GB2620724A

GB2620724A - Multi-stage vacuum pump with improved low vacuum pressure performance

Info

Publication number: GB2620724A
Application number: GB2207237.5A
Authority: GB
Inventors: Turner Neil; Thomas Chrisp David
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2022-05-18
Filing date: 2022-05-18
Publication date: 2024-01-24
Also published as: GB202207237D0

Abstract

A multi-stage vacuum pump 200 comprises a stator assembly defining at least two chambers 100a-f and a rotor assembly 110 having at least two rotors 112a-f housed in respective chambers to define at least two pump stages, including at least an inlet stage 106 and an outlet stage 108. The rotor assembly also includes a shaft 114 supported on a first, axially moveable, bearing 120, and a second, fixed, bearing 130. The inlet stage 106 is positioned closer to the fixed bearing than at least one of the other pump stages. Alternatively, a set axial clearance Ca for the rotor assembly is defined between the set side wall 105a and the rotor 112a of the inlet stage. These embodiments permit tighter control of clearances in the inlet stage to improve lower or “rough” vacuum pressure performance.

Description

MULTI-STAGE VACUUM PUMP WITH IMPROVED LOW VACUUM PRESSURE PERFORMANCE

TECHNICAL FIELD

This disclosure relates to a multi-stage vacuum pump. This disclosure also relates to a method of assembling the multi-stage vacuum pump.

BACKGROUND

Vacuum systems commonly utilise pumps in order to evacuate gases from the system One type of vacuum pump used in such systems is a Roots vacuum pump A Roots vacuum pump generally includes two counter-rotating shafts with rotors mounted on each shaft. The rotors include a series of lobes and recesses defined between the lobes. The rotors are mounted such that a lobe of a rotor on one shaft cooperates with a corresponding recess of a rotor on the other shaft. As the shafts and rotors rotate, gas is trapped and compressed between the cooperating lobes and the recesses. The repeated trapping and compression of gas between the rotors can generate a pumping action that can be used to pump gas from an inlet on one side of the rotors to an outlet on the opposite side to evacuate gas from a system.

It is common for Roots vacuum pumps to feature a plurality of stages of cooperating rotors, with each stage being axially spaced apart along the shafts and separated by a stator structure that defines a series of chambers of decreasing volume that house the rotors of each stage. By having multiple stages of decreasing volume, progressive increases of gas compression can occur across the pump, allowing it to provide a higher degree vacuum for the system in an efficient manner.

Clearances are defined between the rotors and walls of the chambers in each stage of the vacuum pump. The clearances can differ in each stage depending on a variety of factors, such as rotor assembly positioning/calibration during assembly, manufacturing tolerances, and thermal expansion during operation.

When the vacuum pump is first assembled, the rotor assembly will be positioned to provide a set axial clearance between a rotor and a wall of one of the chambers. This is known as "setting off' the rotor assembly. The set axial clearance is known as the "set off distance", which is the minimum axial distance present between any of the rotors and chamber walls in the pump after assembly of -2 -the pump. The chamber wall from which the rotor assembly is "set off' is known as the "set face" or "set side wall" of the chamber.

The set off distance will define the total clearance available between the rotors and the chambers of the pump stages during operation. In particular, it will dictate the axial clearance available between the rotor and the opposing side wall of the chamber. This clearance typically allows for thermal expansion of the rotor assembly in the axial direction during operation of the pump, and so the opposing chamber wall to which it is defined may be known as the "expansion side wall".

Typically, only one stage in a multi-stage vacuum pump will have a clearance equal to the set off distance. Manufacturing variability makes it virtually impossible to guarantee that more than one stage has the same clearance from the set side wall of the chambers. The designer of a multi-stage vacuum pump can choose which of its stages becomes the datum for the setting off process.

Historically, the set off distance between the rotors and chambers has been defined between the rotor and the chamber in the outlet stage (or low vacuum end) of the pump. It has also been the historical norm to position the outlet stage closer to the fixed side of the rotor assembly and the inlet stage closer to the expanding side of the rotor assembly.

It has been found that this can lead to increases in clearances (and the tolerances thereof) between the rotors and chambers in the inlet or high vacuum stages that can have negative implications on the performance and efficiency of the pump for particular vacuum pressure regimes.

Accordingly, there is a need to provide a multi-stage vacuum pump that enables improvements in performance and efficiency for different target vacuum pressure ranges.

Although the present disclosure is exemplified with regards to a multi-stage Roots vacuum pump, it is to be appreciated that this disclosure and its benefits are applicable and extend to any other suitable type of multi-stage vacuum pump that includes a plurality of rotors interacting within a plurality of chambers in a stator.

