Abstract:
A system for preventing stall in a centrifugal compressor. The compressor includes an impeller rotatably mounted in a housing and a nozzle base plate fixed to the housing adjacent the impeller. The nozzle base plate cooperates with the housing to define a diffuser gap. The base plate includes a plurality of mechanism support blocks positioned on the backside of the nozzle base plate. A drive ring, mounted to the support blocks, is rotationally moveable with respect to the support blocks and the nozzle base plate between a first position and a second position. Connected to the drive ring is a diffuser ring that moves in response to movement of the drive ring. Diffuser ring moves between a retracted position that is not within the diffuser gap and an extended position extending into the diffuser gap to constrict the gap opening and reduce the flow of fluid through the diffuser gap. The diffuser ring can be positioned at any location between the retracted and extended position to control the amount of fluid flowing through the diffuser gap.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is directed to centrifugal compressors, and more particularly to a system for controlling the flow in the diffuser of a variable capacity turbo compressor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Centrifugal compressors are useful in a variety of devices that require a fluid to be compressed. The devices include, for example, turbines, pumps, and chillers. The compressors operate by passing the fluid over a rotating impeller. The impeller works on the fluid to increase the pressure of the fluid. Because the operation of the impeller creates an adverse pressure gradient in the flow, many compressor designs include a diffuser positioned at the impeller exit to stabilize the fluid flow.  
           [0003]    It is often desirable to vary the amount of fluid flowing through the compressor or the pressure differential created by the compressor. However, when the flow of fluid through the compressor is decreased, and the same pressure differential is maintained across the impeller, the fluid flow through the compressor often becomes unsteady. Some of the fluid stalls within the compressor and pockets of stalled fluid start to rotate with the impeller. These stalled pockets of fluid are problematic in that they create noise, cause vibration, and reduce the efficiency of the compressor. This condition is known as rotating stall or incipient surge. If the fluid flow is further decreased, the fluid flow will become even more unstable, in many cases causing a complete reversal of fluid flow. This phenomenon, known as surge, is characterized by fluid alternately surging backward and forward through the compressor. In addition to creating noise, causing vibration, and lowering compressor efficiency, fluid surge also creates pressure spikes and can damage the compressor.  
           [0004]    A solution to the problems created by stall and surge is to vary the geometry of the diffuser at the exit of the impeller. When operating at a low fluid flow rate, the geometry of the diffuser can be narrowed to decrease the area at the impeller exit. The decreased area will prevent the fluid stalling and ultimately surging back through the impeller. When the fluid flow rate is increased, the geometry of the diffuser can be widened to provide a larger area for the additional flow. The variable geometry diffuser can also be adjusted when the pressure differential created by the compressor is changed. When the pressure differential is increased, the geometry of the diffuser can be narrowed to decrease the area at the impeller exit to prevent fluid stall and surge. Similarly, when the pressure differential is decreased, the geometry of the diffuser can be widened to provide a larger area at the impeller exit.  
           [0005]    Several devices for varying the geometry of the diffuser are disclosed in the prior art. For example, U.S. Pat. No. 5,116,197 to Snell discloses a variable geometry diffuser for a variable capacity compressor. This device, and others like it, include a moveable drive ring that may be selectively adjusted to vary the geometry of the diffuser at the impeller exit. The ring is positioned adjacent to one wall of the diffuser and can be moved out into the flow of fluid to decrease the area of the diffuser to account for a lower fluid flow or an increased pressure differential.  
           [0006]    When the ring is positioned in the fluid flow, the known devices create an opening between the ring and the wall into which fluid exiting the impeller will flow. When attempting to move the ring out of the fluid flow, the fluid must be cleared from between the ring and wall. Displacing this fluid so the ring can be moved requires a significant amount of force, since the fluid acts to oppose the motion of the wall.  
           [0007]    Devices such as set forth in Snell are expensive, as the drive ring pilots on a nozzle base plate. The nozzle base plate includes precision-machined tracks machined into its cylindrical outer surface. The drive ring includes corresponding spherical pockets on its inside diameter. Balls are mounted between the nozzle base plate and the drive ring, sliding in the tracks and pockets, the arrangement converting the rotational movement of the drive ring into axial movement while preventing the drive ring and the nozzle base plate from becoming disconnected. This assembly, however, is expensive to fabricate, as close tolerances must be maintained between the inner diameter of the drive ring and the outer diameter of the nozzle base plate. In addition, the spherical pockets on the drive ring must be matched to the tracks on the nozzle base plate. Furthermore, wear will ultimately result in the replacement of both the drive ring and the nozzle base plate.  
           [0008]    Another approach is set forth in Publication US 2002/0014088A1 to Seki et al. In this approach, the ring which is positioned in the fluid flow is supported by the casing. Three protrusions from the casing are fitted into grooves on the outer peripheral face of the diffuser ring. A bearing may be used with each protrusion to suppress rubbing contact between the casing and the diffuser ring. The diffuser ring is connected to a shaft. Rotation of the shaft causes the diffuser ring via a bracket to rotate in the circumferential direction. The circumferential movement causes the diffuser ring to move axially as the protrusions guide the axial movement of the diffuser ring along the grooves. While effective, the approach is expensive, as the protrusions must be accurately placed in the casing. The threaded shaft and motor for shaft rotation also add expense to this assembly.  
           [0009]    In light of the foregoing, there is a need for a variable geometry diffuser for a variable capacity compressor that may be easily opened and closed during the operation of the compressor. The variable geometry diffuser should be inexpensive to manufacture, easy to assemble, simple to repair or replace and provide positive engagement for accurate position determination in response to signals or commands from the controller.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a system for a variable capacity centrifugal compressor for compressing a fluid. The compressor includes an impeller rotatably mounted in a housing. The system includes a nozzle base plate fixed to the housing adjacent the impeller. The nozzle base plate has an elongated surface that cooperates with an opposed interior surface on the housing to define a diffuser gap or outlet flow path. The base plate includes a plurality of mechanism support blocks mounted to the backside of the nozzle base plate. A drive ring is mounted to the support blocks and is rotationally moveable with respect to the support blocks and the nozzle base plate. The drive ring is selectively moveable between a first position and a second position. Connected to the drive ring is a diffuser ring that moves in response to movement of the drive ring. Diffuser ring moves between a retracted position corresponding to a first position of the drive ring and an extended position corresponding to a second position of the drive ring. In the open or retracted position, the diffuser ring is retracted into a groove so that the face diffuser ring is flush with the face of the nozzle base plate, and the diffuser gap is unobstructed to permit the maximum fluid flow therethrough. In the closed or extended position, the diffuser ring extends outward into the diffuser gap to constrict the gap opening and reduce the flow of fluid through the diffuser gap. The diffuser ring can be positioned at any location between its retracted and extended positions to control the amount of fluid flowing through the diffuser gap.  
           [0011]    The drive ring includes a plurality of cam tracks fabricated into its outer periphery surface, each cam track corresponding in position to a mechanism support block. Assembled to the mechanism support block is a drive pin having a cam follower that is assembled into the cam track. An actuating rod is attached to the drive ring. The actuating rod can move in an axial direction, thereby causing the drive ring to rotate. As the drive ring rotates, the cam followers in the cam tracks cause the drive pins to move in an axial direction. The diffuser ring, connected to the drive ring as a result of being attached to the opposite end of the drive pins, moves with motion of drive pins between its retracted position corresponding to the first position of the drive ring to an extended position corresponding to a second portion of the drive ring. Drive ring, and hence diffuser ring, may be stopped at any intermediate position between a first position (fully retracted) and a second position (fully extended).  
           [0012]    An advantage of the present invention is that the rotational motion of the drive ring can be converted to axial motion by the mechanism of the present invention. This axial motion can be achieved rapidly and effectively in response to appropriate signals from the controller by an axially movable actuating rod.  
