Abstract:
The invention relates to a water-injected screw compressor, comprising a compressor element ( 2 ) with a housing ( 7 ) which borders a compression chamber ( 8 ) in which two rotors ( 9, 10 ) are installed, which, by means of an axle ends ( 13, 14; 15, 16 ), are borne in the housing ( 7 ) by means of water-lubricated slide bearings ( 1 ), an electric motor ( 3 ) comprising a housing ( 18 ) which carries a stator ( 19 ) at the inside, which stator surrounds a rotor ( 21 ) with a rotor shaft ( 22 ). An axle end ( 13 ) of one of the rotors ( 9 ) is directly coupled to or forms one piece with the rotor shaft ( 22 ) of the motor ( 3 ), said rotor shaft being located in the prolongation of said axle end. The rotor shaft ( 22 ) of the motor ( 3 ) is borne in at least one water-lubricated slide bearing ( 23 ).

Description:
FIELD OF THE INVENTION 
   This invention relates to a water-injected screw compressor, comprising a compressor element with a housing, which borders a compression chamber with two rotors therein, and an electric motor for driving the compressor element. 
   BACKGROUND 
   Water-injected screw compressors often are chosen above oil-injected or oil-free screw compressors. 
   With oil-injected screw compressors, the produced air in fact is not free of oil. 
   With classic oil-free screw compressors without an injection of lubricating liquid, the compressor part is connected to the motor by means of a gearwheel case with oil lubrication. Here, the rotational speed of the rotors is higher. 
   Compared to such classic oil-free screw compressor, a water-injected screw compressor has a number of advantages. First, an outlet pressure up to 15 bar can be obtained in one stage instead of in two or three stages. Therefore, the compressor can be realised less expensive and more compact. Also, due to the lower working temperature, coolers may be sufficient which are simpler than with the classic oil-free screw compressors. Moreover, the water-injected screw compressor has a lower noise production and a higher efficiency. Moreover, as no oil or grease is used, no additional maintenance is necessary and the compressor is more environmental-friendly as there is no waste oil or grease. 
   With the water-injected screw compressor, water is injected onto the rotors in order to cool, seal and lubricate the rotors, whereby the male rotor directly drives the female rotor. 
   It is obvious that in this application, by “water” not necessarily 100% pure water is intended. This water may comprise additives, such as anti-corrosive means and/or freezing point depressing means. 
   Water is also injected for lubricating the slide bearings in which the male and the female rotors are borne by means of axle ends. 
   In the compression chamber, the air is compressed and driven out together with the water through an outlet. The air-water mixture then is transported off to a vessel/water separator, where the major part of the water is separated. The water is collected in the vessel, at the bottom thereof, and the compressed air is transported off at the top. 
   The speed range of a water-injected screw compressor is higher than that of an oil-injected screw compressor, amongst others, due to the lower viscosity and the higher heat capacity of water. If a water-injected screw compressor should be driven directly by an electric motor, this motor thus would have to run faster than with an oil-injected screw compressor, which leads to problems for the bearing of the motor as well as for the cooling of the motor. 
   SUMMARY 
   The present invention aims at providing a water-injected screw compressor which solves the aforementioned problems and which allows a direct drive without a gearwheel case, as a result of which said compressor becomes more compact and less expensive. 
   To this aim, the invention consists of a water-injected screw compressor, comprising a compressor element with a housing which borders a compression chamber in which a male rotor and a female rotor are installed, which, by means of axle ends, are borne in the housing by means of water-lubricated slide bearings, an electric motor for driving said compressor element, which motor comprises a housing which carries a stator at the inside, which stator surrounds a rotor with a rotor shaft, a pressure conduit which connects to the compression chamber, a vessel, which also is a water separator, in the pressure conduit, and a water return between the vessel and the compression chamber, the characterising feature of which consists in that an axle end of one of the rotors is directly coupled to or forms one piece with the rotor shaft of the motor, said rotor shaft being located in the prolongation of said axle end, and that also the rotor shaft of the motor is borne in at least one water-lubricated slide bearing. 
   The fact that the axle of the compressor element is coupled directly, in other words, without gearwheel transmission, and thus with a transmission ratio of 1/1, or forms one piece with the shaft of the motor situated in the prolongation thereof, offers the advantage that a bearing can be economised. 
   U.S. Pat. No. 5,222,874 describes an oil-lubricated screw compressor in which the axle end of one of the rotors forms a whole with the rotor shaft of the electric motor. The rotor, which is connected to this rotor shaft, is borne with its axle ends at both extremities in the housing of the compressor element, however, the rotor shaft of the motor in itself is not borne. Therefore, amongst others, the compressor necessarily must be installed vertically. The bearings-also are no slide bearings, but roller bearings, which renders water lubrication impossible. Oil flows downward by gravity from the top of the motor towards the inlet side of the compressor element, where a small under-pressure exists 
   DE-A-197 45 616 describes a vacuum pump. There is, amongst others, no injected compressor element, vessel or return conduit. The pump part exclusively comprises a two-part rotor, an extremity of which forms a whole with the rotor shaft of the electric motor. This rotor shaft is borne at both extremities, contrary to the rotor of the pump part. Also in this vacuum pump, the bearings are no slide bearings, but ball bearings. Although the use of water as a lubrication agent is stated as possible, oil clearly is preferred and only an example with oil cooling is represented. The cooling agent comes into contact with the electric motor, such that water is not suitable. 
   According to the invention, the housing of the motor and the housing of the compressor element preferably also are integrated to form a whole. 
   Of course, there is a risk that water, used for the lubrication of the slide bearings, gets into the inner space of the housing of the motor and causes a short circuit there. 
   In order to avoid this, in one form of embodiment the inner space of the housing around the whole formed by the rotor shaft of the motor and the axle end connected thereto, at the inside of each of the slide bearings situated at opposite sides of the rotor, is sealed by a lip seal which is directed with its free extremity towards the slide bearing, whereas the inner space of the housing of the motor, by means of at least one conduit, is in connection with a source of blocking gas under pressure. 
   In the compressor according to U.S. Pat. No. 5,222,874, around the shaft no sealing is present between the housing of the compressor element and the motor housing. Cooling agent comes into contact with the rotor of the motor, such that also for this reason, no water can be used as a cooling agent. 
   In the vacuum pump according to DE-A-19745 616, around the shaft two shaft sealings are provided between the housing of the pump part and the housing of the motor, however, the inner space of the housing of the motor can not be connected to a source of blocking gas under pressure. The sealings always press against the shaft, also during rotation of the shaft. 
   According to the invention, sealing only takes places during a standstill, however, during rotation of the shaft, blocking gas is added to the space, as a result of which the lip seal is lifted and the friction is minimum. 
   According to the invention, this source of blocking gas preferably is the pressure conduit or the vessel, whereby then in the conduit between this pressure conduit or vessel and the inner space of the motor, a water separator and preferably also a restrictor are installed. 
   When the compressor is operating, the lip seals are lifted, as a result of which wear and tear of these lip seals is avoided. When the compressor stands still, the lip seals clamp around the shaft of the motor, such that the sealing remains guaranteed. 
   In another form of embodiment, the windings of the stator and the rotor are treated with an electrically insulating material, for example, silicones, and instead of seals, one or more drain conduits are provided at the side of each of the water-injected slide bearings directed towards the rotor, which slide bearings are situated on opposite sides of the rotor. 
   In this case, a limited amount of humidity in the motor compartment is tolerable. Humid air, which penetrates into the inner space of the motor or is blown therein, improves the cooling and efficiency of the motor. 
   This form of embodiment has as advantages that fewer parts are necessary and the design thus becomes simpler. The lip seals are redundant, as a result of which also the maintenance thereof is omitted and the mantle of the motor no longer must be provided with channels for cooling. 
   The water-lubricated slide bearing of the rotor shaft of the motor preferably is in connection with a water source, by means of a conduit, which water source is formed in particular by said water return, in particular said vessel/water separator therein. 
   Due to the water lubrication of the slide bearing of the rotor shaft of the motor, not only a good bearing, but also a certain cooling is realised, however, preferably the compressor is provided with extra cooling means for cooling the stator. 
   In a form of embodiment, these cooling means, amongst others, comprise at least one channel, which is provided through the housing and is connected to a water source, in particular said water return, including the vessel/water separator. 
   Therefore, no additional water source is necessary. 
   The channel in the housing may also be connected to an external source of cooling water when such source is available. In this case the requirements for the material of the housing are less severe. No water enters the compression chamber or the bearings so that the water must not be completely free from small particles. 
