Title: Pumping stage and dry vacuum pump

The present invention relates to a pumping stage and a dry vacuum pump that, using two rotors configured to rotate synchronously in opposite directions in at least one pumping stage, draws in, transfers and then delivers a gas that is to be pumped.

Dry vacuum pumps have one or more pumping stages in series in which a gas that is to be pumped circulates. Among known vacuum pumps, a distinction is made between those with rotary lobes, also known as “Roots” pumps, or those with claws, also known as “claw” pumps, or else those of the screw type. Such vacuum pumps have two parallel shafts bearing a respective rotor, which are configured to rotate synchronously in opposite directions in the pumping stages. These vacuum pumps are called “dry” since, in operation, the rotors rotate inside a compression chamber of the stator without any mechanical contact between them or with the stator, this making it possible not to use oil in the pumping stages.

In use, these vacuum pumps are subjected to a number of constraints.

Vacuum pumps are frequently employed for pumping chambers in which processes take place that are known as “powder” processes since they involve gases that generate large quantities of solid by-products. This is the case for example for some methods for manufacturing semiconductors. Solid compounds can settle on the internal surfaces of the vacuum pumps and form agglomerates that can limit the passage dimensions for the gases and result in losses of pumping capacity.

Furthermore, these vacuum pumps are generally disposed in a horizontal position close to the chambers that they place under vacuum, and this can have a number of drawbacks. First, the solid compounds of the powder processes have a tendency to stagnate in the bottom parts of the compression chambers and, secondly, this horizontal configuration takes up a lot of space on the ground.

The vacuum pumps are also subjected to numerous internal mechanical stresses. Forces that are mainly radial are exerted notably on rotors of the claw and Roots types, and axial forces are exerted on rotors of the screw type as a result of the compression of the gases in the compression chambers, as a result of the difference in pressure between the inlet and the outlet of the compression chamber but also as a result of the force of gravity for vacuum pumps of which the axes of rotation are inscribed in a horizontal plane. These forces cause relatively significant bending of the shafts of the pumps. The document EP2042739B1 proposes a single-stage vacuum pump intended to be mounted upstream of a rough-vacuum pump. The single-stage vacuum pump has an inlet situated in a first transverse end surface of the stator and the two rotors have helical torsion of a quarter of a turn, i.e. they rotate 90° on themselves. This vacuum pump can be arranged vertically, and this makes it possible to reduce the footprint on the ground. The vertical arrangement also contributes to the evacuation of the powders under gravity. Furthermore, the helical shape of the rotors means that the forces are no longer only radial but also have an axial component. This makes it possible to limit the shaft bending phenomena.

A drawback of the embodiment described in this document is that the inlet and outlet flanges are relatively bulky and poorly optimized for transfers of gas.

An aim of the present invention is to propose a vacuum pump that resolves at least one of the drawbacks described above, notably by proposing a vacuum pump that is more compact in the longitudinal dimension.

To this end, the subject of the invention is a pumping stage of a dry vacuum pump, having: a stator defining at least one compression chamber of which the shape is inscribed in two parallel cylindrical cavities of which the respective axes define a longitudinal direction, the cylindrical cavities overlapping transversely, two rotors configured to rotate synchronously in opposite directions in the cylindrical cavities so as to drive a gas that is to be pumped between an inlet and an outlet of the compression chamber, the rotors respectively having at least two lobes having helical torsion about a longitudinal axis, characterized in that the inlet and the outlet are provided in cylindrical peripheral surfaces of the compression chamber at the overlaps of the cylindrical cavities, on opposite sides of a transverse median plane of the compression chamber and on either side of a longitudinal plane of the compression chamber that is defined by the axes of the rotors.

The compressions of the gases are smoothed over a rotation linked to the angle of helical torsion instead of being sudden as in a configuration with straight lobes. This generates a restrained pulse at the delivery of the vacuum pump and in the transfer channels. Specifically, when the rotors rotate, the pockets of gas are transmitted from the inlet to the outlet with a frequency f*N (with f the frequency of rotation and N the number of lobes of the rotors), and this generates a pulse at the delivery. The helical torsion of the rotors makes it possible to “smooth” these pulses. The forces are therefore better distributed and of lower intensity since the duration of transfer of the gases is lengthened. Likewise, the helical shape of the rotors improves the entrainment of the powders towards the outlet by reproducing the entrainment function of the extruder screws as can be seen in the field of transporting granules or powders. The vacuum pump is thus more robust over time, both mechanically, since the shaft bending forces are lesser, and also functionally, since the powder pumping capacity is improved. The inlet and the outlet that are provided in the cylindrical peripheral surfaces of the compression chamber also make it possible to obtain greater filling and more rapid expulsion of the volumes of gas to be transferred without breaking the seals between the rotors and the stator.

