Turbomolecular vacuum pump

Abstract

A turbomolecular vacuum pump 10 comprises a rotor 25 with axially spaced rotor blade rows, a stator 35 with stator blade rows, each comprising blades 31 and an outer annular portion 32 for axially spa

Description

FIELD OF THE INVENTION The field of the invention relates to vacuum pumps and in particular, to turbomolecular vacuum pumps. BACKGROUND Turbomolecular pumps are used for evacuating high or ultra-high vacuum systems and comprise axially spaced stator blade rows interspersed with axially spaced rotor blade rows. In operation the outer tips of the rotor blades approach the molecular speed of the gas being pumped and when a molecule strikes the rotor, a significant component of momentum is transferred to the molecule. The angle of the blades means this transferred momentum causes the molecule of gas to move from the inlet side of the pump towards the exhaust side of the pump. In order to achieve this sort of tip speed, turbomolecular pumps are characterized by a high rotational speed of 20,000 to 90,000 rpm and for this require a lightweight motor and robust bearings. In order to increase pumping speed or capacity of a turbomolecular pump TMP a size of the pump and / or a rotational speed can be increased. Increasing the size of the rotor increases its weight and makes increasing the speed more difficult. Furthermore, the temperature of the pump increases with increasing gas friction and a larger sized pump has a lower surface area to volume ratio and may be more difficult to cool. SUMMARY A first aspect provides a turbomolecular vacuum pump for pumping gas from an inlet to an outlet, the turbomolecular pump comprising: a rotor comprising a plurality of axially spaced rotor blade rows; a stator comprising a plurality of stator blade rows comprising multiple blades and an outer annular portion for axially spacing the multiple blades of adjacent stator blade rows; and a housing comprising an inner side wall surrounding an outer surface of the stator blade rows; wherein the plurality of rotor blade rows and the plurality of stator blade rows comprise blade rows of at least two different diameters, larger diameter blade rows being towards the inlet of the turbomolecular vacuum pump; and the housing comprises at least two portions, the at least two portions comprising inner cylindrical surfaces of different diameters, the diameters of the portions decreasing from the inlet to the outlet; wherein the stator blade rows are mounted between inlet and outlet radially extending surfaces of the housing; and at least one intermediate stator blade row is mounted on a corresponding at least one intermediate radially extending surface of the housing, a contact surface area between the at least one intermediate stator blade row and the at least one intermediate radially extending surface being at least 10% higher, in some cases more than 15% higher, than a contact surface area between the inlet stator blade row and the inlet radially extending surface. A pressure at an inlet to a vacuum pump is lower than that towards an outlet, and thus, as a gas moves through a pump it becomes more compressed. This may allow the size of the pumping chamber to be reduced from the inlet to the outlet. Although conventionally turbomolecular pumps may be formed with a uniform diameter from inlet to outlet, both for ease of manufacture and assembly, reducing the diameter of the rotor towards the outlet reduces the weight of the rotor. This may allow for an increase in speed of the rotor without the need to provide bearings of increased robustness and / or an increase in diameter of the rotor towards the inlet without increasing the overall weight of the rotor. It was also recognised that a reduction in the diameter of the rotor at a point along its axis will lead to a corresponding reduction in diameter of the stator, and that this affords an opportunity to provide an additional thermal conductivity path between a radially extending surface of the housing and a surface of an intermediate stator blade row. This may provide an additional and effective way of cooling the stator and thereby the rotor of the pump. Thus, a TMP may be provided with an increased pumping capacity that can be effectively cooled. In some embodiments, the inlet radially extending surface contacts an outer annular portion of an inlet stator blade row and the outlet radially extending surface contacts an outer annular portion of an outlet stator blade row. The stack of stator blade rows may be sandwiched between an inlet radially extending surface and an outlet radially extending surface, the inlet stator blade row being adjacent to and contacted by the inlet radially extended surface and the outlet stator blade row being adjacent to and contacting the outlet radially extended surface. In some embodiments, the outlet radially extending surface and the intermediate radially extending surface extend from an inner surface of a sidewall of the housing. In some embodiments, the inlet radially extending surface also extends from an inner surface of a sidewall of the housing, while in other embodiments the housing comprises an inlet cover portion mounted on the sidewalls of the housing, the inlet radially extending surface comprising a surface of the inlet cover portion facing towards the outlet. In some embodiments, the at least one intermediate radially extending surface delimits a portion of the inner surface of the housing with a first diameter and a portion of an inner surface of the housing with a second diameter, the first diameter being larger than the second diameter. The intermediate radially extending surface may be a surface of the housing between two portions of the housing with different inner diameters, this surface provides a surface adjacent to which the intermediate stator blade row may be mounted, the contact area between the intermediate stator blade row and this radially extending surface providing a thermal conductivity path at a point in a middle portion of the stator