Patent Publication Number: US-11378096-B2

Title: Centrifugal compressor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a centrifugal compressor. 
     BACKGROUND ART 
     Patent Document 1 discloses, as an example of a conventional centrifugal compressor, a centrifugal compressor including a plurality of stages of impellers arranged in the axial direction and a plurality of diaphragms disposed at the radially outer side of the impellers. 
     This type of centrifugal compressor includes a scroll flow passage that communicates with a discharge outlet. The scroll flow passage normally has an inner peripheral wall formed by the outer peripheral surface of the diaphragm at the discharge side, and an outer peripheral wall formed by the inner peripheral surface of an annular spacer disposed between the discharge-side diaphragm and a diaphragm positioned next to the discharge-side diaphragm in the axial direction. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2016-180400A 
     SUMMARY 
     Problems to be Solved 
     Meanwhile, when the diameter of the casing of a compressor is reduced in response to the need to reduce the size of compressors, the diameter of the diffuser also decreases and the flow velocity of the fluid at the outlet of the diffuser increases, which leads to an increase in the centrifugal force of the fluid. Furthermore, when the diameter of the casing of a compressor is reduced, the diameter of the scroll flow passage also decreases, which leads to an increase in the centrifugal force of the fluid in the vicinity of the scroll termination portion of the scroll flow passage. 
     Thus, due to size reduction of a compressor, the fluid may separate from the wall surface of the flow passage in a region from the vicinity of the scroll termination portion of the scroll flow passage to the outlet of the fluid from the compressor. When such separation occurs, the performance of the compressor deteriorates. 
     In view of the above, an object of at least one embodiment of the present invention is to suppress performance deterioration of the compressor due to size reduction of the compressor. 
     Solution to the Problems 
     (1) According to at least one embodiment of the present invention, a centrifugal compressor includes: an impeller fixed on an outer periphery of a rotary shaft; a diffuser disposed at a radially outer side of the impeller; a casing accommodating the impeller and the diffuser; a scroll flow passage connected to an outlet of the diffuser, the scroll flow passage being formed into a scroll shape by a scroll inner peripheral wall and a scroll outer peripheral wall positioned at a radially outer side of the scroll inner peripheral wall; and a discharge pipe connected to the casing so as to form a discharge flow passage for guiding a fluid from the scroll flow passage to outside of the casing. The scroll inner peripheral wall is positioned at an inner side, in a radial direction, of the outlet of the diffuser, and the discharge pipe includes an inner wall surface which has a radially inner region continuing to the scroll inner peripheral wall, the radially inner region being positioned, when viewed in an axial direction of the rotary shaft, closer to a scroll termination portion of the scroll flow passage from a first line segment which passes a center of the rotary shaft and which is parallel to a center axis of an outlet portion of the discharge pipe, at a connection position of the discharge pipe to the casing. 
     With the above configuration (1), the discharge pipe has an inner wall surface which has a radially inner region continuing to the scroll inner peripheral wall, and the radially inner region is positioned closer to the scroll termination portion of the scroll flow passage from the first line segment which passes the center of the rotary shaft and which is parallel to the center axis of the outlet portion of the discharge pipe, at the connection position of the discharge pipe to the casing. Thus, it is possible to orient the direction of the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe outward in the radial direction. Accordingly, it is possible to suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion and suppress separation of the fluid from the wall surface of the flow passage, thereby suppressing the performance deterioration of the compressor. 
     (2) In some embodiments, in the above configuration (1), a second line segment obtained by extending a center line of a width, in the radial direction, of the scroll flow passage in an extension direction at the scroll termination portion passes through an opening of the outlet portion of the discharge pipe. 
     With the above configuration (2). the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe is less bended, and it is possible to suppress pressure loss at the flow passage. 
     (3) In some embodiments, in the above configuration (1) or (2), the radially inner region has a linear shape portion formed into a linear shape from an inlet portion of the discharge pipe toward the outlet portion of the discharge pipe in at least a partial region between the inlet portion and the outlet portion, the partial region including the inlet portion. 
     With the above configuration (3), the radially inner region of the inner wall surface of the discharge pipe continuing to the scroll inner peripheral wall is formed into a linear shape in at least a partial region, and thus the discharge flow passage is less bended, which makes it possible to suppress pressure loss at the discharge flow passage. 
     (4) In some embodiments, in the above configuration (3), an intersecting angle between an extension direction of the linear shape portion from the inlet portion toward the outlet portion and an extension direction of the first line segment is not greater than 30 angular degrees when viewed in the axial direction of the rotary shaft. 
     If the intersecting angle between the above extension direction of the linear shape portion and the extension direction of the first line segment exceeds 30 angular degrees when viewed in the axial direction of the rotary shaft, the direction of the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, with the above configuration (4), the above intersecting angle is not greater than 30 angular degrees. Thus, it is possible to orient the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion of the scroll flow passage, thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     (5) In some embodiments, in the above configuration (4), the extension direction of the linear shape portion from the inlet portion toward the outlet portion coincides with the extension direction of the first line segment, when viewed in the axial direction of the rotary shaft. 
