EP1571340A1 - Dry running positive displacement vacuum pump with internal compression - Google Patents

Abstract

The dry vacuum pump has several successive chambers closed off leaving leakage openings where the cross-sectional area of the opening is greater on at least one chamber on the suction side than on the chamber on the pressure side. The pump can be a screw spindle pump with the chambers formed on the same spindles. The pump is set up for a lower speed with an average and higher suction-side pressure than with a low suction-side pressure.

Description

Mechanical vacuum pumps according to the displacement principle are operated over a wide pressure range. On the one hand, the suction pressure when starting the pump and at the beginning of the evacuation of the connected equipment is equal to the atmospheric pressure. On the other hand, it is the main task of the pump to maintain the vacuum at low suction pressure below 100 mbar down to the order of 10 ^-3 mbar. This results in the operation of the pump external pressure ratios between 1 and 10 ⁶ . Efficient compression is particularly important in the main application area with a high pressure ratio, as this is directly reflected in the investment and operating costs. Of great influence on the efficiency is the internal compression of the pump. This is understood to mean that the volume of the chambers moving from the suction side to the pressure side decreases with increasing approach to the pressure side. The efficiency is better, the closer the internal compression comes to the external pressure ratio. High internal compression is therefore an important design goal for vacuum pumps.

So far, run dry positive displacement pumps with built an internal compression by a factor of 2 to 5. The Limitation arises from the fact that a higher internal compression to thermal and mechanical difficulties at high and medium suction pressures leads. Especially in the mean pressure range from 200 to 300 mbar, in which the theoretical compaction performance is highest due to the large heat generation high temperatures and is a high drive power to apply. Therefore It is customary to design the machines for this area. That means drive power and cooling are essential must be dimensioned larger than it is for the main operating area would be required. In addition, in purchase taken that at the much lower pressures of the Main operating area the internal compression is unfavorable. For non-dry running vacuum pumps can be a higher achieve inner compression by being in the Pumps valves provides that only when the outlet pressure to open. Such valves are oil overlaid, but Modern vacuum processes are increasingly demanding dry conditions running vacuum pumps, partly because the re-evaporation of Operating or lubricating fluid in the evacuated Equipment must be avoided, partly from environmental Establish. Dry valves are dry running machines too short life.

Nevertheless, even with dry running vacuum pumps a higher To be able to achieve internal densifications, one has additional ones Measures for limiting heat generation or the torque in the critical Saugdruckbereichen considered. In EP-A-1111243 a lowering of the speed recommended in case of higher suction pressures. This could to achieve an internal compression ratio of 10 or more. It is overlooked that the applied Torque not from the speed but from the compression ratio and the suction pressure depends and thus by a speed reduction can not be significantly reduced can. - After the W0 208609 can be an internal compression up achieved to a factor of 10 in a screw compressor when the shape of the rotors is geometrically optimized becomes. However, this can not be used when used as a vacuum pump Solve the problems outlined, because the heat generation and the torque requirement is not below the compression ratio theoretically assigned limit can. - The construction according to EP-B-1108143 allows a high internal compression in that one within the Compressor at high intake pressure resulting overpressure is relaxed by a spring-loaded valve. As already noticed, however, has shown that such valves have only a short life during dry operation. DE-A-10129341 is based on the observation that the most of the compaction performance of a screw machine implemented in the last output side Rotorendbereich will, so there correspondingly high temperatures occur during compaction. This disadvantage should thereby be remedied that the power conversion and heat generation on a larger section of the displacement spindle is distributed by the profile contour of the rotors to the outlet side towards with increasingly larger flank gap openings is performed. This should allow a gas backflow between the individual working chambers with appropriate pressure reduction done so that the heat dissipation is cheaper. Thereby However, the achievable vacuum and efficiency deteriorated. Because of the leakage losses low To keep it, it is necessary for dry-running positive displacement vacuum pumps narrow gaps between the displacers and the Housing or between cooperating Verdrängerpaaren.

