Patent Publication Number: US-7581925-B2

Title: Diffuser for a centrifugal compressor

Description:
RELATED APPLICATION DATA 
   This application claims benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Application No. 60/716,600 filed Sep. 13, 2005, which is fully incorporated herein by reference. 

   BACKGROUND 
   The invention relates to centrifugal compressors. More particularly, the invention relates to a diffuser for use in a centrifugal compressor. 
   Compressors are used throughout industry to compress fluids that are generally in a gaseous or vapor state. The most common types of compressors include reciprocating compressors, rotary compressors (e.g., screw, gear, scroll, etc.), and centrifugal compressors. Centrifugal compressors are generally used when a high volume of compressed fluid, such as air is required. 
   Centrifugal compressors employ a rapidly rotating impeller that includes a plurality of aerodynamic blades. The blades interact with the fluid being compressed to accelerate the fluid. The fluid is then discharged from the impeller at a high-velocity. 
   The high-velocity fluid enters a diffuser that includes aerodynamic features that act on the high-velocity flow to reduce the velocity and increase the pressure of the fluid. Because aerodynamic features are employed, inefficiencies can arise due to flow separation, vortices, eddies, and other flow phenomena. In addition, diffusers can be susceptible to choked flow and stall if operated outside of their expected design range. 
   SUMMARY 
   In one embodiment, the invention provides a diffuser for use in a centrifugal compressor that includes an impeller that discharges a high-velocity flow of fluid. The diffuser includes a platform having a blade portion and defining a substantially circular aperture. The impeller is disposed at least partially within the aperture such that the high-velocity fluid exits the impeller in directions that are substantially tangent to the blade portion. A vane extends from the platform and includes a suction side, a pressure side, and a leading edge having a platform portion and a shroud portion. A majority of the shroud portion is disposed on the suction side of a line normal to the platform that passes through a center of the platform portion of the leading edge. A shroud is coupled to the shroud portion of the vane such that the vane, the platform, and the shroud cooperate to at least partially define two flow paths. 
   In another embodiment, the invention provides a diffuser for use in a centrifugal compressor that includes an impeller that discharges a high-velocity flow of fluid. The diffuser includes a platform having a blade portion and defining a substantially circular aperture. The impeller is disposed at least partially within the aperture such that the high-velocity fluid exits the impeller in directions that are substantially tangent to the blade portion. A vane extends from the platform and includes a leading edge having a platform portion, a shroud portion, and a middle portion disposed between the platform portion and the shroud portion. The leading edge is curved such that the middle portion is spaced a non-zero distance from a line that extends through the platform portion and the shroud portion. A shroud is coupled to the shroud portion such that the vane, the platform, and the shroud cooperate to at least partially define two flow paths. 
   In another embodiment, the invention provides a diffuser for use in a centrifugal compressor that includes an impeller that discharges a high-velocity flow of fluid. The diffuser includes a platform having a blade portion and defining a substantially circular aperture. The impeller is disposed at least partially within the aperture such that the high-velocity fluid exits the impeller in directions that are substantially tangent to the blade portion. A shroud is spaced apart from the platform. A vane extends between the platform and the shroud. The vane includes a leading edge, a trailing edge, a suction side, and a pressure side. The suction side includes a platform portion, a shroud portion, and a middle portion. The middle portion is bowed toward the pressure side as compared to the platform portion and the suction portion. 
   In another embodiment, the invention provides a diffuser for use in a centrifugal compressor that includes an impeller that discharges a high-velocity flow of fluid. The diffuser includes a platform having a blade portion and defining a substantially circular aperture. The impeller is disposed at least partially within the aperture such that the high-velocity fluid exits the impeller in directions that are substantially tangent to the blade portion. A chamfered surface is formed as part of the platform and is disposed between the impeller and the blade portion. A shroud is spaced a non-zero distance from the platform. A plurality of vanes is coupled to the platform and to the shroud. The interface between each of the vanes and the platform defines a fillet surface. The interface between each of the vanes and the shroud defines a substantially square corner. 
   Other aspects and embodiments of the invention will become apparent by consideration of the detailed description and accompanying drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a centrifugal compressor embodying the invention; 
       FIG. 2  is a front view of a diffuser of the centrifugal compressor of  FIG. 1 ; 
       FIG. 3   a  is a cross-sectional view of the diffuser of  FIG. 2  taken along line  3 - 3  of  FIG. 2 ; 
       FIG. 3   b  is an enlarged view of the cross-section of  FIG. 3   a  taken along curve b-b of  FIG. 3   a;    
       FIG. 4  is a perspective view of a vane of the diffuser of  FIG. 2 ; 
       FIG. 5  is a side view of the vane of  FIG. 4 . 
