Stator arrangement for a rotary piston vacuum pump

Abstract

Stator arrangement for a rotary piston vacuum pump, wherein the stator arrangement comprises: an intermediate stage stator section for separating a first rotor stage and a second rotor stage, wherein the intermediate stage stator section has a plurality of gas flow channels defined therein, which are designed to establish fluid communication between the first rotor stage and the second rotor stage through the intermediate stage stator section.

Description

TECHNICAL FIELD This disclosure relates to a stator assembly for a Roots vacuum pump. Furthermore, this disclosure relates to a Roots vacuum pump incorporating the assembly. BACKGROUND Vacuum systems typically use pumps to evacuate gases from the system. One type of vacuum pump used in such systems is a Roots vacuum pump. A Roots vacuum pump generally has two counter-rotating shafts, with a rotor mounted on each shaft. The rotors have a series of outer blades and recesses defined between the outer blades. The rotors are mounted so that an outer blade of the rotor on one shaft engages with a corresponding recess of the rotor on the other shaft. As the shafts and rotors rotate, gas is trapped and compressed between the interacting outer blades and recesses. The repeated entrapment and compression of gas between the rotors creates a pumping action that can be used to pump gas from an inlet on one side of the rotors to an outlet on the opposite side, thus evacuating a system. Typically, Roots vacuum pumps have multiple stages of interacting rotors, each axially spaced along the shaft and separated by a stator structure. By providing multiple stages, progressive increases in gas compression are possible throughout the pump, allowing the pump to efficiently create a higher degree of vacuum for the system. The gas must be routed between the inlet, the rotor stages, and the outlet. This is typically achieved by providing a gas flow path between these components, defined by the stator structure separating the rotor stages. Incorporating the gas flow path into the stator structure can adversely increase the length and size of the Roots vacuum pump. Therefore, there is a need to improve the design of the stator structure and the gas flow path within it to enable a more compact pump design without compromising pump efficiency. SUMMARY In one aspect, the present disclosure provides a stator assembly for a Roots vacuum pump according to claim 1. The plurality of gas channels allows the gas compressed by the first rotor stage to be passed on to the next, subsequent rotor stage for further compression. In other words, the interstage stator section allows the gas to be passed from the outlet of the first rotor stage to the inlet of the second rotor stage. The first and second rotor stages are generally axially spaced apart along a longitudinal axis, and the inter-stage stator section is disposed axially between them (i.e., separating them in the axial direction). Providing a plurality of gas flow channels through the inter-stage stator section allows for an increase in the flow area through the inter-stage stator section without requiring an increase in the size of the inter-stage stator section (e.g., its thickness or axial length) (e.g., compared to known configurations of an inter-stage stator section with only one gas flow channel). In this way, minimal resistance to gas flow through the stator assembly can be maintained while maintaining a more compact size. In one embodiment of the above, the intermediate stage stator portion has a first opening for receiving a first rotor shaft and a second opening for receiving a second rotor shaft. The first and second openings allow the rotor shafts to pass through the inter-stage stator section. This means that the rotor shafts can rotate to operate the rotor stages without being obstructed by the inter-stage stator section. The first and second openings are generally spaced apart in a direction transverse to the longitudinal axis. In some embodiments, the openings may be sized to provide sufficient clearance around the shafts to allow them to rotate freely therein, or in other embodiments, they may include bearings to help support the shafts as they rotate within the openings. In a further embodiment of the above, the plurality of gas flow channels comprises a first gas flow channel, a second gas flow channel, and a third gas flow channel. The first gas flow channel extends through a central region of the inter-stage stator section between the first and second openings. The second gas flow channel extends through a first side region of the inter-stage stator section around the first opening. The third gas flow channel extends through a second side region of the inter-stage stator section around the second opening. The side regions are regions on either side of the central region in a direction perpendicular to the longitudinal axis. In other words, there is a left side region, which lies perpendicular to the longitudinal axis to the left of the central region (i.e., around the first opening), and a right side region, which lies perpendicular to the longitudinal axis to the right of the central region (i.e., around the second opening). This design of the gas flow channels creates a balanced flow area around the various regions of the interstage stator