     With the above configuration (5), the intersecting angle between the above extension direction of the linear shape portion and the extension direction of the first line segment is zero angular degrees, when viewed in the axial direction of the rotary shaft, and thus it is possible to further suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion of the scroll flow passage, and further suppress separation of the fluid from the wall surface of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe. 
     (6) In some embodiments, in any one of the above configurations (1) to (5), a separation distance between the first line segment and the radially inner region at the connection position of the discharge pipe to the casing is not smaller than 0.2 times a minimum curvature radius of the scroll inner peripheral wall, when viewed in the axial direction of the rotary shaft. 
     If the separation distance between the radially inner region at the connection position of the discharge pipe to the casing and the first line segment is less than 0.2 times the minimum curvature radius of the scroll inner peripheral wall, when viewed in the axial direction of the rotary shaft, the direction of the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the radially inner region at the connection position becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, with the above configuration (6), the separation distance between the radially inner region at the connection position and the first line segment is not less than 0.2 times the minimum curvature radius of the scroll inner peripheral wall. Thus, it is possible to orient the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the radially inner region at the connection position less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion, thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     (7) In some embodiments, in the above configuration (6), the separation distance between the first line segment and the radially inner region at the connection position of the discharge pipe to the casing is equal to the minimum curvature radius of the scroll inner peripheral wall, when viewed in the axial direction of the rotary shaft. 
     With the above configuration (7), it is possible to further suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion of the scroll flow passage, and further suppress separation of the fluid from the wall surface of the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the radially inner region at the above connection position. 
     (8) In some embodiments, in any one of the above configurations (1) to (7), a separation distance between the first line segment and a center axis of the outlet portion of the discharge pipe when viewed in the axial direction of the rotary shaft is not smaller than 0.3 times a minimum curvature radius of the scroll inner peripheral wall. 
     If the separation distance between the center axis of the outlet portion of the discharge pipe and the first line segment is less than 0.3 times the minimum curvature radius of the scroll inner peripheral wall, when viewed in the axial direction of the rotary shaft, the direction of the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, with the above configuration (8), the separation distance between the center axis of the outlet portion of the discharge pipe and the first line segment is not less than 0.3 times the minimum curvature radius of the scroll inner peripheral wall. Thus, it is possible to orient the flow passage of the fluid from the scroll termination portion of the scroll flow passage to the outlet portion of the discharge pipe less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion, thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     (9) In some embodiments, in any one of the above configurations (1) to (8), an inner wall surface of the discharge pipe has a changing portion whose cross-sectional shape viewed in an extension direction of the discharge flow passage has a rectangular shape at an inlet portion of the discharge pipe and a circular shape at the outlet portion, the cross-sectional shape gradually changing from the rectangular shape toward the circular shape from the inlet portion toward the outlet portion. The inner wall surface of the discharge pipe at the changing portion has an inner side wall surface continuing to the scroll inner peripheral wall and an outer side wall surface continuing to the scroll outer peripheral wall and facing the inner side wall surface. The radially inner region includes a region of the inner side wall surface. 
     With the above configuration (9), the cross-sectional shape gradually changes from a rectangular shape toward a circular shape at the change portion from the inlet portion toward the outlet portion of the discharge pipe, and thus the cross-sectional shape does not change abruptly, which makes it possible to suppress separation of the fluid from the inner side wall surface in the discharge pipe. 
     (10) In some embodiments, in any one of the above configurations (1) to (9), the radially inner region includes a protruding portion formed so as to protrude toward an inner side of the discharge flow passage, in at least a partial region between an inlet portion of the discharge pipe and the outlet portion. 
     With the above configuration (10), the protruding portion is formed in a region of the discharge flow passage where separation of the fluid is likely to occur, and thus it is possible to suppress separation of the fluid from the wall surface of the discharge flow passage. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to suppress performance deterioration of a compressor due to size reduction of a compressor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a centrifugal compressor according to some embodiments, taken along the axial direction of the rotary shaft of the centrifugal compressor. 
         FIG. 2  is a cross-sectional view at a discharge outlet of a centrifugal compressor according to an embodiment, taken along the radial direction. 
         FIG. 3  is a cross-sectional view of the centrifugal compressor depicted in  FIG. 1 , showing an enlarged view mainly including the first diaphragm and the second diaphragm. 
         FIG. 4  is a cross-sectional view at a discharge outlet of a centrifugal compressor according to another embodiment, taken along the radial direction (i.e. viewed in the axial direction). 
         FIG. 5  is a cross-sectional view at a discharge outlet of a centrifugal compressor according to yet another embodiment, taken along the radial direction. 
         FIG. 6  is a cross-sectional view at a discharge outlet of a centrifugal compressor according to yet another embodiment, taken along the radial direction. 
         FIG. 7  is a diagram for describing how the cross-sectional shape changes at the change portion according to the embodiment depicted in  FIG. 2 . 
         FIG. 8  is a diagram for describing how the cross-sectional shape changes at the change portion according to the embodiment depicted in  FIG. 4 . 
         FIG. 9  is a cross-sectional view at a discharge outlet of a conventional centrifugal compressor. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components. 
     Hereinafter, as an example of a centrifugal compressor, a multi-stage centrifugal compressor including a plurality of stages of impellers will be described. 
