Abstract:
A centrifugal compressor is equipped with a resin housing, that is made in a manner such that compression efficiency is not lost and merits such as weight and cost reduction are not cancelled out. The turbo charger housing comprises a volute section, a channel formation section, and an impeller that is fixed in the center of the channel formation section to a rotating shaft. An annular shroud is disposed in a recess formed on the inner wall of the channel formation section. The annular shroud forms the outer walls of a channel and a diffuser for compressed gas. The edge of the annular shroud spaced from a partition wall, which forms a portion of a bearing housing. If the resin housing undergoes thermal deformation, the dimensions of a gap between the annular shroud and the curved profile of a blade is maintained.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a centrifugal compressor provided with a housing made of resin which is used in a turbocharger or the like. 
       BACKGROUND ART 
       [0002]    A turbocharger installed in a vehicle or the like, drives a compressor by a exhaust turbine which is driven by energy of exhaust gas, compresses intake air in the compressor and supplies the compressed air to an engine. This type of turbocharger is provided with a bearing housing between a turbine housing of the exhaust turbine and a compressor housing for a turbo compressor. The bearing housing movably supports the rotating shaft rotatably. On the rotating shaft arranged through the bearing housing, an impeller of the turbo compressor and a wheel of the exhaust turbine are fixed. 
         [0003]    The bearing housing houses a bearing mechanism for movably supporting the rotating shaft rotatably. Between the bearing housing and the compressor housing, a seal wall is interposed to form a flow path for gas which is enclosed in the compressor housing and is to be compressed. 
         [0004]    A spiral-shaped housing such as a housing for a turbocharger, is normally made of cast aluminum, cast iron or the like. In recent years, for the purposes of making the housing lighter and at lower cost, a housing made of resin is used. 
         [0005]    Disclosed in Patent Literature 1, is a housing for a turbo charger, which is made of thermoplastic resin. Disclosed in Patent Literature 2, is a housing for a centrifugal compressor, which has a double-wall structure made of thermoset resin and thermoplastic resin. 
         [0006]    Patent Literature 3 discloses the invention to improve compressor efficiency and to prevent an impeller from being damaged. The inner wall of the compressor housing, which faces a flow path of the gas to be compressed, is made of resin material which is highly machinable. A clearance between the inner wall of the compressor housing and a curved profile of the impeller of the compressor is set small to improve compressor efficiency and to prevent the impeller from being damaged when the impeller comes in contact with the inner wall of the compressor housing. 
         [0007]    In Patent Literature 4, an inner wall of a path forming section of a compressor housing of a turbocharger, is made of resin material which is highly machinable so as to reduce a cost thereof. 
         [0008]    In Patent Literature 5, for the same reason as Patent Literature 3, an inner wall of a path forming section of a compressor housing of a turbocharger, is made of resin material. 
         [0009]      FIG. 10  is a schematic view of a compressor of a turbo charger  100  provided with a resin housing.  FIG. 10  shows an impeller  104  fixed to a rotating shaft  102  and a plurality of blades  106  extending radially from the impeller  104 . An outer edge of the blade  106  forms a curved profile  106   a.  A housing  110  is arranged around the blades  106 . The housing  110  includes a volute section  110   a  which forms a volute part s and a path forming section  110   b  which forms a flow path c for the gas to be compressed. 
         [0010]    The path forming section  110   b  is arranged to surround the blades  106 . The flow path c is formed by a hub face  108  of the impeller and an inner wall of the path forming section  110   b.  The flow path c curves from an axial direction of the rotating shaft  102  (a direction of an arrow a) into a radial direction of the impeller  104  (a direction of an arrow b). A section extending from the flow path c where the blades  106  are arranged to the flow path d which is arranged on an outlet side of the flow path c, functions as a diffuser (static-pressure increasing region). 
         [0011]    By rotation of the impeller  104 , the gas to be compressed is drawn into the flow path c in the direction of the arrow a, and reaches the blades  106  to increase an absolute flow velocity of the gas. The intake gas whose velocity is increased in the flow path c, is directed into the direction of the arrow b to enter the diffuser d. As the intake gas advances in the diffuser d, the intake gas is compressed and then exhausted to the volute part s. 
