Patent Publication Number: US-8118543-B2

Title: Centrifugal compressor having switchable two passages

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a centrifugal compressor having an impeller. 
     A centrifugal compressor is known as one of compressors for compressing gas. Japanese unexamined patent publication No. 2005-194933 discloses a centrifugal compressor which has a fluid passage for communicating with a diffuser, a pair of scrolls for receiving gas from the diffuser and discharging out of the compressor, and a changing means for changing the width of the passage in the diffuser. The changing means changes the width of the passage so that the passage is changed into a narrow passage state where the diffuser is in communication with only one of the scrolls, or into a wide passage state where the diffuser is in communication with both of the scrolls. 
     The above-described centrifugal compressor has a first operational mode and a second operational mode, which are set selectively. The first operational mode is set so that the width of the passage in the diffuser is widened to utilize two scrolls. The second operational mode is set so that the width of the passage in the diffuser is narrowed to utilize one scroll. That is, in the centrifugal compressor, the first operational mode and the second operational mode are set alternatively by switching the state between the narrow passage state and the wide passage state. Accordingly, the centrifugal compressor can achieve high compression efficiency in a substantially wide range of the flow rate with its simple structure. 
     The above-described compressor can set the width of the passage in the diffuser variably by the changing means for changing the width of the passage. However, diffuser stall still exists continuously. When the flow rate in the centrifugal compressor is extremely low, specifically, diffuser stall occurs inevitably in the compressor. Diffuser stall invites problems such as vibration of the compressor, and thereby prevents the stable operation of the compressor. 
     The present invention is directed to providing a centrifugal compressor in which diffuser stall is prevented when the flow rate of the gas is low, so as to obtain stable operation in a wide range of the flow rate. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a centrifugal compressor for compressing gas has a casing; a rotary shaft supported by the casing; an impeller rotatably fixed to the rotary shaft, wherein the impeller sends the gas radially outward to a flow path downstream thereof; a diffuser formed around the impeller and defined by a pair of diffuser walls having the diffuser therebetween; a first scroll formed around the diffuser; a second scroll formed between the diffuser and the impeller; a first passage formed from the impeller to the first scroll through the diffuser; a second passage formed from the impeller to the second scroll; and a switching member for switching the flow path between the first passage and the second passage by opening and closing the diffuser, wherein the switching member includes a movable diffuser wall which is at least one of the pair of the diffuser walls, wherein the movable diffuser wall is moved closer to and away from the other of the pair of the diffuser walls so as to switch the flow path between the first passage and the second passage, wherein a wall surface of the movable diffuser wall facing the diffuser includes a diffuser wall surface and a scroll wall forming surface, wherein the scroll wall forming surface forms part of an inner surface of the second scroll. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a side cross-sectional view of a centrifugal compressor according to a first preferred embodiment of the present invention; 
         FIG. 2  is a cross-sectional view that is taken along the line I-I in  FIG. 1 ; 
         FIG. 3  is an enlarged fragmentary cross-sectional view illustrating the relation between a first casing and a movable diffuser wall; 
         FIG. 4  is a side cross-sectional view of the centrifugal compressor when a second passage is formed; 
         FIG. 5  is a cross-sectional view that is taken along the line II-II in  FIG. 4 ; 
         FIG. 6  is a schematic view illustrating an operation of a movable cam and a fixed cam in the centrifugal compressor; 
         FIG. 7  is a side cross-sectional view of a centrifugal compressor according to a second preferred embodiment of the present invention; 
         FIG. 8  is a fragmentary cross-sectional view of a centrifugal compressor according to a third preferred embodiment of the present invention; 
         FIG. 9A  is an enlarged fragmentary cross-sectional view of a centrifugal compressor having a modified movable diffuser wall according to an alternative embodiment; and 
         FIG. 9B  is an enlarged fragmentary cross-sectional view of a centrifugal compressor having a modified movable diffuser wall according to an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe a first preferred embodiment of a centrifugal compressor according to the present invention with reference to  FIGS. 1 through 6 . The centrifugal compressor  10  according to the first preferred embodiment has a first casing  11 , a second casing  12 , an impeller  14 , a diffuser  20 , a first scroll  22 , and a second scroll  32 , as shown in  FIG. 1 . The first casing  11  and the second casing  12  are coupled to each other to form a casing assembly. The impeller  14  is rotatably received in the first and the second casings  11 ,  12  so as to send the gas radially outward to a flow path downstream thereof. The diffuser  20  is formed around the circumference of the impeller  14 . The first scroll  22  is formed around the circumference of the diffuser  20 . The second scroll  32  is formed between the impeller  14  and the diffuser  20 . The centrifugal compressor  10  includes a first passage, a second passage, and a switching member. The first passage is formed from the impeller  14  to the first scroll  22  through the diffuser  20 . The second passage is formed from the impeller  14  to the second scroll  32 . The switching member selectively switches the flow path downstream of the impeller  14  between the first passage and the second passage by opening and closing the diffuser  20 . 
