Abstract:
A vacuum pump ( 100 ) comprises a first set ( 106 ) of turbo-molecular stages, a molecular drag stage ( 112 ), a first inlet ( 120 ) through which fluid can pass through the first set ( 106 ) of stages and the molecular drag stage ( 112 ) towards a pump outlet ( 116 ), second and third sets ( 108, 110 ) of turbo-molecular stages located between the first set ( 106 ) and the molecular drag stage ( 112 ), a second inlet ( 122 ), the second and third sets ( 108, 110 ) being arranged such that fluid entering the pump through the second inlet ( 122 ) is separated into two streams each flowing through a respective one of the second and third sets ( 108, 110 ), and conduit means ( 126 ) for conveying fluid passing through the first set ( 106 ) and one of the second and third sets ( 108, 110 ) towards the outlet ( 116 ).

Description:
FIELD OF THE INVENTION 
     This invention relates to a vacuum pump and in particular a compound vacuum pump with multiple ports suitable for differential pumping of multiple chambers. 
     BACKGROUND OF THE INVENTION 
     In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. One such example is given in  FIG. 1 . With reference to  FIG. 1 , in such a system there exists a high vacuum chamber  10  immediately following first and second evacuated interface chambers  12 ,  14 . The first interface chamber  12  is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which ions are drawn from an ion source into the first interface chamber  12 , and ion optics for guiding ions from the ion source into the second interface chamber  14 . The second, middle chamber  14  may include additional ion optics for guiding ions from the first interface chamber  12  into the high vacuum chamber  10 . In this example, in use, the first interface chamber is at a pressure of around 1 mbar, the second interface chamber is at a pressure of around 10 −3  mbar, and the high vacuum chamber is at a pressure of around 10 −5  mbar. 
     The high vacuum chamber  10  and second interface chamber  14  can be evacuated by means of a compound vacuum pump  16 . In this example, the vacuum pump has two pumping sections in the form of two sets  18 ,  20  of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism  22 ; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set  18 ,  20  of turbo-molecular stages comprises a number (three shown in  FIG. 1 , although any suitable number could be provided) of rotor  19   a ,  21   a  and stator  19   b ,  21   b  blade pairs of known angled construction. The Holweck mechanism  22  includes a number (two shown in  FIG. 1  although any suitable number could be provided) of rotating cylinders  23   a  and corresponding annular stators  23   b  and helical channels in a manner known per se. 
     In this example, a first pump inlet  24  is connected to the high vacuum chamber  10 , and fluid pumped through the inlet  24  passes through both sets  18 ,  20  of turbo-molecular stages in sequence and the Holweck mechanism  22  and exits the pump via outlet  30 . A second pump inlet  26  is connected to the second interface chamber  14 , and fluid pumped through the inlet  26  passes through set  20  of turbo-molecular stages and the Holweck mechanism  22  and exits the pump via outlet  30 . In this example, the first interface chamber  12  is connected to a backing pump  32 , which also pumps fluid from the outlet  30  of the compound vacuum pump  16 . As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump  16  is able to provide the required vacuum levels in the chambers  10 ,  14 . 
     In order to increase system performance, it is desirable to increase the mass flow rate of the sample and carrier gas from the source into the high vacuum chamber  10 , whilst maintaining the desired pressure in the second interface chamber  14 . For the pump illustrated in  FIG. 1 , this could be achieved by increasing the capacity of the compound vacuum pump  16  by increasing the diameter of the rotors  21   a  and stators  21   b  of set  20 . For example, in order to double the capacity of the pump  16 , the area of the rotors  21   a  and stators  21   b  would be required to double in size. In addition to increasing the overall size of the pump  16 , and thus the overall size of the mass spectrometer system, the pump  16  would become more difficult to drive in view of the increased mass acting on the drive shaft due to the larger rotors and stators of set  20 . 
     BRIEF SUMMARY OF THE INVENTION 
     It is an aim of at least the preferred embodiment of the present invention to provide a differential pumping, multi port, compound vacuum pump, which can enable the mass flow rate in a differentially pumped vacuum system to be increased specifically where required without significantly increasing the size of the pump. 
