EP2042739B1 - Vacuum pump with two helical rotors - Google Patents

Abstract

The pump has a cover (1) defining cylindrical chambers (2a, 2b) bounded by cylindrical peripheral surfaces and transverse end surfaces, and helical rotors (A, B) respectively rotated in the chambers, where the rotors include rotor bodies (6a, 6b), respectively. An inlet passage (4) and an outlet passage (5) communicate inside the cover respectively by an inlet orifice (9) and an outlet orifice (11) in the transverse end surfaces. The rotors are held in overhang by bearings located downstream not upstream of the rotor bodies in a flowing direction of a pumped fluid.

Description

The present invention relates to pumping devices capable of generating and maintaining a suitable vacuum in equipment.

Generating and maintaining a vacuum in equipment is commonly used in industrial semiconductor manufacturing processes, with certain manufacturing steps to be performed under vacuum.

During such manufacturing steps, the equipment is connected to a pumping device that lowers the internal pressure of the equipment to a suitable vacuum.

In practice, the known pumping devices generally comprise at least one primary pump, placed at the discharge of the vacuum line, and at least one secondary pump connected in series in the flow path of the gas pumped between the primary pump and the pump. equipment.

A first known pumping device is used for applications requiring that the pressure in the equipment be within a pressure range of about 10 ^-2 mbar to about 10 mbar. It is usually used a secondary pump type ROOTS. This ROOTS pump is connected to the suction of the primary pump. This solution is typically used to quickly pump large volume equipment or large process streams.

Conventional ROOTS pumps have two parallel bean cross section rotors defining intermeshing lobes. The suction and discharge ports are radial, perpendicular to the axes of rotation of the rotors.

However, the pumping devices thus formed are bulky and heavy, and are important generators of vibrations and noise. To limit these disadvantages, the industrialists deport the pumping unit away from the equipment using pipes easily reaching several meters in length. To maintain the desired pumping capacity, these lines must have a high conductance. They are therefore large and bulky and their interior volume is added to that of the equipment: the pumping system is therefore less reactive because it has to pump a large volume to establish the internal gas pressure in the equipment. On the other hand, it is sometimes necessary to maintain the gases at high temperature inside the vacuum line (particular chemistries with gas to maintain in volatile form with high temperatures up to 150 ° C). These pumping groups then require expensive heaters to keep these large pipes at high temperatures.

These disadvantages make it difficult to use in the clean rooms of these known pumping devices. In fact, the room in a clean room is very expensive, and thus losing space occupied by bulky devices and pipes is a real problem.

One could try to place a smaller secondary pump such as a ROOTS pump or a traditional miniaturized screw pump, at the suction of the vacuum line that is to say directly at the outlet of the equipment. pump. This provision would reduce the size of the pipes connecting the primary pumping and secondary pumping. This would also have the effect of considerably reducing the costs of heating the pipes. However, ROOTS pumps with radial input and output always generate vibrations and noise. In addition, placed at the outlet of the equipment to be pumped, they generate retrograde pollution by an effect of returning the particles and powders in the equipment.

In addition, ROOTS pumps, normally used in position in which the axes of the rotors are horizontal, have the disadvantage of being very cumbersome on the ground.

For applications requiring pressures below 10 ^-2 mbar, secondary pumps of the molecular or turbomolecular type may be employed. However, this type of pump can not be used for applications requiring gas heating in the pumping device and / or in higher pressure applications.

Manufacturers want to continually reduce the size and cost of equipment in their manufacturing rooms, particularly in the semiconductor industry where space is very expensive. Chemistries, too, evolve and require pumping devices to always be more efficient in terms of pumping rate. These new congestion and flow criteria require finding new pumping devices that are less cumbersome and less costly, that are clean and have a high pumping rate.

Such a device is disclosed in DE 195 22 557 A1 , considered to describe the closest state of the art.

The problem proposed by the present invention is to design a vacuum pump with high pumping rate, which can be used as a secondary pump, and sufficiently compact, low noise and low pollution for it can be placed in the immediate vicinity of the equipment without disrupting its operation.

