Abstract:
A helical screw compressor includes two rotors which are mounted in the rotor housing. The helical screw compressor includes sealing arrangements ( 11, 11′ ) for sealing the pressure-sided shaft journals of the rotors. Each sealing arrangement includes a plurality of annular seals ( 11   a   , 11   b ) which are arranged in a row adjacent to each other, and an annular-shaped discharge chamber ( 51 ) is associated with the system on an intermediate position and is connected, via a discharge channel ( 53 ), to the chamber in the rotor housing, wherein pressure which is higher than the atmospheric pressure. Preferably, the discharge channel is connected to the suction chamber ( 10 ) of the rotor housing ( 1 ), and is impinged upon by precompressed gas from an upstream compressor step.

Description:
RELATED APPLICATION DATA 
     This application claims priority from German Patent Application No. 10 2005 058 698.8, filed Dec. 8, 2005, and PCT Application No. PCT/LP2006/005559, filed Jun. 9, 2006, both of which are incorporated by reference herein. 
     BACKGROUND 
     The invention pertains to a screw compressor with the features indicated in the preamble of claim  1 . 
     Screw compressors of this type are known from EP 0 993 553 B1 and EP 1 163 452 B1, for example. In these references, a vent channel that is open to the atmosphere is connected to the relief chamber of the sealing arrangement. 
     The present invention has particular advantages when applied to a screw compressor that compresses a gaseous medium such as air to very high pressures, for example in the range of 30 to 50 bar, and in particular where the application involves the high pressure stage of a two or three stage compressor system. The invention relates to such a multi-stage screw compressor system, in particular a three-stage screw compressor system. 
     Due to the high compression in the compressor, the sealing arrangements that seal the pressurized side of the rotor shafts in the rotor housing are subjected to a very high pressure load. Even if the sealing arrangement consists of a large number of sequentially arranged seal rings, the pressure drop across the entirety of the sealing arrangement is not even, but rather it occurs primarily at the seal rings located external to the rotor, i.e. the farthest ones from it. Consequently, they are subjected to a higher mechanical load. 
     The object of the invention is to construct the sealing arrangement on the pressurized side of the shaft of a screw compressor of the type indicated such that the pressure drop along the sealing arrangement can be controlled and smoothed out so that the reliability of the seal can be improved, especially for very high final pressures in the screw compressor. 
     The solution to this objective is indicated in claim  1 . The dependent claims refer to further advantageous features of the invention. 
     According to the invention, it was found that by providing a defined intermediate pressure at a defined intermediate position in the sealing arrangements on the pressurized side of the rotor shafts, the pressure in the sealing arrangement drops in a controlled, even manner. The result is an especially effective and reliable seal, and the minimization of pressure losses as a result of gas leakage. 
     SUMMARY 
     In one construction, the invention provides a screw compressor with a rotor housing ( 1 ) in which two screw rotors ( 3 ,  5 ) are rotatably held with parallel axes. The rotors mesh into one another with screw-shaped ribs and grooves and which convey a gaseous medium during operation, in particular air, from a suction-side end toward a pressurized end of the rotors, thereby compressing it, wherein each of the rotors has a shaft pin ( 7   a ,  7   b ,  9   a ,  9   b ) at its suction-side end and its pressure-side end, respectively. The pins are held in the rotor housing ( 1 ) by means of bearings ( 13 ,  15 ) and are sealed by means of respective sealing arrangements. The sealing arrangement ( 11 ,  11 ′) of each pressure-side shaft pin has an annular relief chamber ( 51 ) to which a vent channel ( 53 ) is connected. The screw compressor characterized in that the vent channel ( 53 ) connects the relief chamber ( 51 ) to a chamber ( 10 ) within the screw compressor in which a pressure exists during operation of the screw compressor that is higher than atmospheric pressure but lower than the outlet pressure of the screw compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One embodiment of the invention is explained in more detail with the help of the drawings. Shown are: 
         FIG. 1  a perspective, partial sectional view of the screw compressor according to one embodiment of the invention 
         FIG. 2  a cross section of the screw compressor of  FIG. 1 , approximately along the sectional line II-II of  FIG. 1 , 
         FIG. 3  a section essentially along line III-III of  FIG. 2 . 
