil as possible from the gas prior to the gas being introduced into the inlet system. This cleaning process may be undertaken by a centrifugal separator mounted on or adjacent to the crankcase, which directs cleaned gas to the inlet system and separated oil back e.g. to the crankcase. An example of such a separator is disclosed e.g. in US 8,657,908. Such separator has a stack of frustoconical separation discs for increasing the separation efficiency.

[0008] Increasing demands in cleaning efficiency and performance of a crankcase gas separator in combination with limited space for a separator when it is positioned on or close to an engine block of a combustion engine makes improvements and development a challenge. Thus, there is a need in the art for crankcase gas separators with improved performance.Summary of the Invention

[0009] It is an object of the invention to at least partly overcome one or more limitations of the prior art. In particular, it is an object to provide a centrifugal separator with an enhanced separation efficiency.

[0010] As a first aspect of the invention, there is provided a centrifugal separator for cleaning gas containing contaminants, said centrifugal separator comprising

[0011] a stationary casing, enclosing a separation space through which a gas flow is permitted;

[0012] a gas inlet chamber for receiving the supply of the gas to be cleaned and in fluid communication with said separation space;

[0013] a rotating member comprising a stack of frustoconical separation discs that is arranged to rotate around an axis (X) of rotation in said separation space;

[0014] a gas outlet arranged through the stationary casing and configured to permit discharge of cleaned gas out from the stationary casing;

[0015] a drainage outlet arranged in the stationary casing and configured to permit discharge of liquid contaminants that have been separated from the gas out from the stationary casing;

[0016] a drive member for rotating the rotating member;

[0017] wherein said separation discs of said stack forms an angle (a) with said axis (X) of rotation that is below 42° and the axial height (H) of said stack (9) is above 90 mm.

[0018] As used herein, the term “axially” denotes a direction which is parallel to the rotational axis (X). Accordingly, relative terms such as “above,” “upper”, “top”, “below”, “lower”, and “bottom” refer to relative positions along the rotational axis (X). Correspondingly, the term “radially” denotes a direction extending radially from the rotational axis (X). A “radially inner position” thus refers to a position closer to the rotational axis (X) compared to “a radially outer position”. A radial plane is a plane having its normal parallel to the axis (X) of rotation. An axial plane is a plane having its normal perpendicular to the axis (X) of rotation.

[0019] The contaminants in the gas may comprise liquid contaminants, such as oil, and soot.

[0020] Consequently, the centrifugal separator may be for separating liquid contaminants, such as oil, from gas. The gas may be crankcase gas of a combustion engine. However, the centrifugal separator may also be suitable for cleaning gases from other sources, for instance the environment of machine toolswhich frequently contains large amounts of liquid contaminants in the form of oil droplets or oil mist.

[0021] In prior art crankcase gas separators, the frustoconical separation discs of the disc stack usually form an angle (a) of 45° with the axis (X) of rotation. In order to improve the performance of such prior art disc stack, it would be natural to just increase the number and / or diameter of the separation discs. However, the first aspect of the invention is based on the insight that when increasing the axial height of the stack to above 90 mm, it is an advantage to use separation discs that form an angle (a) with the axis (X) of rotation that is below 42° (“<42° discs”) instead of the normal 45° discs. This is unexpected since for a given stack height of above 90 mm, using the “<42° discs” lead to a stack having a fewer number of separation discs compared to if a disc stack having the traditional 45° discs would be used. This is important due to the limited space available for the crankcase gas separator at the engine, i.e. it is important to utilize the axial height of the centrifugal separator as efficient as possible. Thus, it is unexpected that the stack of “<42° discs” performs better than a stack of 45° discs, even though the latter stack would comprise a larger number of discs, as is demonstrated in the experimental example herein.

[0022] Further, the inventor has realised that it is an advantage with no or only a moderate increase in the diameter of the separation discs, since an increase in diameter would affect the power consumption of the separator almost exponentially.

