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.

Description:
BACKGROUND 
       [0001]    The invention relates to a screw pump having two screws. Each screw is equipped with a first thread and a second thread, the threads extending 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 the working chambers decreases in each case from the suction side to the delivery side. Moreover, the invention relates to a screw for a pump of this type. 
         [0002]    Screw pumps of this type can be used for generating a vacuum. The space to be evacuated is connected to the suction side of the pump, with the result that the pump can suck in gas from the space. The gas is compressed in the pump and is output again on the delivery side at a higher pressure. 
         [0003]    Said screw pumps have a number of advantageous properties and are therefore widely used. However, the throughput is restricted in comparison with other pumps, that is to say the capability of discharging a great volume of gas from a space within a short time period. For applications, in which this is required, screw pumps have not previously been taken into consideration as a rule on account of their lack of throughput. Instead, other types of pumps are used, such as Roots pumps. 
       SUMMARY 
       [0004]    The invention is based on the object of proposing a screw pump with an increased throughput. Proceeding from the prior art cited at the outset, the object is achieved by the features of claim  1 . Advantageous embodiments are found in the subclaims. 
         [0005]    According to the invention, the threads have in each case two thread turns. The thread turns are preferably symmetrical with respect to one another in the radial direction. The threads then have a point symmetry such that the thread turns can be copied onto themselves by a rotation about the screw axis by 180°. 
         [0006]    The invention has discovered that the reason for the restricted throughput is, inter alia, that conventional screw pumps cannot be operated at any desired high rotational speed. A restriction of the rotational speed results from the fact that conventional screws have a non-uniform distribution of mass in relation to the screw axis. The non-uniform distribution of mass brings about an unbalance which can be kept under control only with difficulty at high rotational speeds. The distribution of mass is non-uniform because the thread turn already ensures an asymmetrical distribution of mass in the case of the normal (single-turn) threads of traditional screw pumps. 
         [0007]    The invention proposes that the threads of the screws are of two-turn configuration. This means that each thread has two thread turns which are interlaced with one another in such a way that they together form a shape in the manner of a double helix. The two-turn threads are preferably designed in each case in such a way that the result is a symmetrical design in relation to the screw axis. For each outwardly projecting element of one thread turn, there is therefore a corresponding element of the other thread turn which lies opposite it in the radial direction in relation to the screw axis. On account of the more uniform distribution of mass of the two-turn threads in comparison with single-turn threads, it becomes possible to operate the screw pump at a higher rotational speed, with the result that the throughput is increased. 
         [0008]    For operation at high rotational speeds, it is desirable to keep the forces as low as possible not only in the radial direction, but also in the longitudinal direction. To this end, the pump is preferably designed in such a way that the two threads of a screw work in the opposite direction. The forces which are exerted by one thread in the longitudinal direction are then compensated for by the other thread. The threads are preferably oriented in such a way that the suction side is arranged in the center of the screw, that is to say between the two threads. The delivery sides are then formed by the outer ends of the threads, which has the advantage, in particular, that the drive elements and bearings are exposed to the higher output pressure. Moreover, the screw can be designed in such a way that it also has a symmetrical design in the longitudinal direction if that section of the screw which is enclosed between the two outer ends of the threads is considered. 
         [0009]    The pump according to the invention comprises a housing, in which the two screws are received. The housing is provided with an inlet opening in the region of the suction side, and there is an outlet opening in the region of the delivery side. It has been shown that it is of significance for a high throughput of the pump to design the inlet opening and the suction side of the pump in such a way that a high volumetric flow can enter into the pump. 
         [0010]    The housing is preferably designed in such a way that it has a first housing section and a second housing section in the region of a thread, there being a suction gap between the housing and the thread in the first housing section, and the housing sealing with the thread in the second housing section. The fact that the housing seals with the thread is to be understood in such a way that the leakage gap which necessarily exists between the housing and the thread in the case of dry-running pumps is as small as possible (minimum radial spacing). Nowadays, a value of less than 0.2 mm, preferably approximately 0.1 mm is the aim for the minimum radial spacing. Since the two screws of the pump are in engagement with one another, the housing in the first housing section does not seal with the thread over the entire circumference of the screw, but rather only in the circumferential section, in which there is no engagement with the other screw. The second housing section preferably adjoins the delivery side of the thread. 
