Patent Publication Number: US-2023147950-A1

Title: System and method for operation of variable geometry diffuser as check valve

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of U.S. Provisional Application Serial No. 62/982,573, entitled “SYSTEM AND METHOD FOR OPERATION OF VARIABLE GEOMETRY DIFFUSER AS CHECK VALVE,” filed Feb. 27, 2020, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     This application relates generally to vapor compression systems incorporated in air conditioning and refrigeration applications, and, more particularly, to flow control of refrigerant in a compressor. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Vapor compression systems are utilized in residential, commercial, and industrial environments to control environmental properties, such as temperature and humidity, for occupants of the respective environments. The vapor compression system circulates a working fluid, typically referred to as a refrigerant, which changes phases between vapor, liquid, and combinations thereof in response to being subjected to different temperatures and pressures associated with operation of the vapor compression system. For example, the vapor compression system utilizes a compressor to circulate the refrigerant to a heat exchanger which may transfer heat between the refrigerant and another fluid flowing through the heat exchanger. Unfortunately, in certain conditions, refrigerant flow through the compressor may induce backspin in the compressor, which may cause undesirable wear and degradation on the compressor and related components. 
     SUMMARY 
     In an embodiment of the present disclosure, a compressor includes a diffuser passage configured to receive refrigerant flow from an impeller of the compressor, where the diffuser passage is at least partially defined by a compressor discharge plate of the compressor. The compressor also includes a variable geometry diffuser positioned within the diffuser passage and configured to adjust a dimension of a refrigerant flow path through the diffuser passage, an actuator coupled to the variable geometry diffuser and configured to adjust a position of the variable geometry diffuser within the diffuser passage, and a controller configured to regulate operation of the actuator. The controller is configured to instruct the actuator to adjust the position of the variable geometry diffuser from a first position to a second position using a first force and to adjust the position of the variable geometry diffuser from the second position to a third position using a second force less than the first force, where the variable geometry diffuser abuts the compressor discharge plate in the third position. 
     In another embodiment of the present disclosure a heating, ventilation, air conditioning, and refrigeration (HVAC&amp;R) system includes a compressor configured to pressurize refrigerant within a refrigerant circuit, where the compressor includes a diffuser passage configured to receive the refrigerant from an impeller of the compressor. The HVAC&amp;R system also includes a variable geometry diffuser of the compressor, where the variable geometry diffuser is configured to be positioned within the diffuser passage and is configured to adjust a dimension of a refrigerant flow path through the diffuser passage, an actuator configured to adjust a position of the variable geometry diffuser within the diffuser passage, and a controller configured to regulate operation of the actuator, where the controller is configured to control the actuator to position the variable geometry diffuser within the diffuser passage and against a compressor discharge plate during stoppage of the compressor. 
     In a further embodiment of the present disclosure, a heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) system controller includes a tangible, non-transitory, computer-readable medium storing computer-executable instructions that, when executed, are configured to cause processing circuitry to control an actuator to position a variable geometry diffuser in a diffuser passage of a compressor within a first range of positions during operation of the compressor, control the actuator to position the variable geometry diffuser in the diffuser passage of the compressor within a second range of positions during stoppage of the compressor, and control the actuator to maintain a position of the variable geometry diffuser within the diffuser passage and against a compressor discharge plate of the compressor during stoppage of the compressor. 
    
    
     
       DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which 
         FIG.  1    is a perspective view of an embodiment of a building that may utilize a heating, ventilation, air conditioning, and refrigeration (HVAC&amp;R) system in a commercial setting, in accordance with an aspect of the present disclosure; 
         FIG.  2    is a perspective view of an embodiment of a vapor compression system, in accordance with an aspect of the present disclosure; 
         FIG.  3    is a schematic of an embodiment of a vapor compression system, in accordance with an aspect of the present disclosure; 
         FIG.  4    is a schematic of an embodiment of a vapor compression system, in accordance with an aspect of the present disclosure; 
         FIG.  5    is a schematic of an embodiment of a vapor compression system having multiple refrigerant circuits in a series counter-flow arrangement, in accordance with an aspect of the present disclosure; 
         FIG.  6    is a cross-section of an embodiment of a portion of a compressor having a variable geometry diffuser that may be included in the systems of  FIGS.  1 - 5   , in accordance with an aspect of the present disclosure; 
         FIG.  7    is a schematic of an embodiment of a portion of a variable geometry diffuser in a compressor, in accordance with an aspect of the present disclosure; and 
         FIG.  8    is a schematic of an embodiment of a control system for a variable geometry diffuser, in accordance with an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers’ specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     Embodiments of the present disclosure are directed toward a heating, ventilating, air conditioning, and refrigeration (HVAC&amp;R) system configured to cool a conditioning fluid. For example, the HVAC&amp;R system may receive a flow of the conditioning fluid, such as from air handling equipment or other terminal devices in a building, and cool the conditioning fluid. The HVAC&amp;R system may then return the conditioning fluid to the air handling equipment for use in cooling or conditioning air in the building. The HVAC&amp;R system may include a vapor compression system configured to cool a refrigerant and place the cooled refrigerant in a heat exchange relationship with the conditioning fluid to absorb heat or thermal energy from the conditioning fluid. In general, the vapor compression system includes a refrigerant circuit configured to circulate the refrigerant through one or more heat exchangers, such as a condenser and an evaporator. The vapor compression system also includes a compressor (e.g., centrifugal compressor) to circulate the refrigerant through the refrigerant circuit. In some embodiments, the HVAC&amp;R system is a chiller system, such as a water-cooled chiller system or air-cooled chiller system. 
