Abstract:
A liquid-ring vacuum pump comprises a pump casing and an impeller that is eccentrically mounted in the pump casing. The impeller is made of a material having a modulus of elasticity of less than 4000 N/mm 2 .

Description:
BACKGROUND 
       [0001]    The invention relates to a liquid-ring vacuum pump having a pump housing and an impeller which is mounted eccentrically in the pump housing. Moreover, the invention relates to an impeller for a pump of this type. Pumps of this type can be used to evacuate containers or other closed spaces. An inlet opening of the pump is connected to the space to be evacuated, the gas which is contained in the space is sucked in through the inlet opening, is compressed in the pump and is output again through an outlet opening. 
         [0002]    In liquid-ring vacuum pumps, a liquid ring is kept in motion by way of the impeller, with the result that the chambers between the vanes of the impeller are closed by the liquid ring. Since the impeller is mounted eccentrically in the pump housing, the liquid ring penetrates to a different extent into the chamber depending on the angular position of the impeller and, as a result, acts as a piston which changes the volume of the chamber. Since the entire force which is required for this purpose is transmitted by the impeller, the impeller is a highly loaded component. 
         [0003]    In particular, the impeller is subjected to a pronounced alternating load, since the force acts on the vanes in different directions, depending on whether the liquid ring is moving into the chambers or is moving out of the chambers. It has been assumed up to now that reliable and low-vibration operation of the pump is possible only when the impeller is designed with high rigidity. The high rigidity achieves a situation where a deformation of the impeller under the alternating loads is avoided. A deformation of the impeller is undesired because then greater tolerances between the impeller and the pump housing become necessary. However, the leakage flow is increased as a result of greater tolerances, which at the same time means a reduction in the degree of efficiency of the pump. 
         [0004]    The impeller is subjected to a plurality of loads. In addition to the centrifugal and acceleration forces, particularly the pressure loading of the vanes comes to the fore. At the transition between the pressure and suction side, a pronounced change in pressure can be determined which leads to an alternating load as a result of bending. High alternating bending stresses occur in the process at the blade root. Said high alternating bending stresses are increased further in the case of condensate also being delivered. Owing to the principle, cavitation cannot be avoided in a liquid-ring vacuum pump. Cavitation not only leads to damage of the surface, but further alternating bending stresses also occur in addition to the abovementioned loads. Materials which withstand said loads have to be selected for the manufacture of the impeller. 
         [0005]    In previous liquid-ring vacuum pumps, the impeller is composed predominantly of metallic materials. For instance, welded steel constructions, gray cast iron wheels and wheels made from stainless steel or bronze are found. The modulus of elasticity of said materials is as a rule greater than 100 000 N/mm 2 . Impellers made from fiber-reinforced plastics are also known (CN 201650734). The modulus of elasticity then lies in the order of magnitude of 20 000 N/mm 2 . Up to now, it was such a matter of course for a person skilled in the art that mechanically highly loaded components are manufactured only from fiber-reinforced plastics and not from non-reinforced plastics that this was not even worth mentioning expressly, cf., for instance, Faragallah W H: “Liquid ring vacuum pumps and compressors”, Jan. 1, 1985, Beltz Offsetdruck, page 187. 
         [0006]    In particular, the strength of the material, the chemical resistance, the resistance to cavitation and the price play a role in the selection of the materials. 
         [0007]    One disadvantage of impellers with high rigidity consists in that jolt-like loads which are experienced by the impeller during operation of the pump are transmitted in a substantially unfiltered manner to further components of the pump. Jolt-like loads of the impeller are to be expected, in particular, if cavitation occurs in the liquid ring. If the impeller has high rigidity, operating states categorically have to be avoided, in which there is a risk of cavitation. Liquid-ring vacuum pumps are therefore usually operated in such a way that a clear distance from the cavitation limit is maintained at all times. However, part of the possible degree of efficiency is sacrificed as a result. 
       SUMMARY 
       [0008]    A liquid-ring vacuum pump is presented, wherein risk of damage as a result of cavitation is reduced. The impeller is composed of a material, the modulus of elasticity of which is smaller than 4000 N/mm 2 . 
         [0009]    An impeller deforms under the influence of forces to a substantially greater extent than corresponding impellers made from conventional materials. On account of its compliance, the material is suitable for yielding to the alternating load and the cavitation forces which occur and for dissipating stresses as a result. The associated disadvantages are balanced by the improved resistance of the pump to cavitation. The jolt-like loads which occur during cavitation are cushioned by the impeller and are not transmitted in an unfiltered manner to the other components of the pump. As a result, it becomes possible to operate the pump closer to the cavitation limit, without the service life being reduced considerably as a result. The degree of efficiency of the pump is increased as a result of operation close to the cavitation limit. 
         [0010]    Damage as a result of cavitation can also occur on the impeller itself. First of all, the surface is attacked as a result of high local loads. Subsequently, the damage can continue further into the structure of the impeller. This occurs, in particular, if the impeller is composed of a fiber-reinforced material. The surface is namely susceptible to first damage where the fibers reach the surface of the impeller. Small cavitation bubbles can accumulate on said open fibers during the occurrence of cavitation and can lead to great surface damage upon implosion. The impeller is therefore preferably composed of a non-fiber-reinforced material. This then results in a homogeneous surface which has fewer points of action for damage. 
         [0011]    The manufacturing is inexpensive if the impeller is manufactured from plastic. In addition, non-reinforced plastic materials have the advantage that the noise emissions are low during cavitation operation, since non-reinforced plastics have a satisfactory damping characteristic. For example, polyoxymethylene (POM), polyether ether ketones (PEEK), polyamides (PA), polybutylene terephthalate (PBT), polycarbonates (PC) or polyphenylene sulfide (PPS) may be suitable. The modulus of elasticity of said materials lies between 2000 N/mm 2  and 4000 N/mm 2 . 
