Abstract:
A multi-stage vacuum pump includes at least one turbo-compressor stage ( 11 ) and is equipped with a circular compressor stage ( 33 ) on the pressure side of the turbo-compressor stage. The pump has small axial dimensions, enabling the compression to be increased without significantly increasing the space requirement.

Description:
BACKGROUND  
       [0001]     The invention refers to a multi-stage friction vacuum pump with at least one axially compressing turbo-compressor stage having a rotor rotating about its axis and comprising rotor discs projecting between stationary stator discs.  
         [0002]     Turbomolecular pumps belong in the group of friction vacuum pumps with which a high vacuum can be created, for example for recipients used in semi-conductor manufacturing or for mass spectrometers. A multi-stage friction vacuum pump described in U.S. Pat. No. 7,011,491 comprises one or a plurality of turbo-compressor stages each formed by a rotor with radially projecting rotor discs and a stator with radially projecting stator discs. The rotor discs and the stator discs mesh in a comb-like manner with little space between them. They cause a molecular flow axial to the rotor axis. In addition to the turbo-compressor stage, a circular compressor stage may be provided which comprises a rotor with axially projecting rotor blades arranged on a circular line and a stator with axially protruding stator blades arranged on a circular line. The rotor blades and the stator blades mesh alternately and cause a molecular flow that is directed either radially inward or radially outward, depending on the sense of rotation and the angle of attack of the blades.  
         [0003]     It is the object of the invention to provide a multi-stage friction vacuum pump with at least one turbo-compressor stage, wherein the stages are arranged in series in the flow path and which is to yield higher compression.  
       SUMMARY  
       [0004]     The vacuum pump comprises a turbo-compressor stage and a circular compressor stage arranged downstream in the flow path. Whereas the turbo-compressor stage is suited to generate a high vacuum, the downstream circular compressor stage serves to effect a pressure increase. As a consequence, since the gas volume is reduced by compression, the circular compressor stage can have small dimensions. The circular compressor stage has a small axial dimension since it is flown through mainly in the radial direction. The overall dimensions of the friction pump are not significantly increased by the circular compressor stage, but the compression is clearly intensified with respect to single-stage friction vacuum pumps. The present combination of an upstream turbo-compressor stage and a downstream circular compressor stage offers the advantage of requiring little space while having a high compression capacity.  
         [0005]     According to a preferred embodiment of the invention, the turbo-compressor stage and the circular compressor stage are integrated in a common combination of rotor and stator. This means that the rotors of both compressor stages are formed by a single combined rotor and the stators of both compressor stages are also formed by a single combined stator. Thus, the dimensions and the weight can be reduced further.  
         [0006]     The present friction vacuum pump is preferably designed as a multiple inlet pump. It comprises at least two axially spaced, serially compressing turbo-compressor stages between which an intermediate inlet is located. A circular compressor stage is arranged on the compressor side of the first turbo-compressor stage and/or the second turbo-compressor stage. Such a pump is particularly suited for use in the context of mass spectrometers. Due to the increased gas flow at the intermediate inlet to which the analyzing means of the mass spectrometer is connected, the gas flow at the intermediate inlet is increased without a negative effect on the pressure at the high vacuum inlet. The increase in the gas flow at the intermediate inlet means an increased sensitivity of the mass spectrometer.  
         [0007]     Depending on the compression ratio, different types and structures may be used for the circular compressor stages, such as described in U.S. Pat. No. 7,011,491.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The following is a detailed description of embodiments of the invention with reference to the drawings. These embodiments should not be seen as limiting the scope of protection of the present invention. Rather, this scope is defined by the claims and the equivalents thereof.  
         [0009]     In the Figures:  
         [0010]      FIG. 1  illustrates a longitudinal section through a friction vacuum pump of the present invention,  
         [0011]      FIG. 2  is an illustration of the circular compressor stage,  
         [0012]      FIGS. 3 and 4  are longitudinal sections through different embodiments of circular compressor stages. 
     
    
     DETAILED DESCRIPTION  
       [0013]     The friction vacuum pump illustrated in  FIG. 1  comprises a housing  10  of substantially cylindrical design, which has a high vacuum port HV at one end. In the housing wall an intermediate inlet ZE 1  is provided that is open to the side. The intermediate inlet ZE 1  is bridged by webs  18  that connect the stator parts with each other.  
