GB2472635A

GB2472635A - Seal-less tip scroll booster pump for spectrometer

Info

Publication number: GB2472635A
Application number: GB0914217A
Authority: GB
Inventors: Ian David Stones; Ian Keech
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2009-08-14
Filing date: 2009-08-14
Publication date: 2011-02-16
Also published as: WO2011018643A3; GB0914217D0; WO2011018643A2

Abstract

A booster pump 10 for use between primary and secondary pumps in evacuating a mass spectrometer comprises two scrolls 20, 22 for pumping fluid from an inlet 24 to an outlet 26 on relative scroll orbiting. Each scroll comprises a scroll base 30 from which a scroll wall 28 extends generally axially towards the base of the opposing scroll, wherein an axial end face 38 of each of the scroll walls forms a sealing surface for sealing between each scroll wall and the base plate 36 of the opposing scroll across a running clearance G. Omitting dedicated tip seals simplifies construction, reduces power requirement and eliminates tip seal dust and is permitted by a relatively modest compression ratio. The scrolls may be metallic and multi start.

Description

BOOSTER PUMP

The present invention relates to a booster pump, to a vacuum pumping arrangement comprising the booster pump and a mass spectrometer system.

A scroll compressor is a suitable type of booster pump capable in suitable arrangements of relatively high pumping capacity but relatively low compression ratio. A prior art scroll compressor, or pump, 100 is shown in Figure 4. The pump 100 comprises a pump housing 102 and a drive shaft 104 having an eccentric shaft portion 106. The shaft 104 is driven by a motor 108 and the eccentric shaft portion is connected to an orbiting scroll 110 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 112 for pumping fluid along a fluid flow path between a pump inlet 114 and pump outlet 116 of the compressor.

The fixed scroll 112 comprises a scroll wall 118 which extends perpendicularly to a generally circular base plate 120. The orbiting scroll 110 comprises a scroll wall 124 which extends perpendicularly to a generally circular base plate 126. The orbiting scroll wall 124 co-operates, or meshes, with the fixed scroll wall 118 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.

A scroll pump is typically a dry pump and not lubricated. In order to prevent back leakage, the space between the axial ends of a scroll wall of one scroll and the base plate of the other scroll is sealed by a tip seal 128. An enlarged cross-section through a portion of the fixed scroll 112 showing the tip seal 128 in more detail is shown in Figure 5.

As shown in Figure 5, the tip seal 128 typically made from a plastics or rubber is located in a channel 132 at the axial end 134 of the fixed scroll wall 118. There is a small axial gap between an axial end of the tip seal 128 and the base of the channel 132 so that in use fluid occupying the gap forces the tip seal axially towards the base plate 126 of the orbiting scroll. Accordingly, the tip seal is supported on a cushion of fluid which serves to urge the seal against an opposing scroll.

By preventing back-leakage with the use of tip seals, higher compression ratios can be achieved and the pump may work more efficiently. However, when bedding in or during use, the tip seals 128 are worn by contact with the opposing scroll base plate 120, 126 generating tip seal dust. When the pump is used for pumping a clean environment such as a vacuum chamber of a silicon wafer processing apparatus, it is desirable that the tip seal dust does not migrate upstream into the vacuum chamber, particularly during pump down times.

The present invention provides a booster pump comprising two scrolls which are co-operable for pumping fluid from an inlet to an outlet on relative orbiting motion of the scrolls, each scroll comprising a scroll base from which a scroll wall extends generally axially towards the base of the opposing scroll, wherein an axial end face of each of the scroll walls forms a sealing surface for sealing between each scroll wall and the base plate of the opposing scroll.

Other preferred and/or optional aspects of the invention are defined in the accompanying claims.

In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically a booster pump; Figure 2 shows a partial cross-section through a fixed scroll of the booster pump shown in Figure 1; Figure 3 shows a mass spectrometer system comprising the booster pump shown in Figure 1; Figure 4 shows schematically a prior art scroll pump; and Figure 5 shows a partial cross-section through a fixed scroll of the booster pump shown in Figure 4.

A scroll compressor, or pump, 10 is shown in Figure 1. The pump 10 is a booster pump and comprises a pump housing 12 and a drive shaft 14 having an eccentric shaft portion 16. The shaft 14 is driven by a motor 18 and the eccentric shaft portion is connected to an orbiting scroll 20 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 22 for pumping fluid along a fluid flow path between a pump inlet 24 and pump outlet 26 of the compressor.