         [0012]    The compressor achieves high compression efficiency with a small clearance T between the inner wall of the path forming section  110   b  and the curved profile  106   a  of the impeller  104 . The temperature of the gas to be compressed is raised by compressing the gas in the compressor. In the case of using the housing  110  made of resin, the resin has a higher thermal expansion rate than materials such as metal and thus, the housing  110  expands being exposed to the heat of the gas and thermally deforms in a direction of an arrow e as indicated by a dotted line  110 ′. By this, the clearance T becomes wider and the gas leaks from the wider clearance T, resulting in a decrease in compression efficiency. 
         [0013]    In the resin housing, if the impeller  104  is damaged and broken pieces thereof fly in the housing, it is necessary to prevent the broken pieces from flying out of the housing. To take measures against the issue, it is possible to increase a thickness of the housing  110  or to provide a reinforcing rib  112  in the housing  110 . However, such measures offset advantages of using the resin housing from the perspective of weight saving and cost reducing. 
       CITATION LIST 
     Patent Literature  
     [PTL 1] 
       [0000]    
       
         DE 10260042 A1 
       
     
       [PTL 2] 
       [0000]    
       
         EP 1830071 A2 
       
     
       [PTL 3] 
       [0000]    
       
         JP 9-170442 A 
       
     
       [PTL4] 
       [0000]    
       
         JP2001-234753 A 
       
     
       [PTL5] 
       [0000]    
       
         JP2002-256878 A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0019]    It is an object of the present invention to provide a centrifugal compressor equipped with a resin housing, which can obtain advantages such as a weight saving and cost reduction without decreasing compression efficiency. 
       Solution to Problem 
       [0020]    To achieve the above object, the present invention provides a centrifugal compressor which may include, but is not limited to: an impeller which includes a plurality of blades fixed radially to a rotating shaft; a resin housing which is arranged around the impeller and which is made of resin. Between the impeller and the resin housing, a flow path for a gas to be compressed may be formed by an outer periphery of the impeller and an inner wall of a path forming section of the resin housing, the gas flowing in the flow path from an axial direction to a radial direction of the impeller. The inner wall of the path forming section of the resin housing may be carved to form a depression in which an annular shroud is provided, the annular shroud being made of one of a metal and a ceramic material and forming an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud may be fixed to a wall facing the annular shroud at the diffuser. 
         [0021]    According to the present invention, the inner wall of the path forming section of the resin housing is carved to form the depression in which the annular shroud made of metal or ceramic material is provided. The annular shroud forms an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud is made of a material which has a higher strength and a lower thermal expansion rate than resin (engineering plastic), such as aluminum and carbon steel, or ceramic. The annular shroud is fixed to the wall facing the annular shroud at the diffuser. 
         [0022]    The annular shroud is made separately from the resin housing and is not joined to the inner wall of the resin housing. Thus, the thermal deformation of the resin housing does not affect the annular shroud, allowing the space between the annular shroud and the curved profile of the impeller to remain the same. Therefore, the compression efficiency is not affected. With use of the annular shroud, it is not necessary to make the resin housing thicker or to provide a reinforcement rib in the resin housing. As a result, it does not offset merits such as weight saving and cost reduction. 
         [0023]    In the present invention, the centrifugal compressor may be provided with a seal ring housed in a groove which is disposed between a rear face of the annular shroud and the depression of the inner wall of the path forming section. By this, even when the resin housing thermally deforms, leaving a gap between the inner wall of the resin housing and the annular shroud, it is possible to prevent the gas from entering the gap and the gas on the downstream side of the impeller from flowing back to the inlet side of the impeller. As a result, the compression efficiency does not decrease. 
         [0024]    At the inlet of the impeller, the temperature of the gas is low as well as the temperatures of the annular shroud and the inner wall of the path forming section of the resin housing. Thus, an inexpensive rubber O-ring can be used as the seal ring. Further, the diameter of the inlet of the impeller is small and thus, the O-ring of a small diameter can be used. Accordingly, it is possible to achieve the cost reduction of the seal ring. 
         [0025]    In addition to the above structure, the path forming section of the resin housing may be separable at a separating plane into an upstream portion and a downstream portion in a direction of flow of the gas, the separating plane being positioned at the groove for housing the seal ring. By this, it is possible to eliminate a thick-walled portion of the resin housing. This prevents air bubbles or the like remaining in the resin housing during injection molding of the resin housing. As a result, the quality of the resin housing is improved and the yield is improved, thereby preventing cost increase. 
