Patent Publication Number: US-2005121916-A1

Title: Integrated microturbine gearbox generator assembly

Description:
BACKGROUND  
      The present invention relates to a system and method for driving a low-speed component using a high-speed prime mover. More particularly, the present invention relates to a system and method for driving a low-speed component using a high-speed microturbine.  
      Microturbine engines are relatively small and efficient sources of power. Microturbines can be used to generate electricity and/or to power auxiliary equipment such as pumps or compressors. When used to generate electricity, microturbines can be used independent of the utility grid or synchronized to the utility grid. In general, microturbine engines are limited to applications requiring 2 megawatts (MW) of power or less. However, some applications larger than 2 MWs may utilize a microturbine engine.  
      Many microturbine engines include a turbine-compressor assembly that rotates at a high rate of speed. To generate electricity, the turbine-compressor assembly, or a separate turbine, is coupled to the generator, which also rotates at a high rate of speed. The generator output is then conditioned to produce a usable electrical current (e.g., 50 Hz or 60 Hz). In other constructions, a gearbox is positioned between the turbine and the generator to allow the generator to operate at a lower speed. However, due to the high speed of the turbine, the gearbox often requires several high-speed bearings to support the various gears. In addition, the dynamic and mechanical issues associated with the gearbox and turbine (e.g., vibration, imbalance, thrust loading, thermal expansion, and the like) can affect the operation of the other component. For example, a slight imbalance in the gearbox can produce a vibration that is transmitted to the turbine. The high speed of the turbine can act to increase the magnitude or the effect caused by the vibration. This can lead to undesirable operating conditions, system instability, and unwanted engine trips or shutdowns.  
     SUMMARY  
      The present invention generally provides a microturbine engine comprising a turbine including a first housing and a turbine rotor. The engine also includes a generator having a second housing and a generator rotor. The generator rotor is supported for low-speed rotation by a low-speed bearing. The engine also includes a gearbox having a third housing connected to the first housing and the second housing, a pinion gear, and a low-speed gear connected to the generator rotor and at least partially supported by the low-speed bearing. A shaft is connected to the turbine rotor and the pinion gear and a first high-speed bearing and a second high-speed bearing are positioned to support the turbine rotor and the shaft for high-speed rotation.  
      In another aspect, the invention generally provides a microturbine engine including a turbine having a turbine housing and a turbine rotor. The engine also includes a compressor having a compressor housing coupled to the turbine housing and a compressor rotor coupled to the turbine rotor. A rotor flange is coupled to the compressor rotor such that the turbine rotor, the compressor rotor, and the rotor flange at least partially define a rotor train. A first high-speed bearing and a second high-speed bearing are coupled to the rotor train and at least partially support the rotor train for rotation. The first high-speed bearing and the second high-speed bearing are positioned to define a space between the bearings and a space that extends beyond the bearings. At least a portion of the rotor train is positioned within the space beyond the bearings to define a cantilever portion having a free end. The engine also includes a synchronous generator having a generator housing and a generator rotor. The generator rotor is supported for low-speed rotation by at least one low-speed bearing. The engine further includes a gearbox having a gearbox housing connected to the compressor housing and the generator housing. The gearbox includes a ring gear connected to the generator rotor, a plurality of planetary gears, and a pinion gear positioned to engage each of the planetary gears. A quill shaft is coupled to the rotor flange and the pinion gear such that the quill shaft is fully supported by the second high-speed bearing, which is coupled to the rotor flange, and the planetary gears.  
      In yet another aspect, the present invention generally provides a method of coupling a rotating element of an engine that operates at a first speed to a driven component that operates at a second speed, the second speed slower than the first speed. The method includes coupling a shaft to the rotating element and supporting the rotating element and the shaft with a first high-speed bearing and a second high-speed bearing. The rotating element is supported such that at least a portion of the rotating element is disposed in a space between the bearings and at least a portion of the rotating element is disposed in a space beyond the bearings. The method also includes engaging a second end of the shaft with a plurality of planetary gears and supporting the driven component with at least one low-speed bearing. The method also includes coupling a low-speed gear to the driven component, the low-speed gear coupled to each of the planetary gears. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The description particularly refers to the accompanying figures in which:  
       FIG. 1  is a perspective view of a portion of a microturbine engine;  
       FIG. 2  is a sectional view of the turbine-compressor-generator portion of the engine of  FIG. 1 ;  
       FIG. 3  is an enlarged view of the turbine-compressor and gearbox portion;  
       FIG. 4  is an enlarged view of the gearbox portion of the engine of  FIG. 1 ;  
       FIG. 5  is an enlarged sectional view of a quill shaft of the microturbine engine of  FIG. 1 ; and  
       FIG. 6  is a perspective view of the quill shaft of  FIG. 4 . 
