VACUUM APPARATUS CASINGS

Field of the Invention

The invention relates to cooling vacuum apparatus and particularly, but not exclusively, to vacuum apparatus casings and methods of manufacturing vacuum apparatus casings.

Background to the Invention

Vacuum systems may comprise vacuum apparatus including one or both of vacuum pumps and abatement units. The maximum temperature at which equipment connected to a vacuum apparatus can be safely operated may be limited by the cooling capacity of the cooling system associated with the vacuum apparatus. For example, it is often the case that the operating temperature of baking chambers in vacuum systems has to be limited to the maximum operating temperature of a vacuum pump, or pumps, that is a part of the system. In known vacuum systems, baking chambers may be used to condition the vacuum system prior to commencing productive operation. A baking chamber may be operated at 200°C to remove contamination and water vapour from the vacuum system. Baking at 200°C may, for example, last for 48 hours. If the baking temperature were increased, the conditioning time may be reduced significantly, for example halved. This would allow the vacuum system to become productive much faster than is currently possible. Thus, for example, if a baking chamber run at 200°C takes 48 hours to condition a vacuum system and running the baking chamber at 300°C halves the conditioning time, there is the possibility of bringing the vacuum system into productive use a day earlier. Accordingly, at least for some applications it may be desirable to increase the hot working capacity of vacuum apparatus such as vacuum pumps and abatement units.

To permit higher vacuum system operating temperatures, it may be desirable to improve the extraction and transfer of heat from a vacuum pump, or pumps, that is a part of the system. Known vacuum pumps, such as turbomolecular pumps, the pump may be connected into a vacuum system via a flange located at one end of the pump casing and cooled by a cooling fan arranged to provide a cooling airflow directed at the opposite end of the casing. In this example the cooling capability of the vacuum pump is primarily limited by the thermal properties of the pump casing; specifically, the ability of the casing to conduct heat from one end of the casing to the other. In many examples, at least a part of the casing is made of stainless steel, which is a relatively poor thermal conductor. In many other examples, at least a part of the pump casing is made of aluminium, or aluminium alloy. While aluminium is a better thermal conductor than stainless steel, elevated operating temperatures may give rise to creep. Creep can be a major problem where moving parts operate with fine tolerances.

The above-described problem of cooling vacuum pumps generally may not be the only cooling problem pump designers face. Alternatively, or additionally, designers may be faced with local cooling problems within a pump. For example, pump bearings may be sensitive to high temperatures. This is particularly the case with greased bearings. For a greased bearing that is held in a bearing holder, or directly by a part of the pump casing, the primary heat removal mechanism is conduction through the bearing carrier/pump casing to the exterior of the casing. During pumping operations, the interior of the pump will be in vacuum, so cooling from local air is negligible, although there may be some additional heat loss through thermal radiation. Thus, problems arising from the poor thermal conductivity of metals like stainless steel and the creep issues associated with metals such as aluminium that are described above, may make it difficult to provide effective cooling of local hotspots in a pump.

Summary of the Invention

The invention provides a vacuum apparatus casing as specified in claim 1.

The invention also includes vacuum apparatus as specified in claim 13.

The invention also includes a vacuum system as specified in claim 16.

The invention also includes a method of manufacturing a vacuum apparatus casing as specified in claim 17.

Brief Description of the Drawings

In the following disclosure, reference will be made to the drawings, in which: Figure l is a schematic representation of a vacuum system;

Figure 2 is a schematic representation of a turbomolecular pump of the vacuum system shown in Figure 1;

Figure 3 is an enlargement of the portion of the turbomolecular pump that is circled in Figure 2;

Figure 4 is an enlargement of the portion of the turbomolecular pump that is circled in Figure 3;

Figure 5 is a schematic illustration of internal channelling of the turbomolecular pump of Figure 2;

Figure 6 is a schematic illustration of another internal channelling arrangement for a turbomolecular pump; and

Figure 7 is a schematic illustration of a part of a pump showing a pump bearing unit. Detailed Description

