Vacuum pump

Abstract

A vacuum pump 5 provides both a drag and regenerative pumping effect. The vacuum pump comprises at least one pumping channel 22a, 22b, a stator 40, and a rotor 30 mounted on a rotatable shaft 54. A pl

Description

FIELD OF THE INVENTION The field of the invention relates to vacuum pumps. 5 BACKGROUND Different types of vacuum pump that operate on different principles are known. The pressure range across which a vacuum pump operates effectively may be limited by the pumping principle under which it operates. Drag pumps for 10 example operate by adding momentum to molecules in a fluid within the pump in a direction from an inlet towards an outlet, using relative rotation of two closely spaced surfaces to push gas molecules along the channels, they do not work effectively at higher pressures. Regenerative pumps operate by rotating blades in a channel imparting momentum to molecules that they repeatedly impact and 15 operate effectively at higher pressures than a drag pump does. SUMMARY A first aspect provides a vacuum pump comprising: at least one pumping channel; a stator; a rotor mounted on a rotatable shaft, said rotor comprising a 20 plurality of blades extending from a surface of said rotor and into said at least one pumping channel, said surface forming a first wall of said pumping channel, a second wall of said pumping channel facing said first wall comprises a surface of said stator, a distance between said first and second walls providing a depth of said pumping channel and being between 3 and 12mm; and wherein a surface 25 area of said pumping channel provided by said rotor is between 30 and 70% of a total surface area of said pumping channel. A vacuum pump is provided with a pumping channel that has rotating blades and a rotating surface that is close to a static surface. This configuration allows the 30 rotating blades to provide a regenerative pumping effect, while the proximity of the rotating pumping channel surface to the static pumping channel surface provides a drag effect on the gas being pumped. Thus, the pump provides a fluid with two different pumping mechanisms, and this enables the pump to pump across a wider pressure range that includes both molecular and viscous flow regimes. In some cases the vacuum pump may operate at pressures between 20mbarand 1 XlCPmbar. 5 The surface area of the pumping channel provided by the rotor is between 30 and 70% of a total surface area of the pumping channel. Pumping using the drag effect requires closely spaced static and rotating surfaces and for it to operate effectively it is advantageous if the surface area of the pumping channel provided io by the stator and that provided by the rotor are similar. In some embodiments, said pumping channel further comprises at least one curved side wall extending between said first wall and said second wall. 15 The pumping channel may comprise curved sidewalls. These curved sidewalls direct the spiralling flow of the gas being pumped that occurs due to the impact of the rotating blades and provides the regenerative pumping effect whereby the gas molecules impacted by a blade are thrown radially outwards towards the at least one curved sidewall, where they are directed radially inwards and along the 20 channel to be impacted by a further blade. This process is repeated until the gas molecules reach the outlet. In some embodiments, said pumping channel comprises two side walls one extending from said first wall and being formed by said rotor, and one extending 25 from said second wall and being formed by said stator. Although, the curved sidewall may be formed by only one of the rotor or stator, it may be advantageous if it is formed by both and indeed in some cases it is advantageous if the pumping channel has a substantially symmetrical cross 30 section such that the sidewalls meet at the widest point, such an arrangement being easy to machine and to assemble. ln some embodiments, said rotor comprises at least one disc, at least one radial surface of said at least one disc facing at least one corresponding surface of said stator, said at least one pumping channel being partially formed by a recess within at least one of: said radial surface of said rotor and said corresponding 5 surface of said stator. The pumping channels may be formed by a disc rotating opposite a corresponding static surface, with a pumping channel being formed by a recess within at least one of the two surfaces. 10 In some embodiments, said at least one pumping channel is formed by recesses in both of said radial and corresponding surfaces. Having the recesses in both of the surfaces may provide a pumping channel that is easier to machine and provides an effective drag force. 