GB2500603A

GB2500603A - Vacuum pump stators and vacuum pumps

Info

Publication number: GB2500603A
Application number: GB1205230.4A
Authority: GB
Inventors: Ian David Stones
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2012-03-26
Filing date: 2012-03-26
Publication date: 2013-10-02
Also published as: US20140093412A1; GB201205230D0

Abstract

Combined booster and primary pump arrangements can be bulky and require separate supplies of purge gas and cooling water. In order to overcome this problem invention provides a vacuum pump in which two or more pumping mechanisms, i.e. the booster pump 52 and main pump 54, are housed in the same stator 56 forming a multi stage vacuum pump 50 with a clam-shell casing. The invention further provides a vacuum pump stator comprising at least two cavities 64 operatively interconnected by a through hole 70 such that the outlet of booster pump 52 is connected to the inlet of the main pump 54 , wherein at least two of the cavities each comprise at least one rotor-receiving portion shaped to receive two or more at least partially intermeshing rotors 66, 68, and wherein an axis of a rotor-receiving portion of a first one of the cavities is offset with respect to an axis of a rotor-receiving portion of a second one of the cavities.

Description

VACUUM PUMP STATORS AND VACUUM PUMPS

Description:

The present invention relates to a stator for a vacuum pump and to a vacuum pump comprising the stator. In particular, but not exclusively, the present invention relates to a stator that is used in conjunction with intermeshing vacuum pump rotor mechanisms, such as Roots or screw configured pump mechanisms, or a combination thereof, and a stator having a clam-shell configuration.

Vacuum pumps are used in many industrial applications, such as steel production, power generation and the semiconductor and electronic device production industry (including solar panel, flat panel display, Li-ion battery and silicon wafer device production). The vacuum pumps are configured according to the application in which they are utilised, the level of vacuum (or pressure) or pumping capacity required by the application and the nature of the gases that a pump system encounters during operation.

Many industrial processes either require or are moving towards using so-called dry pumps in which a pump comprises a stator and rotor operating within a pumping volume of the stator. The rotor operates within tight clearance tolerances of the stator and seal is maintained between the rotor and stator by virtue of extremely small running clearances. Liquid, oil or other hydrocarbon compounds are not used to maintain a seal -hence the term "dry pump" -because the oil can become contaminated by pumped gases or react with the gas with undesirable effects on the pump's running performance and characteristics. Furthermore, hydrocarbons from pump sealant can cause contamination of a process chamber, which is especially undesirable in ultra-clean process environments such as those required by the semiconductor and electronic device manufacturing industries.

In certain applications, a series of pumps can be required to evacuate a chamber and the pumps can be configured in series, parallel or a combination of both. For example, a booster pump might be disposed between a main pump and an evacuated chamber whereby the booster pump operates to improve the operational characteristics of the evacuation system comprising the pumps. The booster pump is used to improve the throughput of gases and also the ultimate pressure to which the pumping system will evacuate.

Such an arrangement is shown in Figure 1 and is also known from W02011/O1 8370, for example. Typically, a main booster pump 12 is disposed above a dry pump 14 and a conduit 16, 20 connects the outlet of booster with the inlet of the dry pump. For example, each of the booster and dry pumps can be configured as a clam-shell type pump, which is known in the vacuum pump technical field and described in published patent documents EP2071 191, US6572351 or EP1 398507. In such an arrangement, the stator comprises two- halves of a clam-shell, a top half 28, and a bottom half 32 respectively. Head-plates 44 are disposed at each end of the clam shells and serve to maintain clam-shell's position, accommodate bearings for rotor shafts or provide additional sealing of internal pumping chambers from external atmospheric pressure. Motor drives 42 are disposed on each pump and an end cover can accommodate other mechanisms, such as a timing gear arranged to ensure synchronous rotation of intermeshing rotor parts. In addition, each of the pumping units might comprise cooling and sealing plates 26 disposed on top of, and underneath, the stator clam shell halves (and in other areas as required to maintain the required thermal profile of the pump). An example of such cooling and sealing plates is described in more detail in EP2071 191.