     Unfortunately, in certain conditions, the compressor may be susceptible to spin (e.g., backspin) due to flow of the refrigerant through the refrigerant circuit. For example, when operation of a chiller system is suspended, the conditioning fluid (e.g., water) may still flow through the evaporator and/or a cooling fluid (e.g., water) may still flow through the condenser disposed along the refrigerant circuit. The temperature of the water may cause boiling of refrigerant in the condenser and/or condensing of the refrigerant in the evaporator. As a result, natural refrigerant migration through the refrigerant circuit (e.g., from the condenser to the evaporator via the compressor) may be induced, which may cause undesirable spin (e.g., backspin) of the compressor. 
     The compressor may also be susceptible to spin or backspin via refrigerant flow in embodiments of the chiller system having multiple refrigerant circuits (e.g., in a series counter-flow arrangement), and therefore multiple compressors, when one of the refrigerant circuits is idle or not operating. As will be appreciated, spin or backspin of a non-operating compressor can cause wear and degradation to the motor of the non-operating compressor. Additionally, bearing support systems (e.g., oil pumps, magnetic bearings, etc.) of the non-operating compressor may also be inactive, thereby exposing the non-operating compressor and/or the bearing support systems to premature wear and degradation during instances of compressor spin or backspin. Unfortunately, conventional systems and methods to reduce compressor spin or backspin, such as automated discharge isolation valves, are expensive. 
     Accordingly, embodiments of the present disclosure are directed to systems and methods for utilizing a variable geometry diffuser (VGD), such as a variable geometry diffuser ring, as a flow check valve to substantially reduce, block, or prevent undesirable refrigerant flow across the compressor and thereby mitigate spin and/or backspin of the compressor. Specifically, present embodiments include an actuator and/or actuation system (e.g., a two-stage actuator) configured to operate in multiple modes to actuate and move the VGD within a diffuser passage of the compressor. For example, the actuator may be configured to operate in a first mode by applying a first force to move the VGD and to operate in a second mode by applying a second force that is less than the first force to move the VGD. In accordance with present techniques, a control system is configured to selectively regulate operation of the actuator between the first mode and the second mode, for example, based on an operational state of the compressor and/or based on a position of the VGD within the diffuser passage. The control system may operate the actuator in the first mode when the compressor is operating in order to move the VGD within the diffuser passage and adjust a size of a flow path (e.g., refrigerant flow path) through the diffuser passage, such as for surge or capacity control of the compressor. The control system may operate the actuator in the second mode when the compressor is not operating, during a fault sequence, and/or during a shutdown sequence in order to move the VGD within the diffuser passage and abut an opposing surface of the diffuser passage, thereby substantially completely blocking or closing the flow path through the diffuser passage. In this way, the VGD may block or prevent refrigerant flow through the compressor so as to reduce spin and backspin of the compressor when the compressor is not operating. Details of the operation of the control system and the actuator are discussed in further detail below. 
     It should be noted that the disclosure herein describes the present techniques used with a VGD ring of a compressor. However, the present techniques may also be utilized in embodiments of a compressor that utilize other types of VGDs, such as variable vane diffusers, variable wall diffusers, or other types of diffusers. Moreover, the discussion below describes the present techniques implemented in a water-cooled chiller system, but the systems and methods disclosed herein may also be implemented in other HVAC&amp;R systems. 
