GB2558626A

GB2558626A - A multiple stage vacuum pump and pump configuring method

Info

Publication number: GB2558626A
Application number: GB1700462.3A
Authority: GB
Inventors: North Phillip
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2017-01-11
Filing date: 2017-01-11
Publication date: 2018-07-18
Also published as: WO2018130819A1; GB201700462D0

Abstract

Multiple stage vacuum pump comprising multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust and comprising gas flow paths between the stages; a bypass gas flow path for conducting a flow of gas diverted from an output of an intermediate one of the stages to the exhaust; wherein at least one of the multiple vacuum stages has a capacity that is smaller than an adjacent upstream stage; and the pump further comprises means for inserting a gas flow diverter into the gas flow path between the adjacent stages, the gas flow diverter being configured to divert the gas flow output from the upstream stage of the adjacent stages to the bypass gas flow path, such that flow is impeded from entry to any subsequent stage or stages. A method of configuring a multiple stage low power vacuum pump to operate at a higher temperature is also claimed.

Description

(71) Applicant(s):

Edwards Limited

Innovation Drive, BURGESS HILL, Sussex, RH15 9TW, United Kingdom (72) Inventor(s):

Phillip North (56) Documents Cited:

JP 2011202604 A US 20040247465 A

US 20060222506 A US 20030223897 A (58) Field of Search:

INT CL F04B, F04C, F04D Other: Online: WPI & EPODOC (74) Agent and/or Address for Service:

Edwards Limited

Innovation Drive, BURGESS HILL, Sussex, RH15 9TW, United Kingdom (54) Title of the Invention: A multiple stage vacuum pump and pump configuring method

Abstract Title: A configurable multiple stage vacuum pump and pump configuring method (57) Multiple stage vacuum pump comprising multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust and comprising gas flow paths between the stages; a bypass gas flow path for conducting a flow of gas diverted from an output of an intermediate one of the stages to the exhaust; wherein at least one of the multiple vacuum stages has a capacity that is smaller than an adjacent upstream stage; and the pump further comprises means for inserting a gas flow diverter into the gas flow path between the adjacent stages, the gas flow diverter being configured to divert the gas flow output from the upstream stage of the adjacent stages to the bypass gas flow path, such that flow is impeded from entry to any subsequent stage or stages. A method of configuring a multiple stage low power vacuum pump to operate at a higher temperature is also claimed.

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- 1 A MULTIPLE STAGE VACUUM PUMP AND PUMP CONFIGURING METHOD

FIELD OF THE INVENTION

The field of the invention relates to multiple stage vacuum pumps.

BACKGROUND

There is an increasing requirement for vacuum pumps to be operated at lower powers in order to achieve a low cost of ownership for the user. The power of operation is generally measured at ultimate or base pressure. Generally the smaller the capacity of the final or exhaust stage of the pump the lower the power requirements. Thus, multiple stage pumps with decreasing capacity stages and with, in particular, a low capacity exhaust stage have been developed to meet these low power requirements.

In some applications, e.g. in semiconductor manufacturing processes, there is a need for the pumps to operate at a relatively high temperature to avoid or at least reduce condensation of process by-products. Low power pumps do not generate enough heat in themselves to achieve the required pump temperatures and so where such low power pumps are being used, additional heat sources have been added, usually in the form of electric heater elements to the pumps.

It would be desirable to provide a pump that was suitable for both lower power and higher temperature applications as required.

SUMMARY

A first aspect of the present invention provides a multiple stage vacuum pump comprising: multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust with gas flow paths between said stages; a bypass gas flow path for conducting a flow of gas diverted from an output of an intermediate one of said stages to said exhaust; wherein at least one of said multiple vacuum stages has a capacity that is smaller than an adjacent upstream stage; and said

-2pump further comprises means for inserting a gas flow diverter into at least one of said gas flow paths between adjacent stages, said gas flow diverter being configured to divert said gas flow output from an upstream stage of said adjacent stages to said bypass gas flow path, such that flow is impeded from entry to any subsequent stage or stages.