     In a first aspect, the present invention provides a vacuum pump comprising a first pumping section, a first pump inlet through which fluid can enter the pump and pass through the first pumping section towards a pump outlet, second and third pumping sections, a second pump inlet through which fluid can enter the pump, the second and third pumping sections being arranged such that fluid entering the pump through the second inlet is separated into a first stream passing through the second pumping section towards the pump outlet and a second stream passing through the third pumping section away from the pump outlet, means for conveying fluid passing through the third pumping section towards the outlet, and at least one additional pumping section downstream from the first, second and third pumping sections for receiving fluid therefrom and outputting fluid towards the outlet. 
     By effectively replacing the second pumping section  20  of the known pump by two pumping sections, one on either side of the second inlet and with blade angles generally reversed, fluid entering the pump through the second inlet can be split into two streams flowing in different directions. One stream passes through the second section in the direction of the outlet, whilst the other stream passes through the third section away from the outlet (and thus against the usual flow direction) to conveying means, which conveys that stream towards the outlet. This can enable, for example, the mass flow rate at the second inlet, where required, to be effectively doubled in comparison to the pump illustrated in  FIG. 1  for an increase in pump size/length of only around 25-30%. 
     Minimising the increase in pump size/length whilst increasing the system performance where required can make the pump particular suitable for use as a compound pump for use in differentially pumping multiple chambers of, for example, a bench-top mass spectrometer system requiring a greater mass flow rate at, for example, the middle chamber to increase the flow rate into the analyser with a minimal increase in pump size. 
     In one arrangement, the conveying means is arranged to convey fluid passing through the third pumping section to a location intermediate the second pumping section and said at least one additional pumping section. Thus, fluid passing through the second pumping section can be combined with the fluid passing through the third pumping section upstream of the outlet. This can enable the fluid passing through the third pumping section against the usual flow direction to be connected to a similar vacuum point as the fluid passing through the intermediate pumping section  20  in the pump illustrated in  FIG. 1 . 
     In the preferred embodiments, the second and third pumping sections are located between the first pumping section and said at least one additional pumping section. In such embodiments, the above-mentioned conveying means would additionally convey fluid passing through the first pumping section to a location intermediate the second pumping section and said at least one additional pumping section. 
     In an alternative arrangement of the conveying means, the conveying means comprises a first conduit for conveying fluid passing through the first pumping section to a position intermediate the second and third pumping sections, and a second conduit for conveying fluid passing through the third pumping section to a location intermediate the second pumping section and said at least one additional pumping section. This can also enable the fluid passing through the first pumping section to be connected to a similar vacuum point as the fluid passing through the pumping section  18  in the pump illustrated in  FIG. 1 . Preferably, the pump comprises baffle means for directing fluid passing through the first pumping section and the third pumping section to a respective said conduit. 
     Each of the pumping sections preferably comprises a dry pumping section. Said at least one additional pumping section preferably comprises at least one molecular drag stage, such as a Holweck stage, and/or a regenerative pumping stage, downstream from the first to third pumping sections for receiving fluid therefrom and outputting fluid towards the outlet. Preferably, each of the first to third pumping sections comprises a set of turbo-molecular stages. Preferably, each of these pumping sections comprises at least three turbo-molecular stages. The second and third pumping sections may comprise a similar number of stages, or, alternatively, the second pumping section may comprise a greater number of stages than the third pumping section, in order to overcome any conductance losses in the conduit means. The first pumping section may be of a different size/diameter than the second and third pumping sections. This can offer selective pumping performance. 
     The pump preferably comprises a drive shaft having mounted thereon at least one rotor element for each of the various pumping sections. The rotor elements for at least some of the turbo-molecular stages may be located on a common impeller mounted on the drive shaft. The molecular drag stage may comprise a Holweck stage comprising at least one rotating cylinder mounted for rotary movement with the rotor elements of the turbo-molecular stages. The cylinder may be mounted on a disc located on the drive shaft, which is preferably integral with the impeller. 
