TWIN-SHAFT PUMPS

Abstract

A twin-shaft pump comprising: a pumping chamber; two rotatable shafts each mounted on bearings is disclosed. Each of the two rotatable shafts comprises at least one rotor element, the rotor elements being within the pumping chamber and the two rotatable shafts extending beyond the pumping chamber to a support member. The support member comprises mounting means for mounting the bearings at a predetermined distance from each other, the predetermined distance defining a distance between the two shafts. A thermal break between the pumping chamber and the support member is provided for impeding thermal conductivity between the pumping chamber and the support member, such that the pumping chamber and support member can be maintained at different temperatures. The support member and the rotor elements are formed of different materials, a coefficient of thermal expansion of a material forming the support member being higher than a coefficient of thermal expansion of a material forming the rotor elements.

Claims

1. A twin-shaft pump comprising: a pumping chamber; two rotatable shafts each mounted on bearings; each of the two rotatable shafts comprising at least one rotor element, the rotor elements being within the pumping chamber and the two rotatable shafts extending beyond the pumping chamber to a support member; the support member comprising mounting means for mounting the bearings at a predetermined distance from each other, the predetermined distance defining a distance between the two rotatable shafts; and at least one thermal path along structural elements connecting the pumping chamber and the mounting means; a thermal break in at least one of the at least one thermal path for impeding thermal conductivity between the pumping chamber and the mounting means, such that the pumping chamber and mounting means can be maintained at different temperatures; the thermal break comprising a portion of the thermal path where at least one physical property is different to a physical property of an adjoining portion of the thermal path such that thermal conductance of the thermal break portion is more than 20% lower than the thermal conductance of an equivalent thermal path length of the adjoining portion; wherein the thermal break comprises a hollow portion of each of the rotatable shafts between the pumping chamber and the bearing.

2. The twin-shaft pump according to claim 1, wherein the support member and the rotor elements are formed of different materials, a coefficient of thermal expansion of a material forming the support member being higher than a coefficient of thermal expansion of a material forming the rotor elements.

3. The twin-shaft pump according to claim 2, wherein the coefficient of thermal expansion of the material forming the support member is more than a third higher than the coefficient of thermal expansion of the material forming the rotor elements.

4. The twin-shaft pump according to claim 2, wherein the coefficient of thermal expansion of the material forming the support member is more than twice as high as the coefficient of thermal expansion of the material forming the rotor elements.

5. The twin-shaft pump according to claim 1, wherein the support member comprises a headplate of the twin-shaft pump.

6. The twin-shaft pump according to claim 1, comprising a further thermal break, the further thermal break comprising a gap between the mounting means and an end wall of the pumping chamber.

7. The twin-shaft pump according to claim 1, wherein the thermal break in the at least one thermal path comprises at least one of: a material of a lower thermal conductivity than a material forming an adjoining portion of the thermal path and a portion of the structural element that is hollow.

8. The twin-shaft pump according to claim 7, wherein the thermal break comprises the material of lower thermal conductivity, the material comprising a ceramic.

9. The twin-shaft pump according to claim 8, wherein the thermal break comprises ceramic separators between the mounting means and the pumping chamber.

10. The twin-shaft pump according to claim 7, wherein the thermal break comprises a portion of each of the rotatable shafts between the pumping chamber and the bearing being formed of a material of a lower thermal conductivity than a rest of each of the rotatable shafts.

11. The twin-shaft pump according to claim 1, the pump further comprising temperature control means for controlling a temperature of the support member.

12. The twin-shaft pump according to claim 11, the temperature control means being operable to control the temperature of the support member in dependence upon a temperature of the pumping chamber and a ratio of said coefficients of thermal expansion of the material forming the support member and the material forming the rotor elements, the temperature of the support member being controlled to provide an expansion of the rotor elements within the pumping chamber that is substantially the same as an expansion of the support member.

13. The twin-shaft pump according to claim 1, wherein the bearings comprise rolling elements within a housing.

14. The twin-shaft pump according to claim 1, further comprising a means for supplying a flow of oil sufficient to lubricate and cool the bearings.

15. The twin-shaft pump according to claim 1, wherein the mounting means comprises recesses in the support member in which the bearings are mounted.

16. The twin-shaft pump according to claim 1, wherein the mounting means comprise housings extending from the support member at a far side of the support member from the pumping chamber, the housings being configured to house the bearings, and wherein the housings are separated from the support member by low thermal conductivity separating members.

17. (canceled)

18. The twin-shaft pump according to claim 1, wherein a length of the rotatable shafts is such that the support member is at a predetermined distance from the pumping chamber, the bearings providing radial control of the rotatable shafts being mounted towards at least one end of the rotatable shafts, the pump comprising further bearings for providing axial control of the rotatable shafts, the further bearings being closer to the pumping chamber than the bearings providing radial control.

