Patent Publication Number: US-2023160317-A1

Title: Turbo generator with separable shroud

Description:
BACKGROUND 
     This disclosure relates to Rankine cycle turbine generator systems and more particularly to a Rankine cycle turbine generator configured to reduce the cost and complexity of manufacture and repair. 
     Turbines for driving a generator in a Rankine cycle system require precision assembly to ensure efficient and reliable long-term operation. The turbine and generator may operate at very high rotational speeds exceeding 80,000 rpm, which subject the bearings supporting the generator shaft and turbine wheel to high forces. The turbine wheel is subjected thermal stress from large temperature changes, in addition to the stresses from very high rotational speeds. The generator and/or turbine may require service during the useful life of the Rankine cycle system. 
     There is a need for a turbo generator for use in a Rankine cycle system where the turbo generator is configured to reduce the cost and complexity of manufacture and repair. 
     There is a need for a turbo generator for use in a Rankine cycle system that can be serviced in the field without the need for service personnel with highly specialized skills, training, and equipment. 
     SUMMARY OF THE INVENTION 
     A turbo generator is configured to allow the turbine to be assembled to the generator, calibrated, and then shipped, stored, and installed as a generator/turbine unit. According to aspects of the disclosure, the turbine shroud is formed as a separate component from a turbine casing that defines an inlet volute and inlet and outlet connections to working fluid conduits of the Rankine cycle system. Separating the shroud from the turbine casing allows the turbine to be assembled to the generator and tested before the generator/turbine assembly is connected to the turbine casing. According to aspects of the disclosure, the inlet and outlet on the turbine casing can be permanently connected to the associated working fluid conduits by welding or other low-cost, sealed, permanent connection, and the generator/turbine assembly can be separated from the turbine casing while the turbine casing is permanently connected to the working fluid conduits. 
     A disclosed embodiment of a turbine driven generator comprises a generator having a shaft, a generator housing and a generator end plate including a seal surrounding the shaft, the generator shaft extending along an axis of rotation through the generator end plate and coupled to components of the generator that generate electrical current when the shaft is rotated. A turbine wheel is secured to the shaft so the turbine wheel and shaft rotate together, the turbine wheel including a plurality of blades extending from a body of the turbine wheel to blade tips. A nozzle ring including a plurality of vanes surrounds a periphery of the turbine wheel. A turbine shroud is secured to the generator end plate with the nozzle ring axially between the turbine shroud and generator end plate. The turbine shroud including a shroud inside surface defining a working clearance between the blade tips and the shroud. The turbine shroud defines a turbine exhaust outlet surrounding an axis of rotation of the turbine wheel and shaft. A turbine casing includes an inlet opening for vapor phase working fluid and an outlet for vapor phase working fluid passing through the turbine exhaust outlet. The turbine casing at least partially defining an inlet volute directing vapor phase working fluid through the vanes of the nozzle ring and onto the turbine wheel where energy is transferred from the vapor phase working fluid to the turbine wheel. The turbine shroud is separate from the turbine casing and the turbine shroud and turbine casing are each independently attached to the generator end plate and the generator end plate can be separated from the turbine casing while the turbine shroud remains connected to the generator end plate. 
     The nozzle ring may be a separate component trapped between a periphery of the turbine shroud and the generator end plate. Alternatively, the nozzle ring may be formed as an integral part of the generator end plate or as an integral part of the shroud. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a longitudinal sectional view through a turbine-driven generator showing the inlet volute separated from the generator end plate, turbine wheel, nozzle ring and shroud according to aspects of the disclosure; 
         FIG.  2    is an exploded perspective view of the components of the turbine assembly of the turbine-driven generator according to aspects of the disclosure; and 
         FIG.  3    is a longitudinal sectional view through the generator end plate, turbine wheel, nozzle ring, and shroud, with the inlet volute and exhaust diffuser mounted to the generator end plate according to aspects of the disclosure; 
         FIG.  4    is a longitudinal sectional view through a second embodiment of a turbine-driven generator according to aspects of the disclosure; 
         FIG.  5    is an exploded perspective view of the turbine-driven generator of  FIG.  4   ; 
         FIG.  6   , is a longitudinal sectional view through a third embodiment of a turbine driven generator according to aspects of the disclosure; and 
         FIG.  7    is an exploded perspective view of the turbine-driven generator of  FIG.  6   . 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are now described with reference to the drawings, wherein like referenced numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a more thorough understanding of the one or more embodiments. It is evident, however, in various cases, that the one or more embodiments can be practiced without these specific details. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated or if not so described explicitly. Repetitive description of same or similar functionalities of like components described in prior embodiments is omitted for sake of brevity. 
