GB2418958A

GB2418958A - Vacuum pump with enhanced exhaust heat transfer to stator

Info

Publication number: GB2418958A
Application number: GB0422193A
Authority: GB
Inventors: Allan Edmiston Mcdiarmid
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2004-10-06
Filing date: 2004-10-06
Publication date: 2006-04-12
Also published as: GB0422193D0

Abstract

A vacuum pump comprises a stator 12 housing a pumping mechanism 56, 58 which may be of roots, claw, Northey or screw type and defining an exhaust conduit 26 for exhausting gas from the pump. Baffles 60 are located in the exhaust conduit for transferring to the stator heat from gas passing through the exhaust conduit. The baffles change the gas flow direction causing gas impact on the conduit wall to improve local heat transfer. This can enable the temperature of the stator to closely match that of the pumping mechanism at the point of highest temperature and thus reduce the likelihood of contact between the stator and the pumping mechanism. Further baffles may be located in a passage 34 entering the last pumping chamber 22 and the conduit may be integral with the stator.

Description

24 1 8958 - 1

VACUUM PUMP

This invention relates to vacuum pumps, and is directed to improvements in the operational efficiency of such pumps. s

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 lo 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. Dry pumps incorporating Roots and/or Northey mechanisms are commonly multi-stage positive displacement pumps employing intermeshing rotors in each pumping Is chamber. The rotors are located on contra-rotating shafts, and may have the same type of profile in each chamber or the profile may change from chamber to chamber.

In vacuum pumps operating near ultimate pressure, the majority of the pressure differential occurs near the exhaust end of the pump. This is due to the high volumetric efficiency necessary to achieve good ultimate pressures. As a result of this high volumetric efficiency, a significant proportion of the theoretical displaced gas is continually conveyed back and forth in the exhaust line, generating heat in the following manner.

As the pumping mechanism forces the gas down the exhaust line, the kinetic energy of the gas molecules local to the surface of the rotor is increased. This increase in kinetic energy is then dissipated down the exhaust line, causing a net movement of gas away from the rotor surface. This transfer of kinetic energy so along the exhaust line generates a pressure difference. Because of the short duration of time over which this process occurs, a temperature gradient is also generated from the rotor surface. If the process were to occur slowly then the - 2 differential pressure gradient would be reduced and the temperature gradient would be lower and more evenly distributed through the exhaust gas.

When the pumping mechanism reaches the end of its displacement phase, the s following, lower pressure, vacuum chamber is then open to exhaust line pressure, and so exhaust line gas accelerates into the lower pressure chamber. The gas expands into the lower pressure chamber and impacts the surfaces of the chamber. Surfaces normal to the direction of flow see the maximum impact velocity, and so the impacting of gas molecules on these surfaces gives excellent lo heat transfer. As the pumped gas generally moves parallel to the surfaces of the stator, the gas does not impact on the surfaces of the stator as much as it does on the surfaces of the rotor, and so the stator has a lower heat transfer rate than the rotor. Unless there is shaft cooling or other means of dissipating the heat then the temperature of the shaft will increase until it approaches the pumped gas Is temperature. There will be very little transfer of heat from the pumped gas through the shaft at steady state conditions since there is not a ready heat path out of the shaft.

In view of this, the stator and rotor will undergo differential thermal expansion, so especially during transient pumping conditions. This can lead to contact between the stator and the pumping mechanism, which can impair or, in severe conditions, stop the operation of the pump.

It is an aim of at least the preferred embodiment of the present invention to inhibit us contact between the stator and pumping mechanism of a pump even under large differential pressure gradients.

The present invention provides a vacuum pump comprising a stator housing a pumping mechanism, an exhaust conduit in thermal contact with the stator for so exhausting gas from the pump, and means located in the exhaust conduit for transferring to the stator heat from gas passing through the exhaust conduit. - 3

The maximum temperature in the gas being conveyed through the pump occurs in the exhaust conduit, generally immediately outside the swept volume of the pumping mechanism. Because it is easier to cool the stator than the rotor, the stator can be used as the major heat path for the pump when running at ultimate.

The present invention aims to increase the local heat transfer between the exhaust gas and the stator by providing means within the exhaust conduit for transferring to the stator heat from gas passing through the exhaust conduit, thus allowing an increased amount of heat in the exhaust to be absorbed by the stator. By improving the heat transfer rate to the stator, the rate of change of temperature of lo the stator can now closely match the rate of change of temperature of the rotor at the portions of the pump where the highest temperature differential had previously been encountered. This can enable the temperature of the stator to closely match the temperature of the pumping mechanism, and thus reduce the likelihood of contact between the stator and the pumping mechanism. Consequently, pump reliability can be improved.

In order to maximise heat transfer between the exhaust conduit and the stator, the exhaust conduit is preferably defined, at least in part, by the stator.

The heat transfer means preferably comprises a plurality of baffles located in the exhaust conduit, preferably adjacent the pumping mechanism, for causing gas passing through the exhaust conduit to impact upon the internal surfaces of the exhaust conduit. In order to simply pump manufacture, the baffles are preferably integral with the stator, which is generally formed from cast iron.

The pump may be a dry pump, in which the pumping mechanism comprises first and second intermeshing rotors adapted for counter-rotation within the stator. In the preferred embodiment, the rotors have a Roots profile, although they could have a Northey or screw profile as required. The pump may be in the form of a so multi-stage pump in which the stator defines a plurality of interconnected pumping chambers arranged in series and each housing respective rotors, and wherein additional heat transfer means are located between an exhaust stage pumping - 4 chamber and a preceding pumping chamber for transferring to the stator heat from gas passing between the pumping chambers. This can improve the overall heat transfer between the gas and the stator.

s Preferred features of the present invention will now be described, by way of example only, with reference to the following drawings, in which: Figure 1 is a cross-section through a multi-stage dry pump; and lo Figure 2 is a view along line A-A in Figure 1.