19. The twin-shaft pump according to claim 18, wherein the further bearings are located adjacent to the pumping chamber.

20. The twin-shaft pump according to claim 18, wherein the further bearings comprise air bearings.

21. The twin-shaft pump according to claim 1, the pump comprising two support members on either side of the pumping chamber, the rotatable shafts being supported by bearings mounted on each of the support members, and each of the support members being separated from the pumping chamber by a thermal break.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the present disclosure will now be described further, with reference to the accompanying drawings.

[0049] FIG. 1 illustrates one end of a twin shaft pump.

[0050] FIG. 2 illustrates housings for bearings supporting the twin shafts of a pump.

[0051] FIG. 3 illustrates a twin shaft pump with extended shafts according to an embodiment.

[0052] FIG. 4 illustrates temperature control for the bearing housing of a twin shaft pump.

DETAILED DESCRIPTION

[0053] Before discussing the embodiments in any more detail, first an overview will be provided.

[0054] It is often desirable to maintain different portions of a pump at different temepratures. Pumping chambers may need to be maintained at a high temperature while bearings and gears may operate better at lower temperatures. Maintaining different portions of a pump at different temperatures results in different portions expanding by different amounts.

[0055] In this regard process reliability is the biggest limiting factor for pump life in semiconductor applications. Increasing pump temperature is key to improving this. However, it is preferable that this is not be achieved at the expense of reducing the intrinsic reliability of the machine, and therefore gearbox and bearing temperatures should not be increased with the temperature of the pumping chamber. This leads to differential expansion which unless addressed separately requires additional clearances. These additional clearances may impair the chances of simultaneously achieving low power and good vacuum performance.

[0056] The present technique provides a temperature difference between different portions of a pump to provide the desired operating conditions using thermal breaks.

[0057] In some embodiments the issues that arise due to different thermal expansion amounts of the different temperature regimes is addressed by using different materials of construction to synchronise thermal expansion at the different temperatures. In this way materials with different coefficients of thermal expansion and different thermal conductivities are selected to allow one portion of a twin-shaft pump to be maintained at a lower temperature than the pumping chamber of the pump while still providing similar expansion to that experienced by the rotor elements within the pumping chamber. This allows the clearances between rotor elements mounted on different shafts in a twin-shaft pump to be maintained substantially constant by configuring the pump such that the rotational axes of the rotors move apart at the same rate as the rotor elements increase in size, despite the difference in temperature changes at the two locations.

[0058] In other embodiments these issues are addressed by mounting the bearings in mounting means separated from the support member by a thermal break. In such an arrangement the support member temperature can more closely follow that of the pumping chamber so that differential expansion between the two is reduced. The bearings can however, be maintained at a lower operating temperature.

[0059] In preferred embodiments a material with reduced thermal conductance is used to isolate the bearings themselves from the support member that supports them allowing the part of the bearing support between the shaft axes to be at an elevated temperature and thereby expand more, while the individual bearings are at a lower temperature.

[0060] In some embodiments, the shaft may be extended such that the bearings can be mounted at a distance from the pumping chamber this distance contributing to the thermal isolation between the bearings and pumping chamber. In such a case the increased length of the shaft may lead to problems with expansion of the shaft. The bearings on which the shafts are mounted provide for both radial control and axial control of the shaft. The increased axial expansion can lead to clearance problems between the rotor and the end of the pumping chamber. Thus, in some cases in order to address this the functions of radial and axial positional control are separated, the axial control being provided in proximity to the pumping chamber so that the effect of axial expansion of the shaft is reduced. A bearing here however, must be able to operate at the high temperatures of the pumping chamber and thus, a bearing that provides axial control is achieved with a non-contacting pressurised air-bearing which can easily be located in a high temperature region. The radial control is a conventional rolling element bearing which is located in a remote, cooler location.

[0061] Different positions for the bearings within the structure may be used to provide the desired different operational temperatures provided that there is a low thermal conductance between the bearing and pumping chamber and a means of establishing a thermal gradient. This, when provided in conjunction with a difference in thermal expansivity of the materials in the two temperature zones, allows bearings in a twin-shaft pump to be maintained at a lower temperature than the pumping chamber while the pump can be manufactured with small radial clearances.

[0062] FIG. 1 shows a twin-shaft pump according to an embodiment. The pump has two shafts 10 mounted on bearings 20 within recesses 32 in a headplate 30. The shafts 10 each have rotor elements 12 that are located within pumping chamber 40. There is a clearance distance c between the rotor elements. This clearance distance is dependent on the distance d between the bearings 20 mounting the two rotatable shafts 10. As the temperature in the pumping chamber 40 increases the temperature of the rotor elements 12 will increase and they will expand acting to reduce the clearance distance c. If at the same time the headplate's 30 temperature increases then this will expand increasing distance d which acts to move the shafts further apart, acting to increase the clearance distance c. If the pump can be configured such that the increase in the distance d can be set to compensate for the expansion of the rotor elements then the distance c will not change, or at least any change will be reduced.