         [0011]    The inlet opening of the housing is as a rule also arranged in the region of the first housing section which preferably adjoins the suction side of the thread. The screw is then surrounded by the housing only in the circumferential section which still remains next to the inlet opening and the second screw. If there is a suction gap between the housing and the thread in the first housing section, this is to be understood in such a way that there is a radial spacing between the thread and the housing in at least one part section of said circumferential section, which radial spacing is greater than the minimum radial spacing. The radial spacing in the region of the suction gap is preferably greater than the minimum radial spacing by at least the factor 50, preferably the factor 100, further preferably the factor 200. 
         [0012]    The suction gap has the effect that the gas which is sucked in can enter the working chambers not only in the radial direction, but can also move through the suction gap from one working chamber into the next working chamber. By an additional path into the working chamber being offered to the gas, the working chamber can be filled more rapidly, which has a positive effect on the throughput. 
         [0013]    The greater the suction gap, the more gas can enter into the working chambers along this path. The suction gap preferably extends next to the inlet opening in the circumferential direction over at least 10%, preferably at least 20%, further preferably at least 30% of the circumferential section, with which the housing in the second housing section surrounds the screw. In the region, in which there is no longer any overlap between the suction gap and the inlet opening, the suction gap can extend over a correspondingly greater circumferential section of, for example, at least 50%. 
         [0014]    In the longitudinal direction, the suction gap preferably extends over at least 20%, further preferably over at least 30%, further preferably over at least 40% of the length of the thread. Accordingly, the second housing section is considerably shorter than the length of the thread and extends, for example, over no more than 80%, preferably no more than 70%, further preferably no more than 60% of the length of the thread. In contrast to conventional pumps, a comparatively long section of the thread therefore serves to fill the working chambers, whereas the section, in which the compression takes place, that is to say in which the housing seals with the thread, is comparatively short. The extent of the suction gap in the longitudinal direction can correspond substantially to the screw section which is assumed by the first 360° winding of the thread. The thread therefore has a great lead in the inlet region. The first 360° winding as viewed from the suction side preferably assumes at least 20%, preferably at least 30%, further preferably at least 40% of the length of the thread. Overall, each thread turn of the two-turn thread preferably comprises at least three, further preferably at least four complete 360° windings. 
         [0015]    A transition edge can be formed between the first housing section and the second housing section and therefore at the transition from the suction gap to the region[ ] in which the housing seals with the thread. As soon as the thread seals with the transition edge, the working chamber is sealed and the actual compression begins. If the transition edge were oriented parallel to the thread turn, by way of which the sealing takes place, the chamber would be sealed suddenly. This would be positive for the degree of efficiency of the pump, but also increases the noise level. The transition edge is therefore preferably oriented in such a way that it includes an angle with the circumferential direction in accordance with the thread lead, the angle being smaller than the thread lead. 
         [0016]    In order for it to be possible to suck in great volumes, it is advantageous, furthermore, if the housing is provided with a large inlet opening. For example, the inlet opening can be greater than 60%, preferably than 80%, further preferably than 100% of the cross-sectional area of the screw. The cross-sectional area of the screw denotes the contour which is defined by the screw. Using said contour which is as a rule cylindrical, the radial spacings between the thread and the housing can also be determined. 
         [0017]    In order to improve the filling of the working chambers further, a spacing can be provided between the inner ends of the two threads of a screw. As a result, additional space is obtained, through which the gas can also enter into the working chambers in the longitudinal direction. 
         [0018]    The delivery sides are as a rule formed by the outer end of the threads, which means that the delivery sides are at a spacing from one another. A line is preferably provided which extends from the delivery side to an outlet opening of the pump. In one advantageous embodiment, the line is a bore which is formed between the two screws of the pump in the pump housing, the bore further preferably being arranged at least partially within a tangential face which rests on both screws. 
         [0019]    The pump can be designed in such a way that the two screws can be detached together with the drive as one unit from the pump housing. This affords the possibility of installing the pump fixedly in a relatively large plant, it being possible, in particular, for the inlet opening and the outlet opening of the pump housing to be connected fixedly to corresponding pipelines of the plant. If maintenance or repair becomes necessary, the connections between the pump housing and the plant remain in existence and merely the unit comprising screws and drive is detached from the pump housing and replaced by another unit. As a result, long down times during maintenance and repair are avoided. 
         [0020]    For this purpose, the screws are preferably equipped in each case with a bearing at the end which faces away from the drive, which bearing is received slidingly in a bearing seat of the pump housing. When the unit comprising screws and drive is pulled out of the pump housing, the bearing is released from the bearing seat and is also removed from the pump housing. 
