Abstract:
A molecular spiral-type vacuum pumping stage comprises a smooth surfaces rotor disk cooperating with a stator body. The stator body comprises a plurality of spiral channels on at least one surface facing the rotor disk. The cross-section area (σ) of these channels are reduced from the center to the outer periphery of the stator body so that the condition is satisfied according to which the internal channel speed, i.e. the product of the channel cross-section area and half the rotor velocity normal to the aforesaid area, is constant throughout the channels. Due to this arrangement, it is possible to avoid the risk of internal compression or re-expansions, this limiting the power losses. The present invention also refers to a vacuum pump comprising at least one pumping stage as described above.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is related to the application of Varian S.p.A. (dock.08-44 US) entitled “CENTRIPETAL PUMPING STAGE AND VACUUM PUMP INCORPORTING SUCH PUMPING STAGE” 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a spiral pumping stage for vacuum pump. More particularly, the present invention relates to an improved spiral molecular pumping stage and to a vacuum pump comprising the pumping stage. 
       BACKGROUND OF THE INVENTION 
       [0003]    Molecular drag pumping stages produce pumping action by momentum transfer from a fast-moving surface (moving at speed comparable to thermal speed of the molecules) directly to gas molecules. Generally, these pumping stages comprise a rotor and a stator cooperating with each other and defining a pumping channel therebetween. Collisions of gas molecules in the pumping channel with the rotor rotating at a very high speed cause gas in the channel to be pumped from the inlet to the outlet of the channel itself. 
         [0004]    With reference to  FIG. 1 , between 1920-1930 Karl Manne Georg Siegbahn developed a molecular pumping device  10 , wherein the pumping action is obtained through the cooperation of a rotor disk  20  having smooth surfaces integral with a rotating shaft  30  with a pair of stator bodies  40 ,  50 , each facing a rotor disk surface and provided with a corresponding spiral-shaped groove  60  open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel. 
         [0005]    The Siegbahn patent GB 332,879 discloses an arrangement of the above-mentioned kind. The gas to be pumped, entering through an inlet  70  at the outer periphery of each pumping groove, flows in both spiral channels in centripetal direction, i.e. from the outer periphery towards the center of the pumping grooves, as indicated by arrows CP. In this case two spiral pumping channels in parallel are to be considered; the gas flows in both channels in centripetal direction. 
         [0006]    According to Siegbahn, in order to control the resistance of the gas pumped through the spiral channels  60 , the cross-section area of these channels is reduced from the outer periphery of the stator bodies towards their center, in accordance with the reduction of the tangential speed of the disk, in the direction of the gas flow. 
         [0007]    U.S. Pat. No. 6,394,747 (M. Hablanian) discloses a vacuum pump having reduced overall size and weight utilizing for this purposes a pair of Siegbahn-type pumping stages connected in series rather than in parallel. 
         [0008]    According to U.S. Pat. No. 6,394,747 disclosure, a rotor disk having smooth surfaces is placed between a first stator disk and a second stator disk. Each stator disk is provided with a spiral groove open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel. At the beginning, the gas to be pumped flows between the first stator disk and the rotor disk in centrifugal direction, from the center to the outer periphery of the rotor disk, and then between the second stator disk and the rotor disk in centripetal direction, i.e. from the outer periphery to the center of the rotor disk. 
         [0009]    The cross-section area of the groove defining the pumping channel in the first stator disk, where the gas flows in centrifugal direction, is reduced from the center to the outer periphery, while the cross-section area of the groove defining the channel in the second stator disk, where the gas flows in centripetal direction, is reduced from the outer periphery to the center. In this way the cross-section area of the grooves is always reduced in the direction of the flow and in this way, the U.S. Pat. No. 6,394,747 aims at optimizing both the pumping speed and the compression ratio. 
         [0010]    In known Siegbahn-type pumping stage, having the above-mentioned geometric configuration generates the risk of internal compressions and successive re-expansions and corresponding power losses, especially in applications with important flow rates. Therefore, the main object of the present invention is to provide a spiral pumping stage for vacuum pump, which allows to overcome the above-mentioned drawback and to reduce power losses, especially when several stages are connected in series. This and other objects are achieved by a spiral pumping stage as claimed in the appended claims. 
       SUMMARY OF THE INVENTION 
       [0011]    A pumping stage according to the present invention comprises a spiral pumping channel that is designed so that the volumetric channel speed (L/s), given by the product of the channel cross-section area and half the rotor velocity normal to the aforesaid area, is substantially constant throughout the pumping channel. 