           [0013]    Another advantage of the present invention is that the diffuser ring of the present invention can be placed anywhere within the compressor as long as it can be extended into and retracted from the diffuser gap. Because the support blocks carry the load of the diffuser ring, the diffuser ring can assume any position, provided of course, that it can be extended or retracted into the diffuser gap. Thus, unlike prior art devices, the diffuser ring may be placed further downstream in the diffuser, if desired. Since the diffuser ring does not have to be carefully match machined to mate with structures such as the inner diameter of the nozzle base plate and is not supported on the casing, and requires only the extension or retraction of the diffuser ring into the diffuser gap to control the flow of fluid in the diffuser gap, the diffuser ring tolerancing can be loosened thereby reducing its costs.  
           [0014]    Still a further advantage of the present invention is that not only is the diffuser ring less expensive to manufacture and easy to replace, but also the mechanisms for controlling the movement of the diffuser ring are easier and cheaper to replace, as the parts wear.  
           [0015]    Yet another advantage of the present invention is that the mechanism for controlling the diffuser ring includes allowances for over travel, so that the diffuser ring can be quickly moved into the completely extended or retracted position without concerns about excessive wear at these end points.  
           [0016]    Another advantage of the present invention is that the over travel allows the control logic not to be affected by the actual positioning of the diffuser ring. The control logic instead can react solely to noise associated with surge, closing fully the diffuser ring until the condition has abated.  
           [0017]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a cross section view of a prior art centrifugal compressor having a variable geometry diffuser.  
         [0019]    [0019]FIG. 2 is a cross section view of the variable geometry diffuser of the present invention in a centrifugal compressor.  
         [0020]    [0020]FIG. 3 is a cross section view of the variable geometry diffuser of the present invention in a centrifugal compressor in which the diffuser ring of the present invention is in the extended or closed position.  
         [0021]    [0021]FIG. 4 is a cross section view of the variable geometry diffuser of the present invention in a centrifugal compressor in which the diffuser ring of the present invention is in the retracted or open position.  
         [0022]    [0022]FIG. 5 is a perspective view of a drive pin of the present invention.  
         [0023]    [0023]FIG. 6 is a perspective view from above of a diffuser ring of the present invention.  
         [0024]    [0024]FIG. 7 is a perspective view of drive pins assembled to a diffuser ring of the present invention.  
         [0025]    [0025]FIG. 8 is a perspective view of the front of the nozzle base plate.  
         [0026]    [0026]FIG. 9 is the rear of the nozzle base plate, showing support blocks assembled thereto.  
         [0027]    [0027]FIG. 10 is an enlarged view of FIG. 9 depicting a support block assembled to the nozzle base plate.  
         [0028]    [0028]FIG. 11 is an enlarged view of FIG. 9 depicting a drive pin assembled to the support block on the nozzle base plate.  
         [0029]    [0029]FIG. 12 is a side view of FIG. 9 depicting the pin with a cam follower assembled thereto.  
         [0030]    [0030]FIG. 13 is a perspective view of a drive ring of the present invention.  
         [0031]    [0031]FIG. 14 is a perspective view of an assembly comprising the nozzle base plate with support blocks attached thereto and a drive ring assembled thereon.  
         [0032]    [0032]FIG. 15 is a perspective view of the inner circumferential surface of the drive ring assembled to a support block with a radial bearing assembly and an axial bearing assembly installed in the support block.  
         [0033]    [0033]FIG. 16 is a perspective view of an axial bearing assembly.  
         [0034]    [0034]FIG. 17 is an exploded view of a radial bearing assembly.  
         [0035]    [0035]FIG. 18 is a perspective view of an actuator assembled to a drive ring.  
         [0036]    [0036]FIG. 19 is an overhead view of the axial bearing adjustment to drive ring.  
