The invention relates to a vacuum pump and in particular to a vacuum pump for differentially evacuating a vacuum system.

In a differentially pumped scientific instrument system, such as a mass spectrometer, a sample and a carrier gas are introduced for analysis. One such example is given in Figure 1. With reference to Figure 1, in such a system there exists a high vacuum chamber 110 immediately following first, (depending on the type of system) second, and third evacuated interface chambers 111, 112, 114. The first interface chamber is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which ions are drawn from the ion source into the first interface chamber 111. The second, optional interface chamber 112 may include ion optics for guiding ions from the first interface chamber 11 into the third interface chamber 114, and the third chamber 114 may include additional ion optics for guiding ions from the second interface chamber into the high vacuum chamber 110. In this example, in use, the first interface chamber is at a pressure of around 1-10 mbar, the second interface chamber (where used) is at a pressure of around 10 ^_1 -1 mbar, the third interface chamber is at a pressure of around 10 ^~2 - 10 ^~3 mbar, and the high vacuum chamber is at a pressure of around 10 ^~5 -10 ^~6 mbar. Differentially pumped vacuum system may have different pressures dependent on requirements. The high vacuum chamber 110, second interface chamber 112 and third interface chamber 114 can be evacuated by means of a compound vacuum pump 116. In this example, the vacuum pump has two pumping sections in the form of two sets 118, 120 of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism 122; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set 118, 120 of turbo-molecular stages comprises a number (three shown in Figure 1, although any suitable number could be provided) of rotor 119a, 121a and stator 119b, 121b blade pairs of known angled construction. The Holweck mechanism 122 includes a number (two shown in Figure 1 although any suitable number could be provided) of rotating cylinders 123a and corresponding annular stators 123b and helical channels in a manner known per se.

In this example, a first pump inlet 124 is connected to the high vacuum chamber 110, and fluid pumped through the inlet 124 passes through both sets 118, 120 of turbo- molecular stages in sequence and the Holweck mechanism 122 and exits the pump via outlet 130. A second pump inlet 126 is connected to the third interface chamber 114, and fluid pumped through the inlet 126 passes through set 120 of turbo-molecular stages and the Holweck mechanism 122 and exits the pump via outlet 130. In this example, the pump 116 also includes a third inlet 127 which can be selectively opened and closed and can, for example, make the use of an internal baffle to guide fluid into the pump 116 from the second, optional interface chamber 112. With the third inlet open, fluid pumped through the third inlet 127 passes through the Holweck mechanism only and exits the pump via outlet 130.

In this example, in order to minimise the number of pumps required to evacuate the spectrometer, the first interface chamber 111 is connected via a foreline 131 to a backing pump 132, which also pumps fluid from the outlet 130 of the compound vacuum pump 116. The backing pump typically pumps a larger mass flow directly from the first chamber 111 than that from the outlet 130 of the compound vacuum pump 116. As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump 116 is able to provide the required vacuum levels in the chambers 110, 112, 114, with the backing pump 132 providing the required vacuum level in the chamber 111. The vacuum pumping arrangement shown in Figure 1 is also described in for example US5,733,104.

The performance and power consumption of the compound pump 116 is dependent largely upon its backing pressure, and is therefore dependent upon the foreline pressure (and the pressure in the first interface chamber 111) offered by the backing pump 132. This in itself is dependent mainly upon two factors, namely the total mass flow rate entering the foreline 131 from the scientific instrument and the pumping capacity of the backing pump 132. Many compound pumps having a combination of turbo-molecular and molecular drag stages are only ideally suited to relatively low backing pressures, and so if the pressure in the foreline 131 (and hence in the first interface chamber 111) increases as a result of increased mass flow rate or a smaller backing pump size, the resulting deterioration in performance and increase in power consumption can be rapid. In an effort to increase mass spectrometer performance, manufacturers often increase the mass flow rate into the spectrometer, thus requiring increased size or number of backing pumps in parallel to accommodate for the increased mass flow rate. This increases costs, size (footprint) and power consumption of the overall pumping system required to differentially evacuate the mass spectrometer.