     Turning now to the drawings,  FIG.  1    is a perspective view of an embodiment of an environment for a heating, ventilation, air conditioning, and refrigeration (HVAC&amp;R) system  10  in a building  12  for a typical commercial setting. The HVAC&amp;R system  10  may include a vapor compression system  14  that supplies a chilled liquid, which may be used to cool the building  12 . The HVAC&amp;R system  10  may also include a boiler  16  to supply warm liquid to heat the building  12  and an air distribution system which circulates air through the building  12 . The air distribution system can also include an air return duct  18 , an air supply duct  20 , and/or an air handler  22 . In some embodiments, the air handler  22  may include a heat exchanger that is connected to the boiler  16  and the vapor compression system  14  by conduits  24 . The heat exchanger in the air handler  22  may receive either heated liquid from the boiler  16  or chilled liquid from the vapor compression system  14 , depending on the mode of operation of the HVAC&amp;R system  10 . The HVAC&amp;R system  10  is shown with a separate air handler on each floor of building  12 , but in other embodiments, the HVAC&amp;R system  10  may include air handlers  22  and/or other components that may be shared between or among floors. 
       FIGS.  2  and  3    illustrate embodiments of the vapor compression system  14  that can be used in the HVAC&amp;R system  10 . The vapor compression system  14  may circulate a refrigerant through a circuit (e.g., a refrigerant loop) starting with a compressor  32 . The circuit may also include a condenser  34 , an expansion valve(s) or device(s)  36 , and a liquid chiller or an evaporator  38 . The vapor compression system  14  may further include a control panel  40  that has an analog to digital (A/D) converter  42 , a microprocessor  44 , a non-volatile memory  46 , and/or an interface board  48 . 
     Some examples of fluids that may be used as refrigerants in the vapor compression system  14  are hydrofluorocarbon (HFC) based refrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin (HFO), “natural” refrigerants like ammonia (NH3), R-717, carbon dioxide (CO2), R-744, or hydrocarbon based refrigerants, water vapor, or any other suitable refrigerant. In some embodiments, the vapor compression system  14  may be configured to efficiently utilize refrigerants having a normal boiling point of about 19° C. (66° F.) at one atmosphere of pressure, also referred to as low pressure refrigerants, versus a medium pressure refrigerant, such as R-134a. As used herein, “normal boiling point” may refer to a boiling point temperature measured at one atmosphere of pressure. 
     In some embodiments, the vapor compression system  14  may use one or more of a variable speed drive (VSDs)  52 , a motor  50 , the compressor  32 , the condenser  34 , the expansion valve or device  36 , and/or the evaporator  38 . The motor  50  may drive the compressor  32  and may be powered by a variable speed drive (VSD)  52 . The VSD  52  receives alternating current (AC) power having a particular fixed line voltage and fixed line frequency from an AC power source, and provides power having a variable voltage and frequency to the motor  50 . In other embodiments, the motor  50  may be powered directly from an AC or direct current (DC) power source. The motor  50  may include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source, such as a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or another suitable motor. 
     The compressor  32  compresses a refrigerant vapor and delivers the vapor to the condenser  34  through a discharge passage. In some embodiments, the compressor  32  may be a centrifugal compressor. The refrigerant vapor delivered by the compressor  32  to the condenser  34  may transfer heat to a cooling fluid (e.g., water or air) in the condenser  34 . The refrigerant vapor may condense to a refrigerant liquid in the condenser  34  as a result of thermal heat transfer with the cooling fluid. The liquid refrigerant from the condenser  34  may flow through the expansion device  36  to the evaporator  38 . In the illustrated embodiment of  FIG.  3   , the condenser  34  is water cooled and includes a tube bundle  54  connected to a cooling tower  56 , which supplies the cooling fluid to the condenser  34 . 
     The liquid refrigerant delivered to the evaporator  38  may absorb heat from another cooling fluid (e.g., a conditioning fluid), which may or may not be the same cooling fluid used in the condenser  34 . The liquid refrigerant in the evaporator  38  may undergo a phase change from the liquid refrigerant to a refrigerant vapor. As shown in the illustrated embodiment of  FIG.  3   , the evaporator  38  may include a tube bundle  58  having a supply line  60 S and a return line  60 R connected to a cooling load  62 . The conditioning fluid of the evaporator  38  (e.g., water, ethylene glycol, calcium chloride brine, sodium chloride brine, or any other suitable fluid) enters the evaporator  38  via return line  60 R and exits the evaporator  38  via supply line  60 S. The evaporator  38  may reduce the temperature of the conditioning fluid in the tube bundle  58  via thermal heat transfer with the refrigerant. The tube bundle  58  in the evaporator  38  can include a plurality of tubes and/or a plurality of tube bundles. In any case, the vapor refrigerant exits the evaporator  38  and returns to the compressor  32  by a suction line to complete the cycle. 