     A space  17  which has a funnel shape is defined In the casing assembly formed by the first casing  11  and the second casing  12 . The impeller  14  is disposed in the space  17  and fixed to a rotary shaft  15 . The rotary shaft  15  extends through a shaft hole  13  in the first casing  11 . The rotary shaft  15  is rotatably supported by the first casing  11  through a bearing  16  which has a sealing function. The left-hand side of the centrifugal compressor  10  corresponds to the front side and the right-hand side corresponds to the rear side as viewed in  FIG. 1 . The rear end of the rotary shaft  15  is connected to a drive source, such as a motor or the like (not shown), to be rotated therewith. The second casing  12  has a passage  18  with a constant diameter adjacent to the front side of the space  17 . A suction port  19  is formed adjacent to the front side of the passage  18  to increase the diameter thereof so as to be flared out frontward. 
     The impeller  14  has a plurality of blades formed radially. The impeller  14  draws gas from the suction port  19  through the passage  18  in the axial direction, and sends the gas radially outward to the flow path downstream of the impeller  14 . In the first embodiment, the first passage is formed to guide the gas from the impeller  14  to the first scroll  22  through the diffuser  20 . The first passage is utilized when the flow rate of the centrifugal compressor  10  exceeds a predetermined level. When the first passage is formed, the impeller  14  sends the gas radially outward to the diffuser  20 . 
     The diffuser  20  functions as a gas passage so as to decrease the velocity of the gas flowing out from the impeller  14  while increasing the pressure, and to send the gas to the first scroll  22 . In other words, the diffuser  20  converts the velocity energy (kinetic energy) of the gas from the impeller  14  into the pressure energy. The diffuser  20  according to the first embodiment is defined by a pair of diffuser walls, which are formed in the first casing  11  and the second casing  12 , respectively. The first casing  11  includes a movable diffuser wall  27 , which will be described later. The second casing  12  includes a fixed diffuser wall  21 . 
     The fixed diffuser wall  21  is formed by a planar surface perpendicular to the axis of the rotary shaft  15 . The fixed diffuser wall  21  faces the movable diffuser wall  27  in the first casing  11 . The first scroll  22  is formed in the second casing  12  so as to surround the fixed diffuser wall  21 . The first scroll  22  is in communication with the diffuser  20 , and also is in communication with an outlet port (not shown). A curved wall  31  is formed between the fixed diffuser wall  21  and the impeller  14  in the second casing  12  so as to form a concave in the surface adjacent to the fixed diffuser wall  21 . The cross section of the curved wall  31  is formed with a hemispherical shape. The curved wall  31  constitutes a part of the second scroll  32 . 
     In the rear side of the impeller  14  (in the right side in  FIG. 1 ), an annular inner space  23  is defined in the front surface of the first casing  11  and an annular outer space  24  is defined around the inner space  23 . An annular rotation support plate  25  is disposed in the inner space  23 . The rotation support plate  25  is rotatably supported by the first casing  11  through a bearing  26  which has a sealing function. The length of the outer space  24  in the axial direction of the rotary shaft  15  is set larger than that of the inner space  23 . The annular movable diffuser wall  27  is disposed in the outer space  24  so as to face the fixed diffuser wall  21 , with the diffuser  20  therebetween. 