A pump is provided that is designed with multiple stages at least some decreasing in capacity in the direction of gas flow. The lower capacity of the stage towards the exhaust provides for a lower power operation. The pump is however, configured such that at least one of the later stages can be bypassed and an earlier stage then acts as the exhaust stage. Bypassing one or more of the later stages results in a pump that operates at a higher power in this configuration. Operating at a higher power provides an increased temperature of operation. In effect the pump is adapted from a low power operation mode to a higher power, higher temperature mode simply by the insertion of a gas diverting means.

In some embodiments, the adjacent stages have different capacities and the diversion of the gas flow provides a pump with a higher capacity final stage which operates with higher power requirements. In other embodiments the two adjacent stages, which may be the two final stages, may have equal capacity but removing one of them will also increase the power of operation. In this regard, generally multistage vacuum pumps have smaller size stages from inlet to outlet and the size of the final stage determines the power at ultimate. The power is proportional to the amount of gas that can leak back through the stage, usually across the rotor tip in a roots mechanism. For a 2 or 3 lobed roots then at least one of the tips is always sealing and with a 5 lobed roots at least 2 of the tips are always sealing. The 5 lobed design has a lower power at ultimate than the 3 lobed design of the same capacity. So where there are 2 adjacent stages of the 3 lobed design, both of the same capacity, then this is in effect the same as the 5 lobed design but split into 2 parts each part providing one of the 2 seals. Bypassing the final of these two stages will result in an increase in power at

-3ultimate. In either case, the increased power of the pump will provide additional heating to the gases being pumped. This is a more efficient way of supplying heat to the pumped fluids than the conventional use of external heating elements.

In this way a single pump can be adapted for use in different situations depending on requirements. Not only does this mean that a pump’s operation can effectively be tuned according to requirements, but also that a single type of pump can be manufactured and stocked for different functions, the pump being configured at installation.

In this regard pumps with a high temperature capability are required for a small but significant number of applications. Providing such pumps by reconfiguring pumps manufactured primarily for low power operation reduces the number of variants manufactured helping to reduce the cost of supply and spares held by users. Furthermore, when configured for higher temperature operation the presence of the unused final pump stage(s) and the bypass gas path around the exhaust stage(s) provides a longer thermal path between the new exhaust stage and the components of the pump such as the bearings and the seals on the shaft than would be the case if the exhaust was directly connected to this higher temperature exhaust stage. Thus, although the temperature of operation of the pump may be increased many of the components of the pump are protected from this increased temperature.

Although the means for inserting a gas flow diverter into the gas flow path can be configured in different ways, in some embodiments it comprises a valve. A valve such as a manual changeover valve is an easy to operate and effective way of providing the gas diversion. Furthermore, as the valve is fitted to the pump at manufacture, it will have been leak tested at this point obviating the need for further leak tests when the pump is reconfigured.

-4ln other embodiments, said means comprises a removable component between said adjacent stages, said component providing one of: at least a portion of said gas flow path between said stages and said gas flow diverter.

An alternative to a valve might be a removable and/or replaceable component which can be inserted or removed between the stages. The component may in some embodiments be a plug while in others it may be a plate. A removable component is not as simple to operate as a valve but has the advantage of having no moving parts and thus, particularly where the process gases provide harsh conditions, it may be more advantageous to have such a gas diverting mechanism. It should be noted that the component may be removed and replaced by a different component albeit one with the same interfacing dimensions, or it may be removed and replaced by itself mounted a different way round, so that the blocking and gas flow paths provided are in different directions and form different links.

The removable component may be attached to the pump in a number of ways provided that it is removable at installation. It may for example be attached to the pump by removable screws. This is a simple way to fix the component firmly in position and yet allow its removal.

In some embodiments, said means comprises a plug comprising a flow path, said plug being housed within a recess in said pump such that one end of said flow path connects to a portion of said gas flow path extending from said upstream stage.

In one embodiment said plug is configured to be removable and when inserted comprises one of:

a flow path where both ends of said flow path connect to different portions of said gas flow path between said stages, such that said flow path of said removable plug provides at least a portion of said gas flow path between said stages; and a flow path form said upstream stage to said exhaust.

-5The plug may be removably housed in a recess such that the plug can be removed and changed such that it either provides a flow path that completes the flow path between stages or it provides a path from the upstream stage to the exhaust. Thus, replacing the plug changes the gas flow, from flowing between stages to flowing from the upstream stage via a bypass gas flow path to the exhaust.

