GB2416822A

GB2416822A - Inlet valve for vacuum pump

Info

Publication number: GB2416822A
Application number: GB0417580A
Authority: GB
Inventors: Matthew Wickes
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2004-08-06
Filing date: 2004-08-06
Publication date: 2006-02-08
Also published as: GB0417580D0

Abstract

An inlet valve for a vacuum pump comprises a housing 41 having an inlet 42, an outlet 44 spaced from the inlet, and a passageway 46 extending between the inlet and and the outlet. A moveable valve member 48 is located in the passageway for controlling a flow of fluid between the inlet and the outlet. The valve member comprises a first permanent magnet 56 and is moveable between a closed position at which the valve member sealingly engages a cooperating valve seat 50 and an open position spaced from the valve seat. The valve also comprises a second permanent magnet 62 which repels the valve member towards the valve seat. The repulsion force between the magnets serves to retain the valve member in the closed position until a pressure difference between the inlet and the outlet is of sufficient magnitude to cause the valve member to move towards the open position against the repulsion force between the magnets.

Description

24 1 6822

VALVE

The present invention relates to a valve, and in particular to an inlet valve for a vacuum pump.

Scroll-type apparatus is known for operating compressors or vacuum pumps. In both cases the apparatus comprises two scroll members each comprising a flat end plate on which are upstanding strips defining a spiral (or scroll) type structure which interact by placing the scroll members together with the respective spiral lo type structures inter-engaging and allowing one scroll member to "orbit" relative to the other. In this way it is possible to trap a volume of gas between the scroll members thereby to urge the gas from one end of the respective spirals to the other.

Traditionally, one of the scroll members remains stationary, whilst the other scroll member is caused to "orbit" relative to the stationary member. Another known form of scroll-type apparatus replaces the fixed scroll member and orbiting scroll member with two co-rotating scrolls synchronized to each other by means of an Oldham coupling.

In a vacuum pump in particular the gas enters via a pump inlet and exits via a pump outlet. When the pump is switched off, any gas that has been compressed to a higher pressure than the inlet pressure will tend to expand back towards the pump inlet, which potentially can raise the pressure in the apparatus being subjected to a vacuum. This is generally referred to as "suckback". Suckback is not unique to scroll compressors; any pump having a pumping mechanism in which compressed gas is retained within the pumping mechanism when the pump is switched off is prone to some degree of suckback.

so In particular relation to scroll compressors, the erosion of tip seals provided at the free edges of the scroll members generates dust. During use of the pump, this dust is conveyed through the pump exhaust together with the exhaust gas, but - 2 when the pump is switched off, this dust could migrate through the pump inlet towards the evacuated apparatus, leading to unwanted contamination of the evacuated apparatus.

In order to protect the evacuated apparatus from suckback, and from any dust carried thereby, it is known to employ an inlet valve. Ideally, the inlet valve should be closed sufficiently quickly after the pump is switched off that there is virtually no venting of gas from the pump inlet. As a result, such valves normally comprise electrically operated solenoid valves arranged to close as the power is switched lo off. However, such valves tend to be relatively expensive.

It is an aim of the present invention to provide a relatively inexpensive inlet valve that can reliably prevent the escape of gas from a pump inlet when the pump has been switched off.

The present invention provides a valve comprising a housing having an inlet, an outlet spaced from the inlet, and a passageway extending between the inlet and the outlet; a valve member located in the passageway for controlling a flow of fluid between the inlet and the outlet, the valve member comprising a first permanent so magnet and being moveable between a closed position at which the valve member sealingly engages a cooperating valve seat and an open position spaced from the valve seat; and a second permanent magnet which repels the valve member towards the valve seat to retain the valve member in the closed position until a pressure difference between the inlet and the outlet is of sufficient magnitude to cause the valve member to move towards the open position against the repulsion force between the magnets.

The use of repelling magnets to close the valve is advantageous over the use of a spring to perform the same function, as such a spring would be relatively so expensive to manufacture with an appropriately low spring constant, and would be somewhat unreliable. Furthermore, with a low spring constant, the spring would be readily deformed by handling during valve assembly, leading to a short lifetime. - 3

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through a vacuum pump; Figure 2 is an enlarged view of the inlet valve of Figure 1; and Figure 3 is an isometric view of the housing of the inlet valve of Figure 2.

