VACUUM PUMP SYSTEM

Abstract

A vacuum pump system comprises two vacuum pumps which are connected to each other. In order to create vacuum pump system of a more compact size, the connection between the two vacuum pumps is performed via connection elements directly attached to the pump housing and forming connection sites, and the outlet flange of the upper pump is placed directly on the inlet flange of the lower pump without thereby causing a transmission of larger forces or moments.

Description

BACKGROUND

1. Field of the Invention

The present disclosure relates to a vacuum pump system comprising a plurality of vacuum pumps which are connected to each other.

2. Discussion of the Background Art

Vacuum pump systems of the above type comprise two or more vacuum pumps, which usually are arranged in series. The medium to be conveyed, usually gas, will thus be sucked in by a first pump via the inlet of said pump and will then be conveyed, via the pump outlet, to the adjacent second vacuum pump. Normally, in a system comprising two vacuum pumps, the second vacuum pump will perform the conveying process against the ambient pressure. Optionally, it is also possible to arrange a plurality of vacuum pumps in series or also partly parallel to each other, wherein, usually, the last vacuum pump in the direction of conveyance will carry out the compression against the ambient pressure. In such an arrangement, fore vacuum pumps for compression against the ambient pressure are frequently provided in the form of rotary vane vacuum pumps, sliding vane vacuum pumps, claw vacuum pumps, multi-stage Roots vacuum pumps and screw vacuum pumps. In such pump systems, the first pump in the conveying direction, at whose inlet the lowest pressure is generated, is frequently provided as a Roots pump.

Known vacuum pump systems comprise a supporting rack or frame on which the individual vacuum pumps are arranged. Mounting the vacuum pumps on said rack can be performed e.g. with the aid of the feet provided on the pump housing. Then, the inlets and the outlets of the vacuum pumps will be connected to each other by rigid or flexible conduits or by special adapters. The weight forces and gas forces occurring during pump operation will largely be taken up by said rack. In such pump systems, the rack can be lifted and moved by use of a forklift, i.e. from below, or by use of a crane with the aid of crane eyelets. On the one hand, such vacuum pump systems of the type comprising a rack for support of the individual pumps have the advantage that, in most cases, the individual pumps can be exchanged independently from each other. An essential disadvantage of such systems, however, resides in the considerable space requirement for the rack. Further, the rack as well as the required connection elements will incur additional expenses.

In another type of known vacuum pump systems, vacuum pumps—usually two pumps—are directly connected to each other. In this case, no separate rack is provided. Connection of the two vacuum pumps is effected by connecting the outlet flange of the first vacuum pump, directly or via an adapter, to the inlet flange of the second vacuum pump. In this embodiment, the weight and gas forces occurring during operation have to be taken up by the flanges and to be transmitted to the pump housings. Here, it is normally not possible to provide crane eyelets for transport of the vacuum pump system because, during transport, the flanges would be subjected to massive stresses and thus would run a high risk of damage. Further, connecting two pumps via inlet and outlet flanges has the disadvantage that the flanges and the adapter, respectively, must be very solid and be provided with free areas for screw connections, to make it possible to take up and transmit the corresponding forces and, first of all, to assemble the arrangement. This will cause additional costs. Further still, it often happens that the outlines of the two interconnected vacuum pumps are not congruent, with one pump thus projecting beyond the outline of the other one. This will lead to considerable space requirements.

It is an object of the disclosure to provide a compact vacuum pump system which merely has a minimum space requirement.

SUMMARY

The vacuum pump system of the disclosure comprises at least two vacuum pumps. In a first case, a pump outlet of a first vacuum pump is arranged in fluid connection to the pump inlet of a second vacuum pump arranged downstream in the flow direction. The connection is effected by connecting the outlet flange to the inlet flange. Preferably, said connection is established directly via the flanges. Alternatively, also an indirect connection is possible via tube conduits or other intermediate elements. The disclosure provides that, on both pump housings, at least three connection sites are mechanically connected to each other for force transmission and/or moment transmission. According to the disclosure, at least two of said connection sites are flange-independent.