blade stack. This provides an additional path for heat to be conducted away from the stator blade rows and towards the housing which may be cooled. In some embodiments, a portion of the housing with an inner surface comprising the first diameter comprises an inlet portion of the housing, the inlet portion of the housing and the portion of the housing with an inner surface of a second diameter are attached together to provide a preloaded force on the stator blade rows between the inlet radially extending surface and the intermediate radially extending surface. The housing may be formed of two or more portions or components that are attached together in some cases by bolts. The attachments means may be tightened to provide a preloaded force on the elements of the stator blade row stack such that they are compressed together. This helps keep them in place and also aids thermal conductivity between the elements and between radially extending surfaces of the housing and the contacting stator blade row surfaces. In some embodiments, said vacuum pump further comprises at least one elastic element mounted between facing surfaces of an outer annular stator portion adjacent to and at an outlet side of the intermediate stator blade row and the intermediate stator blade row. One effect of the intermediate stator blade row is that the stator blade stack is not aligned radially along its axial length and this provides additional challenges when providing the preloaded force on the stator blade rows. Furthermore, as the number of stator blades in the stack increases, the effect of tolerances may make it more difficult to provide a predetermined preloaded force. Embodiments, provide an elastic element between facing surfaces of the outer annular portions of the intermediate stator blade row and the outer annular stator portion which may be the outer annular portion of an adjacent stator blade row or may be an annular stator spacer, which elastic element allows for some compensation for these tolerances. This also allows for a good contact force between the radially extending surface and the intermediate stator blade while still providing a transmission of the preloaded force to the stator blade rows towards the outlet. In some embodiments, the portion of the housing with an inner surface of a second diameter comprises an outlet portion of the housing. The turbomolecular pump may have two portions, an inlet portion and an outlet portion, the inner surface of the housing wall of the inlet portion having the first diameter and the inner surface of the outlet portion of the housing having the second diameter. In some embodiments, an outer annular portion of the intermediate stator row blade extends radially inwardly beyond the radially extending surface and contacts the adjacent outer annular stator portion. In order for the intermediate stator row blade to transmit the preloaded force towards the adjacent stator blade row the outer annular portion of the intermediate stator blade row may extend radially inwards beyond the radially extending surface and provide a contact surface area for transmitting this preloaded force to the adjacent outer annular stator portion. In this way, the stator blade rows between the adjacent outer annular stator portion and the outlet radially extending surface are held together by a preloaded force on the stator blade rows between the intermediate stator blade row and the outlet radially extended surface. In some embodiments, the inlet end of the portion of the housing with an inner surface of a second diameter comprises an inner surface of an increased diameter a further radially extending surface extending from the increased diameter inner surface to the inner surface comprising the second diameter, the adjacent outer annular stator portion comprising the increased diameter and being mounted between the intermediate stator blade row and the further radially extending surface. In order to provide a sufficient area for the force to be transmitted effectively between the intermediate stator blade and the adjacent outer annular stator portion and also to provide space for the elastic ring, the adjacent outer annular stator portion may be mounted within an increased diameter portion or recess in the housing allowing it to contact the outer annular portion of the intermediate stator blade row and also the outer annular portion of the stator blade row on its outlet side. In some embodiments, the adjacent outer annular stator portion comprises a further elastic element between a surface of an outer annular portion of the adjacent outer annular stator portion and a further radially extending surface of the housing. In this regard, where the adjacent outer annular stator portion is mounted within a larger diameter recess in the portion of the housing having a second diameter inner surface then it will be mounted such that it does not contact the radially extended surface and the preloaded force exerted by the intermediate stator blade on this stator blade is transmitted to the adjacent outer annular stator portion and in this way down through the stack of stator blade rows towards the outlet radially extending surface. In order to ensure that it does not contact the further radially extending surface there may be a further elastic element mounted on this side of the adjacent stator blade row such that the preloaded force is transmitted towards the stator blade rows towards the outlet of the vacuum pump. The outer annular portion of the adjacent outer annular stator portion extends radially inwardly beyond the further radially extending surface. In some embodiments, the plurality of stator blade rows comprise an outer annular portion and a stator blade portion, a radial length of said outer annular portion being different for at least some of said stator blade rows. In some embodiments, at least some of the stator blade rows are made up of multiple components comprising stator blades and a spacer ring that forms the outer annular portion of the stator blade