Abstract:
Combined booster and primary pump arrangements can be bulky and require separate supplies of purge gas and cooling water. In order to overcome this problem invention provides a vacuum pump in which two or more pumping mechanisms, i.e. the booster pump and main pump, are housed in the same stator. The invention further provides a vacuum pump stator comprising at least two operatively interconnected cavities, wherein at least two of the cavities each comprise at least one rotor-receiving portion shaped to receive two or more at least partially intermeshing rotors, and wherein an axis of a rotor-receiving portion of a first one of the cavities is offset with respect to an axis of a rotor-receiving portion of a second one of the cavities.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a stator for a vacuum pump and to a vacuum pump comprising the stator. In particular, but not exclusively, the present invention relates to a stator that is used in conjunction with intermeshing vacuum pump rotor mechanisms, such as Roots or screw configured pump mechanisms, or a combination thereof, and a stator having a clam-shell configuration. 
         [0002]    Vacuum pumps are used in many industrial applications, such as steel production, power generation and the semiconductor and electronic device production industry (including solar panel, flat panel display, Li-ion battery and silicon wafer device production). The vacuum pumps are configured according to the application in which they are utilised, the level of vacuum (or pressure) or pumping capacity required by the application and the nature of the gases that a pump system encounters during operation. 
         [0003]    Many industrial processes either require or are moving towards using so-called dry pumps in which a pump comprises a stator and rotor operating within a pumping volume of the stator. The rotor operates within tight clearance tolerances of the stator and seal is maintained between the rotor and stator by virtue of extremely small running clearances. Liquid, oil or other hydrocarbon compounds are not used to maintain a seal—hence the term “dry pump”—because the oil can become contaminated by pumped gases or react with the gas with undesirable effects on the pump&#39;s running performance and characteristics. Furthermore, hydrocarbons from pump sealant can cause contamination of a process chamber, which is especially undesirable in ultra-clean process environments such as those required by the semiconductor and electronic device manufacturing industries. 
         [0004]    In certain applications, a series of pumps can be required to evacuate a chamber and the pumps can be configured in series, parallel or a combination of both. For example, a booster pump might be disposed between a main pump and an evacuated chamber whereby the booster pump operates to improve the operational characteristics of the evacuation system comprising the pumps. The booster pump is used to improve the throughput of gases and also the ultimate pressure to which the pumping system will evacuate. 
         [0005]    Such an arrangement is shown in  FIG. 1  and is also known from W02011/018370, for example. Typically, a main booster pump  12  is disposed above a dry pump  14  and a conduit  16 ,  20  connects the outlet of booster with the inlet of the dry pump. For example, each of the booster and dry pumps can be configured as a clam-shell type pump, which is known in the vacuum pump technical field and described in published patent documents EP2071191, U.S. Pat. No. 6,572,351 or EP1398507. In such an arrangement, the stator comprises two-halves of a clam-shell, a top half  28 , and a bottom half  32  respectively. Head-plates  44  are disposed at each end of the clam shells and serve to maintain clam-shell&#39;s position, accommodate bearings for rotor shafts or provide additional sealing of internal pumping chambers from external atmospheric pressure. Motor drives  42  are disposed on each pump and an end cover can accommodate other mechanisms, such as a timing gear arranged to ensure synchronous rotation of intermeshing rotor parts. In addition, each of the pumping units might comprise cooling and sealing plates  26  disposed on top of, and underneath, the stator clam shell halves (and in other areas as required to maintain the required thermal profile of the pump). An example of such cooling and sealing plates is described in more detail in EP2071191. 
         [0006]    Such a known pump arrangement is common and can comprise different configurations of pumps according to the application of the pump. For instance, the booster pump  12  might be a single stage booster or a multiple stage booster, both of which are well understood and need no further explanation here. Also, the main dry pump  14  can comprise a Roots mechanism, configured in either a single or multiple stages, or a screw pump. Northey (“hook and claw”) mechanisms might also be used. In all cases, the main dry pump and the booster pumps are separate entities located in close proximity to one another. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Market forces are driving vacuum pump manufacturers to provide pump systems that are more efficient to manufacture and operate without reducing pump performance. Therefore, the present invention aims to provide a vacuum pump arrangement that is more cost efficient to manufacture and transport to the end-user, and a pump that has improved running efficiencies. Furthermore, the present invention aims to simplify pump installation in an industrial facility and reduce complexities associated with customer-specific systemisation and maintenance operations. Further still, the present invention aims to reduce the footprint or space needed to accommodate the pumps when a pumping system is installed. 