     With reference to  FIG.  1   , a turbo generator  10  is configured to reduce the cost and complexity of manufacture and also to reduce the cost and complexity of maintenance should service be required. A radial inflow turbine assembly  12  is arranged to drive an electric generator  14  which may be an induction generator or synchronous generator. The design and operation of induction and synchronous generators is well-understood and will not be discussed in detail. The turbo generator  10  includes a generator  14 , a turbine assembly  12  and a turbine casing  16 . The turbine casing  16  includes an inlet  18 , an inlet volute  20  and a turbine exhaust outlet  22 . The inlet  18  and inlet volute  20  receive energetic vapor phase working fluid and direct the working fluid through a nozzle ring  24  to increase the velocity of the working fluid and orient the flow of working fluid in a specific trajectory toward a turbine wheel  26  as is known in the art. The working fluid may be water, organic refrigerant, or any other material compatible with the Rankine cycle. Energy is transferred from the working fluid to the turbine wheel  26 , which is arranged to rotate a shaft  30  connected to a rotor  31  including magnets that generate electrical current when rotated relative to stator coils  33 . A turbine shroud  28  includes an inside surface  32  spaced from the turbine wheel  26  so that the working fluid is directed through the blades  34  of the turbine  12 . A working clearance is defined between tips of the blades  34  on the turbine wheel  26  and the inside surface  32  of the shroud  28 . The working clearance is as small as possible while avoiding any contact between the turbine blades  34  and the inside surface  32  of the shroud  28 . The working clearance between the turbine blades  34  and the inside surface  32  of the shroud  28  must take into account the dimensional tolerances of the turbine wheel  26  and shroud  28 , dimensional tolerance of components positioning the turbine wheel  26  and shroud  28  relative to each other, and changes in the dimensions of the turbine wheel  26 , shroud  28  and adjacent structures due to expansion and contraction as temperature of the turbine assembly  12  changes during operation of the turbo generator  10 . 
     According to aspects of the disclosure, the turbine casing  16  includes an inflow volute  20  and outlet  22  but does not include the shroud  28 , which is a separate component. The turbine casing  16  is configured to mount to a radially extended end plate  36  of the generator  14 . The generator end plate  36  includes an opening  38  for a shaft  30  that extends from the turbine wheel  26  into the generator  14 . The opening  38  in the generator end plate  36  includes a seal  40  to prevent working fluid from passing along the shaft  30  and into the generator body  42 . The turbine wheel  26  is secured to the shaft  30  so that rotation of the turbine wheel  26  rotates the rotor  31  of the generator  14  resulting in a flow of electrical current. The construction and operation of shaft driven generators and alternators is well-understood and will not be discussed in detail here. A nozzle ring  24  includes vanes  44  that direct working fluid from the inflow volute  20  onto the turbine wheel  26  at an increased velocity and in a pre-determined angular orientation relative to blades  34  on the turbine wheel  26  as is known in the art. The generator end plate  36  defines a circular shoulder  46  within which the nozzle ring  24  is mounted, with a base side of the nozzle ring  24  against an outside surface of the generator end plate  36  and a radially outer side of the nozzle ring  24  against the circular shoulder  46  as shown in  FIGS.  1  and  3   . 
     According to aspects of the disclosure, the shroud  28  is separate from the turbine casing  16  and is secured to the generator end plate  36  by fasteners  48  that extend through the nozzle ring  24  into the end plate  36 . Making the shroud  28  separate from the turbine casing  16  and mounting the shroud  28  and nozzle ring  24  to the generator end plate  36  allow the generator  14 , along with its end plate  36 , turbine wheel  26 , nozzle ring  24  and shroud  28  (the turbine assembly  12 ) to be separated from the turbine casing  16  which includes the inlet  18 , inflow volute  20 , and integrally formed outlet  22  as shown in  FIG.  1   . The inlet  18  to the inflow volute  20  and the outlet  22  can be permanently attached to the associated working fluid conduits by welding or other permanent connection. The generator  14  and turbine assembly  12  can be separated from the turbine casing  16  as shown in  FIG.  1    without disconnecting the inlet  18  and outlet  22   from the working fluid conduits, should service of the generator  14  or turbine assembly  12  be necessary. 