[0023] Without being bound by any theory, the technical effect may be due to a tradeoff between the g-forces experienced by the contaminant particles and the height of the disc stack. The “<42° discs” would lead to a smaller number of discs for a low disc stack height - and thus a lower separation capacity - but when the disc stack height gets high enough - i.e. above 90 mm - the benefit of using the “<42° discs” overcomes the negative separation effects caused by the lower number of discs. The benefits may be due to the higher g-forces experienced by the contaminants at the lower disc angle (a).

[0024] A frustoconical separation discs may comprise a radially inner flat portion that extend perpendicularly to the axis (X) of rotation, and a radially outer conical portion that extend outwardly and downwardly or upwardly from the flat portion. The “inner radius” of a frustoconical separation disc is the inner radius of the frustoconical portion, and the “outer radius” is the outer radius of the whole disc.

[0025] In embodiments of the first aspect, the separation discs of the stack have an outer radius (R) of 50-60mm.In embodiments of the first aspect, the separation discs of the stack have an inner radius (r) of 20-30 mm.

[0026] The radially inner flat portion usually comprises a central opening for receiving a spindle or shaft that also forms part of the rotating member of the separator.

[0027] Openings in the flat portion may further form part of a central space within the separation space into which gas to be cleaned is guided from the gas inlet chamber. Thus, gas to be cleaned may be guided from the inlet into the central space and then to the interspaces formed between the discs in the disc stack.

[0028] A plurality of separation discs is axially stacked to form a disc stack, with small interspaces formed between the discs in the stacks. Therefore, the frustoconical portion may comprise spacing elements, such as elongated spacing elements or spot-formed spacing elements, on an inner and / or outer surface. As an example, the inner or outer surface of a separation disc may comprise a plurality of spot-formed spacing members.

[0029] The imaginary cone formed by the frustoconical portion of the disc has a certain opening angle. Thus, the angle a formed between separation disc and the axis of rotation is half the opening angle of the imaginary cone formed by the frustoconical portion of the disc.

[0030] In embodiments of the first aspect, the separation discs of the stack form an angle (a) with said axis (X) of rotation that is below 40°, such as below 39°, such as around 38°.

[0031] The disc stack is usually axially stacked on a rotatable spindle or shaft between a lower end plate and an upper top disc, both of which having a larger thickness than a single disc of the plurality of separation discs.

[0032] In the context of the present disclosure the “axial height (H)” of the disc stack is the axial height excluding the lower end plate and the upper top disc, i.e. the axial height of all the separation discs having a separating function. Thus, the axial height H is the axial distance from the lowermost position of the conical portion of the lowermost disc to the uppermost position of the conical portion of the uppermost disc in the disc stack.

[0033] In embodiments of the first aspect, the axial height (H) of the stack is above 100 mm.

[0034] In embodiments of the first aspect, the axial height (H) of the stack is above 120 mm.In embodiments of the first aspect, the axial height (H) of the stack is below 200 mm, such as below 150 mm.

[0035] Thus, the axial height of the stack may be between 90-200 mm, such as between 90-150 mm, such as between 90-120 mm, such as between, 100-200 mm, such as between 100-150 mm, such as between 120-200 mm.

[0036] In embodiments of the first aspect, the stack comprises at least 45 separation discs.

[0037] In embodiments of the first aspect, the stack comprises at least 65 separation discs.

[0038] In embodiments of the first aspect, the stack comprises less than 110 separation discs.

[0039] In embodiments of the first aspect, the stack comprises less than 100 separation discs.

[0040] In embodiments of the first aspect, the stack comprises less than 90 separation discs.

[0041] Thus, the stack may comprise between 45-110 separation discs, such as between 50-100 separation discs, such as between 65-90 separation discs.

[0042] The stationary casing of the centrifugal separator may comprise a surrounding side wall, and first and second end walls, which enclose the separation space. The stationary casing may have a cylindrical shape with circular cross-section having a radius R from the axis (X) of rotation to the surrounding side wall. This radius R may be constant at least with respect to a major part of the circumference of the surrounding side wall. The first and second end walls may form an upper end wall and a lower end wall of the cylindrical shaped casing. The stationary casing may also be slightly conical.