   The aforementioned cooling means may comprise cooling fins, which are provided on the housing of the motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the intention of better showing the characteristics of the invention, hereafter, as an example without any limiting character, a preferred form of embodiment of a water-injected screw compressor according to the invention is described, with reference to the accompanying drawings, wherein: 
       FIG. 1  schematically represents a water-injected screw compressor according to the invention; 
       FIG. 2  schematically represents a water-injected screw compressor analogous to that of  FIG. 1 , however, in respect to another form of embodiment. 
   

   DETAILED DESCRIPTION 
   The water-injected screw compressor  1  represented in  FIG. 1  substantially consists of a water-injected compressor element  2 , an electric motor  3 , a pressure conduit  4 , which connects to the compressor element  2 , a vessel  5  which also forms a water separator in the pressure conduit  4 , and a water return  6  between the vessel  5  and the compressor element  2 . 
   The compressor element  2  substantially comprises a housing  7  in which the compression chamber  8  is situated, and two mutually engaging screw-shaped rotors situated therein, namely a male rotor  9  and a female rotor  10 . 
   The housing  7  is provided with an air inlet  11 , to which a not-represented inlet conduit is connected, and an air outlet  12 , to which the pressure conduit  4  connects. 
   Both rotors  9  and  10  are provided with axle ends  13  and  14 ,  15  and  16 , respectively, which are borne in the housing  7  by means of water-lubricated slide bearings  17 . 
   The motor  3  substantially is composed of a housing  18 , to the inner side of which a stator  19  is attached which borders a closed inner space  20  inside which a rotor  21  having a rotor shaft  22  is situated. 
   The motor  3  is directly connected to the compressor element  2 . This means that the rotor shaft  22  of the motor  3  is directly attached to the axle end  13  of the male rotor  9 , for example, by means of a conical extremity, which fits into a conical recess and is retained therein by means of a pin. Theoretically, the rotor shaft  22  and the axle end  13  can form one piece; however, this is less practical for the construction of the compressor. 
   As the axle end  13  and the rotor shaft  22  are connected to each other or form one piece, a bearing of the rotor shaft  22  can be omitted. 
   In the represented example, the rotor shaft  22 , at its extremity most remote from the compressor element  2 , is borne in the housing  18 , to wit by a water-lubricated slide bearing  23 . This increases the stability of the rotating whole, also when the motor  3  is relatively heavy, and it allows using rotors  9  and  10  with a body made of fibre-reinforced synthetic material, so-called composite, around a metal shaft. With such rotors, the resistance against the bending of the shaft in the rotor  9  with the axle end  13  is lower. 
   This permits to place the compressor element  2  horizontally. However, by means of appropriate water-lubricated slide bearings, the compressor element  2  can also be placed vertically instead of horizon-tally, such as represented. 
   The water-lubricated slide bearings  17  and  23  are provided with water-injection points  24  which, by means of branches  25 , connect to the water return  6  which connects the bottom side of the vessel  5  to a number of injection points  26  which give out in the compression chamber  8  and provide for the injection of water into the compression chamber  8 . 
   This water return  6  extends through the housing  18 , which to this end is provided with a number of mutually connected channels  27 . 
   By means of an inlet  28 , these channels  27  are in connection with the vessel  5 , and they connect to the injection points  26  by means of an outlet  29  and, by means of the branches  25 , to the injection points  24 . Between the vessel  5  and the inlet  28 , in the water return  6  successively a cooler  30  and a water filter  31  are provided. 
   The branches ( 25 ) may in a variant connect to an external source of water. 
   Special measures are taken to prevent the penetration of water in the electrical motor  3 . 
   Consequently, in order to prevent that water penetrates into the inner space  20 , the common shaft, formed by the axle end  13  and the rotor shaft  22 , at both extremities of the housing  18 , is surrounded by a lip seal  32 , for instance in PTFE, which is attached in the housing  18  and is directed with its free extremity away from the inner space  20 . 
   The inner space  20  is connected to the topside of the vessel  5  by means of two inlets  33  and conduits  34  connected thereto, which conduits turn into a common conduit  35 . In the conduit  35 , a water separator  36  and a restrictor  37  are provided. 
   To the spaces  38  between each lip seal  32  and the slide bearing  17  situated opposite thereto, a drain conduit  39  for leaked-out air and water of the slide bearing  17  is connected, which conduit gives out into the compression chamber  8 , at the inlet side. 
   With normal operation, the motor  3  directly drives the male rotor  9  of the compressor element  2 . The female rotor  10 , which engages therewith, then rotates along therewith, in reverse sense. Thereby, air is suctioned via the air inlet  11  towards the compression chamber  8  and is compressed. Together with water injected through the water injection points  26  for lubricating and cooling the rotors  9  and  10 , and water originating from the slide bearings  17  through conduits  39 , the compressed air leaves the compression chamber  8  through the air outlet  12 . 