row and provides the axial displacement between the stator blades. In some embodiments at least some of the stator blade rows are formed as a single component, the outer annular portion of the stator blade row and the stator blades being formed as a unitary piece. In some cases, the turbomolecular pump may be made up of an assortment of these types of stator blade rows. In some embodiments, the at least one intermediate stator blade row comprises an outer annular portion with an increased radial length compared to a radial length of an outer annular portion of a stator blade row at an inlet end of a same portion. The at least one intermediate stator blade row comprises a reduced length stator blade compared to the inlet stator blade row but has the same diameter. In some embodiments, a stator blade row adjacent to, and on an outlet side of the at least one intermediate stator blade row comprises an outer annular portion with a reduced radial length and a stator blade of a same radial length as the intermediate stator blade row. The turbomolecular pump has a housing with inner surfaces of different diameters, in some cases two different diameters and in some cases more than two. The diameter of the pumping chamber may change more frequently along the axial length of the pump than the diameter of the housing inner surface does and these additional changes may be provided by changing the length of the outer annular portion compared to the length of the blades while retaining the same overall diameter of the stator blade row. In effect along the axial length from the inlet towards the outlet, the diameter of the pumping chamber may be reduced, by increasing the outer annular portion of the stator blade rows until an intermediate radially extending surface is encountered at which point the increased sized outer annular portion provides a good contact area with the intermediate radially extending surface. The housing inner surface diameter decreases following the intermediate radially extending surface and the subsequent stator blade rows towards the outlet may then have a correspondingly reduced overall diameter and the radial length of the outer annular portion may be reduced, such that the stator blade lengths are the same or similar between the intermediate stator blade row and the adjacent stator blade row. Subsequent stator blade rows towards the outlet may have increased outer annular portions and reduced stator blade radial lengths as the diameter of the pumping chamber decreases further. Again this may allow for an increased contact surface area between the outlet radially extending surface and the outlet stator blade row. In some embodiments, a contact surface area between the at least one intermediate stator blade row and the at least one intermediate radially extending surface comprises more than 50% but less than 80% of a surface area of the outer annular portion of the at least one intermediate stator blade row. A good contact surface area between the intermediate stator blade row and radially extending surface provides for improved thermal conductivity and it is therefore advantageous if a good percentage of the intermediate stator blade row surface area contacts this surface. However, the preloaded force is also transmitted via the stator blade row to the adjacent outer annular stator portion and thus, the outer annular portion of the at least one intermediate stator blade row should extend radially inwardly beyond the radially extending surface to some extent. A value of between 50 and 80% may provide a good compromise between improved thermal conductivity and acceptable transmission of preloaded force. In some embodiments, the outer annular portion of the intermediate stator blade row comprises between 10 and 40% of a radial length of the stator blade row. As noted above, the outer annular portion of the intermediate stator blade row may comprise a significant proportion of the radial length of the stator blade row so as to provide an effective contact surface to provide good thermal conductivity to cool the stator and hence the rotor. However, the portion of the stator blade row that comprises the stator blades governs the conductance area for the gas flow and thus, a length of between 10 and 40% of the radial length of the stator blade row has found to be a good compromise for good conductance of gas and good thermal conductivity. In some embodiments, the turbomolecular vacuum pump comprises at least two intermediate stator blade rows mounted on corresponding at least two intermediate radially extending surfaces of the housing, the housing and stator blade rows comprising at least three different diameters. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 shows a section through a portion of a turbomolecular vacuum pump according to an embodiment; Figure 2 shows a further section through a portion of the turbomolecular vacuum pump comprising the intermediate stator blade row; and Figure 3 shows a stator blade row. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. There are various options for mounting and tensioning the stator stack in a turbomolecular pump TMP. Generally, in single inlet pumps the stator stack is only in axial contact with the housing at two points. This can be disadvantageous for optimum heat dissipation from the stator as the heat generated in the centre of the stack has to travel a long way to reach the pump housing and thus ultimately the cooling device. Embodiments therefore enable thermal contact between the stator and housing at (at least) 3 points. In some embodiments, tolerance compensation is provided by an elastic element which is located towards the centre of the stator stack. In turbomolecular pumps, particularly those designed for high gas flow rates, significant amounts of heat are introduced into the stator. In order to transport this heat to the cooling device, solid-state heat conduction between the stator disks takes place in the first instance. High contact surface and high surface pressure between