     In the first embodiment, the switching member for switching the flow path between the first passage and the second passage includes the movable diffuser wall  27 , which is moved closer to and away from the fixed diffuser wall  21 . The movable diffuser wall  27  is supported by the rotation support plate  25  through an annular flexible member  28  at the inner circumferential surface of the movable diffuser wall  27 . The flexible member  28  is formed by a diaphragm. With the annular flexible member  28  in the form of the diaphragm, the movable diffuser wall  27  is protruded frontward in the direction to the diffuser  20  at the low flow rate, and is caved rearward in the direction to the outer space  24  at the high flow rate. 
     The flexible member  28  is formed with an annular shape and a hole is formed at the center thereof, as shown in  FIG. 3 . The flexible member  28  is made of a material which has predetermined rigidity in the radial direction, and has deformable flexibility in the direction perpendicular to the radial direction (axial direction of the rotary shaft  15 ). An inner periphery  28   a  of the flexible member  28  is retained in an annular groove formed in the rotation support plate  25 , and an outer periphery  28   b  of the flexible member  28  is retained in an annular groove formed in the movable diffuser wall  27 . Thus, the rotation support plate  25 , the flexible member  28  and the movable diffuser wall  27  are formed integrated. Accordingly, the movable diffuser wall  27  is rotatable in the circumferential direction together with the rotation support plate  25  and the flexible member  28 , and is movable in the axial direction of the rotary shaft  15  by the flexible member  28 . 
     As shown in  FIG. 2 , a movable cam  35  is fixed to the movable diffuser wall  27  on the rear side to be disposed in the outer space  24 . The movable cam  35  includes four inclined portions which have inclined movable cam surfaces on the rear side thereof, respectively. The movable cam  35  is formed with an arc shape coaxially with the movable diffuser wall  27 , and the inclined portions are positioned circumferentially to be separated with each other. The movable cam surfaces of the movable cam  35  are formed with inclined surfaces whose height are gradually decreased in the clockwise direction in  FIG. 2  (upward direction in  FIG. 6 ). A fixed cam  36  is formed in the front surface of the first casing  11  to be disposed in the outer space  24 . The fixed cam  36  is formed with an arc shape, and includes four inclined portions which have fixed cam surfaces on the front side thereof, respectively. The position of the four fixed cam surfaces correspond to the movable cam surfaces of the movable cam  35 . The cam surfaces of the fixed cam  36  are formed with inclined surfaces whose height are gradually decreased in the counter-clockwise direction so as to be formed reversely to the movable cam  35 . The movable cam  35  and the fixed cam  36  are positioned so that the movable cam surfaces and the fixed cam surfaces are continuously in contact with each other. According to the first embodiment, the movable cam  35  is urged to the fixed cam  36  by the gas pressure in the diffuser  20 , but may be continuously in contact with each other by a spring and the like. 
     A connecting pin  37  is connected to the back surface (rear surface) of the movable diffuser wall  27  and is projected therefrom. The connecting pin  37  is longer than the moving distance of the movable diffuser wall  27  in the axial direction of the rotary shaft  15 . An actuator  40  is attached to the first casing  11 . The actuator  40  has a rod  41 , which is rotatably connected to the connecting pin  37 . The rod  41  is slidable in the longitudinal direction of the connecting pin  37 . Accordingly, when the actuator  40  is actuated and the rod  41  is moved in its longitudinal direction, the movable diffuser wall  27  is rotated by a predetermined degree. The actuator  40  is a driving source to move the rod  41  forward and backward, and is actuated in accordance with the flow rate of the centrifugal compressor  10 . According to the first embodiment, the actuator  40  is actuated when the flow rate is determined to exceed a predetermined level. The actuator  40  may be preferably a fluid pressure cylinder or an electric motor, or the like. 