section. It also provides better utilization of the total space available in the interstage stator section, maximizing the gas flow area passing through. Accordingly, this design could enable even more compact interstage stator sections while maintaining pump efficiency. In a further embodiment of the above, the second and third gas flow channels branch off from the first gas flow channel and are fluidly connected thereto at both their beginning and their end. In other words, the second and third gas flow channels originate from the first gas flow channel, extend through the respective side region (i.e. around the respective opening) and then flow back into the first gas flow channel. This design provides a compact arrangement of gas flow channels that direct the gas flow around the various regions of the inter-stage stator section. In a further embodiment of the above, the second and third gas flow channels each form a generally C-shaped gas flow path around the respective first or second opening. The C-shape of the second and third gas flow channels allows them to curve around the first and second openings, respectively. This, in turn, helps maximize the space available for gas flow through the interstage stator section. In a further embodiment of the above, the stator arrangement is formed from two interconnected stator halves. This allows the stator assembly to be manufactured more easily by casting the two halves separately and then assembling them together. The halves can be attached to each other using a detachable device (e.g., threaded fasteners). The halves also allow for easier inspection, maintenance, and replacement processes for the stator assembly and the rotor components housed therein during pump operation. In a further embodiment of the above, the inter-stage stator section is formed by two opposing, generally w-shaped sections which are joined together when the two stator halves are connected together. In other words, each stator half defines a corresponding half of the inter-stage stator section, and these half inter-stage stator sections are generally w-shaped in a transverse direction relative to the longitudinal axis. By generally w-shaped, it is meant that the halves of the inter-stage stator sections generally form the shape of two transversely connected U's. When the w-shaped inter-stage stator section halves are joined together, they generally form an ∞ shape (or the shape of two transversely connected O's). The joined region corresponds to the central region of the inter-stage stator section, and the unjoined side of the O shapes correspond to the side regions of the inter-stage stator section, respectively. The U / O shapes are generally defined by cutouts in the w-shaped sections that form the first and second openings when joined together. The cutouts may be generally semicircular, and the resulting openings may be generally circular. In yet another embodiment of the above, the plurality of gas flow channels are formed by a respective plurality of cavities defined by the two generally w-shaped sections which are fluidly connected when the two generally w-shaped sections are joined together. By having the w-shaped sections form halves of the inter-stage stator section and using the cavities therein to form the gas flow channels when joined together, a simpler mold can be used to manufacture the stator assembly (i.e., compared to inter-stage sections that are not split into two halves). In a further embodiment of the above, the inter-stage stator section has an inlet opening for receiving fluid from a first rotor stage and an outlet opening for passing fluid from the inter-stage stator section to a second rotor stage. The plurality of gas flow channels are arranged between the inlet and outlet openings and fluidly connect the inlet opening to the outlet opening. In this way, the majority of gas flow channels are fed from a common inlet, and the conveying gas flows to a common outlet. This, in turn, can help simplify the mold for manufacturing the stator assembly and maintain a balanced gas flow distribution through the interstage stator section. In a further embodiment of the above, the stator assembly comprises a plurality of intermediate stage stator sections. This allows the stator assembly to separate additional rotor stages. For example, a second interstage stator section may be used to separate the second rotor from a third rotor and to transfer gas between the two. Within the scope of the present disclosure, the stator assembly may include any number of interstage stator sections, depending on the number of adjacent rotor stages that need to be separated, while still allowing fluid communication between them. In a further aspect, the present disclosure provides a (multi-stage) Roots vacuum pump comprising the stator arrangement according to the above aspect or one of its embodiments. The pump further comprises a first (upstream) rotor stage and a second (downstream) rotor stage spaced apart along a longitudinal axis of the pump. The interstage stator section of the stator assembly separates the first rotor stage from the second rotor stage along the longitudinal axis, and the plurality of gas flow channels are configured to receive fluid from the first (upstream) rotor stage and to convey the fluid to the second (downstream) rotor stage. In one embodiment