In some embodiments, said plug is rotatably mounted in said recess to rotate between two positions, said plug being configured such that in one of said two positions said flow path connects to different portions of said gas flow path between said stages, such that said flow path of said plug provides at least a portion of said gas flow path between said stages and in the other of said two positions said flow path connects said upstream stage to said bypass gas flow path.

Rather than being removable the plug may be rotatable between two positions, in one of which it acts as part of the gas flow path between stages and in the other of which it acts as the gas flow diverter, diverting the flow of gas output from the upstream stage to the exhaust. Such a rotatable plug forms a plug valve, where the plug is cylindrical or conical.

In some embodiments, each subsequent stage of said multiple stage vacuum pump has a smaller capacity than a preceding upstream stage.

Although in some embodiments some of the stages of the vacuum pump may have a same capacity, in some embodiments each vacuum pump has a successively smaller capacity towards the exhaust.

As noted previously, a low capacity pumping chamber at the exhaust stage provides for a lower power of operation. In order to arrive at this small capacity pumping stage and maintain a reasonable pumping volume, it may be

-6advantageous to step down through decreasing capacity stages such that each stage decreases in capacity by an incremental amount towards the low capacity exhaust stage.

In some embodiments, said pump comprises a plurality of said means for inserting a gas flow diverter between a plurality of adjacent stages.

Although in some cases there may be a single means for inserting the gas flow diverter between two adjacent stages, such that the pump can be configured either for the gas to flow through all the stages of the pump to the exhaust or can be diverted at a particular point to the bypass flow path. In other embodiments, there may be a plurality of means between a plurality of adjacent stages, such that the point of diversion can be selected depending on conditions. This provides more choice in the configuration as the gas may be diverted before reaching the final stage or before reaching the next to last stage or before reaching some other stage further upstream. The earlier the diversion occurs the higher the power requirements of the pump and thus, the higher the temperature of operation. Thus providing a plurality of these means provides for a greater choice in the configuration of the pump. The vacuum that can be generated by a pump with fewer stages is not as high as one with more stages although the volumetric capacity will stay approximately constant.

In some embodiments, said means for inserting a gas flow diverter is between a final exhaust stage of said pump and an adjacent upstream stage.

In some cases, the gas diversion may be between the penultimate and the final exhaust stage of the pump. This provides an increased power without unduly affecting the vacuum created by the pump.

In some embodiments, the vacuum pump comprises five or more stages.

-7Although embodiments of this invention may be applicable to vacuum pumps with fewer stages, vacuum pumps with five or more stages provide reduced power operation while providing a reasonable volumetric flow. The multiple stages mean that the changes in capacity between stages is not too high and they can be converted for higher temperature operations as required without the vacuum produced being too greatly reduced.

In some embodiments, said vacuum pump comprises configuration means for configuring said pump to be operable either as a lower power pump or as a higher temperature pump, said configuration means comprising said means for inserting said gas flow diverter, said pump being operable as said higher temperature pump when said gas flow is diverted to said exhaust and away from at least a final stage of said pump and as said lower power pump when said gas flow is not diverted.

As noted earlier, the gas flow diverter means can be used to configure the pumps for different types of operations such that where the gas flow diverter is diverting the flow away from the exhaust stage, the pump is configured to operate at a higher temperature while where this gas flow diverter is not in place for diverting the gas flow, the pump will operate at a lower power.

Although embodiments are applicable to different type of pumps, they are particularly applicable to dry pumps. These pumps often operate in multiple stages and their temperature is difficult to control.

In some embodiments, at least one of said intermediate stages of said pump comprising a blow off valve, said blow off valve being configured to be enabled or disabled.

Some pumps may comprise a blow off valve, which may take the form of a valve in one of the mid-stages. These are used for clean load lock applications where rapid pump down is required. Providing a blow off valve on a process application

-8vacuum pump with its own gas flow diverter allows the pump to be further configured either for load lock or process applications. In this way one pump can be configured for low power, higher temperature and load lock applications, allowing one spare pump in the service pool. This feature can be used in conjunction with or independently of the low or high temperature configuration.

In some embodiments, said pump further comprises a means for inserting a seal means between a final stage of said pump and said exhaust.