Such types of pump include, amongst others, claw type vacuum pumps with two or more stages. In some such pumps, different types of rotors can be combined in the same pump, so the pump is a mixture of types. For example, some stages in the pump may be of a Roots type and other stages in the pump may be of a claw type.

SUMMARY -3 -

From one aspect, the present disclosure provides a multi-stage vacuum pump in accordance with claim 1.

This arrangement will place the inlet stage a closer axial distance to the fixed bearing than in historical designs, which permits tighter control of the total clearance in the inlet stage. This can reduce gas leakage and backflow in the inlet stage of the pump to provide improved pumping performance and efficiency therein. This results in more efficient pump operation at low (or "rough") vacuum pressure ranges (e.g., above 10 mbar, such as between 50 to 1,000 mbar).

The inlet stage is the pump stage that includes the inlet for the pump and has the largest volume chamber of the pump stages. The outlet stage is the pump stage that includes the outlet for the pump and has the smallest volume chamber of the pump stages.

In embodiments, there may be any suitable number of intervening stages (i.e., one or more) fluidly connected to the inlet and outlet stages. Such intervening stages will have chamber volumes between that of the inlet and outlet stage, and will be fluidly connected in order of progressively decreasing chamber volume between the inlet and outlet stage.

The outlet stage and intervening stage(s) Of present) are the "other pump stages".

In an embodiment of the above, the multi-stage vacuum pump includes the features of claim 2.

When the pump stages are provided in between axially opposed fixed and moveable bearings, this arrangement will allow the aforementioned advantages to be realised.

This distinguished from an alternative "cantilevered" pump arrangement, where the pump stages and the moveable bearing are positioned on opposite sides of the fixed bearing. It will be appreciated that in such an arrangement the inlet stage will be positioned closer to the fixed bearing than the movable bearing in all positions compared to the other pump stages.

In a further embodiment, the multi-stage vacuum pump includes the features of claim 3.

In these embodiments, the inlet stage will be a closer distance to the fixed bearing than the outlet stage and any intervening pump stages Cif present). This will permit the clearance tolerances to be more tightly controlled in the inlet stage to provide larger pumping efficiency improvements therein. -4 -

In an alternative embodiment, the inlet stage is positioned closer to the fixed bearing than at least two of the other pump stages (such as the outlet stage and at least one intervening stage).

In an alternative embodiment, the multi-stage vacuum pump includes the features of claim 4.

In this embodiment, the outlet stage will be a closer distance to the fixed bearing than the inlet stage, but the inlet stage will still be closer to the fixed bearing than in historical designs. This will permit a compromise of improved clearance tolerances and pumping efficiency between both the inlet and outlet stages.

In a further embodiment, the multi-stage vacuum pump includes the features of claim 5.

In this manner, when present, there will be no intervening stages positioned between (i.e., axially separating) the inlet stage and the outlet stage.

Again, compared to historical designs, this can permit a compromise of improved clearance tolerances and pumping efficiency between both the inlet and outlet stages. It also means heat generated in the outlet stage during operation can be better distributed to the inlet stage that is relatively cooler during operation This may advantageously reduce temperature gradients across the pump during operation.

Alternatively, in other embodiments, the one or more intervening pump stages are positioned axially between the inlet and outlet stages in a more traditional manner.

In a further embodiment of any of the above, the multi-stage vacuum pump includes the features of claim 6.

From another aspect, the present disclosure provides a multi-stage vacuum pump in accordance with claim 8.

In both of the above, the set axial clearance (i.e., "set off distance") is defined during assembly and is the minimum axial clearance set for the rotor assembly (i.e., the minimum axial clearance between a chamber set side wall and rotor in the pump). It dictates the amount of clearance left between the rotor and the expansion side wall of the chamber available during operation. This clearance is necessary to allow for thermal expansion of the rotor assembly during operation, but if it is too large it may promote gas leakage/backflow that can cause inefficiencies in the pump stage. -5 -

Setting this axial clearance at the inlet stage can improve the tolerance of the expansion clearance therein compared to historical designs. This means excess clearance in the stage (beyond what is required to account for thermal expansion) can be reduced to reduce gas leakage/backflow effects in the inlet stage. This can further improve the pump efficiency in the inlet stage and operation at low (or "rough") vacuum pressure ranges (e.g., above 10 mbar, such as between 50 to 1,000 mbar).

In a further embodiment of both of the above, the set axial clearance (i.e., "set off distance") is between 7 to 1,000 pm, for example, between 30 to 150 pm. In one particular example, it is between 30 to 80 pm.

These values are set axial clearance ranges that have generally been found to be suitable across different vacuum pump applications and sizes.

In a further embodiment of the above aspect, the multi-stage vacuum pump includes the features of claim 10. In one particular example, the inlet stage is positioned further from the fixed bearing than any of the other pump stages.

