Abstract:
A centrifugal compressor unit includes a driving means for rotatably driving a rotor, and at least one compressor including a statoric body and an arrangement of blade wheels mounted on a shaft which is rotatably driven by the rotor in the statoric body. The group formed by the motor and/or each compressor being mounted in a common housing is sealed from the gas used by the compressor. The compressor unit also includes an arrangement of active bearings for axially and radially guiding the rotor and the driven shaft, and a means for cooling the driving means and the guiding bearings by withdrawing the gas used by the compressor at the outlet of a first compression stage. The cooling means includes a set of internal conduits for supplying the driving means and the bearings with cooling gas. The cooling gas flow in the motor and the cooling gas flow in the bearing is separated and then converge upstream of the first compression stage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a centrifugal compressor unit. More specifically, the invention relates to a centrifugal compressor unit of the integrated type, that is to say of the type in which the compressor and a motor means for driving the compressor are mounted in a common housing sealed against the gas handled by the compressor.  
         [0003]     2. Brief Description of the Related Art  
         [0004]     With reference to  FIG. 1 , a conventional integrated compressor unit includes a motor means, generally including an electric drive motor  10  and a centrifugal compressor  12  including, for example, several compression stages, the entity being mounted in a common housing sealed against the gas handled by the compressor.  
         [0005]     As can be seen in  FIG. 1 , the motor  10  rotationally drives a rotor  16  itself rotationally driving a driven shaft  18  supporting a set of compression impeller wheels  20 ,  22 ,  24  and  26 .  
         [0006]     In the example of a compressor unit illustrated in  FIG. 1 , the compressor includes four compression stages which together compress a gas taken in via an inlet pipe  28  to deliver it on the outlet side  30 , passing via a volute casing  32 .  
         [0007]     The rotor  16  and the driven shaft  18  are connected via a flexible coupling  34 . In this case, the rotor  16  and the driven shaft  18  are each supported by radial bearings  36 ,  38 ,  40 , and  42 . An opening  44  formed in the housing  14  and closed off by a blanking means  46  allows access to the flexible coupling  34  for assembling the compressor.  
         [0008]     Finally,  FIG. 1  shows that an axial thrust bearing  48  limits the axial displacement of the driven shaft  18 , while an equalizing piston  49  allows the axial pressure applied to the driven shaft while the compression unit is in operation to be equalized.  
         [0009]     Producing a compressor unit in the form of a motor and of a compressor which are located in a pressurized sealed common housing makes it possible to eliminate sealing gaskets needed by the compressor. These are elements liable to compromise the reliability of the compressor unit and to be the source of gas leaks into the atmosphere. In this case, the motor is located actually within the gas handled by the compressor. In order to avoid an excessive increase in the mechanical losses of the motor as a result of ventilation, the motor is arranged in such a way that it is at the intake pressure of the compressor. Thus gas is circulated through the motor in order to remove the losses, that is to say in the stator in order to remove the losses through a Joule effect in the windings, and in the air gap between the rotor and the stator in order to remove the ventilation losses and the eddy current losses in the rotor.  
         [0010]     This is why centrifugal compressor units are generally provided with means for cooling the motor means and the guide bearings by tapping off gas leaving the first compression stage in order to cool the motor and the bearings. In this respect, reference may be made to documents EP-A-1 069 313 and U.S. Pat. No. 6,390,789, which describe various types of motor-compressor units in which the motor and the bearings are cooled by tapping cooling gas from the outlet side of the first compression stage.  
         [0011]     However, this type of cooling technique presents a certain number of major disadvantages, particularly it does not allow optimum cooling of the motor and of the bearings. There is a need to alleviate this disadvantage and to provide a motor-compressor unit that exhibits improved cooling means.  