In some embodiments it may be advantageous to seal off the exhaust of the low power stage for high temperature operation to prevent back migration of process gases. In order to do this, the pump may be provided with a means for inserting a seal for sealing between the final stage of the pump and the exhaust. Such a means may comprise a replaceable member such as a plate or plug that either provides a flow path between the final stage and the exhaust or no such flow path, or it may comprise a rotatable valve operable to rotate between a position in which it provides the flow path between the final stage and the exhaust and a position in which it provides a seal between the two.

A second aspect of the present invention provides a method of configuring a multiple stage low power vacuum pump to operate at a higher temperature, said multiple stage vacuum pump comprising multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust and comprising gas flow paths between said stages, at least one of said multiple vacuum stages having a capacity that is smaller than an adjacent upstream stage said method comprising: inserting a gas flow diverter into a gas flow path between adjacent stages of said pump, said adjacent stages comprising an upstream stage and a downstream stage, said gas flow diverter being operable to impede gas from said upstream stage entering said adjacent downstream stage and to divert said gas flow output from said upstream stage to a bypass gas flow path operable to conduct gas to an exhaust.

-9ln some embodiments, the method further comprises supplying power to an external heating means connected to said pump.

As noted previously, the pump can be configured to operate at a higher temperature by providing a diversion of the gas flow. In some cases where the temperature of operation required is very high, then in addition to this, further methods of heating the pump may be used such as providing the pump with an insulation jacket and/or providing it with an external heating means.

In some embodiments, the pump may have a blow off valve which can be enabled or disabled.

In some embodiments it may be advantageous to seal between the exhaust and exhaust stage of the pump where this stage is not operational as this will impede any back migration of the process gases.

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 illustrates a conventional multiple stage vacuum pump;

Figure 2 illustrates a multiple stage vacuum pump according to an embodiment; Figure 3 illustrates the final stages of a roots pump according to an embodiment configured for low power operation;

- 10Figure 4 illustrates the final stages of the roots pump of Figure 3 configured for higher temperature operation;

Figure 5 illustrates a gas diverting means in the form of a replaceable plug; and Figure 6 illustrates a valve arrangement to seal the final stage exhaust.

DESCRIPTION OF THE EMBODIMENTS

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

A pump that can be mechanically reconfigured at installation for example, to bypass the final stage(s) of the vacuum pumping mechanism is provided. This allows the power and thus, the temperature of the pump to be selected depending on requirements. Ideally this by-pass will prevent any of the gases passing into the exhaust stage of the pump where they have the potential to induce failure modes (by condensation for example) and reduce the reliability of the pump. For example, if the pump has 6 stages in low power configuration and the 6^th stage is the exhaust stage then in the high power configuration the 5^thstage becomes the exhaust stage. Where this 5th stage is of a bigger capacity than the 6^th then it will do more work and so will be a source of heat for the vacuum pump.

One significant cause of poor reliability of such devices is contamination or degradation by the process chemistry. At installation it should be known that the pump is required to be hot and the mechanical reconfiguration of the gas flows can be achieved by a manual changeover valve or fixed flow diverter.

It is true that the reconfigured pump will no longer be as low power as the preconfigured pump however the cost of ownership for the reconfigured pump would need to be compared to that of the pre-configured pump plus the heaters and control system required to achieve the same pump temperatures.

-11 For very high temperature pumps then a combined approach can be taken where some of the heat is generated by the reconfigured pump and the remainder is provided by additional heat sources and insulation where appropriate.

Figure 1 schematically shows a multiple stage conventional dry pump 1 where gas flows from an input through decreasing capacity stages to an exhaust outlet. The low capacity of the final stage 2 provides a low power pump.

The gas flow is indicated by the arrows. The final exhaust stage 2 is usually designed to be as small a capacity as is reasonable for the required gas flow in order to achieve the lowest power at ultimate (base) pressure.

Figure 2 schematically shows a multiple stage dry pump according to an embodiment. This diagram shows a bypass conduit 4 for conducting the gas around the exhaust stage 5. This bypass flow path 4 can be opened or closed with a simple mechanical replaceable component or a manual valve. The valve only needs to be operated once at installation of the pump by the user. The valve gives the benefit of not requiring a leak test after reconfiguring the pump, whereas the use of a replaceable component such as a plug or plate in place of a valve would require a leak test.