         [0008]    As a result, a first aspect of the invention provides a vacuum pump in which two or more pumping mechanisms, i.e. the booster pump and main pump, are housed in the same stator. 
         [0009]    A second aspect of the invention provides a vacuum pump stator comprising a plurality of stator cavities. 
         [0010]    A third aspect of the invention provides a vacuum pump stator comprising at least two operatively interconnected cavities, wherein at least two of the cavities each comprise at least one rotor-receiving portion shaped to receive two or more at least partially intermeshing rotors, and wherein an axis of a rotor-receiving portion of a first one of the cavities is offset with respect to an axis of a rotor-receiving portion of a second one of the cavities. 
         [0011]    The stator cavities are preferably operatively interconnected by a conduit, which conduit may advantageously extend through the body of the stator to interconnect two of the stator cavities. 
         [0012]    Advantageously, the invention enables a multi-stage vacuum pump, that is to say, a vacuum pumping system comprising a number of pumps connected in series (for example a main vacuum pump and a booster pump), to be rationalised. This is achieved by forming a number of pumping stages in a single unit by two or more stages of the vacuum pumping system sharing a common stator. 
         [0013]    The stator cavities may therefore form part of a number of different types of vacuum pump, for example one cavity may form part of a booster pump, whereas a second one of the cavities may be adapted for receiving the plurality of interconnected pumping stages of, say, a multi-sage Roots pump. 
         [0014]    In such a situation, one of the cavities may comprise a plurality of axially aligned, and interconnected, rotor-receiving portions that may be adapted, in use, to receive a pair of intermeshing rotors. A first rotor of each pair may be mounted on a first shaft and second rotor of each pair may be mounted on a second shaft. Moreover, by interconnecting the rotor receiving portions of the cavity in series, for example, using interconnecting conduits, each pair of intermeshing rotors and its corresponding rotor-receiving portion of the cavity can form a separate pumping stage of a multi-stage vacuum pump. 
         [0015]    Additionally or alternatively, another one of the cavities may simply comprise a single rotor-receiving portion adapted, in use, to receive a single pair of intermeshing rotors. Again, the rotors can be mounted separate shafts such that the intermeshing rotors and rotor-receiving portion of the cavity together form a single pumping stage of a vacuum pump, such as a booster pump. 
         [0016]    In order to obtain the maximum benefit from the invention, it is preferable that the stator cavities of the stator be operatively interconnected, for example, by interconnecting the outlet of a first stator cavity to the inlet of a second stator cavity, and so forth. This may be achieved by providing a conduit that extends through the body of the body of the stator (to interconnect two stator cavities directly) or via a conduit, channel or pipe that interconnects two stator cavities via a passage extending outside the body of the stator. 
         [0017]    The stator cavities are preferably adapted to receive at least two intermeshing rotors, such as a pair of intermeshing Roots or Northey rotors. The vacuum pump is preferably a “dry” pump, that is to say, that the clearance between the stator and rotors, in use, is sufficiently small to form an effective seal therebetween. In other words, a very small running clearance between an exterior surface of a rotor and the interior surface of the stator cavity is so small as to impede or minimise the backflow of pumped gasses, or the circumvention of a particular pumping stage. 
         [0018]    The stator cavities of the stator may be adapted to receive the same type of rotors, or different types. For example, one cavity may be adapted to receive the rotors of a roots-type pump, whereas another cavity may be adapted to receive a set of intermeshing Archimedean screw-type rotors. The various possible combinations of rotors will be apparent to those familiar with vacuum pumping technology. 