[0043] The gas inlet chamber may be in fluid communication and attached to an end wall of the stationary casing, so that gas entering through the gas inlet chamber may be directed to the separation space. The gas inlet chamber may be arranged above or below the rotating member of the centrifugal separator, i.e. above or below the separation space.

[0044] The rotating member is arranged for rotation during operation of the separator by means of the drive member. The rotating member comprises the stack of separation discs discussed above.The rotating member may be journaled within the stationary casing via at least one bearing, such as via an upper and lower bearing arranged axially above and below the plurality of separation members, respectively.

[0045] In embodiments of the first aspect, the rotating member comprises an axial shaft that is supported by the at least one bearing. The axial shaft may thus be centered at the axis (X) of rotation. The separation members may be arranged around such axial shaft.

[0046] The gas outlet may be in the form of a gas outlet tube extending through a wall of the stationary casing, such as through a lower portion or upper portion of a surrounding side wall of the stationary casing. The gas outlet tube may be a generally cylindrical tube or have another cross-section, such as an oval crosssection.

[0047] The drainage outlet may be arranged in the lower portion of the stationary casing, such as arranged in a lower at the bottom of the separator. The drainage outlet may be arranged centrally in the end wall opposite the end wall through which, or at which, the inlet is arranged. The drainage outlet of the centrifugal separator may further be formed by several spot shaped through holes in the stationary casing or by a single drainage passage. The drainage outlet may be arranged at the axis of rotation or centered around the axis of rotation. The drainage outlet may also be in an annular collection groove at the inner end wall of the stationary casing. The drainage outlet may be arranged such that contaminants, such as oil, are drained though a bearing, or radially outside a bearing, arranged for journaling the rotating member.

[0048] The drive member may for example comprise a turbine wheel, rotated by means of an oil jet from the lubrication oil system of the combustion engine or a free jet wheel comprising a blow-back disk. However, the drive member may also be independent of the combustion engine and comprise an electrical motor, a hydraulic motor, or a pneumatic motor.

[0049] During operation, gas to be cleaned may be directed centrally through the stack of separation discs. In such a set-up, the rotating member may further define a central space formed by at least one through hole in the separation discs. This central space is connected to the gas inlet chamber and configured to convey the gas to be cleaned from the gas inlet chamber to the separation members, such as to the interspaces between the separation discs of the stack. Thus, the centrifugal separator may be configured to convey gas to be cleaned, such as crankcasegases, from the gas inlet chamber into a central portion of the rotating member. In this manner the crankcase gases may be "pumped" from the central portion of the rotating member through the disc stack by the rotation of the rotating member. Thus, the centrifugal separator may work according to the concurrent flow principle, in which the gas flows from a radial inner part to a radial outer part of the separation space, which is opposite to a separator operating according to the counter-current flow principle, in which the gas is conducted into the separation space at the periphery and conveyed towards a central part of the separation space.

[0050] As a second aspect of the invention, there is provided a method for cleaning gas containing contaminants, the method comprising

[0051] guiding gas containing contaminants to a centrifugal separator according to the first aspect above during rotation of the rotating member,

[0052] discharging cleaned gas from the gas outlet, and

[0053] discharging contaminants from the drainage outlet.

[0054] The contaminants in the gas may comprise liquid contaminants, such as oil, and soot.

[0055] This aspect may generally present the same or corresponding advantages as the former aspects. Effects and features of the third aspect are largely analogous to those described above in connection with the first and second aspects.

[0056] Embodiments mentioned in relation to the first aspect are largely compatible with the third aspect.

[0057] Brief description of the Drawings

[0058] The above, as well as additional objects, features, and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0059] Figure 1 shows a schematic drawing of the cross-section of an embodiment of a centrifugal separator for cleaning gas.

[0060] Figure 2 shows a schematic drawing of an embodiment of a disc stack of frustoconical separation discs.

[0061] Figure 3 shows the result from an experimental example comparing the cleaning efficiency of two different disc stacks.Figure 4 shows a perspective view of an embodiment of a frustoconical separation disc.