The present invention provides a vacuum pump for differentially pumping a plurality of chambers, the vacuum pump comprising: a housing which houses a plurality of compound pumping arrangements supported for independent rotation one from another on respective drive shafts by separate motors; a first housing inlet for receiving fluid from a first chamber; a second housing inlet for receiving fluid from a second chamber; a first of the compound pumping arrangements comprising a first pumping section comprising a turbo molecular pumping mechanism and a second pumping section downstream from the first pumping section, the sections being arranged such that fluid entering the compound pump from the first inlet passes through the first and second pumping sections and fluid entering the compound pump from the second inlet passes through, of said sections, only the second section; the vacuum pump further comprising: a third housing inlet for receiving fluid from a third chamber; a fourth housing inlet for receiving fluid from a fourth chamber; a second of the compound pumping arrangements comprising a third pumping section comprising a turbo molecular pumping mechanism and a fourth section downstream from the first pumping section, the sections being arranged such that fluid entering the second compound pumping arrangement from the third inlet passes through the third and fourth pumping sections and fluid entering the second compound pump from the fourth inlet passes through, of said sections, only the fourth section. In another aspect there is provided a vacuum pump for differentially pumping a plurality of chambers at different pressures, the vacuum pump comprising: a housing comprising first and second housing inlets for connection to respective vacuum chambers, the housing comprising a bore for receiving a cartridge casing of a compound vacuum pumping arrangement, the compound vacuum pumping arrangement comprising a first pumping section and a second pumping section downstream from the first pumping section, the sections being arranged such that fluid entering the compound pump from the first housing inlet passes through the first and second pumping sections and fluid entering the compound pumping arrangement from the second inlet passes through, of said sections, only the second section, the cartridge casing comprising a first fluid inlet arrangement exposed to receive fluid from the first housing inlet when the cartridge is located in the housing and a second fluid inlet arrangement exposed to receive fluid from the second housing inlet when the cartridge is located in the housing, the first fluid inlet arrangement comprising at least one radial fluid inlet for receiving fluid in a generally radial direction into the casing and at least one axial fluid inlet for receiving fluid in a generally axial direction into the casing.

The second fluid inlet arrangement may be formed in a portion of the cartridge casing which protrudes into a volume in gas communication with the first housing inlet and the volume may encircle an axis of the compound pumping arrangement. The first fluid inlet arrangement may comprise a plurality of said radial inlets spaced about the circumference of the casing and exposed to the volume.

Said at least one axial fluid inlet may be formed at least in part by a bearing mount supporting a bearing of the compound pumping arrangement and supported by the cartridge casing. The bearing mount, or spider, may be received in the volume. At least one turbo molecular stage (or array of rotor blades) may be located at the axial fluid inlet for drawing gas through the inlet from the volume. The first section of the compound vacuum pumping arrangement may be spaced from the spider or said at least one turbo molecular stage by an amount generally equal to or greater than an axial width of the radial fluid inlets.

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

In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described in more detail, with reference to the accompanying drawings, in which:

Figure 1 is a simplified view of a prior art vacuum pumping arrangement and differentially pumped vacuum system;

Figure 2 is a perspective view of a vacuum pump;

Figure 3 is a further perspective view of the vacuum pump shown in Figure 2;

Figure 4 shows a section of the vacuum pump taken along a central longitudinal and vertical plane; Figure 5 is a perspective view of the vacuum pump prior to full assembly; and

Figure 6 is a part section for showing a cartridge casing in the vacuum pump.

Referring to Figure 2, a vacuum pump 10 is shown for differentially pumping a plurality of chambers. As described above in relation to the prior art, the chambers may form part of a mass spectrometer system and comprise a high vacuum chamber 110 immediately following first, (depending on the type of system) second, and third evacuated interface chambers 111, 112, 114. Alternatively, the vacuum system may comprise a plurality of chambers as disclosed in more detail in US2015/0056060.

The vacuum pump 10 comprises a housing 12. The housing 12 may be cast or machined from a suitable metallic material, such as steel or iron, and has a one-piece construction for housing the various pumping mechanisms of the pump. In this example the housing is aluminium and machined from a solid block, but could be extruded or cast. Aluminium is preferred because it is light-weight. The housing comprises a plurality of housing inlets 14, 16, 18, 20, 22, 24 and can be fixed to the vacuum system by suitable fastening members and sealed to allow fluid communication between the different pressure chambers of the system and selected pumping mechanisms of the pump. The fluid inlets are surrounded by grooves 26 for receiving sealing members (not shown), such as o-rings, for sealing against the vacuum system to avoid or resist the leakage of ambient air into the pump. Inlets 18, 20, 22, 24 are surrounded by a single groove and sealing member. The housing comprises a plurality of flow paths which guide fluid from the housing inlets towards the inlets of the different pumping mechanisms, as shown in more detail in Figure 4. These flow paths are formed by the internal structure of the housing and may be cast in the manufacturing process or machined into the housing.

