Roots pumps

A Roots vacuum pump stator is arranged to house a pair of intermeshing rotors, said stator being characterized in that it comprises a director or deflector arranged to direct solid material entrained in a gas pumped through the pump towards the outlet. In other words, the stator according to the present invention has a means for directing or deflecting powder or solid material entrained in a gas being pumped directly towards an outlet of the pump or out of the pump. The directing means can comprise a channel disposed between the arcuate surface and the outlet, said channel comprising a portion that engages the arcuate surface prior to the bottom-dead-centre position and which extends away from the rotor's axis of rotation towards the pump outlet. Thus, powder entrained in the pumped gas is effectively removed from pumping volume in a way which reduces the likelihood of any solid material causing the pump to seize: the channel can be arranged so that the solid material is “flung” towards the outlet or away from a meshing zone where rotors mesh with one another.

CROSS REFERENCE

This application claims priority to a foreign patent application no. 08167555.5 filed with the European Patent Office on Oct. 24, 2008.

FIELD OF THE INVENTION

The present invention relates to improvements to Roots pumps, including single stage and multi stage roots pumps. In particular, the present invention relates to a stator component and a rotor component of a Roots vacuum pump suitable for use in industrial processes.

BACKGROUND

Vacuum pumps find wide usage throughout industry. For instance, vacuum pumps are used in the semiconductor industry to evacuate a process chamber. The by-products of the process taking place in the chamber can pass through the pump as the gases are evacuated from the chamber. These by-products include substances in vapour, liquid or solid phase and are often “harsh”, by which it is meant that the by-products can cause corrosion or wear of pump components exposed to the by-products.

Efforts have been made to improve vacuum pump design so that a pump can better handle by-products of harsh processes. For instance, certain pump components can be made from non-corrosive materials. Furthermore, certain vacuum pump configurations, such as the so-called “hook and claw” or Northey pumps are known to be relatively effective at handling powder entrained in the pumped gases.

A pump's ability to handle powder is an important factor when considering which type of pump should be used for certain processes known to produce powdered by-products. This is a particular problem with some semi-conductor processes where excessive amounts of silica powder are formed in a process chamber and which then pass into the pump evacuating the chamber. In the worse case scenario, the powder can cause a pump to seize and completely malfunction, resulting in potential loss of semiconductor components in the chambers and the chamber having to be taken off-line whilst a replacement pump is fitted and tested. Moreover, the effective operational lifetime of the pump is shortened by excessive exposure to powders.

SUMMARY

The present invention aims to ameliorate the problems of the prior art and provide a Roots-configuration pump suitable for use in the semiconductor industry (but not limited to use in the semiconductor sector, of course) with improved powder-handling capabilities.

To achieve this aim, the present invention provides a Roots pump stator arranged to house a pair of intermeshing rotors, said stator being characterised in that the stator comprises director or deflector arranged to direct solid material entrained in a gas pumped through the stator towards the outlet. In other words, the stator according to the present invention has a means for directing or deflecting powder or solid material entrained in a gas being pumped directly towards an outlet of the pump or out of the pump. This may be achieved by providing a channel disposed in the stator wall that tangentially engages the arc surface of the stator arranged to cooperate with the tip of the Roots rotor. The channel can have a generally flat or linear profile that is angled towards to the pump outlet so that solid material or powder in the pump chamber is encouraged towards the outlet as the rotor rotates. Thus, powder entrained in the pumped gas is effectively removed from the pumping volume in a way which reduces the likelihood of any solid material causing the pump to seize: the channel can be arranged so that the solid material is “flung” towards the outlet or away from a meshing zone where rotors mesh with one another. Alternatively, or in addition, this can be achieved by providing a member that extends above the outlet and has an underside facing the outlet which is so profiled to direct solid material into the outlet.

