GB2440341A

GB2440341A - Vacuum pump with purge gas channel

Info

Publication number: GB2440341A
Application number: GB0614616A
Authority: GB
Inventors: David Paul Manson; Tristan Richard Ghisla Davenne
Original assignee: BOC Group Ltd; Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2006-07-24
Filing date: 2006-07-24
Publication date: 2008-01-30
Anticipated expiration: 2026-07-24
Also published as: GB0614616D0; GB2440341B

Abstract

A vacuum pump comprises a stator having formed therein a pumping chamber 22, 32 and a purge gas channel 26, 36 suitable for conveying purge gas from a purge gas source 50 to the pumping chamber for control of vapour pressure in the pumped flow. To maximize thermal transfer from the stator to the purge gas, the channel has a hydraulic diameter of less than 4.5 mm and may have an aspect ratio greater than 1. The channel may be formed in or under the faces of adjacent stator plates 20, 30 in a multistage machine. Permits preheating of the purge gas to control condensation within the pump caused by introduction of the purge gas.

Description

-1-2440341

VACUUM PUMP

This invention relates to the field of vacuum pumps.

Purge gas is conventionally introduced into a pumping chamber of a vacuum pump in order to control the concentration or the vapour pressure of a waste stream being conveyed from an inlet to an outlet of the vacuum pump.

However, the temperature of the delivered purge gas is generally lower than that of the waste stream. If the purge gas is introduced towards the outlet in the higher pressure region of the pumping chamber, condensates are likely to form as a result of the introduction of the purge gas.

io This problem is addressed in turbomolecular vacuum pumps in JP1237397 by using a heating element to warm the purge gas prior to injection :.:::. into the pumping chamber. Such a system introduces complexity into the system together with an increased cost and potential reduction in reliability that would *:*::* preferably be avoided.

:. is It is, therefore, desirable to provide a simplified configuration that achieves heating of the purge gas to an extent that is sufficient to avoid condensation of S...

* the waste stream and thus inhibit formation of deposits within the pumping S.....

* chamber.

According to one aspect of the present invention there is, therefore, provided a vacuum pump comprising a stator having formed therein a pumping chamber and a purge gas channel for conveying purge gas from a purge gas source to the pumping chamber, the channel having a hydraulic diameter of less than 4.5 mm.

By providing a purge gas channel with such dimensions within the bulk of the stator any purge gas travelling through the channel experiences a direct thermal transfer from the stator through the surfaces of the channel. The purge gas may thus be heated without the need for additional heating elements and their associated costs and reliability issues.

The channel may be configured such that, at steady state, any purge gas to be conveyed along the channel reaches a temperature within 90%, preferabty 95%, of the operating temperature of the stator prior to entering the pumping chamber. The channel may have a hydraulic diameter of less than 3 mm.

The cross section of the channel may have an aspect ratio greater than 1, preferably greater than 1.5.

The vacuum pump may be a multi-stage vacuum pump, the stator comprising a plurality of stator plates. A pumping chamber may be formed between each adjacent pair of stator plates and the channel may be formed in an axial surface of one of the stator plates. A further purge gas channel may be formed in another stator plate, each purge gas channel being in fluid communication with a respective pumping chamber. * *.

Each pumping chamber may be in fluid communication with a respective *** channel, each respective channel being formed in a stator plate adjacent to its respective pumping chamber. If more than one purge gas channel is provided, the purge gas channels may have different hydraulic diameters. In this way, separate channels having different dimensions can be provided within the stator to deliver purge gases having different thermal properties to different locations of the pumping chamber as required.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic cross-sectional view of a multi-stage vacuum pump having purge gas channels located within the stator; Figure 2 illustrates a stator plate from the vacuum pump of Figure 1; Figure 3 illustrates another stator plate from the vacuum pump of Figure 1; Figure 4 shows a graph representing the thermal effect of variation of dimensions of a purge gas channel; Figure 5 shows a graph representing the thermal effect of variation of the hydraulic diameter of a purge gas channel at different purge flow rates; and Figure 6 shows a graph representing the variation in flow rate required to achieve appropriate thermal conditions as the hydraulic diameter is varied.

A vacuum pump 10 is illustrated in Figure 1. In this example, the vacuum pump 10 is a multi-stage vacuum pump having a stator which comprises a number of stator plates 20, 30. Each stator plate 20, 30 has formed therein a pumping chamber 22, 32 and each stator plate is connected to an adjacent stator plate or an end plate to form the stator of the vacuum pump 10. Seals are provided between each pair of adjacent plates to prevent transfer of fluid between the pumping chambers of the vacuum pump 10 and the environment external to io the vacuum pump. Ports 35 are formed within the stator plates to bring the pumping chambers into fluid communication with one another so that a fluid path is provided through the vacuum pump 10.