The fixed scroll 22 comprises a scroll wall 28 which extends perpendicularly to a generally circular base plate 30. The orbiting scroll 20 comprises a scroll wall 34 which extends perpendicularly to a generally circular base plate 36. The orbiting scroll wall 34 co-operates, or meshes, with the fixed scroll wall 28 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.

As indicated above with reference to the prior art, a scroll pump is typically a dry pump and not lubricated. Therefore, in order to prevent back leakage, the space between the axial ends of a scroll wall of one scroll and the base plate of the other scroll is sealed by tip seals. However, in the present embodiment, the scroll pump is a booster pump which is adapted for achieving relatively high pumping capacity and relatively low compression ratio.

It will be appreciated that back-leakage across a scroll wall is a more significant problem when the pressure differential across the wall is relatively high. When a scroll pump is adapted to achieve high compression ratios, the pressure differential across a scroll wall is increased and therefore back-leakage becomes a problem. Tip seals alleviate this problem but suffer from the generation of tip seal dust. In the present embodiment, the booster pump is adapted to achieve high capacity but low compression and therefore the problem of back-leakage is reduced as the pressure differential across a scroll wall is reduced. Accordingly, it is possible to provide a booster pump without tip seals without significant loss in pump performance. The absence of tip seals means that tip seal dust is not generated and does not contaminate the system. Further, in a normal scroll pump tip seals require replacement at regular intervals after they become worn. Also, the channel 132 shown in Figure 5 must be machined in order to locate the tip seals and machining adds to the cost and time of manufacture. Additionally, tip seals are contact seals and create friction during use, thereby increasing power consumption.

Figure 2 shows an enlarged cross-section taken through a portion of the fixed scroll 22 in Figure 1. Scroll 22 comprises scroll base plate 30 from which scroll wall 28 extends generally axially towards the base plate 36 of the opposing, orbiting, scroll 20. The position of the base plate 36 is shown by broken lines in Figure 2. An axial end face 38 of the scroll wall 28 forms a sealing surface for sealing between the scroll wall 28 and the base plate 36 of the opposing scroll. A scroll wall without a tip seal is advantageous in a booster as explained above.

The axial end face 38 of scroll wall 28 forms a planar sealing surface across a width at least in a radial direction R of the scroll wall. The planar surface 38 is located close to the opposing base plate 36 by a gap G (or running clearance) which is selected to be as small as can be achieved within manufacturing and operating tolerances. Preferably, the gap is less than 100 tm and more preferably, less than 50 pm.

The sealing surface is formed by the axial end face of the scroll wall. In order to simplify the end face is formed from a metallic material and preferably formed from the same material as the scroll wall. In this regard, the end face is unitary and integral with the scroll wall.

The orbiting scroll may have equivalent features as the fixed scroll as described above in relation to Figure 2. Alternatively, one scroll may be provided with a tip seal and one scroll without a tip seal, although this is not preferred.

A vacuum pumping arrangement 40 comprising booster pump 10 is shown in Figure 3. The pumping arrangement 10 is for differentially pumping a plurality of vacuum chambers in a mass spectrometer system 42. The vacuum chambers are connected in series to provide a sample flow path 44 starting from a first vacuum chamber 46 through a second vacuum chamber 48, a third vacuum chamber 50 to a fourth vacuum chamber 52. The pressure decreases along the sample flow path which flows to the right as shown in the Figure 3 from atmosphere at the inlet of the first chamber 14 to high vacuum at the fourth chamber 52. For example, the first chamber 46 may be at a high pressure (low vacuum) such as 10 mbar. The second vacuum chamber may be at a relatively lower pressure of 1 mbar.

The first and second vacuum chambers in this example are considered to be at a viscous, or non-molecular, regime or condition. The third vacuum chamber 50 may be at a low pressure of i0 mbar. The fourth vacuum chamber 52 is at a lower pressure of 106 mbar. The third and fourth chambers in this example are considered to be at a molecular flow regime or condition.

The vacuum pumping arrangement 40 is designed to differentially pump the vacuum chambers and maintain a relatively increased sample flow rate through the mass spectrometer.

The vacuum pumping arrangement 40 comprises a primary, or backing, pump 54 connected in series with booster pump 10 SO that the booster pump exhausts at a pressure less than atmosphere. The booster pump 10 exhausts at about 10 mbar in the arrangement shown in Figure 3. In the prior art scroll pump, back-leakage in the pump is relatively greater at the exhaust and relatively less at the inlet because back-leakage is greater at higher pressures. In this regard, the leakage across the scroll walls relates to the pressure drop across the wall and the pressure drop is typically highest at or towards the exhaust. Conversely, if the absolute pressure is low, then the pressure drop can only be small. Accordingly, as booster pump 10 is at a pressure less than atmosphere, back-leakage is decreased. Therefore, when booster pump 10 exhausts at less than atmosphere, the absence of tip seals constitutes a less significant problem and causes less back-leakage than when the booster pump exhausts at atmosphere.