of the above, the pump further comprises a pair of rotor shafts extending along the longitudinal axis, and each rotor stage comprises first and second outer-bladed rotors mounted to rotate with a corresponding one of the rotor shafts. The respective rotor shafts are arranged to pass through the first and second openings. The openings can be sized to allow the shafts to pass through and rotate freely within them. In further examples, the opening could have bearings inside to support the shaft for rotation. The pump may also include a motor operatively connected to the shafts and configured to drive their rotation. By using the stator arrangement in the Roots vacuum pump, the overall length and size of the pump can be reduced for a given capacity. This opens up new applications for the Roots vacuum pump where a more compact pump is required. Embodiments of the present arrangement are designed to transfer gas from a first (upstream) rotor stage to a second (downstream) subsequent rotor stage in a (multi-stage) Roots vacuum pump. The provision of a plurality of gas flow channels through the inter-stage stator section enables an increase in the flow area through the inter-stage stator section without having to increase the size of the inter-stage stator section (e.g., its thickness or axial length) (e.g., compared to known configurations of an inter-stage stator section with only one gas flow channel). In one embodiment of the above, the plurality of gas flow channels comprises a first gas flow channel, a second gas flow channel and a third gas flow channel. This design of the gas flow channels creates a balanced flow area around the various regions of the interstage stator section. It also provides better utilization of the total space available in the interstage stator section, thus maximizing the gas flow area. In one embodiment of the above, the plurality of gas flow channels is defined by forming a corresponding plurality of cavities in two opposite stator halves. This can enable easier manufacturing of the inter-stage stator sections and their associated gas flow channels, as well as simpler inspection, maintenance, and replacement processes. Although certain advantages have been explained above with reference to certain features, further advantages of certain features may become apparent to those skilled in the art in accordance with the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS One or more non-limiting embodiments according to the present disclosure will now be described with reference to the accompanying figures. In the drawings: Fig. 1 is a top perspective view of an example of a Roots vacuum pump according to the prior art; Fig. 2 is a perspective cross-sectional view of the pump of Fig. 1 along line AA; Fig. 3 is an axial cross-sectional view of the pump of Fig. 2 along line BB; Fig. 4A is a perspective view of the upper half of the stator assembly of the pump of Fig. 1; Fig. 4B is a perspective view of the lower half of the stator assembly of the pump of Fig. 1; Fig. 5A is a perspective view of the upper half of a stator assembly for a Roots vacuum pump according to an embodiment of the present disclosure;5B is a perspective view of the lower half of a stator assembly for a Roots vacuum pump according to an embodiment of the present disclosure; FIG. 6 is an axial cross-sectional view of a stator assembly for a Roots vacuum pump according to an embodiment of the present disclosure. DETAILED DESCRIPTION To better understand embodiments of the present disclosure, a known multi-stage Roots vacuum pump and a stator assembly thereof are illustrated in Figs. 1 to 4B; they are explained below for comparison. Fig. 1 shows a Roots vacuum pump 100 defined along a longitudinal axis L. As is well known, the pump 100 includes a motor 102 attached to an outer casing 104 at one axial end 104a and an end cap 106 attached to the outer casing 104 at an opposite axial end 104b. The motor 102 is operatively connected to two rotor shafts, to which multiple rotor stages are mounted. As explained above as background, the shafts operate to rotate the rotor stages for pump operation. The outer casing 104 has a gas inlet 108 defined at the axial end 104b, which is used to conduct gas from the system or the environment to be evacuated to the pump 100. The outer casing 104 also has a gas outlet 109 (at the bottom of the pump 100) at the opposite axial end 104a, through which the evacuated gas can be discharged, for example, into the environment or atmosphere. The gas inlet 108 and the gas outlet 109 are openings extending through the outer casing 104 and may have any suitable shape. They may also include mounting bosses and surrounding parts for attaching a suitable inlet and outlet system (e.g., inlet and outlet piping). Fig. 2 shows a cross-sectional view of pump 100 taken along line AA. Pump 100 includes seven rotor stages 110a-110g axially spaced along the longitudinal axis L (however, any number of stages may be used depending on the application). Although seven rotor stages 110a-110g are illustrated, it should be understood that the present disclosure is equally suitable for application to a multi-stage Roots vacuum pump 100 having any number of stages, i.e., a plurality of stages / two or more stages. Although not shown, the rotor stages 110a-110g, as is well known and described above, comprise