         [0037]    [0037]FIG. 20 is a perspective view of an eccentrically drilled mounting hole  320  in a flanged race  300  of a radial bearing.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    The present invention is a variable geometry diffuser mechanism for a centrifugal compressor. FIG. 1 depicts a prior art variable capacity centrifugal compressor having a different diffuser configuration. The system of the prior art utilizes a movable wall as an annular ring positioned adjacent to the exit of the impeller. The wall is movable into the diffuser space, as is typical, to control the flow of fluid through the diffuser. The annular ring is disposed on the base plate. The ring is connected to an intricate support structure for moving the wall that includes an annular push ring and pins connected to the wall. A drive ring is mounted on the base plate via a ball bearing arrangement. The drive ring pushes annular push ring which in turn moves the wall. The ball bearing arrangement rides in a race in the drive ring and in inclined races in the base plate. The rotational motion of the drive ring by any suitable mechanism thus results in an axial movement of the moveable wall into and out of the diffuser space. A more detailed description of the assembly and operation of this arrangement can be found in U.S. Pat. No. 6,139,262 issued Oct. 31, 2000, assigned to the assignee of the present invention and incorporated herein by reference.  
         [0039]    [0039]FIG. 2 is a cross section view of a centrifugal compressor  100  having the variable geometry diffuser  110  of the present invention. As illustrated in FIG. 2, compressor  100  includes a housing or diffuser plate  120 , an impeller  124 , and a nozzle base plate  126 . A diffuser ring  130 , part of the variable geometry diffuser  110  of the present invention, is assembled into a groove  132  machined into nozzle base plate  126 . Diffuser ring  130  is movable away from groove  132  and into diffuser gap  134  that separates diffuser plate  120  and nozzle base plate  126 . In the completely retracted position, diffuser ring  130  is nested in groove  132  in nozzle base plate  126  and diffuser gap  134  is in a condition of maximum flow. In the completely extended position, diffuser ring  130  extends substantially across diffuser gap  134 , essentially closing diffuser gap  134 . The diffuser ring  130  can be moved to any position intermediate the completely retracted position and the completely extended position.  
         [0040]    The directional flow of fluid into the compressor is controlled by the inlet guide vanes, shown as item  26  in FIG. 1, which can be rotated about their axis in a limited fashion to control the direction and to adjust the flow of fluid through the compressor. The inlet guide vanes  26  are not shown in any of the other Figures, as their location will not vary significantly from one centrifugal compressor to another, being positioned upstream of the impellers, and their location is not critical to the operation of this invention. The rotation of the vanes  26  through the range of rotation changes the capacity of the compressor. The vanes  26  typically include a means for determining their relative position, such as a position sensor, so that the amount of fluid flow through the compressor can be determined and the flow can be adjusted as desired by the actuator.  
         [0041]    After passing the inlet vanes  26 , the fluid typically in the form of a refrigerant or a refrigerant mixed with a lubricant mist flows over impeller  24  (FIG. 1) or  124  (FIG. 2). The rotation of the impeller  124  imparts work to the fluid, thereby increasing its pressure. As is well-known in the art, a fluid of higher pressure exits the impeller and passes through diffuser gap  134  as it ultimately is directed to the compressor exit.  
         [0042]    As the compressor load decreases, the inlet guide vane  26  rotate to decrease the fluid flow exposed to impeller  124 . However, as the same pressure is maintained across impeller  124 , the fluid flow exiting the compressor can be come unsteady and may flow backwards to create the surge condition discussed above. In response to the lower flow, to prevent the surge condition, the diffuser gap  134  is reduced to decrease the area at the impeller exit and stabilize fluid flow. The diffuser gap  134  is controlled by moving diffuser ring  130  into the gap  134  to decrease its area, as shown in FIG. 3 or to increase the area by moving the diffuser ring  130  back into groove  132 , shown in the maximum flow condition in FIG. 4.  
         [0043]    The arrangement and operation of the variable geometry diffuser  110  of the present invention will now be described in detail with further reference to the drawings.  
         [0044]    The variable geometry diffuser  110  of the present invention comprises diffuser ring  130 . Diffuser ring  130  is attached to drive pin  140 . Referring now to FIG. 5, drive pin  140  has a first end  142  and a second end  144  to mate with diffuser ring  130 . At first end  142  of drive pin  140  is a cam follower aperture  146 . At second end  144  of drive pin  140  is a means for attachment of drive pin  140  to diffuser ring  130 . In the preferred embodiment, means for attachment is at least one aperture  148 , which as shown, includes a pair of threaded apertures.  
         [0045]    Diffuser ring  130 , shown in FIG. 6, has a has a first face,  150 , a second opposed face  152 , an inner circumferential wall  154  extending between first face  150  and second face  152  and an outer circumferential wall  156  extending between first face  150  and second face  152 , substantially concentric to inner circumferential wall  154 . Diffuser ring  130  has a predetermined thickness, the thickness determined by the distance between inner circumferential wall  154  and outer circumferential wall  156 , and a predetermined axial length, the axial length determined by the distance between first face  150  and opposed second face  152 . A plurality of apertures  158  extend through the axial length of diffuser ring  130  and form part of the attachment means between the drive pin  140  and diffuser ring  130 . As shown in the preferred embodiment, the plurality of apertures includes three pair of apertures  158 . Each pair of apertures  158  is located on ring  130  to correspond to apertures  148  in drive pin. Second face  152  (not shown in FIG. 5) of diffuser ring  130  is assembled adjacent to face of drive pin  140 . Second face  152  may optionally include counterbores opposite apertures  158  to accept drive pin  140 , if desired. In FIG. 7, a plurality of drive pins  140  are shown assembled to diffuser ring  130 . Threaded fasteners extending through apertures  158  into apertures  148  of drive pin  140  secure the drive pin  140  to diffuser ring. As shown, the means of attachment of the drive pin  140  to diffuser ring  130  includes threaded fasteners extending through apertures  158  into apertures  148 . However, the means of attachment is not so limited, as any known means of mechanical fastening may be utilized. For example, drive pin second end  144  may be threaded and be threadably received by diffuser ring. Alternatively, pin  140  may be secured to ring  130  by, for example, tack welding. The means of securing the pin  140  to the diffuser ring  130  is not critical, as any means of securing these parts together is acceptable.  
         [0046]    [0046]FIG. 8 depicts a perspective view of the front side  160  of nozzle base plate  126 . Groove  132  extends around the circumference of nozzle base plate  126 . A plurality of apertures  162  penetrate nozzle base plate  126  in groove  132 . These apertures accommodate drive pin  140 , to which is attached diffuser ring  130 . In the preferred embodiment as shown in FIG. 8, there are three apertures  162  located about 120° apart. Large central aperture  164  accepts the drive shaft (not shown) of compressor  100  to which is mounted impeller  124 .  
         [0047]    [0047]FIG. 9 depicts the rear side  170  of nozzle base plate  126 . Attached to the rear side  170  of nozzle base plate  126  are a plurality of support blocks  180 . The support blocks  180  may be separate pieces assembled to base plate  170 , which is most useful for retrofit applications. Alternatively, support blocks  180  may be an integral part of nozzle base plate  170 . Most typically, these blocks may be configured into the cast base plate geometry. In the preferred embodiment, depicted in FIG. 9, there are three support blocks  180 . Each support block includes a main aperture  182  that penetrate support blocks  180 . Support blocks  180  are assembled to rear side  170  of base plate  126  so that each main aperture  182  through support block  180  is coaxial with each aperture  162  through nozzle base plate  126 . These coaxial apertures  162 ,  182  each accept a drive pin  140 , as will become more apparent.  
         [0048]    [0048]FIG. 10 is an enlarged perspective view of a support block  180  assembled to base plate  126 . A bushing  184  is assembled into aperture  182 . In a preferred embodiment, this bushing  184  is TEFLON®-coated and press fit into aperture  182 . A drive pin  140  slides into bushing  184  as shown in FIG. 11, an enlarged view of support block  180  assembled to base plate  126  with drive pin  140  assembled therein.  
         [0049]    Referring to FIG. 11 and FIG. 12, drive pin first end  142  extends above support block  182 . As depicted in FIG. 12, drive pin first end  142  has flat surfaces  190  perpendicular to the axis of cam follower aperture  146 . While any geometry may be utilized, this geometry permits ease of assembly of cam follower  200  to drive pin first end  142 . Cam follower  200  is assembled through aperture  146  and secured to drive pin  126  with a nut  202 . Any means, such as a lock pin arrangement, of securing cam follower  200  to drive pin  126  may be used, as long as cam follower  200  is free to rotate. Preferred means include those that can be readily assembled and disassembled.  
         [0050]    [0050]FIG. 13 is a perspective view of drive ring  250 . Drive ring  250  includes an outer circumferential surface  252  and an inner circumferential surface  254 , both extending between its top surface  256  and its bottom surface  258 . The axial length of drive ring  250  is the axial distance between top surface  256  and bottom surface  258 , the axis of the drive ring  250  being an imaginary line extending through and perpendicular to planes extending through the top and bottom surfaces  256 ,  258 , generally the axis being located in the geometric center of drive ring  250 . Located along inner circumferential surface  254  is an inner circumferential groove  260 . Groove  260  is of preselected width to accept an axial bearing, as will be explained below. As shown in FIG. 13, inner circumferential groove  260  extends 360° around the inner circumferential surface  254  for ease of manufacturing. As will become apparent, groove  260  does not have a limitation of extending 360°. Located on outer circumferential surface  252  are a plurality of cam tracks  262 , although only one is shown. These cam tracks  262  are grooves fabricated into the outer circumferential surface  252  at a preselected depth and at a preselected width to receive cam follower  200 . Ideally, each cam track  262  should correspond to and mate with a support block  180 . Thus, in the preferred embodiment as depicted in FIG. 9, which depicts three support blocks  160 , drive ring  250  would have three corresponding cam tracks  262 . Cam tracks  262  comprise the groove that extends along outer circumferential surface at a preselected angle to the axis of the drive ring between top surface  256  and bottom surface  258 . At either end of cam track  262 , the groove includes a circumferential portion  264  that is substantially parallel to the top surface  256  and bottom surface  258  to allow for overtravel. At the end of cam track groove proximate bottom surface  258 , groove includes a portion  268  that extends to bottom surface  258  to provide access for assembly of cam follower  200  into groove. Although portion  268  is shown substantially parallel to the main axis of drive ring  250 , any configuration that assists in assembly may be used. For example, portion  268  may also extend upward into top surface  256  from horizontal. Cam track  262  has two components, one of which is parallel to the axis of drive ring  250  and one that extends circumferentially about drive ring  250  in a direction radial to the axis of drive ring  250 . The distance that cam track  262  extends parallel to the axis of drive ring  250  corresponds substantially to the width of diffuser gap  134 . The angle of the cam shaft groove can be any preselected angle. As the angle becomes shallower, the more precise is the control of drive ring  250  and hence diffuser ring  130 . However, there is a lower limit to this angle, which is dictated by the diameter of drive ring  250  and the number of cam followers in the outer diameter of drive ring  250 . If the angle becomes too large, drive ring  250  can become difficult to position. Preferably the angle of the cam shaft groove is between about 5°-45° to the axis of the drive ring  250 , and most preferably, the angle is in the range of about 7° to about 14°.  
         [0051]    [0051]FIG. 14 is a perspective view of drive ring  250  assembled onto support blocks  180 . The support blocks  180  extend underneath drive ring  250 . Support blocks  180  are assembled to nozzle base plate  126 . Drive pins  140  are assembled into support blocks as shown in FIG. 11, drive pins extending down through nozzle base plate  126 . Cam followers  200 , not visible in FIG. 14 but constructed as shown in FIG. 12, are assembled into cam track  262 . As can be seen in FIG. 14, support blocks  180  extend under bottom surface  258  of drive ring  250 .  
         [0052]    Referring now to FIG. 15, which is a perspective view of one of support blocks  180  extending under drive ring  250 . This view shows inner circumferential surface  254  and inner circumferential groove  260  of drive ring  250 . Assembled to bearing block  180  is an axial bearing assembly  280  and a radial bearing assembly  290 .  
         [0053]    A perspective view of axial bearing assembly  280  is provided in FIG. 16. Axial bearing assembly  280  comprises a support structure  282  for axial bearing  284  and attachment means  286  to secure the support structure  282  to support block  180 . A shaft (not shown) extends through support structure  282 . At one end of the shaft is a bushing  285  which is preferably eccentric. As shown in the preferred embodiment, attachment means  286  is substantially a pair of threaded members that are captured in mating holes in support block  180 . Any other well-known means of securing the support structure  282  to support block  180  may be utilized. Referring back to FIG. 15, axial bearing  284  is installed onto support block  282  by a means for securing  288 . As shown in FIG. 15, means for securing axial bearing  284  to support block  282  is a nut fastened to a threaded end of the shaft extending through support block  282 . Bushing  285  is free to rotate about the opposite end of this shaft. Again, any other arrangement for securing axial bearing  284  in position opposite inner circumferential groove  260  may be used. As depicted in FIG. 15, axial bearing  284  (hidden from view) is assembled into inner circumferential groove  260 . Axial bearing  284  resists axial movement of drive ring  250  as it rotates. In addition to resisting axial movement of drive ring  250 , the axial bearing  284  also allows for small adjustments of the axial location of the drive ring  250 . This adjustment is necessary to account for the variation in the length of the drive pins  140 . The adjustment is possible due to an eccentric bushing  285  on the shaft of axial bearing  284 . Following the assembly of axial bearings  284  into drive ring  250 , drive ring  250  is rotated such that drive pin cam follower  200  is at the end of travel in cam track  262  next to aperture  266 . This aligns axial bearing  284  with aperture  266  adjacent to cam track  262 . In this position, as shown in FIG. 19, a tool such as a hexagon (Allen) wrench can be inserted through aperture  266  into a feature matching the wrench head, here a hex hole to match the wrench hex head located on axial bearing  284 . Axial bearing  284  is rotated clockwise or counterclockwise as necessary to adjust the axial position of drive ring  250  with respect to bushing  285 . Once the position is correct, axial bearing  284  is secured by tightening nut on the opposite end of shaft. The preferred adjustment of drive ring  250  is such that the face of diffuser ring  130  is flush with the face of nozzle base plate  125  when diffuser ring is in the fully retracted position.  
         [0054]    [0054]FIG. 15 also shows radial bearing assembly  290  installed onto support block  180 . FIG. 17 provides an exploded view of radial bearing assembly  290 . Radial bearing assembly  290  comprises a roller  292  and at least one bushing  294  installed in the roller  292 , and preferably two flanged bushings  294 , one on either side of roller  292 . A flanged race  300  is assembled into the at least one bushing  294 . In a preferred embodiment, the pair of flanged bushings  294  comprise two TEFLON®-flanged bushings, one installed into either end of roller  292 . A partially threaded shaft  296  extends through race  300  to secure the assembly to support block  180 . A washer  298  may be added between roller  292  and support block  180 . One of the radial bearing assemblies  290  employs an eccentrically drilled mounting hole  320  in the flanged race  300  as shown in FIG. 20. The eccentric mounting hole allows for adjustment of the radial bearing  290 . This adjustment is necessary to compensate for variations in the inside diameter of drive ring  250 . The preferred adjustment is to have all radial bearings just contacting the inner surface of drive ring  250 . The radial bearing assembly  290  resists radial movement of drive ring  250  as it rotates. Any other suitable radial bearing assembly may be utilized that can resist radial movement of the drive ring  250  as it rotates.  
         [0055]    Operation of the mechanism can now be described by reference to FIGS. 2, 3 and  4  as well as to FIG. 18. FIG. 18 is a perspective view of an actuating means  310  attached to top surface  256  of drive ring  250 . As shown in FIG. 18, actuating means  310  is a mechanical actuator that moves only in an axial direction and is attached to a motor that causes it to move. Although a mechanical actuator is used, any other well-know means for rotating the drive ring  250  may be used, including hydraulic actuators, pneumatic actuators, a screw mechanism attached to the drive ring  250  or other systems that can cause rotation of the ring  250 . The direction and length of its stroke is limited. The axial motion of the actuator causes the drive ring to rotate. The motor is activated in response to a control means such as described in provisional application identified as Attorney Docket 20712-0059 entitled SYSTEM AND METHOD FOR DETECTING ROTATING STALL IN CENTRIFUGAL COMPRESSORS. However, any other control means for an actuator may be used. As the compressor operates in its normal mode with the diffuser ring in its retracted position, as shown in FIG. 4, if the onset of stall or incipient surge is detected by a sensor, a signal is sent to the controller which activates the motor in a direction to cause the diffuser gap  134  to close. The motor moves the actuating means  310  which causes drive ring  250  to rotate. Drive ring  250  is restricted to rotational movement in the plane in which it resides over support blocks  180 . As drive ring  250  rotates, each of cam followers  200  moves from a first position in cam tracks  262  where the cam track grooves are proximate the top surface  256  of drive ring  250  along the tracks toward bottom surface  258  of drive ring  250 . As the drive ring  250  and cam tracks  262  rotate, cam followers  200  are forced downward along the tracks  262 . As the followers move downward, drive pins  140  move into support block  180 . Since diffuser ring  130  is attached to the opposite end of drive pin  140  on the opposite side of nozzle base plate  126 , the movement of drive pin  140  into support block  180  moves the opposite side of drive pin  140  away from nozzle base plate, causing diffuser ring  130  to move into diffuser gap  134 . If cam followers  200  move in cam tracks  262  completely from a position proximate top surface  256  to a position proximate bottom surface  258 , then diffuser gap  134  is in a substantially fully choked or closed position. The horizontal groove portions  264  of cam tracks  262  allow for overtravel of the actuating means  310  and cam followers  200 , so that some additional movement of these elements can be accommodated without further movement of the diffuser ring  130  which could cause damage to any one of or all of the compressor  100 , the drive ring  250 , the actuating means  310  and the actuating means motor.  
         [0056]    Depending upon the control system, the actuating means  310  may stop drive ring  250  rotation at any position intermediate between the fully extended position and fully retracted position of actuating means  310 . It can do this in response to a signal from the control means. This in turn results in the diffuser ring  130  being stopped in any position, such as an intermediate position shown in FIG. 2 between fully retracted, as shown in FIG. 4 to fully extended as shown in FIG. 3. It will remain in this position until a signal from control means causes additional movement of the drive ring  250  which causes a repositioning of diffuser ring  130 .  
         [0057]    In a preferred embodiment, once a signal is sent to the control means indicating the detection of the onset of surge or incipient stall, a command (or series of commands) is activated which causes the drive ring  250  to rotate as described above, thereby causing diffuser ring  130  to move to an extended position (substantially choking the flow of fluid through diffuser gap  134 ) an amount necessary to eliminate the surge or incipient stall or prevent the formation of a surge or stall condition. In one embodiment, a timing function may be activated in the controller which maintains the diffuser ring  130  at the required position. At the end of a preselected time period, the drive ring  250  is rotated in the opposite direction, thereby causing diffuser ring  130  to move to a retracted position until the onset of surge or incipient stall is again detected. Repeating the above process in response to a sensor signal causes a command (or series of commands) to be again activated which causes the drive ring  250  to rotate, thereby causing diffuser ring  130  to move or extend, again choking the flow of fluid through diffuser gap  134  the amount necessary to eliminate the surge or incipient stall condition. This process repeats as long as a surge or incipient stall condition is detected. If no surge or incipient stall condition is detected when diffuser ring  130  is retracting, the diffuser ring  130  will continue to retract to the fully retracted or open position, thereby allowing full flow of refrigerant through diffuser gap  134 . It will remain in this position until the control means activates the command or series of commands in response to a signal indicative of the onset of surge or incipient stall.  
         [0058]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.