It can also be provided that the pump inlet of the second vacuum pump is connected to the pump inlet of the first vacuum pump. In such an embodiment, two identical vacuum pumps, arranged parallel to each other, are connected to each other in such a manner that one pump will suck fluid from the suction region of the other pump so that both pumps will be running under identical operating conditions, while the free suction flange of the other pump can be used for mounting thereon a still further vacuum pump, optionally a vacuum pump of a different type. Instead of using a direct flange connection, both suction regions of said identical pumps can also be connected via an external tube conduit.

Thus, according to a first preferred embodiment, there is effected—in accordance with the state of the art—a first mechanical connection via said two interconnected flanges, so that the connection of the outlet flange to the inlet flange will form a first connection site for transmitting a part of the occurring forces and/or moments. Additionally, in accordance with the disclosure, at least two further connection sites are provided on the two pump housings. Herein, said two connection sites preferably comprise respectively one connection element on both pump housings wherein, according to a preferred embodiment, the two connection elements of a connection site are arranged confronting each other. Also on said at least two further connection sites, a transmission of forces and/or moments will occur. As compared to the direct connection of two vacuum pumps via said pump flanges, the additional provision of at least two further connection sites has the advantage that only a part of the forces and moments has to be taken up by the flange connection. This in turn offers the advantage that the flanges themselves are allowed to be less robust. Particularly, it is possible to arrange the three connection sites at a largest possible distance from each other so as to achieve an advantageous transmission of forces and/or moments. Thus, according to a particularly preferred embodiment, at least two connection sites are located outside the flange surfaces, i.e. those surfaces where the two flanges are lying against each other and where they are optionally connected to each other by screw connections or the like. Preferably, at least two of said connection sites are arranged outside the flange surfaces defined by standards. In large flange systems, it may be sufficient to provide just one additional connection site for achieving a sufficient distribution of forces and moments.

With particular preference, said at least two flange-independent connection sites are able to take up and respectively transfer at least 25%, and more preferably 40%, of the forces and moments occurring during operation. Thereby, the flanges on the pump inlet and the pump outlet connected thereto can be given a less massive and thus a correspondingly less expensive design.

According to a further preferred embodiment, at least three flange-independent connection sites are provided. Particularly, this has the advantage that the fluid connection via the inlet flange and the outlet flange can be given a more simple configuration. Particularly, it is made possible to design the flange connection to the effect that it will substantially serve only as a fluid connection and that there will be substantially no force transmission. Such a design is advantageous particularly in cases where the flange connection is accessible only with difficulties. In this embodiment, for instance, the flange connection between the pump outlet and the pump inlet can be designed in such a manner that only a sealing function is effected and, if required, there is additionally provided a positioning via register pins or the like (e.g. housing sealings with centering rings). This has the advantage that the mechanical connection between the two vacuum pumps can be provided at more-conveniently accessible sites.

Thus, in this embodiment, the pump outlet and the pump inlet are particularly not connected to each other in a force-locking manner. Particularly, no screw connections are provided.

It is especially preferred that the connection elements are at least partly formed as outward projections of a housing wall of the corresponding vacuum pump. Advantageously, such projections are more easily accessible and, particularly, they can also be processed more easily. Thereby, especially, it is rendered possible to process the contact faces of the projections in an easy manner and to give them the most planar shape possible. Preferably, in this regard, two mutually confronting connection elements comprise, at a connection site, two mutually plane-parallel contact faces. In this manner, particularly, it is avoided that tensions might be introduced into the housings of the individual vacuum pumps as a result of the assembly process of the vacuum pump system. Further, in this regard, it is particularly preferable that at least two—and, more preferably, all—contact faces of a vacuum pump are arranged in one plane. Preferably, this plane corresponds to a flange contact face, this being preferred particularly in embodiments wherein the flange connection also serves for transmission of forces and moments. When using three or more connection sites, it may be advantageous to keep the flange faces of the outlet and inlet flanges at a distance of about 0.1 mm, provided that the sealing system allows for such a measure, so that the flange system will not have to take up any connection forces or moments. It can thus be avoided that a pump housing might be deformed due to connection forces, with the possible consequence of reduced running gaps for the pump rotors.

