SCREW ROTOR

Abstract

A screw rotor for screw vacuum pumps, comprising a rotor shaft which bears at least two displacement elements. The displacement element is conical in the conveying direction and the adjoining conveying element has a cylindrical design.

Description

BACKGROUND

1. Field of the Disclosure

The disclosure relates to a screw rotor for a screw vacuum pump.

2. Discussion of the Background Art

Screw vacuum pumps comprise a suction chamber in a housing, in which suction chamber two screw rotors are arranged. Each screw rotor comprises at least one displacement element with a helical recess. Thereby, a plurality of windings is formed. With screw vacuum pump, the goal is always to achieve an internal volume ratio that is as high as possible. The internal volume ratio is the ratio of the volume at the inlet of the vacuum pump to the volume at the outlet of the vacuum pump. Screw vacuum pumps of the first generation, such as e.g. the pumps LEYBOLD Screwline or BUSCH Cobra, have an internal volume ratio of approx. 3 to 4. With vacuum pumps currently on the market, such as e.g. the screw vacuum pumps LEYBOLD DRYVAC or Edwards GKS, the volume ratio is 5 to 7.

To achieve low pressures at the pump inlet, a high energy input is required and the power consumption of a corresponding vacuum pump is very high, respectively.

It is an object of the disclosure to provide a screw rotor for screw vacuum pumps with which it is possible to reduce energy consumption.

The disclosure is based on the finding that it is possible to reduce energy consumption by increasing the internal volume ratio.

For a high internal volume ratio the outlet stages of the pump must have a small delivery volume. However, small outlet stages have a disadvantageous ratio between transport flow and return flow, i.e. they are relatively leaky. Thus, only a rather low pressure build-up can be generated with each single stage. In order to still realize the major part of the compression in the small outlet stages, a great number of outlet windings becomes necessary.

Basically, two designs of screw rotors are known. These are screw rotors with either a cylindrical or a conical outer dimension.

With cylindrical rotors, the tooth space width at the outlet must be chosen to be small for high internal volume ratios. Thereby, the tooth height becomes rather large relative to the tooth space width, which can be realized in manufacture only with great effort and at high costs. However, with this rotor design, it is readily possible to integrate a great number of small outlet stages in the rotor (if the ratio of tooth height/tooth space width allows for an economically feasible manufacture). Thus, the geometrically installed volume ratio also becomes effective thermodynamically.

With conical rotors, in contrast, the chamber volume decreases steadily towards the outlet due to the tapering tooth height, so that it is possible to produce small outlet volumes with a ratio between the tooth height and the tooth space width that is favorable under manufacturing aspects. However, it is difficult to manufacture a plurality of stages with the low delivery volume, since the tooth height decreases continuously due to the cone shape. Although high geometric volume ratios are thus conceivable, these hardly show the desired effect, since the compression occurs in the larger stages due to the return flow through the gap.

It is further known to design staged rotors with two cylindrical displacement elements having different diameters. However, a great disadvantage of these staged rotors is the discontinuous transition which is extremely difficult to manufacture. Therefore, staged rotors are not common on the market.

SUMMARY

The screw rotor of the disclosure comprises a rotor shaft connected with at least two displacement elements, each displacement element comprising at least one helical recess. According to the disclosure a suction-side displacement element, i.e. a displacement element arranged in the direction of the pump inlet, in particular at the pump inlet, is designed to taper in the conveying direction. The displacement element is arranged such that is tapers in the conveying direction, i.e. towards the pump outlet. It is particularly preferred that the suction-side displacement element has an outer contour that is designed to constantly decrease in the conveying direction. A conical design of the suction-side displacement element, tapering in the conveying direction, is particularly preferred. The cone angle is preferably in a range from 2° to 8°.

Further, a pressure-side displacement element, i.e. a displacement element provided in the direction of the pump outlet, in particular at the pump outlet, is designed substantially cylindrically. The pressure-side displacement element may also be designed to be slightly conical or slightly decreasing constantly in the conveying direction. The substantially cylindrically designed pressure-side displacement element has, in particular, a diameter ratio from 1.1 to 1.0 between the suction-side diameter directed towards the pump inlet and the pressure-side diameter directed towards the pump outlet.