     Separating the shroud  28  from the turbine casing  16  allows the generator  14  to be assembled to the turbine assembly  12  to form a turbo-generator  10  that can be handled as a unit. In the illustrated embodiment, the turbine assembly  12  is built on the generator end plate  36  with the axial position of the turbine wheel  26  defined by a shoulder on the shaft  30 . The shroud  28  is spaced apart from the generator end plate  36  by the nozzle ring  24 , with the axial position of the shroud inside surface  32  relative to the tips of the turbine wheel blades  34  defining the working clearance. Together, the generator end plate  36 , nozzle ring  24 , and shroud  28  define a chamber surrounding the turbine wheel  26 . In some embodiments, one or more annular shims (not shown) may be arranged between the nozzle ring  24  and the generator end plate  36  to adjust the axial position of the shroud  26  to calibrate the working clearance between an inside surface  32  of the shroud  26  and the tips of the blades  34  of the turbine wheel  26 . With the shroud  28  securely mounted to the generator end plate  36 , the working clearance of the turbine assembly  12  is fixed and is not disturbed when the generator/turbine assembly is handled as a unit as shown in  FIG.  1   . 
     In the disclosed Rankine cycle turbine generator system, the fluid flow path for working fluid passing through the turbine is closed and sealed, so the connections between working fluid conduits and the inlet  18  and outlet  22  on the turbine casing  16  must be hermetically sealed to prevent leakage of working fluid. The disclosed turbo generator  10  is configured to be separated from the turbine casing  16  without disturbing the position of the shroud  28  relative to the turbine wheel  26 . Providing the turbine shroud  28  as a separate component from the turbine casing  16  allows the connections of working fluid conduits to the inlet  18  and outlet  22  to be permanent, welded connections. Welded connections are relatively inexpensive, sealed, and permanent. According to aspects of the disclosure, the inlet  18  and outlet  22  connections can be welded and inspected while the turbine casing  16  is separated from the generator  14  and turbine assembly  12  (the turbo-generator  10 ) as shown in  FIG.  1   . With the turbine casing  16  separated from the turbo-generator, heat from the welding process cannot distort the shroud  28  or other components of the turbine assembly  12 . The generator  14  and turbine assembly  12  can be connected to the turbine casing  16  after the welded connections are made, cooled, and inspected. Further, the permanent connections of working fluid conduits to the inlet  18  and outlet  22  can remain in place while the turbo-generator  10  is separated from the turbine casing  16  to permit service of the generator  14  and/or turbine assembly  12 . A replacement, pre-assembled and pre-calibrated turbo-generator  10  can be exchanged for a turbo-generator in need of service. The exchange of one turbo-generator  10  for another is relatively simple and does not require specialized skills or equipment. 
     As shown in  FIG.  2   , the shroud  28  is mounted to the generator end plate  36  by fasteners  48  that pass through the shroud  28 , the nozzle ring  24  and into threaded apertures in the generator end plate  36 . The fasteners  48  ensure accurate location of the shroud  28  on the generator end plate  36  concentric with the axis of rotation of the turbine wheel  26  and shaft  30 . In some embodiments, the nozzle ring  24  may include protruding nubs or pins  50  that are received in corresponding detents  52  in the generator end plate  36  and shroud  28  to ensure accurate placement of the shroud  28  relative to the axis of rotation of the shaft  30  and turbine wheel  26 . The shroud  28  must be positioned accurately relative to the turbine wheel  26  to allow the smallest possible working clearance between the inside surface  32  of the shroud and the tips of the blades  34  on the turbine wheel  26 . The working clearance must allow for expansion and contraction of the turbine assembly components, and any increase in working clearance necessary to also accommodate inaccurate positioning of the shroud  28  would decrease the efficiency of the turbine. The generator end plate  36 , nozzle ring  24 , turbine wheel  26  and shroud  28  are all constructed of stainless steel and have substantially equal coefficients of expansion, so these components should all expand and contract at approximately the same rate, keeping temperature-related changes in dimensions to a minimum. Providing the shroud  28  as a separate component from the much larger and more massive turbine casing  16 , allows the shroud  28  to heat and cool at a different rate relative to heating and cooling of the turbine casing  16 . As shown in  FIG.  3   , when the generator end plate  36  is mounted to the turbine casing  16 , the casing  16  and shroud  28  are in contact with each other at the periphery of the shroud  28 , but are not coupled to each other by fasteners or other means, which allows the shroud  28  and casing  16  to expand and contract independently without exposing each other to thermal stresses. The annular joint  53  between the periphery of the shroud  28  and the turbine casing  16  is formed by two planar surfaces clamped together when the turbine casing  16  is mounted to the generator end plate  36 . In a disclosed embodiment, this connection  53  is not sealed since leakage of working fluid from joint  53  remains contained within turbine casing  16 . As can be seen in  FIG.  3   , when the generator end plate  36  is mounted to the turbine casing  16 , an annular space  54  is defined between the turbine exhaust outlet  56  on the shroud  16  and the exhaust outlet  22  on the turbine casing  16 . During operation of the turbo generator  10 , vapor phase working fluid leaving the shroud  28  will fill the space  54  between an outside surface of the shroud  28  and an inside surface of the turbine casing  16 . Exposing both the inside and outside surfaces of the shroud  28  to heating from contact with vapor phase working fluid leaving the turbine should allow the shroud  28  to heat more uniformly and reduce thermal stresses within the shroud  28 . It will be apparent to those skilled in the art that a uniformly heated shroud  28  will experience reduced thermal distortion and this should permit a reduced working clearance between the shroud  28  and the turbine blades  34 . 
