Abstract:
An improved variable geometry diffuser (VGD) mechanism for use with a centrifugal compressor. This VGD mechanism extends substantially completely into the diffuser gap so that the VGD mechanism may be used more fully to control other operational functions. The VGD mechanism may be used to minimize compressor backspin and associated transient loads during compressor shut down by preventing a reverse flow of refrigerant gas through the diffuser gap during compressor shutdown, which is prevented because the diffuser gap is substantially blocked by the full extension of the diffuser ring. During start-up, transient surge and stall also can be effectively eliminated as gas flow through the diffuser gap can be impeded as load and impeller speed increase, thereby alleviating the problems caused by startup loads at low speeds. The VGD mechanism can be used for capacity control as well so as to achieve more effective turndown at low loads.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from and the benefit of U.S. Provisional Application No. 61/724,684, entitled VARIABLE GEOMETRY DIFFUSER HAVING EXTENDED TRAVEL, filed Nov. 9, 2012. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to centrifugal compressors, and more particularly to an improved variable geometry diffuser mechanism allowing improved control over the complete operating range of a centrifugal compressor including startup and shutdown. 
       BACKGROUND OF THE INVENTION 
       [0003]    Centrifugal compressors are useful in a variety of devices that require a fluid to be compressed, such as 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, some compressor designs include a variable geometry diffuser positioned at the impeller exit to stabilize the fluid flow during stall events, thereby mitigating stall. Stall results as refrigerant flow decreases while the pressure differential across the impeller is maintained. Stall undesirably creates noise, causes vibration and reduces compressor efficiency. 
         [0004]    Since stall conditions are present only a very small percentage of the time that the compressor operates, the operation of the variable geometry diffuser similarly has been limited, so that wear and tear, loadings and other functions that affect the overall life integrity of a diffuser mechanism has been limited. However, increasing usage of a variable geometry diffuser mechanism would dramatically affect the overall reliability and life of a diffuser mechanism. 
         [0005]    A diffuser design that has been effective is set forth in U.S. Pat. No. 6,872,050 issued on Mar. 29, 2005, to Nenstiel (the &#39;050 Patent). The &#39;050 Patent discloses a variable geometry diffuser that is opened and closed during the operation of the compressor, is inexpensive to manufacture, is easy to assemble, is simple to repair or replace, and provides positive engagement for position determination in response to signals or commands from the controller in response to incipient stall conditions. 
         [0006]    The variable geometry diffuser design of the &#39;050 Patent utilizes a diffuser ring movable between a first retracted position in which flow through a diffuser gap is unobstructed and a second extended position in which the diffuser ring extends into the diffuser gap to alter the fluid flow through the diffuser gap in response to detection of stall. This is accomplished by extending the diffuser ring substantially across the diffuser gap to alter fluid flow. This mitigation can be accomplished by extending the diffuser ring across about 75% of the diffuser gap. The diffuser ring is driven by a drive ring movable from a first position corresponding to the first retracted position of the diffuser ring, a second position corresponding to the second extended position of the diffuser ring, and any intermediate position between the first position and the second position. The second position is an extended position that stabilizes the system at about 75% of the diffuser gap so that stall is mitigated. The drive ring in turn is mounted to support blocks, and the drive ring is rotationally movable with respect to the support blocks, which are mounted to the backside of a nozzle base plate. The nozzle base plate is fixed to the housing adjacent the impeller of the centrifugal compressor. While the variable geometry diffuser design is effective during compressor operation in altering flow through the diffuser gap when the diffuser ring is in its second extended position, the diffuser ring does not sufficiently block flow during compressor shutdown to retard compressor backspin and associated transient loads or to avoid transient surge and stall during start-up as the compressor ramps up from low loads and low speeds to high speed. 
         [0007]    Use of the variable geometry diffuser generates a load on the diffuser ring due to a pressure differential on the overall ring area. When the ring is in its retracted position, the compressed refrigerant passes over the ring surface and very little load is encountered. However, as the ring moves to its extended position into the diffuser gap, high velocity gas passes over the face of the diffuser ring creating a low pressure area. Higher pressure gas in the groove of the nozzle base plate exerts a force on the back side of the ring. The load on the ring, and the rest of the variable geometry diffuser mechanism, can be calculated. It is the difference in gas pressure on either side of the ring multiplied by the area of the ring. The variable geometry diffuser of the present invention includes a relatively large diffuser ring, the operation of which must overcome substantial forces and which must withstand substantial forces in operation. Thus, the mechanisms are substantial and the energy required to operate these mechanisms to overcome these forces is also substantial. However, because the variable geometry diffuser is engaged for only a small percentage of the overall life of the compressor, the loads and wear and tear experienced by the variable geometry diffuser have been acceptable. 
         [0008]    There is a desire to increase the usage of the variable geometry diffuser ring so that it can be used for more than as just a stall mitigation device. The variable geometry diffuser may be used for not only stall mitigation, but also for capacity control, surge control, improved turndown, minimization of compressor backspin and associated transient loads during compressor shut down as well as for minimization of start-up transients. Due to the increased usage of such a variable geometry diffuser, an improved device is required to provide desirable control enhancements to overall centrifugal compressor operation, while providing longevity to the variable geometry diffuser experiencing increased usage. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a variable geometry diffuser (VGD) mechanism. The VGD mechanism includes a diffuser ring extending into a diffuser gap that mitigates stall, as expected of a VGD mechanism. However, the VGD mechanism of the present invention extends further into the diffuser gap than prior art VGD mechanisms so that the VGD mechanism of the present invention may be used to control other operational functions. Thus the VGD mechanism may be used to minimize compressor backspin and associated transient loads during compressor shut down by preventing a reverse flow of refrigerant gas through the diffuser gap during compressor shutdown. The reverse flow of refrigerant gas is prevented because the diffuser gap is substantially blocked by the full extension of the diffuser ring. The VGD mechanism further provides for better and more efficient compressor turn-down, reducing the need for significant hot gas bypass during low cooling capacity operation. During start-up, transient surge and stall also can be effectively eliminated as the variable geometry diffuser ring can be positioned to impede gas flow through the diffuser gap as load and impeller speed increase, thereby alleviating the problems caused by startup loads at low speeds. The VGD mechanism of the present invention can be used for capacity control as well, so as to achieve more effective turndown at low loads. 
         [0010]    While the diffuser ring extends across the diffuser gap to accommodate reduced gas flow through the diffuser gap during normal operation under certain conditions, the diffuser ring must extend substantially completely across the diffuser gap during shut-down and start-up since the gas flow is significantly reduced as the impeller ramps up to speed during start-up or decreases its speed during shut-down. The outer edge of the diffuser ring comprises a flange that, when fully extended across the diffuser gap, substantially impedes gas flow through the diffuser gap. The axial force on the diffuser ring is a function of the pressure differential on either side of the ring and the area of the ring. When the diffuser ring is extended into the diffuser gap, high velocity gas passes over the outer face of the ring creating a low pressure area. Higher pressure gas on a first side of the ring provides a force on the first side of the ring. The overall axial force on the ring is the difference in gas pressure between the first side of the ring and the second, opposite side of the ring multiplied by the radial face area of the ring. The axial force on the ring may be minimized by reducing the area of the ring. By reducing the radial width of the ring extending into the diffuser gap, the axial force on the ring is reduced proportionally to the width of the ring. While the width (thickness) of the ring may be reduced to lower the load, the ring must be sufficiently thick to accommodate the increased radial forces from flow past the ring or it will not act to block gas flow effectively and may be subject to operational failures. The thickness of the ring will vary among compressors depending upon the capacity of the compressor, the thickness of the ring being relative, that relation depending on several factors, the most important being the net radial flow forces acting on the first, inner cylindrical surface and second, outer cylindrical surface of the diffuser ring, particularly as the impeller slows from operational speed during shut-down or ramp-up to operational speed during start-up. Larger compressors with larger impellers will generate higher flow forces and experience higher loads, requiring thicker rings. But, regardless of compressor size, reducing the axial forces on the ring reduces the forces necessary to operate the VGD mechanism. 
         [0011]    The resulting axial load on the ring ultimately is transmitted to an actuator mechanism. The actuator mechanism of the present invention includes improvements that allow it to be operated in an oil free environment, although its operation is not so restricted. The actuator mechanism also is modified so that the position of the diffuser ring with respect to the opposed interior face of the housing can be monitored and adjusted by a controller as needed. The associated cam track mechanism also has been modified so that the position of the ring in the diffuser gap can be determined at any time. 
         [0012]    Not only must the ring be sufficiently thick to handle the radial loads over the life of the compressor, the ring must also interface with the opposed housing to provide a gap that is uniform around its circumference and must effectively mate with an interior face of the housing that also must be dimensioned to be uniform. If the gap is not substantially uniform, that is, outside of allowable tolerances, pressurized gas will leak through the gap at locations where the gap is larger than allowable, defeating the purpose of the closed diffuser ring without reducing the problems related to capacity control, surge, that occurs during shutdown and start-up, and other operational improvements associated with the improved VGD mechanism. Whereas elimination of such leakage around the diffuser ring during shut down and start-up was not an imperative with prior art designs, to be effective, both the diffuser ring and the opposed interior face of the housing of the present invention must have carefully controlled mating surfaces so that proper operation of the VGD mechanism can be accomplished over a range of conditions. 