The vacuum pump of the invention will also be able to pump powders as well as other particles generated in the equipment.

In another aspect, the invention provides a more responsive pumping system for the efficient generation and maintenance of a vacuum in equipment.

• une enveloppe définit deux chambres cylindriques parallèles, se chevauchant transversalement, limitées par des surfaces périphériques cylindriques et des surfaces transversales d'extrémités, et ayant des axes respectifs définissant une direction longitudinale, les surfaces transversales d'extrémités définissant une direction transversale,
• un passage d'entrée et un passage de sortie traversent l'enveloppe en deux positions respectives, ces positions étant généralement opposées,
• deux rotors sont disposés chacun de façon à pouvoir tourner dans une chambre cylindrique respective, les rotors ayant des corps de rotor à lobes complémentaires engrenés et des arbres de rotor, chaque lobe de chaque rotor ayant une surface de portée radiale coopérant de façon étanche avec la surface périphérique cylindrique de la chambre cylindrique respective, les corps de rotor ayant des première et seconde surfaces de portée transversales coopérant chacune de façon étanche avec une surface transversale d'extrémité respective de chambre,
• chaque corps de rotor présente une torsion hélicoïdale autour d'un axe longitudinal entre la première surface de portée transversale et la seconde surface de portée transversale,
• le passage d'entrée communique avec l'intérieur de l'enveloppe par un orifice d'entrée prévu dans la première surface transversale d'extrémité,
• le passage de sortie communique avec l'intérieur de l'enveloppe par un orifice de sortie prévu essentiellement dans la seconde surface transversale d'extrémité,
• chaque rotor est tenu en porte-à-faux par des moyens de guidage de rotor situés en aval du corps de rotor dans le sens d'écoulement du fluide pompé, la pompe étant dépourvue de moyens de guidage de rotor en amont des corps de rotor.

To achieve these and other goals, the invention provides a vacuum pump in which:

• an envelope defines two transversely overlapping parallel cylindrical chambers, bounded by cylindrical peripheral surfaces and transverse end surfaces, and having respective axes defining a longitudinal direction, the transverse end surfaces defining a transverse direction,
• an inlet passage and an outlet passage through the envelope in two respective positions, these positions being generally opposite,
• two rotors are each rotatably disposed in a respective cylindrical chamber, the rotors having lobed complementary lobe rotor bodies and rotor shafts, each lobe of each rotor having a radial bearing surface cooperating sealingly with the cylindrical peripheral surface of the respective cylindrical chamber, the rotor bodies having first and second transverse bearing surfaces each sealingly cooperating with a respective transverse end surface of a chamber,
• each rotor body has a helical twist around a longitudinal axis between the first transverse bearing surface and the second transverse bearing surface,
• the inlet passage communicates with the interior of the envelope through an inlet orifice provided in the first transverse end surface,
• the outlet passage communicates with the interior of the casing through an outlet orifice provided essentially in the second end transverse surface,
• each rotor is held cantilevered by rotor guide means located downstream of the rotor body in the flow direction of the pumped fluid, the pump being devoid of rotor guide means upstream of the rotor bodies .

According to the invention, each of the two rotor bodies has a ROOTS type transverse profile, and is designed with a quarter-turn helical twist between the first rotor transverse bearing surface and the second rotor transverse bearing surface. , the inlet orifice being located along a first side of the plane defined by the axes of the rotors, and the outlet orifice being located along a second side of the plane defined by the axes of the rotors.

This quarter-turn helical twist provides the best compromise between rotor diameter. A pumping rate of about 4000 m ³ / h can be achieved.

It is also possible to provide control and supply means for controlling the speed of rotation of the rotors. The pumping rate and the upstream pressure can then be easily adjusted according to the equipment and the processing steps.

The single-stage vacuum pump thus formed has the advantages of ROOTS pumps or screw pumps, ie a large pumping rate. Its special design also gives it the advantage of generating less vibrations and fewer noises, and having a large flow rate at a smaller volume thanks to the possibility of faster rotation.

