EP2567096A2

EP2567096A2 - Screw vacuum pump

Info

Publication number: EP2567096A2
Application number: EP11718056A
Authority: EP
Inventors: Thomas Dreifert; Magnus Janicki; Peter Birch; Roland Müller
Original assignee: Oerlikon Leybold Vacuum GmbH
Current assignee: Leybold GmbH
Priority date: 2010-05-04
Filing date: 2011-05-03
Publication date: 2013-03-13
Anticipated expiration: 2031-05-03
Also published as: TWI568935B; EP2567096B1; DE102010019402A1; CN102884324B; TW201200733A; JP5860035B2; CN102884324A; KR20130100911A; WO2011138318A3; JP2013525690A; KR101855398B1; WO2011138318A2

Abstract

The invention relates to a screw vacuum pump comprising screw rotors (12, 14) disposed in a suction chamber (10). The screw rotors (12, 14) are each supported by means of two bearing elements (20) in the pump housing and comprise a ratio of rotor length (I) to rotor axis spacing (d) that is greater than 3.0. The screw rotors (12, 14) further comprise a variable pitch, at least 7 turns, and an integral compression ratio of at least 4.5. The pitch after half of the turns is less than twice the pitch on the pressure side rotor outlet (24).

Description

Screw vacuum pump

The invention relates to a screw vacuum pump.

Screw vacuum pumps have two screw rotors in a pump chamber formed by a pump chamber. The screw rotors are usually mounted on two sides and can have different pitch profits. The rotors may have a symmetrical or asymmetrical tooth profile as described, for example, in "Wutz", Handbuch Vakuumtechnik, 10th Edition, 2010 pp. 270-277. Such rotors usually have a built-in compression ratio, ie a ratio of the chamber volume of the suction-side chamber to the pressure-side chamber of less than 4. Higher compression ratios lead to very high power consumption at high suction pressures. This would require the use of disproportionately large drive motors (see "Wutz", loc. Cit., Page 276). Furthermore, there is the problem in increasing the compression that high temperatures occur in the pressure-side region of the screw rotors. The heat dissipation is no longer possible via the pump housing, so that a heat dissipation via an internal cooling of the screw rotors would have to be done. This is technically complicated and increases the manufacturing and maintenance costs of the screw vacuum pump.

In order to enable high built-in compression ratios, the gap height is to be varied in VDI Report No. 1932, 2006. Here, the gap height, d, h, in particular the distance between the screw rotor and the pump housing, is described in such a way that it is greater on the suction side than on the pressure side. Due to the pressure-dependent flow types, which are viscous or molecular, a larger gap on the suction side is acceptable. In combination with a reduction in the rotor speed, this causes a reduction in internal compression at high intake pressures. This results in a lower compression ratio resulting in lower heat generation. The disadvantage, however, is that the reduction in internal compression also has a reduction in the pumping capacity result.

Furthermore, one-sided, or overhung rotors are known. This has the significant advantage that only one bearing must be provided. This is arranged on the pressure side, or on the side of the transmission. The second bearing, which is arranged on the suction side in the region of low pressures, can be omitted here. Flying rotors mounted on the fly must, however, have short rotors, otherwise there is a risk of contact between the rotors during operation. The relatively short length of the rotors has the consequence that the number of turns is small. Furthermore, overhung rotors have a relatively large diameter. The ratio of the rotor length to the distance between the rotor axes is usually less than 2.5.

The object of the invention is to provide a screw vacuum pump with a "built-in compression ratio" of at least 4.5, wherein a simple heat dissipation is realized.