Referring to Figure 1, a vacuum system 1 comprises a vacuum chamber 2, a chamber heating system 3, vacuum apparatus in the form of a vacuum pump 4, a temperature control unit 5 and a cooling system 6. Although not essential, in this example the vacuum system 1 further comprises further vacuum apparatus in the form of a second vacuum pump 7 and an abatement unit 8. in this example the vacuum pump 4 is a turbomolecular pump and the vacuum pump 7 is a roughing pump. The roughing pump 7 may comprise any suitable pump connected into the vacuum system 1 downstream of the turbomolecular pump 4. The roughing pump 7 may, for example, comprise a positive displacement pump such as a diaphragm, rotary vane or scroll pump. The abatement unit 8 may comprise a casing 9 and at least one abatement device 10 such as a combustor or a plasma burner. The chamber heating system 3 may comprise a heating jacket or other suitable form of heating unit or units. The chamber heating system 3 is associated with the vacuum chamber 2 and is operable to heat the interior of the vacuum chamber. Although not essential, the turbomolecular pump 4 may comprise a casing 12 having a flange 13 that defines an upstream end of the pump. The turbomolecular pump 4 may be directly connected to the vacuum chamber 2 via the flange 13. The flange 13 may be fitted with a flange heating unit 14. The cooling system 6 may be a forced air or water cooling system configured to cool the turbomolecular pump 4. The abatement unit 8 may also be cooled by the cooling system 6 or by a separate cooling system. The temperature control unit 5 may be configured to control the operation of the heating system 3, flange heating unit 14 and cooling system 6, or cooling systems, to maintain a desired heating profile in the vacuum system 1. As will be readily understood by those skilled in the art, the temperature control unit 5 may comprise a processor, electronic circuitry and suitably positioned temperature sensors. In some examples, the temperature control unit 5 may comprise individual controllers for the chamber heating system 3, flange heating unit 14 and cooling system 6, or cooling systems, and such individual controllers may be controlled by a master controller.

Referring to Figure 2, the turbomolecular pump 4 comprises a pumping mechanism 15 disposed in the casing 12, an inlet 16 and an outlet 18. The inlet 16 may take the form of an aperture defined in the flange 13. The pumping mechanism 15 may comprise a turbomolecular pumping mechanism comprising a plurality of rotor blades 20 di sposed in interleaving relationship with a plurality of stator discs 22. The rotor blades 20 are mounted on, or integral with, a rotor shaft 24 that has a longitudinal axis (axis of rotation) 26. The rotor shaft 24 is driven to rotate about the axis of rotation 26 by a motor 28. The turbomolecular pump 4 may comprise at least one further pumping mechanism 30. The at least one further pumping mechanism 30 may comprise a molecular drag pumping mechanism, which may be a Gaede mechanism, a Holweck mechanism or a Siegbahn mechanism. There may be additional, or alternative, pumping mechanisms downstream of the molecular drag pumping mechanism such as an aerodynamic pumping mechanism comprising a regenerative mechanism. The rotor shaft 24 is supported by a plurality of bearings 32, 34. The plurality of bearings may comprise two bearings 32, 34. The bearings 32, 34 may be positioned at, or adjacent, respective ends of the rotor shaft 24 as shown, or alternatively, intermediate the ends. In the example illustrated by Figure 2, a rolling bearing 32 supports a first end portion of the rotor shaft 24 and a magnetic bearing 34 supports a second end portion of the rotor shaft 24, although this is not essential as a second rolling bearing may be used in place of the magnetic bearing 34. When a magnetic bearing 34 is used, a back-up rolling bearing (not shown) may be provided. The rolling bearings 32, 34 may be packed with a lubricating grease.

The turbomolecular pump 4 additionally comprises a lubricant supply system 36 and a lubricant transfer device 38 provided on the rotor shaft 24. The lubricant device 38 is configured to transfer lubricant from the lubricant supply system 36 to the rolling bearing 32. The lubricant supply system 36 may comprise a lubricant reservoir comprising layers of felt, or a material with similar wicking properties, and one or more fingers by which lubricant is conducted from the reservoir onto the lubricant transfer device 38. The lubricant transfer device 38 may comprise a conical nut or sleeve secured to the rotor shaft 24.

The rolling bearing 32 may be fixed directly to the casing 12 or mounted to the casing via a bearing support, or carrier, 40. The bearing support 40 may be an essentially inflexible part that is fixed to the casing 12. Alternatively, the bearing support 40 may be configured to provide limited flexing in both radial and axial directions of the rolling bearing 32. A flexing bearing support 40 may be configured to damp vibrations of the rotor shaft 24 and rolling bearing 32 during use of the turbomolecular pump 4.