Its design also avoids the generation of retrograde particulate pollution in adjacent equipment, since the pumped particles enter the pump axially through the inlet passageway and are not likely to be returned to the equipment by rebound on rotor portions. upstream movement. Its design also avoids retrograde pollution due to the absence of bearings and lubricants in the low pressure zone upstream of the rotors.

This vacuum pump can therefore be placed in the immediate vicinity of equipment in which it is desired to generate and maintain a vacuum, and can correctly fulfill the functions of secondary pump.

In addition, the absence of mechanical guide elements of the upstream rotors leaves room for choosing the shape and position of the suction port of the pump which ensure in particular the best flow of the pump.

To ensure a satisfactory mechanical strength of the cantilevered rotors, allowing a high speed of rotation without risk of contact of the rotors with the pump body, it is advantageous to choose relatively short rotors bodies, that is to say say whose ratio of the height and the overall diameter of the rotor bodies is less than 1.

Good results can be obtained with a ratio between the height and the overall diameter of the rotor bodies of about 0.6.

According to a first embodiment, the pump according to the invention comprises two rotor bodies each having lobes which have a cross section whose contour has a profile in conventional type ROOTS pumps.

This profile provides the best compromise between the external size of the pump according to the invention and the flow rate of the order of 4000 m ³ / h that is desired.

For example, the rotor bodies may each comprise two lobes.

According to a second embodiment, each rotor body comprises at least three lobes. One advantage is a better dynamic balance, for the reduction of noise and vibrations. Another advantage is a better compression ratio.

A disadvantage, however, is a reduction in the pumping rate at the same size, and a greater complexity of machining.

According to an advantageous embodiment, the vacuum pump of the invention may comprise means for maintaining it in a position in which the axes of the rotors are oriented, with respect to a vertical direction, at an orientation angle less than 90 °.

Recall that in known pumping devices comprising secondary pumps ROOTS type radial input and output, particles in the pumped gas could be trapped in dead zones. The particular design of the vacuum pump of the invention, associated with a non-horizontal orientation of the axes of the rotors of the vacuum pump, has the effect of preventing stagnation of the particles.

Advantageously, the orientation angle may be chosen less than 45 ° with respect to the vertical. Such an angle can further reduce the footprint of the vacuum pump of the invention, and promotes the expulsion of particles.

Indeed, the particular design of the vacuum pump of the invention allows its use along the vertical axis. Congestion is then minimum.

Preferably, the vacuum pump according to the invention may comprise means for maintaining it in a position in which the inlet orifice is higher than the outlet orifice.

This particular position of the inlet orifice with respect to the outlet orifice further reduces the possible stagnation of certain particles in the pump. empty. Indeed, the action of gravity is added to the action of the gas stream to expel the particles to the outlet of the pump.

According to an advantageous embodiment, the vacuum pump according to the invention is made from materials which are chosen to withstand up to a temperature of about 150 ° C.

Thus, the choice of materials makes it possible to use the vacuum pump of the invention at the temperatures usually required to make certain gases volatile in the vacuum lines. Such a temperature can also be achieved by a judicious choice of the materials constituting the insulation part between the pumping part and the mechanical part of the pump.

According to one embodiment of the invention, the vacuum pump comprises a motor mounted on one of the drive shafts between the guide means.

According to another embodiment, the vacuum pump comprises two synchronized motors each mounted on a respective drive shaft between the guide means.

• une pompe primaire ayant une entrée d'aspiration primaire et une sortie de refoulement primaire,
• une pompe secondaire, ayant une entrée d'aspiration secondaire raccordée à une sortie de l'équipement par une canalisation d'entrée, et ayant une sortie de refoulement secondaire raccordée à l'entrée d'aspiration primaire par une canalisation intermédiaire, et dans lequel :

• la pompe secondaire est une pompe monoétagée du type tel que défini ci-dessus,
• la pompe secondaire est positionnée à proximité immédiate de l'équipement, -- a pompe primaire est déportée à l'écart de l'équipement.

