Baffle configurations for molecular drag vacuum pumps

A molecular drag compressor includes a rotor disk coupled to a drive shaft for rotation about an axis, a stator disposed about the rotor disk, the stator defining a tangential flow channel, an inlet to the tangential flow channel and an outlet from the tangential flow channel, and a stationary baffle disposed in the tangential flow channel adjacent to the outlet. The baffle and the rotor disk have a gap between them. A surface of the baffle facing the rotor disk has cavities configured to produce turbulent gas flow through the gap between the baffle and the rotor disk and to thereby reduce leakage.

FIELD OF THE INVENTION

This invention relates to vacuum pumps used for evacuating an enclosed vacuum chamber and, more particularly, to baffle configurations for molecular drag vacuum pumping stages of a vacuum pump. The molecular drag pumping stages can be utilized in hybrid turbomolecular vacuum pumps, but are not limited to such applications.

BACKGROUND OF THE INVENTION

Conventional turbomolecular vacuum pumps include a housing having an inlet port, and interior chamber containing a plurality of axial pumping stages, and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high speed to provide pumping of gases between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include 9 to 12 axial pumping stages.

Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid vacuum pumps, are known in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages which form a molecular drag compressor. This configuration is disclosed in U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 to Casaro et al. A hybrid vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian, Inc. Other hybrid vacuum pumps are disclosed in U.S. Pat. No. 5,074,747 issued Dec. 24, 1991 to Ikegami et al.; U.S. Pat. No. 5,848,873 issued Dec. 15, 1998 to Schofield; and U.S. Pat. No. 6,135,709 issued Oct. 24, 2000 to Stones.

Molecular drag compressors include a rotor disk and a stator. The stator defines a tangential flow channel and an inlet and an outlet of the tangential flow channel. A stationary baffle, often called a stripper, is disposed in the tangential flow channel and separates the inlet and the outlet. As known in the art, the momentum of the rotor disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. The rotor disk and the stator of the molecular drag compressor are separated by a small gap, typically on the order of 0.005 inch, selected to permit unrestricted rotation of the disk, while limiting leakage through the gap.

Prior art vacuum pumps which include an axial turbomolecular compressor and a molecular drag compressor provide generally satisfactory performance under a variety of conditions. Nonetheless, improvements are desired. One source of performance degradation that occurs in the molecular drag stages is backward leakage through the gaps between the rotor disk and the stator. In a specific example, gas may leak from the outlet of the molecular drag stage through the gap between the stationary baffle and the rotor disk to the inlet, thus reducing the achievable pressure ratio of the pumping stage. Leakage can be reduced by reducing the dimension of the gap between the stationary baffle and the rotor disk. However, a reduction in gap dimension requires increased precision and thereby increases cost. Furthermore, very small gaps increase the risk of undesired contact between the rotor disk and the stator during operation.

Accordingly, there is a need for improved molecular drag vacuum pumps which have a low level of backward leakage.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a molecular drag compressor comprises a rotor disk coupled to a drive shaft for rotation about an axis, a stator disposed about the rotor disk, the stator defining a tangential flow channel, an inlet to the tangential flow channel and an outlet from the tangential flow channel, and a stationary baffle disposed in the tangential flow channel adjacent to the outlet. The baffle and the rotor disk have a gap between them. A surface of the baffle facing the rotor disk has cavities configured to produce turbulent gas flow through the gap between the baffle and the rotor disk and to thereby reduce leakage.

According to a second aspect of the invention, an integral high vacuum pump comprises a pump housing having an axis, an axial turbomolecular compressor disposed in the housing and coupled to a motor drive shaft, and a molecular drag compressor disposed in the housing and coupled to the motor drive shaft. The molecular drag compressor includes at least one molecular drag stage comprising a rotor disk coupled to the motor drive shaft for rotation about an axis, a stator disposed around the rotor disk, the stator defining a tangential flow channel, an inlet to the tangential flow channel, an outlet from the tangential flow channel, and a stationary baffle disposed in the tangential flow channel adjacent to the outlet. The baffle and the rotor disk have a gap between them. A surface of the baffle facing the rotor disk has cavities configured to produce turbulent gas flow through the gap between the baffle and the rotor disk and to thereby reduce leakage.

