US20080253903A1

US20080253903A1 - Vacuum pumps with auxiliary pumping stages

Info

Publication number: US20080253903A1
Application number: US11/786,358
Authority: US
Inventors: Marsbed Hablanian
Original assignee: Varian SpA
Current assignee: Varian SpA
Priority date: 2007-04-11
Filing date: 2007-04-11
Publication date: 2008-10-16

Abstract

Vacuum pumping apparatus includes a housing, one or more primary pumping stages disposed in the housing, a motor coupled to the primary pumping stages, the motor including at least one stationary surface and at least one moving surface when the motor is energized, the stationary and moving surfaces of the motor defining an auxiliary pumping stage coupled in series with the primary pumping stages to pump gas through a pumping channel between the stationary and moving surfaces of the motor, the housing having a first exhaust port between the primary pumping stages and the auxiliary pumping stage, and a second exhaust port at an outlet of the auxiliary pumping stage, and a valve connected in series with the first exhaust port and configured to close when an inlet pressure of the vacuum pumping apparatus is below a predetermined pressure.

Description

FIELD OF THE INVENTION

This invention relates to high vacuum pumps and, more particularly, to high vacuum pumps and methods for vacuum pumping that include one or more auxiliary pumping stages. The invention relates to 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 six to twelve axial pumping stages.
Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid turbomolecular vacuum pumps, have been disclosed 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. Molecular drag stages and regenerative stages for hybrid vacuum pumps are disclosed in U.S. Pat. No. 5,358,373, issued Oct. 25, 1994 to Hablanian. 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 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 the 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 vacuum pump which utilizes an inverted motor to achieve a compact structure is disclosed in U.S. Pat. No. 6,179,573, issued Jan. 30, 2001 to Hablanian. This patent also discloses a vacuum pump structure wherein the rotor of the motor is provided with a molecular drag groove and gas is pumped through the molecular drag groove when the rotor is rotated at high speed.
Known vacuum pump structures have certain drawbacks, including but not limited to the need for a roughing vacuum pump in many applications and limited efficiency under certain operating conditions. Accordingly, there is a need for improved vacuum pumping methods and apparatus, particularly in regard to increasing the maximum compression ratio of the pump.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, vacuum pumping apparatus comprises a housing, one or more primary pumping stages disposed in the housing, a motor coupled to the primary pumping stages, the motor including at least one stationary surface and at least one moving surface when the motor is energized, the stationary and moving surfaces of the motor defining an auxiliary pumping stage coupled in series with the primary pumping stages to pump gas through a pumping channel between the stationary and moving surfaces of the motor, the housing having a first exhaust port between the primary pumping stages and the auxiliary pumping stage, and a second exhaust port at an outlet of the auxiliary pumping stage, and a valve connected in series with the first exhaust port and configured to close when an inlet pressure of the vacuum pumping apparatus is below a predetermined pressure.
According to a second aspect of the invention, a method is provided for operating vacuum pumping apparatus of the type including a housing, one or more primary pumping stages disposed in the housing and a motor to operate the primary pumping stages, the motor including at least one stationary surface and at least one moving surface when the motor is energized. The method comprises defining an auxiliary pumping stage, coupled in series with the primary pumping stages, to pump gas through a pumping channel between the stationary and moving surfaces of the motor, exhausting gas from the vacuum pumping apparatus through a first exhaust port between the primary pumping stages and the auxiliary pumping stage when an inlet pressure of the vacuum pumping apparatus is above a predetermined pressure, and exhausting gas from the vacuum pumping apparatus through a second exhaust port at an outlet of the auxiliary pumping stage when an inlet pressure of the vacuum pumping apparatus is below the predetermined pressure. Thus, in accordance with embodiments of the invention, close-proximity rotating and stationary surfaces are utilized to provide auxiliary pumping.