       SUMMARY OF THE INVENTION  
       [0012]     A centrifugal compressor unit of the type includes a motor means rotationally driving a rotor and at least one compressor including a stator body and a set of impeller wheels which are mounted on a driven shaft rotationally driven by the rotor in the stator body, the entity including the motor and the or each compressor being mounted in a common housing sealed against the gas handled by the compressor unit, the compressor unit further including a set of active bearings for axially and radially guiding the rotor on the driven shaft and cooling means for cooling the motor means and the guide bearings by tapping off some of the gas handled by the compressor at the outlet from a first compression stage, passing said gas through the motor means and through the bearings and reinjecting the gas into the inlet side of the compressor.  
         [0013]     In some embodiments of a centrifugal compressor unit, the cooling means includes a set of internal passages for feeding the motor means and the bearings with cooling gas. The flow of cooling gas in the motor means is separate from the flow of cooling gas in the bearings and converging upstream of the first compression stage.  
         [0014]     It has been found that such an arrangement improves considerably the cooling within the compressor unit. Specifically, it has been found that the size of the air gap of the magnetic bearings compared with the size of the air gap of the motor detracts from cooling when the same flow is used to cool the bearings and the motor, by preventing the cooling gas from circulating correctly. What happens is that the motor generates greater losses than the bearings and therefore requires a higher flow rate. Likewise, the temperature of the cooling gas leaving the motor detracts from the cooling of the bearings when the flow of cooling gas leaving the motor is used to cool the bearings.  
         [0015]     In some embodiments, the cooling means further includes a set of external lines collecting the gas on the outlet side of the first compression stage and feeding the internal passages in parallel.  
         [0016]     In one embodiment, the internal passages for feeding the motor means are fed in parallel with the internal passages for feeding the bearings with cooling gas.  
         [0017]     The cooling means are equipped with filtering means for filtering the gas handled by the compressor.  
         [0018]     In certain embodiments of the compressor unit, with the driven shaft of the compressor supported by two end radial bearings, the cooling means includes an axial passage running from one bearing to the other and fed at one of its ends by the external lines, said axial passage globally running longitudinally and radially externally through the compressor. For example, the internal passages for feeding the bearings includes a set of directional passages directed radially externally in the compressor and each feeding one bearing.  
         [0019]     In some embodiments, the motor is fed with cooling gas via an orifice formed in an end cover and in communication with an external line. The flow of cooling gas may be mixed with the flow of cooling gas leaving the bearings cooled by the gas coming from the internal passages. For example, the motor-compressor unit includes means for regulating the refrigeration flow rate for the motor on the one hand and for each bearing on the other.  
         [0020]     In some embodiments, the compressor unit includes means for collecting flows of cooling gas from members situated on the same side as an equalizing piston.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Other objects, features and advantages of the invention will become apparent from reading the following description, given solely by way of a nonlimiting example and made with reference to the attached drawings in which:  
         [0022]      FIG. 1  illustrates an embodiment of the overall structure of a conventional integrated motor-compressor unit;  
         [0023]      FIG. 2  is a block diagram of an embodiment of a centrifugal compressor unit;  
         [0024]      FIG. 3  illustrates one embodiment of a centrifugal compressor unit;  
         [0025]      FIG. 4  illustrates one embodiment of a compressor unit;  
         [0026]      FIG. 5  illustrates another embodiment of a compressor unit; and  
         [0027]      FIG. 6  illustrates an embodiment of a compressor unit.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0028]      FIG. 2  depicts just one compression stage for reasons of clarity, the other compression stages not having been depicted. It will, however, be appreciated that it may be provided with any number of compression stages, as will be discussed later with reference to FIGS.  3  to  6 .  
         [0029]     The compressor unit illustrated in this  FIG. 2  includes a motor means  50 , consisting for example of a variable high-speed electric motor rotationally driving a rotor  52 , itself driving, at the same speed, a driven shaft  54  on which an impeller wheel  56  is mounted. The rotor  52  and the driven shaft  56  are connected via a flexible coupling  58 . As a result, the rotor  52  and the driven shaft  54  are each supported by two end radial bearings  60 ,  62  and  64 ,  66 , respectively. A thrust bearing  67  limits the axial displacement of the rotor  54  when the compressor is in operation, this displacement being caused by the appearance of axial forces that are due to the appearance of a differential pressure across the impeller wheel  56 .  