According to a third aspect of the invention, a method is provided for operating a molecular drag compressor, which includes a rotor disk coupled to a drive shaft, stator disposed around the rotor disk, the stator defining a tangential flow channel, an inlet to the tangential flow channel and an outlet from the tangential flow channel, and a stationary baffle disposed in the tangential flow channel adjacent to the outlet, the baffle and the rotor disk having a gap between them. The method comprises producing turbulent gas flow through the gap between the baffle and the rotor disk to thereby reduce leakage.

DETAILED DESCRIPTION OF THE INVENTION

An integral high vacuum pump suitable for incorporation of the present invention is shown inFIG. 1. A housing10defines an interior chamber12having an inlet port14and an exhaust port16. The housing10includes a vacuum flange18for sealing the inlet port14to a vacuum chamber (not shown) to be evacuated. The exhaust port16is typically connected to a roughing vacuum pump (not shown). In cases where the vacuum pump is capable of exhausting to atmospheric pressure, the roughing pump is not required. Located within housing10is an axial turbomolecular compressor20, which typically includes several axial turbomolecular stages, and a molecular drag compressor22, which typically includes several molecular drag stages. In general, axial turbomolecular compressor20includes one or more axial turbomolecular stages, and molecular drag compressor22includes one or more molecular drag stages.

Each stage of the axial turbomolecular compressor20includes a rotor24and a stator26. Each rotor and stator has inclined blades as known in the art. Each stage of the molecular drag compressor22includes a rotor disk30and a stator32. The molecular drag compressor22is described in more detail below. The rotor24of each turbomolecular stage and the rotor30of each molecular drag stage are attached to a drive shaft34. The drive shaft34is rotated at high speed by a motor located in a motor housing38.

A first configuration of the molecular drag compressor22is shown inFIGS. 2–4. In the molecular drag compressor, the stator is provided with one or more tangential flow channels. Each tangential flow channel has an inlet and an outlet separated by a stationary baffle. When the disk is rotated at high speed, gas is pumped through the tangential flow channel by molecular drag produced by the rotor disk.

As shown inFIGS. 2–4, a molecular drag stage includes a rotor disk100, an upper stator portion102and a lower stator portion104mounted within a housing105. The upper stator portion102is located in proximity to an upper surface of disk100, and lower stator portion104is located in proximity to a lower surface of disk100. The upper and lower stator portions102and104together constitute the stator for the molecular drag stage. The rotor disk100is attached to a shaft106for rotation at high speed.

The upper stator portion102has an upper tangential flow channel110located in opposed relationship to the upper surface of disk100. The lower stator portion104has a lower tangential flow channel112located in opposed relationship to the lower surface of disk100. In the configuration ofFIGS. 2–4, the tangential flow channels110and112are circular and are concentric with the disk100. The upper stator portion102includes a stationary baffle114which blocks tangential flow channel110at one circumferential location. The channel110receives gas from a previous stage through an inlet116on one side of baffle114. The gas is pumped through the tangential flow channel110by molecular drag produced by the rotor disk100. At the other side of baffle114, a conduit120, formed in stator portions102and104, interconnects channels110and112around the outer peripheral edge of disk100. The lower stator portion104includes a stationary baffle122which blocks lower tangential flow channel112at one circumferential location. The lower channel112receives gas on one side of baffle122through conduit120from the upper surface of disk100and discharges gas to the next stage through a conduit124on the other side of baffle122.

In operation, gas is received from the previous stage through conduit116. The previous stage can be a molecular drag stage, an axial turbomolecular stage, or any other suitable vacuum pumping stage. The gas is pumped around the circumference of upper tangential flow channel110by molecular drag produced by rotation of disk100. The gas then passes through conduit120around the outer periphery of disk100to lower tangential flow channel112. The gas is then pumped around the circumference of lower tangential flow channel112by molecular drag and is exhausted through conduit124to the next stage or to the exhaust port of the pump. In the configuration illustrated inFIGS. 2–4, upper channel110and lower channel212are connected such that gas flows through the upper and lower channels in series. Also in the configuration ofFIGS. 2–4, the upper tangential flow channel100and the lower tangential flow channel212are spaced inwardly from the outer peripheral edge of disk100. This configuration limits leakage between channels110and112around the outer edge of disk100, except through conduit120.