The pumping stage may also have one or more of the features that are described below, considered on their own or in combination.

According to one exemplary embodiment, the inlet and the outlet are also open on the transverse end walls of the compression chamber. The inlet and the outlet are thus simultaneously open radially and axially, and this makes it easier to connect the pumping stage to the pipes or other pumping stages arranged axially upstream or downstream of the pumping stage since no bend is necessary.

According to one exemplary embodiment, the inlet is inscribed in an isosceles triangular shape, such as an isosceles trapezoidal shape, in a longitudinal section of the pumping stage, so as to align a first and a second lateral edge of the inlet with an end generatrix of a lobe of a respective rotor during rotation of the rotors. An inlet that is inscribed in an isosceles triangular shape, such as an isosceles trapezoidal shape, makes it possible to obtain greater filling of the volumes of gas to be transferred without breaking the seals between the rotors and the stator. The filling coefficient is then optimal.

According to one exemplary embodiment, the outlet is inscribed in an isosceles triangular shape, such as an isosceles trapezoidal shape, in a longitudinal section of the pumping stage, so as to align a first and a second lateral edge of the outlet with an end generatrix of a lobe of a respective rotor during rotation of the rotors. An outlet having a base of isosceles triangular shape, such as an isosceles trapezoidal shape, makes it possible to expel the volume of gas that is to be pumped more rapidly, without breaking the seals between the rotors and the stator.

The acute angles of the base of the isosceles trapezoidal or triangular shape are for example respectively between 50° and 80°. The length of the lateral edges of the inlet and/or of the outlet extends for example from a first transverse end wall of the compression chamber as far as more than a third of the length of an end generatrix of a lobe of a rotor.

The lengths of the lateral edges of the inlet are for example equal to the lengths of the lateral edges of the outlet.

The cylinders of the cylindrical cavities may be truncated by longitudinal claws with a flat end, the compression chamber having two longitudinal claws in opposite surfaces. This truncation of the cylindrical cavities by longitudinal claws makes it possible to avoid impacts of the rotors on the peaks at the intersection of the two diameters of the cylindrical cavities.

The angle of helical torsion between the two transverse end surfaces of the rotors is for example between 5° and 85°, such as between 65° and 85°, and more particularly between 70° and 80°.

The rotors have for example a cross section of two-lobe Roots type (the number of lobes per rotor is equal to two).

At least one transfer channel in communication with the outlet may be provided in the stator next to the compression chamber for example so as to connect the outlet to an inlet of an additional pumping stage. The transfer channels integrated in the stator on either side of the compression chamber make it possible to reduce the thickness of each of the transfer channels compared with a single channel.

The pumping stage may have a device for recirculating the gases comprising: a duct provided in the stator, connecting the inlet to the outlet, and a valve urged to close a passage of the duct, the valve being configured to open as soon as the difference in pressure between the inlet and the outlet exceeds a predefined threshold.

The device for recirculating the gases that is integrated in the stator makes it possible to cause the gases to be recirculated into the pumping stage in the event of overpressure so as to avoid congesting the following pumping stage.

The invention also relates to a vacuum pump having a pumping stage as described above.

The rotors may be cantilever-mounted.

The vacuum pump may be configured so that the axes of the rotors are vertical when it is placed on the ground. The vertical arrangement of the vacuum pump notably makes it possible to significantly reduce the footprint on the ground. The vacuum pump may have at least one additional pumping stage mounted in series with and downstream of the pumping stage and a mechanical driving part that is common to the pumping stage and to the additional pumping stage(s).

The vacuum pump may be disposed vertically on the ground with the axes of the rotors vertical, the pumping stage being situated at the top and the mechanical driving part at the bottom.

The vacuum pump may be configured to be connected in series with and upstream of a rough-vacuum pump.

The vacuum pump may be configured to deliver the pumped gases at atmospheric pressure.