Figure 1 illustrates a vacuum pump 2 which includes a housing 4 to which is mounted a fixed scroll member 6. Complimenting the fixed scroll member 6 and intermeshing therewith in a manner known per se is an orbital scroll member 8.

The orbital member 8 is mounted on bearings 10, which are supported on a crank Is end 12 of a drive shaft 14 that extends from a motor 16. Bearings 17 serve to support the drive shaft 14 relative to the housing 4. Each scroll member 6, 8 includes an end plate to which is attached and from which extends outwardly therefrom a spiral wrap 18, 20. The height of each wrap 18, 20 is substantially the same as the distance between the opposed end plates so that the free edge of so each wrap 18, 20 forms a seal against the face of the complimentary scroll member end plate. To assist in the sealing process, tip seals (not shown) may be provided at the free edge of each wrap 18, 20. In order to seal the vacuum space in the housing 4 from ambient atmosphere and the pump drive system, a metal bellows 22 is disposed substantially co-axially with the drive shaft 14. The metal us bellows is attached at one end by bolts 24 to the orbital scroll member 8 and at its opposite end to a stationary part 26 of the pump by bolts 28. O-rings 30, 32 serve to seal the attachment of the metal bellows 22. By virtue of the torsional stiffness of the metal bellows 22, the bellows 22 also serves to limit undesirable travel in the circular translation of the orbital scroll member 8 and thus maintain the correct so angular position between the fixed scroll member 6 and the orbital scroll member 8.

The pump inlet 34 is located at the periphery of the scroll members 6, 8. An inlet non-return valve 36 is provided at the pump inlet 34. In use, orbiting motion of the orbital scroll member 8 relative to the fixed scroll member 6 causes the inlet non return valve 36 to open and permit gas to enter the pump 2 through the pump inlet 34. A volume of gas is trapped between the scroll members 6, 8 and is urged from the outer end of the wraps 18, 20 to the inner end for discharge via pump outlet 38. Outlet non-return valve 40 prevents gas from entering the pump through the pump outlet 38. As the pressure reduces above the inlet valve 36 due to pumping, the inlet valve 36 gradually closes until there is no further gas to lo pump. The pump 2 may then be switched off.

The inlet valve 36 is illustrated in more detail in Figure 2. The inlet valve 36 comprises a housing 41 having an inlet 42 and, spaced therefrom, an outlet 44.

The housing 41 is preferably formed from aluminium. Located in a passageway 46 extending between the inlet 42 and the outlet 44 is a moveable valve member 48 and a cooperating valve seat 50 for controlling the flow of a fluid, for example a gas, between the inlet 42 and the outlet 44.

The valve member 48 comprises a valve stem 52 and a flange 54 extending go radially outwardly from the valve stem 52. In this embodiment, the valve stem 52 and flange 54 are integral, and are formed from non- magnetic material, such as aluminium. Attached to the lower (as shown) end of the valve stem 52 is a first permanent magnet 56, which may, as illustrated, be conveniently located in a recess 58 located at the lower end of the valve stem 52. As shown in Figure 2, the lower end of the valve stem 48 is located in a bore 60 formed in the housing 41 and extending from the outlet 44 towards the inlet 42. The bore 60 serves to guide the movement of the valve member 48. A second permanent magnet 62 is located towards the bottom of the bore 60. As illustrated, the second permanent magnet 62 may be conveniently located in a recess 64 located proximate the so outlet 44, with an air gap 66 being formed between the bore 60 and the recess 64.

The magnets 56, 62 are preferably formed from samarium cobalt, SmCO5.

The valve seat 50 is also formed from non-magnetic material, preferably aluminium. A first O-ring 68 is provided on the side of the valve seat 50 opposite the flange 54. This O-ring 68 serves to provide a fluid-tight seal between the valve member 48 and the valve seat 50. A second O-ring 70 is provided on the s opposite side of the valve seat 50. This second Oring 70 provides a seal with a stainless steel retaining ring 72 located proximate the inlet 42 of the housing 41.

The polarities of the magnets 56, 62 are chosen such that the magnets 56, 62 mutually repel each other so that the upward (as shown) magnetic repulsion force to between the magnets 56, 62 serves to urge the valve member 48 towards the valve seat 50. The strength of the repulsion force is chosen such that when the pump 2 is switched off, the valve 36 is in the fully closed position shown in Figure 2.

