Abstract:
A centrifugal refrigerant compressor system includes an impeller connected to a shaft. A diffuser is arranged on a downstream side of the impeller and is configured to regulate refrigerant flow exiting the impeller. A magnetic bearing supports the shaft. A sensing element is configured to produce an output relating to a shaft condition. A controller is configured to receive the output and determine an undesired impeller operating condition based upon the shaft condition. The controller is configured to command the diffuser to a desired state in response to the undesired impeller operating condition.

Description:
BACKGROUND 
     This disclosure relates to a centrifugal refrigerant compressor with a magnetic bearing assembly. More particularly, the disclosure relates to such a refrigerant compressor having a variable geometry diffuser. 
     Refrigerant compressors are used to circulate refrigerant to a chiller via a refrigerant loop. One type of typical refrigerant compressor operates with a set of variable inlet guide vanes arranged upstream from the impeller for capacity control. The variable inlet guide vanes are actuated during operation of the refrigerant compressor to regulate its capacity during various operating conditions. In one example, the impeller is supported on a rotor shaft by magnetic bearings. Vibrations detected by the magnetic bearing control systems have been used to detect instability in the fluid caused by stall and surge conditions and then regulate the flow through the impeller by controlling the inlet guide vane position. 
     Variable Geometry Diffusers (VGD) have been suggested for centrifugal refrigerant compressor systems. One typical approach of detecting impeller instability measures the pressure with pressure sensors at either side of the impeller. An undesired pressure differential at a given operating condition indicates impeller instability. The VGD position is then manipulated to regain impeller stability. 
     SUMMARY 
     A centrifugal refrigerant compressor system includes an impeller connected to a shaft. A diffuser is arranged on a downstream side of the impeller and is configured to regulate refrigerant flow exiting the impeller. A shaft assembly is supported by a active magnetic bearing system. The magnetic bearing system equipped with position sensors for its feedback control keeps the shaft in the desired position. Under the conditions of stall or surge, the disturbances from the fluid instability will act on the shaft to cause vibration. Sensing elements from magnetic bearing control system are configured to receive the vibration. A controller is configured to use this information to control the diffuser to gain fluid stability. No additional sensing devices like pressure sensors are needed for the diffuser control. 
     A method of controlling a centrifugal refrigerant compressor includes sensing a shaft condition of a shaft supporting an impeller. Whether an undesired impeller operating condition exists is determined based upon the sensed shaft condition. A diffuser is effectively closed on a downstream side of the impeller in response to an undesired impeller operating condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a highly schematic view of a refrigerant system having a refrigerant compressor with a magnetic bearing. 
         FIG. 2  is a highly schematic view of a shaft-mounted impeller supported by magnetic bearings. 
         FIG. 3  is a schematic view of an example centrifugal refrigerant compressor control system. 
         FIG. 4  is an example method of controlling a centrifugal refrigerant compressor. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a refrigeration system  12  includes a refrigerant compressor  10  for circulating a refrigerant. The refrigerant compressor  10  includes a housing  14  within which an electric motor  16  is arranged. The housing  14  is schematically depicted and may comprise one or more pieces. The electric motor  16  rotationally drives an impeller  18  via a shaft  20  about an axis A to compress the refrigerant. 
     The impeller  18  includes a inlet end  42  and an outlet end  44  in fluid communication with a refrigerant loop  26  that circulates the refrigerant to a load, such as a chiller  28 . In the example illustrated in  FIG. 1 , the compressor contains the impeller  18 , which is centrifugal. Although only one impeller is illustrated, multiple impellers can be used. That is, the refrigerant inlet  22  is arranged axially, and the refrigerant outlet  24  is arranged radially. The refrigerant loop  26  includes a condenser, an evaporator, and an expansion device (not shown). 
     An oil-free bearing arrangement is provided for support of the shaft  20  so that oil-free refrigerant can be used in the refrigerant compressor  10 . In the example, the shaft  20  is rotationally supported relative to the housing  14  by a magnetic bearing assembly  30 . The magnetic bearing assembly  30  may include radial ( 30   R1 ,  30   R2 ) and/or axial ( 30   A ) magnetic bearing elements, for example, as illustrated in  FIG. 2 . Position sensors  66  (in the example, two radial sensors  66 R 1  and  66 R 2 ) are used to sense the shaft position for control feedback system and vibration monitoring. 
     Returning to  FIG. 1 , a controller  32  communicates with the magnetic bearing assembly  30  providing a magnetic bearing command to energize the magnetic bearing assembly  30 . The magnetic bearing assembly creates a magnetic field levitating the shaft  20  and controls its characteristics during operation of the refrigerant compressor  10 . The controller  32  is depicted schematically, and may include multiple controllers that are located remotely from or near to one another. The controller  32  may include hardware and/or software. 
     The electric motor  16  includes a rotor  34  supporting multiple magnets  36  about its circumference in one example of permanent magnet motors. A stator  38  is arranged about the rotor  34  to impart rotational drive to the shaft  20  when energized. In one example, the controller  32  communicates with the stator  38  and provides a variable speed command to rotationally drive the impeller  18  at a variable speed depending upon compressor operating conditions. The controller  32  communicates with multiple sensors (not shown) to monitor and maintain the compressor operating conditions. 
