FIELD OF THE INVENTION

The field of the invention relates to clam-shell stators for a multistage pump and a multistage vacuum pump.

BACKGROUND

Multistage pumps such as the multistage Roots pump have a clamshell stator design that enables the rotors to be placed within the stator. A clamshell stator design consists of two stator halves split horizontally through the centreline of the two rotor shafts. For assembly the two rotor shafts are laid in one stator clam and an upper stator clam is placed on top of the lower one. These are bolted together to form the main pump housing. The upper and lower clams are sealed by two gaskets running along the outer sides of the clams.

There are problems with this design particularly, for high temperature operation (>200°C). Firstly, the gasket material needs to be able to withstand such temperatures and this tends to require an expensive sealing material such as a perfluoroelastomer FFKM. Secondly the seal running along the length of the clamshells seals with an 0-ring at the end cover or head plates, the so-called T- seal junction between the two seals is difficult to seal effectively. Furthermore, where corrosive process gases such as fluorine are being pumped the clams may need to be plated to inhibit corrosion. Plating of the surfaces of the complex claim design with inter-chamber channels may not be straightforward.

It would be desirable to provide an improved stator design.

SUMMARY

A first aspect provides a stator for a multistage pump according to claim 1 .

The inventors of the present invention recognised that stators formed of half shell components such as clam-shell stators are difficult to seal effectively particularly at the junction between the longitudinal seal running along the long sides of the half shells and the O-ring between the half shells and the end or head plates. They have addressed this by providing an outer casing enclosing the two half shell components and comprising the end plates. In use this outer casing can be sealed to provide a substantially gas-tight environment for the half-shell stator and thus, there is no longer the same requirement for a completely gas-tight seal along the edges of the half shells and between this seal and the end plates as any leakage of gas from the inside of the pump will be retained within the outer casing. This is particularly advantageous where the pump is configured to operate at high temperatures as providing seals that are effective and resistant to high temperatures requires expensive materials to be used. Furthermore, when the pump is taken apart during servicing or after testing such seals need to be replaced which, where expensive seals are used, increases the costs of the pump considerably. Seals in the outer casing that have a standard acceptable leak rate that is less than 10’ ³ mbar litres/sec, preferably less than 10’ ⁵ mbar litres/sec, are found to be acceptable.

Forming a pump that is provided with a substantially gas-tight outer casing that seals around the pump and inhibits leakage of process gases pumped by the pump to the atmosphere and indeed atmospheric gases such as oxygen leaking into the pump may allow the seals between the clam shell components or between the clamshell components and the end plates to be dispensed with or fewer seals with lower specifications to be used. In effect the seals on the clam shell no longer have a safety critical function as any gas leakage is to a sealed volume.

The outer casing is substantially gas tight in that a pressure difference between the interior of the housing and atmosphere is maintained when the pump is operational.

In some embodiments, said outer casing defines an end and side walls of an exhaust pumping chamber of said multistage pump. Although, the outer casing may simply encase conventional half shell stator components which define all of the pumping chambers of the multistage pump, in some embodiments, the outer casing itself defines the end and side walls of one or more of the end pumping chambers of the multistage pump. This arrangement may be advantageous as the end pumping chambers require ports to allow gases to either enter the pump or be exhausted from the pump. Where an outer casing surrounds a half shell stator component that defines all the pumping chambers then such ports need to pass through the outer casing walls and through the half shell stator walls in order to reach the pumping chambers, and this can present problems with alignment of the ports and with gas leakage. A particularly elegant solution to this problem is to use the outer casing to define at least some of the walls of one or both of the end pumping chambers allowing ports to pass through the outer casing and into these end pumping chambers.

In some embodiments, said outer casing is formed of multiple parts, said multiple parts being arranged so that junctions between adjacent parts are between two parts sealed together in a substantially gas tight manner with a single sealing surface.

Forming the outer casing in two or more parts allows the pump to be assembled by mounting the half shell components within the outer casing. The outer casing parts are sealed together to provide the gas-tight enclosure. The seal between the at least two parts of the outer casing can be located in a position favourable of achieving a sealing function.