         [0021]    The pump according to the invention is preferably dimensioned in such a way that it achieves a throughput of more than 5000 m 3 /h and in the process can compress the gas from 1 mbar to 100 mbar. To this end, the diameter of the screws is preferably greater than 20 cm. The pump can be designed for operation at a rotational speed of more than 10 000 rpm. 
         [0022]    By virtue of the fact that the screw pump according to the invention combines a high throughput with great compression, possible applications are opened up which were not accessible previously to the screw pumps. In order to generate a vacuum at low pressure with a simultaneously large volumetric flow, a pump arrangement comprising two pumps connected behind one another is usually used, the first pump usually being called a booster pump and the following pump being called a forepump. Connecting two pumps behind one another is expedient because, according to the gas law (pressure*volume=constant; under the assumption of a constant temperature), the forepump can be designed for a substantially smaller volumetric flow than the booster pump. 
         [0023]    As a result of the greatly increased throughput in comparison with classic screw pumps, it becomes possible to use the screw pump according to the invention as a booster pump. As a consequence, the invention relates to a pump arrangement comprising a booster pump and a forepump, in which pump arrangement the booster pump is a screw pump according to the invention. A pump arrangement, in which a screw pump is used as booster pump, has independent inventive content, even without the threads of the screws being of two-turn configuration. 
         [0024]    In comparison with Roots pumps which have usually been used up to now as booster pump, the screw pump according to the invention produces a considerably higher compression. If a steady-state operating state of the pump arrangement is considered, in which operating state the booster pump can suck in substantially the maximum possible volumetric flow and the pressure is kept constant at a low value of, for example, less than 1 mbar, classic single-stage Roots pumps produce merely a compression by the factor 10. The volumetric flow through the following forepump is, as a consequence, merely smaller than the volumetric flow through the booster pump by the factor 10 in accordance with the gas law. 
         [0025]    In the steady-state operating state, in which substantially the maximum possible volume is sucked in and the pressure is kept constant below 1 mbar, the screw pump according to the invention produces a compression by at least the factor 50 or even the factor 100. This results in completely new options for the design of the pump arrangement. For instance, in the steady-state operating state which is described, the volumetric flow through the forepump can be smaller than the volumetric flow through the booster pump by at least the factor 50, preferably at least the factor 100. The volumetric flow at the inlet of the booster pump in the steady-state operating state is preferably greater than 1000 m 3 /h, further preferably greater than 5000 m 3 /h. 
         [0026]    Moreover, the use of the screw pump according to the invention as booster pump opens up the option to use a liquid ring vacuum pump as forepump. The liquid ring vacuum pumps are not suitable for pressures which lie below the vapor pressure of the operating liquid. In general, said pumps can therefore not be used for pressures below 30 mbar. The screw pump according to the invention achieves an output pressure of more than 30 mbar even if the input pressure lies below 1 mbar. As a consequence, it becomes possible by way of the invention to use a liquid ring vacuum pump as forepump. 
         [0027]    Moreover, the invention relates to a screw for a screw pump of this type. The screw comprises two threads which extend in each case from a suction side to a delivery side. According to the invention, the screw is distinguished by the fact that the threads in each case have two thread turns, the thread turns preferably being symmetrical with respect to one another in the radial direction. The screw can be developed by way of further features which are described with reference to the pump according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    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: 
           [0029]      FIG. 1  shows a perspective, partially cut-away illustration of a screw pump according to the invention, 
           [0030]      FIG. 2  shows a detail of the pump from  FIG. 1  in an enlarged illustration, 
           [0031]      FIG. 3  shows the view from  FIG. 2  in another state of the pump, 
           [0032]      FIG. 4  shows a diagrammatic cross-sectional view of a screw pump according to the invention along the axis of a screw, 
           [0033]      FIGS. 5A and 5B  show sections along the lines A-A and B-B in  FIG. 4 , 
           [0034]      FIG. 6  shows the view from  FIG. 4  in another state of the screw pump, and 
           [0035]      FIG. 7  shows a block diagram of an arrangement according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    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 . 
         [0037]    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. 
         [0038]    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 . 
         [0039]    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. 
         [0040]    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. 
         [0041]    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 . 
         [0042]    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. 
         [0043]    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. 
         [0044]    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. 
         [0045]    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. 
         [0046]    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 . 
         [0047]    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. 
         [0048]    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. 
         [0049]    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. 
         [0050]    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 . 
         [0051]    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. 
         [0052]    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 . 
         [0053]    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 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. 
         [0054]    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 3 /h for the forepump  33 . The forepump  33  compresses said volumetric flow to atmospheric pressure and discharges it to the surroundings.