       FIG. 1  is a cross-sectional view of a centrifugal compressor according to some embodiments, taken along the axial direction.  FIG. 2  is a cross-sectional view at a discharge outlet of a centrifugal compressor according to an embodiment, taken in the radial direction. 
     As depicted in  FIG. 1 , the centrifugal compressor  1  includes a casing  2 , and a rotor  7  supported rotatably inside the casing  2 . The rotor  7  has a rotary shaft  4 , and a plurality of stages of impellers  8  fixed to the outer surface of the shaft  4 . 
     Inside the casing  2 , accommodated is a plurality of diaphragms  10  arranged in the axial direction. The plurality of diaphragms  10  are disposed so as to surround the impellers  8  from the radially outer side. Furthermore, casing heads  5 ,  6  are disposed on both sides of the plurality of diaphragms  10  in the axial direction at the radially inner side of the casing  2 . 
     The rotor  7  is supported rotatably by the radial bearings  20 ,  22  and the thrust bearing  24 , and is configured to rotate about the center O. 
     An introducing inlet  16  through which a fluid flows in from outside is disposed on the first end portion of the casing  2 , and a discharge outlet  18  for discharging a fluid compressed by the centrifugal compressor  1  to the outside is disposed on the second end portion of the casing  2 . Inside the casing  2 , a flow passage  9  is formed so as to bring the gaps between the plurality of stages of impellers  8  into communication. The introducing inlet  16  and the discharge outlet  18  are in communication with one another via the plurality of impellers  8  and the flow passage  9 . 
     The first end  50   a  of a discharge pipe  50  connected to the casing  2  is connected to the discharge outlet  18 . 
     The discharge pipe  50  has a discharge flow passage  51  formed inside, for guiding the fluid from the scroll flow passage  30  to the outside of the casing  2 . An inlet portion  55  at the side of the first end  50   a  of the discharge flow passage  51  is in communication with an outlet flow passage  19  formed on the casing  2 . A flange portion  53  for connecting to an external pipe, for instance, is formed at the radially outer side of the outlet portion  52  at the side of the second end  50   b  of the discharge flow passage  51 . 
     In the depicted embodiment, the flow passage  9  inside the casing  2  is formed at least partially by the plurality of diaphragms  10 . 
     As depicted in  FIGS. 1 and 2 , a scroll flow passage  30  is formed between the discharge outlet  18  of the casing  2  and the final-stage impeller  8 A disposed most downstream of the plurality of stages of impellers  8 . The scroll flow passage  30  is an annular flow-passage disposed such that the flow-passage cross-sectional area changes along the circumferential direction. Furthermore, the scroll flow passage  30  and the discharge outlet  18  are connected to one another via the outlet flow passage  19  of the casing  2 . 
     The fluid flows into the centrifugal compressor  1  via the introducing inlet  16 , and then flows from the upstream toward the downstream through the plurality of stages of impellers  8  and the flow passage  9 . When passing through the plurality of stages of impellers  8 , a centrifugal force of the impellers  8  is applied to the fluid, and the fluid is compressed in stages. The compressed fluid after passing the final-stage impeller  8 A disposed most downstream of the plurality of stages of impellers  8  is guided to the outside of the casing  2  via the scroll flow passage  30  and the discharge outlet  18 , and is discharged from the outlet portion  52  of the discharge flow passage  51  via the discharge pipe  50 . 
     Furthermore, for the penetration portions at which the shaft  4  penetrates the casing heads  5 ,  6 , a shaft sealing device may be provided to prevent leakage of the fluid through the penetration portion. In the embodiment depicted in  FIG. 1 , a shaft sealing device  26  is disposed on the casing head  6  at the side of the introducing inlet  16 . 
     As depicted in  FIG. 1 , the plurality of diaphragms  10  include the first diaphragm  12  having a surface that forms the scroll flow passage  30 , and a second diaphragm  14  disposed next to the first diaphragm  12  in the axial direction. 
       FIG. 3  is a cross-sectional view of the centrifugal compressor  1  depicted in  FIG. 1 , showing an enlarged view that mainly includes the first diaphragm  12  and the second diaphragm  14 . 
     In the embodiment depicted in  FIGS. 1 to 3 , the first diaphragm  12  and the second diaphragm  14  are fastened by bolts  34  and thereby connected. The first diaphragm  12  and the second diaphragm  14  have bolt holes  41 ,  42  (see  FIG. 3 ) each of which has a female thread formed thereon. As the bolts  34  are screwed into the bolt holes  41 ,  42 , the first diaphragm  12  and the second diaphragm  14  are fastened to one another. 
     In the embodiment depicted in  FIGS. 1 to 3 , the axial-directional spacer  32  positioned between the first diaphragm  12  and the second diaphragm  14  includes a bolt insertion hole  33  (see  FIG. 3 ) on which a female thread is formed. With the above described bolts  34  screwed into the bolt holes  41 ,  42  and the bolt insertion hole  33 , the first diaphragm  12  and the second diaphragm  14  are fastened in a state where the axial-directional spacer  32  is interposed between the first diaphragm  12  and the second diaphragm  14 . By providing the axial-directional spacer  32 , it is possible to determine the position of the first diaphragm  12  with respect to the second diaphragm  14  in the axial direction. 