15 In some embodiments, said vacuum pump comprises multiple pumping channels. The configuration of the pumping channels as recesses within a surface of one or more of the rotor and stator allows for the provision of multiple pumping channels 20 which enables the vacuum pump to increase its pumping capacity. In some embodiments, said at least one disc comprises two radial surfaces facing in opposite directions, each facing a corresponding surface of said stator, said multiple pumping channels being partially formed by a recess within at least 25 one of the following: said two radial surfaces of said rotor and said two corresponding surfaces of said stator. The disc may have pumping channels on both surfaces and not only does this increase the capacity of the pump but it also provides for a better balanced disc. 30 In some embodiments, said at least one pumping channel comprises an annular channel that is at a given radius along its length from said shaft. ln some embodiments, there may be at least two pumping channels on the same radial surface of one or more discs, the two pumping channels may be concentric annular channels having different radii. 5 In some embodiments, at least some of said pumping channels are arranged in series. Where there are multiple pumping channels then some may be arranged in series, having pumping channels arranged in series will increase the compression provided by that pumping channel. 10 In some embodiments, where there are two annular channels of different radii on the same surface these may be arranged in series. This arrangement is simple to manufacture and provides for increased compression without the requirement to provide channelling of the gas between different disc surfaces. 15 In some embodiments, at least some of said pumping channels are arranged in parallel. Arranging channels in parallel increases the pumping capacity of the vacuum pump. 20 In some embodiments, said rotor comprises multiple discs at least one pumping channel on one surface of one of said discs being arranged in parallel with at least one pumping channel on another surface of said one of said discs. Arranging pumping channels in parallel on either side of a disc allows for 25 increased pumping capacity in a space efficient and easy to design fashion. Furthermore, the disc will tend to be pressure balanced when there are pumping channels pumping in parallel on either side such that there will be little or no net thrust on the disc. 30 In some embodiments, said rotor comprises multiple discs at least one pumping channel on one surface of one of said discs being arranged in series with at least one pumping channel on another surface of another of said discs. Where there are multiple discs then there may be pumping channels in series on different ones of these discs. In some cases there may be pumping channels arranged in series on one surface of a disc and then further pumping channels 5 arranged in series with these on another surface of another disc. In this way pumping compression can be significantly increased. In other cases there may be multiple pumping channels arranged in parallel on one surface of a disc and then further multiple pumping channels arranged downstream and in series with each other and with these parallel pumping channels on another surface of a 10 disc. In this way the cross section and capacity of the downstream channels is reduced compared to that of the upstream channels. Although the width and depth of the channel may be equal, it is preferable if the channel is wider than it is deep as this increases the capacity of the pumping 15 channel. The more elongated that the pumping channel is the more the capacity is increased, however there is a limit for the two pumping mechanisms to operate efficiently. In some embodiments, a width of said pumping channel is between 1.5 and 4 times said pumping channel depth. 20 In some embodiments, said pumping channel comprises at least three strippers configured to segment said channel into segments, each segment comprising a respective inlet and outlet at either end of said segment. The channels may be segmented by the use of strippers with inlets and outlets 25 being formed on either side of the stripper. This again may increase the capacity of the vacuum pump. Where the pumping channel is an annular channel then the segments will be sectors of that circular channel. The strippers are arranged to push fluid from the pumping channel into the outlet and there is an inlet of a subsequent segment at the other side of the stripper. The strippers have 30 apertures that allow for the passage of the rotating blades through the strippers. There may be more than three strippers and in some cases there may be between three and eight, in some cases preferably five. Although multiple strippers increase the capacity providing multiple inlets and outlets, they do reduce the channel length and thus the compression. Although, the blades may be straight in some embodiments they are curved in a 5 direction of rotation. In some embodiments, said blades extend from said first wall in a radial plane in a region that is between a radial mid-point of said pumping channel and a side of said pumping channel closest to said shaft. 