         FIG. 4  a perspective representation of a three-stage screw compressor system, the third stage of which is a screw compressor according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The screw compressor shown in  FIG. 1  has a rotor housing  1 , shown in a sectional view, in which two rotors  3  and  5  are rotatably held with parallel axes. The rotating axes of the rotors  3 ,  5  lie in a common vertical plane that is also the sectional plane used to illustrate the rotor housing  1 . Each rotor has a profile section  7 ,  9  with a profile exhibiting screw-shaped ribs and grooves, wherein the ribs and grooves of the two profile sections  7 ,  9  mesh with one another such that a seal is created. On both sides of the profile sections  7 ,  9  are shaft pins  7   a ,  7   b ,  9   a ,  9   b , the surfaces of which cooperate with seal arrangements  11 ,  12  to seal the rotor in the rotor housing  1 . The shaft pins  7   a ,  7   b ,  9   a ,  9   b  are also rotatably held in the rotor housing  1  by bearings  13 ,  15 . 
     The upper rotor  3  in  FIG. 1  is the main rotor, at the left end of which in  FIG. 1  is an extension  7   c  of its shaft pin provided to hold a drive gear (not shown) that meshes with a corresponding gear in a drive transmission (not shown) in order to turn the rotor  3 . At the right end in  FIG. 1 , the two rotors  3 ,  5  have two gears  17 ,  19  that mesh with one another, thus forming a synchronization unit (synchronizing transmission) that conveys the rotation of the upper rotor  3  to the lower rotor  5 , which is the secondary rotor, at the desired RPM ratio. 
     When the screw compressor shown in  FIG. 1  is operated, the gas to be compressed, in particular air, is fed to its intake chamber  10 , which is located at the left end of the profile sections  7  and  9  in the rotor housing  1  in  FIG. 1  and is connected to an inlet nozzle (not shown). It is preferable if the incoming gas has already been pre-compressed to an intermediate pressure by one or more upstream compressor stages (not shown), for example a pressure in the range of 10 to 15 bar, preferably about 12 bar. This pre-compressed gas is conveyed to the right in  FIG. 1  through the profile sections  7 ,  9  of the two rotors  3 ,  5  and in the process compressed to a final pressure, which is preferred to be in the range of 30 to 50 bar, in particular about 40 bar. The compressed gas leaves the rotor housing  1  through an outlet (not shown) at the right, pressurized end of the profile sections  7 ,  9  in  FIG. 1 . 
     Rotor housing  1  is surrounding by a cooling jacket or cooling housing  21 , which is for the most part designed as one-piece together with rotor housing  1 , surrounding the same at a distance. Above and below, the cooling housing  21  has large openings that are closed off using a cover plate  23  and a base plate  25  fastened with bolts. Between the rotor housing  1  and the cooling housing  21 ,  23 ,  25  is an annular cooling space  27  that surrounds the rotor housing  1 . 
       FIG. 2  shows a simplified schematic illustration of a cross section approximately along line II-II of  FIG. 1 . The rotor housing  1  that houses the screw rotors (not shown) is surrounded by the cooling jacket or cooling housing  21 , the side walls  21   a ,  21   b  of which are preferably designed in one piece together with the rotor housing  1  and which is closed above and below by cover  23  and by base plate  25 . Together with the rotor housing  1 , the cooling housing  21  forms an essentially completely annular cooling chamber  27  that surrounds the rotor housing  1 ; this chamber is only interrupted at one point by a separating wall  29  that connects the rotor housing  1  to the side wall  21   b  of the cooling housing  21 . The separating wall  29  runs horizontally approximately half way between the center points of the axes M 1 , M 2  of the screw rotors that are arranged perpendicular one above the other. 