     The front surface of the movable diffuser wall  27  (a wall surface facing the diffuser  20 ) has a diffuser wall surface  29  and a scroll wall forming surface  30 . The diffuser wall surface  29  is formed with a surface perpendicular to the axial direction of the rotary shaft  15 . The scroll wall forming surface  30  is formed correspondingly to a curved wall  31  so that the scroll wall forming surface  30  and the curved wall  31  form the inner surface of the second scroll  32 . The diffuser wall surface  29  faces the fixed diffuser wall  21 , and is moved closer to and away from the fixed diffuser wall  21  in accordance with the displacement of the movable diffuser wall  27 . When the diffuser wall surface  29  is moved to the closest to the fixed diffuser wall  21 , the diffuser wall surface  29  and the fixed diffuser wall  21  are in close contact with each other, and the diffuser  20  is closed. When the diffuser wall surface  29  is in close contact with the fixed diffuser wall  21 , the second scroll  32  is formed by the curved wall  31  and scroll wall forming surface  30 , and the second passage is formed from the impeller  14  to the second scroll  32 . An outlet of the second scroll  32  is in communication with the first scroll  22 . 
     The operation of the centrifugal compressor  10  according to the first embodiment of the present invention will be described. When the centrifugal compressor  10  is stopped, the actuator  40  is not actuated, or in a non-operational state. When the actuator is in the non-operational state, the rod  41  is maintained at a position as shown in  FIG. 5 . The movable cam  35  is at a position shown by solid lines, and the movable diffuser wall  27  is at a position shown by solid lines as shown in  FIG. 6 . In this state, the diffuser wall surface  29  is in close contact with the fixed diffuser wall  21 , and the second passage is formed as shown in  FIG. 4 . 
     When the flow rate of the centrifugal compressor  10  is equal or lower than a predetermined level during the operation of the centrifugal compressor  10 , the actuator  40  is not actuated, and the second passage is maintained. The flow rate which does not exceed a predetermined level includes a flow rate at which diffuser stall in the diffuser  20  is inevitable if the gas flows through the diffuser  20  of the first passage. The gas drawn into the impeller  14  during the operation of the centrifugal compressor  10  flows through the second passage which is formed from the impeller  14  to the second scroll  32 . At this time, the diffuser  20  is closed, and the gas is not drawn to the first scroll  22 . The gas from the impeller  14  is guided by the scroll wall forming surface  30  and the curved wall  31  while flowing swirlingly, and is discharged out from the outlet port through the second scroll  32  and the first scroll  22 . In this case, the gas from the impeller  14  does not flow through the diffuser  20 , and stall in the diffuser  20  does not occur. 
     The operation when the flow rate of the centrifugal compressor  10  exceeds a predetermined level is now described. The flow rate which exceeds a predetermined level includes a flow rate at which diffuser stall in the diffuser  20  does not occur when gas flows through the diffuser  20  in the first passage. When the flow rate in the centrifugal compressor  10  exceeds a predetermined level, the actuator  40  is actuated and the rod  41  is moved. In accordance with the movement of the rod  41 , the connecting pin  37  is drawn in the circumferential direction (left side in  FIG. 2 ), and the movable diffuser wall  27  is rotated in the clockwise direction by angle θ, as shown in  FIG. 2 . In accordance with the rotation of the movable diffuser wall  27 , as shown in  FIG. 6 , the movable cam  35  is moved along the inclined surface of the fixed cam  36  from the position indicated by the solid line to a position indicated by an imaginary line (two-dot chain line), and the movable diffuser wall  27  is moved from the position indicated by the solid line to a position indicated by an imaginary line, by distance D. Thus, the movable diffuser wall  27  is moved away from the fixed diffuser wall  21  by the displacement of the flexible member  28 , and is moved parallelly in the direction to the outer space  24  to be caved in. 
     The movable diffuser wall  27  is moved away from the fixed diffuser wall  21 , and the diffuser  20  is opened as shown in  FIG. 1 . By opening the diffuser  20 , the first passage is formed from the impeller  14  to the first scroll  22  through the diffuser  20 , and the gas from the impeller  14  is drawn into the first scroll  22  through the diffuser  20 . At the time, the flow rate of the centrifugal compressor  10  exceeds a predetermined level, therefore, stall in the diffuser  20  does not occur, and the centrifugal compressor  10  is operated stably. 