Such a known pump arrangement is common and can comprise different configurations of pumps according to the application of the pump. For instance, the booster pump 12 might be a single stage booster or a multiple stage booster, both of which are well understood and need no further explanation here. Also, the main dry pump 14 can comprise a Roots mechanism, configured in either a single or multiple stages, or a screw pump. Northey ("hook and claw") mechanisms might also be used. In all cases, the main dry pump and the booster pumps are separate entities located in close proximity to one another.

Market forces are driving vacuum pump manufacturers to provide pump systems that are more efficient to manufacture and operate without reducing pump performance. Therefore, the present invention aims to provide a vacuum pump arrangement that is more cost efficient to manufacture and transport to the end-user, and a pump that has improved running efficiencies. Furthermore, the present invention aims to simplify pump installation in an industrial facility and reduce complexities associated with customer-specific systemisation and maintenance operations. Further still, the present invention aims to reduce the footprint or space needed to accommodate the pumps when a pumping system is installed.

As a result, a first aspect of the invention provides a vacuum pump in which two or more pumping mechanisms, i.e. the booster pump and main pump, are housed in the same stator.

A second aspect of the invention provides a vacuum pump stator comprising a plurality of stator cavities.

A third aspect of the invention provides a vacuum pump stator comprising at least two operatively interconnected cavities, wherein at least two of the cavities each comprise at least one rotor-receiving portion shaped to receive two or more at least partially intermeshing rotors, and wherein an axis of a rotor-receiving portion of a first one of the cavities is offset with respect to an axis of a rotor-receiving portion of a second one of the cavities.

The stator cavities are preferably operatively interconnected by a conduit, which conduit may advantageously extend through the body of the stator to interconnect two of the stator cavities.

Advantageously, the invention enables a multi-stage vacuum pump, that is to say, a vacuum pumping system comprising a number of pumps connected in series (for example a main vacuum pump and a booster pump), to be rationalised. This is achieved by forming a number of pumping stages in a single unit by two or more stages of the vacuum pumping system sharing a common stator.

The stator cavities may therefore form part of a number of different types of vacuum pump, for example one cavity may form part of a booster pump, whereas a second one of the cavities may be adapted for receiving the plurality of interconnected pumping stages of, say, a multi-sage Roots pump.

In such a situation, one of the cavities may comprise a plurality of axially aligned, and interconnected, rotor-receiving portions that may be adapted, in use, to receive a pair of intermeshing rotors. A first rotor of each pair may be mounted on a first shaft and second rotor of each pair may be mounted on a second shaft.

Moreover, by interconnecting the rotor receiving portions of the cavity in series, for example, using interconnecting conduits, each pair of intermeshing rotors and its corresponding rotor-receiving portion of the cavity can form a separate pumping stage of a multi-stage vacuum pump.

Additionally or alternatively, another one of the cavities may simply comprise a single rotor-receiving portion adapted, in use, to receive a single pair of intermeshing rotors. Again, the rotors can be mounted separate shafts such that the intermeshing rotors and rotor-receiving portion of the cavity together form a single pumping stage of a vacuum pump, such as a booster pump.

In order to obtain the maximum benefit from the invention, it is preferable that the stator cavities of the stator be operatively interconnected, for example, by interconnecting the outlet of a first stator cavity to the inlet of a second stator cavity, and so forth. This may be achieved by providing a conduit that extends through the body of the body of the stator (to interconnect two stator cavities directly) or via a conduit, channel or pipe that interconnects two stator cavities via a passage extending outside the body of the stator.

The stator cavities are preferably adapted to receive at least two intermeshing rotors, such as a pair of intermeshing Roots or Northey rotors. The vacuum pump is preferably a "dry" pump, that is to say, that the clearance between the stator and rotors, in use, is sufficiently small to form an effective seal therebetween. In other words, a very small running clearance between an exterior surface of a rotor and the interior surface of the stator cavity is so small as to impede or minimise the backflow of pumped gasses, or the circumvention of a particular pumping stage.

The stator cavities of the stator may be adapted to receive the same type of rotors, or different types. For example, one cavity may be adapted to receive the rotors of a roots-type pump, whereas another cavity may be adapted to receive a set of intermeshing Archimedean screw-type rotors. The various possible combinations of rotors will be apparent to those familiar with vacuum pumping technology.