         [0012]    The pumping stage comprises a stator body having at least one spiral channel on a first surface, the cross-section area of this channel is reduced from the center to the outer periphery of the body so as to maintain the product of the channel cross-section area and the rotor velocity normal to the aforesaid area (i.e. the internal gas flow velocity) constant, irrespective of whether the gas flows through the channel in a centripetal or centrifugal direction. 
         [0013]    According to a preferred embodiment of the invention, the pumping stage comprises a stator body having at least one spiral channel on a first surface, wherein the gas flows in a first direction, and at least one further spiral channel on its opposite surface, wherein the gas flows in a second direction opposite to the first direction, the cross-section area of both these channels is reduced from the center to the outer periphery of the disk so as to maintain the constant internal channel speed. Thus, the variation of the cross-section area of the grooves defining the spiral channel of the pumping stage stator body is designed on the grounds of purely geometrical reflections, independently from the advancing direction of the gas flow. 
         [0014]    It is evident to the person skilled in the art that the above-mentioned structural feature, in addition to reducing power losses, also constitutes a remarkable advantage with respect to simplicity and cost reduction during the manufacturing process, since all the stator bodies can be made identical, except for the winding direction of the spiral, without regard to whether they are used in centripetal or centrifugal pumping stages. 
         [0015]    Advantageously, the pumping stage according to the invention can be used in a vacuum pump in combination with other pumping stages, of the same kind or of a different kind. For example, the pumping stage can be provided downstream of a plurality of turbomolecular axial pumping stages. Also, the pumping stage according to the invention can be provided upstream of a Gaede pumping stage and/or regenerative pumping stage. 
         [0016]    According to first preferred application of the invention to a vacuum pump, a plurality of pumping stages are connected in series so that the gas flows through the pumping stages in centripetal and centrifugal direction alternately. 
         [0017]    According to a second preferred application of the invention to a vacuum pump, a plurality of pumping stages are connected in parallel so that the gas to be pumped flows through these channels in parallel in centrifugal direction. 
         [0018]    According a third preferred application of the invention to a vacuum pump, a plurality of pumping stages are connected in parallel so that the gas to be pumped flows through these channels in parallel in centripetal direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Further advantages and features of the invention will be evident from the detailed description of some preferred embodiments of the invention, given by way of non-limiting example, with reference to the attached drawings, wherein: 
           [0020]      FIG. 1  is a cross-sectional view of a known Siegbahn-type pump; 
           [0021]      FIG. 2   a  is a perspective view of a stator body of a pumping stage according to the present invention; 
           [0022]      FIG. 2   b  is a cross-sectional view of a first pumping stage incorporating the stator body of  FIG. 2   a;    
           [0023]      FIG. 2   c  is a cross-sectional view of a first pumping stage incorporating the stator body of  FIG. 2   a;    
           [0024]      FIG. 3  is a cross-sectional view of a vacuum pump according to a first embodiment of the present invention; 
           [0025]      FIG. 4  is an enlarged view of a detail of the vacuum pump of  FIG. 3 ; 
           [0026]      FIG. 5  is a cross-sectional view of a vacuum pump according to a second embodiment of the present invention; 
           [0027]      FIG. 6  is a cross-sectional view of a vacuum pump according to a third embodiment of the present invention; 
           [0028]      FIG. 7  is a perspective view of a stator body of a pumping stage for different embodiments of the vacuum pump according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    With reference to  FIGS. 2   a  though  2   c,  the pumping stage comprises a rotor disk  7 ,  7 ′ having smooth surfaces cooperating with a stator body  1 , which is provided with a plurality of spiral channels  3   a,    3   b,    3   c,    3   d,  on the surface facing said rotor disk  7 , 7 ′. These spiral channels are connected in parallel and separated from each other by corresponding spiral ribs  5   a,    5   b,    5   c,    5   d.    
         [0030]    The cross-section area  6  of channels  3   a,    3   b,    3   c,    3   d  is reduced from the center to the outer periphery of disk  1 , i.e. as the distance R from the center of stator body  1  increases. More particularly, as known, the rotor velocity V T =ωR is reduced concordantly with radius R from the outer periphery towards the center of the stator body. 
         [0031]    According to the invention, the cross-section area  6  of channels  3   a,    3   b,    3   c,    3   d  varies so that, the volumetric channel speed S is constant, according to which 
         [0000]        S=V   n σ=constant   (1) 
         [0000]    wherein V n  is half the rotor velocity normal to area σ. 
         [0032]    More particularly, according to a preferred embodiment of the invention, the shape of the spiral channels of the stator body  1  is defined so that along each spiral channel the following condition is always satisfied: 
         [0000]    
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       2 
                        
