Patent Publication Number: US-6220824-B1

Title: Self-propelled vacuum pump

Description:
FIELD OF THE INVENTION 
     This invention relates to high vacuum pumps used for evacuating an enclosed vacuum chamber and, more particularly, to compact, low cost vacuum pumps. The invention relates to improvements in prior art vacuum pumps of the type which incorporate an electric motor, such as for example turbomolecular pumps, molecular drag pumps and hybrid pumps. 
     BACKGROUND OF THE INVENTION 
     Conventional turbomolecular vacuum pumps include a housing having an inlet port, an 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 by a motor to pump gas between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include nine to twelve axial pumping stages. 
     Variations of the conventional turbomolecular vacuum pump are known in the art. In one prior art configuration, one or more of the axial pumping stages are replaced with disks which rotate at high speed and function as molecular drag stages. This configuration is disclosed in U.S. Pat. No. 5,238,362 issued Aug. 24, 1993 to Casaro et al. A turbomolecular vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian, Inc. under Model No. 969-9007. Turbomolecular vacuum pumps utilizing molecular drag disks and regenerative impellers are disclosed in German Patent No. 3,919,529 published Jan. 18, 1990. 
     Molecular drag compressors include a rotating disk and a stator. The stator defines a tangential flow channel and an inlet and an outlet for the tangential flow channel. A stationary baffle, often called a stripper, disposed in the tangential flow channel separates the inlet and the outlet. As is known in the art, the momentum of the rotating disk is transferred to gas molecules within the tangential flow channel thereby directing the molecules toward the outlet 
     Another type of molecular drag compressor includes a cylindrical drum that rotates within a housing having a cylindrical interior wall in close proximity to the rotating drum. The outer surface of the cylindrical drum is provided with a helical groove. As the drum rotates, gas is pumped through the groove by molecular drag. 
     A prior art high vacuum pump is shown in FIG. 4. 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 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. Each stage of the axial turbomolecular compressor  20  includes a rotor  24  and a stator  26 . Each rotor and stator has inclined blades as is known in the art. Each stage of the molecular drag compressor  22  includes a rotor disk  30  and a stator  32 . 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 . 
     Turbomolecular vacuum pumps and related types of vacuum pumps are used in a wide variety of applications. In many applications, the physical size of the vacuum pump is an important system design consideration. For example, vacuum pumps are frequently used in semiconductor processing equipment that is located in or adjacent to clean room facilities, and strict limitations are placed on the size of the equipment. Another application requiring small size is portable instruments, such as miniature mass spectrometers. In such applications, the electric motor adds significantly to the size, weight and cost of the vacuum pump. 
     A prior art large capacity turbomolecular pump has been driven by a gas turbine, which in turn was driven by an air compressor. Because of the need for an air compressor, the prior art pump was expensive and required a tight rotary seal between the pump and turbine sections. 
     Accordingly, there is a need for vacuum pumps which are compact, which are low in cost and which are simple to manufacture. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, a vacuum pump is provided. The vacuum pump comprises a housing having an inlet port and an exhaust port for coupling to a backing pump, one or more vacuum pumping stages disposed in the housing, each of the vacuum pumping stages comprising a stationary member and a rotating member, and a gas turbine. The gas turbine comprises a gas inlet, a gas outlet coupled to the exhaust port and a rotor coupled to the rotating members of the vacuum pumping stages. A gas flow, produced by the backing pump, through the gas turbine causes the rotor and the rotating members of the vacuum pumping stages to rotate, wherein gas is pumped by the vacuum pumping stages from the inlet port to the exhaust port. 
     The exhaust port of the vacuum pump may be adapted for direct coupling to a backing pump or may be adapted for coupling to a centralized vacuum system having a remotely-located backing pump. 
     The gas turbine may include a valve for controlling gas flow through the gas turbine. The gas turbine may include a nozzle for directing the gas flow to the rotor of the gas turbine. The nozzle inlet may operate at or below atmospheric pressure. 