Having an outer casing allows for greater flexibility in the choice of configuration of the multiple parts and allows for the location of the junction between the parts that need to be sealed to be selected. In particular, the configuration can be selected so that any junction is between no more than two adjoining parts. This allows the sealing surface between the two parts to be a single sealing surface, which may be sealed with a single seal and avoids the problems that arise where perhaps three or more parts meet, and there are junctions between seals such as at the end of the conventional claim shell stator, where there is the T-seal between the longitudinal seals between the clam shells and the 0-ring between the shells and the end plates.

It may be advantageous for the seal between the surfaces of the different parts of the outer casing to be at a position on the machine that makes achieving a seal easier. Where for example, the pump is configured to run with a high temperature exhaust, the seal location can be chosen nearer to the cooler inlet.

The provision of an outer casing surrounding the clam shell, provides for greater flexibility in where the seals can be placed and allows locations to be selected that are favourable to achieving a sealing function, this location may vary with the configuration of the pump.

In some embodiments, at least a portion of one part of said outer casing is integral with one of said half shell components, and at least a portion of a second part of said outer casing is integral with another of said half shell components, said outer casing parts having a different form to said half shell components.

In some embodiments, the outer casing may not be a separate outer casing but may be integral with the half shell components but of a different shape. This different shape allows the sealing surfaces to be located in different positions to the sealing surfaces between the two half shell portions, which may eliminate the T-seal.

In some embodiments, said outer casing is formed of cooperating wedge-shaped parts.

Wedge-shaped parts allow corresponding sloped surfaces to form the sealing surfaces which surfaces can be sealed with an O-ring component. In some embodiments, the outer casing has windows in the end plates that allow the rotor to be slotted into the lower part of the outer casing. These windows will be filled with a plate which comprises the apertures for receiving the rotor shafts. In other embodiments there may not be windows and the rotors may fit within the outer casing and have stub axles attached to them after assembly.

In other embodiments, said outer casing is attached to at least a portion of said two half shell components by at least one of a sealing material and an adhesive material.

Where the outer casing is of a similar size to the half shell components it may be stuck to the half shell components by an adhesive material or there may be a sealing material between them to impede leakage of process gases from the pumping chamber within the half shell components. In other embodiments where the outer casing fits closely to the half shell component there may be an interference fit between the two. This latter may make it more difficult to take the machine to pieces during servicing.

In other embodiments, there is a gap between said outer casing and said two half shell components.

An alternative may be to configure the outer casing such that it is mounted at a distance from the two half shell components and there is a gap between them. This gap provides a thermal break such that where a pump operates at high temperatures the outer casing remains at a lower temperature providing a safer environment for engineers and also allowing the sealing material sealing the parts of the outer casing to be made of a material with a lower heat resistance than might otherwise be the case. The gap may be greater than 0.5mm, preferably greater than 1 mm. In some embodiments, said outer casing comprises a larger box part and a lid part, said lid part sealing an aperture in said box part, said aperture being large enough to receive said two half shell components.

One arrangement of the outer casing may be for it to comprise a box type arrangement with an open top, the open top being large enough to receive the half shell stator components, the outer casing comprising a lid that seals to the box part. In such an arrangement the shaft of the rotor may be configured to receive stub axles at either end, so that once the rotor is mounted within the half shells within the outer casing, the stub axles may be inserted through the apertures in the end plates of the outer casing.

In some embodiments, said outer casing comprises two parts, each part comprising one of said end plates and substantially cylindrical side walls extending from said end plates, said substantially cylindrical side walls of said two parts being configured to seal together to form said outer casing.

One arrangement for the outer casing may be for it to comprise two parts each comprising an end part and side walls and being arranged to seal at the side walls. The two parts may in some embodiments be symmetrical and substantially cylindrical.

In some embodiments, said outer casing comprises a central part and two end parts, each end part comprising one of said end plates and side walls extending from said end plates, said side walls being configured to seal to said central part.