       FIG. 6  is a front view of the vane of taken along line  6 - 6  of  FIG. 5 ; and 
       FIG. 7  is the front view of the vane of  FIG. 6  coupled to a shroud. 
   

   DETAILED DESCRIPTION 
   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     FIG. 1  illustrates a fluid compression system  10  that includes a prime mover, such as a motor  15  coupled to a compressor  20  and operable to produce a compressed fluid. In the illustrated construction, an electric motor  15  is employed as the prime mover. However, other constructions may employ other prime movers such as but not limited to internal combustion engines, diesel engines, combustion turbines, etc. 
   The electric motor  15  includes a rotor  25  and a stator  30  that defines a stator bore  35 . The rotor  25  is supported for rotation on a shaft  40  and is positioned substantially within the stator bore  35 . The illustrated rotor  25  includes permanent magnets  45  that interact with a magnetic field produced by the stator  30  to produce rotation of the rotor  25  and the shaft  40 . The magnetic field of the stator  30  can be varied to vary the speed of rotation of the shaft  40 . Of course, other constructions may employ other types of electric motors (e.g., synchronous, induction, brushed DC motors, etc.) if desired. 
   The motor  15  is positioned within a housing  50  which provides both support and protection for the motor  15 . A bearing  55  is positioned on either end of the housing  50  and is directly or indirectly supported by the housing  50 . The bearings  55  in turn support the shaft  40  for rotation. In the illustrated construction, magnetic bearings  55  are employed with other bearings (e.g., roller, ball, needle, etc.) also suitable for use. In the construction illustrated in  FIG. 1 , secondary bearings  60  are employed to provide shaft support in the event one or both of the magnetic bearings  55  fail. 
   In some constructions, an outer jacket  65  surrounds a portion of the housing  50  and defines cooling paths  70  therebetween. A liquid (e.g., glycol, refrigerant, etc.) or gas (e.g., air, carbon dioxide, etc.) coolant flows through the cooling paths  70  to cool the motor  15  during operation. 
   An electrical cabinet  75  may be positioned at one end of the housing  50  to enclose various items such as a motor controller, breakers, switches, and the like. The motor shaft  40  extends beyond the opposite end of the housing  50  to allow the shaft  40  to be coupled to the compressor  20 . 
   The compressor  20  includes an intake housing  80  or intake ring, an impeller  85 , a diffuser  90 , and a volute  95 . The volute  95  includes a first portion  100  and a second portion  105 . The first portion  100  attaches to the housing  50  to couple the stationary portion of the compressor  20  to the stationary portion of the motor  15 . The second portion  105  attaches to the first portion  100  to define an inlet channel  110  and a collecting channel  115 . The second portion  105  also defines a discharge portion  120  that includes a discharge channel  125  that is in fluid communication with the collecting channel  115  to discharge the compressed fluid from the compressor  20 . 
   In the illustrated construction, the first portion  100  of the volute  95  includes a leg  130  that provides support for the compressor  20  and the motor  15 . In other constructions, other components are used to support the compressor  20  and the motor  15  in the horizontal position. In still other constructions, one or more legs, or other means are employed to support the motor  15  and compressor  20  in a vertical orientation or any other desired orientation. 
   The diffuser  90  is positioned radially inward of the collecting channel  115  such that fluid flowing from the impeller  85  must pass through the diffuser  90  before entering the volute  95 . The diffuser  90  includes aerodynamic surfaces (e.g., blades, vanes, fins, etc.) arranged to reduce the flow velocity and increase the pressure of the fluid as it passes through the diffuser  90 . 
   The impeller  85  is coupled to the rotor shaft  40  such that the impeller  85  rotates with the motor rotor  25 . In the illustrated construction, a rod  140  threadably engages the shaft  40  and a nut  145  treadably engages the rod  140  to fixedly attach the impeller  85  to the shaft  40 . The impeller  85  extends beyond the bearing  55  that supports the motor shaft  40  and, as such is supported in a cantilever fashion. Other constructions may employ other attachment schemes to attach the impeller  85  to the shaft  40  and other support schemes to support the impeller  85 . As such, the invention should not be limited to the construction illustrated in  FIG. 1 . Furthermore, while the illustrated construction includes a motor  15  that is directly coupled to the impeller  85 , other constructions may employ a speed increaser such as a gear box to allow the motor  15  to operate at a lower speed than the impeller  85 . 
   The impeller  85  includes a plurality of aerodynamic surfaces or blades  150  that are arranged to define an inducer portion  155  and an exducer portion  160 . The inducer portion  155  is positioned at a first end of the impeller  85  and is operable to draw fluid into the impeller  85  in a substantially axial direction. The blades  150  accelerate the fluid and direct it toward the exducer portion  160  located near the opposite end of the impeller  85 . The fluid is discharged from the exducer portion  160  in at least partially radial directions that extend 360 degrees around the impeller  85 . 