     The gas turbine may be positioned adjacent to a last stage of the vacuum pumping stages and may be located in the same housing with the vacuum pumping stages. The rotor of the gas turbine and the rotating members of the vacuum pumping stages may be coupled to a common shaft. 
     In a first embodiment, at least one of the vacuum pumping stages comprises an axial turbomolecular stage wherein the rotating member and the stationary member have inclined blades. In a second embodiment, at least one of the vacuum pumping stages comprises a molecular drag stage having a stationary member that is provided with a tangential flow channel having an inlet and an outlet separated by a stationary baffle, and a rotating member comprising a disk. In a third embodiment, at least one of the vacuum pumping stages comprises a regenerative stage. In a fourth embodiment, the vacuum pumping stages comprise one or more axial turbomolecular stages and one or more molecular drag stages. 
     According to another aspect of the invention, a vacuum pumping system is provided. The vacuum pumping system comprises a vacuum pump including a housing having an inlet port and an exhaust port, one or more vacuum pumping stages disposed in the housing, each of the vacuum pumping stages having a stationary member and a rotating member, and a gas turbine. The gas turbine comprises a gas inlet, a gas outlet coupled to the exhaust port and a rotor coupled to the rotating members of the vacuum pumping stages. The vacuum pumping system further comprises a backing pump coupled to the exhaust port of the vacuum pump, wherein a gas flow, produced by the backing pump, through the gas turbine causes the rotor and the rotating members of the vacuum pumping stages to rotate, wherein gas is pumped by the vacuum pumping stages from the inlet port to the exhaust port 
    
    
     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 block diagram of a vacuum pumping system incorporating a self-propelled vacuum pump in accordance with an embodiment of the invention; 
     FIG. 2 is an elevation view, partly in cross section, of a self-propelled vacuum pump in accordance with an embodiment of the invention; 
     FIG. 2A is a cross-sectional elevation view of a molecular drag vacuum pumping stage. 
     FIG. 2B is a partial cross-sectional view of a regenerative vacuum pumping stage. 
     FIG. 3 is a simplified plan view of an example of the gas turbine used in the vacuum pump of FIG. 2; and 
     FIG. 4 is a elevation view, partly in cross-section, of a prior art vacuum pump. 
    
    
     DETAILED DESCRIPTION 
     A block diagram of a vacuum pumping system in accordance with an embodiment of the invention is shown in FIG. 1. A vacuum pump  110  includes an inlet port  112  and an exhaust port  114 . In use, inlet port  112  is sealed to a vacuum chamber (not shown) to be evacuated. Exhaust port  114  is connected by a suitable conduit to a backing pump  120 . Backing pump  120  may be a roughing vacuum pump that is configured for operation at a relatively low vacuum level, i.e. near one tenth of atmospheric pressure. 
     Vacuum pump  110  includes one or more vacuum pumping stages, each having a stationary member and a rotating member, as described below. Examples of such vacuum pumps include turbomolecular pumps, molecular drag pumps and hybrid pumps. Vacuum pump  110  further includes a gas turbine  130  located adjacent to exhaust port  114 . The gas turbine is preferably driven by atmospheric air. Gas turbine  130  includes a gas inlet  132 , a gas outlet coupled to exhaust port  114  and a rotor (not shown in FIG. 1) coupled to the rotating members of the vacuum pumping stages. 
     In operation, backing pump  120  pumps air through gas turbine  130  from gas inlet  132  to gas outlet  115  coupled to exhaust port  114 , thereby causing the rotor of the gas turbine  130  to rotate. The rotation produced by backing pump  120  in turn causes rotation of the rotating members of vacuum pump  110 , so that gas is pumped by the vacuum pumping stages from inlet port  112  to exhaust port  114 . Thus, vacuum pump  110  operates without an electric motor. 
     Backing pump  120  may have any convenient location with respect to vacuum pump  110 . Thus, backing pump  120  may be located in close proximity to vacuum pump  10  or may be at a remote location. For example, exhaust port  114  of vacuum  110  may be connected to a centralized vacuum system in a hospital, laboratory or other facility. The centralized vacuum system may be driven by a backing pump that is connected by suitable conduits to various locations in the facility. 
