Patent Publication Number: US-2023137972-A1

Title: Centrifugal compressor with reverse overhung volute

Description:
BACKGROUND 
     Field of the Invention 
     The present invention generally relates to a centrifugal compressor adapted for use in a chiller system. More specifically, the present invention relates to a centrifugal compressor with a volute having a reverse overhung configuration. 
     Background Information 
     A chiller system is a refrigerating machine or apparatus that removes heat from a medium. Commonly, a liquid, such as water, is used as the medium and the chiller system operates in a vapor-compression refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to the ambient surroundings or, for greater efficiency, recovered for heating purposes. A conventional chiller system often utilizes a centrifugal compressor, which is often referred to as a turbo compressor. Thus, such chiller systems can be referred to as turbo chillers. Alternatively, other types of compressors, e.g. a screw compressor, can be utilized. 
     In a conventional (turbo) chiller, refrigerant is compressed in the centrifugal compressor and sent to a heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as a condenser because the refrigerant condenses in this heat exchanger. As a result, heat is transferred to the medium (liquid) so that the medium is heated. Refrigerant exiting the condenser is expanded by an expansion valve and sent to another heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as an evaporator because refrigerant is heated (evaporated) in this heat exchanger. As a result, heat is transferred from the medium (liquid) to the refrigerant, and the liquid is chilled. The refrigerant from the evaporator is then returned to the centrifugal compressor and the cycle is repeated. The liquid utilized is often water. 
     A conventional centrifugal compressor basically includes a casing (housing), an inlet guide vane, an impeller, a diffuser, a volute, a motor, various sensors and a controller. Refrigerant flows in order through the inlet guide vane, the impeller and the diffuser. Thus, the inlet guide vane is coupled to a gas intake port of the centrifugal compressor while the diffuser is coupled to a gas outlet port of the impeller. The inlet guide vane controls the flow rate of refrigerant gas into the impeller. The impeller increases the velocity (kinetic energy) of refrigerant gas. The diffuser works to transform the velocity of refrigerant gas (dynamic pressure) discharged from the impeller into (static) pressure. The volute receives the refrigerant exiting the diffuser and guides the refrigerant to a discharge pipe connected to the centrifugal compressor while allowing the velocity of the refrigerant to be maintained. The motor rotates the impeller. The controller controls the motor, the inlet guide vane and the expansion valve. In this manner, the refrigerant is compressed in a conventional centrifugal compressor. The inlet guide vane is typically adjustable and the motor speed is typically adjustable to adjust the capacity of the system. In addition, the diffuser may be adjustable to further adjust the capacity of the system. In addition to controlling the motor, the inlet guide vane and the expansion valve, the controller can further control any additional controllable elements, such as the diffuser. 
     Some centrifugal compressors for chillers have multiple compression stages to achieve a higher degree of compression. Some multistage centrifugal compressors have an in-line configuration in which the impellers are disposed adjacently along the axial direction of the centrifugal compressor and the motor is disposed on one side of the compressor housing (e.g., the discharge side). There are also two-stage centrifugal compressors in which the motor is disposed between the two stages of the centrifugal compressors. 
     In a conventional in-line centrifugal compressor for a chiller, an output shaft of the motor is typically, connected to the impellers through a gear mechanism and a secondary shaft that is connected to the impellers. A motor housing of the motor is typically disposed on a discharge side of the compressor housing and the gear mechanism is disposed between the motor housing and the compressor housing. The secondary shaft is typically offset from the output shaft of the motor in a radial direction of the output shaft and arranged to extend in a direction parallel to an axial direction of the output shaft of the motor (see  FIG.  8   ). The volute is typically biased away from the motor and the gear mechanism (toward the first stage side of the centrifugal compressor) and disposed outboard of or surrounding the second stage impeller. 
     SUMMARY 
     There is a need to shorten the axial length of in-line centrifugal compressors adapted for use in chiller systems. A smaller footprint provides the advantage of enabling the centrifugal compressor to be installed in a wider variety of locations. This is particularly true in the case of multistage centrifugal compressors and multistage centrifugal compressors having an injection nozzle for introducing refrigerant from an economizer or other portion of the refrigeration circuit to an intermediate stage of the multistage centrifugal compressor. The additional stages, the injection nozzle, and an injection space into which refrigerant is introduced from the injection nozzle each contribute to the axial length of the centrifugal compressor. Conventionally, as mentioned above, a centrifugal compressor used in a chiller has a forward overhung volute on the discharge side (second stage side in a two-stage centrifugal compressor). That is, in a conventional centrifugal compressor, the volute is biased or offset toward the impeller with respect to the diffuser and competes with other components for space in the area surrounding the outer periphery of the impeller (second-stage impeller in a two-stage centrifugal compressor). 