     The cooling housing  21  has an inlet opening  31  and an outlet opening  33  for coolant fluid, e.g. cooling water or oil. The inlet opening  31  opens up into a perpendicular entrance channel  35  that runs vertically upward, the upper exit opening  35 ′ of which is situated opposite the bottom of the separating wall  29  at a distance. Prior to the outlet opening  33  is a perpendicular exit channel  37 , the lower entrance opening  37 ′ of which is situated opposite the top of the separating wall  29  at a distance. 
     The black arrow in  FIG. 2  identifies the flow path of the coolant fed to the inlet opening  31 . It is directed through the entrance channel  35  perpendicular upward toward the bottom of the separating wall  29 , turns sharply away from the wall and then flows downward and around the entire periphery of the rotor housing  1 , clockwise in  FIG. 2 , until it meets the top of the separating wall  29 , where it turns sharply away from the wall upward and is withdrawn through the exit channel  37  and the outlet opening  33 . 
     There is a small vent opening  41  in the wall  39  that separates the exit channel  37  from the cooling chamber  27  at a height that roughly corresponds to the upper edge of the outlet opening  33 . While filling the cooling chamber  27  with coolant, this vent opening  41  allows air to escape, as indicated in  FIG. 2  by the upper dotted arrow, so that the cooling chamber  27  can be filled up to the height of the vent opening  41 , i.e. up to the fluid level indicated by line  43 , and so that the volume of the included residual air above the fluid level  43  is very low. 
     A very small bleed opening  47  is placed in the wall  45  that separates the entrance channel  35  from the cooling chamber  27  at the level of the lower edge of the inlet opening  31 . When the cooling fluid is emptied from the cooling chamber  27 , cooling fluid can drain out (as indicated by the lower dotted arrow in  FIG. 2 ) through the bleed opening  47  and the inlet opening  31  until the cooling fluid level in the cooling chamber  27  has reached the level of the bleed opening  47 , i.e. until it has dropped to the level indicated by line  49 . The amount of cooling fluid remaining below line  49  is therefore very low when the cooling chamber  27  is emptied. 
       FIG. 3  shows other details of the invention that relate to the seal arrangement  11  shown in  FIG. 1  to seal the shaft pins  7   b ,  9   b  of the rotors  3 ,  5  in the rotor housing on the pressurized side. As shown, the seal arrangement  11  consists of a number of radial seal rings  11   a ,  11   b  in series. In the embodiment shown, eight radial seal rings  11   a ,  11   b  are arranged one after the other. These radial seal rings  11   a ,  11   b  can be lip seal rings, as is preferred, and as are known from EP 0 993 553, for example. The sealing arrangement  11  is surrounded by a first annular relief chamber  51  to capture any gas that has leaked through the seals  11   a , said chamber placed at a suitable location between a first number of radial seal rings  11   a  and a second number of radial seal rings  11   b . In the embodiment of  FIG. 3  with eight radial seal rings, it can be advantageous to place the relief chamber  51  between the first number of five radial seal rings  11   a , seen as beginning from the rotor profile  7 , and the last three, in other words the outer radial seal rings  11   b.    
     The relief chamber  51  is connected to the intake chamber  10  of the screw compressor via a connection channel  53  incorporated into the rotor housing  1  running parallel to the rotor axis. The annular relief chamber  51  is thus exposed to the intake pressure of the screw compressor present in the intake chamber  10 . In the preferred use of the screw compressor as a high pressure stage of a multistage compressor system, the air fed to the intake chamber  10  can have already been pre-compressed by the upstream compressor stages to a pressure of between 10 and 15 bar, for example, in particular about 12 bar. This, then, is the pressure that is present in the relief chamber  51 . As the compressor is operated, the high final pressure produced by the rotors, for example 40 bar, must drop to zero through the sealing arrangement  11   a ,  11   b . It has been shown that this pressure drop is not linear, but concentrates primarily on the outer radial seal rings  11   b  that are some distance away from the profile section  7 ,  9  and therefore these seals are very heavily loaded mechanically. A defined intermediate pressure is established, by way of the first relief chamber  51  being exposed to the pressure at the inlet to the compressor, at a defined point of the sealing arrangement, and thus the pressure drop along the entire sealing arrangement  11   a ,  11   b  is smoothed out. This mechanically relieves the seals  11   b.    
