Abstract:
An annular radially disposed split ring diffuser for a centrifugal compressor includes an inner ring and an outer ring. A drive positioning mechanism includes a pinion gear on a pinion axle driven by an actuator. A rack gear is mounted to the inner ring and adapted to engage in meshing arrangement with the pinion gear. The actuator is operable to position the inner ring between a fully open position and a partially closed position with respect to the outer ring. A travel limiter is provided to positively limit the travel of the inner ring at the fully open and the partially closed positions. A compliant mechanical stop on a side of the rack gear touches an outer ring stop on the outer ring when the inner ring is in the fully open position to prevent the gear teeth from being stripped by the actuator torque output.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of centrifugal compressors, and more particularly to a mechanical stop for limiting split ring diffuser travel.  
         BACKGROUND OF THE INVENTION  
         [0002]    One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the volumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide range of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates.  
           [0003]    Many schemes have been devised to maintain high machine efficiencies over a wide operation range. In U.S. Pat. No. 4,070,123, the entire impeller wheel configuration is varied in response to load changes in an effort to match the machine performance with the changing load demands. Adjustable diffuser flow restrictors are also described in U.S. Pat. No. 3,362,625 which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates.  
           [0004]    A common technique for maintaining high operating efficiency over a wide flow range in a centrifugal machine is through use of the variable width diffuser in conjunction with fixed diffuser guide vanes.  
           [0005]    U.S. Pat. Nos. 2,996,996 and 4,378,194 describe variable width vaned diffusers wherein the diffuser vanes are securely affixed, as by bolting to one of the diffuser walls. The vanes are adapted to pass through openings formed in the other wall thus permitting the geometry of the diffuser to be changed in response to changing load conditions.  
           [0006]    Fixedly mounting the diffuser blades to one of the diffuser walls presents a number of problems particularly in regard to the manufacture, maintenance and operation of the machine. Little space is afforded for securing the vanes in the assembly. Any misalignment of the vanes will cause the vane to bind or rub against the opposite wall as it is repositioned. Similarly, if one or more vanes in the series has to be replaced in the assembly, the entire machine generally has to be taken apart in order to effect the replacement.  
           [0007]    The efficiency of a compressor could be greatly enhanced by varying the outlet geometry of the diffuser. A variable geometry pipe diffuser is disclosed in U.S. Pat. No. 5,807,071. A variable geometry pipe diffuser (which may also be termed a split-ring pipe diffuser) splits the diffuser into a first, inner ring and a second outer ring. The inner and outer rings have complementary inlet flow channel sections formed therein. That is, each inlet flow channel section of the inner ring has a complementary inlet flow channel section formed in the outer ring. The inner ring and outer ring are rotatable respective one another. The rings are rotated to improve efficiency for varying pressure levels between a fully open position and a partially closed position. In the partially closed position the misalignment of the exit pipes of the diffuser causes an increase in noise. Rotation of the rings past an optimum design point results in excessive noise and efficiency degradation.  
           [0008]    The geometrical tolerances within a centrifugal compressor are small. At the same time the loads within the compressor are large and dynamic in nature. In a split ring pipe diffuser the problem of maintaining tolerances in the face of the dynamic loading becomes quite onerous. There are both axial (thrust) loads and circumferential loads on the ring pair that need to be managed. The diffuser rings must be able to rotate relative to one another and at the same time tight control over their relative position must be maintained in order to ensure proper alignment of the flow channels and the ultimate efficiency of the compressor. The cost of maintaining the necessary tolerances in a split ring diffuser is generally very high.  
           [0009]    Another problem with split ring diffusers is premature part wear. Lubricants are generally not used within the gas flow regions of centrifugal compressors to preclude contamination of the gases. The dynamic loads imposed upon the split ring diffuser by the gas flow exiting the impeller cause wear in the components of the diffuser to be accelerated by the absence of lubricating oil.  