       FIG.  3    illustrates the turbine assembly  12  coupled to the turbine casing  16  and omits the generator  14  for ease of reference. According to aspects of the disclosure, it is not possible to separate the turbine assembly  12  from the generator  14 , which are constructed to be assembled and handled as a generator/turbine assembly (turbo-generator  10 ) as shown in  FIG.  1   . The generator shaft  30  is supported on bearings in the generator  14 , and the generator shaft  30  in turn supports the turbine wheel  26  in a pre-determined axial position. Further, the generator end plate  36  defines part of the chamber within which the turbine wheel  26  rotates. The shroud  28  can only be accurately positioned relative to the turbine wheel  26  after the turbine wheel  26  is secured to the generator shaft  30 . The generator  14  includes a second, rear end plate  37  that connects to an end of the generator body  42  and includes a bearing  39  supporting one end of the generator shaft  30 . In one embodiment, the generator body  42 , end plate  36  and second end plate  37  cooperatively define coolant channels  41  through which coolant can be circulated to remove heat from the generator  14 . The generator body  42 , end plate  36  and second end plate  39  include seals to contain coolant in the coolant channels  41  and separate the coolant channels  41  from the “dry” area at the center of the generator  14 . According to aspects of the disclosure, the generator  14 , including end plate  36  and second end plate  37  is assembled before attachment of the turbine wheel  26 , nozzle ring  24  and shroud  28  to form the turbo-generator  10  shown in  FIG.  1   . 
     As shown in  FIG.  3   , the inlet volute  20  is defined by portions of the turbine casing  16 , the periphery of the shroud  28 , and an annular portion  58  of the generator end plate  36  radially between the nozzle ring  24  and the turbine casing  16 . The generator end plate  36  defines an annular, outward facing shoulder  60  configured to mate with an inward facing rim  62  of a circular opening in the turbine casing  16 . In a disclosed embodiment, the generator end plate  36  includes grooves that accommodate one or more annular seals  63  compressed between the end plate  36  and the turbine casing  16  to seal the connection between the end plate  36  and turbine casing  16  to contain vapor phase working fluid. The seals  63  may be axially compressed, radially compressed or may include both an axial seal and a radial seal.  FIG.  1    illustrates annular glands and O-ring type seals, but other seal configurations are compatible with the disclosed turbo generator  10 . Since the connections of the turbine casing inlet  18  and outlet  22  to conduits carrying working fluid are permanent, only one sealed connection is needed between the turbine casing  16  and the disclosed turbo-generator  10  is needed at the periphery of the generator end plate  36  and turbine casing  16 . It is possible to replace the seals  63  or gaskets at this connection 60/62 whenever the turbo-generator  10  is separated from the turbine casing  16 . 