Preferably, the individual connection sites are additionally provided with holding elements. The holding elements comprise e.g. screws, clamping elements or the like for safeguarding a mechanical connection of the individual vacuum pumps of the vacuum pump system. The individual holding elements are e.g. integrated into said projections forming the connection elements. For instance, a holding site can be configured in such a manner that one of the two vacuum pumps to be connected to each other comprises a cylindrical projection, with a foot-shaped projection resting thereon which relative to the second pump housing extends laterally outward. It is then possible to use a screw as a holding element, which will be screwed through the foot-shaped projection of the upper pump into the cylindrical projection of the lower pump. It is also possible to provide separate holding elements; in this case, there are provided e.g. two projections arranged cylindrically confronting each other and being in abutment with each other while, however, not being mechanically connected to each other. With the aid of a claw-like connection, an eyelet connection or the like, the two housings of the vacuum pump can then be connected to each other, preferably outside on their sidewalls.

According to a further preferred embodiment, at least one connection element of a connection site is configured to allow for displacement of the two connection elements relative to each other at this connection site. Preferably, in this case, one of the two connection elements is of a rail-like type so that the second connection element of this connection site can be displaced within the first, rail-like connection element. With particular preference, two connection sites are configured in this manner, wherein the two displacement means are parallel to each other. This has the advantage, for instance, that manufacturing tolerances can be easily compensated for, and that an occurrence of tensions due to the assembly process is avoided. Further, this makes it possible to connect vacuum pumps of different sizes to each other. By the provision of such rail-like connection elements, also the assembly and disassembly processes are facilitated, especially under conditions of narrow space. In case that said rail-like connection elements are suitably configured, also a compensation of different thermal expansions of the two pumps is possible.

Further, it is possible to provide spacer elements between connection elements for level equalization. Such a provision is particularly advantageous since it offers the possibility to connect different pumps to each other, thus making it possible to compile a modular system. The corresponding spacer elements can be rigid or elastic.

According to a particularly preferred embodiment, the position of the individual connection elements is at least partially selected to the effect that the connection elements are arranged in the region of the side walls of the pump housing and/or in the region of support flanges. Advantageously, this allows for a good introduction of forces and/or moments into the pump housing. Thereby, it is safeguarded that the introduced forces and/or moments will cause only a slightest possible deformation of the pump housings. This is advantageous particularly because of the very narrow play between the pump housings and the pump elements such as e.g. the rotors.

According to a further preferred embodiment of the disclosure, or a possible modification of the above described embodiments, an adapter element is provided between the two vacuum pumps which are to be connected. This makes it possible, e.g., to connect very different vacuum pumps to each other. Particularly, thereby, a vacuum pump which does not comprise separate connection elements can be connected, via the outlet flange, to a further vacuum pump comprising a plurality of connection elements. For this purpose, the adapter element preferably comprises at least two adapter feet. A connection of the above type is possible particularly if the flange of the upper pump is sufficiently dimensioned for taking up forces and/or moments. The lower pump in turn would be stressed in such a manner that at least two flange-independent connection sites would take up and respectively transfer at least 25% and preferably at least 40% of the forces occurring during operation.

The above described individual embodiments of the inventive vacuum pump system with at least two vacuum pumps have the special advantage that, by corresponding adaptation of the positions of the inlet and outlet flanges and of the connection sites, a very compact vacuum pump system can be realized. Particularly, this vacuum pump system has a short constructional length since it is possible, for instance, to arrange the smaller one of the two vacuum pumps within the outline of the larger vacuum pump so that the smaller pump does not project beyond the larger pump. Particularly in case of Roots pumps, the position of the outlet flange can be freely selected in the axial direction, at least within certain limits. Consequently, there can be performed a corresponding geometric adaptation and, thus, the position of the pumps relative to each other can be improved so as to create a compact vacuum pump system.

Particularly, the inventive provision of connection elements on the individual vacuum pumps makes it possible to provide a modular system including different vacuum pumps, particularly fore vacuum pumps and high-vacuum pumps. Different pumps of different performance levels can be combined with each other in a simple manner, thus making it easily possible to realize vacuum pump systems with very different performance spectra. Thereby, the system costs can be kept very low. Further, the various embodiments of the disclosure have the advantage of allowing for a compact vacuum pump system which additionally is highly robust. Further, the assembly process can be significantly simplified. By said preferred modular system, high flexibility can be achieved, it being possible to realize a scalable modular system. Further, it is of course possible to combine more than two pumps with each other in a vacuum pump system. Further, as a result of the advantageous force and/or moment transmission, it is rendered possible to transport the entire vacuum pump system with the aid of a crane or a floor conveyer system (e.g. a forklift).