     An example of an implementation of vacuum pump  110  is shown in FIG. 2. A housing  210  defines an interior chamber  212  having inlet port  112  and exhaust port  114 . The housing  210  includes a vacuum flange  214  for sealing inlet port  112  to a vacuum chamber (not shown) to be evacuated. Exhaust port  114  is adapted for coupling to a backing pump as shown in FIG.  1  and described above. Located within housing  210  is an axial turbomolecular compressor  220 , which typically includes several axial turbomolecular stages, and a molecular drag compressor  222 , which typically includes several molecular drag stages. Each stage of the axial turbomolecular compressor  220  includes a rotor  224  and a stator  226 . Each rotor and stator has inclined blades as is known in the art. Each stage of the molecular drag compressor  222  includes a rotor disk  230  and a stator  232 . The rotor  224  of each turbomolecular stage and the rotor disk  230  of each molecular drag stage are attached to a drive shaft  234 . The drive shaft  234  is rotated at high speed by gas turbine  130 . A bearing housing  240  may contain bearings for supporting drive shaft  234 . 
     Gas turbine  130  is illustrated by way of example in FIGS. 2 and 3. Gas turbine  130  includes gas inlet  132 , a rotor  250  and a gas outlet coupled to exhaust port  114 . Rotor  250  is coupled to drive shaft  234  and includes a rotor body  252  and peripheral blades  254 . Rotor  250  may be located within housing  210 . Gas inlet  132  may be coupled through a valve  260 , which functions as a flow restrictor, to a nozzle  262 . 
     In operation, the backing pump connected to exhaust port  114  produces an air flow through gas inlet  132 , valve  260 , nozzle  262  and the interior of housing  210 . The air flow is directed by nozzle  262  against blades  254  causing rotation of rotor  250 . Because rotor  250  is connected to drive shaft  234 , rotors  224  of turbomolecular compressor  220  and rotor disks  230  of molecular drag compressor  222  rotate. The rotation of the rotating elements of turbomolecular compressor  220  and molecular drag compressor  222  causes gas to be pumped by the vacuum pumping stages from inlet port  112  to exhaust port  114 . Therefore, vacuum pump  110  is driven by gas turbine  130 , and an electric motor is not required. 
     Gas turbine  130  is preferably located within housing  210  adjacent to a last vacuum pumping stage before exhaust port  114  and is preferably located near exhaust port  114 . Gas turbine  130  may be located within the interior chamber  212  of housing  210  with the vacuum pumping stages or may be located in a separate compartment, depending on design considerations. However, in each case the vacuum pump and the gas turbine have a common connection to backing pump  120 . 
     As described above, the gas outlet of gas turbine  130  is coupled to exhaust port  114 . The last stage of vacuum pump  110 , the gas outlet of gas turbine  130  and the inlet to backing pump  120  are connected together and must have compatible operating pressure levels. The pressure level at exhaust port  114  is preferably in a range of about 10 torr to 100 torr. Gas turbine  130  rotates the rotating elements of vacuum pump  110  at the speed required for operation of the vacuum pump, typically in a range of about 20,000 to 100,000 RPM. 
     Gas turbine  130  may have a variety of different configurations within the scope of the present invention. Different configurations of rotor  250  are known to those skilled in the art The gas turbine may include one or more nozzles for directing air at the rotor  250 , or no nozzle. Valve  260  is optional and may have a permanent setting or may be manually adjustable or electrically programmable in accordance with operational conditions. Thus, inlet  132  of gas turbine  130  is at atmospheric pressure, and, depending on the setting of valve  260 , the inlet to nozzle  262  is at or below atmospheric pressure. 
     By way of example, assume that the backing pump has a pumping speed of 5 liters per second (approximately 11 cubic feet per minute) and operates at a pressure of 50 torr. The air flow into the backing pump will be 50 torr×5 liters per second=250 torr liters per second. This air flow can be directly converted to units of power, giving 33 watts. Assuming 60% efficiency, 20 watts are available for driving the vacuum pump. 