         [0030]     The impeller wheel  56  draws in a compressed gas delivered from an intake pipe  68  to increase its static pressure and thus increase its kinetic energy. A diffuser  70  ( FIG. 3 ) slow the gas coming from the impeller wheel  56  to increase its pressure. Downstream, a return duct  72  leads the gas towards compression stages  74 , . . .  76  located downstream.  
         [0031]     As shown in  FIGS. 2 and 3 , in order to cool the motor  50  and the bearings  60 ,  62 ,  64  and  66  and the thrust bearing  67  that limits the axial displacement of the rotor  50 , some of the gas leaving the first compression stage  56  is tapped off and used as a cooling gas. These various elements, such as the motor, the bearings and the thrust bearing are cooled using separate flows of cooling gas. Flows in parallel are delivered by passages  80 - 1 ,  80 - 2 , . . .,  80 - 6  forming part of a set of external lines collecting the gas leaving the first compression stage  56 , after passing through a set of filter cartridges such as  82 . As will be appreciated, this arrangement, whereby the motor, on the one hand, and the bearings, on the other, are fed in parallel with separate cooling flows, makes it possible to get around the constraints associated with the size of the air gap of the magnetic bearings, on the one hand, and of the motor, on the other.  
         [0032]     As can be seen in  FIG. 3 , according to one embodiment, the stator part of the compressor includes a tubing  84  collecting the fluid leaving the diffuser  70 . The tubing passes through the casing  86  of the compressor. For actually cooling the motor and the bearings, the motor-compressor unit includes a set of internal feed ducts fed respectively from the external pipes  80 - 1 , . . . ,  80 - 6 . Having passed through the motor and the bearings, the cooling gas is collected by a generally longitudinal central duct  88  which opens into the intake pipe  68  upstream of the first compression stage  56 .  
         [0033]     To cool the motor  50  and the end bearings  60  and  62  supporting the rotor  52 , the corresponding end cover  90  closing the casing  86  is provided with an orifice  92  which communicates with the corresponding external pipe  80 - 1 . Part of this cooling flow is used to cool the bearing  60 . This flow is then collected for cooling the motor, by passing it through the motor air gap. Another part of this flow is used directly to cool the motor. A second internal passage  94  is fed from the external lines to cool the second bearing  62  of the motor. Downstream, the flow of cooling gas used to cool the bearings  60  and  62  and the motor  50  is collected in a cavity  95  in which the flexible coupling  58  is located and which is closed off by sealed blanking means  96 . Downstream, as depicted by the arrows F, the gas is collected by the internal passage  88  to be reinjected upstream of the first compression stage  56 .  
         [0034]     Furthermore, in some embodiments, the bearings and the thrust bearing are cooled using a flow of cooling gas delivered through an end cover  98  blanking off the corresponding end of the casing  86 . In  FIG. 3 , cover  98  is provided with an orifice  100  which communicates with a corresponding external pipe  80 - 6 . This flow of cooling gas cools, on the one hand, the end bearing  66  situated at the same end as this cover  98  and the opposite-end end bearing  64 , via an axial pipe  104  which runs longitudinally and radially externally between these bearings  64  and  66 , through the compressor stator elements. This axial pipe is also shaped in such a way as also to cool the thrust bearing  78 . The flow of gas is therefore reinjected into the passage  88 . In some embodiments, withdrawing cooling gas on the outlet side of the first compression stage makes it possible to tap off gas that is not as hot as it would have been had it been taken from the outlet side of the compressor, making the cooling more effective, while at the same time limiting the compression power that has to be developed in order to raise the pressure of this gas. In addition, this tapping represents an autonomous feed once the motor has been started, means  105  for adjusting the cooling flow rate for the motor, on the one hand, and for each of the bearings, on the other, being provided so as to create appropriate and controlled pressure drops in the external lines. These adjusting means may be active, of the regulating valve type, or passive, of the fixed orifice type.  
