GB2625295A

GB2625295A - Vacuum pump and cooling fan for vacuum pump

Info

Publication number: GB2625295A
Application number: GB2218701.7A
Authority: GB
Inventors: Krumpolc Zdenek; Zahradka Martin; Milner Paul; Fnukal Martin
Original assignee: Edwards sro
Current assignee: Edwards sro
Priority date: 2022-12-13
Filing date: 2022-12-13
Publication date: 2024-06-19
Also published as: GB202218701D0; WO2024126246A1

Abstract

A fan 10 for circulating gas for cooling a vacuum pump comprises an annular hub 11, which may comprise an inclined or frustoconical surface, configured for mounting on a motor shaft of the vacuum pump 20 and a plurality of blades 14 extending radially out from said hub. A radially outer portion of the blades is curved in a direction of rotation of the blades such that, the blades direct the gas towards a direction perpendicular to the direction of rotation. The radially inner portion of the blades may be substantially straight. A vacuum pump is also disclosed; the vacuum pump may be a scroll pump. The vacuum pump and fan arrangement also comprises a cowling 24 mounted on the vacuum pump, which may complement the curvature of the fan and extend around the pumping mechanism of the scroll pump. A method for replacing a fan in a vacuum pump is also disclosed.

Description

VACUUM PUMP AND COOLING FAN FOR VACUUM PUMP

FIELD OF THE INVENTION

The field of the invention relates to vacuum pumps and in particular fans for circulating gas for cooling vacuum pumps.

BACKGROUND

Vacuum pumps, such as scroll pumps, often have a fan for circulating gas for cooling the vacuum pump. The gas used is typically the air surrounding the io vacuum pump. The fans used are conventionally radial fans with a central hub and radially extending straight blades. The fans are often noisy and the cooling may not be very effective.

It would be desirable to provide an improved fan for cooling a vacuum pump that is less noisy and provides improved cooling.

SUMMARY

According to an aspect of the invention, there is provided a fan for circulating gas for cooling a vacuum pump said fan comprising: an annular hub configured for mounting on a motor shaft of said vacuum pump; and a plurality of blades extending radially out from said hub; wherein a radially outer portion of said blades is curved in a direction of rotation of said blades such that, in use, said blades direct said gas towards a direction perpendicular to said direction of rotation.

The direction perpendicular to said direction of rotation may be referred to as axial. Providing a fan with curved radially outer portions to direct gas towards the axial direction enables gas to be redirected around an end of a vacuum pump to which the fan is mounted. The fan is configured to be mounted on the motor shaft of the pump and to rotate with the motor, in this way no additional driving means is required and although the speed of the fan is dictated by the speed of the motor, this is effective as a higher speed of pump requires increased cooling. -2 -

The gas is dragged in towards the fan, much of it towards the centre part where the annular hub steers the air outwards towards the blades where the curve turns the air again back to the axial direction. This provides for a more efficient and effective cooling and quieter fan. . In some embodiments, a radially inner portion of said blades is substantially straight. Although curved blades provide axial flow towards the pump, the flow of air that is required for cooling is around the outside of pump, such that the axial direction of flow is more advantageous towards the outer diameter of the fan. The straight radially inner portion helps push air radially outward feeding the curved radially outer portion of the blades. Such fans are sometimes referred to as diagonal fans because they can impart both radial and axial momentum to incoming gas. It should also be noted that the straight radial inner portions of the blades which may overlap with the annular hub facilitate removal of the fan from a mould during manufacture.

In some embodiments, said radially outer portion comprises between 15% and 40% of a radius of said fan. The radius of the fan is defined between a radial centre of the fan and an outer edge of the blades. The smaller the proportion of blade that is curved the easier the fan is to remove from a mould for making the fan. However, the larger the proportion of blade that is curved, the stronger the flow generated by the fan in the axial direction, thereby facilitating a strong flow of gas for cooling the vacuum pump. Having curved radially outer portions comprising between 15% and 40% of a radius of the fan allows the fan to be readily removed from a mould whilst still producing improved flow over the conventional straight blade radial fans. Furthermore it is towards the outer diameter of the fan that axial flow is more advantageous.

In some embodiments, the hub comprises between 50% and 70% of said radius of said fan. In some embodiments, said radially outer portion comprises about 30% of the radius of the fan, the hub comprises about 60% of the radius of the -3 -fan and the blades are straight between the hub and the radially outer portion. In this disclosure, about and approximately mean within a tolerance of 10%.