The object is achieved according to the invention by the features of claim 1. The screw vacuum pump according to the invention has a pump housing forming a pump chamber. In the pump housing two screw rotors are arranged. Since the screw rotors according to the invention are long, it is in each case mounted on both sides screw rotors, so that each screw rotor two bearing elements are provided. Furthermore, the screw rotors have a relatively small diameter, so that the ratio of the length of the screw rotor to the spacing of the rotor axes is greater than 3.0, in particular greater than 3.5 and particularly preferably greater than 4.0. Furthermore, the screw rotors according to the invention have a variable pitch and at least 7, in particular at least 9 and particularly preferably at least 11 turns. The compression ratio according to the invention is at least 4.5, preferably at least 5. In order to avoid overheating of the rotor in a large compression ratio according to the invention, the rotor has several windings on the pressure side, whose pitch varies only slightly or not. According to the invention, therefore, the slope after half of the turns less than twice the slope at the rotor outlet. In particular, the pitch after half of the turns is smaller than the 2-fold pitch, more preferably smaller than the 1.5-fold pitch at the rotor outlet. Due to the small pitch change according to the invention on the pressure side of the rotors and the preferably correspondingly selected gap height, the compression takes place over a relatively long region of the rotor. This has the significant advantage according to the invention that better heat dissipation is possible. This is due to the fact that the compression work and thus the waste heat essentially occurs in the region of high pressures and, due to the invention significantly extended range in which these high pressures occur, the housing surface for heat absorption is greater. In a preferred embodiment of the screw vacuum pump according to the invention screw rotors are provided, each having only one thread. Due to the invention provided on the pressure side long area in which the screw rotors have a small slope change, it is thus possible to realize compression ratios of at least 4.5 and dissipate the heat occurring here, so that overheating of the rotors is avoided. It must be taken into account here that the heat removal can only take place in the pressure-side region, since in areas of low pressures or high vacuum due to the low gas density sufficient heat transfer to the housing is not possible »

The inventive design of the screw rotors with a high volume ratio built-in also has the advantage that at low pressures, the power consumption is low. As a result, a power consumption based on the pumping speed of less than 12 W / (m ³ h) for outlet pressures below 10 mbar can be realized.

In a particularly preferred embodiment, the heat dissipation takes place exclusively via the pump housing. In addition to the heat dissipation, which takes place via the medium itself, the heat dissipation therefore preferably takes place exclusively via the pump housing. A rotor internal cooling, which is technically complex, must therefore not be provided.

Furthermore, the provision according to the invention of providing a plurality of turns with a small change in pitch in the pressure-side region of the rotors has the advantage that the noise development is markedly reduced. This is due to the fact that the compression takes place over a longer range and thus the pressure difference between the last chamber and the region of the gas outlet is lower. As a result, the re-venting is reduced, with the re-venting pressure waves that cause the noise. Due to the lower recirculation, the noise of free blowing is reduced by 3 to 6 dB (A). This has the significant advantage that a smaller sound-damping element can be provided. Due to the possibility of the construction volume of To reduce dampers, thus, the increase in the length of the vacuum pump due to the singer screw rotors can be at least partially compensated again.

Further, it is preferable that the profile of the screw rotors be substantially symmetrical. For example, trapezoidal profits, cycloidal profiles or involute profiles are preferred here. It is preferred that the gap height, i. H. in particular, the distance between the screw rotors and the housing inner wall is selected such that the compression extends over a relatively long area on the outlet side of the rotor. Particularly preferred here in the cold state of the turbomolecular pump is a ratio cold gap height / axial distance> 2/1000. Furthermore, it is preferred that in the operating state, d. H. According to the invention, the gap heights are preferably selected so that at final pressure operation an average chamber pressure of 100 mbar is not reached until after about 20% of the rotor length, measured from the starting side.

In a preferred embodiment, the screw vacuum pump according to the invention has a rated speed of more than 5000 revolutions per minute. Furthermore, to avoid over-compression in the pressure-side region of the screw rotors, a pressure relief valve may be provided. Instead of or in addition to the provision of a relief valve, it is possible to provide a speed control. By a corresponding lowering of the speed also over-compression can be avoided. By both measures, the power consumption at high intake pressures and thus the installed engine power can be effectively reduced.

