Abstract:
Provided is a rotating machine that is provided with: a rotating shaft ( 2 ); a plurality of impellers ( 3 ) that are affixed to the rotating shaft ( 2 ) and that rotate with the rotating shaft ( 2 ); and a casing ( 5 ) that surrounds the rotating shaft ( 2 ) and the impellers ( 3 ), and that forms diffuser channels ( 19 ) through which is circulated a fluid (G) that is discharged from the impellers ( 3 ) in the radial direction toward the outside and return channels ( 20 ) that guide the fluid (G) circulated in the diffuser channels ( 19 ) in the radial direction toward the inside and lead the fluid (G) to downstream impellers ( 3 ). As regards the plurality of impellers ( 3 ), the further downstream the arrangement thereof, the smaller the cross-sectional areas of the fluid (G) flow paths are formed to be, and among the diffuser channels ( 19 ) that correspond to each pair of neighboring impellers ( 3 ), the diffuser channel ( 19 ) arranged on the upstream side is a vaneless diffuser, and the diffuser channel ( 19 ) arranged on the downstream side is a vaned diffuser.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rotating machine including a rotating shaft, and multiple impellers which are fixed to the rotating shaft and rotate together with the rotating shaft. 
         [0002]    Priorities of this application are claimed based on Japanese Patent Application No. 2013-193390 filed on Sep. 18, 2013, the content of which is incorporated herein by reference. 
       BACKGROUND ART 
       [0003]    As well known, a rotating machine such as a centrifugal compressor is configured to allow gas to pass through a rotating impeller in a radial direction of the rotating impeller, and to compress the gas by centrifugal force occurring during rotation. A multistage centrifugal compressor including impellers of multiple stages in an axial direction, and compressing gas in stages is known as this type of centrifugal compressor. 
         [0004]    The impellers are rotatably supported by a rotating shaft in a casing of the centrifugal compressor. The centrifugal compressor suctions a fluid such as air or gas via a suction port of the casing, and applies centrifugal force to the fluid by rotating the impellers via the rotating shaft. Kinetic energy induced by the centrifugal force is converted into compression energy by diffusers and a scroll portion, and the compressed gas is sent from a discharge port of the casing. 
         [0005]    Among the aforementioned rotating machines, particularly, a centrifugal compressor, in which many impellers are installed on the same shaft, and each of the impellers has one outlet for gas, is referred to as a straight type centrifugal compressor among single-shaft multistage centrifugal compressors. 
         [0006]    PTL 1 discloses an example of a single-shaft multistage centrifugal compressor in which a stage having a vaneless diffuser and a stage having a vaned diffuser are combined together. This centrifugal compressor aims to maintain high efficiency in the stage having the vaned diffuser, and to secure a wide operating range in the stage having the vaneless diffuser. 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2010-31777 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In a rotating machine including impellers of multiple stages, the temperature of gas is further increased as the gas passes through a further downstream stage. In contrast, when the impellers of all of the stages have the same diameter, the impellers of all of the stages rotate at the same rotational speed, and the speed of sound is further increased due to an increase in the temperature as the gas passes through a further downstream stage. Accordingly, an upstream stage has a high machine Mach number (a value obtained by dividing the circumferential speed of the impeller by the speed of sound), and a downstream stage has a low machine Mach number. 
         [0009]    As with the rotating machine disclosed in PTL 1, when a vaned diffuser is provided in an upstream stage having a high machine Mach number, an operating range (flow rate range) may be narrowed compared to when a vaneless diffuser is provided. 
         [0010]    Since gas is compressed and a volumetric flow rate is decreased as the gas passes through a further downstream stage, the width of a flow path of a downstream stage becomes narrower than an upstream stage. When a vaneless diffuser is provided in a downstream stage having a narrow width of a flow path, efficiency may be decreased. 
         [0011]    That is, when the rotating machine has a configuration in which a vaned diffuser of an upstream stage and a vaneless diffuser of a downstream stage are combined together, it may not be able to not only satisfactorily maintain high efficiency but also satisfactorily secure a wide operating range. 
         [0012]    An object of the present invention is to provide a rotating machine in which not only high efficiency is maintained but also a wide operating range is secured. 