: *. Fluid, for example a waste stream from a process chamber being e.

evacuated by the vacuum pump 10, is drawn through the pumping chambers 22, 32 by a pumping mechanism (not shown) located within each pumping chamber 22, 32. The waste stream is conveyed through the vacuum pump 10 from an inlet end 40 to an outlet end 45.

In moving from the inlet end 40 towards the outlet end 45 of the vacuum pump, the pressure of the waste stream increases. Consequently, more work is done in conveying and compressing the waste stream and so the temperature of the waste stream increases towards the outlet end 45 of the vacuum pump 10.

The waste stream often contains material which has a tendency to form liquid or solid deposits when the pressure of the purge gas is elevated, i.e. towards the outlet end 45 of the vacuum pump 10. Purge gas is therefore often introduced into the waste stream to retain a tower vapour pressure to inhibit the formation of these deposits. However, when the purge gas is delivered at a reduced temperature compared to that of the waste stream, this reduces the overall temperature of the waste stream and increases the tendency for these materials to condense out. It is therefore desirable to elevate the temperature of the purge gas prior to its delivery into the waste stream. -.4-

In this example, with reference to Figures 2 and 3, purge gas channels 26, 36 are formed in the stator plates 20, 30. The dimensions of these purge gas channels 26, 36 are chosen to effect a particular thermal transfer between the stator and the purge gas under normal operating conditions. Parameters affecting the extent of heating of the purge gas by the interaction between the channel walls, i.e. the bulk material of the stator plate, and the gas stream are flow rate of the purge gas through the channel, length of the channel and cross sectional shape and dimensions of the channel. An additional constraint to be considered is the quantity of purge gas that is required to be delivered to the ic waste stream, this quantity is governed, in turn, by a combination of the pressure and the flow rate at which the purge gas is delivered.

Figure 2 illustrates a stator plate 20 located towards the inlet end 40 of the vacuum pump 10. Purge gas channel 26 is formed in an axial surface of stator plate 20 and extends around the pumping chamber 22. The precise locus of the * .. e purge gas channel 26 is not necessarily as depicted but it is generally located in the axial surface of the stator plate 20 for ease of manufacture. The purge gas * : channel 26 is connected to the pumping chamber 22 via duct 24 and is connected to a purge source 50, external to the pump 10, via duct 28. These ducts 24, 28 may be formed in the axial surface of the stator plate 20 or, as : * 20 shown, may be formed under the axial surface, within the material of the stator plate 20. Ducts 24, 28 may be formed by using casting techniques. Purge gas channel 26 is a relatively short conduit (when compared to that in Figure 3), extending only about part of one side of the pumping chamber 22. The cross sectional area of the purge gas channel is relatively large when compared to that of a purge gas channel provided in a downstream stator plate 30 as depicted in Figure 3.

Figure 3 illustrates a stator plate 30 located towards the outlet end 45 of the vacuum pump 10. Due to its axial location the stator plate 30 will reach a higher operational temperature, as the waste stream passing therethrough will itself be hotter. A purge gas channel 36 is formed in an axial surface of the stator plate 30, and ducts 34, 38 are provided to connect purge gas channel 36 to the pumping chamber 32 and the purge source 50 respectively. The path of purge gas channel 36 is longer than that of the purge gas channel 26 illustrated in Figure 2. Purge gas channel 36 also has a smaller cross sectional area than purge gas channel 26. This increase in channel length and reduction in channel cross sectional area permits the temperature of any purge gas passing through the channel to tend towards the temperature of the stator in which it is formed.

This phenomena is described in further detail below. Example dimensions of the channels illustrated in Figures 2 and 3 are given in the table below: purge gas channel hydraulic channel lengti1 L channel diameter 26 3mm 400mm 36 2mm 650mm * .. 10 The measure of cross sectional area used here is that of "hydraulic diameter". The hydraulic diameter h of any particular two dimensional duct is defined as: * *.

h4A'P *.

* where A = cross sectional area of the duct; and P = distance around the perimeter of the duct. S....

* Figure 4 illustrates how the steady state thermal characteristics of the purge gas vary depending on the length of a channel formed in the stator plate.

This graph also illustrates how these thermal characteristics vary with hydraulic diameter, h. As this graph illustrates, given a sufficiently long channel length adequate heating of the purge gas can be achieved for a wide range of cross sections of purge gas channel. However, as illustrated in Figures 2 and 3 there may be a limited space available in which to extend the channel length. Whilst convolutions in the channel path may be introduced to extend the channel length to some extent, it is preferable to retain simple lad for the purge gas channels 26.