     As shown in  FIG.  2   , the nozzle ring  24  has a base with an axial height equal to an axial height of the annular shoulder  46  on the generator end plate  36 . Above the base, the nozzle ring  24  defines vanes  44  that direct vapor phase working fluid onto the turbine wheel  26  at a predetermined trajectory. Although vanes  44  are illustrated only on the lower portion of nozzle ring  24 , it will be understood that the vanes extend about the entire circumference of the nozzle ring  24 . As shown in  FIG.  3   , the configuration of the inlet volute  20 , the generator end plate  36  and the nozzle ring  24  form a surface of uniform height to guide vapor phase working fluid toward and through the vanes  44  on the nozzle ring  24 . The bottom surface of the shroud  28  sits on top of the vanes  44  to form an upper boundary of openings between the vanes  44 , each of which has a flat top surface to support the shroud  28  relative to the generator end plate  36  and turbine wheel  26 . The circumference of the inner rim  64  of the inlet volute  20  and outer circumference  66  of the shroud  28  are substantially identical and define an inner boundary of the inlet volute  20 . The surfaces of the generator end plate  36 , nozzle ring  24 , shroud  28  and turbine casing  16  that define the inlet volute  20  are selected and arranged to form a smooth, uninterrupted surface for guiding vapor phase working fluid from the inlet  18  through the vanes  44  of the nozzle ring  24 . 
       FIGS.  4  and  5    illustrate an alternative embodiment of a turbo-generator  10   a  and turbine casing  16   a  according to aspects of the disclosure. The basic structure and function of the turbo-generator  10   a  and turbine casing  16   a  of  FIGS.  4  and  5    are similar to that of the turbo-generator  10  and turbine casing  16  described with regard to  FIGS.  1 - 3    and will be discussed only to clarify differences between the embodiments.  FIGS.  4  and  5    illustrate a turbo-generator  10   a  including a generator end plate  36   a  where the nozzle ring  24   a  is formed as an integral part of the generator end plate  36   a . Reducing the number of parts in the generator  14   a  that determine the axial position of the generator shaft  30  and turbine wheel  26  relative to the generator end plate  36   a  allow the nozzle ring  24   a  to be formed integrally with the generator end plate  36   a  and eliminate the need for shims described with respect to the turbo-generator  10  described with respect to  FIGS.  1 - 3   . Outer and inner generator body parts  42   a ,  42   b  are sealed to each other to define a coolant channel  41   a  through which coolant is circulated to remove heat from the generator  14   a . Outer generator body part  42   a  defines coolant openings  72  allowing coolant to circulate in the coolant channel  41 . Coolant openings  72  are provided with durable, sealed connectors to coolant conduits (not shown). A forward end of generator body part  42   a  includes an outward projecting flange  68  by which the generator  14   a  is secured to the generator end plate  36   a  with the generator shaft  30  projecting through a shaft seal  40  supported by the generator end plate  36   a . The generator shaft  30  is supported by a rear bearing  70  seated in inner generator body part  42   b  and forward bearings  71  seated in the forward end of the outer generator body part  42   a . The forward end of the outer generator body part  42   a  receives and is sealed to a central projection  74  of the generator end plate  36   a  that extends axially to abut a shoulder  76  on the outer generator body part  42   a  that determines the position of the generator end plate  36   a  relative to the generator  14   a  when the generator  14   a  is secured to the generator end plate  36   a . A generator rear end plate  37   a  closes the rear end of the generator housing  42   a ,  42   b  and biases inner generator housing part  42   b  into contact with a shoulder on generator outer housing part  42   a , holding the outer generator part  42   a  and inner generator housing part  42   b  in a fixed, predetermined location relative to each other. The generator rear end plate  37   a  may define an outlet  43  for condensed vapor phase working fluid that has leaked into the central “dry” area of the generator casing  42   a ,  42   b , e.g., the region defined between the generator end plate  36   a , forward end of outer generator housing part  42   a , inner generator housing part  42   b  and generator rear end plate  37   a . Some vapor phase working fluid may get past the shaft seal  40  and condense within the central working region of the generator  14   a  and providing a drain  43  for this liquid phase working fluid will prevent it from accumulating within the generator and also allow the recovery of the working fluid for return to a working fluid reservoir (not shown). 