In another aspect, the invention also provides a pumping system for generating and maintaining a vacuum in equipment, comprising:

• a primary pump having a primary suction inlet and a primary discharge outlet,
• a secondary pump, having a secondary suction inlet connected to an outlet of the equipment through an inlet pipe, and having a secondary discharge outlet connected to the primary suction inlet through an intermediate pipe, and wherein :

• the secondary pump is a single-stage pump of the type as defined above,
• the secondary pump is positioned in the immediate vicinity of the equipment, - the primary pump is remote from the equipment.

Preferably, the secondary suction inlet is disposed facing the outlet of the equipment, and the inlet pipe directly connects the secondary suction inlet to the outlet of the equipment. The inlet pipe can then be very short or non-existent.

Very advantageously, the single-stage vacuum pump as designed according to the present invention has the advantage of being positioned in the immediate vicinity of the equipment. Thus, the pipes are smaller than in the pumping devices of the prior art. Fewer pipelines means less space in the clean room, and less volume to pump, so more responsiveness to the pumping system.

According to an advantageous embodiment, the pumping system according to the invention comprises control and supply means for controlling the speed of the secondary pump.

The control and supply means control the secondary pump so as to adjust its speed in a speed range allowing optimal control of the suction pressure by the discharge pressure.

According to a first variant, the pumping system further comprises a valve placed at the discharge of the secondary pump, and control and supply means for controlling the opening of the valve.

According to a second variant, the pumping system comprises a valve placed at the discharge of the secondary pump, the control and supply means acting on the speed of the secondary pump and / or on the opening of the valve so as to regulate the pressure in the equipment.

• la figure 1 est une vue de face en coupe longitudinale dans le plan des axes de rotation d'une pompe selon un mode de réalisation de l'invention ;
• la figure 2 est une vue de face de la pompe de la figure 1, en coupe longitudinale partielle selon le plan des axes de rotation ;
• la figure 3 est une vue de dessus en coupe transversale selon le plan C-C de la figure 1 ;
• la figure 4 est une vue de dessus de la pompe de la figure 1 ;
• la figure 5 est une vue en perspective illustrant un couple de rotors à profil transversal ROOTS selon un mode de réalisation de l'invention ;
• la figure 6 est une vue frontale du couple de rotors de la figure 5 ; et
• la figure 7 est un schéma d'ensemble d'un système de pompage selon un mode de réalisation de l'invention.

Other objects, features and advantages of the present invention will become apparent from the following description of particular embodiments, with reference to the accompanying figures, in which:

• the figure 1 is a front view in longitudinal section in the plane of the axes of rotation of a pump according to one embodiment of the invention;
• the figure 2 is a front view of the pump the figure 1 , in partial longitudinal section along the plane of the axes of rotation;
• the figure 3 is a top view in cross-section along the plane CC of the figure 1 ;
• the figure 4 is a top view of the pump of the figure 1 ;
• the figure 5 is a perspective view illustrating a pair of ROOTS transverse profile rotors according to one embodiment of the invention;
• the figure 6 is a frontal view of the couple of rotors of the figure 5 ; and
• the figure 7 is a block diagram of a pumping system according to an embodiment of the invention.

We consider the figure 1 , which illustrates a vacuum pump according to one embodiment of the invention. This pump comprises a pump body in which there are two main parts. A first main part comprises the mechanical drive elements 19 of the pump of the invention. A second main portion comprises an envelope 1 sealingly enclosing the elements constituting the pump portion 22 of the pump.

The first main portion includes a first drive shaft 21a and a second drive shaft 21b, parallel to each other. The two drive shafts 21a and 21b are held by bearings 29a, 29b, 29c and 29d. A motor rotor 30 is attached to the second drive shaft 21b between the bearings 29c and 29d and rotates in a fixed motor stator 31 in the pump body between said bearings 29c and 29d. Electrical conductors 20 supply the motor stator 31 with electrical energy to drive the second drive shaft 21b in rotation.

The first drive shaft 21a defines a first axis I-I and the second drive shaft 21b defines a second axis II-II.

A driving gear 32 is keyed on the second drive shaft 21b and meshes with a driven gear 33. This driven gear 33 is keyed to the first drive shaft 21a.