Although positioning the inlet stage further from the fixed bearing can negatively increase the tolerance of the expansion clearances left therein, the setting off of the inlet stage still provides improvements therein compared to historical designs. It may also do so with less redesigning of the pump (e.g., the stage order and/or bearing arrangements) being needed.

In an alternative further embodiment, the multi-stage vacuum pump includes the features of claim 11. In one particular example, the inlet stage is positioned closer to the fixed bearing than any of the other pump stages.

This embodiment combines the backflow benefits of defining the set off distance at the inlet stage with those of the inlet stage being positioned closer to the fixed bearing discussed above. This may require more modification of historical vacuum pump designs, but will provide the further improvements to the low vacuum performance thereof.

In a further embodiment of any of the above aspects or their embodiments, the multi-stage vacuum pump includes the features of claim 12.

In a further embodiment of any of the above aspects or their embodiments, the multi-stage vacuum pump includes the features of 13.

In this manner, the fixed bearing and/or stop can be conveniently used to vary the set axial clearance during assembly of the pump. -6 -

Alternatively, the fixed bearing can remain fixed in one position and other means, such as shims, can be used to move rotor assembly to different fixed axial positions during assembly.

In a further embodiment of any of the above aspects or their embodiments, the multi-stage vacuum pump includes the features of claim 14.

The biasing force will resist the movement of the rotor assembly to help absorb the movement from thermal growth thereof.

The biasing force can be provided in any suitable manner. For example, by a biasing member attached between the moveable bearing and a fixed stop positioned a predetermined axial distance away. The biasing member can be any suitable member for generating an appropriate biasing force, such as a spring or resilient member.

In a further embodiment of any of the above aspects or their embodiments, the multi-stage vacuum pump includes the features of claim 15.

In this manner, the fixed and moveable bearings are axially outboard of the pump stages, and thus define opposing extremities of the pump. The pump stages will be positioned between the opposing ends.

In alternatively embodiments, the fixed and moveable bearings can be provided at any suitable position along the shaft. For example, as mentioned above, the pump stages may be "cantilevered" from the fixed bearing. In such an arrangement the pump stages and the moveable bearing are positioned on opposite axial sides of the fixed bearing.

It is to be understood that within the scope of this disclosure, the multi-stage vacuum pump can be a Roots pump, or any other suitable type of pump, such as claw pump, or a mixture of such types (i.e., combining pump stages of different types in the same pump).

As will be appreciated by the skilled person, vacuum pumps according to the present disclosure include two of the rotor assemblies adjacent each other, with respective rotors of each rotor assembly interacting within the stator chambers to provide the pump stages.

Although certain advantages have been discussed in relation to certain features above, other advantages of certain features may become apparent to the skilled person following the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS -7 -

One or more non-limiting embodiments according to the present disclosure will now be described, with reference to the accompanying figures in which: Figure 1 shows a cross-sectional view of a known multi-stage vacuum pump; Figure 2 shows a cross-sectional view of one example of a multi-stage vacuum pump in accordance with the present disclosure; Figure 3A shows a graph comparing simulations of pump inlet pressure vs pumping speed for the vacuum pumps of Figures 1 and 2; Figure 3B shows a graph comparing simulations of pump inlet pressure vs pumping efficiency for the vacuum pumps of Figures 1 and 2; Figure 4 shows a cross-sectional view of another example of a multi-stage vacuum pump in accordance with the present disclosure; Figure 5 shows yet another example of a multi-stage vacuum pump in accordance with the present disclosure.

DETAILED DESCRIPTION

With reference to Figure 1, a known multi-stage vacuum pump 100 is shown in cross-section. The vacuum pump 100 includes a stator assembly 102 and a rotor assembly 110.

The rotor assembly 110 comprises a series of rotors 112a-112f that are mounted to a shaft 114 that extends axially along a central axis X-X. The rotors 112a-112f are spaced axially apart along the shaft 114 and protrude radially therefrom.

The rotors 112a-112f may be integrally formed with the shaft 114 (e.g., via casting) or may be formed and connected thereto separately (e.g., via welding).

Moreover, although a single shaft 114 is shown, separate sections of shaft 114 may be provided and connected together instead. Such separate sections may include one or more rotors 112a-112f attached thereto.

Each rotor 112a-112f is housed within a respective chamber 104a-104f defined in the stator assembly 102. Each respective rotor 112a-112f and chamber 104a-104f combination defines a stage 100a-100f of the vacuum pump 100. In particular, an inlet stage 100a and an outlet stage 100f fluidly connected by a plurality of intervening stages 100b-100e.