           [0010]    The drive system for accurately positioning the rings relative to one another must, among other things, be rigid to avoid any fretting of components. Because of circumferential loading on the rings there is a propensity for the inner ring to oscillate relative to the outer ring which could cause compressor instability, part wear and could adversely affect efficiency. This causes several problems that need to be overcome. A drive system is needed that is capable of preventing the relative movement between the inner and outer rings. A bearing concept is also needed which would allow for the relative rotation of the two rings and also be capable of withstanding the circumferential and thrust loads while maintaining tight geometric tolerances between the rings. There is also a need to provide a positioning system that includes positive minimum and maximum stops to avoid unnecessary noise and efficiency degradation as well as simple field retrofit. In addition, there is a need for the drive and bearing systems have a long operating life and be easy to install and adjust properly. U.S. Pat. Nos. 5,895,204; 5,988,977; and 6,015,259 address these concerns.  
         SUMMARY OF THE INVENTION  
         [0011]    Briefly stated, an annular radially disposed split ring diffuser for a centrifugal compressor includes an inner ring and an outer ring. A drive positioning mechanism includes a pinion gear on a pinion axle driven by an actuator. A rack gear is mounted to the inner ring and adapted to engage in meshing arrangement with the pinion gear. The actuator is operable to position the inner ring between a fully open position and a partially closed position with respect to the outer ring. A travel limiter is provided to positively limit the travel of the inner ring at the filly open and the partially closed positions. A compliant mechanical stop on a side of the rack gear touches an outer ring stop on the outer ring when the inner ring is in the fully open position to prevent the gear teeth from being stripped by the actuator torque output.  
           [0012]    According to an embodiment of the invention, in a centrifugal compressor having a casing and an impeller rotatably mounted therein for bringing a working fluid from an inlet to the entrance of an annular radially disposed split ring diffuser, the diffuser including an inner ring, the inner ring having a plurality of first channel sections formed therein, an outer ring, the outer ring having a plurality of second channel sections formed therein, each second channel section having a complementary first channel section; the compressor including a drive positioning mechanism for rotating the inner ring circumferentially within the outer ring between a first, fully open position wherein the complementary first and second channel sections are aligned to allow a maximum flow of fluid through the complementary channel sections, and a second, partially closed position, wherein the first and second complementary flow guide channels are misaligned to restrict flow of fluid through the complementary channel sections, the drive positioning mechanism includes an actuator; a pinion axle rotationally driven by the actuator at a first end of the pinion axle; a pinion gear mounted to a second end of the pinion axle; a rack gear fixedly mounted to the inner ring extending radially outwardly from the inner ring and adapted to engage in meshing arrangement with the pinion gear; first and second limit stops in the actuator for limiting travel of the inner ring between the first position and the second position; and a compliant mechanical stop on a side of the rack gear which touches an outer ring stop on the outer ring when the inner ring is in the first position, so as to prevent actuator stall. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 shows a cross-sectional side view of a compressor according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 2 shows an exploded perspective view of a variable pipe diffuser according to an embodiment of the invention.  
         [0015]    [0015]FIG. 3 shows a partial cross-sectional view of part of the variable pipe diffuser of FIG. 2 in full open position.  
         [0016]    [0016]FIG. 4 shows a partial cross-sectional view of part of the variable pipe diffuser of FIG. 2 in partially closed position.  
         [0017]    [0017]FIG. 5 shows a top view of the compressor of FIG. 1.  
         [0018]    [0018]FIG. 6 shows a partial cross-sectional view of a ring support mechanism taken along the line  6 - 6  in FIG. 5.  
         [0019]    [0019]FIG. 7 shows a partial cross-sectional view of a ring support mechanism taken along the line  7 - 7  in FIG. 6.  
         [0020]    [0020]FIG. 8 shows a partial cross-sectional view of a drive positioning mechanism of the present invention.  
         [0021]    [0021]FIG. 9 shows a top view of a portion of the drive positioning mechanism of FIG. 8.  
         [0022]    [0022]FIG. 10 shows a perspective view of a rack gear of the present invention.  