     In some embodiments, the first diaphragm  12  and the second diaphragm  14  may be joined by welding. 
     Furthermore, a pair of diaphragms  10  other than the pair of the first diaphragm  12  and the second diaphragm  14  may be joined by welding. 
     The first diaphragm  12  has a first end surface  37  and a second end surface  38  which are the opposite end surfaces in the axial direction. The first end surface  37  is an end surface next to the casing head  5  positioned at the side of the discharge outlet  18 , and the second end surface  38  is an end surface next to the second diaphragm  14 . Furthermore, in the positional range in the axial direction between the first end surface  37  and the second end surface  38 , a recess portion  31  recessed inward in the radial direction from the outer peripheral surface  11  of the first diaphragm  12  is formed. The recess portion  31  has a pair of side surfaces  15 ,  17  along the radial direction and a bottom surface  13  along the circumferential direction. That is, the bottom surface  13  is a surface positioned at the radially inner side of the outer peripheral surface  11 . 
     As depicted in  FIGS. 2 and 3 , the scroll flow passage  30  is formed into a scroll shape by a scroll inner peripheral wall  30   a  being the wall surface at the radially inner side, and a scroll outer peripheral wall  30   b  being the wall surface at the radially outer side, positioned at the radially outer side of the scroll inner peripheral wall  30   a . Furthermore, the scroll inner peripheral wall  30   a  is formed by the bottom surface  13  of the above described recess portion  31  of the first diaphragm  12  (surface of the first diaphragm positioned at the radially inner side of the outer peripheral surface  11 ), and the scroll outer peripheral wall  30   b  is formed by the inner peripheral surface of the axial-directional spacer  32  (spacer inner peripheral wall  35 ). 
     As depicted in  FIGS. 2 and 3 , the pair of side surfaces  15 ,  17  of the recess portion  31  of the first diaphragm  12  each form a wall surface along the radial direction of the scroll flow passage  30 . 
     That is, the scroll flow passage  30  has a rectangular cross-sectional shape when viewed in the extension direction of the scroll flow passage  30 , that is, the circumferential direction. In the following description, a virtual curve along the circumferential direction passing through the center of the cross section viewed in the circumferential direction of the scroll flow passage  30  will be referred to as the center line ax 1  of the scroll flow passage  30 . 
     Although not depicted, the axial-directional spacer  32  may be disposed on the diffuser  36  disposed at the radially outer side of the final-stage impeller  8 A. That is, the axial-directional spacer  32  may be disposed between the end surfaces of the first diaphragm  12  and the second diaphragm  14  that face one another (i.e., the second end surface  38  of the first diaphragm  12  and the end surface  29  of the second diaphragm  14 ). In this case, the scroll outer peripheral wall  30   b  is formed by the inner peripheral surface  3  of the casing  2 . 
     The scroll flow passage  30  is connected to the outlet  43  of the diffuser  36 . 
     The inner peripheral surface  3  of the casing  2  forming the scroll outer peripheral wall  30   b  may be a cylindrical shape centered at the rotational center (the center O of the rotary shaft  4 ) of the centrifugal compressor  1 . 
     As described above, in a case where the inner peripheral surface  3  of the casing  2  forming the scroll outer peripheral wall  30   b  has a cylindrical shape centered at the rotational center of the centrifugal compressor  1 , it is possible to form the scroll flow passage  30  easily by utilizing the inner peripheral surface  3  having a cylindrical shape. 
     That is, while the scroll inner peripheral wall  30   a  is formed by the bottom surface  13  (surface) of the recess portion  31  of the first diaphragm  12 , it is possible to form the scroll outer peripheral wall  30   b  with the inner peripheral surface  3  of the casing  2  having a simple cylindrical shape. Thus, it is possible to form the scroll flow passage  30  relatively easily without forming a complicated flow passage shape on the casing  2  by machining. 
     Furthermore, since the inner peripheral surface  3  of the casing  2  forming the scroll outer peripheral wall  30   b  has a cylindrical shape centered at the center O and is coaxial with the rotor  7 , it is possible to simplify the structure of the centrifugal compressor  1 . 
     Meanwhile, when the diameter of the casing  2  of the centrifugal compressor  1  is reduced in response to the need reduce the size of compressors, the diameter of the diffuser  36  also decreases and the flow velocity of the fluid at the outlet  43  of the diffuser  36  increases, which leads to an increase in the centrifugal force of the fluid. Furthermore, when the diameter of the casing  2  of the centrifugal compressor  1  is reduced, the diameter of the scroll flow passage  30  also decreases, and thus the centrifugal force of the fluid increases in the vicinity of the scroll termination portion  45  of the scroll flow passage  30 . 
     Thus, due to size reduction of the centrifugal compressor  1 , the fluid may separate from the wall surface of the flow passage in a region from the vicinity of the scroll termination portion  45  of the scroll flow passage  30  to the outlet of the fluid from the centrifugal compressor  1 . When such separation occurs, the performance of the centrifugal compressor  1  deteriorates. The region E surrounded by the two-dotted chain line in  FIG. 2  and  FIG. 4  described below is a region where the above separation is likely to occur. 