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be described hereunder in greater detail with reference to the accompanying drawings.

In the drawings, the following is shown:

FIGS. 1 to 5 plan views of different embodiments of a vacuum pump of a vacuum pump system;

FIG. 6 an enlarged sectional view as seen in the direction of arrow VI in FIG. 5;

FIG. 7 a schematic plan view of a preferred configuration of a connection site;

FIG. 8 a schematic sectional view taken along line VIII in FIG. 7;

FIGS. 9 and 10 schematic lateral views of preferred embodiments of connection sites;

FIG. 11 a schematic lateral view of an embodiment of a vacuum pump system;

FIG. 12 a view of the bottom side of the upper vacuum pump shown FIG. 11, as seen in the direction of arrow XII in FIG. 11;

FIG. 13 a schematic sectional view of an embodiment of the flange connection; and

FIG. 14 a schematic lateral view of a further embodiment of a vacuum pump system, provided with an adapter element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Each of FIGS. 1 to 5 is a strongly schematic plan view of a vacuum pump such as e.g. a screw-type vacuum pump. Onto the respective upper side 10 of the respective vacuum pump housing, there will be mounted the second vacuum pump (not shown in the Figures) of the vacuum pump system and be connected to the first vacuum pump 12 (which is shown). In all vacuum pumps 12 depicted in FIGS. 1 to 5, the upper side 10 is illustrated with an inlet flange 14 surrounding an inlet opening 16. Said inlet flange 14, which in the illustrated embodiment has a quadrangular cross section, can of course also be circular. In the illustrated embodiment, inlet flange 14 is provided with four fastening bores 18. In cooperation with corresponding fastening bores on the outlet flange of the second pump placed thereon, which pump is to be connected to the first pump 12, the two flanges can be mechanically fastened to each other by screws. Further, the vacuum pumps 12 shown in FIGS. 1 to 5 comprise a lateral outlet 20.

For creating connection sites, the embodiment shown in FIG. 1 is provided with six cylindrical connection elements 22 extending upward from said housing side 10. Via these cylindrical, columnar connection elements 22, a connection to the second, or upper, vacuum pump can be established by use of various connection options, as will still be explained with reference to FIGS. 6 to 10. In the embodiment shown in FIG. 1, there are thus provided, outside said flange 14, six flange-independent connection elements 22 for forming up to six connection sites. The connection of first pump 12 with the second pump thereabove can thus be mechanically effected already by said connection sites alone. This has the advantage that only small forces and/or moments have to be transmitted via flange 14. Therefore, it will not be necessary to design the flange 14 in an especially robust or stable manner.

Particularly in small vacuum pumps, it may be sufficient to provide only two connection elements 22 (FIG. 2). The connection with the second vacuum pump arranged thereabove will then be performed by the two connection elements 22 as well as by flange 14.

According to a further possible embodiment, three flange-independent connection sites 22 (FIG. 3) are provided. Thus, there exists a three-point connection via three connection elements 22. In this embodiment, flange 14 does not need to be capable of taking up forces and/or moments.

In the embodiments illustrated in FIGS. 1 to 3, the individual cylindrical connection sites 22 comprise threaded bores 24 for receiving fastening screws. Also in these embodiments, a different connection of the two pumps can be provided, as described hereunder with reference to FIGS. 7 to 10.

One such alternative connection is schematically shown in FIG. 4. In this embodiment, the connection is not performed via the connection elements 22. Here, the connection sites 22 are of a cylindrical shape and comprise a planar surface 26 for placing thereon cylindrical foot-shaped elements of the vacuum pump arranged thereabove, said elements e.g. corresponding to the connection elements 22. Thus, in this embodiment, the connection elements 22 serve only for support but not for establishing a mechanically fixed connection. The mechanical connection is performed via separate holding elements 28. In the illustrated embodiment, there is provided, for each connection element 22, one respective holding element 28 arranged on an outer wall 30 of the pump housing. These holding elements are designed as projections with U-shaped recesses, thus allowing for attachment to a corresponding holding element located on the second vacuum pump by use of a threaded bar or the like (in this regard, cf. FIG. 9 to be discussed later on).