         [0023]    [0023]FIG. 11 shows a partial sectional view of the compressor of the present invention.  
         [0024]    [0024]FIG. 12 shows an expanded view of a mechanical stop according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    U.S. Pat. No. 5,895,204 is incorporated herein by reference. Referring now to FIG. 1, the invention is shown as installed in a centrifugal compressor  10  as part of an HVAC system (not shown) having an impeller  12  for accelerating refrigerant vapor to a high velocity, a variable geometry pipe diffuser  14  for decelerating the refrigerant to a low velocity while converting kinetic energy to pressure energy, and a discharge plenum in the form of a collector  16  to collect the discharge vapor for subsequent flow to a condenser. Power to impeller  12  is provided by an electric motor (not shown) which is hermetically sealed in the other end of the compressor and which operates to rotate a high speed shaft  19 . The refrigerant enters an inlet opening  29  of a suction housing  31 , passes through a blade ring assembly  32  and a plurality of guide vanes  33 , and enters a compression suction area  23  which leads to a compression area defined on its inner side by impeller  12  and on its outer side by a housing  34 . After compression, the refrigerant flows into diffuser  14 , collector  16 , and a discharge line (not shown).  
         [0026]    Referring also to FIGS.  2 - 4 , diffuser  14  includes an inner ring  40  concentrically disposed inside an outer ring  42 . Inner and outer rings  40 ,  42  have complementary flow channel sections  44  and  46  formed therein. That is, each flow channel section  44  of inner ring  40  has a complementary channel section  46  formed in outer ring  42 . Inner ring  40  and outer ring  42  are rotatable with respect to one another. In a preferred embodiment, inner ring  40  rotates circumferentially within a stationary outer ring  42 .  
         [0027]    When one ring is rotated with respect to the other, the alignment between each pair of complementary inlet flow channels of the inner and outer rings changes. Rings  40  and  42  are adjustable between a fully open position, as shown in FIG. 3, wherein complementary channel sections are aligned and a maximum amount of fluid passes through inner and outer rings  40  and  42 , and a partially closed position, as shown in FIG. 4, wherein complementary channels are misaligned so that flow through channel sections  44  and  46  is restricted.  
         [0028]    The flow of fluid (flow rate) through diffuser  14  in the partially closed position in relation to the fully open position is determined by the ratio of the minimum cross-sectional area of the flow channel of diffuser  14  in the partially closed position to the minimum cross-sectional area of the flow channel (defined by complementary channel sections  44  and  46 ) in the fully open position. This minimum flow channel area, known as the “throat area”, is generally be determined by the smallest diameter of a flow passage  52  of inner ring channel section  44  when diffuser  14  is in a fully open position, and is controlled by a width  53  at an interface between inner and outer rings  40  and  42  when diffuser  14  is in the partially closed position. The flow rate of fluid through compressor  10  when diffuser  14  is in the partially closed position is generally between about 10% and 100% of the flow rate of the fluid through compressor  10  when diffuser  14  is in the fully open position.  
         [0029]    In the partially closed position, at least about 10% the volume of flow as compared to the fully open position should flow through diffuser  14  so as to prevent excessive thermodynamic heating, excessive noise and degradation in the efficiency of the compressor. To this end, the amount of relative rotation between the two ring sections should be limited to an amount of rotation necessary to effect the second partially closed position. In other words, the rings should not be adjustable to completely close off the flow of fluid there between. The degree of allowable rotation between rings  40 ,  42  is determined by the desired flow between rings  40 ,  42  in the fully closed position and the number and volume of inlet flow channel sections  44 ,  46  in rings  40 ,  42  in relation to the volume of ring sections  40 ,  42 .  
