KR20130100911A - Screw vacuum pump - Google Patents

Abstract

The screw-type vacuum pump includes screw rotors 12, 14 arranged in the pump housing 10. Each of the screw rotors 12, 14 is supported in the pump housing via two bearing elements 20 and has a ratio between the rotor length l greater than 3.0 and the distance d of the rotor axes. In addition, the screw rotors 12, 14 have a variable pitch, seven or more windings, and a built-in compression ratio of 4.5 or more. After half of the windings, the pitch is two times smaller than the pitch on the pressure side rotor outlet 24.

Description

Screw Type Vacuum Pumps {SCREW VACUUM PUMP}

The present invention relates to a screw type vacuum pump.

Screw-type vacuum pumps comprise two screw rotors arranged in a pumping chamber formed by a pump housing. The screw rotors are supported normally on both sides and may have different pitch profiles. The rotors can have a symmetric or asymmetric tooth profile as described, for example, in "Wutz-Handbuch Vakuumtechnik", 2010, 10th edition, pages 270-277. Such rotors typically have a built-in compression ratio of less than 4, that is, a chamber volume ratio between the suction side chamber and the pressure side chamber. Higher compression ratios will result in very high power input when the suction pressures are high. This would inevitably require the use of large drive motors (see page 276 of a book such as "Wutz" above). The increase in compression will further cause the problem that high temperatures occur in the pressure side region of the screw rotors. In this case, heat dissipation through the pump housing is no longer possible, so that heat dissipation will have to be carried out by internal cooling of the screw rotors. This is technically complicated and will lead to an increase in the manufacturing and maintenance costs of the screw-type vacuum pump.

In order to allow for high built-in compression ratios, in "VDI-Bericht" 2006 1932, it was proposed to modify the gap height. It is described that the gap height, in particular the distance between the screw rotor and the pump housing, should be greater on the suction side than on the pressure side. Due to the viscous and molecular pressure dependent flow types, a larger gap on the suction side will be acceptable. If the suction pressures are high, this gap will cause a decrease in the internal compression in conjunction with a lowering of the rotational speed of the rotor. This results in lower compression power, resulting in lower heat generation. However, the reduction in internal compression also has the result that the suction capacity is reduced.

In addition, rotors are known which are supported on only one side, ie in a cantilevered manner. This has the significant advantage that only one bearing should be provided. This bearing is arranged on the pressing side, ie on the transmission side. To that end, the second bearing arranged in the region of low pressures on the suction side can be omitted. However, the screw rotor assemblies supported in a cantilevered manner must have rotors of structurally short length, because otherwise there may be a risk of mutual contact between the rotors during operation. Structurally relatively short lengths of the rotors result in fewer numbers of windings. In addition, cantilevered rotors have a relatively large diameter. The ratio between the rotor length and the distance of the rotor axes is normally less than 2.5.

It is an object of the present invention to provide a screw-type vacuum pump having a "built-in pressure ratio" of 4.5 or more in which heat dissipation can be realized in a simple manner.

According to the invention, this object is achieved by the features defined in claim 1.

The screwed vacuum pump of the present invention comprises a pump housing forming a pumping chamber. In the pump housing, two screw rotors are arranged. Since the screw rotors of the present invention have a large length, these rotors are of a type supported on both sides, so two bearing elements are provided per screw rotor. In addition, the screw rotors have a relatively small diameter such that the ratio between the distance of the screw rotor and the rotor axes is greater than 3.0, preferably greater than 3.5, and more preferably 4.0. In addition, the screw rotors of the present invention comprise a variable pitch and at least 7, preferably at least 9 and more preferably at least 11 windings. The compression ratio of this invention is 4.5 or more, Preferably it is 5 or more. In order to avoid overheating of the rotor at high compression ratios as provided by the present invention, the rotor comprises a plurality of windings on the pressing side with a pitch that only slightly changes or does not change at all. Thus, according to the invention, the pitch after half of the windings is less than twice the pitch at the rotor outlet. Although the pitch after half of the windings is less than twice the pitch at the rotor outlet, it is particularly preferred that the pitch is less than 1.5 times the pitch at the rotor outlet. Compression will occur along the longer area of the rotor, due to the small change in pitch on the pressing side of the rotors as provided by the present invention and the gap height that is preferably selected correspondingly. By this, the present invention provides a significant advantage allowing for improved heat dissipation. This is because the compression operation, and hence waste heat, occurs substantially in the region of high pressures, and also due to the clearly extended region in which such high pressures occur in the present invention, As the housing surface area for up is now larger, it becomes possible. According to a preferred embodiment of the screwed vacuum pump of the present invention, screw rotors are provided which each comprise only one thread.