       FIG.  4    is a schematic of an embodiment of the vapor compression system  14  with an intermediate circuit  64  incorporated between condenser  34  and the expansion device  36 . The intermediate circuit  64  may have an inlet line  68  that is directly fluidly connected to the condenser  34 . In other embodiments, the inlet line  68  may be indirectly fluidly coupled to the condenser  34 . As shown in the illustrated embodiment of  FIG.  4   , the inlet line  68  includes a first expansion device  66  positioned upstream of an intermediate vessel  70 . In some embodiments, the intermediate vessel  70  may be a flash tank (e.g., a flash intercooler). In other embodiments, the intermediate vessel  70  may be configured as a heat exchanger or a “surface economizer.” In the illustrated embodiment of  FIG.  4   , the intermediate vessel  70  is used as a flash tank, and the first expansion device  66  is configured to lower the pressure of (e.g., expand) the liquid refrigerant received from the condenser  34 . During the expansion process, a portion of the liquid may vaporize, and thus, the intermediate vessel  70  may be used to separate the vapor from the liquid received from the first expansion device  66 . Additionally, the intermediate vessel  70  may provide for further expansion of the liquid refrigerant due to a pressure drop experienced by the liquid refrigerant when entering the intermediate vessel  70  (e.g., due to a rapid increase in volume experienced when entering the intermediate vessel 70). The vapor in the intermediate vessel  70  may be drawn by the compressor  32  through a suction line  74  of the compressor  32 . In other embodiments, the vapor in the intermediate vessel may be drawn to an intermediate stage of the compressor  32  (e.g., not the suction stage). The liquid that collects in the intermediate vessel  70  may be at a lower enthalpy than the liquid refrigerant exiting the condenser  34  due to the expansion in the expansion device  66  and/or the intermediate vessel  70 . The liquid from intermediate vessel  70  may then flow in line  72  through a second expansion device  36  to the evaporator  38 . 
     As mentioned above, the systems and methods disclosed herein may be utilized in HVAC&amp;R systems  10  and/or vapor compression systems  14  having multiple refrigerant circuits. For example,  FIG.  5    is a schematic of an embodiment of the vapor compression system  14  with multiple refrigerant circuits  80  (e.g., refrigerant loops). In particular, the illustrated embodiment includes a first refrigerant circuit  82  and a second refrigerant circuit  84  arranged in a series counter-flow arrangement. The first refrigerant circuit  82  includes a first compressor  32 A, a first condenser  34 A, a first expansion device  36 A, and a first evaporator  38 A. The second refrigerant circuit  84  includes a second compressor  32 B, a second condenser  34 B, a second expansion device  36 B, and a second evaporator  38 B. Each of the refrigerant circuits  80  is configured to circulate a respective refrigerant therethrough and is configured to operate in a manner similar to that described above with reference to the vapor compression system  14  shown in  FIGS.  2 - 4   . It should be noted that each of the refrigerant circuits  80  may also include components in addition to those shown in  FIGS.  2 - 4   . 
     In the illustrated embodiment, the first and second refrigerant circuits  82  and  84  of the vapor compression system  14  are arranged in a series counter-flow arrangement. Specifically, the first and second evaporators  38 A and  38 B define a portion of a conditioning fluid flow path or circuit  86  that extends from a cooling load  88  (e.g., air handlers  22 ), sequentially through the second evaporator  38 B and the first evaporator  38 A, and back to the cooling load  88 . Similarly, the first and second condensers  34 A and  34 B define a portion of a cooling fluid flow path or circuit  90  that extends from a cooling fluid source  92  (e.g., cooling tower  56 ), sequentially through the first condenser  34 A and the second condenser  34 B, and back to the cooling fluid source  92 . Thus, conditioning fluid is directed through the vapor compression system  14  first through the second evaporator  38 B and then through the first evaporator  38 A, while cooling fluid is directed through the vapor compression system  14  first through the first condenser  34 A and then through the second condenser  34 B, thereby providing the series counter-flow arrangement. 