With reference to Figures 1 and 2, the pump 10 comprises a stator 12 having a series of walls that define a plurality of pumping chambers 14, 16,18, 20, 22. An inlet conduit 24 for conveying gas to be pumped to the inlet pumping chamber 14, and an exhaust conduit 26 for exhausting pumped gas from the exhaust pumping chamber 22, are also formed in the stator 12. Circumferential passages 28, 30, 32 and 34 formed in the stator 12 connect the pumping chambers 14,16,18, 20, 22 in series.

so The stator 12 houses a first shaft 36 and, spaced therefrom and parallel thereto, a second shaft 38. Bearings 40 for supporting the shafts 36, 38 are provided in the end plates 42, 44 of the stator 12. One of the shafts 36 is connected to a drive motor 46, the shafts being coupled together by means of timing gears 47 so that in use the shafts 36, 38 rotate at the same speed but in opposite directions, as indicated by arrows 48 and 50 in Figure 2. A gear box 52 attached to the side of the pump 10 contains oil 54 for lubricating the timing gears 47.

Within each pumping chamber, the shafts 36, 38 support respective rotors 56, 58.

In this embodiment, the rotors 56, 58 have a Roots-type profile within each pumping chamber, although a mixture of Roots and Northey-type profiles may be provided within the pump 10. The rotors 56, 58 are located in each pumping - 5 chamber relative to an internal surface of the stator 12 such that the rotors 56, 58 can act in an intermeshing manner known per se.

In use, gas is urged into the pump 10 through the inlet conduit 24 and passes into s the inlet pumping chamber 14. The gas is compressed by the rotors 56, 58 located within the inlet pumping chamber 14, and is fed by passage 28 into the next pumping chamber 16. The gas fed in the pumping chamber 16 is similarly compressed by the rotors 56, 58 therein, and fed by the passage 30 to the next pumping chamber 18. Similar gas compressions take place in the pumping to chambers 18, 20 and 22, with the pumped gas finally being exhaust from the pump 10 through exhaust conduit 26.

As the gas is pumped from the inlet conduit 24 to the outlet conduit 26, heat is generated. Part of this heat is transferred to the pumping mechanism due to Is impact between the pumped gas molecules and the surfaces of the rotors 56, 58 as the gas passes through the pump 10. However, as the pumped gas generally moves parallel to the surfaces of the stator 12, the gas does not impact on the surfaces of the stator 12 as much as it does on the surfaces of the rotors 56, 58, and so the stator 12 has a lower heat transfer rate than the rotor 56, 58.

In view of this, the stator 12 and rotors 56, 58 will undergo differential themmal expansion, especially during transient pumping conditions. The maximum pumped gas temperature occurs in the exhaust conduit 26, generally immediately outside the exhaust pumping chamber 22, and so this is where the greatest risk of contact between the stator 12 and the rotors 56, 58 lies. In order to increase the local heat transfer between the exhaust gas and the stator 12, and thereby enable the local temperature of the stator 12 to closely match the temperature of the rotors 56, 58 of the exhaust pumping chamber 22, a series of baffles 60 are provided in the exhaust conduit 26 immediately adjacent the exhaust pumping so chamber 22, and preferably integral with the stator 12. These baffles 60 cause the direction of flow of the exhaust gas passing through the exhaust conduit 26 to be continually changed, thus causing the gas to impact the walls of the exhaust - 6 conduit 26, and thus the stator 12, to improve the local transfer of heat between the exhaust gas and the stator 12. This can enable the local thermal expansion, and contraction, of the stator 12 to closely match the local thermal expansion, and contraction of the rotors 56, 58 in the exhaust pumping chamber 22, and thus s reduce the risk of pump seizure due to contact between the stator and the rotors 56, 58 during transient conditions.

In addition to the provision of baffles 60 in the exhaust conduit, further baffles may be located in the passage 34 connecting the exhaust pumping chamber 22 with lo the preceding pumping chamber 20. This can assist in improving the overall heat transfer between the pumped gas and the stator 12. - 7

Claims

CLAIMS 1.

A vacuum pump comprising a stator housing a pumping mechanism, s an exhaust conduit in thermal contact with the stator for exhausting gas from the pump, and means located in the exhaust conduit for transferring to the stator heat from gas passing through the exhaust conduit.

lo
2. A pump according to Claim 1, wherein the exhaust conduit is defined, at least in part, by the stator.
3. A pump according to Claim 1 or Claim 2, wherein the heat transfer means comprises a plurality of baffles located in the exhaust conduit is for causing gas passing through the exhaust conduit to impact upon the internal surfaces of the exhaust conduit.
4. A pump according to Claim 3, wherein the baffles are integral with the stator.
5. A pump according to any preceding claim, wherein the heat transfer means are located adjacent the pumping mechanism.
6. A pump according to any preceding claim, wherein the pumping mechanism comprises first and second intermeshing rotors adapted for counterrotation within the stator.
7. A pump according to Claim 6, wherein the rotors have a Roots profile.
8. A pump according to Claim 6 or Claim 7, in the form of a multi-stage pump in which the stator defines a plurality of interconnected - 8 pumping chambers arranged in series and each housing respective rotors, and wherein additional heat transfer means are located between an exhaust stage pumping chamber and a preceding pumping chamber for transferring to the stator heat from gas passing between the pumping chambers.
9. A pump according to Claim 8, wherein the additional heat transfer means comprises a second plurality of baffles located in a conduit connecting the exhaust stage pumping chamber to the preceding lo pumping chamber.