         [0014]     In the front portion  10   a  of the housing  10 , a first turbo-compressor stage  11  formed by a stator  12  and a rotor  13  is arranged. The stator  12  has a plurality of stator discs  15  directed radially inward from a circumferential wall  14 . The rotor  13  has a plurality of rotor discs  16  projecting radially outward between the stator discs  15 . A drive  17  including a fast rotating electric motor drives the rotor  13  at a number of rotations between 30,000 and 60,000 rpm.  
         [0015]     A second turbo-compressor stage  21  is arranged on the compressor side of the first turbo-compressor stage  11  and has its inlet connected with the intermediate inlet ZE 1 . The turbo-compressor stage  21  is formed by a stator  22  and a rotor  23 . The stator  22  comprises a plurality of stator discs  25  directed radially inward from a circumferential wall  22 . The rotor  23  comprises a plurality of rotor discs  26  projecting radially outward between the stator discs  25 . The rotors  13  and  23  are fixedly interconnected and are driven together by the drive  17 .  
         [0016]     In the housing  10 , a further compressor stage  30  follows the second turbo-compressor stage  21 , this further compressor stage being additionally connected with an intermediate inlet ZE 2 . For example, the compressor stage  30  is a Holweck stage or another molecular pump, for example a Gaede pump, a Siegbahn pump, an Englander pump or a side channel pump.  
         [0017]     In the present embodiment, a circular compressor stage  33  is provided following the first turbo-compressor stage  11 . It comprises a rotor disc  34  which is a part of the rotor  13  of the turbo-compressor stage  11 , and a stator disc  32  which is a part of the stator  12 . The rotor disc  34  comprises rotor blades  35  arranged on concentric circles, and the stator disc  32  comprises stator blades  36  also arranged on concentric circles and engaging in gaps between the rotor circles, as is illustrated in  FIG. 2 . The stator blades and the rotor blades are inclined oppositely with respect to the radial direction. Depending on the sense of rotation of the rotor, the circular compressor stage  33  conveys either radially outward or radially inward. In the present embodiment, the conveying direction is indicated by the arrow  37 . The gas transport passes from the high vacuum inlet HV through the turbo-compressor stage  11  and radially inward from the circumference thereof through the circular compressor stage  33  and from there through a gap  38  to the intermediate inlet ZE 1 . From the intermediate inlet ZE 1 , the turbo-compressor stage  21  conveys the gas to the compressor stage  30 . The second intermediate inlet ZE 2  also opens into the compressor stage  30 . The compressor stage  30  conveys to an outlet (not illustrated).  
         [0018]     One of the rotor discs  16  of the turbo compressor stage  11  is the supporting disc for the rotor blades of the circular compressor stage  33 . The stator disc of the circular compressor disc simultaneously forms the end wall of the pressure-side end of the turbo-compressor stage  11 .  
         [0019]     It is a special advantage that the circular compressor stage  33  is quasi integrated in the turbo-compressor stage  11 . The only additional effort required are the rotor and stator blades  35 ,  36  additionally provided at the rotor and the stator of the turbo compressor stage.  
         [0020]     As an alternative to the present embodiment, a circular compressor stage  33  may also be provided behind the second turbo-compressor stage  21 . The circular compressor stage arranged on the pressure side of the respective turbo-compressor stage and integrated in the turbo-compressor stage increases the gas flow on the pressure side. For a mass spectrometer connected thereto, this means an increase in sensitivity.  
         [0021]      FIG. 3  illustrates the gas flow  40  through the circular compressor stage  33  flowing radially from the outside inward.  
         [0022]     In the embodiment of  FIG. 4 , the blade surface of the rotor disc  34  is conical. The rotor blades  35  have an axial length that decreases as the radius of the circular path decreases.  
         [0023]     It is also possible to use a circular compressor stage with a plurality of discs and alternately outward and inward directed flow paths, as is generally illustrated in FIG. 7 of U.S. Pat. No. 7,011,491.  
         [0024]     The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.