In other embodiments, the radially outer portion comprises between 60% and 80% of said radius of said fan. In some other embodiments, the blades are curved along substantially their entire length. As mentioned above, providing a larger curved portion of the blades improves the flow of gas generated by the fan, thereby improving cooling of the vacuum pump.

In some embodiments, said blades comprise a radially extending edge configured in use to face incoming gas and a radially extending edge configured in use to face away from incoming gas, wherein a radially outer portion of said radially extending edge configured in use to face incoming gas is more curved in said direction of rotation than a radially outer portion of said radially extending edge configured in use to face away from incoming gas.

By curving the radially extending edge facing incoming gas (sometimes referred to as the top edge) more than the radially extending edge facing away from incoming gas (sometimes referred to as a bottom edge), an angled surface is formed which is configured to direct gas in a direction perpendicular to said direction of rotation.

In some embodiments, the radially extending edge configured in use to face incoming gas does not extend over the radially extending edge configured in use to face away from incoming gas of the immediately adjacent blade. In other words, when looking at the fan in an axial direction, the blades do not overlap. In this way, the fan can be made using a mould.

In some embodiments, said radially outer portion defines an angle of between 40 and 50 degrees relative to an axis of said fan. Blades angled within this range are proficient at directing gas towards a direction substantially perpendicular to said direction of rotation. Blades with too steep an angle limit the number of -4 -blades that the fan can have and thus, reduce gas flow and are where they overlap are impractical to make using a mould. Blades with a smaller angle allow the fan to have more blades, providing increased flow. However inlet velocity may be reduced with the increased number of blades and noise increases.

In some embodiments, said hub comprises an inclined surface configured in use to direct gas radially outward and wherein said blades extend from said inclined surface. In some embodiments, the inclined surface defines an angle of between 40 and 60 degrees relative to the axis of the fan. In some embodiments, the inclined surface is angled at 50 degrees relative to the axis of the fan.

The inclined surface of the hub is configured to direct gas radially outwards towards the curved radially outer portion of the blades. Therefore, gas arriving substantially axially at the central region of the fan is directed radially out and gas at the radially outer portion of the blades is directed axially. In this way, the efficiency of the fan to direct gas around an end of a vacuum pump to which it is mounted is improved.

In some embodiments, the hub comprises a concave surface. Although the inclined surface may be straight in some embodiments it has a concave shape, the concave shape also directing gas radially outwards towards the curved portion of the blades.

In some embodiments, said annular hub is frustoconical. In some embodiments, 25 the hub has an axial bore for accommodating said motor shaft.

In some embodiments, said radially outer portion of said blades is radially outside of said hub.

In some embodiments, said radially inner portion of said blades is radially within said hub. -5 -

In some embodiments, said plurality of blades comprises at least 20 blades. In some embodiments, said plurality of blades comprises 21 blades. An increase in the number of blades may increase the efficiency and flow generated by the fan but will also increase noise, while a reduced number of blades may reduce the effectiveness, unless the fan working surface increases (larger blades), but will reduce noise. In some embodiments 21 blades provides for the most efficient operation (work out to power in).

In some embodiments, said plurality of blades comprises at least 15 blades.

In some embodiments, said plurality of blades comprises fewer than 12 blades. In some embodiments, said plurality of blades comprises 11 blades. Providing fewer blades reduces the noise created by the fan. There is therefore a balance between improved efficiency with more blades and reduced noise with fewer blades. An increased number of blades does produce the effect of forcing more air, but if the number of blades becomes too high then the effectiveness may decrease as the system becomes choked. This balance should be considered when designing a fan depending on the requirements of the vacuum pump in which it is to be used. A further consideration is that having more blades reduces the possible curvature of the radially outer portion of the blades whilst still being practical to make the fan using a mould. This is because the blades will be closer together and should not overlap when looking at the blades in an axial direction.

In some embodiments, said plurality of blades comprises fewer than 15 blades, preferably fewer than 12 blades.

In some embodiments, at least one of said radially extending edges of said blades is serrated. The at least one serrated edge allows the blades to smoothly move through gas, thereby reducing noise. Noise can be reduced further if both radially extending edges of the blades are serrated. However, this may increase manufacturing costs. -6 -

In a further aspect of the invention, there is provided a vacuum pump comprising: a motor having a shaft; a fan according to a first aspect of the invention mounted on said shaft.