     In some circumstances, one of the refrigerant circuits  80  may be in an operating state, while the other of the refrigerant circuits  80  may be in a non-operating state. As will be appreciated, the compressor  32  of the refrigerant circuit  80  that is not operating may be idle (e.g., the motor  50  associated with the compressor  32  is not powered or energized). Thus, the compressor  32  of the non-operating refrigerant circuit  80  does not operate to circulate refrigerant through the non-operating refrigerant circuit  80 . Nevertheless, the non-operating refrigerant circuit  80  may still be susceptible to natural refrigerant migration therethrough. For example, if the first refrigerant circuit  82  is in an operating state and the second refrigerant circuit  84  is in a non-operating state, cooling fluid may still circulate through the second condenser  34 B along the cooling fluid circuit  90  (e.g., from the first condenser  34 A, through the second condenser  34 B, and to the cooling fluid source  92 ). Similarly, conditioning fluid may still circulate through the second evaporator  38 B along the conditioning fluid circuit  86  (e.g., from the cooling load  88 , through the second evaporator  38 B, and to the first evaporator  38 A). In some circumstances, the flow of cooling fluid through the second condenser  34 B and/or the flow of conditioning fluid through the second evaporator  38 B may induce natural refrigerant migration through the second refrigerant circuit  84 . As discussed above, natural refrigerant migration may induce undesirable spin or backspin in the second compressor  32 B that is not operating. 
     Accordingly, present embodiments include a flow control system  94  configured to improve operation and control of the compressor  32 , such as by reducing, blocking, and/or preventing undesirable spin and/or backspin of the compressor  32 . As described in further detail below, the flow control system  94  may be incorporated with (e.g., integrated with) the compressor  32  (e.g., one or both of compressors  32 A,  32 B) and may include a variable geometry diffuser (VGD) of the compressor  32 , an actuation system configured adjust a position of the VGD within the compressor  32 , and a control system configured to control operation of the actuation system. In some applications, the VGD is utilized to adjust a flow path through a diffuser passage of the compressor  32  in order enable surge and/or capacity control of the compressor  32  during operation of the compressor  32 . Additionally, the VGD may be controlled via the actuation system and control system to position the VGD within the diffuser passage to completely or substantially completely block the flow path through the diffuser passage by positioning the VGD against an opposing wall of the diffuser passage and thus block refrigerant flow through the diffuser passage when the compressor  32  is not operating. In this way, the VGD may function as a flow check valve to mitigate or reduce spin and/or backspin of the compressor  32  that may be caused by natural refrigerant migration when the compressor  32  is not operating. As discussed in further detail below, the actuation system is configured to move the VGD within the diffuser passage for capacity and/or surge control using a first force and to move the VGD within the diffuser passage to abut the opposing surface and completely block the flow path through the diffuser passage using a second force that is less than the first force. 
       FIG.  6    is a cross-section of an embodiment of a portion of the compressor  32  which may be included in any of the systems described with reference to  FIGS.  1 - 5    or in any other suitable HVAC&amp;R system  10 . A refrigerant flow path  100  is illustrated through the compressor  32 , whereby refrigerant travels through blades  102  of an impeller  104  of the compressor  32 , toward a diffuser passage  106  defined by and extending between a nozzle base plate  109  (e.g., compressor casing) and a compressor discharge plate  116  (e.g., diffuser plate) From the diffuser passage  106 , the refrigerant is directed into a collector  108  (e.g., volute). The blades  102  of the impeller  104  rotate (e.g., via operation of the motor  50 ) to accelerate the refrigerant outwardly from a center of rotation of the impeller  104 . The accelerated refrigerant may travel along the illustrated refrigerant flow path  100  toward the diffuser passage  106 , which is designed to convert kinetic energy of the refrigerant into pressure, for example, by gradually reducing a velocity of the refrigerant. 
     As noted above, the compressor  32  may include the flow control system  94  to regulate refrigerant flow through the compressor  32 . The flow control system  94  may include a variable geometry diffuser (VGD)  110  disposed in, or proximate to, a lower portion of the diffuser passage  106  (e.g., between the impeller  104  and the collector  108  and proximate the impeller  104 ), an actuator  112 , and a controller  114  (e.g., a control system). For example, the VGD  110  may be positioned at least partially within or adjacent the nozzle base plate  109  (e.g., within a groove formed in the nozzle base plate  109 ). In the illustrated embodiment, the VGD  110  is a VGD ring. However, in other embodiments, the VGD  110  may be a variable vane diffuser, a variable wall diffuser, or other type of variable diffuser. The position of the VGD  110  within the diffuser passage  106  is adjustable in order to improve control and operation of the compressor  32 . For example, the VGD  110  may be coupled to the actuator  112  (e.g., a two-stage actuator, an actuation system, etc.), which, upon instruction by the controller  114  (e.g., a control system), actuates or moves the VGD  110  from a previous position to a desired position. In some embodiments, the actuator  112  may be an electromechanical actuator, a magnetic actuator, a hydraulic actuator, or any other suitable type of actuator. As described herein, the flow control system  94  (e.g., the actuator  112  and/or the controller  114 ) is configured to operate in two or more stages or modes. For example, the actuator  112  may actuate the VGD  110  in a first stage or mode (e.g., high torque mode) by applying a first force to the VGD  110  and in a second stage or mode (e.g., low torque mode) by applying a second force to the VGD  110  that is less than the first force. 