[0062] Figure 5 shows a top view of the frustoconical separation disc of Fig. 4.

[0063] Figure 6 shows a cross section of the frustoconical separation disc of Fig. 5 along line A.

[0064] Detailed Description

[0065] The centrifugal separator according to the present disclosure will be further illustrated by the following description with reference to the accompanying drawings.

[0066] Fig. 1 shows a cross-section of a centrifugal separator 1 according to the present disclosure. The centrifugal separator 1 comprises a stationary casing 2, which is configured to be mounted to a combustion engine (not disclosed), especially a diesel engine, at a suitable position, such as on top or at the side of the combustion engine.

[0067] It is to be noted that the centrifugal separator 1 is also suitable for cleaning gases from other sources than combustion engines, e.g. the environment of machine tools which frequently contains large amounts of liquid contaminants in the form of oil droplets or oil mist.

[0068] The stationary casing 2 encloses a separation space 3 through which a gas flow is permitted. The stationary casing 2 comprises, or is formed by, a surrounding side wall 4, an upper end wall 5 and a lower end wall 6.

[0069] The centrifugal separator 1 comprises a rotating member 7, which is arranged to rotate around an axis (X) of rotation within the separation chamber 3. The stationary casing 2 is stationary in relation to the rotating member 7, and preferably in relation to the combustion engine to which it may be mounted.

[0070] The stationary casing 2 has a radius from the axis (X) of rotation to the surrounding side wall 4 that is constant at least with respect to a major part of the circumference of the surrounding side wall 4. The surrounding side wall 4 thus has a circular, or substantially circular, cross-section in a radial plane.

[0071] The rotating member 7 comprises a rotatable shaft, i.e. spindle 8, and a plurality of frustoconical separation discs 50 arranged in a disc stack 9 that is mounted on the spindle 8. All the separation discs 50 of the stack 9 are provided between a top disc 10 and a lower end plate 11. The spindle 8, and thus the rotating member 7, is rotatably supported in the stationary casing 2 by means of an upper bearing 12 and a lower bearing 13, the bearings being arranged one on each axial side of the discstack 9. However, as an alternative, the bearings could both be arranged axially below or above the stack 9.

[0072] The separation discs 50 of the disc stack 9 are frustoconical and extend outwardly and upwardly from the spindle 8. An individual separation disc will be described in further detail in relation to Figs. 4-6 below.

[0073] The centrifugal separator 1 further comprises a drive member 22 in the form of a turbine wheel that is driven by a jet of pressurized oil from an oil nozzle 24. This nozzle 24 is arranged for being connected to an engine oil circuit of an internal combustion engine. During running of the internal combustion engine, oil is pumped through the oil nozzle 24 onto a turbine wheel 22, which is arranged in turbine housing 26. Since turbine wheel 22 is connected to the spindle 8, the rotating member 7, and thus the stack 9 of separation discs, also rotate upon rotation of wheel 22. As an alternative, the centrifugal separator 1 may comprise a drive member in the form of an electric motor arranged to rotate the rotating member 7. As a further alternative, the centrifugal separator 1 may comprise a turbine wheel connected to the spindle 8, wherein the turbine wheel is arranged to be driven by exhaust gases from the internal combustion engine. The rotating member 7 may also be arranged for being rotated by a mechanical drive unit. Thus, the centrifugal separator 1 may comprise a mechanical drive unit for rotating the rotating member.

[0074] The rotating member 7 defines a central space 15 within the separation space 3. The central space 15 is formed by through holes 62 evenly spaced around the axis (X) of rotation in each separation disc 50 of the stack 9. In the embodiments of Fig. 1, the through holes 62 extend through the top disc 10 and through each of the separation discs 50, but not through the lower end plate 11.