   The intake housing  80 , sometimes referred to as the intake ring, is connected to the volute  95  and includes a flow passage  165  that leads to the impeller  85 . Fluid to be compressed is drawn by the impeller  85  down the flow passage  165  and into the inducer portion  155  of the impeller  85 . The flow passage  165  includes an impeller interface portion  170  that is positioned near the blades  150  of the impeller  85  to reduce leakage of fluid over the top of the blades  150 . Thus, the impeller  85  and the intake housing  80  cooperate to define a plurality of substantially closed flow passages  175 . 
   In the illustrated construction, the intake housing  80  also includes a flange  180  that facilitates the attachment of a pipe or other flow conducting or holding component. For example, a filter assembly could be connected to the flange  180  and employed to filter the fluid to be compressed before it is directed to the impeller  85 . A pipe would lead from the filter assembly to the flange  180  to substantially seal the system after the filter and inhibit the entry of unwanted fluids or contaminates. 
   Turning to  FIG. 2 , the diffuser  90  is illustrated in greater detail. The diffuser  90  includes a platform  185  and a plurality of vanes  190 . Other constructions may include more vanes or less vanes than the amount illustrated. 
   As illustrated in  FIG. 2 , the platform  185  includes a blade portion  195 , an outlet portion  200 , an inlet portion  205 , and an aperture  210 . The blade portion  195  supports the vanes  190  and may include a single surface (e.g., planar, conical, irregular, etc.) or multiple surfaces that interconnect to define the blade portion  195 . In the construction illustrated, the blade portion  195  includes a planar surface  215 , shown in  FIG. 3   b , that supports a leading edge  220  of the vanes  190  and a conical portion  225  disposed radially outward from the planar surface  215  that supports a trailing edge  230  of the vanes  190 . 
   The outlet portion  200  is positioned radially outward of the blade portion  195 . As illustrated in  FIGS. 3   a  and  3   b , the outlet portion  200  is substantially planar. However, other constructions could employ conical or irregular surfaces in addition to combinations of these surfaces to define the outlet portion  200 . 
   The inlet portion  205  is disposed radially inward of the blade portion  195  and at least partially defines an inlet  235  to the diffuser  90 . In the construction illustrated in  FIG. 4 , the inlet portion  205  includes a conical or chamfered surface  240 . The chamfered surface  240  is angled at about 45 degrees with respect to a rotational axis X-X, with other angles also being possible. In still other constructions, the inlet portion  205  includes curved surfaces, multiple surfaces, and/or a combination of surfaces. 
   The aperture  210  is disposed adjacent the inlet portion  205  and extends through the platform  185 . As such, the inlet portion  205  is a transition between the impeller  85  disposed at least partially within the aperture  210  and the diffuser  90 . 
   The vanes  190  extend from the blade portion  195  of the platform  185  and include the leading edge  220 , the trailing edge  230 , a suction side  245 , a pressure side  250 , and a shroud portion  260 . The vanes  190  are securely mounted (e.g., by welding, etc.) on the platform  185 . In a preferred construction, the vanes  190  are integrally-formed as a single homogeneous component with the platform  185 . In these constructions, the vanes  190  are generally machined from the same piece of material as the platform  185 . 
   A fillet surface  265 , shown in  FIGS. 6 and 7 , is disposed at the interface between the vanes  190  and the platform  185  to smoothly transition from the vanes  190  to the platform  185 . 
   The leading edge  220  is adjacent the aperture  210  of the platform  185  and includes a cut-hack and a forward lean (i.e., the vane leans toward the incoming fluid).  FIG. 5  illustrates the cut-hack of the leading edge  220 . The cut-back causes a middle portion  268  of the leading edge  220  to be spaced a non-zero distance  269  from a line  270  extending between the platform  185  and the shroud portion  260 . In the illustrated construction, the cut-back is a curve such that the line  270  contacts the leading edge  220  at both the platform  185  and the shroud portion  260 . However, the cut-back may take other forms (e.g., linear, etc.) such that the leading edge  220  is not symmetrical. 
   The forward lean, as illustrated in  FIG. 6 , causes the shroud portion  260  of the leading edge  220  to be closer to an adjacent vane on the suction side  245  than another adjacent vane  190  on the pressure side  250 . In other words, a line  270  drawn through the center of the leading edge  220  normal from the platform  185  crosses the leading edge  220  near the shroud portion  260  such that a majority of the leading edge  220  near the shroud portion  260  is on the leading or suction side of the line  270 . In the construction illustrated, the forward lean is a result of a curved leading edge  220 . In other constructions, the forward lean may result from a leading edge that is linear, parabolic, etc. 