cooperating pairs of outer-bladed rotors mounted on respective counter-rotating shafts. The shafts may be supported at both axial ends 104a, 104b by bearing assemblies 103a, 103b housed in the motor 102 and end cap 106, respectively. The outer cover 104 includes a stator assembly 120 with interstage stator sections 122a-122f interposed between the individual rotor stages 110a-110g. In this manner, the stator sections 122a-122f serve to separate the successive rotor stages 110a-110g. The number of interstage stator sections 122a-122f varies depending on the number of rotor stages 110a-110g used in a particular design. The stator sections 122a-122f each define a gas flow channel 124 internally, which allows fluid communication between axially adjacent (i.e., consecutive) rotor stages 110a-110g. In this way, the stator sections 122a-122f serially fluidly connect the consecutive rotor stages 110a-110g. The gas flow channels 124 extend generally radially upward relative to the longitudinal axis, thus receiving compressed gas at the exit of one rotor stage (at the "bottom" of the stator assembly 120) and directing it to the beginning of the next consecutive rotor stage (at the "top" of the stator assembly 120). This results in a gas flow path G from the gas inlet 108 through the pump 100, which passes sequentially through each rotor stage 110a-110g before being expelled from the gas outlet 109. This gas flow path G ensures that the gas is processed sequentially by each of the rotor stages 110a-110g, allowing the pump to efficiently produce higher compression ratios. Fig. 3 is an axial cross-section through stator section 122a, in which aspects of the structure of stator assembly 120 can be seen more clearly. The other stator sections 122b-122f also have the same structure and features. The following description therefore also applies to them. The stator assembly 120 is formed of two halves: an upper stator half 120a and a lower stator half 120b. The upper and lower stator halves 120a, 120b are secured together by fasteners 130a, 130b and contact each other along an interface 123. The fastening elements 130a, 130b extend through holes in mounting flanges 132a, 132b defined on each stator half 120a, 120b and are threadedly received therein. This removable fastening device facilitates the removal and replacement of the stator halves 120a, 120b for pump assembly, as well as for inspection and maintenance. However, any other suitable fastening device may be used; for example, welding the stator halves 120a, 120b along the interface 123. The upper and lower stator halves 120a, 120b define internal gas flow cavities 125a, 125b that define the gas flow channel 124 when the stator halves 120a, 120b are joined together. The cavities 125a, 125b are hollowed-out portions of the stator halves 120a, 120b. The upper and lower stator halves 120a, 120b are also complementarily shaped to form two shaft openings 121. The shaft openings 121 are formed by two generally semicircular shaft recesses 121a, 121b in the upper and lower stator halves 120a, 120b. When the stator halves 120a, 120b are joined together, the recesses 121a, 121b accordingly form the two circular shaft openings 121. The openings 121 are arranged and dimensioned so that the two counter-rotating rotor shafts (not shown) pass through the stator arrangement 120 and can rotate the rotor stages 110a - 110g for pump operation. The stator assembly 120 and the halves 120a, 120b may be made of any suitable material; for example, a metal material such as cast iron or graphite iron. Other suitable metal materials include (but are not limited to) steel alloys and aluminum alloys. As can be appreciated, the particular material may be selected depending on its particular application and operating conditions. The stator assembly 120 and the halves 120a, 120b may be formed in any suitable manner. For example, they may be cast. In another example, additive manufacturing could be used. Although a particular construction of the stator assembly 120 has been explained above, it should be understood that the stator assembly 120 could also be constructed in any other suitable manner. For example, instead of the stator halves 120a, 120b, the stator assembly 120 could be formed from a single piece. The specific shape of the gas flow channel 124 and the openings 121 (as well as the recesses 121a, 121b and cavities 125a, 125b forming them) can also be easily modified and varied depending on the requirements of a particular application. Fig. 4A and Fig. 4B show perspective views of the stator halves 120a and 120b, respectively. For each of the successive stator sections 122a-122f, the upper stator half 120a has a respective gas flow cavity 125a defining an opening 126a at the interface 123, and the lower stator half 120b has a corresponding gas flow cavity 125b providing an opening 126b at the interface 123. As can be seen, with the stator halves 120a, 120b joined together, the openings 126a, 126b of each stator section 122a-122f are joined at the interface 123, and the cavities 125a, 125b thereof are accordingly fluidly connected to form the respective gas flow channels 124. As shown, each stator section 122a-122f is formed by the combination of two generally W-shaped sections 140a, 140b of the upper and lower stator halves 120a, 120b. By generally W-shaped, it is meant