     The invention also provides a differentially pumped vacuum system comprising two chambers and a pump as aforementioned for evacuating each of the chambers. This system may be a mass spectrometer system, a coating system, or other form of system comprising a plurality of differentially pumped chambers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a simplified cross-section through a known multi port vacuum pump suitable for evacuating a differentially pumped, mass spectrometer system; 
         FIG. 2  is a simplified cross-section through a first embodiment of a multi port vacuum pump suitable for evacuating the differentially pumped mass spectrometer system of  FIG. 1 ; 
         FIG. 3  is a simplified cross-section through a second embodiment of a multi port vacuum pump suitable for evacuating the differentially pumped mass spectrometer system of  FIG. 1 ; and 
         FIG. 4  is a simplified cross-section through a third embodiment of a multi port vacuum pump suitable for evacuating the differentially pumped mass spectrometer system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 2 , a first embodiment of a vacuum pump  100  suitable for evacuating at least the high vacuum chamber  10  and intermediate chamber  14  of the differentially pumped mass spectrometer system described above with reference to  FIG. 1  comprises a multi-component body  102  within which is mounted a shaft  104 . Rotation of the shaft is effected by a motor (not shown), for example, a brushless dc motor, positioned about the shaft  104 . The shaft  104  is mounted on opposite bearings (not shown). For example, the drive shaft  104  may be supported by a hybrid permanent magnet bearing and oil lubricated bearing system. 
     The pump includes at least four pumping sections  106 ,  108 ,  110  and  112 . The first pumping section  106  comprises a set of turbo-molecular stages. In the embodiment shown in  FIG. 2 , the set of turbo-molecular stages  106  comprises four rotor blades and three stator blades of known angled construction. A rotor blade is indicated at  107   a  and a stator blade is indicated at  107   b . In this example, the rotor blades  107   a  are mounted on the drive shaft  104 . 
     The second pumping section  108  is similar to the first pumping section  106 , and also comprises a set of turbo-molecular stages. In the embodiment shown in  FIG. 2 , the set of turbo-molecular stages  108  also comprises four rotor blades and three stator blades of known angled construction. A rotor blade is indicated at  109   a  and a stator blade is indicated at  109   b . In this example, the rotor blades  109   a  are also mounted on the drive shaft  104 . 
     The third pumping section  110  also comprises a set of turbo-molecular stages, with blade angles generally reversed in relation to those of the second pumping section  108 . In the embodiment shown in  FIG. 2 , the third pumping section  110  contains the same number of stages as the second pumping section  108 , that is, the set of turbo-molecular stages  110  also comprises four rotor blades and three stator blades of known angled construction. A rotor blade is indicated at  111   a  and a stator blade is indicated at  111   b . In this example, the rotor blades  111   a  are also mounted on the drive shaft  104 . 
     As shown in  FIG. 2 , downstream of the first to third pumping sections is a fourth pumping section  112  in the form of a Holweck or other type of drag mechanism. In this embodiment, the Holweck mechanism comprises two rotating cylinders  113   a ,  113   b  and corresponding annular stators  114   a ,  114   b  having helical channels formed therein in a manner known per se. The rotating cylinders  113   a ,  113   b  are preferably formed from a carbon fibre material, and are mounted on a disc  115  that is located on the drive shaft  104 . In this example, the disc  115  is also mounted on the drive shaft  104 . Downstream of the Holweck mechanism  112  is a pump outlet  116 . 
     As illustrated in  FIG. 2 , the pump  100  has two inlets; although only two inlets are used in this embodiment, the pump may have three or more inlets, which can be selectively opened and closed and can, for example, make the use of internal baffles to guide different flow streams to particular portions of a mechanism. For example, an inlet may be located interstage the second pumping section  108  and the fourth pumping section  112 . 
     In this embodiment, a first, low fluid pressure inlet  120  is located upstream of all of the pumping sections. A second, high fluid pressure inlet  122  is located interstage the second pumping section  108  and the third pumping section  110 . A conduit  126  has an inlet  128  located interstage the first pumping section  106  and the third pumping section  110 , and an outlet  130  located interstage the second pumping section  108  and the fourth pumping section  112 . 