     In the turbo-generator  10   a  of  FIGS.  4  and  5   , the shroud  28   a  is secured to the generator end plate  36   a  by fasteners (not shown) extending through holes in the periphery of the shroud  28   a  in a manner similar to that shown with respect to the turbo-generator  10  illustrated in  FIGS.  1 - 3   .  FIG.  5    illustrates the shroud  28   a  separated from the generator end plate  36   a  to show the nozzle ring  24   a  integrally formed as part of the generator end plate  36   a . When manufactured and calibrated according to aspects of the disclosure, the turbo-generator  10   a  will include the shroud  28   a  mounted to the generator end plate  36   a . The turbo-generator  10   a  is separable from the turbine casing  16   a  by removal of fasteners (not shown in  FIGS.  4  and  5   ) from the periphery of the generator end plate  36   a  that secure the turbine casing  16   a  to the generator end plate. Although not shown in  FIGS.  4  and  5   , the turbine casing  16   a  includes an inlet in addition to the outlet  22   a . As in the embodiment illustrated in  FIGS.  1 - 3   , the inlet and outlet  22   a  of the turbine casing  16   a  can be permanently joined to fluid conduits for energetic vapor phase working fluid provided from an evaporator (or other source) and for turbine exhaust from the outlet  22   a  to flow to a condenser. In the embodiment illustrated in  FIGS.  4  and  5   , the outlet  56   a  of the shroud  28   a  includes a gland and seal that mates with an inside surface of the turbine casing outlet  22   a . The points of contact between the turbine casing  16   a  and the shroud  28   a  allow for relative movement between the shroud  28   a  and the turbine casing  16   a , allowing the shroud  28   a  to expand and contract independently of the turbine casing  16   a  when heated during use and cooling when not being used. Allowing relative movement between the shroud  28   a  and the turbine casing  16   a  eliminates transfer of stress between the shroud  28   a  and the turbine casing  16   a  as the components heat up and cool down. According to aspects of the disclosure, the same material is used for the generator end plate  36   a , turbine wheel  26   a , shroud  28   a  and turbine casing  16   a  to reduce stress that might be caused by using different materials that have different rates of thermal expansion. If different materials are used, the materials may be selected to have similar rates of thermal expansion. 
       FIGS.  6  and  7    illustrate a further embodiment of a turbo-generator  10   b  and turbine casing  16   b  that is substantially identical to the turbo-generator  10   a  and turbine casing  16   a  illustrated in  FIGS.  4  and  5   . Turbo-generator  10   b  differs from turbo-generator  10   a  in that the nozzle ring  24   b  is formed as an integral part of the periphery of the shroud  28   b  and not generator end plate  36   b . In all other respects, the turbo-generator  10   b  and turbine casing  16   b  of  FIGS.  6  and  7    are identical to the turbo-generator  10   a  and turbine casing  16   a  of  FIGS.  4  and  5   . In  FIG.  7   , the vanes on the nozzle ring  24   b  are shown at the bottom of the figure and not at the top, but it will be understood that the nozzle ring  24   b  includes vanes extending around its entire circumference. 
     In the turbo-generator embodiments of  FIGS.  4 - 7   , determining the axial position of the generator shaft  30  and turbine wheel  26  relative to the generator end plate  36   a ,  36   b  will be described with reference to  FIG.  4   . The turbine wheel  26  is seated on a wheel shoulder  29  of the generator shaft  30 , so the axial position of the turbine wheel  26  depends upon the axial position of the generator shaft  30 . The generator shaft  30  includes a bearing shoulder  35  that seats against the forward bearings  71 . The embodiments of  FIGS.  4 - 7    employ an assembly of two forward bearings  71  separated by a spacer  73 , although a single bearing with no spacer may also be used. The central projection  74  extending rearwardly from the generator end plate  36   a  defines a bearing stop surface against which the forward bearings  71  are seated. A biasing element between the rear bearing  70  and the rear end plate  37   a  biases the generator shaft  30  toward the generator end plate  36   a , ensuring that the forward bearings  71  are in contact with the bearing stop surface on the central protrusion  74  of the generator end plate  36   a . In this configuration, the axial position of the turbine wheel  26  is determined by the distance between the bearing shoulder  35  and wheel shoulder  29  on the generator shaft  30 , the axial dimensions of the bearings  71  and spacer  73  and the distance between the bearing stop surface and outside surface of the generator end plate  36   a . By precisely controlling these dimensions, the position of the turbine wheel  26  can be controlled within a small range that eliminates the need for a separate nozzle ring  24  and shims as described in the embodiment of  FIGS.  1 - 3   . Controlling the position of the turbine wheel  26  relative to the generator end plate  36   a ,  36   b  allows the nozzle ring  24   a ,  24   b  to be integrally formed with either the generator end plate  36   a  as shown in  FIGS.  4  and  5    or with the shroud  28   b  as shown in  FIGS.  6  and  7   .