                       πω 
                        
                       
                           
                       
                        
                       
                         H 
                          
                         
                           ( 
                           R 
                           ) 
                         
                       
                        
                       
                         R 
                         2 
                       
                        
                       
                         
                           
                              
                             R 
                           
                           
                             R 
                              
                             
                                
                               
                                   
                               
                                
                               φ 
                             
                           
                         
                         
                           1 
                           + 
                           
                             
                               [ 
                               
                                 
                                    
                                   R 
                                 
                                 
                                   R 
                                    
                                   
                                      
                                     
                                         
                                     
                                      
                                     φ 
                                   
                                 
                               
                               ] 
                             
                             2 
                           
                         
                       
                     
                     = 
                     CONSTANT 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    wherein ω=V T /R is the rotor angular velocity;
       H(R) is the height of the channel, possibly variable as a function of R;   φ is the winding angle of the channel spiral.
 
It will be evident to an expert in the field that a spiral pumping stage whose channel has a shape determined by the values of R and φ, which—although they do no represent an exact solution of the equations (1) and (2)—are in any case a good approximation thereof, still falls within the scope of protection of the present invention. In particular, a spiral pumping stage wherein R and φ have a deviation not higher than ±10% with respect to the exact solution of the equations (1) and (2) set forth above or has a channel speed S which is CONSTANT within a deviation of ±10% along the channel itself, allows to effectively reach the objects of the present invention.
       
 
         [0035]    According to a first order approximation of the above equation and in order of the manufacturing simplification for a channel with constant height H, the channel shape is defined by: 
         [0000]    
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       2 
                        
                       πω 
                        
                       
                         
                           R 
                            
                           
                              
                             R 
                           
                         
                         
                            
                           φ 
                         
                       
                     
                     = 
                     
                       CONSTANT 
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0036]    By integration, it is obtained 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     2 
                   
                   - 
                   
                     R 
                     1 
                     2 
                   
                 
                 
                   
                     R 
                     2 
                     2 
                   
                   - 
                   
                     R 
                     1 
                     2 
                   
                 
               
               = 
               
                 φ 
                 
                   φ 
                   o 
                 
               
             
             , 
           
         
       