The invention will be explained in more detail with reference to a preferred embodiment with reference to the accompanying drawings. Show it :

Fig. 1 is a schematic plan view of two inventively designed screw rotors and

Fig. 2 is a schematic representation of a screw rotor after

Prior art compared to the screw rotor according to the invention in conjunction with a schematic diagram of the pressure curve,

The two screw rotors shown in Fig. 1 are arranged in a pump housing, not shown. Through the pump housing of the pump chamber 10 is formed, in which the two screw rotors 12, 14 are arranged. The two screw rotors have shaft projections 16, 18 on both sides, which are each rotatably supported by bearing elements 20 in the pump housing. To drive the two screw rotors 12, 14 is usually a shaft extension 18 or alternatively a shaft extension 16 is connected directly or via a transmission with a drive motor. The second screw rotor is driven by a corresponding toothing (not shown) by the same drive motor, so that the two screw rotors 12, 14 are synchronized with each other and rotate in the opposite direction. The screw rotors suck the medium to be delivered on a suction side (arrow 22) and eject the medium on a pressure side (arrow 24).

The pitch of the screw rotors is represented by the oblique lines 26. From Figure 1 it can be seen that the slope varies over the length I of the rotor. In the pressure-side region 28, the slope is significantly lower than in the suction-side region 30. The gradient in the pressure-side region 28 is inventively designed such that the slope in the region 31 at half of the turns at most twice the slope at Rotor Ausiass 24 is. This has the consequence that a relatively long pressure-side region 28 is formed by the slope changes only slightly. In the pressure-side region 28, the compression takes place over the majority of the pressure difference between inlet and outlet. Thus, in the area 28 also a large part of the compaction work is directed. As a result, the heat to be dissipated is generated essentially in this area. In this case, the heat is dissipated according to the invention by the housing surrounding the screw rotors 12, 14 in the pressure-side region.

The ratio of the length l of the screw rotors 12, 14 to the distance d of the rotor axes is therefore greater than 3.0 according to the invention.

In Fig. 2, an inventive screw rotor 12 is shown in the upper region, which corresponds to the screw rotor 12, 14 in Fig. 1. Below is a screw rotor 32 is shown in the prior art. The screw rotor 32 is shorter and has a smaller number of turns in the pressure-side area, in which the pitch changes only slightly. In the rotor 32 according to the prior art results in a pressure curve, as shown schematically by the line 34. It can be seen that in the pressure-side region 36 of the screw rotor 32, a strong increase in pressure occurs.

Due to the inventive design of the rotor 12, the pressure-side region 28 is significantly longer. Furthermore, the gap height is selected accordingly (cold gap height / center distance> 2/1000 and

Hot gap height / center distance> 12/1000). This results in the pressure increase corresponding to the line 38 in the diagram, which runs flatter.

Claims

claims

1. Screw vacuum pump, with a pump chamber forming a pump chamber (10), two screw rotors (12, 14) arranged in the pump chamber (10), which are mounted in the pump housing via two bearing elements (20) and their rotor length ratio (I). to the distance of the rotor axes (d) is greater than 3.0, wherein the screw rotors (12, 14) have a variable pitch, at least 7 turns and a built-in compression ratio of at least 4.5 and wherein the slope after half of the turns less than is the double slope on the pressure-side Rotorausiass (24).

2. Screw vacuum pump according to claim 1, characterized in that each screw rotor (12, 14) has only one thread.

3. Screw vacuum pump according to one of claims 1 or 2, characterized in that the profile of the screw rotors (12, 14) is substantially symmetrical or asymmetrical.

4. Screw vacuum pump according to one of claims 1-3, characterized in that the heat dissipation exclusively the pump housing is actively cooled.

5. Screw vacuum pump according to one of claims 1-4, characterized in that the ratio of the rotor length (I) to the distance of the rotor axes (d) is greater than 3.5, in particular greater than 4.

6. Screw vacuum pump according to one of claims 1-5, characterized in that at least 9, preferably at least 11 turns per screw rotor (12, 14) are provided.

7. screw vacuum pump according to one of claims 1-6, characterized in that the compression ratio is at least 5, preferably at least 6,

8. Screw vacuum pump according to one of claims 1-7, characterized in that the gap heights are selected so that there is a final pressure of the vacuum pump of at least 5 Pa.

9. Screw vacuum pump according to one of claims 1-8, characterized in that the maximum speed is more than 5000 revolutions per minute.

10. Screw vacuum pump according to one of claims 1-9, characterized in that to avoid over-compression in the pressure-side region (28) one or more pressure relief valves are arranged and / or the rotor speed is controllable by a speed control.