Referring to Figures 2 to 5, the pump casing 12 may be provided with internal channelling 60 defining at least one closed heat transfer pathway along which, in use, heat is conducted through the casing from a first position to a second position at which it is rejected from the casing to an external cooling system, such as the cooling system 6 shown in Figure 1. The internal channelling 60 may be configured such that the heat transfer pathway has a higher thermal conductivity, or higher thermal conductivity potential, than the pump casing 12, thereby increasing the heat transfer capability of the casing. In this example, the first position is in, or adjacent, the flange 13 at the upstream end of the turbomolecular pump 4 and the second position is towards the opposite end of the pump casing 12 at the downstream end of the turbomolecular pump.

The internal channelling 60 may comprise a first annular, or endless, channel 62 defined in the flange 13 at, or adjacent, the first position, a second annular, or endless, channel 64 defined at, or adjacent, the second position and at least one pipe-like elongate channel 66 connecting the first annular channel with the second annular channel. There may be a plurality of elongate channels 66 and these may be disposed in equi-spaced apart relation about the pump casing 12. In examples in which the pump casing 12 has a circular cross- section, or at least extends in a generally circular fashion about the pumping mechanism 14 and motor 28, the elongate channels 66 may be disposed in equi-spaced relation on a pitch circle diameter that extends in a circumferential direction within the pump casing about the pumping mechanism or motor.

The first and second annular channels 62, 64 may have a relatively larger volume than the elongate channels 66. At least one of the first and second annular channels 62, 64 may have a relatively large cross-section area when compared with the or each elongate channel 66. Thus, in examples in which the first and second annular channels 62, 64 and the or each elongate channel 66 have a generally circular cross section, the annular channels may have a relatively larger diameter than the or each elongate channel 66. The or each elongate channel 66 may be sized to provide a capillary effect.

Referring to Figure 3, it is not essential that one or both annular channels 62, 64 is an open channel; that is a channel that comprises an uninterrupted open, or empty, passage or pathway. One or both of the annular channels 62, 64 may be at least partially defined by a portion of the pump casing 12 that while not devoid of structure has greater porosity than a surrounding portion of the pump casing sufficient to allow a fluid to flow through it. Thus, as shown in Figure 3, the flange 13 may comprise a first portion 68 that is a substantially non-porous structure capable of providing the degree of gas impermeability and strength needed to fulfil its casing and load supporting functions and a second portion 70 that has a relatively greater porosity providing a relatively more open structure that defines a plurality of mini-passages that in combination define the annular channel 62. Thus, the first annular channel 62 may be defined by a region of relatively higher porosity surrounded by a substantially non-porous structure. Configuring the first annular channel 62 as a porous zone within a substantially non-porous structure, may provide improved heat transfer as compared with an open channel. As shown in Figure 3, the portion of higher porosity 70 that defines the first annular channel 62 may be defined by a regular mesh-like or honeycomb structure, an amorphous structure or a combination of the two.

Referring to Figure 4, although in some examples the or each elongate channel 66 may comprise an open pipe-like structure dimensioned to provide a capillary effect, an elongate channel 66 may instead comprise an outer portion defined by grooves 71 extending in the lengthways direction of the channel or a porous structure 70 that may, for example, correspond to any of the porous structures shown in the first annular channel 62 and an at least substantially uninterrupted passage disposed inwardly of the grooves 71 or porous structure 70. In examples, comprising grooves 71 or a porous structure 70, the or each elongate channel 66 is not necessarily sized to provide a capillary effect as the capillary effect is provided by the grooved or porous portion of the channel.

As shown by way of example in Figure 2, openings 72 may be provided in the pump casing 12 to allow filling of the internal channelling 60 with a suitable heat conducting medium. Once the internal channelling 60 has been filled with heat conducting medium, the openings 72 may be sealed with suitable closing devices 74. The openings 72 may comprise tapped holes and the closing devices 74 may comprise screw fasteners, for example grub screws. Seals or sealant (not shown) may be provided to ensure that the internal channelling 60 is at least substantially closed to the turbomolecular pump’s surroundings.