The gas flow path from the penultimate stage 6 to the final exhaust stage 5 is blocked when the gas diversion is in place (shown as a cross on the figure) and gas is instead sent to the bypass conduit 4. In this way the exahust stage is now the former penultimate stage 6 and this has a larger capacity and thus, the pump operates with increased power requirements and reaches a higher operating temperature.

Figure 3 shows the final and penultimate stages of a roots type pump with a gas diverting means 30 in the form of a rotatable plug. The rotatable plug forms a rotary valve, which along with path A provides a gas flow path between the penultimate stage 6 and the final exhuast stage 5 when in the position shown in

- 12Figure 3. This arrangement means that gas flows through all the stages of the pump and it operates in its low power mode.

Figure 4 shows the higher power, higher temperature mode of operation where the value is rotated to provide a gas flow path between the penultimate stage 6 and the exhaust via bypass path B. Thus, penultimate stage 6 operates as the exhaust stage in this arrangement and the pump operates at a higher temperature than it does in the arrangement of Figure 3. It should be noted that although a rotary type valve is shown as the gas diverting means in this embodiment, other types of valve such as a poppet valve could be used. In this regard any valve or indeed replaceable part which can provide the required change in flow path would be suitable as the gas diverting means.

The plug of the rotatable valve can be sealed to the pump body with a static seal, e.g. an O-ring. In some embodiments an additional seal on the plug may be provided to reduce gas leaking around the outside of the plug.

In the embodiment of Figures 3 and 4, the exhaust stage is changed by simple rotation of the plug valve 30. In other embodiments the plug may not be rotatable but may be a replaceable plug that can be removably inserted and fixed to the pump. Figure 5 shows one such plug 35. In this embodiment, when inserted one way up, with the outlet of the plug facing upwards then the pump is configured for low power operation with exhaust stage 5 being part of the pump stages. When it is inserted the other way up with the gas outlet facing down towards the bypass path 4, then the pump is configured for higher temperature operation with the gas bypassing the previous exhaust stage 5 and the penultimate stage 6 forming the new exhaust stage. In some embodiments, a seal may be made between the outlet of stage 5 and the exhaust when the pump is configured for high temperature operation to impede any back flow of gases into this stage, such a sealing means is shown in Figure 6.

- 13As can be seen the plug has fixing holes 37 through which screws can be inserted to secure the plug to the pump body.

Figure 6 shows a simple plug valve that may be used for sealing between the 5 outlet of the final stage 5 and the exhaust. In this example the valve has a straight through flow path and rotating this plug through 90 degrees blocks the flow path. Thus, the plug is in the path between the exhaust stage 5 and the exhaust of the pump. In low power operation the plug provides a path for gas flow, while in higher temperature operation, the plug is rotated through 90 io degrees to provide a seal between the pump and the exhaust.

Although not shown in the figures an intermediate stage may comprise a blow off valve for use in clean load lock applications where rapid pump down is required. A means to disable the valve is associated with the valve, such that where the pump is configured for process operations then the blow off valve may be disabled such that it is not operational in this configuration. However, where a pump for load lock applications is required then the blow off valve may be enabled and the pump is then suitable for such applications.

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.

- 14REFERENCE SIGNS

Conventional multiple stage vacuum pump 2, 5 exhaust pump stage

3 multiple stage vacuum pump

4, B bypass gas path 6 penultimate stage 30 rotatable plug 35 replaceable plug io 37 fixing holes for replacable plug A inter-stage gas flow path

Claims

1. A multiple stage vacuum pump comprising:

multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust and comprising gas flow paths between said stages;

a bypass gas flow path for conducting a flow of gas diverted from an output of an intermediate one of said stages to said exhaust; wherein at least one of said multiple vacuum stages has a capacity that is smaller than an adjacent upstream stage; and said pump further comprises means for inserting a gas flow diverter into at least one of said gas flow paths between adjacent stages, said gas flow diverter being configured to divert said gas flow output from an upstream stage of said adjacent stages to said bypass gas flow path, such that flow is impeded from entry to any subsequent stage or stages.