     
         [0000]    wherein R 1  and R 2  are the inner radius and the outer radius of the stator channel, respectively; and φ 0  is the overall winding angle of the spiral (360° in the example of  FIG. 2   a ). Therefore, as stated above, by maintaining the volumetric channel speed constant, the risk of internal expansions or compressions is avoided and the power losses are limited. 
         [0037]    With reference to  FIGS. 2   b  and  2   c,  the geometrical configuration of the pumping stage according to the invention is advantageously independent from the flow direction of the gas to be pumped, since it is defined by the cited mathematical law, whichever the gas flow direction is. 
         [0038]      FIG. 2   b  shows a pumping stage where the gas flows through the channel in a centripetal direction. The pumping stage comprises a gas inlet  6  at or close to the outer periphery of the stator body  1  and a gas outlet  8  at or close to the center of the stator body, so that the gas to be pumped flows through channels  3   a,    3   b,    3   c,    3   d  in a centripetal direction, as indicated by arrow CP. According to the invention, the cross-section area of said channels is reduced from the center to the outer periphery of the stator body so that the internal volumetric channel speed is constant along the pumping stages and the equation (1) or (2) or (3) is satisfied. 
         [0039]      FIG. 2   c  shows a pumping stage where the gas flows through the channel in a centripetal direction. The pumping stage comprises a gas inlet  6 ′ at or close to the center of the stator body  1  and a gas outlet  8 ′ at or close to the outer periphery of the stator body, so that the gas to be pumped flows through channels  3   a,    3   b,    3   c,    3   d  in a centrifugal direction, as indicated by arrow CF. As in the pumping stage shown in  FIG. 2   b,  the cross-section area of these channels is reduced from the center to the outer periphery of the stator body so that the internal volumetric channel speed is constant along said pumping stages and the equation (1) or (2) or (3) is satisfied. 
         [0040]    Comparing embodiments shown in  FIGS. 2   b  and  2   c,  it is evident that the stator bodies can be made identical irrespective of whether they are intended to be used in centripetal or centrifugal pumping stages. 
         [0041]    With reference to  FIGS. 3 and 4  a vacuum pump P is shown according to the present invention. Vacuum pump P comprises an inlet for the gas to be pumped at lower pressure, an outlet for the pumped gas at higher pressure and a plurality of pumping stages provided between said inlet and said outlet. More particularly, it comprises: a first region A at low pressure provided with a plurality of turbomolecular axial pumping stages connected in series; a second region B at intermediate pressure provided with a plurality of spiral pumping stages according to the invention; and a third region C at high pressure provided with one or more Gaede pumping stages (which can possibly be followed or replaced by regenerative stages). 
         [0042]    More particularly, the intermediate region B of the vacuum pump P comprises one or more centripetal pumping stages  301   a,    301   b,    301   c  according to the invention (three in the example shown in  FIG. 3 ) connected in series with as many centrifugal pumping stages  303   a,    303   b,    303   c  according to the invention, alternated with the centripetal stages. 
         [0043]    With reference to  FIG. 4 , a first centripetal pumping stage S 1  and a second centrifugal spiral pumping stage S 2  according to the invention connected in series are shown in detail. 
         [0044]    To this aim, a stator body  11  is provided on both surfaces  11   a,    11   a ′ with spiral channels  13   a,    13   b,    13   c,    13   d  and  13   a ′,  13   b ′, 13   c ′, 13   d ′, separated by corresponding spiral ribs  15   a,    15   b,    15   c,    15   d  and  15   a ′,  15   b ′  15   c ′,  15   d ′, respectively. 
         [0045]    A first rotor disk  17  having smooth surfaces is located opposite to a first surface  11   a  of the stator  11  and cooperates therewith for forming a first pumping stage S 1  according to the invention. A second rotor disk  17 ′ having smooth surfaces is located opposite to a second surface  11   a ′ of the stator  11  and cooperates therewith for forming a second pumping stage S 2  according to the invention. 
         [0046]    The gas, coming from an inlet  21  placed at the outer periphery of the first pumping stage S 1  flows through the first pumping stage S 1  in centripetal direction (as indicated by arrow CP), passes through the passage  23  provided at or close to the center of said stator body  11  that connects the two stages S 1  and S 2  and then flows through the second pumping stage S 2  in centrifugal direction (as indicated by arrow CF), successively exiting through an outlet  25  placed at the outer periphery of the second pumping stage S 2 . 
         [0047]    With reference again to  FIG. 3 , it is evident that the inlet  21  can put a turbomolecular pumping stage or a previous centrifugal spiral pumping stage or a pumping stage of other kind in the region A in communication with the first pumping stage S 1  of the region B. The same way, the outlet  25  of the last pumping stage of the region B can put the pumping stage S 2  in communication with a successive pumping stage according to the invention or with a Gaede pumping stage or even with a regenerative pumping stage or with a pumping stage of other kind in the region C. 
         [0048]    As described above, according to the invention, the cross-section area of channels  13   a,    13   b,    13   c,    13   d  of the first pumping stage S 1  and of channels  13   a ′,  13   b ′,  13   c ′,  13   d ′ of the second pumping stage S 2  is reduced from the center to the outer periphery of the stator body  11  and varies so that the internal pumping speed is constant along the pumping stages S 1  and S 2  and the condition of equation (1) or (2) or (3) is satisfied. 