    
    
      Before any embodiments of the invention are explained, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements 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 is meant to encompass the items listed thereafter and equivalence thereof as well as additional items. The terms “connected,” “coupled,” and “mounted” and variations thereof are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings.  
     DETAILED DESCRIPTION  
      With reference to  FIG. 1 , a microturbine engine system  10  that includes a turbine section  15 , a generator section  20 , and a control system  25  is illustrated. The turbine section  15  includes a radial flow turbine  35 , a compressor  45 , a recuperator  50 , and a combustor  55 . The recuperator  50  may be of the plate-fin variety with the combustor  55  in the inlet manifold as disclosed in U.S. Pat. No. 5,450,724, the entire contents of which is incorporated herein by reference.  
      The engine  10  includes a standard Brayton cycle combustion turbine with the recuperator  50  added to improve engine efficiency. The engine shown is a single-spool engine (one set of rotating elements). However, multi-spool engines are also contemplated by the invention. The compressor  45  is a centrifugal-type compressor having a compressor rotor  56  that rotates in response to operation of the turbine  35 . The compressor  45  shown is a single-stage compressor. However, multi-stage compressors can be employed where a higher pressure ratio is desired. Alternatively, compressors of different designs (e.g., axial-flow compressors, reciprocating compressors, and the like) can be employed to supply compressed air to the engine.  
      The turbine  35  is a radial flow single-stage turbine having a turbine rotor  57  directly coupled to the compressor rotor  56 . In other constructions, multi-stage turbines or other types of turbines may be employed. The coupled rotors  56 ,  57  of the turbine  35  and the compressor  45  engage the generator section  20  through a gearbox  60 .  
      The recuperator  50  includes a heat exchanger employed to transfer heat from a hot fluid to the relatively cool compressed air leaving the compressor  45 . One suitable recuperator  50  is described in U.S. Pat. No. 5,983,992 fully incorporated herein by reference. The recuperator  50  includes a plurality of heat exchange cells stacked on top of one another to define flow paths therebetween. The cool compressed air flows within the individual cells, while a flow of hot exhaust gas passes between the heat exchange cells.  
      During operation of the microturbine engine system  10 , the compressor rotor  56  rotates in response to rotation of the turbine rotor  57 . The compressor  45  draws in atmospheric air and increases its pressure. The high-pressure air exits the air compressor  45  and flows to the recuperator  50 .  
      The flow of compressed air, now preheated within the recuperator  50 , flows to the combustor  55  as a flow of preheated air. The preheated air mixes with a supply of fuel within the combustor  55  and is combusted to produce a flow of products of combustion. The use of a recuperator  50  to preheat the air allows for the use of less fuel to reach the desired temperature within the flow of products of combustion, thereby improving engine efficiency.  
      The flow of products of combustion enters the turbine  35  and transfers thermal and kinetic energy to the turbine  35 . The energy transfer results in rotation of the turbine rotor  57  and a drop in the temperature of the products of combustion. The energy transfer allows the turbine  35  to drive both the compressor  45  and the generator  20 . The products of combustion exit the turbine  35  as a first exhaust gas flow.  
      In constructions that employ a second turbine, the first turbine  35  drives only the compressor, while the second turbine drives the generator  20  or any other device to be driven. The second turbine receives the first exhaust flow, rotates in response to the flow of exhaust gas therethrough, and discharges a second exhaust flow.  
      The first exhaust flow, or second exhaust flow in two turbine engines, enters the flow areas between the heat exchange cells of the recuperator  50  and transfers excess heat energy to the flow of compressed air. The exhaust gas then exits the recuperator  50  and is discharged to the atmosphere, processed, or further used as desired (e.g., cogeneration using a second heat exchanger).  
      Turning to  FIGS. 2 and 3 , the portion of the engine of  FIG. 1  that includes the generator  20 , the turbine  35 , the compressor  45 , and the gearbox  60  is illustrated. The components are arranged and coupled to one another such that the turbine  35 , the compressor  45 , and the generator  20  rotate on a common axis A-A. In other constructions, the generator  20  may be offset such that it rotates on an axis different from that of the turbine  35  and the compressor  45 .  
