Patent Publication Number: US-6709228-B2

Title: Vacuum pumps

Description:
FIELD OF THE INVENTION 
     This invention relates to vacuum pumps and in particular to compound vacuum pumps which employ at least one turbo-molecular stage and one molecular drag stage. 
     BACKGROUND OF THE INVENTION 
     There is described in European patent publication number 0 919 726 a vacuum pump comprising first and second turbo-molecular stages and a molecular drag (Holweck) stage. This known pump has a first inlet through which fluid being pumped passes through all the pump stages and a second inlet through which fluid enters the pump between the two turbo-molecular stages and passes only through one turbo-molecular stage and the Holweck stage. The turbo-molecular stage upstream of the second inlet is sized differently from the turbo-molecular stage downstream of the second inlet so that the vacuum pump suits the pressure requirements or pumping capacities of the chambers/systems being evacuated and attached to the first and second inlets respectively. 
     This known “split flow” turbo-molecular pump facilitates the differential pumping of, for example, two chambers of a scientific instrument. For example, in well known types of mass spectrometer that part of the apparatus known as the detector commonly has to be operated at, for example, 10 −6  mbar whereas that part known as the analyser has to be operated at a different level of vacuum for example 10 −3  mbar. The chamber requiring the lower pressure (higher vacuum) is attached to the first inlet so that the fluid being evacuated is subject to all the stages of the pump whilst the chamber requiring the higher pressure (lower vacuum) is attached to the second inlet so that the fluid being evacuated is subject only to the pump stages downstream of the second inlet. 
     With liquid phase mass spectrometer (MS) systems there is increasingly a demand for higher gas loads to be consumed by the vacuum system. The highest gas load handling capacity is required at the comparatively “high” pressure end of the pump. As the gas load diminishes in the subsequent chambers so to the system pressure decreases until eventually it reaches a level acceptable for analysis. 
     OBJECTS OF THE INVENTION 
     It is an aim of the present invention to provide an improved compound vacuum pump including at least one turbo-molecular stage and downstream therefrom a multi-channel molecular drag stage in which one or more channels of the molecular drag stage are devoted to pumping a high pressure fluid inlet whilst the remainder of the channels are devoted to “back” the turbo-molecular stage. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a compound vacuum pump comprises at least one turbo-molecular stage and downstream therefrom a multi-channel molecular drag stage, a first inlet through which fluid can pass through the turbo-molecular stage and the molecular drag stage towards a pump outlet, a second inlet through which fluid can enter the pump at a location between the turbo-molecular and the molecular drag stages to pass only through the molecular drag stage towards the pump outlet, in which one or more channels of the molecular drag stage are adapted to communicate directly with the second inlet whilst the remaining channel or channels communicate with the turbo-molecular stage. 
     Preferably, the multi-channel molecular drag stage is a Holweck stage with a plurality of channels arranged as a plurality of helixes. 
     In one embodiment, a baffle directs one or more of the helical channels of the Holweck stage towards the second inlet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: 
     FIG. 1 is a vertical cross-section through a known compound vacuum pump employing two turbo-molecular stages, a Holweck stage, a low pressure inlet and a high pressure inlet; 
     FIG. 2 is a vertical cross-section through a compound vacuum pump according to the present invention; 
     FIG. 3 is a cross section on the line X—X of FIG. 2; 
     FIG. 4 is an enlarged detail of the compound vacuum pump of FIGS. 2 and 3 at a high pressure inlet interstage between a turbo-molecular stage and a Holweck stage; 
     FIG. 5 is a detail in perspective of the Holweck stage and a baffle member forming part of the compound vacuum pump of FIG. 2; and 
     FIG. 6 is a schematic illustration of an embodiment of the invention in which a single channel of the Holweck stage is directed to communicate directly with a high pressure fluid inlet whilst the remaining four channels communicate with a turbo-molecular stage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1 there is shown a known compound vacuum pump having a multi-component body  1  within which is mounted a shaft  2 . Rotation of the shaft  2  is effected by a motor  3  positioned about the shaft  2 . The shaft  2  is mounted in lower and upper bearings  4 ,  5  respectively. 
     The pump includes two sets of turbo-molecular stages  6 ,  7 . The first set of turbo-molecular stages  6  comprises four rotor and stator blade pairs of known angled construction, a rotor blade stage is indicated at  8  and a stator blade stage is indicated at  9 . 
     The second set of turbo-molecular stages  7  comprises a further six rotor and stator blade pairs of angled construction, a rotor blade stage is indicated at  12  and a stator blade stage is indicated at  13  in the drawing. 
     The pump is provided with a low pressure inlet  10  and a higher pressure inlet  16 . 
     Downstream of the turbo-molecular stage  7  are a number of Holweck stages. These Holweck stages comprise two rotating cylinders  17 ,  18  and corresponding annular stators  19 ,  20  having helical channels formed therein all in a manner known per se. Downstream of the Holweck stages is a pump outlet  22 . 
     In this known pump, the inlet  10  is connected to a chamber/system requiring a relatively high vacuum (low pressure) and the fluid pumped through the inlet  10  passes through both the turbo stages  6 ,  7  and also the Holweck stages and exits the pump via the outlet  22 . The inlet  16  is connected to a chamber/system requiring less vacuum (higher pressure) and the fluid pumped through the inlet  16  passes only through the turbo-molecular stages  7  and the Holweck stages and exits the pump via the outlet  22 . 
     Referring now to FIGS. 2 to  4  where like reference numerals denote like parts, a compound vacuum pump  30  comprises a body  1  within which is mounted a vertical shaft  2  supported by lower and upper bearings  4 ,  5 . Rotation of the shaft  2  is effected by a motor  3 . 
     The pump  30  has two sets of turbo-molecular stages  6 ,  7  and a Holweck stage  32 . 
     The pump  30  has three inlets and an outlet  22 . The first inlet  34  (mid gas pressure) is located interstage the two turbo-molecular stages  6 ,  7 ; the second inlet  36  (high gas pressure) is located interstage the turbo-molecular stage  7  and the Holweck stage  32 ; and the third inlet  38  (low gas pressure) is located upstream of all three stages 
     The Holweck stage  32  includes two rotating cylinders  17 ,  18  and corresponding annular stators  19 ,  20  and helical channels formed thereon all in a manner known per se. 
     According to the invention, and as shown in FIG. 6, one or more channels of the Holweck stage  32  is adapted to communicate directly with the high pressure inlet  36  whilst the remaining channel or channels communicate and serve to back turbo-molecular stage  7 . 
     Referring also to FIG. 5, the Holweck stage  32  is provided with a baffle member  40  having a radially inwardly directed flange  42  which allows only one channel (as shown) of the Holweck stage to communicate with the high pressure gas inlet  36  and prevents back streaming of the high pressure gas in to the turbo blades of the turbo-molecular stage  7 . 
     The remaining channels of the Holweck stage are used to “back” the turbo blade stages of the turbo-molecular stage  7 . 
     It will be apparent that the flange  42  could be extended so that more than one channel could be directed at the high pressure gas inlet  36 . 
     A particular advantage of the embodiment described above is that the use of varying numbers of Holweck molecular drag stage channels for the purpose of either pumping the high pressure inlet or “back” the turbo blades stages  7  is matched to individual scientific instrument applications.