Patent Description:
A known vacuum pump <NUM> is shown in <FIG> which comprises a pumping mechanism <NUM>. The pumping mechanism comprises a plurality of pumping stages <NUM> for pumping fluid along a fluid flow path <NUM> between an inlet <NUM> for fluid at high vacuum and an outlet <NUM> for fluid at low vacuum to atmospheric pressure. Five pumping stages <NUM> are shown. A motor <NUM> drives rotation of the rotors R relative to the stators S in each of the pumping stages <NUM>.

If the fluid being pumped comprises a corrosive agent, such as fluorine, corrosion is caused to the pumping mechanism <NUM>. Over time, corrosion causes build up of deposits on the surface of components of the pumping mechanism which causes a reduction in the running clearances between the rotors R and the stators S of the pumping stages <NUM>. After continued operation of the pump over many hours the corrosion can bring the rotors and stators of the pumping stages into contact causing pump failure.

It is possible to reduce corrosion in vacuum pumps by manufacturing the pumping mechanism from a corrosion resistant material, as disclosed in <CIT>, but typically such materials are expensive.

The present invention provides a dry vacuum pump for pumping corrosive fluid, the pump comprising the features as defined in independent claim <NUM>, having a first and second section and a first and second material as defined therein.

The first material may be spheroidal graphite iron. The second material may be one of the group of Nickel rich spheroidal graphite iron, cast stainless steel, Nickel alloy and Ni-res D5S.

The respective materials of the first section and the second section may be such that build-up of corrosive deposits at the first section is less than or equal to build-up of corrosive deposits at the second section. The first section and the second section may be defined by a respective plurality of said pumping stages. The first section may be adjacent to the second section along said fluid flow path.

The, or each, pumping stage of the second section may comprise components made from a Nickel rich iron. The, or each, pumping stage of the first section may comprise components made from Spheroidal graphite iron.

The components of said pumping stages are fabricated from selected materials such that the build-up of corrosion deposits in each stage during pumping of corrosive gas is generally equal one stage to another stage.

Other preferred and optional aspects of the invention are defined in the accompanying claims.

In order that the present invention may be well understood, an embodiment thereof, which is given by way of example only, will now be described with reference to the accompanying drawings, in which:.

Referring to <FIG>, a multi-stage vacuum pump <NUM> is shown for pumping corrosive fluid. A pumping mechanism <NUM> comprises a plurality of pumping stages <NUM> for pumping fluid along a fluid flow path <NUM> between an inlet <NUM> for fluid at high vacuum and an outlet <NUM> for fluid at low vacuum to atmospheric pressure. Five pumping stages <NUM> are shown in this example. A motor <NUM> drives rotation of a rotor R relative to a stator S in each of the pumping stages <NUM>.

In arriving at the present invention, an analysis of the prior art pump shown in <FIG> was conducted and <FIG> shows a graph in which corrosion build-up over time (<NUM>,<NUM> hours in this example) is plotted against pump reference temperature for the prior art pump. In the graph a first line shows build-up at the faces of the rotor and stator of a middle pumping stage along the flow path <NUM> and a second line shows build-up at the faces of the rotor and stator of a final pumping stage along the flow path <NUM>. The middle pumping stage in this example is the <NUM>rd stage. Corrosive build-up is measured in microns and temperature is measured in degrees centigrade. The temperature used in the graph is a pump reference temperature taken at the final pumping stage. It will be appreciated that the temperature of the middle pumping stage is less than that shown in the graph but for simplicity has not been shown. The pump reference temperature increases during operation and the increase is dependent on a number of factors such as the type of fluid which is pumped and the work exerted by the pump.

In <FIG>, the material from which the pumping mechanism is made is relatively non-corrosion resistant. An example of such a material is SG iron. It will be seen from <FIG> that corrosive build-up of the final stage is considerably greater than that of the middle pumping stage, particularly when the pump reference temperature is at <NUM>. At this reference temperature, corrosive build-up at the final stage is just lower than <NUM>, whereas at the middle stage, build-up is only just higher than <NUM>.

<FIG> shows an equivalent graph as shown in <FIG>, except in this analysis the material from which the pump is made is relatively corrosion resistant. An example of such a material is Ni-rich SG iron. In <FIG>, corrosive build-up at both the middle and final pumping stages is reduced, but build-up at the final pumping stage has been reduced by around <NUM> to just lower than <NUM> whereas build-up at the middle pumping stage has been reduced by only around <NUM> to around <NUM>.

When considering the pump shown in <FIG>, the pressure of fluid along the flow path <NUM> increases from the inlet <NUM> to the outlet <NUM> as fluid is compressed by each pumping stage <NUM>, typically with the compression ratio increasing from one pumping stage to the next pumping stage along the flow path. The temperature of the fluid and the pumping mechanism also increases along the flow path.

Accordingly, if the fluid being pumped comprises a corrosive agent, such as fluorine, the amount of corrosion caused to the pumping mechanism <NUM> increases along the flow path <NUM> as temperature and pressure increase. Increased pressure increases the amount of corrosive molecules available for corroding the pumping mechanism and increased temperature increases corrosive reaction. Therefore, corrosive build-up is greater at the final pumping stage than at the middle pumping stage. Accordingly, pump failure occurs because of the reduction in running clearance at the final stage of the pumping mechanism where build-up is greatest. Whilst corrosion resistance can be increased as shown in <FIG>, pump failure often occurs at the final stage of the pumping mechanism.

In the pump shown in <FIG>, the pumping mechanism comprises a first section <NUM> and a second section <NUM>. The second section <NUM> is downstream of the first section <NUM>. During operation, the temperature and pressure of the downstream section <NUM> is greater than the temperature and pressure of the upstream section <NUM>. Therefore, when pumping a corrosive fluid, corrosion of section <NUM> is less than corrosion of section <NUM>. As the build-up of corrosion deposits on the rotors and stators in section <NUM> is less than in section <NUM>, the first section <NUM> is required to be less resistant to corrosion than the second section <NUM>. The first section <NUM> can therefore be made from a material which is relatively less expensive than the material of the second section.

The first section <NUM> and the second section <NUM> comprise a respective plurality of pumping stages <NUM>. The first section is adjacent to the second section along the fluid flow path. In <FIG>, the first section comprises <NUM>st, <NUM>nd and <NUM>rd pumping stages whilst the second section <NUM> comprises <NUM>th and <NUM>th pumping stages. In another arrangement, one of the first and the second sections may comprise a single pumping stage. For instance, the first section may comprise <NUM>st to <NUM>th pumping stages whilst the second section may comprise the <NUM>th pumping stage. As temperature and pressure increase to the greatest extent at the final downstream pumping stage it may be desirable to manufacture this stage from a corrosion resistant material whereas stages <NUM> to <NUM> may be manufactured from a material which is less corrosion resistant. Alternatively, the first section may comprise the <NUM>st pumping stage and the second section may comprise the <NUM>nd to <NUM>th pumping stages.

A graph equivalent to the graphs shown in <FIG> and <FIG> is shown in <FIG>, which plots corrosive build-up against pump reference temperature for the pump shown in <FIG>. <FIG> shows at <NUM> that corrosive build-up at the final stage of the pumping mechanism has been reduced from around <NUM> as shown in <FIG> to just lower than <NUM> as is the case with a corrosion resistant pump according to <FIG>. However, in <FIG>, corrosive build-up at the middle, or <NUM>rd, stage is the same as that shown in <FIG> for a non-corrosion resistant pump. In this regard, the corrosive build-up of the middle stage is around <NUM> which is less than that of the final stage even though the middle stage is made from a non-corrosion resistant material (e.g. SG iron) and the final stage is made from a corrosion resistant material (e.g. Ni-rich iron). Therefore, there is no benefit to be gained from making both the first section and the second section of a pumping mechanism from a corrosion resistant material and doing so would unnecessarily add to the cost of a pump.

Materials should be selected for the first and second sections so that the build-up of corrosive deposits at the first section is less than or equal to the build-up of corrosive deposits at the second section. The components of said pumping stages are fabricated from selected materials such that the build-up of corrosion deposits in each stage during pumping of corrosive gas is generally equal one stage to another stage. In this way, the materials for the various pumping stages can be selected in a cost efficient manner whilst maintaining acceptable resistance to corrosion. In <FIG>, the rotor R and stator S of each pumping stage <NUM> of the second section <NUM> is made from a Nickel rich iron, whilst the rotor R and stator S of each pumping stage <NUM> of the first section <NUM> is made from an SG (spheroidal graphite) iron.

Examples of these materials are shown in <FIG>, although other materials may be selected according to requirements. For instance, Nickel is resistant to fluorine but if the fluid being pumped contains other corrosive agents it would be desirable to select an appropriately resistant material. Further, the first section of the pumping mechanism may be made from materials other than SG iron.

If the pump is to be used for pumping particularly corrosive fluid, the first section may be made from a corrosion resistant material for instance NI-rich SG iron, whilst the second section may be made from a more corrosion resistant material such as cast stainless steel or nickel alloy.

<FIG> shows three examples of both SG iron and Ni-rich SG iron. It is preferable to select materials for the first and second section with similar linear expansion coefficients. In this regard, Ni-res D-<NUM> is a preferred corrosion resistant material as its linear expansion coefficient is <NUM>/mK which is similar to the coefficient of SG iron of <NUM>/mK.

The constituents of an Ni-rich SG iron material, which exhibits good corrosion resistant properties and also good strength and stiffness in high temperature conditions are shown in <FIG>. In such a material, it will be seen that the Nickel content is relatively high between <NUM> % and <NUM> % by weight.

The same or similar material is used for the rotor R and stator S in each section to avoid problems associated with having components of different thermal expansion coefficients.

Claim 1:
A dry vacuum pump (<NUM>) for pumping corrosive fluid, the pump comprising: a dry pumping mechanism (<NUM>) comprising a plurality of pumping stages (<NUM>) along a fluid flow path (<NUM>) between an inlet (<NUM>) for fluid at high vacuum and an outlet (<NUM>) for fluid at low vacuum, each of said pumping stages (<NUM>) comprising a stator (S) and a rotor (R), wherein the pumping stages comprise a roots pumping mechanism or a claw pumping mechanism, characterised in that the rotor and the stator of a pumping stage at a first section (<NUM>) of said flow path is formed from a first material and the rotor and the stator of a pumping stage at a second section (<NUM>) of said flow path, downstream of said first section, is formed from a second material, the first and second materials being configured such that the build-up of corrosion deposits in each respective stage of the pump are generally equal, one stage to the other stage, during operation of the pump.