To facilitate manufacture, assembly of a pump and subsequent servicing, the stator is preferably of a clamshell type (that is to say the stator comprises a plurality of separable stator portions that are adapted to sealingly mate with one another). Where a clamshell-type construction is employed, at least one of the separable stator portions may comprise a recess forming, in use, at least part of one of the stator cavities, or a number of recesses forming, in use, at least part of a number of respective stator cavities To aid with thermal management of the pump, the stator preferably comprises a cooling circuit, which may take the form of a channel in the stator for conveying, in use, a flow of coolant fluid to it. Where a clamshell-type stator is used, the cooling circuit channel may comprise a number of discrete cooling circuit channel portions that arranged, in use, to align when the separable stator portions are assembled: the discrete cooling circuit channel portions together forming at least one continuous cooling circuit channel within in the stator.

Additionally or alternatively, the cooling circuit may comprise an actively-cooled heat sink affixed to an exterior surface of the stator, which may be cooled by a liquid cooling circuit or a forced air cooling system.

A fourth aspect of the invention provides a unitary, multi-sage vacuum pump comprising a stator as described herein.

The unitary, multi-sage vacuum pump preferably comprises a number of pumping stages, which may be formed by sets of rotors that are configured to rotate within separate cavities of the stator. As the stator preferably comprises a plurality of operatively interconnected stator cavities for receiving the rotors, it is possible to provide a more compact and rationalised multi-stage pump: the stator being shared by a number of pumping stages.

The unitary, multi-stage vacuum pump may further comprise a head plate that is seal ingly affixable to the stator, which head plate may comprise a channel or recess forming a conduit for the flow of gas between first and second stator cavities of the stator. Such an arrangement enables the axial orientation of the stator cavities to be parallel or non-parallel, and avoids having to form conduits in the stator itself for interconnecting the stator cavities. The head plate may additionally comprise apertures for receiving the rotor shafts, or bearings for the rotor shafts of the rotors.

More precisely, the invention provides a vacuum pump stator comprising, a longitudinal member and end members disposed at opposing ends of the longitudinal member, wherein at least two pumping volumes are defined by the longitudinal and end members respectively and each pumping volume being arranged to accommodate a rotatable pump element disposed on a shaft, and wherein a rotatable pump element shaft is disposable in one of the pumping volumes between end members such that a shaft in a first volume is parallel to a shaft in a second volume, wherein a portion of the longitudinal member that defines the first volume comprises a main body and a second body attachable to the main body. Therefore, the first pumping volume is defined by the main and second bodies and a second pumping volume is defined by the main body. The end members can be integrally formed together to provide a unitary end plate.

The longitudinal members can be formed to provide a stator having a clam-shell configuration.

Embodiments of the present invention are described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic orthographic drawing of a known multi-stage vacuum pump from the side and in partial cross-section on II; Figures 2 to 6 are schematic orthographic drawings of various embodiments of a multi-stage vacuum pump in accordance with the invention as viewed from the side and in partial cross-section; Figure 6 is a schematic perspective view of a further alternate embodiment of a stator in accordance with the invention; and Figure 7 is a schematic perspective view of a yet further alternate embodiment of one portion of a stator in accordance with the invention for a two-stage pump comprising a booster and a multi-stage Roots pump.

In Figure 1, a known multi-stage vacuum pump 10 comprises separate booster 12 and main pump 14. The right-hand side of Figure lisa schematic side view of the multi-stage pump 10 and the left-hand side is a schematic cross-section on I-I. The inlet 16 of the booster pump 12 is connected to a vessel to be evacuated (not shown) and its output 18 is connected via a conduit 20 to the inlet 22 of the main pump 14. The outlet 24 of the main pump 14 vents to atmosphere, or to a further backing pump (not shown).

As can be seen from the right-hand side of Figure 1, each of the pumps 12 has its own cooling circuit 26, which comprises a cooling plate, which is thermally coupled or bonded to the pump 12, 14, for instance using a thermal compound and a clamp or by other means. A detailed discussion of the construction of the cooling plates 26 is not necessary here, although it will be appreciated that the cooling plates 26 typically comprise internal channels through which a cooling fluid can flow.

