Screw pump

Abstract

The invention relates to a screw pump having two screws, in which screw pump each screw has a first thread and a second thread. The threads extend in each case from a suction side to a delivery side. The threads are in engagement with one another, with the result that the threads are divided into a plurality of working chambers, the volume of which decreases from the suction side to the delivery side. According to the invention, the threads have two thread turns. Moreover, the invention relates to a screw for a pump of this type. On account of the uniform distribution of mass of the two-turn threads, the pump can be operated at a high rotational speed, with the result that the throughput of the pump is increased.

Claims

1. A screw pump comprising: two screws each having first and second ends, each screw having a first thread and a second thread, each of which threads has two thread turns and extends from a suction side at a center of the screw to a delivery side at an outer end of each thread, the first thread and second thread work in opposite directions, with the delivery side of the first thread at the first end of the screw and the delivery side of the second thread at the second end of the screw, the first and second threads of one screw engage with the first and second threads of the other screw to define a plurality of working chambers having a volume that decreases from the suction side to the delivery side; and a housing in which the screws are received, said housing having a first housing section in the region of the suction side at the center of the screws and a second housing section surrounding the delivery side of the first and second threads of each screw, said first housing section defining an inlet, there being a suction gap between the housing and at least one of the first and second threads in the first housing section, and there being a minimum radial spacing between the housing and the thread in the second housing section.

2. The screw pump of claim 1, wherein the screws have a design which is symmetrical in the longitudinal direction between the two outer ends of the threads.

3. The screw pump of claim 1, wherein a radial spacing between the housing and the thread in the region of the suction gap is at least 50 times greater than the radial minimum spacing.

4. The screw pump of claim 1, wherein the extent of the suction gap in the circumferential direction corresponds to at least 10% of a circumference of the housing surrounding the screw in the first housing section.

5. The screw pump of claim 1, wherein an extent of the suction gap in a longitudinal direction corresponds to at least 20% of a length of the thread.

6. The screw pump of claim 1, wherein a transition edge is formed between the first housing section and the second housing section.

7. The screw pump of claim 1, wherein the housing is provided with an inlet opening having a cross sectional area greater than 60% of a cross-sectional area of the thread.

8. The screw pump of claim 1, wherein an inner end of the two threads of a screw are spaced apart from one another.

9. The screw pump of claim 1, wherein said housing includes a bore between the two screws which connects the delivery sides to an outlet opening, the bore being arranged at least partially within a tangential plane which rests on both screws.

10. The screw pump of claim 1, wherein the two screws and a drive form a unit that is releasably connected to the pump housing.

11. A pump arrangement comprising the screw pump of claim 1 and a forepump arranged at an outlet of said screw pump.

12. The pump arrangement of claim 11, wherein, in a steady operating state in which the screw pump sucks in substantially a maximum possible volumetric flow and a pressure at an inlet of the screw pump is kept constant at a value of less than 1 mbar, a volumetric flow through the forepump is smaller than the volumetric flow through the booster pump by at least a factor of 50.

13. The pump arrangement of claim 11, wherein the forepump is a liquid ring vacuum pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following text, the invention will be described by way of example using one advantageous embodiment with reference to the appended drawings, in which:

(2) FIG. 1 shows a perspective, partially cut-away illustration of a screw pump according to the invention,

(3) FIG. 2 shows a detail of the pump from FIG. 1 in an enlarged illustration,

(4) FIG. 3 shows the view from FIG. 2 in another state of the pump,

(5) FIG. 4 shows a diagrammatic cross-sectional view of a screw pump according to the invention along the axis of a screw,

(6) FIGS. 5A and 5B show sections along the lines A-A and B-B in FIG. 4,

(7) FIG. 6 shows the view from FIG. 4 in another state of the screw pump, and

(8) FIG. 7 shows a block diagram of an arrangement according to the invention.

DETAILED DESCRIPTION

(9) A pump according to the invention in FIG. 1 comprises two screws 14 which are received in a pump housing 15. One of the screws 14 can be seen over the entire length on account of the pump housing 15 which is not shown completely, whereas substantial parts of the other screw 14 are covered by the pump housing 15. The two screws 14 are in engagement with one another, which means that the thread projections of one screw 14 engage into the depression between two thread projections of the other screw 14.

(10) The pump comprises a control and drive unit 16, in which an electronically controlled drive motor 17 is arranged for each of the screws 14. The electronic controller of the drive motors 17 is set up in such a way that the two screws 14 run completely synchronously with respect to one another, without the thread projections of the screws 14 coming into contact. As an additional safety measure against damage to the screws 14, the two screws 14 are equipped in each case with a gearwheel 18. The gearwheels 18 are in engagement with one another and bring about positive coupling of the two screws 14 for the case where the electronic synchronization of the screws 14 fails.

(11) Each screw 14 is equipped with two threads 19, with the result that the pump has four threads 19 overall. The threads 19 extend in each case from a suction side 20 in the center of the screw 14 to a delivery side 21 at the outer ends of the screw 14. The two threads of one screw 14 are oriented in opposite directions, with the result that they operate from the suction side 20 toward the delivery side 21.

(12) Each of the threads 19 comprises a first thread turn 22 and a second thread turn 23. The threads 19 are therefore two-turn in the sense that the thread turns 22, 23 are interlaced with one another, with the result that they together form a shape in the manner of a double helix. The two thread turns 22, 23 are shaped in such a way that the threads 19 are symmetrical in the radial direction. If the screw 14 is considered from the delivery side of the first thread 19 as far as the delivery side of the second thread 19, the screw 14 has, moreover, a symmetry in the longitudinal direction.