that the shape is generally that of two connected U-shapes. This shape results in part from the shaft recesses 121a, 121b defined in the upper and lower halves of the stator sections 122a-122f, and the gas inlet and outlet openings 128, 129 located therein (as explained in more detail below). However, the stator halves 120a, 120b need not be limited to this shape, and any other suitable shape is possible to enable any particular formation of inlets, outlets and flow channels through the stator sections 122a - 122f. As shown in Fig. 3, the cavities 125a, 125b and the gas flow channel 124 formed thereby extend radially through the W-shaped sections 140a, 140b in a central region defined between the shaft recesses 121a, 121b. Accordingly, the openings 126a, 126b open at the interface in the central region of the W-shaped sections 140a, 140b. In contrast, the curved U-shaped regions on either side of this central region (i.e., outside each shaft recess 121a, 121b) are side regions of the W-shaped sections 140a, 140b (i.e., the left and right side regions of the W-shaped sections 140a, 140b). These side regions are solid and, unlike the central region, have no cavities or openings. Each cavity 125b has an inlet opening 128 through which gas from the outlet of a respective rotor stage 110a-110f is directed into the respective gas flow channel 124. Each cavity 125a has an outlet opening 129 through which the gas exits the respective gas flow channel 124 to be passed on to the next of the successive rotor stages 110b-110g. In the example shown, the lower stator half 120b is contoured such that a vane 127 is located below each inlet opening 128, which helps direct the gas flow exiting the rotor stages 110a-110f into the gas flow channel 124. A corresponding vane (not visible) may also be present in the upper stator 120a above each outlet opening 129 to help direct the gas flow exiting the gas flow channels 124 into the inlet of the next of the successive rotor stages 110b-110g. Both the upper and lower stator halves 120a, 120b have an end plate 131 at the axial end 104a, which acts to close off the last rotor stage 110g. As a result, the gas flow exiting the last rotor stage 110g is directed to the gas outlet 109 of the pump 100, which is defined by the lower stator half 120b at the axial end 104a (i.e., axially between the last stator section 122f and the end plate 131). The upper and lower stator halves 120a, 120b both have mounting flanges 133a, 133b at the axial ends 104a and 104b, respectively. The mounting flange 133a at the axial end 104a is intended for mounting the stator assembly 120 to the motor 102. The mounting flange 133b at the opposite axial end 104b is intended for mounting the stator assembly 120 to the end cap 106. The gas inlet 108 extends axially between the mounting flange 133b and the first stator section 122a through the upper stator half 120a. The openings 126a, 126b are shown as substantially rectangular and having an axial length and width. However, they may have any suitable regular or irregular shape (e.g., circular or oval), depending on the radial cross-section of the cavities 125a, 125b forming them. The gas flow channels 124 must have a sufficient flow area to ensure that the flow resistance is minimal as the gas flows forward through the rotor stages 110a-110g and the stator sections 122a-122f during pump operation. If the flow area through a particular stator section 122a-122f is insufficient for the amount of gas pumped through it, a pressure drop could occur, negatively impacting the pump's efficiency. Therefore, the cavities 125a, 125b and openings 126a, 126b are designed to have sufficient volume and flow area to allow gas to be pumped therethrough with minimal flow resistance (i.e., at least up to the maximum designed pumping capacity of the pump 100). The principle by which this is achieved is to dimension the cavities 125a, 125b and openings 126a, 126b such that the gas flow channels 124 have sufficient volume and flow area to accommodate the maximum operating gas flow from each rotor stage 110a-110f. As the gas flows from the high-vacuum side of pump 100 at gas inlet 108 to the low-vacuum side of pump 100 at gas outlet 109 and is progressively compressed by rotor stages 110a-110f, its volume is reduced. As shown, the size and volume of gas flow channels 124 also progressively decrease to compensate. The dimensioning of the gas flow channels 124 for sufficient gas flow capacity after each rotor stage 110a-110f affects the size of the stator sections 122a-122f and the corresponding W-shaped sections 140a, 140b. In particular, these sections must have a sufficient axial length X to accommodate the required size of the cavities 125a, 125b and the openings 126a, 126b. Accordingly, the overall length and size of the pump 100 is limited by the required axial length X of each stator section 122a - 122f. Figures 5A, 5B, and 6 illustrate embodiments of the present disclosure intended to overcome this pump design limitation. In particular, the embodiments of Figures 5A, 5B, and 6 include additional gas flow channels that allow for achieving a sufficient gas flow area through the stator sections 122a-122f while simultaneously allowing a reduction in the axial length X of the stator sections 122a-122f. The essential structure and assembly of the pump and stator arrangement (and other suitable examples) explained above with reference to Figs. 1 to 4B also apply to the embodiments of Figs. 5A, 5B and 6; they are therefore not repeated below. Unless expressly stated otherwise, it can be assumed that the features of Figs. 1 to 4B also apply to the embodiments of Figs. 5A, 5B, and 6. Indeed, the reference numerals for features explained above with reference to Figs. 1 to 4B remain the same in Figs. 5A, 5B, and 6, provided these same features apply and need to be designated for explanation. Different features are provided with a new number in the form 2xx instead of 1xx. Fig. 5A and Fig. 5B show perspective views of the upper and lower stator halves 220a and 220b, respectively. As shown, the stator halves 220a, 220b correspond to the stator halves 120a, 120b discussed above and also have the features already mentioned. However, the stator halves 220a, 220b have additional cavities 225a, 225b, 235a, 235b (and corresponding openings 226a, 226b, 236a, 236b) that form further gas flow channels 224, 234 for passing gas between the rotor stages 110a-110g. Fig. 6 shows an axial cross-section through the stator section 222a with the halves 220a, 220b joined together. Although the stator section 222a is shown, the same features may apply to any of the other stator sections 222b-222f. In Figs. 5A and 5B, only the cavities 225a, 225b, 235a, 235b and the openings 226a, 226b, 236a, 236b, which extend through the first three stator sections 222a, 222b, 222c, are explicitly shown. This is because, in the particular design shown, the gas after the fourth rotor stage 110d is compressed to such a small volume that the additional gas flow channels 224, 234 in further stator sections 222d, 222e, 222c are no longer required to maintain the gas flow without creating a flow resistance. However, it should be understood that the additional gas flow channels 224, 234 may be used in any number of stator sections 222, depending on the gas flow and compression characteristics of a particular design. As shown in Fig. 6, the cavities 125a, 125b extending through the central regions of the w-shaped sections 140a, 140b form a first gas flow channel 124 extending through the center of the stator sections 122a-122f (i.e., between the shaft openings 121). The cavities 225a, 225b and 235a, 235b (with corresponding openings 226a, 226b and 236a, 236b) extend through the aforementioned side regions of the w-shaped sections 140a, 140b to provide additional second and third gas flow channels 224, 234 extending through opposite sides of the stator sections 122a-122f (i.e., outside the shaft openings 121). As can be seen, in this way the side region cavities 225a, 225b and 235a, 235b form two opposite, generally C-shaped gas flow channels 224 and 234 which extend circumferentially outwardly around a respective shaft opening 121. It can be seen that the central gas flow channel 124 thus forms a first gas flow path G1 through the center of the stator section 222a and the second and third gas flow channels 224, 234 form second and third gas flow paths G2, G3 through the sides of the stator section 222a. The additional side-region gas flow channels 224, 234 cooperate with the central-region gas flow channels 124 to increase the flow area available for passing gas through the stator sections 122a-122f. This additional flow area means that the axial length X of the stator sections 122a-122f can be reduced while maintaining minimal resistance to gas flow. This allows the overall size of the pump to be reduced, which can advantageously lead to a more compact design. This can also open up new applications for the Roots vacuum pump where more compact pumps are desirable. As shown in Fig. 6, the side region gas flow channels 224, 234 are in fluid communication with the inlet opening 128 at one end via the central region gas flow channel 124 and in fluid communication with the outlet opening 129 at the other end via the central region gas flow channel 124. In this way, the side region gas flow channels 224, 234 begin at the central region gas flow channel 124 downstream of the inlet 128, curve around the openings 121 and then rejoin or terminate with the central region gas flow channel 124 upstream of the outlet 129. In other words, the side region gas flow channels 224, 234 branch off from the central region gas flow channel 124 and are fluidically connected thereto at both their starting and end points. It should be understood, however, that many different configurations of additional gas channels 224, 234 may be provided within the scope of the present disclosure. For example, the lateral gas flow channels 224, 234 may not be C-shaped or may be connected to or branch off from the central gas flow channel 124. Instead, each lateral gas flow channel 224, 234 could have its own gas inlet and outlet, and / or they could be substantially straight channels (or another shape) that extend around the sides of the stator sections 122 / shaft openings 121. Similarly, the lateral gas flow channels 224, 234 (and the corresponding cavities 225a, 225b, 235a, 235b and openings 226a, 226b and 236a, 236b) can be designed in any suitable cross-sectional shape (e.g., rectangular, square, circular, oval, etc.) according to the design requirements of a particular application. Although the illustrated design provides three gas flow channels 