     An object of the present invention is to reduce the axial length of a centrifugal compressor for a chiller, particularly an in-line multistage centrifugal compressor. 
     In view of the state of the known technology, one aspect of the present disclosure is to provide a centrifugal compressor for a chiller. The centrifugal compressor includes a first stage impeller, a first stage diffuser, a second stage impeller, a second stage diffuser, and a second stage volute. The first stage impeller is arranged to receive refrigerant from an inlet. The second stage volute is disposed downstream of the second stage diffuser to receive the refrigerant after the refrigerant has been compressed. The second stage volute has a reverse overhung configuration. 
     Another aspect of the present disclosure is to provide a centrifugal compressor for a chiller in which the centrifugal compressor includes a compressor housing and a volute forming member. The compressor housing encloses at least one impeller. The compressor housing has an inlet side and a discharge side along an axial direction of the centrifugal compressor. The volute forming member defines a volute having a reverse overhung configuration on the discharge side of the compressor housing. The volute forming member is attached to an exterior of the compressor housing. 
     Another aspect of the present disclosure is to provide a multiple stage centrifugal compressor for a chiller. The multiple stage centrifugal compressor includes a compressor housing, at least a first stage impeller and a second stage impeller, and a discharge volute. The compressor housing has an inlet side and a discharge side along an axial direction of the multiple stage centrifugal compressor. The first stage impeller and the second stage impeller are arranged in the compressor housing. The first stage impeller has a first radius in a direction perpendicular to the axial direction, and the first stage impeller is disposed between the second stage impeller and the inlet side of the compressor housing. The discharge volute is disposed on the discharge side of the compressor housing and having a reverse overhung configuration. A ratio of the first radius to a distance between the first stage impeller and the second stage impeller is equal to or larger than 0.5 and smaller than or equal to 1.0. 
     These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG.  1    illustrates a chiller system including a circumferential compressor according to an embodiment of the present invention; 
         FIG.  2    is a side view of a discharge side of the centrifugal compressor as viewed along an axial direction of the centrifugal compressor; 
         FIG.  3    is a longitudinal cross-sectional view of the centrifugal compressor as viewed according to the section line shown in  FIG.  2   ; 
         FIG.  4    is a longitudinal cross-sectional view of the centrifugal compressor in a section plane perpendicular to the section plane of  FIG.  3   ; 
         FIG.  5    is an enlarged partial cross-sectional view extracted from  FIG.  3   ; 
         FIG.  6    is an exploded cross-sectional view showing the motor, the second-stage volute, the second stage impeller, and the insert; 
         FIG.  7    illustrates a forward overhung volute configuration and a symmetrical volute configuration; and 
         FIG.  8    is a cross-sectional view of a conventional in-line multistage centrifugal compressor for a chiller. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     Referring initially to  FIG.  1   , illustrated is a chiller system  10  that includes at least a centrifugal compressor  12  in accordance with an exemplary embodiment of the present invention. The chiller system  10  is preferably a water chiller that utilizes cooling water and chiller water in a conventional manner. The chiller system  10  illustrated herein is a two-stage chiller system. However, it will be apparent to those skilled in the art from this disclosure that the chiller system  10  could be a single stage chiller system or a multiple stage chiller system including three or more stages. 
     The chiller system  10  basically includes the centrifugal compressor  12 , a chiller controller  14 , a condenser  16 , an economizer  18 , expansion valves  20  and  22 , and an evaporator  24  connected together in series to form a loop refrigeration cycle. In addition, various sensors S and T may be disposed throughout the circuit of the chiller system  10 . The chiller system  10  may include orifices instead of the expansion valves  20  and  22 . 