Alternatively, the outer casing may comprise a central part and two end parts the end parts having side walls which attach to the central part. In some embodiments, the central part may be substantially cylindrical. This requires additional seals but may be easier to manufacture. In some embodiments (owing to the outer casing), there are no seals disposed between the end plates and the half shell components. For example, at least 0- ring seals between the end plates and the half shell components are omitted. In this manner, the problematic T-seals of previous designs (where longitudinal seals between the half shell components meet the 0-ring seals) are avoided.

In some embodiments, at least one of said half shell components comprises a plurality of ports each comprising an opening of a gas transfer passage in an outer surface of said at least one half shell component, said gas transfer passage extending from said outer surface of said at least one half shell component to a portion of said void forming one of said plurality of pumping chambers, said outer casing closing said port and containing said gas transfer passage.

The passages between the pumping chambers generally run within the half shell components and as such are formed during casting. Multistage pumps may be used for pumping corrosive process gases and thus, it may be advantageous to coat surfaces of the stator components that contact the process gases with a corrosion resistive coating. In this regard, the half shell components of stators are typically made from cast SG iron and for successful plating or coating the surfaces should be machined. Where there are internal passages within the components machining is not viable. Changing the design to have ports to the passages in an outer surface of the clam shell improves access to the gas transfer passages for machining and allows the surfaces that are exposed to process gases to be machined and therefore successfully coated.

In effect the provision of an outer casing surrounding the half shell components provides an opportunity for passages between pumping chambers to run externally to the half shell components and thus, allows the ports from these pumping chambers to be machined.

In some embodiments, surfaces of said half shell components configured to contact gases during pumping are coated. As noted previously, gases pumped by multistage pumps may be process gases which may be corrosive. Coating the surfaces that contact the gases will protect them from such gases. In some embodiments, the surfaces that contact the gases are machined and this allows effective coating and provides an improved stator component.

In some embodiments the end plates comprise head plates.

The head plates of a pump mount the bearings for supporting the rotor. In some embodiments, these are part of the outer casing forming the end plates of the outer casing, while in other embodiments, they may be separate to the outer casing. Where the outer casing forms a part of the inlet and/or exhaust pumping chamber then having the head plate separate to the outer casing provides a thermal break between the outer casing and head plates allowing the bearings to be maintained at a cooler temperature.

A second aspect provides a multistage vacuum pump comprising a stator according to a first aspect and two rotors mounted within said stator.

In some embodiments, said vacuum pump comprises one of a Roots type pump or a claw pump.

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 A schematically shows a cross section through a stator for a multistage pump according to an embodiment;

Figure 1 B shows the clamshells and rotors of such a pump;

Figure 2 shows the stator of Figure 1 with the casing lid in place;

Figure 3A shows a cross section through the rotor showing a central bore for receiving the axle;

Figure 3B shows a multistage pump having the stator of Figures 1 and 2 and with the axle and headplates in position;

Figures 4A shows a cross section of a multistage vacuum pump according to a second embodiment;

Figure 4B shows a top view of a multistage vacuum pump according to a second embodiment;

Figure 4C shows an overview of a multistage vacuum pump according to a second embodiment;

Figure 5 shows an overview of the assembled clamshells components of a stator according to an embodiment without the outer casing in place;

Figure 6 shows a further embodiment of a stator for a multistage pump; and Figure 7 shows an alternative embodiment of a stator for a multistage vacuum pump.

DESCRIPTION OF THE EMBODIMENTS

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

Embodiments provide a multistage pump that has clamshells components housed within a hermetically sealed outer casing. The outer casing is formed in two or more parts and the seal between the two or more parts does not run parallel to the junction between the two clamshells and in some embodiments runs substantially perpendicular to the junction. This reduces, and in some cases, eliminates the need for a gasket or sealant between the clamshell halves and also eliminates the hot ‘T’ seal between the longitudinal and O-ring seals.