     Thus, according to some embodiments, the centrifugal compressor  1  is configured to suppress the above described separation with the configuration described below. 
     In some embodiments, the scroll termination portion  45  of the scroll flow passage  30  refers to the position of the scroll flow passage  30  corresponding to the position  75  where the scroll inner peripheral wall  30   a  has a curvature center at the radially inner side of the scroll inner peripheral wall  30   a  viewed in the axial direction and where the distance from the center O is the shortest, of the scroll inner peripheral wall  30   a  whose curvature radius gradually decreases along the rotational direction of the final-stage impeller  8 A. 
     Thus, as in  FIG. 4  described below, of the scroll inner peripheral wall  30   a , at the side closer to the discharge outlet  18  from the position  75 , the position  76  whose distance from the center O is the shortest in the region formed in a linear shape when viewed in the axial direction is not a position that corresponds to the scroll termination portion  45 . 
       FIG. 4  is a cross-sectional view at the discharge outlet of a centrifugal compressor according to another embodiment, taken along the radial direction (i.e. viewed in the axial direction).  FIG. 5  is a cross-sectional view at the discharge outlet of a centrifugal compressor according to yet another embodiment, taken along the radial direction.  FIG. 6  is a cross-sectional view at the discharge outlet of a centrifugal compressor according to yet another embodiment, taken along the radial direction. Hereinafter, the description below will mainly refer to  FIGS. 2, 4 to 6 . 
     According to some embodiments, the centrifugal compressor  1  includes, as described above, impellers  8  fixed to the outer periphery of the rotary shaft  4 , a diffuser  36  (see  FIG. 3 ) disposed at the radially outer side of the impellers  8  (see  FIG. 1 ), and a casing  2  that accommodates the impellers  8  and the diffuser  36 . According to some embodiments, the centrifugal compressor  1  includes, as described above, the scroll flow passage  30  connected to the outlet  43  of the diffuser  36  and formed into a scroll shape by the scroll inner peripheral wall  30   a  and the scroll outer peripheral wall  30   b  positioned at the radially outer side of the scroll inner peripheral wall  30   a , and the discharge pipe  50  connected to the casing  2  so as to form the discharge flow passage  51  for guiding the fluid from the scroll flow passage  30  to the outside of the casing  2 . According to some embodiments, as described above, the scroll inner peripheral wall  30   a  of the centrifugal compressor  1  is positioned at the radially inner side of the outlet  43  of the diffuser  36 . 
     In some embodiments, as depicted in  FIGS. 2, 4 to 6 , the discharge pipe  50  of the centrifugal compressor  1  has an inner wall surface  60  which has a radially inner region  61  continuing to the scroll inner peripheral wall  30   a , the radially inner region  61  being positioned, when viewed in the axial direction of the rotary shaft  4 , closer to the scroll termination portion  45  of the scroll flow passage  30  from the first line segment  71  which passes the center O of the rotary shaft  4  and which is parallel to the center axis  52   a  of the outlet portion  52  of the discharge pipe  50 , at a connection position  54  of the discharge pipe  50  to the casing  2 . 
     Thus, it is possible to orient the overall direction of the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the outlet portion  52  of the discharge pipe  50  outward in the radial direction. Accordingly, it is possible to suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45  and suppress separation of the fluid from the wall surface of the flow passage, thereby suppressing the performance deterioration of the centrifugal compressor  1 . 
     According to some embodiments, as depicted in  FIGS. 2 and 4 , the centrifugal compressor  1  is configured such that the second line segment  72  obtained by extending a center line (the center line ax 1 ) of the width, in the radial direction, of the scroll flow passage  30  in the extension direction at the scroll termination portion  45  passes through the opening of the outlet portion  52  of the discharge pipe  50 . The embodiments depicted in  FIGS. 5 and 6  have a similar configuration. 
     Accordingly, the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the outlet portion  52  of the discharge pipe  50  is less bended, and it is possible to suppress pressure loss in the flow passage. 
     Each of the flow passages  30 ,  19 ,  51  will be described specifically. 
     In the scroll inner peripheral wall  30   a  according to the embodiments depicted in  FIGS. 2, 5, and 6 , the region  81  closer to the discharge outlet  18  from the position  75  whose distance from the center O is the shortest extends linearly in the same direction as the extension direction of the tangent to the scroll inner peripheral wall  30   a  at the position  75  whose distance from the center O is the shortest. 
     In the scroll inner peripheral wall  30   a  according to the embodiment depicted in  FIG. 4 , the region  81  closer to the discharge outlet  18  from the position  75  whose distance from the center O is the shortest extends so as to pass through the radially inner side of the tangent  77  of the scroll inner peripheral wall  30   a  at the position  75  whose distance from the center O is the shortest, that is, extends linearly in a region at the left side of the tangent  77  in  FIG. 4 . 
     Accordingly, it is possible to suppress separation of the fluid in the region  81  compared to a case where the region  81  closer to the discharge outlet  18  from the position  75  is formed to have a shape that further curves in the circumferential direction toward the discharge outlet  18 . 