The vacuum pump comprises two vacuum pumping arrangements 28, 30. In this example, vacuum pumping arrangement 30 is arranged to evacuate pressure chambers through housing inlets 18, 20, 22, 24 at a relatively higher vacuum (lower pressure) compared to vacuum pumping arrangement 28 which is arranged to evacuate pressure chambers through housing inlets 14, 16 at a relatively lower vacuum (higher pressure).

The vacuum pumping mechanisms for the two arrangements are housed within the housing 12. In each case, part of the vacuum pumping arrangements extends externally to the housing and these external parts may include the motor and electrical connections for connecting the arrangements to a source of electrical power. Locating the motors at partially external to the housing allows heat generated during use to escape from the pump more easily. Figure 3 shows the vacuum pump 10 from a different perspective, underneath the pump. The first vacuum pumping arrangement 30 comprises an inlet port 32 and the second vacuum pumping arrangement 28 comprises an exhaust port 34. A foreline pipe 36 connects the exhaust port with the inlet port for fluid communication. As described in more detail below gas is exhausted from the second vacuum pumping arrangement through the foreline to the first vacuum pumping arrangement, where it is pumped by at least one pumping mechanism. The first vacuum pumping arrangement comprises an exhaust port (not shown in this Figure) for exhausting gas through another foreline pipe to a separate backing, or primary, pump. In this way, the second vacuum pumping arrangement is backed in series by the first vacuum pumping arrangement and a primary pump, and the first vacuum pumping arrangement is backed by the primary pump. This can reduce the overall power consumption of the system or of the first vacuum pumping arrangement 28. Alternatively the reduced backing pressure could be used to enhance the overall compression of arrangement 28.

In an alternative, the housing could be configured to include internal structure defining the flow path between the vacuum pumping arrangements, thereby eliminating the need for additional fittings and pipes. In other examples, the vacuum pump can be backed independently by a single or multiple backing pumps. Figure 4 shows a section through the vacuum pump 10. The first vacuum pumping arrangement is inserted into a bore 38 of the housing 12 through an opening 40 in the underside of the housing and fastened in position with fastening members (not shown). An o-ring 42 seals the arrangement when in position. The first vacuum pumping arrangement comprises in this example seven vacuum pumping sections each having a pumping mechanism and one or more pumping stages. The number of sections, stages in each section and type of pumping mechanism may be selected as required depending on pumping requirements, such as capacity and compression. The arrangement 30 comprises a drive shaft 58 supported for rotation by upper and lower bearings 60, 62. Typically the upper bearing is a magnetic bearing (with a back-up bearing) and the lower bearing is a roller bearing. The upper bearing is supported by a spider 63 having a central hub from which three arms extend in a radial direction. The arms are fixed to the housing to provide support for the bearing and the vacuum pumping arrangement, whilst allowing space for gas to enter the most upstream pumping mechanism 44. The spider could also be machined into and form part of the housing (i.e. integral with the housing).

A motor 64 drives rotation of the drive shaft and is connected to a source of electrical power. The first pumping arrangement is a high speed pump and is typically rotated at speeds of between about 10,000 and 100,000 rpm.

The sections 44, 46, 48, 50 of vacuum pumping arrangement 30 each comprise a turbo molecular pumping mechanism. These sections comprise respectively four stages, three stages, three stages and two stages, but more or fewer stages may be provided as required. Although at least one section comprising a turbo molecular pumping mechanism is required to generate the required vacuum pressure, the other sections may be replaced with other types of pumping mechanism. Sections 52 and 54 are each four stage drag pumping mechanisms, and section 56 is a two stage regenerative pumping mechanism. The regenerative pumping mechanism is otherwise known as an aerodynamic mechanism in which an array of blades on a rotor disc extend into respective channels of a stator generating a vortex in the channels on rotation which compress the gas being pumped. Different types of pumping mechanisms may be used in the latter sections, as well as different numbers of stages, depending on pumping requirements.