     Referring to  FIGS.  1  and  3   , the centrifugal compressor  12  is a two-stage in-line centrifugal compressor in the illustrated embodiment. The centrifugal compressor  12  illustrated herein is a two-stage centrifugal compressor that includes two impellers. However, the centrifugal compressor  12  can be a single stage centrifugal compressor or a multiple stage centrifugal compressor including three or more impellers. The two-stage in-line centrifugal compressor  12  of the illustrated embodiment includes a first stage impeller  26  and a second stage impeller  28 . The first stage impeller  26  is arranged to receive refrigerant from an inlet  30 . The centrifugal compressor  12  also includes a first stage diffuser  32  and a second stage diffuser  34 . The first stage diffuser  32  is disposed on a downstream side of the first stage impeller  26  and an upstream side of the second stage impeller  28 . The second stage diffuser  34  is disposed on a downstream side of the second stage impeller  28 . A second stage volute  36  is disposed downstream of the second stage diffuser  34  to receive the refrigerant after the refrigerant has been compressed. The second stage volute  36  has a reverse overhung configuration. The centrifugal compressor  12  further includes a first stage inlet guide vane  38 , a second stage inlet guide vane  40 , a compressor motor  42 , and various sensors (only some shown). In some embodiments, the compressor motor  42  may include a magnetic bearing assembly  44 . The magnetic bearing assembly  44  magnetically supports an output shaft  48  of the compressor motor  42 . Alternatively, the bearing system may include a roller element, a hydrodynamic bearing, a hydrostatic bearing, an oil bearing, and/or a magnetic bearing, or any combination of these. The structure of the centrifugal compressor  12  will be discussed in more detail later. The compressor motor  42  includes a motor housing  50 . 
     The chiller controller  14  receives signals from the various sensors and controls the inlet guide vanes  38  and  40 , the compressor motor  42 , and the magnetic bearing assembly  44 , as explained in more detail below. Refrigerant flows in order through the first stage inlet guide vane  38 , the first stage impeller  26 , the first stage diffuser  32 , the second stage inlet guide vane  40 , the second stage impeller  28 , the second stage diffuser  34 , and the second stage volute  36 . The inlet guide vanes  38  and  40  control the flow rate of refrigerant gas into the impellers  26  and  28 , respectively. The impellers  26  and  28  increase the velocity of refrigerant gas, generally without changing pressure. The speed of the compressor motor  42  determines the amount of increase of the velocity of refrigerant gas. The first and second stage diffusers  32  and  34  increase the refrigerant pressure. The first and second stage diffusers  32  and  34  are non-movably fixed relative to a compressor housing  46 . The compressor motor  42  rotates the impellers  26  and  28  via a shaft, e.g., the output shaft  48  of the compressor motor  42  or a second shaft coupled to the output shaft  48 . In this manner, the refrigerant is compressed in the centrifugal compressor  12 . 
     More specifically, in operation of the chiller system  10 , the first stage impeller  26  and the second stage impeller  28  of the centrifugal compressor  12  are rotated by the compressor motor  42 , and the refrigerant of low pressure in the chiller system  10  is drawn through the inlet  30  by the first stage impeller  26 . The flow rate of the refrigerant is adjusted by the first stage inlet guide vane  38 . The refrigerant drawn by the first stage impeller  26  is compressed to intermediate pressure, the refrigerant pressure is increased by the first stage diffuser  32 , and the refrigerant is then introduced to the second stage impeller  28 . The flow rate of the refrigerant is adjusted by the second stage inlet guide vane  40 . The second stage impeller  28  accelerates and compresses the refrigerant, and the refrigerant pressure is increased from an intermediate pressure to a high pressure by the second stage diffuser  34 . The high-pressure gas refrigerant is then discharged through the second stage volute  36  to the chiller system  10 . 
     Referring to  FIG.  1   , in the chiller system  10 , the economizer  18  is disposed between the condenser  16  and the evaporator  24 . The economizer  18  includes an inlet port  18   a , a liquid outlet port  18   b , and a gas outlet port  18   c . The inlet port  18   a  is provided to introduce the two-phase refrigerant from the condenser  16  into the economizer  18 . The liquid outlet port  18   b  is provided to discharge liquid refrigerant separated from the two-phase refrigerant to the evaporator  24 . The gas outlet port  18   c  is provided to discharge the gas refrigerant separated from the two-phase refrigerant to an intermediate stage of the centrifugal compressor  12 . The gas outlet port  18   c  is connected to an injection nozzle  52  of the centrifugal compressor  12 . The flow rate of the refrigerant flowing into the inlet port  18   a  is controlled by the expansion valve  20  which is disposed between the condenser  16  and the economizer  18 . 