If so desired, further cylindrical and/or conical displacement elements can be arranged on the rotor shaft. The combination of a suction-side tapering, in particular conical displacement element and a pressure-side substantially cylindrical displacement element is essential according to the disclosure. This allows to combine the advantages of both screw rotor designs. The tooth height is reduced by the preferably conically designed suction-side displacement element, so that it is possible to provide the substantially cylindrical displacement element, which adjoins the former element in the flow or conveying direction, with a large number of outlet windings with small delivery volumes at a small ratio between the tooth height and the tooth width. In a particularly preferred embodiment it is thus possible to realize internal volume ratios greater than 6, in particular greater than 8 and particularly preferred greater than 10.

Although more than two displacement elements may be provided, a conical displacement element arranged on the suction side and a cylindrical displacement element arranged on the pressure side are provided in a particularly preferred embodiment. Hereinafter, the disclosure will be described with reference to this preferred embodiment, while further displacement elements may be provided in each case.

It is preferred that the adjacent displacement elements in particular abut against each other or contact each other by their end faces directed towards each other, and that they are substantially equal in diameter. Thus, a substantially stepless transition is realized. When providing two displacement elements, the diameter of the conical displacement element, starting from the pump inlet, decreases to a diameter that corresponds to the diameter of the cylindrical displacement element.

It is further preferred that the diameter of the cylindrical displacement element is smaller by 5-35%, preferably 10-25%, than the suction-side diameter of the conical displacement element, i.e. in particular the diameter of the conical displacement element provided at the inlet of the pump.

In a particularly preferred embodiment the lengths and the diameters of the conical and the cylindrical displacement element are chosen such that the greater part of the compression is performed at a low suction pressure by the cylindrical displacement element. In particular more than 70% of the compression performance is provided by the cylindrical displacement element.

As such it is further preferred that the conical displacement element has an internal volume ratio of more than 4, in particular more than 8.

In a preferred embodiment the cylindrical displacement element has an internal volume ratio greater than 1, in particular greater than 3. The internal compression is preferably effected by a decrease in pitch.

In the transition region between the in particular conical displacement element and the substantially cylindrical displacement element no continuous transition of the winding pitch has to be provided. A step in the winding pitch is also possible so that an internal compression is caused thereby. The internal compression can thus be caused already in the transition and/or in the cylindrical part.

For further improvement it is preferred according to the disclosure that the ratio between the tooth height and the tooth width is less than 3 in the outlet region of the vacuum pump. A favorable manufacture is possible. In particular the ratio is in a range from 1.8-2.2. In another preferred embodiment of the disclosure the length of the substantially cylindrical displacement element is 25-50% of the total profile length of the screw rotor.

In another preferred embodiment the ratio between the outer diameter of the displacement element at the pump outlet and the outer diameter of the displacement element at the pump inlet is less than 0.9, in particular less than 0.85.

Moreover it is particularly preferred that the diameter of the tapering displacement element is 80-300 mm in the region of the pump inlet. In the transition region between the tapering displacement element and the substantially cylindrical displacement element, the diameter is preferably 65-180 mm. Correspondingly, in a preferred embodiment, the outer diameter of the substantially cylindrical displacement element is also 65-180 mm, wherein in case of a cylindrical displacement element also tapering slightly this diameter may also be somewhat smaller than the diameter in the transition region.

In a preferred development of the disclosure, the number of windings of the cylindrical displacement element is at least 6 or preferably at least 10 and particularly preferred at least 12. By providing a great number of windings of the cylindrical displacement element the major part of the compression can be performed by the same.

In a preferred embodiment the in particular conical suction-side displacement element has 3-6 windings.

The individual displacement elements are preferably formed as separate components and are connected to the rotor shaft e.g. by being pressed thereon. However, it is also possible that individual or all displacement elements are formed integrally with the rotor shaft.

The rotor shaft further preferably comprises cylindrical subs at both ends that serve as bearing seats. However, it is also possible to support the screw rotor in an overhung manner, i.e. on one side.

Basically the screw rotor of the disclosure can be manufactured from known materials, such as steel, cast iron or aluminum, with the advantage of the disclosure being realizable in particular with screw rotors of steel or cast iron.

In a preferred development of the disclosure a further displacement element is provided on the suction side. The further displacement element is thus arranged upstream of the in particular conical displacement element that tapers in the conveying direction. The further displacement element preferably is an also substantially cylindrical displacement element. Here, it is preferred that the pitch of the windings of this further displacement element decreases in the conveying direction.