Presentation of the drawings

Other advantages and features will become apparent on studying the following description of a particular, but in no way limiting, embodiment of the invention, and also the appended drawings, in which:

[Fig.1] Figure 1 is a partial perspective view of elements of a pumping stage of a dry vacuum pump.

[Fig.2] Figure 2 is a view similar to Figure 1 in which an end flange of the stator has been removed.

[Fig.3] Figure 3 shows a cross-sectional view of the elements of the pumping stage in Figure 2, on a longitudinal section plane passing through the inlet of the compression chamber.

[Fig.4] Figure 4 shows another cross-sectional view of the elements of the pumping stage in Figure 2, rotated 180°, on a longitudinal section plane passing through the outlet of the compression chamber.

[Fig.5] Figure 5 is a perspective view of a casing of the stator of the pumping stage in Figure 1.

[Fig.6] Figure 6 shows another viewing angle of the casing of the stator in Figure 5.

[Fig.7] Figure 7 is a perspective view of the rotors of the pumping stage in Figure 2.

[Fig.8] Figure 8 is a front view of a rotor in Figure 7.

[Fig.9] Figure 9 is a schematic front view of the cylindrical cavities of the stator in Figure 2.

[Fig.10a] Figure 10a is a front view of elements of the pumping stage in Figure 2 in which only one of the two rotors is shown, the rotor being oriented in an initial angular position. [Fig.10b] Figure 10b is a rear view of the elements in Figure 10a.

[Fig.11a] Figure 11a is a view similar to Figure 10a, the rotor having pivoted 10° in the anticlockwise direction with respect to the initial angular position in Figure 10a.

[Fig.11b] Figure 11b is a rear view of the elements in Figure 11a.

[Fig.12a] Figure 12a is a view similar to Figure 10a, the rotor having pivoted 35° in the anticlockwise direction with respect to the initial angular position in Figure 10a.

[Fig.12b] Figure 12b is a rear view of the elements in Figure 12a.

[Fig.13a] Figure 13a is a view similar to Figure 10a, the rotor having pivoted 45° in the anticlockwise direction with respect to the initial angular position in Figure 10a.

[Fig.13b] Figure 13b is a rear view of the elements in Figure 13a.

[Fig. 14] Figure 14 shows an exemplary embodiment of a vacuum pump.

The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment or that the features apply only to a single embodiment. Individual features of different embodiments can also be combined or interchanged to provide other embodiments.

An “upstream” element is to be understood as one that comes before another in relation to the direction of circulation of the gas that is to be pumped. By contrast, a “downstream” element is to be understood as one that comes after another in relation to the direction of circulation of the gas that is to be pumped.

The invention applies to any type of dry vacuum pump, single-stage or multistage, i.e. having one or more stages, such as comprising one to nine pumping stages. This vacuum pump can be a multistage rough-vacuum pump configured to deliver the pumped gases at atmospheric pressure (Figure 14) or a dry vacuum pump with one to three pumping stages that, in use, is connected upstream of a rough-vacuum pump and of which the delivery pressure is that obtained by the rough-vacuum pump.

The dry vacuum pump has at least one pumping stage 1 comprising a stator 2 and two rotors 3, 4. It also has, in a manner known per se, a mechanical driving part comprising a motor for driving the rotors, gears for synchronizing the rotors, and bearings supporting the shafts of the rotors.

Figures 1 and 2 show the dry pumping part of the pumping stage 1 in which the gases circulate, the vacuum pump also comprising sealing means that allow the rotation of the shafts in the dry pumping part while at the same time limiting the transfers of lubricant between the mechanical driving part and the dry pumping part.

It can be seen in Figures 1 and 2 that the stator 2 can be formed by a plurality of stator elements 2a, 2b. It has for example at least one end flange 2a and a casing 2b.

As can be seen more clearly in Figures 3 to 6, the stator 2 defines at least one compression chamber 5 of which the shape is inscribed in two parallel cylindrical cavities 5a, 5b. The respective axes of the cavities 5a, 5b define a longitudinal direction. The transverse direction is the direction perpendicular to the longitudinal direction.

The cylindrical cavities 5a, 5b overlap transversely.

The cylinders of the cylindrical cavities 5a, 5b can be truncated by longitudinal claws 6 with a flat end forming a protruding longitudinal end between the two cavities 5a, 5b, the compression chamber 5 having two longitudinal claws 6 in opposite surfaces. These truncations of the cylindrical cavities 5a, 5b by the longitudinal claws 6 make it possible to avoid impacts of the rotors on the peaks at the intersection of the two diameters of the cylindrical cavities 5a, 5b.