     In the first embodiment, the movable diffuser wall  27  opens and closes the diffuser  20  by switching the flow path downstream of the impeller  14  between the passages. That is, the movable diffuser wall  27  switches the flow path downstream of the impeller  14  between a first operational state where the gas from the impeller  14  is drawn to the first scroll  22  through the diffuser  20 , and a second operational state where the gas from the impeller  14  is drawn to the second scroll  32  without flowing through the diffuser  20 . Concretely, at the flow rate at which stall in the diffuser  20  may occur (generally extremely low flow rate), the movable diffuser wall  27  is brought into contact with the fixed diffuser wall  21 . Therefore the diffuser  20  is closed, and the second passage is utilized so that the gas does not flow through the diffuser  20 . On the contrary, at the flow rate at which stall in the diffuser  20  may not occur, the movable diffuser wall  27  is moved away from the fixed diffuser wall  21  to open the diffuser  20 , thereby the first passage is utilized so that the gas flows through the diffuser  20 . Therefore, even when the flow rate is extremely low, stall in the diffuser  20  is prevented, and the centrifugal compressor  10  is operated stably. When the flow rate is sufficient, the diffuser effect is fully obtained, and effective compression is performed. 
     According to the first embodiment, the following advantageous effects are obtained.
     (1) The movable diffuser wall  27  switches the flow path downstream of the impeller  14  between the first passage and the second passage. That is, the movable diffuser wall  27  switches the flow path between the first operational state where the gas from the impeller  14  is drawn to the first scroll  22  through the diffuser  20 , and the second operational state where the gas from the impeller  14  is drawn to the second scroll  32  without flowing through the diffuser  20 . Therefore, in the state where the gas flows from the downstream of the impeller  14  to the second scroll  32 , the gas does not flow through the diffuser  20 , and stall in the diffuser  20  does not occur even when the flow rate of the centrifugal compressor  10  is extremely low. On the other hand, when the flow rate is sufficient, the gas is drawn to the first scroll  22  through the diffuser  20 . As a result, the centrifugal compressor  10  is stably operated in a wide range of the flow rate.   (2) The movable diffuser wall  27  is moved closer to and away from the fixed diffuser wall  21  to change the cross-sectional area of the diffuser  20  in accordance with the flow rate. Therefore, when the gas flows through the diffuser  20 , diffuser effect is obtained sufficiently, and effective compression is achieved.   (3) When the movable diffuser wall  27  closes the diffuser  20 , the curved wall  31  and the scroll wall forming surface  30  form part of the second scroll  32 . Therefore, the scroll wall forming surface  30  in the movable diffuser wall  27  can introduce the gas from the impeller  14  to the second scroll  32  smoothly.   

     The following will describe a centrifugal compressor according to a second preferred embodiment of the present invention with reference to  FIG. 7 . Some parts or elements are in common with that of the first embodiment. For the sake of convenience of explanation, like or same parts or elements will be referred to by the same reference numerals as those which have been used in the first embodiment, and the description thereof is omitted. 
     Referring to  FIG. 7 , a centrifugal compressor  50  includes the second casing  12 , the impeller  14 , and the rotary shaft  15 , which are substantially the same as that of the first embodiment. A first casing  51  has an annular accommodation space  52  and a shaft hole  53 . A movable diffuser wall  57  is disposed in the accommodation space  52 . The rotary shaft  15  extends through the shaft hole  53 . The accommodation space  52  is in communication with a communication passage  54  which is connected to the outside of the centrifugal compressor  50 . The front surface of the movable diffuser wall  57  includes a movable diffuser wall surface  58  and a scroll wall forming surface  59 , which are similar to the first embodiment. The scroll wall forming surface  59  in the movable diffuser wall  57  constitutes a part of the wall surface of the second scroll  32 , and also constitutes a pressure receiving surface for receiving the internal pressure downstream of the impeller  14 . 
     The inner periphery of the movable diffuser wall  57  and the first casing  51  is connected through a first flexible member  55  in the form of a diaphragm. The outer periphery of the movable diffuser wall  57  and the first casing  51  is connected through a second flexible member  56  in the form of a diaphragm. The flexible members  55 ,  56  function as members for allowing the movable diffuser wall  57  to move, and also function as pressure receiving surfaces for receiving the internal pressure downstream of the impeller  14 . The diffuser  20  and the accommodation space  52  are separated from each other by the movable diffuser wall  57  and the flexible members  55 ,  56 . The accommodation space  52  is in communication with the outside of the centrifugal compressor  50  through the communication passage  54 , therefore, the accommodation space  52  is in an atmospheric pressure. 