         [0026]    Further, by placing the separating plane to the groove for housing the seal ring, there is no need for carving to form the groove and thus, the molding manufacturing of the resin housing becomes easy, resulting in reducing the processing cost. 
         [0027]    In the present invention, the resin housing may have a slit in one of the path forming section and a separated part of the path forming section, the split being formed in an axial direction of the rotating shaft and opening to outside. By this, the thick-walled portion of the resin housing can be eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the resin housing. As a result, the quality of the resin housing is improved and the yield is enhanced, thereby preventing cost increase. 
         [0028]    In the present invention, between the annular shroud and the path forming section of the resin housing, a circulation space for the gas may be formed, and at least two communication openings may be formed along a flow direction of the gas such that the communication space and the flow path are in communication with each other through the communication openings to allow the gas to flow in the communication space. In this manner, when the flow rate of the gas is low, a portion of the gas enters the communication space through the opening on the downstream side and then returns to the flow path from the opening on the upstream side, thereby forming a circulation flow. As the circulation flow joins the flow of the gas at the inlet of the impeller, it is possible to keep the flow rate of the gas at the inlet of the impeller not less than a stall limit. As a result, it is possible to lower a lower limit of the flow rate of the compressor. 
         [0029]    In contrast, when the flow rate of the gas is high, a portion of the gas enters the communication space from the opening on the upstream side and then returns to the flow path from the opening on the downstream side. Thus, the upper limit of the flow rate of the gas is enhanced. As a result, the upper limit and the lower limit of the flow rate of the gas, which is allowable for operation of the centrifugal compressor, can be expanded. 
         [0030]    In the present invention, the path forming section of the resin housing may be reinforced by an annular reinforcement layer covering an outer wall of the path forming section, the reinforcement layer being made of glass fiber. By providing on the outer wall of the path forming section the reinforcement layer made of glass fiber with high tension strength, when the impeller is damaged, the broken pieces of the impeller or the broken impeller does not penetrate through the wall of the path forming section and thus, the wall thickness of the path forming section, reinforced by the reinforcement layer, can be thinner. As a result, it is possible to reduce the production cost of the resin housing. 
       Advantageous Effects of Invention 
       [0031]    According to the centrifugal compressor of the present invention, the centrifugal compressor is provided with: an impeller which includes a plurality of blades fixed radially to a rotating shaft; a resin housing which is arranged around the impeller and which is made of resin. Between the impeller and the resin housing, a flow path for a gas to be compressed is formed by an outer periphery of the impeller and an inner wall of a path forming section of the resin housing, the gas flowing in the flow path from an axial direction to a radial direction of the impeller. The inner wall of the path forming section of the resin housing is carved to form a depression in which an annular shroud is provided, the annular shroud being made of one of a metal and a ceramic material and forming an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud is fixed to a wall facing the annular shroud at the diffuser. 
         [0032]    Therefore, the clearance between the annular shroud and the curved profile of the blade can be kept at a set dimension, thereby maintaining a high compression efficiency, and by providing the annular shroud, there is no need to make the resin housing thicker or to provide a reinforcement rib in the resin housing and thus, the benefits such as a lighter weight or cost reduction of the resin housing can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0033]      FIG. 1  is a side view of a centrifugal compressor in relation to a first embodiment, to which the present invention is applied. 
           [0034]      FIG. 2  is a side view of a centrifugal compressor in relation to a second embodiment, to which the present invention is applied 
           [0035]      FIG. 3  is a side view of a centrifugal compressor in relation to a third embodiment, to which the present invention is applied. 
           [0036]      FIG. 4  is a side view of a centrifugal compressor in relation to a fourth embodiment, to which the present invention is applied. 
           [0037]      FIG. 5  is a side view of a centrifugal compressor in relation to a fifth embodiment, to which the present invention is applied. 
           [0038]      FIG. 6  is a side view showing a modified example of the fifth embodiment. 
           [0039]      FIG. 7  is a side view of a centrifugal compressor in relation to a sixth embodiment, to which the present invention is applied. 
           [0040]      FIG. 8  is a side view showing a modified example of the sixth embodiment. 
           [0041]      FIG. 9  is a side view of a centrifugal compressor in relation to a seventh embodiment, to which the present invention is applied. 
           [0042]      FIG. 10  is a side view showing a part of a conventional centrifugal compressor. 