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 an elevation view, partly in cross-section, of a prior art vacuum pump;

FIG. 2 is a cross-sectional diagram of a vacuum pump in accordance with a first embodiment of the invention;

FIG. 3 is a partial cross-sectional view of the vacuum pump motor, configured for vacuum pumping;

FIG. 4 is a partial cross-sectional view of the lower end of the vacuum pump, showing a bearing preload nut used for vacuum pumping;

FIG. 5 is a partial cross-sectional view of the vacuum pump, showing a shaft extension used for vacuum pumping; and

FIG. 6 is a block diagram of vacuum pumping apparatus in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A prior art high vacuum pump is shown in FIG. 1. An upper housing 10 defines an interior chamber 12 having an inlet port 14. The housing 10 includes a vacuum flange 18 for sealing the inlet port 14 to a vacuum chamber (not shown) to be evacuated. A motor housing 15 includes an exhaust port 16. 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 vacuum 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 motor housing 15.
A high vacuum pump in accordance with a first embodiment of the invention is shown in FIG. 2. An upper housing 110 defines an interior chamber 112 having inlet port 114. Upper housing 110 includes a vacuum flange 118 for sealing the inlet port to a vacuum chamber (not shown) to be evacuated. A motor housing 115 includes a first exhaust port 116 and a second exhaust port 128. The upper housing 110 and the motor housing 115 may be implemented as a single, integral pump housing or as two housing elements which are fastened together and sealed vacuum tight to form a pump housing.
Located within upper housing 110 may be an axial turbomolecular compressor 120, which typically includes several axial turbomolecular stages, and a molecular drag compressor 122, which typically includes several molecular drag stages. The pumping stages of axial turbomolecular compressor 120 and molecular drag compressor 122 constitute primary pumping stages of the vacuum pump. Each stage of the axial turbomolecular compressor 120 includes a rotor and a stator. Each rotor and stator has inclined blades as is known in the art. Each stage of the molecular drag compressor 122 includes a rotor disk and a stator. The rotor of each turbomolecular stage and the rotor disk of each molecular drag stage are attached to a drive shaft 134. The drive shaft 134 is rotated at high speed by a motor 140 located in motor housing 115.
Motor 140 includes a rotor 150 positioned on a central axis 152 and a stator 154 including stationary motor windings 160 disposed around rotor 150. Rotor 150 includes drive shaft 134 and a magnetic element 162 disposed on drive shaft 134. Drive shaft 134 is mounted for rotation in bearings 164 and 166. When motor windings 160 are energized, rotor 150 rotates about axis 152.
Motor 140 includes stationary surfaces and, when energized, moving surfaces in close proximity to the stationary surfaces. The stationary surfaces include surfaces of motor housing 115 and stator 154. Moving surfaces include surfaces of rotor 150 and drive shaft 134. The motor can be configured to provide auxiliary vacuum pumping to supplement the primary pumping stages of turbomolecular compressor 120 and molecular drag compressor 122. Auxiliary vacuum pumping is achieved by configuring selected surfaces of the motor 140 to perform vacuum pumping. This is achieved by providing a pumping channel in the moving surface or in the stationary surface, in regions of the motor where the stationary and moving surfaces are in close proximity.
As best shown in FIG. 3, rotating magnetic element 162 has a generally cylindrical outer surface, and stationary motor windings 160 have a generally cylindrical inner surface in close proximity to magnetic element 162. The cylindrical outer surface of magnetic element 162 or the cylindrical inner surface of motor windings 160 may be provided with a pumping channel 180. In one embodiment, molecular drag pumping channel 180 has a helical configuration as shown in FIG. 3. When rotor 150 is rotated at high speed, gas is pumped from an inlet region 184 through pumping channel 180 to second exhaust port 128 (FIG. 2). The pumping channel 180 between magnetic element 162 and motor windings 160 defines an auxiliary pumping stage 182 that is connected in series with the primary pumping stages of turbomolecular compressor 120 and molecular drag compressor 122. Gas is pumped from the outlet of the primary pumping stages through auxiliary pumping stage 182 to second exhaust port 128. The pumping channel 180 can have a variety of configurations within the scope of the invention. As noted above, pumping channel 180 may be formed on the outer surface of rotor 150 or on the inner surface of stator 154 of motor 140. In one embodiment, the pumping channel 180 has a helical configuration on the outer surface of rotor 180, as shown in the FIG. 3. In another embodiment, pumping channel 180 may be formed as multiple circumferential grooves in the rotor 150. The circumferential grooves are interconnected by axial passages, and a baffle or stripper is provided in each channel to separate the inlet and the outlet. In further embodiments, two or more pumping channels are provided in parallel.