   The trailing edge  230  is situated near the outlet portion  200  of the platform  185  and is positioned such that a vector pointing from the leading edge  220  to the trailing edge  230  generally corresponds in direction to the rotation of the impeller  85 . 
   The suction side  245  of each of the vanes  190  is defined by a surface between the leading edge  220 , the trailing edge  230 , the platform  185 , and the shroud portion  260  and facing the inlet portion  205 . The suction side  245  is bowed toward the pressure side  250  between the platform  185  and the shroud portion  260  as shown in  FIG. 6 . In other words, a middle portion  275  of the vane  190  between the platform  185  and the shroud portion  260  is spaced a non-zero distance  278  from a plane that passes through a plurality of straight lines  280  (one shown) that extend from the platform  185  to the shroud portion  260  and are substantially normal to the flow of fluid through the diffuser  90 . 
   The pressure side  250  of each of the vanes  190 , shown in  FIG. 4 , is defined by a surface between the leading edge  220 , the trailing edge  230 , the platform  185 , and the shroud portion  260  and facing the outlet portion  200 . The pressure side  250  is convex away from the suction side between the leading edge  220  and the trailing edge  230  as shown in  FIG. 6 . In other constructions, the pressure side  250  is not convex between the platform  185  and the shroud  100 . 
   The shroud portion  260  is located on a surface of the vane  190  opposite the platform  185 . The shroud portion  260  may be machined, molded, etc. such that the shroud portion  260  defines a sharp edge  285  along the perimeter of the vane  190 . The shroud portion  260  couples to a shroud of the compressor  20 , defining a substantially square corner  288  as illustrated in  FIG. 7 . In some constructions, the shroud is fixedly attached to the vanes  190 , while other constructions include a shroud closely spaced from the vanes  190  or in contact with, but not attached to, the vanes  190 . The construction illustrated in  FIG. 1  uses the first portion  100  of the volute  95  as the shroud. In other constructions, the shroud may be a distinct disc not serving another purpose for the compressor  20 . 
   A diffuser channel  290 , shown in  FIG. 2 , is formed at each pair of adjacent vanes  190  around the diffuser  90 . Each diffuser channel  290  is defined as an area between the suction side  245  of one vane  190 , the pressure side  250  of an adjacent vane  190 , the platform  185 , and the shroud  100 . Each diffuser channel  290  includes an inlet area  295  and an outlet area  300 . The inlet area  295  is disposed between the leading edges  220  of two adjacent vanes  190 . The outlet area  300  is disposed between the two adjacent vanes  190  near the trailing edge  230  of one of the vanes  190 . The cross-sectional area of the diffuser channel  290  increases such that the outlet area  300  is greater in size than the inlet area  295 . 
   In operation, power is provided to the motor  15  to produce rotation of the shaft  40  and the impeller  85 . As the impeller  85  rotates, fluid to be compressed is drawn into the intake housing  80  and into the inducer portion  155  of the impeller  85 . The impeller  85  accelerates the fluid from a velocity near zero to a high-velocity at the exducer portion  160 . The fluid passes out of the impeller  85 , passes over the chamfered surface  240 , and enters the diffuser  90  at the inlet area  295  of the diffuser channel  290 . The diffuser channel  290  maintains a guided flow pattern for the fluid that expands from the inlet area to the outlet area so as to reduce the flow velocity. The increasing cross-sectional area of the diffuser channel  290  acts to convert the dynamic energy of the flow of the fluid into potential energy or high-pressure. The now high-pressure fluid exits the diffuser  90  at the outlet area  300  of the diffuser channel  290  and enters the volute  95  via the inlet channel  110 . The high-pressure fluid then passes into the collecting channel  115  which collects fluid from any angular position around the inlet channel  110 . The collecting channel  15  then directs the high-pressure fluid out of the volute  95  via the discharge channel  125 . 
   During operation, the efficiency of the compressor  20  may drop due to various undesirable flow phenomena such as flow separation, vortices, or eddies. The leading edge is cut-back and forward leaning to help reduce or minimize these phenomena. 
   The diffuser  90  also increases the efficiency of the compressor  20  by expanding the operational range of the compressor  20 . The operational range spans from the maximum allowable stable pressure increase, above which the diffuser is susceptible to surge, to the maximum allowable flow at which the diffuser is choked. The cut back  270  of the leading edge  220  effectively increases the inlet area  295  of the diffuser channel  290 , thus increasing the maximum allowable flow through the diffuser  90 . 
   Thus, the invention provides, among other things, a new and useful diffuser  90  for use in centrifugal compressors. The constructions of the diffuser  90  described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the invention. Various features and advantages of the invention are set forth in the following claims.