The first vacuum pumping mechanism comprises four fluid inlets to the various sections. Section 44 has a fluid inlet 66 connected for fluid communication with the housing inlet 24 by internal flow path 67. Section 46 has a fluid inlet 68 connected for fluid communication with the housing inlet 22 by internal flow path 69. Section 48 has a fluid inlet 70 connected for fluid communication with the housing inlet 20 by internal flow path 71. Section 50 has a fluid inlet 72 connected for fluid communication with the housing inlet 18 by internal flow path 73. Flow paths 69, 71, 73 extend away from the housing inlets and through 90 degrees to their respective fluid inlets, but may alternatively be direct in line with the pumping mechanism (i.e. not through 90 degrees). Fluid entering through housing inlet 24 passes through all of the pumping sections of the first vacuum pumping arrangement, namely sections 44 to 56. Fluid entering through housing inlet 22 passes through sections 46 to 56 only. Fluid entering through housing inlet 20 passes through sections 48 to 56 only and fluid entering through housing inlet 18 passes through sections 50 to 56 only. In this example, housing inlet 24 is evacuated to the lowest pressure and the evacuation pressure gradually increases as gas passes through fewer sections.

Pumping sections 52, 54, 56 provide a backing pressure for the turbo molecular sections of the first vacuum pumping arrangement 30, since a turbo molecular mechanism cannot, or at least cannot efficiently, exhaust at atmosphere. Whilst in some examples the most downstream section 56 may exhaust at atmosphere, typically it exhausts below atmosphere and is itself backed by a separate primary pump, or alternatively another similar turbo molecular pump then a primary pump.

As discussed above, the exhaust 34 of the second vacuum pumping arrangement 28 is connected to an inlet 32 of the first vacuum pumping arrangement 30. The section of Figure 4 does not show these ports, however the exhaust 34 is located downstream of the most downstream pumping section of the second arrangement and the inlet 32 is located upstream of at least one of sections 52, 54, 56 and downstream of at least section 50. In this way, the exhaust 34 is backed by one, two or three pumping sections. The backing section is preferably configured as a booster mechanism whereby the compression ratio of the section is between 10:1 and 1:1 (for example) so that pumping capacity is increased. A section configured as a booster is capable of pumping a greater amount of gas which is useful in the event that a large amount of gas is input to the mass spectrometer. The second vacuum pumping arrangement 28 comprises a cartridge in this example, although a cartridge envelope is not essential for arrangement 28 (or both pumps) and alternatively it may be integrated directly into the housing. The cartridge 75 comprises a casing 74 for supporting the pumping mechanisms of the cartridge. The - in casing is configured so that the cartridge can be inserted into and engage with a bore 76 of the housing 12 to expose fluid inlets 78, 80 of the pumping mechanisms to respective housing inlets 16, 14. Figure 5 shows a perspective view of the vacuum pump 10 with the cartridge prior to insertion in bore 76 through housing opening 82. The inner surface of the bore 76 guides the cartridge 75 towards the fully inserted position shown in Figure 4. The casing has an outwardly projecting flange 83 which forms with housing end surface 84 abutment surfaces for locating the cartridge in the correct position and limiting the extent to which the cartridge 75 can be inserted into the housing 12. Fastening members 86 fix the cartridge in position when it has been inserted. The members engage in closed bores 88 of the housing, typically by threaded engagement.

As shown in Figure 4, the housing 12 is shaped so as to expose the bore at a number of locations to allow fluid to enter the fluid inlets 78, 80 when the cartridge 14 is in the fully inserted position. Flow paths 79, 81 manufactured in the housing guide flow from the housing inlets 14, 16 to respective fluid inlets 78, 80 of the cartridge. The housing also defines a volume 89 for gas flow from housing inlet 16. Gas from inlet 16 may therefore pass into the cartridge radially through fluid inlet 78 or axially through a fluid inlet 85. The provision of radial and axial fluid inlets increases conductance into the pumping arrangement. It is preferable in this case as shown that at least one turbo molecular pumping stage 87 (or array of rotor blades) is located at the fluid inlet 85 to draw gas into the cartridge. The provision of two fluid inlets at the housing inlet 16 produces low conductance resistance to flow.