   This mixture of compressed air and lubricating water is pressed, through pressure conduit  4 , towards the vessel  5 , where the major part of the water is separated and, by means of the water return  6 , is fed back to the compression chamber  8 . 
   This water flows through the channels  27  and thereby cools housing  18  and so the stator  19 . 
   A part of this water or water from an external source flows, through branches  25 , towards the slide bearings  17  and the slide bearing  23 , which thereby are lubricated and cooled. 
   Through conduits  34  and  35 , blocking air under pressure is fed into the inner space  20 . 
   As air under pressure from the vessel  5  is brought into the inner space  20 , this latter is put under slight overpressure, as a result of which the lip seals  32  are lifted somewhat and there is no wear and tear of these lip seals  32  against the co-operating contact ring of the rotor shaft  22  when the shafts  22  starts and rotates, also when water is used for lubricating. 
   When the compressor stands still and the pressure in the vessel  5  and, thus, also in the inner space  20  has dropped to almost the atmospheric pressure, the lip seals  32  no longer are lifted, but rest against the combination of the axle end  13  and the rotor shaft  22 . 
   These lip seals  32  are called reverse lip seals, on account of the fact that they thus effect in a reverse manner in respect to the classic lip seals. These classic lip seals rest also during rotation of the shaft against this shaft and often will rest against it even stronger than during standstill, what would give excessive wear if not oil but water would be used as lubricant. 
   Air leaking through the lip seal  32  towards the slide bearing  17  is transported off together with water from the slide bearings  17 , through drain conduit  39 , towards the compression chamber  8 . 
   The restrictor  37  provides for the blocking air to expand in the conduit  35 , which provides for a reduction of the relative humidity thereof. As the temperature in the motor  3  always is relatively high the relative humidity will be reduced even more, such that the remaining water vapour in the air does not tend to condense. 
   Although in the form of embodiment described here, the water, for lubricating the various slide bearings  17  and  23  and rotors  9  and  10 , flows over the stator  19 , the stator  19  may be cooled by a separate water circuit. 
   Within the scope of the invention, it is also possible that the blocking air fed to the inner space  20  originates from an external source instead of, as in the form of embodiment described here, from the compressor element  2  itself. 
   The form of embodiment according to  FIG. 2  substantially differs from the form of embodiment described in the foregoing in that the channels  27  of said means for cooling the stator  19  are connected by means of conducts  40  to an external source of cooling water, schematically indicated by  41  in  FIG. 2  and the housing  18  at the bottom is provided with one or more drain openings  42 . 
   The fact that water from an external source  41  is used for cooling the housing  18  and thus the stator  19  instead of the internal source, this is the circuit used for lubricating the compressor element  2 , has the advantage that this water contain less oxygen than the water of said circuit, is much colder and has less requirements as to the purity. 
   As the electrical motor  3  is situated adjacent a compressor element  2  under pressure and injected with water, and although already measures are taken to prevent the entry of water in the electrical motor  3 , the form of embodiment according to  FIG. 2  comprises extra measures to make the motor  3  more water resistant. 
   Therefore, the windings of the stator  19  and the rotor  21  are treated with an electrically insulating material, for example, silicones, against humidity. 
   The drain openings  42  discharge into a so-called “floating valve”  43 . Such valve comprises a container, wherein the water is collected, and a float opening the outlet of the container when the liquid in it reaches a determined level. 
   The cooling water from the external source  41  is used as cooling medium in the cooler  30 . 
   Due to the insulating material, for example, the silicones, a limited amount of humidity in the inner space  20  can be allowed. 
   Water entering accidentally the inner space  20  is transported off through one or more of said drain openings  42 . 
   In both forms of embodiment, there is no gearwheel transmission between the motor  3  and the compressor element  2 , and the housings  7  and  18  are integrated to form a whole. In total, only three bearings are required for the rotor shaft  22  and the axle ends  13  and  14  of the rotor  9 . Consequently, the whole is relatively compact and inexpensive. 
   The heat produced by the high speed of the motor  3  can be transported off by means of the above-described means for cooling. 
   The present invention is in no way limited to the form of embodiment described as an example and represented in the figure; on the contrary may such water-injected screw compressor be realised in different forms and dimensions, without leaving the scope of the invention.