The compression chamber 5 is thus delimited by the two cylindrical peripheral surfaces that are the internal surfaces of the cylindrical cavities 5a, 5b, in this case formed by the casing 2b, and also by two transverse end walls, one of which is in this case formed by the end flange 2a. The compression chamber 5 also has an inlet 7 through which the gases that are to be pumped enter, and an outlet 8 through which the pumped gases exit.

The rotors 3, 4 are configured to rotate synchronously in opposite directions in the cylindrical cavities 5a, 5b so as to drive a gas that is to be pumped between the inlet 7 and the outlet 8 of the compression chamber 5. The rotors 3, 4 each rotate in a respective cylindrical cavity 5a, 5b.

The axes l-l of the rotors 3, 4 are mutually parallel and extend in the longitudinal direction of the cylindrical cavities 5a, 5b. In operation, the rotors 3, 4 are driven in rotation by the motor of the vacuum pump, which is for example mounted on one of the rotors 3, 4.

The rotors 3, 4 can be cantilever-mounted, i.e. the shafts passing through the rotors 3, 4 are supported by the bearings situated on only one side of the stator 2. There is for example no guiding means on the other side. The vacuum pump is for example configured so that the axes l-l of the rotors 3, 4 are vertical when it is placed on the ground. The vertical arrangement of the vacuum pump makes it possible to significantly reduce the footprint on the ground. This vertical arrangement also makes it easier to dispose the vacuum pump as close as possible to the equipment, and this makes it possible to notably reduce the losses of conductance. This vertical configuration also makes it easier to pump the powders that fall towards the outlet under gravity. In addition, the shaft bending force generated by the force of gravity is reduced. Likewise, the oil retained in the oil sump, situated at the bottom of the vacuum pump is easier to cool and it is easier to remove the rotors and stator during maintenance.

Two rotors 3, 4 are illustrated in Figures 7 and 8. The rotors 3, 4 respectively have at least two lobes. They are for example two-lobe, i.e. the cross section of the rotors 3, 4 is of the conventional Roots type, i.e. comprising two opposite lobes of which the outline is substantially in the shape of an “8” (Figure 8). They can also be three-lobe or have four, five or more lobes that are regularly angularly distributed.

The lobes of the rotors 3, 4 have an angle of helical torsion a about a longitudinal axis I that is also the axis of rotation of the rotor. The helical torsions of the lobes of the rotors 3, 4 are conjugate, in particular they have opposite directions, and they are complementary to the cylindrical peripheral surfaces and transverse end walls of the compression chamber 5 so that the rotation of the rotors 3, 4 in the compression chamber 5 transfers the gases from the inlet 7 towards the outlet 8 in a sealed manner. The vacuum pump is called “dry” since, in operation, the rotors 3, 4 rotate inside the stator 2 without any mechanical contact between them or with the stator 2 but via very small clearances, this making it possible for there to be no oil in the compression chamber 5.

The rotors 3, 4 also have two parallel transverse end surfaces 9.

The maximum angle of helical torsion a of the lobes of the rotors 3, 4 can be defined by the formula 360 - (2*g) - 360/N considering that the isolation time of the volume 17 of gas trapped by the lobes of the rotors 3, 4 is zero, with cylindrical cavities 5a, 5b that are not truncated by the longitudinal claws and with N the number of lobes of the rotor and g the value of the angle in the compression chamber 5 defined by the formula cos g = a/b where a is the value of half an inter axis distance and b is the radius of a cylindrical cavity 5a, 5b (see Figure 9).

Considering for example an inter-axis distance of 98 mm and a diameter of the cylindrical cavity 5a, 5b of 145 mm for rotors 3, 4 with two lobes, a maximum angle of helical torsion a of 85.44° is obtained. According to another example, considering an inter-axis distance of 76 mm and a diameter of the cylindrical cavities 5a, 5b of 145 mm for rotors 3, 4 with two lobes, a maximum angle of helical torsion a of 81.02° is obtained.

When the cylindrical cavities 5a, 5b are truncated, the angle of helical torsion a is smaller than the maximum angle of helical torsion and more particularly smaller than 90°. It is for example between 5° and 85°, such as between 65° and 85°, and more particularly between 70° and 80° (Figure 8).

