Abstract:
A turbo-molecular vacuum pump of increased pumping capacity has parallel access toward two initial impeller-rotors. An additional annular space is provided around the periphery of the first rotor and the conventional first stator disc is omitted, thus creating an accelerated annual gas flow entering directly into the second rotor without an accumulation of pressure.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates to axial flow stages of rarefied gas compressors commonly used in turbo-molecular vacuum pumps and, more particularly, to modifications of rotor/stator arrangements at the inlet of such pumps. The purpose of the invention is to increase the volumetric flow rate, as related to the property called ‘pumping speed’ in high-vacuum technology. The basic idea may also be of use for very high altitude aircraft for increasing the mass flow of rarefied air into the engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Axial flow pumping stages used in turbo-molecular vacuum pumps are essentially rarefied gas compressors that pump and compress the gas by a displacement process, resulting from sweeping the gas through angled blades attached to a rotating shaft. The peripheral velocity of the blades must be very high because the displacement process is non-positive and back flow must be prevented as much as possible. In vacuum pumps, usually operating in molecular flow conditions, this velocity must be commeasurable to the normal thermal molecular velocities of the pumped gases. This is necessary for effective capture probability of molecules entering into the space between passing blades. 
         [0003]    It becomes immediately apparent that the maximal pumping rate (pumping speed) will be limited by the maximum peripheral velocity of the pumping blades. This in turn is limited by the strength of the material of the blades. Peripheral velocity is proportional to the product of the diameter of the rotor (the distance between outer edges of opposite blades) and the rotational speed (RPM). To obtain highest possible pumping speed, the first bladed rotor is placed near the inlet plane of the pump. To prevent backflow, the inner diameter of the pump body is made to be very close to the tip of the blades. Thus, the pumping speed is limited for a given rotor diameter and given RPM. 
         [0004]    A secondary limitation arises from lower peripheral velocity at the base of the impeller blades because the diameter (or radius) is smaller at that location. 
         [0005]    Thus, for consideration of maximizing pumping speed, the blades cannot be made too long, and the effectiveness of molecular capture will depend on the average peripheral velocity of blade surfaces. The pumping speed will depend on this average and on the annular area of the inlet plane traversed by the pumping blades. 
         [0006]    Accordingly, there is a need for improving the pumping speed without substantially changing the overall size, cost and power consumption of the pump. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the first aspect of the invention, a vacuum pump is provided with multiple axial flow stages in series, consisting of rotor/stator pairs placed at the inlet of the pump. Typically, the angles of the blades in the stator disks are opposite to the blade angles of the rotors. Regardless of the shape and size of the pump inlet structure, the limitation of the pumping speed at the inlet plane of the first stage is the property that the invention desires to increase. Thus, additional annular space is provided at the periphery of the inlet rotor through which gas molecules can have access to the second stage rotor. This additional flow, under molecular flow conditions, does not interfere with the flow arriving from the exit side of the first rotor, because the molecules do not collide and are “unaware” of the presence of others. In prior art operation mode, the presence of the first stator will increase the pressure (or density), due to compression, which may result in backflow and negate the desired effect. Therefore, it is proposed to remove the first stator disk entirely and thus use the first rotor only for the pumping speed effect and free it from producing compression. This renders the first rotor to become an auxiliary capture mechanism which sends an annular molecular beam into the second rotor. The first rotor can be confined at its periphery by a thin annular guard which will prevent radial spreading of the exit flow. The pumping speed of the second rotor is increased by receiving an additional flow from the added peripheral access space. This flow is only limited by the width of the annular space and the distance of the second rotor to the inlet plane of the pump, i.e. by conductance of that passage. 
         [0008]    Thus, the basic invention provides an arrangement of sharing the initial gas flow capture among the first two rotors. Depending on detailed design, as much as 50% improvement in pumping speed can be achieved. Conversely, the blades can be made shorter to increase their average peripheral velocity, which is desirable for efficient pumping of gases having a low molecular weight. It is not effective to extend the new arrangement to the third stage rotor because of an additional loss of compression due to the omission of stator discs and due to the resultant longer distance to the inlet plane of the pump. Conductance of annular cylindrical conduits rapidly diminishes with the growing ratio of length to width of the duct. 
         [0009]    The principle of the invention remains valid whether or not the first stator is entirely removed or kept in its usual place with any blade angle, including tilted backward (i.e. at the same tilt as the rotor blades), or made to be 90 degrees to the plane of rotation. It also remains valid regardless what kind of impellers follow the first two (or three) axial pumping stages, including axial-flow, or molecular drag impellers (with Gaede, Holweck, Siegbahn type pumping channels), spiral or concentric, or the regenerative/centrifugal kind. It also remains valid whether or not the shield around the blades of the first rotor is present. It remains also valid regardless of the shape of the conduit leading from the inlet plane of the pump to the entry into the second rotor; it can be a concentric or non-concentric annular cylinder, or it can have a cross-section of elliptical, ovoid, or polygonal shape. 