     A second annular relief chamber  55  is provided at the far end of the sealing arrangement  11  away from the rotor. This chamber is connected to the atmosphere in a known fashion. The purpose of this second relief chamber  55  is to maintain the oil system that lubricates the bearings  15  and the synchronization gears  17 ,  19  at zero pressure and to prevent bleed gas from passing through the sealing arrangement  11  through to the oil-lubrication areas. 
     As can be seen from  FIG. 1 , the sealing arrangement  11 ′ for shaft pin  9   b  of the lower rotor  5  is designed in the same manner as the sealing arrangement  11  of shaft pin  7   b  and also has an annular relief chamber  51 ′ that is connected to the intake chamber  10  of the screw compressor through a vent channel. The vent channel  53  shown in  FIGS. 2 and 3  is preferred to be a common connection channel that is connected to both relief chambers  51 ,  51 ′ of the sealing arrangements  11 ,  11 ′ and that connects them to the intake chamber  10 . 
     As shown in  FIG. 2 , the connection channel  53  that connects relief chamber  51  to the intake chamber  10  runs inside the rotor housing  1 , preferably in the direct vicinity of the separating wall  19  that connects the rotor housing  1  to the cooling housing  21 . Thanks to the intensive cooling of the separating wall  29 , which acts like a cooling rib, by the coolant that is redirected by it, the connecting channel, and thus the bleed gas flowing through it to the intake chamber  10 , is also subjected to especially intensive cooling. 
       FIG. 4  shows a perspective view of a three-stage screw compressor system with three screw compressors  60 ,  70 ,  80  that are attached to a gearbox  90  via flanges, said gearbox having essentially the shape of a perpendicular plate, and said screw compressors cantilevered parallel to one another. They are driven by a common drive gear held in the gearbox  90 , said drive gear driven by a motor. This arrangement is known for two-stage compressor systems from DE 299 22 878.9 U1 and DE-A-16 28 201. In the compressor system shown, screw compressor  60  is the initial stage (low pressure stage), with inlet opening  61  and outlet opening  63 , screw compressor  70  is the second or intermediate stage with inlet opening  71  and outlet opening  73 , and screw compressor  80  is the final stage or high pressure stage with inlet opening  81  and an outlet opening on the side opposite the inlet opening  81  that is not shown in  FIG. 4 .  FIG. 4  also shows an oil sump housing  95  that is flanged to the base of the gearbox  90  and that is connected to the synchronizing gears of screw compressors  60 ,  70 ,  80  and to the drive gear located in the gearbox  90 . 
     Not shown in  FIG. 4  are the connection lines for the gas to be compressed, in particular air, which connect the inlets and outlets  61 ,  63 ,  71 ,  73 ,  81  of the three screw compressors  60 ,  70 ,  80  together. These lines can be designed in the usual fashion and can be equipped with filters, intercoolers, and/or mufflers, for example. 
     The screw compressor  80  of the third stage is a screw compressor according to the invention according to  FIGS. 1 through 3 . The three-stage compressor system according to  FIG. 4  is preferred to be designed such that the outlet pressure of the first stage  60  is about 3 to 6 bar, in particular about 3.5 bar, the second stage (intermediate stage)  70  produces an outlet pressure of about 10 to 15 bar, in particular about 12 bar, and the third stage (high pressure stage) produces an outlet pressure in the range of 30 to 50 bar, in particular about 40 bar. The outlet pressure produced by the second stage  70  of about 12 bar is thus the pressure present in the intake chamber  10  of the third stage  80  and thus is the pressure present in the relief chambers  51 ,  51 ′ of the sealing arrangements  11 ,  11 ′ for the shaft pins on the pressurized side according to  FIG. 1  and  FIG. 3 .