Abstract:
A centrifugal compressor that includes a low speed shaft connected to a drive motor and a high speed shaft containing an impeller. The high speed shaft is coupled to the low speed shaft by a gear train. The low speed shaft is supported by hydrodynamic, oil lubricated, bearings while the high speed shaft is supported by roller and ball bearings. Any vibrational stresses developed in the high speed shaft are dampened by the low shaft system through the gear train coupling.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a centrifugal compressor and, in particular, to a hybrid bearing arrangement for a centrifugal compressor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many centrifugal compressors, such as those employed in refrigeration systems, are driven by electric motors. The motor shaft is typically connected to the rotor shaft of the compressor which supports one or more impeller wheels by means of a gear train. The gear train is designed to increase the rotor speed above that of the motor shaft to obtain optimum efficiency. In refrigeration applications, single stage centrifugal compressors are generally favored because this type of single stage machine produces a large pressure ratio for the single stage of compression and it is more easily manufactured when compared to the more complex multi-stage machines.  
           [0003]    Generally, the low speed motor shaft and the high speed impeller shaft are supported by oil lubricated hydrodynamic journal or sleeve bearings. In addition to radial loads exerted upon the shafts, gas pressures and the gearing arrangement also create relatively high axial loads upon the shafts. These axial loads require the further use of oil lubricated hydrodynamic thrust bearings to help carry the extra loading on the shafts.  
           [0004]    Hydrodynamic bearings have higher friction losses when compared to ball and roller bearings, particularly at startup. To reduce friction losses and prevent damage at startup and coast down, hydrodynamic bearings are equipped with soft metal linings that are about 0.020″ to 0.040″ thick. In the case of an impeller shaft, the clearance between the impeller blades and the shroud that encompasses the impeller is oftentimes less than the thickness of the protective liner. In the case when there is damage or wear to thrust bearings that support the impeller shaft, the impeller can be displaced significantly with the shaft whereupon the impeller blades will rub against the shroud resulting in serious damage being done to the impeller. Although hydrodynamic bearings have higher power losses when compared to roller and ball bearings, they are better able to dampen out troublesome vibrations when compared to other bearings.  
         SUMMARY OF THE INVENTION  
         [0005]    It is, therefore, an object of the present invention to improve centrifugal compressors and, in particular, single stage centrifugal compressors of the type employed in air conditioning and refrigeration systems.  
           [0006]    It is a further object of the present invention to protect the impeller of a single stage centrifugal compressor from damage.  
           [0007]    It is still a further object of the present invention to enhance the performance of a single stage centrifugal compressor of the type employed in an air conditioning or refrigeration system.  
           [0008]    Another object of the present invention is to increase the bearing reliability of a centrifugal compressor.  
           [0009]    Yet another object of the present invention is to reduce bearing frictional losses in a centrifugal compressor.  
           [0010]    These and other objects of the present invention are attained by a centrifugal compressor having a low speed shaft connected to a drive motor and a high speed shaft coupled to the low speed shaft by a gear train. An impeller is mounted upon the high speed shaft and the shaft is supported upon ball and roller bearings which reduce friction losses within the machine and prevent rubbing damage to the impeller due to axial loading of the shaft. The low speed shaft is supported by oil lubricated hydrodynamic bearings which dampen vibrational stresses induced in both the interconnected low speed shaft and high speed shafts. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    For a further understanding of these and other objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, wherein:  
         [0012]    [0012]FIG. 1 is a side elevation in section illustrating a single stage centrifugal compressor embodying the teachings of the present invention; and  
         [0013]    [0013]FIG. 2 is an enlarged partial sectional view illustrating the high speed and low speed shaft assemblies of the centrifugal compressor shown in FIG. 1; and  
         [0014]    [0014]FIG. 3 is a further enlarged partial view in section illustrating the ball and roller bearing unit for supporting the high speed shaft. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Turning initially to FIG. 1, there is illustrated a single stage centrifugal compressor  10  of the type employed in many air conditioning and refrigeration systems embodying the teachings of the present invention. The compressor includes a main casing  11  having a motor housing  12  secured to the back of the compressor the casing is closed by means of front cover  14  that contains a fluid inlet  15  through which a fluid to be compressed enters the compressor. A series of inlet guide vanes  44  are mounted in a control unit  16  positioned in the inlet to the compressor.  
         [0016]    An electric motor generally referenced  17  is mounted within the motor housing. The motor stator windings  19  are stationarily secured to the inner wall of the motor housing and the motor rotor unit  20  is mounted upon a low speed shaft  22 . The low speed shaft is mounted within the motor housing upon suitable bearings ( 27 ,  28 ) in a conventional manner. One end of the low speed shaft extends outwardly beyond the motor housing and passes through a bearing housing  25  that forms part of the back wall  13  of the compressor casing.  