     In the embodiments depicted in  FIGS. 2 and 4 , the region  19   a  of the outlet flow passage  19  formed on the casing  2  continuing to the region  81  of the scroll inner peripheral wall  30   a  and the radially inner region  61  of the inner wall surface  60  of the discharge pipe  50  continuing to the region  81  of the scroll inner peripheral wall  30   a  via the region  19   a  extend linearly in the same direction as the extension direction of the region  81  of the scroll inner peripheral wall  30   a . In the embodiments depicted in  FIGS. 2 and 4 , the region  81  of the scroll inner peripheral wall  30   a , the region  19   a  of the outlet flow passage  19 , and the radially inner region  61  of the discharge pipe  50  are disposed on the same line when viewed in the axial direction of the rotary shaft  4 . 
     In the embodiment depicted in  FIG. 5 , of the radially inner region  61 , the region closer to the second end  50   b  from the protruding portion  85  described below extends linearly in the same direction as the extension direction of the region  81  of the scroll inner peripheral wall  30   a . That is, in the embodiment depicted in  FIG. 5 , the region  81  of the scroll inner peripheral wall  30   a , and the region of the radially inner region  61  closer to the second end  50   b  from the protruding portion  85  are disposed on the same line when viewed in the axial direction of the rotary shaft  4 . 
     As described above, in the embodiments depicted in  FIGS. 2, 4, and 5 , the radially inner region  61  has a linear shape portion  63  formed into a linear shape from the inlet portion  55  of the discharge pipe  50  toward the outlet portion  52  of the discharge pipe  50  in at least a partial region between the inlet portion  55  and the outlet portion  52 , the region including the inlet portion  55 . 
     Accordingly, the radially inner region  61  of the inner wall surface  60  of the discharge pipe  50  continuing to the scroll inner peripheral wall  30   a  is formed into a linear shape in at least a partial region, and thus the discharge flow passage  51  is less bended, which makes it possible to suppress pressure loss at the discharge flow passage  51 . 
     The protruding portion  85  according to the embodiment depicted in  FIG. 5  protrudes toward the inner side of the outlet flow passage  19  and the discharge flow passage  51 , in the entire region  19   a  of the outlet flow passage  19  and in a region of the radially inner region  61  of the discharge pipe  50  at the side of the first end  50   a.    
     In the embodiment depicted in  FIG. 6 , the protruding portion  86  protrudes toward the inner side of outlet flow passage  19  and the discharge flow passage  51 , in the entire region  19   a  of the outlet flow passage  19  and from the first end  50   a  to the second end  50   b  of the radially inner region  61  of the discharge pipe  50 . 
     The protruding portions  85 ,  86  each have a curvature radius at the side closer to the first line segment  71  from the radially inner region  61  when viewed in the axial direction of the rotary shaft  4 . 
     In the embodiments depicted in  FIGS. 2, 4, and 5 , the intersecting angle θ (see  FIG. 4 ) between the extension direction of the linear shape portion  63  from the inlet portion  55  toward the outlet portion  52  and the extension direction of the first line segment  71  is not greater than 30 angular degrees when viewed in the axial direction of the rotary shaft  4 . 
     If the intersecting angle θ between the above extension direction of the linear shape portion  63  and the extension direction of the first line segment  71  exceeds 30 angular degrees when viewed in the axial direction of the rotary shaft  4 , the direction of the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the outlet portion  52  of the discharge pipe  50  becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion  45  increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, in the embodiments depicted in  FIGS. 2, 4, and 5 , the above intersecting angle θ is not greater than 30 angular degrees. Thus, it is possible to orient the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the outlet portion  52  of the discharge pipe  50  less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45 , thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     In the embodiments depicted in  FIGS. 2 and 5 , the extension direction of the linear shape portion  63  from the inlet portion  55  toward the outlet portion  52  and the extension direction of the first line segment  71  coincide with one another, when viewed in the axial direction of the rotary shaft  4 . 
     That is, in the embodiments depicted in  FIGS. 2 and 5 , the intersecting angle θ between the above extension direction of the linear shape portion  63  and the extension direction of the first line segment  71  is zero angular degrees, and thus it is possible to further suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45 , and further suppress separation of the fluid from the wall surface of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the outlet portion  52  of the discharge pipe  50 . 
     In the embodiments depicted in  FIGS. 2 and 4 to 6 , the separation distance d 1  between the first line segment  71  and the radially inner region  61  at the connection position  54  (that is, the position  54   a ) is not smaller than 0.2 times the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , when viewed in the axial direction of the rotary shaft  4 . 
     If the separation distance d 1  between the position  54   a  and the first line segment  71  is smaller than 0.2 times the minimum curvature radius Rmin of the scroll inner peripheral wall, for instance, the direction of the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the above position  54   a  becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion  45  increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, in the embodiments depicted in  FIGS. 2, 4, and 6 , the above separation distance d 1  is not smaller than 0.2 times the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , and thus it is possible to orient the flow passage of the fluid from the scroll termination portion  45  of the scroll flow passage  30  to the position  54  less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45 , thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     In the embodiment depicted in  FIG. 2 , the above described separation distance d 1  is equal to the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , when viewed in the axial direction of the rotary shaft  4 . 