Each rotor 3, 4 has end generatrices 15, 16 that are formed by helical lines situated at the respective tops of the lobes. The two-lobe rotors 3, 4 thus have a first and a second end generatrix 15, 16 (Figure 7).

The compressions of the gases are thus smoothed over a rotation linked to the angle of the helical torsion instead of being sudden as in a configuration with straight lobes. This generates a restrained pulse at the delivery of the vacuum pump and in the transfer channels. Specifically, when the rotors 3, 4 rotate, the volumes 17 of gas that are trapped by the lobes of the rotors are transmitted from the inlet 7 to the outlet 8 with a frequency f*N (with f the frequency of rotation and N the number of lobes of the rotors), and this generates a pulse at the delivery. The helical torsion of the rotors 3, 4 thus makes it possible to “smooth” these pulses. The forces are therefore better distributed and of lower intensity since the duration of transfer of the gases is lengthened. Likewise, the helical shape of the rotors 3, 4 improves the entrainment of the powders towards the outlet 8 by reproducing the entrainment function of the extruder screws as can be seen in the field of transporting granules or powders. The vacuum pump is thus more robust over time, both mechanically, since the shaft bending forces are lesser, and also functionally, since the powder pumping capacity is improved.

The inlet 7 and the outlet 8 are provided in the cylindrical peripheral surfaces of the compression chamber 5 at the overlaps of the cylindrical cavities 5a, 5b (Figures 2 to 6).

According to one exemplary embodiment, the inlet 7 and the outlet 8 are also open on the transverse end walls of the compression chamber 5. More specifically, the transverse end walls of the compression chamber 5 respectively delimit a transverse edge 22 of the inlet 7 and a transverse edge 23 of the outlet 8 (Figures 3 and 4). The inlet 7 and the outlet 8 are thus simultaneously open radially and axially, and this makes it easier to connect the pumping stage 1 to the pipes or other pumping stages arranged axially upstream or downstream of the pumping stage 1 since no bend is necessary. The inlet 7 and the outlet 8 are also provided respectively on opposite sides of a transverse median plane P (or vertical median plane with reference to the position of the pumping stage 1 in Figure 1) of the compression chamber 5, on either side of a longitudinal plane L (or horizontal plane with reference to the position of the pumping stage 1 in Figure 1) of the compression chamber 5. The longitudinal plane L of the compression chamber 5 is defined by the axes l-l of the rotors 3, 4. It is median and perpendicular to the transverse plane P.

According to one exemplary embodiment, the inlet 7 and/or the outlet 8 is inscribed in an isosceles triangular shape in a longitudinal section of the pumping stage 1 (or horizontal section with reference to the position of the pumping stage 1 in Figures 3 and 4).

In order to make it easier to realize the inlet 7 and/or the outlet 8, it is possible to truncate the triangle such that the inlet 7 and/or the outlet 8 is inscribed in an isosceles trapezoidal shape in the longitudinal section of the pumping stage 1.

The inlet 7 and/or the outlet 8 being inscribed in a trapezoidal shape makes it possible to align a first and a second lateral edge 10, 11, 12, 13 of the inlet 7 and/or of the outlet 8 with an end generatrix 15, 16 of a lobe of a rotor 3, 4 during rotation of the rotors 3, 4 (Figure 3).

More specifically, the inlet 7 and/or the outlet 8 has at least a first lateral edge 10, 12 forming a first acute angle b of the trapezium and a second lateral edge 11, 13 forming a second acute angle b of the trapezium (Figures 3 and 4). The acute angles b are for example respectively between 50° and 80°.

The length of the lateral edges 10, 11, 12, 13 extends for example from the first transverse end wall of the compression chamber 5 as far as a third or more of the length of the first or second end generatrix 15, 16. The lengths of the first and second lateral edges 10, 11 of the inlet 7 are for example equal to the lengths of the first and second lateral edges 12, 13 of the outlet 8.