The internal channelling 60 may be filled with a non-fluid heat conducting medium having relatively high thermal efficiency and, in particular, a thermal conductivity that is greater than the thermal conductivity of the material from which the pump casing 12 is made. Examples of non-fluid heat conducting media that may be used include thermally conductive greases and resins or solder. Alternatively, or additionally, the internal channelling may be filled with a fluid heat conducting medium. Suitable fluid heat conducting media may include acetone, ammonia, caesium, ethanol, Freon 11, Freon 21, Freon 113, heptane, mercury, methanol sodium and water (preferably distilled water). The selection of the fluid heat conducting media may be influenced by the operating temperatures the turbomolecular pump 4 is intended to work at or the materials from which the pump casing 12 is made. As explained in more detail below, a fluid heat conducting media be selected such that, in use, it will evaporate and condense as it circulates around the internal channelling 60, thus providing the enhanced heat transfer capability obtainable in evaporating and condensing cooling systems.

In at least some examples, the pump casing 12 may be manufactured with integral internal channelling 60 by a generative production process, popularly known as 3D printing or additive manufacturing (AM). This may be preferable to manufacturing a casing comprising separate components that are joined together to define the internal channelling or forming at least some of the channelling by a metal cutting, or other metal removal, process. Suitable generative production processes may include vat photopolymerisation, material jetting, material extrusion (including fuse deposition modelling (FDM)), binder jetting, powder bed fusion, directed energy deposition and sheet lamination. Of these, powder bed fusion or fuse deposition modelling where the casing is built up leaving internal channelling for the heat conducting media or sintered powder to be removed to form the internal channelling may be particularly suitable. For example, a fuse deposition moulding may use a suitable metal to form the pump casing and an epoxy to define the internal channelling so that when the structure is heated to fuse the metal powder, the epoxy is melted. This method has the advantage that no metal powder has to be removed to clear the internal channelling.

While not limited to these materials, the pump casing may be made of aluminium, aluminium alloy or stainless steel.

The heat conducting medium may be introduced into the internal channelling 60 via the openings 72 by evacuating air from the internal channelling and then introducing the heat conducting medium after which, the openings 72 may be sealed by the closing devices 74. Seals or sealant may be applied to the closing devices to ensure the openings 72 are closed to the turbomolecular pump’s surroundings. In other examples, a fluid heat conducting medium may be heated to cause it to vapourise so that it fills the internal channelling and any air in the channelling is expelled.

As best seen in Figure 5, the internal channelling 60 illustrated by Figures 2 to 5 comprises a plurality of joined, or linked, channels. As illustrated by the example shown in Figure 6, this is not essential. The internal channelling 60 may comprise a plurality of separate channels 66 defining discrete, or individual, heat transfer pathways. A plurality of discrete heat transfer pathways may take the form of a plurality of pipe-like elongate channels 66 extending between a first position, such as the flange 13, from which heat is collected, and a second position at which heat is rejected to an external cooling system. However, it may be preferable that at one or both of the first and second positions, each heat transfer pathway defined by the internal channelling 60 comprises a portion 80, 82 that extends transversely of the elongate channel 66 to respectively provide an additional volume for heat collection at the first position or heat rejection at the second position. A structure as shown in Figure 6 in which there are breaks, or gaps, in the internal channelling may be desirable to allow for the siting, or routing, of other pump features, for example, ducts that need to extend from the exterior of the pump casing to a location within the pump casing.

In use, when the internal channelling 60 is filled with a fluid heat conducting medium, heat present in the flange 13 may cause the fluid in the first annular channel 62 to evaporate. The vapour thus created may flow rapidly from the first annular channel 62 along the elongate channels 66 under the influence of the pressure differential resulting from the difference in the temperature at the first and second annular channels 64. When the vapour reaches the second annular channel 64, it may condense due to the cooling effect of the external cooling system 6. The liquid condensate may then flow back to the first annular channel 62 through the elongate channels due to capillary force. In examples in which the elongate channels 66 are configured the same as, or similarly to, the channel 66 shown in Figure 4, the vapour will tend to flow down the open central portion of the channel and the liquid will tend to flow through the grooves 71 or porous structure 70, which provide the capillary effect. Referring to Figure 7, a pump 100 comprises a casing 102 that comprises a side wall 104, a dividing wall 106 and an end cap 108. The side wall 104, dividing wall 106 and end cap 108 are secured one to another to at least partially form the pump casing 102. The side wall 104 may be annular and have a generally circular cross section, at least over a part of its length. The dividing wall 106 extends across an end of the side wall 104. The side wall 104 and dividing wall 106 at least partially define a pumping chamber 110 that may house a motor and at least one pumping mechanism (not shown). The pump 100 may be a turbomolecular pump comprising a plurality of pumping mechanisms, an electric motor, an inlet and an outlet in similar fashion to the turbomolecular pump 4 shown in Figure 2. A rotor shaft 112 having a longitudinally extending axis of rotation 114 extends from the pumping chamber 110 through an aperture 116 defined by the dividing wall 106 to a chamber 118 disposed between the dividing wall and the end cap 108. It will be understood that in examples in which the pumping mechanism, or mechanisms, and, if provided within the casing 102, the electric motor, can be fitted into the pumping chamber 110 by a route other than through the end of the side wall 104 that is covered by the dividing wall 106, the dividing wall may be integral with the side wall and not a separate component.

