DRAG PUMP

Abstract

A drag pump for pumping fluid from an inlet to an outlet includes a stator and a rotor. One of the stator or rotor includes a disc having a plurality of channels each of the channels extending from an inlet portion of the disc at or close to an inlet edge towards an outlet portion at or close to an outlet edge. The plurality of channels each has walls for guiding fluid flow from the inlet edge to the outlet edge in response to relative motion between the stator and the rotor. The disc further includes a plurality of protrusions extending from the channels, each of the protrusions being arranged to divide a channel at the inlet or the outlet end of the channel, into sub-channels that extend for a portion of a length of the channel and do not extend for a whole length of the channel.

Claims

1. A drag pump for pumping fluid from an inlet to an outlet said drag pump, comprising: a stator and a rotor; one of said stator or rotor comprising a disc comprising a plurality of channels, each of said channels extending from an inlet portion of said disc at or close to an inlet edge towards an outlet portion at or close to an outlet edge, said plurality of channels each comprising walls for guiding fluid flow from said inlet edge to said outlet edge of said disc in response to relative motion between said stator and said rotor; said disc further comprising a plurality of protrusions extending from said channels towards said other of said rotor or said stator, each of said protrusions being arranged to divide a channel at said inlet or said outlet end of said channel, into sub-channels that extend for a portion of a length of said channel and do not extend for a whole length of said channel.

2. The drag pump according to claim 1, wherein said protrusions do not extend along a mid portion of said channel.

3. The drag pump according to claim 1, wherein said protrusions have a length that is less than 60% of a length of one of said walls which said protrusion is adjacent to,

4. The drag pump according to claim 1, wherein said plurality of protrusions are arranged in each channel at an inlet end of said channels.

5. The drag pump according to claim 1, wherein said plurality of protrusions are arranged in each channel at an outlet end of said channels.

6. The drag pump according to claim 1, wherein said plurality of protrusions arranged at said inlet end of said channels extend from an inlet edge of said channel to a point beyond a line extending perpendicularly from a trailing wall of said channel.

7. The drag pump according to claim 6, wherein 50% or less of said protrusion extends beyond a line perpendicular to said trailing wall of said channel.

8. The drag pump according to claim 1, wherein said plurality of protrusions are arranged such that said sub-channels have substantially the same cross section.

9. The drag pump according to claim 1, wherein said plurality of protrusions are arranged such that said sub-channels each have a different cross sectional area.

10. The drag pump according to claim 1, said drag pump comprising a plurality of protrusions arranged between each of said channel walls such that said plurality of protrusions divides said channel into a plurality of three or more sub-channels.

11. The drag pump according to claim 1, wherein said protrusions have a thickness that varies along a length of said protrusions.

12. The drag pump according to claim 11, wherein said protrusions are configured to be thicker at an end adjacent to an edge of said disc and thinner towards a middle of said disc.

13. The drag pump according to claim 1, said inlet portion of said disc comprising an outer circumference of said disc.

14. The drag pump according to claim 1, wherein said drag pump comprises a Siegbahn drag pump, said channels being formed on a surface of a disc shaped stator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0045] FIG. 1 schematically shows molecules being pumped through a channel in a stator of a drag pump, where for the sake of the Figure the channel has been unwrapped;

[0046] FIG. 2 shows an “unwrapped” channel of a drag pump according to an embodiment;

[0047] FIG. 3 shows an overview of a stator of a drag pump according to related technique;

[0048] FIG. 4 shows an overview of an outlet end of a stator of a drag pump according to a related technique;

[0049] FIG. 5 shows an annular washer for providing blocking of a portion of an outlet of channels of a stator according to a related technique;

[0050] FIG. 6 schematically shows the flow of molecules within a channel of a pump according to an embodiment; and

[0051] FIG. 7 schematically shows the stator of a Siegbahn pump according to embodiment.

DETAILED DESCRIPTION

[0052] Before discussing the embodiments in any more detail, first an overview will be provided.

[0053] Embodiments provide the addition of short vanes or sealing lands at the outlet and/or or inlet of a drag pump either on the stator or the rotor between the walls forming the channels to provide reduced cross sectional sub-channels and impede recirculation and provide more pumping surface.

[0054] For a drag pump such as a Holweck stage the compression ratio increases as a function of channel length L and velocity v.Math.cos α, where α is the angle between the channel and the direction of rotation, v.Math.cos α being the component of drag velocity along the channel.

[0055] Increasing rotational velocity for a drag pump has negative impacts on durability and balance, while increasing L, which is done in a Holweck pump by increasing the Holweck rotor & stator heights is in many applications undesirable as a pump's space claim is all too often limited.

[0056] The physical length of a channel can alternatively be increased by using a shallower channel angle, but as the angle reduces, the problems of recirculation of gases at the input and output, where there is a region of the channel that is in effect single sided, increases. This recirculation means that the flow back towards the inlet increases and we “lose” a considerable proportion of the extra length gained.

[0057] The mechanism by which a drag pumps works, and specifically a Holweck stage, is to influence the rate of relative flow of molecules (M12 from inlet to outlet, M21 from outlet to inlet) by adding a degree of momentum in the M12 direction. The geometry of the channels in conjunction with the direction of rotation of the rotor tend to bias the molecules towards the downstream or trailing wall as molecules pass through the stage, see FIG. 1.