124, 224, 234, it should also be noted that the reduction in the axial length X of the stator sections 222 can also be achieved by using two gas flow channels (e.g., only two of the channels 124, 224, 234) or more than three gas flow channels instead. The number and design depend on the gas flow range and the pump size requirements for a specific application. However, it should be noted that by using gas flow channels 224, 234 that extend around opposite sides of the stator sections, the gas flow around the (left and right) sides of the stator sections 222 is balanced. This can advantageously provide a balanced directional flow of the gas exiting the stator sections 222 for the next receiving rotor stage. LIST OF REFERENCE SYMBOLS For a better overview, a list of reference symbols used in the attached Figs. 1 to 4 is presented here. 100 Roots vacuum pump 102 Motor 103a, 103b Bearing assemblies 104 Outer casing 104a, 104b Axial ends 106 End cap 108 Gas inlet 109 Gas outlet 110a - 110g Rotor stages 120 Stator assembly 120a, 120b Stator halves 121 Shaft opening 121a, 121b Shaft recesses 122a - 122f Inter-stage stator sections 123 Interface 124 Central gas flow channel 125a, 125b Central gas flow cavities 126a, 126b Central gas flow openings 127 Inlet vane 128 Gas inlet opening 129 Gas outlet opening 130a, 130b Fasteners 131 End plate 132a, 132b fastening flanges 133a, 133b mounting flanges 140a, 140b w-shaped sections 220a, 220b stator halves 222a - 222f inter-stage stator sections 224 lateral gas flow channel 225a, 225b lateral gas flow cavities 226a,226b lateral gas flow openings 234 lateral gas flow channel 235a, 235b lateral gas flow cavities 236a, 236b lateral gas flow openings G gas flow path G1 first gas flow path G2 second gas flow path G3 third gas flow path X axial stator section length L central longitudinal axis AA cross-sectional view line BB cross-sectional view line,

Claims

A stator assembly for a Roots vacuum pump, the stator assembly comprising:an inter-stage stator section for separating a first rotor stage and a second rotor stage, the inter-stage stator section having a plurality of gas flow channels defined therein configured to establish fluid communication between the first rotor stage and the second rotor stage through the inter-stage stator section. The stator assembly of claim 1, wherein the intermediate stage stator portion has a first opening for receiving a first rotor shaft and a second opening for receiving a second rotor shaft. The stator assembly of claim 2, wherein the plurality of gas flow channels comprises a first gas flow channel, a second gas flow channel, and a third gas flow channel, the first gas flow channel extending through a central region of the inter-stage stator section between the first and second openings, the second gas flow channel extending through a first side region of the inter-stage stator section outwardly around the first opening, and the third gas flow channel extending through a second side region of the inter-stage stator section outwardly around the second opening. Stator arrangement according to claim 3, wherein the second and third gas flow channels branch off from the first gas flow channel and are fluidly connected thereto at both their beginning and their end. A stator assembly according to claim 3 or 4, wherein the second and third gas flow channels each form a generally C-shaped gas flow path around the respective first or second opening. Stator arrangement according to one of the preceding claims, wherein the stator arrangement is formed from two interconnected stator halves. Stator assembly according to claim 6, wherein the intermediate stage stator section is formed by two opposing generally w-shaped sections which are joined together when the two stator halves are connected together. The stator assembly of claim 7, wherein the plurality of gas flow channels are formed by a respective plurality of cavities defined by the two generally w-shaped sections which are fluidly connected when the two generally w-shaped sections are joined together. Stator assembly according to one of the preceding claims, wherein the inter-stage stator section has an inlet opening for receiving fluid from a first rotor stage and an outlet opening for passing fluid from the inter-stage stator section to a second rotor stage, and wherein the plurality of gas flow channels are arranged between the inlet and outlet openings and fluidly connect the inlet opening to the outlet opening. Stator assembly according to one of the preceding claims, wherein the stator assembly comprises a plurality of intermediate stage stator sections. A Roots vacuum pump comprising:a first rotor stage and a second rotor stage spaced apart along a longitudinal axis of the pump;the stator assembly of any preceding claim, wherein the inter-stage stator section separates the first rotor stage from the second rotor stage along the longitudinal axis, and the plurality of gas flow channels are configured to receive fluid from the first rotor stage and to communicate the fluid to the second rotor stage. A roots vacuum pump according to claim 11, wherein the pump further comprises a pair of rotor shafts extending along the longitudinal axis, and each rotor stage comprises first and second outer-bladed rotors mounted to rotate with a corresponding one of the rotor shafts.

Images