According to a further preferred embodiment (FIG. 5), rail-like connection elements 32 are provided instead of said cylindrical connection elements 22. The connection elements 32 comprise a rail 33 having a rectangular cross section and being fixedly connected to the housing of the lower pump 12. On said rail 33, a carriage 37 fixedly connected to the housing of the upper pump, is guided for displacement in the longitudinal direction 39 of the pump housing. For adjustment, there can further be provided a lateral guidance 33a. Thereby, the upper pump can be easily adjusted relative to the lower pump. Particularly, in this manner, it is safeguarded that no tension can be introduced into flange 14 due to the assembly process. For mechanical connection of the two pumps, also this embodiment is provided with holding elements 28 on the lateral walls 30 of the lower pump as well as on the upper pump (said upper pump being not shown).

The connection between two or more pumps of a vacuum pump system can be provided in the manner explained hereunder with reference to FIGS. 7 to 10.

In a first embodiment of a connection site 34 (FIGS. 7, 8), the upper side 10 of lower pump 12 is provided with the cylindrical connection element 22. A housing 36 of an upper pump comprises a lateral eyelet-like projection 38. This projection is supported by its underside on the plane upper side 26 of connection element 22. Further, said connection element 38 of said upper pump housing 36 is formed with a bore 40 which is in alignment with the threaded bore 24 of lower connection element 22. Thus, the fixation can be performed by a screw 42 (FIG. 8), not shown in FIG. 7.

Further, it is possible to realize a connection site 34 by connecting two lug-like connection elements 38 (FIG. 9) to each other. In the embodiment shown in FIG. 9, the two lug-like connection elements 38 are not in immediate abutment with each other but are provided with an intermediate spacer element 44. Said element 44 can be rigid or elastic. Depending on the configuration of the connection elements, it can also be omitted.

The two lug-like connection elements 38, preferably arranged on an outer wall of the pump housing, can also serve as holding elements. These holding elements will thus effect a substantially mechanical connection of the two pumps, as described above in the context of said holding elements 28 (FIGS. 4 and 5). Additionally, connection elements are provided, on which the two pumps are in abutment above each other, however without a mechanical connection. These connection elements can be designed e.g. as already described with reference to FIGS. 4 and 5.

Further, a connection site 34 can also be designed in the manner illustrated in FIG. 10. Optionally, herein, a spacer element 44 is arranged between a lug-like connection element 38 and a cylindrical connection element 22, said elements 38 and 22 being again fastened to each other by a screw 42.

FIG. 11 is a lateral view of an embodiment of a pump system according to the disclosure. In this pump system, the lower, first pump 12 is provided e.g. in the form of a screw-type vacuum pump. The second, or upper, pump 35 is provided in the form of a Roots vacuum pump. The connection of the two pumps 12,35 is provided in such mutual adjustment that the vacuum pump system will be of the most compact design possible. Particularly, when seen in lateral view, the upper pump 35 does not laterally project beyond the lower pump 12. As depicted on the right-hand side in FIG. 11, the connection of the two pumps 12,35 is performed via two connection sites 34 arranged behind each other. These connection sites 34 are designed as illustrated in FIGS. 9 and 10, with a spacer element 44 being provided for height adjustment. Provided on the left side in FIG. 11 are two further connection sites 34, again arranged behind each other, wherein these connection sites are designed as shown in FIGS. 7 and 8. Here, no spacer element is provided.

Thus, in a bottom view of said Roots vacuum pump 35 (FIG. 12), the four connection elements 38 of the four connection sites 34 are visible. Further shown in this view is an outlet flange 46 connected to the inlet flange 14 of lower pump 12. The connection can be performed via screws in corresponding bores 18 provided for this purpose. Since, however, it is not absolutely required to bring about a transmission of forces and/or moments via said flanges 46,14, it is also possible to provide a purely fluidic connection with corresponding sealing elements. This has the advantage that, on the flanges 14,46 which possibly are accessible only with difficulties, no attachment needs to be performed by screws.

A flange connection between two flanges 14,46 is illustrated by way of example in FIG. 13. In such a flange connection, substantially no transmission of forces and/or moments takes place between the two flanges 14,46. The present connection is a purely fluidic connection between the two flanges 14,46. For this purpose, flange 14 is provided with an annular groove 48 surrounding the inlet 16 of lower pump 12. On said annular groove 48, a sealing element 52 is arranged, generating a sealing effect against the upper flange 46. The two housings will be connected to each other by screws 42 on connection sites 34 and will be aligned with each other. Thus alignment can also be performed via register pins.