     In operation, the refrigerant cooled to condense in the condenser  16  is decompressed to an intermediate pressure by the expansion valve  20  and then introduced into the economizer  18 . The two-phase refrigerant introduced from the inlet port  18   a  into the economizer  18  is separated into gas refrigerant and liquid refrigerant by the economizer  18 . Under some conditions, the gas refrigerant is injected from the gas outlet port  18   c  of the economizer  18  into injection nozzle  52  of the centrifugal compressor  12  via a pipe. Under some conditions, the liquid refrigerant is guided from the liquid outlet port  18   b  to the evaporator  24 , or can be stored in a liquid storage portion of the economizer  18 , or can be injected into the first stage diffuser  32  and/or the second stage diffuser  34  of the centrifugal compressor  12  via a pipe. Liquid refrigerant from the condenser  16  can also be injected into the first stage diffuser  32  and/or the second stage diffuser  34  of the centrifugal compressor  12  via a pipe. 
     The gas refrigerant injected into the injection nozzle  52  enters an injection space  54  of the centrifugal compressor  12  and is mixed with the refrigerant of intermediate pressure compressed by the first stage impeller  26  of the centrifugal compressor  12 . The mixed refrigerant flows to the second stage impeller  28  to be further compressed. 
     The compressor housing  46  includes an inlet portion (inlet side)  46 A and an outlet portion (discharge side)  46 B. The inlet portion  46 A includes the inlet  30  and houses the first stage impeller  26 . The second stage portion  46 B houses the second stage impeller  28  and mates with the second stage volute  36  (described in more detail later). The first stage impeller  26  is rotatable about a first rotation axis A 1 , and the second stage impeller  28  is rotatable about a second rotation axis A 2 . In the illustrated embodiment, the first and second rotation axes A 1  and A 2  are collinear as shown in  FIG.  3   , but in some embodiments the rotation axes may be radially offset. The second stage diffuser  34  is disposed downstream from the second stage impeller  28  and upstream from the second stage volute  36 . 
     With reference to  FIGS.  2 - 6   , the centrifugal compressor  12  according to the illustrated embodiment will now be discussed in more detail. The centrifugal compressor  12  is a multiple-stage in-line centrifugal compressor provided with a volute having a reverse overhung configuration. More specifically, the centrifugal compressor  12  according to this embodiment has two stages, a first stage and a second stage. In some embodiments, the centrifugal compressor  12  may have more than two stages. As mentioned above, the centrifugal compressor  12  includes the first stage impeller  26 , the second stage impeller  28 , the first stage diffuser  32 , the second stage diffuser  34 , and the second stage volute  36 . The first stage diffuser  32  is disposed on a downstream side of the first stage impeller  26  and an upstream side of the second stage impeller  28 . The second stage diffuser  34  is disposed on a downstream side of the second stage impeller  28 . A second stage volute  36  is disposed downstream of the second stage diffuser  34  to receive the refrigerant after the refrigerant has been compressed. The second stage volute  36  has a reverse overhung configuration. That is, the second stage volute  36  bulges axially outward toward the compressor motor  42  instead of axially inward toward the inlet side of the compressor housing  46 . 
     In some embodiments, the second stage volute  36  is configured and arranged such that the second stage diffuser  34  is disposed between the second stage impeller  28  and an axial-direction center C of the second stage volute  36 . In other words, in a cross-sectional view including an axial center line of the centrifugal compressor (i.e., the rotational axes A 1  and A 2  in the illustrated embodiment), a geometric center (axial-direction center C, see  FIG.  5   ) of the cross-sectional shape of the second stage volute  36  is on the opposite side of the second stage diffuser  34  as the second stage impeller  28  along the axial direction of the centrifugal compressor  12 . 
     The compressor housing  46  encloses the first stage impeller  26  and the second stage impeller  28 . The compressor motor  42  is arranged to drive the first stage impeller  26  and the second stage impeller  28 . The compressor motor  42  disposed on the second-stage side of the compressor housing  46  such that the second stage impeller  28  is positioned between the compressor motor  42  and the first stage impeller  26  along the axial direction of the centrifugal compressor  12 . Thus, the axial-direction center C of the second stage volute  36  is disposed toward the compressor motor  42  with respect to the second stage diffuser  34 . 