In more detail and as shown additionally in Figure 6, the cartridge casing defines a plurality of apertures 91 spaced about its circumference forming radial fluid inlets into the vacuum pumping arrangement 28. Four apertures 91 are provided in this example at 90 degrees to one another but other configurations are possible. The apertures are located in part of the housing that extends into volume 89 and therefore all apertures are exposed to the volume, including the apertures located furthest from the housing inlet 16. The array of rotor blades 87 and the pumping section 90 are spaced apart in an axial direction by an amount which is approximately equal to the axial width of the apertures 91. This configuration allows gas to penetrate into the space 93 between the pumping mechanisms towards the drive shaft to enable pumping by both radially outer and radially inner portions of the turbo molecular mechanism 90. Without such a space 93 the gas would interact only with the radially outer portions of the mechanism, or to a much greater extent than with the radially inner portions, and less efficient pumping would be achieved.

The fluid inlet 85 of the cartridge casing is formed by a generally circular aperture which is open in the axial direction (to the right in Figures 4 and 6). The casing comprises a shoulder portion 95 which is located upstream of the apertures 91 and fully within volume 89. The shoulder portion seats a bearing mount, or spider, 97 for mounting bearing 98 in position, which is likewise received in volume 89. The spider comprises an outer rim fixed to the casing, an inner hub for supporting the bearing and three radial arms (or spokes) connecting the rim with the hub. This configuration provides three apertures extending through approximately 120 degrees forming the axial fluid inlet 85 into the vacuum pumping arrangement 28. The spider may have fewer or more radial arms as is not restricted to three.

The cartridge casing further comprises apertures 99 forming a fluid inlet to the pumping section 92. Unlike apertures 91 located in volume 89, only that part of the casing proximate the housing inlet 14 is exposed to receive gas and therefore apertures 99 are provided only in an upper part of the casing.

The pumping arrangement as shown fits horizontally into the housing, but could be arranged vertically - similarly the first arrangement could be horizontal - hence providing four configurations. That is horizontal-horizontal, horizontal-vertical, vertical- horizontal, and vertical -vertical.

In this example, the second vacuum pumping arrangement 28 comprises three pumping sections 90, 92, 94. Section 90 comprises three turbo molecular stages. Section 92 comprises three Holweck stages. Section 94 comprises two Seigbahn stages. Different types of molecular drag or other pumping mechanisms may be used in sections 92 and 94, or there may only two sections in total. The number of stages in the sections may be varied as required dependent on pumping requirements. A drive shaft 96 supports the pumping mechanisms for rotation and is itself supported by bearings 98, 100. In this example, bearing 98 is a dry magnetic bearing with roller bearing back-up and bearing 100 is a lubricated roller bearing. The drive shaft is driven by motor 102 connected to a source of electrical power at high rotational speeds for example between about 10,000 and 100,000 rpm. Rotation of the pumping sections 90, 92, 94 causes gas to flow from housing inlet

16 through fluid inlets 78, 85 passing through section 90 (and turbo molecular stage 87), section 92 and section 94. Gas is caused to flow from housing inlet 14 through fluid inlet 80 passing through, of the sections, only the downstream sections 92, 94. The pressure chambers in fluid communication with the housing inlets 14, 16 are evacuated at different pressures; housing inlet is evacuated at a lower pressure because gas passing through it is conveyed through more pumping sections and section 90 is a configured to pump at lower pressures. More than two housing inlets may be evacuated by the cartridge.

Gas is exhausted from the cartridge 75 through exhaust port 34 and passes through a booster section (one or more of sections 52, 54, 56) of the first vacuum pumping arrangement prior to a separate backing or primary pump. If the first pump power is high the second pump could be used to back the first (switching the backing arrangement shown).

The cartridge type configuration of the second vacuum pumping arrangement 28 is advantageous in that it can readily be removed and inserted into the housing, allowing easy maintenance, repair or replacement. Since the casing of the cartridge provides the support required for operation (and rotation at high speeds) the vacuum pump 10 can be more easily manufactured and assembled. The various components of the cartridge are assembled outside the housing 12, without the need to manufacture typically intricate structures for supporting the mechanism inside the housing. In the example of vacuum pump 10, vacuum pumping arrangement 30 does not have a cartridge type configuration and cannot be operated without support inside the housing, particularly without spider 63 and bearing assembly 60. However, vacuum pumping arrangement 30 may alternatively be formed as a cartridge type configuration similarly to vacuum pumping arrangement 28.

The vacuum pumping arrangements may have drive shafts having respective axes of rotation which are angled one relative to another. In the example shown the drive shafts 58, 96 have first and second axes of rotation, and the first axis is

perpendicular to the second axis. The axis 96 intersects axis 58, but in other examples the axes may be offset. In an earlier document of the present applicant (EP2273128) an axis of a cartridge type vacuum pumping arrangement is inclined by an angle Θ to the direction of gas flow through the pressure chambers of a differentially pumped system and in further examples of the present invention a similar construction can be adopted whereby a pumping axis of one pumping arrangement is at an angle Θ as is the case with the earlier document.