10 The blades are preferably towards the axis of the machine such that rotation of the blades throws the gas radially outwards. In some embodiments, the height of the blade is between 40 and 60% of a depth 15 of the pumping channel. The height of the blade should be such that there is sufficient area of the blade to provide effective pumping of the fluid while leaving sufficient space for the flow of the gas back round in its spiral flow allowing it to be impacted by a subsequent blade. The width of the blades is governed by competing factors, a wide blade provides an effective pumping action, however it 20 requires corresponding larger aperture in the stripper and this provide a leakage path. A blade that is of a sufficient size for effective pumping, but not too large is preferred. In embodiments the width of the blade is less than half the width of the pumping channel perhaps between 20 and 45% of the width of the pumping channel. 25 In some embodiments, the blades are quadrant blades that extend from the first wall and from a sidewall formed by the rotor. The sidewall being the sidewall closest to the shaft. 30 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, 5 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 10 reference to the accompanying drawings, in which: Figure 1 shows a vacuum pump according to an embodiment; Figure 2 shows a view of an annular pumping channel of an embodiment; Figures 3A and B show a radial cross section of a pumping channel according to an embodiment; 15 Figure 4 shows a radial cross section of a pumping channel according to another embodiment; and Figure 5 shows a vacuum pump according to a further embodiment; DESCRIPTION OF THE EMBODIMENTS 20 Before discussing the embodiments in any more detail, first an overview will be provided. A vacuum pump that combines the performance of a drag stage with that of a regenerative stage to give effective pumping performance over a wide pressure 25 range that includes molecular and viscous flow regimes is disclosed. The vacuum pump comprises one or more pumping channels designed to pump fluid according to both a drag and a regenerative pumping principle. Embodiments provide one or more pumping channels, with pumping channel 30 walls formed from both the surface of a stator and the surface of a rotor. The walls are close together so that their relative motion provides a drag effect on gas molecules within the channel. There are multiple blades extending from the rotor surface along the length of the channel to impact gas molecules as the rotor rotates and provide a regenerative pumping effect. In this regard, the rotor may carry a row of blades similar to those of a 5 regenerative or side channel pump. The blades act on the gas, imparting momentum to the gas and swirling the gas in the channel. The channel is designed to accommodate the swirling gas by having rounded sides or ends. Typically a regenerative channel has similar width and depth dimensions, i.e. it has a roughly round section. In embodiments, the channel is elongated in the 10 width y direction to increase capacity while maintaining the depth dimension x at a few mm to provide the drag effect. The dimension y may be about 1.5 to 4 times dimension x. The blades may be mounted at a radius that is slightly less than the mid point of the channel to encourage the gas to swirl in the whole channel and not to leave a dead area at the inside end. The blades may be 15 simple straight blades or curved blades to better catch the swirling gas and impart more velocity to the gas. In some embodiments the pumping channels may have multiple starts, thereby increasing pumping capacity. The pumping channels may have up to 6 or 8 20 starts. Each section of the pumping channel being is separated by a ‘stripper’ to inhibit gas from circulating back to the inlet. The exhaust can be arranged to be radially inwards and in this way can connect to the inlet of a second stage. The second stage has the same number of starts 25 as the first stage. Multiple stages in some cases 3 or 4, may be connected in this way to increase compression. Each stage may have a reduced cross section and capacity. The final stage exhausting to a plenum space at the centre of the rotor. The rotor may have similar channels on each side of the rotor disc. The rotor may have holes or slots at a radius inside of the final stage to allow the 30 exhaust gas to pass through the disc to an exhaust port. Alternatively the gas could pass to a second rotor in series, to enable further compression. A pump may have several inlet rotors to increase capacity with a common 5 exhaust at one end, or an exhaust at each end of a long stack of inlet rotors. A further way of increasing capacity may be to configure the pump such that pumping channels, perhaps those on either side of a disc are arranged in parallel. In summary the configuration is such that the channels may be arranged 10 in