       Solution to Problem 
       [0013]    According to a first aspect of the present invention, there is provided a rotating machine including: a rotating shaft; multiple impellers fixed to the rotating shaft, and rotating together with the rotating shaft; and a casing configured to surround the rotating shaft and the impellers, and to form diffuser channels allowing the flowing through of a fluid which is discharged from the impellers to an outward side in a radial direction, and return channels by which the fluid flowing through the diffuser channels is guided to an inward side in the radial direction, and is introduced into the impellers of downstream stages. The multiple impellers are formed such that the sectional area of a flow path of the impeller for the fluid is smaller when the impeller is disposed at a further downstream stage. The diffuser channel of an upstream stage of the diffuser channels respectively corresponding to a pair of adjacent impellers is a vaneless diffuser, and the diffuser channel of a downstream stage is a vaned diffuser. 
         [0014]    In this configuration, it is possible to secure a wide operating range by providing a vaneless diffuser in an upstream stage having a high machine Mach number, and it is possible to maintain high efficiency by providing a vaned diffuser in a downstream stage having a small sectional area of a flow path. As a result, it is possible to provide the rotating machine in which not only high efficiency is maintained but also a wide operating range is secured. 
         [0015]    In the rotating machine, all the diffuser channels of upstream stages disposed further upstream of the diffuser channel disposed in the upstream stage may be vaneless diffusers. All the diffuser channels of downstream stages disposed further downstream of the diffuser channel disposed in the downstream stage may be vaned diffusers. 
         [0016]    In the rotating machine, multiple pairs of the impellers may be connected to each other in an axial direction of the rotating shaft. 
       Advantageous Effects of Invention 
       [0017]    According to the present invention, it is possible to secure a wide operating range by providing a vaneless diffuser in an upstream stage having a high machine Mach number, and it is possible to maintain high efficiency by providing a vaned diffuser in a downstream stage having a small sectional area of a flow path. As a result, it is possible to provide a rotating machine in which not only high efficiency is maintained but also a wide operating range is secured. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a schematic sectional view of a centrifugal compressor in a first embodiment of the present invention. 
           [0019]      FIG. 2  is an enlarged view of impellers of the centrifugal compressor in the first embodiment of the present invention. 
           [0020]      FIG. 3  is a performance curve graph for the centrifugal compressor in the first embodiment of the present invention, and a centrifugal compressor in the related art. 
           [0021]      FIG. 4  is a schematic sectional view of a centrifugal compressor in a second embodiment of the present invention. 
           [0022]      FIG. 5  is a schematic sectional view of a centrifugal compressor in a third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0023]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the embodiment, a single-shaft multistage centrifugal compressor including multiple impellers is exemplarily described. 
         [0024]    As illustrated in  FIG. 1 , a centrifugal compressor  1  in the embodiment is configured to include the following main components: a rotating shaft  2  rotating around an axial line  0 ; an impeller  3  attached to the rotating shaft  2 , and compressing a fluid G (for example, air) by centrifugal force; and a casing  5  which rotatably supports the rotating shaft  2 , and is provided with a flow path  4  through which the fluid G flows from an upstream side to a downstream side. 
         [0025]    The casing  5  has a substantially tubular outline. The rotating shaft  2  is disposed to penetrate through the center of the casing  5 . Journal bearings  7  are respectively provided at both ends of the casing  5  in an axial direction of the rotating shaft  2 . A thrust bearing  8  is provided at one of both ends. The journal bearings  7  and the thrust bearing  8  rotatably support the rotating shaft  2 . That is, the rotating shaft  2  is supported by the casing  5  via the journal bearings  7  and the thrust bearing  8 . 
         [0026]    A suction port  9  is provided on a first side of the casing  5  in the axial direction, and the fluid G flows into the casing  5  from the outside via the suction port  9 . A discharge port  10  is provided on a side opposite to the first side, and the fluid G flows to the outside via the discharge port  10 . An inner space  11  communicating with the suction port  9  and the discharge port  10  is provided in the casing  5 , and increases and decreases in the diameter of the inner space  11  are repeated. 
         [0027]    The inner space  11  serves as not only a space accommodating the impellers  3  but also the flow path  4 . That is, the suction port  9  communicates with the discharge port  10  via the impellers  3  and the flow path  4 . The casing  5  is configured to include a shroud casing  5   a  and a hub casing  5   b . The inner space  11  is formed by the shroud casing  5   a  and the hub casing  5   b.    