     The vacuum pump  110  shown in FIG.  2  and described above is a hybrid pump which includes both axial turbomolecular stages and molecular drag stages. The present invention, wherein the vacuum pumping stages are driven by a gas turbine rather than an electric motor, may be applied to any vacuum pump which has one or more rotating members. 
     In a first example, the vacuum pumping stages are axial turbomolecular stages. Each axial turbomolecular stage includes a rotating member and a stationary member. Each rotating member and each stationary member has inclined blades, with the blades of the rotating and stationary members being inclined in opposite directions. The blades of the rotating members are rotated at high speed to pump gas. The construction of axial turbomolecular stages is well known to those skilled in the vacuum pump art. 
     In a second example, each of the vacuum pumping stages may comprise a molecular drag stage, which includes a rotating disk and a stationary member. The stationary member is provided with one or more tangential flow channels. Each tangential flow channel has an inlet and an outlet separated by a stationary baffle. Referring to FIG. 2A, at least one molecular drag stage of the molecular drag compressor  222  may include the stator  232  that is provided with a tangential flow channel. A stationary baffle  238  blocks tangential flow channel  236  at one circumferential location. The tangential flow channel  236  receives gas from a previous stage through an inlet  239  on one side of the stationary baffle  238 . When the rotor disk  230  is rotated at high speed, gas is pumped through the tangential flow channel  236  by molecular drag produced by the rotating rotor disk  230  and pumped gas is discharged to the next pumping stage on the other side of the stationary baffle  238 . 
     In a third example, the vacuum pump includes a molecular drag compressor wherein the rotating member comprises a cylindrical drum and the stationary member has a cylindrical interior wall in closely spaced relationship to the cylindrical drum. The rotating member may be provided with a helical groove on its outer surface. As the drum is rotated, gas is pumped through the groove by molecular drag. 
     In a fourth example, one or more of the vacuum pumping stages may comprise a regenerative vacuum pumping stage, which includes a regenerative impeller and a stationary member. The regenerative impeller is configured as a disk having spaced-apart radial ribs at or near its outer periphery. The stationary member is provided with a tangential flow channel which has an inlet and an outlet separated by a stationary baffle. Referring to FIG. 2B, the regenerative vacuum pumping stage includes a regenerative impeller  280  which operates with a stator  282 . The impeller  280  comprises a disk  284  with spaced-apart radial ribs  286 . Two portions of the stator  282  define a conduit  288  adjacent to the blockage  290 . In operation, the disk  284  is rotated at high speed about the shaft (not shown). The rotation of the disk  284  and the ribs  286  causes the gas to be pumped as shown by arrows in FIG.  2 B. When the regenerative impeller is rotated at high speed, gas is pumped through the tangential flow channel by the rotation of the disk and the radial ribs. Additional details regarding axial turbomolecular stages and regenerative stages are disclosed in U.S. Pat. No. 5,358,373 issued Oct. 25, 1994 to Hablanian, which is hereby incorporated by reference. 
     In a fifth example, the vacuum pump includes a combination of two or more types of vacuum pumping stages. For example, the vacuum pump may include axial turbomolecular stages and molecular drag stages as shown in FIG.  2  and described above. In each case, the rotating member of each vacuum pumping stage is attached through drive shaft  234  to gas turbine  130 . 
     An advantage of the vacuum pumping system shown in FIGS. 1-3 and described above is that the vacuum pump is very compact. The pump length may be limited to the length required for the vacuum pumping stages and any length required for gas turbine  130  and bearing housing  240 . In addition, the cost of the vacuum pump is reduced in comparison with prior art vacuum pumps by elimination of the electric motor. The invention is particularly advantageous in small and miniature vacuum pumps where size and weight are significant factors and where the cost of the electric motor may be a significant fraction of the total cost of the vacuum pump. 
     According to a further feature of the invention, the air flow for driving the gas turbine  130  may be channeled through the space where the pump bearings are located and/or through the stationary members of the vacuum pump for cooling before it is directed to the gas turbine. 
     While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.