MULTI-STAGE TURBOMOLECULAR PUMP

Abstract

A vacuum pump comprising a turbomolecular stage and a drag stage, the vacuum pump comprising a stator and a rotor. The rotor comprises a turbomolecular rotor and a drag rotor attached together. The turbomolecular rotor comprises a hub from which a plurality of blades extend, the hub comprising a mounting portion for mounting to a spindle of a motor and a hollow cylindrical portion, the hollow cylindrical portion extending from the mounting portion towards an outlet end of the turbomolecular stage. The drag rotor comprises a cylindrical skirt and an attachment part extending away from the cylindrical skirt, the attachment part extending within the hollow cylindrical portion of the hub of the turbomolecular rotor and being attached thereto at a point that is closer to the mounting portion than to the outlet end of the turbomolecular rotor.

Claims

1. A vacuum pump comprising a turbomolecular stage and a drag stage, said vacuum pump comprising a stator and a rotor, said rotor comprising a turbomolecular 1 rotor and a drag rotor and a spindle of a motor; wherein said turbomolecular rotor comprises a hub from which a plurality of blades extend, said hub comprising a mounting portion for mounting to a member of said rotor and a hollow cylindrical portion, said hollow cylindrical portion extending from said mounting portion towards an outlet end of said turbomolecular stage; and said drag rotor comprises a cylindrical skirt and an attachment part extending away from said cylindrical skirt, said attachment part extending within said hollow cylindrical portion of said hub of said turbomolecular rotor and being attached to the member of said rotor at a point that is closer to said mounting portion than to said outlet end of said turbomolecular rotor.

2. The vacuum pump according to claim 1, wherein said drag rotor is formed of a material that is resistant to higher temperatures than a material forming said turbomolecular rotor.

3. The vacuum pump according to claim 1, wherein said drag rotor is formed of a material with a lower thermal conductivity than a material forming said turbomolecular rotor.

4. The vacuum pump according to claim 1, wherein said drag rotor is formed of steel.

5. The vacuum pump according to claim 1, wherein said drag rotor is formed of stainless steel.

6. The vacuum pump according to claim 1, wherein said turbomolecular rotor is formed of aluminium.

7. The vacuum pump according to claim 1, wherein said attachment part is attached to said mounting portion of said turbomolecular rotor.

8. The vacuum pump according to claim 1, wherein said mounting portion extends substantially perpendicular to said cylinder.

9. The vacuum pump according to claim 1, wherein said attachment part has a thermal conductivity of less than 50 W/mK, preferably less than 20 W/mK.

10. The vacuum pump according to claim 1, wherein said attachment part is thin and has a thickness of 3 mm or less.

11. The vacuum pump according to claim 1, wherein said attachment part comprises a cylinder of a smaller diameter than said hollow cylindrical portion of said hub of said turbomolecular rotor such that there is a gap between said cylinder of said attachment part and said cylindrical portion of said hub.

12. The vacuum pump according to claim 1, wherein said turbomolecular rotor comprises a high emissivity coating.

13. The vacuum pump according to claim 1 wherein said turbomolecular stator comprises a high emissivity coating.

14. The vacuum pump according to claim 1, wherein said stator 1 comprises a turbomolecular stage and a drag stage stator, said turbomolecular stage stator extending around said rotor and said drag stage stator being mounted within and thermally isolated from said turbomolecular stage stator.

15. The vacuum pump according to claim 14, wherein said vacuum pump comprises a heater for heating said drag stage stator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0046] FIG. 1 schematically illustrates a vacuum pump according to an embodiment.

DETAILED DESCRIPTION

[0047] Before discussing the embodiments in any more detail, first an overview will be provided.

[0048] A vacuum pump is provided with a turbomolecular stage and a drag stage the rotor for which is formed in two parts. The drag stage rotor is attached to the turbomolecular stage rotor by an attachment part that extends upwardly from the drag stage skirt inside the turbomolecular stage rotor. The attachment part is configured to have a low thermal conductivity such that the drag stage can run at higher temperatures than the turbomolecular stage thereby impeding condensation of process gases. Heat flow from the hotter drag stage rotor to the turbomolecular rotor is constrained by the low thermal conductivity of the attachment part connecting the two. In order for the turbomolecular rotor not to heat up, any heat flow that does pass along the attachment part should be less than, or of an amount that can be dissipated from the turbomolecular rotor. In this regard owing to the high vacuum operation of this stage of the pump most of the heat dissipated from the turbo rotor is through radiation and is thus, quite small. A high emissivity coating to the turbo rotor may increase radiation heat loss. This coating may in some embodiments take the form of a black coating.

[0049] FIG. 1 shows a vacuum pump according to an embodiment. This vacuum pump comprises a turbomolecular stage and a drag stage. The vacuum pump has a main turbo rotor 20 which is mounted by a drive spindle 22 within a motor and magnetic bearings 70. The magnetic bearings allow the rotor to rotate at high speeds with very low friction such that lubricants are not required. The main turbo rotor 20 comprises turbo pump blades 10 and a central cylindrical hub 12 from which the blades extend. The turbo stage of the rotor has a stator 80 which also has blades corresponding to the rotor blades. Turbo stator 80 extends around the whole of the vacuum pump to form a part of the pump housing. Within this pump housing is the stator of the drag stage 40 that is mounted to the turbomolecular stator 80 via thermal insulating members 50. The drag stage 40 is heated to maintain it at a temperature selected to be sufficient to inhibit condensation of the process gasses being pumped. The drag stage of the pump has a drag stage stainless steel rotor 60 which in this embodiment is a Holweck drag stage rotor. This drag stage rotor has a skirt form and extending from the upper surface is a thin attachment part 30. The thin attachment part 30 extends up into the cylindrical hub 12 of the turbomolecular rotor and is attached to the under surface of the upper part of the cylindrical hub. In some cases it may be braised or welded to the upper part, in other cases it may be attached with some bolting means and there may be a thermal insulator between the attachment piece and turbomolecular part of the rotor.

[0050] The attachment piece 30 is in the form of a cylinder that has a smaller diameter than the inner diameter of the cylindrical hub 12 of the turbomolecular rotor. In this way there is an air gap between the two.

[0051] During operation the drag stage of the vacuum pump will operate at a higher temperature and pressure than the turbomolecular stage. As it operates at a higher pressure there is an increased likelihood of condensation of particles from process gasses being pumped. Maintaining the drag stage at a higher temperature reduces the chance of such condensates appearing. The use of a stainless steel rotor 60 that is more robust to higher temperatures allows this higher temperature operation while the attachment piece 30 having a significant length moving up into the turbomolecular rotor and being formed of a material with a low thermal conductivity, provides low thermal conduction between the higher temperature drag stage rotor and the lower temperature turbomolecular stage rotor allowing them to operate at different temperatures.

[0052] Conventionally the drag stage and turbomolecular stage have been formed as a single piece such that differences in temperatures between the two are difficult to maintain. Embodiments of the present invention form the rotor in two parts such that different materials can be used. Furthermore, although the two parts are attached together this is done in a way that despite the two parts of the rotor being adjacent to each other they are attached using a long attachment piece that extends within the turbo stage rotor. In this way, a certain degree of thermal isolation between the two stages of the rotor is provided allowing different temperatures of operation.

[0053] Although illustrative embodiments of the invention have been disclosed in detail herein, with 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.

[0054] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0055] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.