         [0019]    To facilitate manufacture, assembly of a pump and subsequent servicing, the stator is preferably of a clamshell type (that is to say the stator comprises a plurality of separable stator portions that are adapted to sealingly mate with one another). Where a clamshell-type construction is employed, at least one of the separable stator portions may comprise a recess forming, in use, at least part of one of the stator cavities, or a number of recesses forming, in use, at least part of a number of respective stator cavities 
         [0020]    To aid with thermal management of the pump, the stator preferably comprises a cooling circuit, which may take the form of a channel in the stator for conveying, in use, a flow of coolant fluid to it. Where a clamshell-type stator is used, the cooling circuit channel may comprise a number of discrete cooling circuit channel portions that arranged, in use, to align when the separable stator portions are assembled: the discrete cooling circuit channel portions together forming at least one continuous cooling circuit channel within in the stator. 
         [0021]    Additionally or alternatively, the cooling circuit may comprise an actively-cooled heat sink affixed to an exterior surface of the stator, which may be cooled by a liquid cooling circuit or a forced air cooling system. 
         [0022]    A fourth aspect of the invention provides a unitary, multi-sage vacuum pump comprising a stator as described herein. 
         [0023]    The unitary, multi-sage vacuum pump preferably comprises a number of pumping stages, which may be formed by sets of rotors that are configured to rotate within separate cavities of the stator. As the stator preferably comprises a plurality of operatively interconnected stator cavities for receiving the rotors, it is possible to provide a more compact and rationalised multi-stage pump: the stator being shared by a number of pumping stages. 
         [0024]    The unitary, multi-stage vacuum pump may further comprise a head plate that is sealingly affixable to the stator, which head plate may comprise a channel or recess forming a conduit for the flow of gas between first and second stator cavities of the stator. Such an arrangement enables the axial orientation of the stator cavities to be parallel or non-parallel, and avoids having to form conduits in the stator itself for interconnecting the stator cavities. The head plate may additionally comprise apertures for receiving the rotor shafts, or bearings for the rotor shafts of the rotors. 
         [0025]    More precisely, the invention provides a vacuum pump stator comprising, a longitudinal member and end members disposed at opposing ends of the longitudinal member, wherein at least two pumping volumes are defined by the longitudinal and end members respectively and each pumping volume being arranged to accommodate a rotatable pump element disposed on a shaft, and wherein a rotatable pump element shaft is disposable in one of the pumping volumes between end members such that a shaft in a first volume is parallel to a shaft in a second volume, wherein a portion of the longitudinal member that defines the first volume comprises a main body and a second body attachable to the main body. Therefore, the first pumping volume is defined by the main and second bodies and a second pumping volume is defined by the main body. The end members can be integrally formed together to provide a unitary end plate. The longitudinal members can be formed to provide a stator having a clam-shell configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0026]    Embodiments of the present invention are described by way of example, with reference to the accompanying drawings, of which: 
           [0027]      FIG. 1  is a schematic orthographic drawing of a known multi-stage vacuum pump from the side and in partial cross-section on II; 
           [0028]      FIGS. 2 to 6  are schematic orthographic drawings of various embodiments of a multi-stage vacuum pump in accordance with the invention as viewed from the side and in partial cross-section; 
           [0029]      FIG. 6  is a schematic perspective view of a further alternate embodiment of a stator in accordance with the invention; and 
           [0030]      FIG. 7  is a schematic perspective view of a yet further alternate embodiment of one portion of a stator in accordance with the invention for a two-stage pump comprising a booster and a multi-stage Roots pump. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In  FIG. 1 , a known multi-stage vacuum pump  10  comprises separate booster  12  and main pump  14 . The right-hand side of  FIG. 1  is a schematic side view of the multi-stage pump  10  and the left-hand side is a schematic cross-section on I-I. The inlet  16  of the booster pump  12  is connected to a vessel to be evacuated (not shown) and its output  18  is connected via a conduit  20  to the inlet  22  of the main pump  14 . The outlet  24  of the main pump  14  vents to atmosphere, or to a further backing pump (not shown). 
         [0032]    As can be seen from the right-hand side of  FIG. 1 , each of the pumps  12  has its own cooling circuit  26 , which comprises a cooling plate, which is thermally coupled or bonded to the pump  12 ,  14 , for instance using a thermal compound and a clamp or by other means. A detailed discussion of the construction of the cooling plates  26  is not necessary here, although it will be appreciated that the cooling plates  26  typically comprise internal channels through which a cooling fluid can flow. 