Thus, as a result of the long area of the present invention on the pressing side where the screw rotors have a slight change in pitch, it is possible to realize overheating of the rotors by realizing compression ratios of 4.5 or more and dissipating the heat thus generated. In the present application, in low pressure or high vacuum areas, heat dissipation should be considered to occur only in the pressurized region, since heat transfer to the housing may not be sufficient due to the low gas density.

The inventive design of screw rotors with very high built-in volume ratios further has the advantage that the power input is small when the pressures are low. Thus, for output pressures less than 10 mbar, for a suction capacity, a power input of less than 12 W / (m 3 h) can be realized.

According to a particularly preferred embodiment, heat dissipation is carried out only through the pump housing. Apart from the heat dissipation carried out through the medium itself, the heat dissipation is preferably carried out like this only through the pump housing. Thus, there is no need to provide internal cooling of the rotor which can be technically complex.

In addition, the provision of the present invention of a plurality of windings with a slight pitch change in the pressing side region of the rotor has the advantage of apparent noise reduction. This is the case because the compression occurs over a longer region, whereby the pressure difference between the last chamber and the gas outlet region is smaller. Thereby, the return aeration is reduced, which causes pressure waves that cause noise generation. As a result of this low return aeration, the generation of noise in the case of free blow-out emissions is reduced to 3-6 dB (A). This provides a significant advantage that a smaller amount of noise damping element can be provided. Because of this possibility to reduce the structural volume of the damping element, an increase in the structural length of the vacuum pump due to longer screw rotors can be at least partially compensated.

In addition, the profile of the screw rotors is preferably substantially symmetrical. Trapezoidal profiles, cycloid profiles or evolvent profiles are preferred herein. Preferably, the gap height, ie the distance between the screw rotors and the inner wall of the housing, is chosen to affect the expansion of the compression along the relatively long area on the outlet side of the rotor. In particular herein, it is preferred that the "height / axis distance of the cold gap" ratio in the cooled state of the turbomolecular pump is greater than 2/1000. In addition, in the operating state, ie when the operating temperature is reached, it is preferred that the "height / axis distance of the cold gap" ratio is greater than 12/1000. According to the present invention, it is preferred that the gap heights are selected such that during final pressure operation, the chamber pressure drops below an average chamber pressure of only 100 mbar after about 20% of the rotor length, as measured from the outlet side. .

According to a preferred embodiment, the screwed vacuum pump of the present invention has a nominal rotational speed of more than 5000 revolutions per minute. In addition, in order to avoid overcompression, an overpressure valve may be provided in the pressing side region of the screw rotors. As an alternative or in addition to the provision of an overpressure valve, a rotational speed control can be provided. In addition, overpressure can be avoided by appropriately reducing the rotational speed. Both measures are suitable for effectively reducing power input at high suction pressures and thus built-in motor performance.

The invention will be explained in more detail below by means of a preferred embodiment with reference to the accompanying drawings.

1 is a schematic plan view of two screw rotors designed according to the invention.
Figure 2 is a schematic drawing comparing a screw rotor according to the invention and a screw rotor according to the prior art with a schematic diagram of pressure development.

The two screw rotors illustrated in FIG. 1 are arranged in a pump housing (not shown). The pump housing forms a pumping chamber 10 in which two screw rotors 12, 14 are arranged. On both sides of these sides, the two screw rotors comprise shaft stubs 16, 18 rotatably arranged in the pump housing through the bearing elements 20. To drive the two screw rotors 12, 14, a shaft stub 18 or generally a shaft stub 16 is connected to the drive motor in a conventional manner, either directly or via a transmission. The second screw rotor is driven by the same drive motor, through a corresponding toothing arrangement (not shown), so that the two screw rotors 12, 14 can be synchronized with one another and rotate in opposite directions. The screw rotors 12, 14 act to cause suction of the medium to be conveyed on the suction side (arrow 22) and discharge of the medium on the pressurized side (arrow 24).