Although only one rotor assembly 110 is shown in the sectional view of Fig. 1, it will be understood by the skilled person that a second cooperating rotor -8 -assembly is positioned adjacent the rotor assembly 110 within the stator assembly 102 and will interact therewith to provide the vacuum pump 100. The depicted example is a Roots vacuum pump, and so the rotors 112a-112f will include a series of interacting lobes and recesses (not shown) as discussed above. Nonetheless, as discussed above, in other examples within the scope of this disclosure, the rotors 112a-112f and/or rotor assembly 110 may include different geometries (e.g., claw type etc.) depending on the type of multi-stage vacuum pump that is implemented.

The chambers 104a-104f are arranged serially between an inlet 106 and an outlet 108. The inlet 106 and outlet 108 can be formed by openings or ports defined through the stator assembly 102.

As is known, the chambers 104a-104f are in fluid communication with each other via passages (not shown) defined through the stator assembly 102. The passages may be defined in any suitable manner, such as by being drilled through the stator assembly 102, cast into the stator assembly 102 or provided by ducts or pipes extending between chambers 104a-104f. This provides a path for a gas flow G to pass from the inlet 106 (or high vacuum side of the pump 100) to the outlet 108 (or low vacuum side of the pump 100) via each stage 100a-100f of the vacuum pump 100.

As will be appreciated, this configuration provides a multi-stage vacuum pump 100 that can evacuate a system or space by ingesting gas at the inlet 106 and progressively compressing the gas through the pump stages 100a-100f (by rotating the shaft 114 and rotors 112a-112f within the chambers 104a-104f) before the gas is exhausted at the outlet 108. The rotor assembly 110 is rotated during operation by any suitable means, for example, shaft 114 can be operatively connected to a motor (not shown).

The chambers 104a-104f progressively decrease in volume as gas is progressively compressed to a greater extent in each subsequent stage 100a-100f. Accordingly, a larger volume chamber provides a so-called 'higher vacuum' stage than that of an adjacent chamber of smaller volume downstream thereof, which is thus a so-called 'lower vacuum' stage.

The shaft 114 is supported for rotation by first and second bearings 120, 130. The first bearing 120 is disposed adjacent and axially outboard of the inlet 106 relative to central axis X-X. The second bearing 130 is disposed adjacent and axially outboard of the outlet 108 relative to central axis X-X. -9 -

The bearings 120, 130 are operatively connected to the rotor assembly 110 such that movement of the rotor assembly 110 during operation (e.g., axial movement or growth) is transferred to the bearings 120, 130. This operative connection can be provided in any suitable manner, e.g., via a tube or sleeve (not shown) around the shaft 114 rigidly connecting the bearings 120, 130 to the rotor assembly 110 and/or a firm fit being provided between the shaft 114 and inner bearing races.

In the depicted example, the bearings 120, 130 are ball bearings, and opposing axial ends 114a, 114b of the shaft 114 pass through respective apertures defined by inner races of the bearings 120, 130, whilst outer races of the bearings 120, 130 are connected to the stator assembly 102.

The first bearing 120 is a moveable bearing 120 that is axially moveable relative to the stator assembly 102 to allow axial expansion of the rotor assembly 110 during operation.

In the depicted embodiment, the moveable bearing 120 is supported against a first stop 122 attached to the stator assembly 102. In the depicted embodiment, a biasing member 124 is attached between the bearing 120 and the stop 122, and provides a biasing force that opposes movement of the first bearing 120. Accordingly, the axial movement of bearing 120 and rotor assembly is absorbed and resisted by the biasing member 124. The biasing force may be provided by any suitable biasing member 124, such as a spring or other resilient member.

In contrast to the first bearing 120, the second bearing 130 is a fixed bearing 130 that is fixed in axial position relative to the stator assembly 102 to react against the axial expansion of the rotor assembly 110 during operation.

In the depicted embodiment, the fixed bearing 130 is supported directly against (i.e., in direct contact with) a second stop 132 that is fixedly attached to the stator assembly 102. Accordingly, the axial movement of bearing 130 is restricted to a fixed axial position limited by the stop 132.

The fixed axial position can be varied by the stop 132 being moved axially and locked back in place. In the depicted example, this is achieved by the stop 132 being threadably engaged with the stator assembly 102, such that rotation of the stop 132 advances the stop 132 a desired amount in the axial direction to provide different fixed axial positions for the bearing 130. In this manner, the stop 132 may be provided in the form of a threaded nut.

-10 -Nonetheless, within the scope of this disclosure any other suitable configuration and/or mechanism for achieving the provision of different fixed positions for the rotor assembly 110 can be used, for example, by positioning shims between the fixed bearing 130 and the rotor assembly 110.