[0075] The separator 1 further comprises a gas inlet chamber 20 arranged for receiving the gas to be cleaned. The gas inlet chamber 20 is in this example arranged axially above the separation space 30 and also above and in fluid contact with the central space 15 via channels 21. The channels 21 are arranged radially outside the upper bearing 12. Thus, the gas inlet chamber 20 communicates with the central space 15 so that the gas to be cleaned is conveyed from the inlet 20 via the central space 15 to the interspaces 14 of the stack 9 of separation discs. The gas inlet chamber 20 is configured to communicate with the crankcase of the combustion engine, or any other source, via a gas inlet tube 35 permitting the supply of gas from the crankcase to the separator 1.The centrifugal separator 1 comprises a drainage outlet 29 arranged in the lower portion of the stationary casing 2 and configured to permit discharge of liquid contaminants separated from the gas. The drainage outlet 29 is in this embodiment in the form of through holes arranged in the lower end wall 6 so that separated liquid contaminants flow through the lower bearing 13 as they are drained from the separation space 3 to the turbine housing 26. Drainage outlet 29 may as an alternative be arranged radially outside the lower bearing 13. The separated oil, and other particles and / or substances, is led to an oil outlet 25 of the centrifugal separator 1 , which together with oil from the oil nozzle 24 used to drive the wheel 22, may be led back to the engine oil circuit of an internal combustion engine.

[0076] The gas outlet tube 28 of the centrifugal separator 1 is arranged through the stationary casing 2 and is configured to permit discharge of cleaned gas.

[0077] The gas outlet tube 28 forms in this example part of a stationary insert 30, which is arranged in a lower portion of the stationary casing 2. The stationary insert 30 is configured to form a sort of barrier between cleaned gas and separated contaminants prior to the cleaned gas and separated contaminants exit the stationary casing 2. For this purpose, the stationary insert 30 is arranged in the stationary housing 2 so that an outer annular vertical channel 32 for the separated contaminants is formed between the inner wall 40 of the stationary casing 2 and an annular wall member 31 of the insert 30. The insert 30 further comprises a central frustoconical cup member 33 with conically extending sidewalls. The annular wall member 31 extends axially downwards from the outer edge of the cup member 33. In this example, the lower part of the end plate 11 is arranged within the central cup member 33. With the design of the insert 30, separated liquid particles flowing down on the inner wall 40 of the casing 2 may be shielded from the rotating gas, so as to decrease the risk of separated oil being pulled from the inner wall 40 on its way down to drainage outlet 29.

[0078] During operation of the centrifugal separator as shown in Fig. 1, the rotating member 7 is kept in rotation by the oil nozzle 24 supplying oil against the wheel 22. As an example, the rotational speed may be in the range of 7.500-12.000 rpm. Contaminated gas, e.g. crankcase gas from the crankcase of an internal combustion engine, is supplied to the gas inlet chamber 20 via gas inlet tube 35. This gas is led further into the central space 15 and from there into and through interspaces 14 formed between the separation discs 50 of the stack 9. As a consequence of the rotation of the rotating member 7, the gas is brought to rotate, whereby it is pumpedfurther on radially outwardly through the gaps or interspaces 14. During the rotation of the gas in the interspaces 14, solid or liquid particles such as oil suspended in the gas are separated therefrom. The particles settle on the insides of the separation discs 50 and slide or run after that radially outwardly thereon. When the particles and / or liquid drops have reached out to the radial outer edges of the separation discs 50, they are thrown away from the rotating member 7 and hit the inner surface 40 of the surrounding side wall 4. Separated oil particles may form a film on the inner surface of the stationary casing 2. From there, oil may be pulled by gravity downwardly to bottom end wall 6 and then and leave the separation space 3 through the drainage outlet 29. For this, the inner wall of the bottom end wall 6 may be tilted radially inwards, so that oil leaving the surrounding inner wall 40 of the stationary casing 2 may be pulled by gravity towards drainage outlet 29. The path of the contaminants in the gas is schematically illustrated by arrows “D” in Fig. 1.

[0079] Cleaned gas freed from particles and exiting from the stack of separation discs 9 leaves the stationary casing 2 through the gas outlet tube 28. The path of the gas through the centrifugal separator 1 is schematically shown by arrows “C” in Fig. 1.