The invention is therefore based on the object, a dry running positive displacement vacuum pump according to the preamble of claim 1, which provides improved performance characteristics without the mentioned difficulties shows higher suction pressures. The solution according to the invention consists in the features of claim 1 and preferably in those of the subclaims.

Thus, a dry displacement vacuum pump has several successively arranged chambers whose volume for the purpose the internal compression from the suction side to the pressure side decreases. It is assumed that the chambers Have leak openings, namely in particular the inevitable Column between the displacers and the housing as well as between the displacers. The measure according to the invention is that the leak port cross section at least a suction-side chamber is greater than the one pressure side chamber. On the one hand, the higher suction-side effect Leakage losses at high intake pressure a reduction the internal compression and thus a reduction associated with heat generation and power consumption Problems. On the other hand - this is the core of the inventive Knowledge - are the leakage at low Suction pressure much lower. That's based on the fact that at high intake pressure, the return flow in the Leak openings according to the laws of viscous flow expires, while at low suction pressure molecular or Knudsen's flow characteristic is present. From the difference the flow conditions follows that the Leak openings at low suction pressures much denser are as at higher suction pressures. They allow at low Intake pressures better compression values than at high Suction pressures. On the one hand, this achieves the desired Relief at high suction pressures, without the other corresponding disadvantages at low suction pressures to have to accept.

This dimensioning of the leak openings contradicts earlier Intuition that one is particularly good on the suction side To achieve compression through narrow sealing gaps.

Molecular flow takes place when the middle free path of the molecules is not significantly lower as the characteristic cross-sectional dimension of the opening. viscose Flow takes place when the mean free path is much smaller than the cross-sectional dimension. The term Knudsenströmung refers to the transition region between the two types of flow mentioned.

The teaching according to the invention generally amounts to that the gap widths between displacer and housing and between displacer pairs in the suction side of the Pump be made much larger than in the pressure side Range, where in the pressure-side area the columns - like Generally common - as close as possible. Same thing for everyone Positive displacement pumps with several consecutively arranged Chambers and primarily for screw pumps, Roots pumps and claw pumps.

In addition, the speed - as known - at Medium to high suction pressure lowered below the speed which is provided at low suction pressure. Thereby is the time taken for the backflow from a chamber is available, compared to the normal speed in Ratio of speeds increased. During this effect in known pumps is not sufficient because there the small gap even at low speed one in the weight prevents falling backflow, leads the speed reduction in connection with the inventively larger Gap widths to a synergistic effect in the sense of a considerable reduction of heat production and of Torque requirement at medium and high intake pressure.

The invention effectively avoids the over-compression in the Range from 200 to 300 millibars and thereby makes oversizing of cooling and drive unnecessary. It will a high internal compression allows at low Suction pressures reduce power and cooling or the pumping speed elevated.

Although the displacer with a high geometric compression ratio is in the middle and high pressure range (over 200 millibar) the internal compression on the suction side so low that the effective inner Compaction and thus the power consumption significantly behind the theoretical compression that one has due to the calculated geometric Verdrängerverhältnisse remains. Although this increases the pumping speed at high Press down; however, at low pressures, at which it is mainly used, a much higher one Suction capacity achieved at low drive power become. Because the inner compression is no longer alone the geometry of the displacer, but also by the ratio the suction side gap currents to the theoretical Suction capacity is determined, there is also the possibility by reducing the speed (the one with a reduction equate to the theoretical pumping speed) the internal compression (albeit at the expense of the pumping speed) in addition to influence. Also the problem of Over-compression at atmospheric pressure is essential reduced.

Claims (4)

Dry Verdrängervakuumpumpe with a plurality of successively arranged, leaving leak openings closed chambers whose volume decreases from the suction side to the pressure side, characterized in that the leak port cross-section of at least one suction-side chamber is greater than that of a pressure-side chamber. Vacuum pump according to claim 1, characterized in that it is a screw pump and the chambers are formed on the same spindles. Vacuum pump according to claim 1 or 2, characterized in that it is set up for a lower speed at medium and higher suction-side pressure than at low suction-side pressure. Vacuum pump according to one of claims 1 to 3, characterized in that the different leak port cross section is based at least partially on different gap width.