         [0033]    Each of the pumps  12 ,  14  comprises a stator  28  formed in two or more parts  30 ,  32 , that is having a “clamshell” construction, whereby the stator parts  30 ,  32  each comprise a recess forming part of the stator cavity  34  and a mating surface  36  that can be clamped to the mating surface  36  of the opposite part to form a seal. A sealant can be applied to the mating surfaces  36  to improve the seal, where this is necessary. Within the stator cavities  34  of each pump  12 ,  14 , there is provided a pair of intermeshing rotors  38  that rotate in opposite directions about a rotor shaft  40 . The rotor shafts  40  are driven by motors  42  and typically, but not exclusively, via a gearbox (not shown), which comprises gears to cause rotor shafts  42  to synchronise. Where gears are not present this can be achieved by driving both shafts from individual motors and synchronising the position by other means, for example, magnetic couplings. 
         [0034]    The ends of the rotor shafts  40  are mounted in bearings (not shown) which are typically set into the recesses of the pump head plates  44 . The headplates  44  must be accurately seated on to the end faces of the stator parts  30 ,  32  to ensure an airtight seal and to ensure correct running clearances between the ends of the rotors  38  and the interior surfaces of the stators and head plates  44 , given that the components of the pumps  12 ,  14  are subject to thermal and stress-induced expansion/traction during use. 
         [0035]    In a multi stage vacuum pump of this known type  10 , there are a relatively large number of components, many of which, for example the cooling circuits  26 , the head plates  44 , the stator component  30 ,  32 , and so on are duplicated. In addition, given that the booster pump  12  and the main pump  14  are separated, the volume occupied by the multi-stage pump  10  is relatively large. Moreover, given that there are a great number of sealingly mating surfaces, for example the head plate to the stator parts, the mating of the stator parts themselves and the connection of the conduit  20  between the pumps  12 ,  14  the likelihood of a leak and a consequential reduction in the efficiency of the pump is increased. 
         [0036]    A multi stage vacuum pump  50  in accordance with the present invention is shown schematically in  FIG. 2  in which it will be noted that the booster pump  52  and main pump  54  share a common stator  56  of a three-part clamshell type. The stator  56  is made up of three parts  58 ,  60 ,  62 , which are a fixed to one another to form a stator  56  having two stator cavities  64  for receiving the rotors  66 ,  68  of the booster pump  52  and main pump  54 . The stator cavities  64  are interconnected by a through hole  70  forming a conduit between stator cavities  64 . As such, the outlet of the booster pump  52  is directly connected to the inlet of the main pump  54  meaning that an interconnecting conduit ( 20 , as in  FIG. 1 ) is not required. The location and configuration of the through hole  70 , can of course, be varied, for example, it could extend through the middle portion  56  of the stator (not shown), or could, indeed, be formed as a channel in one of the head plates  72 . 
         [0037]    In  FIG. 2  the booster pump stage  52  is located on top of a dry pump stage  54 . The main booster  52  comprises a single stage Roots mechanism housed in a single stator body  56 . The dry pump section comprises a multiple stage Roots pump (as is well known in the vacuum pump technical domain). The dry pump section stator  56  is formed of a clam-shell design, wherein a first part  58  of the clam-shell is formed integrally with the Booster stator and a second part  60  of the clamshell is an independent component that is attached to the first part  58  to form the complete stator  56 . 
         [0038]    The right-hand side of  FIG. 2  shows a side view of the multi-stage pump of the invention  50  in which a first motor  51  is shown for driving the Booster pump stage  52 , and a second motor  53  drives the dry pump mechanism  54 . A single pair of head plates  72  is used, whereby one head plate  72  is disposed at each end of the pump stator and is configured to accommodate necessary mechanisms needed for booster and main pump operation. Thus, only two head plates are required by both the booster pump and main pump stator and a single pair of head plates can replace the multiple pairs of head plates used by prior pumping systems shown in  FIG. 1 . A single end cover  55  can be disposed over a head plate  72  and can accommodate timing gears, bearings, lubrication systems or the like. 
