WO2006061558A1

WO2006061558A1 - Vacuum pump with heat sink on rotor shaft

Info

Publication number: WO2006061558A1
Application number: PCT/GB2005/004339
Authority: WO
Inventors: Nigel Paul Schofield; Stephen Dowdeswell
Original assignee: The Boc Group Plc
Priority date: 2004-12-08
Filing date: 2005-11-11
Publication date: 2006-06-15
Also published as: GB0426959D0

Abstract

A vacuum pump comprises a stator and a rotor comprising a shaft supported for rotation relative to the stator. On end of the shaft is located in an enclosure provided adjacent the stator and having mounted thereon a heat sink (128) for dissipating heat from the shaft to the enclosure. The enclosure has a plurality of water-cooled fins (134) located on the inner surface of the enclosure for receiving the heat dissipated from the heat sink.

Description

VACUUM PUMP WITH HEAT SINK ON ROTOR SHAFT

This invention relates to vacuum pumps, and is directed to the cooling of one or more components of a vacuum pump.

Dry pumps are widely used in industrial processes to provide a clean and/or low - pressure environment for the manufacture of products. Applications include the pharmaceutical, semiconductor and flat panel manufacturing industries. Such pumps include an essentially dry (or oil free) pumping mechanism, but generally also include some components, such as bearings and transmission gears, for driving the pumping mechanism that require lubrication in order to be effective.

Examples of dry pumps include Roots, Northey (or "claw") and screw pumps. A typical screw pump mechanism comprises two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body (which acts as a stator) cause volumes of gas entering at an inlet to be trapped between the threads of the rotors and the internal surface and thereby urged towards an outlet of the pump as the rotors rotate.

During use, heat is generated as a result of the compression of the gas by the rotors acting in combination with one another. Consequently, the temperature of the rotors rapidly rises. By comparison, the bulk of the stator is large and heating thereof is somewhat slower. As the rotors of the pump heat up, heat is conducted from the rotors to the shafts. Unless there is shaft cooling or other means of dissipating the heat then the temperature of the shafts will increase until it approaches the pumped gas temperature. This can cause undesirable heating of other components of the pump that are in contact with the shaft, such as static and dynamic seals, bearings, gears and lubricating oils. Figure 1 illustrates schematically one known arrangement for cooling an outlet section of a screw pump. Ip this arrangement, a central cavity 10 is formed in each end of the threaded body 12 of the rotor (one end only shown in Figure 1 ), the cavity 10 being co-axial with the body 12, the longitudinal axis of which is indicated at 14. A shaft 16 is attached to the body 12 by means of bolts 18 such that the shaft 16 extends into the cavity 10 and rotates with the body 12 of the rotor during use. The shaft 16 has a first central bore 20 formed therein. The first bore 20 houses a coolant supply tube 22 for supplying coolant pumped from a source thereof into a second central bore 24 of the shaft 16, the second bore 24 being co-axial with the first bore 20. The coolant flows from the second bore 24 into the cavity 10, wherein the coolant flows radially outwards between the end 26 of the shaft 16 and the end wall 28 of the cavity 10, and then flows away from the end wall 28 within a narrow annular gap 30 located between the cylindrical wall 32 of the shaft 16 and the cylindrical wall 34 of the cavity 10. Radial bores 36 formed in the shaft 16 allow the coolant to flow into the first bore 20 of the shaft 16 and back towards the end 38 of the shaft 16, from which it is discharged into a reservoir (not shown) with a pumping mechanism for returning the coolant to the supply tube 22.

Whilst such an arrangement can provide efficient cooling of the rotors and shafts of a relatively large screw pump, there is a cost increase associated with the provision of a coolant recirculation system and a coolant pumping mechanism. In addition, such an arrangement is not particularly suitable for the cooling of components of a relatively small screw pump.

It is an aim of at least the preferred embodiment of the present invention to provide a relative simple and low cost system for cooling the shaft of a vacuum pump.

The present invention provides a system for cooling a shaft of a vacuum pump, the system comprising an enclosure for receiving one end of the shaft, the enclosure housing a heat sink mounted on the shaft and a plurality of fins for receiving heat dissipated from the heat sink, the system further comprising means for cooling the fins.

The fins are preferably located on at least one of the walls of the enclosure, with the cooling means being located adjacent said at least one of the walls. The fins may be mounted on said at least one of the walls, or integral with said at least one of the walls.

The cooling means preferably comprises means for conveying a coolant about the enclosure.

The heat sink preferably comprises a plurality of heat dissipating fins for generating a fluid flow within the enclosure with rotation of the shaft to promote the transfer of heat from the heat sink to said plurality of fins. The heat dissipating fins are preferably arranged radially on the heat sink.

The present invention also provides a vacuum pump comprising a stator, a rotor comprising a shaft supported for rotation relative to the stator, part of the shaft being located in an enclosure provided adjacent the stator and having mounted thereon a heat sink for dissipating heat from the, shaft to the enclosure, the enclosure housing a plurality of fins for receiving the heat dissipated from the heat sink, the pump further comprising means for cooling the fins.

The heat sink is preferably located proximate a bearing assembly for rotatably supporting the shaft relative to the stator. The bearing assembly is preferably located in a plate separating the stator from the enclosure.

The rotor may comprise a threaded body located on the shaft for rotation relative to the stator.

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-section through part of a known rotor of a screw pump; and

Figure 2 is a cross-section through an embodiment of a vacuum pump.