[0080] Fig. 2 shows a side view of the stack 9 of frustoconical separation discs 50 that are stacked onto spindle 8. The number of separation discs 50 in the stack 9 is normally higher than indicated in Fig. 2 and may be for instance 50 to 100 separation discs 50 depending on the size of the centrifugal separator 1. Thus, the stack 9 may comprise at least 45, such as at least 65 separation discs, and for clarity reasons, not all of them are shown in the stack 9.

[0081] A separation disc 50 in the stack has a conical portion 70 - in the form of a frustum of a cone - extending from an inner radius 56 to an outer radius 55. Radially within the inner radius 56 there is a flat portion 60 that will be described in more detail in relation to Fig. 4-6 below.

[0082] The axial height H of the disc stack 9 is at least 90 mm, which is higher compared to prior art centrifugal separators for cleaning crankcase gas. In this example, in which the conical portion 70 extends axially upwards, the axial height H of is defined as the axial distance from the inner radius 56 of the lowermost disc 50 of the stack 9 to the outer radius 55 of the uppermost disc 50 in the stack 9.

[0083] Consequently, in examples in which the conical portion 70 extends axially downwards, the axial height H of would be defined as the axial distance from the outer radius 55 of the lowermost disc 50 of the stack 9 to the inner radius 56 of the uppermost disc 50 in the stack 9.As an example, the axial height H of the stack 9 could be above 100 mm, such as above 120 mm. However, the axial height H of the stack 9 could be above 90 mm but below 200 mm, such as below 150 mm.

[0084] Figs. 4-6 show some more details of a separation disc 50. As seen in the perspective view in Fig. 4, the disc 50 comprises an outer conical portion 70 and an inner flat portion 60. The inner flat portion 60 is arranged in a plane perpendicular to the axis (X) of rotation when stacked onto the spindle 8. The flat portion 60 comprises a central opening 61 for receiving the spindle 8. Further, the flat portion 60 comprises a number of ribs 57 connected to the inner radius 56 of the conical portion 70. Between these ribs, opening 62 are formed. When formed into a stack, the openings or through holes 62 are aligned to form axial channels, that thus form part of the central space 15 of the separation space 3 (see Fig. 1).

[0085] The conical portion 62 extend outwardly and upwardly from the flat portion 60. It should be noted that in other embodiments, the conical portion could extend outwardly and downwardly. The conical portion 70 has an inner 51 and an outer 52 surface. In this example, the outer surface 52 is smooth, whereas the inner surface 51 comprises a plurality of spacing elements 54 - in this case spot-formed spacing elements 54. Due to the spacing elements 54, interspaces 14 between adjacent separation discs 50 are formed when the discs 50 are stacked to form the disc stack 9. The spacing elements 54 may have a dimension so that the axial thickness of each interspace 14 in a formed stack 9 is in the order of 0.5-2 mm, such as 1-2 mm.

[0086] The spot-formed spacing elements 54 may be shaped like raised points or half spherical protrusions. The spot-formed shape may further have either a circular or an oval cross-section, e.g. with a diameter of 5 mm or less, such as of 2 mm or less. Further, the spot-formed spacing elements 54 may provide for flow of gas in the circumferential direction in the disc stack. In the present example, the spot-formed spacing elements 54 are distributed in rings around the center of the disc 50.

[0087] As an alternative, the spacing members 54 may be elongated spacing members, such as radial spacing members.

[0088] The spacing elements 54 may be made integrally with the separation disc, e.g. integrally with a plastic separation disc during an injection moulding production method.

[0089] The top view of the separation disc 50 in Fig. 5 further illustrates the inner radius r of the separation disc, which is the radius from the center of the disc to the inner periphery 56 of the conical portion 70. Also illustrated in the outer radius R,which is thus the radius from the center of the disc to the outer periphery 55 of the conical portion 70.

[0090] As an example, the separation discs 5 of the stack 9 may have an outer radius (R) of 50-60 mm.