In operation, the first lateral edge 10 of the inlet 7 of trapezoidal shape is aligned with an end generatrix 15, 16 of a lobe of a first rotor 3 so as to trap a volume 17 of gas that is to be pumped in a sealed manner between the stator 2 and the first rotor 3 in a predetermined angular range of the first rotor 3. The second lateral edge 11 is aligned with an end generatrix 15, 16 of a lobe of a second rotor 4 so as to trap a volume 17 of gas that is to be pumped in a sealed manner between the stator 2 and the second rotor 4 in a predetermined angular range of the second rotor 4. An inlet 7 that is inscribed in an isosceles triangular shape, such as an isosceles trapezoidal shape, makes it possible to obtain greater filling of the volumes 17 of gas to be transferred without breaking the seals between the rotors 3, 4 and the stator 2. The filling coefficient is then optimal.

On the side of the outlet 8, in operation, the first lateral edge 12 of the outlet 8 of trapezoidal shape is aligned with an end generatrix 15, 16 of the lobe of the first rotor 3 so as to release the volume 17 of gas that is to be pumped. The second lateral edge 13 is aligned with an end generatrix 15, 16 of the lobe of a second rotor 4 so as to release a volume 17 of gas that is to be pumped.

An outlet 8 having a base of isosceles triangular shape, such as an isosceles trapezoidal shape, makes it possible to expel the volume 17 of gas that is to be pumped more rapidly, without breaking the seals between the rotors 3, 4 and the stator 2.

This will be better understood with reference to the example illustrated in Figures 10a to 13b that shows various successive angular positions of the first rotor 3, seen from the front (Figures 10a, 11a, 12a, 13a) and the rear (Figures 10b, 11b, 12b, 13b) during pumping.

The first rotor 3 has a helix pitch that is oriented in the clockwise direction when facing the horizontal rotors 3, 4 with the inlet 7 in the upper part (Figure 10a). The first rotor 3 rotates in the anticlockwise direction.

It can be seen in Figure 10a that the first rotor 3 has almost finished trapping the volume 17 between the first rotor 3 and the stator 2 on the front face at the first end generatrix 15 of the first lobe. This volume 17 is furthermore closed in a sealed manner in the cylindrical cavity 5a of the compression chamber 5 at the second end generatrix 16 of the second lobe. This same volume 17 extends in the space between the two rotors 3, 4 on the rear face at the first end generatrix 15 of the first lobe while it is closed in a sealed manner at the second end generatrix 16 of the second lobe (Figure 10b). The gas that is to be pumped can therefore continue to enter into the volume 17 in contrast to an inlet that is inscribed in a rectangular shape in the longitudinal plane.

In Figures 11a and 11b, the first rotor 3 has pivoted 10° in the direction of anticlockwise rotation with respect to Figures 10a and 10b. It can be seen in Figure 11a that the first lateral edge 10 of the inlet 7 is aligned with the first end generatrix 15 of the first lobe of the first rotor 3. The volume 17 is now sealed on the front face at the two end generatrices 15, 16 of the first rotor 3. By contrast, this same volume 17 is still not trapped with the stator 2 on the rear face at the first end generatrix 15 of the first lobe (Figure 11b).

In Figures 12a and 12b, the first rotor 3 has pivoted 35° in the direction of anticlockwise rotation with respect to Figures 10a and 10b. The volume 17 is now trapped in a sealed manner between the stator 2 and the first rotor 3 by the two end generatrices 15 and 16 with respect to the inlet 7 and the outlet 8 (whether seen from the front or the rear). The volume 17 can therefore be displaced from the inlet 7 towards the outlet 8.

In Figures 13a and 13b, the first rotor 3 has pivoted 45° in the direction of anticlockwise rotation with respect to Figures 10a and 10b. It can be seen in Figure 13b that the same volume 17 is still trapped with the stator 2 in a sealed manner on the rear face and that it starts to communicate with a space between the rotors 3, 4 at the second end generatrix 16 of the second lobe that has moved away from the longitudinal claw 6 (figure 13a). As soon as the first lateral edge 12 of the outlet 8 has passed alignment with the second end generatrix 16 of the lobe of the first rotor 3, then the gas that is to be pumped can be expelled towards the outlet 8. The gas that is to be pumped can therefore exit earlier than with an outlet inscribed in a rectangular shape in the longitudinal plane.

The volume 17 is trapped in a sealed manner in Figures 12a and 12b over an angular range of the first rotor 3 of around 10° (or 10°+/- 5°).

On each revolution, each rotor 3, 4 with two lobes thus displaces two volumes of gas. The filling and the expulsion of the four volumes 17 of gas transferred by the two rotors 3, 4 are thus optimized.