         [0030]    R 2  defines the radius of the impeller tip, R 3  defines the outside radius of inner ring  40 , and R 4  defines the outside radius of outer ring  42 . By making the difference R 3 −R 2 , i.e., the preferred thickness of inner ring  40 , no larger than is necessary to block a desired portion (e.g. 50% of flow) of flow through outer ring channel sections  46 , the flow of fluid through diffuser  14  is efficiently controlled. Rotating inner ring  40  with respect to outer ring  42  reduces the diffuser throat area before any diffusion has taken place, thus preventing flow acceleration after diffusion. The smaller the inner ring thickness, the smaller the turning angles of the flow through diffuser  14  in the partially closed position. Both of the above-described effects tend to improve compressor efficiency under partial-load operating conditions.  
         [0031]    Referring to FIGS.  5 - 7 , a ring support mechanism  35  according to an embodiment of the present invention is shown. The embodiment shown illustrates the use of three such mechanisms spaced circumferentially equidistant about the diffuser. Ring support mechanism  35  includes an inner bearing slot  41 , a cutout  43  disposed in a surface  28  of inner ring  40 , a roller assembly  54 , a roller axle assembly  36 , and an outer bearing slot  45  disposed in outer ring  42 . Axle assembly  36  includes an axle  37  and an axle bolt  39 .  
         [0032]    Outer ring  42  is stationary with respect to suction housing  31  and three sets of ring support mechanisms  35  are preferably installed into outer ring  42  by positioning roller assembly  54  within bearing slot  45  of outer ring  42 , passing axle  37  through a mounting hole  58  and roller assembly  54 , and then installing axle bolt  39  through axle  37  and loosely threading axle bolt  39  into threaded holes  59  in outer ring  42 . Inner ring  40  is installed inside outer ring  42  with cutouts  43  of inner ring  40  circumferentially aligned with bearing slot  45  and roller assemblies  54  before rotating inner ring  40  clockwise as shown in FIG. 7 to position roller assemblies  54  within bearing slot  41 . With inner ring  40  installed within outer ring  42 , ring support mechanisms  35  are employed to properly center and position the inner ring by rotating axle  37  by using a wrench (not shown) placed on hex head  38 . An axle body centerline  48 , on which a roller  55  of roller assembly  54  is mounted, is offset from an axle bore centerline  50 . Rotating hex head  38  causes roller assembly  54  to be radially displaced relative to outer ring  42 . Once inner ring  40  is properly centered within outer ring  42 , hex head  38  is further rotated preferably to preload an outer bearing surface  56  of roller assemblies  54  against inner ring  40 . Axle bolt  39  is then tightened. The preload conditioned is preferred because it prevents inner ring  40  from movement due to tangential and circumferential loads.  
         [0033]    Referring to FIGS.  8 - 10 , a positioning drive mechanism  121  rotates inner ring  40  circumferentially within outer ring  42 . A rack gear  123 , which extends radially outwardly from outer ring  42 , is fixedly attached to outer ring  42 . Rack gear  123  includes first and second gear faces  144  and  145 , respectively, as well as a plurality of mounting holes  142 . A pinion gear  124  is in gearing relation with rack gear  123 . Pinion gear  124  is driven via pinion axle  126  by an actuator  128 . Actuator  128  is selected and controlled to effect movement of inner ring  40  in relation to outer ring  42  between the fully open position and the partially closed position and any number of intermediate positions therebetween. Pinion axle  126  is housed in a containment housing  130  which hermetically seals axle  126  from compressor interior  132  and which prevents leakage of fluid out of compressor  10  through containment housing  130 .  