Figure 5 illustrates how the steady state thermal characteristics of the purge gas vary depending on the hydraulic diameter of the purge gas channel.

The stator temperature (60) and 90% stator temperature (65) are marked for comparison. This graph also illustrates how these thermal characteriSticS vary with flow rate of the purge gas, (70 denotes increasing flow rate 23 slm, 49 slm, 69 slm and 104 slm). The example flow rates shown in the graph, each represent reasonable operating flow rates for purge gas delivery to different capacity vacuum pumps. For example, in a I 00m3/hr capacity vacuum pump the flow rate of purge gas into the final, low vacuum, stage ought to be in the region 0123 slm whereas in a 450m3/hr capacity vacuum pump the flow rate of purge gas into the final, low vacuum, stage is preferably in the region of 104 slm. As the hydraulic diameter of the channel decreases the rate of change of io temperature of the purge gas decreases and becomes less sensitive to changes in flow rate. It is, therefore, preferable to use a purge gas channel having a hydraulic diameter, h of less than 4.5 mm in order to achieve a purge gas temperature in excess of 90% of the value of the stator temperature1 especially in : ** the final, low vacuum stage of a vacuum pump. In the higher capacity vacuum S..' pumps where the purge gas flow rate is consequently greater, it is preferable to use a hydraulic diameter, h, of less than 3 mm in order to achieve appropriate purge gas temperatures approaching those of the stator temperature. S..

* As described above, in order to avoid condensation, it is preferable to raise the temperature of the purge gas to be as close to that of the waste stream as possible. The maximum temperature that can be achieved by embedding the purge gas channel within the stator plate 20, 30 is the temperature of the stator plate itself. It is desirable for the purge gas temperature to be raised to at least 90% of this value. Preferably, the temperature of the purge gas should be raised to at least 95% of the value of the temperature of the stator plate through which it is conveyed. Figure 6 represents the variation in flow rate required to achieve appropriate thermal conditions as the hydraulic diameter of the channel is varied.

In particular. the graph shows that if the hydraulic diameter of a channel is large (e.g. 10 mm) the flow rate would need to be exceptionally low, for example 0.5 slm, in order to achieve adequate steady state thermal characteristics. Whilst such a low flow rate may be appropriate in the higher vacuum stages of a vacuum pump this low flow rate would result in inadequate purge flow being delivered to the pumping chamber of the lower vacuum stages towards the outlet of the vacuum pump.

The shape of the cross section of the channel also has an effect on the thermal characteristics of the purge gas being conveyed through the purge gas channel. A purge gas channel having a cross sectional shape with an aspect ratio greater than 1, for example 1.51 will have an elevated temperature when compared to a purge gas channel having the same channel length and hydraulic diameter having an aspect ratio of 1. * * S.. * S*. S... * S. * . * .*

S

S * S S....

Claims

CLJMS

1. A vacuum pump comprising a stator having formed therein a pumping chamber and a purge gas channel for conveying purge gas from a purge gas source to the pumping chamber, the channel having a hydraulic diameter of less than4.5mm.

2. A vacuum pump according to Claim 1, wherein the channel has a hydraulic diameter of less than 3 mm.

3. A vacuum pump according to Claim 1 or Claim 2, wherein the channel is configured such that, at steady state, any purge gas to be conveyed along the ia channel reaches a temperature within 90% of the operating temperature of the stator prior to entering the pumping chamber.

4. A vacuum pump according to Claim 3, wherein the channel is configured such that, at steady state, any purge gas to be conveyed along the channel reaches a temperature within 95% of the operating temperature of the stator prior to entering the pumping chamber.

S

5. A vacuum pump according to any preceding claim, wherein the cross section of the channel has an aspect ratio greater than 1. a,... * a

6. A vacuum pump according to Claim 5, wherein the cross section of the channel has an aspect ratio greater than 1.5.

7. A vacuum pump according to any preceding claim, wherein the vacuum pump is a multi-stage vacuum pump and the stator comprises a plurality of stator plates, a pumping chamber being formed between each adjacent pair of stator plates and the channel being formed in an axial surface of one of the stator plates.

8. A vacuum pump according to Claim 7, wherein a further purge gas channel is formed in another stator plate, each purge gas channel being in fluid communication with a respective pumping chamber.

9. A vacuum pump according to Claim 8, wherein each pumping chamber is in fluid communication with a respective channel, each channel being formed in a stator plate adjacent to its respective pumping chamber.

10. A vacuum pump according to Claim 8 or Claim 9, wherein the purge gas channelS have different hydraulic diameters. * * *.* **.* **** * .* * * * **

S *** *

.. S.. * . S.... * .