the components involved in heat transport are advantageous for this. When the stator disks of a stator stack of a TMP have different outer diameters (at least 2) the protruding area between the two different diameters can be used as a radially extending contact surface between the housing and stator stack and thus improve heat dissipation from the stator stack. Any tolerance compensation may be provided by an elastic element. Figure 1 shows a section through a turbomolecular pump according to an embodiment. Turbomolecular pump 10 comprises a rotor 25 rotatably mounted within a stator 35 which comprises a stack of stator blade rows retained within housing 45. The rotor 25 has a corresponding plurality of axially spaced rotor blade rows interspersed between the stator blade rows. The rotor blade rows comprise a plurality of rotor blades that are shaped to pump gas from an inlet port 50 towards an outlet end of the pump 52. The rows of stator blades are arranged axially interspersed with the rows of rotor blades and the blades are angled such that molecules of gas are hit by the rotating rotor blades onto the stator blades and are preferentially directed towards the outlet. In this embodiment, the housing 45, the stator 35 and the rotor 25 have diameters that decrease from a wider diameter at the inlet 50 end to a smaller diameter at the outlet end 52 of the pump. The stator blade rows are formed of an outer annular portion 32 and a blade portion 31. Outer annular portion 32 supports the blade portion 31 and provides the axial separation between the stator blade rows. The outer annular portion may be integral with the stator blade or it may be formed as a separate spacer ring. In this embodiment, the annular outer portion 32 increases in length while the blade portion 31 reduces in length between an inlet in this case upper stator blade row 38 and an intermediate stator blade row 33 which rests on an intermediate surface 40 of the housing 45. The outer annular portion 32 of the intermediate stator blade row 33 has an increased surface area owing to its increased radial length compared to the outer annular portion of the inlet stator row blade 38 and this provides an increased contact surface area between the outer annular portion 32 of the intermediate stator blade row 33 and the intermediate radially extending or supporting surface 40. This provides a heat conduction path allowing heat to be removed from the stator to the cooled housing at a point somewhere towards the axial centre of the stator. As can be seen, the stator blades 31 decrease in length from the inlet 50 to the outlet 60. The outer annular portion 32 of the stator blade rows increases in length in different portions of the housing such that at the inlet the outer annular portion 32 is small while at the intermediate surface it is significantly larger. It then decreases again as the diameter of the housing decreases to become larger again towards the outlet end 52. In this embodiment, there is stator spacer 34 adjacent to and at the outlet side of the intermediate stator blade row 33. This adjacent stator spacer 34 comprises elastic mounting rings 41 between it and the intermediate stator blade row 33, which allows tolerances to be compensated for. The stator blade rows continue to the outlet stator blade row 37 which provides a further thermal conductivity path 62 between it and the radially extending outlet surface 47. Cooling of the housing is shown schematically at arrow 60, while heat flow from the stator through the housing to the cooled portion is shown schematically by arrows 62. The stator blade rows are mounted as a stack and are held together by inlet radially extending surface 42 and outlet radially extending surface 47. The housing 45 has an inlet portion 45I and an outlet portion 450 that are bolted together by attachment means 49. This compresses the stator blade rows together and holds them in place. Figure 2 shows a section through the pump showing the junction between the inlet portion 45I and the outlet portion 450. Tightening of the attachment means or bolt 49 compresses the two portions of housing 45 together and provides a preloaded force on the stator blade rows held between radially extending surfaces of these two portions of the housing. Intermediate stator blade row 33 can be seen mounted on radially extending intermediate surface 40. Adjacent stator spacer 34 is mounted in a recess at the top of the outlet portion 450 of the housing. The recess has a radially extending further surface 43 which the adjacent stator spacer 34 is mounted above and does not contact. This allows force exerted on the intermediate stator blade row 33 by tightening the bolt 49 to be transmitted to adjacent stator spacer 34 and then on to subsequent stator blade rows towards the outlet of the pump. There is an elastic ring 41 between the adjacent stator spacer 34 and the intermediate stator blade row 33, which elastic ring allows for tolerances. Figure 3 schematically shows a stator blade row 36 between two rotor blade rows 26. The stator 5 blade row 36 has a radial length C, comprising the radial length A of the outer annular portion 32, which in this example comprises a spacer ring, and a radial length B comprising the radial length of the stator blade 31. Although illustrative embodiments of the invention have been disclosed in detail herein, with io reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 10 turbomolecular pump 25 rotor 26 rotor blade row 31 stator blade portion 32 outer annular portion 33 intermediate stator blade row 34 adjacent stator spacer 35 stator 36 stator blade row 37 outlet stator blade row 38 inlet stator blade row 40 intermediate radially extending surface 41 elastic ring 42 inlet radially extending surface 43 further intermediate radially extending surface 45 housing 45I inlet portion of housing 450 outlet portion of housing 47 outlet radially extending surface 49 attachment means 50 Inlet 52 outlet end of the pump 60 cooling thermal pathway 62 heat thermal pathway