     The controller  114  may control the position of the VGD  110  such that the VGD  110  adjusts a size of a flow path through the diffuser passage  106 . For example, the controller  114  may control operation of the actuator  112  to increase or decrease a size of the flow path (e.g., refrigerant flow path  100 ) through the diffuser passage  106  without completely blocking the flow path through the diffuser passage  106  during operation of the compressor  32  (e.g., to control surge and/or capacity of the compressor  32 ). The controller  114  may also control operation of the actuator  112  to position the VGD  110  within the entire diffuser passage  106 , such that the VGD  110  abuts the compressor discharge plate  116  (e.g., a diffuser plate) of the compressor  32 , thereby completely blocking the diffuser passage  106  and preventing flow of refrigerant therethrough. In this manner, the VGD  110  is used as a flow check valve to mitigate or prevent spin and/or backspin (e.g., of the impeller  100 ), such as during non-operational periods or stoppage of the compressor  32 . 
     The controller  114  may include processing circuitry  118  and a memory  120 . The memory  120  may include a tangible, non-transitory, computer-readable medium that may store instructions that, when executed by the processing circuitry  118 , may cause the processing circuitry  118  to perform various functions or operations described herein. To this end, the processing circuitry  118  may be any suitable type of computer processor or microprocessor capable of executing computer-executable code, including but not limited to one or more field programmable gate arrays (FPGA), application-specific integrated circuits (ASIC), programmable logic devices (PLD), programmable logic arrays (PLA), and the like. For example, the controller  114  may control an operating capacity of the compressor  32  based at least in part on certain operating and/or environmental conditions (e.g., refrigerant temperature). The controller  114  may also include data stored on the memory  120  indicating a desired position of the VGD  110  based on the operating capacity of the compressor  32 . Further, the controller  114  may be configured to control a stage or actuating force of the actuator  112  based on a position of the VGD  110  within the diffuser passage  106  and/or based on an operational state of the compressor  32 . For example, the controller  114  may control the actuator  112  to adjust a position of the VGD  110  using a first force or torque when the VGD  110  is within a first range of positions within the diffuser passage  106  and using a second force or torque, less than the first force or torque, when the VGD  110  is within a second range of positions within the diffuser passage  106 . Control of the VGD  110  via the actuator  112  and the controller  114  is described in further detail below. 
       FIG.  7    is a cross-section of an embodiment of a portion of the compressor  32  of  FIG.  6    having the VGD  110  in a partially blocking position. As shown in  FIGS.  6  and  7   , the VGD  110  is generally configured to travel within the diffuser passage  106   along a direction  130  (e.g., axis) and, as shown in  FIG.  7   , may restrict a portion (e.g., a flow path) of the diffuser passage  106  to a width  132  (e.g., dimension) that is less than a total width  134  (e.g., dimension) of the diffuser passage  106 . As discussed, the actuator  112  is configured to actuate and move the VGD  110  within the diffuser passage  106  to, for example, adjust a size of the width  132  of the diffuser passage  106  through which refrigerant may flow. In some embodiments, the actuator  112  may be coupled to the VGD  110  via a linkage  136 , such as a mechanical linkage, configured to transfer force applied by the actuator  112  to the VGD  110 . 
     In the illustrated embodiment, the VGD  110  is shown in a home or “zero” position  138 . For example, the home position  138  of the VGD  110  may be a threshold position (e.g., a lower threshold position) within the diffuser passage  106  beyond which the actuator  112  and/or controller  114  does not adjust the VGD  110  (e.g., further into the diffuser passage  106  and/or further towards the compressor discharge plate  116 ) during operational periods of the compressor  32 . In other words, when the compressor  32  is operating, the actuator  112  and/or controller  114  is configured to move the VGD  110  within a first range of positions  140  in the diffuser passage  106  and does not position the VGD  110  beyond the home position  138  (e.g., closer to the compressor discharge plate  116 ). Thus, when the compressor  32  is operating, a gap  142  remains between a distal surface  144  of the VGD  110  and the compressor discharge plate  116 , where a dimension (e.g., width) of the gap  142  from the distal surface  144  to the compressor discharge plate  116  is greater than or equal to the width  132  shown in  FIG.  7   . As will be appreciated, the presence of the gap  142  allows for thermal growth of the VGD  110  and blocks contact between the VGD  110  and the compressor discharge plate  116  during operation of the compressor  32  that may otherwise cause undesirable transfer of force to the linkage  136  or other components of the compressor  32 . 