      The turbine  35  is located at one end of the engine  10  and includes the turbine rotor  57  and a first, or turbine housing  65 . The turbine housing  65  remains stationary and provides support to the turbine rotor  57  as well as the stationary flow elements such as inlet guide vanes  70 , a shroud/diffuser cone  75 , and a central body  80 . The turbine housing  65  includes a scroll  85  that receives the flow of products of combustion from the combustor  55 . The shroud/diffuser cone  75  and scroll  85  direct the flow of products of combustion to the inlet guide vanes  70  that then direct the products of combustion into the turbine rotor  57  along the desired path. After passing through the turbine rotor  57 , the flow of products of combustion enters a diffuser  90  made up of the diffuser cone  75  and the central body  80 . The diffuser walls, defined by the cone  75  and the central body  80 , gradually diverge from one another to allow the exhaust gas exiting the turbine  35  to efficiently decelerate the flow as it is discharged from the engine  10 . In other constructions another diffuser may be attached to the turbine  35  to further decelerate the flow.  
      The compressor  45  includes the compressor rotor  56  and a second, or compressor housing  95 . The compressor housing  95  is directly connected to the turbine housing  65  to fix the position of the two components  65 ,  95  relative to one another. The compressor housing  95  includes an inlet plenum  100  that receives atmospheric air for use in the engine  10 . The air is drawn in by operation of the compressor  45  and may be filtered before it enters the plenum  100 , as it enters the plenum  100 , or before it enters the compressor rotor  56 . The air enters the compressor  45  in a substantially axial direction (i.e., parallel to the rotational axis A-A of the compressor  45 ) and exits in a substantially  5  radial direction (i.e., centrifugal). After passing through the compressor rotor  56 , the air collects in a volute chamber  105  defined within the compressor housing  95  and is directed from the compressor housing  95  to the recuperator  50 .  
      The compressor rotor  56  is directly coupled to the turbine rotor  57  such that the two components  56 ,  57  rotate in unison about the rotational axis A-A. A first high-speed bearing  1   10  supports the compressor end of the turbine-compressor rotor assembly  115 . Thus, the turbine-compressor rotor assembly  115  is supported in a cantilever fashion with all of its support points being on the air inlet side (i.e., adjacent the inlet plenum  100 ) of the compressor  45 . This arrangement supports the rotors  56 ,  57  for rotation without subjecting the first high-speed bearing  110  (or any other bearings) to the high operating temperatures associated with the turbine  35 .  
      While many different types of bearing can be used (e.g., roller, ball, needle, journal, and the like), angular contact bearings are preferred. Angular contact bearings support the rotors  56 ,  57  for rotation, while simultaneously carrying the thrust load produced by the engine  10 . Thus, the need for an independent thrust bearing is avoided. In addition, angular contact bearings provide increased efficiency over many other types of bearings.  
      The generator  20 , also shown in  FIG. 2 , includes a generator housing  120  that contains a stator and a generator rotor  125  (shown in  FIG. 4 ). The rotor  125  is supported for rotation by two low-speed bearings (not shown) located within the generator housing  120  and positioned at either end of the rotor  125 . The generator rotor  125  includes a shaft  130  that extends out of the generator housing  120  and supports a ring gear  135 . While a synchronous generator that operates at 3600 RPM or 1800 RPM to output 60 Hz is preferred, other types of generators will also function with the invention. For example, a synchronous generator that operates at 3000 RPM or 1500 RPM to output 50 Hz will also work with the present invention. In addition, asynchronous or high-speed generators (alternators) can also be driven by the present invention. These generators may operate at 4,000 RPM or higher in some cases. Furthermore, components other than generators (e.g., pumps, compressors, conveyors, and the like) can also be driven by the present invention. Some of these components may operate in excess of 15,000 RPM.  
      The gearbox  60  is positioned between the compressor  45  and the generator  20  and includes a gearbox housing  140  and a plurality of gears. The gearbox housing  140  attaches to the compressor housing  95  to align the components  95 ,  140  and maintain their positions relative to one another. The generator housing  120  also attaches to the gearbox housing  140  to fix the positions of the stationary components of the engine  10  relative to one another. With all of the housings  65 ,  95 ,  120 ,  140  attached, a relatively rigid structural backbone is established that supports the rotating components  56 ,  57 ,  125  in their desired locations. The relatively rigid housings  65 ,  95 ,  120 ,  140  also reduces undesirable movement between components and/or vibration during engine operation.  