[0091] As an example, the separation discs 5 of the stack 9 may have an inner radius (r) of 20-30 mm.

[0092] Fig. 6 shows the cross-section of the disc 5 as seen at line “A” in Fig. 5. As can be seen, the separation disc - or more precisely the conical portion 70 of the separation disc 50 - forms an angle a with the axis (X) of rotation. According to the present invention, this angle is below 42°, such as below 40°, such as between 37° and 39°, such as about 38°. The surprising effect of having this unusually low angle a is shown in the Experimental Example below.

[0093] The angle a is thus half of the opening angle of the imaginary cone formed by the frustoconical portion 70.

[0094] The separation discs 50 of the stack 9 may be made of plastic or metal.

[0095] Experimental Example

[0096] The separation efficiency Eff(%) was evaluated for two different disc stacks; a prior art disc stack (1) and a disc stack according to the present invention (2). The disc stacks had the following characteristics:

[0097]

[0098] Thus, the stack height H was the same but the number of discs in the stacks was different.The separation efficiency Eff(%) was measured in a set up comprising an oil mist generator and an optical particle counter, set up in a test rig to measure the separating efficiency of each particles size down to ca 0.2micrometer.

[0099] Illustrated in the graph of Fig. 3 is the measured Eff(%) for different particle sizes (pm) for disc stacks 1 and 2. As clearly seen, the disc stack 2 according to the inventive concept lead to an Eff(%) that is higher, especially for particle sizes less than 0.4 pm, even though the number of discs are fewer. For example, the prior art disc stack 1 had only between 95-95.5 cleaning efficiency for particle sizes around 0.25 pm, whereas the inventive disc stack 2 had a cleaning efficiency of about 98% for this size. Thus, the better performance of the inventive disc stack is unexpected, since the prior art disc stack with an angle a of 45° have a larger number of separation discs.

[0100] The invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the claims set out below. The invention is not limited to the orientation of the axis (X) of rotation disclosed in the figures. The term “centrifugal separator” also comprises centrifugal separators with a substantially horizontally oriented axis of rotation. In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claims

CLAIMS1. A centrifugal separator (1) for cleaning gas containing contaminants, said centrifugal separator comprisinga stationary casing (2), enclosing a separation space (3) through which a gas flow is permitted;a gas inlet chamber (20) for receiving the supply of the gas to be cleaned and in fluid communication with said separation space (3);a rotating member (7) comprising a stack (9) of frustoconical separation discs (50) that is arranged to rotate around an axis (X) of rotation in said separation space (3);a gas outlet (28) arranged through the stationary casing (2) and configured to permit discharge of cleaned gas out from the stationary casing (2);a drainage outlet (29) arranged in the stationary casing (2) and configured to permit discharge of liquid contaminants that have been separated from the gas out from the stationary casing;a drive member (22) for rotating the rotating member (7);wherein said separation discs (5) of said stack (9) form an angle (a) with said axis (X) of rotation that is below 42° and the axial height (H) of said stack (9) is above 90 mm.

2. A centrifugal separator (1) according to claim 1, wherein said separation discs (5) of said stack (9) form an angle (a) with said axis (X) of rotation that is below 40°.

3. A centrifugal separator (1) according to claim 1 or 2, wherein the axial height (H) of said stack (9) is above 100 mm.

4. A centrifugal separator (1) according to claim 3, wherein the axial height (H) of said stack (9) is above 120 mm.

5. A centrifugal separator (1) according to any previous claim, wherein the axial height (H) of said stack (9) is below 200 mm, such as below 150 mm.

6. A centrifugal separator (1) according to any previous claim, wherein said separation discs (5) of said stack (9) have an outer radius (R) of 50-60 mm.

7. A centrifugal separator (1) according to any previous claim, wherein said separation discs (5) of said stack (9) have an inner radius (r) of 20-30mm.

8. A centrifugal separator (1) according to any previous claim, wherein said stack (9) comprises at least 45 separation discs.

9. A centrifugal separator (1) according to claim 8, wherein said stack (9) comprises at least 65 separation discs.

Images