         [0039]    The first and second pump volumes each accommodate rotor pump elements disposed on shafts. Each volume has a longitudinal axis that runs along the length of the volume and the longitudinal axis of the first second pumping volumes are in the same plane and parallel to one another. In the arrangement shown in  FIG. 2 , the plane in which the respective longitudinal axes are disposed is vertical—in other words, the first pumping volume is arranged on top of the second pumping volume. Furthermore a second and third plane is defined by each axis of rotation for each rotor pair, respectively. In this embodiment, the second plane defined by the booster rotors&#39; axis is parallel to the third plane defined by the main pump rotors&#39; axis. In other words, the planes are spaced apart and do not cross one another. Additionally, the headplates are spaced apart by a distance substantially equal to the overall length of the rotor pumping mechanism. As such the headplates are arranged to form a portion of the stator and define a surface of a swept volume occupied by the rotor mechanism. 
         [0040]    It will be appreciated that one of the main advantages of the invention is an overall reduction in the number of parts, crucially, reduction in the number of surfaces that must be sealed to one another. In addition, it will be noted, in particular, from the right-hand side of  FIG. 2 , that a single cooling circuit  74  can be used to cool the common stator  56  shared by the two pump stages, as opposed to having to have a separate cooling circuit for each of the pumping stages, as shown in  FIG. 1 . Furthermore, it will be noted that there are just two head plates  72 , as opposed to the four headplates shown in  FIG. 1 . As such, the complexity, “part count” and physical volume of the multistage vacuum pump  50  of the invention has been greatly reduced. 
         [0041]    In  FIG. 2  it will be seen that the shared stator  56  comprises three stator parts  58 ,  60 ,  62 , the upper and lower parts  58 ,  62  having a single recess therein forming one half of each of the stator cavities  64 , whereas the middle part  60  has to recesses located on opposite sides thereof forming the remaining half of each of the stator cavities  64 . 
         [0042]    However, as can be seen in  FIG. 3 , a similar pump arrangement can be fabricated from a two-part stator  56  in which the upper portion  76  comprises an elongated through hole forming one entire stator cavity  64  and a recess forming one half of another stator cavity  64 ; the lower part  78  comprising a single recess forming the other half of the lower stator cavity  64 . 
         [0043]    Similarly, as can be seen from  FIG. 4 , a one-piece stator  56  comprises two elongate through holes forming a pair of complete stator cavities  64 . The arrangement is shown in  FIGS. 3 and 4 , in particular, may be used where the rotors are of a unitary type that can be inserted and removed axially, that is to say, lengthwise, into the cavity  64  as a pair. However, where the rotors  68  need to be, or are more easily installed individually, rather than as a pair, the stator  56  is preferably of the clamshell type as shown in  FIG. 2  or  FIG. 3  to enable them to be inserted and removed by vertical placement, rather than by axial insertion. 
         [0044]    A variant of the invention shown in  FIG. 5  differs from the embodiment shown in  FIGS. 2 ,  3  and  4  in as much as this multi-stage pump  80  has two inlets  82  and one outlet  84 . Such a multistage pump may be used for evacuating different parts of a system simultaneously, or for simultaneously evacuating two different systems altogether. In any event, the rotors  66  of the two pumping stages share a common stator  52 , which is of a three-part clamshell design as previously described. In this instance, however, it is the outlets of the two-stage cavity  64  which are interconnected, rather than the outlet of one of the stator cavities being connected to the inlet of the other stator cavity. 
         [0045]    A further variant of a stator for a multistage pump  90  in accordance with the invention shown in  FIG. 6 , in which the stator cavities  64  are not parallel, but rather perpendicular to one another. The stator  56  is made up from one, two or three portions  58 ,  60 ,  62  that fit together as previously described. The upper portion  58  has a recess forming one half of the upper cavity  64 , and the middle portion  60  has an upper recess forming the other half of the upper cavity  64 . The lower surface of the middle portion  60  also has a recess, oriented at right angles to the recess in its upper surface forming the upper half of the lower cavity  64  and the lower portion  62  has a recess in its upper surface forming the other half of the lower cavity  64 . In use, the portions  58 ,  60 ,  62  are clamped together such that their mating surfaces  36  form a gas-tight seal—a gasket or other means of sealing (not shown) is typically provided between the mating surfaces  36  to help form a seal. 