series and / or parallel to increase compression and / or capacity as desired. Many different arrangements are possible with little change required to the design. Figure 1 shows a vacuum pump 5 according to an embodiment. Vacuum pump 5 15 comprises a rotor formed of two discs 30 mounted on a rotatable shaft 54. Rotatable shaft 54 is mounted on bearings 52 and driven by motor 50. The surfaces of discs 30 face and are close to a corresponding surface of stator 40. In this embodiment, each surface of each rotor disc 30 comprises two annular recesses 32 of different radii that extend around the disc shaped rotor. These 20 recesses 32 comprise a radially extending first wall 33 and curved side walls 35 that together form one side of respective pumping channels 22, the other side of the pumping channels are formed by corresponding annular recesses 42 formed in the facing surface of stator 40. These annular recesses 42 are formed of a radially extending second wall 43 and sidewalls 45. There are multiple blades 34 25 extending from the first wall 33 of the recess 32 at different points along the length of the annular recess. The two annular recesses of different radii form two pumping channels 22a, 22b that are arranged in series, such that fluid entering the outer pumping channel flows along pumping channel 22a and into the inner pumping channel 22b and then out towards exhaust 12. The two pumping 30 channels in series increases the compression applied by the pump. Thus, a pump with a desired compression may be designed by arranging different pumping channels in series. The pumping channels on each side of each disc are arranged in parallel, such that the capacity of the vacuum pump is increased by a factor of four compared to a vacuum pump with pumping channels on just one side of one disc. In this 5 embodiment the annular pumping channels are each divided into segments or sectors by strippers (not shown) such that each pumping channel has multiple inlets and corresponding outlets. Where there are five strippers and five inlets and outlets for each channel, then the capacity is increased by a factor of five, although the compression is reduced as the channel length is reduced. Thus, 10 overall for this embodiment the capacity of the vacuum pump is increased by a factor of 20 compared to a single pumping channel with a single inlet and outlet on one disc. As can be understood, increasing pumping capacity by adding discs and / or splitters, is a scalable, compact and straightforward way of increasing capacity, making the design of the pumps very adaptable to different 15 compression requirements (by arranging pumping channels in series and / or increasing their length) and different capacities (providing more splitters, more discs and / or more pumping channels in parallel). In effect the configuration of the vacuum pump with the ability to increase the number of rotors and pumping channels allows vacuum pumps of different capacities and different 20 compressions to be provided. In operation fluid entering at inlet 10 flows around the outer edge of the rotor discs 30 and into one of the multiple pumping channel inlets on the outer circumference of the rotor discs, the fluid then flows through the outer and inner 25 pumping channels 22a, 22b towards the centre of the disc and then out to exhaust 12. Figure 2 shows a plan view of a section through an annular pumping channel 22 formed by the rotor 30 and stator 40 of Figure 1. Figure 2 shows the blades 34 30 on the rotor that extend from a surface of the rotor that forms the first wall of the pumping channel, the facing second wall being formed by a surface of the stator not shown. There are two strippers 20 in this example, that separate the annular pumping channel 22 into two different sectors such that the fluid flows from one of two inlets 10 to a corresponding outlet 12. In this embodiment, the blades 32 are curved in the direction of rotation (anticlockwise), so that the outer edges are in front of the inner portion of the blades as they rotate. In other embodiments, 5 the blades may be straight. Straight blades are easier to machine but curved blades provide a better pumping performance. Figure 3A shows a radial section through a pumping channel according to an embodiment. Figure 3A shows blades 34 located on the inner side of the io pumping channel towards the shaft 54. Rotation of the blades 34 throws gas radially outwards as shown by the arrow. The curved side walls of the pumping channel 22, then guide the gas around such that it flows back towards the inner side of the pumping channel and then around to be impacted by a subsequent blade. In effect the gas moves with a spiral motion along pumping channel 22. 15 The height of the blade is such that it extends to about half the depth X of the channel, so that it pushes fluid radially outwards, but does not impede the radially inward fluid flow. The width of the blade 34 is sized to be large enough to do useful work, but not too large as the apertures in the stripper that allow passage of the blades provide a leakage path that increases with the size of the blade. 