     In use, each inlet is connected to a respective chamber of the differentially pumped mass spectrometer system. Fluid passing through the first inlet  120  from the low pressure chamber  10  passes through the pumping section  106 , enters the conduit  126  at conduit inlet  128 , passes out of the conduit  126  via conduit outlet  130 , passes through the fourth pumping section  112  and exits the pump  100  via pump outlet  116 . Fluid passing through the second inlet  122  from the middle pressure chamber  14  enters the pump  100  and “splits” into two streams. One stream passes through the second pumping section  108  and fourth pumping section  112  and exits the pump via the pump outlet  116 . The other stream passes through the third pumping section  110  and enters the conduit  126  at conduit inlet  128  to combine with the fluid passed through the first pumping section  106 . This enables the fluid passing through the third pumping section  110  against the “usual” flow direction (i.e. away from the outlet) to be connected to a similar vacuum point as the fluid passing through the intermediate pumping section  20  in the pump illustrated in  FIG. 1 . Fluid passing through a third inlet  124  from the high pressure chamber  12  may be pumped by a backing pump  150  which also backs the pump  100  via outlet  116 . 
     A particular advantage of the embodiment described above is that, by providing two pumping sections (namely the second and third pumping sections  108 ,  110 ) on either side of the inlet to the middle chamber  14  of the differentially pumped mass spectrometer system, the mass flow rate of fluid entering the pump from the middle chamber  14  can be at least doubled in comparison to the known arrangement shown in  FIG. 1 , without varying the level of the vacuum in the middle chamber. Thus, the flow rate of sample and carrier gas entering the high vacuum chamber  10  from the middle chamber can also be increased, increasing the performance of the differentially pumped mass spectrometer system. 
     With reference to  FIG. 3 , a second embodiment of a vacuum pump  200  suitable for evacuating the high vacuum chamber  10  and intermediate chamber  14  of the differentially pumped mass spectrometer system is similar to the first embodiment, save that the conduit  126  is replaced by a first conduit  202  and a second conduit  208 . The first conduit  202  has an inlet  204  located interstage the first pumping section  106  and the third pumping section  110 , and an outlet  206  located interstage the second pumping section  108  and the third pumping section  110 . 
     The second conduit  208  has an inlet  210  located interstage the first pumping section  106  and the third pumping section  110 , and an outlet  212  located interstage the second pumping section  108  and the fourth pumping section  112 . A baffle member  220  ensures that fluid passing through the first pumping section  106  enters the first conduit  202  and the fluid passing through the third pumping section  110  enters the second conduit  208 . This arrangement can enable both the fluid passing through the third pumping section against the usual flow direction to be connected to a similar vacuum point as the fluid passing through the intermediate pumping section  20  in the pump illustrated in  FIG. 1 , and the fluid passing through the first pumping section to be connected to a similar vacuum point as the fluid passing through the pumping section  18  in the  FIG. 1  pump. 
     With reference to  FIG. 4 , a third embodiment of a vacuum pump  300  suitable for evacuating the high vacuum chamber  10  and intermediate chamber  14  of the differentially pumped mass spectrometer system is similar to the first embodiment, with the exception that the rotors of the various pumping sections are located on a common impeller  302 . In this embodiment, the rotor blades  107   a ,  109   a  and  111   a  of the first, second and third pumping sections  106 ,  108  and  110  are integral with the impeller  302 , and the disc  115  of the fourth pumping section  112  is also integral with the impeller  302 . However, only one or more of these rotor elements may be integral with the impeller  302 , with the remaining rotor elements being mounted on the drive shaft  204 , as in the first embodiment, or located on another impeller, as required. The right (as shown) end of the impeller  302  may be supported by a magnetic bearing, with permanent magnets of this bearing being located on the impeller, and the left (as shown) end of the drive shaft  104  may be supported by a lubricated bearing.