Fans that are mounted on a motor shaft must be designed bearing in mind the range of speeds of rotation of the fan is limited by the speed of the motor shaft. In practice, the rotation of the motor shaft will be constrained to speeds which enable proper vacuum pump operation.

In some embodiments, a radius of said hub is between 80% and 100% of a radius of an end of said vacuum pump facing said fan such that said hub directs cooling gas around said vacuum pump. In this way, the fan is suitably dimensioned such that the hub directs gas to a radial edge of the end of the vacuum pump at which the fan the curved radially outer portion of the blades directs gas axially such that the gas is blown around the end of the vacuum pump.

In some embodiments, said vacuum pump comprises a scroll pump. Scroll pumps typically comprise a generally circular end comprising the motor of the pump. Therefore, fans according to an aspect are well designed to direct air around the end of a scroll pump.

In another aspect of the invention, there is provided a vacuum pump arrangement comprising: a vacuum pump according to a further aspect of the invention; and a cowling mounted on said vacuum pump, said cowling having an inlet for receiving gas for cooling said vacuum pump; wherein said cowling surrounds said fan such that said inlet is aligned with said fan.

The cowling is sometimes referred to as a fan cover. Surrounding the fan with a cowling facilitates guiding gas towards the direction perpendicular to the direction of rotation of the fan. The cowling also acts to retain and direct the gas for circulation about the vacuum pump. -7 -

In some embodiments, a portion of said cowling compliments the curvature of said radially outer portion of said blades such that a clearance between said portion of said cowling and said radially outer portion is substantially constant. In this way, the cowling follows the curve of the radially outer portion of the blades to maintain a substantially constant space between the radially outer portion of the blades and the cowling. The cowling therefore works with the curved radially outer portion of the blades to more efficiently direct the gas in a direction substantially perpendicular to the direction of rotation of the fan. Moreover, io having a substantially constant clearance reduces stagnant or reduced flow areas, thereby improving overall flow and efficiency. In some embodiments, the portion of the cowling which compliments the curvature of the radially outer portion of the blades is a corner of the cowling.

In some embodiments, a clearance between said inlet and said fan is larger than said clearance between said portion of said cowling and said radially outer portion. In this way, more space is provided for incoming gas which can improve gas intake and reduce the noise created by gas intake.

In some embodiments, said clearance between said inlet and said fan is at least 3 times larger than said clearance between said portion of said cowling and said radially outer portion. In other embodiments, the difference is at least 5 times.

In some embodiments, said clearance between said portion of said cowling and said radially outer portion is between 'I mm and 2mm. In some embodiments, said clearance between said inlet and said fan is between 2mm and 6mm.

In some embodiments, said cowling extends around a pumping mechanism of said scroll pump. In this way, the gas for cooling the vacuum pump may be used 30 to cool the pumping mechanism of the vacuum pump -8 -In some embodiments, the plurality of blades comprises an uneven number of blades and the inlet of the cowling comprises an even number of inlet segments. Providing this mismatch reduces resonance between the inlet and the fan, thereby reducing noise.

In some embodiments, each inlet segment comprises a plurality of inlet portions. In some embodiments, the plurality of inlet portions comprises four inlet portions.

According to an additional aspect of the invention, there is provided a method of 10 replacing a fan in a vacuum pump comprising: removing a fan from a motor shaft of said vacuum pump; and mounting a fan according to an aspect of the invention on said motor shaft.

In some embodiments, said method further comprises removing a cowling of the 15 vacuum pump and replacing the cowling after mounting the fan according to the aspect of the invention on the motor shaft.

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 perspective view of a fan according to an embodiment; Figure 2 shows a perspective view of the fan of Figure 1 mounted on a vacuum pump according to an embodiment; -9 -Figure 3 shows the curved edges of the blades of the fan of Figure 1; Figure 4 shows a perspective view of a cowling for use with the vacuum pump of Figure 2 according to an embodiment; and Figure 5 shows a perspective view of a fan according to another embodiment.

DESCRIPTION OF THE EMBODIMENTS

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

io Vacuum pumps, such as scroll pumps, use fans to help circulate gas for cooling the vacuum pump and the motor of the vacuum pump. To cool the vacuum pump, gas arriving at an end of the pump at which the fan is mounted is directed around the motor and then on around a pumping mechanism. In known vacuum pumps, radial fans are used to blow gas radially outward and a cowling surrounding the fan is used to redirect the gas flow around the motor. This arrangement can be noisy and inefficient at directing the gas around the end of the vacuum pump.