Although one particular example of providing a moveable bearing 120 and fixed bearing 130 have been described, it is to be understood that any other suitable method of providing bearings 120, 130 with such functionality is envisaged within the scope of this disclosure. This includes alternative arrangements to those depicted, in which the pumping stages are "cantilevered" from the fixed bearing (i.e. the pump stages and the moveable bearing are on opposite sides of the fixed bearing).

Each chamber 104a-104f defines a set side wall (or face) 105a-105f closest to the fixed bearing 130 and an expansion side wall (or face) 107a-107f opposite the set side wall (or face) 105a-105f. The expansion side wall 107a-107f is so called because it is the wall of the chambers 104a-104f towards which the rotor assembly 110 will expand owing to thermal effects during operation. In this way, each rotor 112a-112f is disposed in its respective chamber 104a-104f axially between the set and expansion side walls 105a-105f, 107a-107f thereof.

Clearances Ca-Cf are defined axially between the set side walls 105a-105f of the chambers 104a-104f and the rotors 112a-112f therein. The size of the set side clearances Ca-Cf will also define the amount of clearance left on the opposing expansion side of the chambers 104a-104f (i.e., the clearance between the expansion side walls 107a-107f and the rotors 112a-112f). The expansion side clearances are necessary to allow thermal expansion of the rotor assembly 110 during operation without the rotors 112a-112f making contact with the chamber walls/stator assembly 102.

When the pump 100 is assembled, the rotor assembly 110 is positioned and secured within the stator assembly 102. In one example, a series of portions of the stator assembly 102 may be slid over the rotor assembly 110 and stacked on top of each other to surround the rotors 112a-112f, and then secured together. In another example, the stator assembly 102 may be defined by two stator clamshell halves that are positioned around the rotors 112a-112f and secured together.

In the depicted example, during assembly, the rotor assembly 110 is positioned such that the rotor 112f is stacked or placed in contact against the set side wall 105f of the outlet chamber 104f. The stator and rotor assemblies 102, 110 are designed so that each of the other rotors 112a-112e have some (small) clearance left between them and their corresponding set side walls 105a-105e to facilitate assembly.

In order to tune the expansion side clearance that is left between the rotors 112a-112f and the chambers 104a-104f for operation, the rotor 112f is then moved apart from the set side wall 105f to provide a set axial clearance (i.e., a desired minimum clearance) Cf between the rotor 112f and the wall 105f. In the depicted embodiment, this is achieved by stop 132 being moved axially.

This procedure of moving the rotor assembly 110 relative to the stator assembly 102 is known as "setting off". Accordingly, in Fig. 1, the rotor assembly is said to be "set-off' relative to the outlet stage 100f of the pump 100 with the wall 105f providing a so-called "set face" and the set axial clearance Cf providing a "set off distance".

Once the pump is set in this manner, each of the other clearances Ca-Ce will be larger than Cf. This variation in clearance can be particularly pronounced for clearances Ca-Cb in stages 100a-100b compared to clearances Ce-Cf in stages 100e-100f.

These increased clearances can provide a larger cross-sectional area for the back-flow of gas in a pump stage. Back-flow gases are generated due to higher pressure gas leaking upstream across a pump stage (e.g., from a pump stage outlet to pump stage inlet) and/or to an adjacent pump stage containing lower pressure gas (e.g., see gas leakage path L). The accumulation of such back-flow gas in a particular stage can negatively affect the throughput of gas being compressed through that stage and thus negatively affect the performance and efficiency of the pump 100.

It has been found that setting the rotor assembly 110 off the set side wall 105f as in Fig. 1 causes the pump 100 to act more efficiently when it is being driven to maintain higher vacuum levels in a system (e.g., less than 10 mbar, such as between 1.0 to 0.1 mbar) and less efficiently when it is being driven to maintain lower vacuum levels in a system (e.g., above 10 mbar, such as between 50 to 1,000 mbar). This is thought to be due to the setting off procedure providing tighter clearance control in the downstream-most stages (e.g., stages 100e-100f) reducing back-flow accumulation therein and improving their gas throughput compared to the stages upstream thereof (e.g., stages 100a-100b).

-12 -Although these lower vacuum pressure range inefficiencies may be acceptable for a pump 100 that is primarily intended to operate to maintain higher vacuum pressures, it does not make the pump 100 that useful for applications that require the maintenance of lower (or "rougher") vacuum pressure ranges (e.g., 50 to 1,000 mbar or more specifically 100 to 500 mbar).

As will be appreciated by the skilled person, there are numerous examples of such applications, which can include conveying, lifting, steel degassing, medical applications and chemical/pharmaceutical manufacturing.

It would therefore be advantageous if pump 100 could be adapted to improve its efficiency and performance at these lower vacuum pressure ranges to enhance is usage in such applications.

With reference to Figure 2, a multi-stage vacuum pump 200 in accordance with one embodiment of the present disclosure is shown in cross-section. As explained below, the pump 200 is adapted to improve its low vacuum pressure range efficiency and performance compared to pump 100.