The chamber 118 houses a bearing unit comprising a bearing 120. The bearing 120 may be mounted directly to a seat, or housing, 122 defined by the dividing wall 106. Alternatively, the bearing unit may comprise a bearing carrier, or mounting, via which the bearing is fitted to the dividing wall 106. The bearing carrier may be a rigid component of a flexible as described above. Although not shown, the bearing unit may additionally comprise one or more seals arranged to form a seal between the pumping chamber 110 and chamber 118. The bearing 120 may be a greased bearing and may, for example, be a rolling bearing packed with a lubricating grease.

The pump casing 102 is provided with internal channelling 124 that defines a closed heat transfer pathway along which, in use, heat is conducted through the casing from a first position to a second position to be rejected from the casing to an external cooling system. In the illustrated example, the internal channelling 124 is provided in the dividing wall 106, the first position is adjacent the seat 122 and the second position is adjacent the external surface 126 of the dividing wall. The internal channelling 124 may comprise an annular channel disposed at one or both of the first and second positions and a plurality of pipe-like elongate channels extending radially with respect to the longitudinal axis 114 between the first and second positions. Alternatively, the internal channelling 124 may comprise a plurality of separate channels that define discrete heat transfer pathways in analogous fashion to the arrangement shown in Figure 6. The internal channelling 124 may comprise a porous structure or combination of open and porous portions as illustrated by Figures 3 and 4. The internal channelling 124 may be filled with a heat conducting medium in the same, or similar, fashion as the internal channelling 60 described above.

It is to be understood that although the described examples have internal channelling filled with a heat conducting media post production of the pump casing, this is not essential. It is envisaged that examples may be produced in which the internal channelling formed and filled as the casing is formed. Thus, a heat conducting medium may be deposited alongside the casing material during production of the pump casing so that filled internal channelling is produced simultaneously. For example, steel and copper powder deposition may be arranged such that the finished product has copper-filled internal channels. However, for many applications, having internal channelling filled with a heat conducting media that goes through phase changes as it moves around the channelling may be advantageous. This is because the latent heat of vapourisation allows the heat conducting medium to absorb and carry more heat, or energy, than the metals normally used to make vacuum apparatus casings.

It is to be understood that although the examples shown in the drawings show internal channelling conducting heat to a position in the pump casing at which the heat is rejected to an external cooling system in the form of apparatus such as a water cooling or forced air cooling system, this is not essential. For some examples, ambient air may be a sufficient external cooling system.

The provision of internal channelling as described above may improve heat transfer in casings and interior structural members of vacuum apparatus made of metals such as, for example, aluminium, aluminium alloy and stainless steel. Such internal channelling may also enable the use of plastics to form vacuum apparatus casings. Thus, a casing may at least in part be made of a plastics material, allowing the possibility of reducing pump weight. A plastics casing may be made of known engineering plastics materials suitable for generative production processing. For some applications, reinforcement with, for example aramid or carbon fibres, may be required. In some examples, a plastics casing may be provided with a metal body portion to act as a heat reservoir to which the internally defined heat transfer pathway, or pathways, conduct heat that is then rejected to an external cooling system.

It is to be understood that although the internal channelling shown in the drawings is described primarily in connection with a turbomolecular pump, it or similar such channelling employing the same or similar heat transfer processes may be applied to transferring heat in other forms of pump, for example, screw or scroll pumps or abatement units and vacuum apparatus generally. For example, such internal channelling may be provided in the casing of an abatement unit such as the abatement unit shown in Figure 1.