     In accordance with present embodiments, the actuator  112  and/or controller  114  is also configured to selectively move the VGD  110  beyond the home position  138  and into contact with the compressor discharge plate  116 . For example, during stoppage (e.g., non-operating periods, a fault sequence, and/or a shutdown sequence) of the compressor  32 , the controller  114  may instruct the actuator  112  to move the VGD  110  beyond the home position  138  (e.g., further away from the nozzle base plate  109 ), such that the VGD  110  contacts the compressor discharge plate  116  to block (e.g., completely block) the discharge passage  106  and thereby block or prevent refrigerant flow through the discharge passage  106 . In other words, during non-operational periods, a fault sequence, and/or a shutdown sequence of the compressor  32 , the controller  114  may instruct the actuator  112  to move the VGD  110  within a second range of positions  146 , such that the VGD  110  is positioned beyond the home position  128  (e.g., relative to the nozzle base plate  109 ). As illustrated in  FIG.  7   , the first range of positions  140  and the second range of positions  146  may cooperatively extend across (e.g., equal) the total width  134  of the diffuser passage  106 . In certain embodiments, the first range of positions  140  and the second range of positions  146  do not overlap with one another and are separated by the home position  138 . By positioning the VGD  110  within the second range of positions  146  (e.g., in abutment with the compressor discharge plate  116 ), the VGD  110  may function as a flow check valve that does not allow natural migration of the refrigerant through the compressor  32  (e.g., from the condenser  34  to the evaporator  38  and/or in a direction  148 ) that may be induced when the compressor  32  is not operating. 
     As mentioned above, the flow control system  94  (e.g., the actuator  112 ) is configured to operate in two or more modes or stages. In a first mode or stage, the controller  114  may control the actuator  112  to adjust the position of the VGD  110  by applying a first force or torque (e.g., a large force and/or a force above a threshold amount) to the VGD  110 , and in the second mode or stage the controller  114  may control the actuator  112  to adjust the position of the VGD  110  by applying a second force or torque (e.g., a small force and/or a force below a threshold amount) to the VGD  110  that is less than the first force or torque. For example, the controller  114  may be configured to instruct the actuator  112  to operate in the first mode or stage when the VGD  110  is within the first range of positions  140  and to instruct the actuator  112  to operate in the second mode or stage when the VGD  110  is within the second range of positions  146 . By utilizing the first or large force to move the VGD  110  across the first range of positions  140  when the compressor  32  is operating, a position of the VGD  110  may be quickly and effectively adjusted during operation of the compressor  32  to control surge and/or capacity. By utilizing the second or small force to move the VGD  110  across the second range of positions  146  when the compressor  32  is not operating, the VGD  110  may be positioned to contact the compressor discharge plate  116 , and therefore block natural refrigerant migration through the diffuser passage  106 , while avoiding transfer of undesirable forces to the VGD  110 , the linkage  136 , the actuator  112 , or other components of the compressor  32 . 
     As an example, the compressor  32  may operate with the VGD  110  positioned in the diffuser passage  106  within the first range of positions  140 , and the controller  114  may receive an indication (e.g., feedback) of a fault or shutdown of the compressor  32  (e.g., from the control board  40 ). To this end, the controller  114  may be communicatively coupled to other control components of the vapor compression system  14  and/or HVAC&amp;R system  10  that regulate system operations. Based on the indication, the controller  114  may instruct the actuator  112  to adjust the position of the VGD  110  to the home position  138  in the first mode or stage of the actuator  112  (e.g., using the first or large force). Once the VGD  110  reaches the home position  138 , the controller  114  may instruct the actuator  112  to adjust the position of the VGD  110  from the home position  138  to a position in contact with the compressor discharge plate  116  in the second mode or stage of the actuator  112  (e.g., using the second or small force). As discussed further below, once the VGD  110  is in sufficient contact with the compressor discharge plate  116 , the controller  116  may instruct the actuator  112  to maintain the position of the VGD  110  against the compressor discharge plate  116  to block or prevent refrigerant flow through the discharge passage  106 . For example, the actuator  112  may maintain the position of the VGD  110  in contact with the compressor discharge plate  116  until a command to operate the compressor  32  or to unblock the diffuser passage  106  is received by the controller  114  (e.g., from the control board  40 ). 