Each of the pumps 12, 14 comprises a stator 28 formed in two or more parts 30, 32, that is having a "clamshell" construction, whereby the stator parts 30, 32 each comprise a recess forming part of the stator cavity 34 and a mating surface 36 that can be clamped to the mating surface 36 of the opposite part to form a seal. A sealant can be applied to the mating surfaces 36 to improve the seal, where this is necessary.. Within the stator cavities 34 of each pump 12, 14, there is provided a pair of intermeshing rotors 38 that rotate in opposite directions about a rotor shaft 40. The rotor shafts 40 are driven by motors 42 and typically, but not exclusively, via a gearbox (not shown), which comprises gears to cause rotor shafts 42 to synchronise. Where gears are not present this can be achieved by driving both shafts from individual motors and synchronising the position by other means, for example, magnetic couplings.

The ends of the rotor shafts 40 are mounted in bearings (not shown) which are typically set into the recesses of the pump head plates 44. The headplates 44 must be accurately seated on to the end faces of the stator parts 30, 32 to ensure an airtight seal and to ensure correct running clearances between the ends of the rotors 38 and the interior surfaces of the stators and head plates 44, given that the components of the pumps 12, 14 are subject to thermal and stress-induced expansion/traction during use.

In a multi stage vacuum pump of this known type 10, there are a relatively large number of components, many of which, for example the cooling circuits 26, the head plates 44, the stator component 30, 32, and so on are duplicated. In addition, given that the booster pump 12 and the main pump 14 are separated, the volume occupied by the multi-stage pump 10 is relatively large. Moreover, given that there are a great number of sealingly mating surfaces, for example the head plate to the stator parts, the mating of the stator parts themselves and the connection of the conduit 20 between the pumps 12, 14 the likelihood of a leak and a consequential reduction in the efficiency of the pump is increased.

A multi stage vacuum pump 50 in accordance with the present invention is shown schematically in Figure 2 in which it will be noted that the booster pump 52 and main pump 54 share a common stator 56 of a three-part clamshell type. The stator 56 is made up of three parts 58, 60, 62, which are a fixed to one another to form a stator 56 having two stator cavities 64 for receiving the rotors 66, 68 of the booster pump 52 and main pump 54. The stator cavities 64 are interconnected by a through hole 70 forming a conduit between stator cavities 64. As such, the outlet of the booster pump 52 is directly connected to the inlet of the main pump 54 meaning that an interconnecting conduit (20, as in Figure 1) is not required.

The location and configuration of the through hole 70, can of course, be varied, for example, it could extend through the middle portion 56 of the stator (not shown), or could, indeed, be formed as a channel in one of the head plates 72.

In Figure 2 the booster pump stage 52 is located on top of a dry pump stage 54. The main booster 52 comprises a single stage Roots mechanism housed in a single stator body 56. The dry pump section comprises a multiple stage Roots pump (as is well known in the vacuum pump technical domain). The dry pump section stator 56 is formed of a clam-shell design, wherein a first part 58 of the clam-shell is formed integrally with the Booster stator and a second part of the clamshell is an independent component that is attached to the first part 58 to form the complete stator 56.

The right-hand side of Figure 2 shows a side view of the multi-stage pump of the invention 50 in which a first motor 51 is shown for driving the Booster pump stage 52, and a second motor 53 drives the dry pump mechanism 54. A single pair of head plates 72 is used, whereby one head plate 72 is disposed at each end of the pump stator and is configured to accommodate necessary mechanisms needed for booster and main pump operation. Thus, only two head plates are required by both the booster pump and main pump stator and a single pair of head plates can replace the multiple pairs of head plates used by prior pumping systems shown in Figure 1. A single end cover 55 can be disposed over a head plate 72 and can accommodate timing gears, bearings, lubrication systems or the like.

The first and second pump volumes each accommodate rotor pump elements disposed on shafts. Each volume has a longitudinal axis that runs along the length of the volume and the longitudinal axis of the first second pumping volumes are in the same plane and parallel to one another. In the arrangement shown in Figure 2, the plane in which the respective longitudinal axes are disposed is vertical -in other words, the first pumping volume is arranged on top of the second pumping volume. Furthermore a second and third plane is defined by each axis of rotation for each rotor pair, respectively. In this embodiment, the second plane defined by the booster rotors' axis is parallel to the third plane defined by the main pump rotors' axis. In other words, the planes are spaced apart and do not cross one another. Additionally, the headplates are spaced apart by a distance substantially equal to the overall length of the rotor pumping mechanism. As such the headplates are arranged to form a portion of the stator and define a surface of a swept volume occupied by the rotor mechanism.