Some embodiments have a channel in the outside surface of the clamshell components, which, when surrounded by the outer casing, forms a gas transfer passage between pumping chambers of the multistage pump. This allows gas transfer ports between pumping chambers to simply extend through the clamshells, providing a fluid communication path from the pumping chambers to the passage between the outer surface of the clamshells and inner surface of the outer casing. This geometry allows these ports and transfer passages to be machined and these machined surfaces can be effectively coated and thereby protected from corrosive gases being pumped.

Some embodiments have gas transfer passages that are substantially internal to, but which have openings in the outside surface of the clamshell components.

The openings provide access to machine all the surfaces of the gas transfer passage and the ports communicating between the passages and the pumping chambers of the multistage pump. The outer casing closes the openings in the outside surface of the clamshell components and isolates the gas transfer passages from each other, in this way the cover forms a surface of the gas transfer path between the pumping chambers.

Figure 1A shows a stator for a multistage pump according to an embodiment. The stator comprises two clamshell components 10 that enclose the pumping chambers and rotors of the multistage pump. Figure 1 B shows how the two clamshells 10 are assembled to hold and enclose the rotors 32.

The clamshells 10 are mounted within an outer casing 20 which in this embodiment is formed in two portions comprising a main box casing 22 having a base and end plates or pieces and a lid portion 24 which is not yet in place in Figure 1 A. The lid 24 seals in a substantially gas type manner with the main box forming outer casing 20 such that when the pump is in operation there is a pressure difference between the inside of the outer casing and atmosphere. In this regard, the inside of the outer casing is at an intermediate pressure between the lower pressure within the vacuum pump and atmosphere. In effect, the vacuum pump or vacuum generator is contained within an outer container which is substantially hermetically sealed. This allows seals between the clamshells and in particular seals between the clamshells and the head plates to be dispensed with or where retained to be less critical to safety and thus, of lower specification.

The T-seal that is conventionally between the longitudinal seals sealing the clamshells and the O-ring sealing the clamshells to the head plate is always a difficult seal to provide in a gas type manner and is no longer present.

The seals used to hermetically seal the outer casing are located in places governed by the configuration of the outer casing and thus with appropriate design the seals may have a more convenient form and T-seal junctions between seals can be avoided. In this embodiment seals are provided between the lid 24 and the main box portion of the outer casing 20. There is an inlet 40 which extends from the clamshells through the outer casing and an exhaust 42 which does likewise. There are apertures within the end pieces 22 of the outer casing which allow the shaft of the rotor to extend out of the pump. In this regard, in some embodiments the shaft may be cantilevered and then there may only be apertures 25 on one end piece.

The shafts are supported on bearings which in some embodiments are incorporated into the end pieces of the outer casing. In other embodiments the bearings are incorporated in a separate head plate 62 which seals to the outer casing. There may be a seal 64 around the apertures 65 in the head plate 62 which receives the rotors.

Figure 2 shows the stator of Figure 1 with the lid 24 in place thereby forming a hermetically sealed outer casing 20. Figure 3A shows an example of a rotor that may be mounted in the pump shown in Figure 3B. In this embodiment the rotor is a shortened rotor with a bore through it. The rotor is mounted in the clam shells and the clam shells placed within the outer case prior to the longer axle being inserted through the bore. Once the rotor is in position the axle may be pushed through the bored rotor and be located at either end using an expanding device to lock it into the rotor. Once axles are installed in both rotors a further sleeve may be used to increase the diameter of the axle’s exterior to the outer casing so that the assembly resembles a typical clamshell design.

An alternative design would be to use stub axles in shortened rotors, wherein the rotors would be mounted in the clam shells and the clam shells within the outer casing prior to pressing the stub axles into either end of the shortened rotors.

Figure 3B shows the outer casing 20 with the head plates 62 supporting the rotor shaft 30 attached to the end plates 22 or surfaces of the outer casing 20. The head plates 62 comprise bearing within the apertures supporting the rotor shaft allowing it to rotate when driven by a motor.