A second configuration of the molecular drag stage is shown inFIGS. 5A and 5B. A partial cross-sectional view of the molecular drag stage near the outer periphery of the rotor disk is shown. In the configuration ofFIGS. 5A and 5B, a rotor disk150is positioned between an upper stator portion152and a lower stator portion154. The upper stator portion152defines an upper tangential flow channel160above rotor disk150, and the lower stator portion154defines a lower tangential flow channel162below rotor disk150. A peripheral stator portion156is spaced from the outer periphery of rotor disk150, so that upper and lower tangential flow channels160and162are effectively connected in parallel. As shown inFIG. 5B, a stationary baffle166is positioned in tangential flow channels160and162at one circumferential location so as to substantially block gas flow between the inlet and outlet, except through each tangential flow channel.

A third configuration of the molecular drag stage is shown inFIGS. 6A and 6B. A partial cross-sectional view of the molecular drag stage near the outer periphery of the rotor disk is shown. A rotor disk180is positioned between an upper stator portion182and a lower stator portion184. The upper stator portion182defines an upper tangential flow channel190, and the lower stator portion184defines a lower tangential flow channel192. A small gap194between the outer periphery of rotor disk180and a peripheral stator portion186permits rotation of rotor disk180but substantially blocks gas flow between tangential flow channels190and192. Thus, tangential flow channels190and192may be connected in series. As shown inFIG. 6B, a stationary baffle196is positioned in upper tangential flow channel190at one circumferential location, and a stationary baffle198is positioned in lower tangential flow channel192at one circumferential location. Each of the stationary baffles196and198is positioned between the inlet and the outlet of the respective tangential flow channel and substantially blocks gas flow between the inlet and the outlet, except through each tangential flow channel.

It will be understood that the tangential flow channels of a molecular drag stage may have a variety of configurations and shapes. However, in each case, a stationary baffle is typically positioned at one circumferential location of the tangential flow channel to substantially block direct gas flow between the inlet and the outlet, except through the tangential flow channel. Nonetheless, some gas leaks through the gap between the rotor disk and the stationary baffle. Such backward leakage through the gap between the rotor disk and the stationary baffle degrades the performance of the vacuum pump.

An aspect of the invention is illustrated with reference toFIGS. 7 and 8. Partial schematic elevation and plan views, respectively, of a molecular drag stage are shown. A rotor disk300rotates about an axis302. A stator304positioned above rotor disk300defines a tangential flow channel306. The stator304further defines an inlet308to tangential flow channel306and an outlet310from tangential flow channel306. A stationary baffle320is disposed in tangential flow channel306adjacent to outlet310. The baffle320may, but is not required to be, an integral part of stator304.

A surface324of baffle320facing rotor disk300is provided with cavities330. Rotor disk300is spaced from surface324by a gap332and moves relative to surface324during operation of the vacuum pump. Cavities330extend from surface324into stationary baffle320and are configured to reduce gas flow through gap332between rotor disk300and stationary baffle320in comparison with the case where surface324is flat. Cavities330effectively produce turbulence in the gas flow through gap332and thereby reduce the volume of gas flow. Cavities330may have a variety of configurations within the scope of the invention.

The cavities in the surface of baffle320reduce the transfer of pumped gas through gap332. By providing cavities in the surface of the baffle, the gas flow in the gap becomes turbulent and therefore is reduced. The cavities can be configured using multiple grooves, holes, or dimples in the surface the baffle facing the rotor disk.

The shape of cavities330depends on the dimension of gap332, i.e., the spacing between rotor disk300and surface324of baffle320. The gap is typically in a range of 0.125 to 0.250 millimeter, but is not limited to this range. The total area of cavities330is preferably in a range of 30 to 70 percent of the total area of surface324facing rotor disk300. The cavities330preferably have dimensions that are 1 to 10 times larger than the gap between baffle320and rotor disk300. The ratios of the typical depths of the cavities to their lateral dimensions should preferably be near unity, although the depth can be larger without significant effect.

The cavities can be simple cylindrical holes in staggered rows, as shown inFIG. 8. Thus, rows340are offset from rows342in a direction orthogonal to the direction of rotation of rotor disk300. In other embodiments, the cavities330can be semi-circular, semi-oval, triangular, rectangular or square in cross-section.FIG. 9shows elongated cavities350having long dimensions oriented generally orthogonally to the direction of rotation of rotor disk300.FIG. 10shows rectangular cavities360arranged in staggered rows362and364. The lateral dimension of the cavities is preferably in a range of 0.25 to 1.25 millimeters.

Having described several embodiments and an example of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and the scope of the invention. Furthermore, those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the system of the present invention is used. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined by the following claims and their equivalents.