It will be appreciated that one of the main advantages of the invention is an overall reduction in the number of parts, crucially, reduction in the number of surfaces that must be sealed to one another. In addition, it will be noted, in particular, from the right-hand side of Figure 2, that a single cooling circuit 74 can be used to cool the common stator 56 shared by the two pump stages, as opposed to having to have a separate cooling circuit for each of the pumping stages, as shown in Figure 1. Furthermore, it will be noted that there are just two head plates 72, as opposed to the four headplates shown in Figure 1. As such, the complexity, "part count" and physical volume of the multistage vacuum pump of the invention has been greatly reduced.

In Figure 2 it will be seen that the shared stator 56 comprises three stator parts 58, 60, 62, the upper and lower parts 58, 62 having a single recess therein forming one half of each of the stator cavities 64, whereas the middle part 60 has to recesses located on opposite sides thereof forming the remaining half of each of the stator cavities 64.

However, as can be seen in Figure 3, a similar pump arrangement can be fabricated from a two-part stator 56 in which the upper portion 76 comprises an elongated through hole forming one entire stator cavity 64 and a recess forming one half of another stator cavity 64; the lower part 78 comprising a single recess forming the other half of the lower stator cavity 64.

Similarly, as can be seen from Figure 4, a one-piece stator 56 comprises two elongate through holes forming a pair of complete stator cavities 64. The arrangement is shown in Figures 3 and 4, in particular, may be used where the rotors are of a unitary type that can be inserted and removed axially, that is to say, lengthwise, into the cavity 64 as a pair. However, where the rotors 68 need to be, or are more easily installed individually, rather than as a pair, the stator 56 is preferably of the clamshell type as shown in Figure 2 or Figure 3 to enable them to be inserted and removed by vertical placement, rather than by axial insertion.

A variant of the invention shown in Figure 5 differs from the embodiment shown in Figures 2, 3 and 4 in as much as this multi-stage pump 80 has two inlets 82 and one outlet 84. Such a multistage pump may be used for evacuating different parts of a system simultaneously, or for simultaneously evacuating two different systems altogether. In any event, the rotors 66 of the two pumping -14-stages share a common stator 52, which is of a three-part clamshell design as previously described. In this instance, however, it is the outlets of the two-stage cavity 64 which are interconnected, rather than the outlet of one of the stator cavities being connected to the inlet of the other stator cavity.

A further variant of a stator for a multistage pump 90 in accordance with the invention shown in Figure 6, in which the stator cavities 64 are not parallel, but rather perpendicular to one another. The stator 56 is made up from one, two or three portions 58, 60, 62 that fit together as previously described. The upper portion 58 has a recess forming one half of the upper cavity 64, and the middle portion 60 has an upper recess forming the other half of the upper cavity 64. The lower surface of the middle portion 60 also has a recess, oriented at right angles to the recess in its upper surface forming the upper half of the lower cavity 64 and the lower portion 62 has a recess in its upper surface forming the other half of the lower cavity 64. In use, the portions 58, 60, 62 are clamped together such that their mating surfaces 36 form a gas-tight seal -a gasket or other means of sealing (not shown) is typically provided between the mating surfaces 36 to help form a seal.

The stator 56 has an inlet port 16 and an outlet port 24 communicating with the recesses of the upper 58 and lower 62 stator portions, respectively, and a through hole 17 forming a conduit between the two recesses 64 such that the outlet of the upper pump stage discharges directly into the inlet of the lower pump stage. In use, head plates (not shown) are fitted to the exterior surfaces 19 of the stator 56 to close-off the ends of the cavities to define elongate, internal stator cavity volumes for receiving the pumps' rotors (not shown).