         [0012]    The impeller is preferably provided with a hub, via which a tight connection to a shaft of the pump can be produced. The shaft is mounted eccentrically in the pump housing, whereas the hub is centered in the impeller. A plurality of vanes extend radially to the outside from the hub. The number of vanes can lie, for example, between 10 and 20. 
         [0013]    Together with the liquid ring, the chambers which are enclosed in each case between two vanes form the working chambers of the pump. The chambers are open toward one end side, in order to make the supply and discharge of the gas to be delivered possible. By way of said end side, the impeller adjoins a control disk of the pump, in which control disk inlet openings and outlet openings are provided in suitable positions. The gap between the vanes and the control disk is kept as small as possible, in order to minimize the leakage flow. The vanes can be inclined relative to the axial direction, with the result that the impeller is pressed in the direction of the control disk by the flow force. 
         [0014]    The chambers are preferably closed on the opposite end side of the impeller. For this purpose, the impeller can comprise a disk-shaped projection which extends radially to the outside from the hub to such an extent that the disk-shaped projection protrudes over its entire circumference into the liquid ring during operation of the pump. For an effective transmission of force to the liquid ring, the vanes preferably protrude further into the liquid ring than the disk-shaped projection. 
         [0015]    An impeller for a liquid-ring vacuum pump of this type comprises a hub for a tight connection to an eccentrically mounted shaft of the pump. A plurality of vanes extend radially to the outside from the hub. On one end side, the vanes are covered by a disk-shaped projection over at least half of their radial extent. The impeller is composed of a material, the modulus of elasticity of which is smaller than 4000 N/mm 2 . 
         [0016]    The impeller is preferably manufactured in one piece from a plastic material, the plastic material further preferably being non-reinforced. The impeller can be developed with further features which are described above in relation to the pump. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the following text, the invention will be described by way of example using advantageous embodiments with reference to the appended drawings, in which: 
           [0018]      FIG. 1  shows a diagrammatic sectional illustration of a liquid-ring vacuum pump, 
           [0019]      FIG. 2  shows the pump from  FIG. 1  in a side view, and 
           [0020]      FIG. 3  shows a perspective view of an impeller. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In a liquid-ring vacuum pump which is shown in  FIG. 1 , an impeller  14  is mounted eccentrically in a pump housing  20 . Liquid in the interior of the pump is driven by the impeller  14  which is in rotation, and forms a liquid ring which extends radially to the inside from the outer wall of the pump housing  20 . On account of the eccentric mounting, the vanes of the impeller  14  protrude into the liquid ring to different depths depending on the angular position. The volume of a chamber  22  which is enclosed between two vanes changes as a result. The liquid ring therefore acts like a piston which moves up and down in the chamber during a revolution of the impeller  14 . 
         [0022]    A duct leads from an inlet opening  16  into the interior of the pump, in which the impeller  14  rotates. The duct  16  opens in the region, in which the vanes of the impeller  14  emerge from the liquid ring, in which region the chamber which is enclosed between two vanes is therefore enlarged. As a result of the enlarging chamber, gas is sucked into the chamber through the inlet opening  16 . After the chamber has reached its maximum volume, the liquid ring penetrates into the chamber again during the further rotation of the impeller  14 . When the gas is compressed sufficiently as a result of the liquid ring which penetrates further, it is output again through an outlet opening  17  at atmospheric pressure. A liquid-ring vacuum pump of this type serves to evacuate a space which is connected to the inlet opening  16  to a pressure of, for example, 50 millibar. 
         [0023]    According to  FIG. 2 , the impeller  14  is connected via a shaft  18  to a drive motor  19 . The pump is of modular design, and the drive and the impeller  14  are therefore accommodated jointly in the pump housing  20 . Via a control unit  21  which is arranged on the pump housing  20 , electrical energy is fed to the drive  19  and the rotational speed of the pump is set. 
         [0024]    According to  FIG. 3 , the impeller  14  has fifteen vanes  23  which extend radially to the outside from a central hub  24 . Via the hub  24 , the impeller  14  is connected to the shaft  18  of the pump. The vanes  23  have a three-dimensional shape which includes a curve in relation to the radial direction. In the installed state, that end side of the impeller  14  which is visible in  FIG. 3  points in the direction of the control disk of the pump. The chambers  22  which are arranged between in each case two vanes  23  are therefore open toward the control disk, with the result that the gas to be delivered can be supplied and discharged through openings in the control disk. 
         [0025]    On its opposite end side, the impeller  14  has a disk-shaped projection  25  which extends radially to the outside from the hub  24 . The radial extent of the disk-shaped projection  25  is such that the disk-shaped projection  25  dips into the liquid ring over its entire circumference when the pump is in operation. The vanes  23  protrude somewhat beyond the disk-shaped projection  25  in the radial direction, with the result that an effective transmission of force between the vanes  23  and the liquid ring is achieved. 
         [0026]    The impeller  14  is manufactured in one piece from a non-fiber-reinforced plastic material. The modulus of elasticity of the material lies between 2000 N/mm 2  and 4000 N/mm 2 . The material is therefore comparatively flexible, with the result that jolt-like loads on the impeller can be absorbed partially by the material. 
         [0027]    Since the material is non-fiber-reinforced, the impeller has a homogeneous surface. Even if large pressure and speed spikes occur locally as a consequence of cavitation in the operating liquid, the surface withstands the loads and damage to the impeller does not occur. As a result of the impeller, the liquid-ring vacuum pump can be operated closer to the cavitation limit for these reasons, with the result that the degree of efficiency of the pump is increased.