(13) The threads 19 are designed in such a way that a greater volume between two adjacent thread projections is enclosed in the region of the suction side 20 than in the region of the delivery side 21. The volume of the working chambers which corresponds to the volume which is enclosed between the thread projections is therefore reduced from the suction side to the delivery side, with the result that gas which is contained in the working chamber is compressed on the path from the suction side to the delivery side.

(14) The housing 15 of the pump is provided with an inlet opening 24 which is arranged in such a way that it affords access to the suction sides 20 of all four threads 19. In order to make a great volumetric flow into the pump possible, the inlet opening 24 has a great cross section. In the exemplary embodiment, the cross-sectional area of the inlet opening 24 is greater than the circular contour which is defined by a screw 14.

(15) In order to further improve the volumetric flow into the working chambers, a suction gap 25 is formed on the housing 15 of the pump, which suction gap 25 adjoins the inlet opening 24 and follows the contour of the screw 14 in the circumferential direction. In the longitudinal direction, the suction gap 25 extends approximately over half the length of the thread 19 between the suction side 20 and the delivery side 21. In the circumferential direction, the dimension of the suction gap 25 varies with the inlet opening; the further the inlet opening 24 extends to the side at the relevant point, the shorter the extent of the suction gap 25 in the circumferential direction at said point. At the widest point of the inlet opening 24, the suction gap 25 extends over a circumferential angle of approximately 45°. In the region, in which the inlet opening 24 no longer overlaps the suction gap 25, the suction gap 24 extends over a circumferential angle of approximately 120°. The dimension of the suction gap 25 in the radial direction corresponds to the spacing between the pump housing 15 and the contour of the screw 14 in said region. This spacing lies in the order of magnitude of approximately 10 mm.

(16) As a result of the suction gap, the gas is not restricted to entering into the working chambers in the radial direction, but rather the gas can also move beyond a thread projection through the suction gap into the working chamber. The volumetric flow into the working chamber is increased further as a result.

(17) A further contribution to increasing the volumetric flow into the working chamber is achieved by the fact that there is a spacing between the suction side 20 of the first thread 19 of a screw 14 and the suction side 20 of the second thread 19 of the screw 14. As a result, space remains free in the center of the screw 14, through which space the gas can also enter into the working chamber in the radial direction.

(18) The region, in which the suction gap 25 extends (=first housing section 26), serves to fill the working chambers. In the adjoining second housing section 27, the spacing between the housing and the contour of the screw 14 is as small as is technically possible (minimum radial spacing). The compression takes place in the second housing section and a leakage flow from one working chamber into the next working chamber is not desired.

(19) A transition edge 28 is formed at the transition from the first housing section 26 to the second housing section 27. The transition edge 28 extends in the circumferential direction over the entire suction gap 25 and defines the transition from the suction gap 25 to the second housing section 27, in which the minimum radial spacing exists between the housing 15 and the screw 14.

(20) The compression begins as soon as the working chamber has moved into the second housing section, that is to say as soon as the thread projection which delimits the working chamber towards the suction side has sealed with the transition edge 28. The transition edge 28 is arranged in such a way that the seal between the thread projection and the transition edge 28 takes place at an instant[ ] at which the working chamber still has its maximum volume.

(21) As viewed in the circumferential direction, the transition edge 28 includes an angle with the transverse direction which is smaller than the lead of the thread projection which seals with the transition edge 28. This achieves a situation where the seal between the thread projection and the transition edge 28 does not take place suddenly, but rather extends over a short time period. The operating noise of the pump is reduced as a result.

(22) The actual volume compression takes place in a short section of the thread immediately after the seal of the working chamber. The adjoining further windings of the thread serve for sealing and also bring about a thermodynamic compression.

(23) The gas is discharged from the working chamber on the delivery side 21 of the thread 19. The compressed gas is combined by a bore 29 in the pump housing 15 from the outer delivery sides 21 to a central outlet opening. The outlet opening which cannot be seen in the figures is arranged opposite the inlet opening 24. As FIGS. 2, 3 and 5 show, the bore 29 is integrated into the pump housing 15 and extends between the two screws 14, the line 29 being arranged partially within a tangential plane 35 which rests on both screws 14.

(24) According to FIG. 6, the pump according to the invention is constructed in such a way that the control and drive unit 16, together with the screws 14, forms one structural unit which can be pulled as such out of the housing 15. If maintenance or repair is required, the structural unit can be exchanged, without it being necessary for the pump housing 15 to be detached from the plant surroundings.

(25) A bearing 31 is arranged at that end of the screw 14 which faces away from the control and drive unit 16, which bearing 31 is seated fixedly on the shaft and is received slidingly in a bearing seat 34 of the pump housing 15. If the structural unit is pulled out of the housing 15, the bearing 31 is released from the bearing seat 34 and is likewise removed from the housing 15.

(26) One application example for a screw pump according to the invention is shown in FIG. 7, where a pump arrangement comprising a booster pump 30 and a forepump 33 is connected to a space 32 to be evacuated. The booster pump 30 is a screw pump according to the invention, and therefore the forepump 33 is a liquid ring vacuum pump. The pump arrangement is dimensioned in such a way that a volumetric flow of 4000 m.sup.3/h can be sucked out of the space 32, in order to keep the pressure in the space 32 constant at 0.5 mbar.

(27) To this end, the booster pump 30, the screws 14 of which have a diameter of approximately 25 cm, is operated at a rotational speed of approximately 15 000 rpm. A pressure of approximately 50 mbar prevails at the outlet of the booster pump 30 and therefore at the inlet of the forepump 33. According to the gas law, this means a volumetric flow of 400 m.sup.3/h for the forepump 33. The forepump 33 compresses said volumetric flow to atmospheric pressure and discharges it to the surroundings.