Is The inlet valve 36 operates between two stable positions, namely the fully closed position and a fully open position. With the valve 36 in the fully closed position, fluid is prevented from flowing through the passageway between the inlet 42 and the outlet 44 of the housing 41 since the flange 54 sealingly engages the valve seat 50. The valve 36 will remain closed until the pressure differential between the inlet 42 and the outlet 44 increases to such an extent that a downward (as shown) force is produced on the flange 54 which, together with the weight of the valve member 48, overcomes the repulsive force between the magnets 56, 62. In this event, the flange 54 separates from the valve seat 50 to allow the passage of gas through the inlet 42, into the passageway 46, and towards the outlet 44 through channels 74 located in the housing 41 between the outlet 44 and the passageway 46. These channels 74 are shown more clearly in Figure 3. In this embodiment the housing 41 comprises six channels 74 equally spaced about the axis 76 of the valve 36, although any suitable number of such channels 74 may be provided.

The magnetic force between the magnets 56, 62 can be adjusted to ensure that the repulsion force is greater than the weight of valve member 48 and such that a - 6 relatively low pressure differential between the inlet 42 and the outlet 44, for example, between 0.01 mbar and 0.1 mbar, most preferably around 0.02mbar, is sufficient to causes the flange 54 to separate rapidly from the valve seat 50 when the pump 2 is switched on. This prevents the inlet valve 36 from adversely affecting the pump ultimate; in the preferred embodiment the reduction in pump ultimate is less than 0.5mbar is comparison to a pump absent the inlet valve 36.

The magnetic force may be most readily adjusted by adjusting the spacing between the magnets 52, 56, for example, by adjusting the location of the second permanent magnet 62 within the recess 64. In the preferred embodiment, the lo spacing between the magnets when the valve is fully closed is between 10 and 30mm, preferably between 15 and 25mm, with the exact value of the spacing depending, inter alla, on the strength of the magnets, the weight of the valve member, and the required sensitivity of the valve.

Is The gas flow between the flange 54 and the housing 41 produces a downward force which propels the valve member 48 towards the fully open position, at which the lower (as shown) side of the flange 54 abuts the upper (as shown) end of the bore. As the amount of gas available for pumping reduces, due to the evacuation of the pumped environment connected to the pump inlet 34, the force produced by ho the gas flow through the valve 36 also slowly reduces. Eventually, the magnetic force between the magnets 56, 62 is able to push the valve member 48 gradually upwards, until the flange 54 engages the valve seat 50 to close the valve. At this point, the pump 2 may be switched off. As the inlet valve 36 was closed before the pump was switched off, the inlet valve 36 can reliably prevent any suckback gas from escaping back to the pumped environment connected to the pump inlet 34, and thus prevent any dust generated by erosion of tip seals provided at the free edges of the wraps 18, 20 from passing back to this environment.

Whilst the inlet valve 36 has been described for use on a pump incorporating a so scroll mechanism, the valve 36 can be used on any other type of pump. The valve 36 may be attached to the pump inlet, as shown in Figure 1, or may be integral with the pump.

Claims

1. A valve comprising a housing having an inlet, an outlet spaced from s the inlet, and a passageway extending between the inlet and the outlet; a valve member located in the passageway for controlling a flow of fluid between the inlet and the outlet, the valve member comprising a first permanent magnet and being moveable between a closed position at which the valve member sealingly engages a lo cooperating valve seat and an open position spaced from the valve seat; and a second permanent magnet which repels the valve member towards the valve seat to retain the valve member in the closed position until a pressure difference between the inlet and the outlet is of sufficient magnitude to cause the valve member to move towards the open position against the repulsion force between the magnets.

2. A valve according to Claim 1, wherein the housing comprises a guide for guiding the movement of the valve member between the closed and open positions.

3. A valve according to Claim 2, wherein the guide comprises a bore formed in the housing, the valve member comprising a stem located within the bore and moveable therein.

4. A valve according to Claim 3, wherein the first permanent magnet is located on the stem, and the second permanent magnet is located at or towards one end of the bore.

so

5. A valve according to Claim 3 or Claim 4, wherein the valve member comprises a flange extending radially from the stem for engaging the valve seat. - 8

6. A valve according to any of Claims 2 to 5, wherein the guide is co axial with the housing.

7. A valve according to any of Claims 2 to 6, wherein the guide is integral with the housing.

8. A valve according to any preceding claim, wherein the valve is an inlet valve for a vacuum pump.

9. A vacuum pump comprising an inlet valve according to any preceding claim.