Patent Publication Number: US-2007110601-A1

Title: Compressor Unit Comprising a Centrifugal Compressor and an Electric Motor

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation of copending U.S. application, Ser. No. 10/480,011, filed Apr. 16, 2004, which is a national phase application based on PCT/NL2002/00336 filed on May 24, 2002, which claims priority to and the benefit of Application NL 1018212, filed on Jun. 5, 2001, which applications are entirely incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to a compressor unit, comprising a centrifugal compressor for compressing a gas, having a rotor with one or more compressor impellers, and an electric motor having a stator and a rotor for driving the rotor of the compressor, the compressor and the electric motor being accommodated in a common gas-tight housing which is provided with a gas inlet and a gas outlet, and the rotor of the compressor and the rotor of the electric motor being arranged on a common rotor shaft which is mounted in magnetic bearings.  
     BACKGROUND OF THE ART  
      A compressor unit of this type is known, for example, from WO-A-94/29597 and EP-A 1 074 746.  
      In the compressor unit which is known from WO-A 94/29597, a compressor impeller is arranged on both sides of the electric motor. The rotor shaft is mounted in two radial magnetic bearings, which are each arranged between the electric motor and a compressor impeller, and an axial magnetic bearing, which is likewise arranged between the electric motor and one of the compressor impellers.  
      In the compressor unit which is known from EP-A 1 074 746, in particular  FIG. 2 , the compressor impellers are arranged on one side of the electric motor. The rotor shaft comprises two parts which are connected to one another by means of a coupling. The rotor shaft is mounted in three radial magnetic bearings which are arranged at both ends of the rotor shaft and between the electric motor and the centrifugal compressor, and one axial magnetic bearing, which is arranged between the electric motor and the centrifugal compressor.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide an improved compressor unit of the type described in the preamble which can be of compact design and in which the bearings are readily accessible without it being necessary to remove the housing of the compressor unit.  
      This object is achieved by the fact that the rotor shaft comprises a single unit and is mounted in two magnetic radial bearings, each in the vicinity of one end of the common rotor shaft, and one axial magnetic bearing which is arranged in the vicinity of the one of the radial bearings.  
      Preferred embodiments of the compressor unit according to the invention are defined in the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will be explained in more detail in the following description of a number of embodiments of the compressor unit according to the invention with reference to the drawing, in which:  
       FIG. 1  diagrammatically depicts a first embodiment of the compressor unit according to the invention, and  
       FIG. 2  diagrammatically depicts a second embodiment of the compressor unit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The compressor unit shown in  FIG. 1  comprises a centrifugal compressor  1  for compressing a gas, for example process gas, having a rotor  2  with one or more, in this case three, compressor impellers  3 , and an electric motor  4  with a stator  5  and a rotor  6  for driving the rotor  2  of the compressor. The compressor  1  and the electric motor  4  are accommodated in a common gas-tight housing  7  which is provided with a gas inlet  8  and a gas outlet  9 . The housing  7  is divided in the customary way and comprises a plurality of parts which are fixedly connected to one another.  
      The rotor  2  of the compressor  1  and the rotor  6  of the electric motor  4  are arranged on a common rotor shaft  10  which comprises a single unit. The rotor shaft  10  is mounted in two radial magnetic bearings  11  and  12  which are each arranged in the vicinity of one end  13  and  14 , respectively, of the rotor shaft  10 , and one axial magnetic bearing  15 , which is arranged in the vicinity of the radial bearing  11 . The rotor shaft  10  is not supported between the electric motor  4  and the centrifugal compressor  1 . The magnetic bearings  11 ,  12  and  15  will generally be electromagnetic bearings.  
      The design of the compressor unit with the rotor shaft which comprises a single unit and the bearings arranged only in the vicinity of the ends of the rotor shaft means that the compressor unit can be of compact structure and that the bearings are readily accessible for maintenance without the housing of the compressor unit having to be removed.  
      The compressor unit is preferably arranged vertically. This has the advantage that in particular the radial bearings  11  and  12  can be of relatively lightweight design, since these bearings merely have to centre the rotor shaft.  