         [0028]    Multiple impellers  3  are arranged along the axial direction of the rotating shaft  2  while being spaced apart from each other. The centrifugal compressor  1  in the embodiment has five compressor stages of a first compressor stage  31  to a fifth compressor stage  35 . In the illustrated example, five impellers  3  are provided; however, at least two impellers  3  may be provided. 
         [0029]    As illustrated in  FIG. 2 , each of the impellers  3  is configured to include a hub  13 ; a blade  14 ; and a shroud  15 . The hub  13  is formed to have a substantially disc shape such that the diameter of the hub  13  gradually increases toward the discharge port  10 . The blades  14  are radially attached to the hub  13 , and multiple blades  14  are disposed side by side in a circumferential direction. The shroud  15  is attached in such a way as to cover tip sides of the multiple blades  14  in the circumferential direction. 
         [0030]    While meandering in a radial direction of the rotating shaft  2 , the flow path  4  progresses in the axial direction to connect the impellers  3  such that the fluid G is compressed in stages by the multiple impellers  3 . The flow path  4  is configured to include a suction channel  17 ; a compression channel  18 ; a diffuser channel  19 ; and a return channel  20  as main components. The diffuser channel  19  is a channel which converts kinetic energy applied to the fluid G by the impeller  3  into pressure energy. 
         [0031]    The impeller  3  is formed such that the sectional area of a flow path of the impeller  3  for the fluid G is smaller when the impeller  3  is disposed at a further downstream stage. In other words, the compression channel  18  is formed to become narrower as the fluid G approaches the downstream side. 
         [0032]    The suction channel  17  is a channel which allows the fluid G to flow inward from an outward side of the channel in the radial direction, and then changes the direction of the fluid G to the axial direction of the rotating shaft  2  when the fluid G reaches a region immediately before the impeller  3 . Specifically, the suction channel  17  is configured as a straight channel  21  and a corner channel  22 . The straight channel  21  is a straight channel which allows the fluid G to flow inward from the outward side of the channel in the radial direction. The corner channel  22  is a curved channel which changes the flow direction of the fluid G (flowing from the straight channel  21 ) at a radial inward side of the channel to the axial direction, and allows the fluid G to flow toward the impeller  3 . 
         [0033]    Multiple return blades  23  are radially disposed around the axial line  0  in the straight channel  21  positioned between two impellers  3  such that the multiple return blades  23  divide the straight channel  21  in a circumferential direction of the rotating shaft  2 . 
         [0034]    The compression channel  18  is a channel which compresses the fluid G (sent from the suction channel  17 ) in the impeller  3 . The compression channel  18  is surrounded by a blade installation surface of the hub  13 , and an inner wall surface of the shroud  15 . 
         [0035]    A radial inward side of the diffuser channel  19  communicates with the compression channel  18 . The diffuser channel  19  serves as a channel which allows the fluid G compressed by the impeller  3  to flow to a radial outward side of the channel. The radial outward side of the diffuser channel  19  communicates with the return channel  20 . The diffuser channel  19 , which is connected to a radial outward side of the impeller  3  (a fifth stage impeller  3  in  FIG. 1 ) positioned on the furthest downstream side of the flow path  4 , communicates with a discharge scroll  12  (to be described later). 
         [0036]    The return channel  20  is formed to have a substantially U-shaped section. The diffuser channel  19  communicates with an upstream end of the return channel  20 , and the straight channel  21  of the suction channel  17  communicates with a downstream end of the return channel  20 . The return channel  20  reverses the flow direction of the fluid G (flowing to an outward side in the radial direction via the diffuser channel  19  by the impeller  3  (the impeller  3  of an upstream stage)) to the inward side in the radial direction, and then sends the fluid G to the straight channel  21 . 
         [0037]    As described above, the impeller  3  is formed such that the sectional area of a flow path of the impeller  3  for the fluid G is smaller when the impeller  3  is disposed at a further downstream stage. Accordingly, the width of the flow path  4  is to become narrower as the fluid G approaches the downstream side (downstream stage). For example, the diffuser channel  19  is formed to become narrower when the diffuser channel  19  is positioned closer to the downstream side. 
         [0038]    The discharge scroll  12  is provided in the casing  5  so as to discharge the fluid via a discharge port. The discharge scroll  12  includes a scroll flow path  25  which is formed to surround the entire circumference of an outlet of the diffuser channel  19  positioned in the outer circumference of a final stage impeller  3 . 