      The gearbox housing  140  includes an adapter plate  145  that facilitates the attachment of the generator housing  120  and the gearbox housing  140 . The adapter plate  145  directly attaches to both the gearbox housing  140  and the generator housing  120 . The use of the adapter plate  145  allows for the use of a standard gearbox housing  140  with multiple generators  20  or other components. For example, engines  10  of various sizes can use the same gearbox housing  140  with different adapter plates and generators. Similarly, different speed engines (e.g., 60 Hz vs. 50 Hz) can use the same gearbox housing  140  with different rotating components and generators. Furthermore, with a new adapter plate, a standard gearbox housing  140  can be mated to a pump or compressor.  
      The gearbox  60 , shown in  FIG. 4 , includes a plurality of gears that are sized and arranged to step down a high input speed (e.g., 40,000 RPM or higher) to a low output speed (e.g., 15,000 RPM or lower to drive pumps or compressors and 3600 RPM or lower to drive synchronous generators). The gearbox  60  includes three planetary gears  150 , each supported by a planetary shaft  155  for rotation about a planetary axis B-B. The planetary axes B-B are spaced about 120 degrees apart around the rotational axis A-A and are spaced a parallel distance from the rotational axis A-A, thereby allowing a pinion gear  160  to engage all three planetary gears  150  simultaneously. Each planetary shaft  155  supports a drive gear  165  and is supported for rotation by two bearings  170 .  
      Each of the drive gears  165  is sized and positioned to engage the ring gear  135  and drive the generator rotor  125 . As illustrated in  FIG. 4 , the drive gears  165  are formed as part of the planetary shafts  155 . In other constructions, separate gears  165  attach to the shafts  155  in a known manner (e.g., pinned, shrunk on, welded, bolted, and the like).  
      The gear ratio between the pinion gear  160  and each of the planetary gears  150  allows for the use of relatively low-speed bearings  170  (e.g., about 9,000 RPM) rather than high-speed bearings (e.g., 10,000 RPM or higher). In addition, the size of the drive gears  165  and the ring gear  135  are chosen to assure that the generator rotor  125  operates at the proper speed when the turbine-compressor shaft assembly rotates at its operating speed.  
      A quill shaft  175 , shown in  FIGS. 4-6 , extends from the gearbox  60  and forms at least a portion of the connection between the turbine-compressor rotor assembly  115  and the gearbox  60 . The quill shaft  175  is a substantially hollow member having a pinion end  180  and a diaphragm end  185 . The pinion gear  160  is positioned at the pinion end  180  of  5  the quill shaft  175 . In most constructions, the pinion gear  160  is formed as part of the quill shaft  175 . However, other constructions include a pinion gear  160  that is attached to the quill shaft  175  (e.g., shrunk-on, pinned, screwed, welded, and the like). The pinion gear  160  and the pinion end  180  of the quill shaft  175  are supported for rotation by the three planetary gears  150 . As such, no additional high-speed bearing is needed to support the pinion end  180  of the quill shaft  175 .  
      The diaphragm end  185  includes a diaphragm portion  190  that forms an attachment surface  195 . The attachment surface  195  facilitates the attachment of the quill shaft  175  to a rotor flange  200 .  
      The rotor flange  200  includes a planar surface  205  that engages the diaphragm  190  and a central bore that includes a tapered surface  210 . A second high-speed bearing  215  supports the rotor flange  200  for high-speed rotation. The various rotating components (i.e., turbine rotor  57 , compressor rotor  56 , rotor flange  200 ) at least partially define a rotor train that is supported by the first and second high-speed bearings  110 ,  215 . The bearings  110 ,  215  are positioned to define a space between the bearings  110 ,  215  and a space beyond the bearings  110 ,  215 . A portion of the rotor train is positioned within the space between the bearings  110 ,  215  and a portion is disposed in the space beyond the bearings  110 ,  215 . In the illustrated construction, a portion of the compressor rotor  56  and the entire turbine rotor  57  are disposed in the space beyond the bearings  110 ,  215 . As such, the compressor rotor  56  and the turbine rotor  57  are supported in a cantilever fashion with the turbine rotor  57  defining a free end  57   a.    
      The rotor flange also defines a catcher  216  that includes a first diameter bore  217  and a second diameter bore  218 . The first diameter bore  217  is disposed adjacent the planar surface  205  and is sized to snuggly receive the diaphragm  190 . Thus, the first diameter acts as a guide to assure that the quill shaft  175  is aligned with the rotor flange  200 . The second diameter bore  218  is positioned adjacent the first diameter bore  217  and extends for the remaining length of the catcher  216 . The second diameter bore  218  is also larger than the first diameter bore  217 .  
