Patent Publication Number: US-7223064-B2

Title: Baffle configurations for molecular drag vacuum pumps

Description:
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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is a cross-sectional elevation view of a high vacuum pump which includes an axial turbomolecular compressor and a molecular drag compressor; 
         FIG. 2  is a cross-sectional elevation view of a first configuration of a molecular drag vacuum pumping stage; 
         FIG. 3  is a cross-sectional plan view of the molecular drag stage, taken along the line  3 — 3  of  FIG. 2 ; 
         FIG. 4  is a partial, cross-sectional elevation view of the molecular drag stage, taken along the line  4 — 4  of  FIG. 3 ; 
         FIG. 5A  is a partial cross-sectional view of a second configuration of a molecular drag vacuum pumping stage, showing the tangential flow channel; 
         FIG. 5B  is a partial cross-sectional view of the molecular drag stage of  FIG. 5A , showing the stationary baffle between the inlet and the outlet; 
         FIG. 6A  is a partial cross-sectional view of a third configuration of a molecular drag vacuum pumping stage, showing the tangential flow channel; 
         FIG. 6B  is a partial cross-sectional view of the molecular drag stage of  FIG. 6A , showing the stationary baffle between the inlet and the outlet; 
         FIG. 7  is a partial, schematic, cross-sectional elevation view of a molecular drag stage, showing a baffle having cavities for producing gas turbulence in accordance with a first embodiment of the invention; 
         FIG. 8  is a partial, cross-sectional plan view of the molecular drag stage of  FIG. 7 ; 
         FIG. 9  is a partial, schematic plan view of a baffle, showing a pattern of cavities in accordance with a second embodiment of the invention; and 
         FIG. 10  is a partial, schematic cross-sectional plan view of a molecular drag stage, showing a stationary baffle having a pattern of cavities in accordance with a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An integral high vacuum pump suitable for incorporation of the present invention is shown in  FIG. 1 . A housing  10  defines an interior chamber  12  having an inlet port  14  and an exhaust port  16 . The housing  10  includes a vacuum flange  18  for sealing the inlet port  14  to a vacuum chamber (not shown) to be evacuated. The exhaust port  16  is 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 housing  10  is an axial turbomolecular compressor  20 , which typically includes several axial turbomolecular stages, and a molecular drag compressor  22 , which typically includes several molecular drag stages. In general, axial turbomolecular compressor  20  includes one or more axial turbomolecular stages, and molecular drag compressor  22  includes one or more molecular drag stages. 
     Each stage of the axial turbomolecular compressor  20  includes a rotor  24  and a stator  26 . Each rotor and stator has inclined blades as known in the art. Each stage of the molecular drag compressor  22  includes a rotor disk  30  and a stator  32 . The molecular drag compressor  22  is described in more detail below. The rotor  24  of each turbomolecular stage and the rotor  30  of each molecular drag stage are attached to a drive shaft  34 . The drive shaft  34  is rotated at high speed by a motor located in a motor housing  38 . 
     A first configuration of the molecular drag compressor  22  is shown in  FIGS. 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 in  FIGS. 2–4 , a molecular drag stage includes a rotor disk  100 , an upper stator portion  102  and a lower stator portion  104  mounted within a housing  105 . The upper stator portion  102  is located in proximity to an upper surface of disk  100 , and lower stator portion  104  is located in proximity to a lower surface of disk  100 . The upper and lower stator portions  102  and  104  together constitute the stator for the molecular drag stage. The rotor disk  100  is attached to a shaft  106  for rotation at high speed. 
     The upper stator portion  102  has an upper tangential flow channel  110  located in opposed relationship to the upper surface of disk  100 . The lower stator portion  104  has a lower tangential flow channel  112  located in opposed relationship to the lower surface of disk  100 . In the configuration of  FIGS. 2–4 , the tangential flow channels  110  and  112  are circular and are concentric with the disk  100 . The upper stator portion  102  includes a stationary baffle  114  which blocks tangential flow channel  110  at one circumferential location. The channel  110  receives gas from a previous stage through an inlet  116  on one side of baffle  114 . The gas is pumped through the tangential flow channel  110  by molecular drag produced by the rotor disk  100 . At the other side of baffle  114 , a conduit  120 , formed in stator portions  102  and  104 , interconnects channels  110  and  112  around the outer peripheral edge of disk  100 . The lower stator portion  104  includes a stationary baffle  122  which blocks lower tangential flow channel  112  at one circumferential location. The lower channel  112  receives gas on one side of baffle  122  through conduit  120  from the upper surface of disk  100  and discharges gas to the next stage through a conduit  124  on the other side of baffle  122 . 