A further embodiment of the auxiliary pumping stage is described with reference to FIG. 4. Motor 140 includes bearing preload nut 170 secured to drive shaft 134 in order to maintain bearing 164 in a desired position on drive shaft 134. The bearing preload nut 170 rotates about axis 152 during motor operation. Motor housing 115 or other stationary component of motor 140 may be positioned in close proximity to bearing preload nut 170. A moving outer surface of bearing preload nut 170 or a stationary inner surface of motor housing 115 adjacent to bearing preload nut 170 may be provided with a pumping channel 210. The configuration of bearing preload nut 170, adjacent motor housing 115 and pumping channel 210 defines an auxiliary pumping stage 212. Gas is pumped from an inlet region 214 through pumping channel 210 to second exhaust port 128. As described above, pumping channel 210 may have a variety of configurations within the scope of the invention.
As shown in FIG. 5, drive shaft 134 at the bottom of the vacuum pump may be utilized to provide additional pumping capacity. The drive shaft 134 may be extended, if necessary to provide sufficient length. A drive shaft extension 134 a is positioned in close proximity to motor housing 115 or other stationary component of motor 140. A moving outer surface of drive shaft extension 134 a or a stationary inner surface of motor housing 115 adjacent to drive shaft extension 134 a is provided with a pumping channel 220 to form an auxiliary pumping stage 222. Gas is pumped from an inlet region 224 through pumping channel 220 to second exhaust port 128. As described above, pumping channel 220 may have a variety of configurations within the scope of the invention.
The auxiliary pumping stage configurations shown in FIGS. 3-5 and described above may be utilized separately or in any combination in a particular vacuum pump. For example, a first configuration may utilize only the rotor and the stator of the motor to form an auxiliary pumping stage, whereas a second configuration may utilize the rotor and stator; the bearing preload nut and motor housing; and the drive shaft extension and motor housing to form several auxiliary pumping stages.
The auxiliary pumping stage may have a small mass flow capacity as a result of the limited volume between the stationary and moving components of the motor. The conductance through the auxiliary pumping stage may not be sufficient for rough pumping of the vacuum chamber. A configuration for overcoming this drawback is shown in FIG. 6. First exhaust 116 is coupled through a valve 250 to a roughing vacuum pump 252. Second exhaust 128 is coupled directly to roughing vacuum pump 252. Valve 250 may be opened when the pressure at inlet port 114 is above a predetermined level, thereby bypassing the secondary pumping stage for rough vacuum pumping. When the pressure at inlet port 114 is below the predetermined pressure level, valve 250 is closed, and all gas is exhausted through second exhaust port 128. When the pressure level at inlet port 114 is below the predetermined pressure level, a small mass flow is sufficient to maintain the desired pressure level. Control of valve 250 may be manual or automatic in response to pressure sensing at inlet port 114.
The vacuum pump shown in FIG. 2 is described as having primary pumping stages including turbomolecular compressor 120 and molecular drag compressor 122. However, the primary pumping stages of the vacuum pump can have any configuration that is driven by a motor. For example, the primary pumping stages may include only turbomolecular stages or only molecular drag stages. Furthermore, the primary pumping stages may include regenerative centrifugal stages or may have any hybrid configuration known in the art or yet to be developed. In each case, the primary pumping stages are used in combination with one or more auxiliary pumping stages associated with the stationary and moving surfaces of the motor.
Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