     Accordingly, it is possible to further suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45 , and further suppress separation of the fluid from the wall surface of the flow passage of the fluid from the scroll termination portion  45  to the above position  54   a.    
     In the embodiments depicted in  FIGS. 2 and 4 to 6 , the separation distance d 2  between the first line segment  71  and the center axis  52   a  of the outlet portion  52  of the discharge pipe  50  is not smaller than 0.3 times the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , when viewed in the axial direction of the rotary shaft  4 . 
     If the separation distance d 2  between the center axis  52   a  of the outlet portion  52  of the discharge pipe  50  and the first line segment  71  is smaller than 0.3 times the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , for instance, the direction of the flow passage of the fluid from the scroll termination portion  45  to the outlet portion  52  of the discharge pipe  50  becomes more inward in the radial direction, and thereby the centrifugal force of the fluid in the vicinity of the scroll termination portion  45  increases, which makes separation of the fluid from the wall surface of the flow passage more likely to occur. 
     In this regard, in the embodiments depicted in  FIGS. 2, 4, and 6 , the above separation distance d 2  is not smaller than 0.3 times the minimum curvature radius Rmin of the scroll inner peripheral wall  30   a , and thus it is possible to orient the flow passage of the fluid from the scroll termination portion  45  to the outlet portion  52  of the discharge pipe  50  less inward in the radial direction, and suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45 , thereby suppressing separation of the fluid from the wall surface of the flow passage. 
     In the embodiments depicted in  FIGS. 2 and 4 to 6 , the maximum width d 3  of the centrifugal compressor  1  from the side surface of the flange portion  53  of the outlet portion  52  of the discharge pipe  50  is not greater than 1.2 times the casing outer shape D. 
     In the embodiments depicted in  FIGS. 2 and 4 to 6 , to suppress the centrifugal force of the fluid in the vicinity of the scroll termination portion  45  of the scroll flow passage  30 , as described above, the radially inner region  61  is positioned closer to the scroll termination portion  45  of the scroll flow passage  30  from the first line segment  71  at the connection position  54 . As a result, the outlet portion  52  of the discharge pipe  50  is positioned in the width direction, that is, in a direction orthogonal to the center axis  52   a  of the outlet portion  52  compared to a conventional centrifugal compressor, and thus the side surface of the flange portion  53  may extend further from the width of the casing  2 . 
     In this regard, in the embodiments depicted in  FIGS. 2 and 4 to 6 , the maximum width d 3  of the centrifugal compressor  1  from the side surface of the flange portion  53  of the outlet portion  52  of the discharge pipe  50  is not greater than 1.2 times the casing outer shape D, and thus it is possible to suppress a size increase of the centrifugal compressor  1  including the discharge pipe  50 . 
     Herein,  FIG. 9  is a cross-sectional view at a discharge outlet of a conventional centrifugal compressor  1 A. 
     In the embodiments depicted in  FIGS. 2 and 4 , the region  19   b  of the outlet flow passage  19  continuing to the scroll outer peripheral wall  30   b  and the radially outer region  62  of the inner wall surface  60  of the discharge pipe  50  continuing to scroll outer peripheral wall  30   b  via the above region  19   b  are arranged on the same line, when viewed in the axial direction of the rotary shaft  4 . Furthermore, in the embodiments depicted in  FIGS. 2 and 4 , the region  19   b  and the radially outer region  62  are arranged such that the width of the flow passage of the outlet flow passage  19  and the discharge flow passage  51  viewed from the axial direction of the rotary shaft  4  increases toward the second end  50   b  of the discharge pipe  50 . 
     In the embodiments depicted in  FIGS. 5 and 6 , the shapes of the region  19   b  and the radially outer region  62  when viewed from the axial direction of the rotary shaft  4  are the same as the shapes of the region  19   b  and the radially outer region  62  in the embodiments depicted in  FIGS. 2 and 4 . 
     That is, in the embodiments depicted in  FIGS. 5 and 6 , the shapes of the flow passages of the outlet flow passage  19  and the discharge flow passage  51  are the same as the shapes of the outlet flow passage  19  and the discharge flow passage  51  of the embodiment depicted in  FIG. 2 , except for the presence or absence of the protruding portion  85  or the protruding portion  86 . 
     In the embodiments depicted in  FIGS. 2 and 4 , the outlet flow passage  19  has a rectangular cross-sectional shape when viewed in the extension direction of the outlet flow passage  19 , that is, in a direction of the main flow of the fluid passing through the outlet flow passage  19 . In the following description, a virtual line passing through the center of the cross section will be referred to as the center line ax 2  of the outlet flow passage  19 . 
     Furthermore, in the embodiments depicted in  FIGS. 2 and 4 , the discharge flow passage  51  has a rectangular cross-sectional shape at the side of the first end  50   a  and a circular shape at the side of the second end  50   b , when viewed in the extension direction of the discharge flow passage  51 , that is, in a direction of the main flow of the fluid passing through the discharge flow passage  51 . In the following description, a virtual line passing through the center of the cross section will be referred to as the center line ax 3  of the discharge flow passage  51 . 