The invention advantageously provides a single dual pump configuration, such as a booster/dry pump configuration, that can be shipped to an end-user as a single entity and which has a reduced volume when installed for use in an industrial process. A single cooling circuit 74 can be utilised for both pumping sections 52, 54 making thermal management system much simpler and less expensive. Also, the number of components needed to manufacture such a pump is reduced thereby saving manufacturing effort and costs.

Gas pathways or conduits between the main booster section and dry pump sections, and pump inlet and outlet are not shown in all of the drawings for clarity. However, it is clearly understood that such features are required for the normal operation of the pump. These features can be incorporated into the stator during the manufacture of the stator components, during stator casting and/or machining processes for instance.

Figure 7 is a perspective view of a middle stator portion 60 of a 3-piece clamshell stator 50 according to the invention, which comprises stator cavities 64 for a unitary multi-stage vacuum pump comprising a booster pump 80 and a multi-stage Roots pump 82. The clamshell portion 60 is manufactured from a solid, machined block of material and has an upper cavity (as shown) 82 shaped to receive the shafts and rotors (not shown) of a two-stage Roots pump, and a lower cavity (as shown) 80 shaped to receive a pair of elongate Roots rotors (not shown).

The upper cavity 82 is formed from a pair of parallel shaft receiving portions 84, which receive the shafts of the rotors (not shown), and wider rotor-receiving portions 86, which are shaped to receive the overlapping/intermeshing rotors themselves (not shown). The rotor-receiving portions 86 are fluidly interconnected by a conduit 88 that extends from the lower surface of one rotor- receiving portion and which feeds into the upper part of an adjacent rotor-receiving portion via a top clamshell portion (not shown). As such, pumped gas can be transferred from one rotor-receiving portion 86 to the next, in series.

The inlet 90 of the Roots pump 82 connects to the outlet 92 of the booster pump 80 via a cavity interconnecting conduit 94, which is a through-hole extending between the respective rotor-receiving portions 86 of each of the cavities 64. The booster pump cavity 80 is similar to the Roots pump cavity 82 except that there is only one rotor-receiving portion 86 and no rotor-receiving portion interconnecting conduits 88.

Of course, the stator portion of Figure 7 could be modified such that both of the cavities 64 are shaped to receive the shafts and rotors of a multi-stage pump, in which case both cavities would have a similar shape. Furthermore, although a two-stage Roots pump has been illustrated for simplicity, any number of pumping stages could be employed. The booster pump could also be replaced by an Archimedean screw-type pump: the various options being a matter of design preference and pumping requirements.

The present invention is not limited to the arrangements shown and the pumping volumes can be arranged side-by-side or such that the longitudinal axes (as shown in Figure 6, for example) are not in the same plane. Alternatively, the embodiments of the present invention shown in the figures could be adapted to have a number of inlets and outlet ports. In this way, two or more booster pumps can be arranged in a unitary pump stator, each having their own independent inlets and outlets respectively. This configuration allows for a compact pumping arrangement that facilitates efficient switching between pumping lines to account for different process gases passing through the vacuum system without mixing or reacting with deposits that might be found in the pump or ducting. Furthermore, multi-stage booster pump configurations can be utilised alongside multistage main pump.

Claims

Claims: 1. A vacuum pump stator comprising at least two cavities, wherein at least two of the cavities each comprise at least one rotor-receiving portion shaped to receive at least one pair of intermeshing rotors, and wherein an axis of a rotor-receiving portion of a first one of the cavities is offset with respect to an axis of a rotor-receiving portion of a second one of the cavities.
2. A vacuum pump stator as claimed in claim 1, wherein the stator cavities are operatively interconnected by a conduit.
3. A vacuum pump stator as claimed in claim 3, wherein the conduit extends through the body of the stator to interconnect two stator cavities.
4. A vacuum pump stator as claimed in any of claims 1, 2 or 3, wherein at least one of the cavities comprises a plurality of axially aligned, and interconnected, rotor-receiving portions.
5. A vacuum pump stator as claimed in claim 4, wherein each rotor receiving portion is adapted, in use, to receive a pair of intermeshing rotors, a first rotor of each pair being mounted on a first shaft and second rotor of each pair being mounted on a second shaft, and wherein the rotor receiving portions of the cavity are interconnected in series by interconnecting conduits such that each pair of intermeshing rotors and its corresponding rotor-receiving portion of the cavity forms a separate pumping stage of a multi-stage vacuum pump.
6. A vacuum pump stator as claimed in any of claims 1, 2 or 3, wherein at least one of the cavities comprises a single rotor-receiving portion adapted, in use, to receive a pair of intermeshing rotors, a first one of which rotors being mounted on a first shaft and second one of which rotors being mounted on a second shaft, the intermeshing rotors and rotor-receiving portion of the said cavity together forming a single pumping stage of a vacuum pump.
7. A vacuum pump stator as claimed in any preceding claim, wherein, in use, the clearance between the stator and rotors is sufficiently small to form an effective seal therebetween.
8. A vacuum pump stator as claimed in any preceding claim, wherein the rotors have a profile from any one or more of the group comprising: a Roots profile, a Northey profile, and screw profile.
9. A vacuum pump stator as claimed in any preceding claim, wherein the stator comprises a plurality of separable stator portions adapted to sealingly mate with one another.
10. A vacuum pump stator as claimed in claim 8, wherein at least one of the separable stator portions comprises a recess forming, in use, at least part of one of the stator cavities.