According to a further embodiment of a pump system, an upper or second pump 56 (FIG. 14) is fastened to the lower pump 12 exclusively via its outlet flange 46. In order to realize a good reception of the occurring forces and/or moments, outlet flange 46 is not connected directly to inlet flange 14. Instead, an adapter element 58 is provided. Said adapter element is configured to realize a mechanic and fluidic connection between the two flanges 46,14 and, further, it comprises two adapter feet 60. Thus, when seen in plan view, the adapter element 58 of the illustrated embodiment is substantially Y-shaped so that the adapter feet 60 are arranged behind each other. The two adapter feet 60 are connected to connection elements 22 of lower pump 12 wherein, according to the disclosure, this connection can be provided in the manner described in the various examples herein.

In order to mount the upper vacuum pump, which possibly has a heavy weight, to the pump housing of the lower pump, and to do so preferably without transmission of forces and moments, it is particularly advantageous to design said adapter element 58 in such a manner that it can be fixed in an X-shaped configuration over the four connection sites 22 and that the inlet flange 14 of lower pump 12 will remain nearly unstressed.

Of course, pump systems of the disclosure may comprise not only two pumps but also more than two pumps.

Claims

1. A vacuum pump system, comprising

a first vacuum pump comprising a pump inlet and a pump outlet,

a second vacuum pump comprising a pump inlet and a pump outlet,

the pump outlet and/or the pump inlet of the second vacuum pump being fluidically connected, with the aid of flanges, to the pump inlet of the first vacuum pump,

wherein a first and a second pump housing are mechanically connected to each other on at least three connection sites so as to allow for force transmission, at least two of said connection sites being flange-independent.

2. The vacuum pump system according to claim 1, wherein each connection site comprises two mutually confronting connection elements.

3. The vacuum pump system according to claim 1, wherein at least two connection sites are arranged outside of the surfaces of said flanges.

4. The vacuum pump system according to claim 1, wherein at least two flange-independent connection sites are configured or arranged in a manner enabling them, in combination, to transfer 25%, and preferably at least 40%, of the forces and moments occurring in operation.

5. The vacuum pump system according to claim 1, wherein at least three connection sites are flange-independent.

6. The vacuum pump system according to claim 5, wherein the connection of the pump inlet to the pump outlet is substantially exclusively a fluidic connection.

7. The vacuum pump system according to claim 6, wherein no force-locking connection, and particularly no screw connection, is provided between the pump inlet and the pump outlet.

8. The vacuum pump system to according claim 1, wherein the connection elements comprise projections extending from a housing wall.

9. The vacuum pump system according to claim 1, wherein mutually confronting, mutually connected connection elements of a connection site comprise two mutually plane-parallel support faces.

10. The vacuum pump system according to claim 9, wherein the support faces of at least two connection elements of a vacuum pump are arranged in one plane, preferably in the plane of a flange-abutment face.

11. The vacuum pump system according to claim 1, wherein the connection sites comprise holding elements, said holding elements preferably being integrated into said projections or being provided separately.

12. The vacuum pump system according to claim 2, wherein the connection elements of at least one connection site are displaceable relative to each other.

13. The vacuum pump system according to claim 2, wherein a spacer element is provided to compensate for level differences between mutually confronting connection elements, particularly between their abutment faces.

14. The vacuum pump system according to claim 2, wherein the connection elements are at least partially arranged in the region of side walls of the pump housing and/or in the region of support flanges.

15. The vacuum pump system according to claim 2, wherein an adapter element arranged between two vacuum pumps connected to each other, for connection of connection elements which are not confronting each other, one of said vacuum pumps optionally comprising only one connection site which is formed as a flange.

16. The vacuum pump system according to claim 15, wherein, for receiving the weight of an upper vacuum pump, said adapter element comprises at least two adapter feet.

17. The vacuum pump system according to claim 15, wherein, for receiving the weight of an upper vacuum pump, said adapter element comprises at least three, preferably four adapter feet, to the effect that the inlet flange preferably remains stress-free.

18. The vacuum pump system according to claim 1, comprising more than two vacuum pumps.