Embodiments provide a fan comprising an annular hub that may be mounted on a shaft of a vacuum pump motor and a plurality of blades extending from the hub.

An outer portion of the blades is curved such that gas is directed in an axial direction around the motor. In this way, the fan helps redirect gas around the motor with reduced or no reliance on a cowling, thereby improving flow and reducing noise.

The blades may comprise an inner portion that is straight to facilitate removal from a mould used to make the fan. Alternatively, the blades may curve along substantially the entire length of the blades to improve efficiency and flow. Where the blades curve along substantially their entire length they may not overlap radially with the hub but instead extend from an outer diameter of the hub.

-10 -To further improve the efficiency of the fan, the hub may have an inclined surface configured to direct incoming gas radially outward towards the curved portions of the blades. The radially outer edge of the hub may be configured to be approximately the same size as an end of the vacuum pump to which the fan is attached. In this way, the gas is directed radially beyond the outer edge of the motor before being directed axially by the curved portion of the blades.

The fan may be used in conjunction with a cowling attached to the vacuum pump. The cowling is designed to help redirect the air around the motor by complimenting the curved portion of the blades.

Figure 1 shows an embodiment of a fan 10 comprising a hub 11 and a plurality of (in this case twenty-one) blades 14. The hub 11 has an annular cross section and has a bore 12 through an axial centre of the fan 10. The hub 11 comprises an inclined surface 13 defining a radially outer portion of the hub 11.

The blades 14 extend radially out from the hub 11, in this embodiment from the inclined surface 13 of the hub. Each blade 14 comprises a radially outer portion 15 and a radially inner portion 16. The radially outer portion 15 is radially outside the hub 11 and is curved forward in a direction of rotation of the fan 10. A radially extending edge 17 of the blades 14 which in use faces towards incoming gas is curved more than a radially extending edge 18 which in use faces away from incoming gas. In this way, the blades 14 comprise an angled surface configured to direct gas towards a direction that is perpendicular to the direction of rotation of the fan 10 and substantially parallel to incoming gas.

The fan 10 is configured to be mounted at an end 23 of a vacuum pump 20 as shown in Figure 2. The end 23 of the vacuum pump 20 comprises a motor within a motor housing. The bore 12 is configured to receive a motor shaft 21 of the motor that extends outside the motor housing. The fan 10 is configured to direct gas around the edge of the end 23 of the vacuum pump 20. To this end, a radius of the hub 11 is dimensioned to be between 80% and 100% of the radius of the end 23 of the vacuum pump 20 such that the hub 11 can direct gas radially outwards towards the edge of the end 23 of the vacuum pump 20. At this point, the curved radially outer portion 15 of the blades 14 is configured to redirect the gas towards the axial direction. The vacuum pump 20 comprises cooling fins 22 extending from the motor housing towards which the gas is directed to help cool the vacuum pump 20.

Figure 3 shows the angle of the blades 14 created by the top radially extending edge 17 being more curved at a radially outer portion 15 than the bottom radially io extending edge 18 of the fan 10. The blades 14 are angled at 45 degrees relative to the axis of the fan 10 (represented by the vertical dashed line in Figure 3). It will be appreciated that other similar angles will also be suitable for redirecting gas towards a direction substantially perpendicular to a direction of rotation of the fan 10. In some embodiments, the radially outer portion 15 defines an angle of between 40 and 50 degrees relative to the axis of the fan 10.

Figure 3 also shows how the top radially extending edge 17 does not extend over the bottom radially extending edge of the blade immediately adjacent in a direction of rotation (to the right in Figure 3). In this way, the fan can be created using a mould.

Figure 4 shows a view of the inside of a cowling 24 for use with the vacuum pump 20 and the fan 10 to provide a vacuum pump arrangement. The cowling 24 is mounted about at least a portion of the vacuum pump 20 including the fan 10.