         [0010]    There are three preferred embodiments apparent. First, for a given size of the inlet flange (or inlet area of the pump), create as great as possible peripheral access from the inlet plane of the pump to the inlet area of the second rotor. Second, use a smaller rotor, rotating at a higher RPM to obtain a pumping speed equivalent to same size pumps made according to the prior art. Third, enlarge the body of the pump near the inlet to provide a higher conductance passage to the vicinity of the second rotor. The use of a smaller rotor assembly reduces the weight with advantages of lower rotational moment of inertia and lower load for the bearings. This arrangement can also be achieved by placing the first rotor (as an auxiliary rotor attached to the main shaft) above the conventional inlet plane of the pump, protruding into the vacuum chamber, as long as sufficient space is available for pumped gas to enter directly into the second rotor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated by reference and in which: 
           [0012]      FIG. 1  is a simplified cross-sectional schematic diagram of the inlet section of a turbo-molecular vacuum pump according to the prior art. 
           [0013]      FIG. 2  is simplified cross-sectional schematic diagram of a turbo-molecular vacuum pump in accordance with an embodiment of the invention. 
           [0014]      FIG. 3  is a cross-sectional drawing demonstrating typical area relationships of annular gas access channels in accordance with the invention. 
           [0015]      FIG. 4  is a simplified cross-sectional diagram showing an enlarged inlet part of the pump body, in accord with the second embodiment of the invention 
           [0016]      FIG. 5  is a schematic diagram showing an additional feature of the invention where longer blades are used in the first rotor to enhance gas capture capacity without increasing the projected diameter and the peripheral velocity. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    A simplified cross-sectional diagram of a conventional turbo-molecular pump is shown in  FIG. 1 . The inlet plane ( 110 ) defines the plane where the rarefied pumped gas enters the pump and where the onset of pumping mechanism occurs by the action of the first bladed rotor ( 111 ). Compression occurs when the accelerated gas enters the stationary bladed disk, the stator ( 112 ). There follow several sets of rotor-stator pairs (connected in series) to continue the compression process. The pump housing body ( 113 ), enclosing the spacers ( 114 ) between stages, is placed in very close proximity to the rotors in order to avoid backflow. The compressed gas is exhausted at ( 118 ) where connection is typically made to a different type of pump that is more effective at higher pressures and can exhaust directly to atmosphere. The pumping stages that follow the axial pumping section can be of any type. The entire rotor assembly ( 115 ) is attached to the shaft ( 116 ) which turns at high rotational speed around the axis ( 117 ). 
         [0018]    The basic embodiment of the invention is shown in  FIG. 2 , where a passage of adequate conductance ( 210 ) is created at the periphery of the first rotor ( 211 ), to provide pumped gas access into the second rotor ( 212 ). The first stator ( 112 , in  FIG. 1 ) is absent to eliminate compression, which is undesirable at this point. An additional axial distance between the first and second rotors can be made as necessary for adequate access for entry into the second rotor ( 212 ). The stationary annular ring ( 213 ) placed in proximity of the blade tips of the first rotor ( 211 ) can be omitted but may be preferably used to prevent spraying of the pumped gas in centrifugal directions. In this arrangement, according to the invention, the capacity of capturing the gas molecules entering the inlet plane of the pump ( 110 , in  FIG. 1 ) is shared by the two leading rotors, while in a conventional (prior art) pumps, this capacity is entirely dependent on the performance of the first rotor alone. 
         [0019]      FIG. 3  illustrates the comparative cross-sectional areas, at the inlet plane of the pump of the active and inactive elements. The active area of the rotor is the bladed section ( 310 ), where collisions occur between gas molecules and rotating blades. The inactive area which is useless for pumping effect is the circle ( 311 ), where the peripheral velocities are too low for effective pumping action. The additional annular access conduit ( 312 ) for reaching the second rotor is shown at the outer periphery of the figure. The width of this duct should not be greater than is adequate for the desired improvement of overall pumping speed. Making it too wide will quickly reach diminishing returns. This space can be created, for example, by omitting the few spacers used for proper axial positioning of stators and replacing them by a thin cylindrical spool ( 214  in  FIG. 2 ). The pump body position is outlined by the circle ( 313 ). 
         [0020]      FIG. 4  is a schematic representation of the modified entrance arrangement according to the second embodiment of the invention. The space between the first rotor ( 410 ) and the second rotor ( 411 ) is enlarged to provide an adequate access ( 413 ) for the gas entering directly toward the second rotor. The conventional first stator disk is omitted. The blades of the first rotor are surrounded by a thin cylindrical ring ( 412 ) which is supported from the body by a few straps. This thin cylinder can be in the form of a converging or diverging cone or it can have a curved shape. In addition, such an arrangement provides the possibility of using a smaller pump with an enlarged inlet body section that may reach a pumping speed of the next larger conventional pump. 
         [0021]      FIG. 5  is schematic diagram of the inlet section of a pump with elongated blades in a conical arrangement. This permits to shift a longer section of the blades into the region of a greater peripheral velocity without enlarging the projected rotor diameter. The resulting additional bending forces, in such arrangement, must be compensated by reinforcing the blade structure. The outer edges of the blades ( 510 ) are immediately below the inlet plane ( 511 ), the central section ( 512 ) rotating around shaft ( 513 ) is at a lower level. The conventional first stator is omitted.