     A coil spring  60  as an urging member is disposed between the rear surface of the movable diffuser wall  57  and the first casing  51 . The coil spring  60  applies an urging force to the movable diffuser wall  57  in the direction to close the diffuser  20 . The flexible volume of the coil spring  60  is set equal to or above the length of the diffuser  20  in the axial direction. The accommodation space  52  has a hole  61  with a bottom for retaining the coil spring  60 . The hole  61  regulates the misalignment of the coil spring  60  in the radial direction. Preferably, a plurality of the holes  61  and the coil springs  60  may be formed in the circumferential direction. 
     In the centrifugal compressor  50  of the second embodiment, the displacement of the movable diffuser wall  57  is determined in accordance with the internal pressure downstream of the impeller  14 , instead of the actuator  40  in the first embodiment. When the flow rate of the gas from the impeller  14  is low, the internal pressure downstream of the impeller  14  is low. In this case, the internal pressure acting on the scroll wall forming surface  59  as the pressure receiving surface applies the load to the movable diffuser wall  57  in the reverse direction of the urging force of the coil spring  60 . The load is small and the movable diffuser wall  57  is not moved in the direction to the first casing  11  against the urging force of the coil spring  60 . The movable diffuser wall  57  is retained in contact with the fixed diffuser wall  21  by the urging force of the coil spring  60 . In this state, the diffuser  20  is closed, and the gas from the impeller  14  flows through the second passage. 
     When the flow rate of the gas from the impeller  14  increases, the internal pressure downstream of the impeller  14  increases. When the internal pressure acting on the scroll wall forming surface  59  as the pressure receiving surface exceeds a predetermined level, the load acting on the movable diffuser wall  57  overcomes the urging force of the coil spring  60 . At that time, the movable diffuser wall  57  is moved to the first casing  51  against the urging force of the coil spring  60 . By the movement of the movable diffuser wall  57  to the first casing  51 , the diffuser  20  is opened and the first passage is formed. The gas from the impeller  14  is introduced into the first scroll  22  through the diffuser  20 . 
     According to the second embodiment, the same advantageous effects as (1) through (3) of the first embodiment are obtained. In addition, the scroll wall forming surface  59  receives the load based on the internal pressure downstream of the impeller  14 , and the movable diffuser wall  57  can be moved away from the fixed diffuser wall  21  by utilizing the internal pressure downstream of the impeller  14 . Furthermore, the movable diffuser wall  57  can be moved autonomously by the load based on the internal pressure downstream of the impeller  14  and the urging force of the coil spring  60 . Therefore, the centrifugal compressor  50  does not need an independent drive force to move the movable diffuser wall  57  closer to and away from the fixed diffuser wall  21 . Thus, the simple structure of the centrifugal compressor  50  is obtained, compared to a centrifugal compressor having an actuator. The scroll wall forming surface  59  in the movable diffuser wall  57  forms part of the wall surface of the second scroll  32 , and also functions as the pressure receiving surface, thereby the movable diffuser wall  57  does not need an independent pressure receiving surface. 
     The following will be describe a centrifugal compressor according to a third preferred embodiment of the present invention with reference to  FIG. 8 . Since some parts of the centrifugal compressor of the third embodiment are common to those of the centrifugal compressor  10  of the first embodiment, the common or similar reference numerals of the first embodiment are applied to those of the third embodiment to incorporate the common or similar description of the first embodiment into that of the third embodiment. 
     A centrifugal compressor  70  has a movable diffuser wall  75  which moves autonomously in accordance with the internal pressure downstream of the impeller  14 , similar to the second embodiment. Referring to  FIG. 8 , an accommodation space  72  is defined in the first casing  71  for accommodating the movable diffuser wall  75 . The movable diffuser wall  75  is retained at the inner and the outer periphery by flexible members  73 ,  74 , and is movable in the axial direction of a rotary shaft (not shown). In  FIG. 8 , an urging member is not shown, but an urging member similar to the coil spring  60  of the second embodiment may be utilized. The movable diffuser wall  75  has a diffuser wall surface  76  and a scroll wall forming surface  77  in the side of the second casing  12  (or the wall surface facing the diffuser  78 ) The diffuser wall surface  76  is formed with a tapered surface which is not parallel to the fixed diffuser wall  21 . 