         [0013]    Thus, in the present invention, in order to affect control of gas flow through the diffuser gap, physical changes extending the travel of the diffuser ring into the diffuser gap are required for the VGD mechanism. In addition to extending the length of the diffuser ring into the diffuser gap to allow substantially full closure of diffuser gap, the radial area of the diffuser ring is reduced to reduce the axial forces on the ring in response to the pressure forces. Also, by inclusion of sensors, a controller can now monitor the position of the diffuser ring accurately and direct the actuator mechanism to accurately move the diffuser ring between positions that are fully open and fully closed in response to compressor operating conditions. Faster-acting mechanisms can be used to achieve better control of the ring position and respond to chiller system transients such as startup with pressure differential across the compressor or power failure shutdowns. 
         [0014]    An additional benefit of the improved variable geometry diffuser of this invention is the elimination of the need for pre-rotation vanes for capacity control and startup management. Pre-rotation vanes and their mechanisms are complex, expensive, and require their own drive mechanisms and controls. 
         [0015]    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 
         [0016]      FIG. 1  is a cross-sectional view of a prior art variable geometry diffuser in a centrifugal compressor utilizing a movable diffuser ring. 
           [0017]      FIG. 2  provides a perspective view of a prior art diffuser ring. 
           [0018]      FIG. 3  is cross sectional view of the variable geometry diffuser of the present invention. 
           [0019]      FIG. 4  is a top view of the diffuser ring of the present invention. 
           [0020]      FIG. 5  is a cross sectional view showing load distributions on the diffuser ring of the present invention. 
           [0021]      FIG. 6  generally depicts the drive ring operation of a variable geometry diffuser. 
           [0022]      FIG. 7  depicts the arrangement of the linear actuator to the drive ring of the present invention. 
           [0023]      FIG. 8  depicts the cam track in the circumference of the drive ring of the present invention. 
           [0024]      FIG. 9  depicts the cam track in the circumference of the prior art drive ring. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The present invention sets forth an improved VGD mechanism for a centrifugal compressor.  FIG. 1  depicts generally, in cross-section, a prior art variable capacity centrifugal compressor  100  utilizing a VGD mechanism having a movable diffuser ring  130  to control the flow of fluid through a diffuser gap  134  such as disclosed in U.S. Pat. No. 6,872,050, assigned to the assignee of the present invention and incorporated herein in its entirety by reference.  FIG. 1  generally represents current state-of-the-art variable capacity centrifugal compressors. 
         [0026]    As illustrated in  FIG. 1 , compressor  100  includes diffuser plate  120  which, as shown, is integral with the compressor housing, an impeller  122 , and a nozzle base plate  126 . A diffuser ring  130 , part of the variable geometry diffuser  110 , is assembled into a groove  132  machined into nozzle base plate  126  and mounted onto a drive pin  140 . Also shown in the  FIG. 1  cross section is a cam follower  200  that is inserted into cam track  262  which is located in drive ring  250 . Cam follower  200  is connected to drive pin  140 . These mechanisms, as is fully discussed in the &#39;050 patent, transform rotational movement of drive ring  250  into axial movement of diffuser ring  130 . Inner circumferential groove  260  supports an axial bearing (not shown), which resists axial movement of drive ring  250  as it rotates. 
         [0027]    Diffuser ring  130  is movable away from groove  132  and into diffuser gap  134  that separates diffuser plate  120  and nozzle base plate  126 . Refrigerant passes through diffuser gap  134 , which is intermediate between impeller  122  and volute (not shown) that receives refrigerant exiting diffuser  110 . Refrigerant may pass through the volute to an additional stage of compression or to a condenser (also not shown). In the completely retracted position, diffuser ring  130  is nested in groove  132  in nozzle base plate  126  and a diffuser gap  134  is in a condition to allow maximum refrigerant flow. In the completely extended position, diffuser ring  130  extends across diffuser gap  134 , reducing clearance for refrigerant to pass through diffuser gap  134 . Diffuser ring  130  can be moved to any position intermediate the retracted position and the extended position. 
         [0028]    The rotation of impeller  122  imparts work to the fluid, typically a refrigerant, entering at the impeller inlet  124 , thereby increasing its pressure. As is well-known in the art, refrigerant of higher velocity exits the impeller and passes through diffuser gap  134  as it is directed to a volute and ultimately to the compressor exit. Diffuser  110 , comprising diffuser plate  120 , nozzle base plate  126  and diffuser gap  134  formed between diffuser plate  120  and nozzle base plate  126 , as well as diffuser ring  130  used to adjust diffuser gap  134 , reduces the velocity of the refrigerant from impeller  122 , thereby increasing the pressure of the refrigerant at the diffuser exit. 