The pitch of the screw rotors is visualized by the oblique lines 26. In figure 1 it can be seen that the pitch changes over the length l of the rotor. In the pressure side region 28, the pitch is significantly better than the pitch in the suction side region 30. Here, according to the invention, the pitch in the suction side region 28 is provided such that the pitch in the region 31 which is half of the windings is at most twice the pitch at the rotor outlet 24. As a result, a relatively long pressing side region 28 is formed in which the pitch only changes slightly. In the pressure side region 28, compression may occur over most of the pressure difference between the inlet and the outlet. Accordingly, in this area 28, most of the compression work is also executed. Subsequently, heat to be dissipated will be generated substantially in this region. According to the invention, heat dissipation is carried out through a housing which surrounds the screw rotors 12, 14 in the pressure-side region.

According to the invention, the screw rotors 12, 14 have a structurally large length. Thus, as provided by the present invention, the ratio between the length l of the screw rotors 12, 14 and the distance d of the rotor axes is greater than 3.0.

In the upper part of FIG. 2, the screw rotor 12 according to the invention is depicted as corresponding to the screw rotors 12, 14 of FIG. 1. Below that, the prior art screw rotor 32 is shown. The screw rotor 32 is shorter and contains fewer windings with only a slight change in pitch in its pressing side region. In the rotor 32 of the prior art, the pressure generation will take a course schematically represented by line 34. From this, it is apparent that a strong pressure will occur in the pressure side region 36 of the screw rotor 32.

Due to the configuration of the rotor 12 according to the invention, the pressing side region 28 is obviously longer. In addition, the gap height is correspondingly selected (the distance of the height / axis of the cold gap> 2/1000 and the height / axis of the warm gap> 1/1000). From that, as shown by line 38 of the diagram, an increase in pressure taking a flatter course is caused.

Claims (10)

A pump housing forming a pump chamber 10, and
Two screw rotors 12, 14 arranged in the pump housing 10,
The screw rotors 12, 14 are each supported in the pump housing via two bearing elements 20 and have a ratio between the rotor length l greater than 3.0 and the distance d of the rotor axes,
The screw rotors 12, 14 have a variable pitch, seven or more windings, and a built-in compression ratio of 4.5 or more,
The pitch after half of the windings is characterized in that it is twice as small as the pitch on the pressure side rotor outlet 24,
Screw type vacuum pump.
The method of claim 1,
Characterized in that each of the screw rotors 12, 14 has only one thread,
Screw type vacuum pump.
3. The method according to claim 1 or 2,
The profile of the screw rotors 12, 14 is characterized in that it is substantially symmetrical or asymmetrical,
Screw type vacuum pump.
The method according to any one of claims 1 to 3,
For heat dissipation, only the pump housing is actively cooled,
Screw type vacuum pump.
The method according to any one of claims 1 to 4,
The ratio between the rotor length l and the distance d of the rotor axes is greater than 3.5, preferably greater than 4,
Screw type vacuum pump.
6. The method according to any one of claims 1 to 5,
Characterized in that at least 9 and preferably at least 11 windings are provided per screw rotor 12, 14,
Screw type vacuum pump.
7. The method according to any one of claims 1 to 6,
The compression ratio is 5 or more, preferably 6 or more,
Screw type vacuum pump.
The method according to any one of claims 1 to 7,
The gap heights are selected such that the final pressure of the vacuum pump is at least 5 Pa,
Screw type vacuum pump.
The method according to any one of claims 1 to 8,
The maximum rotation speed is characterized in that more than 5000 revolutions / minute,
Screw type vacuum pump.
10. The method according to any one of claims 1 to 9,
In order to avoid overcompressure, it is characterized in that one or a plurality of overpressure valves are provided in the pressurizing side region 28 and / or the rotational speed is controllable by the rotational speed control unit,
Screw type vacuum pump.

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