           [0043]      FIG. 11  is a side view of a centrifugal compressor showing a comparison between a thick resin housing and a reinforcement rib. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0044]    An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
       First Embodiment 
       [0045]    A first embodiment of a centrifugal compressor to which the present invention is applied is explained in reference to  FIG. 1 . In  FIG. 1 , a rotating shaft  12  is arranged in a center of a resin housing  20 , and an impeller  14  having a plurality of blades  16  fixed radially is fixed to an outer periphery of the rotating shaft  12 . A hub face  18  of the impeller  14  curves from an inlet side to an outlet side in a flow direction of a gas to be compressed (a direction indicated with an arrow a) from an axial direction to a radial direction of the rotating shaft  12 . The resin housing  20  includes a volute section  20   a  which forms a volute part s and a path forming section  20   b  which forms a flow path c for the gas to be compressed. 
         [0046]    The outer edge of the blade forms a curved profile  16   a.  Along the curved profile  16   a,  an inner wall  20   b   1  of the path forming section  20   b  of the resin housing  20  is formed. The inner wall  20   b   1  is carved to form a depression  20   b   2  in which an annular shroud is inserted. The rotating shaft  12  is rotatably and movably supported by a bearing provided in a bearing housing  30 . The bearing is not shown in the drawing. The spiral-shaped volute section  20   a  forms a volute part s. A wall  32  constituting a part of the bearing housing  30  is arranged facing the volute part s formed by the spiral-shaped volute section  20   a.  An end of the volute section  20   a  is fixed to the wall  32  via a seal ring  34 . 
         [0047]    In the depression  20   b   2  formed in the inner wall  20   b   1  of the path forming section  20   b,  an annular shroud  22  is inserted. A downstream end of the annular shroud  22  which is on a downstream side in a direction of the gas flow, projects into the volute part s. The downstream end of the annular shroud  22  is fixed to the wall  32  by a bolt  26  via a spacer  24 . The annular shroud  22  is made of a metal which has a higher strength and a lower thermal expansion rate than resin, such as aluminum and carbon steel, or ceramic. The annular shroud  22  and the resin housing  20  are not connected. The clearance between the annular shroud  22  and the curved profile  16   a  of the blade  16  is set as small as possible to maintain decent compression efficiency. 
         [0048]    In this manner, the annular shroud  22  forms an outer surface of the flow path c and an outer surface of the diffuser d. The flow path c directs the gas from an axial direction of the impeller  14  (the direction of the arrow a) into a radial direction of the impeller (a direction of an arrow b). The diffuser d is arranged on an outlet side of the flow path c to convert the kinetic energy of the gas into a static pressure. An inner surface of the flow path c is formed by the hub face  18  of the impeller  14 , whereas an inner surface of the diffuser d is formed by the wall  32 . 
         [0049]    With the above structure, the rotation of the impeller  14  causes the gas to be drawn in from an inlet side of the blade  16 , through the flow path c and the diffuser d and then converted into the static pressure. The compressed gas flows through the diffuser d and then enters the volute part s. A plurality of bolts  26  are arranged at intervals in the circumferential direction of the rotating shaft  12 . However, the bolts  26  do not interfere with the flow of the gas. 
         [0050]    According to the first embodiment, the annular shroud  22  is fixed to the wall  32  of the bearing housing  30  solely by the bolts  26  and the annular shroud  22  and the resin housing  20  are separated from each other. Thus, heat deformation of the resin housing  20  does not reach the annular shroud  22 . Therefore, even when the resin housing  20  thermally deforms, the clearance between the annular shroud  22  and the blade  16  does not change. As a result, the compression efficiency is not affected. 
         [0051]    By providing the annular shroud  22 , it is no longer necessary to make the resin housing  20  thicker or to provide a reinforcement rib or the like in the resin housing  20 . As a result, it does not offset merits such as a lighter weight and cost reduction. 
       Second Embodiment 
       [0052]    A second embodiment of the centrifugal compressor to which the present invention is applied is explained in reference to  FIG. 2 . In the second embodiment, as shown in  FIG. 2 , the downstream end  22   a  of the annular shroud  22  has two bending sections. The annular shroud  22  bends at a first bending section to extend to a position where the downstream end comes in contact with the wall  32  of the bearing housing  30  and bends at a second bending section to form a flange part  22   a   1 . The wall  32  is formed with a depression  32   a  in which the flange part  22   a   1  is fitted. The flange part  22   a   1  is connected to the depression  32   a  by a bolt  40 . 