Claims

1. Vacuum pumping apparatus comprising:

a housing;

one or more primary pumping stages disposed in the housing;

a motor coupled to the primary pumping stages, the motor including at least one stationary surface and at least one moving surface when the motor is energized, the stationary and moving surfaces of the motor defining an auxiliary pumping stage coupled in series with the primary pumping stages to pump gas through a pumping channel between the stationary and moving surfaces of the motor;

the housing having a first exhaust port between the primary pumping stages and the auxiliary pumping stage, and a second exhaust port at an outlet of the auxiliary pumping stage; and

a valve connected in series with the first exhaust port and configured to close when an inlet pressure of the vacuum pumping apparatus is below a predetermined pressure.

2. Vacuum pumping apparatus as defined in claim 1, wherein the motor includes a stator and a rotor and wherein the stationary and moving surfaces comprise stator and rotor surfaces, respectively.

3. Vacuum pumping apparatus as defined in claim 1, wherein the motor includes a bearing preload nut and wherein the stationary and moving surfaces comprise housing and bearing preload nut surfaces, respectively.

4. Vacuum pumping apparatus as defined in claim 1, wherein the motor includes a shaft and wherein the stationary and moving surfaces comprise housing and shaft surfaces, respectively.

5. Vacuum pumping apparatus as defined in claim 1, wherein the auxiliary pumping stage comprises a screw-type pumping stage

6. Vacuum pumping apparatus as defined in claim 1, wherein the auxiliary pumping stage comprises a molecular drag stage.

7. Vacuum pumping apparatus as defined in claim 2, wherein the auxiliary pumping stage includes a helical groove in the rotor.

8. Vacuum pumping apparatus as defined in claim 2, wherein the auxiliary pumping stage includes a helical groove in the stator.

9. Vacuum pumping apparatus as defined in claim 1, wherein the primary pumping stages comprise one or more turbomolecular pumping stages.

10. Vacuum pumping apparatus as defined in claim 1, wherein the primary pumping stages include one or more molecular drag pumping stages.

11. Vacuum pumping apparatus as defined in claim 1, wherein the primary pumping stages include one or more turbomolecular pumping stages and one or more molecular drag pumping stages.

12. Vacuum pumping apparatus as defined in claim 1, wherein the valve is automatically closed when the inlet pressure of the vacuum pumping apparatus is below the predetermined pressure.

13. Vacuum pumping apparatus as defined in claim 1, further comprising a roughing vacuum pump coupled to the first exhaust port.

14. A method for operating vacuum pumping apparatus of the type including a housing, one or more primary pumping stages disposed in the housing and a motor to operate the primary pumping stages, the motor including at least one stationary surface and at least one moving surface when the motor is energized, the method comprising:

defining an auxiliary pumping stage, coupled in series with the primary pumping stages, to pump gas through a pumping channel between the stationary and moving surfaces of the motor;

exhausting gas from the vacuum pumping apparatus through a first exhaust port between the primary pumping stages and the auxiliary pumping stage when an inlet pressure of the vacuum pumping apparatus is above a predetermined pressure; and

exhausting gas from the vacuum pumping apparatus through a second exhaust port at an outlet of the auxiliary pumping stage when an inlet pressure of the vacuum pumping apparatus is below the predetermined pressure.

15. The method as defined in claim 14, further comprising automatically closing a valve coupled to the first exhaust when the inlet pressure of the vacuum pumping apparatus is below the predetermined pressure.

16. The method as defined in claim 14, wherein the motor includes a stator and a rotor and wherein the stationary and moving surfaces comprise stator and rotor surfaces, respectively.

17. The method as defined in claim 14, wherein the motor includes a bearing preload nut and wherein the stationary and moving surfaces comprise housing and bearing preload nut surfaces, respectively.

18. The method as defined in claim 14, wherein the motor includes a shaft and wherein the stationary and moving surfaces comprise housing and shaft surfaces, respectively.