The second main part comprises the envelope 1 which defines two parallel cylindrical chambers 2a and 2b, centered on the axes I-I and II-II, overlapping transversely. These parallel cylindrical chambers 2a and 2b are limited by cylindrical peripheral surfaces 3a and 3b and transverse end surfaces 10 and 12.

An inlet passage 4 is adapted to be connected to equipment in which the vacuum must be made and to allow the fluids to enter the pump according to the invention.

The inlet passage 4 communicates with the interior of the casing 1 via an inlet orifice 9 provided essentially in the first transverse end surface 10.

Two parallel rotors A and B are each rotatably disposed in a respective cylindrical chamber 2a or 2b about a respective axis II or II-II. The rotors A and B each respectively have a rotor body 6a and 6b and a downstream coaxial shaft 62a and 62b. The rotor bodies 6a and 6b are axially limited by first coplanar transverse surface surfaces 7a and 7b and by coplanar second cross-surface bearing surfaces 8a and 8b. Each rotor A or B is fixed cantilevered respectively by its downstream coaxial shaft 62a or 62b, at the end of the first drive shaft 21a or the second drive shaft 21b of the first main portion. Thus, the rotors are held cantilevered by guide means (the bearings 29a-29d) located downstream of the rotor bodies 6a and 6b in the direction of flow of the pumped fluids. There is no guiding means in the low gas pressure zone upstream of the rotor bodies 6a and 6b.

The guide means 29a-29d may be plain bearings, or magnetic bearings, or gas bearings, for example.

A thermally insulating wall 100 separates the first main portion from the second main portion. In this way, it is possible to heat the second main part which contains the pumped gases, in order to prevent their deposition on the pumping elements, while maintaining a lower temperature in the first main part provided with holding means and rotor drive.

A motor, for example consisting of a rotor 30 and a stator 31, can be mounted directly on one of the drive shafts 21a or 21b between two guide means 29a and 29b, or 29c and 29d. This makes it possible to increase the compactness of the pump with respect to a motor mounted at the end of the shaft after the gears.

However, the latter solution makes it possible, if necessary, to use a larger and more powerful motor if the space between the pairs of guide means 29a and 29b, or 29c and 29d is not sufficient.

It can also be provided that the vacuum pump according to the invention operates with two synchronized motors each mounted on a respective drive shaft 21a or 21b between the guide means. This allows to have more power in a given size.

We consider the figure 2 , on which the same essential elements are marked by the same numerical references as on the figure 1 .

An outlet passage 5 passes through the casing 1 and is positioned so that the inlet passage 4 and the outlet passage 5 pass through the casing 1 in two generally opposite positions.

The outlet passage 5 communicates with the interior of the casing 1 through an outlet orifice 11 provided in the second transverse end surface 12.

It is easily seen that, on the pump according to the invention, the inlet and the outlet of the fluids are effected axially.

It can be seen from this figure that the inlet orifice 9 and the outlet orifice 11 are, however, offset with respect to each other, while being oriented axially: the inlet orifice 9 is cut by the cutting plane, while the outlet orifice 11 is in front of the cutting plane.

The figure 3 is a section according to the plane CC of the figure 1 . The same essential elements are identified by the same numerical references as on the figures 1 and 2 .

This figure illustrates the two rotors A and B. The rotor A comprises a rotor body 6a having two opposing lobes 60a and 61a. The rotor B comprises a rotor body 6b having two opposing lobes 60b and 61b. The rotor bodies 6a and 6b are each rotatably disposed in a respective cylindrical chamber 2a and 2b. Each lobe 60a, 61a, 60b, 61b of each rotor body 6a and 6b has a respective radially extending surface 25a or 26a and 25b or 26b, cooperating sealingly with the cylindrical peripheral surface 3a or 3b of the chamber respective cylindrical 2a or 2b during a portion of the rotary stroke of the rotor A or B corresponding.

The cross sections of the rotor bodies 6a and 6b have contours similar to the contours of the conventional ROOTS profiles.