[0058] Further at the inlet and particularly for shallow angle Holwecks, the opportunity for reverse transmission of molecules (to re-exit the inlet) remains until they are shrouded by the “upper” channel wall & thus have no direct path back out of the stage.

[0059] With a steep angled channel design the length of the channel is severely limited by the Holweck's height or in a Siegbahn disc by the Siegbahn's diameter, thus a shallower angle is preferred to increase the channel length. However, on a shallower angled channel (of the same channel width) though the channel length can be greatly increased, a significant length of the channel at both the inlet & exhaust has only one side wall, see FIG. 2, giving increased opportunity for a molecule having entered the pumping stage to re-exit, thus the effective working length of such a channel is much shorter than its physical dimensions.

[0060] FIG. 1 schematically show flow in a drag pump as it progresses through a channel 14, between walls 12 and 13. The walls are the walls of a channel on a static stator, the arrow 5 showing the direction of rotation of the rotor, so that wall 12 is the upstream or leading wall as this meets the rotor first, while wall 13 is the downstream or trailing wall. The movement of the rotor drags gas towards downstream or trailing wall 13, which deflects the gas towards the outlet. Owing to this movement the molecules become more concentrated close to the downstream or trailing wall 13 towards the outlet.

[0061] FIG. 2 shows how the effective channel length Le can be increased by an amount La by the use of an inlet splitter vane 10, which protrudes from the channel surface and acts as an additional wall to the walls 12, 13 of channel 14 and in effect provides two subchannels 14a and 14b at the inlet of the pump.

[0062] In effect this protrusion or splitter vane creates an extension to the upper or leading channel wall 13 and provides positive blockage extending the effective channel length and thus reducing back leakage. FIG. 1 also shows a protrusion 11 at the outlet end of the channel.

[0063] FIG. 3 shows the channels 14 and protrusions 10 as an overview in a Holweck pump of a related technique.

[0064] In the embodiment of FIG. 2 the protrusions at the inlet and outlet end are the same. However, in other embodiments, the protrusions at the outlet end may be different and may act to block a portion of the outlet adjacent to the leading wall 12 of the channel as opposed to dividing the channel In this regard in a Holweck pump of a related technique there is a bias for a skewed molecule density in the lower regions of a Holweck channel and thus, the portion of the channel adjacent to the leading wall 12 has a lower density of gas molecules and a corresponding lower pressure. This makes it not particularly effective at pumping the gas and also provides a path for the re-entry of gas molecules at the higher pressure of the pump exhaust. Thus, in some embodiments, there may be a protrusion 16 that extends to block a portion of the outlet adjacent to the leading wall 12. This can be provided by a washer 18 that has protrusions 16 on it as shown in FIG. 5 and which is mounted onto the outlet end of the drag stage or it can be formed by an extension to the end of wall 12 at the channel exit as is shown in FIGS. 4 and 6.

[0065] The protrusions of FIGS. 4 and 6 are formed as an integral machined feature of the Holweck stator, while in FIG. 5 they are formed as a separate entity by means of a simple thin “castellated washer”.

[0066] This design not only extends the effective Holweck channel length, but also adds a positive block to aid reverse transmission of molecules that have left the Holweck stage.

[0067] FIG. 7 shows an embodiment, where the pump is a Siegbahn drag stage. The Siegbahn mechanism, while relying on a similar operating principle to the Holweck mechanism has the added challenge of increased difficulty controlling inlet and outlet areas as the outer edge of the Siegbahn stator, whether inlet or outlet, is necessarily larger than the inner edge. This can cause problems in controlling inlet/outlet area ratio and also in managing the gas flow at the outer edge, which is likely to have significant recirculation, particularly at low flows. This means that it can be difficult to manage stage volume ratios and to control the recirculation of gas, particularly at the outside edge of the blade stator.

[0068] FIG. 7 shows an embodiment configured to mitigate these effects. In this embodiment stator 1 is modified, by adding short, thin splitter sealing lands 2 in the channels at the outer edge and optionally adding the splitter sealing lands 3 at the inner edge. The addition of these lands or protrusions will have the following benefits: [0069] 1. The large recirculation at the outer edge, particularly at low flow will be reduced. [0070] 2. There will be extra pumping due to the drag on the additional sealing lands.

[0071] In summary a relatively short blade or splitter land will help address the recirculation and area ratio problems encountered by Siegbahn stages.

[0072] Although in FIG. 7 the protrusions are shown at both the inner and outer edges, there may only be protrusions at one of the edges. Furthermore, there may be more than one protrusion or sealing land within each channel, particularly at the outer edge where the channels are wider. The sealing lands or protrusions 2 are shown as being of a uniform width, but in some embodiments, they may have a tapered shape, such as a wedge type shape which tapers towards the middle of the stator, allowing improved control of inlet and outlet areas.

[0073] In summary advantages of embodiments include [0074] Maintaining capacity at the inlet & compression at the outlet; [0075] Enhanced pump performance within a pump particular space envelope.

[0076] 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.

[0077] 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.

[0078] 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.