         [0035]     In the embodiment illustrated in  FIG. 3 , the intake of gas into the compressor is situated on the electric motor side. The aforementioned cooling principle may also be applied to an arrangement in which the delivery side of the compressor is on the motor side. In this case, it is the flow of cooling gas leaving the motor or, in general, leaving components situated on the same side as the equalizing piston  107 , which is mixed with the flow of gas from this equalizing piston  107  to then be injected into the intake pipe  86  via an equalizing line  108 .  
         [0036]     For maintenance, in an embodiment, the sealed blanking means  96  allows access to the flexible coupling  58 . Extracting the rotor from the motor is, for its part, done by removing the end cover  90 , which for example is bolted on to the casing. Removal of the internal part of the compressor is, for its part, done by extracting the corresponding cover  98 , which for example is fixed to the casing by a shear ring  110 . In an embodiment, the entity is arranged in such a way that the rotor-diaphragms assembly, that is to say the entirety of the compressor, can be withdrawn from the casing at the same time as the cover  98  without having to detach the casing from its baseplate and from the process gas pipework and the cooling lines. It will be noted that, during these assembly-disassembly phases, the rotors rest on their bearing, which makes the coupling and uncoupling operations easier, without the risk of damaging the rotating parts and the stator parts which might otherwise come into contact with the rotors during these operations.  
         [0037]     It will be noted that the invention is not restricted to the embodiments described. Specifically, whereas  FIGS. 2 and 3  depict a centrifugal compressor unit equipped with a multi-stage compressor incorporated in-line with a single compression section having multiple stages, the invention also applies to other types of compressor unit, for example those having two in-line sections S 1  and S 2 , for example each including two stages, each compressing a process gas, as depicted in  FIGS. 4 and 5 . In the embodiment depicted in  FIG. 4 , two inlets E′l and E′ 2  and two outlets S′ 1  and S′ 2  are provided in the casing such that the inlet E′ 2  of the second section is near the outlet S′ 1  of the first section. Thus, in this case, as can be seen in  FIG. 4 , the first compression stage of one of the sections S 2  is located facing the second compression stage of the other section S 1 . By contrast, as in  FIG. 5 , for a configuration known by the name “back-to-back”, the first compression stages of each of the sections S 1  and S 2  may be placed side by side. In this case, the outlets S′ 1  and S′ 2  of these compression stages are located side by side and the inlets E′ 1  and E′ 2  are located at opposite ends from one another. In  FIG. 6 , that the invention also applies to an arrangement in which use is made, arranged in a common casing, of a motor  50  and two compressor units G 1  and G 2  each provided with respective compression stages S 3 , S 4 , S 5  and S 6  and S′ 3 , S′ 4 , S′ 5  and S′ 6  each mounted on a respective driven shaft  54  and  54 ′, these shafts being fixed at two mutually opposed ends of the rotor  52  using flexible couplings  58  and  58 ′. Of course, this arrangement with two compression units may use any arrangement, such as either one of the arrangements described hereinabove with reference to  FIGS. 4 and 5 .  
         [0038]     In these various embodiments, use is made of means for cooling the motor and the bearings that use parallel flows of cooling gas. As will be appreciated, separate fluids for cooling the motor and the bearings are not required. Furthermore, the electric motor, the bearings and the magnetic thrust bearing are cooled autonomously with the flow rate needed for each distributed to each, these various flows then being collected to be reinjected into the inlet side of the first compression stage. These features make it possible to minimize the size of the machine and to simplify the installation. Furthermore, producing internal passages makes it possible to limit the space required for the external lines.  
         [0039]     In some embodiments, it is possible to limit gas leaks to the outside. The reliability is also improved in that integrated filtration of the motor cooling gas. Furthermore, the use of several filter cartridges arranged in parallel and associated with a set of valves allows these cartridges to be changed during running.  
         [0040]     In the description given hereinabove, the filtering means are produced in the form of cartridges mounted on the external pipes, however, it is equally possible, as a variant, to mount the cartridges within the compressor unit housing, at a location that makes them readily accessible, in this instance, for example, in the cavity  95  that provides access to the coupling, preferably mounting them on the blanking means.