         [0039]    The scroll flow path  25  is formed to surround the entire circumference of the outlet of the diffuser channel positioned in the outer circumference of the final stage impeller  3 . The scroll flow path  25  is formed in such a way that the sectional area of the scroll flow path  25  gradually and continuously increases along a rotation direction of the impeller  3 . 
         [0040]    The diffuser channel  19  and the discharge scroll  12  serve as an outlet flow path  6  through which the fluid sent from the outlet of the impeller  3  flows, and by which the pressure of the fluid is increased as the fluid approaches the downstream side. 
         [0041]    In the centrifugal compressor according to the embodiment, the diffuser channels  19 , which are respectively connected to the outlets of the impellers of the first compressor stage  31 , the second compressor stage  32 , and the third compressor stage  33 , are vaneless diffusers. That is, vanes (diffuser vanes or blades) are not formed in the diffuser channels through which the fluid G (discharged to the radial outward side from the impellers  3  of the first compressor stage  31  to the third compressor stage  33 ) flows. 
         [0042]    The diffuser channels  19  respectively connected to the outlets of the impellers  3  of the fourth compressor stage  34  and the fifth compressor stage  35  are vaned diffusers. That is, multiple vanes  29  are formed in the diffusers through which the fluid G (discharged to the radial outward side from the impellers  3  of the fourth compressor stage  34  and the fifth compressor stage  35 ) flows. 
         [0043]    Hereinafter, the compression of the fluid G by the centrifugal compressor  1  with such a configuration will be described. 
         [0044]    When the impellers  3  rotate together with the rotating shaft  2 , the fluid G flowing into the flow path  4  via the suction port  9  sequentially flows from the suction port  9  to the suction channel  17 , the compression channel  18 , the diffuser channel  19 , and then the return channel of the impeller  3  of the first compressor stage  31 . Thereafter, the fluid G sequentially flows from the suction channel  17  to the compression channel  18 , . . . of the impeller  3  of the second compressor stage  32 . The fluid G flowing up to the discharge scroll  12  immediately after the diffuser channel  19  positioned on the furthest downstream side of the flow path  4  flows to the outside via the discharge port  10 . 
         [0045]    The fluid G is compressed by the impellers  3  while flowing through the flow path  4  in the aforementioned sequence. That is, in the centrifugal compressor  1 , the fluid G is compressed in stages by the multiple impellers  3 , and thus it is possible to easily obtain a large compression ratio. 
         [0046]      FIG. 3  illustrates performance test results of a centrifugal compressor in the related art, a centrifugal compressor disclosed in PTL 1, and the centrifugal compressor  1  in the embodiment. 
         [0047]    The centrifugal compressor in the related art is configured to include vaneless diffusers in all stages. The centrifugal compressor disclosed in PTL 1 is configured to include vaned diffusers of the first compressor stage to the third compressor stage, and vaneless diffusers of the fourth compressor stage and the fifth compressor stage. 
         [0048]    In the graph illustrated in  FIG. 3 , the horizontal axis represents the suction volumetric flow rate, and the vertical axis represents the isentropic head (outlet pressure of a centrifugal compressor) and the efficiency. 
         [0049]    As illustrated in  FIG. 3 , both the efficiency and the operating range (flow rate range) of the centrifugal compressor are better than those of the centrifugal compressor in the related art. According to the centrifugal compressor in the embodiment, it is possible to satisfactorily obtain a wide operating range and the maintenance of high efficiency which cannot be satisfactorily obtained by the centrifugal compressor disclosed in PTL 1. 
         [0050]    It is possible to secure a wide operating range by providing a vaneless diffuser in an upstream stage having a high machine Mach number. When the rotational speed of the impeller, the outer diameter of the impeller, and the speed of sound are assumed to be N, D, and a, respectively, a machine Mach number M is a value calculated by Expression (1). 
         [0000]        M=π×D×N/ 60/ a   (1)
 
         [0051]    When the temperature of a gas, the specific heat ratio of a gas, and a gas constant are assumed to be T, κ, and R, respectively, the speed of sound can be calculated by Expression (2). 
         [0000]        a =√(κ× R×T )  (2)
 
         [0052]    That is, it is possible to eliminate a limitation to the operating range specified by the vanes by providing a vaneless diffuser in an upstream compressor stage having a high machine Mach number, and thus it is possible to secure a wide operating range. 