The pump 200 is assembled in the same manner and includes many of the same features as pump 100, and these same features are denoted with the same numerals as in Fig. 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Fig. 1 and so will not be repeated.

In contrast to pump 100, in pump 200 the fixed bearing 130 is disposed adjacent and axially outboard of the inlet 106 rather than the outlet 108, and the moveable bearing 120 is disposed adjacent and axially outboard of the outlet 108 instead of the inlet 106. In this manner, the inlet stage is positioned closer to the fixed bearing 130 than the moveable bearing 120, and in particular, is now positioned closer to the fixed bearing 130 than any of the other pump stages 100b-100f.

In further contrast to pump 100, in pump 200 the rotor assembly 110 is setoff against the set side wall 105a of the inlet stage 100a (i.e., the pump stage that includes the inlet 106 of the pump 100 and the largest volume chamber 104a) instead of the set side wall 105f of the outlet stage 100f.

Accordingly, the set axial clearance is defined by the clearance Ca between the set side wall 105a and the rotor 112a. The clearances Cb-Cf therefore tend to become less tightly controlled progressively downstream in the pump 200.

-13 -Due to the rearrangement of the inlet stage 100a closer to the fixed bearing 130 and the setting off of the rotor assembly 110 relative to the set side wall 105a of the inlet stage 100a, the expansion clearances in the upstream stages (e.g., stages 100a-100b) are more tightly controlled than those in the downstream stages (e.g., stages 100e-100f). Accordingly, the backflow accumulation in these upstream stages is reduced during operation, and thus the throughput, efficiency and performance thereof is improved compared to that of pump 100 at higher pressures.

The set axial clearance (i.e., set off distance) Ca can be varied to suit the particular pump application and operating requirements. In the depicted example, the clearance Ca is set to a minimum of 50 pm; however, in other examples it may be set to between 30 to 80 pm, or 30 to 150 pm, or more widely between 7 to 1,000 pm (e.g., depending on the application and pump size).

Although the depicted pump 200 shows the inlet stage 100a being positioned closer to the fixed bearing 130 than any of the other pump stages 100b- 100f (as will be better appreciated from Fig. 5 discussed below) improved clearance control and low vacuum pressure performance can be realised as long as the inlet stage 100a is positioned closer to the fixed bearing 130 than at least one of the other pump stages 100b-100f present in the pump 200.

Moreover, although the feature of moving the inlet stage 100a closer to the fixed bearing 130 compared to pump 100 has been combined with the feature of setting off the rotor assembly 110 in the inlet stage 100a in pump 200, the two provide different ways of improving clearance control and low vacuum pressure performance that can be provided independently of each other. Thus, in addition to being combined in the same pump 200, in other embodiments (and as discussed further below in relation to Fig. 4), only one need be employed in a given pump to achieve the aforementioned advantages.

Figures 3A and 3B show the different operating characteristics for the pumps 100 and 200 taken from computer simulations thereof (as well as pump 300 discussed below in relation to Fig. 4). The X-axis in both Figs 3A and 3B refers to a logarithmic scale of the vacuum pressure at the pump inlet 106. This corresponds to the vacuum pressure for the system that the pump is operating to achieve/maintain. In Fig. 3A, the Y-axis refers to the relative pumping speed or flow rate of gas passing through the pumps (in m3/hr). In Fig. 3B, the Y-axis refers to the relative amount of power used per unit gas flow rate (in W/(m3/hr), which can be -14 -used to indicate a relative efficiency of the pump (i.e., a lower value is more efficient).

With reference to Fig. 3A, it can be seen that pump 200 has improved flow rate performance at lower vacuum pressure ranges 10 to 1,000 mbar compared to pump 100. With reference to Fig. 3B, it can also be seen that pump 200 provides a particular improvement in efficiency at lower vacuum pressure ranges 50 to 500 mbar compared to pump 100.

As discussed above, this makes pump 200 advantageously suited to a variety of vacuum pump applications for which pump 100 was not. This can lead to improved performance and cost benefits by utilising pump 200 in such applications.

With reference to Figure 4, a multi-stage vacuum pump 300 in accordance with another embodiment of the present disclosure is shown in cross-section.

Again, the pump 300 is assembled in the same manner and includes many of the same features as pump 100. These same features are denoted with the same numerals as in Fig. 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Fig. 1 and so will not be repeated.

In contrast to pump 200, pump 300 provides the same configuration of bearings 120, 130 as pump 100; however, the rotor assembly 100 is instead set off against the set side wall 105a of the inlet stage 100a. In this manner, the set axial clearance Ca is provided in the inlet stage 100a.