     The inner wall surface  60  of the discharge pipe  50  has a change portion  56  whose cross-sectional shape gradually changes from a rectangular shape to a circular shape from the inlet portion  55  toward the outlet portion  52 . 
       FIG. 7  is a diagram for describing how the cross-sectional shape changes at the change portion  56  according to the embodiment depicted in  FIG. 2 , showing cross-sectional shapes perpendicular to the center lines ax 1 , ax 2 , and the center axis  52   a .  FIG. 8  is a diagram for describing how the cross-sectional shape changes at the change portion  56  according to the embodiment depicted in  FIG. 4 , showing cross-sectional shapes perpendicular to the center lines ax 1 , ax 2 , and the center axis  52   a.    
     In  FIG. 7 , depicted are the shape of the cross section  111  of the scroll flow passage  30  at the first position  101 , the shape of the cross section  112  of the outlet flow passage  19  at the second position  102 , and the shapes of the cross sections  113  to  115  of the discharge flow passage  51  at the third position  103  to the fifth position  105 . 
     The first position  101  is a position slightly closer to the outlet flow passage  19  from the scroll termination portion  45 , and the second position  102  is a position inside the outlet flow passage  19 . The third position  103  to the fifth position  105  are positions inside the discharge flow passage, and arranged from the first end  50   a  toward the second end  50   b  in the following order: the third position  103 , the fourth position  104 , and the fifth position  105 . 
     As depicted in  FIG. 7 , the shape of the cross section  111  of the scroll flow passage  30  at the first position  101  and the shape of the cross section  112  of the outlet flow passage  19  at the second position  102  are substantially rectangular. The shapes of the cross sections  113  to  115  of the discharge flow passage  51  at the third position  103  to the fifth position  105  change from a rectangular shape to a circular shape gradually from the inlet portion  55  toward the outlet portion  52 . 
     In  FIG. 8 , depicted are the shape of the cross section  131  of the scroll flow passage  30  at the first position  121 , the shape of the cross section  132  of the outlet flow passage  19  at the second position  122 , and the shapes of the cross sections  133  to  135  of the discharge flow passage  51  at the third position  123  to the fifth position  125 . 
     The first position  121  is a position closer to the outlet flow passage  19  from the scroll termination portion  45 , and the second position  122  is a position inside the outlet flow passage  19 . The third position  123  to the fifth position  125  are positions inside the discharge flow passage  51 , and are arranged from the first end  50   a  toward the second end  50   b  in the following order: the third position  123 , the fourth position  124 , and the fifth position  125 . 
     As depicted in  FIG. 8 , the shape of the cross section  131  of the scroll flow passage  30  at the first position  121  and the shape of the cross section  132  of the outlet flow passage  19  at the second position  122  are substantially rectangular. The shapes of the cross sections  133  to  135  of the discharge flow passage  51  at the third position  123  to the fifth position  125  change from a rectangular shape to a circular shape gradually from the inlet portion  55  toward the outlet portion  52 . 
     The inner wall surface  60  of the discharge pipe  50  at the change portion  56  has an inner side wall surface  141  continuing to the scroll inner peripheral wall  30   a  and an outer side wall surface  142  continuing to the scroll outer peripheral wall  30   b  and facing the inner side wall surface  141 . Further, the radially inner region  61  includes a region of the inner side wall surface  141 . 
     Accordingly, the cross-sectional shape gradually changes from a rectangular shape toward a circular shape at the change portion  56  from the inlet portion  55  toward the outlet portion  52  of the discharge pipe  50 , and thus the cross-sectional shape does not change abruptly, which makes it possible to suppress separation of the fluid from the inner side wall surface  141  in the discharge pipe  50 . 
     In the embodiments depicted in  FIGS. 5 and 6 , the radially inner region  61  includes protruding portions  85 ,  86  protruding toward the inner side of the discharge flow passage  51 , in at least a partial region between the inlet portion  55  and the outlet portion  52  of the discharge pipe  50 . 
     Accordingly, the protruding portions  85 ,  86  are formed in a region of the discharge flow passage  51  where separation of the fluid is likely to occur, and thus it is possible to suppress separation of the fluid from the wall surface of the discharge flow passage  51 . 
     Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented. 
     For instance, while the centrifugal compressor  1  is a multi-stage centrifugal compressor including a plurality of stages of impellers in the above described embodiments, the centrifugal compressor  1  may be a single-stage centrifugal compressor with a single stage of impeller. 
     REFERENCE SIGNS LIST 
       1  Centrifugal compressor 
       2  Casing 
       4  Rotary shaft 
       8  Impeller 
       19  Outlet flow passage 
       30  Scroll flow passage 
       30   a  Scroll inner peripheral wall 
       30   b  Scroll outer peripheral wall 
       36  Diffuser 
       43  Outlet 
       45  Scroll termination portion 
       50  Discharge pipe 
       51  Discharge flow passage 
       52  Outlet portion 
       54  Connection position 
       55  Inlet portion 
       56  Change portion 
       60  Inner wall surface 
       61  Radially inner region 
       63  Linear shape portion 
       71  First line segment 
       72  Second line segment 
       85 ,  86  Protruding portion