         [0029]    If the compressor flow rate decreases to accommodate, for example, a reduction in cooling demand for a chiller, and the same pressure is maintained across impeller  122 , the fluid flow exiting impeller  122  can become unsteady and may flow alternately backward and forward to create the stall and/or surge condition discussed above. In response to a lower refrigerant flow, to prevent a surge condition from developing, the diffuser gap  134  is reduced to decrease the area at the impeller exit and stabilize fluid flow. The diffuser gap  134  can be changed by moving diffuser ring  130  into gap  134  to either decrease the cross-sectional area of gap  134  or increase the cross-sectional area of gap  134  by moving the diffuser ring within groove  132 . However, because of the mechanism used to drive diffuser ring  130 , the exact position of diffuser ring in gap  134  is not known except at the extreme positions of the diffuser ring, that is, when fully extended or fully retracted. Furthermore, because the geometry of both the diffuser ring and the diffuser plate have not been carefully controlled in the invention of the &#39;050 patent, even when the diffuser ring  130  is fully extended, a gap permitting leakage past the diffuser ring may still exist. The prior art diffuser ring  130  is set forth in  FIGS. 6 and 7  of the &#39;050 Patent,  FIG. 6  of the &#39;050 Patent being reproduced herein as  FIG. 2 . The features are fully described in the &#39;050 Patent, wherein  150  is a first face of diffuser ring  130 ,  152  is a second opposed face of diffuser ring  130 ,  154  is an inner circumferential wall of diffuser ring  130 ,  156  is an outer circumferential wall of diffuser ring  130 , and  158  are apertures used to assemble the diffuser ring to mating parts to facilitate its movement. However, since the VGD mechanism of the &#39;050 Patent is utilized for control of stall based on related noise and vibration, the configuration is acceptable for its intended purpose, but its use for other functions is restricted. 
         [0030]    The improved variable geometry diffuser (VGD) mechanism of the present invention will now be described in detail with further reference to the drawings. The VGD mechanism of the present invention performs functions in addition to controlling rotating stall and thus requires a different configuration as well as a different control mechanism. 
         [0031]    The VGD mechanism  810  of the present invention is set forth in  FIG. 3 . It has many similarities to the previous VGD mechanism; however, it also has significant differences, which differences may affect operation of the compressor. Diffuser ring  830  of the present invention has a different cross-sectional profile than prior art diffuser ring  130 . Diffuser ring  130  is shown in perspective view in  FIG. 2  and has a rectangular cross-section. By contrast, diffuser ring  830  of the present invention has an L-shaped cross-section as shown in the cross-section of  FIG. 3  and in  FIG. 4 . Diffuser ring  830  includes a pair of substantially orthogonal flanges, a first flange  833  extendable into diffuser gap  134  and a second flange  835  substantially perpendicular to the first flange, the second flange  835  extending substantially parallel to the diffuser gap and the direction of gas flow. By substantially orthogonal flanges is meant flanges that extend within a range that includes 90°±15° to each other where orthogonal flanges extend 90° to each other. The second flange extending substantially parallel to the diffuser gap and the direction of the gas flow means that the orthogonal flanges extend within a range that includes 0°±15°, where 0° is parallel. When diffuser ring  830  is assembled into the compressor as an element of VGD mechanism  810 , first flange  833  extends toward an opposed face of diffuser plate  120 . It will be noted that first flange  833  provides diffuser ring  830  with the ability to extend further into diffuser gap  134  than prior art diffuser ring  130 , as flange  833  provides an extended dimension in the axial direction, that is, into diffuser gap  134 . The axial force on diffuser ring  830  is the result of the pressure differential across first flange  833 . When diffuser ring  830  is fully retracted, the axial force is at its minimum since no pressure differential exists. However, when first flange  833  is extended into diffuser gap  134 , high velocity gas passes over the face of first flange  833  of the ring creating a low pressure area. Higher pressure gas in the groove of nozzle base plate  126  applies a pressure to second flange  835 . The force on ring  830  and on the mechanism that causes the ring to move into and out of diffuser gap  134  is the difference in gas pressure multiplied by the face area of diffuser flange  833 , as previously discussed. 
         [0032]    The axial force on ring  830  is reduced by reducing the overall radial thickness of first flange  833 , which is the portion of diffuser ring  830  that extends into diffuser gap  134  when first flange  833  is extended, the radial thickness of first flange being perpendicular to the direction of gas flow in diffuser gap  134 . Referring to  FIG. 3  and diffuser ring  830 , the area of first flange  833  that protrudes into diffuser gap  134  is reduced as compared to the design of prior art diffuser ring  130 . The radial thickness of first flange  833  has been reduced by about  2 / 3 , thereby reducing the load on diffuser ring proportionally, that is, by about ⅔, since load is proportional to the face area of first flange  833  within diffuser gap  134 . 