Furthermore, it can be provided that at least one transfer channel 14 in communication with the outlet 8 is provided in the stator 2 next to the compression chamber 5 of the pumping stage 1 for example so as to connect the outlet 8 situated on one side of the pumping stage 1 to an inlet of an additional pumping stage situated on the opposite side. There are for example two transfer channels 14 provided respectively on either side of the compression chamber 5 (Figures 4 and 5). The transfer channels 14 are in the form of a circular arc, substantially semicircular, partially surrounding a cylindrical cavity 5a, 5b.

The pumping stage 1 can also have a device for recirculating the gases comprising a duct 20 provided in the stator 2, for example in the casing 2b of the stator (Figure 2). The duct 20 connects the inlet 7 of the compression chamber 5 to the outlet 8. The outlet 8 communicates in this case with an inlet of an additional pumping stage via the transfer channels 14 (Figure 5). Consequently, the duct 20 can be provided in the longitudinal direction, on one and the same side of the stator 2, behind the inlet 7.

The device for recirculating the gases also has a valve 21, which has for example a disc shape (Figure 5). The valve 21 is urged to close a passage of the duct 20. The recirculation device can also have a spring urging the valve 21 into the closure position.

The valve 21 is configured to open as soon as the difference in pressure between the inlet 7 and the outlet 8 of the pumping stage 1 exceeds a predefined threshold. Opening the valve 21 makes it possible to cause the gases to be pumped to be recirculated into the same pumping stage 1 in the event of a strong gas flow being pumped. The threshold is predefined notably according to the ratio of the swept-volume outputs and according to the mechanical safety settings.

The vacuum pump can be single-stage, the stator 2 defining a single compression chamber 5. The vacuum pump is then connected in series with and upstream of an independent rough-vacuum pump.

According to another example, the vacuum pump can have two or three pumping stages and can be configured to be connected in series with and upstream of an independent rough-vacuum pump.

According to another example, the vacuum pump 100 has at least one additional pumping stage 18 mounted in series with and downstream of the pumping stage 1, the at least one additional pumping stage 18 comprising rotors driven in rotation by the same motor as drives the rotors 3, 4 of the pumping stage 1, the mechanical driving part 24 of the vacuum pump being common to the pumping stage 1 and to the additional pumping stage(s) 18.

In the example illustrated in Figure 14, the vacuum pump 100 has between three and nine additional pumping stages 18, such as six additional pumping stages, forming a rough pumping assembly since the last additional pumping stage 18 is configured to deliver the pumped gases at atmospheric pressure.

The pumping stage 1 and the additional pumping stages 18 are mounted in series between the intake orifice 19 and a delivery orifice 25 of the vacuum pump 100.

The rotors of the pumping stage 1 and of the additional pumping stages 18 can be cantilever-mounted. The vacuum pump 100 can be disposed vertically, the pumping stage 1 being situated at the top and the mechanical driving part 24 at the bottom, i.e. placed on the ground and supporting the pumping stages 1, 18. Each additional pumping stage 18 defines a compression chamber accommodating two mating rotors of the vacuum pump 100, the chambers comprising a respective inlet and outlet. The rotors are for example of the straight two-lobe Roots type or are straight three-lobe rotors. During rotation, the gas drawn in through the inlet is trapped in the volume created by the rotors and the compression chamber of the additional pumping stage 18, and is then compressed and driven towards the outlet and towards the following stage.

The compression chambers of the successive pumping stages 1, 18 are connected in series one after another by at least one respective inter-stage channel connecting the outlet of the compression chamber of the preceding pumping stage to the inlet of the compression chamber of the following stage. The inter-stage channels are for example provided in the body of the stator, for example next to the compression chambers. There are for example two inter-stage channels per pumping stage, connected in parallel between the outlet and the inlet of the compression chamber, arranged on either side of the compression chamber.

The pumping stage 1 and the additional pumping stages 18 have a swept volume, i.e. a volume of pumped gas, that decreases (or remains the same) with the pumping stages, the pumping stage 1 having the highest swept-volume output and the last additional pumping stage 18 having the lowest swept-volume output. The transfer channels 14 integrated in the stator 2 on either side of the compression chamber 5 and the device for recirculating the gases integrated in the stator 2 of the pumping stage 1 are particularly suitable for a two-stage vacuum pump or for a rough-vacuum pump 100 that combines the pumping stage 1 with additional pumping stages 18.