     In some embodiments, the motor housing  50  of the compressor motor  42  is connected to the second-stage side of the compressor housing  46 , and at least a portion of the second stage volute  36  overlaps the motor housing  50  when viewed along a direction perpendicular to the axial direction, i.e., perpendicular to the rotational axes A 1  and A 2 . Also, in some embodiments, the first stage impeller  26  and the second stage impeller  26  are connected to the output shaft  48  of compressor motor  42  such that the output shaft  48  passes through the second stage impeller  28  and partially into the first stage impeller  26 . The output shaft  48  is fixed to the first stage impeller  26  and the second stage impeller  28  such that the output shaft  48  can rotate both impellers simultaneously. 
     In some embodiments, the compressor housing  46  encloses the first stage impeller  26  and the second stage impeller  28  and the second stage volute  36  is attached to an exterior of the compressor housing  46  such that at least a portion of the second stage volute  36  overlaps the motor housing  50  when viewed along a direction perpendicular to an axial direction of the centrifugal compressor  12 , i.e., perpendicular to the rotational axes A 1  and A 2 . 
     Referring to  FIGS.  4 - 6   , in the illustrated embodiment, the centrifugal compressor  12  includes a volute forming member  56  (second stage volute forming member) that is separate from the compressor housing  46  and the motor housing  50  and defines the second stage volute  36 , and the volute forming member  56  is interposed between the motor housing  50  and the outlet portion  46 A on a second stage side (discharge side) of the compressor housing  46 . The centrifugal compressor  12  includes an insert  58  disposed around an outer periphery of the second stage impeller  28  and interposed between the volute forming member  56  and the compressor housing  46 . One side of the insert  58  includes a diffuser defining wall surface  58   a  that opposes an inner surface  56   a  of the volute forming member  56 . The second stage diffuser  34  is defined between the volute forming member  56  and the one side of the insert  58 , and the injection space  54  is defined between the opposite side of the insert  58  (i.e., the side opposite the diffuser defining wall surface  58   a  along the axial direction of the centrifugal compressor  12 ) and an interior wall of the compressor housing  46 . At least a portion of an interior space  36   a  of the second stage volute  36  is disposed toward the compressor motor  42  with respect to the insert  58 . In other words, the volute forming member  56  defines a reverse overhung configuration such that the second stage volute  36  bulges away from the compressor housing  46  and toward the motor housing  50  such that at least a portion of the interior space  36   a  overlaps the motor housing  50 . 
     In the illustrated embodiment, the volute forming member  56  is configured to be mated against a flange  60  formed around an outer circumference of the compressor housing  46 . The mating portion of the volute forming member  56  may have an internal circumferential surface that mates with an external circumferential surface of the compressor housing  46 . The mating portion of the volute forming member  56  may also include an axially facing mating surface that mates against the flange of the compressor housing  46  in the axial direction of the centrifugal compressor  12 . The volute forming member  56  may be secured to the flange  60  with fasteners  62 . The insert  58  may be configured to be held in place by being clamped between the volute forming member  56  and the compressor housing  46 . For example, the insert  58  may include an annular protrusion  64  configured to be clamped between the mating portion of the volute forming member  56  and the compressor housing  46  (e.g., the flange  60 ). Other attachment configurations can be used, but preferably the volute forming member  56  is a separate piece from the compressor housing  46  and the insert  58 , and preferably the volute forming member  56  defines the entire interior space  36   a.    
     In some embodiments, the volute forming member  56  and the insert  58  may be configured and arranged such that the internal space  36   a  of the second stage volute  36  does not overlap the insert  58  when viewed along a direction perpendicular to the axial direction (rotational axes A 1  and A 2 ). In other words, the entire interior space  36   a  of the second stage volute  36  is defined by the volute forming member  56  and only the second stage diffuser  34  is defined by opposing surfaces of the volute forming member  56  and the insert  58  (i.e., the inner surface  56   a  and the diffuser defining wall surface  58   a ). Also, in some embodiments, the second stage volute  36  has an asymmetrical cross-sectional shape in a cross section lying in a plane that includes a rotational center axis of the second stage impeller (i.e., the rotational axes A 1  and A 2 ). That is, unlike conventional centrifugal compressors for use in a chiller system, which may have a symmetrical or a forward overhung configuration (see  FIGS.  7  and  8   ), the second stage volute  36  according to this disclosure has a reverse overhung configuration (i.e., a shape that is biased toward the compressor motor  42 ) and may have a non-circular or other asymmetrical shape in the cross-sectional view. 