The cowling 24 comprises an inlet 25 which when mounted on the vacuum pump 20 is aligned with the fan 10 such that the fan acts to draw in air surrounding the vacuum pump 20 through the inlet 25 for circulation around the vacuum pump 20. The inlet 25 comprises a plurality of, in this case eight, equally sized inlet segments 26 about an axial centre of the cowling 25 (aligned in use with the axial centre of the fan 10). Each inlet segment 26 is split into circumferential inlet portions 27. Splitting the inlet into smaller inlet portions 27 improves the strength of the cowling 25 and inhibits objects from being sucked into the fan 10.

-12 -The fan 10 and the cowling 24 combine a plurality of blades 14 comprising an uneven number of blades 14 (twenty-one) with an inlet 25 of the cowling 24 comprising an even number of inlet segments 26 (eight) which helps reduce blade resonance, thereby reducing the noise created by the fan 10. It will be appreciated that numbers of blades 14 and inlet segments 26 other than those shown can be used.

The corner 28 comprises a complimentary shape to the radially outer portion 15 of the blades 14 such that curve of a corner 28 of the cowling 24 substantially matches the curve of the blades 14. In this way, the cowling 24 works with the fan 10 to help direct air towards the direction perpendicular to the direction of rotation of the fan 10. The complimentary shape of the corner 28 ensures a clearance between the cowling 24 and the radially outer portion 15 of the blades 14 is substantially constant. In some embodiments, the clearance is between 1mm and 2mm. The substantially constant clearance reduces turbulent flow and stagnant regions which reduction can reduce noise and improve gas flow.

When mounted to the vacuum pump 20, a clearance between the inlet 25 and the fan 10 is larger than the clearance between the corner 28 and the radially outer portion 15 of the blades 14 to improve gas intake through the inlet 25. However, vacuum pumps are often used in places where space is at a premium, for example, semiconductor processing plants. Therefore, a balance is required between improving gas intake and the size of the vacuum pump arrangement. In some embodiments, the clearance between the inlet 25 and the fan 10 is three times larger than the clearance between the corner 28 and the radially outer portion 15. In some embodiments, the clearance between the inlet 25 and the fan 10 is between 2mm and 6mm.

In use, a gas such as air is drawn through the inlet 25 of the cowling 24 by the fan 10. Incoming air approaches the fan 10 approximately perpendicularly to the direction of rotation of the fan. The air is directed radially out by the inclined -13 -surface 13 of the hub 11 and the straight radially inner portion 16 of the blades 14. Once the air reaches the curved radially outer portion 15 of the blades 14, the air is redirected once more towards a direction substantially perpendicular to the direction of rotation of the fan 10, i.e., a direction similar to the incoming air of the fan 10. The cowling 24 helps with this second redirection by conforming to the shape of the blades 14. Thus, incoming air is directed through two substantially right angle turns by the fan Figure 5 shows an alternative embodiment of a fan 110 for circulating gas for cooling a vacuum pump. Like the fan 10 of Figure 1, the fan 110 comprises a hub 111 having a central axial bore 112 and an inclined surface 113 for directing gas radially outward. However, the fan 110 comprises blades 114 which curve substantially over their entire length with the angle of the blades becoming steeper towards the radially outer end of the blades. Having blades 114 that are curved substantially over their entire length increases the velocity of the gas flow generated by the fan. However, fewer blades must be used to maintain the ability to mould the fan 110. In this embodiment, there are 9 blades. In some embodiments, there are fewer than 12 blades.

To further reduce noise, the radially extending edge which in use faces incoming gas is serrated, thereby allowing the blades 14 to cut through gas more smoothly. Additionally or alternatively, the radially extending edge facing away from incoming gas may be serrated.

In summary, scalable fan geometry has been developed for attachment to motors of vacuum pumps. The fans can reduce the noise emission from 63dB(A) to less than 53dB(A) (measured in pump assembly) and flow can be increased by approximately 20%.

The fan geometry is more silent and delivers more air flow than conventional radial fans used with vacuum pump motors. The geometry of fan of embodiments is designed to enable injection moulded manufacturing technology. Alternatively, -14 -the fans could be made using additive manufacturing and in which case there may be more flexibility in the curve and number of blades.

Embodiments of the fan are designed to fit onto and be driven by the motor shaft of the pump and use the fan cover (cowling) for better steering of air. However, even without the cowling, the fan works well and obtains the advantages of reduced noise and improved flow compared to a conventional radial fans.

Embodiments of the fan have a straight blade geometry above the cone area io (hub with inclined surface) for easier moulding but having continuous curving from base to tip to provide a stronger gas flow. The air is dragged in (gas intake) mainly in the centre part where the cone steers it to the outer diameter of the fan. Along the path to the outer diameter, the aggressive curving of the fan blade turns the air towards the axial direction.