Claims

1. A turbomolecular vacuum pump for pumping gas from an inlet to an outlet, the turbomolecular pump comprising:a rotor comprising a plurality of axially spaced rotor blade rows;a stator comprising a plurality of stator blade rows comprising multiple blades and an outer annular portion for axially spacing the multiple blades of adjacent stator blade rows; anda housing comprising an inner side wall surrounding an outer surface of the stator blade rows; whereinthe plurality of rotor blade rows and the plurality of stator blade rows comprise blade rows of at least two different diameters, larger diameter blade rows being towards the inlet of the turbomolecular vacuum pump; andthe housing comprises at least two portions, the at least two portions comprising inner cylindrical surfaces of different diameters, the diameters of the portions decreasing from the inlet to the outlet; whereinthe stator blade rows are mounted between inlet and outlet radially extending surfaces of the housing; andat least one intermediate stator blade row is mounted on a corresponding at least one intermediate radially extending surface of the housing, a contact surface area between the at least one intermediate stator blade row and the at least one intermediate radially extending surface being at least 10% higher than a contact surface area between an inlet stator blade row and the inlet radially extending surface.

2. A turbomolecular vacuum pump according to claim 1, wherein the inlet radially extending surface contacts an outer annular portion of the inlet stator blade row and the outlet radially extending surface contacts an outer annular portion of an outlet stator blade row.

3. A turbomolecular vacuum pump according to any preceding claim, wherein the at least one intermediate radially extending surface delimits a portion of the inner surface of the housing with a first diameter and a portion of an inner surface of the housing with a second diameter, the first diameter being larger than the second diameter.

4. A turbomolecular pump according to claim 3, wherein a portion of the housing with an inner surface comprising the first diameter comprises an inlet portion of the housing, the inlet portion of the housing and the portion of the housing with an inner surface of a second diameter are attachedtogether to provide a preloaded force on the stator blade rows between the inlet radially extending surface and the intermediate radially extending surface.

5. A turbomolecular pump according to claim 4, wherein an outer annular stator portion adjacent to and at an outlet side of the intermediate stator blade row comprises at least one elastic element between facing surfaces of outer annular portions of the intermediate stator blade row and the adjacent outer annular stator portion.

6. A turbomolecular pump according to any one of claims 3 to 5, wherein the portion of the housing with an inner surface of a second diameter comprises an outlet portion of the housing.

7. A turbomolecular pump according to claim 5 or 6, wherein an outer annular portion of the intermediate stator row blade extends radially inwardly beyond the intermediate radially extending surface and contacts the adjacent outer annular stator portion.

8. A turbomolecular pump according to claim 5, or claims 6 or 7 when dependent upon claim 5, wherein the inlet end of the portion of the housing with an inner surface of a second diameter comprises an inner surface of an increased diameter a further radially extending surface extending from the increased diameter inner surface to the inner surface comprising the second diameter, the adjacent outer annular stator portion comprising the increased diameter and being mounted between the intermediate stator blade row and the further radially extending surface.

9. A turbomolecular pump according to claim 8, wherein the adjacent outer annular stator portion extends radially inwardly beyond said further radially extending surface.

10. A turbomolecular vacuum pump according to any preceding claim, wherein the plurality of stator blade rows comprise an outer annular portion and a stator blade portion, a radial length of said outer annular portion being different for at least some of said stator blade rows.

11. A turbomolecular vacuum pump according to any preceding claim, wherein the at least one intermediate stator blade row comprises an outer annular portion with an increased radial length compared to a radial length of an outer annular portion of a stator blade row at an inlet end of a same portion12. A turbomolecular vacuum pump according to claim 11, wherein a stator blade row adjacent to, and on an outlet side of the at least one intermediate stator blade row comprises an outerannular portion with a reduced radial length and a stator blade of a same radial length as the intermediate stator blade row.

13. A turbomolecular vacuum pump according to claim 11 or 12, wherein a contact surface area 5 between the at least one intermediate stator blade row and the at least one intermediate radiallyextending surface comprises more than 50% but less than 80% of a surface area of the outer annular portion of the at least one intermediate stator blade row.

14. A turbomolecular vacuum pump according to any one of claims 11 to 13, wherein the outer io annular portion of the intermediate stator blade row comprises between 10 and 40% of a radiallength of the stator blade row.

15. A turbomolecular vacuum pump according to any preceding claim comprising at least two intermediate stator blade rows mounted on corresponding at least two intermediate radially15 extending surfaces of the housing, the housing and stator blade rows comprising at least three different diameters.s

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