         [0049]      FIG. 5  shows a second embodiment of a vacuum pump P′ according to present invention. The pump P′ comprises: a first region A′ at low pressure that is provided with a plurality of centrifugal pumping stages connected in parallel (five in the example shown in  FIG. 5 ); a second region B′ at intermediate pressure that is provided with a plurality of pumping stages according to the invention connected in series; and a third region C′ at high pressure that is provided with one or more Gaede pumping stages (which can possibly be followed or replaced by regenerative stages). 
         [0050]    More particularly, the second region B′ at intermediate pressure of vacuum pump P′ comprises one or more centripetal pumping stages  501   a,    501   b,    501   c  according to the invention (three in the example shown in  FIG. 5 ) connected in series with as many centrifugal pumping stages  503   a,    503   b,    503   c  according to the invention, alternated with said centripetal stages. 
         [0051]    Regarding the first region A′ at low pressure, for obtaining the centrifugal pumping stages  505   a,    505   b,    505   c,    505   d,    505   e  connected in parallel, the wall of the central cavity D′ of the rotor E′ comprises radial through-holes F′, so that the gas arriving from inlet G′ penetrates inside the cavity D′ of the rotor E′, passes through the through-holes F′ and is subdivided between the several pumping stages of this first region A′, being successively collected in a collector defined by holes H′. 
         [0052]    With reference to  FIG. 5 , a further region can be provided upstream to the first region A′. This further region, for example, may comprise a plurality of turbomolecular axial pumping stages. In this case, the outlet of the last turbomolecular stage is connected to the inlet G′ of the pumping stages of the first region A′. 
         [0053]      FIG. 6  shows a third embodiment of a vacuum pump P″ according to the present invention. The pump P″ comprises: a first region A″ at low pressure, provided with a plurality of pumping stages according to the invention connected in parallel (five in the example shown in  FIG. 6 ); a second region B″ at intermediate pressure, provided a plurality of pumping stages according to the invention connected in series; and a third region C″ at high pressure, provided with one or more Gaede pumping stages (which can possibly be followed or replaced by regenerative stages). 
         [0054]    More particularly, the second region B″ at intermediate pressure of vacuum pump P″ comprises one or more centripetal pumping stages  601   a,    601   b,    601   c  according to the invention (three in the example shown in  FIG. 6 ) connected in series with as many centrifugal spiral pumping stages  603   a,    603   b,    603   c  according to the invention, alternated with said centripetal stages. 
         [0055]    In the first region A″ being at low pressure, the wall D″ of the rotor E″ comprises one or more radial through-holes F″ and is closed on its upper side by a closing member J″, so as to define a collector for the gas. The gas arriving from the inlet G″ passes through the radial through-holes H″ suitably formed in the wall of the stators of the pumping stages  605   a,    605   b,    605   c,    605   d,    605   e  is subdivided among the several pumping stages of the first region A″, flows through these pumping stages in centripetal direction and converges into the cavity D″ of the rotor E″, from which it enters successively the region B″ at intermediate pressure of the pump P″, through a centrifugal pumping stage  607   a.    
         [0056]    With reference to  FIG. 6 , a further region can be provided upstream to the first region A″, the further region may comprise, for example, a plurality of turbomolecular axial pumping stages. In this case, the outlet of the last turbomolecular stage is connected to the inlet G″ of the pumping stages of the first region A″. 
         [0057]    From embodiments shown in  FIGS. 3 ,  5  and  6 , it is evident to the person skilled in the art that the pumping stages can be made substantially identical in structure (except for the spiral winding direction), not depending on the direction of the gas flow whether the gas to be pumped flows through them in centripetal or centrifugal direction. This feature remarkably simplifies the manufacturing of the pumps with a corresponding reduction of their manufacturing costs. 
         [0058]    With reference to  FIG. 7 , a stator  21  of a pumping stage that is particularly suitable for applications of the kind of the one shown in  FIGS. 5  or  6 , where a pair of pumping stages are defined on opposite surfaces of the same stator and are connected in parallel. In this case, instead of providing separate channels on the opposite surfaces of the stator body, it is possible to provide a stator body  21  comprising an outer ring  27  that carries cantilever curved vanes  25   a,    25   b,    25   c,    25   d,    25   e,    25   f  defining there between corresponding spiral channels  23   a,    23   b,    23   c,    23   d,    23   e,    23   f.  The stator body  21  can be located between two rotor disks having smooth lo surfaces and cooperate therewith for forming a pair of either centripetal or centrifugal spiral pumping stages according to the invention connected in parallel through which the pumped gas flows. 
         [0059]    It is evident that the described examples and embodiments are in no way limiting. Many modifications and variants are possible without departing from the scope of the invention as defined by the appended claims.