With reference to Figure 2, a vacuum pump 100 comprises a stator 102 defining a pumping chamber 104. The stator 102 houses a plurality of parallel, mutually spaced shafts, one of which is indicated at 106 in Figure 2. Each shaft 106 is supported by respective bearings 108, 110 provided in the end plates 112, 114 of the stator 102. One of the shafts is connected to a drive motor (not shown), the shafts being coupled together by means of timing gears 116 so that in use the shafts 106 rotate at the same speed but in opposite directions. A gear box 118 attached to one side of the pump 100 contains oil for lubricating the timing gears 116. '

Within the pumping chamber 104, the shafts 106 support respective rotors 120. In this embodiment, the rotors 120 have a threaded screw-type profile within the pumping chamber 104, although the rotors 120 may have a Roots or Northey-type profile as required. The rotors 120 are located in the pumping chamber 104 relative to an internal surface of the stator 102 such that the rotors 120 can act in an intermeshing manner known per se.

In use, gas is urged into the pump 100 through an inlet (not shown) and passes into the pumping chamber 104. The gas is compressed by the rotors 120 and exhaust from the pump 10 through an outlet (not shown). As the gas is pumped from the inlet to the exhaust, heat is generated. Part of this heat is transferred to the pumping mechanism due to impact between the pumped gas molecules and the surfaces of the rotors 120 as the gas passes through the pump 100. As the rotors 120 heat up, heat is conducted from the rotors 120 to the shafts 106. This can cause heat to be transferred from the shafts 106 to the rolling bearings 108 provided at the exhaust end of the pump 100, at which the highest temperatures are generated. Excessive heating of the rolling bearings 108 could damage the bearings, which could result in pump failure and/or excessive pump repair time.

In order to reduce the amount of heat that is transferred from the shafts 106 to the bearings 108, the pump 100 includes a system 122 for cooling the shafts 106 of the pump 100. The system 122 comprises an enclosure 124 adjacent the pumping chamber 104 and housing one end 126 of each of the shafts 106; alternatively, where the motor is connected to that end of the other shaft, the enclosure 124 may house the end 126 of just one of the shafts 106. A heat sink 128 is mounted on the end 126 of each shaft 106. The heat sink 128 is preferably mounted proximate the bearing 108, and may provide a clamp 130 for the bearings 108. The heat sink 128 may be formed from any suitable material having a relatively high thermal conductivity, preferably a metallic material such as aluminium, so that heat is readily transferred from the shaft 106 to the heat sink 128.

The heat sink 128 comprises a plurality of heat dissipating radial fins 132 for generating a flow of air or nitrogen within the enclosure 124 with rotation of the shafts 106 so that heat is drawn from the radial fins 132 by the forced flow of air circulating within the enclosure 124. The enclosure 124 is shaped such that the hot air flowing from the heat sink 128 flows over a plurality of cooling fins 134 located on the walls 136 of the enclosure 124 so that heat is transferred from the hot air to the surfaces of the cooling fins 134. The thus-cooled air is then circulated back to the heat sink 128 for re-heating. The cooling fins 134 may be mounted on the walls 136, or may be integral with the walls 136. For example, the cooling fins 134 may be formed during casting of the walls 136, or they may be machined from the walls 136. The cooling fins 134 and walls 136 of the enclosure 124 are also preferably formed from any materia) having a relatively high thermal conductivity.

A coolant conveyed about the outside of the enclosure 124 provides the cooling of the cooling fins 134. For example, as shown in Figure 2, the system 122 comprises a plurality of water pipes 138 provided in thermal contact with the external surfaces 140 of the walls 136 of the enclosure 122 so that, in use, heat is transferred from the cooling fins 134 by conduction through the walls 136 of the enclosure 124 to the water flowing within the pipes 140. The system 122 can thus maintain the shafts 106, and therefore the bearings 108 and any other components in thermal contact with the shaft 106, at a temperature lower than that inside the pumping chamber 104. For example, when the temperature inside the pumping chamber 104 is around 200⁰C, the bearings 108 can be maintained at a temperature below 100⁰C. This can increase the life time of the bearings 106, and can enable the pumping speed to be increased.

Claims

1. A system for cooling a shaft of a vacuum pump, the system comprising an enclosure for receiving one end of the shaft, the enclosure housing a heat sink mounted on the shaft and a plurality of fins for receiving heat dissipated from the heat sink, the system further comprising means for cooling the fins.

2. A system according to Claim 1 , wherein the fins are located on at least one of the walls of the enclosure, the cooling means being located adjacent said at least one of the walls.

3. A system according to Claim 2, wherein the fins are mounted on said at least one of the walls.

4. A system according to Claim 2, wherein the fins are integral with said at least one of the walls.

5. A system according to any preceding claim, wherein the cooling means comprises means for conveying a coolant about the enclosure.

6. A system according to any preceding claim, wherein the heat sink comprises a plurality of heat dissipating fins for generating a fluid flow within the enclosure with rotation of the shaft to promote the transfer of heat from the heat sink to said plurality of fins.

7. A system according to Claim 6, wherein the heat dissipating fins are arranged radially on the heat sink.

8. A vacuum pump comprising a stator, a rotor comprising a shaft supported for rotation relative to the stator, and a system according to any preceding claim for cooling the shaft.

9. A vacuum pump comprising a stator, a rotor comprising. a shaft supported for rotation relative to the stator, part of the shaft being located in an enclosure provided adjacent the stator and having mounted thereon a heat sink for dissipating heat from the shaft to the enclosure, the enclosure housing a plurality of fins for receiving heat dissipated from the heat sink, the pump further comprising means for cooling the fins.

10. A vacuum pump according to Claim 8 or Claim 9, wherein the heat sink is located proximate a bearing assembly for fotatably supporting the shaft relative to the stator.

11. A vacuum pump according to Claim 10, wherein the bearing assembly is located in a plate separating the stator from the enclosure.

12. A vacuum pump according to any of Claims 8 to 11 , wherein the rotor comprises a threaded body located on the shaft for rotation relative to the stator.