         [0053]    It is possible to maintain high efficiency by providing a vaned diffuser in a downstream stage having a narrow width of a flow path. That is, it is possible to increase the pressure of the fluid G by the vane  29  of the diffuser channel  19 . 
         [0054]    In this configuration, it is possible to secure a wide operating range by providing a vaneless diffuser in an upstream stage having a high machine Mach number, and it is possible to maintain high efficiency by providing a vaned diffuser in a downstream stage having a small sectional area of a flow path. As a result, it is possible to provide the centrifugal compressor  1  in which not only high efficiency is maintained but also a wide operating range is secured. 
       Second Embodiment 
       [0055]    Hereinafter, a centrifugal compressor  1 B in a second embodiment of the present invention will be described with reference to the accompanying drawings. The points of difference of the embodiment with respect to the first embodiment will be mainly described, and the description of the same portions will be omitted. 
         [0056]    As illustrated in  FIG. 4 , in the centrifugal compressor  1  according to the embodiment, the diffuser channels  19  of the first compressor stage  31 , the third compressor stage  33 , and the fifth compressor stage  35  are vaneless diffusers. In contrast, the diffuser channels  19  of the second compressor stage  32  and the fourth compressor stage  34  are vaned diffusers. 
         [0057]    In the embodiment, the diffuser channel  19  of an upstream stage of the diffuser channels  19  respectively corresponding to a pair of adjacent impellers  3  is a vaneless diffuser, and the diffuser channel  19  of a downstream stage is a vaned diffuser. In this configuration, it is possible to partially maintain high efficiency, and to obtain a partially high operating range. 
       Third Embodiment 
       [0058]    Hereinafter, a centrifugal compressor  1 C in a second embodiment of the present invention will be described with reference to the accompanying drawing. 
         [0059]    As illustrated in  FIG. 5 , in the centrifugal compressor  1  according to the embodiment, the diffuser channels  19  of the first compressor stage  31  is a vaneless diffuser. In contrast, the diffuser channels  19  of the second compressor stage  32 , the third compressor stage  33 , the fourth compressor stage  34 , and the fifth compressor stage  35  are vaned diffusers. That is, the diffuser channel  19  of only the first compressor stage  31  is a vaneless diffuser, and the diffuser channels  19  of the second compressor stage  32  and the following compressor stages are vaned diffusers. 
         [0060]    It is possible to adopt this type of disposition so as to balance the maintenance of efficiency against the securing of a wide operating range. That is, it is possible to appropriately adjust the disposition of vaneless diffusers and vaned diffusers according to required efficiency and a required operating range. 
         [0061]    The technical scope of the present invention is not limited to the embodiments, and changes can be made to the embodiments in various forms insofar as the changes do not depart from the purport of the present invention. 
         [0062]    For example, with regard to the diffusers of the centrifugal compressor, the disposition of the vaneless diffusers and the vaned diffusers is different in each of the embodiments, and these different dispositions can also be applied to other rotating machines, for example, multistage blowers. 
       INDUSTRIAL APPLICABILITY 
       [0063]    The present invention can be applied to a rotating machine including a rotating shaft; multiple impellers fixed to the rotating shaft, and rotating together with the rotating shaft; and a casing configured to surround the rotating shaft and the impellers, and to form diffusers allowing the flowing through of a fluid which is discharged from the impellers to an outward side in a radial direction, and return channels by which the fluid flowing through the diffusers is guided to an inward side in the radial direction, and is introduced into the impellers of downstream stages. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 ,  1 B,  1 C: CENTRIFUGAL COMPRESSOR 
               2 : ROTATING SHAFT 
               3 : IMPELLER 
               4 : FLOW PATH 
               5 : CASING 
               6 : OUTLET FLOW PATH 
               9 : SUCTION PORT 
               10 : DISCHARGE PORT 
               11 : INNER SPACE 
               12 : DISCHARGE SCROLL 
               13 : HUB 
               14 : BLADE 
               15 : SHROUD 
               17 : SUCTION CHANNEL 
               18 : COMPRESSION CHANNEL 
               19 : DIFFUSER CHANNEL 
               20 : RETURN CHANNEL 
               21 : STRAIGHT CHANNEL 
               22 : CORNER CHANNEL 
               23 : RETURN VANE 
               25 : SCROLL FLOW PATH 
               29 : VANE 
               31 ,  32 ,  33 ,  34 ,  35 : COMPRESSOR STAGE