Further, the disclosure relates to a screw vacuum pump with two screw rotors arranged in a suction chamber defined by a housing. Here, the two screw rotors are designed or developed according to the disclosure, as described above.

The disclosure will be explained hereinafter in more detail with reference to a preferred embodiment and to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic side view of an embodiment of a screw rotor according to the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The screw rotor illustrated comprises a rotor shaft 10 supporting two displacement elements 12, 14. The two cylindrical ends 16, 18 of the rotor shaft serve to receive bearings for supporting the screw rotor in a pump housing. It is also possible to support the rotor shaft in an overhung manner, i.e. on one side.

The displacement element 12 on the right in the FIGURE is conical and tapers in the conveying direction 22 from a pump inlet 20 which is arranged on the right in the FIGURE but not illustrated therein, towards a pump outlet 24 which is arranged on the left in the FIGURE but not illustrated therein. A helical recess 26 of the conical displacement element 12 is designed such that the volume decreases. This is achieved on the one hand due to the conical outer shape of the displacement element 12 and on the other hand due to the inner portion 28 of the displacement element 12 widening in the conveying direction. Individual chamber volumes formed by the two meshing screw rotors thus reduce their respective volume in the conveying direction 22.

In the embodiment illustrated in which only two displacement elements 12, 14 are provided, an end face 30 of the displacement element 12 which is directed towards the pump outlet 24 or towards the pressure side of the pump, abuts on an end face 32 of the cylindrical displacement element 14. The end face 32 is directed towards the pump inlet or in the direction of the suction side of the vacuum pump. The diameters of the two displacement elements 12, 14 are substantially the same in the region of the end faces 30, 32.

The cylindrical displacement element 14 has a helical recess 34 as well.

In the embodiment illustrated the same has a constant pitch, wherein a decrease in pitch is also possible in the conveying direction 22 for further compression. Due to the recess 34 8 windings are formed in the embodiment illustrated.

Claims

1. Screw rotor for screw vacuum pumps, comprising a rotor shaft,

at least two displacement elements connected with the rotor shaft, each having at least one helical recess,

wherein a suction-side displacement element is designed tapering in the conveying direction,

wherein a pressure-side displacement element is designed substantially cylindrically.

2. Screw rotor of claim 1, wherein the suction-side displacement element is designed to constantly decrease in the conveying direction.

3. Screw rotor of claim 1, wherein the interior volume ratio is greater than 8.

4. Screw rotor of claim 1, wherein the displacement element have substantially the same diameter at the end faces directed towards each other.

5. Screw rotor of claim 1, wherein the diameter of the substantially cylindrical displacement element is smaller by 5-35% than the suction-side diameter of the conical displacement element.

6. Screw rotor of claim 1, wherein the tapering displacement element has a volume ratio greater than 4.

7. Screw rotor of claim 1, wherein the at least cylindrical displacement element has a volume ratio of 1 to 3.

8. Screw rotor of claim 1, wherein the diameter of the tapering displacement element is 80 to 300 mm in the region of the pump inlet.

9. Screw rotor of claim 1, wherein the diameter of the tapering displacement element is 65 to 180 mm in the transition region to the cylindrical displacement element.

10. Screw rotor of claim 1, wherein the diameter of the substantially cylindrical displacement element is 65 to 180 mm in the region of the outlet.

11. Screw rotor of claim 1, wherein the number of windings of the cylindrical displacement element is greater than 6.

12. Screw rotor of claim 1, wherein the number of windings of the tapering displacement element is 3 to 6.

13. Screw rotor of claim 1, wherein a further displacement element is provided that is arranged upstream of the tapering displacement element in the flow direction, the further displacement element being preferably substantially cylindrical in shape.

14. Screw vacuum pump with a housing defining a suction chamber and two screw rotors of claim 1 arranged in the suction chamber.

15. Screw rotor of claim 5, wherein the diameter of the substantially cylindrical displacement element is smaller by 10-25% than the suction-side diameter of the conical displacement element.

16. Screw rotor of claim 6, wherein the tapering displacement element has a volume ratio greater than 8.

17. Screw rotor of claim 11, wherein the number of windings of the cylindrical displacement element is greater than 10.

18. Screw rotor of claim 17, wherein the number of windings of the cylindrical displacement element is greater than 2.

19. Screw rotor of claim 3, wherein the interior volume ratio is greater than 10.

20. Screw rotor of claim 19, wherein the interior volume ratio is greater than 12.