         [0053]    The flange part  22   a   1  is formed partially around the impeller  14 . Thus, the downstream end  22   a  of the shroud  22  does not block the diffuser d or interfere with the flow of the gas in the diffuser. The rest of the structure is substantially the same as that of the first embodiment. The same parts are indicated by the same reference numerals. 
         [0054]    According to the second embodiment, the annular shroud  22  is a single-piece member which extends to the flange part  22   a   1  and is processed by pressure forming. Therefore, in addition to the function effects achieved in the first embodiment, the annular shroud  22  is easy to manufacture by pressure-forming and it is easy to connect the annular shroud  22  to the wall  32  as there is no need for the spacer  24  which is provided in the case of the first embodiment, in order to connect the annular shroud  22  to the wall  32 . 
       Third Embodiment 
       [0055]    Next, a third embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to  FIG. 3 . In the third embodiment, the depression  20   b   2  carved in the inner wall  20   b   1  of the path forming section  20   b  of the resin housing  20 , is carved to form a groove  52 . In the groove  52 , a rubber or resin seal ring  50  is inserted. The rest of the structure is substantially the same as that of the second embodiment. 
         [0056]    According to the third embodiment, even when the resin housing  20  thermally deforms, creating a gap between the depression  20   b   2  and a rear face of the annular shroud  22 , it is possible to prevent the entry of the gas between the depression  20   b   2  and the annular shroud  22  by means of the seal ring  50 . Therefore, the compression efficiency of the compressor does not decrease. 
         [0057]    At an inlet of the impeller  14 , the temperature of the gas is approximately the same as the ambient air and is low. At the inlet of the impeller, the temperatures at the annular shroud  22  and the path forming section  20   b  are also low and thus, an inexpensive rubber O-ring can be used. Further, at the inlet of the impeller  14 , the gas path in which the gas flows does not curve in the radial direction and the diameter of the impeller  14  is small and thus, the O-ring of a small diameter can be used. Therefore, it is possible to achieve the cost reduction of the seal ring  50 . 
       Fourth Embodiment 
       [0058]    Next, a fourth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to  FIG. 4 .  FIG. 4  shows the resin housing  20  of the fourth embodiment. The path forming section  20   b  of the resin housing  20  is separable into two resin separated portions  60  and  62 . The path forming section  20   b  is separated into the separated portions  60  and  62  at a separating plane  64  having a stepped portion  64   a  in a center of a thick-walled portion thereof. The separating plane  64  is disposed such that one end there of is positioned at the groove where the seal ring  50  is housed. The rest of the structure is substantially the same as that of the third embodiment. 
         [0059]    According to the fourth embodiment, the path forming section  20   b  is separated into the separated portions  60  and  62  and thus, the thickness of the path forming section  20   b  is reduced. This prevents, during molding manufacturing of the resin housing  20 , air bubbles from remaining in the path forming section  20   b.  Therefore, the quality of the resin housing  20  is improved and with improved yield rate, the production cost of the resin housing is reduced. 
         [0060]    Further, by placing the separating plane  64  to the groove  52 , there is no need for carving the path forming section  20   b  to form the groove  52 , and a corner of the separated portion  60  is processed and thus, the molding manufacturing of the separated portion  60  becomes easy, resulting in reducing the processing cost. 
       Fifth Embodiment 
       [0061]    Next, a fifth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to  FIG. 5 . In the fifth embodiment, the thick-walled portion of the path forming section  20   b  of the resin housing  20 , has a slit V in a direction perpendicular to the thickness of the path forming section, i.e. in a direction approximately parallel to the rotational axis of the impeller  14 . The slit V opens to outside. The rest of the structure is substantially the same as that of the third embodiment. 
         [0062]    According to the fifth embodiment, by forming the slit V in the path forming section  20   b,  the thick-walled portion of the path forming section  20   b  can be eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the resin housing  20 . As a result, the quality of the resin housing  20  is improved and the yield is improved, thereby preventing cost increase. 