         [0033]    The reduction of the radial thickness of first flange  833  reduces available space to attach the actuating means that moves diffuser ring  830  from its retracted position to its extended position. Second flange  835  is provided to allow such attachment as shown in  FIG. 3 . Second flange  835  resides in groove  837  in nozzle base plate, second flange  835  moving in groove  837  allowing diffuser ring flange  833  to move into and out of diffuser gap  134 . Groove  837  in nozzle base plate  126  is also required to permit assembly of diffuser ring  830  to the VGD mechanism. A large radial gap around second flange  835  allows high pressure gas which enters groove  837  to equalize on either side of the second flange  835 , thereby not contributing to the load associated with the gas pressure on diffuser ring  830 . Thus, the overall pressure loading on the diffuser ring  830  is the pressure of the refrigerant acting on the area of the exposed portion of first flange  833  when extending into diffuser gap  134 . A removable cover plate  839  is assembled to nozzle base plate  126  and is provided to facilitate assembly of the diffuser ring drive mechanism. Cover plate  839  provides a smooth, aerodynamic surface for flow of refrigerant gas as it flows to the compressor discharge, reducing the likelihood of turbulence in this area. 
         [0034]    In forming flange  833 , care must be taken to provide flange  833  with a preselected radial thickness. As depicted in  FIG. 5 , which shows a cross-section of diffuser ring  830  assembled to nozzle base plate  126 , high pressure refrigerant impacts first flange  833  when diffuser ring  830  is extended into diffuser gap  134 , as indicated by refrigerant flow  863 .  FIG. 5  indicates a radial pressure force on first flange  833 . Another factor to be considered in determining the radial thickness of flange  833  is the fatigue life of diffuser ring  830  which is exposed to sizable pressure fluctuations. In addition, in the present invention, diffuser ring  830  must extend as closely as possible to diffuser plate  120  in order for the VGD mechanism to increase its capabilities for capacity control, improved turn down, surge control and minimization of compressor transient loads at start up and shut down. In order to reduce the gap as much as possible, diffuser plate  120  has carefully controlled dimensions and flange  833  must have carefully controlled tolerancing in terms of flatness of the face of flange  833  as well as the face of mating diffuser plate  120 . If flange  833  is too thin, it may not be possible to maintain these geometric features within the desired tolerances, as mechanisms such as spring-back may occur which can adversely affect tolerances. Deviations from tolerances will increase leakage around flange and through the diffuser gap, and prevent the VGD mechanism from being used effectively for capacity control, turn down, transient control during start up and turn down and surge, even though the VGD mechanism may retain its ability for use in stall mitigation. As can be seen, diffuser ring  830 , and in particular diffuser ring flange  833  ideally must have a flange thickness as small as possible to minimize the forces acting on it, but must have sufficient thickness to avoid spring back during fabrication and satisfy fatigue during operation while resisting the forces of gas pressure applied to it. 
         [0035]    It is an important aspect to operation of this movable diffuser ring to maintain the geometric tolerances so as to minimize leakage around diffuser ring  830  and through diffuser gap  134  when diffuser ring  830  is fully extended. Compressors having higher refrigeration capacities may require additional increases to the flange thickness to accommodate higher pressure forces over wider diffuser widths to satisfy the competing design requirements cited above. 
         [0036]    Other considerations also affect the overall design of the variable geometry diffuser mechanism of the present invention. Recent compressor designs utilize electromagnetic bearings rather than mechanical bearings commonly used in previous designs. Compressors utilizing electromagnetic bearings eschew the use of oil. However, some of the oil in compressors utilizing mechanical bearings assists in lubricating the actuator mechanism used to move diffuser ring  130  in prior art designs from a retracted position to an extended position in diffuser gap  134 . 
         [0037]    The variable geometry diffuser  810  of the present invention also utilizes an improved mechanism design that is operable in either a conventional centrifugal compressor that employs mechanical bearings with standard lubrication, or with centrifugal compressors utilizing electromagnetic bearings in a substantially lubrication-free environment. Generally, the mechanism that moves diffuser ring  830  is depicted in  FIG. 6  and includes a drive pin  140  that travels in cam track  862 . Drive pin  140  connects second flange  835  to drive ring  850  so that the rotational movement of drive ring  850  results in the translational motion of diffuser ring  830  from a reversible retracted position to a reversible extended position within diffuser gap  134 . Drive ring  850  corresponds to drive ring  250  in  FIG. 1 . The arrangement of drive pin  140  to cam follower  200  in the variable geometry diffuser  810  of the present invention is also identical to the arrangement of prior art geometry diffuser  110 , shown in  FIG. 1 . Cam follower  200  attached to drive pin  140  follows cam track  862  in drive ring  850  as drive pin  140  moves within cam track  862 . Drive ring  850  of the present invention is identical to drive ring  250  of  FIG. 1  except for important differences in cam track geometry  262  of drive ring  250 , best shown in  FIG. 9  and cam track geometry  862  of drive ring  850 , shown in  FIGS. 6 and 8 . The attachment of drive ring  850  to diffuser ring  830  is identical to the attachment of drive ring  250  to diffuser ring  230 , except for the points of connection of drive pin  140  to the respective diffuser rings  130  and  830 . Diffuser ring  830  of the present invention has a flange shaped configuration and drive pin  140  connects to second flange  835  of diffuser ring  830 . Of course, second flange  830  is not present in diffuser ring  130  as it is a simple cylindrical ring, as shown in cross-section in  FIG. 1 . 