20 The depth of the channel X is between 3 and 12 mm, that is small enough to provide a drag effect on the gas within the channel when the rotor 30 that forms the end 33 and side 35 walls rotates. The width of the channel Y is between 1.5 and 4 times the depth. Elongating the channel in this way increases pumping capacity while still enabling the drag pumping mechanism to operate effectively. 25 In this embodiment, the pumping channels 22 with the curved side walls and elongated end walls have a lozenge type cross section shape. The blades 34 will provide a regenerative type pumping action on the fluid pushing it forward as they rotate such that the fluid forms a spiral type motion as it processes forward and 30 around the pumping channel and is accelerated by multiple blades 34 until it meets a stripper and exits via an exhaust. The configuration of the pumping channel 22 allows both a regenerative pumping effect and a drag pumping effect and provides for effective pumping across a wide pressure range. Figure 3B shows a radial section through a pumping channel 22 similar to that of 5 Figure 3A but showing a radial seal 37, for sealing between the rotor and stator. This radial seal 37 is formed by corresponding steps 36 and 46 in the rotor and stator, and these provide a narrow radially extending gap that acts as a radial seal. This radial seal may be in addition to the axial seal formed by the narrow axial gap between the rotor and stator. The axial gap and therefore seal, is io sensitive to axial movement of the shaft, while the radial gap and therefore seal is sensitive to radial movement of the shaft. The shaft is less prone to radial movement due to its mounting on bearings and as such providing a radial seal 37 can be advantageous in reducing pumping channel leakage. 15 Figure 4 schematically shows a radial section through a pumping channel 22 according to an embodiment. The pumping channel 22 is formed by recesses within the stator 40 and rotor 30 and has an annular length and the lozenge shaped cross section that is shown. In this embodiment as in the embodiment of Figure 3, the pumping channel is substantially symmetrical and the surface area 20 of the walls formed by the rotor and stator are substantially equal. The pumping channel does not need to be arranged in this way, however, the surface area of the rotor walls should be between 30 and 70% of the total surface area of the channel outer wall to provide the desired drag effect on the fluid being pumped. In this embodiment, the blade is a quadrant blade 34 mounted in the corner of the 25 recess 32 in rotor 30, and extending from the sidewalls 35 and radial wall 33. Figure 5 shows an alternative embodiment of a vacuum pump 5, where the two rotor discs 30a, 30b each have pumping channels on either side. The left hand rotor disc 30a comprises four inlet pumping channels arranged in parallel, two on 30 each side of the disc. The right-hand rotor disc 30b has two pumping channels arranged in series on each surface, the inlet of the two pumping channels on each surface being towards the centre of the disc. The pumping channels on each surface are arranged in parallel with each other. The parallel pumping channels on either surface of right hand rotor disc 30b are arranged in series with the four parallel pumping channels on the left hand rotor disc 30a. Thus, the fluid flows into inlet 10, around the outside of rotor disc 30a and into the outer and 5 inner pumping channels on either side of rotor disc 30a. The fluid flows through the four parallel arranged pumping channels 22 and into an inner flow path in the centre of the rotor discs 30a, 30b. Rotor disc 30b has pumping channel inlets towards the centre of the disc that link to this inner flow path such that the fluid output by rotor disc 30a, flows into the inlets to the pumping channels on disc io 30b, through the serially arranged inner and outer pumping channels 22 on each side of the disc and out to exhaust 12. In this way the flow from four pumping channels is received by two pumping channels and this provides a reduced cross section and capacity for the later pumping stage. The pumping channels 22 on either side of the two rotor discs are arranged in parallel and are symmetrical with 15 each other. This provides balanced discs with little or no nett thrust on the discs. In summary, embodiments may provide a compact, high speed booster pump, that operates with a single shaft across a wide pressure range. 20 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 25 equivalents. REFERENCE SIGNS vacuum pump inlet exhaust / outlet stripper pumping channel rotor disc rotor recess rotor end wall blade rotor side wall rotor step radial seal stator stator recess stator end wall stator side wall stator step motor bearing shaft