         [0017]    The extended end  24  of the low speed shaft is supported within spaced apart, oil lubricated, hydrodynamic bearings  27  and  28  that are arranged to take up both axial and radial loads. These types of oil lubricated bearings are well known in the art and have been used for some time to support both the low speed shaft and the high speed shaft in single stage centrifugal compressors such as the compressor illustrated in FIG. 1.  
         [0018]    As noted above, hydrodynamic bearings of this type have higher power losses when compared to antifriction bearings containing rollers or balls. However, ball and roller bearings, up until recently, have exhibited a shorter life span than the hydrodynamic bearings. However, most hydrodynamic bearings contain a soft metal coating that surrounds the shaft and serves to reduce frictional loads on the bearings during start up and coast down. In the case of a centrifugal compressor where the high speed shaft rotatably supports the impeller within a shroud, the clearance between the impeller blades and the shroud may be less than the thickness of the bearing&#39;s soft metal coating. As a consequence, the bearings can wear to a point where the impeller blades come into rubbing contact with the shroud. This, in turn, can damage the impeller and cause compressor failure. The oil lubricated hydrodynamic bearings have one decided advantage over the low friction roller and ball bearings in that the hydrodynamic bearing provides a damping effect for the shaft, and thus absorbs vibratory stress acting upon the shaft related system.  
         [0019]    With further regard to FIG. 1, a drive gear  30  is secured to the outboard end of the low speed shaft which protrudes outwardly from the bearing housing  25 . The drive gear is arranged to mesh with a smaller diameter driven gear  32  that is secured to the compressor&#39;s high speed shaft  35 . As will be explained in further detail below, the high speed shaft  35  is mounted in a pair of roller bearings  37  and  38  for taking up radial loads exerted on the high speed shaft and a pair of roller bearings  40  and  41  that are arranged to take up axial loads exerted on the high speed shaft. As is conventional, the impeller wheel contains a series of radially turned vanes  43  that form with the shroud a series of converging passages through which the fluid that is being compressed is forced as the wheel turns at high speeds. The impeller wheel is centered inside of a shroud  45  so that little clearance is provided between the impeller blades and the shroud. As noted above, the clearance between the impeller blades and the shroud is very tight and even slight wear on the bearings can cause the blades to rub against the shroud.  
         [0020]    Roller bearing  38  is mounted in the compressor casing and is arranged to support the distal end of the high speed shaft. As best illustrated in FIG. 3, roller bearings  37  and the two ball bearings  40  and  41  are mounted within a bearing block  50  that is secured to an intermediate vertical wall  51  of the compressor casing. Because of recent advances in roller and ball bearing materials, the bearings can be manufactured using wear resistant materials that considerably extend the usable life of the bearing when employed in high speed applications such as herein described. The two wall bearings  40  and  41  have their inner races press fitted upon the high speed shaft and are placed in side by side contact upon the shaft. The outer races of the two ball bearings are contained within a horizontally disposed cavity  54  formed in the bearing block. The cavity opens to one side of the bearing block and empties into a smaller diameter blind hole  55  at its other end. Roller bearing  37  and an oil ring  57  are mounted in the blind hole with the oil ring  57  separating roller bearing  37  and ball bearing  40  in assembly. A spacer  59  is passed over the shaft so that it can pass over the shaft into contact with roller bearing  37 . The spacer is held in place by a snap ring  60  that is fitted in a groove formed in the shaft to prevent axial movement of the bearing and the oil seal to the left, not shown, as viewed in FIG. 3.  
         [0021]    The open end of the cavity is closed by an end cap  62  that is held in place by screws  63  that are threaded into the bearing block. A spring in the form of a belville washer  65  is placed between the end cap and ball bearing  41 . The spring serves to provide a preload upon the axial bearings  40  and  41 , thus preventing the bearing stack from rattling, particularly during surge conditions.  
         [0022]    Oil is provided to the stack by an oil line  66  which has a discharge port adjacent to the oil ring  57 . The oil ring, in turn, is arranged to distribute the oil axially in two directions to provide lubrication to the roller bearing  37  and the ball bearing pair  40  and  41 . Although not shown, an oil discharge passage is provided through which the distributed oil is able to pass out of the casing back to the oil sump.  
         [0023]    A temperature sensing probe  70  is mounted within the bearing block  50  and is arranged to ride in contact with the outer race of ball bearing  40 . The probe contains an outlet line  71  that provides a temperature related signal to a controller  72  (FIG. 2) which is adapted to shut down the compressor in the event an over-temperature condition is sensed. This, in turn, protects the compressor&#39;s impeller from being damaged in the event of a high-speed shaft bearing failure.  
         [0024]    As should be evident from the disclosure above, the present hybrid bearing arrangement reduces bearing frictional losses in a centrifugal compressor as well as preventing the compressor&#39;s impeller from becoming damaged in the event of a high speed shaft bearing failure. The use of hydrodynamic bearings to support the low speed shaft provides vibration damping to the low speed shaft as well as to the high speed shaft through the gear train that couples the two shafts together in assembly.  
         [0025]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.