The figure 4 is a top view of the pump according to the invention. The same essential elements are identified by the same numerical references as on the figures 1 , 2 and 3 . The Figures 5 and 6 illustrate the pairs of rotor bodies 6a and 6b respectively in perspective and in plan view.

In these figures, it can be seen that each rotor body 6a and 6b has a helical twist about a respective longitudinal axis II or II-II between the first transverse bearing surface 7a or 7b and the second transverse bearing surface 8a or 8b, the helical torsions of the rotors being in opposite directions.

This helical twist of the ROOTS type rotor profiles makes it possible to have suction and discharge ports on the walls perpendicular to the axes and thus to have axial pumping.

The figure 4 also illustrates the offset between the inlet passage 4 and the outlet passage 5.

In the illustrated embodiment on the Figures 1 to 7 each of the two rotor bodies 6a and 6b is designed with a quarter-turn helical twist between the first and second transverse bearing surfaces 7a, 7b, 8a and 8b. The inlet orifice 9 is located along a first side of the plane defined by the axes II and II-II of the rotor bodies 6a and 6b, and the outlet orifice 11 is situated along the second side of the plane defined by the II and II-II axes of the rotor bodies 6a and 6b.

The bodies of rotors 6a and 6b illustrated in the figures are relatively short, their axial height H being less than or equal to their overall diameter D.

The H / D ratio is less than 1, preferably about 0.6.

In the illustrated embodiment, the motor formed by the rotor 30 and the stator 31 is positioned on the second drive shaft 21b between the bearings 29c and 29d, increasing the compactness of the pump.

We will now describe the operation of the pump according to the invention.

The stator 31 is fed through the electrical conductors 20. This has the effect of rotating the second drive shaft 21b which rotates, via the gears 32 and 33, the first drive shaft 21a. The rotors A and B mechanically coupled to drive shafts 21a and 21b are then rotated in opposite directions from each other.

The fluids to be pumped continuously enter the pump of the invention through the inlet orifice 9 and fill a volume between the rotor bodies 6a and 6b.

The rotor bodies 6a and 6b, while still rotating on themselves, move said fluid-filled volume towards the outlet port 11. During a part of its displacement, the fluid-filled volume is isolated from both the fluid and the fluid. inlet port 9 and outlet port 11. Then, the fluid-filled volume is in connection with outlet port 11, and the fluids are expelled. The pumping continues with the following volumes.

The table below validates certain qualities of a single-stage vacuum pump of the invention compared to a conventional single-stage ROOTS type pump. This table compares the performance of the pump of the invention (I) compared to a conventional ROOTS type pump (R) of the same nominal flow, in terms of speed, dimensions, volume, weight and power. Nominal flow (m ³ / h) Rotation frequency (Hz) Dimensions (mm) Volume (liters) Weight (kg) Motor power (kW) I 000 200 540 450 300 70 150 6 R 4000 70 1240 570 400 280 430 11 1/4 1/3 1/2

It is noted that for a desired flow rate, the pump of the invention is less bulky, has a much higher rotational speed, and its engine consumes less power.

The design of the pump according to the invention (high rotational speed, vertical position, principle of assembly of the lobes, optimized lobe profiles, choice of materials) makes it possible, compared to a conventional ROOTS pump of the same flow rate, to reduce the volume. of the pump by a factor of 4, its weight by a factor of 3 and its power consumption by a factor of about 2.

In addition, the pump according to the invention can operate efficiently over a wide range of flow rates, for example from 1,000 m ³ / h to more than 4,000 m ³ / h by varying its speed of rotation and at low energy.

In the embodiment described on the figures 1 , 2 , 3 and 4 , the rotor bodies 6a and 6b have a helical twist of a quarter turn. However, without departing from the scope of the invention, one can choose a helical twist of different angular value, thereby lengthening the rotors.

Twisting a half turn would form a second intermediate fluid passage and would constitute a second pumping stage. The resultant pump would be two-stage.

The figure 7 illustrates the pumping system according to the invention. This system consists of equipment 13 in which it is desired to ensure an appropriate vacuum.