         [0038]    Referring now to  FIG. 7 , an actuator  811  of the present invention operates in conjunction with a controller, so that its operation may be programmed. Actuator  811  is a linear actuator and includes a drive rod  896  attached to a drive motor  898 . Drive rod  896  is directly attached to the operating lever  901  attached to drive ring  850 . Linear movement of drive rod  896  in turn rotates drive ring  850 . 
         [0039]    Referring now to  FIG. 8 , cam tracks  862 , located on the outer circumferential surface  252  of drive ring  850 , have a preselected width and depth to accept cam follower  200 . Generally, there are three cam tracks  862  located in circumferential surface  252  of drive ring  850 , although only one is shown in  FIG. 8 . Cam tracks  862  extend from a bottom surface  258  of drive ring  250  toward a top surface  256  of drive ring  850 , extending at an angle between these surfaces, and preferably in a substantially straight line. The shape of cam track  862  is now a ramp having a substantially preselected linear slope, as distinguished from the prior art cam tracks  262  shown in  FIG. 9  having flats  267  and  269  at each end of the ramp. The flats in prior art cam tracks  262  account for inaccurate positioning and travel capabilities of the original damper motor and to accommodate adjustment of the mechanism at the fully retracted position. The flats prevent damage to the mechanism at as the flats eliminate the possibility of jamming at either extreme of travel, and the inaccurate positioning was not a factor in the operation and capabilities of prior art cam tracks. 
         [0040]    By contrast, actuator  811 , in one embodiment a linear actuator, operating in conjunction with the linear cam tracks  862  to control drive ring  850 , which in turn positions diffuser ring  830  in diffuser gap  134 , provides faster action, variable speed, positional accuracy and precise feedback of the position of the location of first flange  833  in diffuser gap  134 . The system of the present invention allows for ready calibration of diffuser ring  830  with respect to diffuser gap  134  at the extremes of diffuser ring  830 , allowing diffuser ring  830  to be used for more than merely stall mitigation. Of course, the simplification of the connections between the levers and linkages of the actuator and the operating lever  901  attached to drive ring  250  provides further advantages. 
         [0041]    During initial set up of VGD mechanism  810  of the present invention, or whenever a follow-up calibration is desired, the actuator simply operates to rotate drive ring  250 , moving cam follower  200  from one end of travel in cam track  862  toward the opposite end of travel in cam track  862 . Any actuator or motor that can accomplish this task may be used, although a device that moves cam follower  200  quickly in cam track  862  is preferred. While a rotary actuator is one variation that may be used, a linear actuator is preferred. The ends of travel at either end of cam track  862  correspond to the fully extended position of first flange  833  and fully retracted position of first flange  833 . The maximum dimension of diffuser gap  134  at first flange  833 , which is the distance between diffuser plate  120  to the outer surface of cover plate  839 , is a known distance that can be determined or measured based on manufacturing and assembly. Programming functions of a controller include the ability to store and save the extreme positions of diffuser ring  830 , the maximum dimension of diffuser gap  134  at first flange  833  and specifically first flange  833  with respect to diffuser plate  120 , cover plate  839  and actuator  811  so that not only the extreme positions are known, but also the opening of diffuser gap  134  at any time (based on the position of first flange  833 ) so that the opening at diffuser gap  134  can be adjusted quickly based on changing operating conditions of compressor  100 . The position of diffuser ring  830  at the extremes of travel can be calibrated, and the position of diffuser ring anywhere within these extremes can be determined without the use of additional sensors. A signal from the actuator may be used as part of the calibration procedure as well as to determine the position of diffuser ring  830  after calibration. Furthermore, if a question as the accuracy of the position of diffuser ring  830  should arise in the course of operation, recalibration can be accomplished as desired. The programming functions allow actuator  811  to operate and move diffuser ring  830  in a normal mode, the movements based on normal transients of compressor  100 . However, actuator  811  also may operate in a rapid mode, which permits diffuser ring  830  to move to a fully extended position in which diffuser gap  134  is fully restricted as required if impending surge or stall is detected. As used herein, a fully restricted diffuser gap  134  is one in which diffuser ring  830  is fully extended so that the opening of diffuser gap  134  is at a minimum. While the design of VGD mechanism  810  does not provide a 100% gas seal when diffuser ring  830  is in the fully extended position, it does provide a substantial improvement over the prior art VGD mechanisms that provided only about a 75% reduction in diffuser gap  134  when diffuser ring  130  was in the fully extended position. The improvement of the present invention allows for leakage to be minimized to such an extent that it no longer impacts chiller control of turndown or start up and shut down surge. Thus, a fully restricted diffuser gap  134  and/or a fully extended diffuser ring  130  functionally is one that does not impact chiller control of turndown or start up and shut down surge. 