Although setting off the rotor assembly 110 relative to the wall 105a in this manner improves the control of clearances in the upstream stages (e.g., stages 100a-100b) in the pump 300 compared to pump 100, it does not do so to the same extent as in pump 200. This is because the inlet stage 100a is in closer proximity to the moveable bearing 120 and so total control of the clearances therein are more limited. This does, however, also mean that pump 300 does not reduce control of the clearances in the downstream stages (e.g., stages 100e-100f) as much as pump 200 either.

As shown by comparison in Figs 3A and 3B, this results in pump 300 providing a 'half-way house', by providing a milder improvement in lower vacuum pressure range performance, but with less of a negative impact on higher vacuum pressure range performance compared to pump 200. Although the performance improvement is lower, it will be appreciated that it is realised with fewer structural modifications to the pump 100 design, which can have cost benefits.

-15 -Wth reference to Figure 5, a multi-stage vacuum pump 400 in accordance with another embodiment of the present disclosure is shown in cross-section.

Again, the pump 400 is assembled in the same manner and includes many of the same features as pump 100. These same features are denoted with the same numerals as in Fig. 1. Unless otherwise stated below, the same method steps are implemented and the same features function and interact with each other as discussed above with regard to Fig. 1, and so will not be repeated.

Pump 400 has the same configuration as pump 100 except that the inlet stage 100a has been rearranged to be axially closer (i.e., closer in positional stage order) to the outlet stage 100f. In the depicted example, the inlet stage 100a has been rearranged to be positioned directly adjacent to the outlet stage 100f such that it is the stage closest to the outlet stage 100f (i.e., with no other pump stages 100b-100e positioned between them). This also results in the outlet stage 100f being positioned closer to the fixed bearing 130 than the inlet stage 100a, but it will be appreciated that the inlet stage 100a is still positioned closer to the fixed bearing than (at least one of) the other pump stages 100b-100e.

In this manner, the positional stage order for the pump 400 from left to right in Fig. 5 is 100b, 100c, 100d, 100e, 100a, 100f. However, the gas flow G through the pump 400 still follows the path from the inlet 106 to the outlet 108 via each stage 100a-100f in series. In other words, stages 100a-f are all still serially fluidly connected.

In the depicted embodiment, the inlet stage 100a is positioned axially inboard of the outlet stage 100f (i.e., between stages 100e and 100f); however, in another example it could be positioned axially outboard (to the right in Fig. 5) of the outlet stage 100f. In other examples, the outlet stage 100f may be repositioned relative to the inlet stage 100a instead.

It will be appreciated that the rearrangement of stage 100a will necessitate a longer and/or more complicated gas flow path (shown schematically in a dotted line) between the outlet of stage 100a and the inlet of stage 100b, as well as between the outlet of stage 100e and the inlet of stage 100f. This path can be provided in any suitable manner, such as by drilling or casting an appropriate fluid passage in the stator assembly 102 or providing ducting or piping between the stages.

The rotor assembly 110 is set off the set side wall 105a of the inlet stage 100a to provide the set axial clearance Ca. However, owing to the outlet stage 100f -16 -being axially closer to the inlet stage 100a, the control of clearances in the outlet stage 100f is improved compared to that of pumps 200, 300. This can provide a reduction in back flow accumulation for both the lower and higher vacuum stages 100a, 100f to provide the pump 400 with more optimal performance and efficiency across both lower and higher vacuum ranges.

Although this improvement in performance and efficiency across a broad range of vacuum pressures is advantageous for many applications, it will need to be balanced with the increases in costs and manufacture time associated with providing the aforementioned more complicated gas flow paths between stages.

Although inlet stage 100a and outlet stage 100f have been placed directly adjacent each other in the depicted embodiment, it should be understood that the same advantages (albeit to a lesser extent) can be achieved by positioning inlet stage 100a in a different position that is closer to the fixed bearing 130. For example, by placing inlet stage 100a axially in between other intervening stages (e.g., stages 100d and 100e). In another example embodiment, the rotor assembly could be set off from the set side wall 105f of the outlet stage 100f instead, and an improvement in low vacuum pressure performance of pump 400 would still be found owing to the closer proximity of the inlet stage 100a to the fixed bearing 130 compared to historical designs.

Accordingly, it should be understood a variety of rearrangements of the inlet stage 100a relative to the outlet stage 100f or vice versa can be made within the scope of this disclosure.

Although the depicted pumps illustrate six pump stages, it should be understood the present disclosure and its advantages apply to a multi-stage vacuum pumps with any number of stages of two and over. In other words, pumps having at least an inlet stage and an outlet stage. In further examples, the pumps can feature one or more intervening stages fluidly connected between the inlet and outlet stages.