     The impeller  18  includes blades  40  that extend from an inlet end  42  generally radially outwardly along an arcuate path to an outlet end  44 . The housing  14  includes an upstream region  23  at the refrigerant inlet  22 . A diffuser  48  is provided downstream from the outlet end  44  in a passage  46 , upstream from volute  25 , to regulate the flow and pressure across the impeller  18  without the need for or use of inlet guide vanes, for example. Although one type of mechanical variable geometry diffusers is illustrated in the example, it should be understood that the diffuser  48  may be any mechanical diffuser, such as an annular ring diffuser, a pipe diffuser or an adjustable variable stator vane diffuser, of the type disclosed in International Application No. PCT/US10/61754 for example. It should also be understood that the diffuser  48  may be a fluid injector, for example, of the type disclosed in International Application No. PCT/US10/55201, used to effectuate refrigerant flow control by effectively changing the fluid flow through the passage  46 . 
     Referring to  FIG. 2 , an example magnetic bearing configuration is shown for supporting the shaft  20  to which impeller  18  is mounted. In one type of magnetic bearing configuration, a pair of radial bearings  30   R1 ,  30   R2  support either end of the shaft  20 . An axial magnetic bearing  30   A  may be provided adjacent to a thrust feature on the shaft  24  limiting its axial movement. Although the axial bearing  30   A  is illustrated at a terminal end of the shaft  20 , it should be understood that the axial bearing may be located adjacent to a thrust runner and may be integrated with one of the radial bearings, for example. It should also be understood that the shaft  20  may incorporate multiple impellers, for example, an impeller at either end of the shaft  20 . 
     The primary control variable to adjust compressor capacity is the speed of the variable-speed centrifugal compressor. For example, if the chilled water temperature exiting the chiller is lower than its set point value (for example, 4° C. instead of the required set-point value of 5° C.) the controller will reduce the compressor speed to diminish the amount of cooling generated by the chiller which will then bring to chilled water temperature exiting the chiller back to its desired set point value. Under certain chiller operating conditions, further slowing down the speed may drive the compressor to a stall or surge conditions (too low a flow rate for a given pressure ratio) to limit the turn-down capability. In that case, variable geometry diffuser closure as opposed to compressor speed reduction will occur. At incipient surge conditions, the high-frequency rotating stall pressure and flow fluctuations can be seen in bearing orbit signals from position sensors. Using this information, the variable geometry diffuser position can be adjusted to prevent surge or harmful stall. 
     An example compressor control system  60  is illustrated in  FIG. 3 . In the example, the radial bearing  30   R1 , which is located closest to the impeller  18 , is used to detect a shaft condition. The shaft condition, for example, vibration, can be used to determine an undesired impeller operating condition, such as stall or surge. In a stall or surge condition, for example, undesired vibrations are imparted to the magnetic bearings and will be picked up by their sensors that also used for the position control feedback system. 
     Active magnetic bearing system equipped with position sensing capability integrated with the magnetic bearing. In the example illustrated, the radial bearing  30   R1  includes position sensors  66   X ,  66   Y  that respectively detect the position of the shaft  20  relative to the magnetic bearing  30   R1  in the X and Y directions. The shaft position is communicated to the controller  32 , as indicated by the arrows. Similarly, the axial bearing  30   A  includes a position sensor  66   Z  that communicates the position of the shaft  20  relative to the axial bearing  30   A  to the controller  32 . Radial bearing position sensors  66   R1 ,  66   R2  also communicate with the controller  32 . 
     A bearing power source  62  supplies power to the bearings  30   R1 ,  30   A . The undesired impeller operating condition may also manifest itself by an additional amount of current drawn from the bearing power source  62  as the magnetic bearings attempt to stabilize the shaft  20  during vibrations induced by stall and/or surge conditions. Accordingly, the electrical circuit providing power to the magnetic bearings may include current sensors  64   X ,  64   Y ,  64   Z  in communication with the controller  32 , which indicate the amount of current drawn by the magnetic bearings respectively in the X, Y and Z directions. 
     The controller  32  is in communication with the diffuser  48 , in particular, an actuator, which manipulates the diffuser  48  to a desired state to regulate the refrigerant flow exiting the impeller  18 . In the case of a mechanical diffuser, the actuator may be a linear actuator. In the case of an air injection diffuser, the actuator may be a fluid control valve. 
     An example method  70  of controlling the centrifugal refrigerant compressor  10  is illustrated in  FIG. 4 . The method  70  includes detecting an impeller vibration based upon whether an undesired vibration in the shaft  20  exists, as indicated in block  72 . The detection is achieved by at least one of magnetic bearing position sensing or current sensing, as described above. The measured position and/or current is compared to a reference position and/or current, which may be determined empirically for a given compressor. The reference may define a surge or stall line for compressor operating conditions. 
     For compressor systems in which a variable speed motor is used, the compressor is most susceptible to surge and stall when the motor speed is decreased and the diffuser fully opened. Thus, stall or surge detection may be initiated, for example, once a predetermined minimum shaft speed is reached, as indicated in block  74 . In this manner, continuous vibration detection is unnecessary. 
     If desired, a verification of the impeller vibration may be used as a check on the detection step, as indicated by block  78 . For example, if bearing position sensing is used in the detection step, bearing current sensing can be used as a verification as a double check that a undesired shaft condition does indeed exist. 
     The diffuser is commanded to a desired state, for example, by closing the diffuser a predetermined increment, in response to the detected undesired impeller operating condition, as indicated at block  76 . The impeller shaft condition is again checked to verify that the new diffuser state was sufficient to mitigate the undesired impeller operating condition, as indicated at block  80 . If the verification was not successful, then the diffuser is closed an additional predetermined increment. If the verification is successful, then a further reduction in motor speed may be performed at the current diffuser state, as indicated at block  82 . 
     Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.