An output 13a of the equipment 13 is connected to the secondary suction inlet 15a of a secondary pump 15 via an inlet pipe 16. The secondary pump 15 is of a type as described previously, with two rotors A and B ( figure 1 ) with helical twist.

The secondary pump 15 has a discharge outlet 15b which is connected to the suction inlet 14a of a primary pump 14 via an intermediate pipe 17. The discharge outlet 14b of the primary pump 14 allows the system according to the invention to repress at atmospheric pressure.

Advantageously, the secondary suction inlet 15a is disposed opposite the outlet of the equipment 13a, and the inlet pipe 16 directly connects the secondary suction inlet 15a to the outlet of the equipment 13a. Thus, the inlet pipe 16 is as short as possible, and may be non-existent in the case of a direct coupling of the secondary pump 15 to the equipment 13. This avoids the additional volumes to be pumped, and the losses of conductance due to pipes. We can then further reduce the size of the secondary pump 15.

On the other hand, the primary pump 14 is deported away from the equipment, for example outside the manufacturing room, while being connected to the pump secondary 15 by a relatively long intermediate pipe 17, limiting the size of the pumping system in the manufacturing room, and avoiding disturbing the equipment 13 by vibration, noise, or other nuisance.

Due to the action of the secondary pump operating from 10 ^-3 mbar to 10 mbar, the gas transferred to the exhaust is at a pressure 10 to 100 times higher than direct output of the process chamber, which allows to reduce the diameter of the pipelines transferring the process gases to the primary pump and therefore the cost of connection.

Another consequence is that the particle traps may be smaller and placed downstream of the secondary pump 15 instead of being at the chamber outlet, thus avoiding phenomena of retro-diffusion of particles in the process chamber.

The removal of the intermediate lines between the equipment 13 and the secondary pump 15 reduces the costs of the connections, and reduces the power consumption.

Maintenance and cleaning are also facilitated.

The rotation speed of the pump rotors A and B can be controlled by control and supply means 18 which supply the electrical energy to the motor (30-31, figure 1 ) of the secondary pump 15 of the invention. Due to the position of the secondary pump 15 in the immediate vicinity of the equipment 13, a secondary pump speed variation 15 or a variation in pressure at its discharge quickly react on the equipment.

It is thus possible to envisage controlling by the secondary pump the treatment method used in the equipment 13.

For example, the control and supply means 18 can act on the speed of the secondary pump 15 so as to regulate the pressure in the equipment 13.

A disadvantage is the inertia of the pump.

Alternatively or in addition, a valve 40 can be provided placed at the discharge of the secondary pump 15. The control and supply means 18 then drive the opening of the valve 40, which modifies the discharge pressure. The modification of the discharge pressure causes the modification of the compression ratio of the pump and therefore the modification of the suction pressure, which itself is the pressure in the equipment 13. The control means and power supplies 18 can thus controlling the opening of the valve 40 so as to regulate the pressure in the equipment 13, for example.

It is also possible to combine the two systems, rotational speed of the pump and discharge valve. In this case, the control and supply means 18 control the secondary pump 15 so as to adjust its speed in a speed range allowing optimal control of the suction pressure by the discharge pressure, and the control means and feed 18 control the opening of the discharge valve 40 so as to control the suction pressure by the discharge pressure. The document EP-1 475 535 teaches how to pilot the secondary pump for this.

In all cases, it is possible to dispense with a control valve between the secondary pump 15 and the equipment 13, a turbulence and particle generating valve in the equipment 13.

Claims (11)

1. Vacuum pump comprising:

   - a casing (1) defining two parallel cylindrical chambers (2a, 2b), overlapping transversally, limited by peripheral cylindrical surfaces (3a, 3b) and transverse end surfaces (10, 12), and with respective axes (I-I, II-II) defining a longitudinal direction, the transverse end surfaces (10, 12) defining a transverse direction,

   - an inlet passage (4) and an outlet passage (5), crossing the casing (1) in two respective positions,

   - the inlet passage (4) connecting with the inside of the casing (1) via an inlet orifice (9) provided in the first transverse end surface (10),