     In operation, gas is received from the previous stage through conduit  116 . 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 channel  110  by molecular drag produced by rotation of disk  100 . The gas then passes through conduit  120  around the outer periphery of disk  100  to lower tangential flow channel  112 . The gas is then pumped around the circumference of lower tangential flow channel  112  by molecular drag and is exhausted through conduit  124  to the next stage or to the exhaust port of the pump. In the configuration illustrated in  FIGS. 2–4 , upper channel  110  and lower channel  212  are connected such that gas flows through the upper and lower channels in series. Also in the configuration of  FIGS. 2–4 , the upper tangential flow channel  100  and the lower tangential flow channel  212  are spaced inwardly from the outer peripheral edge of disk  100 . This configuration limits leakage between channels  110  and  112  around the outer edge of disk  100 , except through conduit  120 . 
     A second configuration of the molecular drag stage is shown in  FIGS. 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 of  FIGS. 5A and 5B , a rotor disk  150  is positioned between an upper stator portion  152  and a lower stator portion  154 . The upper stator portion  152  defines an upper tangential flow channel  160  above rotor disk  150 , and the lower stator portion  154  defines a lower tangential flow channel  162  below rotor disk  150 . A peripheral stator portion  156  is spaced from the outer periphery of rotor disk  150 , so that upper and lower tangential flow channels  160  and  162  are effectively connected in parallel. As shown in  FIG. 5B , a stationary baffle  166  is positioned in tangential flow channels  160  and  162  at 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 in  FIGS. 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 disk  180  is positioned between an upper stator portion  182  and a lower stator portion  184 . The upper stator portion  182  defines an upper tangential flow channel  190 , and the lower stator portion  184  defines a lower tangential flow channel  192 . A small gap  194  between the outer periphery of rotor disk  180  and a peripheral stator portion  186  permits rotation of rotor disk  180  but substantially blocks gas flow between tangential flow channels  190  and  192 . Thus, tangential flow channels  190  and  192  may be connected in series. As shown in  FIG. 6B , a stationary baffle  196  is positioned in upper tangential flow channel  190  at one circumferential location, and a stationary baffle  198  is positioned in lower tangential flow channel  192  at one circumferential location. Each of the stationary baffles  196  and  198  is 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 to  FIGS. 7 and 8 . Partial schematic elevation and plan views, respectively, of a molecular drag stage are shown. A rotor disk  300  rotates about an axis  302 . A stator  304  positioned above rotor disk  300  defines a tangential flow channel  306 . The stator  304  further defines an inlet  308  to tangential flow channel  306  and an outlet  310  from tangential flow channel  306 . A stationary baffle  320  is disposed in tangential flow channel  306  adjacent to outlet  310 . The baffle  320  may, but is not required to be, an integral part of stator  304 . 
     A surface  324  of baffle  320  facing rotor disk  300  is provided with cavities  330 . Rotor disk  300  is spaced from surface  324  by a gap  332  and moves relative to surface  324  during operation of the vacuum pump. Cavities  330  extend from surface  324  into stationary baffle  320  and are configured to reduce gas flow through gap  332  between rotor disk  300  and stationary baffle  320  in comparison with the case where surface  324  is flat. Cavities  330  effectively produce turbulence in the gas flow through gap  332  and thereby reduce the volume of gas flow. Cavities  330  may have a variety of configurations within the scope of the invention. 
     The cavities in the surface of baffle  320  reduce the transfer of pumped gas through gap  332 . 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 cavities  330  depends on the dimension of gap  332 , i.e., the spacing between rotor disk  300  and surface  324  of baffle  320 . 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 cavities  330  is preferably in a range of 30 to 70 percent of the total area of surface  324  facing rotor disk  300 . The cavities  330  preferably have dimensions that are 1 to 10 times larger than the gap between baffle  320  and rotor disk  300 . 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 in  FIG. 8 . Thus, rows  340  are offset from rows  342  in a direction orthogonal to the direction of rotation of rotor disk  300 . In other embodiments, the cavities  330  can be semi-circular, semi-oval, triangular, rectangular or square in cross-section.  FIG. 9  shows elongated cavities  350  having long dimensions oriented generally orthogonally to the direction of rotation of rotor disk  300 .  FIG. 10  shows rectangular cavities  360  arranged in staggered rows  362  and  364 . 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.