         [0046]    The stator  56  has an inlet port  16  and an outlet port  24  communicating with the recesses of the upper  58  and lower  62  stator portions, respectively, and a through hole  17  forming a conduit between the two recesses  64  such that the outlet of the upper pump stage discharges directly into the inlet of the lower pump stage. In use, head plates (not shown) are fitted to the exterior surfaces  19  of the stator  56  to close-off the ends of the cavities to define elongate, internal stator cavity volumes for receiving the pumps&#39; rotors (not shown). 
         [0047]    The invention advantageously provides a single dual pump configuration, such as a booster/dry pump configuration, that can be shipped to an end-user as a single entity and which has a reduced volume when installed for use in an industrial process. A single cooling circuit  74  can be utilised for both pumping sections  52 ,  54  making thermal management system much simpler and less expensive. Also, the number of components needed to manufacture such a pump is reduced thereby saving manufacturing effort and costs. 
         [0048]    Gas pathways or conduits between the main booster section and dry pump sections, and pump inlet and outlet are not shown in all of the drawings for clarity. However, it is clearly understood that such features are required for the normal operation of the pump. These features can be incorporated into the stator during the manufacture of the stator components, during stator casting and/or machining processes for instance. 
         [0049]      FIG. 7  is a perspective view of a middle stator portion  60  of a 3-piece clamshell stator  50  according to the invention, which comprises stator cavities  64  for a unitary multi-stage vacuum pump comprising a booster pump  80  and a multi-stage Roots pump  82 . The clamshell portion  60  is manufactured from a solid, machined block of material and has an upper cavity (as shown)  82  shaped to receive the shafts and rotors (not shown) of a two-stage Roots pump, and a lower cavity (as shown)  80  shaped to receive a pair of elongate Roots rotors (not shown). 
         [0050]    The upper cavity  82  is formed from a pair of parallel shaft receiving portions  84 , which receive the shafts of the rotors (not shown), and wider rotor-receiving portions  86 , which are shaped to receive the overlapping/intermeshing rotors themselves (not shown). The rotor-receiving portions  86  are fluidly interconnected by a conduit  88  that extends from the lower surface of one rotor-receiving portion and which feeds into the upper part of an adjacent rotor-receiving portion via a top clamshell portion (not shown). As such, pumped gas can be transferred from one rotor-receiving portion  86  to the next, in series. 
         [0051]    The inlet  90  of the Roots pump  82  connects to the outlet  92  of the booster pump  80  via a cavity interconnecting conduit  94 , which is a through-hole extending between the respective rotor-receiving portions  86  of each of the cavities  64 . The booster pump cavity  80  is similar to the Roots pump cavity  82  except that there is only one rotor-receiving portion  86  and no rotor-receiving portion interconnecting conduits  88 . 
         [0052]    Of course, the stator portion of  FIG. 7  could be modified such that both of the cavities  64  are shaped to receive the shafts and rotors of a multi-stage pump, in which case both cavities would have a similar shape. Furthermore, although a two-stage Roots pump has been illustrated for simplicity, any number of pumping stages could be employed. The booster pump could also be replaced by an Archimedean screw-type pump: the various options being a matter of design preference and pumping requirements. 
         [0053]    The present invention is not limited to the arrangements shown and the pumping volumes can be arranged side-by-side or such that the longitudinal axes (as shown in  FIG. 6 , for example) are not in the same plane. Alternatively, the embodiments of the present invention shown in the figures could be adapted to have a number of inlets and outlet ports. In this way, two or more booster pumps can be arranged in a unitary pump stator, each having their own independent inlets and outlets respectively. This configuration allows for a compact pumping arrangement that facilitates efficient switching between pumping lines to account for different process gases passing through the vacuum system without mixing or reacting with deposits that might be found in the pump or ducting. Furthermore, multi-stage booster pump configurations can be utilised alongside multistage main pump.