      The axial bearing can also be of relatively lightweight design if it is ensured that the axial force on the rotor shaft which is produced by the compressor impellers acts counter to the force of gravity.  
      The vertical arrangement of the rotor shaft has the further advantage that the compressor unit takes up relatively little space in the horizontal direction.  
      Obviously, it is also possible for the rotor shaft to be arranged horizontally. However, the design of the compressor unit, and in particular the design of the bearings, has to be adapted to this arrangement.  
      The compressor impellers  3  are advantageously an integral part of the rotor shaft  10 . This contrasts with traditional designs, in which separate compressor impellers are mounted on the shaft, for example by thermal shrinking. If the compressor impellers and the rotor shaft are produced from a single unit, separate pieces of impeller material, in which a rotor-shaft part has already been integrated (impeller-shaft segments), are welded to one another. The impeller-shaft segments which have been welded to one another together form the basic compressor rotor, which has to be processed further in order to form the final compressor rotor.  
      Preferably, the rotor  6  of the electric motor  4  is likewise an integral part of the rotor shaft.  
      The compressor unit is provided with a cooling system for cooling the magnetic bearings  11 ,  12 ,  15  and the electric motor  4 . This cooling system comprises a line  16  which runs from the compressor and branches into a line  17  which runs to the electric motor  4  and lines  18  and  19  which run to the magnetic bearings  11 ,  12 ,  15 . A filter  20  is incorporated in the line  16  which runs from the compressor. To cool the electric motor  4  and the magnetic bearings  11 ,  12 ,  15 , compressed gas is tapped off at an intermediate stage of the compressor  1 , is passed through the line  16  and the filter  20  and is fed in metered fashion, via the lines  17 ,  18  and  19 , to the stator  5  of the electric motor  4  and, via the lines  18  and  19 , to the magnetic bearings  11  and  15  and  12 . The cooling gas is collected again inside the compressor unit and is guided to the inlet section of the compressor.  
       FIG. 2  shows a slightly altered embodiment of the compressor unit according to the invention. This embodiment differs from the embodiment illustrated in  FIG. 1  in that the stator  5  of the electric motor  4  has a dedicated cooling system. This is to prevent the stator  5  of the electric motor being affected by any aggressive constituents which may be present in the gas which is to be compressed.  
      The cooling system for the magnetic bearings  11 ,  12 ,  15  is otherwise identical to the cooling system for the magnetic bearings  11 ,  12 ,  15  of the embodiment shown in  FIG. 1 .  
      The stator  5  of the electric motor  4  is incorporated in a stator chamber  21 , which is separate from the remainder of the interior of the compressor unit and is delimited by that section of the wall of the housing  7  of the compressor unit which surrounds the stator  5  and a partition  22 , which adjoins this wall section and extends in the radial direction on both sides of the stator  5  and also between the stator  5  and the rotor  6  of the electric motor  4 . This partition  22  is also known as a can. The stator chamber  21  is provided with a feed  23  and a discharge  24  for a separate cooling medium which is circulated in a cooling circuit  26  by a pump  25 . A heat exchanger  27  is incorporated in the cooling circuit  26 .  
      The partition (can)  22  may be designed in two ways.  
      The partition  22  may be designed in such a manner that the wall of the stator chamber  21  is able to withstand the design pressure of the compressor.  
      In another embodiment of the partition  22 , at least that section of the partition  22  which extends between the stator  5  and the rotor  6  of the electric motor  4  is of thin-walled design. The stator chamber is incorporated in the closed cooling circuit, which is completely filled with a cooling liquid, in such a manner that the cooling system of the stator  5  of the electric motor  4  as a whole is able to withstand the design pressure of the compressor unit. This design principle is based on a combination of a form fit of the thin-walled partition  22  and the poor compressibility of the cooling medium.  
      The advantage of the latter embodiment, in which the partition  22  is of thin-walled design in the area between the stator  5  and the rotor  6 , is that a greater output per unit area of the motor can be achieved, with reduced eddy current losses.