Referring to Figure 3, primary pump 54 is connected to the first vacuum chamber 46 and exhausts to atmosphere. The primary pump 54 may also be a scroll pump adapted for relatively high compression ratio. The capacity of the primary pump may be relatively low as it is connected in series with the relatively high capacity booster pump 10. The booster pump 10 is connected to the second chamber 48. The booster pump exhausts to the inlet of primary pump 54 and not to atmosphere.

In order to increase pumping capacity, the booster pump 10 may have a multi-start scroll pumping arrangement. In one arrangement of scroll pump 10, the scroll pumping mechanism may have for example 6 starts to maximise capacity (75-lOOm3i'h for example) but only one stage of compression (10:1 for example). For comparison, backing pump 54 may be of the same size and have only 1 or 2 starts for nominal capacity (35m3/h for example) but 4 or more stages of compression (100,000:1 for example).

The vacuum pumping arrangement 40 further comprises first and second secondary pumps 60, 62 which may be turbomolecular pumps. Although two secondary pumps are shown only one such pump may be provided. The secondary pumps are arranged in parallel and are connected for pumping vacuum chambers 50, 52 respectively. The secondary pumps are connected in series with the primary pump 54 and the booster pump 10. As the secondary pumps cannot exhaust to atmosphere, the booster pump 10 is connected to the exhausts of the secondary pumps and the booster pump exhausts to the primary pump 54 which in turn exhausts to atmosphere.

The provision of booster pump 10 in series with a primary pump 54 for differentially pumping a plurality of vacuum chambers 46, 48 is advantageous for example in a mass spectrometer system. The booster pump can not only provide backing for secondary pumps 60, 62 but also provides high sample gas flow, particularly in the viscous pressure regime, and in more than one chamber in that regime.

In Figure 3, the combination of a primary pump and a booster pump connected in series provides a number of advantages over the prior art. First, increased sample flow rate is achieved because the combination provides increased pumping capacity. Secondly, both the primary pump 54 and the booster pump 10 can be connected for pumping two vacuum chambers 46, 48. In this latter regard, the primary pump and booster pump combination is capable of pumping lower pressures at the inlet of the booster pump than in the prior art.

Therefore, the inlet of the booster pump can be connected both to a vacuum chamber and back the secondary pumps. A further advantage is that an additional differentially pumped chamber can be provided in the system compared to the prior art whilst using the same

number of pumps as in the prior art.

In known scroll pumps, the tip seals prevent back leakage of gas from a high pressure side of a scroll wall to a low pressure side of a scroll wall. As back leakage is reduced, higher compression ratios can be achieved. However, tip seals are contact seals and therefore increase power requirement of a pump caused by friction between moving surfaces. Further, the absence of tip seals increases back leakage, which reduces the power required by the pump, especially at higher inlet pressures. -10-

Claims

CLAIMS1. A booster pump comprising two scrolls which are co-operable for pumping fluid from an inlet to an outlet on relative orbiting motion of the scrolls, each scroll comprising a scroll base from which a scroll wall extends generally axially towards the base of the opposing scroll, wherein an axial end face of each of the scroll walls forms a sealing surface for sealing between each scroll wall and the base plate of the opposing scroll.
2. A booster pump as claimed in claim 1, comprising the axial end face of each scroll wall forms a planar sealing surface across a width of the scroll wall.
3. A booster pump as claimed in claim 1 or 2, wherein the scroll wall and the axial end face are formed from a metallic material.
4. A booster pump as claimed in any one of the preceding claims, wherein the scroll wall and the axial end face are formed from the same material.
5. A booster pump as claimed in any one of the preceding claims, wherein the scroll pumping mechanism has a multiplicity of starts for increasing pumping capacity. -11 -
6. A vacuum pumping arrangement comprising a booster pump as claimed in any preceding claim in series with a backing pump such that the backing pump can exhaust to atmosphere and the booster pump can exhaust to an inlet of the backing pump.
7. A vacuum pumping arrangement as claimed in claim 6, wherein the booster pump is adapted to exhaust at pressure no greater than 10 mbar.
8. A mass spectrometer system comprising a plurality of vacuum chambers connected in series and a vacuum pumping arrangement as claimed in claims 6 or 7 for differentially pumping the vacuum chambers, wherein the backing pump is connected to a low vacuum chamber for pumping same and the booster pump is connected to a relatively lower pressure chamber for evacuating same.