More specifically, there is provided a Roots pump stator comprising: a pumping volume arranged to accommodate a pair of intermeshing contra-rotating multi-lobed rotors, each being rotatable about an axis such that a tip portion of a rotor lobe can cooperate with the arcuate surface of the stator, and the lobes pass through a top-dead-centre and bottom-dead-centre position with respect to the axis; an inlet disposed above the axis for receiving gas into the pumping volume; and an outlet disposed below the axis for exhausting gas from the pumping volume; characterised in that the stator comprises directing means arranged to direct material entrained in a pumped gas towards the outlet. Also, there is provided a Roots pump stator comprising: a pumping volume arranged to accommodate a pair of intermeshing contra-rotating multi-lobed rotors, each being rotatable about a horizontal axis such that a tip portion of a rotor lobe can cooperate with the arcuate surface of the stator, the arcuate surface having a depth dimension in the plane of the horizontal axis, and the lobes pass through a top-dead-centre and bottom-dead-centre position with respect to the horizontal axis; an inlet disposed above the horizontal axis for receiving gas into the pumping volume; and an outlet disposed below the horizontal axis for exhausting gas from the pumping volume; characterised in that the directing means comprises a channel disposed between the arcuate surface and the outlet, said channel comprising a portion that engages the arcuate surface prior to the bottom-dead-centre position. Thus, powder entrained in the pumped gas is effectively removed from pumping volume in a way which reduces the likelihood of any solid material causing the pump to seize: the channel can be arranged so that the solid material is “flung” towards the outlet or away from a meshing zone where rotors mesh with one another.

Additionally, or alternatively, there is provided a Roots pump stator comprising: a pumping volume arranged to accommodate a pair of intermeshing contra-rotating multi-lobed rotors, each being rotatable about an axis such that a tip portion of a rotor lobe can cooperate with the arcuate surface of the stator, and the lobes pass through a top-dead-centre and bottom-dead-centre position with respect to the axis; an inlet disposed above the axis for receiving gas into the pumping volume; and an outlet disposed below the axis for exhausting gas from the pumping volume; characterised in that the stator comprises a deflector disposed at the outlet which is arranged to direct material passing through the pumping volume towards the outlet. Also, there is provided a Roots pump stator comprising: a pumping volume arranged to accommodate a pair of intermeshing contra-rotating multi-lobed rotors, each being rotatable about a horizontal axis such that a tip portion of a rotor lobe can cooperate with the arcuate surface of the stator, the arcuate surface having a depth dimension in the plane of the horizontal axis; an inlet disposed above the horizontal axis for receiving gas into the pumping volume; and an outlet disposed below the horizontal axis for exhausting gas from the pumping volume; characterised in that the directing means comprises a deflector disposed at the outlet which is arranged to direct material passing through the pumping volume towards the outlet. Thus, powder entrained in the pumped gas is effectively removed from pumping volume in a way which reduces the likelihood of any solid material causing the pump to seize: the deflector is arranged so that the solid material is directed or deflected towards the outlet or away from a meshing zone where rotors mesh with one another.

Additionally, the portion of the channel can be disposed to tangentially engage the arcuate surface at between 5 to 45 degrees in advance of bottom-dead-centre, or between 5 to 25 degrees in advance of bottom-dead-centre or at a position 15 degrees in advance of bottom-dead-centre. As a result, any entrained powder can be thrown radially away from the rotors so that it does not become trapped between the intermeshing rotors.

Additionally, at least one deflector surface can be disposed between the first portion of the channel and the outlet. The deflector surface can be angled towards the outlet and arranged to direct material passing through the pumping volume towards the outlet. The deflector surface also can be arranged to extend across width of the channel to form a closed channel.

Additionally, the deflector has an upper surface which forms a portion of the arcuate surface between the channel and the outlet.

The present invention also provides a multi-staged vacuum pump comprising a stator as described above and a rotor component comprising a plurality of rotor elements arranged to cooperate with a respective plurality of stator stages, wherein each rotor element comprises a lobe portion arranged to cooperate with a portion of the stator, each lobe portion having at least one axial groove disposed at or near its tip, furthest from an axis of rotation.

DETAILED DESCRIPTION

FIG. 1shows a cross section of a Roots pump10known in the art and described in WO2007/088103. The pump comprises a pumping volume11defined by a stator body12. A pair of contra-rotating intermeshing multi-lobed rotors16,17are arranged to rotate about respective horizontal axes14and15. The pump inFIG. 1has two lobes on each rotor and tip portions20and21of the lobes are arranged to cooperate with an arcuate inner surface24of the stator, thereby trapping a volume of gas26between the rotor and stator12. Gas is pumped from an inlet30to an outlet40by the counter rotational movement of the rotors.

Each tip portion20,21of the lobes pass through a top-dead-centre position51and bottom-dead-centre position52during its rotation cycle. Typically, the top-dead-centre position is after or coincides with a point where the arcuate inner surface24meets the pump's inlet30. Furthermore, the bottom-dead-centre position52is in advance of a point where the inner arcuate surface24meets the outlet. This arrangement provides efficient pumping of gas because volume of gas26remains trapped between the rotor and stator for sufficient time to allow proper operation of the pump. Conventionally, for a two-lobed rotor the points where the stator inner wall24meets the inlet and outlet should be more than 180° apart otherwise effective compression of the pumped gases may not occur.

FIG. 2shows a cross section of another known Roots vacuum pump described in U.S. Pat. No. 7,226,280. This figure shows a three-lobed rotor configured Roots pump where the same reference numerals have been used to indicate the same or similar components. In addition, the inner arcuate surface24comprises a plurality of constant depth grooves60cut into the inner surface24. The grooves are formed at a point41prior to the bottom-dead-centre position52and extend to the outlet40. The purpose of these grooves is to release of portion of the pumped gas26trapped between the rotor and stator at a relatively early stage of the rotation cycle in an attempt to reduce any noise associated with the pumped gases exhausting from the output.

Both of the prior art pumps described above can suffer from problems when the gas being pumped contains a powder substance. The powder tends to become trapped between the pump components and ultimately causes the pump to seize. Thus, the limit of powder that can be handled by Roots vacuum pumps without pump seizure is relatively small and of the order of 50-100 grams of a silica powder load for a pump having a pump-rating of 100 cubic meters per hour.

FIG. 3shows a cross sectional diagram of a pump100embodying the present invention. The pump comprises a stator110embodying the invention which provides a pumping volume and which is arranged to accommodate a pair of intermeshing contra-rotating multi-lobed rotors116and117, each rotatable about horizontal axes114and115. The rotors inFIG. 3are shown to have three lobes, but it is understood that the invention is not limited to this arrangement and the inventive concept can apply to any number of rotor lobe configurations.

The stator110comprises an inner wall or surface124which follows an arc path between points A and B. During use, the rotor's lobe tip portions120and121cooperate with the inner surface124to trap pumped gas in a volume126between the rotor and stator. In practice, during use the tips of the rotors have a relatively small clearance between the tip and stator arc surface. Point A of the inner surface120is arranged to be in front of the top-dead-centre position T with respect to the direction of rotation for the rotor.

However, in contrast to the known Roots pump systems described previously, at least a portion125of the inner surface, between point B and the outlet of the pump, is arranged to follow a different path which does not continue along or parallel with the arcuate path of the inner surface124. In the embodiment shown inFIG. 3, this stator outlet portion125follows a path that is substantially linear, it follows a tangent of the arcuate inner surface, and extends away from the axis of rotation towards the output140of the pump. In other words, point B is disposed prior to the bottom-dead-centre position of the rotor lobe and the depth of a channel forming the outlet portion125increases from point B (where the channel engages the arcuate inner surface124of the stator) towards the outlet. In the embodiment shown inFIG. 3, the outlet portion of the stator, or channel, extends away from the axis of rotation in a linear manner, or having a linear cross-section when viewed in the plane of the rotor's rotation. It is understood that the channel can extend away from the axis is a non-linear manner, for instance by following a radius centred inside or outside of the pumping volume or by following a step profile or the like.

Point B can be arranged to be at bottom-dead-centre or within a range of angles in advance of bottom-dead-centre. For instance, Point B can be arranged to be between 5 to 45 degrees in advance of bottom-dead-centre, as indicated by angle α inFIG. 3. Preferably, α can be between 5 to 25 degree, and more preferably α=15°. If the initial part of the outlet portion125of the stator inner surface124is tangential to arcuate portion of the inner surface, then an angle β between an imaginary line I (passing through the top-dead-centre and bottom-dead-centre positions) and the tangent formed at point B follows the equation β=90+α. Thus, it follows that 135°≦β≦90°.

It has been found that outlet portion125of the stator acts as a means for directing particulates or powder passing through the pump towards the outlet140. Any particulates entrained in the pumped gas are flung by centrifugal forces towards the arcuate surface124as the rotors drive the pumped gas through the pump. Thus, particulates in the gas are directed towards the outlet140and away from a zone145where the rotors mesh with one another because the outlet portion125generally slopes towards the outlet140of the pump or pumping stage.

As a result of this arrangement embodying the present invention, we have determined that conventional Roots pumps as described above have poor powder handling capabilities because of the profile of the stator's outlet portion: a conventional Roots pump stator can cause at least some particulates entrained in the pumped gas to be forced or thrown towards a meshing zone of the rotors. Thus, a portion of the powder entrained in the pumped gas passing through a conventional pump can be re-circulated through the pump. If significant amounts of powder enter the zone where the rotors mesh, then pump seizure can occur.

FIG. 4is an isometric view of a portion of a Roots pump stator110embodying the present invention. The stator is formed of a plurality of pumping stages, as is known in the art. In this embodiment, the stator is formed in a “clam-shell” configuration and only the bottom portion of the complete stator is shown for clarity. It is also understood that stator end-plates (not shown) are utilised in the complete pump.

Imaginary line I is shown projected against a stator side wall126. The start of the outlet portion125of the stator wall124is advanced from a position where a rotor passes through a bottom-dead-centre position. Furthermore, the floor127of the outlet portion125has a width dimension δ that is less than a width dimension D of the arcuate stator inner surface124. It is envisaged that the ratio of D:δ can be in the range of 2:1 to 10:11 (that is 50% to 110%). In other words, if excessive amounts of powder in the pump are anticipated then having width dimension that is greater than the stator depth dimension could assist with improving effective handling of powder.

Other configurations of stator output or outlet portions are envisaged, either in addition to the above described embodiments or as an alternative, and the present invention is not limited to linear configuration described above. For instance, the outlet portion can be arranged to follow a path having a radius that is greater than the radius of the inner stator surface124. It is preferable that the distance between the floor or bottom of the outlet portion and the contact portion of the rotor lobe increase from a few microns at point B to at least 1 cm at the outlet. Furthermore, the width of the outlet portion can be arranged to vary and preferably increase towards outlet. The outlet portion can be configured to include a plurality of grooves cut into stator inner surface.

Further means for directing dust, powder or particulates entrained in the pump gas towards the outlet of the pump or pump stage are now described with reference toFIG. 5. In one embodiment vein member132is disposed above the outlet portion of the stator. The vein extends across the outlet portion, either completely or partially across the outlet port, and has a surface facing the outlet that is angled towards the outlet140in order to deflect particulates entrained in the pumped gas towards to outlet140. A relatively small clearance is provided between the vein and the path134taken by the tip portion120of the rotor's lobe. In another embodiment, a directing member133can be configured as a central component disposed directly above the outlet140having two opposed surfaces facing the outlet, each arranged to direct powdered material towards the outlet from each rotor respectively. In this way, at least a portion of particulates entrained in the gas being pumped can be deflected towards the outlet140and before they would otherwise enter a meshing zone145where the rotors mesh. An upper surface135of the vein or deflector can be arranged to form a portion of the arcuate surface124. The upper surface can be arranged to extend to, but not enter, the meshing zone145.

Our experiments have shown that powder handling capabilities of a pump having a stator configuration embodying the present invention are greatly improved without affecting the pumping capability to a significant degree. For instance, we have noted improvements of up to 400%, where a pump rated at 100 meters cubed per hour can efficiently handle a powder load of 400 grams without seizure.

Referring now toFIG. 6, a pump100embodying the present invention is shown. One half of a stator clamshell110is shown and is described above with reference toFIG. 3,4or5. In addition, a pair of rotors200is shown. Each rotor comprises a shaft210and rotor elements having multiple lobes216,217. The tip portions of the lobes comprise axial grooves220, which we have found to improve the powder handling capability of Roots pumps. By disposing the grooves on each stage of a multi-staged Roots pump, the powder handling can be further improved. In the embodiment shown inFIG. 6, each tip portion comprises three grooves. However, it is understood that other embodiments can comprise any number grooves, which can be arranged into any configuration where a number of grooves are disposed fore or aft of the point where the tip portion of the rotor lobe cooperates with the stator inner arcuate surface124. For instance, a middle groove can be disposed adjacent the point where the rotor lobe and stator cooperate, with one groove disposed in a leading position and another groove disposed in a trailing position, respectively. Other combinations of numbers of grooves (one, two, or more) per lobe and their respective positions will be envisaged by the skilled person. Also, the number of grooves is likely to depend on the pump application or process being pumped.

Other embodiments of the present invention will be envisaged by the skilled person without departing from the scope of inventive concept.