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11. A vacuum pump stator as claimed in claim 8 or claim 9, wherein at least one of the separable stator portions comprises first and second recesses, the first and second recesses forming, in use, at least part of a first and second stator cavity, respectively.
12. A vacuum pump stator as claimed in any preceding claim further comprising a cooling circuit.
13. A vacuum pump stator as claimed in claim 11, wherein the cooling circuit comprises a channel in the stator for conveying, in use, a flow of coolant fluid to cool the stator.
14. A vacuum pump stator as claimed in claim 12, when dependent on any of claims 8 to 10, wherein the cooling circuit channel comprises a plurality of cooling circuit channel portions arranged, in use, to align when the separable stator portions are assembled, to together form at least one continuous cooling circuit channel in the stator.
15. A vacuum pump stator as claimed in claim 11, wherein the cooling circuit comprises an actively-cooled heat sink affixed to an exterior surface of the stator.
16. A vacuum pump stator as claimed in claim 14, wherein the actively-cooled heat sink is operatively connectable, in use, to a liquid cooling circuit.
17. A vacuum pump stator as claimed in claim 14, wherein the actively-cooled heat sink is operatively connectable, in use, to a forced air cooling circuit.

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18. A unitary, multi-sage vacuum pump comprising a vacuum pump stator according to any preceding claim.
19. A unitary, multi-stage vacuum pump according to claim 18, further comprising at least one head plate sealingly affixable to the stator.
20. A unitary, multi-stage vacuum pump according to claim 19, wherein the head plate comprises a channel or recess forming a conduit for the flow of gas from a first stator cavity to a second stator cavity.
21. A unitary, multi-stage vacuum pump according to claim 19 or claim 20, wherein the head plate comprises apertures for receiving the, or bearings for the rotor shafts of the rotors.
22. A vacuum pump stator comprising, a longitudinal member arranged to cooperate with end members disposable at opposing ends of the longitudinal member such that at least two pumping volumes are defined by portions of the longitudinal and end members respectively, each pumping volume having a longitudinal axis and being arranged to accommodate a pump rotor disposed on a shaft that can be disposed parallel to the longitudinal axis, the longitudinal axis of the first pumping volume being parallel to and offset from the second pumping volume longitudinal axis, wherein a portion of the longitudinal member that defines the first volume comprises a main body and a second body attachable to the main body.

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23. A vacuum pump stator according to claim 22, wherein the second pumping volume is defined only by a portion of a main body, and respective portions of the end members.
24. A vacuum pump stator according to claim 22 or claim 23, wherein the second pumping volume is defined by the portion of a main body, a portion of a third body attachable to the main body, and respective portions of the end members.
25. A vacuum pump stator according to claim 22, wherein a first end member that partially defines the first pumping volume is integrally formed with a second end member that partially defines the second pumping volume.
26. A vacuum pump stator according to claim 25, wherein the first and second end members are formed by a single plate.
27. A vacuum pump stator substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawings.
28. A unitary, multi-stage vacuum pump substantially as hereinbefore described, with reference to, and as illustrated in, the accompanying drawings.