As shown in the Figures the axis 58 is vertical and the axis 96 is horizontal, with respect to gravitational force. It is preferable in the present construction that the axis 58 is vertical in order to cancel out the effect of gravity acting on the moving parts (rotor parts) of the pumping arrangement. Particularly, upper bearing 60 is a non-contact bearing and therefore allows a small amount of radial movement of the rotor (limited by the back-up bearing). Therefore the drive shaft can to an extent be considered cantilever, but as gravitational force is largely cancelled and does not cause significant radial movement. It will also be appreciated that there are many pumping sections, seven in total and four turbo molecular sections (requiring high speed rotation), thereby contributing to the overall length of the drive shaft. Hence in structures with a multiplicity of pumping sections (more than four) or a multiplicity of turbo molecular sections (more than two) it is desirable, but not essential, that the drive shaft is vertical for improved rotor dynamics.

The drive shaft 96 of the second vacuum pumping arrangement is horizontal, because the problems explained above in relation to the first vacuum pumping arrangement are less apparent, and a horizontal orientation is advantageous for conserving foot-print or pump size. In a one-piece housing construction there is also the issue of mass as the vacuum pump is typically mounted by suspending it from the underside of a vacuum system. A horizontal drive shaft reduces the size of the housing and also its mass. As described above in detail with reference to the Figures, the housing 12 houses a plurality of compound pumping arrangements 28, 30 supported for independent rotation one from another on respective drive shafts 58, 96 by separate motors 64, 102. Independent rotation and drive allow improved versatility for application to multiple different pumping requirements. For example, operators of mass spectrometer systems are increasing the sample size of gas introduced to upstream pressure chambers of the system and accordingly there may be a cyclical or transient requirement during operation for the second vacuum pumping arrangement to pump large quantities of gas. In these circumstances, the power drawn from the motor 102 is increased to overcome increased resistance to rotation in sections 90, 92, 94. However, the first vacuum pumping arrangement 30 is independent and continues operation without interruption or increased power requirement.

The power supply for the vacuum pumping arrangements 28, 30 may be shared so that electrical power is supplied to the motors 64, 102 by a single supply unit (not shown). In this case, the unit may provide the electrical power and control of the units may be performed at the pumping arrangement or in the unit. That is two frequency converters may be provided in the unit or one in each of the pumping arrangements. The advantage of a single supply unit is that the requirement for electrical power is spread across or shared by both pumping arrangement so if one pumping arrangement is subjected to high load conditions and the other to low load conditions the overall power consumption does not increase. Such conditions may occur for example if both vacuum pumping arrangements are operating at ultimate pressure and a sample gas is introduced to the vacuum system - the pressure and amount of gas in the upstream chambers increases, with a commensurate increase in the load on the second vacuum pumping arrangement 28, but there is a delay prior to increased pressure in the downstream chambers and increased load on the first vacuum pumping arrangement 30. Alternatively, each vacuum pumping arrangement can be provided with a dedicated source of electrical power. In some known vacuum pumping systems, a plurality of pumps are provided in respective housings for differentially evacuating a vacuum system. However, in these known systems the pumps are arranged so that gas exhausted from a lower pressure pump is conveyed to an inlet of the higher pressure pump. Therefore the pumps are arranged in series. In vacuum pump 10, however, the two vacuum pumping

arrangements are not in series and instead evacuate the pressure chambers in parallel. A series relationship is partly provided but only between the exhaust of the second vacuum pumping arrangement and the booster section 52, 54, 56.

The housing 12 is manufactured to include the required internal structure both for seating the vacuum pumping arrangements in the correct positions and for guiding fluid flow from the housing inlets to the fluid inlets of the vacuum pumping

arrangements. The internal structure is configured to have a relatively simple shape which permits formation by casting or moulding without the requirement for extensive subsequent machining. The internal structure is configured to include a plurality of shaped flow conduits defined by internal partitioning walls for directing flow to the fluid inlets of both vacuum pumping arrangements. The provision of a single one-piece housing for directing gas flow removes the requirement for forelines and other pipework thereby avoiding the use of additional components, the time required for assembly and a potential cause of leakage.