LIST OF REFERENCE NUMERALS

A list of reference numerals used in the accompanying Figures 1 to 5 is provided for ease of reference: Multi-stage vacuum pump -17 - 100a-100f Pump stages 102 Stator assembly 104a-104f Chambers 105a-105f Set side chamber walls 106 Inlet 107a-107f Expansion side chamber walls 108 Outlet Rotor assembly 112a-112f Rotors 114 Shaft 114a,114b Opposing shaft ends First ('moveable') bearing 122 Stop 124 Biasing member Second ('fixed') bearing 132 Stop Pump 300 Pump 400 Pump Ca-Cf Clearances Gas flow path Gas leakage path X-X Shaft central axis

Claims

-18 -CLAIMS1. A multi-stage vacuum pump comprising: a stator assembly defining at least two chambers; a rotor assembly having at least two rotors housed in respective ones of the chambers to define at least two pump stages, a shaft rotatable about a central axis and on which the rotors are mounted, and a first and second bearing supporting the shaft for rotation relative to the stator assembly, wherein: the at least two pump stages provide at least an inlet stage and an outlet stage; the first bearing is a moveable bearing that is axially moveable relative to the stator assembly to allow axial expansion of the rotor assembly during operation; the second bearing is a fixed bearing that is fixed in axial position relative to the stator assembly to react against the axial expansion of the rotor assembly during operation; and the inlet stage is positioned closer to the fixed bearing than at least one of the other pump stages.
2. The multi-stage vacuum pump of claim 1, wherein the inlet stage is positioned closer to the fixed bearing than the moveable bearing.
3. The multi-stage vacuum pump of claim 1 or 2, wherein the inlet stage is positioned closer to the fixed bearing than any of the other pump stages.
4. The multi-stage vacuum pump of claim 1 or 2, wherein the outlet stage is positioned closer to the fixed bearing than the inlet stage.
5. The multi-stage vacuum pump of any preceding claim, wherein the inlet stage is positioned directly adjacent to the outlet stage such that there are no other pump stages positioned between them.
6. The multi-stage vacuum pump of any preceding claim, wherein the chamber of each pump stage defines a set side wall closest to the fixed bearing and an expansion side wall opposite the set side wall, and the rotor of each pump stage is disposed between the set side wall and the expansion side wall; and -19 -wherein a set axial clearance for the rotor assembly is defined between the set side wall and the rotor of the inlet stage.
7. The multi-stage vacuum pump of claim 6, wherein the set axial clearance is between 7 to 1,000 pm, for example, 30 to 150 pm or 30 to 80 pm
8. A multi-stage vacuum pump comprising: a stator assembly defining at least two chambers; a rotor assembly having at least two rotors housed in respective ones of the chambers to define at least two pump stages, a shaft rotatable about a central axis and on which the rotors are mounted, and a first and second bearing supporting the shaft for rotation relative to the stator assembly, wherein: the at least two pump stages provide at least an inlet stage and an outlet stage; the first bearing is a moveable bearing that is axially moveable relative to the stator assembly to allow axial expansion of the rotor assembly during operation; the second bearing is a fixed bearing that is fixed in axial position relative to the stator assembly to react against the axial expansion of the rotor assembly during operation; the chamber of each pump stage defines a set side wall closest to the fixed bearing and an expansion side wall opposite the set side wall, and the rotor of each pump stage is disposed between the set side wall and the expansion side wall; and a set axial clearance for the rotor assembly is defined between the set side wall and the rotor of the inlet stage.
9. The multi-stage vacuum pump of claim 8, wherein the set axial clearance is between 7 to 1,000pm, for example, 30 to 150pm or 30 to 80 pm.
10. The multi-stage vacuum pump of claim 8 or 9, wherein the inlet stage is positioned further from the fixed bearing than at least one other pump stage, for example, further from the fixed bearing than any of the other pump stages.
11. The multi-stage vacuum pump of claim 8 or 9, wherein the inlet stage is positioned closer to the fixed bearing than at least one of the other pump stages, for example, closer to the fixed bearing than any of the other pump stages.-20 -
12. The multi-stage vacuum pump of any preceding claim, wherein the fixed bearing is movable between different fixed axial positions relative to the stator assembly.
13. The multi-stage vacuum pump of claim 12, wherein the fixed bearing includes a stop that is fixedly attached to the stator assembly and moveable to the different fixed axial positions, for example, by being threadably engaged with the stator assembly.
14. The multi-stage vacuum pump of any preceding claim, wherein the moveable bearing is slideably engaged with the stator assembly, and axial movement of the moveable bearing is opposed by a biasing force.
15. The multi-stage vacuum pump of any preceding claim, wherein the shaft defines axially opposed ends of the rotor assembly, and each opposed end is supported by a respective one of the fixed and moveable bearings.