Claims

1. A vacuum pump comprising:at least one pumping channel;5 a stator;a rotor mounted on a rotatable shaft, said rotor comprising a plurality of blades extending from a surface of said rotor and into said at least one pumping channel, said surface forming a first wall of said pumping channel, a second wall of said pumping channel facing said first wall comprises a surface of said stator, io a distance between said first and second walls providing a depth of said pumping channel and being between 3 and 12mm; and whereina surface area of said pumping channel provided by said rotor is between 30 and 70% of a total surface area of said pumping channel.15 2. A vacuum pump according to claim 1, wherein said pumping channelfurther comprises at least one curved side wall extending between said first wall and said second wall.

3. A vacuum pump according to claim 2, wherein said pumping channel20 comprises two side walls one extending from said first wall and being formed by said rotor, and one extending from said second wall and being formed by said stator.4 A vacuum pump according to any preceding claim ,wherein a width of25 said pumping channel is between 1.5 and 4 times said pumping channel depth.

5. A vacuum pump according to any preceding claim, wherein said rotor comprises at least one disc, at least one radial surface of said at least one disc facing at least one corresponding surface of said stator, said at least one30 pumping channel being partially formed by a recess within at least one of: said radial surface of said rotor and said corresponding surface of said stator.

6. A vacuum pump according to claim 5, wherein said at least one pumping channel is formed by recesses in both of said radial and corresponding surfaces.

7. A vacuum pump according to claim 5 or 6, wherein said vacuum pump 5 comprises multiple pumping channels.

8. A vacuum pump according to claim 7, wherein said at least one disc comprises two radial surfaces facing in opposite directions and each facing a corresponding surface of said stator, said multiple pumping channels being io partially formed by a recess within at least one of the following: said two radialsurfaces of said rotor and said two corresponding surfaces of said stator.

9. A vacuum pump according to any one of claims 5 to 8, wherein said at least one pumping channel comprises an annular channel that is at a given 15 radius along its length from said shaft.

10. A vacuum pump according to claim 9, comprising at least two pumping channels on a same radial surface of said at least one disc, said at least two pumping channels comprising annular channels, said at least two annular 20 channels being at different radii from said shaft.

11. A vacuum pump according to any one of claims 7 or 8 to 10 when dependent on claim 7, wherein at least some of said pumping channels are arranged in series.2512. A vacuum pump according to claim 11 when dependent on claim 10, wherein said at least two annular channels form pumping channels arranged in series.30 13. A vacuum pump according to any one of claims 7 or 8 to 12 whendependent on claim 7, at least some of said pumping channels being arranged in parallel.

14. A vacuum pump according to any one of claims 5 to 13, wherein said rotor comprises multiple discs at least one pumping channel on one surface of one of 5 said discs being arranged in parallel with at least one pumping channel on an other surface of said one of said discs.

15. A vacuum pump according to any one of claims 5 to 14, wherein said rotor comprises multiple discs at least one pumping channel on one surface of one of io said discs being arranged in series with at least one pumping channel on an other surface of an other of said discs.

16. A vacuum pump according to any preceding claim, wherein said pumping channel comprises at least three strippers configured to segment said channel 15 into segments, each segment comprising a respective inlet and outlet at either end of said segment.

17. A vacuum pump according to any preceding claim, wherein said blades are curved in a direction of rotation.2018. A vacuum pump according to any preceding claims, wherein said blades extend from said first wall in a radial plane in a region that is between a radial mid-point of said pumping channel and a side of said pumping channel closest to said shaft.

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