     In some embodiments, a ratio of a radius of the first stage impeller  26  to a distance between the first stage impeller  26  and the second stage impeller  28  is equal to or larger than 0.5 and smaller than or equal to 1.0. More preferably, the ratio is equal to or larger than 0.65 and smaller than or equal to 8.5. The distance is measured, for example, between back sides (i.e., inlet sides) of the impellers  26  and  28 . It has been found ratios in these ranges can be achieved with a centrifugal compressor having a reverse overhung second stage volute in accordance with this disclosure. Smaller values of the ratio indicate a more compact structure with the impellers  26  and  28  being closer together. In some embodiments, the radii (diameters) of the first stage impeller  26  and the second stage impeller  28  are substantially the same, but the centrifugal compressor  12  is not limited to a configuration in which the radii of the impellers are the same. 
     There is a general trend to transition to so-called “low global warming potential (low GWP)” refrigerants in chiller systems and other HVAC applications to reduce the impact on the environment caused by the release of refrigerants into the atmosphere. GWP is a measure of a greenhouse gas when it is released into the atmosphere and benchmarked against CO 2 , which is defined to have a GWP equal to one. Thus, GWP is a measure of the potential for a refrigerant or other gas to behave as a greenhouse gas, which can contribute to global warming. The lower the GWP rating (or “GWP value”, the lower the potential of the refrigerant to behave as a greenhouse gas when released into the atmosphere. Examples of low-GWP refrigerants for HVAC applications include R1233zd, R1234ze and R1234yf. Each of R1233zd, R1234ze and R1234yf has a global warming potential (GWP)&lt;10. In this application, “low-GWP refrigerant” shall be defined as a refrigerant having a GWP value smaller than 10. 
     In some embodiments, the centrifugal compressor  12  may be particularly configured to be used with a low GWP refrigerant. Low GWP refrigerants are being used more and more frequently in chiller systems. However, the centrifugal compressor  12  is not limited a configuration optimized for use with a low GWP refrigerant. 
     The reverse overhung configuration of the second stage volute  36  enables the distance between the first stage impeller  26  and the second stage impeller  28  to be reduced because the second stage volute  36  is biased toward the compressor motor  42  and away from the compressor housing  46  and the second stage impeller  28 . Consequently, some of the space conventionally occupied by the second stage volute  36  can be utilized to move the first stage impeller  26  and the second stage impeller  28  closer together without sacrificing space needed for other features, such as the injection nozzle  52 . Moreover, the compact arrangement of the impellers achieved due to the reverse overhung configuration of the second stage volute facilitates a configuration in which the impellers are driving directly by the output shaft  48  of the compressor motor  42  without using a gear mechanism and a secondary shaft (e.g., compare  FIG.  3    and  FIG.  8   ). Put another way, eliminating the gear mechanism and sizing the motor housing appropriately frees up space on the motor side (discharge side) of the centrifugal compressor  12  and enables the reverse overhung configuration of the second stage volute  36  to be utilized. This, in turn, enables the distance between the first and second stage impellers to be shortened, the length of the portion of the shaft that supports the first and second stage impellers to be shortened, and the overall axial length of the centrifugal compressor  12  to be shortened. 
     Although the centrifugal compressor  12  of the illustrated embodiment is a two-stage centrifugal compressor, similar advantages can be obtained in a centrifugal compressor having any number of impellers. So long as the centrifugal compressor has a compressor housing enclosing at least one impeller and a volute having a reverse overhung configuration on a discharge side of the compressor housing, the reverse overhung configuration can contribute to shortening the axial length of the centrifugal compressor. 
     GENERAL INTERPRETATION OF TERMS 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. 
     The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
     The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. 
     Additionally, the term “low global warming potential (GWP) refrigerant” used herein refers to any refrigerant or blend of refrigerants that is suitable for use in the refrigeration circuit of a chiller system and has a low potential for contributing to global warming as benchmarked against CO 2  gas. The refrigerants R1233zd, R1234ze, and R1234fy are cited in this application as examples of low-GWP refrigerants. However, a person of ordinary skill in the refrigeration field will recognize that the present invention is not limited to these refrigerants. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.