         [0042]    The ability to rapidly position diffuser ring  830  by actuator  811  also allows for capacity control of the centrifugal compressor during normal operation. In addition, the ability to control the positioning of diffuser ring  830  so that the flow of refrigerant through diffuser gap  134  is limited permits for greater chiller turndown before the use of a hot refrigerant gas bypass is needed. Chiller turndown is defined as the minimum capacity that can be achieved by the compressor while still allowing for continuous operation without having to shut the compressor down. This is advantageous because hot gas bypass, or other similar means, is a highly inefficient means for achieving low compressor capacity because it requires artificially loading the compressor with refrigerant flow. 
         [0043]    The rapid positioning of diffuser ring  830  by actuator  811  also allows for swift control of gas flow through diffuser gap  134  during shut down. The refrigeration cycle of a chiller requires mechanical work (compressor/motor) to create a refrigerant pressure rise and move refrigerant from evaporative conditions to condensing conditions. During normal “soft” shut downs, the compressor speed is reduced in a controlled manner to allow equalization of the pressure in evaporator and condenser shells, thereby eliminating large transient or upset conditions during shut downs. However, when the system requires for an immediate shut down, such as due to loss of electrical power to the motor (power interruption, faults, safeties, etc.), there are no means to maintain the high pressure in the condenser shell. The only mechanism for the system pressures to balance is through a back flow of refrigerant from the high pressure condenser to the low pressure evaporator through the compressor. With no electrical power to the compressor, the impeller undesirably behaves as a turbine with an energy transfer from the high pressure fluid in the condenser to the compressor as the refrigerant pressure equalizes, flowing to the low pressure (evaporator) side, spinning the compressor impeller backwards (opposite of design intent). In circumstances of loss of electrical power, battery backup to power actuator  811  may be provided to assure that VGD remains operational at shutdown. In addition, bearing loads can be at their highest levels during shutdown, if backspin, stall or surge occurs. The fast-acting closure of diffuser gap  134  by VGD mechanism  810  avoids bearing stability issues at shutdown. It also relieves a portion of these higher loads so lower load bearings can be used, which also translates into a cost savings because such bearings are less expensive. Closing diffuser gap  134  creates a resistance to back flow of refrigerant through compressor  100 . 
         [0044]    The rapid positioning of diffuser ring  830  by actuator  811  also allows for rapid control of gas flow through diffuser gap  134  during start up. During start up, there may already be a substantial load on the compressor if water pumps are already running with cold water flowing through the evaporator and warm water flowing through the condenser. In this case, a compressor can pass through stall and surge until it achieves sufficient speed to overcome the system pressure differences. Starting with a closed VGD can avoid transient surge under these conditions. Thus, prior to start-up, a controller may automatically instruct actuator  811  to move diffuser ring  830  to a fully extended position, closing diffuser gap  134 . The controller may then instruct actuator  811  to retract diffuser ring  830 , in accordance with a preprogrammed algorithm if desired, from its fully extended position based on a sensed condition, such as sensed pressure or compressor speed. 
         [0045]    Much of the assembly of the variable geometry diffuser may remain unchanged from the previous design. However, in the present invention, the design is modified so that a precise position of diffuser ring  830  with respect to diffuser plate  120  is known at any time during normal compressor operation, allowing the precise opening of diffuser gap  134  to be known at any time. This is accomplished with a mechanism that does not require or utilize additional process lubrication. VGD mechanism  810  of the present invention, unlike prior art VGD mechanisms, preferably may be used in oil-free compressors such as those utilizing electromagnetic bearings. However, it also may be used in compressors that utilize oil-lubricated bearings. 