         [0063]    A modified example of the fifth preferred embodiment is explained in reference to  FIG. 6 . According to the modified example shown in  FIG. 6 , the separated portion  60  of the separated portions  60  and  62  of the resin housing  20 , is formed with a slit Va inside the thick-walled portion of the path forming section  20   b.  The slit Va is formed in the direction perpendicular to the thickness of the path forming section, i.e. in the direction approximately parallel to the rotational axis of the impeller  14 . And, an end of the slit Va opens to outside. The rest of the structure is substantially the same as that of the fourth embodiment. 
         [0064]    According to the modified example, in addition to the function effects achieved in the fourth embodiment, by providing the slit Va in the path forming section  20   b  of the separated portion  60 , the thick-walled portion of the separated portion  60  is eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the separated portion  60  of the resin housing  20 . As a result, the quality of the separated portion  60  is improved and the yield is improved, thereby preventing cost increase. 
       Sixth Embodiment 
       [0065]    A sixth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to  FIG. 7 . In the sixth embodiment, the inner wall  20   b   1  of the path forming section  20   b  is carved to form the depression  20   b   2  in the flow path c and the depression  20   b   2  is further carved to form a circulation space  70  on a rear side of the annular shroud  22 . The annular shroud  22  includes an upstream through-hole  72  and a downstream through-hole  74  where the annular shroud faces the circulation space  70 . The rest of the structure is substantially the same as that of the second embodiment. 
         [0066]    According to the sixth embodiment, when the flow rate of the gas is low, a portion of the gas flowing in the flow path c enters the circulation space  70  from the downstream through-hole  74  and then returns to the flow path c from the upstream through-hole  72 , thereby forming a circulation flow. As the circulation flow joins the flow of the gas at the inlet of the impeller  14 , thereby preventing the impeller  14  from stoll. As a result, it is possible to lower a lower limit of the flow rate of the compressor. 
         [0067]    In contrast, when the flow rate of the gas is high, a portion of the gas flowing in the flow path c enters the circulation space  70  from the upstream through-hole  72  and then returns to the flow path c from the downstream through-hole  74 . By this, it is possible to increase an upper limit of the flow rate of the gas. As a result, the range of the flow rate of the gas, which is allowable for operation of the centrifugal compressor, can be expanded. 
         [0068]    Next, a modified example of the sixth embodiment is explained in reference to  FIG. 8 . In a manner similar to the sixth embodiment, the circulation space  70  is formed on the rear side of the annular shroud  22 . At the entrance to the flow path c, a circular slit opening  76  is formed between an end of the annular shroud  22  which is on an inlet side and the inner wall  20   b   1  of the path forming section  20   b.  On a downstream side of the slit opening  76 , the through-hole  74  which is the same as the through-hole  74  of the sixth embodiment, is formed in the annular shroud  22  path c. The rest of the structure is substantially the same as that of the sixth embodiment. 
         [0069]    In comparison with the sixth embodiment, this modified example has an advantage of easier manufacturing of the annular shroud  22  and the path forming section  20   b  as the number of through-holes formed in the annular shroud  22  is reduced and there is no need to carve the inner wall surface  20   a  of the path forming section  20   b  to form the depression  20   b   2  in which the annular shroud  22  is fit. Further, the annular slit opening  76  is formed and thus, the opening area of the slit opening can be increased and it is easy to form the slit opening. 
       Seventh Embodiment 
       [0070]    A seventh embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to  FIG. 9 . In the seventh embodiment, the path forming section  20   b  of the resin housing  20  is made thinner, and behind the path forming section  20   b,  an annular glass fiberboard  80  is provided. The rest of the structure is substantially the same as that of the second embodiment. 
         [0071]    When the impeller  14  is damaged, the path forming section  20   b  must have enough strength to prevent the broken pieces of the impeller or the broken impeller  14  from breaking through the resin housing  20 . According to the seventh embodiment, the glass fiberboard  80  is provided on the rear side of the path forming section  20   b  for reinforcement and thus, the path forming section  20   b  can be thin. With reduced thickness of the path forming section  20   b,  it is possible to eliminate air bubbles or the like remaining in the resin housing  20  during injection molding of the resin housing  20 . As a result, the quality of the resin housing  20  is improved, thereby improving the yield during the molding manufacturing of the resin housing and preventing cost increase. 
       INDUSTRIAL APPLICABILITY 
       [0072]    According to the present invention, it is possible, in a centrifugal compressor which can be used in a turbocharger or the like, to achieve a lighter weight and cost reduction of the housing without a decrease in compression efficiency even when the housing is thermally deformed.