      To connect the diaphragm  190  to the rotor flange  200 , a plurality of coupling members, such as bolts  220  and shear pins  225 , extend between the diaphragm  190  and the flange  200  to allow for the transmission of torque therebetween. The bolts  220  and shear pins  225  are sized to transmit torque generated by the turbine rotor  57  to the generator rotor  125 . To protect the engine  10  from damage caused by high-torque transients that may periodically arise, the pins  225  and bolts  220  are sized to fail at a predetermined torque level. Thus, if the torque between the quill shaft  175  and the compressor shaft  56  exceeds that predetermined level, the bolts  220  and pins  225  will fail in shear. Generally, following the shear failure of the bolts  220  and pins  225  the quill shaft  175  moves axially away from the rotor shaft  232 . The gearbox housing  140  limits the extent of this axial movement such that the diaphragm  190  remains within the axial extent of the catcher  216 . The second diameter bore  218  acts as a guide to the diaphragm  190  as the two separated portions of the rotor train decelerate at different rates. Thus, the second diameter bore  218  substantially maintains the alignment of the quill shaft  175  as it decelerates.  
      The diaphragm  190  is flexible enough to allow slight misalignments between the rotor assembly  115  and the planetary gears  150  (i.e., angular misalignment) as well as slight axial displacements of the rotor flange  200  relative to the quill shaft  175 . In addition, the ability of the diaphragm  190  to flex allows the arrangement to tune the vibrations transmitted by the turbine-compressor rotor assembly  115  or the gearbox  160 .  
      A tie-bolt  230  extends through the rotor flange  200 , a rotor shaft  232 , and the compressor rotor  56  to the turbine rotor  57 . A first end  235  of the tie-bolt  230  engages the turbine rotor  57  (e.g., threads, pins, shrink-fit, and the like) or an intermediate component such as a threaded sleeve. A second end  240  of the tie-bolt  230 , shown in  FIG. 5 , includes a threaded portion  245  that extends beyond the tapered surface  210  disposed within the rotor flange  200 . The rotor shaft fits between the compressor rotor and the rotor flange. A tapered nut  250  engages the threaded portion  245  and the tapered surface  210  within the rotor flange  200  to lock the rotor flange  200 , the rotor shaft  232 , the compressor rotor  56 , and the turbine rotor  57  for rotation, while pretensioning the tie-bolt  230 . The pretensioning of the tie bolt  230  applies a compressive force to the rotor flange  200 , the rotor shaft  232 , and the compressor rotor  56 . It should be noted that the tie-bolt arrangement described herein is but one way of coupling the rotating elements (i.e., turbine rotor  57 , compressor rotor  56 , rotor shaft  232 , rotor flange  200 ) that define the rotor train. One of ordinary skill will realize that there are many other methods available to couple the rotating elements to one another. As such, the invention should not be limited to the tie-bolt arrangement just described.  
      In operation, the turbine-compressor rotor assembly  115  rotates at a high speed (e.g., typically in excess of 40,000 RPM). The quill shaft  175  also rotates at a high speed. The high-speed components are coupled to one another via the tie-bolt  230  and are completely supported for rotation by two high-speed bearings  110 ,  215  and the planetary gears  150  of the gearbox  60 .  
      The rotating quill shaft  175  drives the pinion gear  160 , which engages and drives the planetary gears  150 . The planetary gears  150  are sized relative to the pinion gear  160  to achieve a rotational speed that is low enough to allow for the use of low-speed bearings  170  to support the planetary gears  150 . In preferred constructions, the planetary gears  150  rotate at about 9,000 RPM with slower or higher speeds being possible. By properly sizing the gears  150 ,  160 , the need for additional high-speed bearings in the gearbox  60  can be avoided. High-speed bearings are susceptible to wear and have a higher instance of failure due to the significantly increased stress levels under which they operate. In addition, high-speed bearings are much more sensitive to vibration and imbalance issues than are their low-speed counterparts. As such, it is desirable to avoid the use of high-speed bearings where possible.  
      The drive gears  165  rotate with the planetary gears  150  and drive the ring gear  135  that is attached to the generator rotor  125 . The ring gear  135  is sized to rotate at the desired low speed (e.g., typically 3600 RPM or 1800 RPM for 60 Hz generators and 3000 RPM or 1500 RPM for 50 Hz generators) when the turbine-compressor rotor assembly  115  is rotating at the high speed. The ring gear  135  and the generator rotor  125  are supported for rotation by low-speed bearings. Thus, as one of ordinary skill will realize, the present arrangement allows for the support and rotation of several high-speed and low-speed components using only two high-speed bearings  110 ,  215 . The close coupling of the engine, the generator, and the gearbox allows the bearings  110 ,  215  to support various other high-speed components.  
      Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.