         [0034]    The tangential and circumferential loading on rings  40 ,  42  by the refrigerant flow within diffuser  14  causes inner ring  40  to have the propensity to chatter back and forth within outer ring  42 . Excess movement or chattering of inner ring  40  causes rack gear  123  and pinion gear  124  to fret and also causes other parts to wear. Preloading inner ring  40  via roller assemblies  54  as discussed earlier prevents movement of inner ring  40  as well as chattering under normal operating conditions. In cases of abnormal conditions, such as operating in a surge, a secondary mechanism is needed to prevent unwanted motion of inner ring  40 . A drive mounting system prohibits adverse movement and chattering of inner ring  40  via adjustment of the relative center positions of pinion gear  124  and rack gear  123  utilizing axle containment housing  130 . The axle housing outer surface  125  is concentric about housing centerline  127  while housing bore  129  is concentric about housing bore centerline  131 . In one embodiment, housing centerline  127  and housing bore centerline  129  are offset by 0.060 inches. Wrench flats  135  and adjustment slots  134  of the positioning drive mechanism are shown in FIG. 9. After installation of positioning drive mechanism  121  into suction housing  31 , the backlash between rack gear  123  and pinion gear  124  is removed by rotating positioning drive mechanism  121  by placing a wrench (not shown) across wrench flats  135 . Once minimal backlash is achieved, positioning drive mechanism  121  is fixed in place by the tightening of cap screws  133 . Once the backlash is eliminated, the tendency for inner ring  40  to move is discharged directly by actuator  128  through the gear system.  
         [0035]    Referring also to FIG. 5, an embodiment is shown having a mechanism to provide positive positioning of inner ring  40  corresponding to the fully open position and the partially closed position. A cavity  137  is machined in outer ring  42  to accommodate rack gear  123  (FIG. 10). Rack gear  123  is accurately mounted to inner ring  40  in a tongue and groove fashion wherein rack gear  123  is provided with a circumferential groove  143  adapted to receive a tongue section  139  of inner ring  40 . To determine the fully open position, inner ring  40  is positioned within outer ring  42 , after which rings  40 ,  42  are rotated relative to one another until flow passages  52  are fully aligned with outer flow channels  46 . With rings  40 ,  42  in this position, and with ring support mechanisms  35  adjusted as described above, rack gear  123  is mounted to inner ring  40  with second gear face  145  in contact with full open stop  140  of cavity  137 . Bolts  152  (FIG. 11) are then installed through gear mounting holes  142  and securely tightened into threaded holes  138  in inner ring  40 . Rack gear  123  and cavity  137  are sized to provide for a predetermined amount of closure of the pipe diffuser. For example, an embodiment is sized such that difference between the rack gear angular width and the cavity provide for a 10% open position. In this example, the required travel of rack gear  123  is 10 degrees, the rack gear angular width is 35 degrees, and the corresponding cavity angular width is 45 degrees. With rack gear  123  so positioned, a positive stop is created between rack gear  123  and cavity  137  to accurately and repeatably position rings  40 ,  42  at points corresponding to the fully open position and the partially closed position. The positive stops also allow for field retrofit of actuator  128  without the need to readjust the position of inner and outer rings  40 ,  42 .  
         [0036]    Referring to FIGS.  11 - 12 , a mechanical stop  154  is affixed to rack gear  123  by a fastener such as a bolt  156 . Mechanical stop  154  is preferably of a deformable or compliant material so that actuator  128  (FIG. 8) does not stall, thereby avoiding actuator failure. Preferable materials for mechanical stop  154  include Teflon® and ultra high molecular weight (UHMW) plastic.  
         [0037]    Referring back to FIGS. 3, 4,  8 , and  11 , actuator  128  includes first and second electrical limit switches to control total travel. The position of rack gear  123  in inner ring  40  is adjusted so that when inner ring channel sections  44  and outer ring channel sections  46  are collinear, mechanical stop  154  on rack gear  123  is against outer ring stop  140 . In this position, actuator  128  is against one the first limit switch. During the course of operation, it is possible for rack gear  123  to touch outer ring  42  before actuator  128  hits the first limit switch. In this mode, actuator  128  drives rack gear  123  a few more degrees before hitting the first limit switch. Without mechanical stop  154 , because actuator  128  puts out very high torque, known as stall torque, the gear teeth in the actuator drive train break. The compliant, deformable material in mechanical stop  154  between rack gear  123  and outer ring  42  limits the actuator torque output, thereby protecting the gear teeth.  
         [0038]    While the present invention has been described with reference to a particular preferred embodiment and the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the preferred embodiment and that various modifications and the like could be made thereto without departing from the scope of the invention as defined in the following claims.