As noted above the shafts of the rotors have been shortened and may have a through ground hole to receive stub axles or a through bore to receive a whole axle, allowing the rotor shafts to be assembled within the clamshells and the clamshells then placed within the outer casing and the head plates then attached, without projecting shafts. No 0-rings are required at the end of the clamshells 20 as this is now within the outer casing 20. The outer casing may be a cast outer casing and the clams may be located by dowels within it. The exhaust and inlet seal to the outer casing. O-rings and grooves are used to seal the head plates 62 to the outer casing 20. The lid 24 is fixed and sealed to the edges of the main box portion of the outer casing and to the inlet 40. Figures 4A to 4C schematically show an alternative embodiment where the outer casing is formed of two portions which meet at a sealed junction 28 which surrounds the clamshells 10. This junction may be sealed by an 0-ring where the clamshells have a circular cross section. In this embodiment, the inlet pumping chamber 44 and the exhaust pumping chamber 46 have outer walls formed by the outer casing 20 and this allows the inlet and exhaust ports 40, 42 to provide access directly to these pumping chambers through the outer casing rather than needing to travel through the clamshell walls too which would require accurate alignment of the clamshells and outer casing and additional sealing. The middle pumping chambers are formed by the clamshells in a conventional manner and have passages 15 that allow gas to travel between them.

In this arrangement, the outer casing 20 can be seen as inlet and exhaust stage buckets that meet at a single flange towards the middle of the pump which is sealed by O-ring 28. An alternative embodiment, not shown, would be to have shorter inlet and outlet buckets each sealing to a central tube. In this case the outer casing would have three parts an inlet and an exhaust part which form part of the inlet and exhaust pumping chambers and a central cylindrical part. This would require an additional seal such that there would be two O-ring seals but would avoid the need to produce the deep bores in the end buckets and might therefore be easier to manufacture.

Figure 4B shows a top view showing the two rotor shafts 30, while Figure 4C shows an overview of the exterior of the assembled pump. In some embodiments (not shown) the outer casing may be configured with retractable cooling plates arranged to contact and cool the outer surface of the casing 20.

Figure 5 shows a similar design without the outer casing in place, so that the interstage ports 15 providing gas transfer between the pumping chambers are visible. With suitable sealing means between the clamshells 10 and the outer casing, gas transfer channels between the pumping chambers via neighbouring ports 15 are provided. This design allows the ports to be machined rather than requiring the ports and passages to be cast. Machined surfaces are easier to coat or plate which allows the clamshells to be protected from aggressive process gases that are being pumped.

Figure 6 shows an alternative cylindrical design similar to that of Figure 4 but where there are three portions, two end portions which comprise end plates 22 and a cylindrical middle portion 20. The end portions seal via an O-ring seal with the cylindrical portion 22 at either end. There is therefore no requirement for a T- seal as the O-ring seal is around the outer circumference of the end pieces and does not cross the junction between the clamshells.

Figure 7 shows an alternative design where the clamshell components 10 are integral with the outer casing 20. In this embodiment the outer casing 20 is formed of two half wedge shaped pieces which each accommodate one half clamshell. Thus, the seal between the two halves travels along the edge between the two wedge shaped pieces. The end plates 22 incorporate windows at the ends of the wedges which accommodate the shafts 30 to allow the rotors to be angled into position. Separate headplates cover the windows and contain the bearings supporting the rotor shafts. Alternatively, axles may be inserted into the rotor shafts once they are in the stator, in which case there is no need for a large window, and the bearings can be incorporated into the end piece of the outer casing.

A continuous O-ring seal (not shown) is provided between the two parts of the wedge shaped outer casing 20 and this avoids the need for the T-section seal that is difficult to achieve between the two clam halves and a headplate. In this embodiment the end plates 22 may comprise the head plates.

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.

REFERENCE SIGNS

10 clamshells stator

15 inter-stage ports

20 outer housing 22 end plate

25 apertures for receiving rotor

28 outer housing seal

30 rotor shaft

32 rotor 40 inlet

42 exhaust

44 inlet pumping chamber

46 exhaust pumping chamber

62 head plate 64 0-ring

65 rotor shaft support