[0063] In the embodiment of FIG. 1, the headplate 30 is formed of a metal of high thermal expansivity such as aluminium. The rotor elements are made of cast iron that has a lower thermal expansivity. In this embodiment there is a thermal break 33 between the pumping chamber 40 and headplate 30 to thermally isolate the two to some extent. This thermal break 33 is provided by material of low conductivity within the shafts 10 and between the stator 42 of the pump and the headplate 30 that mounts the shafts 10. There is also an air gap 48 between the headplate 30 and stator 42. The shafts may in addition to having a material of low conductivity have a portion (not shown) that is hollow.

[0064] In the example above, the temperature of the region where the bearings are located increases by approximately half the increase in the temperature of the pumping chamber owing to the thermal break. Manufacturing the headplate 30 of a material with a thermal expansion coefficient that is twice that of the rotor material, allows the increase in rotor separation to match the increase in rotor diameter. In this example the rotors are made of cast iron (linear expansivity 1.210-5/K) while the bearing housing is made of aluminium (linear expansivity 2.310-5/K). The bearing housing is thermally isolated from the pump body by gap 48 and by the material of low thermal conductivity 33. Furthermore, the headplate 30 also has some cooling (not shown) that helps maintain a temperature gradient between these parts. The air gap 48 is sized (i.e. is sufficiently narrow) so as to avoid setting up any significant convective heat transfer between the two parts.

[0065] FIG. 2 shows a different technique for maintaining a substantially constant distance c between rotor elements during temperature changes within the pumping chamber 40. Here the bearings 20 are housed in housings 50 that are separated from the headplate 30 by a path of low thermal conductance. In this case this low thermal conductance path is provided by inserting a low thermal conductivity material in the form of ceramic gaskets 60 between the elements. The thermal conductivity of this path is further reduced by using a bearing housing 50 that has walls of a thin cross-section. Cooling on the individual bearing housings 50 can also be used to establish a large temperature gradient between it and the headplate 30. If, however, the thermal conductance is reduced enough, then only a small amount of cooling is required and this may be achieved with only the splashing of oil on the bearings 20. There is additionally a portion of the shaft formed of a material of low thermal conductivity 17 which again helps to thermally isolate the bearings from the pumping chamber. As for FIG. 1 the shafts may additionally have a portion that is hollow.

[0066] The separation c of the rotor elements 12 is controlled by the expansion of the headplate 30 with associated variations in the distance d, along with the expansion of the rotor elements 12 themselves. In the example shown the headplate 30 holding the shafts 10 is the stator of the high temperature pump and thus, to a large extent follows the temperature of the pumping chamber 40 and thus, its expansion follows the expansion of the rotor elements and the distance c is controlled by this. The bearings meanwhile are maintained at a lower temperature by the thermal break between the pumping chamber and the bearing housing and the cooling of the bearings.

[0067] However, in other embodiments the headplate 30 may be maintained at a slightly lower temperature than the interior of the pumping chamber perhaps by being slightly removed from the stator and in such a case a material of a higher thermal expansivity to that of the rotor elements can used for the headplate to compensate for the differences in temperature. In this regard the distance c can be maintained across a large temperature range by a combination of a material forming the headplate 30 of increased thermal expansivity compared to that of the rotor elements 12, and a temperature gradient between the headplate and the bearings, which temperature gradient allows the headplate 30 to be maintained at a higher temperature closer to that of the pumping chamber 40 than the temperature that the bearings 20 are maintained at.

[0068] FIG. 3 shows a further embodiment where the required thermal break between the headplate 30 mounting the shaft bearings 20 and the stator 42 of the pump is achieved at least in part by providing an increased distance between the two. Here, the radial location control in the form of bearings 20 is positioned at the far end of the oil box of the pump. However, if the axial control were also located there, then the pump axial clearances would need to be increased to account for the additional length of the shaft between the fixed axial point and the first rotor. Hence, the radial and axial position control functions are separated. The axial control is achieved using an air bearing 70 located adjacent to the pumping chamber 40. The air bearings 70 rely on pressurised air to maintain the distances and can easily operate in a high temperature environment. The radial control seeks to maintain radial clearances such as c, while the axial control seeks to maintain axial clearances shown here as e. The temperature difference between the headplate 30 and the pumping chamber 40 is further increased by the shafts 10 having hollow portions 14 between the pumping chamber 40 and headplate 30.

[0069] FIG. 4 schematically shows a system similar to that of FIG. 3, but in this embodiment there is controlled cooling of the headplate 30. Temperature sensors 80 within the pumping chamber and those 82 on the headplate 30 are used as inputs to control circuitry 90 which controls cooling element 95 which acts to cool headplate 30 and maintain an appropriate temperature difference between the pumping chamber 40 and headplate 30. This temperature difference is determined based on a knowledge of the materials of the rotor elements 12 and headplate 30 and is selected such that their relative expansions are similar and the clearance c between rotor elements 12 is maintained substantially constant.

[0070] In summary, it is very important for some pumps to operate at very high internal temperatures in order to improve process reliability. This technique enables this and in some embodiments provides a solution that does not require additional clearances that would otherwise degrade the pump performance.

[0071] Although illustrative embodiments of the disclosure have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the disclosure 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 disclosure as defined by the appended claims and their equivalents.