       FIG.  8    is a schematic of the flow control system  94  including the controller  114 , the actuator  112 , and the VGD  110  and illustrating additional features that may be incorporated with systems utilizing the disclosed techniques. For example, the actuator  112  includes a sensor  150  and a locking system  152 . The sensor  150  is configured to detect an operating parameter of the actuator  112  and may communicate the feedback indicative of the operating parameter to the controller  114 . For example, in one embodiment, the actuator  112  may be an electromechanical motor, and the sensor  150  may be configured to detect a torque acting on the motor (e.g., acting on a shaft of the motor coupled to the VGD  110 ). The controller  114  may reference the torque feedback from the sensor  150  to determine when the VGD  110  is positioned in sufficient contact with the compressor discharge plate  116  to block refrigerant flow through the discharge passage  106 . As discussed above, the controller  114  may also be configured to receive input and/or feedback from other components (e.g., control board  40 ) and may operate the actuator  112  based on the feedback. In some embodiments, the input and/or feedback may be indicative of an operating mode or capacity of the compressor  32 , vapor compression system  14 , and/or HVAC&amp;R system  10 . 
     When the controller  114  determines that the VGD  110  is positioned in sufficient contact with the compressor discharge plate  116  (e.g., based on feedback from the sensor  150 ), the controller  114  may instruct the actuator  112  to activate the locking system  152  to maintain the position of the VGD  110  within the diffuser passage  106  and may discontinue operation of the actuator  112  to move the VGD  110 . In some embodiments, the locking system  152  may include a mechanical locking system configured to maintain a position of the actuator  112  and the VGD  110 . The mechanical locking system may include, for example, a mechanical interlocking device, a key, a pin, a tapered ring, a spring lock, a brake mechanism, a piston, another suitable locking device, or any combination thereof. In some embodiments, the locking system  152  may include an electric locking system configured to block electrical power supplied to the actuator  112  and thereby retain a position of the actuator  112  and the VGD  110 . Other embodiments of the locking system  152  may include additional or alternative components, such as a pneumatic lock, a hydraulic lock, a magnetic lock, an electromechanical lock, or any combination thereof. 
     It should be appreciated that embodiments in accordance with the present techniques may utilize additional and/or alternative sensors  150  configured to provide feedback to the controller  114 . For example, the flow control system  94  may include sensors  150 , such as position sensors, current sensors, temperature sensors, pressure sensors, flow rate sensors, contact sensors or other sensors to enable the functionality described above. In some embodiments, one or more sensors  150  may be coupled to other components of the vapor compression system  14  and/or disposed in other locations along or within refrigerant circuit  80 . 
     As discussed above, embodiments of the present disclosure are directed to systems and methods for utilizing a variable geometry diffuser (VGD) as a flow check valve in a compressor to substantially reduce, block, or prevent undesirable refrigerant flow across the compressor and thereby mitigate spin and/or backspin of the compressor. Embodiments include an actuator configured to operate in multiple modes to actuate and move the VGD within a diffuser passage of the compressor, and the mode of operation may be based on an operational state of the compressor and/or based on a position of the VGD within the diffuser passage. The actuator may operate in a first mode when the compressor is operating in order to move the VGD within the diffuser passage and adjust a size of a flow path through the diffuser passage, such as for surge or capacity control of the compressor. The control system may operate the actuator in a second mode when the compressor is not operating in order to move the VGD within the diffuser passage and abut an opposing surface of the diffuser passage, thereby substantially completely blocking or closing the flow path through the diffuser passage. Thus, the disclosed systems and methods enable the use of the VGD to block or prevent refrigerant flow through the compressor so as to reduce spin and/or backspin of the compressor when the compressor is not operating. 
     While only certain features and embodiments of the disclosure have been illustrated and described, many modifications and changes may occur to those skilled in the art, such as variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, including temperatures and pressures, mounting arrangements, use of materials, colors, orientations, and so forth without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described, such as those unrelated to the presently contemplated best mode of carrying out the disclosure, or those unrelated to enabling the claimed disclosure. It should be noted that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function]... ” or “step for [perform]ing [a function]... ”, it is intended that such elements are to be interpreted under 35 U.S.C. 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. 112(f).