   - the outlet passage (5) connecting with the inside of the casing (1) via an outlet orifice (11) provided essentially in the second transverse end surface (12),

   - two rotors (A, B), each arranged so as to be able to rotate in a respective cylindrical chamber (2a or 2b), the rotors (A, B) having rotor bodies (6a, 6b) with complementary meshed lobes (60a, 61a; 60b, 61b) and rotor shafts (21a, 62a; 21b, 62b),

   - each lobe (60a, 61a; 60b, 61b) of each rotor (A, B) with a radial seating surface (25a, 25b, 26a, 26b) cooperating tightly with the peripheral cylindrical surface (3a, 3b) of the respective cylindrical chamber (2a, 2b), the rotor bodies (6a, 6b) having first and second transverse seating surfaces (7a, 7b; 8a, 8b) each cooperating tightly with a respective transverse end surface (10, 12) of a chamber,

   - each rotor body (6a, 6b) with a helicoidal torsion around a longitudinal axis (I-I, II-II) between the first transverse seating surface (7a, 7b) and the second transverse seating surface (8a, 8b),

   - each rotor (A, B) being held in an overhang by rotor guiding means (29a-29d) located downstream of the rotor body (6a, 6b) in the direction of the flow of the pumped fluid, the pump having rotors guiding means upstream of the rotor bodies (6a, 6b),

   characterized in that each of the two rotor bodies (6a, 6b) has a ROOTS type transverse profile and is designed with a quarter turn helicoidal torsion between the first rotor transverse seating surface (7a, 7b) and the second rotor transverse seating surface (8a, 8b), the inlet orifice (9) is located on a first side of the plan defined by the axes of the rotors (I-I, II-II), and the outlet orifice (11) is located according to a second side of the plan defined by the rotor axes (I-I, II-II).
2. Vacuum pump according to claim 1, in which the ratio between the height (H) and the overall diameter (D) of the rotors bodies (6a, 6b) is less than 1.
3. Vacuum pump according to one of the previous claims, in which each rotor body (6a, 6b) comprises two lobes (60a, 61a; 60b, 61b).
4. Vacuum pump according to one of the previous claims, in which each rotor body (6a, 6b) comprises at least three lobes.
5. Vacuum pump according to one of the previous claims in which the rotor shafts each comprise a respective downstream coaxial shaft (62a, 62b) of a rotor fixed at the end of a respective drive shaft (21a, 21b).
6. Vacuum pump according to one of claims 1 to 5, comprising an engine (30, 31) mounted on one of the drive shafts (21 b) between the guiding means (29c-29d).
7. Vacuum pump according to one of claims 1 to 5, comprising two synchronized engines, each mounted on a respective drive shaft (21a, 21b) between the guiding means (29a-29d).
8. Pumping system for generating and maintaining a vacuum in an equipment item (13), comprising:

   - a primary pump (14) with a primary suction inlet (14a) and a primary discharge outlet (14b),

   - a secondary pump (15) with a secondary suction inlet (15a) connected to an outlet of the equipment (13a) by an inlet pipeline (16) and with a secondary discharge outlet (15b) connected to the primary suction inlet (14a) by an intermediate pipeline (17),

   characterized in that:

   - the secondary pump (15) is a single-stage pump of the type defined by one of claims 1 to 7,

   - the secondary pump (15) is positioned in the immediate proximity of the equipment (13),

   - the primary pump (14) is moved away from the equipment (13).
9. Pumping system according to claim 8, in which the secondary suction inlet (15a) is arranged opposite the outlet of the equipment (13a), and the inlet pipeline (16) directly connects the secondary suction inlet (15a) to the equipment outlet (13a).
10. Pumping system according to one of claims 8 and 9, comprising control and supply means (18) for controlling the secondary pump (15) in order to adjust its speed within a speed range allowing an optimum control of the suction pressure by the discharge pressure.
11. Pumping system according to one of claims 8 to 10, comprising a valve (40) placed at the discharge of the secondary pump (15), the control and supply means (18) acting on the speed of the secondary pump (15) and/or on the opening of the valve (40) so as to regulate the pressure in the equipment (13).

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