Conventional motor fans are noisy and they deliver just enough air to cool the motor. Embodiments of the fan are less noisy and deliver more air.

In the alternative embodiment, the fan has different geometry ratios and "owl wing" teeth on the intake side of the fan (serrated radially extending edge configured in use to face incoming gas).

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.

-15 -

REFERENCE SIGNS Fan

11 Hub 12 Bore 13 Inclined Surface 14 Blades Radially Outer Portion 16 Radially Inner Portion 17,18 Radially Extending Edges io 20 Vacuum Pump 21 Motor Shaft 22 Cooling Fins 23 End of Pump 24 Cowling 15 25 Inlet 26 Inlet Segments 27 Inlet Portions 28 Cowling Corner Fan 111 Hub 112 Bore 113 Inclined Surface 114 Blades 117, 118 Radially Extending Edges

Claims

-16 -CLAIMS1. A fan for circulating gas for cooling a vacuum pump said fan comprising: an annular hub configured for mounting on a motor shaft of said vacuum 5 pump; and a plurality of blades extending radially out from said hub; wherein a radially outer portion of said blades is curved in a direction of rotation of said blades such that, in use, said blades direct said gas towards a direction perpendicular to said direction of rotation.
2. A fan according to claim 1, wherein a radially inner portion of said blades is substantially straight.
3. A fan according to claim 1 or claim 2, wherein said radially outer portion 15 comprises between 15% and 40% of a radius of said fan.
4. A fan according to claim 1, wherein the blades are curved along substantially their entire length.
5. A fan according to any preceding claim, wherein said blades comprise a radially extending edge configured in use to face incoming gas and a radially extending edge configured in use to face away from incoming gas, wherein a radially outer portion of said radially extending edge configured in use to face incoming gas is more curved in said direction of rotation than a radially outer portion of said radially extending edge configured in use to face away from incoming gas.
6. A fan according to any preceding claim, wherein said radially outer portion defines an angle of between 40 and 50 degrees relative to an axis of said fan.
-17 - 7. A fan according to any preceding claim, wherein said hub comprises an inclined surface configured in use to direct gas radially outward and wherein said blades extend from said inclined surface.
8. A fan according to claim 7, wherein said annular hub is frustoconical and has an axial bore for accommodating said motor shaft.
9. A fan according to any preceding claim, wherein said radially outer portion of said blades is radially outside of said hub.
10. A fan according to any preceding claim, wherein said plurality of blades comprises at least 20 blades.
11. A fan according to claim 5 or any one of claims 6 to 10 when dependent on claim 5, wherein at least one of said radially extending edges of said blades is serrated.
12. A vacuum pump comprising: a motor having a shaft; a fan according to any one of claims 1 to 11 mounted on said shaft.
13. A vacuum pump according to claim 12, wherein a radius of said hub is between 80% and 100% of a radius of an end of said vacuum pump facing said fan such that said hub directs cooling gas around said vacuum pump.
14. A vacuum pump according to claim 12 or claim 13, wherein said vacuum pump comprises a scroll pump.
15. A vacuum pump arrangement comprising: a vacuum pump according to any one of claims 12 to 14; and a cowling mounted on said vacuum pump, said cowling having an inlet for receiving gas for cooling said vacuum pump; wherein -18 -said cowling surrounds said fan such that said inlet is aligned with said fan.
16. A vacuum pump arrangement according to claim 15, wherein a portion of said cowling compliments the curvature of said radially outer portion of said blades such that a clearance between said portion of said cowling and said radially outer portion is substantially constant.
17. A vacuum pump arrangement according to claim 16, wherein a clearance io between said inlet and said fan is larger than said clearance between said portion of said cowling and said radially outer portion.
18. A vacuum pump arrangement according to claim 17, wherein said clearance between said inlet and said fan is at least 3 times larger than said clearance between said portion of said cowling and said radially outer portion.
19. A vacuum pump arrangement according to any one of claims 15 to 18 when dependent on claim 13, wherein said cowling extends around a pumping mechanism of said scroll pump.
20. A method of replacing a fan in a vacuum pump comprising: removing a fan from a motor shaft of said vacuum pump; and mounting a fan according to any one of claims 1 to 11 on said motor shaft.