         [0046]    The ability to precisely position diffuser ring  830  allows fine adjustments to be made to diffuser gap  134  during compressor operation based on compressor demand and/or output (i.e., chiller cooling load and pressure difference between the condenser and evaporator), and these fine adjustments can be programmed into the controller during a calibration procedure and stored in the controller. For example, as temperature changes in a conditioned space, diffuser gap  134  can be modified to correspond to the cooling demand on the chiller, the temperature changes corresponding to compressor demand. The demand on the compressor can be compared to actual compressor output. Thus, if demand is increased slightly, such as to cool the space slightly or to maintain the space at a temperature (as outside temperature increases) and if demand requires a slight increase in compressor output, diffuser gap  134  can be increased slightly. If demand is increased dramatically, such as by a demand to lower temperature in the space significantly, and there is a corresponding large increase required in compressor output, diffuser gap  134  can be fully opened to accommodate increased refrigerant flow. The position of diffuser ring  830 , and hence the opening of diffuser gap  134  can be calibrated and the calibration results can be stored in the controller. Thus, when the compressor demand is 100%, diffuser gap  134  can be fully open as diffuser ring  830  is fully retracted. A fully retracted diffuser ring  830  occurs as diffuser ring flange  833  is fully retracted within groove  832 . A fully extended diffuser ring  830  occurs as diffuser flange  833  is fully extended into diffuser gap  134 , such as at compressor shut-down. These two conditions represent the extremes of compressor operation. 
         [0047]    As noted, the controller can be programmed using the position of diffuser ring  830  at these extreme positions and a signal from the actuator that determines the position of diffuser ring  830  between these extreme positions. In addition, operating conditions can be correlated to the position of diffuser ring. Thus, the controller can be programmed to “learn” the position of diffuser ring  830  at, for example, a water temperature leaving the evaporator (cooling load). Other normally monitored and sensed conditions of the system can also be correlated to the position of diffuser ring  830 , and the actuator. In addition, stall and surge preferably can be sensed using acoustic sensors, although sensing surge and stall is not limited to use of such acoustic sensors and other methods may be utilized for determining when surge and stall may be imminent. Of course, in the present invention, since the controller can determine the position of diffuser ring  830  at any time, this position can be used by the controller to move diffuser ring  830  based on refrigerant flow behavior, compressor efficiency and detection of surge or stall, the effect on any of these conditions not being linearly related to the position of diffuser ring  830 . 
         [0048]    For example, on start up, when compressor demand is throttled to 10%, diffuser gap  134  can be opened by moving diffuser ring  830  from the fully extended (closed) position to a first predetermined position. It should be noted that the movement of diffuser ring  830  will not always be the same for a 10% change in compressor demand, due to the nonlinear effect of diffuser ring movement. Movement also depends on the initial and final positions of diffuser ring  830 . Similarly, when compressor demand is required at 50% (an increase of 40% from the 10% demand above), diffuser gap  134  can be further opened by positioning diffuser ring  830  from the first predetermined position to a second predetermined position. In this way, an entire range of values can be stored in the controller, as required, to provide efficient operation of the compressor, and these values can be recalled (or further estimated) as compressor duty changes, and diffuser ring  830  can be repositioned quickly by the controller to achieve steady state operating conditions. 
         [0049]    Once the occurrence of a detrimental event is detected, such as surge or stall detected by acoustic sensors, or loss of electric power to the system, the controller can override the programmed settings and quickly extend diffuser ring  830  into diffuser gap  134  to choke the flow of refrigerant through diffuser gap  134  until stall or surge is mitigated. Although surge or stall also may be detected by monitoring refrigerant flow through diffuser  810  with sensors, the preferred way of monitoring surge or stall is by use of acoustic sensors, as surge or stall generates significant and undesirable noise, the acoustic sensors communicating with the controller. Other methods for detecting surge and stall may utilize algorithms that detect surge or stall such as set forth in U.S. Pat. No. 7,356,999 entitled “System and Method for Stability Control in a Centrifugal Compressor” issued Apr. 15, 2008, U.S. Pat. No. 7,905,102 entitled “Control System” issued Mar. 15, 2011. U.S. Pat. No. 7,905,702 entitled “Method for Detecting Rotating Stall in a Compressor” issued Mar. 15, 2011 utilizes a pressure transducer downstream of the diffuser ring to detect and correct rotating stall. These patents are all assigned to the assignee of the present invention and are incorporated herein by reference. After surge or stall has been corrected, the programmed operation of the positioning of diffuser ring  830  based on compressor demand may be restored by the controller, as discussed above. 
         [0050]    Advantages of the improved variable geometry diffuser mechanism  810  of the present invention include the use of a movable L-shaped flange  833  that reduces forces acting on the mechanism. This L-shaped flange also may be lighter in weight than movable flanges utilized in prior art variable geometry diffuser mechanisms. The reduced forces and reduced weight provide for a VGD that can react faster. It also allows the use of lighter weight and less expensive actuators. Further, the ability of the improved variable geometry diffuser to not only fully close, but also to be calibrated to control compressor operation based on sensed system conditions, allows the variable geometry diffuser to be used for capacity control as well as for surge and stall mitigation. This capacity control feature permits the elimination of pre-rotation vanes (PRV) which have been used in the past. Thus, although the improved variable geometry diffuser will be used more, the lower forces it will experience and its lighter weight will result in reduced wear with longer life, which in turn will provide increased reliability. 
         [0051]    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.