2. A multiple stage vacuum pump according to claim 1, wherein said means for inserting a gas flow diverter is located such that said gas flow diverter is inserted between adjacent stages having different capacities, said upstream stage having a larger capacity than said downstream stage.

3. A multiple stage vacuum pump according to claim 1 or 2, wherein said means comprises a valve.

4. A multiple stage vacuum pump according to claim 1 or 2, wherein said means comprises a removable component between said adjacent stages, said component providing one of: at least a portion of said gas flow path between said stages and said gas flow diverter.

5. A multiple stage vacuum pump according to claim 4, wherein said removable component is attached to said pump by removable screws.

- 166. A multiple stage vacuum pump according to any preceding claim, wherein said means comprises a plug comprising a flow path, said plug being housed within a recess in said pump such that one end of said flow path

5 connects to a portion of said gas flow path extending from said upstream stage.

7. A multiple stage vacuum pump according to claim 6 when dependent on claim 4 or 5, wherein said plug is configured to be removable and when io inserted comprises one of:

a flow path where both ends of said flow path connect to different portions of said gas flow path between said stages, such that said flow path of said removable plug provides at least a portion of said gas flow path between said stages; and

15 a flow path form said upstream stage to said exhaust.

8. A multiple stage vacuum pump according to claim 6 when dependent on claim 3, wherein said valve comprises a plug valve, said plug being rotatably mounted in said recess to rotate between two positions, said plug

20 being configured such that in one of said two positions said flow path connects to different portions of said gas flow path between said stages, such that said flow path of said plug provides at least a portion of said gas flow path between said stages and in the other of said two positions said flow path connects said upstream stage to said bypass gas flow path.

9. A multiple stage vacuum pump according to any preceding claim, wherein each subsequent stage of said multiple stage vacuum pump has a smaller capacity than a preceding upstream stage.

- 1710. A multiple stage vacuum pump according to any preceding claim, wherein said pump comprises a plurality of said means for inserting a gas flow diverter between a plurality of adjacent stages.

5 11. A multiple stage vacuum pump according to any preceding claim, wherein said means for inserting a gas flow diverter is between a final exhaust stage of said pump and an adjacent upstream stage.

12. A multiple stage vacuum pump according to any preceding claim, wherein io said vacuum pump comprises five or more stages.

13. A multiple stage vacuum pump according to any preceding claim, said vacuum pump comprising configuration means for configuring said pump to be operable either as a lower power pump or as a higher temperature

15 pump, said configuration means comprising said means for inserting said gas flow diverter, said pump being operable as said higher temperature pump when said gas flow is diverted to said exhaust and away from at least a final stage of said pump and as said lower power pump when said gas flow is not diverted.

14. A multiple stage vacuum pump according to any preceding claim, wherein said pump is a dry pump.

15. A multiple stage vacuum pump according to any preceding claim, at least

25 one of said intermediate stages of said pump comprising a blow off valve, said blow off valve being configured to be enabled or disable.

- 1816. A multiple stage vacuum pump according to any preceding claim, further comprising a means for inserting a seal between a final stage of said pump and said exhaust.

5 17. A method of configuring a multiple stage low power vacuum pump to operate at a higher temperature, said multiple stage vacuum pump comprising multiple vacuum pumping stages arranged in series between a gas inlet and an exhaust and comprising gas flow paths between said stages, at least one of said io multiple vacuum stages having a capacity that is smaller than an adjacent upstream stage said method comprising:

inserting a gas flow diverter into a gas flow path between adjacent stages of said pump, said adjacent stages comprising an upstream stage and a downstream stage, said gas flow diverter being operable to impede gas from said

15 upstream stage entering said adjacent downstream stage and to divert said gas flow output from said upstream stage to a bypass gas flow path operable to conduct gas to an exhaust.

18. A method according to claim 17, further comprising providing said pump

20 with an insulation jacket.

19. A method according to claim 17 or 18, comprising providing said pump with an external heating means.

25 20. A method according to any one of claims 17 to 19, comprising a further step of disabling a blow off valve located on one of said intermediate stages of said pump.

- 1921.

A method according to any one of claims 17 to 19, comprising a further step of sealing between said exhaust and a final stage of said pump.

Intellectual

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Office

Application No: GB 1700462.3 Examiner: Mr Robert Arnold