     The fixed diffuser wall  21  has a wall surface which is parallel to the radial direction of the centrifugal compressor  70 . Therefore, when the diffuser wall surface  76  is moved closest to the fixed diffuser wall  21 , the edge portion of the movable diffuser wall  75  adjacent to the first scroll  22  is brought into contact with the fixed diffuser wall  21 . In this state, a space exists between the diffuser wall surface  76  and the fixed diffuser wall  21 , however, the diffuser  78  is closed and the communication is shut off. Thus, the diffuser wall surface  76  of the movable diffuser wall  75  functions as a pressure receiving surface. The diffuser wall surface  76  has the tapered surface so that the cross-sectional area of the pressure receiving surface is increased in the movable diffuser wall  75 . The responsiveness of the movement of the movable diffuser wall  75  is improved as the cross-sectional area of the pressure receiving surface in the movable diffuser wall  75  is increased. In the state where the diffuser wall surface  76  is moved closest to the fixed diffuser wall  21 , the gas from the impeller  14  is introduced into the second scroll  32  through the second passage. 
     When the movable diffuser wall  75  is moved to the bottom of the accommodation space  72  in the first casing  51  in accordance with the increase of the internal pressure, the diffuser  78  performs its function, but the cross-sectional area of the passage in the diffuser  78  is decreased as the passage in the diffuser  78  is directed from the impeller  14  toward the first scroll  22 . 
     In the third embodiment, the diffuser wall surface  76  of the movable diffuser wall  75  is formed with the tapered surface. Therefore, the diffuser wall surface  76  functions as the pressure receiving surface and increases the cross-sectional area for receiving the pressure in the movable diffuser wall  75 . Compared to a case where a pressure receiving surface includes only the scroll wall forming surface  77 , the responsiveness of the movement of the movable diffuser wall  75  responding to the internal pressure can be improved by the increase of the cross-sectional area of the pressure receiving surface. 
     The present invention is not limited to the embodiments described above but may be modified into alternative embodiments. 
     In the first and the second embodiments, the diffuser wall surface of the movable diffuser wall is in close contact with the fixed diffuser wall, but in an alternative embodiment, the diffuser wall surface and the fixed diffuser wall may not be in close contact with each other. For example, as shown in  FIG. 9A , a minute clearance K may be set between the movable diffuser wall  57  and the fixed diffuser wall  21 , and the diffuser  20  does not substantially function due to the pressure loss when the gas flows through the clearance K. 
     As shown in  FIG. 9B , the diffuser wall surface  58  of the movable diffuser wall  57  and the fixed diffuser wall  21  are formed with convexes and corresponding concaves. When the diffuser wall surface  58  and the fixed diffuser wall  21  are closer to each other, a minute clearance may be maintained and a labyrinth seal L may be formed. 
     As shown in  FIGS. 9A and 9B , even when the diffuser wall surface  58  of the movable diffuser wall  57  may not be in close contact with the fixed diffuser wall  21 , the diffuser  20  does not perform its function and the state is substantially the same as the state where the diffuser  20  is closed. It is noted that the common numerals are used in  FIGS. 9A and 9B  as the first and the second embodiments for the sake of convenience. 
     In the first through third embodiments, the movable diffuser walls are formed in the first casings, but a movable diffuser wall as a switching member may be formed in a second casing, or formed in both casings. A movable diffuser wall as a switching member can be formed depending on the construction and the condition of a centrifugal compressor. 
     In the first through third embodiments, the scroll wall forming surfaces which constitute part of the wall surfaces of the second scrolls are formed in the movable diffuser walls. Another scroll wall forming surface may be formed in a movable diffuser wall adjacent to a first scroll so as to constitute part of the wall surface of the first scroll, depending on a shape of the movable diffuser wall. In this case, the another scroll wall forming surface not only guides the gas to the first scroll, but also functions as a pressure receiving surface for receiving the internal pressure in the first passage. As in the second and the third embodiments, when the movable diffuser wall is moved by the urging member, the responsiveness of the movement of the movable diffuser wall can be further improved. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims.